PIEZOELECTRIC AND ELECTROSTRICTIVE EFFECTS IN CERAMIC MATERIALS Piezoelectricity, first discovered in Rochelle salt by Jacques and Pierre Curie, is the term used to describe the ability
Trang 20 100 200 300 4000
102030
Figure 13 Time course of the EOT (neffl) of a p-type PSi chip
etched at 440 mA/cm 2 , oxidized by ozone for 20 min, and
function-alized as shown in Scheme 2 a The arrow labeled A identifies the
addition of 10µM streptavidin preincubated in 1 mM biotin
dis-solved in PBS buffer, pH 7.4 (control); B addition of 10µM
strepta-vidin without biotin (washing cycles in between); C washing cycles
with buffer; D addition of dithiothreitol, which was used to reduce
the disulfide bridge and therefore release the bound protein–linker
complex The sample was mounted in a flow cell using a constant
flow rate of 0.5 mL/min [reprinted with permission from (59)].
the silicon walls Using an ethanol–water mixture
in-stead of the protein solution results in a rectangular
sig-nal response upon adding the mixture and rinsing with
water
Specific binding of streptavidin to the
biotin-func-tionalized PSi matrix was measured by monitoring the
changes in EOT time-resolved in a PBS buffer containing
Figure 14 Binding curve (change in
EOT) on a PSi surface functionalized as
shown in Scheme 2b Sequential
addi-tion of streptavidin (1 mg/mL),
biotin-ylated protein A (2.5 mg/mL), and
human IgG (2.5 mg/mL) Reversible
binding of IgG was demonstrated by
binding of IgG followed by a pH-induced
release and a second binding of IgG
to the immobilized protein A layer
[reprinted with permission from (60)].
01020304050607080
t (nm)
Streptavidin b-Protein A IgG Rinse IgG Rinse
As expected, specific binding of streptavidin to thebiotin-derivatized porous layer resulted in an increase inthe measured effective optical thickness The change in theEOT is due to binding of proteins that have a higher refrac-
in the pores and is in direct quantitative agreement withwhat was expected from effective medium approximations
23 nm In a control experiment, in which all streptavidinbinding sites were deactivated by saturating them withbiotin in solution, a change in EOT was not observed, sug-gesting that there is little or no nonspecific protein adsorp-tion to the PSi matrix Rinsing the surface with buffer afterthe protein has bound does not alter the EOT significantly.However, because the biotin recognition element is linked
to the surface via a disulfide bond, the protein–ligand plex could be released from the surface by adding dithio-threitol to the bulk phase The initial red shift of 23 nmupon binding streptavidin to the biotinylated PSi can becompletely reversed and provides further support for theinterpretation that the observed red shift is due to specificbinding of the protein to the functionalized surface More-over, the reversible linkage of the proteins via disulfidebridges to the surface offers the possibility of reusing thefunctionalized PSi chips for further binding experiments.Sailor and co-workers bound protein A to the PSi surfacethrough the BSA-containing linker (60,61) Streptavidinbinds to the biotin-terminated linker and adds three acces-sible free biotin-binding sites to the surface (Fig 14).Adding a solution of biotinylated protein A results inattaching it to the surface This prefunctionalized surfacecan be used for binding studies of aqueous human IgG Theobserved change in EOT for binding IgG required severalminutes to reach a steady-state value, presumably due toslow diffusion of this large molecule into the pores of thePSi film The proteinA/IgG complex was partly dissociated
com-by rinsing with buffer and completely dissociated com-by a pHswitch to a low pH Protonation of the binding sites on
Trang 3protein A by decreasing the pH of the solution releases
IgG from protein A A second binding of IgG after its
re-lease can be demonstrated that shows the reproducibility
of the method The incorporation of BSA in the linker
of-fered two advantages Due to the increased hydrophilicity
of the chemically modified PSi, surface nonspecific
adsorp-tion was not observed, and the addiadsorp-tion of detergent in the
buffer was no longer necessary A second reason for
incor-porating BSA in the linker was to separate binding sites in
the PSi films Sailor and co-workers (61) found that without
BSA the sensor did not scale with the mass of analyte, as
was expected, assuming the same refractive index for all
proteins investigated Larger analytes were consistently
underestimated, indicating crowding of binding sites at the
surface The insertion of BSA in the linker avoided
crowd-ing and thus, the sensor scaled with the analyte mass above
20 kDa (60)
Optical Transduction—Ellipsometry
Optical biosensing is usually based on the interaction
of light with biomolecules Techniques such as surface
plasmon resonance and ellipsometry have focused mostly
on interactions on a macromolecular scale, for example,
antigen–antibody and nucleic acid interactions The optical
detection of small molecules (0.2–2 kDa) that have
biologi-cal receptors is much more difficult due to their small
change in EOT Mandenius and co-workers (64)
demon-strated the advantage of using oxidized PSi as a surface
enlargement for binding small receptor molecules such as
biotin or small peptides They used p-type silicon that had
samples were thermally oxidized to stabilize the porous
structure The PSi surface was functionalized by using
streptavidin, either physisorbed on the silica surface or
cross-linked via glutardialdehyde Streptavidin adsorption
monitored by ellipsometry showed a 10-fold larger
re-sponse compared to a planar surface However, the rate of
adsorption was one order of magnitude lower, probably due
to the long diffusion time of the protein within the pores
Theoretically, the refractive index and the thickness of a
thin layer can be calculated from the measured
parame-tersψ (the ratio of the amplitude change of light polarized
(the phase shift) For PSi, however, the microstructure of
the porous layer is very complicated, and a simple optical
model that allowing quantifying film thickness and surface
concentration is not straightforward to define Therefore,
direct measure of analyte binding without quantification
Using this setup, they detected binding of biotin and an
re-sponse time of 30 s for the oligopeptide at a concentration
CONCLUSIONS
Porous silicon based biosensors may add a new dimension
to conventional technologies due to their unique optical and
electronic properties Tunable properties such as pore size,
porosity, dielectric function, and thickness render poroussilicon a versatile matrix for biological compounds that act
as the receptive layer for molecular recognition of analytes
in solution Interferometry has been successfully employed
to detect changes in the effective optical thickness upon sorption of molecules on the pore walls The large surfacearea of porous silicon that displays a spongelike appear-ance or exhibits ordered cylindrical pores provides a quasithree-dimensional space that increases the signal-to-noiseratio of many transducing principles
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Trang 5In a rapidly developing world, the use of smart materials
becomes increasingly important when executing
sophis-ticated functions within a designed device In a common
definition (1), smart materials differ from ordinary
mate-rials because they can perform two or several functions,
sometimes with a useful correlation or feedback
mecha-nism between them For piezoelectric or electrostrictive
materials, this means that the same component may be
used for both sensor and actuator functions
Piezoelec-tric/electrostrictive sensors convert a mechanical variable
(displacement or force) into a measurable electrical
quan-tity by the piezoelectric/electrostrictive effect Alternately,
the actuator converts an electrical signal into a useful
displacement or force Typically, the term transducer is
used to describe a component that serves actuator
(trans-mitting) and sensor (receiving) functions Because
piezo-electrics and electrostrictors inherently possess both direct
(sensor) and converse (actuator) effects, they can be
consid-ered smart materials The degree of smartness can vary
in piezoelectric/electrostrictive materials A merely smart
material (only sensor and actuator functions) can often be
engineered into a “very smart” tunable device or further,
into an “intelligent structure” whose sensor and actuator
functions are intercorrelated with an integrated
process-ing chip
Recent growth in the transducer market has been
rapid and, it is predicted will continue on its current
pace through the turn of the century The sensor market
alone rose to $5 billion in 1990, and projections are
$13 billion worldwide by the year 2000 and an 8% annual
growth rate during the following decade (2) Piezoelectric/
electrostrictive sensors and actuators comprise a
signifi-cant portion of the transducer market There is a growing
trend due especially to automobile production, active
vibration damping, and medical imaging In this article,
the principles of piezoelectric/electrostrictive sensors and
actuators are considered along with the properties of the
most useful materials and examples of successful devices
PIEZOELECTRIC AND ELECTROSTRICTIVE EFFECTS
IN CERAMIC MATERIALS
Piezoelectricity, first discovered in Rochelle salt by Jacques
and Pierre Curie, is the term used to describe the ability of
certain crystals to develop an electric charge that is directly
proportional to an applied mechanical stress (Fig 1a) (3).Piezoelectric crystals also show the converse effect: theydeform (strain) proportionally to an applied electric field(Fig 1b) To exhibit piezoelectricity, a crystal should belong
to one of the twenty noncentrosymmetric crystallographicclasses An important subgroup of piezoelectric crystals isferroelectrics, which possess a mean dipole moment perunit cell (spontaneous polarization) that can be reversed
by an external electric field Above a certain temperature(Curie point), most ferroelectrics lose their ferroelectricand piezoelectric properties and become paraelectrics, that
is, crystals that have centrosymmetric crystallographicstructures do not spontaneously polarize Electrostriction
is a second-order effect that refers to the ability of all rials to deform under an applied electrical field The phe-nomenological master equation (in tensor notation) thatdescribes the deformations of an insulating crystal sub-jected to both an elastic stress and an electrical field is
mate-x i j = s i jkl X kl + d mi j E m + M mni j E m E n ,
i , j, k, l, m, n = 1, 2, 3, (1)
M mni j are the electrostrictive moduli, and E m and E n arethe components of the external electrical field Here, theEinstein summation rule is used for repeating indexes.Typically, the electrostriction term (∝ Em E n) is more than
an order of magnitude smaller than the piezoelectric term
in Eq (1), that is, the electrostrictive deformations aremuch smaller than the piezoelectric strains In this case,under zero stress, Eq (1) simply transforms to
x i j ≈ d mi j E m , i, j, m = 1, 2, 3. (2a)
relationship in matrix notation (4) expressed as
x i ≈ d mi E m , m = 1, 2, 3,
i = 1, 2, 3, 4, 5, 6,
(2b)
perpen-dicular to the crystal axis resulting from application ofthe electrical field Equations (2a) and (2b) describe theconverse piezoelectric effect where the electrical fieldinduces a change in the dimensions of the sample (Fig 1b).The piezoelectric effect is absent in centrosymmetricmaterials, and the elastic strain is due only to electrostric-tion In ferroelectric crystals that have a centrosymmetricparaelectric phase, the piezoelectric and electrostrictioncoefficients can be described in terms of their polarizationand relative permittivity For example, when the electricalfield and deformation are along the orthogonal axis in a
139
Trang 6−Voltage
+
−Charge
ForceP
Figure 1 Schematic representations of the direct and converse
piezoelectric effect: (a) an electric field applied to the material
changes its shape; (b) a stress on the material yields an electric
field across it.
described in matrix notation as follows (5):
M11 = Q11(ε0ε33)2, (3b)
electrostriction coefficient, which couples longitudinal
strain and polarization (in matrix notation), as described
by the general electrostriction equation,
In matrix notation, the mathematical definition of the
direct piezoelectric effect, where applied elastic stress
causes a charge to build on the major surfaces of the
piezo-electric crystal, is given by
P i = d i j X j , = 1, 2, 3,
where P iis the component of electrical polarization In
elec-trostriction (centrosymmetric crystals), no charge appears
on the surface of the crystal upon stressing Therefore, the
converse electrostriction effect is simply a change of the
inverse relative permittivity under mechanical stress:
1
ε0ε33
The piezoelectric and electrostrictive effects were
de-scribed for single crystals in which spontaneous
polari-zation is homogeneous A technologically important class
of materials is piezoelectric and electrostrictive ceramics,
that consist of randomly oriented grains, separated by
grain boundaries Ceramics are much less expensive to
process than single crystals and typically offer
compa-rable piezoelectric and electrostrictive properties The
piezoelectric effect of individual grains in nonferroelectric
P
PE
Figure 2 Schematic of the longitudinal (a), transverse (a) and
shear deformations (b) of the piezoelectric ceramic material under
an applied electric field.
ceramics is canceled by averaging across the entire ple, and the whole structure has a macroscopic center
sam-of symmetry that has negligible piezoelectric properties.Only electrostriction can be observed in such ceramics Sin-tered ferroelectric ceramics consist of regions that have dif-ferent orientations of spontaneous polarization—so-calledferroelectric domains Domains appear when a material
is cooled through the Curie point to minimize the static and elastic energy of the system Domain boundaries
electro-or domain walls are movable in an applied electric field,
so the ferroelectric can be poled For example, domainsbecome oriented in a crystallographic direction closest tothe direction of the applied electric field Typically, poling isperformed under high electric field at an elevated tempera-ture to facilitate domain alignment As a result, an initiallycentrosymmetric ceramic sample loses its inversion cen-
three independent piezoelectric coefficients: d33, d31, and
defor-mations, respectively, to the applied electric field (Fig 2).Other material coefficients that are frequently used tocharacterize the piezoelectric properties of ceramics are the
piezoelectric voltage coefficients g i j, which are defined inmatrix notation as
where E iare components of the electric field that arise from
external stresses X j The piezoelectric charge d i jand
volt-age g i jcoefficients are related by the following equation:
Trang 7An important property of piezoelectric and
electrostric-tive transducers is their electromechanical coupling
piezoelectric material in converting electrical energy into
mechanical energy and vice versa Energy conversion is
never complete, so the coupling coefficient is always less
than unity
MEASUREMENTS OF PIEZOELECTRIC
AND ELECTROSTRICTIVE EFFECTS
Different means have been developed to characterize the
piezoelectric and electrostrictive properties of ceramic
ma-terials The resonance technique involves measuring
char-acteristic resonance frequencies when a suitably shaped
specimen is driven by a sinusoidal electric field To a first
approximation, the behavior of a poled ceramic sample
close to its fundamental resonance frequency can be
rep-resented by an equivalent circuit, as shown in Fig 3a The
schematic behavior of the reactance of the sample as a
function of frequency is shown in Fig 3b The equations
used to calculate the electromechanical properties are
de-scribed in the IEEE Standard on piezoelectricity (6) The
simplest example of a piezoelectric measurement by the
resonance technique can be shown by using a ceramic rod
(typically 6 mm in diameter and 15 mm long) poled along
this configuration is expressed as a function of the
funda-mental series and parallel resonance frequencies fsand fp,
Frequencyy
fa
frL
Figure 3 (a) Equivalent circuit of the piezoelectric sample near
its fundamental electromechanical resonance (top branch
repre-sents the mechanical part and bottom branch reprerepre-sents the
elec-trical part of the circuit); (b) elecelec-trical reactance of the sample as
a function of frequency.
respectively:
k33 = (π/2)( fs/fp) tan[(π/2)( fp − fs)/2]. (10)
cal-culated using k33, the elastic compliance s33, and the frequency relative permitivityε33:
low-d33 = k33(ε33s33)1/2 (11)Similarly, other coupling coefficients and piezoelectricmoduli can be derived using different vibration modes
of the same ceramic sample The disadvantage of theresonance technique is that measurements are limited tospecific frequencies determined by the electromechanicalresonance Resonance measurements are difficult for elec-trostrictive samples due to the required application of astrong dc bias field to induce a piezoelectric effect in re-laxor ferroelectrics (see next section of the article).Subresonance techniques are often used to evaluate thepiezoelectric properties of ceramic materials at frequenciesmuch lower than their fundamental resonance frequencies.These include the measurement of piezoelectric chargeupon the application of a mechanical force (direct piezoelec-tric effect) and the measurement of electric-field-induceddisplacement (converse piezoelectric effect) when an elec-tric field is introduced It has been shown that piezoelectriccoefficients obtained by direct and converse piezoelectriceffects are thermodynamically equivalent
The electrostrictive properties of ceramics are easily termined by measuring displacement as a function of the
de-electric field or polarization Thus the M and Q
electrostric-tive coefficients can be evaluated according to Eqs (1) and(4), respectively As an alternative, Eqs (3b) and (6) canalso be used for electrostriction measurements
A direct technique is widely used to evaluate the sensorcapabilities of piezoelectric and electrostrictive materials
at sufficiently low frequencies Mechanical deformationscan be applied in different directions to obtain differentcomponents of the piezoelectric and electrostrictive ten-sors In the simplest case, metal electrodes are placed onthe major surfaces of a piezoelectric sample normal to itspoling direction (Fig 1b) Thus, the charge produced onthe electrodes with respect to the mechanical load is pro-
The charge can be measured by a charge amplifier using
an etalon capacitor in the feedback loop Details of directpiezoelectric measurements can be found in a number oftextbooks (7)
Electric-field-induced displacements can be measured
by a number of techniques, including strain gauges, ear variable differential transformers (LVDT), the capaci-tance method, fiber-optic sensors, and laser interferome-try Metal wire strain gauges are the most popular sensors
perform the measurement, the strain gauge is glued to theceramic sample, and the resistance of the gauge changesaccording to its deformation The resistance variation ismeasured by a precise potentiometer up to a frequency ofseveral MHz However, several gauges need to be used toobtain a complete set of piezoelectric and electrostrictivecoefficients of the sample
Trang 8Secondarycoils
Primarycoil
Vin
Vout
Figure 4 Principle of the linear variable differential transformer
(LVDT) used for measuring electric-field-induced deformations in
a piezolectric sample.
Figure 4 illustrates the design of an LVDT The
mov-ing surface of the sample is attached to the magnetic core
inserted into the center of the primary and secondary
elec-tromagnetic coils The change of the core position varies
the mutual inductance of the coils An ac current supplies
the primary coil, and the signal in the secondary coils is
proportional to the displacement of the core The response
speed depends on the frequency of the ac signal and the
mechanical resonance of the coil, which typically does not
exceed 100 Hz Generally the resolution is sufficiently high
of turns
The capacitive technique for strain measurements is
based on the change of capacitance in a parallel-plate
ca-pacitor that has an air gap between two opposite plates
One of the plates is rigidly connected to the moving
sur-face of the sample, and another plate is fixed by the holder
The capacitance change due to the vibration of sample
can be measured precisely by a zero-point potentiometer
and a lock-in amplifier Therefore, high resolution (in the
Å range) can be achieved by this technique The
mea-surement frequency must be much lower than the
fre-quency of the ac input signal, which typically does not
exceed 100 Hz
All of the aforementioned techniques require
mechan-ical contact between the sample and the measurement
unit This, however, limits the resolution and the
maxi-mum operating frequency, which prevents accurate
mea-surement of piezoelectric loss (the phase angle between
the driving voltage and the displacement) The force
ex-erted on the moving surface of the sample (especially on
a thin ceramic film) may damage the sample Therefore,
noncontact measurements are often preferred to determine
the electric-field-induced displacement of piezoelectric and
electrostrictive materials accurately Figure 5 shows the
operating principle of a Photonic fiber-optic sensor, which
can be used to examine the displacement of a flat reflecting
Lamp
Target surface
Probe
Photodetector
Opticalfibers
Gap
Figure 5 Schematic of the fiber-optic photonic sensor used for
nondestructive evaluation of electric-field-induced strains.
surface (8) The sensor head consists of a group of mitting and receiving optical fibers located in the immedi-ate vicinity of the vibrating surface of sample The inten-sity of the reflected light depends on the distance betweenthe moving object and the probe tip This dependence al-lows exact determination of displacement in both dc and
trans-ac modes Using a lock-in amplifier to magnify the outputsignal, which is proportional to the light intensity, a reso-lution of the order of 1 Å can be achieved (8) The frequencyresponse is determined by the frequency band of the pho-todiode and the amplifier (typically of the order of severalhundreds of kHz)
Optical interferometry is another technique that lows noncontact accurate measurement of the electric-field-induced displacements Interferometric methods ofmeasuring small displacements include the homodyne (9),heterodyne (10), and Fabri–Perot (11) techniques The mostcommon technique is the homodyne interferometer thatuses active stabilization of the working point to preventdrift from thermal expansion When two laser beams of the
optical path length between the two beams If one of thebeams is reflected from the surface of a moving object, theintensity of the output light changes, which can later betranslated to the amount of displacement Using a sim-ple Michelson interferometer (12), a very high resolution
limited to a narrow frequency range because the sample
is attached to a rigid substrate and only the displacement
of the front surface of the sample is monitored (12) As
a result of this configuration, the errors arising from thebending effect of the sample can be very high, especially inferroelectric thin films In response to that, a double beam(Mach–Zender) interferometer is used to take into accountthe difference of the displacements of both major surfaces
of the sample (13) The modified version of the double-beam
Trang 9interferometer, specially adapted to measure thin films,
of 10–105Hz and long-term stability (<1%) (14).
COMMON PIEZOELECTRIC AND ELECTROSTRICTIVE
MATERIALS
Single Crystals
A number of single crystals (ferroelectric and
nonferroelec-tric) have demonstrated piezoelectricity However,
nonfer-roelectric piezoelectric crystals exhibit piezoelectric
coeffi-cients much lower than those of ferroelectric crystals The
former are still extensively used in some applications in
which either high temperature stability or low loss is
re-quired The most important nonferroelectric piezoelectric
used mostly in surface acoustic wave (SAW) devices
Re-cent investigations (15) have shown that rhombohedral
single crystals in the Pb(Zn1/3Nb2/3)O3–PbTiO3 system
addi-tion, ultrahigh strain of 1.7% has been observed in these
materials under high electric field These single crystals
are now being intensively investigated and show
signifi-cant promise for future generations of smart materials
Piezoelectric and Electrostrictive Ceramics
As indicated earlier, the randomness of the grains in
as-prepared polycrystalline ferroelectric ceramics yields
non-piezoelectric centrosymmetric material Thus “poling” the
ceramic (Fig 6) is required to induce piezoelectricity Due
to symmetry limitations, all of the domains in a ceramic
can never be fully aligned along the poling axis However,
the end result is a ceramic whose net polarization along the
poling axis has sufficiently high piezoelectric properties
The largest class of piezoelectric ceramics is made up
of mixed oxides that contain corner-sharing octahedra of
in this class are perovskites that have the general
ceramics that have this structure are barium titanate
Unpoled
Ep
Poled
Figure 6 Schematic of the poling process in piezoelectric
ceram-ics: (a) in the absence of an electric field, the domains have random
orientation of polarization; (b) the polarization within the domains
are aligned in the direction of the electric field.
titanate {Pb1−xLax(ZryT1−y)1−x/4O3, or PLZT}, and leadmagnesium niobate{PbMg1/3Nb2/3O3, or PMN}
in the 1940s (3), and it became the first recognizable
Ti4 +) coincides with the center of the negative charge (O2 −)(Fig 7a) When cooled below the Curie point, a tetragonalstructure (Fig 7b) develops where the center of positive
as a capacitor
Lead titanate (PbTiO3) first reported to be ferroelectric
through the Curie temperature, the grains go through a bic to tetragonal phase change that leads to a large strainwhich causes the ceramic to fracture Thus, it is difficult tofabricate pure lead titanate in bulk form This spontaneousstrain has been decreased by adding dopants such as Ca,
coefficient (d33) of 65 pC/N Because of its high piezoelectriccoefficient and low relative permittivity, the voltage piezo-electric coefficient of lead titanate ceramic is exceptionallyhigh Therefore, lead titanate is used in hydrophones andsonobuoys (17)
Lead zirconate titanate (PZT) is a binary solid solution
sites PZT has a temperature-independent morphotropicphase boundary (MPB) between tetragonal and rhom-bohedral phases, when the Zr:Ti ratio is 52:48 (Fig 8).This composition of PZT has efficient poling and excellentpiezoelectric properties because of its large number ofpolarization orientations At the MPB composition, PZT
is usually doped by a variety of ions to form what areknown as “hard” and “soft” PZTs (3) Doping PZT with
Al3 +, or Mn3 +at the B site, creates hard PZT This dopingreduces the piezoelectric properties and makes the PZT
Trang 10FR (LT)
FR(IIT)
8 DomainStates <111>
FT
6 DomainStates < 100 >
Figure 8 Phase diagram of lead zirconate titanate piezoelectric
ceramics (PZT) as a function of mole% PbTiO 3
more resistant to poling and depoling Introducing donor
the B site, makes soft PZT This doping increases the
piezoelectric properties and makes the PZT easier to pole
and depole Table 1 compares the piezoelectric properties
of several major piezoelectric ceramics
perovskite ceramic known as a relaxor ferroelectric Unlike
normal ferroelectrics, which have well-defined Curie points
in their weak-field relative permittivity, relaxor
ferro-electrics exhibit a broad transition peak between
ferroelec-tric and paraelecferroelec-tric phases (18) This kind of transition is
often referred to as a diffuse phase transition The
dis-tinctive features of relaxor ferroelectrics are their strong
frequency dispersion of relative permittivity and a shift of
their maximum relative permittivity with frequency
Lo-cal inhomogeneity of B site ions (e.g., Mg2 +and Nb5 +) in
the perovskite lattice are the proposed cause of relaxor
properties Relaxors do not possess piezoelectricity
with-out a dc bias field to break the paraelectric cubic phase into
the rhombohedral ferroelectric piezoelectric phase
Relax-ors have been used as actuatRelax-ors because of their negligible
hysteresis and large induced polarization (electrostrictive
strain of the order of 10−3) Figure 9 compares the
electric-field-induced strains of typical piezoelectric (PZT) and
elec-trostrictive (PMN) ceramics
Processing of Piezoelectric Ceramics
The electromechanical properties of piezoelectric
ceram-ics are largely influenced by their processing conditions
Table 1 Piezoelectric Properties of Major Piezoelectric Ceramics
Quartz BaTiO3 PZT-4 PZT-5 PbTiO3:Sm
2
S3/10− 3
20 E3/ kVcm− 1
Figure 9 Comparison of the electric-field-induced strain in a
typical piezoelectric (PZT) and relaxor (0.9PMN–0.1PT).
Each step of the process must be carefully controlled toyield the best product Figure 10 is a flowchart of a typi-cal oxide manufacturing process for piezoelectric ceramics.First, high purity raw materials are accurately weighed ac-cording to their desired ratio and then are mechanically orchemically mixed During the calcination step, the solidphases react to form the piezoelectric phase After calcina-tion, the solid mixture is milled to fine particles Shaping
is accomplished by a variety of ceramic processing niques, including powder compaction, tape casting, slipcasting, and extrusion During the shaping operation, or-ganic materials are typically added to the ceramic powder
tech-to improve its flow and binding characteristics The organic
step After organic removal, the ceramic structure is fired
to an optimum density at an elevated temperature
crucibles in an optimized PbO atmosphere to prevent leadloss above 800◦C
PIEZOELECTRIC COMPOSITES
Single-phase piezoelectric/electrostrictive materials arenot ideally suited for hydrostatic and ultrasonic applica-tions where ceramic elements radiate and receive acous-
are exceptionally high in PZT ceramics, their hydrostaticvoltage response is relatively low due to the high di-electric constant and low hydrostatic charge coefficient
dh = d33+ 2d31 Because d31≈ −0.4d33in PZT ceramics (3),their hydrostatic sensor capabilities are rather low In
Trang 11Figure 10 Flowchart for processing piezoelectric ceramics.
addition, the high density of ceramics results in a high
acoustic impedance mismatch between the transducer and
the medium in which the acoustic waves are propagating
On the other hand, piezoelectric polymers have low
den-sity (low impedance), dielectric constant, and piezoelectric
coefficients
In the past three decades, researchers have focused on
methods for combining the best characteristics of
ceram-ics and polymers to overcome the aforementioned
deficien-cies Integration of a piezoelectric ceramic with a polymer
allows tailoring the piezoelectric properties of composites
The mechanical and electrical properties of a composite
de-pend strongly on the characteristics of each phase and the
manner in which they are connected In a diphasic
compos-ite, the materials can be oriented in ten different ways in
a three-dimensional space (19) The possible connectivity
patterns are 0–0, 1–0, 2–0, 3–0, 1–1, 2–1, 3–1, 2–2, 3–2,
and 3–3 As a matter of convention, the first and second
numbers in the connectivity denote the continuity of the
piezoelectric and polymer phases, respectively Figure 11
shows some of the composites made in the past 30 years
(20) The most important connectivity patterns are 0–3,
1–3, 3–3, and 2–2 The 0–3 composites are made of a
ho-mogeneous distribution of piezoelectric ceramic particles
within a polymer matrix The primary advantage of these
composites is that they can be formed into shapes and
still retain their piezoelectricity However, they cannot be
sufficiently poled because the ceramic phase is not
self-connected in the poling direction On the other hand, 3–0
PerforatedComposite(3-1)
Ceramic-Air-PolymerComposite(1-1-3)
Particles in aPolymer(0-3)
PVDF CompositeModel(0-3)
Ceramic Rods
in a Polymer(1-3)
Diced Composite(1-3)
TransverseReinforcement(1-2-3)
PerforatedComposite(3-2)
Sheet Composite(2-2)
Moonie(3-0)
Ceramic-AirComposite(3-0)
HoneycombComposite(3-1)
ReplamineComposite(3-3)
BURPSComposite(3-3)
LadderComposite(3-3)
Figure 11 Schematic of various piezoelectric composites of different connectivities.
composites that are simply the ceramic matrix containing
a low concentration of polymer inclusions or voids can beeffectively poled and exhibit hydrostatic properties supe-rior to those of single-phase PZT (20)
In composites of 3–3 connectivity, the piezoceramic andpolymer phases are continuous in three dimensions andform two interlocking skeletons The first composite of 3–3connectivity was formed by the replamine process using
a coral skeleton (21) Another effective method of ing 3–3 composites is called BURPS (acronym for burnedout plastic spheres) (22) which provides properties similar
mak-to the replamine composites In this process, a mixture ofPZT powder and burnable plastic spheres is used to fabri-cate the PZT/polymer composites Other techniques, such
as relic processing (23) and distorted reticulated ceramics(24) have been developed to fabricate 3–3 composites Re-cently, fused deposition modeling (FDM) and fused deposi-tion of ceramics (FDC) have been used to make ladder and3-D honeycomb composites (25) In the FDM technique, a3-D plastic mold is prepared and filled with PZT slurry.The FDC process deposits a mixture of PZT and polymerdirectly in the form of a three-dimensional ladder struc-ture Either structure is heat treated to burn the organic,sintered, and embedded in epoxy polymer
The composites most extensively studied and used intransducer applications are those that have 1–3 connectivi-ties They consist of individual PZT rods or fibers aligned
in the direction parallel to poling and embedded in a mer matrix The rod diameter, spacing between them,
Trang 12poly-composite thickness, volume% of PZT, and polymer
com-pliance influence the composite’s performance The most
common methods of forming 1–3 composites are the dice
and fill technique (26) and injection molding (27) In the
former method, the composite is fabricated by dicing deep
grooves in perpendicular directions into a solid sintered
block of poled PZT The grooves are backfilled with
poly-mer, and the base is removed via grinding or cutting In
the latter method, a thermoplastic mixture of ceramic
pow-der and organic binpow-der is injected into a cooled mold The
process can be used to form composites that have a variety
of rod sizes, shapes, and spacings This technique has
re-cently been employed by Materials Systems, Inc to mass
APPLICATIONS OF PIEZOELECTRIC/
ELECTROSTRICTIVE CERAMICS
By directly coupling mechanical and electrical quantities,
piezoelectrics and electrostrictives have been extensively
used in a variety of electromechanical devices for both
sen-sor and actuator applications The direct piezoelectric
ef-fect is currently being used to generate charge (voltage)
in applications such gas igniters, acoustic pressure
sen-sors, vibration sensen-sors, accelerometers, and hydrophones
(29) The best known examples of actuators, which take
advantage of the converse effect, are piezoelectric motors,
piezoelectrically driven relays, ink-jet heads for printers,
noise cancellation systems, VCR head trackers, precise
positioners, and deformable mirrors for correcting of
op-tical images (30) Acoustic and ultrasonic vibrations can
be generated by piezoelectrics using an ac field at
res-onance conditions and/or detected by a piezoelectric
re-ceiver Very often, an acoustic sender and receiver are
com-bined in the same piezoelectric devices Transducers have a
variety of applications, including imaging, nondestructive
testing, and fish finders (31) At high frequencies,
piezo-electric transducers also function as frequency control
de-vices, bulk and surface acoustic wave (SAW) resonators,
filters, and delay lines
Ultrasonic transducers operate in a so-called
pulse-echo mode, where a transducer sends an acoustic wave
that is reflected from the interfaces and is received by
the very same transducer These echoes vary in
inten-sity according to the type of interface, which may
in-clude tissue and bone Therefore, the ultrasonic image
that is created clearly represents the mechanical
prop-erties of human tissue Thus, anatomic structures of
dif-ferent organs can be recognized in real time A
sensi-tive ultrasonic transducer that generates low-intensity
acoustic waves can be one of the safest diagnostic
de-vices for medical imaging These transducers are usually
composed of matching and backing layers and the
piezo-electric material itself The matching layers are added
to the transducer to reduce the acoustic impedance
mis-match between the imaged object and the transducer,
and the backing layers dampen the acoustic backwaves
Composite materials instead of single phase materials
are frequently used to increase the performance of
trans-ducers (20)
Simple structures
PiezoelectricMetal
Structures withstrain amplification
BrassDispl
Displ
Displ
Figure 12 Typical actuator designs: simple structures [(a)–(d)]
and structures with strain amplification [(e)–(g)].
When a transducer function is to displace an object, it iscalled an actuator It is desirable for an actuator to generate
a significant displacement and/or generative force under amoderate electric field In addition, actuators must havereproducible displacements when precise positioning is im-portant Thus, electrostrictive materials such as PMN orits solid solution with PT are preferred over PZT materialsdue to their small hysteresis Figure 12 shows several pos-sible designs of piezoelectric/electrostrictive actuators Insimple structures, like those shown in Fig 12a–d, the ac-
tuator displacements are solely due to d33, d31, or d15effects
of the ceramic rod, plate, or tube Because strain is limited
to 10−3, the typical displacement of a 1-cm long actuator is
∼10 µm Multilayer actuators (Fig 12b) use a parallel
con-nection of ceramic plates cemented together In this case,the displacements of many individual sheets of a piezo-electric ceramic are summed The advantage of multilayeracuators is their small operating voltage, fast speed, andlarge generative force A useful design is the piezoelectrictube (Fig 12c) which is poled and driven by the voltageapplied in a radial direction (through the wall width) The
and is proportional to the length/width ratio The radialresponse can be tuned to almost zero by manipulating thegeometry of the tube (32) This configuration is beneficial insuppressing unwanted lateral displacements Another im-portant design is a shear actuator (Fig 12d) which directlytransforms the voltage applied normal to the polarization
vector into a pure rotation due to the d15coefficient (33)
As previously indicated, all of the simple structures are
based on pure d31, d33, or d15 actions, so that ments are limited to tens of microns The amplification ofstrain at the expense of generative force can be achieved
displace-by using monomorph and bimorph structures (Fig 12e–f).These types of actuators produce large displacements (up
to several mm) but have low generative force and slowresponse Another type of strain amplification can beachieved by flextensional transducers One of the designs,
Trang 13SensorBrass
PZT
Brass
Feedbackloop
Figure 13 Example of the smart system using a PZT sensor
in-corporated in the MOONE actuator (36).
called MOONE, is shown in Fig 12g (34) This type of
ac-tuator uses the bending effect of the moon-shaped
metal-lic cap attached to both sides of a multilayer actuator
the metallic cap Other examples of flextensional
actua-tors are RAINBOWs and CYMBALs (not shown in the
fig-ure) (35, 36) Flextensional actuators have characteristics
intermediate between multilayers and bimorphs and are
now extensively used in various actuator applications An
example of a smart structure using flextensional actuator
(MOONE) is shown in Fig 13 The actuator portion of the
device consists of the standard MOONE and a small
piezo-electric ceramic embedded in the upper cap that serves as
a sensor The sensor detects vibrations normal to the
ac-tuator surface and, via a feedback loop, sends a signal of
appropriate amplitude and phase to the actuator, so that
it effectively cancels the external vibration Potential
ap-plications of the smart structure shown in Fig 13 include
active optical systems, rotor suspension systems, and other
noise cancellation devices
Recent trends toward miniaturization have resulted in
extensive use of piezoelectric/electrostrictive materials in
microelectromechanical systems (MEMS) Because
minia-turization of bulk ceramics is limited, these materials are
used in a thin/thick film form Thin film actuators based on
the piezoelectric effect in PZT materials have been
demon-strated They include micromotors (37), acoustic
imag-ing devices (38), components for atomic force microscopes
(AFM) (39), and micropumps (40) Figure 14 shows the
de-sign of an atomic force microscope using PZT film for both
sensing and actuating functions The excitation ac voltage
signal superimposed on the actuation dc voltage is applied
to the PZT film deposited on the Si cantilever The
vibra-tional amplitude, which is sensitive to the atomic force
between the tip and investigated surface, is detected by
measuring the difference between the cantilever current
and the reference current The feedback system maintains
a constant current while scanning in the x, y plane This
system does not require optical registration of the vibration
that makes PZT-based AFM compact and it allows the
mul-tiprobe systems to be achieved Because PZT film is very
sensitive to vibrations, the vertical resolution of such an
AFM approaches that of conventional systems This
elec-tromechanically driven AFM is an excellent example of
us-ing piezoelectric ceramic thin films as smart materials
Feedback controller
Lock-in Amplifier A
PZ Tfilm
Si cantilever
SampleTube scanner(x-y scanning)Differential Current Amplifier
FeedbacksignalFrequency
synthesizer
Ref
+
Figure 14 Schematic of the AFM cantilever sensor and actuator
based on a PZT thin film.
FUTURE TRENDS
Most piezoelectric/electrostrictive ceramics currently rely
on lead oxide based materials due to their excellent erties as sensors and actuators However, due to increasedpublic awareness of health problems associated with leadand environmental protection policies, future research will
prop-be focused on finding lead-free compounds that have electric properties similar to those of PZT Relaxor single-crystal materials that have a giant piezoelectric effect willprobably find a wide range of applications from compositetransducers for medical imaging to microelectromechan-ical systems The current trend of miniaturization willcontinue to give rise to complex sensors and actuators inte-grated directly on a silicon chip Further, batch processingwill effectively reduce the cost of such devices In addi-tion, the research will continue toward the development ofmore resilient piezoelectric/electrostrictive materials usedfor operation under severe external conditions (tempera-ture, pressure, harsh chemical environments) This willfurther improve their potential application in space anddeep ocean exploration, as well as in noise cancellation inairplanes and helicopters
piezo-BIBLIOGRAPHY
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INTRODUCTION Smart Material
Let us start with the “smartness” of a material Table 1lists the various effects that relate the input—electric field,magnetic field, stress, heat and light—to the output—charge/current, magnetization, strain, temperature, andlight Conducting and elastic materials that generate cur-rent and strain outputs, respectively, from input voltage,
or stress (well-known phenomena!) are sometimes called
“trivial” materials Conversely, pyroelectric and tric materials that generate an electric field from the in-put of heat or stress (unexpected phenomena!) are called
piezoelec-“smart” materials These off-diagonal couplings have a responding converse effect such as electrocaloric and con-verse piezoelectric effects, and both “sensing” and “actu-ating” functions can be realized in the same materials
cor-“Intelligent” materials should possess a “drive/control” or
“processing” function which is adaptive to the change inenvironmental conditions, in addition to the actuation andsensing functions Note that ferroelectric materials exhibitmost of these effects, except magnetic-related phenom-ena Thus, the ferroelectrics are said to be very “smart”materials
The “actuator” in a narrow meaning stands for als or devices that generate mechanical strain (or stress)output As indicated by the thick columnar border inTable 1, solid state actuators use converse piezoelectric,magnetostriction, elasticity, thermal expansion, or photo-striction phenomena A shape-memory alloy is a kind
materi-of thermally expanding material On the other hand, a
“sensor” requires charge/current output in most cases.Thus, conducting/semiconducting, magnetoelectric, piezo-electric, pyroelectric, and photovoltaic materials are usedfor detecting electric fields, magnetic fields, stress, heat,and light, respectively (see the thin columnar border inTable 1)
In this sense, piezoelectric materials are most larly used in smart structures and systems because thesame material is applicable to both sensors and actua-tors, in principle We treat mainly piezoelectric transduc-ers, sensors, and actuators in this article Even thoughtransducers, in general, are devices that convert input en-ergy to a different energy type of output, the piezoelectric
Trang 15popu-Table 1 Various Effects in Ferroelectric and Ferromagnetic Materials
Input →MaterialDevice → Output Output Charge
Elec eld Permittivity Elect.-mag Converse Elec caloric Elec.-optic
Conductivity effect piezo-effect effect effect Mag eld Mag.-elect Permeability Magneto- Mag.caloric Mag.optic
Sensor Actuator
aOff-diagonal coupling
= Smart Material
a
“transducer” is often used to denote a device that possesses
both sensing and actuating functions, exemplified by
un-derwater sonar
Piezoelectric Effect
Certain materials produce electric charges on their
sur-faces as a consequence of applying mechanical stress
When the induced charge is proportional to the mechanical
stress, it is called a direct piezoelectric effect and was
dis-covered by J and P Curie in 1880 Materials that show this
phenomenon also conversely have a geometric strain
gen-erated that is proportional to an applied electric field This
is the converse piezoelectric effect The root of the word
“piezo” is the Greek word for “pressure”; hence the
origi-nal meaning of the word piezoelectricity implied “pressure
electricity” (1,2)
Piezoelectric materials couple electrical and mechanical
parameters The material used earliest for its piezoelectric
properties was single-crystal quartz Quartz crystal
res-onators for frequency control appear today at the heart
of clocks and are also used in TVs and computers
Ferro-electric polycrystalline ceramics, such as barium titanate
and lead zirconate titanate, exhibit piezoelectricity when
electrically poled Because these ceramics possess
signif-icant and stable piezoelectric effects, that is, high
elec-tromechanical coupling, they can produce large strains/
forces and hence are extensively used as transducers
Piezoelectric polymers, notably polyvinylidene difluoride
and its copolymers with trifluoroethylene and
piezoelec-tric composites that combine a piezoelecpiezoelec-tric ceramic and a
passive polymer have been developed and offer high
poten-tial Recently, thin films of piezoelectric materials are being
researched due to their potential use in microdevices
(sen-sors and microelectromechanical systems)
Piezoelectric-ity is being extensively used in fabricating various devices
such as transducers, sensors, actuators, surface acoustic
wave devices, and frequency controls
We describe the fundamentals of piezoelectric effectfirst, then present a brief history of piezoelectricity, fol-lowed by present day piezoelectric materials that are used,and finally various potential applications of piezoelectricmaterials are presented
PIEZOELECTRICITY Relationship Between Crystal Symmetry and Properties
All crystals can be classified into 32 point groups according
to their crystallographic symmetry These point groups aredivided into two classes; one has a center of symmetry, andthe other lacks it There are 21 noncentrosymmetric pointgroups Crystals that belong to 20 of these point groupsexhibit piezoelectricity Although cubic class 432 lacks acenter of symmetry, it does not permit piezoelectricity Ofthese 20 point groups, 10 polar crystal classes contain aunique axis, along which an electric dipole moment is ori-ented in the unstrained condition
The pyroelectric effect appears in any material that sesses a polar symmetry axis The material in this cate-gory develops an electric charge on the surface owing tothe change in dipole moment as temperature changes Thepyroelectric crystals whose spontaneous polarization arereorientable by applying an electric field of sufficient mag-nitude (not exceeding the breakdown limit of the crystal)are called ferroelectrics (3,4) Table 2 shows the crystallo-graphic classification of the point groups
pos-Piezoelectric Coefficients
Materials are deformed by stresses, and the resulting
stress X (force per unit area) causes a proportional strain x,
where all quantities are tensors, x and X are second rank, and s is fourth rank Piezoelectricity creates additional
Trang 16Table 2 Crystallographic Classification According to Crystal Centrosymmetry and Polarity
Non-Centro (11) m3m m3 6/ mmm 6/m 4/mmm 4/m 3m 3 mmm 2/m
centro (21)
Polar (pyro- electric) (10)
where E is the electric field and d is the piezoelectric
con-stant which is a third-rank tensor This equation can be
also expressed in a matrix form such as for a poled ceramic:
x1 x2 x3 x4 x5 x6
Another frequently used piezoelectric constant is g
which gives the electric field produced when a stress is
effectiveness of the electromechanical energy conversion is
the electromechanical coupling factor k, which measures
the fraction of electrical energy converted to mechanical
energy when an electric field is applied or vice versa when
a material is stressed (5):
(5)or
= (stored electrical energy/input mechanical energy).
(6)
is applied to a piezoelectric material Because the input
stored mechanical energy per unit volume under zero
ex-ternal stress is given by (1/2) x2/s = (1/2)(dE)2/s, k2 can
be calculated as
k2= [(1/2)(dE)2/s]/[(1/2)ε0ε E2
]
Note that k is always less than one Typical values of k are
ceramic and 0.1–0.3 for PVDF polymer Another important
an inverse value of mechanical loss:
is most desired for ultrasonic actuations (e.g., ultrasonicmotors) to suppress heat generation through the loss
History of Piezoelectricity
As already stated, Pierre and Jacques Curie discoveredpiezoelectricity in quartz in 1880 The discovery of ferro-electricity accelerated the creation of useful piezoelectricmaterials Rochelle salt was the first ferroelectric disco-vered in 1921 Until 1940 only two types of ferroelectricswere known, Rochelle salt and potassium dihydrogen phos-phate and its isomorph In 1940 to 1943, unusual dielectricproperties such as an abnormally high dielectric constant
by Wainer and Salmon, Ogawa, and Wul and Golman
led to improvement in temperature stability and
ceramics became well established in a number of devices
In the 1950s, Jaffe and co-workers established thatthe lead zirconate–lead titanate system (called the PZTsystem) induces strong piezoelectric effects The maximumpiezoelectric response was found for PZT compositions nearthe morphotropic phase boundary between the rhombohe-dral and tetragonal phases Since then, the PZT systemcontaining various additives has become the dominantpiezoelectric ceramic for a variety of applications The de-velopment of PZT-based ternary solid solutions was a majorsuccess of the piezoelectric industry for these applications
In 1969, Kawai et al discovered that certain polymers,notably polyvinylidene difluoride (PVDF), are piezoelec-tric when stretched during fabrication Such piezoelectricpolymers are also useful for some transducer applications
Trang 17Table 3 Material Parameters of Representative Piezoelectric Materials
Parameter Quartz BaTiO 3 PZT4 PZT 5H (Pb,Sm)TiO 3 PVDF-TrFE
In 1978, Newnham et al improved composite
piezoelec-tric materials by combining a piezoelecpiezoelec-tric ceramic and a
passive polymer whose properties can be tailored to the
requirements of various piezoelectric devices
Another class of ceramic material has recently become
important: relaxor-type electrostrictors such as lead
mag-nesium niobate (PMN), typically doped with 10% lead
ti-tanate (PT), which have potential applications in the
piezo-electric actuator field A recent breakthrough in the growth
of high-quality, large, single-crystal relaxor piezoelectric
compositions has created interest in these materials for
applications ranging from high strain actuators to
high-frequency transducers for medical ultrasound devices due
to their superior electromechanical characteristics More
recently, thin films of piezoelectric materials such as zinc
oxide (ZnO) and PZT have been extensively investigated
and developed for use in microelectromechanical (MEMS)
devices
PIEZOELECTRIC MATERIALS
This section summarizes the current status of
piezo-electric materials: single-crystal materials, piezoceramics,
piezopolymers, piezocomposites, and piezofilms Table 3
shows the material parameters of some representative
piezoelectric materials described here (6,7)
Single Crystals
Piezoelectric ceramics are widely used at present for a large
number of applications However, single-crystal materials
retain their utility; they are essential for applications such
as frequency stabilized oscillators and surface acoustic
devices The most popular single-crystal piezoelectric
general, and have different properties depending on the
cut of the materials and the direction of the bulk or
sur-face wave propagation
belongs to the triclinic crystal system with point group 32
not piezoelectric Quartz has a cut with a zero temperature
coefficient at the resonant frequency change For instance,
quartz oscillators using the thickness shear mode of the
AT-cut are extensively used as clock sources in computers
and as frequency stabilized oscillators in TVs, and VCRs
On the other hand, an ST-cut quartz substrate that has X
propagation has a zero temperature coefficient for surface
acoustic waves and so is used for SAW devices that havehighly stabilized frequencies Another distinguishing char-acteristic of quartz is its extremely high mechanical quality
factor Qm> 105.Lithium niobate and lithium tantalate belong to an iso-morphous crystal system and are composed of oxygen oc-
the ferroelectric phase of these single crystals is 3m, and
the polarization direction is along the c axis These terials have high electromechanical coupling coefficientsfor surface acoustic waves In addition, large single crys-tals can easily be obtained from their melts by using theconventional Czochralski technique Thus, both materialsare very important in SAW device applications
ma-Perovskite Ceramics
Most of the piezoelectric ceramics have the perovskite
of a simple cubic unit cell that has a large cation A at thecorner, a smaller cation B in the body center, and oxygens O
in the centers of the faces The structure is a network
of corner-linked oxygen octahedra surrounding B cations.The piezoelectric properties of perovskite-structured mate-rials can be easily tailored for applications by incorporatingvarious cations in the perovskite structure
Barium Titanate Barium titanate (BaTiO3) is one of themost thoroughly studied and most widely used piezoelec-tric materials Figure 2 shows the temperature dependence
Trang 18Figure 2 Dielectric constants of BaTiO3 as
anoma-lies can be observed The discontinuity at the Curie point
paraelectric phase The other two discontinuities are
ac-companied by transitions from one ferroelectric phase to
another Above the Curie point, the crystal structure is
cu-bic and has no spontaneous dipole moments At the Curie
point, the crystal becomes polar, and the structure changes
from a cubic to a tetragonal phase The dipole moment and
the spontaneous polarization are parallel to the
tetrago-nal axis Just below the Curie temperature, the vector of
the spontaneous polarization points in the [001] direction
(tetragonal phase), below 5◦C it reorients in the [011]
(rhombohe-dral phase) The dielectric and piezoelectric properties of
sto-ichiometry, microstructure, and by dopants entering into
contains dopants such as Pb or Ca ions have been used as
commercial piezoelectric materials
Lead Zirconate–Lead Titanate Piezoelectric Pb(Ti, Zr)O3
solid-solution (PZT) ceramics are widely used because of
their superior piezoelectric properties The phase diagram
of the PZT system (Pb(ZrxTi1 −x) O3) is shown in Fig 3 The
crystalline symmetry of this solid solution is determined
by the Zr content Lead titanate also has a tetragonal
fer-roelectric phase of the perovskite structure As the Zr
con-tent x increases, the tetragonal distortion decreases, and
tetrago-nal 4mm phase to another ferroelectric phase of
rhombohe-dral 3m symmetry Figure 4 shows the dependence of
sev-eral d constants on the composition near the morphotropic
phase boundary between the tetragonal and rhombohedral
phases The d constants have their highest values near the
morphotropic phase boundary This enhancement in the
piezoelectric effect is attributed to the increased ease of
reorientation of the polarization in an electric field
Dop-ing the PZT material with donors or acceptors changes the
properties dramatically Donor doping with ions such as
Nb5 +or Ta5 +provides soft PZTs like PZT-5, because of the
facility of domain motion due to the charge compensation
200
PbTiO3
Mole % PbZrO3
100
a ac
a
Rhombohedrala
Tetragonal Morphotropic
phaseboundery
a
Cubicaa
Figure 3 Phase diagram of the PZT system.
of the Pb vacancy which is generated during sintering Onthe other hand, acceptor doping with Fe3 +or Sc3 +leads tohard PZTs such as PZT-8 because oxygen vacancies pin thedomain wall motion
Lead Titanate PbTiO3 has a tetragonal structure at
d15
d33
Figure 4 Piezoelectric d strain coefficients versus composition
for the PZT system.
Trang 19The Curie temperature is 490◦C Densely sintered PbTiO3
ceramics cannot be obtained easily because they break
up into powders when cooled through the Curie
temper-ature This is partly due to the large spontaneous strain
that occurs at the transition Lead titanate ceramics
mod-ified by small amounts of additives exhibit high
(9) has extremely low planar coupling, that is, a large
kt/kp ratio Here, ktand kpare thickness-extensional and
planar electromechanical coupling factors, respectively
tem-perature coefficient of surface acoustic wave delay have
been developed as superior substrate materials for SAW
devices (10)
Relaxor Ferroelectrics Relaxor ferroelectrics differ from
normal ferroelectrics; they have broad phase transitions
from the paraelectric to the ferroelectric state, strong
fre-quency dependence of the dielectric constant (i.e., dielectric
relaxation), and weak remanent polarization at
tempera-tures close to the dielectric maximum Lead-based relaxor
materials have complex disordered perovskite structures
of the general formula Pb(B1, B2) O3(B1= Mg2 +, Zn2 +, Sc3 +;
B2= Nb5 +, Ta5 +, W6 +) The B-site cations are distributed
randomly in the crystal The characteristic of a relaxor is a
broad and frequency dispersive dielectric maximum In
ad-dition, relaxor-type materials such as the lead magnesium
niobate Pb(Mg1/3Nb2/3)O3–lead titanate PbTiO3 solid
so-lution (PMN–PT) exhibit electrostrictive phenomena that
are suitable for actuator applications Figure 5 shows an
electric-field-induced strain curve that was observed for
0.9 PMN–0.1 PT and reported by Cross et al in 1980 (11)
Note that a strain of 0.1% can be induced by an electric
field as small as 1 kV/mm and that hysteresis is negligibly
small for this electrostriction
Because electrostriction is the secondary
electrome-chanical coupling observed in cubic structures, in principle,
Figure 5 Field-induced electrostrictive strain in 0.9PMN–0.1PT.
the charge is not induced under applied stress The verse electrostrictive effect, which can be used for sensorapplications, means that the permittivity (the first deriva-tive of polarization with respect to an electric field) ischanged by stress
con-In relaxor ferroelectrics, the piezoelectric effect can beinduced under a bias field, that is, the electromechani-
changes As the dc bias field increases, the coupling creases and saturates These materials can be used for ul-trasonic transducers that are tunable by a bias field (12).The recent development of single-crystal piezoelectricsstarted in 1981, when Kuwata et al first reported an
solid-solution single crystals between relaxor and normalferroelectrics, Pb(Zn1/3Nb2/3)O3–PbTiO3(13) After about
10 years, Yamashita et al (Toshiba) and Shrout et al.(Penn State) independently reconfirmed that these val-ues are true, and much more improved data were ob-tained in these few years, aimed at medical acoustic ap-plications (14,15) Important data have been accumulatedfor Pb(Mg1/3Nb2/3)O3(PMN), Pb(Zn1/3Nb2/3)O3(PZN), and
(PMN–PT and PZN–PT) for actuator applications Strains
as large as 1.7% can be induced practically for a photropic phase boundary composition of PZN–PT solid-solution single crystals Figure 6 shows the field-inducedstrain curve for [001] oriented 0.92PZN–0.08PT (15) It isnotable that the highest values are observed for a rhom-bohedral composition only when the single crystal is poledalong the perovskite [001] axis, not along the [111] spon-taneous polarization axis
mor-Polymers
piezoelec-tric when stretched during fabrication Thin sheets of thecast polymer are drawn and stretched in the plane of the
00.0
60Electric field (kV/cm)
0.51.01.5
Trang 20sheet in at least one direction and frequently also in the
perpendicular direction to convert the material into its
microscopically polar phase Crystallization from a melt
dipoles are then reoriented by electric poling Large sheets
can be manufactured and thermally formed into complex
shapes Copolymerization of vinylidene difluoride with
trifluoroethylene (TrFE) results in a random copolymer
poly-mer does not need to be stretched; it can be poled directly
as formed A thickness-mode coupling coefficient of 0.30
has been reported Such piezoelectric polymers are used
for directional microphones and ultrasonic hydrophones
Composites
Piezocomposites comprised of piezoelectric ceramics and
polymers are promising materials because of excellent
tai-lored properties The geometry of two-phase composites
can be classified according to the connectivity of each phase
(0, 1, 2, or 3 dimensionality) into 10 structures; 0–0, 0–1,
0–2, 0–3, 1–1, 1–2, 1–3, 2–2, 2–3, and 3–3 (15) A 1–3
piezo-composite, or PZT-rod / polymer-matrix composite is a most
promising candidate The advantages of this composite are
high coupling factors, low acoustic impedance (square root
of the product of its density and elastic stiffness), a good
match to water and human tissue, mechanical flexibility,
a broad bandwidth in combination with a low
mechani-cal quality factor, and the possibility of making undiced
arrays by structuring only the electrodes The
thickness-mode electromechanical coupling of the composite can
almost approaches the value of the rod-mode
acoustic match to tissue or water (1.5 Mrayls) of typical
piezoceramics (20–30 Mrayls) is significantly improved by
forming a composite structure, that is, by replacing a heavy,
stiff ceramic by a light, soft polymer Piezoelectric
compo-site materials are especially useful for underwater sonar
and medical diagnostic ultrasonic transducers
Thin Films
Both zinc oxide (ZnO) and aluminum nitride (AlN) are
sim-ple binary compounds that have Wurtzite type structures,
which can be sputter-deposited in a c-axis oriented thin
film on a variety of substrates ZnO has reasonable
piezo-electric coupling, and its thin films are widely used in bulk
acoustic and surface acoustic wave devices The fabrication
of highly c-axis oriented ZnO films has been extensively
studied and developed The performance of ZnO devices is,
however, limited due to their small piezoelectric coupling
(20–30%) PZT thin films are expected to exhibit higher
piezoelectric properties At present, the growth of PZT
thin film is being carried out for use in microtransducers
and microactuators A series of theoretical calculations on
perovskite type ferroelectric crystals suggests that large
d and k values of magnitudes similar to those of PZN–PT
can also be expected in PZT Crystal orientation
depen-dence of piezoelectric properties was phenomenologically
EE
Figure 7 The principle of enhancement in electromechanical
couplings in a perovskite piezoelectric.
calculated for compositions around the morphotropic phaseboundary of PZT (17) The maximum longitudinal piezo-
the rhombohedral composition were found at angles of 57
polar-ization direction [111], which correspond roughly to theperovskite [100] axis
Figure 7 shows the principle of the enhancement in
the highest in perovskite piezoelectric crystals, the appliedfield should be canted from the spontaneous polarizationdirection to obtain the maximum strain Epitaxially grown,[001] oriented thin/thick films using a rhombohedral PZTcomposition reportedly enhance the effective piezoelectricconstant by four to five times
APPLICATIONS OF PIEZOELECTRICITY
Piezoelectric materials can provide coupling between trical and mechanical energy and thus have been exten-sively used in a variety of electromechanical devices Thedirect piezoelectric effect is most obviously used to generatecharge or high voltage in applications such as the spark ig-nition of gas in space heaters, cooking stoves, and cigarettelighters Using the converse effect, small mechanical dis-placements and vibrations can be produced in actuators
elec-by applying an electric field Acoustic and ultrasonic brations can be generated by an alternating field tuned atthe mechanical resonant frequency of a piezoelectric deviceand can be detected by amplifying the field generated byvibration incident on the material, which is usually usedfor ultrasonic transducers Another important application
vi-of piezoelectricity is frequency control The application vi-ofpiezoelectric materials ranges over many fields, includingultrasonic transducers, actuators, and ultrasonic motors;electronic components such as resonators, wave filters, de-lay lines; SAW devices and transformers and high-voltageapplications; and gas igniters, and ultrasonic cleaning andmachining Piezoelectric-based sensors, for instance, ac-celerometers, automobile knock sensors, vibration sensors,strain gages, and flow meters have been developed becausepressure and vibration can be directly sensed as electric
Trang 21signals through the piezoelectric effect Examples of these
applications are given in the following sections
Pressure Sensor/Accelerometer/Gyroscope
The gas igniter is one of the basic applications of
piezoelec-tric ceramics Very high voltage generated in a piezoelecpiezoelec-tric
ceramic under applied mechanical stress can cause
spark-ing and ignite a gas
Piezoelectric ceramics can be employed as stress sensors
and acceleration sensors, because of their “direct
piezo-electric effect.” Kistler (Switzerland) is manufacturing a
3-D stress sensor By combining an appropriate number
of quartz crystal plates (extensional and shear types), the
multilayer device can detect three-dimensional stresses
(18)
Figure 8 shows a cylindrical gyroscope commercialized
by Tokin (Japan) (19) The cylinder has six divided
elec-trodes; one pair is used to excite the fundamental bending
vibration mode, and the other two pairs are used to
de-tect acceleration When rotational acceleration is applied
around the axis of this gyro, the voltage generated on the
electrodes is modulated by the Coriolis force By
subtract-ing the signals between the two sensor electrode pairs, a
voltage directly proportional to the acceleration can be
ob-tained This type of gyroscope has been widely installed
in handheld video cameras to monitor the inevitable hand
vibration during operation and to compensate for it
elec-tronically on a display by using the sensed signal
Ultrasonic Transducer
One of the most important applications of piezoelectric
materials is based on the ultrasonic echo field (20,21)
Ultrasonic transducers convert electrical energy into a
mechanical form when generating an acoustic pulse and
convert mechanical energy into an electrical signal when
detecting its echo Nowadays, piezoelectric transducers are
being used in medical ultrasound for clinical applicationsthat range from diagnosis to therapy and surgery They arealso used for underwater detection, such as sonars and fishfinders, and nondestructive testing
Ultrasonic transducers often operate in a pulse-echomode The transducer converts electrical input into anacoustic wave output The transmitted waves propagateinto a body, and echoes are generated that travel back to
be received by the same transducer These echoes vary inintensity according to the type of tissue or body structure,and thereby create images An ultrasonic image representsthe mechanical properties of the tissue, such as density andelasticity We can recognize anatomical structures in an ul-trasonic image because the organ boundaries and fluid-to-tissue interfaces are easily discerned Ultrasonic imagingcan also be done in real time This means that we can fol-low rapidly moving structures such as the heart withoutmotional distortion In addition, ultrasound is one of thesafest diagnostic imaging techniques It does not use ion-izing radiation like X rays and thus is routinely used forfetal and obstetrical imaging Useful areas for ultrasonicimaging include cardiac structures, the vascular system,the fetus, and abdominal organs such as the liver and kid-ney In brief, it is possible to see inside the human body byusing a beam of ultrasound without breaking the skin.There are various types of transducers used in ultra-sonic imaging Mechanical sector transducers consist ofsingle, relatively large resonators that provide images bymechanical scanning such as wobbling Multiple elementarray transducers permit the imaging systems to accessdiscrete elements individually and enable electronic focus-ing in the scanning plane at various adjustable penetrationdepths by using phase delays The two basic types of arraytransducers are linear and phased (or sector) Linear arraytransducers are used for radiological and obstetrical exam-inations, and phased array transducers are useful for car-diologcal applications where positioning between the ribs
is necessary
Figure 9 shows the geometry of the basic sonic transducer The transducer is composed mainly of
ultra-Inputpulse
Trang 22matching, piezoelectric material, and backing layers (22).
One or more matching layers are used to increase sound
transmissions into tissues The backing is attached to the
transducer rear to damp the acoustic returnwave and to
re-duce the pulse duration Piezoelectric materials are used
to generate and detect ultrasound In general, broadband
transducers should be used for medical ultrasonic
imag-ing The broad bandwidth response corresponds to a short
pulse length that results in better axial resolution Three
factors are important in designing broad bandwidth
trans-ducers The first is acoustic impedance matching, that is,
effectively coupling the acoustic energy to the body The
second is high electromechanical coupling coefficient of the
transducer The third is electrical impedance matching,
that is, effectively coupling electrical energy from the
driv-ing electronics to the transducer across the frequency
range of interest The operator of pulse-echo transducers
is based on the thickness mode resonance of the
is related to the efficiency of converting electric energy into
acoustics and vice versa Further, a low planar mode
cou-pling coefficient kpis beneficial for limiting energies from
being expended in a nonproductive lateral mode A large
dielectric constant is necessary to enable a good electrical
impedance match to the system, especially in tiny
piezo-electric sizes
Table 4 compares the properties of ultrasonic
trans-ducer materials (7,23) Ferroelectric ceramics, such as lead
zirconate titanate and modified lead titanate, are almost
universally used as ultrasonic transducers The success of
ceramics is due to their very high electromechanical
cou-pling coefficients In particular, soft PZT ceramics such as
PZT-5A and 5H type compositions are most widely used
because of their exceedingly high coupling properties and
because they can be relatively easily tailored, for instance,
in the wide dielectric constant range On the other hand,
modified lead titanates such as samarium-doped
mate-rials have high piezoelectric anisotropy: the planar
to reduced interference from spurious lateral resonances
in longitudinal oscillators, this is very useful in
high-frequency array transducer applications One
disadvan-tage of PZT and other lead-based ceramics is their large
compared to body tissue (1.5 Mrayls) Single or multiple
matching layers of intermediate impedances need to be
used in PZT to improve acoustic matching
On the other hand, piezoelectric polymers, such as
poly-vinylidene difluoride-trifluoroethylene, have much lower
Table 4 Comparison of the Properties of Ultrasonic Transducer Materials
Piezoelectric ceramic/polymer composites are tives to ceramics and polymers Piezocomposites that have2–2 or 1–3 connectivity are commonly used in ultra-sonic medical applications They combine the low acousticimpedance advantage of polymers and the high sensitivityand low electrical impedance advantages of ceramics.The design frequency of a transducer depends on thepenetration depth required by the application Resolu-tion is improved as frequency increases Although a high-frequency transducer can produce a high-resolution image,higher frequency acoustic energy is more readily attenu-ated by the body A lower frequency transducer is used
alterna-as a compromise when imaging deeper structures Mostmedical ultrasound imaging systems operate in the fre-quency range from 2–10 MHz and can resolve objects ap-proximately 0.2–1 mm in size At 3.5 MHz, imaging to adepth of 10–20 cm is possible, and at 50 MHz, increasedlosses limit the depth to less than 1 cm Higher frequencytransducers (10–50 MHz) are used for endoscopic imag-ing and for catheter-based intravascular imaging Ultra-sound microscopy is being done at frequencies higher than
100 MHz The operating frequency of the transducer isdirectly related to the thickness and velocity of sound inthe piezoelectric materials employed As the frequencyincreases, resonator thickness decreases For a 3.5 MHztransducer, the PZT ceramic must be roughly 0.4 mm thick.Conventional ceramic transducers, such as PZT, are lim-ited to frequencies below 80 MHz because of the difficulty
of fabricating thinner devices (24) Piezoelectric thin-filmtransducers such as ZnO have to be used for microscopicapplications (at frequencies higher than 100 MHz, corre-
Resonator and Filter
When a piezoelectric body vibrates at its resonant quency, it absorbs considerably more energy than at otherfrequencies, resulting in a fall of the impedance This phe-nomenon enables using piezoelectric materials as wave fil-ters A filter is required to pass a certain selected frequencyband or to stop a given band The bandwidth of a filter fab-ricated from a piezoelectric material is determined by the
fre-square of the coupling coefficient k Quartz crystals that have very low k values of about 0.1 can pass very narrow
Trang 23frequency bands of approximately 1% of the center
reso-nance frequency On the other hand, PZT ceramics whose
planar coupling coefficient is about 0.5 can easily pass a
band of 10% of the center resonance frequency The
sharp-ness of the passband depends on the mechanical quality
and well-defined frequency of the oscillator
A simple resonator is a thin disk electroded on its
plane faces and vibrating radially for applications in filters
whose center frequency ranges from 200 kHz to 1 MHz and
whose bandwidth is several percent of the center frequency
The disk diameter must be about 5.6 mm for a frequency of
455 kHz However, if the required frequency is higher
than 10 MHz, other modes of vibration such as the
thick-ness extensional mode are exploited, because of its smaller
size disk Trapped-energy type filters made from PZT
ceramics have been widely used in the intermediate
fquency range, for example, 10.7 MHz for FM radio
re-ceivers and transmitters By employing the trapped-energy
phenomenon, the overtone frequencies are suppressed The
plate is partly covered by electrodes of a specific area
and thickness The fundamental frequency of the
thick-ness mode beneath the electrode is less than that of the
unelectroded portion because of the extra inertia of the
electrode mass The longer wave characteristic of the
elec-trode region cannot propagate in the unelecelec-troded region
Higher frequency overtones can propagate into the
unelec-troded region This is called the trapped-energy principle
Figure 10 shows a schematic drawing of a trapped-energy
filter In this structure, the top electrode is split so that
cou-pling between the two parts is efficient only at resonance
More stable filters suitable for telecommunication systems
have been made from single crystals such as quartz or
LiTaO3
Piezoelectric Transformer
The transfer of vibrational energy from one set of
elec-trodes to another on a piezoelectric ceramic body can be
used to transform voltage The device is called a
piezo-electric transformer Recently, office automation
equip-ment that has a liquid crystal display such as notebook
type personal computers and car navigation systems has
been successfully commercialized This equipment that
uses liquid crystal display requires a very thin
trans-former without electromagnetic noise to start the glow of
Ceramic plateElectrode
Figure 10 Trapped-energy filter.
Low voltage input
High voltage output
Figure 11 Piezoelectric transformer.
a fluorescent back-lamp This application has recently celerated the development of piezoelectric transformers.Figure 11 shows the basic structure, where two differentlypoled parts coexist in one piezoelectric plate The plate haselectrodes on half of its major faces and on an edge Theplate is then poled in its thickness direction at one endand parallel to the long axis over most of its length A low-voltage ac supply is applied to the large-area electrodes at
ac-a frequency thac-at excites ac-a length extensionac-al mode nance Then, a high-voltage output can be taken from thesmall electrode and from one of the larger electrodes Fol-lowing the proposal by Rosen mentioned before, piezoelec-tric transformers of several different structures have beenreported (26) Multilayer type transformers are proposed toincrease the voltage step-up ratio (27) The input part has
reso-a multilreso-ayer structure reso-and hreso-as internreso-al electrodes, reso-and theoutput electrodes are formed at the side surface of the half
of the rectangular plate This transformer uses the electric transverse mode for the input and output parts
piezo-SAW Device
A surface acoustic wave (SAW), also called a Rayleigh wave,
is composed of a coupling between longitudinal and shearwaves in which the SAW energy is confined near the sur-face An associated electrostatic wave exists for a SAW on
a piezoelectric substrate that allows electroacoustic pling via a transducer The advantages of SAW technologyare that a wave can be electroacoustically accessed andtrapped at the substrate surface and its velocity is approx-
The SAW wavelength is of the same order of magnitude
as line dimensions that can be photolithographically duced, and the lengths for both short and long delays areachievable on reasonably size substrates (28,29)
pro-There is a very broad range of commercial applications,including front-end and IF (intermediate frequency) filters,CATV (community antenna television) and VCR (video cas-sette recorder) components, synthesizers, analyzers, andnavigators In SAW transducers, finger electrodes providethe ability to sample or tap the wave, and the electrodegap gives the relative delay A SAW filter is composed of
a minimum of two transducers A schematic of a simpleSAW bidirectional filter is shown in Fig 12 A bidirectionaltransducer radiates energy equally from each side of thetransducer Energy not received is absorbed to eliminatespurious reflection
Various materials are currently being used for SAW vices The most popular single-crystal SAW materials arelithium niobate and lithium tantalate The materials havedifferent properties depending on their cuts and the direc-tion of propagation The fundamental parameters are the
Trang 24de-Input SAW Output
Interdigital electrodePiezoelectric substrate
⬃
Figure 12 Typical SAW bidirectional filter that consists of two
interdigital transducers.
SAW velocity, the temperature coefficient of delay (TCD),
the electromechanical coupling factor, and the propagative
loss Surface acoustic waves can be generated and detected
by spatially periodic, interdigital electrodes on the plane
surface of a piezoelectric plate A periodic electric field is
produced when an RF source is connected to the electrode,
thus permitting piezoelectric coupling to a traveling
sur-face wave If an RF source of a frequency f is applied to an
electrode whose periodicity is p, energy conversion from an
electrical to mechanical form will be maximum when
of the device SAW velocity is an important parameter that
determines the center frequency Another important
pa-rameter for many applications is the temperature
sensi-tivity For example, the temperature stability of the center
frequency of a SAW bandpass filter is a direct function of
the temperature coefficient for the velocity and delay time
of the material used The first-order temperature
coeffi-cient of delay time is given by
(1/t)(dt/dT) = (1/L)(dL/dT) − (1/Vs)(dVs/dT), (10)
s is defined
in terms of the change in SAW velocity that occurs when
the wave passes across a surface coated by a thin massless
conductor, so that the piezoelectric field associated with the
wave is effectively shorted-circuited The coupling factor
Table 5 Material Parameters for Representative SAW Materials
and the amount of signal loss between input and outputthat determines the fractional bandwidth versus minimuminsertion loss for a given material and a filter Propagativeloss, one of the major factors that determines the insertionloss of a device, is caused by wave scattering by crystallinedefects and surface irregularities Materials that have highelectromechanical coupling factors combined with smalltemperature coefficients of delay time are likely to be
a function of the cut angle and the propagative direction.The TCD is an indication of the frequency shift expectedfrom a transducer due to a temperature change and is also
a function of the cut angle and the propagative direction.The substrate is chosen on the basis of the device’s designspecifications for operating temperature, fractional band-width, and insertion loss
Table 5 shows some important material parameters ofrepresentative SAW materials Piezoelectric single crys-tals such as 128◦Y-X (128◦-rotated Y cut and X propagation)
sub-strates for VIF filters ZnO thin films c-axis oriented anddeposited on a fused quartz, glass, or sapphire substratehave also been commercialized for SAW devices
Actuators
Currently, another important application of piezoelectricmaterials exists in the actuator field (30) Using theconverse piezoelectric effect, a small displacement can beproduced by applying an electric field to a piezoelectric ma-terial Vibrations can be generated by applying an alter-nating electric field There is a demand in advanced preci-sion engineering for a variety of types of actuators thatcan adjust position precisely (micropositioning devices),suppress noise vibrations (dampers), and drive objects dy-namically (ultrasonic motors) These devices are used inareas, including optics, astronomy, fluid control, and pre-cision machinery Piezoelectric strain and electrostrictioninduced by an electric field are used for actuator applica-tions
Trang 25Bimorph
Moonie
vz
vzz
vz
Figure 13 Structures of ceramic actuators.
Figure 13 shows the design classification of ceramic
ac-tuators Simple devices composed of a disk or a multilayer
type use the strain induced in a ceramic by the applied
elec-tric field directly Complex devices do not use the induced
strain directly but use the amplified displacement through
a special magnification mechanism such as a unimorph,
bimorph or moonie The most popularly used multilayer
and bimorph types have the following characteristics: The
but has advantages in generation force (1 kN), response
coupling factor k33(0.70) The bimorph type has a large
force (1 N), response speed (1 ms), lifetime (108cycles), and
in a 0.65 PMN–0.35 PT multilayer actuator made of 99
displacement is generated by a 100 V voltage, accompanied
by a slight hysteresis The transmited response of the
in-duced displacement after the application of a rectangular
field-induced strain of 0.1% along the length
Unimorph and bimorph devices are defined by the
num-ber of piezoelectric ceramic plates: only one ceramic plate
is bonded onto an elastic shim, or two ceramic plates are
bonded together The bimorph causes bending deformation
because each piezoelectric plate bonded together produces
extension or contraction in an electric field In general,
there are two types of piezoelectric bimorphs: the
antipar-allel polarization type and the parantipar-allel polarization type,
(a)
V
(b)
V
Figure 14 Two types of piezoelectric bimorphs: (a) antiparallel
polarization type and (b) parallel polarization type.
as shown in Fig 14 Two poled piezoelectric plates t/2 thick and L long are bonded so that their polarization directions
are opposite or parallel to each other In the cantilever morph configuration where one end is clamped, the tip dis-
metallic sheet (called a shim) is occasionally sandwichedbetween the two piezoelectric plates to increase reliability;the structure can be maintained even if the ceramics frac-ture Using the bimorph structure, a large magnification ofdisplacement is easily obtainable However, the disadvan-tages include low response speed (1 kHz) and low genera-tive force
A composite actuator structure called a “moonie” hasbeen developed to amplify the small displacements in-duced in piezoelectric ceramics The moonie consists of athin multilayer element and two metal plates that havenarrow moon-shaped cavities bonded together This devicehas characteristics intermediate between the conventionalmultilayer and bimorph actuators; it has an order of mag-
much larger generative force (100 N), and quicker response(100µs) than the bimorph.
Some examples of applications of piezoelectric and trostrictive actuators are described here The piezoelectricimpact dot-matrix printer is the first mass-produced de-vice that uses multilayer ceramic actuators (Fig 15) Theadvantages of a piezoelectric printer head compared toconventional magnetic type are low energy consumption,low heat generation, and fast printing speed Longitudi-nal multilayer actuators do not have a large displacement,and thus a suitable displacement magnification mecha-nism is necessary The displacement induced in a multi-layer actuator pushes up the force point, and its displace-ment magnification is carried out through hinge levers togenerate a large wire stroke When the displacement in
Trang 26WireWire guide
Ink ribbonPaper
Platen
(b)
Figure 15 Impact dot-matrix printer head.
obtained; the magnification rate is 30 times
Bimorph structures are commonly used for VCR
head-tracking actuators because of their large displacements
An autotracking scan system uses a piezoelectric
actu-ator so that the head follows the recording track, even
driven in both still and quick modes As can be anticipated,
the bimorph drive is inevitably accompanied by torsional
motion A special mechanism has to be employed to
ob-tain perfectly parallel motion Piezoelectric pumps for gas
or liquid that use the alternating bending motion of the
bimorph have been developed for intravenous drip
in-jection in hospitals and for medication dispensing for
chemotherapy, chronic pain, and diabetes Piezoelectric
fans for cooling electronic circuits are made from a pair of
bimorphs that are driven out of phase so as to blow
effec-tively Furthermore, piezo-bimorph type camera shutters
have been widely commercialized by Minolta
Lenses and mirrors in optical control systems require
micropositioning, and even the shapes of mirrors are
ad-justed to correct image distortions For instance, a
space-qualified active mirror, called an articulating fold mirror,
uses six PMN electrostrictive multilayer actuators to
posi-tion and tilt a mirror tip precisely to correct the focusing
aberration of the Hubble Space Telescope
Piezoelectric actuators are also useful for vibrational
suppression systems in an automobile An electronic
controlled shock absorber was developed by Toyota
Figure 16 Bearingless rotor flexbeam and attached piezoelectric
strips.
Piezoelectric sensors that detecting road roughness arecomposed of five layers of 0.5-mm thick PZT disks Theactuator is made of 88 layers of 0.5 mm thick disks Under
magnified 40 times by a piston and plunger pin tion This stroke pushes the change valve of the dampingforce down and then opens the bypass oil route, leading to
combina-a decrecombina-ase in flow resistcombina-ance This electroniccombina-ally controlledshock absorber has both controllability and provides com-fort simultaneously
The U.S Army is interested in developing a rotor controlsystem for helicopters because a slight change in the bladeangle dramatically enhances controllability Figure 16shows a bearingless rotor flexbeam that has piezoelectricstrips attached Various types of PZT-sandwiched beamstructures have been investigated for such a flexbeam ap-plication and for active vibrational control
Ultrasonic Motors
An ultrasonic motor (USM) is an example of a piezoelectricactuator that uses resonant vibration Linear motion in ul-trasonic motors is obtained by frictional force from ellipti-cal vibration The motor consists of a high-frequency powersupply, a vibrator, and a slider The vibrator is composed of
a piezoelectric driving component and an elastic vibratorypart, and the slider is composed of an elastic moving partand a friction coat The characteristics of ultrasonic mo-tors are low speed and high torque compared to the highspeed and low torque of conventional electromagnetic mo-tors (30,31)
Ultrasonic motors are classified into two types: thestanding-wave type and the propagating-wave type Thedisplacement of a standing wave is expressed by
xs (x , t) = Acos (kx) cos (ωt); (15)for a propagative wave displacement is given by
xp (x , t) = Acos(kx − ωt)
= Acos(kx) cos(ωt) + Acos(kx − π/2) cos(ωt − π/2).
(16)
Trang 27Vibratory piece Rotor
Figure 17 Vibratory-coupler type ultrasonic motor.
A propagating wave can be generated by superimposing
two standing waves whose phases differ from each other
is sometimes called a vibratory coupler or a “woodpecker”
type; a vibratory piece is connected to a piezoelectric driver,
and the tip portion generates a flat elliptical movement
(Fig 17) The vibratory piece is attached to a rotor or a
slider at a slight cant angle The standing-wave type has
high efficiency, up to 98% of theoretical However, a
prob-lem of this type is lack of control in both clockwise and
counterclockwise directions The principle of the
propaga-tive type is shown in Fig 18 In the propagating-wave type,
also called the “surfing-type,” a surface particle of the
elas-tic body draws an ellipelas-tical locus due to coupling of the
longitudinal and transverse waves This type generally
re-quires two vibrational sources to generate one propagating
wave; this leads to low efficiency (not more than 50%), but
it is controllable in both rotational directions An
ultra-sonic rotary motor is successfully used in an autofocusing
camera to produce precise rotational displacements The
advantages of this motor over the conventional
electromag-netic motor are silent drive (inaudible), thin motor design,
and energy savings
Tiny conventional electromagnetic motors, smaller than
1cm that have sufficient energy efficiency are rather
diffi-cult to produce Therefore, the ultrasonic motor is gaining
widespread attention Ultrasonic motors whose efficiency
is independent of size are superior in the minimotor area
A compact ultrasonic rotary motor as tiny as 3 mm in
dia-meter was developed by Uchino et al (32) The stator
con-sists of a piezoelectric ring and two concave/convex metal
end caps that have windmill-shaped slots bonded together,
so as to generate a coupled vibration of the up-down and
Figure 18 Principle of the propagating-wave type ultrasonic
motor.
torsional type Because the number of components and thefabrication process are minimized, the cost of fabricationwill decrease remarkably, and it can be disposable; this
is very suitable for medical catheter and endoscopic plications When driven at 160 kHz, a maximum speed of
The following key problems should be systematicallystudied in developing reliable ultrasonic motors: (1) de-velopment of low loss and high vibrational velocity piezo-electric materials; (2) piezoactuator designs that accountfor heat generation and degradation mechanisms; (3) USMdesigns, including displacement amplification mechanismsand frictional contact parts
BIBLIOGRAPHY
1 B Jaffe, W Cook, and H Jaffe, Piezoelectric Ceramics
Aca-demic Press, London, 1971.
2 W.G Cady, Piezoelectricity McGraw-Hill, NY, rev ed., Dover,
1964.
3 F Jona and G Shirane, Ferroelectric Crystals Pergamon
Press, London, 1962.
4 M.E Lines and A.M Glass, Principles and Applications of
Ferroelectric Materials Clarendon Press, Oxford, 1977.
5 IEEE Standard on Piezoelectricity IEEE, NY, 1978.
6 Landold and Boernstein, Numerical Data and Functional
Relationships in Science and Technology: Crystal and Solid State Physics, Vol.11 Springer-Verlag, Berlin, 1979.
7 W.A Smith, Proc SPIE 1733 (1992).
8 H Takeuchi, S Jyomura, E Yamamoto, and Y Ito, J Acoust.
Soc Am 74: 1114 (1982).
9 Y Yamashita, K Yokoyama, H Honda, and T Takahashi, Jpn.
J Appl Phys 20 (Suppl 20-4): 183 (1981).
10 Y Ito, H Takeuchi, S Jyomura, K Nagatsuma, and S Ashida,
Appl Phys Lett 35: 595 (1979).
11 L.E Cross, S.J Jang, R.E Newnham, S Nomura, and
18 Kistler, Stress Sensor, Production Catalog Switzerland,
19 Tokin, Gyroscope, Production Catalog Japan,
20 B.A Auld, Acoustic Fields and Waves in Solids 2e, Krieger,
Melbourne, FL, 1990.
21 G.S Kino, Acoustic Waves: Device Imaging and Analog Signal
Processing Prentice-Hall, Englewood Cliffs, NJ, 1987.
22 C.S Desilets, J.D Fraser, and G.S Kino, IEEE Trans Sonics
Ultrasonics, SU-25: 115 (1978).
23 T.R Gururaja, Am Ceram Soc Bull 73: 50 (1994).
24 F.S Foster, L.K Ryan, and D.H Turnbull, IEEE Trans
Ul-trasonics Ferroelectrics Frequency Control 38: 446 (1991).
Trang 2825 Y.Ito, K Kushida, K Sugawara, and H Takeuchi, IEEE
Trans Ultrasonics Ferroelectrics Frequency Control 42: 316,
1995.
26 C.A Rosen, Proc Electron Component Symp., 1957, p 205.
27 S Kawashima, O Ohnishi, H Hakamata, S Tagami, A.
Fukuoka, T Inoue, and S Hirose, Proc IEEE Int Ultrasonic
Symp ’94, France, Nov 1994.
28 C Campbell, Surface Acoustic Wave Devices and Their Signal
Processing Applications Academic Press, San Diego, 1989.
29 H Matthews, Surface Wave Filters Wiley-Interscience, NY,
1977.
30 K Uchino, Piezoelectric Actuators and Ultrasonic Motors.
Kluwer Academic, Boston, 1997.
31 S Ueha and Y Tomikawa, Ultrasonic Motors Clarendon
This article explores piezoelectric ceramics analysis and
characterization The focus is on polycrystalline ceramics;
therefore, single crystals, polymeric materials, and
or-ganic /inoror-ganic composites are outside the scope of this
review To grasp the behavior of a piezoelectric
polycrys-talline ceramic thoroughly, a basic understanding of the
ceramic itself should not be overlooked To this end, we
have presented a brief introduction of the history of
piezo-electricity and a discussion on processing of the ceramic
and the development of the constitutive relationships that
define the behavior of a piezoelectric material We have
at-tempted to cover the most common measurement methods
and to introduce parameters of interest Excellent sources
for more in-depth coverage of specific topics can be found in
the bibliography In most cases, we refer to lead zirconate
titanate (PZT) to illustrate some of the concepts because it
is the most widely used and studied piezoelectric ceramic
to date
PIEZOELECTRIC MATERIALS: HISTORY
AND PROCESSING
Smart materials are materials that undergo
transforma-tions through physical interactransforma-tions An alternate definition
is that a smart material is a material that senses a change
in its environment and adapts to correct or eliminate such
a change by using a feedback system Piezoelectric
materi-als, shape-memory alloys, electrostrictive materimateri-als,
mag-netostrictive materials, and electrorheological fluids are
some examples of currently available smart materials
Piezoelectricity stems from the Greek word piezo for
pressure It follows that a piezoelectric material develops a
potential across its boundaries when subjected to a nical stress (or pressure), and vice versa, when an electricfield is applied to the material, a mechanical deformationensues Therefore, piezoelectric materials fall in the class
mecha-of smart materials Ferroelectricity is a subgroup mecha-of electricity, where a spontaneous polarization exists thatcan be reoriented by applying an ac electric field
piezo-Definition and History
Piezoelectricity is a linear effect that is related to the croscopic structure of a solid Some ceramic materials be-come electrically polarized when they are strained; this
mi-linear and reversible phenomenon is referred to as the
di-rect piezoelectric effect The didi-rect piezoelectric effect is
al-ways accompanied by the converse piezoelectric effect where
a solid becomes strained when placed in an electric field.The microscopic origin of the piezoelectric effect is thedisplacement of ionic charges within a crystal structure
In the absence of external strain, the charge distributionwithin the crystal is symmetrical, and the net electricdipole moment is zero However when an external stress isapplied, the charges are displaced, and the charge distribu-tion is no longer symmetrical A net polarization developsand results in an internal electric field A material can bepiezoelectric only if the unit cell has no center of inversion.Piezoelectricity is a property of a group of materials thatwas discovered in 1880 by Pierre and Jacques Curie dur-ing their study of the effects of pressure on the generation
of electrical charge by crystals such as quartz, tourmaline,and Rochelle salt In 1881, the term “piezoelectricity” wasfirst suggested by W Hankel, and the converse effect wasdeduced by Lipmann from thermodynamics principles Inthe next three decades, collaborations within the Europeanscientific community established the field of piezoelectri-
city, and by 1910, Voigt’s Lerbuch der Kristallphysic was
published and became a standard reference work ing the complex electromechanical relationships in piezo-electric crystals (1) However, the complexity of the science
detail-of piezoelectricity made it difficult for it to mature to anapplication until a few years later Langevin et al (2) de-veloped a piezoelectric ultrasonic tranducer during WorldWar I Its success opened up opportunities for piezoelectricmaterials in underwater applications and a host of otherapplications such as ultrasonic transducers, microphones,and accelerometers In 1935, Busch and Scherrer disco-vered piezoelectricity in potassium dihydrogen phosphate(KDP) The KDP family was the first major family of piezo-electrics and ferroelectrics discovered
During World War II, research in piezoelectric als expanded to the United State, the Soviet Union, andJapan Until then, limited performance by these materi-als inhibited commercialization, but that changed when amajor breakthrough came with the discovery of barium ti-tanate and lead zirconate titanate (PZT) in the 1940s and1950s, respectively These families of materials exhibitedvery high dielectric and piezoelectric properties Further-more, they offered the possibility of tailoring their behavior
materi-to specific responses and applications by using dopants
To date, PZT is one of the most widely used tric materials Most commercially available ceramics (such
Trang 29O
Ti, Zr
Figure 1 Perovskite structure.
as barium titanate and PZT) are based on the perovskite
simplest arrangement where the corner-sharing
oxy-gen octahedra are linked together in a regular cubic
array; smaller cations (Ti, Zr, Sn, Nb, etc.) occupy the
central octahedral B site, and larger cations (Pb, Ba, Sr,
Ca, Na, etc.) fill the interstices beween octahedra in the
larger A site Compounds such as BaTiO3, PbTiO3, PbZrO3,
their high-temperature ferroelectric and antiferroelectric
phases have been extensively exploited This structure also
allows multiple substitutions at the A site and B site that
result in a number of useful though more complex
com-pounds such as (Ba,Sr)TiO3, (Pb,Sr)(Zr,Ti)O3, Pb(Fe,Ta)O3,
and (KBi)TiO3
Starting around 1965, several Japanese companies
fo-cused on developing new processes and applications and
opening new commercial markets for piezoelectric devices
The success of the Japanese effort attracted other nations,
and today the needs and uses extend from medical
applica-tions to the communicaapplica-tions field to military applicaapplica-tions
and the automotive field
A review of the early history of piezoelectricity is found
in the work of Cady (3), and in 1971, Jaffe et al published
the book Piezoelectric Ceramics (4) that is still one of the
most referenced works on piezoelectricity
Piezoelectric Ceramic Processing
The fabrication of most bulk piezoelectric ceramics starts
with powder preparation The powder is then pressed to
the required shapes and sizes, and the green shapes are
in turn processed into mechanically strong and dense
ce-ramics The more important processes that influence the
product characteristics and properties are powder
prepa-ration, powder calcining and sintering The next steps are
machining, electroding, and poling, the application of a dc
field to orient the dipoles and induce piezoelectricity
The most common powder preparation is the
mixed-oxide In this process, powder is prepared from the
ap-propriate stoichiometric mixture of the constituent oxides
Lead oxide, titanium oxide, and zirconium oxide are the
main compounds for, lead zirconate titanate (PZT)
De-pending on the application, various dopants are used to
tai-lor the properties of interest PZT ceramics are rarely used
Wet millingZirconia media + Ethanol
24 hrs
Drying at 80°C, 12 hrsSieving for better mixing andsize reduction
Ready for calcining
PbO, TiO2, ZrO2dopants if neededMixing of oxides:
Figure 2 Mixed-oxide route for preparing PZT.
without adding of dopants to modify some of their ties A-site additives tend to lower the dissipation factor,which affects heat generation, but also lower the piezo-electric coefficients; for this reason they are used mostly inultrasonics and other high-frequency applications B-sitedopants increase the piezoelectric coefficients but also in-crease the dielectric constant and loss B-site doped ceram-ics used are as actuators in vibrational and noise control,benders, and optical positioning applications
proper-Figure 2 shows a flowchart of the mixed-oxide route formaking PZT ceramics The powders can be mixed by dryball milling or wet ball milling; both methods have advan-tages and disadvantages: wet ball milling is faster thandry milling; however, the disadvantage is the added step
of liquid removal The most common method for makingPZT ceramics is wet ball milling; ethanol and stabilizedzirconia media are added for wet milling A vibratory millmay be used rather than a conventional ball mill; Herner(5) showed that this process reduces the risk of contamina-tion by the balls and the jar Zirconia media are used to re-duce the contamination risks further Calcination is a verycrucial step in processing PZT ceramics; it is importantfor crystallization to be complete because the perovskitephase forms during this step The goals are to remove anyorganics, water, or other volatiles left after mixing; to re-act the oxides to form the desired phase composition beforethe ceramic is processed into useful devices; and to reducevolume shrinkage and allow for better homogeneity duringand after sintering
After calcining, a binder is added to the powder, andthen the mixture is shaped usually by dry pressing in a diefor simple shapes, or extrusion, or casting for more compli-cated bodies Next, the shapes are sintered—placed in anoven for binder burnout and densification
Trang 30The major problem in sintering a PZT ceramic is the
prob-lem, the PZT samples are sintered in the presence of a lead
source, such as PbZrO3, and placed in closed crucibles
Sat-uration of the sintering atmosphere with PbO minimizes
lead loss from the PZT bodies Sintering can now be
Despite precautions, usually 2–3% of the initial lead
con-tent is lost
After cutting and machining into desired shapes,
elec-trodes are applied, and a strong dc field is used to orient
the domains in the polycrystalline ceramic Dc poling can
be done at room temperature or at higher temperatures,
depending on the material and the composition The poling
process only partially aligns the dipoles in a polycrystalline
ceramic, and the resulting polarization is lower than that
of single crystals
This processing technique presents many uncertainties;
the existence of a wide number of other fabrication
tech-niques is an indication that there is a great need for the
production of reliable PZT ceramics whose properties and
microstructure are optimal One problem often
encoun-tered is deviation from stoichiometry This problem is often
due to impurities in the raw materials as well as the lead
loss during sintering, and invariably results in substantial
alterations of the PZT properties As a result, the
elas-tic properties can vary as much as 5%, the piezoelectric
properties 10%, and the dielectric properties 20% within
the same batch (6) The piezoelectric and dielectric
prop-erties generally suffer also if there is any lack of
homo-geneity from poor mixing It is important then that the
constituent oxides be intimately mixed In the method
de-scribed before, however, the constituents are solid solutions
and it has been shown that intimate mixing of solid
so-lutions is difficult if not impossible More information on
the preparation of piezoelectric ceramics can be found in
Jaffe et al (4), and Moulson and Herbert (7) Other
pro-cessing methods, including hydrothermal propro-cessing and
coprecipitation methods, are described in (8–10) Noted
that there has been a great deal of development in
pow-der processing, shaping, and sintering (11–13) that has
re-sulted in further expanding the application of piezoelectric
ceramics
Ferroelectricity
Some piezoelectric materials are also ferroelectric A
fer-roelectric material possesses spontaneous polarization
whose direction can be reversed by applying a
realiz-able electric field across some temperature range Most
be-low which they are polar and above which they are not
lin-early decreases above it according to the Curie–Weiss law
(4,7) The very large permittivity values that are
charac-teristic of ferroelectric materials are greatly exploited in
many applications, most widely in the multilayer-capacitor
industry
Applying a large alternating electric field reverses
the polarization, and this gives rise to the ferroelectric
Figure 3 P–E hysteresis loop of a poled piezoelectric ceramic.
hysteretic loop that relates polarization P to applied tric field E A typical field-polarization loop is illustrated
elec-in Fig 3 For large signals, both the electric displacement
D and the polarization P are nonlinear functions of the
field E They are related to each other through the linear
equation
D i = P i + ε0E i , (1)
C/ Vm) The second term in Eq (1) is negligible for most
ferroelectric ceramics, and a D–E loop and P–E loop come interchangeable Two key characteristics of the P–E
is the field at which polarization is zero Pris the value ofthe polarization when the electric field is zero Once thefield is switched off, the material’s polarization is equal to
determined A loop is said to be saturated once the values
of Prand Ecno longer vary
Generally, the existence of the P–E loop is considered
evidence toward establishing that a material is tric A Sawyer–Tower circuit (14), or a modified version of
ferroelec-it, is commonly used to obtain a P–E loop An ac voltage is
applied to the electroded sample; the resulting chargestored on the sample is determined by a large referencecapacitor placed in series with the sample An electrome-ter can be used to detect the voltage across the capacitor; bymultiplying this voltage by the value of the reference capac-itor, the charge across the sample results The capacitance
of the reference capacitor should be 100 to 1000 times thevalue of the capacitance of the sample Note that ferroelec-tric hysteretic loops are both frequency- and temperature-dependent
In addition to the P–E loop, polarization switching leads
to strain–electric field hysteresis A typical strain–field
Trang 31Electric field
Figure 4 Butterfly loop indicating switching.
response curve is shown in Fig 4 The shape resembles
that of a butterfly, and it is often referred to as the
“but-terfly loop.” As the electric field is applied, the converse
piezoelectric effect dictates that a strain results As the
field is increased, the strain is no longer linear with the
field, as domain walls start switching
For more sources on ferroelectricity, the reader should
consult the bibliography (15–19)
PIEZOELECTRIC CONSTITUTIVE RELATIONSHIPS
An understanding of piezoelectricity begins with the
struc-ture of the material To explain it better, let us consider a
in average diameter) from a polycrystalline ceramic This
crystal is made up of negatively and positively charged
atoms that occupy specific positions in a repeating unit
or cell The specific symmetry of the unit cell determines
the possibility of piezoelectricity in the crystal All crystals
can be divided into 32 classes or point groups (from seven
basic crystal systems: triclinic, monoclinic, orthorhombic,
tetragonal, rhombohedral, hexagonal, and cubic) Of the
32 classes, 21 do not possess a center of symmetry, and 20
are piezoelectric (although one class lacks a center of
sym-metry, it is not piezoelectric because of the combination of
other symmetry elements) The lack of a center of
symme-try means that a net movement of positive and negative
ions with respect to each other as a result of stress
pro-duces an electric dipole Because the ceramic is composed
of randomly oriented piezoelectric crystallites, it is
inac-tive, that is, the effects of the individual crystals cancel
each other and no discernable piezoelectricity is present
Regions of equally oriented polarization vectors are known
as domains “Poling” is a commonly used method to orient
the domains by polarizing the ceramic through the
appli-cation of a static electric field Electrodes are applied to
Figure 5 Poling of a piezoelectric, ferroelectric ceramic.
the ceramic, and a sufficiently high electric field is appliedsuch that the domains rotate and switch in the direction ofthe electric field Full orientation of all domains never re-sults; however, the polycrystalline ceramic exhibits a largepiezoelectric effect During this process, there is a smallexpansion of the material along the poling axis and a con-traction in both directions perpendicular to it (see Fig 5).Due to large number of allowable polar directions such asnear the morphotropic phase boundary (where the Zr to
Ti ratio is 48 to 52), the maximum deviation of the polaraxis of a grain from the average polar direction is smaller,and the reduction of polarization is minimized, assumingoptimum alignment
Constitutive Relationships
When writing the constitutive equations for a tric material, account must be taken of changes of strainand electrical displacement in three orthogonal directionscaused by cross-coupling effects due to applied electricaland mechanical stresses Tensor notation is first adopted,and the reference axes are shown in Fig 6 The state of
state of stress is also described by a second-rank tensor
strain tensor, compliance s i jkl , and stiffness c i jkl, are thenfourth-rank tensors The relationship between the electric
D i(also a first-rank tensor) is the permittivity ε i j, which
is a second-rank tensor Therefore the piezoelectric
36
4
1
52
Figure 6 Reference axes.
Trang 32where d i jk , d i jkare the piezoelectric constants (third-rank
tensor) Superscripts T and E indicate that the dielectric
under conditions of constant stress and constant electric
field, respectively In general, a first-rank tensor has three
components, a second-rank tensor has nine components,
a third-rank tensor has 27 components, and a fourth-rank
tensor has 81 components Not all of the tensor components
are independent
Both of these relationships depend on orientation; they
describe a set of equations that relate these properties in
different orientations of the material The crystal
symme-try and the choice of reference axes reduce the number
of independent components A convenient way of
describ-ing them is by usdescrib-ing axis directions, as given by Fig 6
The convention is to define the poling direction as the
3 axis, the shear planes are indicated by the subscripts
4, 5, and 6 and are perpendicular to directions 1, 2, and 3,
respectively This simplifies the notations introduced
be-fore, where a 3-subscript tensor notation (i , j, k = 1, 2, 3)
(i , j = 1, 2, 3) is replaced by a 1-subscript matrix notation
(i = 1, 2, 3, 4, 5, 6) A shear strain such as S4is a measure
of the change of angle between the two initially
orthog-onal axes in the plane perpendicular to axis 1 The first
subscript of the d constant gives the “electrical” direction
(field or dielectric displacement), and the second gives the
component of mechanical deformation or stress The
pla-nar isotropy of poled ceramics is expressed in their
field parallel to the poling axis 3 interacts in the same way
with axial stress along either the 2 axis or the 1 axis) and
d24 = d15(an electric field parallel to the 2 axis interacts in
the same way with a shear in the 2,3 plane as a field along
the 1 axis with a shear in the 1,3 plane) Similar
relation-ships hold for the elastic constants because of isotropy in
the plane perpendicular to the polar axis
Property Matrix for a Poled Piezoelectric Ceramic
A piezoelectric ceramic has only one type of piezoelectric
matrix, regardless of the symmetry of the constituent
crys-tals The ceramic is initially isotropic This isotropy is
destroyed in the poling direction The material is
trans-versely isotropic in the directions perpendicular to the
pol-ing direction The symmetry elements are an axis of
ro-tation of infinite order in the direction of poling and an
infinite set of planes parallel to the polar axis as reflection
planes In crystallographic notation, this symmetry is
crystal class, 6 mm
The elastic, dielectric, and piezoelectric matrices for the
cylindrical symmetry of poled PZT are shown in the
follow-ing equations Matrices analogous to the piezoelectric also
in the next section)
piezoelectric charge coefficients (d31and d33), the
piezoelec-tric voltage coefficients ( g31and g33), and the piezoelectric
coupling factors (k31, k33, kp, and kt) The d coefficient is
the proportionality constant between electric displacementand stress, or strain and electric field [Eqs (2) and (3)]
High d coefficients are desirable in materials used as
actu-ators, such as in motional and vibrational applications The
g coefficient is related to the d coefficient by the following
expression:
d mi = ε T
where m , n = 1, 2, 3 and i = 1, 2, 6 High g coefficients
are desirable in materials to be used as sensors to producevoltage in response to mechanical stress
The piezoelectric coupling factor k is a measurement of
the overall strength of the electromechanical effect It isoften defined as the square root of the ratio of electrical
Trang 33energy output to the total mechanical energy input (in the
direct effect) or the mechanical energy available to the total
electrical energy (in the converse effect) The value of k is,
of course, always less than unity because energy conversion
is always incomplete
Other important properties of PZT are the dielectric
con-stant is a measure of the charge stored on an electroded
material brought to a given voltage The dielectric constant
constant K (often referred to as just “the dielectric
ac field, the dielectric constant has both a real part and an
imaginary part; the loss tangent is defined as the ratio of
the imaginary part to the real part
The values of these constants depend on the PZT
com-position As an example, the d constants, g constants,
and the dielectric constant for compositions near the
mor-photropic phase boundary show their highest values on the
tetragonal side of the transition (4) Then, it is possible to
tune the values of these properties for most compositions;
one way to achieve this is by adding dopants to the PZT
formulation
Resonant Method and Equivalent Circuit
Resonance Method Any body has certain characteristic
frequencies at which it prefers to resonate When excited
at this resonant frequency fr, the body will resonate freely
at a greater amplitude than at other frequencies
Follow-ing this resonant frequency is an antiresonant frequency
the oscillatory amplitude is at a minimum Piezoelectric
ce-ramics are no different, and measuring these characteristic
frequencies provides the means to evaluate the
piezoelec-tric and elastic properties of the ceramic Different modes
of vibration of the ceramic, such as thickness or planar, give
insight to the different constants for that mode A typical
resonance plot of impedance versus frequency for a
piezo-electric ceramic near a resonance is shown in Fig 7
of maximum impedance
At resonance, a piezoelectric element may be modeled
by the equivalent circuit shown in Fig 8 This circuit,
com-monly referred to as Van Dyke’s model, is recommended
by the IEEE Standard on Piezoelectricity (20) An
alter-nate model, proposed by Sherrit et al (21), elimialter-nates
components as complex to characterize better the losses
of certain piezoelectric elements, especially polymers All
between these two frequencies, the ceramic behaves
in-ductively This model is valid only near the resonance
Additionally, the resonance must be sufficiently isolated
from other modes to eliminate the effects of any adjacent
modes To ensure that the resonance is isolated, sample
geometry must be chosen carefully Geometries suitable
for measuring the different piezoelectric and elastic
coef-ficients are presented in Table 1 Fixturing of the sample
Figure 7 Impedance of a piezoelectric ceramic at resonance.
should not impose any constraints on the vibration of theceramic This can be accomplished by using a point holderpositioned at a node of vibration All leads should also beshielded up to the contact point, as much as possible, toavoid any stray capacitances that may arise
Earlier literature has suggested several circuits for
These circuits usually consist of an oscillator for excitingthe sample, a voltmeter or other device for measuring cur-rent through the circuit, and additional discrete compo-
nents To find fr, the frequency of the oscillator is varieduntil the maximum current is detected through the cir-
cuit Similarly, for fa, the frequency of minimum current isdetermined Note that there are actually six characteristicfrequencies that may be identified for a particular reso-
resonant frequency and series resonant frequency IEEEStandard 177 (23) identifies these six frequencies and es-tablishes that for many cases, including piezoelectric ce-
films, this assumption can introduce appreciable errors,
so the six frequencies should be considered separately The
deter-mined by substituting an adjustable resistor into the cuit for the ceramic at the previously identified frequencyand adjusting the resistance until the voltmeter reading
cir-is the same as for the ceramic Today, fully integrated
Trang 34Table 1 Sample Geometries for Measuring Material Properties
Dimensional Long, slender, length Thin, flat plate, thickness poled; Thin flat disc, thickness poled requirements poled rod; l > 3d l > 3 5 t, w d > 10 t
impedance analyzers are commercially available to make
this type of measurement, allow the researcher to choose
an equivalent circuit model, and report the values of the
and fa Commercial off- the shelf software is also available
now which can be used in conjunction with an analyzer
to evaluate the impedance information and calculate the
relative material properties of a piezoelectric device (26)
These tools can aid the researcher in evaluating material
properties, however, a basic understanding of piezoelectric
behavior is an important foundation that should not be
overlooked
Measuring Material Properties Capacitance
measure-ments are usually carried out at 1 kHz and at low
exci-tatory voltages (mV level) Although research has shown
that capacitance and loss vary with excitation voltage and
frequency (27,28), the 1-kHz, low-voltage measurement is
used to determine material properties The free relative
dielectric constant K Tis defined as the ratio of the
permit-tivity of the material to the permitpermit-tivity of free space It is
calculated from
K T= tC
Table 2 Typical Properties of Common Piezoelectric Materials
where t is the distance between electrodes in meters, C
in meters2 The loss tangent, tanδ, is defined as the ratio
of resistance to reactance in the parallel equivalent cuit of Fig 9b It is a measure of the dielectric losses in thematerial and therefore, also a measure of the heat generat-ing capacity of the ceramic when operated under dynamicconditions This is a direct measurement and is usuallyformed at the same conditions as the capacitance measure-ment
33 subscripts are for length extensional and thickness
cal-culated from the frequencies of minimum and maximumimpedance and are given by the equations
Trang 352 fr
and can be approximated by
kp≈ f2− f2r
f2 r
Elastic compliance is the ratio of a material’s change in
dimensions (strain) relative to an externally applied load
(stress) This is the inverse of Young’s modulus For a
piezo-electric material, the compliance depends on whether the
strain is parallel or perpendicular to the poling axis and
the electrical boundary conditions Elastic constants are
calculated from the following equations:
whereρ is the density of the material in kg/m3, l is the
dis-tance between electrodes, and w is the width of the ceramic
The superscripts D and E stand for constant electric
dis-placement (open circuit) and constant electric field (short
circuit), respectively
ap-plied electric field to the strain, can be calculated from
the coupling, the elastic coefficients, and the dielectric
coefficients by the following equations:
do not depend on the dimensions of the material; ever, they vary with the degree of polarization of the ce-ramic They also do not provide the sign of the coefficient,which must be determined by direct measurements The
series equivalent circuit of Fig 9a is given by
a known input to the ceramic, either an electric field or aforce, and record the corresponding output, either a defor-mation or a charge under various conditions These meth-ods are in contrast to the bulk material characterizationusing the electrical resonance techniques described before.Many times, direct measurements are carried out on a cera-mic that has been configured as a sensor or actuator Typi-cal processing may include electroding, laminating, apply-ing preload, mounting, and other assembly procedures toadapt the material effectively for use as a sensor or actu-ator These measurements aid the researcher in modelingthe behavior of the piezoelectric device and allow efficientintegration of the devices into real-world applications.Displacements of piezoelectric actuators are measured
to determine the magnitude and sign of the relationshipbetween the applied electric field and the strain developed,that is, the converse effect For a PZT wafer, this corres-
ponds to the d i jcoefficient; however, for bending type ators, this relationship does not correlate directly with any
actu-of the measured properties for out-actu-of-plane bending usingthe resonance techniques Based on Eqs (10)–(15), it can be
the strain is a function only of the product of the applied
field E i and the d i jcoefficient
Careful attention must be paid to the boundary conditions
of the ceramic to ensure that this assumption is valid
In a plot of the strain as a function of applied field, the
Trang 36slope yields an average value of d i j Typically, these
mea-surements are made by using a noncontacting
displace-ment transducer (29) to reduce the effects of loading on the
actuator Laser-based and other optical or capacitive
dis-placement measurement techniques are most commonly
used (30–32) Displacements may range from submicron
levels for single PZT wafers to the centimeter level for
bending type actuators For very small displacements, an
optical-lever type measurement system or interferometric
techniques (33) have been used to resolve the displacement
of the ceramic Direct application of either foil or optical
strain gages has also been used for measuring the
actu-ator strain These measurements may be either static or
dynamic, depending on the measurement system and the
intended application of the ceramic If dynamic
measure-ments are made, excitatory frequencies should be at least
an order of magnitude less than any resonant frequency of
the device to ensure linear behavior and boundary
condi-tions suitable for the intended measurement
Another direct method used to measure piezoelectric
constants is based on the direct piezoelectric effect (22,34)
Here, a known load is either applied to or lifted off a
cera-mic at rest The resulting charge, which accumulates on
the electrodes, is then measured as a voltage across a
capa-citor in parallel with the ceramic, or the current from the
ceramic can be integrated directly If E iis 0 (short circuit),
then Eq (2) reduces to
Knowing the applied stress and measuring the electric
If a piezoelectric ceramic is immersed in a liquid and the
pressure of the liquid is varied, then the piezoelectric
large capacitor in parallel with the ceramic This coefficient
represents the response of the ceramic to hydrostatic
pres-sure applied equally to all axes Convention has dictated
that electrodes are perpendicular to the 3 direction for the
Figure 10 Strain hysteresis of a
piezoelec-tric ceramic unimorph.
coefficients for a ceramic by the equation,
The frequency response of the device may be obtained byvarying the frequency of the excitatory voltage to the cera-mic while measuring the displacement Typical resonantfrequencies of bulk ceramic material are in the kilohertz
to megahertz range depending on the mode of vibration,whereas resonant frequencies of bender types (unimorph
or bimorph) may be less than 100 Hz For maximum strain,
a piezoelectric actuator can be excited at its natural quency; however, this nonlinear behavior must be takeninto account if the actuator is to be used across a range
fre-of frequencies Careful attention must also be paid tothe instrumentation system’s dynamic response in bothamplitude and phase distortions, when making dynamicmeasurements Measurement systems have their own fre-quency response characteristics which must be separatedfrom the response of the ceramic under test
Hysteresis is a phenomenon that is present in all electric materials Hysteretic behavior is due to the lossynature of the ceramic where the current trails the applied
material For actuators, this means that the absolute placement depends on the excitatory voltage and frequencyand also on whether the voltage is increasing or decreas-ing To characterize the amount of hysteresis in a ceramic,
a sinusoidal voltage is applied to the device, and the placement is recorded By plotting the displacement versusdriving voltage, as shown in Fig 10, the hysteretic behav-ior of the ceramic can be observed The amount of hystere-sis (usually expressed in percent) is defined as the largestdifference between the maximum and minimum displace-ment for any voltage divided by the total displacement Ofnote in Fig 10 is the fact that, as the peak voltage is in-creased, the amount of hysteresis also increases for anygiven voltage
dis-Generally, piezoelectric ceramic actuators exhibit adecrease in their displacement for a given excitatory
Trang 37Stress, F/A
Increasingapplied field
x
Figure 11 Typical stress/strain relationship for a piezoelectric
ceramic.
voltage, as they are loaded This relationship can be seen in
Eq (3), when T k= 0 As the load is increased, the
displace-ment eventually reaches zero, and the actuator provides
only a force output This force is known as the blocked
force, and it is the maximum amount of force that the
ac-tuator can produce at that voltage To characterize this
relationship, the actuator is loaded with a load less than
the blocked force, and the displacement is measured If the
load is varied, then the force/displacement relationship can
be determined (Fig 11) To determine the blocked force,
the actuator must be rigidly held so as not to deform, and
the force output is measured by using a load cell or other
force-measuring device Because the displacement of some
piezoelectric actuators is quite small, this measurement
calculated by the equations
FB= E3d33wl
sE 33
FB= E3d31wt
sE 11
where E is the applied field; and l, w, and t are the length,
width, and thickness of the ceramic, respectively Equation
(34) applies to the thickness extensional mode and Eq (35)
applies to the length extensional mode
Actuators that have greater displacements lend
them-selves better to blocked force measurement (such as domed
prestressed actuators or unimorph/bimorph type
actu-ators) The blocked force may also be determined by
extrapolating the force–displacement relationship to zero
displacement if a true blocked force measurement is not
practical In most applications, actuators operate
some-where between the free (unloaded) state and the
com-pletely constrained state
It has been previously reported that a constant preload
applied to a piezoelectric actuator can actually increase
the displacement of the ceramic, compared to an unloaded
specimen (34–36) This may result from simply reducing
the compliance or mechanical play in a PZT assembly or
may be a real increase in the d coefficient This effect
reaches a maximum and then starts to cause a decrease inthe coefficient as the preload is increased up to the blockedforce
Temperature effects on the piezoelectric coefficients ofceramics may also be evaluated Usually, ceramics must
be used well below their Curie temperatures to maintainpolarization The respective Curie temperatures for hardand soft PZTs are of the order of 360◦C (680◦F) and 330◦C
cryogenic levels, the piezoelectric coefficients generally crease as temperature decreases This effect can be exper-imentally quantified through either resonance techniques
de-or direct measurements across the desired temperaturerange (34)
The power required to drive a piezoelectric ceramic can
be calculated from the following equation:
when the ceramic is modeled as in Fig 9a where f is the
Typically, it is assumed that both the capacitance and losstangent of the ceramic are constant when using Eq (36).Doing so can lead to large errors when estimating thepower consumption of a ceramic To avoid these errors, ei-ther the voltage and current supplied to the ceramic should
be measured to provide the power consumption directly, orthe variation of capacitance and loss of the material asfunctions of applied field and frequency must be quanti-fied and incorporated into Eq (36) (28) A number of re-searchers have investigated the power consumption char-acteristics of PZT actuators used to excite a host structure(27,37,38) and found a coupling between the mechani-cal motion of the structure and the electrical character-istics of the piezoelectric actuator Research by Brennanand McGowan (27) shows that the power consumption ofpiezoelectric materials used for active vibrational control
is independent of the coupling effects of the host structure
when the structure is completely controlled From these
findings, they conclude that the power requirements of thepiezoelectric actuator depend only on its geometry and ma-terial properties and the driving voltage and frequency ofthe control signal Research (23) has indicated that bothcapacitance and resistance are nonlinear functions of thepeak amplitude and frequency of the excitatory voltage
In time, piezoelectric effects imparted through polingdegrade Aging of piezoelectric ceramics, like many othermaterials, is logarithmic with time In most ceramics, ainitial performance levels can be recovered by simply re-poling the sample Aging levels depend on the composition;the coupling coefficient of a soft PZT composition ages at arate of –1% per time decade versus –2% for a hard compo-sition Degradation of piezoelectric behavior also depends
on the level of stress to which the ceramic is subjected.High stress levels can lead to switching of the polarizationand eventually depoling of the ceramic High stresses alsoinduce microcracking, which can lead to ceramic breakageand failure
Trang 38The methods outlined before can be used either
sep-arately or together to investigate the dielectric,
piezoel-ectric, and elastic properties of a ceramic Resonant
tech-niques, which are the preferred method of measurement
in the IEEE standard, are easy to implement, and the
as-sociated frequencies can be measured accurately There is
even commercially available hardware and software to
as-sist in these measurements and the evaluation of material
properties However, these methods do not explain any
nonlinear behavior that is present in the ceramic
Depen-dence of material properties on the frequency and
am-plitude of the applied voltage are among these nonlinear
effects Direct measurements of the piezoelectric
con-stants can quantify the material properties under different
operating conditions and provide insight beyond the
stan-dard linear behavior predicted by resonance techniques
These methods though, are usually more rigorous in their
requirements for material handling and instrumentation
Modeling of Piezoelectric Ceramics
There are a host of applications for piezoelectric
materi-als, and although they have been studied for more than
a century, potential for improvement and innovation still
persists Modeling of piezoelectric ceramics and their
prop-erties affords a way to accelerate materials improvement
and aid in device design and development For that reason,
we would be remiss not to mention it, albeit briefly This
introduction is in no way meant as a comprehensive review
of the vast area of modeling of piezoelectricity; however the
references cited provide a good starting place Care must be
taken to differentiate between modeling the piezoelectric
material and modeling a “piezoelectric structure;” often, a
piezoelectric material is laminated or bonded to a substrate
as a unimorph or bimorph
A number of researchers experimented with
com-mercial packages have limitations Other groups have
written their own codes and achieved varying degrees of
success (40–43) Finite element schemes that combined
piezoelectric and acoustic elements proved useful in
char-acterizing the electromechanical behavior of piezoelectric
transducers (44) Most of these schemes are restricted
because they assume linearity of the coefficients P´erez
et al expanded on these models by including nonlinear
ele-ments in the equivalent circuit (45) Models of the
nonlin-ear hysteretic behavior of piezoelectric materials are
abun-dant in the literature and can be categorized on the basis of
the dimensional scale they probe Microscopic models stem
primarily from energy relationships applied at the atomic
or molecular level (46) Macroscopic models (47–49) often
use empirical relationships to describe the behavior of the
bulk material Both methods have their advantages and
disadvantages; microscopic models require a great number
of parameters, often not available, and macroscopic
mod-els do not consider the underlying physics A number of
authors proposed a third approach, a mesoscale or
semimi-croscopic model that combines the advantages of the
pre-vious methods, thus allowing a better way to model
hys-teretic behavior This is accomplished by starting out from
energy principles applied at the microscopic level, then
using a relatively small number of parameters to simulatethe behavior of bulk ceramics (50,51)
CONCLUSION
Characterization of the elastic, dielectric and mechanical properties of piezoelectric ceramics is crucialfor several reasons First, investigations of the materialproperties provide a link between the manufacturing pro-cess and ceramic performance This enables the developer
electro-of the materials to adjust the manufacturing process electro-of theceramic to produce tailored materials Second, the engi-neer can investigate prospective materials for applicability
to a specific need Material parameters obtained throughcharacterization can also be used to develop and validateanalytical models of the ceramics Insights gained throughcharacterization have led to many new devices and uses.For example, investigation of the hydrostatic coefficients ofPZT and those of the piezoelectric polymer polyvinylidene
figure of merit and led to composite research to combineboth materials in a superior device that fits underwaterand hydrophone applications better More than a centuryafter their discovery, piezoelectric ceramics have becomecommercially viable Researchers continue diligently touncover novel ways to characterize the complex electrome-chanical properties, and as they do so, new processingmethods and applications are revealed Recently, as an ex-ample, researchers at MIT successfully grew piezoelectricsingle crystals (52) that opened opportunities for newerapplications Published articles on composite processingand characterization have also become more abundant.Without question, piezoelectric ceramics have secured
a permanent place in the field of material science andengineering
ACKNOWLEDGMENT
The authors express their sincere appreciation to Dr.Jeffrey A Hinkley (NASA Langley Research Center) forhis review of the manuscript and his helpful comments
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CHEMICAL INDICATING DEVICES
CHRISTOPHERO ORIAKHI Hewlett-Packard Company Corvallis, OR
INTRODUCTION
Most people remember a chemical indicator from their highschool chemistry This kind of indicator is a material thatchanges color to signify the end point of a titration or toprovide a relative indication of the acidity or alkalinity of
a chemical substance The use of indicators extends farbeyond this For example, food, cosmetic, pharmaceutical,and other chemical formulations undergo complex chemi-cal, enzymatic, and microbial interactions when they areexposed to UV light or temperature fluctuations over time.Consequently, product quality may be degraded and maylead to additional safety concerns The challenges facingthe produce industry include successful implementation
of Good Manufacturing Practices (GMP), Hazard Analysisand Critical Control Point (HACCP), Total Quality Man-agement (TQM) programs, and other regulations that de-mand compliance (1)
Trang 40To address these issues and increase consumer
confi-dence in product quality, safety, and authenticity, many
manufacturers incorporate inexpensive monitoring
de-vices into their products during production, packaging, or
storage A large number of consumer-readable indicators
are available commercially Some examples of these types
of indicators are tags, labels, seals, and thermometers
Some give a visual color change in response to degradation
of product quality, tampering, or to detect a counterfeit
They are used extensively in the chemical, food, and
phar-maceutical industries where consumers need assurance of
product integrity, quality, and safety during
postmanufac-ture handling
Generally, chemical indicators may be defined as
stim-ulus responsive materials that can provide useful
infor-mation about changes in their environment Organic dyes,
hydrogels or “smart polymers,” shape-memory alloys,
ther-mochromic or photochromic inks, and liquid crystals are
some examples They may function by forming structurally
altered ionic or molecular complexes with species in their
environment through chemical or physical interactions
involving proton exchange, chelation, hydrogen bonding,
dipole–dipole interactions, or van der Waal forces (1)
The resulting characteristic biochemical, chemical, optical,
magnetic, thermal, or mechanical changes can be tailored
to provide the desired indication response
This article focuses on inexpensive disposable chemical
indicating devices such as pH indicators, temperature
indi-cators, time–temperature indicators (TTI), and tampering
and counterfeit indicators The temperature and TTIs are
widely used in the food and pharmaceutical products where
date coding on a package may sometimes be inadequate
CHEMICAL INDICATING DEVICES ARE SMART
Smart materials or devices are defined as materials that
produce strong visually perceptible changes in a physical
or chemical property in response to small physical or
chemical stimuli in the medium The material
proper-ties measured may include pH, concentration, composition,
solubility, humidity, pressure, temperature, light intensity,
electric and magnetic field, shape, air velocity, heat
capa-city, thermal conductivity, melting point, or reaction rates
(2–6) Chemical indicating devices can respond reversibly
or irreversibly to small changes in the physical or chemical
properties in their environment in a predictable manner
They may be regarded as smart materials because of the
range of materials properties they encompass Typical
ma-terials include shape-memory alloys, piezoelectric
mate-rials, magnetostrictive substances, electrorheological and
magnetorheological fluids, hydrogel polymers, and
photo-and thermoresponsive dyes (2–6)
CLASSIFICATION
Indicators can be classified on the basis of the response
mechanism, operating principles, or application Thus
there are chemical, biological, biochemical, electrical,
mag-netic, and mechanical indicators according to the
re-sponse mechanism Based on the intended application,
indicators can be classified as temperature indicators,
time–temperature indicators, pH indicators, counterfeitindicators, tamper indicators, freeze and thaw indicators,
or freshness indicators
GENERAL OPERATING PRINCIPLES
The response mechanism of most indicators includes one ormore of the following: physical, chemical, physicochemical,electrochemical, and biochemical Physical mechanismsare based on photophysical processes, phase transition, orother critical material properties such as melting, glasstransition, crystallization, boiling, swelling, or changes inspecific volume In most cases these transitions are driven
by changes in the interactive forces (e.g., hydrophilic–hydrophobic forces) within or around the indicator mate-rial The indicator response mechanism can also be based
on chemical, biochemical, and electrochemical reactions.Examples include acid–base, oxidation–reduction, photo-chemical, polymerization, enzymatic, and microbial reac-tions Many of these changes are irreversible, and the onset
or termination can be observed visually as a color change,color movement, or mechanical distortion (1–6)
CHOICE OF INDICATORS
Some factors governing the selection of a given indicatordevice include
should not be more expensive than the product it isprotecting
contain-ers or packages Once installed, the device must main intact and readable during the service life of thepackage
kinetics of the order of seconds to a few hours andmust be reproducible There should be no time de-lay in response to reactions involving a solid, liquid,
or gas Most applications require the response in theindicator to be irreversible to preserve the needed in-dication record
accurate, and easily activated A user-friendly cator that provides useful information when neededwill make both the product manufacturer and the con-sumer happy
to or longer than that of the product it is monitoring
rIt should be technically difficult to duplicate or
coun-terfeit the indicator’s response In this case, the cator is acting as a “smart” locking mechanism
indi-PH INDICATORS
The pH indicator is probably the oldest and simplest smartchemical indicating device known The chemistry of acid–base indicators is well documented (7) and involves proton