Therefore, if the sensitivity of a smart paint is high enough in both frequency ranges,the paint can be used as a vibrational and AE sensor inte-grated into a structural material.. In th
Trang 2NONDESTRUCTIVE EVALUATION 713
00.001
00.010.020.030.040.050.06
Time (microsec)
Figure 53 Noncontact ultrasound transmission through a
hu-man heel using 250-kHz (top) and 500-kHz (bottom) frequency
transducers The first peak corresponds to ultrasound
transmis-sion through air, skin, tissue, and heel bone Other peaks are not
identified.
the material surface in ambient air The ultrasound
re-ceived by this transducer was amplified by a 64-dB gain
Figure 55 shows the time and frequency domain of the
ultrasound detected (heard) by the NC transducer By
sweeping the frequency across a wide range, the
frequency-dependent response from the source (vibrating system) can
be investigated and related to its characteristics or
condi-tion In this mode, we successfully interrogated frequencies
Non-contactpassive “Listener”
3.5 MHz 12.5 mm
diameter
Broadbandamplifier
3 mm Ambient air
25 mmSteel
Figure 55 Time and frequency domains of ultrasound detected
by noncontact transducer, per Fig 54 setup.
as high as 7 MHz in ambient air This opens the door tononcontact acoustic emission, acoustoultrasonics, and anyother situation where detection of high frequency ultra-sound is desired Applications of the passive use of NCtransducers are dynamics of vibration, materials cutting,testing of railroad, highways, bridges, runways, etc
Other Noncontact Ultrasound Applications
Besides the applications of NCU described here, this modecan also be used for level detection; dimensional andproximity analysis; high temperature material evaluation;analysis of liquid-sensitive and hazardous material, andanalysis of gases and liquids Finally, it suffices to say that
if ultrasound can be propagated through a medium or flected from an interface, then much information about themedium and the interface can be obtained
re-CONCLUSIONS
In this paper, we outlined the significance of ultrasound fornondestructive characterization of materials and for non-invasive diagnostic applications in the medical field Wehave also shown the feasibility of noncontact ultrasonicmeasurements in the time, frequency, and image domains,analogous to other wave-based methods
Underscoring the significance of the noncontact sound mode, we presented a detailed discussion about thedifficulty of achieving this mode We have also shown thatthis work ultimately resulted in very high transductionnoncontact transducers, thus making the noncontact ul-trasound mode a reality Applications of these transducers
ultra-in ultra-industry and the medical field have been described byusing documentary evidence
We also provided an introduction to a novel ultrasonicnoncontact analyzer and its applications for characterizingindustrial and biomedical materials and products
We believe that the noncontact ultrasound mode isamong the most significant developments for characteriz-ing and analyzing all states of matter Though we have
Trang 3714 NONDESTRUCTIVE EVALUATION
provided selected examples of its applications, there is no
doubt that the users of this technology will further enhance
its use in materials quality, process control, and health care
in our increasingly complex world This advancement in
the field of ultrasound and materials characterization has
opened much needed and unprecedented opportunities in
research and education
ACKNOWLEDGEMENTS
The author gratefully acknowledges the assistance of
M Langron, Ultran Laboratories, in producing the
trans-ducers used for this paper The enthusiastic support
and valuable suggestions of E Blomme, Katholieke
Hogeschool, Belgium and M Landa, Academy of Sciences,
Czech Republic, are acknowledged in kind The work
pre-sented in this article was supported by the continuing
efforts of SecondWave and Ultran Laboratories for the
ad-vancement of industry and medical science through
inno-vative developments in ultrasound
BIBLIOGRAPHY
1 J Curie and P Curie, Bull no 4 Soc Mineral France 3:90
(1880), C.R Acad Sci Paris 91:294 (1880).
2 Apparatus for Warning a Ship at Sea of its Nearness to Large
Objects Wholly or Partially under Water, Brit Pat tion 11,125, March 27, 1913, R.L Richardson.
Specifica-3 R.E Green, in Materials Analysis by Ultrasonics, A Vary, ed.,
Noyes Data, NJ, 1987, p 6.
4 Z Cho, J.P Jones, and M Singh, Foundations of Medical
Imag-ing Wiley, NY, 1993, pp 477–486.
5 R.M White, J Appl Phys 34: 3559–3567 (1963).
6 A.A Bondarenko, Y.B Drobat, and S.V Kruglov, Soviet J NDT
9 G.A Allers, in Intelligent Processing of Materials and
Ad-vanced Sensors, H.N.G Wadley, P.A Parish, B.B Rath, and
S.M Wolf, eds., Metallurgical Society, PA, 1986, pp 17–27.
10 J.A Brunk, Allied Signal, private communication, 1999.
11 J.A Brunk, C.J Valenza, and M.C Bhardwaj, in Ultrasonics, Theory and Applications, J.C Duke, Jr., ed.,
Acousto-Plenum Press, NY, 1988, pp 231–238.
12 M.C Bhardwaj and A Bhalla, J Mater Sci Lett 10 (1991).
13 N Kulkarni, B Moudgil, and M Bhardwaj, Am Ceram Soc.,
19 D.W Schindel, D.A Hutchins, L Zou, and M Sayer, IEEE
Trans Ultrasonics Ferroelectic Frequency Control 42: 42–51
(1995).
20 I Ladabaum, B.T Khuri-Yakub, and D Spoliansky, Appl.
Phys Lett 68: 7–9 (1996).
21 M Castaings and B Hosten, Ultrasonics 36: 361–365 (1998).
22 M Landa, M.C Bhardwaj, and I Neeson, Institute of momechanics, Academy of Sciences of the Czech Republic, Prague, CZ, Report no Z1266/99 (1999).
Ther-23 M.C Bhardwaj, Mater Res Innovation 1: 188–196 (1997).
24 J.P Jones, D Lee, M Bhardwaj, V Vanderkam, and
B Achauer, Acoust Imaging 23: (1997).
25 M.C Bhardwaj, Proc Am Ceram Soc 89: (1998).
26 T Carneim, D.J Green, and M.C Bhardwaj, Ceram Bull (1999).
27 B.R Tittmann, M.C Bhardwaj, V Vandervalk, and I.
Neeson, Proc 23rd Annu Conf Composites Adv Ceram Mater Struct The American Ceramic Society, Westerville, OH, 1999.
28 M.C Bhardwaj, I Neeson, M.E Langron, and V Vandervalk,
24th Annu Conf Composites Adv Ceram Mater Struct The
American Ceramic Society, Westerville, OH (2000).
29 R.Y Vun, Q Wu, M Bhardwaj, and G Stead, Proc 12th Int Symp Nondestructive Test Wood, University of Western
Hungary, Sopron, Hungary, 2000.
Trang 4Paints are used everywhere in an industrialized society
(1,2) The most important functions of paints are
protec-tion and decoraprotec-tion of a substrate Paints can protect
sub-strates against corrosion, oxidative aging, weathering, and
mechanical damage and can also provide pleasant color
contrasts or a lustrous appearance, hide imperfections in
the substrate such as knots in wood, or enhance the beauty
of the substrate by using a wood grain In other words,
paints can add to the useful life of materials and also to
their attractiveness (1)
Smart paints are an innovative type of paint that has
a sensor function as well as the protective and
decora-tive functions of conventional paints Smart paints can
de-tect abnormal vibration of a structural material by
mon-itoring the natural frequencies and mode shapes of the
material They can also detect damage generated in the
material by monitoring the acoustic emission (AE) wave
traveling from the damage location to the material
sur-face Vibration and AE can be monitored in real time, thus
enabling health monitoring of the material even during
operation
Smart paints are used in large-scale structures such as
vehicles operated at high speeds, civil infrastructures of
huge mass and volume, and special facilities that contain
large amounts of petroleum, nuclear fuel, and explosive
substances An accident in these facilities can be
cata-strophic because an enormous amount of energy stored in
the form of kinetic, potential, or internal energy is released
suddenly by the accident Smart paints can possibly
pre-vent such a disaster by warning of abnormal vibration and
damage generated in a structural material Hence, one
ref-erence goes so far as to say “Brush with disaster—Smart
paint warns of impending doom” (3)
The frequency of health monitoring needed for
struc-tural materials increases steadily as age increases
be-cause the corrosion of steel and concrete progresses
gradu-ally during the service period of several decades Smart
paints can be applied to a structural material at any
time before and after the construction of the structure,
thus making health monitoring quite, easy even for a
structure already in active service Smart paints can
make a significant contribution to increasing the service
life of a structure, and consequently to saving natural
resources
∗Deceased
BASIC CONCEPTS OF SMART PAINTS
The frequency range covered by vibrational measurements
is the low-frequency range below∼20 kHz (4), whereas thatcovered in AE wave monitoring is the ultrasonic frequencyrange above ∼20 kHz (5) Therefore, if the sensitivity of
a smart paint is high enough in both frequency ranges,the paint can be used as a vibrational and AE sensor inte-grated into a structural material Such a sensor function
of a smart paint is analogous to the action of a spongethat discharges and soaks up water in response to theapplication and release of external pressure (6) In thisanalogy, a smart paint is a sponge that repeats the cycle ofreleasing and drawing an electrical charge at the naturalfrequency of a structural material or at a frequency of the
AE wave traveling through the material
A smart paint is applied directly to the surface of a tural material when the material is a conductor like metal
struc-or carbon fiber composite In this case, the conducting terial can be used as a bottom electrode for the smart paint.When the structural material is an insulator like concrete
ma-or ceramic, on the other hand, an electroconductive paint
is first applied to the material surface, thus forming a thinconducting layer as a bottom electrode Then, the smartpaint is applied to the surface of the bottom electrode.Whether the structural material is conducting or insulat-ing, an electroconductive paint is applied to the surface ofthe smart paint film, thus forming a thin conducting layer
as a top electrode Then, a high voltage is applied to thesmart paint film using the top and bottom electrodes, thusmaking the film piezoelectrically active This poling proce-dure is usually performed in air at room temperature.Smart paints are piezoelectric composites that consist ofpiezoceramic and polymer phases (see Characterization ofPiezoelectric Ceramic Materials; Piezoelectricity in Poly-mers) Thus, smart paints and piezoelectric compositeshave essentially the same nature with respect to many fac-tors such as the ceramic/polymer composition, the method
of preparation, the poling procedure, and the mechanical,electrical, and piezoelectric properties An essential differ-ence exists in that a piezoelectric composite is used as adiscrete point sensor or actuator, but a smart paint is used
as a continuously distributed sensor that can cover a largesurface area of a structural material
PIEZOELECTRIC COMPOSITES
Piezoceramics such as barium titanate (BaTiO3) andlead zirconate titanate (PZT) are typical piezoelectricmaterials that have excellent properties such as a highelectromechanical coupling coefficient and a moderatedielectric constant (7,8) Piezoceramics, however, havethe problem that the high density inherent in ceramicsmakes the specific acoustic impedance much higher thanthat of water or human tissue, thus causing impedancemismatch (7) Brittleness common to all ceramics is
754
Trang 5PAINTS 755
another drawback of piezoceramics Piezoelectric polymers
such as poly(vinylidene fluoride) (PVDF), on the other
hand, do not have the problems of brittleness and
impedance mismatch, and furthermore have the excellent
property that they can be formed into thin, broad films
However, the electromechanical coupling coefficients and
the dielectric constants of piezoelectric polymers are much
lower than those of piezoceramics (8)
A solution to these problems is the previously
men-tioned piezoelectric composites that consist of piezoceramic
and polymer phases The polymer phase in the composites
increases the composite toughness and also decreases the
composite density and dielectric constant, thus solving
the problems of piezoceramics and piezoelectric polymers
simultaneously (9–11) The electrical and mechanical
properties of piezoelectric composites are determined
primarily by the fraction of the piezoceramic and polymer
phases and by the properties of these constituent materials
(12–14) Composite properties are affected also by the
con-nectivity pattern of the piezoceramic and polymer phases
(15–20)
COMPOSITION OF SMART PAINTS
The smart paints reported so far are piezoelectric
compos-ites made up of piezoceramic particles dispersed in a
poly-mer matrix The polypoly-mer matrix need not be
piezoelectri-cally active, and hence popular polymers such as alkyd,
acrylic, and epoxy resins can be used as the matrix resin
The preparation of smart paints and the application
pro-cedures are essentially the same as those of conventional
paints, except for poling for a dried film of smart paint As
a result, most of the fundamental characteristics and
func-tions of conventional paints are imparted to smart paints,
thus enabling smart paints to have protective, decorative,
and sensor functions simultaneously
Smart paints can form continuous paint films covering a
large surface area of a structural material Because of the
electrically insulating nature of the paint film, however,
the electrical charge actually detected is only that
gener-ated in a region that has an electrode on the surface of
the paint film Therefore, if a set of separate electrodes is
formed on the paint film surface, the electrical charge
gen-erated in each region can be detected and analyzed
sepa-rately This feature of smart paints enables the application
of the paints as a vibrational modal sensor that can
deter-mine the natural frequencies and mode shapes of a
struc-tural material (21,22) Furthermore, this feature enables
another application of smart paints as an AE sensor that
can determine the damage location in a structural
mate-rial quite easily without using the conventional technique
based on the arrival time difference of an AE wave (5)
Paints in general can be applied to all kinds of
materi-als such as metmateri-als, composites, concrete, and ceramics; the
material surface can be flat, curved, or even irregularly
shaped Furthermore, paints can be applied and reapplied
at any time, when necessary Final dry films of paints are
generally light, flexible, and tough These excellent
prop-erties of paint in general are imparted to smart paints as
well, thus giving the smart paints further useful features
as vibrational and AE sensors integrated into a structuralmaterial
FORMATION OF SMART PAINT FILMS
Paint Preparation, Application, and Curing
Paints in general are made up of three components: ment, binder, and volatile liquid (1,2) The volatile liquid
pig-is a solvent or a nonsolvent that provides a practical vpig-is-cosity for packaging and application and does not normallybecome part of the dried paint film The binder is a film-forming substance which is mostly a polymeric materialsuch as alkyd, acrylic, or epoxy resin The binder is used
vis-as a solution in a solvent or vis-as a dispersion of fine particles
in a nonsolvent Such a solution or dispersion is called avehicle Paint pigments are finely divided, insoluble, solidparticles such as titanium dioxide (TiO2), zinc oxide (ZnO),and calcium carbonate (CaCO3) The pigment particles aredispersed stably in the paint vehicle before application andthe pigment particles are distributed uniformly through-out the binder resin in the dried paint film The decora-tive functions of a paint are due, for the most part, to thepigment
The basic components of smart paints are essentiallythe same as those of conventional paints, except that piezo-ceramics such as PZT and BaTiO3are used as pigments insmart paints The piezoceramics used in the smart paints
so far are PZT (23–30) and lead titanate (PbTiO3) (23), andthe binders used are acrylic resin (23), polyurethane (23),and epoxy resin (25–29) Smart paints made up of thesecomponents are prepared by essentially the same proce-dure as used for conventional paints Smart paints areapplied by using familiar coating tools such as brushes,rollers, or spray guns Smart paints are also cured in theusual way in air at ambient temperature or at elevatedtemperatures
Electrode Formation and Poling
A simple method for forming an electrode on the surface
of a paint film is to apply an electroconductive paint by ing a coating tool such as a brush or roller A more elaboratemethod is to deposit a vapor of gold or aluminum onto thepaint film surface (30) A screen mask technique is also ef-fective for this purpose, especially when the electrode pat-tern is complicated The main advantage of this technique
us-is that leads as well as electrodes can be printed on thepaint film surface, as shown in Fig 1 This technique, how-ever, has the disadvantage that it cannot be used for largestructures such as airplanes, trains, or bridges
For such large structures, an ordinary coating method
by brush, roller, etc may be the most practical for forming
an electrode on the paint film surface As a lead for theelectrode, on the other hand, a thin electrical wire or tape
∼50 µm thick or so may be the most practical choice for a
large structure because such a thin wire or tape is rable in thickness to a paint film and hence, can be buried
compa-in the pacompa-int film or under a topcoat Note that when smartpaints are put into practical use, the electrodes and leadsare covered by a topcoat, thus making the appearance ex-actly the same as that of conventional paints
Trang 6756 PAINTS
Figure 1 Electrodes and leads printed on a PZT/epoxy paint film formed on one surface of an
alu-minum beam The left end of the beam where the leads come together is wrapped in an electrically insulating material The aluminum beam is clamped at this section for vibrational measurements.
Piezoelectric composites are usually poled in an oil bath
at elevated temperatures because poling at a higher
tem-perature achieves saturation poling in a lower poling field
For smart paints, on the other hand, poling is done in air
at room temperature because even room temperature
pol-ing can achieve high enough piezoelectric activity for the
paint application to serve as vibrational and AE sensors
integrated into a structural material (25–29)
EVALUATION OF SMART PAINT FILMS
The sensor function of smart paints relies heavily on the
piezoelectric activity of the poled paint film Usually, the
activity is expressed in terms of a piezoelectric constant
which is the ratio of the charge developed per unit
sur-face area or the voltage developed per unit film thickness
to the stress or strain applied externally The
charge-to-stress, charge-to-stress, charge-to-strain, and
voltage-to-strain ratios are the piezoelectric constants d, g, e, and h,
respectively (7)
Piezoelectric materials are inherently anisotropic, and
hence two subscripts are attached to the piezoelectric
constant to describe the anisotropic properties The first
subscript is used to indicate the direction of the charge or
voltage development, and this is always the film thickness
direction for a piezoelectric film such as PVDF or a smart
paint film The second subscript is used to indicate the
di-rection of the stress or strain applied externally, and this
direction is any of the 1, 2, and 3 axes of the film which
correspond to the length, width, and thickness directions,
respectively (7)
Sensitivity as a Vibrational Sensor
When a structural material is deformed, strain is
devel-oped in all directions of the material, including the
direc-tion tangent to the material surface This is also true when
the structural material is vibrating For a smart paint used
as a vibrational sensor, therefore, one of the most
impor-tant sensitivities to be evaluated is the piezoelectric
con-stant e31 because this constant is the ratio of the charge
per unit surface area to the strain in the direction tangent
to the paint film surface
The e31constant is evaluated from vibrational
measure-ment on a cantilever beam like that shown in Fig 1 A
typical example of the measurement is shown in Fig 2
Figure 2 Frequency spectra of output signals from a PZT/epoxy
paint film formed on one surface of an aluminum beam and from
a strain gauge bonded to the opposite surface of the beam.
for a paint film which has the PZT/epoxy composition of53/47 by volume and is formed on the surface of an alu-minum beam 3.0 mm thick, 30 mm wide, and 460 mm long(350 mm long as a cantilever beam) (27) This example isfor a 109-µm thick paint film cured at room temperature
and poled at 240 kV/cm for 5 min The spectrum shape tained from the paint film is similar to that obtained from
ob-a strob-ain gob-auge which is bonded to the opposite surfob-ace ofthe beam to monitor the strain developed in the direction
of the cantilever length Then, the e31constant is evaluatedfrom the charge-to-strain ratio at a natural frequency of 18
or 112 Hz
The e31constant thus evaluated depends on many tors such as the poling field, the film thickness, the curetemperature, and the PZT/epoxy composition (26,27) Atypical example of the poling-field and film-thickness de-pendence is shown in Fig 3 for paint films cured at roomtemperature that have the PZT/epoxy composition of 53/47
fac-by volume (27) The e31constant increases steadily as thepoling field increases for all of the paint films shown here,and saturation poling is not achieved, even at a high pol-ing field of ∼150 kV/cm The e31 constant obtained at aparticular poling field, say, 100 kV/cm, increases as filmthickness increases from 33 to 152µm, thus exhibiting a
clear film-thickness dependence This point is further scribed later
Trang 7Figure 3 Plots of the piezoelectric constant e31 vs the poling
field for PZT/epoxy paint films cured at room temperature and
evaluated as a vibrational sensor.
Sensitivity as an Acoustic Emission Sensor
In many cases, eventual failure of a structural material
oc-curs after a certain amount of damage accumulates within
the material The generation of such damage is almost
al-ways accompanied by the emission of an AE wave, and
hence the damage generated and accumulated can be
de-tected by monitoring the AE wave (5) The AE wave is
emit-ted in all directions, and consequently, an AE wave that
arrives at the material surface and enters the smart paint
film on the material surface always exists Furthermore,
an AE wave that enters the paint film nearly
perpendi-cularly always exists Such an AE wave develops strain in
the paint film in the direction normal to the film surface
be-cause the AE wave is a compression wave in which particle
motion is in the same direction as the propagation of the
wave For a smart paint used as an AE sensor, therefore,
the sensitivity to be evaluated is the piezoelectric constant
h33because the h33constant refers to the ratio of the
volt-age per unit film thickness to the strain in the direction
normal to the paint film surface
For a conventional AE sensor, the sensitivity s is
usu-ally given by s = V/v0, where V is the output voltage of the
sensor and v0is the velocity amplitude of AE waves (31)
The strain amplitude of AE wavesε0is given byε0= v0 /v,
where v is the phase velocity of AE waves Combining these
equations with h33 = (V/d)/ε0 leads to s = h33d/v, where
d is the film thickness This equation indicates that the
paint film sensitivity as an AE sensor s is independent
of the frequency of AE waves and that the sensitivity
in-creases linearly as film thickness inin-creases This
equa-tion also indicates that the h33constant is calculated from
h33= sv/d.
The paint film sensitivity as an AE sensor is evaluated
from measurement using an ultrasonic transducer to
pro-duce AE waves and a laser Doppler vibrometer to
moni-tor the velocity amplitude of the AE waves (28) A typical
example of the measurement is shown in Fig 4 for a paint
film that has the PZT/epoxy composition of 53/47 by
vol-ume and is formed on the surface of square aluminum plate
Figure 4 Frequency spectra of output signals from a PZT/epoxy
paint film formed on one surface of an aluminum plate and from
a laser Doppler vibrometer that monitors the velocity amplitude
of AE waves.
0.2 mm thick that has 50 mm sides This example is for a
poled at 184 kV/cm for 5 min The spectral shape obtainedfrom the paint film is similar to that obtained from thelaser vibrometer in the frequency range above∼0.3 MHz.Such a similarity of spectral shapes reflects a nearly flatfrequency response of the paint film to AE waves Then,the paint film sensitivity as an AE sensor is evaluatedfrom the average ratio of the output voltage of the paintfilm to the velocity amplitude of AE waves in the frequencyrange 0.3–1.0 MHz
The paint film sensitivity thus evaluated, s can be verted into the h33 constant by using the relationship
con-h33= sv/d, where v is the phase velocity of AE waves in the PZT/epoxy paint film The h33constant calculated by using
an assumed value of v= 2850 m/s (6) is plotted in Fig 5 as
a function of film thickness for paint films cured at temperature that have the PZT/epoxy composition of 53/47
Film thickness, µm
120100806040200
h33
Figure 5 Plots of the piezoelectric constant h33at 50 (◦), 100 (•),
150 ( ), and 250 kV/cm () vs film thickness for PZT/epoxy paint films cured at room temperature and evaluated as an acoustic emission sensor.
Trang 8758 PAINTS
by volume (28) It is seen that the h33 constant obtained
at a poling field of 50, 100, 150, or 250 kV/cm increases
steadily as film thickness increases, thus exhibiting a
clear film-thickness dependence Such a film-thickness
de-pendence is also observed for the e31 constant shown in
Fig 3
FACTORS DETERMINING POLING BEHAVIOR OF SMART
PAINT FILMS
The poling behavior of a PZT/epoxy paint film depends
on the film thickness, as shown in Figs 3 and 5
Fur-thermore, the poling behavior also depends on the cure
temperature and the PZT/epoxy composition (26–29) Such
complicated poling behavior is virtually determined by the
electric field that acts on the PZT particles dispersed in the
epoxy matrix The most important factors that determine
the electric field and, consequently, the poling behavior of
the paint film are the electrical conductivities of the PZT
particles and the epoxy matrix, the connectivity pattern of
the PZT phase, and the space charge accumulated at the
PZT/epoxy interface
Electrical Conductivities of Constituent Materials
It is now well established that in poling a composite
speci-men made of piezoceramic particles dispersed in a polymer
matrix, the electric field that acts on the ceramic
parti-cles is very low compared with that applied externally to
the composite specimen (14,32) This occurs because the
electrical conductivity of polymeric materials in general is
much lower than that of ceramic materials, and hence the
polymer matrix in the composite specimen bears almost all
of the externally applied electric field at the expense of the
electric field that acts on the ceramic particles As a result,
the piezoelectric activity of the ceramic/polymer composite
specimen is very low, compared with a pure piezoceramic
specimen poled in the same electric field This idea explains
why saturation poling is not achieved, even in a high poling
field of ∼150 kV/cm, as seen in Fig 3 Saturation poling
for a pure PZT ceramic specimen, on the other hand is
achieved in a low poling field of ∼10 kV/cm (12)
A promising solution to this problem is to increase the
electrical conductivity of the polymer matrix up to that of
the ceramic particles, so that the electric field distribution
becomes uniform throughout the composite specimen This
can be achieved by adding a small amount of a
semicon-ductor filler such as carbon, germanium, or silicon to the
composite specimen (32) This can also be achieved by
pol-ing at a high temperature where the electrical conductivity
of the polymer matrix becomes equal to that of the ceramic
particles (33)
Connectivity Pattern of Ceramic Phase
Figure 6 is a scanning electron microscopy (SEM) picture
that shows the internal microstructure of a paint film that
has the PZT/epoxy composition of 53/47 by volume (27) It
is seen that the size of PZT particles ranges from∼0.5 to
∼1.5 µm, and that a substantial fraction of the PZT
parti-cles are in contact with each other, so that the PZT phase
10 µm
Figure 6 SEM picture of a paint film that has the PZT/epoxy
composition of 53/47 by volume This example is a 49-µm thick
paint film cured at 150 ◦C.
is practically self-connected in three dimensions Theself-connectivity of the PZT phase is one of the most im-portant factors that determines the poling behavior of aPZT/epoxy paint film In fact, the paint film is hardly poledwhen the PZT volume fraction is decreased to such a levelthat the PZT particles are isolated from one another by thecontinuous phase of the epoxy matrix (26)
Figures 3 and 5 show that the poling behavior of aPZT/epoxy paint film depends on the film thickness evenwhen the PZT volume fraction remains constant at 53%
A SEM picture like that shown in Fig 6, however, detects
no observable difference in the PZT phase connectivity forpaint films that have different thicknesses The difference
in the PZT phase connectivity is reflected much more plicitly in the current–voltage characteristic of the paintfilm rather than in the SEM picture, as described here
ex-Space Charge at the Ceramic/Polymer Interface
The current–voltage characteristic of a PZT/epoxy paintfilm shows that the conduction is ohmic in a low electricfield, whereas in a high electric field, the space-charge-limited (SCL) conduction predominates over ohmic conduc-tion (28) Furthermore, the current–voltage characteristicshows that the critical electric field at which the ohmic-to-SCL transition takes place decreases as the film thicknessdecreases The result is that conduction during the polingprocess is mostly SCL for a thin film, whereas conduction
is mostly ohmic for a thick film
The SCL conduction becomes predominant when aspace charge of electrons is injected into the PZT/epoxypaint film during the poling process The space charge has
a tendency to build up preferentially at the interface tween the PZT and epoxy phases in the paint film (28).The space charge decreases the electric field acting on thePZT phase, and hence decreases the piezoelectric activity
be-of the paint film obtained in a given poling field This fect of the space charge becomes significant, particularlyfor a thin film, because SCL conduction becomes more
Trang 9ef-PAINTS 759
predominant as the film thickness decreases Therefore,
the film-thickness dependence of the piezoelectric constant
shown in Figs 3 and 5 is ascribed to the space charge of
electrons injected into the paint film during the poling
pro-cess
The fact that the current–voltage characteristic of a
PZT/epoxy paint film depends on the film thickness is
closely related to the drying rate of the wet paint film In
fact, it is well known that the thickness of a wet paint film
has a significant influence on the rate of solvent
evapora-tion and, consequently, on film formaevapora-tion during curing (3)
Thus, it is quite possible that the degree of self-connectivity
of the PZT phase depends on the thickness of the dried
paint film Therefore, the drying rate of the wet paint film
is another important factor that determines the poling
be-havior of a PZT/epoxy paint film
TECHNIQUES FOR APPLYING SMART PAINT FILMS
Techniques for applying smart paint films as vibrational
and AE sensors are essentially the same as those for a
PZT ceramic or PVDF film bonded to the surface of a
structural material Theories, models, methods, and
sys-tems constructed for use of the PZT and PVDF sensors
(21,22,34) can also be applied to smart paint films used
as vibrational and AE sensors integrated into a structural
material
Vibrational Modal Sensor
One example of an application of smart paints is a
vibra-tional modal sensor integrated into a structural material
As noted before, the sensitivity of the paint film used for
this purpose is the e31 constant which is the ratio of the
charge per unit surface area to the strain in the
direc-tion tangent to the paint film surface Figure 7 shows a
result of vibrational modal testing of a cantilever beam
like that shown in Fig 1 by using a PZT/epoxy paint film
Figure 7 Modal strain shapes of a cantilever aluminum beam
for the first (◦), second (•), and third modes ( ) determined by a
PZT/epoxy paint film formed on the beam surface.
that has an e31constant of 9.0 × 10−3(C/m2)/(m/m) (26) Aset of vibrational measurements is carried out for all of theelectrodes formed on the paint film surface: an identical ex-citatory force is applied at a fixed point on the cantileverbeam Then, the output charge of the paint film at each
electrode is converted into the strain using the e31constantand is plotted against the distance from the clamped end ofthe beam to the center of each electrode The modal strainshapes thus obtained are shown in Fig 7 for the first threemodes at 18, 112, and 315 Hz
It is worth nothing that the modal strain shapes shown
in Fig 7 can be converted into modal displacement shapes
by d2φ/dx2= −ε/η, where φ is the transverse ment of a uniform cantilever beam, x is the longitudinal
displace-coordinate of the beam,ε is the longitudinal strain in the
beam surface, andη is the half-thickness of the beam (35).
Modal displacement shapes determined by this equationare identical to those determined by a laser Doppler vi-brometer that measures the transverse movement of thebeam surface (26) Thus, smart paints offer an interestingand promising alternative to conventional sensors such asaccelerometers and laser vibrometers (1)
FUTURE DIRECTIONS
Smart Paints
The highest sensitivity of smart paint films achieved so
far is e31= ∼40 × 10−3(C/m2)/(m/m) as a vibrational
sen-sor and h33= ∼100 × 106 (V/m)/(m/m) as an AE sensor,
as shown in Figs 3 and 5 For commercially available
PVDF films, the sensitivity is e31= ∼66 × 10−3 (C/m2)/
(m/m), e32 = ∼6.8 × 10−3 (C/m2)/(m/m), and h33= ∼50 ×
106 (V/m)/(m/m), determined in essentially the same waydescribed before for smart paint films This indicates thatthe sensitivity of smart paint films is comparable to that
of PVDF films So far as sensitivity is concerned, fore, smart paints have already reached a level suitablefor practical use
there-For smart paints to be put into practical use, however,the paints must meet performance requirements such asexterior durability and sensitivity stability Exterior dura-bility is the paint films resistance to environmental factorssuch as uv radiation, heat, moisture, oxygen, and ozone (2).These environmental factors can cause mechanical degra-dation of paint films, thus leading to the failure of the pro-tective and decorative functions of smart paints These en-vironmental factors may also cause electrical degradation
of paint films, thus leading to the failure of the sensor tion of smart paints Considering that smart paints aretruly appreciated when used in severe and isolated envi-ronments, the evaluation of exterior durability and sensi-tivity stability is absolutely necessary for the paints to beput into practical use
func-Smarter Paints
According to a concept of intelligent materials in Japan,the intelligence in materials is classified into three cat-egories; intelligence from the human standpoint, intelli-gence inherent in materials, and intelligence at the most
Trang 10760 PAINTS
primitive levels in materials (36) The intelligence from
the human standpoint is a relative concept based on the
value of a material and its utility in relation to all
as-pects of society such as economy, conservation of resources,
intensiveness of information, human friendliness,
relia-bility, harmony with the environment, and optimum life
span
Water-borne piezoelectric paints are smarter paints
from the standpoint of harmony with environment (37)
A paint that can spontaneously become a piezoelectric film
after the usual drying process will also be a smarter paint
from the standpoint of human friendliness In fact, poling
a paint film at a high voltage is dangerous work and should
be avoided if possible A feasibility study of a poling-free
piezoelectric paint shows that a paint made of PVDF
par-ticles and epoxy resin does not need poling for the final
dry film to be piezoelectrically active (38) At the present
stage, however, the piezoelectric activity is not enough for
practical use of the paint film Studies are currently
un-der way to increase the piezoelectric activity of the paint
film
From the standpoint of intensiveness of information, a
smarter paint of the future will have a sensor function for
material conditions such as vibration and damage
gener-ation and also for atmospheric variables such as
temper-ature, pressure, moisture, and wind velocity Such a paint
resembles human skin in that the skin has a sensor
func-tion for the external stimuli imposed on the human body
and also for the surrounding conditions such as
tempera-ture, humidity, wind, and rain The ultimate goal of smart
paints, therefore, should be to mimic the human skin as
closely as possible
ACKNOWLEDGMENTS
The work in smart paints by S Egusa and N Iwasawa was
supported by the Japan Atomic Energy Research Institute
through the Special Program for Fundamental Researches
(1991–1994) and through REIMEI Research Resources
(1998)
BIBLIOGRAPHY
1 J.H Lowell, in Coatings, J.I Kroschwitz, ed., Encyclopedia of
Polymer Science and Engineering, 2e., Wiley-Interscience, NY,
1985, Vol 3, pp 615–675.
2 Z.W Wicks, Jr., in Coatings, J.I Kroschwitz, ed., Encyclopedia
of Polymer Science and Engineering, 2e., Wiley-Interscience,
NY, 1989, Supplement Vol pp 53–122.
3 O Graydon, New Scientist, p 20, October 17, 1998.
4 D.J Ewins, Modal Testing: Theory and Practice Research
Studies Press, Taunton, 1984.
5 C.B Scruby, J Phys E: Sci Instrum 20: 946–953 (1987).
6 KYNAR Piezo Film Technical Manual, Pennwalt Corporation,
Valley Forge, PA, 1987, p 6.
7 A.J Moulson and J.M Herbert, Electroceramics Chapman &
Hall, London, 1990, Chap 6.
8 M.V Gandhi and B.S Thompson, Smart Materials and
Structures Chapman & Hall, London, 1992, Chap 5.
9 T Kitayama and S Sugawara, Proc Gr Inst Electr Comm Eng Jpn., 1972, CPM 72-17 (in Japanese).
10 L.A Pauer, IEEE Conf Res., pp 1–5 (1973).
11 W.B Harrison, Proc Workshop Sonar Transducer Mater.
Naval Research Laboratories, November 1975, p 257.
12 T Furukawa, K Fujino, and E Fukada, Jpn J Appl Phys.
19 R.E Newnham, Ferroelectrics 68: 1–32 (1986).
20 R.E Newnham and G.R Ruschau, J Am Ceram Soc 74(3):
463–480 (1991).
21 C.-K Lee and F.C Moon, J Appl Mech 57: 434–441 (1990).
22 S.A Collins, D.W Miller, and A.H von Flotow, Sensors for Structural Control—Applications Using Piezoelectric Polymer Film Space Engineering Research Center #12-
90, Massachusetts Institute of Technology, Cambridge, MA, 1990.
23 K.A Hanner, A Safari, R.E Newnham, and J Runt,
27 S Egusa and N Iwasawa, Ferroelectrics 145: 45–60 (1993).
28 S.S Egusa and N Iwasawa, J Appl Phys 78: 6060–6070
(1995).
29 S Egusa and N Iwasawa, J Smart Mater Struct 7: 438–445
(1998).
30 J.M Hale and J Tuck, A Novel Strain Transducer Using
Piezo-electric Paint Proc Mech Eng in press.
31 ASTM E1106-86, Standard Method for Primary Calibration of Acoustic Emission Sensors American Society for Testing and
Materials, Philadelphia, PA, 1986, pp 489–498.
32 G Sa-Gong, A Safari, S.J Jang, and R.E Newnham,
35 S.H Crandall, N.C Dahl, and T.J Lardner, An tion to the Mechanics of Solids McGraw-Hill, NY, 1972,
38 S Egusa, 1998 REIMEI Conf., Japan Atomic Energy
Research Institute, Tokai, Japan, July 14–15, 1999.
Trang 11PEST CONTROL APPLICATIONS 761
PEST CONTROL APPLICATIONS
SHERRYDRAISEY
Good Vibrations Engineering, Ltd
Nobleton, Ontario, Canada
INTRODUCTION
The smart aspects of the piezoceramic ultrasonic
appli-cation being used for pest control are just beginning to
evolve Pest control, using ultrasonics, is based on
devel-oping a pressure environment which is extremely
unpleant or deadly to the pests in question The feedback
as-pect of smart structure applications involves three types of
sensing:
rmotion sensors (designed to power up the ultrasonic
device when large pest groups have been detected)
rpressure sensors (these are used in fluid media to
sense if pressure levels have risen enough to ate structural instability)
gener-rsound sensors (for antinoise generation to stop the
sound from being externally transmitted) that nate the antinoise generation
coordi-Airborne or land pests, such as some insects, spiders,
rodents, and small cats and dogs are driven away by the
unpleasant sound created by the noise generated by the
ceramic elements For fluid-borne pests, the ceramic is
driven to create a pressure field that includes cavitation
The release of energy from the collapse of cavitating
bub-bles provides the source deadly to small microorganisms
Table 1 lists the types of pests that have been
effec-tively deterred by ultrasonic measures The table lists the
frequency range that has been successful for these pests,
as well as the approximate coverage (or flow rate) across
which they are effective The coverage is directly related to
the system size and power
The Environmental Protection Agency (EPA) has
sug-gested that pest control devices have a deterrent effect of
>60% to be considered viable.
SOUND AS A PEST DETERRENT
The control of airborne and land pests is based on
gen-erating high-frequency noise This is done to disturb and
confuse the species, making the environment generally
un-pleasant The sound levels are in the range of 90+ dB at
1 meter from the source
Table 1 Pests Effectively Controlled by Ultrasonic Devices
Coverage (varies with power
Dogs, cats, skunks 14–25 kHz 278.8 m 2 (4000 ft 2 )
The power supplies for the designs varies from plug-inwall units (110 or 220/240 V) to battery operated systems.Motion sensors are used for detecting larger size pests.This reduces power consumption and eliminates unneces-sary noise pollution
Test Results
The test data presented here were provided by theWeitech company, a manufacturer of a variety of ultrasonicdeterring devices designed to produce ultrasonic sound
in air
Mosquitoes At least one company’s test results of the
high-frequency ultrasonic deterrent effect on mosquitoeshas suggested that it does not meet the EPA suggesteddeterrent level
Small Rodents The available test results (1) for small
rodents depend on the particular rodent Two types of dents are considered For each test set, there were six ro-dents in the sample—three males and three females Theywere housed in two adjoining chambers, one exposed to theultrasonic sound (∼90 dB), the other at much lower noiselevels ( 30 to 35 dB or lower)
ro-Two parameters are used to evaluate the influence of theultrasound—food consumption (measurement of the dailyfood consumption in the treated and untreated chambers)and activity (animal track evidence in the treated and un-treated chambers) Before the introduction of ultrasonictreatment, healthy mice that had good hearing (hearingtest—Preyer’s reflex, a reaction to loud noise) are housed
in the two chambers, and their activity and food tion levels are measured
consump-The effect of the ultrasonic deterrent on the Norway rat
(Rattus norvegicus) is more pronounced than on wild house mice (Mus musculus) The average weight of the Norway
rats in the test was 237 grams (8.4 oz) The average weight
of the wild house mice was 17 grams (0.6 oz) The resultsare shown in Figs 1 and 2 as an index (the ratio of thetreated measurements to the total measurements) Foodconsumption influence is shown in black bars, and trackingactivity is shown in gray
Trang 12762 PEST CONTROL APPLICATIONS
Influence on wild norway rat population
Figure 1 The influence of ultrasonic noise on the Norway rat
population.
Figure 1 shows the effect of treatment on the Norway
rat Figure 2 shows the effect of the treatment on wild
house mice The influence on both populations is most
sig-nificant for food consumption The tracking activity of the
wild house mice is not heavily influenced by the ultrasonic
effect
The rodents’ hearing was checked before and after the
testing Only rodents that had good hearing were selected
for the study It has been postulated that the rodents might
eventually become accustomed to the noise, but this was
not the case There were instances where rodents were not
influenced, but this was due to hearing loss
The sound patterns (frequency and amplitude) of four
of the pace electronic pest repeller units were measured
0.8 Pre treatment
Treatment
Post treatmentInfluence on wild housemice population
Figure 2 The influence of ultrasonic treatment on the wild house
mice population.
The primary source of total sound output was at 40 kHzand above The sound output dropped slightly at 31.5 kHz.Sound output below 20 kHz was negligible
CAVITATION AS A DESTRUCTOR
Piezoceramic elements are commonly used to induce tation in fluids in biological applications for scaling in-struments, but killing microorganisms is normally done byhigh-temperature sterilization The erosive effect of cavi-tation is what is useful in removing a variety of type ofscales Cavitation is caused when the localized pressuredrops below the fluid vapor pressure This results in cavi-tating bubbles
cavi-The collapse of cavitating bubbles is accompanied by arapid release of energy It is the collapse of the cavitat-ing bubbles that is used to destroy microorganisms It isnot clear whether the microorganism population is imme-diately killed by the bubble collapse, or if the population isjust weakened enough to limit its viability
The generation of cavitation is limited to areas fairlyclose to the pressure/sound source Cavitation can be ap-plied to a large volume of fluid either by moving the sourcethrough the fluid or by moving the fluid past the source.The application described here moves the fluid past thesource by pumping the volume through tubing to ensurefairly even exposure of the liquid to the pressure field Thisdoes not sterilize the fluid, but it does eliminate a signifi-cant portion of the microorganism population
The biological test results available indicate that tion does significantly reduce the population in both waterand diesel fuel, but the effect varies for the types of microor-ganisms tested The population reduction is of the order of50%
cavita-It is expected that piezoceramically induced cavitationcould be used to reduce zebra mussel population in nuclearreactor water intake tubes by interfering with the zebramussels during an early stage of their development, such
as the larval stage
The specific engineering design that follows was based
on controlling microbial growth in military marine dieseltanks These populations are currently controlled by “goodhousekeeping” of ships’ tanks and by using environmen-tally harmful biocides If an ultrasonic cavitation systemwere to be installed on a ship, it would be necessary to in-clude an antinoise system to cancel the ultrasonic soundthat creates the cavitation This would be needed to mini-mize the likelihood that the vessel would be detected byunfriendly ships
Engineering Application/Design
The cavitation of a fluid is induced when local pressuredrops below its vapor pressure It involves the release ofrelatively small amounts of energy (compared to boiling),
so that though there is a temperature change in the fluid;
it is small (of the order of 1–2◦C, depending on exposuretime and volume)
One of the well-known side effects of cavitation is its sive effects on materials This presents a practical problem
Trang 13ero-PEST CONTROL APPLICATIONS 763
Driverelectronics
Cavitation bubblesInner tube
Working mediumPiezoceramic ringsTransmission medium
Figure 3 Schematic of cavitation concept.
in trying to use cavitation The components used to cause
the cavitation need special consideration to survive the
ero-sive environment
A general requirement for pest control is that it is
needed for large volumes Cavitation is a fairly local
ef-fect To apply it to a large liquid volume, the fluid must
be brought into a fairly local range One way of
achiev-ing this is a flow-through system The liquid is pumped
through tubes that are exposed to the cavitating field Such
an arrangement could involve expenditures of significant
amounts of power
A flow-through configuration was studied analytically
to achieve maximum fluid cavitation at minimum power
consumption The particular system modeled was based
on a two-fluid system to avoid the electrode erosion that
would be induced by cavitation Figure 3 shows the
con-ceptual arrangement The fluid immediately adjacent to
the electrodes is pressurized to eliminate cavitation This
fluid is used to transmit energy through a thin-walled pipe
(stainless steel) into the fluid that contains the
microor-ganism The analytical model of the system was a
piezo-dynamic field modeled by using finite elements It is based
on a finite element formulation of the piezoceramic
ele-ments, the physical piping structure, a liquid
transmis-sion medium, and the sound pressure field experienced
by the microorganism-borne fluid (either water or diesel
fuel)
The model was then test verified before applying it to a
specific design
Finite Element Formulation The finite element method
is an analytic technique for solving general field problems
It offers a number of advantages over competing
meth-ods It can handle arbitrary geometries and both static
and dynamic problems It uses matrix numerical methods
for which very efficient and general algorithms have been
developed
The special purpose FE formulation developed to dle both the fluid characteristics and the electrical input(as well as the normal structural characteristics) was based
han-on the principles of the FE method in (2) The code eled the structural behavior of the elements that representthe piezoelectric components, as outlined in (2, p 22) Thepiezoelectric behavior was included using the approach of(3, p 86) The fluid areas of the model were analyzed usingthe approach described in (2, p 540)
mod-The degrees of freedom of the model are the group of
rnodal displacements of the solid components,
rnodal pressures of the fluid components,
rnodal electrical potentials of the piezoelectric
compo-nents, and
rthe junction voltages of an external electrical circuit
connected to the piezoelectric components (this lattercapability was not used, though it is included for pos-sible future use)
Then, the defining equations of the finite element approachused are
.
Trang 14764 PEST CONTROL APPLICATIONS
I = external circuit inductance
C= external circuit capacitance
R= external circuit resistance
U= solid element nodal displacements
P= fluid element nodal pressures
V = external circuit voltages
F= externally imposed force on solid element nodes
Q= externally imposed charges on piezoelectricelements
Q N = externally imposed charges on external circuit
φ = piezoelectric element nodal potentials
a= speed of sound in fluid
where
[Ns]= shape function matrix for solid elements
[Nf]= shape function matrix for fluid elements
[B ]= shape function derivatives giving strain in solid
elements
[Be]= derivatives of potential shape function in
piezo-electric elements
ρ = mass density (subscript s for solid, f for fluid)
µ = damping (subscript s for solid, f for fluid).
The model assumed axisymmetry which was
imple-mented as described in (2, p 119) The elements describe
the cross section of the complete unit from the centerline
out, that is, that section which is rotated about the axis
of symmetry to sweep out the 3-D geometry of the unit
The elements used were eight-node, isoparametric
quadri-laterals, using quadratic shape functions for all fields (2-D
solid displacements, fluid pressures, and electrical fields)
Third-order Gaussian numerical integration was used for
all element integrals The integrals across volume are
done by the usual finite element approach of integrating
across each element independently, followed by assemblingthe resulting equations into matrix form, as described in(2, p 9)
Damping was included in the model by adding rial damping to the fluid regions, as described in the pre-ceding equations Based on experimental measurements,enough damping was included to give a resonant amplifica-
mate-tion (Q factor) of 5 to 8 Two extreme condimate-tions were used.
In the first, damping was distributed across both the mission and working media In the second, damping wasconcentrated in the working medium The first case corre-sponds most closely to low excitation levels, whereas thesecond should more closely match high excitations whencavitation is occurring Then, the energy dissipation will
trans-be concentrated in the working medium trans-because of thecavitation
The model is linear This is expected to give good sults up to the point at which cavitation begins Beyondthat point, the response of the system is no longer linearbecause the fluid behaves effectively less stiff on the nega-tive side of the pressure wave than on the positive side due
re-to the formation of cavitating bubbles In principle, thiseffect could be modeled using the nonlinear approachesdescribed in (2, p 450) This simplification was acceptedbecause the objective was to compare alternative designs,rather than to analyze the behavior in absolute terms It isassumed that systems that give a greater linear responsewill also give a greater nonlinear response This may not
be true in unusual cases, and it may not represent the fect of changes in the spatial distribution of the acousticfield in all cases (it would be expected that the “softening”nonlinearity which will occur here would tend to make theenergy distribution more uniform in the system, compared
ef-to the linear case)
Figure 4 shows typical results from the model Theseshow the pressure distribution across the fluid cross sec-tion for 100 volt peak–peak excitation of the piezo rings forvarious excitation frequencies It can be seen that the en-ergy in the working medium in all cases is concentrated atthe center At low frequencies, only a single pressure peakoccurs At higher frequencies, when the wavelength of thesound waves in the fluid becomes comparable to the di-mensions of the device, two and then three pressure peaks
Figure 4 Finite element predictions of cavitating field.
Trang 15PEST CONTROL APPLICATIONS 765
Table 2 Finite Element Model Parameters
Inner tubing Stainless steel tube 1.5 in outer diameter
(E= 30E6 psi) 0.012 in wall thickness
0.5 in height Transmission fluid SAE 10W30 motor oil Density,
speed of sound Working fluid Water or diesel fuel Density,
speed of sound
occur axially along the centerline These observations are
consistent with qualitative results These results were
ob-tained by suspending an aluminum foil strip in the
cavi-tating field Because it is known that cavitation erodes
alu-minum, the distribution and degree of perforation provide
an indication of the cavitating intensity
The specific parameters of the model are listed in
Table 2
Test Verification of Analytical Model Modeling a
com-bined electrical/piezoelectric/structural/fluid system is
complex A number of approximations and simplifications
were made For this reason, some model correlation was
done in advance of prototype development (experimental
data taken from breadboard unit) The FE model was done
for a four-ring prototype The experimental testing was
done on a three-ring arrangement
There were two type of measurements made for the
correlation exercise, the current–voltage relationship and
sound pressure measurements The predicted and
mea-sured current versus voltage relationship for the system is
shown in Figure 5 Measured values are shown at 22.7 kHz
Figure 5 Measured and predicted current vs voltage.
which gives the peak piezo current Model values areshown for both this frequency and for 26.5 kHz, which isthe frequency at which the model shows peak current Itcan be seen that the measured values at low voltages areabout 60% of the modeled values This is mainly due tothe four rings in the model versus three in the breadboard.The sound pressure field was measured using the SpecialtyEngineering Associates needle hydrophone, Model SPRH-2-0500
Figure 6 shows the response of the hydrophone at twodifferent excitatory voltage levels, as captured on a digi-tal storage oscilloscope Note that the two cases were
at slightly different frequencies These frequencies spond to the peak responses at each excitatory level Thatthey are different indicates nonlinearity in the model Itcan be seen that the hydrophone response waveform is un-symmetrical and has pressure spikes on the positive volt-age (low pressure) side This is an indication of cavitation
corre-It is more prominent at the higher excitatory voltage.The model predicts that the peak pressure in the unitshould be 1 kPa per volt of excitation The transducer out-put should be 0.25 mV per volt of excitation The results
in Fig 6 show a 20-mV peak-to-peak response at 130-Vpeak-to-peak excitation in (a) and 65 mV response at 240 Vexcitation, or 0.16 mV/ V and 0.27 mV/ V, respectively Thisagreement is reasonable given the uncertainty of the hy-drophone (it was being used somewhat out of its design fre-quency range) The model predicts that the pressure shouldlead the voltage by 10 to 20◦, and it can be seen that this
is reasonable, though the experimental measurements donot really allow testing this
Figure 7 shows the pressure distribution measuredalong the centerline of the device for low voltage excita-tion (where the nonlinearity of the system does not con-fuse the results), and Fig 8 shows the pressure distribu-tion measured across the centerline at the midheight of thepiezo rings The hydrophone readings in these figures havebeen converted to acoustic pressures The model predic-tions are also shown It can be seen that the model and mea-sured values show the same trends and the differences are1–3 dB
Design Studies
Outer Diameter of Transmission Medium A design was
studied to optimize the outer diameter of the transmissionmedium on the sound intensity in the working medium
Trang 16766 PEST CONTROL APPLICATIONS
The integral of acoustic pressure across the volume of the
working medium was used as a performance indicator
Two extremes of damping models were used—damping
concentrated in the working medium and damping
dis-tributed over both working and transmission media
Fig-ure 9 shows the results for both cases (as the integral
of pressure vs the outer diameter, (OD) of the
transmis-sion medium It can be seen that when damping is
concen-trated in the working medium, the optimum occurs at an
OD of 113 mm because the spacing between the outside
of the piezo ring and the OD of the transmission medium
is about one-half an acoustic wavelength Such a condition
would be expected to result in translating the high acoustic
impedance condition at the rigid outer wall to a low
acous-tic impedance at the ring [see (8), p 18 for an example]
This low acoustic impedance of the transmission medium
Rings
Model at 25.0 kHz
13 V P−P ExcitationMeasured at 23.7 kHzMeasured at 26.0 kHz
84828078767472706866
Figure 7 Acoustic pressure distribution along centerline.
at the ring is mismatched to that of the ring so that thecoupling between the ring and transmission medium ispoor at the outside of the ring Little energy is launchedoutward from the ring, leaving more to be launched inward
to the working medium
The figure also shows that when damping is distributedacross both transmission and working media, the optimumoccurs at a lower OD This may be due to the fact thatwhen damping is included in the transmission medium,the increase in transmission medium volume, which oc-curs as its OD is increased, results in more energy losses
in the system, thus biasing the optimum to a smallerdiameter
8482807876
Radial pressure distribution at ring mid-height
Figure 8 Acoustic pressure distribution across diameter at ring
midheight.
Trang 17PEST CONTROL APPLICATIONS 767
0
30252015105
Figure 9.
0
Power Acoustic νs φ.
Electronics Concept Three electronics concepts were
considered, and two were experimentally evaluated:
ra function generator to produce a sinusoidal (or other)
waveform and a power amplifier to generate a finalhigh-power output signal to be sent through a trans-former to the piezo elements in the mechanical module
ra high-power oscillator
ra switching power supply
The first approach was used in prototype testing and
de-velopment It was not continued in the higher power, high
flow-rate evaluation unit because the readily available
Switched voltage source
3 - Pole butterworth low-pass filter
Coil to produce tuned circuit with piezo
Piezo model 1.53 mH
21.2nF 1.91mH
Figure 10 Electronics concept.
power amplifiers are limited in power (so would have to
be ganged to drive the larger system) and the class A plifier action used is relatively inefficient, making cooling
am-of the electronics an issue
The high-power oscillator was not developed because
of concerns of achieving high power without instabilityproblems
The switching power supply was used for designingthe evaluation unit It is in line with current methods ofdriving high-power motors using pulse-width modulation(PWM) Digital circuitry is used to generate square wave-forms These may be duty-cycle modulated and are used
to switch power MOSFET transistors on and off rapidly
so that the average voltage presented to the equipment
as a result of the variable duty-cycle appears sinusoidal.Such an approach is efficient because the transistors arealways completely on or completely off (except during shortswitching transients), and they dissipate little power in ei-ther of these states In our case, the output frequenciesare too high for true PWM, but square waves can be gen-erated at these frequencies and filtered to eliminate higherharmonics
Figure 10 shows an electronic filtering concept ated by analysis A high voltage supply that has positiveand negative polarity and a 33% duty cycle is switched onand off The fundamental frequency of the source is 25 kHz.This is followed by a three-pole low-pass filter that has
evalu-a cutoff evalu-at 62.5 kHz The output from this filter feeds evalu-atuned circuit that represents the piezo rings (21.2-nF ca-
pacitance and a 100-ohm resistor to simulate a system Q
of 3) in series with an inductance chosen to tune the cuit to the 25 kHz fundamental This makes the drivensystem of this tuned circuit appear resistive at the funda-mental frequency and so matches the low-pass filter’s out-put impedance expectation Note that no transformer isshown, though by adding a transformer between the filterand the piezo, lower voltages would exist in the left-hand
Trang 18cir-768 PEST CONTROL APPLICATIONS
Figure 11 Frequency response function of electronics concept.
side of the circuit which would probably ease component
choice
Figure 11 shows the calculated frequency response
func-tion It also shows the spectral content of the voltage out of
the switched power supply and into the piezo The output
from the switched power supply it is assumed, is both
posi-tive and negaposi-tive in the 33% duty cycle and has switching
transients 25% as long as the on-time, that is, 1.67µs
Sum-ming all power above the fundamental to 250 kHz gives a
total harmonic distortion figure of 71% for the switched
power supply output that has this waveform, but only 4%
for the voltage across the piezo
A breadboard of this system was built and tested It was
felt that the advantages of the switching amplifier concept
outweighed its disadvantages for a production application
A commercial supplier (Instruments Inc of San Diego CA)
was found
Implementation Issues The thin walled stainless steel
tube that contains fluid-borne microorganisms was
de-signed to be as thin as possible to maximum the pressure
transmitted through to the fluid The thickness is
limi-ted by the pressure in the transmission medium The thin
walled tube is fairly close to buckling under the pressure
of the transmission medium
In the prototype system, there was no pressure sensor to
ensure that the pressure of the transmission medium was
maintained between 30–100 psi The small temperature
change (1–2◦C) that results from the excitation of the
system causes the pressure to vary The temperature
change is kept to this low level by pumping the working
fluid continuously past the transmission medium During
biological evaluation of the prototype system, the pressuredid drift above 100 psi After completing of prototypetesting, the system was dismantled, and it was discoveredthat the tubing had buckled
The evaluation unit which was built as a follow-on tothe prototype includes both a temperature and pressuresensor as part of the design This ensures that the systemwill shut down before the critical pressure is exceeded In
an early version of the evaluative design (which contained
16 piezo rings, rather than the original four), the stainlesssteel tubing did buckle because the unsupported length ofthe tubing had more than doubled Modifications of the tub-ing boundary conditions were made to ensure that bucklingdid not occur but at the same time maintained as thin aprofile as possible to maximize the energy transfer to themicroorganism-borne fluid
Another significant issue that arose during early ing of the evaluative system relates to the importance oftolerancing the rings themselves After short runs of the16-ring stack system, failures in the rings occurred Theywere failing mechanically—breaking into two pieces Theinitiation of the crack seemed to be associated with a burnmark on the ring It was postulated that the set of rings be-ing used was not sufficiently well toleranced for roundness.The system was rebuilt using rings of improved tolerance(proved by Sensor Technologies of Collingwood, Ontario).There have been no ring failures since the system wasrebuilt
test-The original electronic drive for the system was based onsquare wave input switching When this was implemented,switching noise was feeding back to the input, causingnoise spikes that were outside the acceptable range of themicroprocessor To eliminate this problem, the signal gen-erator was rebuilt to use sine wave excitation
Figure 12 shows a drawing of the cavitation portion
of the system The elements of the figure are as listed inTable 3
Effectiveness of Cavitation in Destroying Microorganisms
The effectiveness of using a cavitation field to destroy croorganisms was measured for two types of fluid hosts(water and diesel fuel) (9) and three types of microorgan-isms:
The fitted results are shown in Fig 13, plotted as a function
of exposure time to the cavitation field Regression analysiswas used to fit the data to the following equation:
log
IrradiatedControl
= (Slope × Time) + const (2)
These test results were for microorganisms exposed tocavitation while the working medium was moving (be-ing pumped) through the cavitation field Earlier test re-sults were performed while the medium was static during
Trang 19PEST CONTROL APPLICATIONS 769
11
121314
15
161718
I
10987
654321
Figure 12 Cavitation unit—16 ring.
exposure to the cavitation field The cavitation effect was
more pronounced on the moving population than on the
static population It was hypothesized that the motion
en-sured improved distribution of the microorganisms in the
cavitation field
There were two different strains of Pseudonomas
aeru-ginosa used in the study Tests in water were done using
ATCC 10145 A strain of Pseudonomas aeruginosa was
isolated from a sample of marine diesel fuel This strain
would not survive at elevated temperatures (37◦C) where
the ATCC 10145 thrived
Table 3 Parts of Cavitation Unit
Exposure time(s)Flow through testing
Saccharomyces(yeast)Pseuds in water
Serratia in waterPseud in diesel
Serratia in dieselPseud 'isolate'
in diesel
Figure 13 Biological test results.
The results were based on a flow-through testing systemthat involved recirculating the population to obtain the re-quired exposure time Figure 14 shows a schematic of theexperimental facility The contaminated working fluid wasrecirculated during testing This eliminated the need fordisposal of large volumes of contaminated fluid The re-circulating effect underestimates the effectiveness of themethod because the population is being gradually reducedfor each pass through the cavitation field
It had been postulated that the pumping action itselfmight influence the microorganism population, but thateffect was studied and found insignificant on either the
Serratia marcescens or the Pseudomonas aeruginosa.
There did seem to be a small effect on the yeast results
An attempt was made to predict the kill efficiency of asingle pass of the population through the cavitation field
Kill efficiency e is the ratio of microorganisms per unit
vol-ume of fluid killed in one pass to microorganisms present
in an untreated unit volume of fluid
6
UDM experimental facility
18
4
32
1 − Cavitator
2 − Tank for treated water
3 − Tank for contaminated water
Trang 20770 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES
NOTATION
Co= initial concentration (microorganism’s/litre)
C n = concentration after n passes through cavitation
field
e= kill efficiency
n= number of times sample passed throughcavitation field
V= volume of cavitation field
X= holding tank volume
When this equation is applied to the yeast test data
ob-tained, the resulting kill efficiency is 0.49 When it is
ap-plied to the test results for Pseudomonas aeruginosa in
diesel fuel, the resulting kill efficiency is 0.45 These
re-sults were based on an exposure time of 3.15 seconds in
the cavitation field
3 K Ragulskis, R Bansevicius, R Barauskas, and G.
Kulvietis, Vibromotors for Precision Microrobots Hemisphere,
NY, 1988.
4 Modern Piezoelectric Ceramics, Morgan Matroc Vernitron
Division, Bedford, OH, 1988.
5 J.R Frederick, Ultrasonic Engineering Wiley, NY, 1965.
6 S.S Save, A.B Pandit, and J.B Joshi, Chem Eng J 55 B67–
B72 (1994).
7 A.J Chapman, Heat Transfer Macmillan, NY, 1967.
8 G.L Gooberman, Ultrasonics: Theory and Application Hart P,
NY, 1969.
9 S Draisey Ultrasonic Destruction of Microorganisms in
Ship-board Fuels: Biology Report Canadian National Defence
Inorganic glasses are the main transparent material,
which people have long used for observation (windows
in buildings, windshields in cars, eyeglasses, prisms and
lenses in optical instruments), light delivery (light bulbs,
projectors, lasers, optical fibers), and fine arts (crockery,
bijouterie, jewelry) The ability of glasses to change
colo-ration after exposure to sunshine was well known since
the last century A new era in glass application was started
in 1949 by S.D Stookey’s publication (12) in which ing a permanent photographic image in silicate glass wasdescribed This two-step process of exposure to UV radia-tion and thermal development that resulted in a crystallinephase precipitation in the exposed areas was similar tothe classical photographic process As a result of inten-sive research during a long period of time, a great number
record-of different photosensitive glasses were developed, whichhave found very wide application in different branches ofindustry and personal use When exposed to optical radia-tion, these glasses (and glass ceramics) change their opticalproperties (absorption, refraction, or scattering) instantly
or after thermal development, permanently or transiently.Among the great variety of photosensitive glasses, we em-phasize only the two most widely used types
The largest commercial application was obtained forso-called “photochromic glasses,” which exhibit reversiblecoloration after exposure to UV or visible light and canvary their absorption depending on the illumination level.Glasses that contained small concentrations of microcrys-tals of silver and copper halides, proposed by Armisteadand Stookey in 1965 became the most widely used forreversible coloration (13) A peculiarity of these materi-als is that they are produced by glassmaking technologywhereas the photochromic processes occur in microcrystalsdistributed in the glass matrix Several hundred originalpapers were dedicated to different aspects of heteroge-neous photochromic glasses in those years The vast biblio-graphy and detailed descriptions of these heterogeneousphotochromic glasses were collected in books (3,4), andtherefore we will not include a list of original publications
in this article
Another type of photosensitive glass, which is beginningits application in optics and photonics right now, is “photo-thermorefractive (PTR)” glass If this glass is exposed to
UV radiation followed by heat treatment, it varies in fractive index A phase hologram in the volume of this glasswas recorded in 1990 by Glebov and coauthors (5) The fea-ture of this process is that homogeneous glass is exposed
re-to light and a microcrystalline phase is produced in thevolume of the glass matrix by a thermodevelopment pro-cess No books have been written on this subject The mainresults concerning phase hologram recording in glassescan be found in a few original papers (5–7) and a survey(8) Similar processes of photoionization followed by ther-moinduced crystallization were studied for single- and full-color photography in polychromatic glasses, as described in(1, 9–12) Thus, these references can also be used forlearning the basic physical phenomena that result fromirradiation and development of PTR glasses Some basicdata concerning intrinsic absorption, electronic excitation,and nonlinear photoionization in multicomponent glassescan be found in (13,14)
PHYSICAL PRINCIPLES OF PHOTOSENSITIVITY
Trang 21PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES 771
Figure 1 Absorption spectra of 25Na2O–75SiO2glass 1:
intrin-sic absorption; 2 and 3: extrinintrin-sic absorption of 0.1 wt.% of Fe 3 +
and Fe 2 +, respectively; and 4: color center generation spectrum
(arbitrary units).
excitation of electrons from ground to upper levels by which
these electrons can be delivered to other places (we will
not consider heating and posterior melting or ablation)
Absorption spectra of solids may be conventionally divided
into three groups Absorption due to electron transitions
in defect-free substances of stoichiometric composition is
called “intrinsic,” “basic,” or “fundamental” absorption The
absorption in atoms or molecules that are present as small
additives is called “extrinsic,” or “dopant,” or “impurity”
ab-sorption The absorption by defects in the host substance
created by chemical or physical effects is called “induced,”
or “additional,” or “defect” absorption
The absorption spectra of widespread alkali silicate
glass, which is the basis of the majority of technical glasses,
are presented in Fig 1 Intrinsic absorption (curve 1) is in
the range of 210 nm (6 eV) and exhibits an exponential
dependence of the absorption coefficient on photon energy
(or wave number) This absorption is caused by basic
struc-tural units of silicate glass (Si–O–Na), which are called L
centers An example of extrinsic absorption in 25Na2O–
75SiO2 glass is shown by curves 2 and 3 for ferric (Fe3 +)
and ferrous (Fe2 +) ions, which determine the actual
ab-sorption of commercial silicate glasses in the near IR,
visi-ble, and near UV spectral regions Induced absorption
pro-duced by UV andγ radiation (Fig 2) is caused by ionization
in the glass matrix and further trapping of electrons and
holes at different glass matrix defects The presence of
dif-ferent dopants and impurities results additional induced
absorption bands Extrinsic absorption can be caused by
additional ions distributed in the glass matrix and also
by bigger units, for example, microcrystals The
absorp-tion spectra of borosilicate glass doped with copper and
chlorine, which has undergone heat treatment, are shown
in Fig 3 Instead of absorption of copper ions in the glass in
the far UV region, a narrow absorption peak near 380 nm
(3.25 eV) is seen in these spectra, which corresponds to
excitons in CuCl crystals precipitated in the glass matrix
as the result of heat treatment Induced absorption can
HEWavelength, nm
Figure 3 Absorption spectra of borosilicate glass doped with
cop-per and chlorine after 2 hours of treatment at T( ◦C): (12) 550, (13)
600, (3) 650.
also be produced by relatively big particles Photoinducedprecipitation of microcrystals of such metals as gold, silver,and copper causes additional absorption, usually called col-loidal coloration
Glass exposure to radiation whose photon energy ismore than the intrinsic absorption edge (curve 1 in Fig 1)causes photoionization in the glass matrix followed by thegeneration of both electron and hole color centers Thedependence of the induced absorption on the photon en-ergy (or wavelength) is called the color center generationspectrum or the spectrum of photosensitivity (curve 4 inFig 1) Photoionization in the glass matrix (generation ofboth electron and hole centers) is impossible if the pho-ton energy of the exciting radiation is less than a bandgap,which is determined by the position of the intrinsic absorp-tion (curve 1 in Fig 1) In other words, the long wavelength
Trang 22772 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES
edge of the color center generation spectrum (curve 4 in
Fig 1) coincides with the intrinsic absorption edge (curve 1
in Fig 1)
The photosensitivity spectrum can be shifted to the long
wavelength side if the glass is doped with some ions in a
lower valence state, and the dopant’s excited level is placed
above the threshold of the charge carrier’s mobility In this
case, a mobile electron can be trapped either by defect at an
intrinsic electron center formation or by another dopant,
that is, to recharge the activators The depth of the dopant
ground level in Na2O–3SiO2glass is 5.2 eV for Fe2 +, 5.0 eV
for Tb3 +, and 3.6 eV for Ce3 + Comparison of these values
with curve 3 in Fig 1 shows that the ionization threshold of
Fe2 +corresponds to the long wavelength edge of the
absorp-tion band whose maximum is at 6.5 eV (191 nm)
Excita-tion using smaller photon energy causes tunnel ionizaExcita-tion
whose efficiency is about one to two orders of magnitude
less than that of over-barrier ionization The thresholds
of tunnel ionization of dopants in Na2O–3SiO2 glass are
3.5 eV for Fe2 +, 3.1 eV for Tb3 +, and 3.1 eV for Ce3 +
Refer-ring Fig 1, one can see that the tunnel ionization of Fe2 +
is obtained at an excitation of the long wavelength bands
whose peaks are at 5.1 and 4.4 eV (243 and 282 nm) up to
3.5 eV (350 nm) Unlike intrinsic ionization that inevitably
produces electron and hole centers, the only hole center
generated from the excitation of dopant absorption bands
is the same (but oxidized) dopant ion All newly created
centers are electron centers (either intrinsic or extrinsic)
The other way to shift photosensitivity to the long
wave-length side is to use nonlinear ionization produced by
pow-erful optical irradiation In silicate glass exposed to pulsed
radiation whose photon energy is more than half of the
bandgap (h ν >3 eV, λ < 400 nm) and whose irradiance is
more than 1 MW/cm2, both electron and hole color centers
appear as a result of two-photon ionization in the glass
matrix The final concentration of color centers is
deter-mined by equilibrium between two-photon generation and
single-photon bleaching of color centers
INDUCED COLORATION OF REVERSIBLE
PHOTOCHROMIC GLASSES
Generally, the term photochromism may be treated as any
variation of color induced by optical radiation, but
usu-ally people use a narrower definition, which excludes
irre-versible color changes So, photochromism is a reirre-versible
variation in color (i.e., of the absorption spectrum or
spec-trum of attenuation) of a material under optical radiation
that relaxes when exposure stops Naturally, when
experi-mental conditions are changed, for example, a temperature
change, the magnitude of the photochromic effect can vary
(even to complete disappearance) Therefore, we shall call
a photochromic material one that, under specified
operat-ing conditions, becomes colored by optical radiation and
restores its transparency after radiation ceases
Relaxation of induced absorption after illumination
ceases is usually caused by thermal fading of color
cen-ters, which are not stable at a given temperature This
is the most important feature of photochromic materials
because reversibility of the photochromic effect means theabsence of any stable induced centers generated by illu-mination A great number of electron and hole color cen-ters in silicate glasses produced by UV radiation (Fig 2)leads to fatigue because of the progressive accumulation ofstable color centers This is the reason that these glassesare not used as photochromic materials, although pho-tochromism was discovered in cerium-doped, reduced sili-cate glasses Glasses doped with microcrystals of silver andcopper halides (Fig 3) show complete reversibility of colo-ration at room temperature and therefore have the widestcommercial application
The main feature of photochromic glasses, variable tical density both observed during exposure and upon itscessation, has to be taken into account to determine charac-teristics such as integral and spectral sensitivity, darken-ing degree and rate, thermal fading, and optical bleachingrates Let us define the main concepts required for pho-tochromic material characterization Light absorption (or,more exactly, light attenuation or losses, that is the sum
op-of absorption and scattering) is characterized by the mittance,τ = Itr/I0(where Itr and I0are the intensities oftransmitted and incident light, respectively), or the opti-
trans-cal density, D= − log10τ The optical density of a sample
before irradiation (original absorption, clear glass) is D0
(Fig 4) The optical density of the sample at the moment
exposure ceases (induced absorption, dark glass) is Dexp The optical density in t seconds of the thermal fading pro- cess (induced absorption, partially relaxed glass) is Dt Thespectral dependences of τ0 and D0 are the transmission
or absorption spectra of clear glass The spectral dences ofτexpand Dexpare the transmission or absorptionspectra of dark glass Glass has a gray color if the absorp-tion (transmission) spectrum is flat in the visible region Abrown color means that the absorption in the blue region
depen-is greater than that in the red region
The dependences of Dexp and Dt on the time of nation or aging are the kinetics of coloration and relax-
illumi-ation, respectively (Fig 4) Dexpincreases when the
expo-sure time increases and comes to the equilibrium level De
Figure 4 Kinetics of photochromic glass darkening under
illu-mination and fading in the aging process D0, Dexp, and Dt are the optical densities of clear, dark, and relaxed glass, respectively.
Trang 23PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES 773
when the rate of color center generation is equal to the
rate of thermal fading The criterion of relaxation
charac-terizes the degree of thermal fading in a certain time after
illumination ceases:
The value of that time interval should be selected on the
basis of the practical applications of a photochromic glass
Thus, for photochromic lenses used as sunglasses, a time
interval of 180 s is recommended From Eq (12), it is
ob-vious that, if a glass has faded completely in that time,
Krel= 1 Contrariwise, if the induced absorption has not
reduced at all in that time, Krel= 0 Now, there are
pho-tochromic glasses whose Krelvary in the entire range from
zero to about one Krelfor a particular glass can be changed
by temperature variation
An important parameter is the spectral sensitivity of
a photochromic material, the dependence of the saturated
photoinduced optical density (De) on the photon energy of
the exciting radiation This dependence is called the color
center generation spectrum The absorption edge of
pho-tochromic glass determines the position of the color
cen-ter generation spectrum because photosensitive crystals
absorb exactly in that region (compare curves 1 and 2 in
Fig 5) The short wavelength edge of the color center
gener-ation spectrum is connected with the decrease of the
thick-ness of the layer containing color centers, that is due to the
increase of the glass absorption coefficient The long
wave-length edge is caused by a decrease in the absorption and
in the efficiency of photosensitive center ionization These
photosensitive centers are usually copper centers in silver
halide crystals or excitons in a crystalline phase of copper
chloride Owing to that, the position of the maximum in
the color center formation spectrum does not coincide with
that of any maximum in the photochromic glass
absorp-tion spectrum Moreover, its posiabsorp-tion is determined by the
spectral shape of the photochromic glass absorption edge,
10001.00.80.6
0.40.20.0
Figure 5 Spectra of glass doped with AgCl(Br) Absorption of
original glass (12) and color centers (3), color center generation
(13) and bleaching (4) efficiency Sample thickness 5 mm.
is a function of the sample thickness, and drifts to the shortwavelength side as the thickness decreases The absorptionspectrum of an exposed glass doped with AgCl microcrys-tals is presented in Fig 5, curve 3 This absorption repre-sents a wide band in the visible spectral range The spec-tral shape of this band is usually ascribed to precipitation
of colloidal silver particles on the surface of halide crystals Curve 4 in Fig 5 shows that excitation of the ab-sorption band of color centers destroys these centers andcauses optical bleaching Thus, optical bleaching by visi-ble light is a process additional to thermal fading, whichaccelerates the relaxation of darkened silver halide photo-chromic glass
micro-The photosensitivity of photochromic glasses dopedwith CuCl can be shifted from the UV region to the longwavelength side Virgin photochromic glass is photosensi-tive only to UV irradiation and cannot be darkened by vis-ible light Excitation of glasses doped with CuCl that areexposed to UV radiation does not produce optical bleach-ing, as shown in Fig 5 (curve 4) for silver halide glasses
On the contrary, initial additional absorption (induced by
UV radiation) can be intensified by additional exposure tovisible and even IR radiation having photon energy muchbelow the ionization threshold of copper centers Note thatthe power density of long wavelength irradiation must behigh enough to produce this intensification It is shown inFig 6 that the spectra of additional absorption produced
in this glass after irradiation at various wavelengths arethe same Consequently, this long wavelength sensitivityresults from generating new color centers by exciting thesame color centers Therefore this process is called “coop-erative breeding of color centers.”
The mechanism of two-photon cooperative breeding is asfollows Initial exposure to UV radiation causes ionization
600800
1000
321
1.5 2.0Photon energy, eV
Wavelength, nm
2.5
Figure 6 Spectra of induced absorption in copper halide
pho-tochromic glass (thickness 5 mm) after exposure to radiation at different wavelengths: (12) 440 nm (2.78 eV), (13) 633 nm (1.96 eV), and (3) 1060 nm (1.17 eV).
Trang 24774 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES
Figure 7 Energy diagram of the first stage
of photochromic glass coloration at (a) short
wavelength coloration, (b) two-photon
coopera-tive breeding, and (c) three-photon cooperacoopera-tive
of a photosensitive center (Cu+) and generates electrons
and hole centers (Cu2 +) Then released electrons produce
color centers by reducing copper (Cu+) or silver (Ag+) ions
The initial concentration of color centers (Fig 7a) is
deter-mined by the number of UV-ionized photosensitive centers
This concentration can be rather small and even invisible
to the naked eye Linear absorption of two photons of
visi-ble light by two color centers causes a transition of these
centers to excited states (Fig 7b) Further, these centers
simultaneously transfer the accumulated energy to the
photosensitive centers (Cu+) and return to their ground
states An excited photosensitive center releases an
elec-tron and converts to its ionized state in the same
man-ner as after linear excitation, as illustrated in Fig 7a The
released electron is trapped by an acceptor, converts to a
reduced state (Cu0), and this is a first stage in
generat-ing a new color center Thus, the number of color centers
increases after each cycle This means that induced
ab-sorption increases in the process of exciting previously
in-duced color centers without altering the spectrum of the
induced absorption The efficiency of this nonlinear
pro-cess is proportional to the squared intensity of the exciting
long wavelength radiation
The coloration caused by exposure to pulsed IR
radia-tion can be explained similarly to the three-photon
cooper-ative breeding of color centers (Fig 7c) The latter process
obeys the cubical dependence of efficiency on the intensity
of the exciting radiation There are several important
fea-tures of cooperative breeding of color centers The first is a
very high level of additional absorption because
photosen-sitivity in this case is not connected with the sharp
absorp-tion edge of glass (Fig 5) and a thick slab can be
homoge-neously colored The second is the opportunity of localizing
colored spots in arbitrary places of the bulk glass The spots
are produced by focusing the exciting beam because
photo-sensitivity is proportional to the squared or cubical
inten-sity of the exciting radiation and therefore, is concentrated
near the focal plane The third is an opportunity to store
a latent image produced by UV radiation that can be
re-vealed by photodevelopment
HETEROGENEOUS PHOTOCHROMIC GLASSES
Photochromic glasses co-doped with silver and copperhalides are heterogeneous materials They representtwo-phase systems that consist of a vitreous host and dis-persed photosensitive microcrystals This is important be-cause microcrystals show a reversible photochromic effectwithout fatigue However, in a two-phase system, light at-tenuation is caused by absorption of each phase and also byscattering produced by the difference between the refrac-tive indexes of the crystalline and vitreous components.Therefore, the parameters of the crystalline phase should
be chosen to prevent strong scattering The size of the ticle of most photosensitive microcrystals, whose refractiveindex is about 2, should be no more than 10–20 nm to keepscattering below the level of acceptability for optical appli-cations
par-The main approach to producing dispersed tals in a vitreous host is crystalline phase growth as aresult of host glass heat treatment at temperatures from500–700◦C, depending on host composition These temper-atures correspond to a viscosity range from 1010–1013poise
microcrys-To secure crystalline phase precipitation, special ments are applied to the host glass First, this glass should
require-be an oversaturated solution of the photosensitive phase(silver and copper halides) that allows effective diffusion
of these components in the temperature range mentioned.Second, the solubility of the photosensitive componentsmust drop quickly when cooling to allow the homogeneousglass to melt at high temperature and the crystalline phase
to precipitate in the secondary heat treatment process Thelast is usually connected with phase separation (immisci-bility) and altered coordination of different components inthe host glass
The best glass, which satisfies the requirements tioned before, is alkaline borosilicate glass This glass ma-trix is the basis for almost all commercial photochromicglasses manufactured by a number of companies in differ-ent countries Halides (Cl, Br, I) of silver and copper arephotosensitive components, which are added to the batch
Trang 25men-PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES 775
Cations such as Mg, Ca, Ba, Zn, Cd, Al, and Pb, or anions
such as P and S are used by different companies as
addi-tions to modify technical and end use properties These
compositional changes lead to variations in
photosensi-tivity, the criterion of relaxation, and induced absorption
spectra Photochromic glasses can be divided into two large
groups: silver halide glasses that have small
concentra-tions of copper, which usually exhibit faster relaxation and
lower sensitivity and copper halide glasses that have small
concentration of silver, which exhibit slower relaxation and
higher sensitivity In silver halide glasses, small additions
of copper are a sensitizer
The traditional schedule for photosensitive phase
cre-ation, “bottom-to-top,” consists of four stages: melting,
rough annealing and cooling to room temperature,
addi-tional heat treatment (roasting), and final annealing Final
annealing is necessary for stress relaxation because
crys-talline phase precipitation occurs at temperatures above
the glass transition temperature The other method of
sen-sitization is “top-to-bottom,” which is used for mass
pro-duction because of heat energy saving In the latter, the
glass casting cools down to roasting temperature but not
to room temperature It requires the other schedule (time
and temperature) because the most effective growth of
nu-cleation centers occurs at temperatures below the roasting
temperature
OPTICAL WAVEGUIDES IN PHOTOCHROMIC GLASSES
The largest commercial application of photochromic
glasses is for sunglasses Tens of millions of photochromic
lenses are produced worldwide each year for this purpose
However, the alkaline borosilicate origin of photochromic
glasses allows some other applications in modern optics
and photonics It is well known that these glasses are
suit-able for ion exchange and, consequently, planar and
chan-nel waveguides can be created on this glass Besides that,
the mildly sloping dependence of photochromic glass
vis-cosity on temperature allows creating of optical fibers The
optical properties of photochromic waveguides compared
with bulk photochromic glasses are unusual because of
structural transformations in the ion-exchanged layers or
in the drawn fibers and the peculiarities of light
propaga-tion in waveguides An important feature of ion-exchanged
glass is incompleteness of structural relaxation The
ex-change of ions that have different radii creates stresses in
glass These stresses produce strong differences between
the refractive indexes of waveguide modes that are
or-thogonally polarized (birefringence) Compression of
sil-ver halide photochromic glass after substituting Na+ by
K+ at temperatures below the glass transition
tempera-ture reaches 1 GPa and produces birefringence up to 20%
of the total refractive index variation, as shown in Fig 8
Exposure of waveguides in photochromic glasses to UV
radiation produces reversible coloration This means that
ion-exchange treatment does not destroy the
photosensi-tive crystalline phase and this technology is available for
photosensitive waveguide fabrication However,
parame-ters of coloration and relaxation of photochromic
wave-guides are different compared to bulk glass For silver
Figure 8 Refractive index profiles of photochromic glass after
or magnetic field oriented along the surface, respectively.
halide glasses, the criterion of relaxation in waveguides ismore than that in bulk glass This means that relaxation
in waveguides occurs faster For copper halide glasses, laxation in the waveguide was not detected, which meansthat the coloration of these waveguides is stable There
re-is a difference in photosensitivity between different guide modes Modes Whith low numbers propagate nearthe surface and have lower sensitivity than modes thathave a large number and propagate in deep layers This dif-ference is caused by copper (which is a sensitizer) depletion
wave-in the surface layer as result of copper exchange for sium or other ions This phenomenon can be used for modeselection
potas-The other feature of photochromic waveguides is sotropy of photosensitivity and induced coloration Thisphenomenon is connected with ion-exchange stresses.Dichroism (the difference between induced absorption fororthogonal polarizations) is proportional to birefringence
ani-in a waveguide It is important to note that tive microcrystals are plastic or melted at the tempera-tures of ion exchange Therefore, dichroism is determined
photosensi-by stresses and also photosensi-by orientation of liquid drops of thephotosensitive phase caused by ion-exchange stresses.The discrete structure of light propagation in photo-sensitive planar waveguides gives one more opportunityfor multiplexing by mode selection If a mode in such awaveguide (Mode #1 in Fig 9) is excited by actinic radi-ation, the waveguide becomes colored The spatial profile
of induced absorption is determined by the spatial profile
of the exciting modes intensity As a result, a sort of tributed absorbing mask will be formed in the waveguidewhose absorption profile is similar to that of the intensitydistribution of actinic radiation in the waveguide Conse-quently, losses for mode #1 increase after excitation of thismode by actinic radiation The attenuation of other modes
is determined by overlapping of their fields by the dis-tributed mask, that is, by the field of the mode that inducedthis absorption Because field profiles for the modes thathave different numbers essentially differ from each other
Trang 26dis-776 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES
Distance from surface
Refractive indexMode field profiles
Incidentbeam
Transmittedbeam Absorption
2
1 2 Mode #
Figure 9 Sketch of a waveguide mode selector The darkened
profile corresponds to the exposed mode, which produces a similar
profile of photoinduced absorption and prevents propagation of
this mode.
(Fig 9), the losses for different modes should be
signifi-cantly different An example of a mode spectrum of a
pla-nar waveguide excited by actinic radiation in the TE0mode
is shown in Fig 10 A mode selection of about 10 dB/cm
can be reached without special effort in planar waveguides
on commercial photochromic glasses The problem of mask
bleaching can be solved by using probe radiation at longer
wavelengths, where bleaching is not effective, or using, as
described earlier, cooperating breeding of color centers for
writing by high-power radiation
Optical fibers were drawn from photochromic glasses It
was found that thermal treatment of these fibers produces
photochromic properties Fiber plates were made from
pho-tochromic glass as a core and a transparent optical glass as
a cladding, or vice versa High contrast was obtained in this
fiber element compared to bulk photochromic glass plate
This feature of photochromic fiber plate is determined by
gradual leakage of actinic radiation from transparent glass
to photochromic glass This effect increases the length
of the interaction of actinic radiation with photochromic
glass and, consequently, increases dramatically the
in-duced absorption and possible contrast of a photochromic
Figure 10 Effect of exposure to powerful excitation of the
funda-mental mode (shown by arrow) on the dependence of photochromic
waveguide transmission on the angle of incidence onto the input
coupler prism (spectrum of waveguide modes) Solid lines before
exposure, dashed lines after exposure.
INDUCED REFRACTION THROUGH IRREVERSIBLE PHOTOINDUCED CRYSTALLIZATION
It is clear that photochromic glasses can be used for ing information Actually some photos and holograms wererecorded in these glasses but no great success was obtainedbecause of small contrast in photography and small diffrac-tion efficiency in holography For highly efficient hologra-phy, it is necessary to produce variation in the refractiveindex but not in the absorption coefficient The refractiveindex in glasses, where color centers are induced by ra-diation, can vary for very small values, less than 10−6.This is not enough for efficient diffraction Recent disco-very of a strong photoinduced refractive index variation inGe-doped silica opened a new very promising approach forefficient Bragg grating recording in optical fibers Anotherapproach, which allows an increase of sensitivity of sev-eral orders of magnitude compared to Ge-doped silica andavoids interaction between writing and diffracted beams,
record-is based on a two-step process of exposure and development
in multicomponent silicate glasses doped with fluorine, ver, and cerium
sil-Phase volume holograms of high diffraction efficiencywere produced in lithium aluminum silicate and sodiumzinc aluminum silicate glasses doped with silver and ce-rium by exposure to UV radiation followed by thermaltreatment Diffraction was caused by a difference in refrac-tive indexes in exposed (enriched by microcrystals) and un-exposed (original glass) areas This phenomenon is calledthe “photo-thermorefractive” process Glasses that possessthese properties are called “photo-thermorefractive” (PTR)glasses This two-step process (exposure and thermal de-velopment that leads to crystallization) was used earlier
to record a translucent image in glass due to light ing caused by a difference between the refractive indexes
scatter-of the precipitated crystalline phase and the glass matrix.Later, colored images were recorded in similar glasses byphotothermal precipitation of a number of complex crystals
of different compositions, sizes, and shapes
The sequence of processes, which occurs in these ses and produces coloration, follows (Fig 11) The first step
glas-is exposure of the glass to UV radiation, which ionizes acerium ion The electrons released from cerium are thentrapped by a silver ion As a result, silver is convertedfrom a positive ion to a neutral atom This second stagecorresponds to latent image formation, and no significantchanges in optical properties of glass occur, except lightcoloration in near UV and blue regions
The next step in the process is obtained by thermal velopment at elevated temperatures The high diffusioncoefficient of silver atoms in silicate glasses leads to thecreation of tiny silver crystals at temperatures from 450–
de-500◦C A number of silver clusters arise in exposed regions
of the glass after aging at these elevated temperatures.This is the third stage of the process Further, these sil-ver particles serve as the nucleation centers for sodiumand fluorine ion precipitation Cubic sodium fluoride crys-tal growth occurs at temperatures from 500–550◦C becausethe PTR glass composition is an oversaturated solution ofthese components This is the last step, which finishes thephoto-thermorefractive process Further heat treatment
Trang 27PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES 777
kT
kT
Ag0
Ag0(1)
(4)
Figure 11 Stages of the photo-thermorefractive process.
leads to the growth of elongated pyramidal complex Na,
Ag–F, Br crystals on the surface of cubic NaF crystals This
mixture of crystals can produce an opal coloration in large
crystal sizes or a yellow coloration caused by colloidal
sil-ver precipitated on the interfaces of dielectric crystals A
second exposure to UV followed by a second heat
treat-ment produces a different coloration because of metallic
silver reduction on the surfaces of the dielectric pyramids
The final resulting coloration depends on the size and
as-pect ratio of these silver particles These two last steps are
used for photography because strong scattering does not
allow using them in holography
A refractive index decrease of about 5×10−4 occurs in
the areas of glasses exposed to nitrogen laser radiation at
337 nm The refractive index of NaF in the red spectral
re-gion is nNaF = 1.32 compared to the refractive index of PTR
glass nPTR = 1.49 The small value of the refractive index
change is due to the small volume fraction of the
precipi-tated crystalline phase, which produces no scattering in the
exposed volume However, it is sufficient to result in highly
efficient Bragg grating recording in samples more than
sev-eral hundreds of microns thick This photo-thermoinduced
refraction is stable up to 400◦C The photosensitivity is
in the range of several tens of mJ/cm2at wavelengths in
the absorption band region of Ce3 +, which has a maximum
near 300 nm and a long wavelength tall up to 400 nm Thismeans that several commercial lasers such as N2, Ar, andHe–Cd, can be used for recording Once developed, holo-grams in PTR glass are not destroyed by further exposure
to visible or UV radiation
PHOTO-THERMOREFRACTIVE GLASS
The composition (mol %) of PTR glass which was usedfor hologram recording is 15Na2O–5ZnO–4Al2O3–70SiO2–5NaF–1KBr–0.01Ag2O–0.01CeO2 Absorption spectra ofPTR glasses are presented in Fig 12 Figure 12a shows the
UV part of the absorption spectrum One can see the wideabsorption band of Ce3 +that has a maximum at 305 nm.The short wavelength absorption in the regionλ < 270 nm
is due to several components, such as Ce4 +, Ag+, Br−, and
Fe3 + The short wavelength edge, at which writing tion is attenuated by two times in the recording medium(optical density about 0.3), is placed at 330 nm for a 1-cmthick plate and at 265 nm for a 1-mm thick plate The range
radia-of photosensitivity radia-of this glass is from 280–360 nm.Absorption of PTR glass is less than 0.01 cm−1in thevisible and near IR regions, which is close to the limit ofmeasurements, and therefore it is not shown in Fig 12.One can see in Fig 12b that detectable absorption occurs
at wavelengths higher than 2700 nm Absorption in thisspectral region is usually ascribed to different vibrations ofhydroxyl groups in the glass network and reaches several
cm−1 in regular silicate glasses Hydroxyl absorption influorine-containing PTR glass is lower compared to similarfluorine-free silicate glass This phenomenon is caused byhigh volatilization of HF molecules, which can result fromthe interaction of fluorine and hydrogen in the glass melt-ing process This decrease of IR absorption in PTR glassresults in an opportunity for PTR use in the middle IR re-gion up to 4300 nm for 1-mm thick specimens
Additional absorption of PTR glass under UV exposurethat is used in hologram recording in this glass is shown
in Fig 12c, curve 1 Detectable photoinduced absorption isseen only in the UV region Even at the recording wave-length, this absorption is less 0.1 cm−1and cannot impactthe recording process significantly The small tail of theinduced absorption spectrum in the blue region can be dis-tinguished by the naked eye as a slight yellow coloration
of the exposed area Thermodevelopment causes colloidalsilver and sodium fluoride precipitation in the glass matrix.Fluoride crystals are colorless and can result in scattering
if the size of the crystals is too large (more than 100 nm)
A shoulder near 450 nm in the additional absorption trum after thermal treatment in Fig 12c (curve 2) is as-cribed to silver particles in glass matrix One can see thatthe visible additional absorption does not exceed 0.3 cm−1and 0.03 cm−1 in the blue and red regions, respectively.This means that losses in this region do not exceed a fewpercent for a 1-mm thick plate Additional absorption inthe whole IR region is not detectable and therefore is notshown in Fig 12c Consequently, this glass can be usedsuccessfully at all wavelengths important for lasersand optical communication in the visible and near IRregions
Trang 28spec-778 PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES
10
12
Figure 12 Absorption spectra of PTR glass: (a) and (b) original
glass in the UV and IR spectral regions, (c) induced absorption
after exposure to 325 nm for 400 mJ/cm 2 (12) and consequent
thermal development for 1 hour at 520 ◦C (13) Arrow shows the
position of the wavelength of the writing He–Cd laser.
Optical microscopy of exposed and developed samples
used for induced absorption measurements has shown
op-tical inhomogeneities in the exposed region The structure
of these inhomogeneities appears as a series of parallel,
continuous, aligned filaments whose widths are tens of
mi-crons oriented in the direction of light propagation in the
glass sample These microscopic features are caused by
structures whose different refractive indexes arise in glass
processing (phase structures) It is proved that these phasepatterns are not an intrinsic feature of PTR glass but arecaused by various defects of the sample bulk and surfaces.Some additional patterns were found in micrographs; theyare combinations of different rings and fringes It wasfound that they are recordings of the interference pat-terns produced by matching propagating beams to beamsconsequently reflected from the back and front surfaces ofdifferent elements in the optical setup Diffraction of theexciting beam on different apertures produces systems ofstraight or curved fringes that have variable periods de-pendent on the shape and position of the aperture It isnecessary to make special adjustments to eliminate theseinterference and diffraction patterns in the plane of therecording to avoid these parasitic structures Therefore,the homogeneity of the photosensitive medium (includingsurface and volume defects) and the writing beam (includ-ing interference and diffraction patterns of low visibility)must be tested to avoid undesirable losses
The pattern of probe radiation transmitted through posed area consists of the zero and first orders of diffrac-tion but exhibit some rings The diameters and positions
ex-of these rings on the screen depend on the incident angle
of the probe beam and on the feature of the writing tern The origin of these rings follows Each medium causesscattering of propagating light Therefore, even for singlebeams propagating in a photosensitive medium, one canobserve an interference pattern produced by matching theoriginal and scattered beams In this case, the probe beamused for hologram reading should be scattered twice Thefirst time is regular scattering by the medium The sec-ond time is scattering produced by a hologram of scatteredlight recorded together with the main hologram This holo-gram can be completely reconstructed only by the readingbeam of the same wavelength and direction as the writ-ing beam When the wavelengths or the directions of thewriting and reading beams are different, the whole holo-gram of scattered light cannot be read out because itswavefronts are not planar At each angle of incidence, thereading beam can read only that part of the hologram, forwhich Bragg conditions are satisfied Because the angulardiagram of scattering has cylindrical symmetry, this partshould be a ring All phase defects mentioned (filaments,fringes, and rings) appear in all materials but they are vis-ible well in PTR glass because of the high homogeneity andtransparency of this material
pat-BRAGG GRATINGS IN PTR GLASS
The dependence of the absolute diffraction efficiency ofBragg gratings recorded in PTR glasses in the thermaltreatment period is shown in Fig 13 The specimen ex-posed for 400 mJ/cm2has undergone consecutive thermaltreatments for 10–15 minutes each at 520◦C and in inter-vals between, was cooled down to room temperature fordiffractive efficiency measurements The absolute diffrac-tion efficiency is
Trang 29PHOTOCHROMIC AND PHOTO-THERMO-REFRACTIVE GLASSES 779
0.60.40.20
0 100 200 300 400 500
0.81
Thermodevelopment time, min
Figure 13 Effect of the period of thermal treatment on the
abso-lute diffraction efficiency of a Bragg grating in PTR glass
Expo-sure 400 mJ/cm 2 at 325 nm, spatial frequency 600 mm −1
Devel-opment at 520 ◦C Specimen thickness 1.42 mm.
where IL and I1 are the intensities of the incident and
diffracted beams, respectively The reflection coefficient (ρ)
is calculated by the Fresnel formulaρ = (n − 1/n + 1)2
The dependence of diffraction efficiency versus
develop-ment time has an inflection point at the beginning of the
process and is saturated at the 85% level after long heat
treatment Note that this multiple heat treatment is not
the same as a regular development for one or several hours
because this procedure includes multiple heating and
cool-ing However, the curve in Fig 13 shows a tendency for the
diffraction efficiency to approach a high value after some
exposure at elevated temperature
The growth of diffraction efficiency in increasing
peri-ods of thermal development is obviously caused by
refrac-tive index changes that result from crystalline phase
preci-pitation Figure 14 shows the dependence of the refractive
index on the thermal treatment period This
photo-thermoinduced refractive index was calculated from
0.0002
0.0001
0
0 100 200 300 400 500
Thermodevelopment time, min
Figure 14 Effect of the period of thermal treatment on the
in-duced refractive index Exposure 400 mJ/cm 2 at 325 nm, spatial
frequency 600 mm −1 Development at 520◦C Specimen thickness
1.42 mm.
Kogelnik’s equation:
√ηR
where λ is the wavelength of the reading beam, is
the Bragg angle, and d is the thickness of the specimen.
The linear dependence of induced refractive index on thethermal treatment period is present in Fig 14 The func-
(Fig 13) The linear dependence ofδn(t) up to the value of
0.00015 allows writing high efficiency holograms in glassplates more than several hundreds of microns thick Theoptical quality of inorganic glass allows using plates up
to several centimeters thick The saturation of the tion efficiency in Fig 13 corresponds to the refractive indexsaturation at about 0.00017 in Fig 14 No oscillations ofdiffraction efficiency were recorded in this experiment inlong development periods up to 13 hours This means that
diffrac-no significant result exceedingπ for the induced phase was
obtained and, consequently, no additional refractive indexgrowth occurred
The effect of the spatial frequency of the interferencepattern on the diffraction efficiency of the grating in PTRglasses is shown in Fig 15 This was measured in athin sample of 1.65 mm in a transmittance configura-tion when writing (325 nm) and reading (633 nm) beamswere directed from the same side of the glass plate Thisconfiguration allows spatial frequency variations below
2500 mm−1 Exposure or development of gratings wasnot optimized for different spatial frequencies No signif-icant dependence of diffraction efficiency on special fre-quency can be observed in the region from 300–2500 mm−1
in Fig 15 The absence of a drop in the frequency sponse at low frequencies is a feature of the PTR process,which requires transport of species in the glass matrix tobuild single crystals (tens of nanometers) and does notrequire transport of species between exposed and unex-posed areas, as is necessary in photorefractive crystals The
re-0
25002000
15001000
5000
0.20.40.60.81
Spatial frequency, mm− 1
Figure 15 Dependence of the absolute diffraction efficiency on
the spatial frequency of the grating Exposure 600 mJ/cm 2 at
325 nm, development 90 min at 520 ◦C Specimen thickness1.65 mm.
Trang 30780 PIEZOELECTRICITY IN POLYMERS
0.40.60.81
0.2
0
20001500
1000Exposure, mJ/cm2500
0
Figure 16 Maximum absolute diffraction efficiencies of Bragg
gratings in PTR glasses for different exposures to the radiation of
a He–Cd laser at 325 nm.
absence of a drop at high spatial frequencies means that
no fringe smearing occurs in the developed interferogram
and, consequently, no detectable diffusion of components at
distances comparable with the half-period of the gratings
studied (up to 200 nm) occurs in PTR glass during
ther-mal processing These data show that diffusion of glass
components in the development process cannot affect the
saturation in Fig 14, which was observed for gratings that
have a spatial period of 1600 nm The lack of drop in the
amplitude–frequency response at low frequencies (Fig 15)
is an advantage of PTR glasses compared to
photorefrac-tive crystals; this results in a distinct opportunity to design
holographic optical elements that have very small
diffrac-tion angles
An interesting consequence of the low level of induced
losses (Fig 12c, curve 2) is the rather low sensitivity
of PTR-grating diffraction efficiency on exposure because
underexposure can be compensated for by
overdevelop-ment, and vice versa Figure 16 illustrates this feature of
PTR glass In this figure, the best diffraction efficiencies
for specimens of different thickness from different melts,
which had undergone different development procedures,
are plotted versus exposure to the radiation of a He–Cd
laser A high absolute diffraction efficiency of 80% and more
is observed in Fig 16 for exposures that ranged between
50 mJ/cm2and 5 J/cm2
SUMMARY
Photochromic glasses that have completely reversible
col-oration are made of borosilicate glasses doped with
micro-crystals of copper and silver halides These glasses are
sensitive to near UV radiation Photosensitivity can be
ex-tended to visible and near IR regions by cooperative
breed-ing of color centers Induced coloration is a wide band
that covers the whole visible region Photocontrolled
wave-guides can be fabricated in photochromic glasses These
waveguides can serve as attenuators and mode
selec-tors Photo-thermorefractive glasses that have irreversible
photoinduced refraction are aluminosilicate glasses dopedwith silver, cerium, and fluorine These glasses are sensi-tive to near UV radiation Their photosensitivity is com-parable with the best organic and inorganic materials, itallows wide variations of exposure because of image am-plification in the thermal development process, and it hashigh diffraction efficiency and high transparency from the
UV to the IR region
BIBLIOGRAPHY
1 S.D Stookey, Ind Eng Chem 41: 856–861 (1949).
2 US Pat 3, 208, 860, 1965, W.H Armistead and S.D Stookey.
3 R.J Araujo and N.F Borrelli, in Optical Properties of Glass,
D.R Uhlmann and N.J Kreidl, eds., Westerville, OH, 1991: 125.
4 A.V Dotsenko, L.B Glebov, and V.A Tsekhomsky, Physics and Chemistry of Photochromic Glasses CRC, Boca Raton, FL,
1997.
5 L.B Glebov, N.V Nikonorov, E.I Panysheva, G.T Petrovskii,
V.V Savvin, I.V Tunianova, and V.A Tsekhomskii, Sov Phys.
Dokl 35: 878 (1990).
6 L.B Glebov, N.V Nikonorov, E.I Panysheva, G.T Petrovskii,
V.V Savvin, I.V Tunimanova, and V.A Tsekhomskii, Opt.
Spectrosc 73: 237 (1992).
7 O.M Efimov, L.B Glebov, L.N Glebova, K.C Richardson, and
V.I Smirnov, Appl Opt in press.
8 L.B Glebov Glass Sci Technol (Glastechnische Berichte), in
11 A.V Dotsenko, A.M Efremov, V.K Zakharov, E.I Panysheva,
and I.V Tunimanova, Fiz I Khim Stekla 11: 592–595 (1985)
(in Russian).
12 E.I Panysheva, I.V Tunimanova, and V.A Tsekhomskii, Glass
Phys Chem 17: 543–549 (1991).
13 V.I Arbuzov, Glass Phys Chem 22: 477–489 (1996).
14 L.B Glebov, O.M Efimov, A.M Mekryukov, and Yu.A.
Matveev, J Opt Technol 62: 780–785 (1995).
PIEZOELECTRICITY IN POLYMERS
ALEKSANDRAVINOGRADOVMontana State University Bozeman, MT
INTRODUCTION
The diverse group of “smart” piezoelectric materials isdistinguished by their ability to react actively to chang-ing stimuli as a result of converting mechanical to elec-trical energy and vice versa Synthetic piezoelectric poly-mers, an integral part of the “smart” materials group,exhibit a type of behavior that is often compared withbiological reactions involving transformations of thesensed information into the desired response Due to such
Trang 31PIEZOELECTRICITY IN POLYMERS 781
special qualities, piezoelectric polymers have been
increas-ingly used in a rapidly expanding range of applications
At present, these materials continue to offer
unprece-dented design opportunities, leading to the belief that the
industry is on the verge of major technological
break-throughs
PIEZOELECTRICITY: AN OVERVIEW
Piezoelectricity is a material property that is observed as
an electric charge or voltage produced by applied
mechani-cal forces or, conversely, as mechanimechani-cal deformation that is
caused by an applied electric field These piezoelectric
ef-fects have been defined, respectively, as “direct” and
“con-verse.” The latter classification provides a convenient basis
for reference purposes, although it is clear that both
phe-nomena have the same physical origin
Rapid progress in piezoelectric investigations was made
at the beginning of the twentieth century after Pierre and
Jacques Curie discovered the direct piezoelectric effect in
tourmaline crystals in 1880 Subsequently, piezoelectric
ef-fects were observed and studied in other crystals, such as
quartz, zincblende and Rochelle salt, providing enhanced
understanding of the piezoelectric phenomenon and
lead-ing to new discoveries of piezoelectric effects in a variety
of materials In the 1940s, research efforts were
partic-ularly focused on the piezoelectric response of
ferroelec-tric polycrystalline ceramics, including lead zirconate
ti-tanate (PZT), lithium niobate, and barium titi-tanate For
several decades, and, increasingly, toward the mid-1960s,
piezoelectricity was investigated as a common property of
biopolymers, including natural biological materials that
form the structures of plants, animals, and humans Since
1969, when the strong piezoelectric effect in polyvinylidene
fluoride (PVDF) was first discovered by Kawai, attention
has been attracted to the piezoelectric properties of
syn-thetic polymers At present, the traditional group of smart
materials involving piezoelectric crystals, ceramics, and
polymers is expanding as a new generation of laminated
composites that have embedded piezoelectric elements has
recently emerged The history of scientific developments in
the dynamic and growing field of smart materials has been
reviewed in (1–3)
In phenomenological terms, piezoelectricity is described
as coupling between a quasi-static electric field and
dy-namic mechanical motion Typically, the direct and
con-verse piezoelectric effects have been treated as reversible
Respectively, the constitutive equations of linear
piezoelec-tricity are based on the principle of energy conservation
The piezoelectric constitutive law can be presented in
sev-eral alternative forms One of the formulations is given by
[ε] = [C][σ] + [d]T
[E] ,
(1)
[D] = [d][σ] + [e][E], where [σ] and [ε] denote, respectively, stress and strain
tensors that satisfy the condition of symmetry, that is, σ i j=
σ ji , and ε kl = ε lk (i
respectively, the electric flux density and the electric field;
[C] is the elastic compliance matrix whose components
sat-isfy the condition c i jkl = c i jlk = c jikl = c kli j; [d] is the matrix
of piezoelectric coefficients d i jk = d ikj; [d]Tis the transpose
of [d]; and [e] represents the dielectric permittivity
ma-trix whose components e i j = e ji (i
Other forms of the linear piezoelectric constitutive tions are given in (4)
equa-In the general case of fully populated matrices [C], [d], and [e], the electromechanical properties of an anisotropic
piezoelectric continuum are defined by 21 independentelastic constants, 18 piezoelectric coefficients, and 6 dielec-tric constants However, the actual number of parametersrequired to characterize the properties of various piezo-electric materials is less than the total of 45 The structure
and content of the matrices [C], [d], and [e] depend on the
type of material microstructure The anisotropic properties
of piezoelectric crystals and, respectively, the composition
of the matrices [C], [d], and [e] are determined by the
type of symmetry in the crystal lattice Because only thosecrystals that possess no center of symmetry on the atomicscale tend to exhibit piezoelectric effects, only 20 out of 32crystallographic classes of crystals are piezoelectric Spe-cific characteristics of various groups of piezoelectric crys-tals and ceramics, their classification, and properties havebeen considered in (1,4,5) The material properties of piezo-electric polymers are discussed in detail in the followingsections
It is important to note that the theory of linear electricity is based on the assumptions of infinitesimal de-formations, linear stress–strain relations, and stationaryelectric fields with respect to an inertial reference frame.Attempts have been made to develop more general nonlin-ear piezoelectric material models that take into account theeffects of higher order electromechanical couplings, such aselectrostriction, nonlinear strain-displacement relations,and the material response to large driving voltages Re-search efforts in this regard have been reviewed (4,6) Asystematic account of anelastic properties of piezoelectricpolymers has been given in (7)
piezo-SYNTHETIC PIEZOELECTRIC POLYMERS
The diverse group of piezoelectric materials includes
a variety of synthetic polymers such as polypropylene,polystyrene, and poly(methyl methacrylate); semicrys-talline polyamides such as nylon-11; and amorphous poly-mers such as vinyl acetate However, piezoelectric effects
in these materials are relatively weak, often unstable,and are considered of limited practical significance Strongpiezoelectricity has been observed only in the syntheticpolymer poly(vinylidene fluoride) (PVDF or PVF2) andPVDF copolymers
Poly(vinylidene fluoride) is a semicrystalline polymerwhose typical crystallinity is approximately 50% Theamorphous phase of the polymer has the properties of asupercooled liquid The glass transition temperature ofthe polymer is about −50◦C The molecular structure ofpoly(vinylidene fluoride) consists of the repeated monomerunit –CF2–CH2– The atoms are covalently bonded, form-ing long molecular chains Because the hydrogen atoms are
Trang 32782 PIEZOELECTRICITY IN POLYMERS
positively charged and the fluoride atoms are negatively
charged with respect to the carbon atoms, PVDF is
in-herently polar However, the net polar moment of the
material in its original state is zero due to the random
orientation of the individual crystallites
Permanent dipole polarization of PVDF is obtained
through a technological process that involves
stretch-ing and polarizstretch-ing extruded thin sheets of the polymer
Stretching aligns molecular chains in the stretch
direc-tion An applied electric field of up to 100 kV/mm at an
elevated, typically, 103◦C temperature causes permanent
polarization that is maintained after the material cools to
room temperature Sessler (8) provides an overview of
poly-mer polarization methods In general, it has been observed
that polarization in PVDF depends on a number of factors,
including polarizing temperature, polarizing time,
polar-izing process, electrode conditions, and the morphology of
the material
Typically, PVDF is produced in thin films whose
thick-nesses range from 9 to 800µm (10−6 m) A thin layer of
nickel, silver, or copper is deposited on both film surfaces
to provide electrical conductivity when an electric field is
applied, or to allow measuring the charge induced by
me-chanical deformation
ELECTROMECHANICAL PROPERTIES OF PVDF
Since the discovery of piezoelectric effects in PVDF (9), the
properties of this material have been studied by many
in-vestigators Research accomplishments in this subject area
have been reviewed in (8,10,11)
Typically, the piezoelectric properties of PVDF are
de-termined within the framework of linear piezoelectric
the-ory An expanded form of the constitutive law defined by
Eqs (1) is formulated for piezoelectric polymers as
to the film surface Axes 1 and 2 are, respectively, lel and normal to the orientation of the polymer’s alignedmolecular chains
paral-According to Eqs (2), coupling of the cal material properties of PVDF is characterized by five
electromechani-piezoelectric coefficients contained in the matrix [d] The
most important coefficients that determine the
magni-tude of piezoelectric effects are the coefficients d3 j , ( j=
1,2,3) Sometimes, the hydrostatic coefficient, dh = d31+
d32+ d33that determines the electric charge generated byhydrostatic pressure is used to represent the degree ofpiezoelectric effects in a material
The values of the piezoelectric coefficients of PVDF pend on the polarization conditions in terms of the polari-
de-zation time tp, polaride-zation temperature Tp, and
polariz-ing field strength Ep(12) In particular, the dependence of the coefficient d31on tp, Tp, and Ep for a PVDF thin filmstretched at a 4:1 ratio, is illustrated in Figs 1–3
30
Ep (MV/m)
Figure 2 Dependence of coefficient d31of PVDF on polarization
temperature T (12).
Trang 33Tp (°C)
Figure 3 Dependence of coefficient d31of PVDF on polarizing
field strength Ep (12).
A number of experimental techniques have been
de-veloped to determine the values of the piezoelectric
co-efficients of PVDF In particular, the response of 20-µm
thick PVDF films has been studied under the conditions
of superimposed static and sinusoidal loads (13) The
elec-tric charge resulting from the mechanical loading has been
measured for various values of the static load and at
vari-ous temperatures; the amplitude (0.15 N) and frequency
(15 Hz) of the dynamic load remained unchanged It has
been determined that the piezoelectric coefficient d31of
PVDF strongly depends on temperature, particularly, in
the range from−40 to −50◦C, close to the glass transition
temperature Tg A similar dependence of coefficient d31on
temperature has been observed in (14)
The electromechanical response of PVDF as a function
of temperature has been studied in (15) using the
piezoelec-tric resonance method By applying an alternating stress
in the material directions 1, 2, and 3 and using polarization
measurements along axis 3, it has been determined that
that the piezoelectric coefficients of PVDF tend to increase
with temperature, as illustrated in Fig 4
factors k31, k32, and k33 These coefficients represent the
ratios between the dissipated and input energies in the spective material directions It has been determined (15)
re-that the electromechanical coupling factor k31of PVDF
tends to increase with temperature, whereas k32 and k33
remain temperature insensitive These results are trated in Fig 5
illus-The shear piezoelectric properties of uniaxially orientedPVDF films have been studied in (16) It has been observedthat polarization of PVDF samples is linearly proportional
to applied shear stresses It has been determined that the
values of the piezoelectric coefficients d15 and d24rangefrom –13 pCN−1 to –27 pCN−1 and from −23 pCN−1 to–38 pCN−1, respectively
The mechanical properties of PVDF have been defined
by the constitutive equations of linear elasticity in the form
of a generalized Hooke’s law For orthotropic materials, the
coefficients of the compliance matrix [C] in Eqs (2) can be
represented such that
c11= 1/Y1, c22= 1/Y2 , c33= 1/Y3 , c44= 1/2G23 ,
c55= 1/2G31 , c66= 1/2G12
c12= −ν12 /Y1= −ν21 /Y2, c13= −ν13 /Y1= −ν31 /Y3, and
where Y1, Y2, and Y 3are the elastic moduli in directions 1,
2, and 3, respectively; G12,G31, and G23denote the shearmoduli; andν12, ν23, andν31are Poisson ratios whose firstindex indicates the direction of contraction or expansionand the second indicates the direction of force action Note
that due to the symmetry of the compliance matrix [C],
the mechanical properties of PVDF thin films are terized by nine independent elastic constants
charac-The elastic response of PVDF has been studied in(14,15,17–21) It has been observed that the experimental
values of the elastic moduli Y1 and Y2 have been tently very close This result has been often interpreted
Trang 34consis-784 PIEZOELECTRICITY IN POLYMERS
Figure 6 Stress–strain response of PVDF
(direc-tion 1) (20).
Sample 1Sample 2Sample 3
as evidence that the mechanical properties of PVDF are
isotropic However, it has been demonstrated in (18–20)
that PVDF thin films exhibit significantly different
re-sponses, depending on the orientation of the aligned
molec-ular chains In the latter studies, 28-µm PVDF samples
were tested under displacement controlled experimental
conditions at a strain rate of 1.27 cm/min The respective
stress–strain diagrams for both in-plane material
direc-tions of PVDF are given in Figs 6 and 7
It is clear that the mechanical properties of PVDF
thin films strongly depend on the orientation of the
poly-mer’s molecular chains aligned in the stretch direction
The diagram in Fig 6 demonstrates that the stress–strain
response of the material in the direction of the aligned
molecular chains (direction 1) is characterized by a
con-tinuous increase of stresses that culminates in sudden
fail-ure This type of response is typical for brittle materials In
Figure 7 Stress–strain response of PVDF
(direc-tion 2) (20).
Sample 1Sample 2Sample 3
mate-Besides the observed differences in the stress–strain
behavior, the ultimate stresses (σ u)i and ultimate strains
(ε u)i (i = 1,2) in the respective in-plane material
direc-tions of PVDF have considerably different values: (σ u)1=
The Poisson ratio for uniaxially stretched PVDF filmshas been measured experimentally in (21) Material sam-ples were subjected to uniaxial tension in the direction ofthe aligned molecular chains The values of the Poissonratiosν31andν21were obtained by measuring the respec-tive deformations in the thickness and width directions of
Trang 35Figure 8 Elastic compliances of PVDF as functions of
tempera-ture (15).
the samples It has been determined that ν21∼ 0.1 and
ν31∼ 0.8 It is important to note that the value of ν31
ex-ceeds 0.5, the theoretical maximum possible value of the
Poisson ratio for isotropic elastic materials This result
in-dicates that PVDF thin films are highly anisotropic
Experimental studies (14,15,22–24) indicate that the
elastic properties of PVDF are temperature-dependent
In particular, according to the results reported in (15),
the elastic compliances of PVDF increase with
tempera-ture The yield stress and yield strain of PVDF are also
temperature-dependent (22) These results are illustrated
in Figs 8, 9, and 10
Due to the fact that the electromechanical response
of PVDF depends on a number of factors, including
polarization conditions, stress/strain rates, temperature,
and hydrostatic pressure, the reported data for the values
of the piezoelectric and elastic constants of the polymer
appear to involve certain inconsistencies Nevertheless, it
is possible to identify the typical values of the
electrome-chanical characteristics of PVDF such as summarized in
273 K=0°C
Figure 9 Temperature dependence of the true yield stress of
PVDF (22).
0.50.40.30.2
273 K = 0°C
Figure 10 Temperature dependence of the true yield strain of
PVDF (22).
NONLINEAR AND TIME-DEPENDENT EFFECTS
The constitutive law of linear piezoelectricity in the form
of Eqs (1) tends to neglect energy dissipation, dependent effects, and various nonlinearities in the elec-tromechanical response of piezoelectric materials How-ever, there is consistent experimental evidence that theseassumptions have certain limitations It has been observedthat, in general, all piezoelectric materials exhibit non-linear effects, as well as dielectric and mechanical energylosses, although to different degrees Thus, energy losses inpiezoelectric crystals and ceramics are negligible (26,27),whereas in piezoelectric polymers such effects are of prac-tical significance (28)
time-One study demonstrates strong nonlinear dependence
of the transverse piezoelectric response of PVDF on the plied stress (29) It has been observed that the piezoelectric
ap-coefficient d32of 22-µm uniaxially oriented PVDF films
be-comes negative under large stresses This effect appearedreversible upon unloading but tended to repeat itself insubsequent loading–unloading cycles
Under cyclic conditions, piezoelectric polymers exhibitenergy losses observed from hysteresis loops formed by the
electric displacement D as a function of electric field E (10,
30–34) Furukawa et al (30) subjected 20-µm thick PVDF
films to high sinusoidal electric fields whose amplitudesranged from 40 to 120 MV/m in the frequency range of
10−4–10−2Hz at temperatures between –100 and 100◦C.These experiments demonstrated a strong dependence of
D on temperature and on the amplitude and frequency of
the electric field At sufficiently high electric fields, D–E
hysteresis loops have been observed, even in the ture range below the glass transition temperature of the
tempera-polymer The D–E response of PVDF samples at different
temperatures is illustrated in Fig 11
D–E hysteresis loops similar to those shown in Fig 11
have been obtained for PVDF copolymers, vinylidenefluoride-trifluoroethylene (VDF-TrFE), and vinylidenefluoride-tetrafluoroethylene (VDF-TFE) (35–37) Simi-larly, the piezoelectric coefficients of PVDF and its copoly-mers have demonstrated hysteresis under variable electricfields (38–41)