Encyclopedia of Smart Materials (Vols 1 and 2) - M. Schwartz (2002) Episode 9 ppt

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 hol

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

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 nents, and

compo-rthe junction voltages of an external electrical circuitconnected 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



+ [A0]{w} + [A−1]



{w}dt + [A−2]





.

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 fluidwhere

[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.

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

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.

30252015105

Figure 9.

 0

PowerAcousticνs 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

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:



= (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

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

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

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

H

EWavelength, 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

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 cess (induced absorption, partially relaxed glass) is Dt Thespectral dependences of τ0 and D0 are the transmission

pro-or abspro-orption 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 Dexpand 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 Dtare the optical densities of clear, dark, and relaxed glass, respectively.

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:

Krel= Dexp− Dt

Dexp− D0

(1)

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,

10001.0

0.80.6

0.4

0.20.0

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

Photon 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).

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

men-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

Naglass–Kmelt ion exchange TE or TM polarizations mean electric

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

dis-Distance from surface

Refractive indexMode field profiles

Incidentbeam

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

kT kT

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

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

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

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:

δn = λ cos  arcsin

√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-tion δn(t) shows no inflection point compared to DE(t)

(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.

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).

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

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 d 3 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, polarization 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).

Tp (°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

d31, d32> 0, and d33< 0 In addition, it has been observed

that the piezoelectric coefficients of PVDF tend to increase

with temperature, as illustrated in Fig 4

that the electromechanical coupling factor k31of PVDF

tends to increase with temperature, whereas k32and k33remain temperature insensitive These results are illus-trated in Fig 5

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 d15and 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 Y3are 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

consis-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=

3.5 × 108Pa, and (σ u)2= 5 × 107Pa

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

Figure 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)

Table 1 Electromechanical Characteristics of PVDF

Range of film thicknesses 9–800µm (10−6m)

Operating temperature range −40 to 80 ◦C

Glass transition temperature, Tg −60 to –20 ◦C

Melting temperature, Tm 170–178 ◦CMaximum operating voltage 30 V/µm (750 V/mil)

Breakdown voltage 100 V/µm (2000 V/mil)

Capacitance 380 pF/cm 2 for 28-µm films at 1 kHZ

Piezoelectric coefficients d31= 21.4 × 10−12C/N; d32= 2.3 × 10−12C/N; d33= −31 × 10 −12C/N;

d24= −35 × 10 −12C/N; d

15 = −27 × 10 −12C/NElectromechanical coupling factors k31= 12%; k32 = 3%; k33= 19% at 1 kHZ

Young’s moduli Y1= 2.56 × 109Pa; Y2= 2.6 × 109 Pa Yield stress (σ y)1= 4.5 × 107Pa; (σ y)2= 3.9 × 107 Pa Yield strain (ε y)1= 1.8%; (ε y)2= 1.4%;

Ultimate stress (σ u)1= 3.5 × 108Pa; (σ u)2 = 5 × 10 7 Pa Ultimate strain (ε u)1= 16.9%; (ε u)2 = 2.5%

One of the most obvious indicators of the

time-dependent behavior of piezoelectric polymers is their

ten-dency to undergo piezoelectric, dielectric, and mechanical

relaxation As an example, relaxation of the piezoelectric

coefficient d31of PVDF thin films stretched at a 5:1 ratio

and polarized for 2 hours at a polarization temperature

Tp= 85◦C and electric field Ep= 50 MV/m is illustrated in

Fig 12

The relaxation properties of PVDF attracted attention

in the early 1960s (42), and since that time, the relaxation

phenomenon in piezoelectric polymers has been studied

extensively (43–47) Typically, piezoelectric, dielectric, and

mechanical relaxation of piezoelectric polymers is

charac-terized by the complex coefficients

range of 0.015 to 0.25, depending on the experimental

con-ditions (25,42,44)

Time-dependent mechanical properties of PVDF have

been studied in (18–20) based on a series of quasi-static

creep tests and dynamic mechanical tests of 28-µm thick,

commercially produced PVDF thin films In the

experi-ments, PVDF samples were tested in two in-plane material

directions, parallel (direction 1) and perpendicular

(direc-tion 2) to the polymer’s aligned molecular chains Creep

experiments were performed at 10 different stress levels

under sustained loading conditions at room temperature

Strain measurements were taken by a linear variable

dif-ferential transformer (LVDT)

The time-dependent response of PVDF thin films has

been described by the constitutive equations of linear

vis-coelasticity in either of two alternative forms (48,49):

ε (t) = σ(0)C i (t)+

t

0

Y i (t − τ) dε dτ dτ, i = 1, 2, (7)

where the functions C i (t) and Y i (t) (i = 1, 2) denote,

respec-tively, the creep compliances and relaxation moduli of the

polymer in both material directions

The creep compliances of PVDF were determined by

us-ing the method of dynamic mechanical testus-ing and analysis

(DMTA), commonly employed for testing and

characteriz-ing polymers and polymer matrix composites (50)

The dynamic experimental program described in (18–

20) was implemented by subjecting PVDF samples to

sus-tained tensile stresses at several levels belowσ Yi (i = 1, 2)

that had a superimposed sinusoidal strain ε = ε◦ sin(ωt).

The response of the material was measured in terms of the

respective stress σ = σ◦sin(ωt + δ), and the phase angle δ

represented the loss of mechanical energy Dynamic tests

were performed in each material direction for 20 different

values of frequency in the range from 1 to 50 Hz at ambient

temperature The viscoelastic properties of the polymer in

both directions were characterized in terms of the

respec-tive storage and loss moduli, Y i, and Y i, and storage and

loss compliances, C iand C i(i = 1, 2).

The experimental frequency range was expanded on the

basis of the temperature-frequency correspondence

prin-ciple that provides a correlation between the viscoelastic

material characteristics at a base temperature Toand the

respective material characteristics at a different

tempera-ture T, such that

Y(ω, To)= Y(ωaT, To), (8)

where aTdenotes the shift factor (48)

The shift factor aTfor PVDF thin films was determined

by repeating the entire set of dynamic experiments at 13

different temperature levels in the range from 25.5 to 81◦C

for the material direction 1 and at 10 different temperature

levels in the range from 24.4 to 81.1◦C for the material

direction 2 On this basis, the loss and storage

relax-ation moduli and the loss and storage creep compliances of

PVDF were determined As an example, the storage moduli

8e+097e+096e+095e+094e+093e+092e+0930405060708090050100150200250300

Y′ (Pa)

T (°C)ω(rad/sec)

Figure 13 Storage modulus of PVDF (direction 1) (18).

6e+095.5e+095e+094.5e+094e+093.5e+093e+092.5e+092e+091.5e+0930405060708090050100150200250300

Y′ (Pa)

T (°C)ω(rad/sec)

Figure 14 Storage modulus of PVDF (direction 2) (18).

of PVDF as functions of frequency and temperature areshown in Figs 13 and 14 in directions 1 and 2, respectively.Using the approximate inverse Fourier transformation

method (48), the creep compliances C i (t) (i = 1, 2) of PVDF

were obtained numerically An analytical approximation of

C i (t) (i = 1, 2) was obtained as a power law

respec-It is important to note that, the time-dependent nical response of PVDF films is independent of stress only

mecha-Y1, Pa2.8 × 109

Figure 16 Relaxation function of PVDF (direction 2) (51).

at stress levels below 57% of the yield stress (σ Y)1 in the

material direction 1 and below 76% of (σ Y)2in the material

direction 2 (18) Beyond these limits, the creep and

relax-ation behavior of PVDF is nonlinear

It is important to note that, as determined by Holloway

(18), the time-dependent mechanical response of PVDF

films is stress independent only at the stress levels

be-low 57% of the yield stress (σY)1in the material direction 1

and below 76% of (σY)2in the material direction 2 Beyond

these limits, the creep and relaxation behavior of PVDF is

nonlinear

It has been shown (52) that the mechanical and

di-electric relaxation responses of piezodi-electric solids are

interrelated The same molecular relaxation mechanisms

that give rise to mechanical relaxation also give rise to

di-electric relaxation It has been suggested (53) that the

in-herent coupling between the time-dependent electrical and

mechanical properties of piezoelectric materials can be

de-scribed by the constitutive equations in a form similar to

the constitutive law of linear viscoelsticity:

ε i j =

t

0

dτ dτ +

t

0

the temperature range from 30 to 100◦C It was observedthat, for temperatures below 40◦C, the behavior of PVDFsamples in the draw direction followed the typical relax-ation pattern However, at elevated temperatures rang-ing from 50 to 100◦C, PVDF samples demonstrated aninverse behavior, an increase of the relaxation modulusafter a certain period of continuous decrease For ex-ample, at 100◦C, the relaxation modulus of PVDF de-creased for about 1 min, reached a minimum, and, subse-quently, increased from about 1.14 GPa to 2.97 GPa during

a period of 36 hours This behavior of PVDF, fied as “inverse stress relaxation,” was accompanied by

identi-a gridenti-aduidenti-al decidenti-ay of the piezoelectric properties of thematerial

To date, significant progress has been made in thedevelopment of various constitutive models of nonlinearpiezoelectricity (7,55–57) However, many aspects of ex-perimentally observed nonlinear and time-dependent phe-nomena that characterize the behavior of piezoelectricpolymers still remain unexplained

APPLICATIONS

Piezoelectric polymers have been increasingly integrated

in structural design as active elements that can sense andrespond intelligently to external stimuli On this basis, anew generation of so-called “smart structures” or “smartmaterial systems” has emerged that can detect changes inloading or environmental conditions, decide rationally on

a set of respective actions, and carry out these actions in

a controlled manner A broad range of applications usingsuch functions include active vibration damping, acousticsuppression, damage detection, shape and position control

of compliant structures, and self-inspection of structuralintegrity Systematic reviews (3,25,58,59) provide a con-sistent account of modern technological developments inthe field of smart material systems

Material selection in designing smart material systemsinvolves considerations of such factors as the maximumachievable strain, stiffness, spatial resolution, frequencybandwidth, and temperature sensitivity Traditionally,piezoelectric ceramics such as lead zirconate titanite (PZT)and barium titanite (BaTiO3) have played a leading role

in many applications due to their dielectric strength andstable electromechanical properties at high temperatures

up to 400◦C However, the potentials of piezoelectric ics are limited because these materials are brittle, some-what heavy, and are difficult to scale to larger applica-tions In this regard, piezoelectric polymers offer definiteadvantages because they are light, flexible, easy to shape,and can be bonded to almost any surface The attractiveproperties of PVDF and PVDF copolymers include stableresponse characteristics in a wide frequency range up to

ceram-109 Hz, low acoustic impedance; a high degree of tance to impact, and resistance to moisture absorption, in-tense ultraviolet, and nuclear radiation However, the ef-fectiveness of piezoelectric polymers tends to decrease in

resis-low-frequency applications, and their use is limited to a

temperature range not exceeding 100◦C

The key factor that defines the application range of

piezoelectric polymers is their use in the design of

trans-ducers, sensors, and actuators An impressive array of

PVDF applications in transducers, including

loudspeak-ers, optical scannloudspeak-ers, light deflectors, and variable

aper-ture diaphragms, modulators for fiber optics,

pyroelec-tric detectors, and capacitors has been delineated in (60)

PVDF based transducers are particularly effective in

high-frequency applications such as acoustics, ultrasonics, and

nondestructive material evaluation (61) Examples of such

applications include medical diagnostics (62), marine

foul-ing prevention (63), acoustic microscopy (64), and damage

detection in fibrous composite materials (65)

Piezoelectric sensors have been typically used for strain

measurements through the readings of voltage, rates of

voltage change, or the frequency spectra of the signal

gene-rated by the sensor Various commercial applications of

piezoelectric film sensors include contact switches,

mu-sical instruments, and vibrational sensing devices (66)

Because they react to temperature changes, PVDF-based

motion sensors are widely used in energy management

sys-tems to control room lights, appliance displays, and HVAC

equipment (67) PVDF thin films have recently been

em-ployed in more advanced sensing technologies, for example,

tactile sensors that recognize objects with a high degree of

precision and also temperature and pressure sensors that

can replicate the functions of human skin (68)

At present, a number of PVDF-based discrete

piezo-electric actuators have been developed for different modes

of operation Traditionally, bending mode actuators have

been designed in the form of bimorph beams consisting

of two piezoelectric layers of opposite polarity bonded

to-gether When an electric field is applied, one of the layers

expands while another contracts, producing bending

de-formation As shown by Wang and Cross (69), piezoelectric

bimorph actuators in the form of cantilever beams can

gen-erate considerable tip displacements, although they tend

to produce low forces Thicker films and multilayer

de-signs can expand the range of forces produced but reduce

the respective displacements A three-layer piezoelectric

actuator with hysteresis has been analyzed and tested in

(70)

The effectiveness of bimorph piezoelectric actuators has

been enhanced by shaping them into a curvilinear

config-uration As an example, a semicircular bimorph C-block

actuator was proposed in (71) and analyzed in (72)

Simi-larly to the beam bending actuator, the C-block design uses

the response of two bonded piezoelectric layers actuated

by equal and opposite electric fields As shown in (72), the

C-block actuator produces an increased stroke or force

out-put, especially when combined in series or in parallel, to

form larger actuator architectures

Concurrently, a double curvature piezoelectric

actua-tor for vibration control in microgravity environments has

been proposed in (73,74) This low force PVDF actuator

uses the bimorph design concept and can be produced in

a variety of sizes, depending on the performance

require-ments The efficiency of the actuator can be enhanced by

in-troducing multilayer configurations and by creating more

complex architectures, as in the C-block design

The bimorph configuration involving PVDF thin filmshas been effectively incorporated (75,76) into the design offlexible mirror systems to control their shape and, conse-quently, provide the required precision of the optical sur-face A similar concept was implemented (77) in designinglarge-scale deployable membrane mirrors for space explo-ration telescopes

In general, integration of the electromechanical erties of piezoelectric polymers into structural design pro-vides the capability of controlling the mechanical charac-teristics of structures in terms of stiffness or damping, ormodifying the structural response in terms of position orvelocity This type of built-in structural intelligence hasbeen particularly effective in applications involving vibra-tion control and damping enhancement of flexible struc-tural elements

prop-Piezoelectricity has played a major role in the velopment of various “passive” and “active” vibrationcontrol strategies Passive damping involves convertingmechanical strain energy into electrical energy, which issubsequently dissipated by a simple resistive element.Active vibrational control is achieved through interac-tive functions of three main components: a sensor thatidentifies the present state of the structure, a cognitiveinterpretation and decision system that controls and opti-mizes, and an actuator that modifies the response of thestructure Active control of sound radiation is based on aconceptually analogous approach

de-Typically, piezoelectric sensors and actuators used invibration control applications are either bonded to the sur-face or embedded within the structure as a patch, a con-tinuous single layer, or multiple material layers Spatialarrangements of segmented piezoelectric elements can beoptimized to achieve the desired effects

The response of intelligent structures that have grated piezoelectric sensors and actuators has been stud-ied extensively In particular, the effectiveness of passivepiezoelectric damping has been examined in (78) Variousproblems of active and passive vibration control of smartbeams using bonded and embedded piezoelectric sensorsand actuators have been investigated in (79–85) The lin-ear vibration theory of piezoelectric plates has been de-veloped in (86) An exact solution has been derived forpiezothermoelastic opthotropic flat panels subjected to ex-ternal pressure, and thermal and electrostatic excitations(87) The vibration response of simply supported elasticrectangular plates excited by two-dimensional patch actu-ators bonded to the plate surface has been investigated in(88) Active vibration control of plates using patches com-posed of a viscoelastic damping layer sandwiched betweentwo piezoelectric layers has been studied in (89) A similarapproach was adopted in (90) to control the vibrations

inte-of cylindrical shells actively The potentials inte-of vibrationalcontrol of cylindrical shells using curved piezoelectricactuators have been studied in (91) The problem of activenoise control of an elastic panel harmonically excited

by multiple piezoelectric actuators has been analyzed in(92)

As an integral part of these efforts, a number of ies specifically focused on using piezoelectric polymers foractive vibration and noise control of structural compo-nents Thus, the problem of vibration control of beams

stud-using distributed PVDF sensors has been examined in

(93,94) The performance of shaped PVDF modal sensors

employed to control specific vibrational modes of

rect-angular plates under steady-state harmonic excitations

has been studied in (95) The results obtained

demon-strated the high effectiveness of PVDF sensors in

ac-tive vibration control applications in both a resonant

and off-resonant range of frequencies A similar

con-clusion was reached (96) based on a study of shaped

PVDF sensors for active structural acoustic control of

rectangular plates One theoretical and experimental

in-vestigation (97) concerns the performance of distributed

PVDF sensors and actuators that can distinguish

be-tween bending and torsional vibration modes of

rectangu-lar plates The results of the study indicate that PVDF

thin films can be effectively used in microactuator

de-vices as well as in modal control applications of larger

continuous structures Studies (98) demonstrated the

ef-fectiveness of distributed piezopolymer actuators for

ac-tive control of sound fields radiated from composite

struc-tures in acoustic control applications Similarly, the high

effectiveness of PVDF-based distributed sensors and

actu-ators used for active vibration control of flexible

manipu-lators was observed (99)

Piezoelectric polymers represent a group of primary

candidates for shape and positional control of flexible

struc-tures in weight-sensitive applications Examples of such

applications are smart skin for airborne structures (100)

and highly compliant smart material systems for space

applications (101) The latter comprise a diverse range of

ultra-lightweight structures such as solar sails, deployable

membrane mirrors, atmospheric balloons, antennae, and

reflectors By incorporating the capabilities of

piezoelec-tric polymers in structural design, it is possible to enhance

the performance of such structures by actively controlling

their shape and stability

At present, a large class of smart composite

struc-tures has been developed that combines the traditional

advantages of laminated composites and the adaptive

capabilities of piezoelectric materials These structures

offer numerous technological benefits However, they

ex-hibit complex electromechanical behavior that depends on

a combination of many factors, such as the individual

prop-erties of active and passive constituents, specific material

lay-ups, interfacial conditions, and the effects of damage

evolution processes These and other related issues have

attracted considerable research interest

Various theories and analyses of laminated composite

structures that have integrated self-sensing, control, and

diagnostic functions have been developed In particular,

the coupled mechanical, electrical, and thermal response of

piezoelectric composite beams has been analyzed in (102)

The development of the general theory of piezoelectric

composite plates involves the efforts of many (103–109)

The response of curvilinear piezoelectric composite

struc-tures has been analyzed in (110–113)

A general finite element formulation for analyzing

dis-tributed thermopiezoelectric sensors and actuators as

el-ements of intelligent structures has been proposed in

(114) A nonlinear three-dimensional constitutive

the-ory of anisotropic piezoelectro-thermoviscoelasticity for

nonhomogeneous layered media has been developed in(115)

An innovative technique for damage diagnostics of inated composites by an integrated sensor-actuator sys-tem in the form of a thin flexible Stanford Multi-Actuator-Receiver Transduction (SMART) layer embedded withinlaminated structures has been described in (116) Appli-cations of the method involve identifying the location andforce of the unknown external impact, estimating the ex-tent of the impact damage, and monitoring the cure condi-tions of composites

lam-Advanced technological developments in intelligent terial systems consistently stimulate the search for smartmaterials that have novel or improved characteristics Cur-rently, attention is focused on the effectiveness of com-posite materials that combine the superior piezoelectricproperties of ceramics and the compliance and flexibility

ma-of various polymers Piezoelectric composites ma-of this typeare typically produced by integrating ceramic fibers or par-ticles of lead zirconate titanate (PZT) or calcium modifiedlead titanate (PbTiO3) within a polymer matrix Fibrouscomposites have been referred to as 1–3 composites be-cause the fibers have unidirectional orientation, whereasthe particulate composites, known as 0–3 composites, areisotropic Fibrous piezoelectric composites usually havebetter piezoelectric properties; however, their fabricationprocesses are complex Particulate composites can be pro-duced as thin films and have the advantage of being lessexpensive The performance and properties of 1–3 and 0–3piezoelectric ceramic/polymer composites have been inves-tigated in (117–120) In general, potential applications ofactive polymeric composite materials (APCM) have beendiscussed in (121)

CONCLUDING REMARKS

At present, the field of intelligent material systems is panding at an unprecedented rate The guiding principlesbehind this progress are structural efficiency, functional-ity, precision, and durability Superior adaptive capabili-ties and other attractive qualities of piezoelectric polymersdetermine their increasingly leading role in the design

ex-of intelligent structures whose applications range fromaerospace, construction and transportation to physics andlife sciences

It is clear that effective implementation of piezoelectricpolymer systems directly depends on the degree to whichtheir behavior and properties are understood To date,despite considerable progress, material characterization

of piezoelectric polymer films is far from complete lenges arise due to the sensitivity of the polymers to vari-ations in fabrication and temperature conditions, time-dependent effects, and material nonlinearities The matter

Chal-is complicated by the technological necessity of providingelectrical conductivity by depositing metallic surface lay-ers Effectively, piezoelectric polymers represent a com-posite material, whose response depends strongly on thethickness and properties of the individual constituents.Due to these factors, stable experimental conditions andhighly precise measurements are required to characterize

the properties of piezoelectric polymers accurately in the

practical range of their operating conditions

In the immediate future, continuing progress in the field

of smart materials will depend on the intensity of research

efforts directed toward the development of piezoelectric

polymer systems that have enhanced adaptive

capabili-ties, formulation of advanced theoretical models, and

im-plementation of innovative testing methodologies On this

basis, the unprecedented opportunities offered by the new

generation of intelligent materials will continue to

stim-ulate further technological progress and, ultimately,

con-tribute to the betterment of humanity

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MICHAELO WOLF

The University of British Columbia

Vancouver, British Columbia

Canada

INTRODUCTION

Poly( p-phenylenevinylene) (PPV) and its derivatives are

polymers that have been widely studied due to their

po-tential applications in optoelectronic devices PPV is a

con-jugated polymer whose backbone consists of alternating

single and double bonds Many conjugated polymers are

known and exhibit remarkably high electrical

conductiv-ities when oxidatively or reductively doped (1) Much of

the early research on PPV focused on the relatively

disap-pointing properties of the doped material; however,

inter-est in this material was reawakened in 1990 when Friend

and co-workers in Cambridge discovered that films of

un-doped PPV could be used as the emitting layer in organic

electroluminescent (EL) devices (2) This discovery

stimu-lated intense research in the area, including many

funda-mental studies of the properties of PPV and its derivatives,

and extensive academic and industrial interest in the

ap-plications of the materials

This article provides an overview of the methods by

which PPV and some of its derivatives may be prepared,

the physical and electronic properties of these materials,

and the applications that are being explored The reader

who seeks a deeper and more detailed understanding of

this fascinating material is referred to several excellent

and comprehensive reviews which have been published on

the synthesis, properties, and applications of PPV (3–7)

METHODS OF PREPARATION

Unsubstituted PPV

Many methods have been devised to prepare PPV for

fun-damental and applied studies Due to its rigid conjugated

backbone, unsubstituted PPV and even short oligomers are

insoluble and intractable materials Therefore, an tant consideration in all preparative routes that are used toprepare high molecular weight material, is the solubility ofthe growing polymer chain One of the most successful app-roaches to high molecular weight (10,000–100,000) PPV isthe Wessling or sulfonium precursor route which proceedsvia a soluble precursor polymer that is subsequently ther-mally converted to fully conjugated PPV Other methodsare available but generally produce low molecular weightmaterial

impor-Sulfonium Precursor Route In this route,

polymeriza-tion of the bis-sulfonium salt 1 with base yields a solublepolyelectrolyte 2 (Scheme 1) (8,9) This intermediate maythen be purified, processed, and finally thermally con-verted to PPV Both the nature of the sulfide used in thesulfonium salt and the counterion affect the conditions re-quired in the preparation, as well as the molecular weightand structure of the resulting polymer (9,10) A modi-fied sulfonium precursor route has also been developed, inwhich the soluble methoxy-substituted polymer 3 is con-verted to PPV in the presence of HCl gas (11)

S+ NaOH

H 2 /N 2

MeOH

S+

S+X

The mechanism of polymerization has been the subject

of some debate in the literature; both radical and anionicmechanisms are proposed The presence of oxygen duringpolymerization results in lower molecular weight polymer

consistent with a radical mechanism (8) A p-xylylene or

p-benzoquinodimethane intermediate is postulated and

has been observed spectroscopically Other studies havesuggested that this intermediate polymerizes via an an-ionic mechanism (12,13) Despite these conflicting results,the polymerization is typically carried out in the absence

of oxygen, and the thermal conversion step is done either

in vacuum or under an inert or forming gas atmosphere

Other Routes Other synthetic routes to PPV include

the Wittig reaction (Scheme 2a) (14), the Pd-catalyzedHeck reaction (Scheme 2b) (15), and the McMurry coupling(Scheme 2c) (16) Reaction ofα, α-dichloro- p-xylene with potassium tert-butoxide yields PPV (Scheme 2d); this pro-

cedure, discovered by Gilch and Wheelwright, is referred to

as the Gilch route (17) All of these methods yield PPV rectly from soluble monomers and thus produce primarilylow molecular weight oligomers

O

HO

KOtBu

Ph3+P

P+Ph3

Br2Cl−

DMF Pd(OCOCH 3 ) 2

Electropolymerization is a convenient method for

preparing insoluble conjugated polymers such as PPV

be-cause it yields thin films directly on an electrode surface

Such a film may then be directly used in an application

that requires a conducting contact, such as

electrolumines-cence Several methods have been reported for the

electro-chemical preparation of PPV films Electroelectro-chemical

reduc-tion of 4–6, it has been shown, yields PPV films on metal

and indium tin oxide (ITO) electrodes (18–20) Another

di-rect route to thin films is chemical vapor deposition (CVD)

from precursors such as 7 or 8 (21,22)

Ph3+P

P+Ph3

Substituted PPV

To increase the solubility and processibility of PPV in the

conjugated form, substituents such as alkoxy or phenyl

groups have been added to the backbone structure In

ad-dition to enhancing the solubility, these substituents also

change the electronic properties of the polymer via both

inductive and conjugative effects An additional benefit of

soluble derivatives of PPV is that techniques for polymer

characterization that require soluble material may be used

and provide direct information about molecular weights

and higher order structure

Sulfonium Precursor Route The Wessling route has also

been used to produce soluble derivatives from monomers

that contain solubilizing substituents on the phenyl ring

For example, dialkoxy substituted monomers yield 11

which is soluble in organic solvents such as chloroform and

chlorobenzene (25), as well as

poly[2-((2-ethylhexyl)oxy-5-methoxy- p-phenylene)vinylene] (MEH-PPV) (12) (Scheme

4) (26) The branched side chains in MEH-PPV improve

the solubility of this derivative versus unbranched analogs,

and this polymer is one of the most popular for

Other Routes Substituted PPV derivatives have also

been prepared by several other routes When the resulting

polymers are soluble, these methods are often successful in

preparing high molecular weight material The Gilch route

has been used to prepare phenyl substituted polymers (13)

(Scheme 5) (27), and the McMurry coupling has been used

to yield a dihexyl substituted polymer (14) that is soluble

in a range of organic solvents (Scheme 6) (28)

Br

Br

KOtBu 18-crown-6

n

13 Scheme 5

H

C6H13

OH

Scheme 6

Copolymers

A number of studies have focused on the preparation ofcopolymers that contain fully conjugated backbones, aswell as those that contain both conjugated and nonconju-gated blocks This work has been motivated by the desire

to prepare materials whose range of electronic propertiescan easily be tuned by variations in the proportion and na-ture of the monomers used in the copolymer synthesis Inaddition, local variations in the π–π∗ energy gap can beintroduced in this way which, it has been shown, result indramatic improvements in the performance of these copoly-mers in electroluminescent devices

The sulfonium precursor route has been used to pare copolymers by using various proportions of differentmonomers in the synthesis This method has yielded bothpartially conjugated (15) and fully conjugated polymers(16) (Scheme 7) (29,30), as well as copolymers that con-tain other aromatic groups in the backbone in addition tophenylenevinylene moieties such as in copolymer 17 (31).The Wittig reaction has also been successfully used

pre-to prepare soluble copolymers using substituents such asalkoxy groups on the backbone (18) (32), as well as CF3sub-stituted copolymers that contain flexible, nonconjugatedspacers (19) (33) Heck chemistry has been applied to pre-pare copolymers such as 20 (34)

Composites and Blends

The mechanical and optoelectronic properties of PPV andits derivatives may be optimized for specific applications

by using polymer blends and composite materials Blends

of MEH-PPV and polyethylene have been used to enhancethe degree significantly to which the conjugated polymerchains are aligned by stretching (35) The density of en-tanglements in gels of polyethylene is much lower than

in spin-cast polymer solutions, and this effect remains ter removing the solvent This allows tensile drawing ofsuch blends to large draw ratios (>200), which causes

af-the conjugated polymers to align to a degree normallyexpected in single crystals Charge transport throughblends can also be controlled by using a host polymer thathas specific properties Blue emitters have been prepared

from blends of poly( p-phenylphenylenevinylene) (PPPV) in

poly(9-vinylcarbazole) (PVK), a hole-transporting polymer(36) In addition to enhancing the processibility of PPPV,PVK blue-shifts the electroluminescence, enhances hole-transport, and increases the probability of radiative recom-bination due to the dilution effect

Composites of PPV and silica (SiO2) and vanadium oxide(V2O5) have been prepared for use in nonlinear optical ap-plications (37–39) These composites are prepared via sol-gel processing methods, and allow combining the superiornonlinear optical properties of the conjugated polymerswith the very low optical losses found in inorganic glasses.Composite films of insulating SiO2, TiO2, and Al2O3nanoparticles and MEH-PPV were prepared and result

in more efficient charge injection and transport in troluminescent devices formed from them, as well as en-hanced emission intensities (40,41) Photovoltaic and time-resolved microwave conductivity measurements were alsoused to study nanocrystalline TiO2/PPV composites; thesestudies show that excitons generated in the polymer are

OROMeMeO

dissociated at the polymer/ TiO2interface and the electrons

are transferred to the nanocrystals (42)

Composites of PPV in films of the polyelectrolyte Nafion

have been synthesized by electrostatically binding the

di-cationic monomer 1 to the film, followed by treatment with

base and thermal conversion (43) Ordered

nanocompos-ites of PPV have been synthesized from mixtures of

poly-merizable lyotropic liquid crystals using PPV precursors

(44) Photopolymerization of the host results in a

hexago-nal architecture, from which thin films and fibers can be

fabricated A significant enhancement in the

photolumi-nescence of the composite relative to PPV was found

PROPERTIES

Absorption and Emission

Films of PPV show three absorption bands whose maxima

are at 6.12, 5.06, and 3.09 eV (45) The two higher energy

bands are attributed to localized molecular states, whereas

the lower energy band is due to a delocalized electronic

ex-citation The emission spectrum of PPV upon excitation

at 355 nm is resolved into three lines whose spacing of

0.16 eV is due to vibronic coupling (46) Migration of the

excited state to the longest conjugation length segments

in the polymer appears to occur before radiative decay

be-cause smearing out of the vibrational fine structure is

ex-pected if emission from a distribution of sites within the

polymer occurs The photoluminescent efficiency varies

be-tween 5 and 25%, depending on the synthetic route used

and the conversion conditions (47)

Two descriptions of the excited state have been

ap-plied to organic semiconductors, the exciton and the band

model The appropriate model depends on the extent of

cou-pling between sites; strong coucou-pling yields uncorrelated

electrons and holes, and weak coupling favors correlated

electron–hole pairs (excitons) Time-resolved fluorescence

and polarized fluorescence experiments suggest that the

exciton model is appropriate in PPV (48) Rothberg andco-workers examined the relative effects of interchain ver-sus intrachain excitations in MEH-PPV by comparing ex-cited state lifetimes and quantum yields in films to those

of dilute solutions (49) They observed significantly lowerquantum yields for emission in films and attributed this tothe formation of nonemissive interchain excitons, that arenot formed in dilute solution They also concluded that filmmorphology can play a significant role in the photophysi-cal behavior of PPV (50) The presence of trace oxygen inthe conversion of precursor polymer to PPV reduces thephotoluminescence of the resulting material (51) It wasshown that this is correlated with the formation of carbonylgroups in the polymer backbone and can be prevented bycarrying out the conversion in a reducing atmosphere

Photoconductivity

Photoconductivity in PPV was first measured threedecades ago (52) A later study revealed low dark conduc-tivities for PPV films (<10−15S/cm) but significant photo-conductivity upon irradiation at 440 nm Significant con-ductivity was also found upon irradiation of the film in thenear-infrared region, despite insignificant optical absorp-tion in this region This was attributed to a charge-transfermechanism that involves trace oxygen (53) Oriented PPVfilms showed enhancements in photoconductivity in lightpolarized parallel to the direction in which the films werestretched (54) Transient photoconductivity measurementshave also been used to address the question of the na-ture of the charge carriers in MEH-PPV films The excitonmodel predicts strong dependence of photoconductivity ontemperature, and this is observed for films thicker than

120 nm In thinner samples, both steady-state and fasttime-resolved photoconductivity measurements demo-nstrate that photoconductivity is independent of temp-erature These results are inconsistent with the excitonmodel (55)

Doping and Electrical Conductivity

Pristine PPV films are insulators at room temperature;

however, exposure to oxygen causes an increase in

con-ductivity to 10−11S/cm, an effect attributed to reversible

doping where the oxygen acts as an electron acceptor

(56) Irreversible doping of PPV films with strong

ox-idants such as FeCl3 or H2SO4 produces black films

whose conductivities are very high relative to the pristine

material (57) Films doped with sulfuric acid showed

con-ductivities of ∼104 S/cm that were only weakly

temper-ature dependent, indicating metallic behavior The

con-ductivity of films doped with FeCl3 was slightly lower

(103 S/cm) and decreased with temperature The doped

films are stable in oxygen but are moisture sensitive

Copolymers of PPV derivatives that contain

electron-donating groups, such as

poly(1,4-phenylenevinylene-co-2,3,5,6-tetramethoxy-1,4-phenylenevinylene) (21), can

also be doped with weaker oxidants such as I2to give

mate-rials whose conductivities are as high as 7× 10−2S/cm (58)

OMe

OMeMeO

MeO

nm

21

Microstructure and Liquid Crystallinity

The degree of structural order in PPV films depends

greatly on the method of preparation The degree of

broadening in electron diffraction patterns has been

used to assess the extent of ordering in different PPV

samples (59) High-resolution transmission electron

mi-croscopy revealed, crystalline regions of approximately

7 nm in oriented PPV films, and these crystallites are

retained upon doping the films with H2SO4 (60)

Sub-stituents also affect the structural order of PPV

deriva-tives Methoxy substituents allow chains to interlock and

n

O

OO

of PPV that bear mesogenic substituents, as well as incopolymers that contain phenylenevinylene segments inthe main chain Copolymers in which some of the phenylrings have alkoxy side chains (20) exhibit a nematic liq-uid crystalline phase which has been characterized bypolarized microscopy and differential scanning calorime-try (34) The temperature range between the melting pointand the nematic–isotropic phase transition, it was found,depends on the length of the alkoxy group A PPV deriva-tive (22) that bears the well-known cyanobiphenyl mesogen

as a side chain has both nematic and smectic mesophases(63) This polymer was oriented by rubbing a film with aTeflon stick, and a significant degree of orientation was ob-served by polarized UV/visible and IR spectroscopies Seve-ral examples of main-chain liquid crystalline polymers thatcontain phenylenevinylene moieties bridged by saturatedlinkers are known The thermotropic polymer 23 was pre-pared by using a Wittig procedure, and it was found that

it melts anisotropically (64) A related main-chain polymer(24) has a mesophase that exists between 218 and 275◦C(65)

Nonlinear Optical Properties

For many optical signal processing applications, it is sirable for materials to have large optical nonlinearitiesand fast response times For example, third-order non-linear optical (NLO) properties result in laser pulse in-duced refractive index changes that occur on the fem-tosecond timescale These changes could be exploited infast optical switches Conjugated polymers are expected

to be good candidates for such applications due to the

de-localization of charge in the polymer backbone (66)

25

PPV has many of the characteristics desired in a NLO

material, including good transparency, high π-electron

density, and optical quality films that may be oriented

and ordered PPV has a third-order nonlinear optical

susceptibility (χ(3)) of 7.8 × 10−12 esu (67), whereas χ(3)

for a substituted derivative, poly(2,5-dimethoxy-

p-pheny-lenevinylene), is 5.4 × 10−11 esu at 1.85 µm (68) The

higher value for the dimethoxy-substituted derivative

may be due to more extended conjugation in this

mate-rial An alternative strategy which has been

inves-tigated is to introduce a NLO-active moiety pendent

to a PPV backbone For example, Disperse Red 1 has been

tethered in this way (25), and the resulting polymer has

aχ(3)value of 2.5 × 10−12esu (69)

APPLICATIONS

Photovoltaics

Heterojunctions between conjugated polymers and films of

electron acceptors behave as rectifying p-n junctions and

may be used in photovoltaic devices Such junctions have

been prepared by vacuum evaporation of n-type

buckmin-sterfullerene (C60) onto spin-cast films of p-type MEH-PPV

on ITO-glass substrates (Fig 1) (70) These devices behave

as rectifiers in the dark and pass a photocurrent when

Figure 1 Schematic of a MEH-PPV photovoltaic cell.

illuminated by visible light The open circuit voltage (Voc)saturates at 0.53 V and has a fill factor of 0.48 and a powerconversion efficiency of 0.04% The Cambridge group alsoreported photovoltaic devices in which a heterojunctionbetween bis(phenethylimido)perylene and PPV is sand-wiched between ITO and Al These devices had a some-

what greater fill factor (0.6), and Voc approached 1 V; thequantum yield was 6% (electrons per incident photon) (71)

A related approach involves using a blend of MEH-PPVand bis(phenethylimido)perylene that gives a fill factor of0.27 at an open circuit voltage of 0.58 V; however, improve-ments in these devices are limited by the poor solubility ofthe bis(phenethylimido)perylene (72)

Photovoltaic cells have also been constructed from emitting electrochemical cells (LECs; see later) (73) Inthese devices, a phase-separated blend of MEH-PPV andcyano-PPV is used along with a solid electrolyte thatconsists of a mixture of polyethylene oxide (PEO) andLiCF3SO3 Sandwich photovoltaic cells using Al and ITO

light-as the electrode contacts were doped using a prebilight-as of

3 V and resulted in a Voc of 1.0 V and a power sion efficiency of 0.1%, assuming a fill factor of 0.25 Thebuilt-in potential is determined by the chemical potentialdifference between the p-doped and n-doped layers, ratherthan the work function of the electrodes; thus, air-stableelectrodes can be used in these cells

conver-Optical Memory

Data have been permanently stored in films of PPV tives by irradiating films of the sulfonium precursor poly-mers using either a Xe arc lamp or Ar ion laser (488 nm)(74) Subsequently heating the films resulted in the for-mation of colored, conjugated films only in regions thatwere not irradiated Photochemical scission of the polymerchains leaves a water-soluble residue which is readily re-moved by rinsing the heat-treated films in water This pro-cess may also be used for lithographic patterning of PPVfilms onto substrates

deriva-Light-Emitting Devices

In the late 1980s, Tang and VanSlyke reported nescent devices that used thin films of 8-hydroxyquinolinealuminum (Alq3), as the emitting material (75) They dis-tinguished these devices from those based on conventionalinorganic semiconductors by calling them “organic,” de-spite the fact that the emissive compound is actually aninorganic coordination complex The devices consisted of

electrolumi-a lelectrolumi-ayer of electrolumi-a hole-trelectrolumi-ansporting electrolumi-aromelectrolumi-atic electrolumi-amine on electrolumi-an ITOelectrode, 600 Å of the luminescent Alq3, and a Mg/Agelectrode The devices behaved like rectifiers and emittedlight whose peak intensity at 550 nm had a forward bias

of as little as 2.5 V In a subsequent publication, Tang andVanSlyke showed that doping the Alq3 layer with otherhighly fluorescent molecules, such as coumarin 540, in-creases the electroluminescent efficiency and allows tuningthe color from blue-green to orange-red (76)

In 1990, the discovery that PPV could be used as theemitter in an electroluminescent device was reported byFriend and co-workers at the Cavendish Laboratory in

ONN

NO

Alq3

Cambridge (2) The devices consisted of a PPV film,

pre-pared by using the sulfonium precursor route, sandwiched

between an indium tin oxide (ITO) and an Al electrode;

green-yellow light was emitted under a forward bias of 14 V

and the quantum efficiency was up to 0.05% Shortly after

this initial publication, Braun and Heeger demonstrated

that MEH-PPV could also be used to fabricate EL devices

in which the polymer was directly cast from solution in the

conjugated form They used both indium and calcium

cath-odes and observed visible light at 4 V forward bias using a

calcium cathode whose efficiency was 1%

Device Operation Single-layer devices consist of an

electroluminescent layer sandwiched between an

electron-injecting cathode (usually a low work-function material

such as Ca or Al) and a hole-injecting anode (most

commonly the transparent conductor indium tin oxide

(ITO) on glass)

The operation of the device under forward bias may be

understood by using a simple band diagram (Fig 2a) The

anode and cathode materials are chosen to provide low

Electrons

Emittedlight

Holes(a)

Holes

ETL = electrontransportinglayer

Figure 2 Band diagram for (a) single-layer and (b) two-layer polymer EL devices.

barriers to electron and hole injection by matching thevalence and conduction band energies of the polymer tothe electrode work functions A disadvantage of the single-layer device configuration is that charge recombination of-ten occurs close to the cathode because most EL polymersare better hole conductors than they are electron conduc-tors The metal electrodes can quench excitons in closeproximity, thus reducing EL efficiency

An approach that has been used successfully to movethe emitting zone away from the electrodes is constructingtwo-layer cells in which recombination occurs at the inter-face between the two organic layers (Fig 2b) Here, thematerial in the layer adjacent to the cathode is selectedfor high electron mobility but hole mobility lower than the

EL polymer which is located adjacent to the anode Thus,electrons and holes are readily injected into the adjacentlayers that contact the respective electrodes and accumu-late at the interface between the two layers A number ofmaterials have been exploited for use as the electron trans-

porting layer (ETL), including

2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (butyl PBD) 26 (77) layer devices constructed from PPV as the emitting layerand butyl PBD dispersed in poly(methylmethacrylate)(PMMA) as the ETL showed a 10-fold improvement inefficiency relative to analogous single-layer devices con-structed only from PPV Polymers that contain oxadiazolemoieties pendent from the backbone and in the main chain,have been synthesized and tested as ETLs and also im-prove external quantum efficiencies in two-layer devices(78) The EL efficiency is temperature independent in thesedevices, suggesting that charge injection from both elec-trodes is well balanced

Two-Hole injection from the anode can be improved by ing a hole-transport layer that functions by improving