poly(vinylidene fluoride) composites

SE@pRS ASTIJA~RS ELSEVIER

Sensors and Actuators

A 65 (1998)

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A

PHYSICAL

Improved piezoelectrics for hydrophone applications based on calcium-modified lead titanate/poly ( vinylidene fluoride) composites Changxing Cui a, Ray H. Baughman a,*, Zafar Iqbal a,Ted R. Kazmar b, Dave K. Dahlstrom b a AlliedSignal, Research and Technology, IOI Columbia Rd, Morristolrw, NJ 07962, USA b AlliedSignal Ocean Systems, 15825 Roxford St, Sylmar, CA 91342, USA Received

24 March

1997; revised 25 June 1997; accepted 21 July 1997

Abstract Piezoelectric composites comprising dispersions of free-flowing calcium-modified lead titanate powders in a poly( vinylidene fluoride) matrix have been developed and characterized as materials for hydrophone applications. Wefind thatthesecomposites providehighlydesirable properties:(1) high figuresof merit, g,d,=50( k.S) X 10-l” mz N-‘, and a relatively low dielectriclosstangent,lessthan 0.018; (2) convenientmeltprocessibility;(3) no significantsensitivitychangesin goingto highpressures ( 14MPaor 2 kpsi); (4) thermalstabilityof thepoledstatefor monthsat 100°C;and(5) easyfabrication.Thisperformance resultsfrommaterialchoicesandfromspecialceramicpowder processingandcomposite preparation methods that optimizethe achievabledegreeof ceramicpoling,the remanentpolarization,andthe transferof mechanicalenergyto the dispersedceramic.Materialsdesignconcepts,compositeprocessing, and device constructionand evaluationaredescribed. 0 1998ElsevierScienceS.A. Keywords:

Hydrophones;

Lead titanate; Piezoelectric

composites;

Poly( vinylidene

1. Introduction Piezoelectric ceramics and polymers have been widely usedfor sensorandactuator applications[ l-41. For example, piezoelectric leadzirconate titanate (PZT) and bariumtitanate (BT) ceramics and piezoelectric poly (vinylidene flueride) (PVDF) and poly( vinylidene fluoride/trifluoethylene) (PVDF-TrFE) polymers have important applications aselectromechanicaltransducers.However, major performance liabilities exist for the use of thesepiezoelectric ceramicsand piezoelectric polymers as hydrophones [ 5,6]. First, although the piezoelectric chargecoefficients (& and &) of PZT and BT are very high (about 100 to 600 pC N- ‘) , their hydrostatic charge coefficients ( dh= d33+ d3, i-d& are relatively low because - (d,, +d,,) nearly equalsdz3. Secondly, since theseceramicshave a high dielectric constant K (about 1000 to 4000)) their piezoelectric voltage coefficients (gh=&lKEO, where l 0 is the vacuum permittivity) are not high (less than 0.004 mV N- ‘1. Thirdly, there is a large difference in acoustic impedance * Corresponding

author. Tel.:

t 1 201 455 23 75. Fax:

t 1 201 455 59

91 0924-4247/98/$19.00

0 1998 Elsevier

PIISO924-4247(97)01646-4

Science S.A. All rights reserved

fluoride)

betweenpiezoelectric ceramicsand water. which can degrade sensorresponsefor either marine or medical applications. Fourthly, in order to usepiezoelectric ceramicssuchasPZT or BT for low-frequency hydrophone applications, mechanical transformer configurations (such asa cylindrical configuration with end caps or a Moonie configuration) are typically utilized in order to increase sensitivity, which increasesdevice manufacturecost. Fifthly, the ceramicsare very brittle and non-flexible. Sixthly, the high mechanical quality factor ( &) of ceramics(i.e., the very low mechanical loss) causesringing within sensorsif external damping layers are not used. On the other hand, the piezoelectric voltage coefficients of PVDF and PVDF-TrFE polymers are very high (above 0.1 mV N-l). Such piezoelectric polymers have an acoustic impedancethat differs little from water and they are flexible. Unfortunately, the relatively low figure of merit (usually less than ghd,,= 20 X lo- I3 m2N- ‘) andhigh dielectric loss tangent (over 2%) of many polymers lead to a low signal-tonoise ratio, i.e., a low loss-correctedfigure of merit. Also, PVDF-TrFE copolymers with the most attractive piezoelectric propertiesmust be stored at below 70°C otherwise they depole.When the degreeof crystallinity of PVDF-TrFE

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copolymers is very high, which is desirable for some applications, these copolymers become brittle. Finally, because of the high poling fields required, it is difficult to fully pole the piezoelectric polymers when the sensor is thick (above about 0.5 mm). In order to obtain optimal material properties for sensor applications, piezoelectric ceramic/polymer composites have been extensively investigated [ 5,6]. Newnham et al. developed the concept of phase connectivity in order to design piezoelectric ceramic/polymer composites more effectively [ 51. While there are many possible phase connectivity patterns, the least expensive composites consist of piezoelectric ceramic particles in a continuous, three-dimensionally connected matrix. If these particles are percolated (so as to form a continuous network) in zero, one, two, or three dimensions, these composites are most rigorously referred to as O-3 composites, l-3 composites. 2-3 composites, or 3-3 composites, respectively. Since it is typically difficult to assess the degree of ceramic particle percolation from literature reports, we shall refer to ceramic particlecomposites in a three-dimensionally connected polymer host as particle-matrix composites independent of the degree of percolation of the ceramic particles. Kitayama [7], Pauer [ 81, and Harrison [ 91 made early attempts at fabricating useful particle-matrix composites. However, their reported c/33values are very low. Yamada et al. [ lo] obtained a maximum piezoelectric charge coefficient ( d33) of 48 pC N- ’ for a particle-matrix composite from a PZT powder and PVDF. Giniewicz and Newnham [ 11,121 described a particle-matrix composite prepared using a 6070 vol.% ceramic powder loading of the PbTiO,-BiFeO, (PT-BF) morphotropic solid solution in epoxy. The maximum reported d,, and t&g,, did not exceed 35 pC N- ’ and 35 X lo-‘” rn’ N-‘, respectively. In order to obtain the reported figure of merit, the ceramic powder waspreparedby repeated grind-calcination (i.e., mechanical crushing followed by calcination) and water-quenchingprocedures.The sameinvestigators [ 121 later reported that a similar material provided largely pressure-independentperformance. The reported figure of merit for this material is moderately high (d,g, = 27 X IO- ” m2N- ’ ). but its high dielectric losstangent (tan??)of 0.06-o. 10 will result in a significant Johnson noise voltage. enolae( eZ,,o,sc = 2kTAf tana/nfC, where k is Boltzmann’s constant, T the absolute temperature,S the frequency, C the capacitance, and Aj” the bandwidth) [ 131. Banno et al. [ 141 preparedflexible piezoelectric composites that are named NTK Piezo-Rubbers PR-303, PR-304, PR305, PR-306, and PR-307. PR-303, PR-304. and PR-306 show a hydrophone figure of merit (d,,g,) in the range 812 X lo- ” m’ N-‘, while PR-305 and PR-307 have a figure of merit of about 50 X 1O- ” m’ N- ‘. Although PR-305 and PR-307 have a very high figure of merit, they are not useful for deep-seahydrophone applicationsbecauseof the dependence of hydrophone sensitivity on hydrostatic pressureand the irreversible damageto the hydrophone causedby high hydrostatic pressures.If another figure of merit (FOM2 =

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d,g,ltan@ is used [ 131, which takesinto account the sensor Johnsonnoise,an additional performanceproblem isevident. Specifically, PR-305andPR-307have dielectric losstangents of 0.03 and 0.05, so that &,g,/tan~ is only 1.7X 10-l’ and 1.0 X lo-” m2 N-l, respectively. Sumita and coworkers [ 151 have reported piezoelectric compositesof Pb0,93La,,07( Zr0,65Ti0,3j)O3 (PLZT) and PZT with carbon powders and PVDF for vibration control applications.For a ceramicpowder loading level lessthan 50 vol.%, the efficiency of vibration damping is low. However, the observedcl,, coefficients are 180 and 440 pC N- ’ for the PLZT and PZT composites, respectively. Thesehigh piezoelectric chargecoefficients can be attributed to the presenceof carbon powders that act as conductive mediato connect individual ceramicparticlesthat are isolated in the polymer matrix, and facilitate poling. Unfortunately, the high loading level of carbon powder makesthesematerialstoo conductive for usein sensorapplications (due to high Johnsonnoise). Han and co-workers [ 16-181 have used a colloidal processing technique to prepare powder-matrix composites from PT-BF morphotropic solid solution, PZT, and undoped lead titanate (PT) ceramic powders and various polymers. The highest hydrophone figure of merit (g&,,) found for a composite of PTBF and epoxy was 56 X lo-l3 m2N- ‘. The noise-corrected figure of merit (FOM2 = g,<an6= 1.7 X lo- lo m’ N- ‘) is low becauseof a high dielectric losstangent.Dias andDasGupta [ 19,201recently reported the fabrication of a particlematrix piezoelectric compositefrom calcium-modified lead titanate (Ca-PT) in a piezoelectric PVDF-TrFE copolymer having a 75/25 mole ratio of vinylidene fluoride to trifluoroethylene. The average particle size of the ceramic powder was 20 p,rn and the highest reported figure of merit (&,gh) was 12 X 1O- I3 m2N- ’ However, becausethis material has a relatively low dielectric loss tangent of 0.014, the noisecorrected figure of merit (FOM2 = d,g,,ltan6= 0.9 X lo-” m2 N-‘) is almost the sameasfor NTK Piezo-Rubber PR307. The above-describedresearchhasprovided particle-matrix compositesthat provide limited advantages.As recently commented by Newnham and Markowski [ 211, the challenge remainsto devise compositesthat showhigh figures of merit for hydrophone applications, little or no variation in static pressuresensitivity under hydrostatic loading, and advantageous features for processingand fabrication. The present paper will describe a new particle-matrix composite that meetsthe above requirements.

2. Experimental The ceramic powdersusedfor composite formation were preparedfrom free-flowing green ceramic powder (containing an organic binder) having an approximate composition (Pb0.&a2J ( (Co,/2Wl/2)0.01Ti0.96)03 (EC-97 of ED0 Acoustics). This green ceramic powder was sintered in an alumina crucible together with a compacted disk ( 13 mm

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diameter and 1.0 mm thick) of the same ceramic powder, which was used for test purposes. Characterization of the piezoelectric and dielectric properties for this test disk indicated when satisfactory sintering is to be expected within the particles of the ceramic powder. Additionally, the powder X-ray diffraction method was used to determine whether or not the metal oxides in the green powder have fully reacted. The optimized sintering process was as follows: ( 1) from 2.5to 600°C at a heating rate of 5°C min- I, (2) 12 h at 600°C (3) from 600 to 1100°C at a heating rate of 5°C min- I, (4) 3 h at 1 lOO”C, and (5) from 1100°C to room temperature without temperature control. After cooling to room temperature, the obtained dark-brownpowderwas shaken in aplastic bottle until it again became free flowing. After sintering, the test disk had a diameter of about 11 mm, a thickness of about 0.75 mm, and a density of 6.7 g cmW3. After electroding both sides with silver ink (Creative Materials), this disk waspoled in silicon oil for 30 min at 120°C under a d.c. voltage of 8 kV. A week after poling, the disk showed a d3a value of 82 pC N- ’ at 100 Hz (Berlincourt .& meter), a dielectric constant of 270, and a dielectric loss tangent of 0.005 at 1 kHz (HP 426 1A LRC meter) . X-ray diffraction analysis of the fired ceramic powder showed the diffraction pattern expected for a tetragonal perovskite structure [ 221. The ceramic powder was classified into three equal-weight portions according to particle size (using either an air-jet separation method or separation sieves with different meshes). The first portion had a particle size distribution between 40 and 70 km, the second between 70 and 90 pm, and the third between 90 and 150 p,m. The above particle size distributions were determined by comparing each portion with 40+2 p,rn diameter glass beads under a microscope

Fig. 1. Scanning

electron micrograph

of Ca-modified

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(Leica Zoom 2000). The error in the determination ofparticle size (i.e., particle diameter) is expected to be f 10 pm. Unless otherwise indicated, the middle portion of the ceramic powder size distribution (70 to 90 p,m) was used for the described experiments. A micrograph of the sintered ceramic powder is shown in Fig. 1. Note that the ceramic particles are nearly spherical and have a granular surface. Ceramic-polymer composites were prepared by two different methods from the above-described sintered ceramic powders and polyvinylidene fluoride (Polyscience, Inc., weight-average molecular weight of 350 000, and a melting point of 166-170°C). Unless otherwise indicated, the ceramic powder loading level was 65 vol.%. In the first method, the PVDF was dissolved in boiling acetone and then the ceramic powder was dispersed in the solution of PVDF/acetone by stirring for 5 min. A flow of nitrogen was introduced to remove acetone, until a wet gel-like soft paste was obtained. This paste were pressed into wet pellets using an IR pellet press at a pressure of about 14 MPa. After drying under vacuum at 100°C for one hour, the pellets were hot-pressed in a die at 200°C and 100 MPa for 15 min using a CARVER laboratory press. In the second method, the ceramic and polymer powders were mixed in hexane to produce a thick slurry. After stirring for 10 min, the hexane was removed by heating and then by dynamic vacuum. The resultant powder, which is nearly free flowing, was used to hot-press various configured sensors (such as pellets and cylinders) at 100 to 170 MPa and 190°C. Both methods provided identical results with respect to the piezoelectric and dielectric properties. The density of the melt-molded piezoelectric ceramic composite was about 4.5 g cmM3.

lead titanate ceramic particles

prepared

by sintering

a spray-dried,

green ceramic powder.

C. Cui et al. /Sensors

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Composite poling was carried out at 110 + 10°C and 125 kV cm-’ in silicon oil for between 20 and 30 min. In all cases, the sample thickness in the poling direction was about 2 mm, so the sample thickness greatly exceeded the average particle size. Also, all measurements of piezoelectric coefficients were conducted at least one week after poling. The hydrostatic voltage coefficient (g,,) was measured using an air-calibrator. Unless otherwise indicated, the reported figures of merit were derived from measurements using this apparatus. The air-calibrator consists of ( 1) an a.c. sweep generator (WAVETEK 164)) (2) a cylindrically shaped air chamber with two speakers at two ends, (3) an ultra-low-noise voltage amplifier ( ITHACO 144N), and (4) a spectrum analyzer (HP3585B). The a.c. sweep generator outputs a sinusoidal a.c. wave (75 Hz) to drive the two speakers to produce sound pressure waves in the air chamber. The acoustic sound pressure in the chamber was detected by either a test sample or a standard that is interfaced with the voltage amplifier. The output of the voltage amplifier was analyzed by the spectrum analyzer at 75 Hz. The free-field hydrophone sensitivity (S,) in dB (re. 1 V FPa- ‘) was calculated using s,=s,+20

log v,-20

+ 20 w

log v,

CA cc, + Ccablr) 1

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The reference standard used was a Navy type I PZT ceramic plate having a d, value of 26.4 pC N- ‘. Measurements made using this apparatus and reference standard are consistent with those made using the air calibrator and associated reference standard. The piezoelectric charge coefficient d,, was measured at 100 Hz using a Berlincourt d33meter. Dielectric constant and dielectric loss tangent were determined using an HP 4261A LRC meter at 1 kHz and room temperature. The coupling constant (k,) , mechanical loss factor ( cp), and sound velocity (G) of the present materials were determined by the method proposed by Koga and Ohigashi [ 241. These parameters were derived by fitting the experimentally measured admittance to Eq. (3) below, which was derived for a free piezoelectric resonator: Y=joC,(l-j@)+jwC,(l-j@)‘k’[j(l-jqd2) X(OX/~L~) coth[j(l-jq/2)(or/2v)]

-(l-j@)Lf]-’

(3)

where C, is the clampedcapacitance,t the samplethickness, @the dielectric lossfactor, and w the angular frequency.

3. Results (1)

where S, is the sensitivity of a standard in dB (re. 1 V p,Pa- ‘), V, and V, are the voltages measured at 75 Hz using the spectrum analyzer for the sample and standard, respectively, C, is the capacitance of the sample, and Ccablr is the total capacitance of the wire connecting the sample to the amplifier, the input of the voltage amplifier, and stray capacitance. The g, value was calculated using S = 201og(tg,l 106), where t is the sensor thickness. Because the present composite has an acoustic impedance that is much higher than that of air? its radiation impedance can be neglected. Thus, this method will give the same low-frequency sensitivity as measured in water or other similar media. The piezoelectric hydrostatic charge coefficient (d,,) was determined using instrumentation at Pennsylvania State University [ 231. A hydraulic oil bath was used for transmission of acoustic pressure waves generated by an actuator at 40 Hz. The charge produced on the sensor surfaces as a result of the the acoustic pressure changes was the input to a current amplifier, which generates an corresponding output voltage that was buffered with a voltage follower. Output voltages from the buffer were measured by a spectrum analyzer (HP 3585B) for both the test sample and an adjacent PZT reference standard. The hydrostatic piezoelectric charge coefficient was calculated using

3.1. Piezoelectric and dielectric properties of the optimized composite

where V, and V, are the voltages of the standard and sample that are measured using the spectrum analyzer. A, and A, are the electrode areas of the sample and standard, respectively?

An unusually high figure of merit is indicated by the aircalibrator measurement(FOM = gh2Kj3e0= 54 X lo- I3 m* N- ‘) andthe hydraulic chambermeasurement(FOM = dh2/ K33~,,= 5 1X lo- I3 m” N- ’ ) . The cylindrical disks usedfor thesedeterminations(2.0 mm thick and 1.4 cm in diameter) were poled for 30 min at 120°C and 125 kV cm-‘. These figures of merit are basedon a measureddielectric constant of K33= 70 at 1 kHz, g, = 0.094 mV N- ’ at 75 Hz from the air-calibrator measurement,and dh= 56 pC N- ’ at 40 Hz from the hydraulic chamber measurement.Essentially the samefigure of merit (54 X lo-l3 m* N- ‘) resultsfrom the useof the value of dhobtainedfrom Berlincourt measurement (58 pC N-’ at 100 Hz) and the approximation that FOM=rl,:lK,,~,,=55x lo-l3 m2 N-‘. Since it is known that d,, nearly equals d, for Ca-PT if the poling field is sufficiently high [ 251, this approximation is quite reasonable for thisparticular composite.Hence,essentiallythe samevery high figure of merit is indicated by three independentmeasurement methods. The dielectric loss tangent measuredfor this composite (tana= 0.020) is somewhathigh. However, we shall later show that the dielectric loss tangent can be decreasedto aslow as0.013. Additional parametersobtained (from the impedancemeasurements,using Eq. (3) ) are a soundvelocity of 1500m s - I, a coupling constantof k, = 0.25 and a mechanicallossof &J= 0.16. Also, measurements made on sensorarrays indicate that the sensorsensitivity is fre-

and d,,,is the hydrostaticcharge coefficient of the standard.

quencyindependentup to at least4 kHz.

dh=dhsVtAs/(

V,A,)

(2)

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3.2. Pressure independence of sensor characteristics The present measurements show that the composites of the present work provide performance that is essentially independent of hydrostatic pressure. Hydrostatic pressure loading was achieved by using a high-pressure oil pump to pressurize the pressure vessel described in Section 2. At each pressure, there was at least a 10 min stabilization period to ensure the reliability of the measurements. No significant changes in d,, resulted from increasing the pressure on the sensor from 1.2 MPa (d,,=56 pC N-l) to 3.8 MPa (cl,,=54 pC N-‘), and then to 7.6 MPa {d,, = 53 pC N- ’ ), followed by a decrease of pressure to 0.8 MPa (dh=54 pC N- ‘). After the sensor underwent this pressure cycle, it was cleaned with hexane to remove oil on the surface and the hydrophone figure of merit was remeasured using the previously described air-calibrator. This measurement confirms that the figure of merit was unchanged by the high-pressure exposure. A further test was performed by placing a polyurethane (HumiSeal2A64) coated, cylindrically configured sensor in the pressure chamber at 4.9 MPa for 48 h and exposing the sensor to numerous cycles at such high pressures. The observed change in d33 was less than 3%. In another test, the sample was pressurized at 13.8 MPa for 24 h. The observed sensitivity change resulting from this high-pressure exposure was less than 0.5 dB, which is within experimental error. 3.3. Properties comparison for the Cu-PT ceramic and the ceramic-polymer composites We find that the present ceramic-polymer composites can provide a & value that is higher than the product of the volume fraction of ceramic and the dg3 value of the neat ceramic. The present experiments use a Ca-PT composite that is based on a ceramic powder having a particle size of 140* 10 pm. Comparison was made between results for a disk-shaped pellet of the neat ceramic and a similar pellet of the composite, where the preparation and fabricationmethods are as described above in Section 2. Electroding was by sputter deposition of gold, which was backed with silver ink (Creative Material). The dielectric and piezoelectric properties were measured one week after poling for the neat ceramic and for the ceramic-polymer composite. Except for a somewhat larger dj3 coefficient, the properties that we measured for the ED0 ceramic ( dxg = 82 pC N- ’ at 100 Hz and K33 = 270 and tans = 0.008 at 1 kHz) are close to those ( 15% errors) reported by ED0 ( dZ3= 68 pC N- ’ at an unreported frequency and Kj3 =270 and tan6=0.009 at 1 l&z). Surprisingly, the measured dx3 for the ceramic-polymer composite (64 pC N- ’ ) is much larger than the product of the & of the neat ceramic (&=82 pC N- ’ from our measurements) and the volume loading of ceramic in the composite (55 pC N- ’ for a ceramic loading of 65 vol.%). In contrast, the measured dielectric constant of the composite is much lower than that of the ceramic (80 versus 270 for the

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neat ceramic at 1 kHz). Also, the dielectric loss tangent of the composite (0.013) is reasonably low. 3.4. ThermaE stabiliry This experiment demonstrates that the dielectric and piezoelectric properties of the composite sensors have high thermal stabilities. One week after poling, one of the piezoelectric composite disks was placed in a temperature-controlled oven at lOO”C, and the dielectric and piezoelectric properties at ambient temperature were monitored as a function of time. After 60 days at 100°C the initially measured free-field hydrophone sensitivity ( - 196.0) decreased by only 0.5 dB, which is within experimental error. Also, the variations in tan6 (between 0.015 and 0.019 at 1 kHz) and sensor capacitance (between 20.4 and 2 1.1 pF at 1 kHz) were sufficiently small and non-uniformly varying over this time period that they were not judged to be significant. 3.5. Combined particle size and loading level effects The effect of ceramic particle size on the dielectric and piezoelectric properties of the composites was investigated for ceramic particle loading levels of 65 and 50 vol.%. While the effects of particle size on figure of merit and d33coefficient were marginal for the investigation at the higher loading level, large increases in these parameters with increasing particle size were observed for the lower particle loading level. The different particle size samples used for these experiments at a loading level of 65 vol.% (corresponding to average particle diameters of 50,60, 120, and 140 + 10 pm) were obtained by the fractionation of a single Ca-PT powder sample by using an air-jet particle displacement method. Excepting this use of different particle sizes, the same preparation and poling methods were used for all preparations. These methods are described above in Section 2. The measurement results at this loading level of 65 vol.% (measured one week after poling) indicate only a moderate apparent effect of particle sizes in the measurement range on either figure of merit (FOM) or tan6 (which varied irregularly between 46X lo-l3 and 50X lo-r3 m2 N-’ and between 0.013 and 0.018. respectively). Upon increasing particle size from the smallest to the largest values, only relatively small apparent increases were measured for dx3 (from 52 to 64 pC N- ‘) and K33 (from 70 to 80). The particle sizes used for the experiments at a 50 vol.% ceramic loading level were 50,80, and 140 ( f 10 pm), Other than this difference in particle size and loading level, all preparative and measurement details are as described above for the 65 vol.% loading level. The FOM (in lo- 13mZ N- ‘) increased from 14 and 24 for 50 pm and 80 &rn particle sizes, respectively, to 37 for a 140 p,rn particle size. Likewise, the d33 coefficient increased from 28 and 35 pC N- ’ for 50 (*m and 80 p,m particle sizes, respectively, to 48 for the 140 pm particle size. Also, the K,, of 42 for the 50 and 80 p,rn particle size samples increased to 52 for the 140 km particle size

sample. In contrast with these strong dependencies of FOM and cl,, on particle size, tan6 varied non-monotonically between 0.014 and 0.016 for the different particle size samples. Interestingly, the ratio of the maximum d,, coefficient (obtained in each case for a 140 Frn particle size) divided by the volume loading level of ceramic was about the same for both the 50 and 65 vol.% samples (96 and 98 pC N-‘, respectively), and substantially larger than the d,, for the neat ceramic ( 82 pC N- ’ from our measurements and 68 pC N- ’ from ED0 literature). Likewise, the ratio of the figure of merit to the ceramic loading level was the same for these samples having a 50 and a 65 vol.% loading of ceramic. 3.6. Particle

size distribution

effect

This experiment demonstratesthe importance of using a narrow particle size distribution for obtaining high-performance ceramic particle-polymer composites.So that comparison can be made with the above results for size-classified Ca-PT ceramic powder at a 65 vol.% loading level, identical methods were usedfor the preparation and characterization of sensorpellets with this loading level that are basedon the unclassified ceramic powder. The observed properties are: &=53 pC N-‘, K33 = 100, g,,=O.O46 mV N-‘. and ,g,d, = 20 X lo- ” m2N- ‘. The figure of merit of this composite is about half that of the composite prepared from ceramic powders with a narrow particle size distribution. A possible explanation for this inferior performance for the unclassifiedceramic powder in compositeswill be given in Section 4. 3.7. Effect of the dielectric ceramics

constmt

of piecoelectric

This experiment suggeststhe importance of using a lowdielectric-constant ceramic componentfor the fabrication of piezoelectric ceramic-polymer composites.For this purpose. comparison is madeof the extent that the d,, coefficient of the parent ceramic is achieved in the d,, coefficient of the ceramic polymer composite. Two casesare considered,the Ca-PT ceramic (with a low dielectric constant of only 270) and PZT (with a very high dielectric constantof 3400). 50 g of the green PZT powder (ED0 EC-76 from ED0 Acoustics), together with several rods preparedby compaction of this powder, wassinteredin analuminacrucible. Other than an increase in the sintering temperature from 1100 to 12OO”C,the thermal processwas the sameas described in Section 2 for Ca-PT. The resultant product (obtained from the uncompactedpowder after post-annealshakingin a plastic bottle) was a yellow-brown free-flowing powder. One of the ceramic rods ( 1.12 cm diameter and 7.0 mm long) was electrodedby applying silver ink (Creative Materials) on the two oppositeendsandcuring the ink at 100°Cfor 5 h. Poling wasperformed at 80°C and 18kV for 30 min. One week after poling, the measuredproperties were d,, = 6 10 pC N- ’ at

100Hz, K33 = 3400, and tan6 = 0.0 15 at 1 kHz. Theseresults for the neat ceramic agreewith the ED0 ( 15%errors) specifications ( dj3 = 583 pC N- ‘, K33 = 3450, and tan6= 0.02 at 1 kHz). The powder particles of the sinteredED-76 ceramic were classifiedinto three equal-weight portions according to particle size using the air-jet separationmethod. The middle portion was usedto preparea compositecontaining 65 vol.% of the ceramic powder, following the methoddescribedpreviously. One week after applying the above-describedpoling process,the measuredd.73value for different disks varied between 50 and 65 pC N- ‘. Values of dj3 in this range are very low compared with the product of the volume fraction of ceramic in the composite and the dx3 of the neat PZT ceramic (d,,(calc) =0.65X610 pC N-l=396 pC N-‘). This result, indicating inferior developmentof the ferroelectric properties of the ceramic, contrastswith the caseof the Ca-PT ceramic composite, where the dj3 of the ceramicpolymer compositeexceeds the product of the volume fraction of ceramic in the compositeand the d33 of the neat CaPT. As will be discussedlater, we believe that the inferior achievementof the piezoelectric propertiesof the PZT likely resultedfrom an inability to pole this high-dielectric-constant ceramic in a low-dielectric-constant, low-conductivity matrix. 3.8. Processing

of cylindrical

sensors

The fabrication of a cylindrically configured sensoris used hereto evaluate the easeof manufacturing high-performance hydrophonesfrom the current compositematerials.A nearly free-flowing mixture consistingof 64 vol.% ceramicpowder (containing particles having an averagediameterof about 80 rJ,m) and 36 vol.% PVDF was prepared using the method describedin Section 2. A cylinder with an inner diameterof 1.Ocm and an outer diameter of 1.42 cm was compression molded at about 100 to 170 MPa and 200°C. The poling of these cylinders was performed for 1 h at 120°C using the maximum voltage that doesnot causedielectric breakdown (about 25-27 kV). The most completely poled cylinder shows the following properties: a capacitance of 89 pF, tan6=0.017, S= -194.5 dB (re. 1 V FPa-‘), g,=O.O89 mVN-‘,K,,=60,andg,d,=42XlO-‘“m’N-’.Asatest of reproducibility, more than 20 of thesewere prepared.All of them provided a free-field hydrophone sensitivity higher than - 196dB (re. 1 V FPa-‘).

4. Discussionand conclusions The practical significance of the present work rests on several aspects.First, hydrophoneshave been achieved that have (a) a FOM of at least 50 X lOpi3 m2 N- ‘, (b) high thermal stability, (c) no significant dependenceof sensitivity on hydrostatic pressureloading up to at least the equivalent of a 1400 m ocean depth, and (d) a frequency-independent response.This FOM can be comparedwith the valuestypical

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of commonly used piezoelectrics: the PVDF-trifluoroethylenecopolymer(17X10-13m2N-‘),PZT(1.7X10-’3m2 N-‘),andPT(19.3X10-‘3m2N-1).Secondly,andequally important, this work has shown that sensors having a freefield sensitivity of higher than - 196 dB (re. 1 V PPa- ‘) can be inexpensively and reproducibly fabricated either as flat plates or cylinders by a simple melt-molding process using inexpensive components. Even though the PVDF used in the present investigation has ferroelectric phases, we believe that any residual piezoelectricity due to this polymer is negligible. The reasons for this assessment are: ( 1) formation of the ferroelectric Pphase normally requires mechanical drawing aftermelt moldingand (2) substantial poling of the ferroelectric state of the polymer requires fields that are far higher than the breakdown voltage of the composites. One can further argue that it is undesirable to cause simultaneously the poling of the polymer host and the ceramic powder, since partial cancellation of the piezoelectric effect would result from the opposite signs of dh for the polymer and the ceramic. The high d3,, dhr and figures of merit of the present compositions suggest that these composites are percolated, so they can be called 3-3 composites. Even though the figures of merit and the d coefficients of the 50 vol.% composite are much lower than for the 65 vol.% composite, we believe that both are percolated. This is suggested by the fact that the product of the d33 coefficient and the volume fraction of ceramic in both composites is larger than that of the neat ceramic. We can compare these results with the results from the prediction of Banno for non-percolated composites in

Fig. 2. Scanning electron matrix.

micrograph

of a cross section of a composite

consisting

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which the ceramic and polymer elements are locally connected in series and in parallel. According to this equation [‘LX-281,

d33=~3~3/[(a+(1-n)K&,)(1-a+a3)]

(4)

where n3 is the volume fraction of the ceramic particle, d33is the piezoelectric charge coefficient of the composite, and 4, is the piezoelectric coefficient of the ceramic particles. Since Ca-PT has a ds3 value of about 82 pC N-r and the ratio of dielectric constants of Ca-PT and PVDF is about Kc/ Kp = 270110 = 27, the d33 values calculated for composites having 50 and 65 vol.% loading are 9 and 15 pC N- I, respectively. These values are much lower than those presently observed (about a maximum of 37 and 64 pC N- ’ at 50 and 65 vol.% loading, respectively). This suggests that Eq. (4) is not applicable, probably since the composites contain percolated ceramic particles. The existence of percolated structures is also suggested by the SEM micrograph of Fig. 2 and by the very fact that the composites can be easily poled by a d.c. field. This conclusion is based on the approximation for an unpercolated system that a d.c. poling field E0 (applied for a period longer than the sample relaxation time) results in an electric field (I?,) acting on the ceramic particles, where E,IE,=

~Ju,

(5)

and where c’p and p’care the d.c. conductivities of the polymer and ceramic phases, respectively [ 291. Since up for PVDF is more than one order of magnitude smaller than for Ca-PT, EC is a small fraction of I&. Therefore, it would be difficult

of 65 vol.% Ca-modified

lead titanate particles

in a poly( vinylidene

difluoride)

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et al. /Sensors

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or impossible to pole non-percolated ceramic particles in the polymer matrix. The high degree of poling of the present composites, evidenced by their piezoelectric coefficients, indicates that these composites contain percolated ceramic particles, so Eq. (5) is not applicable. We believe that the high piezoelectric charge coefficients result because the ceramic particles are percolated and the selected polymer host has a high dielectric constant. More specifically, the relationship between the piezoelectric charge coefficient (d) and composite dielectric permittivity (E) is given by [ 31 d=k(ES)“2

(6)

where k is the electromechanical coupling coefficient and S is the elastic compliance. Hence, if the electromechanical coupling constant is approximately constant, the piezoelectric charge coefficient will vary as the square root of both the dielectric constant and the elastic compliance. The introduction of porosity in the host polymer (which increases the elastic compliance in Eq. (6) ) is expected to increase the d constant. We have not used this method of optimization for the present work, since such porosity would likely cause device sensitivity to depend upon pressure. In fact, we have used very high pressures ( above 100 MPa or 15 kpsi) during molding for the purpose of eliminating porosity (and possibly enhancing the degree of percolation). Our choice of PVDF for the polymer host for the present compositions resulted from a number of considerations. First, PVDF has a high dielectric constant of 8 to 10 for the nonferroelectric phase formed from the melt. which we believe enhances the piezoelectric charge coefficient. Secondly, PVDF has a relatively low dielectric loss, which should lead to a relatively low volumetric contribution to the dielectric loss of the composite. Thirdly, PVDF has a melting point (about 170°C for the CJphase) which is far higher than the upper limit of the normally required temperature stability range of underwater sonar sensors ( 100°C). Also, PVDF has a glass transition temperature of - 45”C, which is at the lower limit of the nonnally required operational temperature range of underwater sonar sensors. Thus, the composite prepared from PVDF is not expected to have any phase transitions or structural changes between about - 45 and 170°C. The fact that the polymer need not be poled is also an advantage from the viewpoint of thermal stability, since one thereby avoids both the possibility of thermal depoling of the polymer and a subtractive piezoelectric contribution due to this polymer [30]. Fourthly, the PVDF is reasonably chemically inert (especially toward commonly used fluids for sensor arrays, such as silicon oil and castor oil) and is not highly hygroscopic. Finally, PVDF is conveniently melt processible, which enables the inexpensive fabrication of sensors. The following considerations lead to our selection of Camodified lead titanate ( Ca-PT) as the ceramic component of the hydrostatic sonar sensors. The piezoelectric ceramic components should have a high piezoelectric charge coefficient d,, a low dielectric loss, and perhaps a low dielectric constant.

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83

Ca-PT [ 3 11, Sm-modified lead titanate (Sm-PT) [ 32,331. and lead metaniobate (PN) [ 31 are especially useful piezoelectric powder compositions for the present sonar sensors. This is because these ceramics have a relatively high d,, value (about 60 to 70 pC N-l), a low dielectric loss tangent ( < 0.005)) and a low dielectric constant (about 250). The present results suggest the use of a low-dielectricconstant piezoelectric ceramic in the polymer-ceramic composites. This conclusion is based on the much larger retention of the d,, of the neat ceramic for Ca-PT (with a dielectric constant of 270) than for PZT (with a dielectric constant of 3400). More specifically, using the same loading level of 65 vol.% in PVDF, the ratio of the d,, coefficient for the composite to that for the neat ceramic was 78% for the Ca-PT and only 11% for the PZT. In contrast with these results, we mentioned in Section 1 that a djj ashigh as440 pC $I- ’ was obtainedfor a carbon-loadedPZT composite [ 1.51.This suggeststhat the presently obtained much lower value of & for the carbon-free PZT compositeresults from the difficulty in poling the high-dielectric-constant PZT in a matrix that has a much lower dielectric constant and electrical conductivity. The choice of the particle size of piezoelectric ceramic powdersis probably important for maximizing thepiezoelectric response of particle-matrix composites. Previous researchers[ 34-361 have typically usedrelatively smallparticle sizesof lessthan 10 pm. The use of suchsmall particle sizes is necessarybecauseof the inability to obtain larger ceramic particles from ceramics such as PT and PT-BF [ 3,35-371. The reasonfor this difficulty is that fragmentation occurs becauseof the large c/a ratio change at the Curie transition temperaturesof neat PT [3] and PT-BF [21]. In contrast, the presentwork usedlargeceramicparticleshaving an averagediameterof over 30 p.m.There are severaladvantages of using larger ceramic particle sizes.First, when the particle size is larger than 30 p,m, the ceramic powder flows freely. Secondly, theseparticles will not easily agglomerate. This facilitates the compositeprocessing,sincethe difficulty of dispersingagglomeratesinto small particles is avoided. Thirdly, a free-flowing piezoelectrically active ceramicpowder with a narrow particle size distribution can be conveniently prepared by sintering a green free-flowing ceramic powder madeby the conventional spray-drying method. Previous investigatorshave preparedpiezoelectric ceramic powders for particle-matrix compositesusing co-precipitation, repeated grind-calcination, water-quenching, or nondisclosed methods. None of these described methods provides a free-flowing ceramic powder with a uniform or very narrow particle size distribution. In fact, Banno and Ogura used a powder prepared by mixing differently sized ceramic particles [ 381. In the presentinvestigation, ceramic powdershaving a narrow particle size distribution are used. The argumentfor using a narrow particle size distribution is as follows. When a ceramic powder-polymer compositeis formed under high pressuresfrom many differently sized ceramic particles, the smallerparticlestend to residein cages formed by larger ceramic particles. Stressesapplied exter-

84

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nally to such a composite will not be efficiently transferred to these smaller particles that are inside cages. As a result, the smaller ceramic particles will not make significant contributions to the piezoelectricity of the composite. In fact, such small ceramic particles will degrade the FOM by contributing disproportionately to the dielectric constant, as opposed to the piezoelectric charge coefficient. Our choice of a ceramic loading volume fraction was guided by the following considerations. The fraction of free space for a periodic close-packed assembly of spherical particles is about 26% [ 391, which is much less than the fraction of free space for a randomly packed sphere assembly (about 35%) [ 40,411. Thus, the use of a ceramic volume filling of much over 65 vol.% must be accompanied either by partial ordering of the ceramic particles or the formation of unfilled interparticle cavities. The presence of such cavities has several effects. First, cavities will decrease the dielectricconstant of the piezoelectric composites. Secondly, although the presence of voids is known to increase figure of merit (&g,,), such void-space-enhanced pieznelectric properties will undesirably depend on hydrostatic pressure. In fact, high hydrostatic pressures can even degrade the performance of void-containing piezoelectric sensors to the point that permanent damage occurs. Fourthly, it may be difficult to pole such composites because they are more susceptible to dielectric breakdown due to the presence of cavities. Thus, the load level of the ceramic powder should be less than 74 vol.% in order to avoid void formation. In the present paper, the loading level is either 50 or 65 vol.%. The best result was obtained at about a 65 vol.% loading level. Poling provides special challenges forparticle-matrixcomposites in which a substantial fraction of the ceramic particles are not part of a percolated network across the inter-electrode dimension. Previous investigators [ 33,341 suggest the use of conductivity enhancement additives such as carbon and semiconductor powders to facilitate poling. One of the drawbacks of the introduction of such additives is the high dielectric loss of the resultant composites. Consequently, the composites can have Johnson noise that is too high for sensor applications. One of the findings of this paper is that a high degree of poling can be achieved without using any conductive enhancement additives. For example, one of the present composites containing 65 vol.% of 150 pm sized Ca-PT ceramic particles shows a d,, value of 64 pC N- ‘, which nearly equals that of a fully poled Ca-PT disk that contains no polymer (which has a da3 value of 82 pC N- ‘) . The presently used poling process was facilitated by the use of high poling temperatures {about 1OO’C). Such temperatures decrease the coercive force of the ceramic component. Also, the use of higher temperatures increases the ratio of polymer conductivity to ceramic conductivity. In accord with the Maxwell-Wagner theory [ 291, this increase will result in a higher electric field passing through the ceramic component, thereby facilitating the poling of non-percolated particles in the composite.

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Acknowledgements We thank Professor L.E. Cross, Professor A. Safari, and their colleagues for useful discussions and assistance with measurements.

References [I]

L.E. Cross, Ferroelectric ceramics: tailoring properties for specific applications, in N. Setter and E.L. Colla (eds.), Ferroelectric Ceramics, Birkhauser Verlag, Basel, 1993, pp. l-85. 121 J.M. Herbert, Ferroelectric Transducers and Sensors, Gordon and Breach, New York, 1982. [ 31 B. Jaffe, W.R. Cook and H. Jaffe, Piezoelectric Ceramics, Academic Press. New York. 1971. [4] H.S. Nalwa. Recent developments in ferroelectric polymers, Rev. Macromol., Chem. Phys., Cl3 ( 1992) 341-432. [5] R.E. Newnham, D.P. Skinner and LE. Cross, Connectivity and piezoelectric and pyroelectric composites, Mater. Res. Bull., 13 ( 1978) 525-536. 163 A. Safari, Development of piezoelectric composites for transducers, J. Phys. III. 4 (1994) 1129-1149. [7] T. Kitayama, Flexible piezoefectric materials, Seramikkusu, 14 ( 1979) 209. [8] L.A. Pauer, Flexible composite materials, IEEE Int, Conv. Rec., 21 ( 1973) l-5. [9] W.B. Harrison, Flexible piezoelectric organic composites, in P.L. Smith and R.C. Pohanka ieds.), Proc. Workshop Sonar Transducer Mater., Navy Research Laboratory, 1976, pp. 257-268. [lo] T. Yamada, T. Ueda and T. Kitayama, Piezoelectricity of a highcontent lead zirconate/polymer composite, J. Appl. Phys., 53 (1982) 43284332. [ 111 J. Giniewicz, (Pb, Bi) (Ti. Fe)O,/polymer O-3 composite materials for hydrophone appiications, M.S. Thesis, Pennsylvania State University, University Park, PA, 1985. [ 121 J. Giniewicz and R.E. Newnham, (Pb, -,,B&)(Ti, -,.Fe,)O,/polymer O-3 composite for hydrophone applications, Ferroelectrics, 73 ( 1987) 405-417. [ 131 (a) A.S. Bhalla and R.Y. Ting, Hydrophone figure of merit, Sensors Mater., 4 (1988) 181-185. (b) J.W. Young, Optimization of acoustic receiver noise performance, J. Acoust. Sot. Am.. 61 (1977) 14711476. (c) CD. Motchenbacher and F.C. Fitchen, Low-Noise Electronic Design, John Wiley, New York, 1973. [ 141 H. Banno, K. Ogura, H. Sobue and K. Ohya, Piezoelectric and acoustic properties of piezoelectric flexible composites, Jpn. J. App. Phya., 26 (1987) 153-155. [ 151 M. Sumita, H. Gohda, S. Asai, K. Miyasaka, A. Furuta, Y. Suzuki and K. Uchino, New damping materials composed of piezoelectric and electro-conductive, particle-filled polymer composites: effect of the electromechanical coupling factor, Makromol. Chem. Rapid Commun., 12 (1991) 657-661. [ 161 K. Han, Effect of processing variables on dielectric and piezoelectric properties of O-3 composites, Ph.D. Thesis, Rutgers University, New Brunswick, NJ, 1992. [ 171 K. Han, A. Safari and R.E. Riman, Dielectric and piezoelectric properties of flexible O-3 piezoelectric composites prepared by coprecipitated (Pb, Bi) (Ti,(Fe, Mn))O, ceramic powder, J. Am. &ram. Sot., 74 (1994) 1699-1702. [IS] A. Saiari, Y.H. Lee and R.E. Newnhdm, O-3 piezoelectric composites prepared by coprecipitated PbTiO, powder, Am. Ceram. Sot. Bull., 66 (1987) 668-670. [ 191 C. Dias and D.K. Das-Gupta, Polymer/ceramic composites for piezoelectric sensors, Sensors and Actuators A, 37-38 ( 1993) 343347.

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H. Banno, Recent developements of piezoelectric ceramic products and composites of synthetic rubber and piezoelectric cramic particles, Ferroelectrics, 50 t 1983) 3-12. H. Banno, Theoretical equations for dielectric. piezoelectric and elastic properties of flexible composite consisting of polymer and ceramic powder of two different materials, Ferroelectrics, 95 ( 1989) 11 l-1 15. A.R. von Hippel, Dielectrics and Waves, John Wiley. New York. 1954. T. Furukawa, K. Ishida and E. Fukada, Piezoelectric properties in the composite systems of polymers and PZT ceramics. J. Appl. Phys., 50 (1979) 49oti912. Y. Yamashita, K. Yokoyama. H. Honda and T. Takahashi, (Pb. Ca) ( (Co, ,2W,,2),Ti)0,3 piezoelectric ceramics andtheirapplications, Jpn. J. Appl. Phys. Suppl., 20-4 (1981) 183-187. D. Damjanovic, T. Gururaja and L.E. Cross, Anisotropy in piezoelectric properties of modified lead titanate ceramics. Am. Ceram. Sot. Bull., 66 (1987) 699-703. L.M. Troilo. D. Damjanovic and R.E. Newnham, Modified lead calcium titanate ceramics with a relatively large dielectric constant for hydrophone applications, J. Am. Ceram. Sot., 77 (1994) 857-859. R.P. Tandon. D.R. Chaubey, R. Sigh and N.C. Soni, Dielectric, piezoelectric and acoustical properties of high performance piezotubber composite hydrophones, J. Mater. Sci. Lett., 12 (1992) 1182-l 184. G. Sagang, A. Safari, S.J. Jang and R.E. Newnham, Poling flexible piezoelectric composites. Ferroelectric Lett.. 5 ( 1986) 131-142. H. Banno. Receiving characteristics of d;, zero piezo-rubber hydrophone. Jpn. J. App. Phys., 32 ( 1993) 23OC2306. S.A. Fedulov. Y.N. Venevstev, G.S. Zhdandov, E.G. Smazhevskaya and I.S. Rez. X-ray and electrical studies of the PbTiO,-BiFeO, system, Soviet Phys. - Crystallography. 7 ( 1962) 62-66. H. Banno and K. Ogura, Piezoelectric properties at polarization reversal process and coercive force of O-3 composite of polymer and ceramic powder mixture of PZT and PbTiOi, Jpn. J. App. Phys., 3 (1991) 225C-2252. D.W. Richerson, Modem Ceramic Engineering, Marcel Dekker, New York, 1982. W.O. Smith, P.D. Foote and P.F. Busang, Packing of homogeneous spheres, Phys. Rev., 34 (1929) 1271-1274. J.A. Hossack and B.A. Auld, Design and modeling of 0:3 composite transducers, Ferroelectrics, 156 ( 1994) 13-18.

Biographies Charzgxing Crli received his B.S. in chemistry ( 1982) from Liaoning University, a Ph.D. ( 1987) in theoretical solid-state

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physics and chemistry from Jiling University, and a Ph.D. ( 1995) in organic and polymer chemistry from Georgetown University. He joined AlliedSignal Inc. in 1995 as a research chemist. His research interests include piezoelectric materials, polymer and organic photochemistry, carbon-based materials, spectroscopic properties of polymers, and conducting polymers. Ray H. Baughman is an Aerospace Fellow at AlliedSignal, where he is involved in research work on sensors and actuators, solid-state synthesis, and the physics and chemistry of non-conventional electronic and optical materials, such as conducting polymers and new carbon phases. He received a B.S. in physics ( 1964) from Carnegie Mellon University and an MS. ( 1966) and Ph.D. ( 1971) from Harvard University in the materials science area. He is a Fellow of the American institute of Physics and has received the Chemical Pioneer Award of the American Institute of Chemists ( 1995) and the Cooperative Research Award (1996) of the Division of Polymeric Materials Science and Engineering of the American Chemical Society. Also, he received AlliedSignal Tech__ _ __ ^ nical Achievement Awards ( 1988 and 1994) for the development and commercialization of time-temperature indicators and the VersiconTM conducting polymer. Zafai- Zqbal received his M.S. in physical chemistry from Dacca University in 1962 and his M.A. and Ph.D. in solidstate physics from Cambridge University in 1967. From 1969 to 1983 he was a member of the technical and academic staff, respectively, of the US Army’s Research and Development Center in New Jersey and the Swiss Federal Institute of Technology (ETH), Zurich and the University of Zurich. He joined the technical staff of AlliedSignal Research and Technology in 1983. His current research interests are in the areas of sensor and nanophase materials, carbon composites, and superconductivity. Ted Kazmar received his diploma in mechanics, mechanical, and aerospace engineering from Illinois Institute of Technology in 1973. From 1973 until 1990 he designed and developed underwater sonar transducers, hydrophones, transduction materials, and systems. Since 1990 Ted has been involved in the technical marketing and development of Advanced Sonar Programs for the US Navy. David Dahlstrom received his master’s degree in mechanical engineering from Syracuse University in 1981. He has been directly involved with transducer IR&D, both sensor and transmitter, at AlliedSignal Ocean Systems since 1982 and is presently the head of the Transducer Development Department. His present interests include new transduction materials for high-performance sensors and transmitters in sonar applications.