role of piezocomposites ultrasonic transducers

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THE ROLE OF PIEZOCOMPOSITES IN ULTRASONIC TRANSDUCERS

WALLACE ARDEN SMITH

Materials Division, Code 1131, Office of Naval Research, Arlington, Virginia 22217-5000

Ceram ics Division, National Institute of Standards and Tech nology, Gaithersburg, M aryland 20899

Combining a piezoelectric ceramic and a passive polymer to

forrii a piezocomposite allows the transducer enginner to

design new piezoelectrics that offer substantial advantagesover the conventional piezoelectric ceramics and polymers.

The rod composite geometry provides materials with

enhanced electromechanical coupling and with acoustic

impedance close to that of tissue; these advantages yield

transducers for medical ultrasonic imaging with high sensi-t iv i ty and compa ct impulse response. The dice-and-fill tech-

nique produces piezocomposites that can be readily formed

into complex shapes to facilitate focusing the ultrasonic

beam. Proper design of the rod spacing yields materials

which exhibit low cross talk between array elements formed

by patterning the electrode alone, without cutting between the

elements. In this way, curved annular arrays have been made

that provide high quality clinical images of substantial diag-

nostic value to physicians. Th is exposition contains an

extensive bibliography of original papers documenting the

role piezoco mposites have in ultrasonic imaging transducers.

INTRODUCTION

This review surveys a class of piezoelectric materials which

offer substantial improvements over the conventionalpiezoelectric ceramics and polymers for making the ultrasonic

transducers used in medical imaging. Piezoelectric ma terials

play a crucial role in medical ultrasonic imaging. The y form

the heart of the transducer -- converting the electrical driving

pulse i n t o an acoustic beam that is projected into the soft tis-

sues of the human body, and then detecting the weak echosreflected by orga n boundaries and internal structures. Th e

piezoelectric must meet severe demands -- interfacing with

the drive/receive electronics, performing the electro-mechanical energy conversion, projecting the strong acoustic

pulse into tissue, and gathering the weak echos. In each ofthese roles, the piezocomposites allow the device engineer to

tailor the material properties -- adjusting the electrical

impedance to that of the electronic chain, enhancing the elec-tromechanical coupling, moving the acoustic impedance closeto that of tissue, and shaping the transducer to focus the

beam. A single material design does not optimize all

material properties simultaneously. The material engineer hasa challenging task in designing a piezocomposite for each

particular device.

0090-5607/89/0000-0755$1 .OO 0 989 IEEE

This paper sets forth a personal perspective on the role of

composite piezoelectric materials in these medical ultrasonic

imaging transducers. I aim to describe the subject i n simple

language for the newcomer; for the cognoscente, an extensive

bibliography is provided. Th e next two sections set the stage

by explaining what a piezocomposites are, and how the typeuseful for medica l ultrasonic transducers are made. Th e fol-

lowing two sections describe the improvements in material

properties that are achievable by proper material design a swell as the technological advantage s piezocomposites p rovide

for making ultrasonic transducers. The next section is

devoted to the spatial scale of the comp osite structure; under-

standing this aspect of the subject is essential to avoid many

pitfalls. Th e final section illustrates how the piezocom posite

materials have been exploited to enhance the performance of

existing devices and make novel devices feasible.

PIEZOELECTRIC COMPOSITES

Simply stated, a piezocomposite is a combination of a

piezoelectric ceramic and a non-piezoelectric polymer to forma new piezoelectric material. Figu re 1 illustrates one such

combination that lies at the focus of this review.

FIGURE 1 Photograph of a rod composite, consisting of long, thin rods

of piezoelectric ceramic held parallel to each other by a passive polymermatrix. This samplewas fabricatedusing the dice-and-fill technique.

1989 ULTRASONICS SYMPOSIUM- 55

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1 1 - 1 1 11-1-1-01

11-11 11 -11 1 s - w 13-11

FIGURE 2 Schematic diagrams of various types of piezocomposites.

Early studies" identified 'PZT ods in a polymer' as most promising for

ultrasonics. (After Gururaja.Safari, Newnham and Cross, 1988)

In general, however, the term piezocomposite applies to any

piezoelectric resulting from combining any piezoelectric -

polymer or ceramic - with other non-piezoelectric materials -

including air-filled voids.'*- 13 * 34* 66-69* 73 *74 * 84 Figure 2 illus-

trates some of the ways in which this combination can be

assembled.

Such composite piezoelectric materials have been studied for

a considerable time. Soon after the appearance of barium

titanate as a useful piezoelectric ceramic, researchers at the

Naval Research Laboratory tried to embed it in a polymer

matrix to make a flexible hydrophone material (P. L. Smith,

private communication). Further attempts to make flexible

piezoelectrics by combining lead zirconate-titanate ceramic

powders with a polymer were made by a number of work-

e r ~ . ~ ~ ,' The report of measurements showing the potential

for improving th e performance of naval hydrophones pro-vided a major impetus to research on piezoelectric compo-

sites!l The subject was soon explored intensely by research-

ers at Pennsylvania State University, opening many avenues

for research and application.

ROD COMPOSITES

While a number of these composite structures have been stu-

died for medical ultrasonic transducer application^:^. most

effort has focused on th e 'PZT rods in a polymer' or 1-3

connectivity. A rich variety of techniques have been

developed to make rod composites. Early samples were pro-

duced at Penn State by forming lead zirconate titanateceramic (PZT) in long cylindrical rods, aligning those

ceramic rods parallel to each other in a fixture, casting a

polymer around them, and then slicing off disks perpendicu-

lar to the rods."* 58 Figure 3 illustrates this approach. This

method is effective for making samples with rod diameters

about two hundred microns or more. Finer spatial scales

become increasingly difficult, as handling vast numbers of

FIGURE 3 Composite fabrication process based on aligning long thincylinders of piezoelectric ceramic, casting a polymer between them, andslicing off the desired composite disk.

thin, delicate ceramic rods is a daunting task. Two methods

have been recently advanced which promise spatial scales

well below fifty microns; the large numbers of fine rods arehandled by forming them in place. In one approach, carbon

fiber is woven into the desired structure by textile techniques

and then the carbon structure is replicated with piezoelectric

ceramic.18*9 In the other approach, a "lost wax" method, a

complimentary structure. is formed in plastic, a ceramic slip

injected into this mold and fired, the plastic mold burns away

during the ceramic firing, and a polymer is cast back into its

place.IM Such approaches promise to produce large area, fine

scale composites at lo w cost.

The dice-and-fill technique illustrated in Figure 4 is perhaps

the most widespread fabrication method for materials used in

medical ultrasonic applications. Deep grooves are cut into a

solid ceramic block, a polymer is cast into these grooves, andthe resulting composite disk sliced off the ceramic base. The

dice-and-fill method also has its origins in early work at Penn

State.75- 6 Mechanical dicing saws are quite effective for rod

scales ranging down to fifty microns; pushing below fifty

micron becomes increasingly difficult, as the rods are quite

fragile and wear on thin saw blades becomes an important

factor. Finer spatial scales can be achieved using laser

machining to cut the grooves; both direct laser ablation (G.

Faber, private communication) and laser-induced chemical

etching3' are viable approaches to reach scales of ten

microns, perhaps lower. A cut-fill-cut-fill strategy alleviates

some of the problems with rod fragility at the cost of making

the second cut through a ceramic-polymer combination more

challenging. Slicing the formed composite off its ceramicbase and polishing to a final thickness also presents problems

in machining a brittle ceramic and soft polymer simultane-

ously. Low temperature machining of polymers is a standard

method that can be called upon, although room temperature

polishing can be done for all but the softest polymers. A

method that eliminates polishing altogether and permits fabri-

cation of very flexible large area material has also been

756- 989 ULTRASONICS SYMPOSIUM



MATERIAL PROPERTIES

FIGURE 4 Dice-a nd-fill composite fabrication. Deep grooves are cutinto a solid ceramic block and a polymer cast into the groove s. Slicingoff the ceramic base leaves the desired 1-3 piezocomposite disk.

developed."' The process starts with a ceramic plate of the

desired final thickness and involves dicing-and-filling halfway

from th e top and bottom successively. Another fabricationproblem associated with the polymer is curing shrinkage; that

problem readily succumbs to the standard plastic molding

technique of curing under pressure.

Polymer issues can be circumvented entirely with the lamina-

tion technique'", '" illustrated in Figure 5. Here alternate

thin plates of piezoelecmc ceramic and a passive material are

glued together into a layered stack. Slicing perpendicular to

that stack results in plates of square ceramic r ods separated

by rods of passive material. Interleaving these composite

plates with another set of passive plates yields a long compo-

site loaf from which many 1-3 thin plates can be sliced. This

technique offers the possibility of building a composite with

a passive material that is not a polymer.

FIGURE 5 Lamination process for comp osite fabrication. Alternateplates of piezoelectric and passive material are glued together to form alayered structure. Slices of the layered structure are again alternated withpassive layers and glued into a final s m c t m from which many thinplates of the desired composite material can be sliced.

These rod composites offer two key advantages -- high elec-

tromechanical coupling constant and an acoustic impedance

close to tissue. Figure 6 shows the variation with ceramic

volume fraction of the material properties important to the

performanceof

thickness-mode transducers: the thickness-

mode electromechanical coupling constant and th e specific

acoustic impedance. These curves are based on a simple

physical model of the thickness-mode oscillations in piezo-

composite plates."

Note that the thickness-mode electromechanical coupling of

the composite can exceed the k, (typically - 40 - 50%) of the

constituent ceramic, approaching almost the value of the

rod-mode electromechanical coupling, k,,, (typically - 7 0 -

80%) of that ceramic. On first blush this appears to be

sophistry: you take a plate of piezoelectric ceramic - optim-

ized as a material for electromechanical energy conversion;

yo u cut away a large fraction of the material, say 75%;

replace that piezoelecmc with a non-piezoelectric polymer;and now you want me to believe that you have a more

effective material for electromechanical energy conversion?

'Less is better' is a slogan heard mainly in political, not

scientific, circles -- and rarely believed even there.

40

1

U

IN 0

I I I I20 4 0 60 80

20

10I00 0

v, VOLUME FRACTION CERAMIC (%)

FIGURE 6 Variation of Ihe com posite's thickness-mode electromechani-cal coupling, k,, and specific acoustic impedance. Z, with volume fraction

piezoceramic. (After Smith,Shaulov and Auld, 1985)

1989 ULTRASONICS SYMPOSIUM - 57



Careful thought reveals that this enhancement of the elec-tromechanical coupling can indeed be accomplished without

resort to magic. First, we note that diag nosti c ultrasonics lies

safely within the small signal limit -- far from the high powerlimit -- of electromechan ical energy conversion. Second , we

consider only rod composites whose lateral spatial scale is‘sufficiently fine’ that the polyme r and c eramic m ove together

on the frequency scales of interest; this restriction will be dis-cussed at length below. Finally, we gain physical insight

from a gedanken experiment. Consider applying a voltage tothe ends of a long thin piezoelectric ceramic rod to shorten it.

Contrast the situation when the rod is completely surrounded

by a soft, light polymer to the case where the piezoceramicrod is surrounded by a stiff, heavy ceram ic. In the case of

the polymer environment, the ceramic rod can bulge at thesides and compress the soft, light polymer. Whe reas, in the

ceramic environment, the piezo rod is tightly confined --

especially if the surrounding ceramic is also a piezoelectricundergoing the same shortening, trying itself to bulge out to

the sides. Our intuition tells us, correctly, that rod sur-

rounded by polymer will shorten more than the one sur-

roun ded by ceramic. This partial relief of the lateral clamp-ing acc ounts for the increased thickness-mode electromechan-

ical coupling constant of a properly designed piezocomposite.Clearly this argument breaks down in the high power limit

when we drive the material to saturation. If we move out ofthe limit of a ‘sufficiently fine’ scaled composite, the argu-

ment breaks down, not because the polymer encased r o d

doesn’t shorten more, but rather because the energy is cou-

pled into complex lateral oscillations of the composite struc-ture, and not into the desired thickness-mode oscillation of

the composite plate as a whole. Note that if the lateral spa-

tial scale is too coarse, yo u will still ‘measure’ an increasedcoupling constant by the standard resonance method, but the

energy is not going into the thickness-mode oscillation.

Thus, the performance of a transducer made from such coarse

scaled material will not live up to th e expectation provided

by the ‘mea sure d’ cou pling constant. A properly designed

piezocornposite produces a transducer whose sensitivity --that is insertion loss -- agrees well with theoretical predic-

t i o n ~ . ’ ~n contrast, too coarse a scale yields a transducer

whose sensitivity falls short.

Recalling that a material’s acoustic impedance is the square

root of the product of its density and elastic stiffness provides

intuitive insight into the improvements achieved by replacingstiff, heavy cer am ic by light, soft polymer. Th e acoustic

match to tissue (1.5 Mrayls) of the typical piezoceramic

(20-30Mrayls) is thus significantly improved by forming a

comp osite structure.

A smaller volume fraction of piezoceramic always reduces

the acoustic impedance. How ever, too small a volume frac-

tion leads to a reduction in the co mposite’s electromechanical

coupling. A trade-off must be made between high elec-

tromechanical c oupling and low acoustic impedance. In

practice, a high electromechanical coupling is most irnpor-

tant; this leads to increased sensitivity and a broaderbandwidth. Th e acoustic impedan ce cannot be reduced

sufficiently to eliminate an acoustic matching layer alto-

gether; once the impedance is reduced by a factor of four or

so, a few percent lowe r provides little additional gain. Th eprincipal benefit of the impedance reduction is the excellent

acoustic matching that can be achieved with a single layer.

Moreover, the manufacturing tolerances in the layer’simpedance and thickness are broadened substantially by the

smaller impedance discontinuity that must be bridgedbetween the piezocomposite and tissue; this lowers produc-

tion costs.

TECHNOLOGICAL ADVANTAGES

Beyond the enhancement of the quantifiable material proper-

ties, piezocomposites offer significant technological advan-

tages for the designe r of ultrasonic transducers. As Figure 7

shows, composite’s can be quite flexible and thus easily

formed into complex shapes for focusing the acoustic beam.

Spherical caps have been made into annular arrays.98 Sector-linear arrays have been made with two curvatures: one to fan

the linear array into a sector, and the second to focus thebeam in the scan plane.65

FIGURE 7 PZT/polyurethane 1- 3 Composite. (After N a h y u .Kaiakwa and Sakamoto.1985)

Takeuchi.

Perhaps the m ost useful technological advantage of the piezo-composites, is the exceedingly low cross talk in arrays

formed on a continuous sheet of piezoelectric by patterning

the electro de alone. Cutting the piezoelectric is not needed

to acoustically isolate the array elements; Figure 8a provides

a schem atic representation of a linear array formed in thisfashion. Figure Xb comp ares the measured CW radiation pat-

tern of a single element of an undiced linear array to the

theoretical value for an isolated element. In addition to theexcellent interelement isolation shown by the agreement with

theory for the main lobe, the side lobes are even further

reduced be low the theoretically expected value. This is du eto goo d desig n. In an undiced array there is refraction a t theinterfaces between the high acoustic speed piezoelectric and

the lower acoustic speed matching layer and again at thema tchin g layer tissue interface. Th e effect of this refraction

is to concentrate the acoustic beam toward the normal to the

interface.29 By proper choice of the velocities for the

piezoelectric and the matching layer, this Snell’s law effect

can be designed to fall just outside the main acoustic lobe,

thus suppressing unwanted side lobes.




WALLACE ARDEN SMITH

The previous sections have alluded to the proper lateral spa-tial scale in rod compo sites; here we treat it at length. Thereare three aspects to the choice of lateral spatial scale. First,

if we are to treat the composite structure as an effectivehomogeneous medium with a new set of material properties,the rod size and spacing must be smaller then all relevantacoustic wavelengths. Then, the acoustic excitations willaverage over the fine scale variations in the compositemedium, much as with the micron sized grain structure inconventional pure ceramic transducers. For medical imaging

transducers, the 'relevant' acoustic wavelengths are thoseassociated with a band of frequencies centered on the thick-ness mode resonance; only these frequencies are excited indiagnostic ultrasonics. Second, the choice of lateral spatialscale can effect cross-talk suppression in undiced arrays.This consideration, as we shall see, suggests an optimum spa-tial scale if cross-talk is to be minimized. Finally, the scaIechoice impacts manufacturing: finer scales are more difficultand costly. Th e second section describes the variety of fabri-

cation techniques in hand or under study. The dice-and-filltechnique is adequate for most purposes, but a cheaperapproach would be highly desirable. We shall not dwellfurther on this point in a paper on the scientific issues, butbear in mind that the practical applications of composites aredetermined at least as much by their manufacturing costs asby their technical charm.

-MEASURED

-CALCULATED

10 30 50

ANGLE (DEG)

FIGURE 8 Acoustic cross talk in an undiced array: (a) schematicrepresentation of a composite l inear array with elements de6ned by theeleceode pattem alone; and (b) measured CW radiation pam h msingle element of an u n d i d composite linear array (circles) and calcu-lated radiation pattern for an i sol ated element of h e same width (solidcurve). (After Smith und Shaulov, 1985)

The right scale choice is best understood by considering whatoccurs when the scale is too coarse: resonances of Lambwaves propagating laterally in the periodic lattice of rods.Figure 9a shows the measured electrical impedance spectrumof a composite plate in the vicinity of its thickness-moderesonance. Just above this desired 620.7lcHz thickness reso-nance, lie two parasitic resonances at 801.9kHz and1033kH z. Thes e resonances correspond to lateral runningwaves which are Bragg reflected by the periodic array of rodsin the composite medium, as indicated schematically in Fig-ure 9b. In direct analogy with electron waves propagating in

a periodic atomic lattice, acoustic Lamb waves propagating in

a periodic lattice of piezoceramic rods are resonantlyreflected when their wavelength is just equal to the rod spac-ing. These two resonances occur well within the typicalbandwidth of a medical imaging transducer. They wouldseverely undermine the performance of a thickness modetransducer made from this material: robbing energy from thedesired 'thickness mode and contributing a long oscillatorytail to the temporal response. Th e frequency of these lateralmodes is inversely proportional to the spacing in the rod lat-

tice, while the thickness mode resonance depends only on theplate thickness. Making the lateral scale finer pushes theselateral resonances to higher frequencies. Positioning thelowest resonance at approximately twice the thickness reso-nance provides a broad band of frequencies around the thick-ness mode resonance in which the composite behaves as aeffective homogeneou s medium free of spurious effects.

46

42 t 60

SO.

FIGURE 9 Resonances in a piezocomposite plate: (a) measured electri-cal impedance spectrum and (b) corresponding standing wave patternsinside a unit cell of the md lattice. (#fer Auld and W m g , 1984)




Another aspect of these lateral resonances is important toultrasonic transducers: the associated stop bands in the spec-

trum of laterally propagating *** lM-la5 Just aswith electrons resonating in an atomic lattice, Lamb wavesresonating in a mechanical lattice are strongIy attenuated fora band of frequencies near the Bragg reflection wavelength.

Figure 10depicts schematically the Lamb w ave frequency asa function of wave number (k=25c /h) for a homogeneousmaterial -- dotted line -- and for a composite -- solid line --with a rod spacing of length d. In the composite's dispersioncurve , the gaps occur ring a t k = d d and k= 2x /d are the stopband resonances corresponding to successive rows of rodsoscillating out-of-phase and in-phase, respectively. The fre-

quency bands corresponding to these gaps is shaded on thevertical axis; for these frequency ranges the Lamb waves arestrongly attenuated. The low est electrically excited stop band

resonance is shown by a heavy round dot on the dispersioncurve. Note that the lowest stop band -- corresponding tosuccessive rows of rods oscillating out of phase. -- does notget excited electrically, but is nevertheless present acousti-cally. A well designed composite material will have its

thickness adjusted so that the thickness mode resonance --

axis -- lies nicely in the middle of this lowest stop band.Then any laterally propagating acoustic waves near the thick-

Y)-

ness mode resonance will be strongly attenuated -- just thoseg "-

waves that carry the unwanted acoustic cross-talk in an m m -

undiced array. Moreov er, since the first electrically excited lb .

frequency, that unwanted mode is conveniently above theY l o -

This positioning for the electrically unseen h / 2 stop-

m

-I

6topband resonance occurs approximately twice as high in

100% bandwidth of interest that surrounds the thicknessmode.

DEVICE PERFORMANCE

0

D ~ % D o * o o . ~ . ~

-10 ,I ID I D IO

The benefits of piemcomposites extend beyond their aestheticappeal for research. Piezocomp osites lead to useful improve-ments in the performance of medical ultrasonic imagingtransducers. Th e higher electromechanical coupling andreduced acoustic impedance lead directly to higher sensitivityand broader bandwidth. Figure 11 illustrates the low inser-tion loss and compact impulse response from a compositetransducer. These improvem ents are obtained with a singleacoustic matching layer, so the manufacturing difficulty andcosts are concomitantly lower.

WAVE NUMBER, k

Perhaps, the most useful device manufactured to date frompiezocomposites is the annular array, schematically depictedin Figure 12. Such devices have low insertion loss and shortringdown times because of the inmnsic improvements in the

material properties noted above. They can be readily madeas spherical caps because of the formability of the composite.Moreover, the elements can be defined by patterning the elec-trodes alone; the piezoelectric layer need not be diced. Thislast feature is more important than it seems prima facie.

Dicing a ceramic annular array to reduce the acoustic cou-pling between elements yields outer elements whose width isnear their thickness. These annu li have spurious lateral oscil-

lations. Subdicing, of course, can suppress these unwantedlateral modes, but is difficult to effect in a circular eeometrvon a spherical ceramic cap. Earlier workers have addressedthis issue by dicing all the array elements in a rectangular

grid;'. 72 this is a good idea -- indeed, it is the first steptoward a properly designed composite device. Such undicedannular arrays lead to high quality medical ultrasonic images,as illustrated in Figure 13. Several transducer manufacturershave introduced such products, including Echo Ultrasound,Precision Acoustic Devices and Acoustic Imaging.

FIGURE 10 Schematic repmkntation of the Lamb wave frequencyverSuS wave number for a composite plate. The CUrYe depicts ahomogeneous material with the effective material properties of the com-P O S i k . The sol id Cm ~ eakes into aon~~the CompOsite'S @ d t Y l d,a'ong the -gation direction. The first excited stopbandresonance (solid circle) and the thickness-mode resome (solid square)in the plate are noted. The shaded on the axis are fresuencybands of attenuated running waves. (Afrer Smith, shadov Md Au ld ,

1989)




COMPOSITE ANNULAR ARRAY COMPOSITEPIEZOELECTRICfCOMPOSITE MATERIAL

CHING LAYER

BACKING Y \ARRAY GROUND ELECTRODE

ELEMENTS

FIGURE 12 Cut-away diagram of undiced sphericauy curved annulararray made from composite piemelectric material. The hiddenfront sur-

face is concave.

FIGURE 3 Ultrasaric image of liver, kidney and@Iladder from a

3.5hII-h compositeannular a m y . Courteg Philips Medical Sysrems)

Piemomposite can also be fruitfully exploited in lineararrays; the sector-linear or convex linear array noted earlier is

a par ticular ly useful appli~ation.~~his device exploitseffectively the ability of piezocomposites to be formed intocomplex shapes.

Novel transducers can be made by defining different arraypatterns on the opposite faces of a piemomposite plate.

Figure14

shows the electrode pattern for a biplane phasedarray?1* 77 Grounding one set of electrodes and scanning onthe other set yields a phased array image in one plane; rev-ersing the roles of the electrodes yields a scan in the orthogo-nal plane, as illustrated in Figure 15. The design of anundiced phased array, however, is hurt by the Snell's laweffect that helps suppress sidelobes in undiced linear andannular compo site arrays. This can be overcome by partiallydicing the different m a y patterns from each side -- alongwith the acoustic matching layer. Indeed the acoustic perfor-mance of such a biplane phased array has been demon-

FIGURE 14 Electrode pattern on a composite plate for a biplane phased

m y . Shaulov. Singer. SmithandDorman. 1988)

FIGURE IS Schematic depiction of the scan pattern for the acousticbeamin a biplanephased array.

This same idea can lead to many novel devicestructures such as crossed linear arrays and combined annularand phased arrays.

CONCLUSION

The potential in medical ultrasonics of composite piezoelec-tric materials was first documented publically at this Sympo-

sium five years ago.". 36* 38* W. 8 An extensive body ofresearch literature has since drawn out many details of theirproperties. Practical devices made from piezocompositeshave been incorporated in commercial ultrasonic imaging sys-tems. The story is not over yet -- at this Symposium twonew aspects have been advanced: varying the ceramic volumefraction across the face of the transducer to apodise th e

a c ous t i c s e ns i t i~ i t y ;~ ~nd forming composites from electros-mctive ceramics so that the sensitivity can be spatially m odu-

lated with an external electric field96




ACKNOWLEDGMENTS

Foremost, I would like to thank Dr. Avner A. Shaulov, my

colleague at Philips Laboratories, who provided me the bulkof my knowledge of this subject. I appreciate the many use-

fu l insights garnered from the other members of the

ultrasound transducer team at Philips Laboratories: John a l a ,Donald Dorman, Michael Athanas, Kevin McKeon, Anthony

Pink Kevin Salott i, John Hannes, and Madeleine R o w ; thesupport of and insightful technical discussions with Dr. Barry

M . Singer are also much appreciated. I have learned much

about composite piezoelectrics from interactions with univer-sity collaborators: Prof. L. Eric Cross, Prof. Robert E. Newn-

ham, Dr. T. R. ‘Raj’ Gururaja, and Dr. Walter A. Schulze at

Pennsylvania State University; and Prof. B. A. ‘Bert’ Auld at

Standord University. My understanding of the devi ce andsystem requirements for medical ultrasonic imaging transduc-

ers stems fro m interactions with colleagues at Philips Medical

Systems: Dr. S. Omar Ishrak, Dr. Pieter J . ’t Hoen, Dr.

Simon Hsu and Dr. E. James Pisa. Valuable insights into the

contribution of Hitachi researchers comes from discussions

with Dr. Hiroshi Takeuchi of the Hitachi Central ResearchLaboratory. I am grateful for information on the perfor-

mance of composite transducers made by their companiesprovided by Mr. Clyde G. Oakley of Echo Ultrasound, Dr.

Charles S . DeSilets of Precision Acoustic Devices, and Mr.

LeRoy Kopel of Acoustic Imaging.

I am indebted to Mr. Fred Rettenmaier, Office of Naval

Research Librarian, for valuable assistance in compiling thebibliography. In preparing the graphics, both for this

manuscript and for the poster presentation, I appreciate the

skillful artistry provided by Mr. Lawrence Behunek, Ms.

Norine J . Davis, Ms. Wanda L. Braxton and Ms. Erin Bohan-non of the Office of Naval Research Visual Services Section.

I31 BLIOGRAPHY

This bibliography contains all the papers known to me that

bear directly upon the use of piezwomposites in ultrasonictransducers. Space limitations force me to omit many other

papers that also contain interesting points; these can be foundas references in these papers. Indeed, I have not been able to

mention the significant results of more than half the papers

listed here. Th e listing is orga nize d alphabetically by auth or

and then chron ologically by date.

1. Alias, P., P. Challandc, C. Kammoun, B. Nouailhas, and F. Pons,

“A Ncw Technique for Realizing Annular Arrays or ComplexShaped Transducers,” in Acoustic Imaging, ed. M. Kaveh, R. K.Mucllcr and J. F. Grcenleaf, vol. 13. pp. 357-368. Plenum Press.New York, New York, 1984. Presented at the Thirteenth Intema-

tional Symposium on Acoustic Imaging, Minneapolis, Minnesola.26-28 October 1983.

Alippi, A., F. Craciun, and E. Molinari, “Stopband-Edge Frcquen-cics in the Resonance Spectra of Piezoelectric Pcriodic CompositePlates,” Proceedings of rhe 1988 IEEE Ultrasonics Symposium, pp.623-626, 1988. Presented at the 1988 IEEE Ultrasonics Symposi-um, Chicago, Illinois, 2-5 October 1988.

2.

3.

4.

5 .

6.

7

8

9.

10 .

11 .

12 .

13 .

14.

15.

Alippi, A., F. Craciun, and E. Molinari, “Stopband Edges in theDispersion C w e s of Lamb Waves Propagating in PiczoelcctricPeriodical Structures,” Applied Physics Letfers, vol. 53, pp. 1806-

1808, 1988.

Alippi. A., F. Craciun. and E. Molinari. “Finite Size Effects in theFrequency Response of Piezoelectric Composite Plates,” Journal of

Applied Physics, vol. in press, 1989.

Auld, B. A., Waves and Vibrations in Periodic Piezoelectric Com-

posite Marerids. Presented at the 1989 European MaterialsResearch Society Conference, Strasbourg, France, 30 Ma y - 2 June

1989

Auld, B. A., “Wave Propagation and RCSOMnCe in Periodic ElasticComposite Materials,” in Proceedings of Inrernational Workihop

on Acourric Nondesrructive Evaluarion, pp. Ml-M22, NanjingUniversity, 1985. Presented at the Acoustic Nondestructive Evalua-tion International Workshop, Nanjing University, Nanjing, China,

4-10 November 1985.

Auld, B. A., “High Frcquency Piczoclectric Resonators,” Proceed-

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