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PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS

R. Stephens

To cite this version:

R. Stephens. PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS. Journal de Physique Colloques, 1972, 33 (C6), pp.C6-4-C6-9. �10.1051/jphyscol:1972602�. �jpa-00215120�

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JOURNAL DE PHYSIQUE Colloque C6, supplCrnent au no 11-1 2, Tome 33, Novembre-Dicembre 1972, page 4

PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS

R. W. B. STEPHENS

Chelsea College, University of London, Grande-Bretagne

Rksumk. - Courte introduction sur la decouverte de la pi6zo6lectricite et son importante appli- cation par P. Langevin. On expose les recherches de nouveaux materiaux, naturels et synthkticlues, en relatant particuli6rement Gurs propriet6s pi6zodectriques et mtcaniques. On dt&it lYappari- tion de nouvelles techniques, et comment les semiconducteurs piezoklectriques peuvent Etre trait&

pour prendre les propri&6s dksirkes, donnant des transducteurs k couche d'amortissement et a couche de diffusion. Indications sur le quartz synthhtique et ses propri&tks, et sur celles de nouveaux matkriaux pi6zoelectriques, comme le tartrate de diamine, le tartrate dipolassique et l'iodate de lithium. Des indications qualitatives avaient Bte donnees dkjk sur la piBzoClectricitC de substances non cristallines, mais c'est seulement au milieu des annQs 1950 que des confirmations quantita- tives ont kt6 obtenues pour des Cchantillons de bois. Leurs auteurs, des chercheurs japonais ont montr6 aussi les proprittts pi6zo6lectriques de divers tissus biologiques comme les os, les tendons, les muscles et les polym6res orient6s.

On insiste sur le rBle croissant des ultra-sons dans beaucoup d'activitks courantes, cornrne leur emploi pour les etudes de laboratoire de divers materiaux, pour le travail industriel de divers mate- riaux, la soudure, etc., ainsi que leur usage trEts repandu pour les essais non destructifs. 11s sont devenus un instrument essentiel dans beaucoup d'applications mkdicales, tant le diagnostic que la chirurgie, et les nouveaux appareils pi6zoklectriques de production d'ondes de surface ont fait progresser la technique des transmissions.

Abstract. - A brief introduction will be given to the discovery of piezoelectricity and the signifi- cant application made by Paul Langevin. Then follows an account of the search for and development of new materials, natural and synthetic, with particular relevance to piezoelectric and mechanical properties. The advent of new techniques such as vapour-deposited transducers will be mentioned and how piezoelectric semi-conductors may be (( tailored )) to achieve desired qualities leading to depletion-layer and diffusion-layer transducers. Reference is made to polorized ceramics and their properties and those of new piezoelectric materials such as diamine tartrate, dipotassium tartrate and lithium iodate. Qualitative statements have been made in the past regarding the existence of piezoelectricity in various non-crystalline materials, but it was not until the mid-1950's that quan- titative confirmation was forthcoming in wood specimens. The Japanese workers concerned also showed the existence of piezoelectricity in various kinds of biological tissues such as bone, tendon, muscle, etc., and in oriented polymers.

The growing impact of ultrasonics on many aspects of everyday activities will be emphasised, from its use in the laboratory for probing various states of matter to industrial processes of metal forming, welding, etc., and its widespread use in non-destructive testing. In many medical appli- cations it has become an essential diagnostic and surgical tool and the use of new piezoelectric devices for mechanical surface wave generation is leading to advances in filter and delayline design for communication purposes.

French scientists have played a leading part in pioneering the science of piezoelectricity for it was the famous French brothers, Pierre and Jacques Curie, who discovered the phenomenon in 1880. However, going back earlier t o the middle of the 17th century a French apothecary of La Rochelle named E. Seignette was the first to synthesise that most common of fer- roelectric materials, Rochelle salt, sometimes designa- ted sel de Seignette. As the next step in the chain of progress it was most fitting therefore that Langevin should have made the first application of piezoelec- tricity to the construction of sound generators and receivers, with particular reference t o propagation in the sea.

My initial acquaintance with the name of Langevin was during my undergraduate days in connection not

with acoustics but with paramagnetism. It is interesting to note that Langevin's classical theory of parama- g n e t i c ~ was later extended by Weiss to include fer- roelectrics, such as Rochelle salt, by assuming the existence of an internal field as could be provided by electric dipole forces.

In the early 1900's the clanging bell still provided the main underwater sound source and bells were even employed in a primitive form of signalling, using a version of the Morse code. However, the main function of acoustic devices up t o the beginning of the first World War had been limited almost entirely t o use as navigational aids. The sinking of the liner Titanic after collision with an iceberg in 1912 and later the submarine blockade of France and England, under- lined the urgency of tackling the problem of under-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1972602

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PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS C6-5 water object detection and location. During World

War 1 the emphasis initially was largely on passive detection methods but during this period Fessenden in USA developed a reciprocating induction motor operating at 540 Hz whose continuous sound output could be pulsed at about 20 words per minute and was actually used with success in testing for icebergs and so was the forerunner of today's active sonar, although because of the low frequency the generator lacked directionality.

The British Admiralty was also active and A. B. Wood, an outstanding pioneer in sonar deve- lopment, describes Rutherford as hopefully, if not very optimistically, scratching small pieces of quartz crystal, with a telephone head piece connected, to discover if the piezoelectric effect of quartz was likely to prove useful. The result was inevitably disap- pointing as no means of amplification was then avai- lable. This era can be regarded as the real beginning of underwater sound technology and Paul Langevin constructed the first underwater acoustic transducer, which was an X-cut quartz crystal set into resonant thickness vibrations. In order to generate frequencies around 20 to 50 kHz however, the required thickness of quartz plate was not feasibly economical and it was in solving this difficulty that Langevin made his very significant contribution to underwater acoustics. He developed the sandwich transducer in which a quartz plate, a few mm. thick, was cemented between two uniform steel plates, so that one of the free surfaces was in contact with the air, so reducing the radiation loss from this face, while the free face of the other steel plate acted as the acoustic emitter. The total thickness of the sandwich corresponded approximately to one half-wavelength of the generated mechanical compressional wave. The device was known at the time as Langevin Supersound Source Projector, for it was not until the late 1940's that the term ultrasonics was adopted, to avoid confusion with the description of the high speeds being then attained by aircraft. Such was the advance in technology that by the late 1940's French industry was marketing the Langevin echo- ranging apparatus to shipowners for use as depth finders or depth sounders.

Langevin found that his transducer output was limited to about 4 watt per sq.cm, and probably the chief besetting practical problem was that of the cement for bonding the quartz and steel. In the early days American workers used shellac which was satis- factory if the voltage applied to the crystal was not greater than 5 kV, and another satisfactory cement was composed of 1 part vermilion, 1 part beeswax, 2 parts asphaltum and 3 parts resin. The need for improvement in quality of the synthetic Rochelle salt crystals, which were used in earlier American tests, was made evident by their tendency to melt at quite low applied voltages ! The growing of synthetic crystals such as quartz, ammonium di-hydrogen phosphate (ADP), (whichw as very widely used during World

War I1 owing to its superiority over Rochelle salt as regards temperature limitations and non-linearity characteristics) and the more modern materials Bis- muth Germanate, Lithium Niobate, etc., has been a notable feature in the developing technology of piezoelectric materials. Some of these new materials have exciting properties, for example if oblique cuts are avoided with the crystal paratellurite (TeO,) then no piezoelectric excitation of longitudinal modes is possible and suggests that very pure shear modes almost completely free of spurious responses may be generated. The element tellurium (Te) itself is a semi- conductor, which is also piezoelectric, but differs from the normal piezoelectric as there are no oppositely charged ions in its lattice. The piezoelectric effect in this case arises from a displacement of the electron distribution in the core of the Te atom.

Many interesting phenomena have been investi- gated in piezoelectric semi-conductors and the solid state physicist has gleaned much fundamental infor- mation about crystal structure by the propagation of elastic waves in such materials. Acoustoelectrics is the general term ascribed to the generation of electrical charges (or currents) in solids during the wave pro- pagation. If an applied electric field results in the motion of the free charge carriers in the direction of the elastic waves, then the ultrasonic attenuation is less.

Furthermore when the velocity of the electron is appre- ciably greater than that of the elastic wave, the inter- action between the conduction electrons and the piezoelectric charges causes an amplification of the ultrasonic wave. This amplification is obtainable with surface as well as with bulk waves. The ability to tailor the electrical conductivity of piezoelectric semi- conductors to any desired value between so-called insulators and conductors has led to the development of the depletion-layer and the diffusion-layer trans- ducers.

Another development arising from improved techno- logy and miniaturisation is the multi-element deposited transducer Such a 21 element has been produced in an area of one cm square. These devices have great possibilities for medical diagnostics since beam steering can be achieved with a stationary transducer.

A notable addition to transducer materials was the introduction in 1947 of the first polarized ceramic, barium titanate. These substances are essentially ferroelectrics and are dealt with in a later lecture, but it should be stated that they permit of higher power generation than previous transducers. High intensities have become a prominent objective during the last few years and the subject has gained the title of Macro- sonics. A piezoelectric discovered at the same time as quartz is tourmaline which however differs in being triaxially sensitive and so finds application in the measurement of pressures from explosions. When quartz crystal resonators are excited at large ampli- tudes they reveal a non-linearity of response and under suitable conditions exhibit the jump-frequency

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C6-6 R. W. B. STEPHENS

effect (Fig. 1 and 2). In fact according as to whether it is an AT or BT cut, the crystal behaves as a hard or a soft spring respectively.

I

'AT' CUT CRYSTAL

fe 1 4.849919 M H z

' 0 : 1, = a.059123 MU2

'BT CUT CRYSTA.

i I

During the mid 1920's a number of observers reported the appearance of electrical charge on sur- faces of materials like rubber, glass, wood, etc. when the media were subjected to mechanical pressure.

These observations were of a very qualitative character however and some thirty years elapsed before investi- gations on a quantitative basis were initiated, largely by Japanese and Russian workers.

The experimental observation of piezoelectricity in wood, whose basic constituent is cellulose, was very significant as previously the phenomenon was only associated with mono-crystals or at least textures with crystalline grains. By this discovery an interesting interdisciplinary field of investigation has been created invoking solid state physics on the one hand and biology on the other, organic chemistry bridging these extremes. Wood is anisotropic but also inhomo- geneous and this structural irregularity has to receive particular attention in the design of physical experi- ment and in the evaluation of the results. There is, for example, a difference in the properties of the spring and the summer wood forming the annual rings thus

giving rise to a variability of physical-mechanical properties along the radius of a trunk. By careful experimentation Shubnikov, Bazhenov and colleagues set out to express the piezoelectric properties of wood in terms of the piezoelectric properties of its consti- tuent cells, taking into consideration the manner of their distribution in normally structured wood. The problem is further complicated by the non-uniformity in size and shape of the cells, the thick-walled cells being concentrated in the summer growth parts and the thin-walled in the spring parts of the annual rings, which produces a layering of the wood structure. The involved details of the analysis is given by Bazhenov but it is more appropriate to mention here the results of impregnating wood with a polar liquid such as xylene, which does not reduce the dielectric properties of an oven-dry wood specimen nor does it lead to appreciable swelling. The mass of such a specimen was found to increase by nearly 42 % and the piezo- electric moduli-dl. and d,, of the wood (ash) increased by over a hundred per cent and by thirty per cent respectively. The radial elastic modulus changed on impregnation from 1.96 to 1.97 x 10'' d y n e ~ . c m - ~ and there was also no significant change in the tangen- tial modulus, indicating no appreciable change in mechanical properties as observed many years earlier by Erickson and Rees. The piezoelectric constants returned approximately to their pre-impregnation values on (( drying-out )) of the specimens. The expla- nation of the observed increase proposed by Bazhenov is that polar liquids like xylene have commensurate dimensions as the porous molecular structure of cellulose and can become oriented inside the fibril into a texture of the same symmetry as that of the oriented component of the cellulose. Such experiments as have been briefly mentioned could provide a new approach to an understanding of the relation between the structure of materials and their engineering and physical-mechanical properties. The piezoelectric data provide quantitative information regarding the orien- tation of the particular constitutive elements but at the same time also they indicate the materials required to obtain desired specific properties.

Just as in the case of wood the piezoelectric effect has been observed in bone which consists essentially of apatite crystals embedded in an organic matrix.

Fukada and Yasuda measured the piezoelectric cons- tants of small specimens cut from human and ox femurs which were completely dried by heating. They carried out three types of experiments, on the static and dynamic direct effects and on the dynamic converse effect, and concluded that the effect was truly piezo- electric. Its origin was suggested as due to the piezo- electric effect of the crystalline micelle of collagen molecules and was produced only when the shearing force is applied to the collagen fibres to cause them t o slip past each other. The magnitude of the piezoelectric constant depends on the angle between the applied stress and the axis of the bone, its maximum value

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PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS C6-7

being about one-tenth of the d l , constant for quartz.

Shamos and Lavine have observed the stress-induced electrical charge in a number of bones both in compres- sion and bending modes. It has been reported that a long bone such as the femur on being subjected to bending polarizes positively in the convex region and negatively in the concave. Now it has long been known that the architectural structure of bones depends on the forces imposed on them so the appearance of surface charges on stressed bone could be a controlling factor in bone formation, the resulting local electric fields influencing the deposition of ions or polarizable molecules.

Eriksson at Imperial College using apparatus (Fig. 3) similar to that of Fukada investigated piezo-

-

mofal frame

I

I "g

m e t a l s h i e l d

lized by ions of opposite polarity absorbed from the solution and so effect a change in the isoelectric point from its normal value for human tendon collagen.

The conclusion reached was that the electrical signals arising from the stressing of wet bone are mainly streaming potentials.

The piezoelectric effect can be observed in uniaxially oriented polymer films, the piezoelectric tensor in most cases being given by

where d,, is the piezoelectric modulus relating the polarisation in the Y direction to the shear stress in the ZX plane, and dl4 the modulus connecting the polarisation in the X direction with the shear stress in the YZ plane. Figure 4 indicates the rectangular

FIG. 3. - Apparatus for measuring converse piezoelectric effect by a dynamical method. Procedure (i) Bone electrodes both earthed. a. c. voltage applied to quartz and Rochelle salt cali- brated for force (ii) Quartz electrodes both eatthed a. c. voltage applied to bone and Rochelle salt measures oscillating force.

electricity of dentin and the d,, coefficient was found to be about one-hundredth that of quartz. Since tendon and bone in vivo are immersed in ionic fluid of high conductivity it is of considerable significance to make measurements under these conditions. Eriksson there- fore investigated the electrical properties of wet bone in the form of human Achilles tendons typically 50 mm long and 5 mm diameter when wet. The tendon consists of almost pure collagen fibres and provided a large number of fine capillary channels throughout the length permitting a movement of the ionic fluid in and out and giving rise to a streaming potential. To differentiate between any piezoelectric potential and the streaming potential the pH of the liquid was varied since at a particular pH, the isoelectric point, there will be no net charge on the macromolecule and the streaming potential will be zero. Any charges appear- ing on the strained collagen fibre would be neutra-

axes assigned to these films, the Z axis giving the direc- tion of the molecular orientation and the ZX plane is that of the film surface.

As most polymers are viscoelastic, a time lag is to be expected between the imposed stress and the result- ing polarisation, so that the piezoelectric modulus is complex and defined by d* = d' + j d , where the phase angle is given by tg 6, = d l d ' . Figure 5 shows the temperature variation of the real and imaginary piezoelectric moduli for a poly-y-methyl-L-glutamate (PMG) film, which had been elongated to approxima- tely double its original length. The variation at 0 OC is typical of piezoelectric dispersion and it is to be noted that this maximum of d' corresponds to the phase reversal point for d", the lagging polarisation with respect to the stress changing to a lead. The loss elastic modulus En also shows maxima around OOC and 100 O C . An explanation for piezoelectric dispersion at these temperatures is forthcoming from NMR and other measurements, the lower temperature maximum being associated with the onset of thermal molecular motion of the side chains in the helix and at the higher temperature with the onset of thermal motion of the disordered parts of the main a-helix. Assuming the

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R. W. B. STEPHENS

piezoelectric effect in oriented polymers is associated with the crystallites, which are embedded within amor- phous regions, then the total effect observed is the summation of the polarisations of the individual crys- tallites. A phenomenological representation of the dis- persion proposed by Fukada is the four element model in figure 6. G1 represents a piezoelectric crystal in series with an amorphous region in the form of the dash-pot q,. The orientation of the crystal dipoles under the strain of the spring gives rise to the piezo- electric polarisation while the surrounding non-piezo- electric amorphous regions (denoted by G , and yo) are connected in parallel with the piezoelectric elements.

The ratio a/& = E* = E' + jE", where E* is the complex elastic modulus, and d* K &,/a, while z, = ql/Gl and z, = qo/G2 define two relaxation times. The phase lag and lead of the piezoelectric polarisation are given respectively by the relaxation of yo and y,. By considering the dielectric properties of the composite model Fukada arrived at the conclu- sion that the piezoelectric relaxation is the sum of the

Material - Quartz (X-cut) Rochelle salt

(450 X-cut) Barium titanate

(Zcut)

Collagen (45kcut) Polymethyl-gluta-

mate (45O-cut)

elastic and dielectric relaxations and that the piezoelec- tric modulus d cc ( E ~ / O ) (Plp), where the observed surface polarisation P is induced by the polarisation p associated with the strain.

The piezoelectric modulus of PMG film can be twice as large as that of quartz and being also thin and flexible its properties suggest application in acoustical devices such as transducer elements in gramophone

<< pick-ups D. (See table of values of physical cons-

tants, Fig. 7.)

Evidence of the growing interest in ultrasonics in the 1940's was reflected by the 600 odd references to be found in that bible of the subject, (( Piezoelectricity >>

by W. G. Cady which appeared in 1946. The many successive editions of Bergmann's Der Ultraschall reveal the continuing development of the subject for the 1954 edition contained over 5 000 references.

Following the decease of Bergmann, Prof. Pohlman produced his first volume, in 1967, of Ultraschall- Dokumentation and the recorded number of ultra- sonic papers had then risen to 16 000.

It would be purposeless to list and describe the many directions in which ultrasonics is making an impact in our everyday life when these are chronicled in many comprehensive texts dealing with applications, and also in a number of excellent review articles such as that of M. Degrois. However an effort has been made Comparison table of physical parameters of some piezoelectrics

Density kg/m3

-

x 103 2.65 1.77 5.5 1.2 1.3

Dielectric constant

-

4.5 350 1500 8 10

Piezoelectric constant d

C/N - x 10-12

2.15 297 168

2.64 4.95

Coupling coefficient

-

0.10 0.73 0.45 0.014 0.017

Elastic modulus E

N/m2

- x 109

85 16 100 2 1

Acoustic impedance

kg/s . m2

x 107 144 0.57 2.3 0.15 0.11

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PIEZOELECTRICITY AND THE GROWTH OF ULTRASONICS C6-9 in figure 8 to compress a great deal of information into a single diagram to give an overall tt picture M of the extent of the field of activity. Ultrasonics has in fact become a subject in its own right and it is signi- ficant to mention that in 1973 the journal <t Ultra- sonics )) celebrates its tenth birthday. Parallel with the rapid growth in literature the numb5: of scientific meetings on the subject has grown and in particular mention should be made of the annual ultrasonics meeting of the Institution of Electrical and Electronic Engineers (USA) which is now an international event of high standing.

Monsieur le Prtsident we have seen that the ultra- sonic beam first propagated underwater by Paul Langevin has proved to be a widely diverging one, developing more and more side lobes with the passage of time and penetrating all types of media. Our civi- lisation has been enriched in many ways by Langevin's invention and these benefits are continuing reminders of the debt that mankind, acousticians in particular,

FIG. 8. owe t o him.

References

FUKADA E. and YASUDA I., On the Piezoelectric Effect of TOURNOIS P., Les lignes B retard acoustiques dispersives Bone. J. Phys. Soc. Jap. (1957). pour la compression d'impulsion. L'Onde Elec- FUKADA E. and SAKURAI T., Piezoelectricity in Polarized trique (1968).

P O ~ Y (vin~lidene fluoride) Films, The Sot. of DEGROIS M., Place occupCe par les ultrasons dans la Polymer Science (Japan) (1971). science et la technique. Revue d'Acoustique (1970).

FUKADA E.. Piezoelectric Disversion in Oriented Polvmers 5th Inti. Congress dn ~heology (1970).

(< Journals >> devoting regular space to Ultrasonics : Jour-

ERIKSSON Thesis Imperial Coll. (1968). C., The Electrical Properties of Wet Bone DIC nal of the Acoustical Society of America, (( Acustica >>,

sEED A., ~~~~i~~ in Q~~~~~ at ~ istrain ~~ h ~ ~ << Ultrasonics l i ~>), Journal of Non-Destructive Testing, ~ d ~ ~ and High Frequencies. Ph. D. Thesis I ~ P . toll. International Congresses on Acoustics (1953-1971). Soviet (1962). ,-- -, Promess in Avvlied Ultrasonics (Consultants Bureau) LANGEVIN P., Les ondes ultrasonores. Revue gCnCrale. soviet Physics i~coustics), IEEE ~iansactions on son&

Electricit6 (1 928). and Ultrasonics, etc.

A bibliography of notable ultrasonic books KIKUCHI Y., Ultrasonic Transducers, Corona Pub. Co.

(Tokyo) (1969).

MATTIAT -0. E., Ultrasonic Transducer Materials, Plenum Press (1 971).

VIGOUREUX P., Ultrasonics, Chapman and Hall (1950).

TIERSTEN H. F., Linear Piezoelectric Plate Vibrations, Plenum Press (1969).

HEISING R. A., Quartz Crystals for Electrical Circuits, Van Nostrand (1946).

TRUELL R., ELBAUM C. and CHICK B., Ultrasonic Methods in Solid State Physics, Academic Press (1969).

BEYER R. T. and LETCHER S. V., Physical Ultrasonics, Academic Press (1 969).

JAFFE B., COOK W. R. and JAFFE H., Piezoelectric Ceramics, Academic Press (1971).

SHUBNIKOV A. C. et al., Etude des Textures PitzoClectriques, Dunod (1958).

FREDERICK J. R., Ultrasonic Engineering, Wiley (1965).

MASON W. P. and THURSTON R. N., Physical Acoustics (Vol. I-VII) Academic Press (1964).

MASON W. P., Crystal Physics of Interaction Processes, Academic Press (1 966).

MUSGRAVE M. J. P., Crystal Acoustics, Holden, Day (1970).

BHATIA A. B., Ultrasonic Absorption, Oxford Clarendon Press (1967).

HERTZFELD K. F. and LITOVITZ T. A., Absorption and Dispersion of Ultrasonic Waves, Academic Press (1959).

RICHARDSON E. G., Relaxation Spectrometry, North.

Holland (1957).

TUCKER G. G. and GAZEY B. K., Applied Underwater Acoustics, Pergamon Press (1966).

BERGMANN L., Der Ultraschall, Hirzel (1954).

HUETER T. F. and BOLT H. R., Sonics, Wiley (1962).

RICHARDSON E. G . , Technical Aspects of Sound, Elsevier- (1957).

VIKTOROV I. A., Rayleigh and Lamb Waves, Plenum Press- (1967).

BROCKELSBY C. F., PALFREEMAN J. S. and GIBSON R. W., Ultrasonic Delay Lines, Iliffe (1967).

WOOD R. W., Supersonics, Brown University (1939).

CRAWFORD, A. E., Ultrasonics Engineering, Butterworths.

(1955).

METHERELL et al., Acoustical Holography, Vol. I to 111,.

Plenum Press (1968-72).

MASON W. P., ~iezoelectric crystals and their Application to Ultrasonics, Van Nostrand (1950).

MASON W. P., Electromechanical Transducers and Wave:

Filters, Van Nostrand (1942).

BAZHENOV V. A., Piezoelectric Properties of Wood, Consultants Bureau (1961).

ROSENBERG L. D., Sources of High Intensity Ultrasound, Vol. I and 11, Plenum Press (1969).

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