We report on recent calculations and experiments on the broadband properties and impulse characteristics of PVF2 transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitation of surface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic substrates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF2 films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency measurements on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60°, and control of acceptance angle profiles, in face plates using multilayer PVF2 films

Chen, W H

Shaw, H J

Weinstein, D G

Zitelli, L T

English

We report on recent calculations and experiments on the broadband properties and impulse characteristics of PVF2 transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitation of surface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic substrates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF2 films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency measurements on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60°, and control of acceptance angle profiles, in face plates using multilayer PVF2 films.</p

Chen, W.

Shaw, H.

Weinstein, D.

Zitelli, L.

Digital Repository @ Iowa State University (ISU)

PVF2 TRANSDUCERS FOR NDE W. H. Chen, H. J. Shaw, D. G. Weinstein, and L. T. Zitelli Ginzton Laboratory, Stanford University Stanford, California 94305 ABSTRACT We report on recent calculations and experiments on the broadband properties and impulse character-istics of PVF? transducers and arrays. Experimental wedge transducers show bandwidths of approximately 100% in the excitationofsurface acoustic waves at 7 MHz on nonpiezoelectric silicon nitride ceramic sub-strates. Computer calculations predict similar bandwidths for interdigital transducer arrays on PVF films for surface acoustic wave excitation on similar substrates. Insertion loss versus frequency m~asure­ments on bulk longitudinal wave transducers in water at frequencies in the 1 to 30 MHz range show good agreement with theory. 0A computer program for multilayer piezoelectric films predicts included angles of acceptance exceeding 60 , and control of acceptance angle profiles, in face plates using multilayer PVF2 films. PVF2 bulk wave transducers are made by bonding 25 11 PVF? film (~ l/2" x 1/2") onto brass backing rods witn V-6 epoxy. The PVF film is obtained from Kureha Corporation in st?etched, poled and electroded form. We etch off the aluminum elec-trodes, which erode quickly when placed in water, and put down a thin layer of chrome followed by a layer of gold (""' 1000 A). These bulk wave transducers have a very flat frequency spectrum up to 20 MHz when radiating into water. 1 This is expected, since the acoustic im-pedance and velocity of longitudinal waves in PVF2 (3.83 x 105 gm/cmZsec and 2.15 x 105 em/sec, re-spectively) match relatively wel~ to the impedance and veloc~ty of water (1.48 x 10 gm/cm2sec and 1.48 x 10 em/sec). As the impedance mismatch at the brass/PVF2 interface is large, little energy is radiated into the brass. These characteristics of PVF? result in a clean, nearly bipolar impulse re-sponse (Fig. 1). PVF transducers have shown a 60 dB two-way insertion foss at their resonance fre-quency (A= 4 x film thickness). UPERIMENTAL ~ """ lOOns THEORET! CAL Fig. 1. Impulse response of a broadband PVF2 transducer. Although the piezoelectric coupling coeffi-381 cients of PVF?. are low compared to PZT, the power output of a PVF transducer remains linear with higher applied ~oltage than PZT. We have carried out power measurements at higher frequencies (13.56 MHz). The power output of a PVF2 transducer re-mained linear up to 750 volts (3 x 105 v/cm), the limit of the signal generator. It is expected that the power-voltage relation will remain linear above 3 x 10° volt/em. Calculations have shown that PVF can produce approxima~ely 5 times the power/volume2 that PZT can produce. Multilayer transducers also have been investi-gated. By folding and bonding an electroded film, PVF transducers can be operated with high applied RF ~lectric fields without requiring high voltage. This provides a multilayer transducer in which the voltages of the individual layers are in parallel while their acoustic fields are in series. This also provides a way of obtaining lower resonant frequencies with a given film thickness. Multi-layer transducers have been constructed and used to observe the movement of the mitral valve of the hu-man heart (see Figs. 2 and 3). · We are currently examining the possibility of a PVF face plate which could receive acoustic ra-diatibn at angles other than normal incidence. For a face plate used as a receiver, broadband frequen-cy response leads to broad angular response.3 Therefore a PVF face plate should have a very broad angular r~sponse. Just as a wide aperture optical lens has better resolution than a narrow aperture lens, a broad angular response transducer will lead to better resolution than a transducer with a relatively narrow angular response. Thus, a PVF face plate promises good resolution. Photo-litho~raphy would be used to produce periodic elec-trode patterns on the PVF2 surface which could then be electronically scanned to achieve imaging. At present, theoretical studies on the angular re-sponse of brass backed PVF? face plates are under-way usin~ a computer program developed by Auld and Roberts. Figure 4 shows the voltage response as a function of angle of a PVF2 brass backed film. The three curves show the effect of the value of the piezoelectric coupling coefficient ezx transducer response. (Shear wave effects nave been suppressed in order to simplify understanding of the result.) As ezx and ezz have opposite signs, it is seen that ezx detracts from the uni-form character of the voltage response from oo to 30°. Calculations have also been made which involve both shear and longitudinal waves. These show a large shear wave resonance near a sso angle of in-cidence. The tail of these resonances contributes to the uniform character of the response from 0° to 300. Fig. 2. Multilayer transducer structure. Fig. 3. Photo shows an echo from a mitral valve leaflet. The right-hand pulse is the mi-tral valve echo; the near-in signals are echoes from inhomogeneities in the brass used as the transducer backing. We have devised a scheme that involves stack-ing three layers of PVF2 in such a way that the re-sulting ezx and ezz of the stacked films have the same sign. By rotating a film goo about an axis perpendicular to the plane of the film we can interchange ezx and ezy , which differ by a factor ~ 5. By inverting a film we change the sign of ezx and ezz . Using these changes, we theo-retically construct a three-layer stack with the properties shown on Fig. 5. This figure also shows the voltage response of such a stack. The theore-tical response of such a stack is more favorable (out to 450) than a single film. Wedge transducers have been constructed for the excitation of surface acoustic waves on high velocity nonpiezoelectric substrates~ These could be used to perform NDE on surface flaws. In this 382 case, a PVF? transducer irradiates the substrate at an angle such as to phase match with a surface a-coustic wave propagating on the substrate. The ex-perimental wedge transducers have a resilent RTV wedge between the PVF transducer and the surface wave substrate allowiftg experimental variation of the wedge angle by distorting the RTV, this being a critical parametar.4 (See Fig. 6). As PVF can be used at frequencies near 20 MHz, the waveleftgth of such a surface wave will be small, and resolution will be good. Using 30 p PVF? films, a bandwidth of approximately 100% has been observed at 7 MHz in initial measurements using two identical wedge transducers on a silicon nitride substrate. 3 • 0,107 X 10-4 ezz Thickness • 0.3 ~ 0 10 20 30 40 50 60 70 80 90 lnciden~e Angle 6 - Degrees Fig. 4. Theoretical PVF2 face plate response. 10 .. .. :l .... 0 I> 5 . Layer 1 Layer 2 Layer 3 e e -A!_ __.E_ -1.0 +1.07 0.2 -1.07 0.2 -1.07 Qs • S Thickness ). 0.07 0.07 0.07 Incidence Angle 6 - Degrees Fig. 5. Theoretical PVF2 face plate response. Interdigital surface wave transducers in which PVF2 film provides coupling between the interdigi-tal array (deposited on the PVF ) and a nonpiezo-electric substrate are being st6died. The effec-tive coupling is determined by evaluating·the fractional difference in velocity (8v/v) of a surface wave propagating in the film substrate com-bination under two conditions: (1) no electrodes are present at the surface which is to contain the interdigital array, and (2) a uniform conductor is placed at that surface. The surface wave veloci-ties have been calculated using a computer program developed by Kino and Wagers,5 are shown in Fig. 7. Focusing on the curve label 10-00, we see that this curve rises to a reasonably high value (0.8%) and is also fairly broad. This implies that an IDT with the 10-00 structure would be capable of a broad frequency response. The resulting transducer will be flexible and mechanically conformable for use on curved surfaces (see Fig. 8). Wedge Angle Adjustment Ceramic Substrate Wedge Fig. 6. Wedge transducer schematic. 2 0 0 .... ,... > 1 ...... :S 0 1 2 3 4 HK 01-00 - lOT on top of PVF2 short at sub-PVF2 interface 10-00 - lOT at PVF2 sub-interface 5 short on top of PVF2 11-01 - lOT at sub-PVF2 interface 11-10- lOT on top of PVF2 (H = film thickness, K = propagation constant) Fig. 7. 6V/V versus film thickness. J Uniform Counterelectrode Fig. 8. lOT schematic. The PVF2 program at Stanford also includes a material synthesis group. C. Frank and S. Bowker of the Chemical Engineering Department are studying the chemistry and synthesis of PVF?, aimed at im-proving the piezoelectric properties of PVF2. R. Reigelson, R. Route, and R. DeMattei are in-volved in melt press production of PVF2 films at the Stanford Center fer Materials Research. References *The work reported in this paper was supported by the Office of Naval Research under Contract N00014-77-C-0582 and the Air Force Office of Scien-tific Research under Grant No. AFOSR-77-3386. l. L. Bui, H. J. Shaw, and L. T. Zitelli, "Experi-mental Broadband Ultrasonic Transducers using PVF2 Piezoelectric Film," Electronics Letters 1.£, 16, 393 (August 5, 1976). 2. John L invill, "PVFrModels Measurements, De-vice Ideas," Stanford Electronics Laboratories Report No. 4834-3, 71 (1978). 3. B. A. Auld, M. E. Drake, and C. G. Roberts, "Monolithic Acoustic Imaging Transducer Struc-tures with High Spatial Resolution," Appl. Phys. Letters 25, 9, 478 (November 1974). 4. 5. 383 J. Fraser, B. T. Khuri-Yakub, and G. S. Kino, "The Design of Efficient Broadband Wedge Trans-ducers," Appl. Phys. Letters 32, 11, 698 (1 June 1978). G. S. Kino and R. S. Wagers, "Theory of Inter-digital Couplers on Non-Piezoelectric Sub-strates," J. Appl. Phys. 44, 4, 1480 (April 1973). 

PVF2 Transducers for NDE

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