BROAD-BAND CERAMIC-POLYMER PIEZOELECTRIC COMPOSITES

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BROAD-BAND CERAMIC-POLYMER PIEZOELECTRIC COMPOSITES

M. Sol Sanchez, F.R. Montero de Espinosa, J. San Emeterio

To cite this version:

M. Sol Sanchez, F.R. Montero de Espinosa, J. San Emeterio. BROAD-BAND CERAMIC-POLYMER PIEZOELECTRIC COMPOSITES. Journal de Physique Colloques, 1990, 51 (C2), pp.C2-587-C2-590.

�10.1051/jphyscol:19902138�. �jpa-00230433�

COLLOQUE DE PHYSIQUE

Colloque C2, supplément au n°2, Tome 51, Février 1990 C2-587 1er Congrès Français d'Acoustique 1990

BROAD-BAND CERAMIC-POLYMER PIEZOELECTRIC C O M P O S I T E S

M . SOL S A N C H E Z , F.R. M O N T E R O D E ESPINOSA and J.L. SAN EMETERIO

Instituto dee Acustlca, CSIC Serrano 144, SP-28006 Madrid, Spain

Résumé - On a travaillé avec différents approximations pour élargir la largeur de bande d'un composé polymère/céramique piézoélectrique. Une nouvelle idée du dessin des composés (1-3) est présenté à cette communication, lequel introduit un élargisement extra de la largeur de bande causé par la dispersion de la fréquence de résonance du composé de barres céramiques.

Abstrac - Different aproaches have been used to enlarge the frequency of (1-3) ceramic polymer piezoelectric composites. In this paper a new concept of (1—3) composite desing is presented which introduces an additional band- width enlargement due to the resonant frequency dispersion of the composite ceramic bars. This frequency dispersion is obtained by chosing bars with different width to thickness ratios.

1 - INTRODUCTION

Different types of ceramic polymer piezoelectric composites have been studied, constructed, and tested during the last ten years (l) (2) (3). Among those designed for pulse-echo imaging operations, the regular (1-3) composites occupy the pole position. These composites consist of piezoceramic bars regularly distributed into a polymer matrix. As it is well known, the two main advantages of these materials in comparison with dense ceramics are their high electromechanical coupling, coefficient and the possibility of tayloring their specific acoustic impedances to match that of the water medium. The main problem of these new materials derives from the spurious lateral resonances which are related to the periodicity of the ceramic bars into the matrix. To avoid the influence of the lateral resonances over the operating thickness frequency, a short spacing between the bars in comparison with the lateral wavelenghts must be used.Another approach to cancel this interference is the elimination of the lateral resonances by designing (1-3) ceramic-polymer composites having the ceramic bars irregularly distributed into the polymer matrix (4)

The limit of the acoustic band-width of (1-3) piezoelectric composites is determined by the internal mechanical losses and the relation between the specific acoustic impedances of tht composite and the water. Q values up to 4 have been obtained (1).

Bowen et al (5) proposed the use of the stepped shape concept to design a composite structure having non parallel faces. This composite is formed by piezoceramic sheets arranged with their polar axis parallel to the polymer interface and in the direction of the desired mechanical resonance. The composite is graunded in wedge shape in such a way that the piezoelectric sheets present different lenght between two limits. These composites present a bandwidth broadening that can be made quite large. Nevertheless, the constructing geometry make this approach not very useful as transducer because the acoustic aperture presents a linear variation of frequency and vibration amplitude along the y direction whereas the frequency and vibration amplitude will be constant along x direction for each y value -Fig.l-.

Bui T. et al (6) proposed a multifrequency composite ultrasonic transducer system that can be understood as another approach to broaden the frequency band of (1-3) composites. This approach consist of using piezoceramic bars with lateral dimensions similar to their length.

In these conditions the ceramic bars vibrate in the zone of high mode coupling and two main different frequencies will appear with the same relative amplitude. This approach leads to degraded impulse responses with a clear signal lenghtering. (6).

This paper deals with a new approach to enlarge the (1-3) piezocomposites frequency band which consist on distributing ceramic bars having values of width to length ratios in the zone of modes uncoupling.

2 -BROAD-BAND COMPOSITE DESIGN

As it is well known, the resonant frequencies of piezoelectric ceramic bars with square

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19902138

C2-588 COLLOQUE DE PHYSIQUE

cross-section are a function of the ratio L/H being L their width and H their thickness. (7).

Fig. 2 shows the solution of the Onoe coupling theory applied to PZT-5 A ceramic bars. From fig. 1 it is possible to distinguish three L/H zones. The first one is the zone of L/H values up to 0.7.In this region only one mode is mainly excited. When L/H ranges from 0.7 up to 1.2, there is a strong coupling between the two lowest resonant frequencies. For L/H values h i ~ h e r than 1.2 there appear a lot of resonances.

As previously mentioned our design criterium consists of putting together ceramic bars with different resonant frequencies. Using ceramic bars with L/H ratios in the first zone, they will vibrate with only one clear resonant frequency. Moreover, using a geome-try as that of Fig. 3, the problem of Bowen's approach will be overcome because the different ceramic bars are regularly distributed all around the matrix.

As shown in fig. 2, the frequency dispersion that can be obtained from L/H=O to L/H=0.7 is of about 10%. So, this is the maximium bandwidth broadening that could be obtained with respect to a normal (1-3) composite,by using this approach.

3 - COMPOSITE CONSTRUCTION.

The geometry of fig.3 can be easily implemented following the typical (1-3) composites constructions (8). A commercial piezoceramic disk is diced part-way through in one direction and again in the perpendicular direction. Instead of maintaining during the cutting process the same pitch between the bars, a series of two consecutive different pitches are used.

After cutting, a resine polymer is embebded into the grooves and then cured. The L/H ratios and plastic volume percents can be varied by changing the cutting width or the pitch or both of them. Finally, the electrical connection between the bars is restored by metalizing the comgosite main surfaces. Following this constructing technique several composites were made and tested. Table I shows their dimensions and geometrical characteristics.

Table I

Composites dimensions and geometrical characteristics composite diameter width/thicknes (L/H) ratios

(mm)

square section rectangular section

4

-

The input impedance behaviour of these composites can be evaluated in a first approximation using a parallel connection of equivalent circuits as that of in Fig. 4 The analytical expressions of the circuit parameters are also shown in this figure.

5 - RESULTS

The simulation of the input conductance and resistance of the composites C1,C2 and R1 are presented in figure 5. The resonant frequency differences correspond to those predicted using the Onoe approach. Figure 6 shows the simulation results of composite CR. This composite was simulated using the parallel connection of the three circuits corresponding to the composites Cl,C2 and R1 with their respective weight. The experimental measured values are also represented in the same figure. As it can be observed, the agreement is quite good.

Comparing Figures 5 and 6 it is easy to observe the broadening on the conductance bands produced by the frequency dispersion. The resulting cpnductance band is the weigthed addition of the frequency bands of the three constitutive regular composites.

To understand better the effect of the conductance band increase on the acoustic behaviour, the impulse response of the composites was measured and then, the acoustic frequency band was obtained by means of spectrum analysis. The acoustic band in pulse-echo conditions was measured by lowering as much as possible the driver output (emission) and input (reception)

complex impedances. The pulse-echo signals of composites C1 and CR in the conductance frequency band are shown in Fig.7 together with their corresponding frequency spectra. The frequency agreement with the respective electrical bands confirms that the excitation conditions were the appropiated. The increase in the acoustic band corresponding to the series resonance is very clear.

6 - CONCLUSIONS

The new design approach that we present to enlarge the frequency band- width of (1-3) ceramic polymer composites, permits a broadening of a 10%. This broadening, produced by the frequency dispersion between the different ceramic bars, is added to the band-width of the composite produced by its internal and acoustical losses. The geometrical pattern used, permits a regular distribution of the different bars in such a way that a certain homogeneity of frequency and vibration amplitude over the composite surface will be obtained. The equivalent circuits model used seems quite good to predict the composite electrical parameters.

7 - ACKNOWLEDGEMENTS

This work has been supported by the CICYT-PRONTIC.

REFERENCES

Newnham, R.Z., Safari, A., Sa-gong, G. and Giniewicz, J., Ultrasonic Symposium (1984) 501.

Gururaja, T.R., Schulze, W.A., Cross, L.E., Newnham, R.E., Auld, B.A. and Wang, Y.J., IZZE Transactions on Sonics and Ultrasonics

E,

Smith, W.A., Shaul~v, A. and Auld, B.A., Ultrasonic Simposium (1985) 642.

Montero de Espinosa, F.R., Pavia, V, Gallego Jusrez, J.A. and Pappalardo, M., Ultrasonics Symposium (1986) 691.

Bowen, L.J. and Gururaja, R., J. Appl. Phys.

51

Bui, T., Chan, H.L.W. and Unsworth, J., IEZE Proc. Ultrasonic Symposium (1988) 627.

Onoe, M. and Tiersten, H.F. IEEE Transactions on Ultrasonics Engineering, July (1963) 32.

Smith, W.A., Shaulov A.A. and Singer, B.M., Ultrasonics Symposium (1984) 539.

Fig. 1. Bowen's broad band rransducer configuration

Fig. 2. Onoe coupling mode theory results for PZT-5A piezoelect-ic bars following Bui T. et a1

Fig. 3. Composite CR with three groups of piezoelect~ic bars. Dotted area shows the elemental composite cell

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Fig. 4. Simplified Mason circuit for thickness piezoelectric resonators

Fig. 5. Conductance and resistance against frequency of composites a) C1, b ) C2 and c ) L1

7ig. 6. Calculated and measured conductance and resistance against frequency of composite CR

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1.6 2.0 2.4 MHz 1.6 2.0 2.1 2.8 MHz

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Fig. 7. Pulse echo responses and frequency spectra of composites a ) C1 and b ) CR

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