### ASA 125th Meeting Ottawa 1993 May

## 3aEA10. Effect of dynamical fluid compressibility and permeability on
porous piezoelectric ceramics.

**O. Lacour
**

**
Michel Lagier
**

**
**
*Thomson-Sintra, DASM, zone des Bouillides, Sophia-Antipolis, 06561
Valbonne, France
*

*
*
**D. Sornette
**

**
**
*Universite de Nice, 06108 Nice Cedex 2, France
*

*
*
For frequencies less than the Biot frequency, the ac piezoelectric
properties of porous ceramics are studied in the presence of slightly
compressive viscous fluids filling the pores inside the ceramics. Two regimes
are found: (1) for f smaller than a cross-over frequency f[sub
c]~((beta)/(eta))L[sup -2], where L is the thickness of the sample and (beta)
the fluid compressibility coefficient, the piezoelectric coefficients d[sub ij]
are close to that of the dense nonporous ceramics weighted by the filling
factor (1-(phi)); for f>f[sub c], the d[sub ij] are those of the empty porous
ceramics as if the fluid was not present. These observations are explained in
terms of a frequency-dependent penetration of the fluid pressure inside the
porous ceramics as a result on the interplay between fluid compressibility and
viscosity. The dependence of the hydrostatic piezoelectric coupling d[sub
H]=d[sub 33]+d[sub 31]+d[sub 32] as a function of frequency observed
experimentally is compared to an effective medium theory for the piezoelectric
properties coupled to a permeability model. The experimental dependence of the
cross-over frequency f[sub c] on sample sizes and on the fluid viscosity are in
excellent agreement with this theory. The study shows that it is thus possible
to obtain the desirable property of a high hydrostatic piezoelectric
coefficient d[sub H], characteristic of light porous piezoelectric ceramics,
and good mechanical properties under high applied pressure with the use of an
internal fluid that allows one to equilibriate the pressure without destroying
the piezoelectric properties of the porous ceramics.