Thermomechanical stress in 1-3 piezoelectric composite

The 1-3 piezoelectric composites have been developed and utilized in many application fields for several decades. For design purposes, constitutive relations have been proposed allowing the prediction of the composite properties. In fact, because the piezoelectric active phase properties are very sensitive with heat treatment or stress, these prediction laws could be inaccurate. When composites are made with a high glass-rubber transition temperature polymer phase, a high curing temperature is needed. In this case, the variation of the composite permittivity is not any more a proportional function of the PZT volume fraction. This points out a decrease of the PZT material permittivity in the composite as a function of the PZT volume fraction instead of a constant. This is interpreted in terms of the non negligible effect of the thermo-mechanical stresses induced by the polymer curing process. In-situ measurements of this stress are described. A thermomechanical model is proposed along with PZT characterization under uni-axial stress, both giving a good numerical interpretation of the composite abnormal behavior. From the relation between the PZT permittivity and polarization variation in a 1-3 composite during the curing and estimated thermal stress by the model, the thermo-mechanical stress effects on the PZT in the composite are investigated.

Introduction

1–3 piezocomposite materials consist of piezoceramic rods aligned in parallel and embedded in a polymer matrix [1]. Piezocomposites are very effective electromechanical devices but they also exhibit different kind of losses. For underwater applications, as high power 1–3 piezocomposite high frequency sonar, as the composite temperature increases as a function of the input power due to the various losses, the polymer glass-rubber transition temperature (Tg) is a critical point for the transducer thermal stability [2], because the polymer phase become a viscous dissipative matrix over Tg. At this point, the use of a high Tg polymer is of great advantage to widen the composite operating temperature range [3]. However, the composite fabrication process with such a polymer requires a high temperature curing.