Microstructure modifications and modulated piezoelectric responses in PLZT/Al2O3 composites

Piezoelectric ceramics for acoustic applications have been prepared by mixing the piezoelectric phase Pb1−1.5xLax□x/2(Ti1−yZry)O3 (PLZT) with variable fractions of Al2O3. The samples are in form of pellets and polarized at high temperature. After thermal treatment, X-ray diffraction and scanning electron microscopy have been used to determine the phase and morphological modifications. The morphotropic PLZT initial phase disproportionates into modified PLZT and ZrO2 phases. Using electrical impedance spectroscopy, the resonance frequencies of the composite system have been determined and analyzed. As the Al2O3 volume fractions increase, the resonance frequency and the amplitude of the electrical response both decrease. An interpretation of the role of Al2O3 additions is proposed in terms of phase and microstructure modifications. Using LRC electrical equivalent circuits, the impedance variations close to the resonance frequency are modeled: the increase of the resistance R (electrical losses) and of the capacitance C are respectively correlated to the change in microstructure and in nature of PLZT initial phase.