Longitudinal piezoelectric coefficient measurement for bulk ceramics and thin films using pneumatic pressure rig

A pneumatic pressure rig was designed to measure the effective d₃₃ coefficient of thin film piezoelectrics by applying a known stress and monitoring the induced charge. It was found that the stress state imposed included components both perpendicular and parallel to the film plane. The later were due to friction and could largely be relieved through sliding of the O-rings to their equilibrium positions for a given pressure. The induced charge stabilized as equilibrium was reached and most of it was produced by the normal component of the stress. By minimizing the surface friction and compensating for the remnant in-plane stress, very good agreement was obtained among the d₃₃ values measured by the Berlincourt method, double-beam interferometry and this method for a bulk lead zirconate titanate (PZT) sample. The d₃₃ value of PZT thin films made by sol-gel processing was also measured. The as deposited films usually showed very weak piezoelectricity with d₃₃ values ranging from 0 to 10 pC/N, indicating little pre-existing alignment of the domains. With increasing poling field, the d₃₃ value also increased and saturated at poling fields exceeding three times the coercive field. Typically, films with thicknesses around 1 μm had d₃₃ values of 100 pC/N. Good agreement between double-beam interferometry and this technique was also obtained for thin films. The small difference between the two measurements is attributed to the effect of mechanical boundary conditions on the effective d₃₃ coefficient.