Thermo-mechanical stress effect on 1-3 piezocomposite power transducer performance

This paper deals with the emission performance of 1-3 piezoelectric composite power transducers made with a hard PZT (Navy III) and epoxy resins with a high glass-rubber transition temperature. Following the "dice and fill" technique, various composite transducers with 30 and 50% PZT volume fractions were fabricated with an air backing and no front matching layer with resonance operating frequencies around 500 kHz. The transducers were first evaluated under isothermal conditions, with a low emission duty cycle. Efficiencies as high as 95% were monitored as a function of the instantaneous input power up to a 60 W/cm(2) density. The effect of the polymer matrix mechanical losses and the fabrication conditions is then discussed. For the transducer thermal stability, the case of long duty cycle or continuous emission was considered in a second evaluation. In this case the transducer working temperature and axial radiated pressure were monitored as functions of the input power density up to 40 W/cm(2). It is shown that the transducer efficiency and working temperature were strongly dependent on the type of resin used but also on the PZT material, even for hard PZT compositions. A composite transducer configuration with strongly improved thermal stability was investigated demonstrating a working temperature higher than 90 degrees C and an extended power range (30-40 W/cm(2)). The composite thermal breakdown mechanism was analyzed and the effect of the curing-induced thermo-mechanical stresses on the PZT mechanical losses was considered in relation to the composite working temperature. Measurements of the composite mechanical losses versus the temperature were obtained and related to the variation of the PZT mechanical losses with the stresses due to the composite transducer temperature change. It is found that the thermally induced stress can strongly influence the PZT ceramic mechanical losses and that it can be the reason for a thermal breakdown taking place at a temperature much lower than the epoxy resin transition.