Enhanced actuation and acoustic transduction by pressurization of micromachined piezoelectric diaphragms

A strategy to increase the performance of diaphragm-type transducers is to provide an initial curvature to the diaphragm, which converts in-plane strains to motion normal to the diaphragm surface. In this paper, we analyze and measure the actuation characteristics of composite piezoelectric diaphragms that are biased by a static pressure that induces diaphragm curvature. To analyze the actuation characteristics of the curved-diaphragm transducer, a mathematical model for thin diaphragms relating the applied static pressure to the actuation characteristics is developed. In this model, the elastic response to applied pressure is caused by contributions of plate and membrane components. A linear relationship between applied voltage and boundary strain fluctuations was assumed, which leads to an analytic expression for the quasi-static diaphragm displacement amplitude in terms of diaphragm properties and biasing pressure. This expression shows that there is an upper limit to the displacement amplitude enhancement under piezoelectric actuation when diaphragms are curved by a biasing pressure. Measurements of displacement amplitude and acoustic pressure as a function of static bias pressure on micromachined piezoelectric diaphragms showed the expected behavior, which is typically displacement amplification, and a null when bending and stretching effects cancel. The model predicts that a decrease in residual stresses will increase performance at all biasing pressure levels. Residual stresses can be beneficial in a way, in that the pressure for peak performance may be varied to suit a design constraint.