Determination of nonlinear electromechanical energy conversion and the field-dependent elastic modulus of Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-based electrostrictors

Energy conversion efficiency is a critical parameter for all electromechanical materials. Although excellent techniques are available for linear materials, nonlinearities complicate the determination of conversion efficiency in electrostrictive Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN). The field dependence of the elastic modulus presents additional problems. A technique combining pulse-echo ultrasound has been developed to approximate the change in the Young´s modulus with applied field. A 30% decrease was observed at 1 MV/m (from /spl sim/100 to /spl sim/70 GPa). The boundary condition for these measurements was a constant electric field as opposed to constant dielectric displacement. In combination with the results from harmonic analysis, the modulus data may be used to determine the electromechanical conversion efficiency. This has been accomplished using an energy balance criterion for a PMN-based composition (with and without dc bias). The resulting longitudinal coupling values are >0.5 for practical electric fields (<1 MV/m peak).