Electrode-ceramic interactions in low-firing PZT multilayer actuators with Ag-Pd electrodes

In the last decade piezoelectric multilayer actuators (MLA'S) have been adapted for use in fuel-injector systems in automobiles. The potential for this technology to replace traditional solenoid injectors has renewed research and development in MLA's. In this work we have focused on some of the remaining technological problems which are holding back universal acceptance of piezo-MLA's. The key issues to address are reducing the actuator cost while maintaining performance and reliability. Cost significantly depends on the amount of Pd in the Ag-Pd electrodes which in turn, is dictated by the sintering temperature. By the use of various fluxing agents we have co-fired multilayers with pure Ag electrodes at temperatures as low as 900°C. Maintaining high d₃₃ piezoelectric coefficients at such low sintering temperatures is challenging, but we have managed to produce samples with high field values in excess of 600pm/V. We have shown that reactions may take place between Pd and the ceramic near the interface when Pd-rich electrode compositions are used. Similarly, the degradation of multilayers under HALT testing appears to be dependent on the electrode composition. We have used a combinatorial approach including techniques such as impedance spectroscopy, Rayleigh analysis and novel voltage-contrast SEM analysis to characterize the effects of Ag diffusion and migration in MLA's.