Electrical conduction mechanisms in piezoelectric ceramics under harsh operating conditions

Piezoelectric ceramics are widely used in industry for sensor and actuator applications. Under an applied electric field over a long duration, the conductivity of the ceramic increases resulting in leakage currents and increased power consumption. This process is accelerated by harsh environments and operating conditions such as high temperature, high humidity and high electric field. In this study, a piezoelectric ceramic was stressed by exposure to high relative humidity, at an elevated temperature, with a continuously applied d.c. electric bias field. Periodically during the test, the bias field was removed and a low frequency a.c. electrical cycle applied, during which current and voltage were measured. Results show that the conductivity is both time and voltage dependent and indicative of a complex breakdown process in the ceramic. The breakdown field strength was observed to vary only slightly during the exposure to humidity and d.c. bias despite large increases in conductivity. The reestablishment of the conduction process on re-application of the d.c. bias field was also studied. Whilst these results are specific to the piezoelectric material system studied, the mechanisms and insights into the interaction of electrodes with a ceramic material under conditions of high humidity and high electric field are likely to have much wider relevance for electro-ceramics in general, including technologically important materials such as dielectrics for energy storage and electronics, electronic oxides, and ferroelectric memory devices.