Electrode effects in positive temperature coefficient and negative temperature coefficient devices measured by complex-plane impedance analysis

Impedance spectroscopy in conjunction with capacitance measurements were utilized to study the electrical contact behavior of commercial positive temperature coefficient and negative temperature coefficient ceramics. The impedance spectroscopy data, measured at frequencies ranging from 100 Hz to 40 MHz, exhibited a double semi-circle shape characteristic of systems which have both barrier and bulk contributions to the impedance of the device. From the impedance data, equivalent capacitive-resistive circuits were assigned to grain, grain boundary, and barrier elements. These barrier capacitances and resistances were measured for Au, Ag, Pt, and In-Ga electrodes. The trends in the magnitude of the barrier resistance are discussed with relevance to the thermodynamic work of adhesion and the effect of metal-oxygen and metal-metal interactions at the interface.