Thermal cycling properties of a lead-free positive temperature coefficient thermistor in the Ba-0.97(Bi0.5Na0.5)(0.03)TiO3 system

Abstract

A Pb-free PTC (positive temperature coefficient thermistor) heater was developed in the Ba-0.97(Bi0.5Na0.5)(0.03)TiO3 system especially for automotive part applications. The reliability was verified by using a thermal cycling test designed on the basis of the result from a quality function deployment (QFD) analysis. We compared the thermal cycling test results from the newly-developed Pb-free PTC heaters with the results from PTC heaters currently on the market, namely, PTC heaters containing Pb. Life prediction and stress-strength relationships were analyzed together with a thermal diffusivity evaluation. We discuss the potential failure mechanisms during the thermal cycling test, focusing on the fact that electrical degradation in PTC materials is closely related to mechanical degradation due to the internal stress in the materials that comes from repeated phase changes. Different grain size distributions on the sintered bulks were considered to a key factor for explaining the different results of the reliability tests between the new Pb-free PTC heaters developed in this study and the commercial PTC heaters containing Pb.