The influence of cooling rate and SiO 2 additions on the grain boundary structure of Mn-doped PTC thermistors

Abstract

BaTiO 3 based positive temperature coefficient of resistance (PTC) thermistors were prepared with 0, 1.0, 2.0 and 3.0 at.% SiO 2 additions. The effects of these SiO 2 additions and cooling rate variations on microstructural development and bulk electrical performance are discussed in the context of the double Schottky barrier (DSB) model. An increase in SiO 2 content increased the proportion of triple junctions containing BaTiSi 2O 8, but reduced the density slightly, without affecting the grain size. The bulk charge carrier density was unaffected either by SiO 2 level or changes to the cooling rate. Adding SiO 2 or increasing the post sintering cooling rate decreased ρ 25 and ρ max, and increased Tρ max. These effects were attributed to a reduction in activated surface state density, while maintaining a fixed acceptor energy depth. By assuming a single acceptor state energy level, satisfactory agreement between theory and experimental PTC behaviour was only obtained at Tρ max. Agreement was extended over a wider range of temperatures when the acceptor states were considered to be spread over a broader energy interval.