The influence of cooling rate and SiO2 additions on the grain boundary structure of Mn-doped PTC thermistors

BaTiO3 based positive temperature coefficient of resistance (PTC) thermistors were prepared with 0, 1.0, 2.0 and 3.0 at.% SiO2 additions. The effects of these SiO2 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 SiO2 content increased the proportion of triple junctions containing BaTiSi2O8, but reduced the density slightly, without affecting the grain size. The bulk charge carrier density was unaffected either by SiO2 level or changes to the cooling rate. Adding SiO2 or increasing the post sintering cooling rate decreased ρ25 and ρmax, and increased Tρmax. 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 Tρmax. Agreement was extended over a wider range of temperatures when the acceptor states were considered to be spread over a broader energy interval.