The effect of SiO2 addition on the development of low-Σ grain boundaries in PTC thermistors

Electron backscatter pattern analysis has been used to characterise, using the coincidence site lattice model, the distribution of grain boundary structures in a series of BaTiO₃ based positive temperature coefficient of resistance (PTC) thermistors, prepared with 0, 1.0, 2.0 and 3.0 at.% SiO₂ additions. As the SiO₂ content was raised, the proportion of random, high-angle grain boundaries in the microstructure increased steadily from 85.7% to 89.6%, while the proportion of grain boundaries indexable in the range Σ3–Σ29 decreased from 14.3% to 10.4%, and the Σ3 grain boundary population fell from 5.9% to 3.6%. At the same time the proportion of Σ3 twin boundaries remained approximately constant at 3.0 ± 0.3%. Significantly more Σ3 grain boundaries than would be expected in a randomly oriented, untextured material were observed in all samples. The variation in grain boundary types with SiO₂ addition is discussed in terms of grain boundary energy and its effect on PTC performance.