Influence of Stoichiometry on the Microstructure and Positive Temperature Coefficient of Resistivity of Semiconducting Barium Titanate Ceramics

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Abstract
The influence of stoichiometry, i.e., Ba/Ti ratio, on the microstructure and positive temperature coefficient of resistivity (PTCR) characteristics of BaTiO3 was investigated. Fine‐grain microstructures are obtained for Ba‐rich, stoichiometric, low‐temperature‐sintered, Ti‐rich materials. The room‐temperature resistivities (ρRT) of the fine‐grain Ti‐rich samples are large (>108Ω·cm). Excess Ba2+ ions can decrease the ρRT, by more than 2 orders of magnitude, because of the compensation of barium vacancies near the grain‐boundary regions. Rapid cooling after sintering can also decrease ρRT (⋍100×) and is ascribed to the suppression of reoxidation. Large‐grain microstructures and low ρRT, on the other hand, are generally observed for Ti‐rich and Al2O3‐SiO2‐TiO2‐added samples after sintering at a temperature higher than the corresponding eutectic point.

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