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The controlled addition of impurities such as lanthanum in barium titanate and barium strontium titanate systems results in thermistor materials which have large positive temperature coefficients of resistance over controlled temperature ranges. The resistivity in these systems is sensitive to the concentration of the trace additives, to undesirable impurities in the raw materials, and to the sintering and maturing conditions. Ceramic procedures well adapted to prepare these thermistors reproducibly have been developed.
Sauer et al. (1960) studied this question.