Anomalous Temperature Coefficient of Permittivity in Barium Titanate

Abstract

WHILE engaged in tests on the ‘ageing’ of the permittivity of a sample of ceramic barium titanate, an observation was made which may be of interest in connexion with the theory of this substance. The sample was held at a temperature of 125° C, about 1° below the Curie point through which the sample had just been cooled. It was found that the temperature cycling due to the operation of the thermostat caused the capacitance of the sample to cycle in the opposite sense, that is, a negative temperature coefficient was observed, whereas the normal permittivity curve has, of course, a high positive slope just below the Curie point. A phase lag between the switching of the thermostat and the sample temperature was at first suspected; but a test made with a thermocouple clamped to the specimen and the on–off cycle very much slowed down gave the same result.