Effect of Hydrostatic Pressure on the Permittivity of Barium Titanate Ceramics

The effect of hydrostatic pressure on the permittivity of barium titanate ceramics was investigated experimentally up to a pressure of 1,125 kg/cm
²
(kilograms per square centimeter) and at temperatures of 0, 11, and 25 C (degrees centigrade) respectively. The pressure system used consisted of a stainless steel chamber in which the specimen was placed under oil, a hydraulic jack, and a Baldwin pressure cell with accompanying strain indicator. No marked changes of permittivity with hydrostatic pressure were observed. The variation of permittivity e with pressure p is linear, or almost linear. The pressure coefficient of permittivity (∂e/∂p)t of the e versus p curves was small: 0.01 to 0.16 cm
²
/kg (square centimeters per kilogram). The curves with the greatest slope are those which refer to data taken at 0 C; those for 12 C and 25 C have a lower and approximately the same slope for any given specimen. Additions of calcium titanate and lead titanate, respectively, in small percentages reduced the pressure coefficient to zero. It was observed during the course of this investigation that if the hydrostatic pressure on a ceramic disk is suddenly released, the capacitance of the disk rises rapidly to a maximum and, thereafter, slowly declines to its normal value. The corresponding rise in permittivity may be as much as 20%. Oscillograms were taken and an empirical equation fitted to the experimental data.