Ceramic dielectric materials for high energy storage capacitors

Abstract

WO 93/11548. Multilayer capacitors comprise layers of a dielectric material interleaved with metallic electrodes. To improve the energy storage of such capacitors when operating in field strengths greater than 20 kV/mm, the dielectric material is a modified ceramic material comprising a base material and an additive. The base material is a ferroelectric ceramic material having (i) its ferroelectric Curie temperature either greater than or lower than the working temperature of the capacitor, (ii) a large value of spontaneous polarisation near 0 °K and (iii) a large Curie constant. The additive forms mixed crystals with the base material without significantly changing its crystal structure, spontaneous polarisation or Curie constant, but reduces its ferroelectric Curie temperature to a value at least 50 °C below the capacitor working temperature. If the base material is barium titanate, the preferred additive is at least 40 atomic per cent strontium titanate.