Energy storage at 77 K in multilayer ceramic capacitors

A ceramic material having a large dielectric constant at 77 K, /spl epsi/=8000-12000, has been developed for capacitive energy storage at this temperature. A large matrix of multilayer ceramic capacitors were fabricated using conventional tape-casting methods to optimize the dielectric breakdown strength at 77 K, and measured energy storage values on these capacitors range up to 6 J/cm/sup 3/ at 77 K. An unfused bank of these capacitors was voltage-cycled 10/sup 5/ times at 77 K without failure, and the heating effects during cycling were immeasurably small (i.e., nitrogen boiloff was monitored). An electrocaloric effect on discharge (/spl Delta/T/spl sim/1 K) contributes to the thermal stability. Measurements of the frequency dependence of the dielectric properties of the ceramic at 77 K indicate a fundamental limit of about 8 /spl mu/s for the switching repetition rate. Improved capacitor-manufacturing methods are discussed which can increase the energy density to the 20-30 J/cm/sup 3/ range.