Energy Storage in Ceramic Dielectrics

Historically, multilayer ceramic capacitors (MLC's) have not been considered for energy storage applications for two primary reasons. First, physically large ceramic capacitors were very expensive and, second, total energy density obtainable was not nearly so high as in electrolytic capacitor types. More recently, the fabrication technology for MLC's has improved significantly, permitting both significantly higher energy density and significantly lower costs. Simultaneously, in many applications, total energy storage has become smaller, and the secondary requirements of very low effective series resistance and effective series inductance (which, together, determine how efficiently the energy may be stored and recovered) have become more important. It is therefore desirable to reexamine energy storage in ceramics for contemporary commercial and near‐commercial dielectrics. Stored energy is proportional to voltage squared only in the case of paraelectric insulators, because only they have capacitance that is independent of bias voltage. High dielectric constant materials, however, are ferroics (that is ferroelectric and/or antiferroelectric) and display significant variation of effective dielectric constant with bias voltage. The common ferroelectric materials, whether based upon barium titanate or lead manganese niobate (PMN), in the high‐field limit, exhibit an energy storage which increases linearly with bias voltage. Mixed phase, ferroelectric plus antiferroelectric, dielectrics from the lead lanthanum zirconate titanate (PLZT) system, as predicted theoretically, show the best energy density at low to moderate fields. Surprisingly, maximum energy storage is not obtained in high dielectric constant materials but in those materials which display intermediate dielectric constant and the highest ultimate breakdown voltages.