(Ba,Sr)TiO3 based capacitors with curie temperatures below room temperature. Barium titanate is the most popular dielectric material used for multilayer ceramic capacitors (MLC), but the dielectric constant drops as a function of electric field.

Antiferroelectrics show an increase in the dielectric constant with increase in the bias field due to switching from the antiferroelectric (AFE) to the ferroelectric (FE) phase. The application of an electric field greater than the switching field (Es) causes a phase transition from the orthorhombic to the tetragonal crystal structure. The AFE/FE property of rare earth doped lead zirconate titanate (PZT) compositions has been exploited to fabricate high energy density capacitors for power electronic inverter applications. Dielectric constants >5000 at a bias field of 5 kV/mm have been observed for proprietary lead lanthanum zirconate stannate titanate (PLZST) compositions.

Ceramic capacitors are made from layers of ceramic material with metallic electrodes applied to the surfaces. Such structures can be arranged to place multiple layers either in series or parallel to increase the voltage withstand or capacitance, respectively. High-voltage capacitors in the range of tens of kVdc are also made from bulk ceramic of substantial thickness and are used in pulsed lasers, and other applications.

HIGH ENERGY DENSITY CAPACITOR STATE OF THE ART

Film/foil capacitors can operate at very high ripple current, but have low energy densities. The best commercial high-voltage capacitors achieve an energy density in the range of 0.6 J/cm3. This has apparently been extended to about 1 J/cm3 using modified forms of common capacitors materials such as metallized BOPP and PET film. Other technologies approaching this energy density include soggy foil capacitors with a high dielectric constant polymer coating applied to the nonmetallized side of the foil. Thus, the present state of the art in high-voltage film capacitors appears to be in the range of 1 J/cm3.

Ceramic capacitors can be efficient at lower voltages, where they can be designed to required voltages below the efficient voltage range of film capacitors. Ceramic capacitors can also operate at very high ripple currents, where metalized film capacitors are limited by the end connections. Greater energy density is the main advantage of nonlinear dielectrics, typically 8 to 15 J/cm3 for modified PZT as compared to 1 to 2 J/cm3 for common dielectrics. However, for multilayer ceramic capacitors, there is substantial loss in energy density due to significant electric field derating and packaging. The energy density for a fully packaged AFE/FE capacitor, manufactured by Medronics, is in the 2 J/cm3 range. In addition, since ceramic capacitors cannot recover from a breakdown in the capacitor structure, they must be designed relatively conservatively.

Double layer “supercapacitors” achieve the largest energy densities presently available in capacitors. However, the operating voltage of such capacitors is limited to a



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