Date of Award
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Energy storage--Equipment and supplies., Ferroelectric crystals., Supercapacitors.
The need for optimal energy storage and power quality is rapidly growing. With the increase in energy demand, there is a directly proportional huge dependence on the price of conventional energy. Ultracapacitor devices are currently preferred due to their high energy density capabilities which range between electrochemical double-layer capacitors and electrolytic capacitors. However, these are not viable for aerospace environments as they typically require hermetically sealed containers which increases mass and volume. A solid-state ultracapacitor would be able to avoid the use of toxic and flammable components and has significant advantages over currently used electrochemical and electrolytic devices including long operational life, low maintenance, a wide temperature withstanding ability, high efficiency, high current output, and environmental friendliness. The ferroelectric ultracapacitors (Fe-Ucaps) presented in this work have a novel dielectric medium comprised of coated or doped Barium titanate (BaTiO3) particles. The high permittivity of the medium is due to the internal barrier layer capacitance (IBLC) effect whose key parameter is the grain boundary interface. These Fe-Ucaps were seen to exhibit energy storage values of approximately 350 to 400 nJ/cm3 for sample area 3.264 cm2. Although the IBLC mechanism has been reported in ceramics (CaCu3Ti4O12), to the best of our knowledge its grain boundary morphology and electrical properties have not been studied for perovskite – BaTiO3 crystals. Presently, the process of developing Fe-Ucap devices is an extremely large and complicated process due to a large number of variables involving complex ferroelectrics and fabrication parameters. The cost of producing these devices in-house is also relatively high. The main goal of this study is to streamline the device parameters through electrical characterization and modeling for effective capacitance and energy storage. First, the hysteresis and discharge characteristics of the Fe-Ucaps were studied for a range of test frequencies and voltages. Consequently, the leakage inherent in the hysteresis curves of these devices was observed. The corresponding effects on energy storage and energy efficiency were analyzed. The leakage had to be compensated to obtain an optimal dataset. The characterization variables for energy storage were consolidated into a MATLAB empirical model. The effects of BaTiO3 particle size in the dielectric, particle processing, particle coating type, coating thickness and individual permittivities of the materials forming the dielectric in the Fe-Ucaps on energy storage parameters were analyzed. An equation for the compound permittivity of the dielectric ink was derived in terms of these variables and the ideal ranges of the variables for optimal energy storage have been detailed.
John, Caroline S., "Characterization and modeling of ferroelectric ultracapacitors for energy storage" (2018). Dissertations. 154.