Graduate Thesis Or Dissertation
 

Development of complex perovskite dielectric ceramics for high temperature capacitor applications

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  • Dielectric ceramics designed for capacitor applications for use in extreme environments, such as high temperatures, high voltages, or high-pressure environments, are in great demand. Existing capacitor technology fails to perform within these extreme conditions or such environments. The development of new dielectric ceramic materials for high temperature capacitor applications is of specific interest in this study. Two major factors that influence the performance of dielectric ceramics, including a high insulation resistance and high permittivity were investigated. Complex perovskites based on BaTiO₃ - Bi(Zn₁/₂Ti₁/₂)O₃ (BT-BZT) compounds are the focus of this study due to the existence of a linear dielectric behavior persisting with high permittivity, which is highly suitable for use in demanding capacitor applications. Compositions of complex perovskite based on BT-BZT compounds were fabricated using a solid-state reaction technique. By understanding conduction mechanisms of pseudo-binary BT-BZT, the high insulation resistance dielectric ceramics can be controlled. Pseudo-ternary BaTiO₃ - Bi(Zn₁/₂Ti₁/₂)O₃ - ABO₃ compounds were also explored, where ABO₃ represents BiInO₃, BiScO₃, and NaNbO₃, in order to develop dielectric ceramics that exhibit a temperature- stable permittivity over a wide temperature range, particularly at high temperatures, while maintaining their high permittivity characteristics. The addition of small concentration of dopants and the control of stoichiometry of the BT-BZT compounds significantly affected the insulation behavior. The improved insulation resistance, especially at high temperatures (> 200 °C), can be obtained via the introduction of Ba-vacancies into the compound. The intrinsic conduction mechanism, the electrically homogeneous microstructure, and the presence of Ba-vacancy - O- vacancy (VBa-VO) defect associates as a trap site were observed to govern the high insulation resistance of the Ba-deficient BT-BZT compounds. In contrast, the electrically inhomogeneous microstructure and an oxygen vacancy-based conduction mechanism were the main factors giving rise to low insulation resistance behavior at high temperatures for the stoichiometric BT-BZT compound. The formation of pseudo-ternary BT-BZT-ABO₃ complex perovskite strongly modified the dielectric characteristics to a more temperature-stable permittivity in the case of ABO₃ is BiInO₃ and BiScO₃. With a combination of pseudo-ternary compositions and control of the non-stoichiometry in the BT-BZT-BiScO₃ system, a temperature stable dielectric response was obtained over a broad temperature ranges (T > 100 °C), with a high permittivity (> 1000), and a high insulation resistance at elevated temperatures (> 200 °C). Additionally, the temperature-stable permittivity could be extended to low temperatures (< -50 °C) to satisfy X9R specifications with pseudo- ternary BT-BZT-NaNbO₃ compounds. The results obtained in this study clearly show that compounds based on BT-BZT complex perovskite are promising candidates for future use as a high permittivity dielectric ceramic for high temperature capacitor applications.
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