Graduate Thesis Or Dissertation
 

Effects of doping, cation stoichiometry, and the processing conditions on the dielectric properties of high-K CaCu₃Ti₄O₁₂ ceramics

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  • Since its discovery, the anomalous dielectric behavior of CaCu₃Ti₄O₁₂ (CCTO) has drawn a great deal of attention for many possible applications in electronic devices. The origin of the giant dielectric constant in CCTO was explained via an internal barrier layer capacitor model, and it has been reported that the dielectric properties of CCTO are sensitive to its microstructure which depends upon the processing conditions. To further explore its unusual dielectric phenomena, the current study focuses on the process-property-structure relationship of the high-K CCTO via doping, cation non-stoichiometry, and sintering conditions. A variety of CCTO pellets were obtained by utilizing the different processing parameters via conventional solid-state synthesis methods. For the doping study, three types of dopants were selected with the variation of doping concentration. The study of undoped CCTO ceramics was carried out by the modification of the CuO and TiO₂ content in CCTO to create the stoichiometric formula CaCu₃+xTi₄+yO₁₂ (x = -0.06, 0, - 0.06; y = 0.08, 0, -0.08). Different processing factors including heating and cooling rates, sintering temperature, and sintering time were applied for the study of stoichiometric CCTO. X-ray diffraction, dielectric measurements, impedance spectroscopy, thermal analysis, and electron microprobe analysis were used to determine the existence of CuO and Cu₂O secondary phases, dielectric constant and loss tangent, electrical resistivity of grains and boundaries, decomposition reactions, and the microstructural changes of CCTO ceramics. It was revealed that the doping method improved the dielectric constant and loss tangent. The similar improvement in dielectric properties was also found in the Cu-deficient and Ti-deficient CCTO. The measurement and characterization results of stoichiometric CCTO clearly indicated that the dielectric properties, evolution of secondary phases, and microstructures were strongly dependent upon the processing parameters. Further, CCTO became Cu-deficient and Ti-excessive regardless of sintering conditions, which provides evidence for its complex behavior created by the reduction/oxidation reactions and the defect chemistry.
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