Synthesis and property study of dielectric metal oxides

Abstract

The dielectric materials have drawn impressive attention from scientists and engineers over several decades because of the various industrial applications. Searching new, commercially viable dielectric materials with excellent dielectric properties, such as the colossal permittivity (CP > 1000), a low dielectric loss and high dielectric strength, is a challenge in this field. This dissertation aims to design and develop new dielectric materials by utilizing a new methodology and strategy whilst studying the mechanisms of underlying dielectric property and exploring process conditions for achieving optimal performance. In the first work, synergistic doping heteroatoms strategy is applied to modify dielectric property of stannate materials, ASnO3 (where A can be Ba, Sr, Ca or their combination), and the CP is achieved by introducing a coupled 'difficult dopant' La3+ and 'easy dopant' Nb5+ ions into BaSnO3 and SrSnO3. In the case of BaSnO3, structural analysis and density functional theory (DFT) proved that defect clusters of electron-rich LaO6 and electron-deficient NbO6 octahedra adjacent along [110] direction are introduced into local structure. These defect clusters locally form a large defect dipoles that contribute to a frequency- and temperature-independent CP (~3300) at the temperature range of 10 K to 450K, over the frequency range from 20 Hz to microwave region (~1 GHz). It is noteworthy that such a CP material also presents the ultra-low dielectric loss, 0.04%. This work overcome the bottleneck limitation of the permittivity (mostly 100-200) of traditional microwave dielectric materials, shedding light on developing advanced microwave dielectric materials to minimize the microwave device size. The defect clusters of electron-rich LaO6 and adjacent electron-deficient NbO6 octahedra are also introduced into SrSnO3, which contribute to CP (>2000) and a low dielectric loss of 0.05 from 10 K to 450K. The work of co-doped stannate materials demonstrates the significant effect of co-doping heteroatoms on modifying dielectric performance by inducing defect clusters into parent structure, and paves a way to explore new CP materials. To achieve the high energy density, as an essential component of capacitors, dielectric materials are required to possess not only CP and a low dielectric loss, but also a high dielectric strength. This initiates the further research to enhance the dielectric strength of (In+Nb) co-doped TiO2 (INTO) by inducing In2TiO5 into INTO grain boundaries. The binary-phase CP (>10000) ceramic was synthesized by a one-step solid-state reaction method. Because of In2TiO5 restriction on the movement of weakly constrained electrons and the formation of dielectric breakdown path, the resistivity of binary-phase ceramics is increased, six times higher than INTO without In2TiO5, implying the enhanced dielectric strength. It suggests its promising application in super capacitors and other high-density energy storage devices. In the last part of this thesis, dielectric and electric properties of Na2Ti6O13 (NTO) were studied, where NTO was sandwiched between different types of metal electrodes including symmetric Ag-Ag electrodes, symmetric Pt-Pt electrodes and asymmetric Ag-Pt electrodes. Dielectric properties of NTO is electrode-independent, while different types of electrodes result in different electric performance. NTO with symmetric electrodes exhibits symmetric cyclic voltametric performance, while asymmetric electrodes lead to asymmetric performance. Galvanostatic charge/discharge study implies that the higher work function of the working electrode is, the longer cycle time and higher specific capacity of NTO, ascribed to holding force from the working electrode on Na+. The low energy density of NTO implies the invalidation of NTO working as energy storage devices but does give a hint of sodium battery study. Show more