|Author:||Tse, Mei Yan|
|Title:||Growth and characterization of electron-pinned defect-dipole-induced colossal permittivity thin films|
|Advisors:||Hao, J. H. (AP)|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
Capacitors -- Materials
Energy storage -- Equipment and supplies
|Department:||Department of Applied Physics|
|Pages:||xvi, 134 pages : color illustrations|
|Abstract:||Propelled by the urgent demand for high energy-storage performance of dielectric materials, the exploration on alternative colossal permittivity (CP)-based capacitors has arisen increasing attention. In order to meet the specifications of the current modern electronic systems and applications, they require high dielectric permittivity, low dielectric loss and relatively weak dependence of frequency and temperature. Research progress on several types of CP materials have been studied, still, it is an arduous task to ameliorate these materials in two aspects: their temperature/frequency dependent properties and high dielectric loss. Recently, remarkable dielectric behaviour has been found in (In+Nb) co-doped rutile titanium dioxide (TiO₂). It exhibits giant temperature and frequency independent CP and low dielectric loss. These superior dielectric properties are attributed to electron-pinned defect-dipole model. This thesis studies synthesis and characterization of the new CP behaviour of rutile TiO₂ material co-doped with niobium and erbium, (Er₀.₅Nb₀.₅)xTi₁-xO₂ in details. The purpose of the work is to investigate the relationship among the processing parameters, structural analysis, electrical and dielectric properties of the material for high-energy-density storage applications. The effects of niobium and erbium dopants on the dielectric properties were examined, making effort for further improving the desired functional properties. Excellent dielectric properties were observed in the ceramics. Large dielectric constant (~ 8.6 × 104), sufficiently low dielectric loss (0.04), as well as relatively stable of frequency and temperature behaviours were achieved in a doping level of 5 mol%(Er+Nb) co-doped TiO₂ ceramics. In addition, the up/down photoluminescence (PL) was observed in the visible (Vis) and near-infrared region (NIR) from the material under 980 nm laser diode excitation. The upconversion emissions are ascribed to the energy transfer between Er ions in the excited states. These findings explored the existence of both interesting luminescence and dielectric characteristics in the modified ceramics.|
As there is a trend towards size reduction of microelectronic devices, optimum dielectric properties of the ceramics target were selected for fabricating the composite films. Studies on (Er+Nb) co-doped TiO₂ ceramic were extended to multi-layer-structured ceramic/polymer composite films processed by solution casting and hot pressing method. Herein, suspending surface modified co-doped TiO₂ ceramic powders as 0-dimensional fillers into poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 55/45 mol% as 3-dimensional continuative copolymer matrix resulted in improved dielectric properties and breakdown strength. At room temperature, it is found that 4-layer composite with 50 wt% surface hydroxylated 2.5%(Er+Nb) ceramic fillers co-doped TiO₂/P(VDF-TrFE) achieved integrated performances with a large dielectric constant (300), approximately 19 and 4 times higher than that of pure copolymer and the single layer composite, respectively. The dielectric loss was down to 0.04 at 1 kHz. It displayed the highest energy density of 8.9 J/cm³ at 82 MV/m. These findings are comparable with other composites. Consequently, it is believed that both co-doped TiO₂ ceramics and composite films might be attractive for potential solid-state capacitors and energy-density storage applications.
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