A wet-chemical approach to perovskite and fluorite-type nanoceramics: synthesis and processing

摘要

In thesis the low-temperature, wet-chemical approach to various functional inorganic oxide materials is described. The main focus of this research is to control the material’s synthesis from liquid precursor to metal oxide powder or thin film; while understanding its formation mechanism. In addition, the synthetic approaches should be compatible with deposition techniques that allow for the upscaling to larger deposited surface areas. The research focuses mainly on the preparation and soft-lithographic patterning of fluorite-type yttria-stabilized zirconia (YSZ) thin films and the synthesis of perovskite-type barium titanate (BTO) and yttrium-doped barium zirconate (BZY) nanocrystalline powders, respectively. Throughout the thesis, sol-gel chemistry is used as a versatile route to prepare nanocrystalline metal oxides. The YSZ thin films are used as thin film electrolyte material in solid oxide fuel cell (SOFC) applications. Due to its high efficiency, SOFCs offer great possibilities as an environmentally friendly energy source. However, the current high operational temperatures limit the economic feasibility and thus its use. The reduction of thin film thickness enables lower operational temperatures, and thus the use of cheaper materials. However, the preparation of a gas-impermeable thin electrolyte film remains a major challenge. A method based on X-ray reflectivity was described to measure the density and to study the densification behaviour of the YSZ thin films on various substrates. Dense films are the key requirement for fuel cell applications, since the fuel and oxidant need to be separated by the electrolyte membrane. Barium titanate is used as a high-k dielectric material in multilayer ceramic capacitors (MLCC), however, the commercially used tape-casting method has reached its limit of downscaling. In order to comply with the current trend of miniaturization, this research is focused on new synthetic routes to yield finer starting powders and compa¬tible deposition techniques. Various instrumental techniques combined with simple computational models were used to investigate the stability and interaction of the precursors and the amorphous-to-crystalline phase transition. Knowledge of the reaction mechanism enabled to expand for the synthesis of proton conducting BZY.