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


ten Elshof, J;






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In Chapter 1, an introduction to the low-temperature wet-chemical approach to various perovskite and fluorite-type materials is presented. The main focus of the research described in this thesis is to control the material’s synthesis route from liquid precursor to ceramic 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 scope of the thesis is presented at the end of the chapter. In Chapter 2 and 3 the reaction mechanism of the low-temperature (23-78 °C) one-pot synthesis of BaTiO3 (BTO) is described. In Chapter 2, the formation of the crystalline phase was studied by investigating the stability and interaction of the precursors with each other and the solvent. Additional computational models could explain the experimental data well, showing that the phenylmethoxy ligand (from the parent alcohol) enhanced the stability of the Ti precursor by steric hindrance, and inhibited the growth of the BTO by forming a dense ‘capping layer’ on the surface of the crystallites. The influence of temperature, water amount, and precursor concentration and stoichiometry on the BTO formation kinetics are described in Chapter 3. Time-resolved small-angle X-ray scattering (SAXS), X-ray diffraction (XRD), and high-resolution transmission electron microscopy (HR-TEM) were used to gain insight in nucleation, growth, and crystallization phenomena. The amount of water in the system (released from the barium hydroxide octahydrate precursor) was found to predominantly determine the rate of hydrolysis and consequently the rate of crystallization. At high precursor concentrations, crystallization followed the apparent zero-th order dehydration kinetics of the Ba precursor. Lower precursor concentrations constituted a decreased water availability, and resulted in impeded crystallization times. Careful adjustments of the total water concentration and precursor stoichiometry enabled to discern three rate-limiting regimes for the crystal phase formation. The knowledge gained from the previous two chapters was used to expand this facile approach for the synthesis of other perovskite ceramics, and the incorporation of dopants. In Chapter 4 the synthesis of proton-conducting yttrium-doped barium zirconate (BZY; BaZrxY1-xO3-δ) is presented. The formation of nano-crystalline BZY powders enhanced the sintering temperature, and dense, phase pure, ceramic bodies were obtained at significantly lower temperatures than necessary for bulk materials. The proton conductivity of the prepared BZY ceramic was mainly determined by the grain boundary contribution to the total resistance. The existence of ultrafine BZY grains after sintering (i.e. lack of grain growth) contributed to an enhanced grain boundary area, which negatively enhanced the proton conduction. A method to measure the thin film density of sol-gel derived YSZ is described in Chapter 5. This facile approach is based on X-ray reflectivity (XRR) in which electron density of the material is determined by the critical angle (of total external reflection). The method describes the mathematical calculation of a so-called pseudo-critical angle. Calibration curves, illustrating the correlation between simulated XRR curves and their corresponding pseudo-critical angles, were used to determine the density of the prepared thin films. The method was validated by determining the thin film density of single crystal substrates and by using a combination of Rutherford backscattering (RBS) and high-resolution scanning electron microscopy (HR-SEM). In Chapter 6, the aforementioned method was applied to investigate the densification behavior of YSZ thin films. In particular, the effect of annealing temperatures and substrate choice on the final density was studied. A final thin film density approximately 95 % was achieved after annealing for only 5 min. Dense thin films are the key requirement for fuel cell applications, since fuel and oxidant need to be separated by the electrolyte membrane. Underlining, once more, the necessity of the method described in Chapter 5. The soft-lithographic patterning of ionically conducting YSZ patterns is described in Chapter 7. A combination of sol-gel chemistry and micromolding in capillaries (MIMIC) was used to obtain (isolated) features with aspects ratios of ~1. The influence of the mold geometry and precursor concentration on the final patterned structures was investigated. At high precursor concentration, the use of a circular mold yielded micrometer-sized isolated features through crack formation during drying, whereas a square mold resulted in a continuous patterned structure. The use of diluted precursor solutions resulted in ring-shaped structures more than an order of magnitude smaller. The soft lithographic patterning of YSZ structures results in an increased electrolyte surface area. The combined knowledge of Chapter 5, 6, and 7 may result in a cost-effective fabrication method for gas-impermeable thin films with increased electrolyte surface area, and present a step towards improved economic feasibility and implementation of solid oxide fuel cell (SOFC) technology. In Chapter 8, general conclusions are drawn. It is stated that sol-gel chemistry can be used as a cheap, and facile approach for the synthesis of high-quality nanoceramics. Although it is often described as an easy-to-use technique, subtle changes often have major consequences. Therefore, a full understanding of relevant synthesis parameters, e.g. the role of modifying ligands, precursor stability, and water concentration, is necessary to fully control the process from molecular precursor to final product. A change in perspective on how sol-gel chemistry is perceived, focusing on possibilities rather than on limitations, may contribute to a larger industrial platform. In addition, various experimental strategies, for future research are described, with a clear focus on the influence of synthesis parameters on the material’s properties.