Nonnenmacher, Katharina (2016)
Microstructure Characterization of Hafnium-Modified Polymer-Derived SiOC and SiCN Ceramics.
Technische Universität Darmstadt
Dissertation, Erstveröffentlichung

Kurzbeschreibung (Abstract)

To feature the development of innovative technologies and efficient usage of conventional energy sources, the application of advanced structural und functional ceramics is indispensable. In search of advanced ceramic materials with high thermo-mechanical performance for high temperature structural applications, research activities in Materials Science have explored thermolytic decomposition (pyrolysis) of organosilicon polymers as a novel process for the manufacturing of nanostructured silicon-based ceramic materials (polymer-derived ceramics, PDCs). Starting from synthetic polymeric organosilicon compounds, cross-linking and a subsequent annealing process, which leads to ceramization, yield the conversion into an amorphous silicon-based ceramic network. The underlying synthesis approach is a “bottom-up” approach which aims at linking organic components to inorganic structures. One of the key questions related to PDCs is whether nano- and microstructure can be tailored in order to achieve attractive structural and functional properties inaccessible by conventional powder-based sintered ceramics. The motivation of the present work was to gather a thorough understanding of the thermal stability of polymer-derived hafnium-modified silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) ceramics. Starting from as-received polymer-derived hafnium-modified SiOC- and SiCN-based ceramic nanocomposites, isothermal annealing experiments were carried out under the same conditions for both material systems. Upon annealing, the development of the local microstructure of both materials was investigated via conventional transmission electron microscopy (CTEM) in conjunction with energy-dispersive X-ray spectrometry (EDS) allowing for parallel microchemical analysis. Placing particular emphasis on the local variation in average hafnia (HfO2) crystallite size observed in a preliminary study using CTEM, gradual variations in average size were typically observed in close proximity to cracks and open pore channels in both materials and are shown to be related to an outward zoning to a carbon-depleted (and nitrogen-depleted) matrix composition. In both materials, HfO2 crystallite growth is time-dependent and proceeds via a coarsening process. The theory derived by Lifshitz, Slyozov and Wagner (LSW theory) for volume diffusion-controlled coarsening, which is based on thermodynamical considerations according to the Gibbs-Thomson equation, was applied to the observed particle coarsening upon annealing in order to calculate the volume diffusion coefficient of hafnium in the host matrix. As it turned out, particle coarsening proceeds at a higher rate in close proximity to internal surfaces as compared to regions closer towards the bulk, which is typically a result of the increased hafnium volume diffusivity in the carbon-depleted (and nitrogen-depleted) matrix within the surface-near regions. As a general trend, in close proximity to internal surfaces, the volume diffusion coefficient of hafnium in the matrix is by three orders of magnitude larger as compared to values derived from regions closer towards the bulk. In the case of the SiOC-based material, bulk regions generally show a narrow particle size distribution of nano-sized HfO2 precipitates. In contrast, HfO2 precipitates were not observed via TEM, even at high resolution, in the bulk volume of the SiCN-based ceramic samples. Here, the local nitrogen content of the bulk matrix is assumed to be beyond a critical value, and may be dominant in determining HfO2 precipitation since the ability of hafnium to diffuse through the SiCN matrix is diminished. Furthermore, for gathering a better understanding of carbon transport in surface-near areas of the ceramic samples, diffusion modelling of measured carbon profiles using an analytical method was carried out for the SiOC-based system. Our modelling results suggest that carbon diffusion stagnates in surface-near areas after surface crystallization of cristobalite since it acts as a diffusion barrier for carbon, according to diffusion data reported in the literature. This surface modification can also be applied to all other PDC material systems, considering their high-temperature stability, and is expected to lend inadequate overall thermo-mechanical and thermo-chemical properties to PDC materials.

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