Strong Influence of Polymer Architecture on the Microstructural Evolution of Hafnium-Alkoxide-Modified Silazanes upon Ceramization

The present study focuses on the synthesis and ceramization of novel hafnium-alkoxide-modified silazanes as well as on their microstructure evolution at high temperatures. The synthesis of hafnia-modified polymer-derived SiCN ceramic nanocomposites is performed via chemical modification of a polysilazane and of a cyclotrisilazane, followed by cross-linking and pyrolysis in argon atmosphere. Spectroscopic investigation (i.e., NMR, FTIR, and Raman) shows that the hafnium alkoxide reacts with the N–H groups of the cyclotrisilazane; in the case of polysilazane, reactions of N–H as well as Si–H groups with the alkoxide are observed. Consequently, scanning and transmission electron microscopy studies reveal that the ceramic nanocomposites obtained from cyclotrisilazane and polysilazane exhibited markedly different microstructures, which is a result of the different reaction pathways of the hafnium alkoxide with cyclotrisilazane and with polysilazane. Furthermore, the two prepared ceramic nanocomposites are unexpectedly found to exhibit extremely different high-temperature behavior with respect to decomposition and crystallization; this essential difference is found to be related to the different distribution of hafnium throughout the ceramic network in the two samples. Thus, the homogeneous distribution of hafnium observed in the polysilazane-derived ceramic leads to an enhanced thermal stability with respect to decomposition, whereas the local enrichment of hafnium within the matrix of the cyclotrisilazane-based sample induces a pronounced decomposition upon annealing at high temperatures. The results indicate that the chemistry and architecture of the precursor has a crucial effect on the microstructure of the resulting ceramic material and consequently on its high-temperature behavio

Carbon Mobility in SiOC/HfO2Ceramic Nanocomposites

Silicon oxycarbide/hafnia (SiOC/HfO2) ceramic nanocomposites were studied by transmission electron microscopy (TEM) upon isothermal annealing at 1300°C for 1–200 h. TEM investigations in conjunction with energy-dispersive X-ray spectroscopy (EDS) analysis revealed a pronounced reduction in the local carbon content in close proximity to internal surfaces. Such small microcracks are a consequence of the polymer-to-ceramic transition and, hence, are commonly formed upon thermal annealing. The profiles of the carbon content between surface and bulk were analyzed employing error functions to yield carbon diffusivities

Investigation of SiCO glasses synthesized with extensive ball milling

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Single-source-precursor synthesis of novel V8C7/SiC(O)-based ceramic nanocomposites

In the present work, novel V8C7/SiC(O) ceramic nanocomposites were synthesized upon thermal transformation of a polymer-derived single-source-precursor, which was obtained by the chemical modification of a polycarbosilane with vanadyl acetylacetonate. High-temperature treatment of the precursor in argon atmosphere first leads to an amorphous SiVOC single-phase ceramic which subsequently undergoes phase-separation, crystallization and finally converts into V8C7/SiC(O) ceramic nanocomposites. Interestingly, the high-temperature stability of V8C7/SiC(O) was shown to strongly depend on the oxygen content present either in the SiC(O) matrix or in the atmosphere during the annealing process. Thus, larger oxygen contents induce a conversion of the V8C7 phase into V5Si3. The specific surface area (SSA) of the obtained nanocomposite powders depends on the processing temperature: The SSA decreases from 64 to 4 m2/g as the pyrolysis temperature increases from 600 to 1300 °C, respectively. Whereas it increases again to ca. 50 m2/g as the sample is exposed to 1700 °C (6 h annealing), due to the evolution of CO. Preliminary results of the catalytic activity of the V8C7/SiC(O)-based materials show that they are active for the decomposition of the ammonia. The maximum ammonia conversion efficiency was found to be 35% at around 650 °C, which is higher than that of the pure vanadium carbide reported in the literature (ca. 13%)