Polymer‐derived SiOC/HfO2 ceramic nanocomposites were prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n‐butoxide). The ceramization process of the starting materials was investigated using thermal analysis and in situ Fourier‐transformed infrared spectroscopy and mass spectrometry. Furthermore, solid‐state NMR, elemental analysis, powder X‐ray diffraction, and electron microscopy investigations were performed on ceramic materials pyrolyzed at different temperatures ranging from 800° to 1300°C, in order to obtain information about the structural changes and phase evolution thereof. The hafnium alkoxide‐modified precursor was shown to convert into an amorphous single‐phase SixHfyOzCw ceramic at temperatures up to 800°C. By increasing the temperature to 1000°C, amorphous hafnia begins to precipitate throughout the silicon oxycarbide matrix; thus, monodisperse hafnia particles with a diameter of <5 nm are present in the ceramic, indicating a homogeneous nucleation of HfO2. At temperatures ranging from 1100° to 1300°C, crystallization of the hafnia nanoprecipitates as well as phase separation of the SiOC matrix occur. The chemical modification of the preceramic precursor with hafnium alkoxide can be considered as a promising method for the preparation of SiOC/HfO2 nanocomposites with well‐dispersed hafnia nanoparticles.
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 behavior.
In this work, dense monolithic polymer‐derived ceramic nanocomposites (SiOC, SiZrOC, and SiHfOC) were synthesized via hot‐pressing techniques and were evaluated with respect to their compression creep behavior at temperatures beyond 1000°C. The creep rates, stress exponents as well as activation energies were determined. The high‐temperature creep in all materials has been shown to rely on viscous flow. In the quaternary materials (i.e., SiZrOC and SiHfOC), higher creep rates and activation energies were determined as compared to those of monolithic SiOC. The increase in the creep rates upon modification of SiOC with Zr/Hf relies on the significant decrease in the volume fraction of segregated carbon; whereas the increase of the activation energies corresponds to an increase of the size of the silica nanodomains upon Zr/Hf modification. Within this context, a model is proposed, which correlates the phase composition as well as network architecture of the investigated samples with their creep behavior and agrees well with the experimentally determined data.
This study presents first investigations on the high‐temperature stability and microstructure evolution of SiOC/HfO2 ceramic nanocomposites. Polymer‐derived SiOC/HfO2 ceramic nanocomposites have been prepared via chemical modification of a commercially available polysilsesquioxane by hafnium tetra (n‐butoxide). The modified polysilsesquioxane‐based materials were cross‐linked and subsequently pyrolyzed at 1100°C in argon atmosphere to obtain SiOC/HfO2 ceramic nanocomposites. Annealing experiments at temperatures between 1300° and 1600°C were performed and the annealed materials were investigated with respect to chemical composition and microstructure. The ceramic nanocomposites presented here were found to exhibit a remarkably improved thermal stability up to 1600°C in comparison with hafnia‐free silicon oxycarbide. Chemical analysis, X‐ray diffraction, FTIR, and Raman spectroscopy as well as electron microscopy (SEM, TEM) studies revealed that the excellent thermal stability of the SiOC/HfO2 nanocomposites is a consequence of the in situ formation of hafnon (HfSiO4), which represents a concurrent reaction to the carbothermal decomposition of the SiOC matrix. Thus, by the annealing of SiOC/HfO2 materials at 1600°C, novel HfSiO4/SiC/C ceramic nanocomposites can be generated. The results presented emphasize the potential of these materials for application at high temperatures.
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