Three crystalline SiC fibers were studied: Tyranno, Hi-Nicalon, and Sylramic.Thermodynamic stability of the SiC fibers was determined by high temperature oxide melt solution calorimetry. Results shed light on the thermodynamic penalty or benefit associated with microstructural modification of the ceramic fibers, and how energetics correlate to mechanical properties. Enthalpies of formation from components (SiC, SiO 2 , Si 3 N 4 , and C, ∆H • f,comp ) for Tyranno, Hi-Nicalon, and Sylramic are −12.05 ± 8.71, −58.75 ± 6.93, and −71.10 ± 8.71 kJ/mol Si, respectively. The microstructure in Sylramic offers the greatest stabilizing effect, thus resulting in its much more exothermic enthalpy of formation relative to elements and crystalline components. In contrast, the microstructure in Tyranno offers the least stabilization. The thermodynamic stability of the fibers increases with increasing mixed bonding (Si bonded to both C and O). From mechanical testing, Young's moduli of Tyranno, Hi-Nicalon, and Sylramic are 112, 205, and 215 GPa, respectively. Greater thermodynamic stability is correlated with a higher Young's modulus.

Accurate chemical analysis of small samples of fine powders in the Si–O–C–H system is challenging. We present a comparison of analysis by X-ray photoelectron spectroscopy (XPS) and combustion analysis, validating XPS as an accurate and simple methodology for Si, C, and O analysis to give bulk and not just surface compositions. The XPS analyses are supported by showing consistency in thermochemical calculations of heats of formation based on high temperature oxide melt solution calorimetry. However, because XPS is not suitable for quantitation of hydrogen, it must be combined with other techniques for samples with substantial H content.

This study presents new experimental data on the thermodynamic stability of SiC(O) and SCN(O) ceramics derived from the pyrolysis of polymeric precursors: SMP‐10 (polycarbosilane), PSZ‐20 (polysilazane), and Durazane‐1800 (polysilazane) at 1200°C. There are close similarities in the structure of the polysilazanes, but they differ in crosslinking temperature. High‐resolution X‐ray photoelectron spectroscopy shows notable differences in the microstructure of all polymer‐derived ceramics (PDCs). The enthalpies of formation (∆H°f, elem) of SiC(O) (from SMP‐10), SCN(O) (from PSZ‐20), and SCN(O) (from Durazane‐1800) are −20 ± 4.63, −78.55 ± 2.32, and −85.09 ± 2.18 kJ/mol, respectively. The PDC derived from Durazane‐1800 displays greatest thermodynamic stability. The results point to increased thermodynamic stabilization with addition of nitrogen to the microstructure of PDCs. Thermodynamic analysis suggests increased thermodynamic drive for forming SiCN(O) microstructures with an increase in the relative amount of SiNxC4−x mixed bonds and a decrease in silica. Overall, enthalpies of formation suggest superior stabilizing effect of SiNxC4−x compared to SiOxC4−x mixed bonds. The results indicate systematic stabilization of SiCN(O) structures with decrease in silicon and oxygen content. The destabilization of PDCs resulting from higher silicon content may reach a plateau at higher concentrations.

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This work systematically investigates the thermodynamic stability of SiaOb(M)cCd structures derived from polymeric precursors incorporating metal fillers: Ta, Nb, and Hf, at 1200 and 1500°C. Structural characterization of the polymer derived ceramics (PDCs) employs X‐ray diffraction, Fourier transform infrared spectroscopy, and X‐ray photoelectron spectroscopy. Enthalpies of formation relative to crystalline components (metal oxide, silica, silicon carbide, and graphite) are obtained from thermodynamic measurements by high temperature oxide melt solution calorimetry. The enthalpies of formation (∆H°f, comp) of Ta‐1200, Hf‐1200, Nb‐1200, Ta‐1500, Hf‐1500, and Nb‐1500 specimens are −137.82 ± 9.72, −256.31 ± 8.97, −82.80 ± 9.82, −182.80 ± 7.85, −292.54 ± 9.38, −224.98 ± 9.60 kJ/mol, respectively. Overall incorporation of Hf results in most thermodynamically stable structures at all synthesis temperatures. SiaOb(M)cCd specimens employing Nb fillers undergo the most stable structural evolution in this temperature range. The results indicate strong thermodynamic drive for carbothermal reduction of metal oxide domains. Incorporation of Ta provides the greatest stabilization of SiO3C mixed bonding environments. Ultimately, the choice of metal filler influences composition, structural evolution, and thermodynamic stability in PDCs.