Polymer-derived Si–Hf–Ta–O–C ceramics: from precursor chemistry to thermal transport

A novel Si–Hf–Ta–O–C polymer-derived ceramic system was developed via alkoxide modification of polymethylsiloxane, enabling the fabrication of monolithic ceramics with controlled composition and porosity. The influence of Hf and Ta incorporation on network structure, phase evolution, and thermal transport was systematically investigated. Elemental analysis reveals that Hf incorporation increases the free-carbon fraction compared to SiOC, whereas additional Ta incorporation further enhances free-carbon formation within the amorphous Si–O–C network. At 1250 °C, the modified compositions showed the onset of HfO$_2$-related crystallization, followed by the formation of HfO$_2$-based phases and subsequent transformation to carbide i.e. (Hf, Ta)C at 1550 °C via carbothermal reduction. Despite comparable porosity (ca. 11–12 vol%), the thermal conductivity varies between compositions, reaching ≈0.68 W m$^{−1}$ K$^{−1}$ for SiOC-Hf and ≈0.85 W m$^{−1}$ K$^{−1}$ for SiOC-Hf$_3$Ta at 900 °C, indicating that in these materials the content of segregated carbon contributes to the observed thermal transport behavior. These are among the lowest values for thermal conductivity recorded for silicon-oxycarbide-related materials, suggesting high potential thereof for thermal insulation applications.