Low-Temperature Synthesis of TaxHf1−xC Solid Solutions via Pectin Gelation: Phase and Morphological Evolution

Ultra-high-temperature ceramics (UHTCs) in the Ta–Hf–C ternary system are of significant interest for extreme aerospace and energy applications due to their melting points near 4000 °C. However, their synthesis typically requires extreme temperatures and pressures. This study reports a pectin-assisted low-temperature route for Ta-rich TaxHf1−xC powder synthesis via carbothermal reduction at 1500 °C. The effect of Ta/Hf molar ratios (2.7/1, 0.9/1, and 0.3/1) on phase evolution, crystallinity, and morphology was systematically investigated. FTIR confirmed the successful formation of homogeneous hybrid organic–inorganic precursors through the chelation of metal ions with pectin functional groups. XRD results demonstrated that the Ta-rich composition (Ta/Hf = 2.7/1) promotes the formation of a high-purity (95.87%) cubic solid solution (lattice parameter a = 4.453 Å) with sharp reflections and improved crystallinity. In contrast, Hf-rich samples exhibited incomplete conversion, leaving unreacted HfO2 and Ta2Hf6O17 oxide phases due to the high thermodynamic stability of hafnia. Microstructural analysis revealed quasi-spherical TaxHf1−xC particles with an average size of approximately 123 nm, together with finer residual oxide particles of about 50 nm. Overall, these results demonstrate that pectin-assisted precursor chemistry is an effective strategy for promoting low-temperature carbide formation in Ta-rich TaxHf1−xC compositions.