Synthesis and Phase Evolution of Ultra-High Temperature MC-Type Carbides (M = Hf, Ta, Nb, Zr, Ti) via a Molecular Precursor Approach

In the present work, a series of single-source precursors were prepared via a one-pot synthesis strategy using transition metal chlorides, acetylacetone, and hydroquinone as raw materials. The molecular structure, cross-linking behavior, and polymer-to-ceramic transformation of the obtained precursors were systematically investigated by Fourier-transform infrared spectroscopy and thermogravimetric analysis. The phase composition, phase-transformation temperature, and grain size of the resulting ceramics were characterized by X-ray diffraction combined with Rietveld refinement. The resulting precursors exhibit good solubility in common organic solvents (e.g., ethanol, propanol, and acetone), rendering them suitable for fabricating ultra-high temperature ceramic matrix composites through polymer infiltration and the pyrolysis method. At 1400 °C, the ceramic yields of the TaC, HfC, ZrC, NbC, and TiC precursors were 62.45%, 57.53%, 48.55%, 45.53%, and 30.32%, respectively. After heat treatment at their respective phase-transformation temperatures, the resulting ceramics exhibited grain sizes of carbides in the range of approximately 80–100 nm. The mechanism governing the different phase-transformation temperatures (T) of the derived ceramics, which follow the order TNbC < TTaC < TTiC < THfC < TZrC, was elucidated through combined thermodynamic and kinetic analyses. This synthesis strategy was extended to the family of ultra-high temperature refractory metal carbides with melting points exceeding 3000 °C, demonstrating promising application potential for ultra-high temperature ceramic matrix composites.