Enhancing oxidation resistance of multicomponent carbides above 2000 ℃ is critical for their thermal protection applications. For this purpose, novel Nb- and Ta-doped (Hf,Zr,Ti)C multicomponent carbides were designed to improve their oxidation resistance at 2500 ℃. The results demonstrated that Nb and Ta doping reduced the oxidation rate constant by 16.67% and 25.17%, respectively, thereby significantly improving the oxidation resistance of (Hf,Zr,Ti)C. This enhancement was attributed to the changes in oxycarbide composition and distribution within the oxide layer by adding Nb and Ta. Owing to the different oxidation tendencies of the constituent elements, a distinctive structure was formed in which (Hf,Zr)O₂ served as a skeleton, and various oxycarbides were dispersed throughout the oxide layer. The doped Nb and Ta were retained within oxycarbides, retarding the diffusion of oxygen into the lattice. More importantly, the addition of Nb and Ta reduced the size of oxycarbides, decreasing both size and quantity of the pores in the oxide layer and facilitating the formation of a more effective oxygen barrier.

Copper (Cu)-containing single-source precursors (SSPs) for the preparation of SiCuCN-based ceramic nanocomposites were successfully synthesized for the first time using polysilazane (PSZ), copper(II) acetate monohydrate (CuAc), and 2-aminoethanol via nucleophilic substitution reactions at silicon (Si) centers of PSZ. The synthesis process, polymer-to-ceramic transformation, and high-temperature microstructural evolution of the prepared ceramics were characterized. Dielectric properties and electromagnetic wave (EMW) absorbing performance of the ceramics were investigated as well. The results show that the polymer-to-ceramic transformation finishes at ca. 900 ℃, and Cu nanoparticles are homogeneously distributed in a SiCN matrix, forming a SiCN/Cu nanocomposite. After annealing at 1200 ℃, the Cu nanoparticles completely transform into copper silicide (Cu₃Si). Interestingly, the thermal stability of the Cu nanoparticles can be strongly improved by increasing the free carbon content, so that a part of metallic Cu nanoparticles can be detected in the ceramics annealed even at 1300 ℃, forming a SiCN/Cu/Cu₃Si/C nanocomposite. Compared with SiCN, the SiCuCN-based nanocomposites exhibit strongly enhanced dielectric properties, which results in outstanding EMW absorbing performance. The minimum reflection coefficient (RC_min) of the SiCN/Cu/Cu₃Si/C nanocomposites annealed at 1300 ℃ achieves −59.85 dB with a sample thickness of 1.55 mm, and the effective absorption bandwidth (EAB) broadens to 5.55 GHz at 1.45 mm. The enhanced EMW absorbing performance can be attributed to an in situ formed unique network, which was constructed with Cu and Cu₃Si nanoparticles connected by ring-like carbon ribbons within the SiCN matrix.

Multicomponent ultra-high temperature ceramics (UHTCs) are promising candidates for thermal protection materials (TPMs) used in aerospace field. However, finding out desirable compositions from an enormous number of possible compositions remains challenging. Here, through elucidating the role of preferential oxidation in ablation behavior of multicomponent UHTCs via the thermodynamic analysis and experimental verification, the correlation between the composition and ablation performance of multicomponent UHTCs was revealed from the aspect of thermodynamics. We found that the metal components in UHTCs can be thermodynamically divided into preferentially oxidized component (denoted as M_P), which builds up a skeleton in oxide layer, and laggingly oxidized component (denoted as M_L), which fills the oxide skeleton. Meanwhile, a thermodynamically driven gradient in the concentration of M_P and M_L forms in the oxide layer. Based on these findings, a strategy for pre-evaluating the ablation performance of multicomponent UHTCs was developed, which provides a preliminary basis for the composition design of multicomponent UHTCs.