Ta-containing MAX phases are promising high-temperature structural ceramics, but high density and concurrent optimization of strength, fracture toughness, and elevated-temperature property retention remains challenging. Entropy engineering can tailor M-site chemistry; however, it remains unclear how M-site combinations control the strength–toughness balance and elevated-temperature retention. Here, (TaNbV)₂AlC, (TaTiNb)₂AlC, (TaTiV)₂AlC, and (TaTiNbV)₂AlC were synthesized by vacuum hot pressing and investigated through phase and microstructural characterization, density functional theory (DFT)-based crystal orbital Hamilton population (COHP) analysis, Deep Potential molecular dynamics (DP-MD), and mechanics-based bridging models. Characterization confirmed a layered M₂AlC-type framework and showed no obvious M-site elemental segregation within the examined regions. Compared with Ta₂AlC, all medium-entropy ceramics showed reduced density and improved room-temperature flexural strength and fracture toughness, with (TaNbV)₂AlC achieving 578 MPa and 8.5 MPa·m^1/2. COHP revealed stronger M–C and M–Al bonding in Ta/Nb-centered environments than in Ti- and V-centered environments, explaining the flexural-strength grouping. However, fracture toughness required path-dependent basal-cleavage descriptors beyond average bond strength. A validated DP-MD model demonstrated structural retention up to 1500 K under inert (oxygen-free) conditions and within the simulation time scale, intrinsic tensile softening, and composition-dependent cleavage-work retention. Qualitative-to-semiquantitative bridging projections indicate that (TaNbV)₂AlC provides the highest absolute strength and toughness, whereas (TaTiNbV)₂AlC has the best elevated-temperature retention. These results reveal that Ta/Nb-rich bonding and basal-cleavage resistance act as complementary, composition-dependent mechanisms governing the strength–toughness balance and its elevated-temperature retention in this Ta-based 211 MAX series, providing a mechanistic basis for designing entropy-engineered MAX ceramics with room-temperature performance and elevated-temperature property retention as distinct targets.

Carbon fiber composites hold significant promise as electromagnetic wave (EMW)-absorbing materials. However, balancing lightweight materials with excellent mechanical properties, low thermal conductivity, and EMW absorption for multifunctional applications remains challenging. Herein, a novel hydrothermal carbon (HC)-coated three-dimensional (3D) needled carbon fiber-reinforced silicon–boron carbonitride (C_f/HC–SiBCN) composite was developed via an optimized precursor infiltration and pyrolysis (PIP) process combined with impregnation–filtration. By adjusting the precursor concentration and number of impregnation‒filtration cycles, a hierarchical C_f/HC–SiBCN composite with the density of 0.32 g·cm⁻³ was obtained, which exhibited remarkable mechanical properties, including flexural strengths of 14.75±0.43 MPa (xy-direction) and 14.45±0.66 MPa (z-direction), along with a compressive strength of 9.36±0.20 MPa (z-direction). It also demonstrated low thermal conductivity (0.145 W·m⁻¹·K⁻¹) and exceptional EMW absorption, with a minimum reflection loss (RL_min) of −58.13 dB and an effective absorption bandwidth (EAB) of 7.38 GHz. Owing to their combination of lightweight, enhanced mechanical properties, low thermal conductivity, and superior EMW absorption capabilities, C_f/HC–SiBCN composites are highly suitable for multifunctional applications.

As a new category of ultra-high-temperature ceramics (UHTCs), multi-anionic high-entropy (HE) carbonitride UHTCs are expected to have better comprehensive performance than conventional UHTCs. However, how to realize the green and low-cost synthesis of high-quality multi-anionic HE carbonitride UHTC powders and prepare bulk ceramics with excellent mechanical properties still faces great challenges. In this work, a green, low-cost, and controllable preparation process of (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders is achieved by sol–gel combined with the carbothermal reduction/nitridation method for the first time. The as-synthesized (Ti_0.2Zr_0.2Hf_0.2Nb_0.2Ta_0.2)C_xN_1−x powders possess high compositional uniformity and controllable particle size. In addition, the obtained bulk ceramics prepared at 1800 ℃ exhibit superior fracture toughness (K_IC) of 5.39± 0.16 MPa·m^1/2 and high nanohardness of 35.75±1.23 GPa, elastic modulus (E) of 566.70±8.68 GPa, and flexural strength of 487±41 MPa. This study provides a feasible strategy for preparing the high-performance HE carbonitride ceramics in a more environmentally friendly and economical manner.