High-entropy MAX phases (HE-MAX) represent a groundbreaking advancement in the design of layered materials, offering unprecedented tunability in mechanical, thermal, and electronic properties. Recent studies have demonstrated that the incorporation of five or more transition metals (e.g., Ti, V, Nb, Ta, Mo) into the M-site of MAX phases (Mn+1AXn) results in a configurational entropy exceeding 1.5R (R = gas constant), stabilizing single-phase structures at high temperatures. For instance, a novel HE-MAX phase (Ti0.2V0.2Nb0.2Ta0.2Mo0.2)2AlC exhibits a hardness of 12.3 GPa and a fracture toughness of 8.7 MPa·m^1/2, surpassing traditional MAX phases by 30% and 25%, respectively. These enhancements are attributed to the cocktail effect of multiple elements, which introduces lattice distortion and solid solution strengthening.
The thermal stability of HE-MAX phases has been extensively investigated, revealing remarkable resistance to oxidation and decomposition up to 1500°C in air. A study on (Ti0.25Zr0.25Hf0.25Nb0.25)2AlC demonstrated a weight gain of only 1.2% after 100 hours at 1200°C, compared to 3.8% for Ti3AlC2 under the same conditions. This superior performance is linked to the formation of dense oxide layers enriched with high-entropy oxides, which act as diffusion barriers against oxygen ingress. Additionally, thermal conductivity measurements show a reduction to 15 W/m·K at room temperature due to phonon scattering from lattice distortions, making HE-MAX phases promising candidates for thermal barrier coatings.
Electronic properties of HE-MAX phases have also been tailored through compositional engineering, opening avenues for applications in energy storage and catalysis. For example, (Cr0.2Mo0.2W0.2V0.2Nb0.2)2AlC exhibits an electrical conductivity of 6 × 10^6 S/m and a specific capacitance of 320 F/g at 1 A/g in supercapacitor tests, outperforming conventional MAX phases by a factor of two. Density functional theory (DFT) calculations reveal that the multi-elemental composition induces localized states near the Fermi level, enhancing charge transfer kinetics and catalytic activity for hydrogen evolution reactions with an overpotential as low as 120 mV at 10 mA/cm^2.
The synthesis of HE-MAX phases has been optimized using advanced techniques such as spark plasma sintering (SPS) and reactive hot pressing (RHP). A recent breakthrough achieved phase-pure (Ti0.3Zr0.3Hf0.3Nb0.1)2AlC with a relative density exceeding 98% using SPS at 1600°C for 10 minutes under a pressure of 50 MPa. This rapid synthesis minimizes grain growth and preserves nanoscale features critical for mechanical performance.
Future research directions include exploring non-equimolar compositions and integrating HE-MAX phases into composite materials for multifunctional applications.
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