Entropy-driven phase regulation of high-entropy transition metal oxide and its enhanced high-temperature microwave absorption by in-situ dual phases

High-temperature microwave absorbers are significant for military equipment which experiences severe aerodynamic heat. In this work, high-entropy oxide（HEO）(FexCoNiCrMn)mO_n with excellent high-temperature microwave absorption is studied. Driven by the effect of entropy, the composition of the oxide can be transformed from spinel-type phase（FexCoNiCrMn）3O4 to corundum-type phase（FexCoNiCrMn）2O3 with the increasing content of iron. Only spinel-type or corundum-type structure composes the oxide when x ≤ 3 or x ≥ 5. But in-situ dual phases can coexist when x equals 4 during phase transition. Interestingly, obliged to abundant heterogeneous interfaces and crystal defects in the dual-phase HEO, magnetic property, dielectric polarization, and microwave loss ability are all well enhanced. The Smith chart analysis demonstrates the impedance matching condition is well improved due to the enhanced loss ability. These findings pave a new way for the adjustment of electromagnetic properties of HEO by entropy-driven phase regulation. Meanwhile, the dual-phase absorber can achieve better than 90% absorption in 9.6–12.4 GHz at 800 °C with a thickness of 2.6 mm, a low thermal diffusivity of 0.0038 cm2/s at 900 °C, and excellent high-temperature stability, which indicates it's promising as a high-temperature microwave absorber.