New frontiers in high-entropy transition metal borides and carbides made using microwave plasma-assisted boro/carbothermal reduction*

A novel approach is demonstrated for synthesis of high entropy transition metal borides and carbides using a single heating step enabled by microwave-induced plasma. For borides, an argon-rich plasma allows rapid boro-carbothermal reduction of a consolidated powder mixture containing five metal oxides blended with graphite and boron carbide (B₄C) as reducing agents. Plasma exposure of the powder bed to temperatures as low as 1800 °C for 45min resulted in the hexagonal AlB₂-type structure characteristic of a high-entropy boride: (Zr,Mo,Hf,Ti,Ta)B₂ with an average particle size of 165 nm and with uniform distribution of the five metal cations in the microstructure. In contrast to primarily convection-based (e.g. vacuum furnace) methods that involve slow thermal reduction steps followed by conversion to the single high-entropy phase at elevated temperature, the microwave plasma approach enables rapid heating rates and reduced processing time as a single heating step. The potential roles that microwave heating combined with plasma surface interactions have on thermal reduction to the high entropy phase will be discussed. More recently, we attempted carbothermal reduction via microwave plasma in order to form high entropy transition metal carbide: (Zr,Mo,Hf,Ti,Ta)C₂. This resulted in a solid solution with FCC structure characteristic of the high entropy carbide phase. The benefits of microwave plasma heating to allow highly efficient reduction reactions for synthesis of high entropy borides and carbides in a single heating step are expected to accelerate progress in the field of high entropy ceramic materials used in extreme conditions such as is needed in hypersonic or atmospheric re‐entry vehicles, rocket propulsion, etc.