Molten-salt reaction media enable high-temperature carbide formation in air

High-temperature formation of carbide ceramics is commonly conducted under inert atmosphere to avoid oxidation. Molten alkali-metal halide salts can kinetically suppress oxidation by isolating reactants within a molten-salt reaction medium, enabling carbide formation under an ambient atmosphere. However, the accessible temperature window is limited by salt volatility, which reduces melt coverage and reintroduces oxidation at elevated temperatures. Here, we extend molten-salt processing in air to 1500 °C using a binary KCl-CaCl 2 molten-halide medium. While CaCl 2 has high O 2- solubility, blending it with KCl reduces effective oxide-ion transport and availability at the reaction interface, thereby kinetically suppressing oxidation and enabling synthesis of oxidation-prone materials at elevated temperatures. We demonstrate this concept by synthesizing ternary MAX-phase carbides (Nb 2 AlC, V 4 AlC 3 , Ta 2 AlC) directly in air.