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. Molten halide salts act as chemically active reaction media that suppress oxidation and enable high-temperature carbide synthesis directly in air. A KCl–CaCl 2 melt provides a wide reaction window up to 1500 °C, allowing formation of ternary MAX-phase carbides (M n+1 AX n ) without inert atmospheres, demonstrating atmosphere-tolerant high-temperature materials chemistry. • High-temperature carbide synthesis achieved directly in air • KCl–CaCl 2 molten halides suppress oxidation up to 1500 °C • Ternary MAX-phase carbides formed without inert atmosphere in air • Oxide-ion solubility in molten salt governs carbide phase synthesis • KCl-CaCl 2 based molten salt expands accessible reaction window for carbides