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Comment Processing & Handling

Protecting oxidation-prone materials with a crust of salt

By Gerald Ondrey |

High-temperature synthesis of ceramics and other materials normally requires a cost-intensive inerting atmosphere to prevent oxidation. Now, researchers from Forschungszentrum Jülich (Germany; www.fz-juelich.de) have developed a process — the molten salt shielded synthesis/sintering (MS3) process — that used molten salts as both a reaction medium and to protect ceramic powders from oxidizing during high-temperature processing in air. Synthesis temperatures are also reduced, and the final product is a highly pure, fine and loose powder that requires no additional milling.

protecting oxidation-prone materials

Described in a recent issue of Nature Materials, the MS 3 process (diagram) has been used for the synthesis of so-called MAX phases, which are ternary transition metal compounds that combine properties of both ceramics and metals — heat resistant and lightweight like ceramics, yet less brittle, enabling them to be deformed like metals. MAX phases are also the building blocks for MXenes — compounds that are similar to graphene and have “extraordinary” electronic properties. These relatively new materials have potential applications in turbines, aircraft and aerospace and medical implants, but making MAX phase powders at the industrial scale has not been possible before.

MAX phases are produced at temperatures above 1,000°C. With MS3, the precursors are first encapsulated with potassium bromide salt, which melts during the production process. The molten KBr protects the material from oxidation — no inert gas or vacuum is required. The salt also prevents the components from agglomerating, resulting in a fine powder. After processing, the salt can simply be washed off with water.

The process has been shown to produce a variety of MAX phases, such as Ti3 SiC23, Ti2 AlN4, MoAlB5), as well as binary carbides (TiC) and for the sintering of titanium.

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