There are three main families:
– Alkaline metal or alkaline-earth carbides;
– Interstitial carbides with transition metals Ti, Zr, Hf, V, Nb, Ta, etc.
– Covalent macromolecular carbides with B, Al, Si.
Only compounds of the two last families are used for preparing ceramics. These products are synthesized industrially according to various methods, the choice of the process depending on certain criteria: purity, crystalline state, morphology of the powder.
?Solid-Solid Reaction
Carbides can be obtained by direct synthesis at high temperature by mixing powders compacted beforehand:
After the carburization stage, the products are ground. This process, which is the oldest, does not yield fine powders. The carboreduction of oxides is the most developed production method:
Grinding is also necessary, followed by a chemical treatment to eliminate oxide traces that very often play a harmful role during sintering. Thus, for the second example, after washing with hydrofluoric acid, the powder contains 98-99% of SiC, with iron as the main impurity.
Sol-gel processes developed recently, in particular in the synthesis of carbosiloxanes, yield ultrafine amorphous powders. The distribution on a nanometric scale of the Si, C, and O elements increases their reactivity.
Amorphous silicon carbide can be synthesized between 1,000 and 1,300°C. It then crystallizes into ?-SiC at about 1,450°C.
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