Silicon nitride powder is an important compound of structural ceramics and is used as cutting tips, engine parts, gas turbine components, nozzles for casting, bearing parts, etc. It is highly recommended for such application because of its excellent strength, toughness and thermal shock resistance.
Direct nitridation method of metallic silicon powder
The process for preparing a silicon nitride powder based on the direct nitridation method includes heating a metal powder to a temperature of 1400 to 1500deg C. or higher, and then, after removing the impurities, treating the powder with an inorganic acid such as a mixture of hydrogen fluoride and sulfuric acid to obtain the desired silicon nitride powder.
By controlling the amount of adsorbed acidic inorganic ions to a predetermined level or less, a silicon nitride powder-containing slurry with a low viscosity and a high powder concentration can be easily obtained even if an organic binder for shaping is added. Therefore, the silicon nitride powder in accordance with the present invention can be used as a starting material for commercially producing a sintered body.
Suitable solvent for the slurry preparation
The slurry prepared with the silicon nitride powder can be easily dispersed in an organic solvent, i.e. acetone, methanol, ethanol or the like having a relatively low viscosity. However, if an organic solvent such as liquid paraffin is used, the dispersibility is poor and the slurry can become discontinuous while being drawn by a scraper.
The slurry can be spray dried to obtain a desired powder by the imide decomposition method of silicon diimide, or a magnetic iron removal treatment or the like is effective for removing coarse particles and metallic foreign particles. The sintering properties of the silicon nitride powder in the present invention are improved by these methods, and thus it can be used as a starting material for preparing a sintered body having a desired structure, mechanical properties and other characteristics.
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