Silicon powder applied in silicon nitride ceramics production
Because the high melting point of silicon nitride ceramics, for metal and the oxide melt have considerable high temperature stability, increasingly used in various fields of thermal, such as ceramic firing batts, roll bars, and high-temperature bearings, turbine blades.
Silicon nitride ceramics used silicon powder as the raw material, used reaction sintering and hot press two kinds of process methods. The first method is making the silicon powder into green-pressing, then make it initially nitride with nitrogen at 1200 high temperature. After the semifinished products through preliminary nitriding repair and processing, then placed in a high temperature of 1350-1450 for the second nitride, thereby producing silicon nitride ceramic. This method is called reaction sintering silicon nitride. The ceramic device produced by this method has a characteristic, not shrink, high precision. Using good crystalline silicon block, wet ball mill, used alcohol as grinding media, grinding to less than 0.07mm. Then purify raw materials, ingredients made into adobe. Molding method can use casting method, compression molding, hot pressing injection method or isostatic compaction methods. When molding should make the biscuit density reaches a certain requirement.
Biscuit first through nitriding treatment in the nitriding furnace, can be used molybdenum wire furnace or molybdenum disilicide rod furnace. Furnace should be sealed tight, to ensure the vacuum and used safety. Silicon powder and nitrogen begin to react at 970-1000, and the reaction rate increases with increasing temperature. However, if the temperature quickly rises above the silicon melting point, then base will collapse because the silicon melting. So it must be pre-nitriding below the melting point.
Nitriding temperature can be lower than the silicon melting point 1420 and above the silicon melting point grading keep warm nitride. One method is nitrided at 1250 for a certain time, make the silicon powder surface form an interleaved ±- silicon nitride single crystal, fills the pores between the silicon particles, the green body has a certain strength. Then at 1350-1400 nitriding for a long time, through the reaction of nitrogen and silicon powder, the original network structure silicon nitride continue to develop, dense. Another method is after keep warm and nitrided at 1250 for a certain time, nitriding at 1450, then silica melt into a liquid, the reaction rate is fast, the resulting silicon nitride are hardness, high density particles, dispersed within the generated network-like silicon nitride which formed in low temperature.
The second method is first nitride silicon powder into silicon nitride powder, then add a small amount of magnesium oxide, placed in the mold, at a high temperature of 1700-1800 hot pressing. The specific method is nitriding silicon powder in the nitriding furnace, pulverized after obtain the silicon nitride, as the hot-press material. In order to improve the purity of silicon nitride powder, can be nitrided first, and then pulverized purification. Do second nitriding, fine grinding, for hot products.
Add about 5% magnesium oxide or magnesium compounds and other additives into silicon nitride powder, alcohol as medium, wet mixing in a ball mill sufficient. The hot die is graphite, coated with a layer of boron nitride powder in the die wall, put the silicon nitride mixture into the mold, at 1750-1850, at 25-50Mpa hot furnace pressing sintering.
According to metal powder supplier, silicon nitride is currently used as a horizontal continuous casting separation ring using excellent materials. Also can be used as a crucible, conveying liquid metal pipes, pumps, valves, thermocouple protection tube, slide gate, high temperature products, ceramic firing batts, roller kiln rollers, and high-temperature ceramic bearings, ceramic turbine leaves, etc. for zinc refining furnace, is a widely used, great potential for developing high-temperature structural and refractory products.
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