Silicon powder applied in silicon nitride ceramics production
by Lee Dora none
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 α- 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.
Source:http://www.mhcmp.com
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