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Boron nitride manufacturing method

Classification:
Technical
2018/11/06 14:09
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[Abstract]:
At present, the three main methods for solidifying boron nitride in practical use are pyrolysis, hot pressing, and reactive sintering of cold pressed billets.
At present, the three main methods for solidifying boron nitride in practical use are pyrolysis, hot pressing, and reactive sintering of cold pressed billets.
 
The pyrolytic boron nitride obtained by vapor phase deposition of boron halide has high purity and high density, and has the highest quality. When heated from 20 ° C to 1400 ° C, it can always maintain and in fact continuously increase its strength. However, pyrolysis is a complex and energy intensive method that involves expensive management to remove harmful exhaust gases. In addition, the maximum practical thickness of the pyrolytic boron nitride article that can be fabricated is about 5 mm.
 
Hot pressing method - a method of simultaneous molding and sintering at 1800-2200 ° C in a graphite stamp - a relatively inexpensive method for producing boron nitride products. These articles have high density and high strength, but contain fusible phases of 10-15% boron oxide, which will actually lose all structural strength above 1000 °C. In addition, the hot-pressed boron nitride material also has a highly anisotropic property, but is weaker than the pyrolysis material.
 
It is apparent that there is a need to invent a boron nitride material that has both the strength of pyrolytic boron nitride at high temperatures, while retaining the economics of hot-pressed boron nitride, low contamination, and the manufacturability of thick samples.
 
A known sintering method of a cold-pressed billet of boron nitride, that is to say, sintering by first molding as an independent operation, is a step in this direction. The resulting material exhibits a considerable porosity (up to 40%) and a strength of no more than 10-20 MPa, which is clearly insufficient for the entire range of use.
 
For this reason, the problem of sintering of boron nitride is directly related to its structure. The basic structural unit of boron nitride is a planar layer composed of hexagonal rings formed by alternating N and B atoms with strong basic covalent bonds; the bonding force between layers is mainly weak intermolecular The result of the interaction. Many layers are stacked together to form a flat sheet up to 50 nm thick with regular angles. In such a structure, only the particles on the crystal face may be incompletely connected, and the hexagonal connection does not actually occur at the corners.
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