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Preparation method of boron nitride
Time：
2023-01-13 16:49
At present, the three main methods of consolidated boron nitride used in practice are pyrolysis, hot pressing and reaction sintering of cold-pressed billets.
Pyrolytic boron nitride obtained by vapor deposition of boron halide has high purity, high density and the highest quality. When heated from 20 ℃ to 1400 ℃, it can always maintain and in fact continuously increase its strength. However, pyrolysis is a complex and energy-consuming method, including expensive management to remove harmful waste gas. In addition, the maximum actual thickness of pyrolytic boron nitride products that can be manufactured is about 5mm.
Hot-pressing method - the method of forming and sintering simultaneously at 1800-2200 ℃ in graphite die - is a cheaper method to produce boron nitride products. These products have high density and high strength, but they will actually lose all structural strength when the temperature is above 1000 ℃ because of the fusible phase containing 10-15% boron oxide. In addition, hot-pressed boron nitride materials also have highly anisotropic properties, but are weaker than pyrolytic materials.
It is obviously necessary to invent a kind of boron nitride material, which not only has the strength of pyrolytic boron nitride under high temperature conditions, but also retains the advantages of hot-pressed boron nitride such as economy, low pollution and the manufacturability of thick samples.
The known sintering method of cold-pressed blank of boron nitride, that is to say, as an independent operation, it is a step in this direction by first molding and then sintering. The resulting material shows considerable porosity (up to 40%) and strength not greater than 10-20MPa, which is obviously insufficient for the entire range of use.
Therefore, the sintering problem of boron nitride is directly related to its structure. The basic structural unit of boron nitride is a plane layer composed of hexagonal rings formed by alternating N and B atoms with strong basic covalent bonds; The binding force between layers is mainly the result of weak intermolecular interaction. Many layers are stacked together to form flat sheets, up to 50 nm thick, with regular angles. In such a structure, only the particles on the crystal plane can have incomplete connection, and the hexagonal connection does not actually occur on the angle.
Pyrolytic boron nitride obtained by vapor deposition of boron halide has high purity, high density and the highest quality. When heated from 20 ℃ to 1400 ℃, it can always maintain and in fact continuously increase its strength. However, pyrolysis is a complex and energy-consuming method, including expensive management to remove harmful waste gas. In addition, the maximum actual thickness of pyrolytic boron nitride products that can be manufactured is about 5mm.
Hot-pressing method - the method of forming and sintering simultaneously at 1800-2200 ℃ in graphite die - is a cheaper method to produce boron nitride products. These products have high density and high strength, but they will actually lose all structural strength when the temperature is above 1000 ℃ because of the fusible phase containing 10-15% boron oxide. In addition, hot-pressed boron nitride materials also have highly anisotropic properties, but are weaker than pyrolytic materials.
It is obviously necessary to invent a kind of boron nitride material, which not only has the strength of pyrolytic boron nitride under high temperature conditions, but also retains the advantages of hot-pressed boron nitride such as economy, low pollution and the manufacturability of thick samples.
The known sintering method of cold-pressed blank of boron nitride, that is to say, as an independent operation, it is a step in this direction by first molding and then sintering. The resulting material shows considerable porosity (up to 40%) and strength not greater than 10-20MPa, which is obviously insufficient for the entire range of use.
Therefore, the sintering problem of boron nitride is directly related to its structure. The basic structural unit of boron nitride is a plane layer composed of hexagonal rings formed by alternating N and B atoms with strong basic covalent bonds; The binding force between layers is mainly the result of weak intermolecular interaction. Many layers are stacked together to form flat sheets, up to 50 nm thick, with regular angles. In such a structure, only the particles on the crystal plane can have incomplete connection, and the hexagonal connection does not actually occur on the angle.
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