Boron nitride

Details Boron nitride Boron nitride

1997-03-07

1998-12-15

Derrington, James

Plastic and nonmetallic article shaping or treating: processes

Outside of mold sintering or vitrifying of shaped inorganic...

Using organometallic or organosilicon intermediate

264604, 264626, 264647, 264649, 501 964, C04B 35583, C04B 355835

Patent

active

058492429

DESCRIPTION:

BRIEF SUMMARY
This invention relates to a method of making ceramic articles, especially from boron nitride (BN), and extends to the articles so made. Boron nitride articles are used in the electronic, electric, radio, metallurgical, atomic and rocket industries as high-temperature dielectric electrical insulation components, crucibles, boats for glass synthesis and smelting, metal alloys, monocrystal growing, evaporator-kettles, various devices for melting and pouring metals and alloys, refractory ceramics and structural components for gas turbine engines.
Three main routes to consolidate boron nitride are currently practised: pyrolysis, hot-pressing and reaction-sintering of cold-pressed blanks.
Pyrolytic boron nitride obtained by vapour deposition from boron halides has high purity and density and is of the highest quality, maintaining and in fact steadily increasing its strength when heated from 20.degree. C. to 1400.degree. C. and beyond. However, pyrolysis is a complicated and energy-consuming process, involving expensive manipulations to filter out harmful effluent gases. Also, the maximum practical thickness of pyrolytic boron nitride articles which can be manufactured is about 5 mm.
Hot-pressing--the process of simultaneous moulding and sintering in graphite press-moulds at 1800.degree.-2200.degree. C.--is a cheaper method of producing boron nitride articles. These show high density and strength but contain 10 to 15% of easy-melting phase of boric anhydride and lose virtually all of their structural strength at temperatures above 1000.degree. C. Besides, hot-pressed boron nitride materials also have highly anisotropic properties, although less so than pyrolytic materials.
Clearly it would be desirable to devise a boron nitride material having the strength at high temperatures of pyrolytic boron nitride while retaining the advantages of hot pressed boron nitride of cheapness, low pollution and fabricability of thick samples.
The known method of sintering cold-pressed blanks of boron nitride, in other words through first shaping and then firing, as separate operations, is a step in this direction. The resulting material shows considerable porosity (up to 40%) and a strength of no more than 10-20 MPa, which is obviously insufficient for a whole range of uses.
Thus Rusanova and Gorchakova have noted (Soviet Powder Metallurgy 1989 28(2) 108) that the problem of sintering of boron nitride is directly related to its structure. Boron nitride has a basic structural element which is flat sheets of hexagonal rings with alternating B and N atoms having strong and primarily covalent bonds; cohesion between the sheets occurs primarily as the result of weak intermolecular interaction. The sheets stack to form flat, thin plates up to 50 nm thick with regular angles. In such a structure, only incomplete linking of the particles at the crystal faces is possible and linking at an angle to the hexagon does not in practice occur.
Production of a self-bonded ceramic has become possible as the result of development of methods of synthesis of turbostratic-structure boron nitride, for example as disclosed in U.S. Pat. No. 3,241,919. Turbostratic boron nitride has a semi-amorphous structure in which groups of approximately parallel sheets are shifted at random or are rotated relative to the normal. The above researchers also established that the existence of packing defects in the stacking of the sheets is caused by the presence of oxygen between the sheets. This fact is of particular importance in the technology of shaping articles by explosive compaction of powder, since the article pressed from powder retains, after the explosion, all its oxygen and hence all its potential for sintering, which is realised in the following way: the oxygen chemically linked to the boron in turbostratic boron nitride powder causes a deficit of nitrogen, and when the temperature is increased sufficiently to rupture the B--O bonds, the oxygen leaves the structure, leaving uncompensated bonds with the help of which boron-to-boron "joining" of the hexagon sh

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Reactive Sintering of Boron Nitride-Authors: L N Rusanova, L I Gorchakova, A G Romashin, Translated from Poroshkovaya Metallurgiya, No. 12(204) pp. 52-56, Dec. 1979.
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