Sintered body of silicon nitride and method of producing the sam

Details Sintered body of silicon nitride and method of producing the sam Sintered body of silicon nitride and method of producing the sam

1996-08-20

1998-05-26

Group, Karl

Compositions: ceramic

Ceramic compositions

Refractory

501 972, 264647, 264649, 264665, 264683, C04B 35591

Patent

active

057564116

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The present invention relates to a sintered body of silicon nitride (Si.sub.3 N.sub.4) and a method of producing the same.


BACKGROUND ART

Reaction sintering has been known as one of the methods for production of a sintered body of silicon nitride wherein a shaped body of silicon (Si) powder is nitrified and sintered at the same time in a nitrogen atmosphere. However, in order to obtain a sintered body of silicon nitride by causing a shaped body composed of pure Si powder to react with nitrogen, it is necessary that the shaped body be subjected to a prolonged process of nitriding which extends for more than 100 hours as stated in, for example, J. Mater. Sci. 22 (1987), pp. 3041-3086, more particularly, pp. 3074-3075. In this way, reaction sintering has involved a large drawback of low productivity.
Methods for accelerating the process of nitriding through addition of a nitriding promotor, such as Ni, Co, Ti, or Zr, to Si powder have been proposed in Japanese Patent Publication No. 61-38149, Japanese Patent Application Laid-Open No. 5-330921, and Japanese Patent Application Publication No. 5-508612. According to these methods, an amorphous SiO.sub.2 layer on the surface of the Si powder reacts with the nitriding promotor to turn into liquid phase so that the rate of nitrogen diffusion is increased. Thus, it is considered that the process of nitriding is accelerated accordingly. According to JPA Laid-Open No. 5-330921 in particular, time required for nitriding can be reduced to about 8 hours.
However, in these methods which use a nitriding promotor, the coefficient of nitrogen diffusion within the Si.sub.3 N.sub.4 layer formed on the inner side of the liquid phase on the surface of Si powder is insignificant so that any further inward nitrogen diffusion and any further Si.sub.3 N.sub.4 creation are prevented. Therefore, any further reduction in the time required for nitriding could not be expected. Further, as described in JPA Laid-Open No. 5-330921, complex procedures of temperature control are required such that nitriding treatment is carried out in several steps at intervals of 50.degree. C. under high temperature conditions on the order of 1200.degree. to 1450.degree. C. As such, it can hardly be said that such method has good productivity advantage.
Further, sintered bodies of Si.sub.3 N.sub.4 as produced under the reaction sintering techniques using such nitriding promoters have been found unsuitable for use as structural material, because they are not sufficiently compact in texture having a porosity of 4 to 11 vol. %, and because their strength characteristic is rated poor with a three-point bending strength of 460 MPa.
In view of these problems of the prior art, it is an object of the present invention to reduce the time required for the process of nitriding in reaction sintering for production of a sintered body of silicon nitride, thus improving productivity, and to provide a sintered body of silicon nitride having sufficient compactness and high strength which can be produced by reaction sintering.


DISCLOSURE OF THE INVENTION

In order to accomplish the foregoing object, the present invention provides a sintered body of silicon nitride produced by reaction sintering of Si powder, characterized in that its unpaired electron density is 10.sup.15 /cm.sup.3 -10.sup.21 /cm.sup.3.
To produce the sintered body of silicon nitride of the present invention, reaction sintering is carried out using a Si powder having an unpaired electron density of 10.sup.15 /cm.sup.3 -10.sup.20 /cm.sup.3. Si powder having unpaired electrons within such a density range can be obtained by annealing any commercially available Si powder at temperatures of 300.degree.-800.degree. C. in other than nitrogen atmosphere for 1 to 5 hours. The atmosphere for this process is preferably air, hydrogen, argon, their mixtures, or a vacuum of not more than 10 torr.
For the sintered body of silicon nitride according to the invention, a sintering assistant and a nitrogen vacancy producing agent powder are added to a Si po

REFERENCES:
patent: 4687655 (1987-08-01), Hunold et al.
patent: 4716133 (1987-12-01), Horiuchi et al.
patent: 5114888 (1992-05-01), Mizuno et al.
patent: 5126294 (1992-06-01), Hirosaki et al.
patent: 5344634 (1994-09-01), Edler
patent: 5387562 (1995-02-01), Dillinger et al.

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