Method of Making .Alpha. Type Silicon Nitride Powder

1. Field of the Invention

The present invention relates to a method for preparing .alpha. silicon nitride (.alpha.Si.sub.3 N.sub.4) powder, and more particularly to .alpha.Si.sub.3 N.sub.4 powder of high quality which is obtained in a high yield.

2. Description of the Prior Art

It is known that sintered silicon nitride - yttrium oxide or magnesium oxide (Si.sub.3 N.sub.4 -Y.sub.2 O.sub.3 or Si.sub.3 N.sub.4 -MgO) materials possess excellent mechanical strength and heat resistance, and therefore have been used in high temperature gas turbine engines. However, when the conventional Si.sub.3 N.sub.4 sintered products are used in practice as materials which are subjected to high temperatures and high stresses, their physical and chemical stabilities and reliability at high temperatures are absolutely essential requirements. Their thermal and mechanical properties, which are particularly important factors, are greatly affected by the nature of the starting materials and the quantities of impurities which these materials contain. Moreover, with regard to the silicon nitride it is desirable that it should contain as much .alpha.Si.sub.3 N.sub.4 powder as possible. Especially desired is a finely divided .alpha.Si.sub.3 N.sub.4 powder for use in sintering materials.

In the past Si.sub.3 N.sub.4 powder has been synthesized by the following methods.

(1) 3Si + 2N.sub.2 .fwdarw. Si.sub.3 N.sub.4

(2) A vapor phase reaction in which silicon tetrachloride or silane is reacted with ammonia as starting materials

and the like;

(3) A method of nitridizing SiO.sub.2 obtained by reducing silica (SiO.sub.2) with carbon in the following stoichiometric ratio

and the like.

In the case of method (1), the nitridization of Si is an exothermic reaction, and therefore the process must be carefully conducted so as to carefully control the generation of heat. For example, the Si which is commercially selected for the reaction is comparatively coarse-grained powder, and therefore, fine grinding is generally conducted after nitridization. Therefore, the admixture of impurities into the product during the grinding process is unavoidable, and although there is no objection to the use of this material for refractory materials in general, such as firebricks, it is not suitable for high temperature gas turbines.

Also, in the case of reaction (2) the product is suitable, for instance, for the surface coating or semiconductor elements and the like, but it cannot be regarded as suitable for the mass production of inorganic refractory materials.

In the case of reaction (3) thoroughly purified SiO.sub.2 powder and C powder must be used as starting materials, and there is also the disadvantage that the product produced by reacting stoichiometric quantities of SiO.sub.2 and C comprises a mixed system of .alpha.Si.sub.3 N.sub.4, .beta.Si.sub.3 N.sub.4, a silicon oxynitride (Si.sub.2 ON.sub.2), SiC and the like. Moreover, the yield of .alpha.Si.sub.3 N.sub.4 is low. In other words, this reaction system has the advantage that the reaction procedure is relatively easy. On the other hand, however, as described above the yield of .alpha.Si.sub.3 N.sub.4 product is low, and therefore the method is not practially preferred.

A need, therefore, continues to exist for a method by which high quality .alpha. silicon nitride can be manufactured in high yield.

Accordingly, one object of the present invention is to provide a method of manufacturing high quality .alpha. silicon nitride in high yield.

Another object of the invention is to provide a high quality .alpha. silicon nitride.

Still another object of the invention is to provide a high quality .alpha.Si.sub.3 N.sub.4 powder which is suitable for use as refractory material in high temperature and high stress environments.

Briefly, these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained in a method for preparing .alpha. silicon nitride powder by

(a) mixing silica powder, carbon powder and metallic silicon powder;

(b) heating said mixture in a non-oxidizing atmosphere containing nitrogen such that reduction and nitridization reactions occur which produce silicon nitride; and

(c) heating said silicon nitride in an oxidizing atmosphere to effect decarbonization of said silicon nitride.

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

The FIGURE is a microphotograph having a manification of 3,000, and showing the fine particle distribution of .alpha. silicon nitride powder manufactured by the method of the present invention.

The essential feature of the discovery of the present invention is that if in the reduction and nitridization of silica (SiO.sub.2), the quantity of carbon (C) which is used is a quantity somewhat in excess of the amount needed to reduce SiO.sub.2 and if a specified quantity of metallic silicon (Si) is also present and the nitridizing reaction and a heat treatment under an oxidizing atmosphere are conducted at specified temperatures, then .alpha. silicon nitride (.alpha.Si.sub.3 N.sub.4) of high quality can be produced in high yield which is extremely fine-grained.

The present invention involves a method manufacturing .alpha. silicon nitride (.alpha.Si.sub.3 N.sub.4) powder, characterised in that a mixture of powdered reactants in which the ratio proportions of the reactants by weight are 1 part silica (SiO.sub.2) powder, 0.4 to 4 parts carbon (C) powder and 0.01 to 2 parts metallic silicon (Si) powder, preferably 0.1 to 2 parts, is heated and fired at 1350.degree. to 1550.degree. C., preferably 1350.degree.to 1480.degree. C. under a non-oxidizing atmosphere containing nitrogen, wherein reduction and nitridization reactions take place and silicon nitride (Si.sub.3 N.sub.4) is produced. Thereafter, the product obtained is subjected to a heat treatment in an oxidizing atmosphere such as air, preferably at 600.degree. to 700.degree. C.

Suitable sources of carbon for the method of the invention beside carbon powders such as carbon black include hydrocarbon materials. Suitable sources of elemental silicon include organosilicon compounds which degrade when heated to metallic silicon.

In the silica-carbon-metallic silicon (SiO.sub.2 -C-Si) mixture used as the starting material in the present process, the preferred SiO.sub.2 :C:Si weight ratio is selected as 1:0.4 to 4:0.1 to 2 for the following reasons. That is, if less than 0.4 parts of C is used to 1 part of SiO.sub.2, then a large quantity of Si.sub.2 ON.sub.2 is formed and the quantity of the .alpha. silicon nitride (.alpha.-Si.sub.3 N.sub.4) produced is small. On the other hand, if more than 4 parts of C are used per 1 part of SiO.sub.2, then .beta. silicon nitride (.beta.-Si.sub.3 N.sub.4) and silicon carbide (SiC) are produced and as a result the purity and the yield of the .alpha. silicon nitride (.alpha.Si.sub.3 N.sub.4) are reduced. Furthermore, if less than 0.1 part of Si is used per part of SiO.sub.2, this has only a slight affect upon increasing the yield of .alpha.Si.sub.3 N.sub.4. On the other hand, if more than 2 parts of Si are used per part of SiO.sub.2, difficulty is achieved in obtaining fine-grained Si.sub.3 N.sub.4 powder having a grain diameter not greater than 1 .mu. m. Furthermore, grinding processes and the like are necessary and because of this other impurities are admixed into the powder and the preferred silicon nitride powder is not obtained. The SiO.sub.2, C and Si starting materials preferably are of high purity of at least 99%. Moreover, concerning grain sizes, SiO.sub.2 and C preferably have an average grain diameter not greater than 1 .mu.m and Si has an average grain diameter not greater than 10 .mu.m.

In the heating and firing of the SiO.sub.2 -C-Si mixture in the process of the present invention, the atmosphere over the reactants can be N.sub.2, NH.sub.3, N.sub.2 and hydrogen (H.sub.2), N.sub.2 and an inert gas such as Ar, He or the like or some other gaseous component such as CO, but the main reaction gas constituent must be at least one of N.sub.2 or NH.sub.3. The reason for this is that it has been confirmed experimentally that one of these gaseous materials has a great effect on the production of highly pure .alpha.Si.sub.3 N.sub.4. On the other hand, the heating and firing temperatures of the silicon nitride under this atmosphere in which the main reaction gas is N.sub.2 or NH.sub.3 is selected within the range from 1350.degree. to 1550.degree. C., preferably 1350.degree. to 1480.degree. C. The reason for this is that if the temperature employed is less than 1350.degree. C., Si.sub.3 N.sub.4 is not formed readily. If the temperature exceeds the upper limit, formation of SiC occurs, and the required .alpha.Si.sub.3 N.sub.4 powder suitable for materials which are to be used under high temperatures and high stresses can not be obtained.

Also, after the silicon nitride product is heated and fired in an atmosphere in which the main reaction gas is N.sub.2 or the like, a heat treatment under an oxidizing atmosphere is conducted for the purpose of removing the residual carbon. The temperature of this treatment is selected within the range of 600.degree. to 800.degree. C., preferably 600.degree. to 700.degree. C. The reason for this is that if in the removal of the carbon from the reaction mixture which has not reacted and is therefore still present, the heat treatment is conducted over the abovementioned temperature range, oxidization of the Si.sub.3 N.sub.4 that has been produced results, and the required .alpha.Si.sub.3 N.sub.4 powder is not obtained.

If, as described above, the reduction and nitridization of SiO.sub.2 by the present invention is employed, in which an excess of carbon far beyond the stoichiometric amount is used and, on the other hand, in particular a specific quantity of Si is also present, then the reduction of the SiO.sub.2 is substantially promoted and also the nitridization of the Si proceeds smoothly and .alpha.Si.sub.3 N.sub.4 powder of high quality, containing a large quantity of Si.sub.3 N.sub.4, is obtained in good yield. When the method of the present invention is used, .alpha.Si.sub.3 N.sub.4 powder suitable for making a Si.sub.3 N.sub.4 sintered materials which are required to withstand high temperatures and high stresses, is easily obtained. The reason for this is believed to be as follow. The primary reaction which occurs is the reduction of silica by carbon SiO.sub.2 + C .fwdarw. SiO + CO. This reaction is a solid phase reaction, and when the C/SiO.sub.2 ratio is high, the reaction becomes relatively rapid and the SiO which is produced reacts with the N.sub.2 or NH.sub.3 more easily. In this reaction the SiO and the N.sub.2 or NH.sub.3 may be present in the vapor state, and therefore it may be said that the proportion of carbon that is present governs the reduction and nitridization reactions of the SiO. In this instance, if the quantity of carbon is the stoichiometric amount of only slightly in excess of this amount, Si.sub.2 ON.sub.2 is formed, and the conversion of Si.sub.2 ON.sub.2 to .alpha.Si.sub.3 N.sub.4 becomes extremely difficult. However, as described above, the quantity of carbon is greatly in excess compared to the stoichiometric amount, and it appears that for this reason the formation of Si.sub.2 ON.sub.2 is inhibited and .alpha.Si.sub.3 N.sub.4 is easily formed.

The presence of an excessive quantity of carbon gives rise to the smooth production of .alpha.Si.sub.3 N.sub.4. However, on the other hand, the presence of carbon also results in the formation and admixture of SiC with the product and the presence of other impurities in the product. Moreover, the .alpha.Si.sub.3 N.sub.4 content of the product becomes relatively low. But in the present invention, a specified quantity of Si powder is also present in the reaction system. The Si powder itself is nitridized, but the reaction of SiO.sub.2 + Si .fwdarw. SiO proceeds to a greater extent and because of this, SiO vapor is easily formed, and therefore the formation of .alpha.Si.sub.3 N.sub.4 by the subsequent SiO + C + N.sub.2 .fwdarw. .alpha.Si.sub.3 N.sub.4 reaction proceeds easily. As a result, the required .alpha.Si.sub.3 N.sub.4 is obtained as a fine grained powder. It appears that for this reason the content and yield of the silicon nitride are vastly improved, and the formation of SiC and the formation and admixture of other impurities are inhibited. In particular, the fact that the formation of SiO is promoted accelerates the subsequent formation of .alpha.Si.sub.3 N.sub.4, and therefore a uniformly shaped fine-grained powder is synthesized as is evident from the microphotograph of the FIGURE. It has been found that a highly realiable, sintered material is obtained by sintering this material.

Thus, in the present invention an .alpha.Si.sub.3 N.sub.4 powder is obtained which is of high quality and which contains a high content of .alpha.Si.sub.3 N.sub.4. Moreover, the nitride contains only small quantities of SiC and other impurities. Thus, the method of the present invention is suitable for making Si.sub.3 N.sub.4 powder which is suitable as a raw material for the preparation of sintered structural materials which are required to withstand high temperature and high stresses, for instance, for gas turbine engine components.

Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

A SiO.sub.2 powder having an average grain diameter of 13 m.mu.m, C (carbon black) powder having an average grain diamter of 29 m.mu.m and Si powder having an average grain diameter of 1.8 .mu.m were mixed in the proportions (parts by weight) shown in Table 1 to form 19 kinds of powdered materials (including the reference examples).

The powdered mixtures were heated and fired at 1350.degree. to 1500.degree. C. for 2 to 5 hours under a N.sub.2, N.sub.2 - H.sub.2, N.sub.2 - Ar or NH.sub.3 atmosphere, and then subjected to a heat treatment under an air atmosphere at 700.degree. C. for 8 hours, and Si.sub.3 N.sub.4 containing powders were thus obtained. For each of the Si.sub.3 N.sub.4 powders thus obtained, the average particle size, the nitrogen content (weight %), the .alpha.Si.sub.3 N.sub.4 content (weight %), which was confirmed by X-ray diffraction patterns, the SiC content (weight %) and the quantities of Si and other metallic impurities (weight %) were determined in each case and the results are all shown in Table 1. In Table 1, specimens 1 to 14 represent embodiments of the present invention, and specimens a to g represent reference examples.

Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.