Method for producing powder of α-silicon nitride
CROSS-REFERENCE TO THE RELATED APPLICATION 
     This application is a continuation-in-part application of the U.S. patent application Ser. No. 250,175 filed Apr. 2, 1981 now abandoned which in turn is a divisional application of U.S. patent application Ser. No. 121,095 filed Feb. 13, 1980, now U.S. Pat. No. 4,264,565. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a method for producing powder of α-silicon nitride and more particularly to a method of producing powder of α-silicon nitride of high quality in high yield. 
     A sintered product of powder mixture containing powder of silicon nitride as a main raw material such as silicon nitride-yttrium oxide (Si 3  N 4  --Y 2  O 3 ) or silicon nitride-magnesium oxide (Si 3  N 4  --MgO) has high mechanical strength and heat resistance. Therefore, an attempt is made to apply such sintered product as a material of a structural member of, for example, a high temperature gas turbine. A sintered product practically used as a material of a structural member withstanding high temperature and great stress is rigidly demanded to have prominent physical and chemical stability at high temperature. The particularly important thermal and mechanical properties of the sintered product are largely affected by the kinds of the powders and the content of impurities therein. The powder of silicon nitride is preferred to contain as much α-Si 3  N 4  as possible. The following three proceses are generally known for the production of powder of Si 3  N 4  : 
     (1) Nitrogenizing powder of metallic silicon; 
     
         3Si+2N.sub.2 →Si.sub.3 N.sub.4 
    
     (2) Carrying out a gaseous reaction between silicon tetrachloride or silane and ammonia; 
     
         3SiCl.sub.4 +4NH.sub.3 →Si.sub.3 N.sub.4 +12HCl 
    
     (3) Reducing silica powder with carbon in a nitrogen atmosphere 
     
         3SiO.sub.2 +6C+2N.sub.2 →Si.sub.3 N.sub.4 +6CO 
    
     With the above-mentioned process of (1) in which an exothermal reaction takes place, it is necessary to control the exothermal reaction, for example, by finely crushing relatively coarse powder of silicon after nitrogenization. During the step of such fine crushing, impurities are unavoidably carried into powder Si 3  N 4  produced. Therefore, the product has many problems for application as a material of, for example, a structural member for a high temperature gas turbine, though usable as a heat-resisting and corrosion-resisting material of low quality. 
     The process of (2) is suitable for production of a material used as a coating on the surface, of for example, a semiconductor element, but meets with a difficulty for the mass production of an inorganic heat-resistant material. 
     The process of (3) has the drawbacks that it is necessary to use fully purified powders of SiO 2  and carbon as raw materials, and that since the resultant product is a mixture, of for example, α-Si 3  N 4 , β-Si 3  N 4 , silicon oxynitride (Si 2  ON 2 ) and SiC, it is difficult to control a yield of α-Si 3  N 4 . Therefore, the process of (3) has practically problems in raw material and reaction conditions, though having the advantage of causing reaction to proceed by an uncomplicated process. 
     SUMMARY OF THE INVENTION 
     It is accordingly the object of this invention to provide a method for producing powder of α-silicon nitride suitable for use as a material of a structural member resistant to high temperature and great stress, in high yield, without the necessity of applying a complicated process and reaction apparatus. 
     According to this invention, there is provided a method for producing powder of α-silicon nitride, comprising the steps of: 
     adding 0.1 to 2 parts by weight of carbon and 0.005 to 1 part by weight of at least one silicon compound selected from the group consisting of Si 3  N 4 , SiC and Si 2  ON 2  to one part by weight, when converted to SiO 2 , of a liquid alkylchlorosilane that forms a precipitate and HCl by hydrolysis which precipitate is convertible to SiO 2  at a baking temperature of 1300° to 1550° C.; 
     hydrolyzing the resultant mixture; 
     washing the mixture to separate a solid component; and 
     baking the solid component at a temperature of 1300° to 1550° C. in an atmosphere mainly consisting of a nitrogen gas or a gas of a nitrogen compound to effect formation of α-silicon nitride. 
     According to this invention, there is also provided a method for producing powder of α-silicon nitride comprising the steps of: 
     hydrolyzing a liquid alkylchlorosilane to form a precipitate and HCl, the precipitate being convertible to SiO 2  at a baking temperature of 1300° to 1550° C.; 
     washing the hydrolyzed mass to separate a solid component; 
     adding 0.1 to 2 parts by weight of carbon and 0.005 to 1 part by weight of at least one silicon compound selected from the group consisting of Si 3  N 4 , SiC and Si 2  ON 2  to one part by weight, when converted to SiO 2 , of said precipitate; and 
     baking the resultant mixture at a temperature of 1300° to 1550° C. in an atmosphere mainly consisting of a nitrogen gas or a gas of a nitrogen compound to effect formation of α-silicon nitride. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This invention relates to a method for manufacturing powder of α-silicon nitride by using liquid alkylchlorosilane as a raw material. In one aspect of this invention the method includes a process for adding carbon and silicon compound to liquid alkylchlorosilane to hydrolize the mixture. In another aspect of this invention the method includes a process for adding carbon and silicon compound to a solid component obtained by hydrolizing alkylchlorosilane. The former process is hereinafter called &#34;a wet process&#34; and the latter process &#34;a dry process.&#34; 
     The method of this invention will be explained in more detail, focusing attention on the wet process. 
     Liquid silane derivatives used in the method of this invention which produce a precipitate and HCl by hydrolysis and cause SiO 2  to be produced by the baking of said precipitate include alkylchlorosilane, and preferably methyltrichlorosilane which allows for ready hydrolysis. In this case, it is possible to replace 1 part of methyltrichlorosilane by, for example, 20% by weight or less of dimethyldichlorosilane. 
     The methyltrichlorosilane in a mixture of powders of carbon and a silicon compound is hydrolyzed by the following reaction formula: 
     
         CH.sub.3 SiCl.sub.3 +H.sub.2 O→CH.sub.3 SiO.sub.3/2 +HCl 
    
     Namely, HCl is produced together with a precipitate of CH 3  SiO 3/2 . The resultant mixture of these products, powders of carbon and silicon compound is thereafter washed and filtered. The HCl thus produced very effectively eliminates impurities such as Ca and Fe in the powders of carbon and silicon compound powders, thereby ensuring the production of α-Si 3  N 4  of high purity. The CH 3  SiO 3/2  is baked into SiO 2 , and reduced and nitrogenized into Si 3  N 4 . 
     In the silicon industry, methyltrichlorosilane is obtained as a by-product with high purity in large quantity. Therefore, application of methyltrichlorosilane makes it possible to produce α-Si 3  N 4  at low cost. 
     The liquid silane compound, powder of carbon and powder of a silicon compound acting as a nucleating agent which are all used as the starting material in the method of this invention are chosen to have the weight ratio (as measured on the basis of the weight of SiO 2 ) of 1 (as converted to SiO 2 ): 0.1 to 2: 0.005 to 1 or preferably 1: 0.4 to 1: 0.01 to 0.1. The reason for this is as follows. Less than 0.1 part by weight of the carbon powder based on 1 part by weight of SiO 2  causes part of the SiO 2  to remain unreacted and results in the noticeable growth of Si 2  ON 2  and a decrease in production of desired α-Si 3  N 4 . Where the carbon powder is added in a larger amount than 2 parts by weight, then β-Si 3  N 4  is formed, deteriorating the purity of the α-Si 3  N 4  and reducing its yield. 
     Less than 0.005 part by weight (based on 1 part by weight of SiO 2 ) of powder of a silicon compound results in a decline in the function of acting as a nucleating agent which enables powder of α-Si 3  N 4  to be produced with the uniform particle size in high yield. Conversely where the powder of a silicon compound is applied in a larger proportion than 1 part by weight, then said nucleating agent powder displays its property too prominently to produce α-Si 3  N 4  having preferred powder characteristics. 
     For the object of this invention, it is preferred to use the liquid silane compound, powder of carbon and powder of the nucleating agent all having a lower impurity metal than 0.1% by weight. The powder of carbon is preferred to have a particle size of 1 micron or less. 
     The powder of the nucleating agent is desired to have a particle size of 2 microns or less. Powder of Si 3  N 4  used as a nucleating agent is preferred to be of the α-type. However, the Si 3  N 4  may be formed of the β-type, or if necessary, contain other elements such as aluminium or oxygen in solid solution. The nucleating agent may be formed of Si 3  N 4 , SiC, or a silicon oxide nitride series compound alone or a mixture thereof, part of which is substituted by metallic silicon. The nucleating agent should preferably be formed of Si 3  N 4  in particular in order to produce α-Si 3  N 4  of high purity. 
     A solid component derived from the washing and filtration of the product of hydrolysis, for example, a mixture of CH 3  SiO 3/2 , carbon and Si 3  N 4  is baked at a temperature of 1300° to 1550° C. in an atmosphere mainly consisting of a nitrogen gas or a gas of a nitrogen compound. The main constituent of said atmosphere includes N 2  gas, NH 3  gas, a mixture of N 2  and H 2  gases and a mixture of N 2  and any other inert gas. Particularly preferred for the production of high purity α-Si 3  N 4  is an atmosphere which contains N 2  or NH 3  acting as a main reaction gas. 
     Where baking is carried out at a lower temperature than 1300° C., then it is difficult to produce the desired powder of Si 3  N 4 . Where baking is carried out at a higher temperature than 1550° C., then SiC is unnecessarily grown. In either case, it is impossible to produce powder of the desired α-Si 3  N 4  which is adapted to be used as a material of a structural member resistant to high temperature and great stress. If the formation of SiC is permissible, baking may be carried out at a higher temperature than 1550° C. 
     After the above-mentioned baking is carried out at a temperature of 1300° to 1550° C., the final product of α-Si 3  N 4  may be further heat-treated at a temperature of 600° to 800° C. in an oxidizing atmosphere to remove the residual carbon. 
     Where a mixture of a liquid silane derivative, carbon and powder of a nucleating agent is hydrolyzed to produce a precipitate, then said precipitate is supposedly deposited on the surface of particles of carbon powder. A solid component containing the product of hydrolysis is separated by washing. The separated solid component should preferably be heat-treated at a temperature of 200° to 300° C. for dehydration. The product of hydrolysis, for example, CH 3  SiO 3/2  is adsorbed to the surface of particles of carbon powder over a broad contact area, thereby effecting the smooth reduction of SiO 2  by carbon. Since, further, the alkyl group is readily substituted by nitrogen, the aforesaid nitrogenization reaction quickly proceeds. Powder of α-Si 3  N 4  produced by the above-mentioned wet process of the invention contains as small an amount of impurities as less than one-tenth of those contained in the prior art similar product in which SiO 2  is used as a starting material. According to the wet process thus explained, since alkylchlorosilane is hydrolized in the presence of carbon powder and silicon compound powder, the hydrolysis of alkylchlorosilane is impeded by carbon powder and silicon compound powder, making it difficult to effect complete hydrolysis. This may lead to an outflow of an effective Si content. For this reason, an inaccurate blend ratio of the material to be baked is involved, producing a low yield and a disadvantageous result from the industrial viewpoint. According to the dry process of this invention, since alkylchlorosilane alone is beforehand hydrolized, complete hydrolysis is effected, permitting accurate selection of the above-mentioned blend ratio and a high yield. The dry process of this invention is substantially similar to the above-mentioned wet process in respect of, for example, a raw material, hydrolizing conditions, conditions of washing and separating the product of hydrolysis, and baking conditions, except that the hydrolizing and mixing processes are reversed in the case of the wet process. In the method of this invention so explained, a solid mixture before baking, for example, a mixture of CH 3  SiO 3/2 , C and silicon compound are normally granular mixtures. An amount of carbon can be lowered to 0.1 to 0.3 weight percent by grinding the granular mixture and nitrogenizing the resultant powder. For this reason, the decarbonizing process can be simplified and thus α-Si 3  N 4  can be obtained in high yield. Such an effect may be regarded as being ascribed to the fact that, by crushing the granular mixture, the capability of the oxidation/reduction of CH 3  radicals in CH 3  SiO 3/2  is prominently manifested, permitting carbon in CH 3  radicals to be used as part of the reducing carbon. It is preferable that the granular mixture be crushed to a powder particle size of 50  μm or less. 
     As described above, the method of this invention can manufacture from inexpensive raw materials powder of α-Si 3  N 4  of high purity and substantially uniform particle size which has sufficiently excellent properties to be applied as a material of a sintered structural member resistant to high temperature and great stress. 
     Where in particular to solid mixture is crushed before baking, the yield of α-Si 3  N 4  can be greatly improved. 
    
    
     This invention will be more fully understood from examples and controls which follow. 
     EXAMPLE 1 
     1 part by weight (as converted to SiO 2 ) of methyltrichlorosilane, 0.4 part by weight of powder of carbon having a mean particle size of 0.029 micron and 0.01 part by weight of powder of Si 3  N 4  having a mean particle size of 0.3 micron were blended together to provide a liquid-solid mixture. A large amount of pure water was added to the mixture. The mass was hydrolyzed with heat generation well controlled, to produce a precipitate. A solid component containing the precipitate was washed with water to thoroughly remove HCl, followed by drying at 110° C. for 3 hours. The dried solid component was roughly crushed in a ball mill. The crushed mass was left 5 hours in streams of N 2  gas at 1450° C. for reaction. The reacted mass was heat-treated 3 hours in the air at 700° C. to remove the residual carbon, producing powder of Si 3  N 4 . 
     The produced powder of Si 3  N 4  had a high purity. Metal impurities totaled less than 0.05%. The produced powder of Si 3  N 4  had a mean particle size of 1.2 microns and contained 98% of α-Si 3  N 4 . 
     EXAMPLES 2 TO 13 AND CONTROLS 1 TO 4 
     Powder of α-Si 3  N 4  was produced in substantially the same manner as in Example 1, except that the kinds and proportions of raw materials and the conditions of reaction were changed, the results of determining the properties of the sample products being set forth in Table 1 below. 
     Note that in controls 1 and 2 a commercially available SiO 2  powder was used. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                  Reaction conditions                                     
                          Material                                        
                               Properties of produced powder              
                          of reac-  Percentage                            
                                          Percentage                      
                                                Percentage                
       Proportions of     tion Particle                                   
                                    nitrogen                              
                                          content of                      
                                                content of                
Sam-   raw materials                                                      
                  tempera-                                                
                       Time                                               
                          atmos-                                          
                               size content                               
                                          α-Si.sub.3 N.sub.4        
                                                impurities                
ples   SiO.sub.2 *                                                        
           C  Si.sub.3 N.sub.4                                            
                  ture (hr)                                               
                          phere                                           
                               (microns)                                  
                                    (%)   (%)   (%)                       
__________________________________________________________________________
Exam-                                                                     
    1  1   0.4                                                            
              0.01                                                        
                  1450 5  N.sub.2                                         
                               1.2  37.4  98    0.05                      
ples                                                                      
    2  1   &#34;  0.02                                                        
                  &#34;    &#34;  &#34;    1.1  &#34;     96    &#34;                         
    3  1   &#34;  0.05                                                        
                  &#34;    &#34;  &#34;    &#34;    37.5  95    0.06                      
    4  1   &#34;  0.1 &#34;    &#34;  &#34;    1.0  37.8  &#34;     0.05                      
    5  1   0.3                                                            
              0.02                                                        
                  &#34;    &#34;  &#34;    1.5  36.8  &#34;     &#34;                         
    6  1   0.6                                                            
              0.01                                                        
                  &#34;    &#34;  &#34;    0.9  37.5  98    0.04                      
    7  1   1  &#34;   &#34;    &#34;  &#34;    0.8  37.8  98    0.06                      
    8  1   2  &#34;   &#34;    &#34;  &#34;    &#34;    37.5  97    0.03                      
    9  1   &#34;  0.1 &#34;    &#34;  &#34;    0.5  37.8  95    &#34;                         
    10 1   1  0.01                                                        
                  1400 &#34;  &#34;    0.8  37.9  98    0.03                      
    11 1   0.4                                                            
              &#34;   1500 10 &#34;    1.3  37.8  97    0.04                      
    12 1   &#34;  &#34;   1450 5  N.sub.2 + H.sub.2                               
                               1.2  37.4  98    0.03                      
    13 1   0.2                                                            
              0.1 1400 &#34;  N.sub.2                                         
                               1.2  35.7  85    0.06                      
Con-                                                                      
    1  1   4  0.1 &#34;    &#34;  N.sub.2                                         
                               1.1  37.5  96    0.13                      
trols                                                                     
    2  1   0.4                                                            
              0.01                                                        
                  &#34;    &#34;  &#34;    2.0  32.1  95    0.09                      
    3  1   &#34;  0   &#34;    &#34;  &#34;    4.5  37.0  92    0.05                      
    4  1    0.05                                                          
              0.1 &#34;    &#34;  &#34;    1.5  10.5  30    0.03                      
__________________________________________________________________________
 *Proportions of methyltrichlorosilane used as a starting material as     
 converted to SiO.sub.2 obtained after hydrolysis and baking              
 
    
     As apparent from Table 1 above, the samples of powder of Si 3  N 4  produced by the method of this invention all contain a smaller amount of impurities than 0.09%, as large an amount of α-Si 3  N 4  as over 85%, as high a content of nitrogen as 35 to 38%, and as small a particle size as less than 1.5 micron. Therefore, powder of Si 3  N 4  produced by the method of this invention has been proved suitable to be used as a material of a structural member resistant to high temperature and great stress. 
     EXAMPLE 14 
     A greater amount of pure water was added to 1 part by weight (as converted to SiO 2 ) of methyltrichlorosilane with heat generation controlled, to produce a precipitate. After the precipitate was thoroughly washed with water to remove HCl, the precipitate was dried 3 hours at 110° C. to obtain the hydrolysis product of a mean particle size of 0.2 μm. 0.13 part by weight of carbon with a particle size of 0.029 μm and 0.01 part by weight of Si 3  N 4  with a particle size of 0.3 μm were added to the product of hydrolysis. The resultant mixture was introduced in a polyethylene pot, blended 5 hours in a ball mill made of quartz and left in streams of nitrogen 5 hours at 1450° C. Then, the resultant reaction product was heat-treated in the air 3 hours at 700° C. to obtain Si 3  N 4  powder. The properties of Si 3  N 4  powder is shown in Table  2. 
     EXAMPLES 5 TO 8 AND CONTROLS 5 TO 7 
     Si 3  N 4  powder was formed in the same way as in Example 14, except that the kinds and blend ratios of the raw materials, blend conditions and reaction conditions were varied. The properties of the Si 3  N 4  are shown in Table 2. 
     As evident from Table 2 it will be appreciated that according to the method of this invention high-purity α-Si 3  N 4  powder is obtained with above 91% α-Si 3  N 4  and about 36 to 38% nitrogen content. It will also be appreciated that α-Si 3  N 4  is obtained in high yield by grinding the granular mixture to a particle size of below 50 μm before a baking process. 
     
                                           TABLE 2                                 
__________________________________________________________________________
               Blend conditions                                           
                          Reactions   Properties of produced powder       
                       Par-                                               
                          conditions      percent-     amount             
       Raw materi-     ti-                                                
                          tem-        par-                                
                                          age ni-      of                 
       al compo-       cle                                                
                          pera-  at-  ticle                               
                                          trogen                          
                                               α-                   
                                                   amount                 
                                                       impu-              
Sam-   nent ratio  hours                                                  
                       size                                               
                          ture                                            
                             hours                                        
                                 mos- size                                
                                          content                         
                                               Si.sub.3 N.sub.4           
                                                   of SiC                 
                                                       rities             
                                                           yield*         
ples   SiO.sub.2 *                                                        
           C   Si.sub.3 N.sub.4                                           
                   (hr)                                                   
                       (μm)                                            
                          (°C.)                                    
                             (hr)                                         
                                 phere                                    
                                      (μm)                             
                                          (%)  (%) (%) (%) (%)            
__________________________________________________________________________
Exam-                                                                     
    14 1   0.13                                                           
               0.01                                                       
                   5   20 1450                                            
                             5   N.sub.2                                  
                                      1.4 37.3 98  0.30                   
                                                       0.04               
                                                           61             
ples                                                                      
    15 &#34;   &#34;   1   &#34;   &#34;  &#34;  &#34;   &#34;    1.1 37.4 91  0.30                   
                                                       0.04               
                                                           77             
    16 &#34;   &#34;    0.005                                                     
                   &#34;   &#34;  &#34;  &#34;   &#34;    1.7 37.4 99  0.30                   
                                                       0.04               
                                                           77             
    17 &#34;   2   0.01                                                       
                   &#34;   &#34;  &#34;  &#34;   &#34;    1.3 37.7 98  0.28                   
                                                       0.14               
                                                           22             
    18 &#34;   0.09                                                           
               &#34;   &#34;   &#34;  &#34;  &#34;   &#34;    1.3 36.6 98  0.41                   
                                                       0.03               
                                                           63             
    19 &#34;   0.13                                                           
               &#34;   &#34;   &#34;  1380                                            
                             &#34;   &#34;    1.4 36.1 98  0.26                   
                                                       0.04               
                                                           60             
    20 &#34;   &#34;   &#34;   &#34;   &#34;  1500                                            
                             2   &#34;    1.5 37.9 98  0.51                   
                                                       0.04               
                                                           60             
    21 &#34;   &#34;   &#34;   &#34;   &#34;  1450                                            
                             5   N.sub.2 + H.sub.2                        
                                      1.4 37.2 97  0.27                   
                                                       0.04               
                                                           60             
    22 &#34;   &#34;   &#34;   &#34;   &#34;  &#34;  &#34;   NH.sub.3                                 
                                      1.3 38.0 98  0.28                   
                                                       0.04               
                                                           60             
    23 &#34;   &#34;   &#34;   &#34;   &#34;  &#34;  &#34;   N.sub.2 + Ar                             
                                      1.4 37.1 97  0.51                   
                                                       0.04               
                                                           60             
    24 &#34;   0.2 &#34;   &#34;   &#34;  &#34;  &#34;   N.sub.2                                  
                                      1.4 37.2 98  0.31                   
                                                       0.04               
                                                           57             
    25 &#34;   0.40                                                           
               &#34;   &#34;   &#34;  &#34;  &#34;   &#34;    1.4 37.2 98  0.31                   
                                                       0.04               
                                                           50             
    26 &#34;   0.13                                                           
               &#34;   0.5 50 &#34;  &#34;   &#34;    1.4 36.0 97  0.30                   
                                                       0.04               
                                                           58             
    27 &#34;   &#34;   &#34;   3   70 &#34;  &#34;   &#34;    1.4 35.9 92  0.30                   
                                                       0.04               
                                                           52             
    28 &#34;   &#34;   &#34;   10  10 &#34;  &#34;   &#34;    1.3 37.9 98  0.30                   
                                                       0.04               
                                                           60             
Con-                                                                      
    5  1   0.13                                                           
               --  5   20 1450                                            
                             5   N.sub.2                                  
                                      3.7 36.6 98  0.30                   
                                                       0.04               
                                                           60             
trols                                                                     
    6  &#34;   0.05                                                           
               0.01                                                       
                   &#34;   &#34;  &#34;  &#34;   &#34;    1.4 30.1 98  0.30                   
                                                       0.04               
                                                           64             
    7  &#34;   4   &#34;   &#34;   &#34;  &#34;  &#34;   &#34;    1.4 37.8 98  0.31                   
                                                       0.34               
                                                           12             
__________________________________________________________________________
 *Proportions of methyltrichlorosilane used as a starting material as     
 converted to SiO.sub.2 obtained after hydrolysis and baking              
 **(amount of α-Si.sub.3 N.sub.4 powder product/amount of ground    
 powder mixture) × 100                                              
 
    
     EXAMPLES 29 TO 43 
     Experiments were conducted, in the same procedure as in Examples 1 to 13, with and without the process of grinding a granular mixture, the results of which are shown in Table 3. As evident from Table 3 it will be appreciated that in Examples 29 to 41 using the grinding process the yields are above 49, while in Examples 42 and 43 using no grinding process the yields are less than 39. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                Crushing                                                  
                time             Properties of produced power             
                   par-                                                   
                      Reaction time per-                                  
       Raw material                                                       
                   ti-                                                    
                      tem-       par-                                     
                                    centage                               
                                         amount                           
       component   cle                                                    
                      pera-  at- ticle                                    
                                    nitro-                                
                                         of α-                      
                                             amount                       
                                                 impu-                    
Sam-   ratio    time                                                      
                   size                                                   
                      ture                                                
                         hours                                            
                             mos-                                         
                                 size                                     
                                    gen con-                              
                                         Si.sub.3 N.sub.4                 
                                             of SiC                       
                                                 rities                   
                                                     yield*               
ples  SiO.sub.2 *                                                         
          C Si.sub.3 N.sub.4                                              
                (hr)                                                      
                   (μm)                                                
                      (°C.)                                        
                         (hr)                                             
                             phere                                        
                                 (μm)                                  
                                    tent (%)                              
                                         (%) (%) (%) (%)                  
__________________________________________________________________________
Exam-                                                                     
    29                                                                    
      1   0.2                                                             
            0.1 5  20 1400                                                
                         5   N.sub.2                                      
                                 1.2                                      
                                    37.8 95  0.3 0.06                     
                                                     60                   
ples                                                                      
    30                                                                    
      1   0.2                                                             
            0.01                                                          
                &#34;  &#34;  &#34;  &#34;   &#34;   1.4                                      
                                    36.5 95  0.3 0.06                     
                                                     58                   
    31                                                                    
      1   0.2                                                             
            0.005                                                         
                &#34;  &#34;  &#34;  &#34;   &#34;   1.8                                      
                                    34.0 96  0.3 0.06                     
                                                     56                   
    32                                                                    
      1   0.2                                                             
            1   &#34;  &#34;  &#34;  &#34;   &#34;   1.0                                      
                                    38.0 90  0.4 0.06                     
                                                     75                   
    33                                                                    
      1   0.2                                                             
            0.1 &#34;  &#34;  1550                                                
                         &#34;   &#34;   1.1                                      
                                    38.6 96  0.5 0.08                     
                                                     64                   
    34                                                                    
      1   0.3                                                             
            0.1 &#34;  &#34;  1400                                                
                         &#34;   &#34;   1.1                                      
                                    36.0 95  0.2 0.06                     
                                                     57                   
    35                                                                    
      1   0.1                                                             
            0.1 &#34;  &#34;  &#34;  &#34;   &#34;   1.1                                      
                                    36.4 96  0.3 0.04                     
                                                     66                   
    36                                                                    
      1   0.2                                                             
            0.1 3  50 &#34;  &#34;   &#34;   1.2                                      
                                    35.0 95  0.3 0.05                     
                                                     58                   
    37                                                                    
      1   0.2                                                             
            0.1 5  20 1380                                                
                         &#34;   &#34;   1.2                                      
                                    37.5 95  0.3 0.06                     
                                                     60                   
    38                                                                    
      1   0.2                                                             
            0.1 &#34;  &#34;  1450                                                
                         &#34;   &#34;   1.2                                      
                                    37.9 96  0.3 0.07                     
                                                     60                   
    39                                                                    
      1   0.2                                                             
            0.1 &#34;  &#34;  1500                                                
                         2   &#34;   1.2                                      
                                    38.0 97  0.3 0.06                     
                                                     60                   
    40                                                                    
      1   0.2                                                             
            0.1 10 10 1400                                                
                         5   &#34;   0.9                                      
                                    38.4 97  0.1 0.06                     
                                                     61                   
    41                                                                    
      1   0.5                                                             
            0.1 5  20 1400                                                
                         5   N.sub.2                                      
                                 1.1                                      
                                    37.0 96  0.2 0.08                     
                                                     49                   
    42                                                                    
      1   2 0.1 0  200                                                    
                      &#34;  &#34;   &#34;   1.3                                      
                                    36.8 95  0.3 0.13                     
                                                     25                   
    43                                                                    
      1   0.3                                                             
            0.1 0  200                                                    
                      &#34;  &#34;   &#34;   0.9                                      
                                    35.6 93  0.2 0.10                     
                                                     39                   
__________________________________________________________________________
 *Proportions of methyltrichlorosilane used as a starting material as     
 converted to SiO.sub.2 obtained after hydrolysis and baking              
 **(amount of α-Si.sub.3 N.sub.4 powder product/amount of ground    
 powder mixture) × 100