JPH0692733A - Production of silicon carbide heating element - Google Patents

Abstract

PURPOSE:To ensure a large amt. of nitrogen solid solution by an easy process without deteriorating the quality or strength of material and to efficiently produce a silicon carbide heating element having low specific resistance. CONSTITUTION:Silicon carbide powder as starting material is mixed with 1-30wt.% silicon nitride powder and further mixed with 5-30wt.% carbon powder basing on the amt. of the silicon nitride powder. The resulting mixture is compacted into a prescribed shape with a binder and sintered at 1,900-2,400 deg.C in an atmosphere of gaseous nitrogen.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a silicon carbide heating element having a large amount of nitrogen solid solution and a low electric resistivity.

[0002]

2. Description of the Related Art Silicon carbide is a compound semiconductor having good conductivity, and has excellent thermal and chemical stability in terms of material. Therefore, it has long been useful as a heating element for a high temperature electric furnace. ing. Generally, a silicon carbide heating element is manufactured by mixing a raw material powder of silicon carbide with an organic binder, shaping it into a predetermined shape, and then performing a sintering treatment to convert the structure into recrystallized SiC. With a wide relationship of about 3 eV, it is necessary to lower the electric resistance to a level at which it can carry electricity. For this purpose, a Group 3 element or 5
Means for forming a solid solution of a group element is effective.

Silicon carbide becomes a P-type semiconductor when a Group 3 element is solid-solved, and becomes an N-type semiconductor when a Group 5 element is solid-solved. Of these, the P-type semiconductor carrier is a hole and the N-type semiconductor carrier is an electron. However, since the electron generally has a higher mobility than the hole, the group 5 element is solid-solved to form an N-type semiconductor. Is effective for lowering the specific resistance. As the Group 5 element capable of forming a solid solution in silicon carbide,
The nitrogen group such as nitrogen, phosphorus, arsenic, antimony or bismuth may be mentioned. Among them, nitrogen is most easily solid-solubilized and has a high solid-solution limit. For this reason, an attempt has been proposed in which nitrogen is solid-dissolved in the tissue in order to reduce the electric resistance of silicon carbide.

For example, Japanese Patent Publication No. 57-18682 discloses a method of sintering silicon carbide in a nitrogen atmosphere. Similarly, Japanese Laid-Open Patent Publication No. 52-110499 discloses hot pressing silicon carbide in a nitrogen atmosphere. A method of sintering is disclosed. However, simply sintering in nitrogen gas does not allow the solid solution of nitrogen to proceed smoothly and the specific resistance cannot be sufficiently reduced. Japanese Patent Publication No. 64-4312 describes a method for increasing the solid solubility of nitrogen so that the nitrogen gas pressure during the sintering of silicon carbide is increased to 80 to 500 atm and nitrogen is forced to form a solid solution. According to this method, the solid solution amount of nitrogen increases, so that the electrical resistivity of silicon carbide can be effectively reduced. However, in order to secure the nitrogen gas pressure under the above conditions, for example, a hot isostatic press ( Since an expensive device such as HIP) has to be applied, there is a problem as an industrial means in terms of equipment and cost.

[0005]

In view of such circumstances, the present inventor has carried out multifaceted research on a simple means for increasing the solid solubility of nitrogen in silicon carbide, and as a result,
When a specific amount of nitride and carbon powder is mixed with the silicon carbide raw material powder when the heating element is manufactured, and the sintering process is performed under the specified conditions, the nitrogen solid solution amount can be reduced without the need for special equipment. It was clarified that the specific resistance of the silicon carbide heating element can be effectively increased and the specific resistance of the silicon carbide heating element can be reduced without deteriorating the material strength.

The present invention was developed on the basis of such findings, and its object is to provide a low specific resistance silicon carbide capable of ensuring a large amount of nitrogen solid solution by a simple process without deteriorating the material strength. It is to provide a method for efficiently producing a heating element.

[0007]

In order to achieve the above object, a method of manufacturing a silicon carbide heating element according to the present invention comprises a raw material powder of silicon carbide having 1 to 30% by weight of a nitride powder and 5 parts by weight of the nitride powder. A characteristic feature is that a carbonaceous powder in an amount corresponding to ˜30 wt% is mixed, shaped into a predetermined shape with a binder, and then sintered at a temperature of 1900 to 2400 ° C. in a nitrogen gas atmosphere.

As the silicon carbide raw material powder of the present invention, α-type crystal system fine powder having a particle size of 100 μm or less, which is used for producing a silicon carbide heating element, is usually used. Nitride powder and carbon powder are previously added and mixed to this silicon carbide raw material powder. As the nitride powder, fine powders such as silicon oxynitride (Si ₂ N ₂ O) and silicon nitride (Si ₃ N ₄ ) are targeted, but if the decomposition temperature is too high, the operation becomes complicated and the nitrogen solid Solubilization becomes difficult. It should be noted that a nitride containing a Group 3 element such as boron nitride (BN) or aluminum nitride (AlN) is not preferable for the purpose of the present invention because it compensates the N-type semiconductor. The most suitable nitride powder for the purposes of the present invention is silicon nitride powder. The reason for this is that silicon nitride has a decomposition temperature (about 1900 ° C).
Is relatively low, and the silicon component produced along with nitrogen during decomposition combines with carbon powder to form silicon carbide, which functions to densify the structure of the silicon carbide base material. This is because it can be achieved. On the other hand, the carbonaceous powder is not particularly limited and, for example, coke powder, carbon powder, graphite powder or carbon black is used.

The nitride powder is 1 to 1 with respect to the silicon carbide raw material powder.
It is added in the range of 30% by weight. This addition amount is 1% by weight
When the amount is less than the above, the amount of nitrogen produced by decomposition becomes small and it is not possible to secure a sufficient amount of solid solution of nitrogen for lowering the specific resistance. Cause deterioration. The addition ratio of the carbonaceous powder to the silicon carbide raw material powder is 5 to 30% by weight of the above-mentioned nitride powder.
Set to the range of the amount equivalent to. This is because when the addition amount is less than 5% by weight, the base material component (for example, silicon) generated by decomposition of the nitride remains as an unreacted material, and when the addition amount exceeds 30% by weight, excess carbonaceous material remains. This is because it adversely affects the material characteristics.

The silicon carbide raw material powder, the nitride powder and the carbonaceous powder are sufficiently stirred and mixed, and then kneaded with an organic binder such as polyvinyl alcohol and water, and formed into a predetermined shape by means such as extrusion and molding. Mold.

Next, the compact is held in a nitrogen gas atmosphere, transferred to a heating furnace, and subjected to a sintering treatment at a temperature of 1900 to 2400 ° C. If the nitrogen gas atmosphere is not maintained in the system of the heating furnace, the nitrogen generated by the decomposition of the nitride diffuses into the furnace, resulting in a decrease in the amount of solid solution of nitrogen. If the sintering temperature is lower than 1900 ° C, decomposition of the nitride does not occur, and the sintering of the compact does not proceed smoothly. If it exceeds 2400 ° C, the silicon carbide itself decomposes to form a sintered body. Will not happen.

[0012]

The function of the solid solution of nitrogen according to the present invention will be described below by taking the case of using silicon nitride as the nitride powder as an example. First, when the silicon nitride powder mixed with the silicon carbide raw material powder is heated to 1900 ° C. or higher in the sintering process, the reaction of the formula (1) occurs and decomposes. Si ₃ N ₄ → 3Si + 4N (1) The generated Si reacts with the mixed carbonaceous powder and is converted into silicon carbide by the formula (2). 3Si + 3C → 3SiC (2) The above equations (1) and (2) can be summarized as the following equation (3). Si ₃ N ₄ + 3C → 3SiC + 4N (3)

The atomic nitrogen decomposed and produced here has a behavior different from that of nitrogen (N ₂ ) existing in the atmosphere system, and SiC is produced in the process of the progress of the sintered grain growth of the silicon carbide raw material and the above reaction. In doing so, it replaces the carbonaceous powder and extremely smoothly forms a solid solution in the silicon carbide sintered structure. Therefore, even if the nitrogen gas atmosphere in the sintering process stage is near atmospheric pressure, a large amount of solid solution of nitrogen becomes possible. At the same time, the reaction-generated SiC disperses inside the structure of the silicon carbide sintered body, contributes to the densification of the structure, and acts to improve the material strength.

These effects synergize with each other to effectively promote solid solution of nitrogen while maintaining a high level of material strength, and effectively reduce the electrical resistivity of the resulting silicon carbide heating element. Is brought about.

[0015]

EXAMPLES Examples of the present invention will be described below in comparison with comparative examples.

Examples 1 to 3 and Comparative Examples 1 to 4 Silicon nitride powder (average particle size 0.5 μm) and carbon powder (particle size 2 μm) were added in different amounts to α-type silicon carbide powder having an average particle size 5 μm. Then, the mixture was stirred and mixed, and 1% by weight of a binder made of polyvinyl alcohol and 20% by weight of water were added thereto and sufficiently kneaded. This kneaded product was extruded into a cylindrical molded body having a diameter of 15 mm and a length of 50 mm. Then, the molded body was dried and then placed in a heating furnace, and the temperature was raised to 2100 ° C. while maintaining the inside of the system in a nitrogen gas atmosphere of 1.3 atm, and sintering treatment was performed at this temperature for 1 hour. The nitrogen solid solution amount, electrical resistivity (normal temperature), bending strength, and other characteristics of the silicon carbide sintered body thus obtained were measured, and the results are shown in Table 1 in comparison with the fluctuation conditions during manufacturing. It was

Comparative Example 5 A silicon carbide heating element was manufactured under the same conditions as in Example 2, except that the silicon nitride powder and the carbon powder were not mixed. Various characteristics of this sintered body were measured, and the results are also shown in Table 1.

Comparative Examples 6 to 7 In Example 2, the sintering temperature was 1800 ° C. and 250.
A silicon carbide sintered body was manufactured by changing the temperature to 0 ° C. and all other conditions being the same. Various characteristics of this sintered body were measured, and the results are also shown in Table 1.

Comparative Example 8 In Example 2, the atmosphere in the furnace during the sintering process was 1.3 at.
A silicon carbide sintered body was manufactured under the same conditions except that the atmosphere was changed to m 2 of argon. Various characteristics of this sintered body were measured, and the results are also shown in Table 1.

[0020]

[Table 1]

From the results shown in Table 1, in each of the silicon carbide sintered bodies according to the examples, the amount of solid solution of nitrogen is obviously increased and the specific resistance is lowered as compared with Comparative example 5 in which the silicon nitride powder and the carbon powder are not mixed. . In addition, since the bending strength is also at a high level, suitable characteristics as a heating element are given. In contrast, in Comparative Example 1 in which the amount of silicon nitride powder added is small, Comparative Example 6 in which the sintering temperature is low, and Comparative Example 8 in which the furnace atmosphere is argon, the solid solution amount of nitrogen is small and the specific resistance is reduced. However, in Comparative Example 2 in which the amount of silicon nitride added was large, the bending strength of the material was extremely reduced. Moreover, in Comparative Example 7 in which the sintering temperature was high, the silicon carbide forming the sintered body was decomposed to such an extent that the shape could not be retained. In Comparative Example 3 in which the amount of carbon powder added was less than the limited range and Comparative Example 4 in which the amount of carbon powder was greater than the limited range, the characteristic data of the sintered body showed good results. In the silicon melting temperature range, a phenomenon of a marked increase in resistance and a decrease in strength were observed. In the latter case, unreacted carbon powder remained, resulting in an increase in resistance due to oxidation of carbon during heat generation by energization. Met.

[0022]

As described above, according to the present invention, the material strength is maintained at a high level through a simple process by adding a specific amount of nitride powder and carbonaceous powder to the silicon carbide raw material powder in advance. However, it is possible to efficiently manufacture a silicon carbide heating element having a large amount of nitrogen solid solution and a low electric resistivity. Therefore, it is useful as an industrial manufacturing technique of a high-performance silicon carbide heating element for a high temperature furnace.

Claims (2)

[Claims] 1. A silicon carbide raw material powder, and nitride powders 1 to 30.
Wt% and an amount of carbonaceous powder corresponding to 5 to 30 wt% of the nitride powder are mixed and shaped into a predetermined shape with a binder, and then 1900 to 2400 in a nitrogen gas atmosphere.
A method for manufacturing a silicon carbide heating element, which comprises performing a sintering treatment at a temperature of ° C. 2. The method for manufacturing a silicon carbide heating element according to claim 1, wherein the nitride is silicon nitride.