JPH0594864A - Manufacture of porous silicon carbide heating element - Google Patents

Abstract

PURPOSE:To provide a method for efficiently manufacturing a porous silicon carbide heating element furished with ventilating holes which are homogeneous while being uniformly distributed without clogging, and with excellent physical strength. CONSTITUTION:Polyurethane foam is impregnated with silicon carbide slurry in the first process, and it is dried and sintered so as to be processed after excess slurry has been removed. In the second place, porous silicon carbide sintered material obtained through the first process is set in a hermetic reaction system, so that silicon carbide is separated while being heated so as to be disposed in tissue holes by means of pulse CVI with trichloromethylsilane supplied intermittently while being accompanied by hydrogen.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently producing a porous silicon carbide heating element having a uniform material structure and uniform ventilation holes and excellent material strength.

[0002]

2. Description of the Related Art As a technique for manufacturing a silicon carbide heating element, a method of mixing silicon carbide powder with an organic binder to form it into a predetermined shape and firing it to convert its structure into recrystallized SiC has long been known. In addition to the above, as a means for producing a silicon carbide heating element having a porous structure for heating and filtering a fluid, collecting and heating particulates, etc., an organic porous foam having a three-dimensional network structure is used. After the silicon carbide slurry is attached to the skeleton surface, there is a method of drying and firing. The silicon carbide heating element produced by this method has a highly porous structure with a porosity of 75 to 95%.
There are drawbacks such as clogging of the tissue and extremely insufficient material strength.

As means for suppressing clogging in the production of this type of ceramics porous body, a method of repeating the steps from deposition to drying by reducing the amount of ceramics slurry deposited by one operation (special feature: (Kaisho 59-3059) or a method in which an organic foam compressed at a specified ratio is filled with a ceramics slurry and then the compressed body is restored to its original volume and dried and fired (Japanese Patent Laid-Open No. 63-156084). Issue gazette). However, in these methods, the voids remain after the organic component forming the skeleton is thermally decomposed and disappeared in the fired tissue, so that the lack of the tissue strength cannot be solved.

As a method for solving the above problems and obtaining a porous ceramic structure having a uniform vent hole without clogging and an excellent skeletal strength, an organic porous foam is impregnated with a ceramic slurry to obtain a surplus slurry. The present applicant has proposed a process consisting of a first step of removing and drying and then calcining, and a second step of re-impregnating the calcined body with ceramics slurry to remove excess slurry, and then drying and calcining. (JP-A-3-83875).

[0005]

According to the above-mentioned prior art, since the voids of the organic component are filled with silicon carbide by re-impregnation, the material strength can be effectively improved. However, there is a difficulty in smoothly and uniformly permeating the ceramics slurry into all the pores inside the structure, and there is a problem that local clogging and uneven strength often occur.

The present invention was developed by changing the structure improvement of the second step in the prior art to the precipitation and deposition of silicon carbide using pulsed CVI, the purpose of which is to obtain a uniform vent hole without clogging or bias. And to provide an efficient manufacturing method of a porous silicon carbide heating element having excellent material structure strength.

[0007]

The method for producing a porous silicon carbide heating element according to the present invention for achieving the above object comprises:
A first step of impregnating an organic porous foam with a silicon carbide slurry, removing excess slurry, and then drying and firing the same.
From the second step of heating the porous silicon carbide sintered body obtained in the step in a closed reaction system and intermittently supplying a halogenated organosilicon compound to deposit and deposit silicon carbide in the tissue vacancy by pulse CVI. It is a feature of the configuration.

The first step of the present invention is a step of forming a skeletal structure made of a porous silicon carbide sintered body, and the details thereof are as follows. As the organic porous foam, for example, a polyurethane foam such as a resin foam having a homogeneous fine three-dimensional network structure which is thermally decomposed and volatilized in a temperature range of 400 to 500 ° C is applied. The silicon carbide slurry is a fine powder of silicon carbide dispersed and suspended in water or an appropriate organic solvent, and is preferably prepared to have a slurry viscosity in the range of 20 to 1000 poise. If the slurry viscosity is less than 20 poise, the skeleton strength of the porous silicon carbide sintered body produced in the first step becomes extremely weak, and if it exceeds 1000 poise, it causes clogging.

The operation of impregnating the organic porous foam with the silicon carbide slurry is carried out by immersing the organic porous foam in the ceramics slurry. The impregnated material treated in this manner is dried after removing excess slurry using a means such as centrifugation or an air gun. Then, it is fired at a temperature of 1900 to 2200 ° C. in a non-oxidizing atmosphere to thermally decompose and eliminate the organic component constituting the organic porous foam and sinter the impregnated silicon carbide component. By this treatment step, vent holes along the cell structure of the organic porous foam,
A porous silicon carbide sintered body having a structure void in which burned holes in which traces of disappearance of organic components are hollowed out and voids intervening between particles are mixed is formed.

The second step is a reforming treatment step in which the voids of the organic components generated in the skeletal structure obtained in the first step are mainly filled with silicon carbide without clogging. It is as follows. Note that this step can be repeated as long as the vent holes are not blocked.

As a halogenated organosilicon compound which is a raw material for pulse CVI, trichloromethylsilane (CH ₃
SiCl ₃ ), trichlorophenylsilane (C ₆ H ₅ SiCl ₃ ), dichloromethylsilane (CH ₃ SiHCl ₂ ), dichlorodimethylsilane ((CH ₃ ) ₂ SiCl ₂ ), chlorotrimethylsilane ((CH ₃ ) ₃ SiC
l), mixed gas of silane (SiH ₄ ) and methane (CH ₄ ), mixed gas of silicon tetrachloride (SiCl ₄ ) and methane (CH ₄ ), and the like. The method of pulse CVI is such that the halogenated organosilicon compound is entrained in hydrogen gas and is supplied and contacted in a gas state to a porous silicon carbide sintered body heated in a closed reaction system, and a tissue is formed by a reduction thermal decomposition reaction. The operation of depositing and depositing silicon carbide in the pores is carried out by a process of intermittently repeating a short cycle. The most suitable reaction condition is trichloromethylsilane (CH ₃ as a halogenated organosilicon compound).
SiCl ₃ ) and the concentration of the trichloromethylsilane is 0.5
.About.5%, pressure 0.133 to 93 kPa, temperature 1000 to 1400.degree. C., pulse holding time 1 to 2 seconds, and pulse number 3000 to 20000. It is especially important to adjust the concentration of trichloromethylsilane. If it is less than 0.5%, it is not possible to fill the disappearing holes by only filling the fine pores, and if it exceeds 5%, the disappearing pores after the organic component disappears are filled. However, since the concentration is too high, a large amount is deposited on the surface, peeling easily occurs, and the vent holes are clogged.

[0012]

According to the process of the present invention, in the stage of the first step, the vent holes along the cell structure of the organic porous foam, the disappearance holes generated by the disappearance of the thermal decomposition of the organic component and the intervening pores between the particles are formed. A mixed porous silicon carbide sintered body is formed.
This sintered body is a fragile structure in which the above-mentioned pores are present throughout the structure, and is a structure that is likely to be a fracture point because the triangular vertexes forming the voids of the foam cell structure are sharp. It has a structure.

This tissue structure is effectively modified in the second step using pulsed CVI. That is, according to the pulse CVI applied in the second step, the step of instantaneously supplying a new reaction gas to the structure of the porous silicon carbide sintered body is repeated for each pulse, so that it depends on the supply gas. The thermal decomposition reaction proceeds smoothly inside the pores of the structure, and is deposited and deposited as silicon carbide in the pores. The silicon carbide deposited and deposited at this time is formed in such a form that it closes up fine pores and burnt-out pores, but uniformly reinforces the skeleton without filling up the ventilation holes, and also penetrates into the fine pores between the SiC particles. Therefore, it has an isotropic structure in which cracks and the like are unlikely to occur even when subjected to thermal stress.

Through the mechanism of the first step and the second step, it is possible to efficiently manufacture a porous silicon carbide heating element having a uniform vent hole without clogging or unevenness and excellent material structure strength. It will be possible.

[0015]

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

Example 1 (1) First Step: A silicon carbide powder having an average particle diameter of 20 μm was dispersed and suspended in water to prepare a slurry having a viscosity of 300 poises. A soft polyurethane foam [“Everlite Scott # 20” manufactured by Bridgestone Co., Ltd.] was dipped in the slurry and pulled up. The excess slurry was removed by centrifugation and then dried at a temperature of 80 ° C. Then transfer to a firing furnace,
The inside of the furnace was maintained in a nitrogen gas atmosphere and subjected to a firing treatment at a temperature of 2000 ° C. to form a porous silicon carbide sintered body processed into a heating element shape having a diameter of 20 mm and a length of 50 mm.

(2) Second step: The porous silicon carbide sintered body obtained in the first step is placed in a quartz reaction tube of a pulse CVI apparatus, the inside of the tube is sufficiently replaced with Ar gas, and then baked by high frequency induction heating. The temperature of the association was raised to 1000 ° C. Then, the pressure inside the reaction tube was reduced to 0.133 kPa by a vacuum pump, and trichloromethylsilane (CH ₃ SiCl ₃ ) was immediately introduced so that the internal pressure became 93 kPa while being accompanied by hydrogen gas so that the concentration became 5%. Hold for 2 seconds. The pulse operation of supplying the reaction gas and reducing the pressure inside the tube was repeated 5000 times to deposit and deposit silicon carbide.

(3) Characteristic evaluation: Various characteristics were measured for the porous silicon carbide heating element thus manufactured, and the results are shown in Table 1. An electrode was attached to the end of this heating element, and the electrode was heated by heating for 20 hours. After that, a heating cycle of natural cooling to room temperature was repeated 30 times, and the electric resistance value after the treatment was measured. For the purpose of comparison, Table 1 also shows the characteristics of the porous silicon carbide sintered body (Comparative Example 1) obtained only in the first step.

Example 2 In the second step of Example 1, the concentration of trichloromethylsilane was changed to 0.5% to prepare a porous silicon carbide heating element,
Further, the second step in which the concentration of trichloromethylsilane was 5% was repeatedly performed. All other conditions were the same as in Example 1. Table 1 also lists various characteristics of the porous silicon carbide heating element produced under these treatment conditions.

Comparative Example 2 The porous silicon carbide sintered body formed in the first step of Example 1 was immersed again in a silicon carbide slurry having a viscosity of 20 poise and impregnated in an autoclave. Then, the excess slurry was removed by centrifugation, dried at a temperature of 80 ° C., and then fired at 2100 ° C. in a furnace maintained in a nitrogen atmosphere.
Various characteristics of the porous silicon carbide heating element thus obtained were measured, and the results are also shown in Table 1.

[0021]

[Table 1]

From the results shown in Table 1, it was confirmed that the porous silicon carbide heating element according to the example has suitable conductivity as a heating element, and has excellent porosity without clogging and high material strength. Was given.

[0023]

As described above, according to the present invention, it is possible to efficiently manufacture a high-performance porous silicon carbide heating element having a uniform vent hole without clogging and unevenness and excellent material structure strength. it can. Therefore, it is useful for purposes such as heating / filtration of a fluid used under particularly severe conditions and heating for collecting particulates.

Claims (2)

[Claims] 1. A first step in which an organic porous foam is impregnated with a silicon carbide slurry, excess slurry is removed, and then drying and firing are performed, and the porous silicon carbide sintered body obtained in the step is a closed reaction system. A method for producing a porous silicon carbide heating element, which comprises a second step of intermittently supplying a halogenated organosilicon compound while heating in a chamber to deposit and deposit silicon carbide in tissue pores by pulse CVI. 2. Pulse CVI is carried out using trichloromethylsilane (CH ₃ SiCl ₃ ) gas as a raw material, and the conditions are trichloromethylsilane concentration 0.5 to 5%, pressure 0.133 to 93.
The method for producing a porous silicon carbide heating element according to claim 1, wherein kPa, temperature 1000 to 1400 ° C., pulse holding time 1 to 2 seconds, and pulse number 3000 to 20000 are set.