JPH0154303B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an ultrafine silicon carbide sintered body, particularly a high purity ultrafine silicon carbide sintered body that is useful as a raw material for a heat-resistant ceramic molded body. (Prior art) Silicon carbide powder has recently attracted attention as a heat-resistant ceramic material, but there is a demand for it to be particularly high in purity and have good sinterability, and to be supplied in the form of submicron-order fine powder. ing. However, what can be obtained by the conventionally known solid phase reaction is α
type silicon carbide, and it also had the disadvantage that it was difficult to obtain a high-purity product because it required a pulverization step for pulverization. In addition, for the production of this type of silicon carbide, the general formula R _o
SiX _4-o (where R is a hydrogen atom or an alkyl group,
(X is a halogen atom, n is a positive number from 1 to 3), or high-temperature thermal decomposition reactions of this silane and hydrocarbon compounds, thermal decomposition reactions of tetramethylsilane, or plasma thermal decomposition reactions of these. A method using a phase reaction is also known, and although this method yields spherical and finely powdered β-type silicon carbide, the former method leaves unreacted silicon halide, and the latter method However, the silicon carbide obtained is difficult to densify, and the sintering process requires the addition of an auxiliary agent. (Structure of the Invention) The present invention relates to a method for producing a silicon carbide sintered body that solves the above disadvantages, and has at least one silicon-hydrogen bond in the molecule, and SiX (X is It is an aggregate of β-type silicon carbide with crystallites of 50 Å or less, obtained by vapor-phase thermal decomposition of organosilicon compounds that do not contain halogen atoms or oxygen atoms) bonds at 750 to 1500 °C, and has an average particle size of 0.01~1μ
Ultrafine particle β-type silicon carbide with a spherical shape is heat treated at 1000 to 1700°C in a non-oxidizing atmosphere, and then heated to 1750 to 2500°C in an inert atmosphere.
It is characterized by being fired at ℃. To explain this, the present inventors first found that the molecule has at least one silicon-hydrogen bond,
If an organosilicon compound that does not contain SiX (X is a halogen atom or an oxygen atom) bond is thermally decomposed at 750°C or higher, ultrafine silicon carbide can be produced in high purity and high yield without going through a pulverization process. found that it is possible to obtain
(See Japanese Patent Application No. 58-155911 and Japanese Patent Application No. 58-201202), the silicon carbide thus obtained can be sintered without the addition of a sintering aid, and is different from conventionally known silicon carbide. It has been found that a mixture of the above can also be sintered with an extremely small amount of sintering aid (see Japanese Patent Application No. 155910/1982 and Japanese Patent Application No. 213477/1983). Further research on this topic revealed that ultrafine β-type silicon carbide obtained by the above-mentioned gas phase pyrolysis method was heat-treated at 1000 to 1700°C in a non-oxidizing atmosphere prior to sintering. Furthermore, they discovered that it was possible to easily obtain a highly densified silicon carbide sintered body, and completed the present invention after repeated studies on the heat treatment conditions. The ultrafine β-type silicon carbide for producing the ultrafine silicon carbide sintered body by the method of the present invention is obtained by a gas phase pyrolysis reaction of an organosilicon compound. contains at least one Si--H bond in its molecule, but no SiX bond, for example, the general formula
R _2o+2 (Si) _o [Here, R is a hydrogen atom, at least one of which is a hydrogen atom, or a monovalent hydrocarbon selected from a methyl group, an ethyl group, a propyl group, a phenyl group, a vinyl group, etc. base, n is a positive number from 1 to 4]
Silanes or polysilanes represented by and the general formula [Here, R is the same as above, R' is a methylene group, ethylene group, or phenylene group, and m is a positive number of 1 to 2. Examples include compounds with a skeleton. and,
This organosilicon compound has the following formula: CH ₃ SiH ₃ , (CH ₃ ) ₂ SiH ₂ , (CH ₃ ) ₃ SiH,
(C ₂ H ₅ ) ₂ SiH ₂ , C ₃ H ₇ SiH ₃ , CH ₂ = CH・
_{CH3SiH2 , C6H5SiH3 , _} Examples of silanes and polysilanes shown by the formula

Methylhydrodienesilanes, which are mainly composed of dimethylpolysilane obtained by thermally decomposing dimethylpolysilane represented by the formula [where n is a positive number] at a temperature of 350° C. or higher, are preferred. These organosilicon compounds can be produced by conventionally known methods, but they can be easily purified by a distillation process, and a pulverization process is not required, so they can be obtained by this reaction. The silicon carbide also has the advantage of being extremely pure. The gas phase thermal decomposition reaction of this organosilicon compound can be carried out by introducing hydrogen gas or an inert gas such as nitrogen, helium, or argon together with a carrier gas into a reaction zone heated to 750 to 1500°C. According to this reaction, ultrafine β-type polycrystalline silicon carbide, which is an aggregate of β-type silicon carbide with crystallites of 50 Å or less and has a spherical shape and an average particle size of 0.01 to 1 μm, is obtained. In the production of the ultrafine-grained silicon carbide sintered body of the present invention, the ultrafine-grained β-type polycrystalline silicon carbide thus obtained is sintered. Therefore, it is necessary to heat-treat this in advance at 1000 to 1700°C in a non-oxidizing atmosphere. This heat treatment completely decomposes the undecomposed organic bonds contained in the ultrafine β-type polycrystalline silicon carbide, and also corrects the sagging of the β-type silicon carbide crystallites in the aggregate. Therefore, this is then sintered to form a sintered body with higher density, but this heat treatment can be performed under normal pressure or reduced pressure, and it can also be performed under mechanical stirring. Good too. However, this heat treatment lowers this temperature to 1000℃.
If the temperature is below 1,700℃, the decomposition rate of undecomposed organic bonds will slow down and the processing time will become longer, and if the temperature is higher than 1700℃, sintering will begin in some parts with rapid growth of crystallites.
It is necessary to keep the temperature in the range of 1000 to 1700℃, and if the gas atmosphere is oxidizing, the ultrafine silicon carbide is highly reactive, so the surface will be oxidized and the density of the sintered body will decrease. This has the disadvantage that the heat resistance is not increased and the heat resistance is deteriorated, so it is necessary to use a non-oxidizing atmosphere such as nitrogen, hydrogen, argon or helium gas. Note that the treatment concentration and treatment time do not need to be particularly limited and are arbitrary.
2 hours at 1000℃ and 30 minutes at 1700℃ are economical. In that case, B ₂ , H ₆ , PH ₃ in the atmospheric gas
It is also optional to adjust the electrical properties and sintering properties of this silicon carbide by adding doping agents such as. The silicon carbide heat-treated as described above may then be shaped and sintered, and this shaping may be performed by a method known in the ceramic industry, such as a die press method. As a binder for this molding, organic compounds that do not leave decomposition products when heated, such as paraffin, low molecular weight cellulose derivatives, phenolic resins, etc., may be used alone or dissolved in acetone etc. , it is also possible to press, mold, and sinter directly without using these binders. In addition, in order to make complex molded products such as tubes and crucibles, it is possible to mold them using a rubber press, but in order to obtain more precise molded products, it is necessary to It is preferable to perform mechanical processing such as grinding or slicing. Note that this molding may be performed by the slip cast method, but in this case, a plasticizer such as polyethylene glycol, a low molecular weight cellulose derivative, or paraffin, and a binder such as polyvinyl butyral are added to the silicon carbide powder, and the molding is performed in water. After dispersing the mixture, pour it into a baked plaster mold.
Further, a moldable paste made of a mixture of a cellulose derivative or the like and water may be subjected to extrusion molding, injection molding, roll molding, or the like. Further, the molded body thus obtained is then sintered to form a sintered body, and the organic compound added is volatilized prior to sintering. This sintering may be performed under normal pressure or under pressure such as gas pressure or press pressure. However, if this heating temperature is too low, sintering will be insufficient.For the purpose of obtaining high-density products, it is better to set the heating temperature as high as possible;
If the temperature is higher than 2500℃, the strength may decrease due to particle growth, and it is also economically disadvantageous.
It is preferable to set the temperature to 2300℃. Furthermore, sintering requires these to be under an inert atmosphere, but this may be done in the presence of argon, nitrogen, or helium gas. In addition, if the above-mentioned molded product is cut prior to this sintering step, it may be calcined if necessary, but this temperature should not exceed 1500℃.
℃ or less, and this temperature may be determined depending on the strength required for machining. On the other hand, the sintered body manufactured by the method of the present invention would be expensive if it were manufactured only from the above-mentioned ultrafine silicon carbide particles, so it is not possible to use commercially available silicon carbide particles with an average particle size of 5μ or less. You may also add
This gives an industrial advantage in terms of price. However, when sintering this type of commercially available silicon carbide, it is necessary to add a sintering aid as described above, for example, 0.15 to 5% by weight.
It is necessary to add 0.1 to 5% by weight of carbon to remove boron and free oxygen contained in the raw material powder, but in the case of the present invention, the above-mentioned silicon carbide is added. Even if the amount of commercially available silicon carbide added to 100 to 50 parts by weight of ultrafine particles is 50 parts by weight, it can be added to 0.15% by weight or 0.1% by weight, which means that the amount added can be reduced as the amount of commercially available silicon carbide is reduced. Therefore, there is an advantage that the disadvantages due to the coexistence of boron and carbon can be minimized. In short, the present invention provides at least one
obtained by vapor-phase thermal decomposition of an organosilicon compound having silicon-hydrogen bonds and no SiX (X is a halogen atom or an oxygen atom) bond at 750 to 1500°C.
Heat treatment of β-type polycrystalline silicon carbide in a non-oxidizing atmosphere at 1000-1700℃ in a non-oxidizing atmosphere of β-type polycrystalline silicon carbide aggregates with crystallites of 50 Å or less and a spherical shape with an average particle size of 0.01-1μ. This invention relates to a method for producing an ultrafine silicon carbide sintered body by sintering the sintered body, and the sintered body thus obtained has a density of 2.89 g/cc or more, which is 90% of the theoretical density. Because it is easily obtained as a substance, it is useful for gas turbine blades and automobile parts that require strength, and because of its high purity, it can be used for various mechanical and electrical parts. It has a nature. Next, examples of the present invention will be given. Examples 1 to 5, Comparative Example 1 A vertical tubular electric furnace equipped with a mullite furnace tube with an inner diameter of 70 mm and a length of 1500 mm was heated to 1150°C, and hydrogen gas containing 10% by volume of tetramethyldisilane was introduced into it.
When introduced at a rate of 200/hour and reacted for 8 hours,
574.3 g (yield 93%) of silicon carbide powder was obtained. This is β-SiC (1, 1, 1) in an electron microscope.
β of less than 50 Å from the measurement results of dark-field images by diffraction.
It was confirmed that the particles were β-type polycrystalline silicon carbide in the form of ultrafine particles with a spherical shape and an average particle size of 0.1 to 0.4μ. Next, 50g of this ultrafine particulate silicon carbide was
After placing it in a graphite container with a diameter of 100 mm and a height of 100 mm and heat-treating it under the conditions shown in Table 1, 3 g of silicon carbide was mixed with 0.3% by weight of boron powder (manufactured by Rare Metallic Co., Ltd.) and 1% by weight. 10 ml of an acetone solution containing paraffin was added and mixed ultrasonically. This mixture was put into a 5mm x 7mm x 5mm mold and 150Kg/
A pressure of 1.5 t/cm ² was applied using ^a rubber press. Next, this sample piece was placed in a carbon mold for pressureless sintering, and placed in an argon gas atmosphere under atmospheric pressure.
Pressureless sintering was performed at 2200°C for 1 hour to produce a sintered body, and the density of the sintered body was measured, and the results shown in Table 1 were obtained. The results for those sintered without heat treatment are also shown.

【table】

[Table] Example 6, Comparative Example 2 When the tetramethyldisilane in Examples 1 to 5 was replaced with dimethylsilane and the same treatment was performed, β-type silicon carbide aggregates of 50A or less with an average particle size of 0.1 to 262.4g of ultrafine particulate β-type polycrystalline silicon carbide with a spherical shape of 0.5μ was obtained.
This was heat-treated at 1300℃ for 1 hour in a nitrogen gas atmosphere in the same graphite container as in the previous example.
0.9g of boron per 30g, methylcellulose/MC-
400 (trade name, manufactured by Shin-Etsu Chemical Co., Ltd.) and 1 g of glycerin were dispersed in a solution of 6 g of water, mixed with three rolls, and taken out as a sheet. Next, this sheet was cut into 50 mm x 2 mm x 50 mm, air-dried, and then pressurized to 1.5 kg/cm ² using a rubber press.
When heated in nitrogen gas at 1200°C for 1 hour to volatilize the organic matter and then sintered in the same manner as in the previous example, a sintered body with a density of 3.04 g/cc (94.7% relative to theoretical density) was obtained. , the density of the sintered body that was not subjected to the above heat treatment for comparison is
It was 2.79 g/cc (Comparative Example 2). Example 7 The ultrafine powdered β-type polycrystalline silicon carbide obtained in Example 6 above was heat-treated at 1400°C for 1 hour in an argon gas atmosphere in a graphite container, and then
When the silicon, carbon, and oxygen contents of this product were analyzed, the results shown in Table 2 were obtained, and it was found that there was no change in the composition even after this heat treatment.

[Table] When this silicon carbide was sintered under the same conditions as in Example 1, the density was 3.15.
g/cc (98.1% relative to theoretical density) and had a strength of 62 Kg/cm ² , whereas the material of Comparative Example 3, which was not heat-treated, had a density of 2.90 g/cc and a strength of 34 Kg/cm ² . Example 8 Hydrogen gas containing tetramethyldisisilane in Example 1 was mixed with dimethylsilane and H
1:1 mixture with (CH ₃ ) ₂ SiCH ₂ Si(CH ₃ ) ₂ H20
When hydrogen gas containing % by volume was pyrolyzed in the gas phase at 1250℃, fine particles of β-type silicon carbide aggregates of 50 Å or less and an average particle size of 0.1 to 0.3 μ were obtained.
type polycrystalline silicon carbide was obtained. Next, this was placed in a graphite container under vacuum at 1250°C.
After heat treatment at ℃ for 2 hours, it was placed in a 40 mm diameter carbon mold for hot press without adding any sintering aid and degassed under reduced pressure.Then, the inside of the system was placed under an argon gas atmosphere and then placed under a pressure of 200 kg/cm ² . When heated at 2100℃ for 50 minutes, the density of the obtained sintered body was 3.19
g/cc (99.4% relative to theoretical density), but the density of a sintered body prepared for comparison without the above heat treatment was 3.10 g/cc (96.6% relative to theoretical density).
%).

Claims (1)

[Claims] 1 Has at least one silicon-hydrogen bond in the molecule, SiX (X is a halogen atom or an oxygen atom)
Organosilicon compounds containing no bonds at 750-1500℃
β with crystallites less than 50 Å obtained by gas phase pyrolysis in
It is an aggregate of silicon carbide with an average particle size of 0.01~
Ultrafine particle β-type polycrystalline silicon carbide with a spherical shape of 1μ is heated to 1000~1μ in a nonoxidizing atmosphere.
Heat treated at 1700℃ and then in an inert atmosphere
A method for producing an ultrafine silicon carbide sintered body, characterized by firing at 1750 to 2500°C.