JPH08337475A - Production of carbon-boron carbide sintered material - Google Patents

Abstract

PURPOSE: To produce a large-sized carbon-baron carbide sintered material contg. uniformly dispersed boron carbide and having high strength. CONSTITUTION: Calcined coke is kneaded with pitch, pulverized and screened to form carbon precursor powder, 100 pts.wt. of this precursor powder is dry- mixed with 1-5 pts.wt. boron carbide powder whose particle size is almost equal to that of carbon precursor powder and the resultant powdery mixture is carbonized by firing at <=2,200 deg.C in a nonoxidizing atmosphere after cold isostatic pressing. The average particle diameter of each of the carbon precursor powder and the boron carbide powder is preferably regulated to 15-50μm and the volatile matter content of the carbon precursor powder is preferably regulated to 13-15%.

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 large carbon / boron carbide sintered material having high strength in which boron carbide is uniformly dispersed.

[0002]

2. Description of the Related Art A composite sintered material in which boron carbide is dispersed in a carbon material is used as a material having excellent oxidation resistance and high strength in many industrial fields including high temperature members.
Further, since the boron component contained in the carbon / boron carbide sintered material has a large neutron absorption cross section, it is also used as a neutron shielding material for a nuclear reactor.

Many methods have been conventionally known for producing a carbon / boron carbide sintered material. For example, Japanese Patent Application Laid-Open No. 62-108767 discloses an optical anisotropic method in which pitches are heat-treated. Of the carbonaceous powder obtained by separating the small spheres from the pitch matrix as a raw material, and adding and mixing boron carbide at a ratio of 1 to 50 parts by weight to 100 parts by weight of the raw material powder, and then molding the mixed powder. , A method of firing has been proposed. Further, in Japanese Examined Patent Publication (Kokoku) No. 5-27589, carbonaceous optically anisotropic small spheres produced from pitches are finely pulverized, and boron carbide and graphite are added thereto and mixed, followed by molding, pre-baking and depressurization. A manufacturing method has been proposed in which high temperature firing under lower or vacuum is sequentially performed.

However, since these methods use expensive optically anisotropic carbonaceous spheres as a carbonaceous raw material, not only is it disadvantageous in terms of cost, but the optically anisotropic carbonaceous spheres are heated. It is difficult to manufacture a large-sized carbon / boron carbide sintered material due to a large volume shrinkage ratio.

Further, Japanese Patent Publication No. 58-38386 discloses a method in which coke having a residual volatile content of 4% by weight or more and ceramic powder are mixed, and then the milled mixed powder is directly pressed and fired. In addition, it is 2-790
No. 7 discloses a method in which a mixed ground powder of raw coke and ceramics powder is preliminarily fired in a non-oxidizing atmosphere at a temperature of 800 ° C. or higher to a temperature at which the ceramics start to be sintered or less, and then molding / firing. There is. However, in these production methods, in order to highly express the mechanochemical effect by the milling treatment, it is necessary to use a semi-coke or a raw coke having a residual volatile component as the coke as a raw material. There is a drawback that the volumetric shrinkage becomes large and it becomes difficult to manufacture a large-sized material, and there is also a problem that it takes a long time for the grinding treatment.

In Japanese Patent Laid-Open No. 4-89355, carbon and boron which are obtained by impregnating a carbon material with boron oxide or (and) a hydrated compound thereof and firing it under a pressure of an inert gas at 1500 ° C. or higher are mainly used. A method of manufacturing a composite material as a component has been proposed. However, since it is difficult to uniformly impregnate the carbon material with boron oxide or a hydrate thereof, there is a drawback that the material composition tends to be inhomogeneous particularly in the case of a large material.

[0007]

In order to solve these problems, the present inventors have used calcined coke as a raw material, added pitch to this, kneaded it, then pulverized it, and specified the particle size obtained by adjusting the particle size. A large-sized sintered material in which boron carbide is uniformly dispersed in carbon can be obtained by using, as a raw material system, a mixed powder obtained by dry-mixing boron carbide powder of the same particle size with boron carbide powder of the same particle size. It was confirmed.

The present invention was developed on the basis of the above findings, and its object is to provide a large-sized carbon having high density, high strength and low specific resistance in which boron carbide is uniformly dispersed in a carbon material. / To provide a method for industrially producing a boron carbide sintered material.

[0009]

A method for producing a carbon / boron carbide sintered material according to the present invention to achieve the above object is to add a pitch to a calcined coke and knead the mixture to pulverize the kneaded product to adjust the particle size. 100 parts by weight of the carbon precursor powder thus obtained was dry-mixed with 1 to 5 parts by weight of boron carbide powder having the same particle size, and the mixed powder was CIP-molded and then at a temperature of 2200 ° C. or lower in a non-oxidizing atmosphere. The constitutional feature is the firing and carbonization treatment.

In the present invention, as a carbon raw material, raw coke is heated in a rotary kiln or other heating furnace at 1300 to 300
A calcined coke that has been heat-treated at a temperature of 1500 ° C., in which moisture and volatile components in the raw coke are removed and shrinkage due to subsequent heat treatment is extremely small, is used.

Pitches such as tar pitch and petroleum pitch are added as a binder to the calcined coke powder, and the mixture is put into a kneading machine and kneaded sufficiently. Pitch is added to the calcined coke at an appropriate ratio so that the kneaded product exhibits appropriate plasticity, but if the amount of pitch added is small, the formability will become insufficient when the boron carbide powder is added and the binder becomes insufficient. On the other hand, if too much, volume shrinkage increases during heat treatment, so it is preferable to add 30 to 80 parts by weight to 100 parts by weight of calcined coke.

Next, the kneaded product is crushed by a crusher and then the particle size is adjusted by sieving to obtain a carbon precursor powder.
The particle size of the carbon precursor powder is preferably adjusted to have an average particle size of 15 to 50 μm. Average particle size is 15μm
When the average particle size is less than 50 μm, the bulk density of the molded product is reduced, and the strength of the sintered product is reduced. Because it will be difficult. Further, it is desirable to adjust the volatile content of the carbon precursor powder to 13 to 15%. When the volatile content is less than 13%, the binder component is insufficient to reduce the density and strength of the sintered material, and when it exceeds 15%, undispersed lumps are generated during mixing of the boron carbide powder and the homogeneity of the mixed powder is deteriorated. This is because.

The particle size of the boron carbide powder mixed with the carbon precursor powder needs to be similar to the particle size of the carbon precursor powder in order to prepare a uniform mixed powder. The equivalent particle size means that the ratio of the average particle diameters of the carbon precursor powder and the boron carbide powder is within the range of 1 ± 0.2, and the average particle diameter of the boron carbide powder is 15 as in the carbon precursor powder. ~
50 μm powder is used. The amount of boron carbide to be mixed is appropriately set according to the material characteristics of the target carbon / boron carbide sintered material, but is added in the range of 1 to 5 parts by weight with respect to 100 parts by weight of the carbon precursor powder. .

The carbon precursor powder and the boron carbide powder thus prepared were thoroughly mixed by dry mixing using a mixer such as a V-type mixer, and the resulting mixed powder was packed in a rubber mold and CIP molded. After being formed into a predetermined shape by the above, the carbon / boron carbide sintered material is manufactured by performing heat treatment at a temperature of 800 ° C. or higher in a heating furnace maintained in a non-oxidizing atmosphere. Since boron carbide begins to decompose at a temperature of 2300 ° C or higher, the heat treatment temperature needs to be set to 2200 ° C or lower, and a preferable heat treatment temperature is 2000 to 2200.
It is in the range of ° C.

[0015]

According to the present invention, since the particle size of the carbon precursor powder and the boron carbide powder prepared in advance are adjusted and dry-mixed, uniform mixing is not required without strong mixing energy such as kneading. A powder can be obtained. Furthermore,
Compared to the method of simultaneously mixing and kneading the coke powder, the binder pitch, and the boron carbide powder, it is possible to effectively suppress the mixing of impurities due to the wear of the mixing equipment due to the hard boron carbide in the mixing process.

When the coke powder, the binder pitch and the boron carbide powder are mixed and kneaded at the same time, it is usually necessary to heat the mixture to a temperature of 200 to 250 ° C. to soften and melt the binder pitch for kneading. Part of the boron carbide powder is oxidized to B ₂ O ₃ , and the generated B ₂ O ₃ is volatilized during the firing and carbonization treatment, so that the composite amount of boron carbide is reduced, and the uniformity of boron carbide dispersion is also reduced. However, in the method of the present invention, since it is not necessary to heat during the mixing, the boron carbide is not oxidized and the volatilization of the boron carbide does not occur during the firing carbonization treatment.

Furthermore, since calcined coke is used as a carbon raw material, a large-sized carbon / boron carbide sintered material having high density and high strength in which volumetric shrinkage during heat treatment is small and boron carbide is uniformly dispersed can be manufactured at low cost. Is possible.

[0018]

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Example 1 Binder pitch 6 was added to 100 parts by weight of calcined pitch coke.
After adding 0 part by weight and kneading with a kneader at a temperature of 250 ° C. for 2 hours, the kneaded product was crushed and adjusted to an average particle size of 20 μm (particle size range of 1 to 48 μm) by sieving to give a carbon precursor powder. Was produced. The volatile content of this carbon precursor powder was 14.2%. Then, with respect to 100 parts by weight of this carbon precursor powder, an average particle diameter of 18 μm (particle diameter range 1 to
40 μm) of boron carbide powder at a ratio of 5 parts by weight, V
Dry mix for 1 hour with a mold mixer. The mixed powder is packed in a rubber mold, and CIP molding is performed to obtain 750 × 650 × 350 mm.
A molded body of was obtained. 100% of the obtained molded body in an inert gas
After heat-treating at 0 ° C for 3 hours, move it to the Acheson furnace and
A carbon / boron carbide sintered material was manufactured by heat treatment at a temperature of 0 ° C. for 1 hour.

Examples 2 to 4, Comparative Examples 1 to 4 Using the same calcined pitch coke and binder pitch as in Example 1, 100 parts by weight of calcined pitch coke and 60 parts by weight of binder pitch were added and the temperature was adjusted by a kneader. And kneading after changing the kneading time, and then pulverizing and sieving to prepare carbon precursor powders having different average particle diameters and volatile contents. Then, the boron carbide powders having different average particle diameters were dry-mixed for 1 hour by a V-type mixer while changing the blending weight part with respect to the carbon precursor powder. The obtained mixed powder was molded and heat treated in the same manner as in Example 1 to produce a carbon / boron carbide sintered material.

Comparative Example 5 Binder pitch 6 was added to 100 parts by weight of calcined pitch coke.
Boron carbide powder 5 with 0 parts by weight and an average particle size of 18 μm
A kneader was added thereto and kneaded at a temperature of 250 ° C. for 2 hours to obtain a kneaded product, which was then molded and heat-treated in the same manner as in Example 1 to produce a carbon / boron carbide sintered material.

Comparative Example 6 A carbon / boron carbide sintered material was produced by the same method as in Comparative Example 5 except that raw coke was used instead of the calcined coke of Comparative Example 5.

Table 1 shows the production conditions of the carbon / boron carbide sintered materials in each of the above examples, and Table 2 shows the results obtained by measuring various characteristics. The measured value is a value obtained by the following method. (1) Bulk density (g / cc): Dimensional weight method (JIS R7202-1965) (2) Bending strength (Kg / cm ² ): Three-point bending method (JIS R7202-1965) (3) Specific resistance (μΩcm) : Voltage drop method (JIS R7202-1965) (4) Boron concentration (%): ICP-AES method (ICP emission spectroscopy) (5) B ₂ O ₃ concentration (%): Curcumin absorption spectrophotometric method (JIS R7223- 1
979) (6) Fe content (ppm): ICP-AES method (ICP emission spectroscopy)

[0024]

[Table 1] Footnote (* 1) 100 parts by weight of carbon precursor powder mixed with 100 parts by weight of boron carbide powder (* 2) 100 parts by weight of coke (coke + pitch) mixed with 100 parts by weight of boron carbide powder (* 3) Volatilization of compound Min (* 4) Weight of raw coke

[0025]

[Table 2] [Table Note] The measurement sample was cut from a position of 40 mm from the surface of the carbon / boron carbide sintered material.

From the results shown in Tables 1 and 2, the carbon / boron carbide sintered materials of Examples 1 to 4 in which the boron carbide powder having the same particle size as the carbon precursor powder were mixed at a ratio of 1 to 5 parts by weight were used. Four
It is understood that the bulk density and bending strength are higher than those of the carbon / boron carbide sintered material, and the boron concentration distribution is narrow and the boron carbide is uniformly dispersed. Further, in Comparative Example 5 in which the calcined coke, the binder pitch and the boron carbide powder were mixed and kneaded at the same time, it was observed that B ₂ O ₃ was produced in a large amount and Fe impurities were mixed in a large amount. In that case, cracking occurred during carbonization by firing.

[0027]

As described above, according to the present invention, a carbon precursor powder obtained by adding a pitch to a calcined coke and kneading, crushing the kneaded product, and adjusting the particle size, and a boron carbide powder having the same particle size By using a mixed powder as a raw material, which is dry-mixed in a specific ratio, a high density in which boron carbide is uniformly dispersed,
It is possible to inexpensively manufacture a large-sized carbon / boron carbide sintered material having high strength characteristics. Therefore, it is extremely useful as a method for producing a carbon / boron carbide sintered material used as a neutron shielding material for nuclear reactors including various high temperature materials.

Claims (3)

[Claims] 1. A pitch is added to a calcined coke, which is then kneaded,
Carbon precursor powder 1 obtained by crushing the kneaded product and adjusting the particle size
It is characterized in that 1 to 5 parts by weight of boron carbide powder of the same particle size is dry mixed with 00 parts by weight, the mixed powder is CIP-molded, and then calcined and carbonized at a temperature of 2200 ° C. or less in a non-oxidizing atmosphere. A method for producing a carbon / boron carbide sintered material. 2. The method for producing a carbon / boron carbide sintered material according to claim 1, wherein the particle diameters of the carbon precursor powder and the boron carbide powder are 15 to 50 μm in average particle diameter. 3. The carbon precursor powder has a volatile content of 13 to 15%.
The method for producing a carbon / boron carbide sintered material according to claim 1 or 2.