JPH0948683A - Boron carbide-coated carbon material, its production and plasma opposing material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a boron carbide-coated carbon material suppressing reduction in thermal conductivity, excellent in heat resistance. SOLUTION: In this carbon material obtained by converting the surface of a carbon fiber-reinforced carbon material containing a carbon matrix 1 and carbon fibers 2 into boron carbide, the surface is a face containing one or more kinds of vertically or approximately vertically orientated carbon fibers. The thickness of the surface converted into boron carbide in at least one kind of the carbon fiber 2 part is smaller than that of the surface converted into boron carbide in the carbon matrix 1 part to give the objective boron carbide- coated carbon material.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a boron carbide-coated carbon material having a surface coated with boron carbide, which is particularly suitable for a plasma facing material of a fusion reactor, and a method for producing the same. The present invention also relates to a plasma facing material that is suitably used as a member of a fusion reactor.

[0002]

2. Description of the Related Art Carbon materials have excellent heat resistance, and particularly carbon fiber reinforced carbon composite materials (C / C composite materials) are extremely useful as various members used at high temperatures. However, carbon materials have problems in chemical stability such as lack of oxidation resistance at high temperatures and large chemical corrosion due to oxygen / hydrogen plasma. Therefore, in order to compensate for such defects of the carbon material, the surface of the carbon material is coated with boron carbide. Boron carbide has excellent heat resistance (melting point about 2400
℃), chemically stable and also excellent in abrasion resistance. Therefore, the carbon material coated with boron carbide is useful as a plasma facing material for a fusion reactor, a heat resistant material for space and aviation, and various sliding materials.

As a method for coating the surface of a carbon material with boron carbide, generally, a CVD method, a plasma spraying method and the like are known. However, since the boron carbide coating film formed by these methods has different thermal expansion coefficients between the boron carbide and the carbon material, there is a problem that cracks due to thermal stress and peeling of the coating film are likely to occur.

On the other hand, the inventors of the present invention disclosed in Japanese Patent Laid-Open No. 5-20
In Japanese Patent No. 1781, a method of forming a boron carbide film by a conversion method was proposed. In this method, a boron compound is chemically reacted with the surface of a carbon material to convert the carbon on the surface into boron carbide. According to this method, boron carbide is sequentially generated from the surface of the carbon material toward the inside, and the difference in coefficient of thermal expansion between the boron carbide coating and the carbon material is relaxed, so that the thermal shock resistance is excellent.

However, since the thermal conductivity of boron carbide is lower than that of carbon materials by one digit, when a high heat load is applied in a short time, the heat release to the carbon base material cannot keep up and the temperature rises, resulting in a significant heat load. The surface temperature exceeds the melting point of boron carbide, and melting of boron carbide occurs. When the surface of the boron carbide is melted, the boron carbide is recrystallized during cooling, but the surface smoothness is impaired during repeated melting and cooling cycles. In particular, in the plasma facing material of the fusion reactor, if the smoothness of the surface is impaired and a convex portion is generated, it becomes easy to receive a heat load from the plasma, and the smoothness is further deteriorated.

[0006]

SUMMARY OF THE INVENTION The present invention has been made as a result of various studies in view of the above problems. The invention according to claim 1 provides a carbon material coated with boron carbide, which is excellent in heat resistance while suppressing a decrease in thermal conductivity. The invention according to claim 2 provides a method capable of producing a carbon material coated with boron carbide, which is excellent in heat resistance while suppressing a decrease in thermal conductivity. The invention according to claim 3 provides a plasma facing material, which is made of boron carbide-coated carbon excellent in heat resistance in which a decrease in thermal conductivity is suppressed and whose surface smoothness is not impaired.

[0007]

The present invention provides a carbon material obtained by converting the surface of a carbon fiber reinforced carbon material containing a carbon matrix and carbon fibers into boron carbide, wherein the surface is vertically or almost vertically oriented. The present invention relates to a boron carbide-coated carbon material having at least one kind of carbon fiber, and having a thickness converted into boron carbide in at least one kind of carbon fiber portion smaller than a thickness converted into boron carbide in a carbon matrix portion. Further, the present invention is to convert a surface of a carbon fiber-reinforced carbon material containing a carbon matrix and a carbon fiber having a higher graphitization degree than the carbon matrix, in which the carbon fibers are vertically or almost vertically oriented, to boron carbide. Relates to a method for producing a boron carbide-coated carbon material. The present invention further provides a plasma facing material obtained by converting the surface of a carbon fiber reinforced carbon material containing a carbon matrix and carbon fibers into boron carbide, wherein the surface has one or more kinds of carbon fibers oriented vertically or almost vertically. A plasma facing material having a boron carbide converted thickness in at least one carbon fiber portion that is less than a boron carbide converted thickness in the carbon matrix portion.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The carbon material coated with boron carbide of the present invention and the method for producing the same will be described. As the base material, a carbon fiber reinforced carbon material (C
/ C composite material). The carbon fiber contained may be one kind or two or more kinds. The arrangement of the carbon fibers of the C / C composite material may be one-dimensional, two-dimensional or three-dimensional, and is not particularly limited, but the thermal conductivity of the C / C composite material is anisotropic, Among them, one-dimensional is preferable because it is excellent in the orientation direction of the carbon fibers. However, in the present invention, the surface of the C / C composite material that is converted to boron carbide is a plane in which one or more kinds of carbon fibers are vertically or substantially vertically oriented. Otherwise, the thermal conductivity of the obtained boron carbide-coated carbon material decreases. The term “substantially perpendicular” means that the minimum angle between the plane and the orientation direction of the carbon fibers is preferably 70 degrees or more, more preferably 80 degrees or more. C / C
The composite material is generally a method of filling a carbon matrix by repeating the steps of (1) impregnating a molded body of carbon fiber with a thermosetting resin, tar pitch, etc. and carbonizing the same.
(2) Obtained by a method of filling a carbon fiber molded body with pyrolytic carbon as a carbon matrix.

As the thermosetting resin in the above method (1), it is preferable to use phenol resin, furan resin or the like. The carbonization is preferably carried out at 700 ° C. or higher, and finally the heat treatment is preferably carried out at a temperature of 900 to 3000 ° C. Further, in the method (2), a hydrocarbon gas such as methane, propane, benzene, and acetylene, together with a carrier gas such as argon and nitrogen, if necessary, is formed into a carbon fiber molded body under reduced pressure or normal pressure. It can be blown into the molded body at a temperature of 700 to 3000 ° C. to form pyrolytic carbon. Further, if necessary, it is preferable to perform heat treatment at a temperature of 900 to 3000 ° C. after impregnation, and the method of (1) may be combined.

The carbon fiber used may be any of PAN (polyacrylonitrile) type, pitch type, rayon type and the like, and the characteristics thereof are not particularly limited. In order to obtain a C / C composite material having a high thermal conductivity suitable for a plasma facing material, it is preferable to use a carbon fiber having a high thermal conductivity or a carbon fiber having a high thermal conductivity by graphitizing the C / C composite material. preferable. 10-70 volume% is preferable, as for the volume ratio (fiber volume ratio) of the carbon fiber in a C / C composite material, 20-70 volume% is more preferable, and 35-65 volume% is still more preferable. If the fiber volume ratio is less than 10% by volume, the effect of compounding carbon fibers tends to be small, and if it exceeds 70% by volume, the filling of hydrocarbons tends to be difficult. In addition, the fiber volume ratio refers to the volume occupied by the fibers when the molded body of the C / C composite material is formed (the total volume of the fibers and the volume of the internal space thereof. For example, the molded body form is filled with the fibers. In this case, it can be determined from the percentage of the substantial volume of the fiber (calculated from the weight of the fiber and the true specific gravity of the fiber) relative to the inner volume of the molded body frame.

It is preferable that the carbon matrix has few pores generated between the carbon fibers because the characteristics of the C / C composite material are good. Specifically, the carbon matrix is filled so as to have an open porosity of 0 to 30% by volume. It is preferably 0 to 20% by volume. The open porosity (volume%) is W1: dry weight (g), W2: weight in water (g), W3: saturated water weight (g).
Is calculated and calculated from the following formula (underwater substitution method).

## EQU1 ## Open porosity P = 100 × (W3-W1) / (W
3-W2)

In the present invention, the surface of the C / C composite material is converted into boron carbide to form a boron carbide conversion layer. In the present invention, the boron carbide conversion layer has a boron carbide conversion thickness of at least one carbon fiber portion oriented perpendicularly or substantially perpendicularly to the surface thereof smaller than a boron carbide conversion thickness of the carbon matrix portion. It needs to be one. If the thickness of the converted boron carbide in all the carbon fiber parts is the same as the thickness of the converted boron carbide in the carbon matrix part, the high heat resistance of the present invention cannot be obtained. Further, when a C / C composite material having a boron carbide conversion layer having a uniform thickness is used, arc discharge may occur, the surface temperature may rise rapidly, and the boron carbide may melt. On the other hand, in the case of the conversion layer of the present invention, the electrical resistivity of the entire coating layer can be lowered, so that arc discharge can be prevented from occurring and melting can be prevented.

FIG. 1 shows an example of a schematic sectional view of the boron carbide-coated carbon material in the above case. FIG. 1 shows the case where the thickness of the converted boron carbide in all the carbon fiber parts is shallower than the thickness of the converted boron carbide in the carbon matrix part. The boron carbide conversion layer is shown to be shallower at the boron carbide conversion portion of the carbon fiber bundle 2 than at the boron carbide conversion portion of the carbon matrix 1. The proportion of the carbon fiber part having a converted thickness of converted boron carbide smaller than that of the carbon matrix part having a thickness smaller than that of the converted boron carbide is determined by the volume of carbon fiber capable of forming such a state after the conversion of boron carbide. You can As will be described later, the carbon fiber is a carbon fiber having a higher degree of graphitization than the carbon matrix, and the ratio thereof can be defined by the fiber volume ratio. In order to obtain high thermal conductivity, the ratio of such carbon fibers is the fiber volume ratio to the volume of the C / C composite material,
5 to 70% by volume is preferable, 10 to 60% by volume is more preferable, and 10 to 55% by volume is further preferable.

The thickness of the converted boron carbide in the carbon fiber portion is preferably 10 μm or more, and more than 30 μm or less than the thickness of the converted boron carbide in the carbon matrix portion from the viewpoint of preventing a decrease in thermal conductivity. Is more preferable. The thickness of the converted boron carbide of the carbon fiber is
From the viewpoint of thermal conductivity and life of the conversion layer, 10 to 800 μm is preferable, and 20 to 500 μm is more preferable. On the other hand, the thickness of the converted boron carbide in the carbon matrix portion is
Again from the viewpoint of thermal conductivity and life of the conversion layer, 20 to 1000
μm is preferable, and 50 to 800 μm is more preferable.

The method for producing such a carbon material coated with boron carbide is not particularly limited, but a C / C composite material containing at least one type of carbon fiber having a higher graphitization degree than the carbon matrix is used as a base material to produce the graphite. It is preferable to produce by a method characterized in that the carbon fiber having a high degree of conversion is converted into boron carbide on the surface in which it is vertically or almost vertically oriented, since a carbon material coated with boron carbide having excellent characteristics can be obtained relatively easily. .
This is because the thickness from the surface where the carbon material is converted to boron carbide changes depending on the reactivity of the carbon material, and this reactivity depends on the crystallinity of the graphite crystal of the carbon material, that is, the degree of graphitization. Therefore, when two or more kinds of carbons having different degrees of graphitization are used as constituent elements, selectivity occurs in the boron carbide conversion reaction, and a boron carbide coating film having an uneven conversion thickness can be obtained. The higher the degree of graphitization, the shallower the conversion thickness. Note that the graphitization degree was compared by measuring the X-ray diffraction pattern of a carbon fiber as a raw material and a sample in which a carbide such as a pitch serving as a carbon matrix was graphitized at the same temperature as in the production of the C / C composite material.
This can be done by comparing the degree of development of graphite crystals.

In the above manufacturing method, a C / C composite material containing at least one carbon fiber having a higher degree of graphitization than the carbon matrix is used as a base material. C / C like this
When a composite material is converted to boron carbide, a carbon matrix having a lower degree of graphitization is more likely to cause a conversion reaction than carbon fiber, and therefore the converted thickness of carbon fiber is smaller than the converted thickness of matrix carbon to boron carbide. Carbon fibers are produced which are thin or in some cases not converted to boron carbide at all. That is, the structure is such that unreacted carbon fibers are present in the boron carbide portion where the matrix carbon is converted. Since the thermal conductivity of carbon fiber is one digit higher than that of boron carbide, when unreacted carbon fiber is left in boron carbide, the thermal conductivity can be improved as compared with the case where only boron carbide is used. The higher the degree of graphitization of the carbon fiber, the higher the thermal conductivity and the poorer the conversion reactivity, so that a boron carbide-converted carbon material having a higher thermal conductivity can be obtained, which is preferable.

Further, as the C / C composite material, a C / C composite material composed of two or more kinds of carbon fibers having different graphitization degrees may be used. In this case, since there is a difference in reactivity depending on the degree of graphitization of the carbon fiber, the thickness from the surface of the portion converted into boron carbide differs depending on the type of fiber. That is, the thickness of the fibers having a high degree of graphitization converted to boron carbide is shallower than the thickness of the fibers having a low degree of graphitization converted. Therefore, by using multiple types of carbon fiber and changing the composition,
The volume of unreacted carbon fibers in the boron carbide layer can be controlled. This method can control the thermal conductivity of the boron carbide layer without reducing the total volume ratio of the carbon fibers in the C / C composite material, and is therefore preferable in that the mechanical strength is maintained.

In the C / C composite material used, the degree of graphitization is higher than that of the carbon matrix, and the proportion of the carbon fibers oriented perpendicularly or almost perpendicularly to the surface is C to obtain high thermal conductivity. The fiber volume ratio of this fiber is preferably 5 to 70% by volume with respect to the volume of the / C composite material, and 10 to 60
The volume% is more preferable, and it is further preferable that the content is 10 to 55% by volume. When the degree of graphitization of the carbon matrix and the carbon fiber are significantly different, depending on the reaction conditions for the conversion of boron carbide, only the carbon matrix may be converted to boron carbide, and the carbon fiber may partially remain unreacted. , Which is also within the scope of the invention. In order to convert the surface of the C / C composite material to boron carbide, it is preferable to use a method of reacting the C / C composite material and a boron compound to generate boron carbide. Specifically, (a) a method of reacting a gas of boron oxide with a C / C composite material, (b) a method of arranging the C / C composite material in a mixture of boron oxide and carbon powder and carrying out the reaction are preferable. As a method.

In the method of producing boron carbide by reacting the C / C composite material with a boron compound, the relationship between the thickness of the boron carbide conversion layer produced by the conversion reaction and the selectivity of the reaction due to the difference in the degree of graphitization is , Affected by reaction conditions.
When the reaction temperature is high, the conversion reaction rate increases and the boron carbide conversion layer becomes thicker, but the selectivity of the reaction decreases due to the difference in graphitization degree. In order to obtain the structure of the present invention in which the conversion thickness to boron carbide varies depending on the constituent elements in the C / C composite material, the reaction temperature is kept low to limit the conversion reaction rate, and the selectivity of the reaction is reduced. It is preferable to perform the treatment under the condition that the value is increased. Further, in the unreacted carbon fiber in the boron carbide conversion layer, boron is diffused even if it is not apparently converted into boron carbide. The diffusion of boron into carbon fibers reduces the thermal conductivity of the fibers. The decrease in thermal conductivity due to the diffusion of boron is
Since the higher the temperature is, the larger the temperature becomes, it is preferable to keep the reaction temperature low also from this point. For the above reasons, the conversion reaction temperature to boron carbide is preferably 2100 ° C or lower, more preferably 1600 to 2000 ° C.

The thickness of the boron carbide conversion layer itself can be controlled by changing the amount of reaction raw material, reaction time and the like. Further, the atmosphere of the reaction for conversion is preferably an atmosphere of an inert gas such as argon or a reduced pressure atmosphere in which oxygen is prevented from entering from the outside. Another method for controlling the thickness of the boron carbide conversion layer is to first form a boron carbide conversion layer which is uniformly and completely converted to boron carbide at a high reaction temperature (preferably 1800 to 2300 ° C.). , Then lower the temperature (preferably 1600 to 200
0 ° C.), there is unreacted carbon fiber in the carbon matrix converted to boron carbide below the layer (ie the thickness of the converted portion of carbon fiber is greater than the thickness of the converted portion of the carbon matrix to boron carbide). A method of forming a (thin) layer may be used.

When high thermal conductivity is prioritized, after forming the coating of boron carbide, the boron carbide is removed from the surface by machining or the like, and the thickness of the portion where the carbon fiber is converted to boron carbide. It may be thin, or in some cases
This portion may be completely removed to expose the carbon fiber on the surface. The thickness of the portion converted to boron carbide in the present invention means the thickness until the boron carbide is substantially eliminated,
The obtained boron carbide-coated carbon material is cut at a right angle to the surface along the orientation direction of the surface and the carbon fiber and the cross section is observed by a scanning electron microscope (SEM), or an X-ray microanalyzer (XMA) is used. Can be used to measure. The thickness is obtained as an average value by measuring the thickness of the portion where the carbon fiber is converted to boron carbide and the thickness of the portion where the carbon matrix is converted at 10 or more points at substantially equal intervals in each portion. be able to. When two or more types of carbon fibers are included, the thickness is determined for each type by the above method. In general, carbon fiber and carbon matrix appear alternately in the cross section. In that case, it is preferable to alternately measure the thickness of each adjacent portion at 10 or more points. In, the measurement is made at 10 or more points at substantially equal intervals.

The carbon material coated with boron carbide manufactured as described above can be used as a plasma facing material useful as a member of a nuclear fusion reactor. In the plasma facing material, the plasma facing surface is converted to boron carbide, and the thickness of the carbon fiber portion converted to boron carbide on this surface is smaller than the thickness of the carbon matrix portion converted to boron carbide. . In this case, it is preferable that the carbon fiber reinforced carbon material is processed into a size and shape suitable for the intended plasma facing material, and then the conversion treatment is performed. The plasma facing surface is preferably a surface in which the carbon fibers are oriented vertically or at an angle closest to the vertical with respect to the surface as a whole, and the boron carbide conversion treatment is preferably performed there.

[0023]

Next, embodiments of the present invention will be described. Examples 1 to 3 and Comparative Examples 1 to 4 Pitch-based carbon fiber having high thermal conductivity (trade name: Carbonic HM-50, manufactured by Petka Co., Ltd., thermal conductivity of about 150 W / m).
K) the fiber bundle (the number of filaments: 6000) is oriented in one direction (one dimension), fixed with a metal jig so that the fiber volume ratio is 55% by volume, and 300 to 3 under vacuum.
Impregnation of petroleum pitch at a temperature of 50 ° C and 1000 ° C
Was fired. Then, the jig was removed, the above-mentioned impregnation and firing were repeated 5 times, and then graphitization treatment was performed at 2800 ° C. to prepare a C / C composite material, which was used as a base material. The open porosity was 8% by volume. The carbon fiber of the raw material and the carbide of the pitch were graphitized at the same temperature as the C / C composite material, and the X-ray diffraction pattern of the sample was measured. As a result, both (002) diffraction line and (004) diffraction line were carbon fiber Shows a sharp peak, and graphite crystals have developed. That is, it was found that the carbon fiber had a higher degree of graphitization than the carbon matrix.

This C / C composite material is 40 mm × 20 mm ×
It was processed into 20 mm, placed in a high-frequency induction furnace, and heated to the temperatures shown in Table 1. On the other hand, in a heating furnace different from this,
1 mixture of boron oxide powder and graphite powder (mixing weight ratio 1: 1)
Boron oxide gas was generated by heating to 700 ° C., and was introduced into the above-mentioned high frequency induction furnace together with Ar gas.
It was held for the processing time shown in. The surface layer of the sample taken out from the furnace after cooling was converted into boron carbide. The surface on which the boron carbide was converted was a surface perpendicular to the orientation direction of the carbon fibers. These samples were cut, and their cross sections were scanned electron microscope (SEM) and X-ray microanalyzer (XM
It was analyzed in A). In the direction perpendicular to the conversion surface, the composition of boron and carbon in the carbon fiber bundle portion and the matrix portion was subjected to line analysis to find the distance from the surface where the atomic percentage of boron was 1% or less. . Since each part was alternately shown in the cross-section, the thickness of each adjacent part was measured alternately, and 10 points were measured at almost equal intervals, respectively, and averaged to obtain the average value of boron carbide conversion thickness. And
Further, the boron carbide conversion surface of each of these samples was processed into a disk-shaped test piece having a diameter of 10 mm and a thickness of 3 mm from the boron carbide conversion surface, and the thermal conductivity was measured by the laser flash method. The results are shown in Table 1.

[0025]

[Table 1]

Examples 4 to 6 and Comparative Examples 5 to 8 Fiber bundles (6000 filaments) of pitch-based carbon fibers (having a thermal conductivity of about 150 W / mK and carbon fibers 1) having the same high thermal conductivity as described above. And a PAN-based carbon fiber (Toray Co., Ltd., T-300, thermal conductivity of about 6 W / mK, carbon fiber 2) fiber bundles (6000 filaments) are oriented in one direction, respectively, and made of metal. The sample was fixed with the jig of No. 1 and impregnated with petroleum pitch and fired at 1000 ° C. as in Example 1. After this, remove the jig and further impregnate
The firing process was repeated 6 times, and graphitization treatment was performed at 2800 ° C. to prepare a C / C composite material, which was used as a base material. The fiber volume ratio is 30% by volume for each type of fiber, and the open porosity is 10%.
% By volume. When the X-ray diffraction pattern was measured by the same method as in Example 1, the graphitization degree was higher in the order of pitch-based carbon fiber, carbon matrix, and PAN-based carbon fiber. This sample was processed into 40 mm × 20 mm × 20 mm and placed in a high frequency induction furnace. At the same time, after arranging boron oxide in a distant part in the furnace, it was held in Ar gas atmosphere under the conditions of temperature and time shown in Table 2, respectively. The surface layer of the sample taken out from the furnace after cooling was converted into boron carbide. The surface on which the boron carbide was converted was a surface perpendicular to the orientation direction of the carbon fibers. These samples were cut and processed, and the conversion thickness and the thermal conductivity were measured in the same manner as in Example 1. The results are shown together in Table 2.

[0027]

[Table 2]

[0028]

The boron carbide-coated carbon material according to claim 1 has high thermal conductivity and excellent heat resistance, and is suitable for a plasma facing material such as a fusion reactor. Further, since the electrical resistivity of the entire coating layer can be lowered, it is possible to prevent the occurrence of arc discharge and prevent melting. The method for producing a boron carbide-coated carbon material according to claim 2 is a method for easily producing a boron carbide-coated carbon material having high thermal conductivity and excellent heat resistance, which is suitable for a plasma facing material such as a fusion reactor. The plasma facing material according to claim 3 has high thermal conductivity and excellent heat resistance. Further, since the electrical resistivity of the entire coating layer can be lowered, it is possible to prevent the occurrence of arc discharge and prevent melting.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of an example of a carbon material coated with boron carbide of the present invention.

[Explanation of symbols]

 1 ... Carbon matrix 2 ... Carbon fiber bundle 3 ... Boron carbide conversion layer

Claims (3)

[Claims] 1. A carbon material obtained by converting the surface of a carbon fiber-reinforced carbon material containing a carbon matrix and carbon fibers into boron carbide, wherein the surface has one or more kinds of carbon fibers oriented vertically or substantially vertically. And the boron carbide-converted thickness of the at least one carbon fiber portion is less than the boron carbide-converted thickness of the carbon matrix portion. 2. A surface of a carbon fiber reinforced carbon material containing a carbon matrix and a carbon fiber having a higher degree of graphitization than the carbon matrix, in which the surface of the carbon fiber oriented vertically or almost vertically is converted to boron carbide. A method for producing a carbon material coated with boron carbide, the method comprising: 3. A plasma facing material obtained by converting the surface of a carbon fiber-reinforced carbon material containing a carbon matrix and carbon fibers into boron carbide, wherein the surface has one or more kinds of carbon fibers oriented vertically or substantially vertically. A plasma facing material, which is a surface and has a thickness converted to boron carbide in at least one carbon fiber portion that is thinner than a thickness converted to boron carbide in the carbon matrix portion.