Carbon fiber-reinforced carbon composite materials, processes for their production, and first walls of nuclear fusion reactors employing them

Abstract: A carbon fiber-reinforced carbon composite material, wherein carbon fibers are oriented substantially in the thickness direction, the ratio of the thermal conductivity in the thickness direction to the thermal conductivity in a direction perpendicular to the thickness direction is at least 2, and the thermal conductivity in the thickness direction is at least 3 W/cm·°C.

Description: The present invention relates to carbon fiber-reinforced carbon composite materials, processes for their production and first walls of nuclear fusion reactors made of such carbon composite materials. Carbon fiber-reinforced carbon composite materials (hereinafter referred to simply as C/C composite materials) are light in weight and highly strong and have a feature that they are excellent in the heat resistance and corrosion resistance. Therefore, they are used, for example, for aerospace materials such as rocket nozzles, nose cones or disk brakes of air planes, for heater elements, for hot pressing molds, for other mechanical parts, and for parts of nuclear reactors. Such C/C composite materials are usually prepared by impregnating or mixing a matrix material such as a thermosetting resin such as a phenol resin or a furan resin, or a thermoplastic resin such as pitch, to long or short carbon fibers of e.g. polyacrylonitrile or pitch type, followed by heating and molding, then baking the molded product in a non-oxidizing atmosphere such as an inert gas atmosphere at a temperature of from 600 to 1,000°C, and further densifying the product by impregnating pitch or a resin thereto, followed by baking, or by a chemical vapor deposition method, or a combination of such methods, followed, if necessary, by graphitization. However, the resulting C/C composite materials were not necessarily satisfactory when they were used for the purposes of conducting or removing heat in one direction i.e. in the thickness direction, and they had problems in their practical application. For example, a first wall of a nuclear fusion reactor represents the entire structure in the nuclear fusion reactor, which is disposed to face the plasma, and includes e.g. limiters, diverters, blankets, and parts thereof. Such first wall is disposed close to the plasma and thus is under a severe environmental condition such that it is subjected to heat from plasma and bombardment of plasma particles. Particularly, the limiters and diverters receive high temperature loads, whereby the heat load conditions are particularly severe. As one of materials used for the first wall under such severe conditions, graphite may be mentioned. Graphite is an excellent low atomic number material from the viewpoint of plasma impurities and also has high thermal shock resistance. Figure 4 in the drawings illustrates the most typical conventional first wall wherein graphite is used. In the illustrated first wall, a graphite tile 11 is secured to a metal substrate 3 by means of a fixing plate 8 and a connector 9. When heat from the plasma enters the graphite tile 11 facing the plasma, the heat is conducted to the substrate by the contact thermal conduction and also dissipated by thermal radiation. In such system, the graphite tile 11 and the substrate 3 are in contact with each other merely by the mechanical connection, and the thermal conductivity at the contact portion is not adequate, and cooling tends to be inadequate when the heat load is high or lasts for a long period of time. The conventional first wall has the following problems. When a high heat load (for example, 2 kW/cm² for 3 seconds, or 4 kW/cm² for at least one second) is exerted to the first wall, the surface temperature will be as high as at least about 2,800°C, and the vapor pressure of the graphite tile will be at least about 10⁻³ atm, whereby the loss in thickness by sublimation from the surface of the graphite tile will be as large as about a few tens µm/sec. As a result, inclusion of carbon atoms in the plasma increases, which brings about a problem that the control of plasma impurities will be thereby seriously adversely affected. Further, the loss of the graphite surface is substantial, whereby there is a problem that the useful life of the first wall is short. In conventional nuclear fusion reactors, it is rare that such high heat load is exerted to the first wall, and the conventional first wall may sufficiently provide its function against the above-mentioned problems. However, in order to further improve the level of safety, or for a furture nuclear fusion reactor for which it is expected that a heat load higher than ever will be exerted to the first wall constantly over a long period of time, it is desired to develop a first wall having the above problems adequately solved. FR-A-2 546 878 describes a composite material comprising a fiber and a metal or metalloid matrix for support of ceramic substrate used in the field of electronics. This composite material is light weight and has a high thermal conductivity which is uniform in all directions. In a first International Symposium on Fusion Nuclear Technology, Tokyo, Japan, 10 to 19 April 1988 high heat load tests of a graphite armor first wall with a water cooling jacket has been reported. Heat load tests were performed on actively cooled graphite/stainless steel laminated structures. The graphite armor tile materials employed in these tests are isographite or carbon fibre-reinforced carbon composite. Under these circumstances, the present inventors have conducted various studies to overcome the above-mentioned drawbacks and to obtain a C/C composite material useful for the above-mentioned first wall or the like, and have finally arrived at the present invention. The present invention provides: A carbon fiber-reinforced carbon composite material in the form of a Plate-shaped part having a thickness direction and a longitudinal direction perpendicular to said thickness direction, said composite material having a density of at least 1.6 g/cm³ and a fiber-reinforcement consisting of carbon fibers comprising pitch-type carbon fibers, characterized in that said fibers are oriented in the thickness direction, the ratio of the thermal conductivity in the thickness direction to the thermal conductivity in the direction perpendicular to the thickness direction is at least 2 and the thermal conductivity in the thickness direction is at least 3 W/cm°C. Preferred embodiments of the present invention are indicated in the subcl aims. The present invention also provides a first wall of a nuclear fusion reactor to be disposed to face a plasma of the nuclear fusion reactor, which is composed essentially of a carbon fiber-reinforced carbon composite material as defined in claim 1, and which is to be disposed so that one side which is substantially perpendicular to the thickness direction, faces the plasma. In a preferred embodiment of said first wall a metal is metallurgically bonded to one side of the carbon composite material which is perpendicular to the thickness direction of the carbon composite material and which does not face the plasma. In the accompanying drawings, Figures 1 to 3 are perspective views illustrating the first to third embodiments of the first wall of the present invention. Figure 4 is perspective view illustrating the conventional first wall. Now, the present invention will be described in detail. The carbon fibers useful for the present invention may be of any type, such as polyacrylonitrile (PAN) type or pitch type carbon fibers or carbon fibers prepared by a gas phase growing method. However, high performance pitch-type carbon fibers having high thermal conductivity in the axial direction of fibers, are particularly preferred. The C/C composite materials of the present invention are obtainable by using such carbon fibers. The carbon fibers are oriented in the thickness direction, the ratio of the thermal coductivity in the thickness direction to the thermal conductivity in a direction perpendicular to the thickness direction is at least 2, and the thermal conductivity in the thickness direction is at least 3 W/cm·°C, such C/C composite material can be prepared by the following method. Firstly, long carbon fibers are impregnated with a thermosetting resin, followed by heating to partially cure the thermosetting resin. Such thermosetting resin includes, for example, a phenol resin, a furan resin, an epoxy resin and an unsaturated polyester resin. A phenol resin, particularly a resol-type phenol resin, is preferred. Such thermosetting resin is used usually as dissolved and diluted with a solvent, for example, an alcohol such as ethanol, a hydrocarbon such as hexane, or acetone. The concentration of the thermosetting resin solution is usually within a range of from 10 to 70% by weight, preferably from 20 to 60% by weight. When a curing agent is required as in the case of a furan resin or an epoxy resin, such curing agent is also added to the solution. Such curing agent is added in an amount suitable for the particular resin used. As a method for impregnating long carbon fibers to such thermosetting resin solution, a simple method of dipping the carbon fibers in the solution, may be employed. However, in the case of a long fiber robing, a method of continuously passing it through a tank filled with the solution, is preferred from the viewpoint of the efficiency of the treatment. Further, in this case, it is preferred to apply supersonic waves of a level of from 10 to 50 KHz to the tank filled with the solution, whereby an adverse effect of irregular treatment due to e.g. air bubbles among monofilaments or among weave openings, can be prevented. The carbon fibers impregnated with the thermosetting resin solution is, for instance, passed through rollers to remove an excess solution and then subjected to heat treatment. By the heat treatment, the thermosetting resin is thermally cured. The conditions for the heat treatment vary depending upon the type of the thermosetting resin used. However, the heat treatment is conducted usually at a temperature of from 50 to 300°C, preferably from 80 to 200°C for from 0.2 to 5 hours, preferably from 0.2 to 2 hours. In such a case, it is preferred to gradually raise the temperature to the prescribed temperature in order to avoid abrupt release of the solvent from the thermosetting resin solution coated on the carbon fibers. The heat treatment is preferably conducted by continuously passing the carbon fibers through a heating furnace, from the viewpoint of the efficiency of the treatment. Then, the obtained fiber/resin composite is cut into pieces having a length longer than the thickness of the desired C/C composite material. The length is selected usually within a range slightly longer than the thickness of the desired product and may, for example, be selected within a range of from 15 to 100 mm. The composite pieces are then aligned in one direction so that they will be substantially parallel to one another. Then, a pressure is exerted to the aligned pieces in a direction perpendicular to the longitudinal direction of the fibers, followed by heating and molding. For example, by supplying the composite pieces to a mold by means of a funnel-form feader, the composite pieces are aligned substantially in parallel to one another in the mold, then a pressure is exerted in a direction perpendicular to the longitudinal direction of the fibers under heating at a temperature required for the curing of the resin, to cure the resin and obtain a molded product. Then, the molded product is introduced into a container so that it is surrounded by coke breeze. Then, the container is introduced into an electric furnace, and the temperature is raised to a level of 1000°C, if necessary, under a nitrogen gas stream, for carbonization. If necessary, the carbonized product is introduced into a graphitization furnace and subjected to heat treatment at a temperature of at least 2,000°C under an inert atmosphere. Then, obtained carbonized product or graphitized product is impregnated with a petroleum-type or coal-type pitch or with a thermosetting resin such as a phenol resin or a furan resin, followed by carbonization, after curing the resin in the case where the thermosetting resin is employed. In such case, the thermosetting resin is used usually as dissolved in a solvent such as an alcohol, acetone or anthracene oil to have a proper viscosity. Further, in such case, it is preferred to employ an impregnation method under pressure. For example, a molded carbonized or graphitized product and pitch are introduced into a low pressure reactor (autoclave), which is then heated under vacuum to dissolve the pitch. After the carbonized or graphitized product is immersed in the molten pitch, nitrogen gas is introduced, and the temperature is raised under a low pressure to a level of from 550 to 600°C. Then, the reactor is cooled, and the densified product of the carbonized or graphitized product is withdrawn, and it is carbonized at a temperature of up to 1000°C in the same manner as described above and, if necessary, graphitized. The above-mentioned so-called densification is repeated to obtain a high density C/C composite material having a specific gravity of at least 1.6 g/cm³. If the densification or the resin content in the fiber/resin composite is inadequate, or if the temperature raising rate for carbonization or graphitization is too high, the strength of the fibers in a direction perpendicular to the longitudinal direction of the fibers, tends to be low. In some cases, the fibers are likely to break. Therefore, it is necessary to select proper conditions. The C/C composite material thus obtained is an anisotropic material having high heat conductivity and electrical conductivity in the direction of the thickness. In order to improve the strength in a direction perpendicular to the thickness direction, the obtained C/C composite material may be wound by long carbon fibers or by a carbon material such as a C/C composite material, depending upon the particular purpose. Further, a plurality of composite materials may be bonded by means of e.g. a resin composed essentially of a phenol resin, and the bonded product is again heated to a temperature range at which the C/C composite materials were finally treated, to firmly bond the plurality of pieces of the C/C composite materials to obtain a composite material having a desired size. In the present invention, this C/C composite material may be reinforced in both the thickness direction and the direction perpendicular thereto, by bonding a metal to one side of the C/C composite material, which is substantially perpendicular to the thickness direction of the composite material. Various metals may be employed depending upon the particular purposes of application of the resulting C/C composite materials. However, it is usual to select the metal from, for example, Ti, Cu, Fe, Ni, Cr and alloys thereof. When heat resistance is required, Ti or an alloy of Ti is preferred. For example, in the case of Ti, such an alloy preferably contains Al or V. Such metal is employed usually in the form of a thin plate having a thickness of at least 5 mm, preferably from 0.1 to 1 mm. The bonding may be conducted by a usual method such as vacuum soldering, diffusion bonding or HIP (hot isostastic press). In the present invention, such C/C composite material is used as the main constituting material for the first wall to be disposed to face a plasma of a nuclear fusion reactor, and it is disposed so that one side which is substantially perpendicular to the thickness direction, faces the plasma. Further, in the present invention, it is preferred to bond or bind, by metallurgical bonding or by mechanical binding, a metal to one side of the C/C composite material which is substantially perpendicular to the thickness direction of the C/C composite material. Particularly preferred is the metallurgically bonding. The metal is usually selected from, for example, Ti, Cu, Fe, Ni, Mo, Cr and alloys thereof. As an alloy system, for example in the case of Ti, it is preferred that the alloy contains Al or V. Such metal is used usually in the form of a thin plate having a thickness of at most about 5 mm, preferably from 0.1 to 0.3 mm. However, a stainless steel plate having a thickness of not more than about 50 mm, may be employed as the substrate directly or with inter-position of the above thin plate. The bonding may be made by a conventional method such as vacuum soldering, diffusion bonding or HIP (hot isotactic press). The C/C composite material of the present invention has high heat conductivity and electrical conductivity in at least one direction including the thickness direction and thus is capable of effectively removing or conducting heat. Further, when a metal is bonded as mentioned above, the thermal shock resistance in both the thickness direction and the direction perpendicular thereto is high. Now, the present invention will be described in further detail with reference to Examples. However, it should be understood that the present invention is by no means restricted to such specific Examples. EXAMPLE 1 Long fibers of pitch-originated carbon fiber ("Dialead" manufactured by Mitsubishi Kasei Corporation, 4,000 filaments, fiber diameter: 10 µm) were immersed in an ethanol solution of a phenol resin, and then they were introduced into a drier to remove ethanol at 70°C and then heated to a temperature of at least 100°C to half-cure the phenol resin. The fiber/resin composite thus obtained (tow prepreg) (carbon fiber/resin = 56/44 in weight ratio) was cut into pieces having a length of 40 mm. The pieces were rod-like and rigid with the fibers fixed with the resin. The cut pieces of the composite were aligned in one direction in a mold so that they were in parallel to one another, so as to fill the mold in the form larger than the size of the desired C/C composite material. Then, a low pressure is exerted at 150°C, and the temperature was raised to 250°C over a period of one hour and held at 250°C for one hour for molding and curing. The size after the molding was 101.9 x 102.5 x 40.1 mm. Then, this molded product was introduced into a container packed with coke breeze and heated to 1,000°C over a period of about 50 hours in a state covered with the coke breeze, for carbonization of the resin. Then, this carbonized composite and solid pitch were introduced into an autoclave, and the temperature was raised to 250°C under a reduced pressure condition. Then, nitrogen was permitted to bring the atmosphere to a positive pressure. Then, the temperature was raised to 500°C in 8 hours and then held at 500°C for 5 hours. The pressure during the temperature raising, was maintained at a constant level by adjusting the valve attached to the autoclave. The autoclave was cooled, and the composite material was taken out and carbonized to a temperature of 1,000°C in the same manner as the carbonization of the molded product. The above autoclave treatment and the subsequent carbonization treatment were repeated in a total of three times. Then, the product was introduced into a graphitization furnace and heated to 2,800°C in an argon atmosphere and then cooled to obtain a C/C composite material. The C/C composite material thus obtained had a bulk density of 1.8 g/cm³, and the thermal conductivities in the thickness direction (the same direction as the fiber axis) and in a direction perpendicular thereto, were measured by a laser flash method thermal coefficient measuring device (manufactured by Shinkuriko). The thermal conductivity in the thickness direction was 3.70 W/cm·°C, and the thermal conductivity in the direction perpendicular to the thickness direction was 0.31 W/cm·°C. The ratio was 12.0. This product did not break even when rapidly introduced into the graphitization furnace heated at a temperature of 2,500°C, and was superior also in the thermal shock resistance. EXAMPLE 2 A C/C composite material was prepared in the same manner as in Example 1 except that the size after the molding was 122 x 137 x 44 mm, and the autoclave treatment and the subsequent carbonization treatment were repeated in a total of 4 times. The bulk density of the C/C composite material thus obtained, was 1.83 g/cm³. The thermal conductivity in the thickness direction was 3.61, and the thermal conductivity in the direction perpendicular to the thickness direction was 0.51. The ratio was 7.08. This C/C composite material did not break even when rapidly introduced into a graphitization furnace heated at a temperature of 2,500°C, and it was excellent also in the thermal shock resistance. EXAMPLE 3 By using the C/C composite material obtained in Example 1, a pure Ti plate was bonded to one side of the composite material which is substantially perpendicular to the thickness direction of the composite material. Namely, on a pure Ti plate (melting point: about 1,675°C having a thickness of about 0.3 mm, a Cu solder was placed, and the above C/C composite material was placed thereon. Then, a weight of about 1 kg was placed thereon. In a vacuum furnace, the temperature was raised over a period of about one hour and maintained at about 1,050°C for 5 hours to obtain a desired C/C composite material having Ti bonded thereto. The obtained C/C composite material had adequate reinforcing effects also in a direction perpendicular to the thickness direction. EXAMPLE 4 A C/C composite material was prepared in the same manner as in Example 3 except that in Example 3, the size after the molding was 122 x 137 x 44 mm and the autoclave treatment and the subsequent carbonization treatment were conducted four times in total. The bulk density of the C/C composite material thus obtained was 1.83 g/cm³. The thermal conductivity in the thickness direction was 3.61, and the thermal conductivity in the direction perpendicular to the thickness direction was 0.51. The ratio of the thermal conductivities was 7.08. This C/C composite material did not break even when introduced rapidly into a graphitization furnace heated to a temperature of 2,500°C, and it was excellent in the thermal shock resistance. Then, a pure Ti plate was bonded to one side of this C/C composite material in the same manner as in Example 14. The C/C composite material thus obtained had adequate reinforcing effects also in a direction perpendicular to the thickness direction. EXAMPLE 5 Figure 1 illustrates a first embodiment of the first wall of a nuclear fusion reactor according to the present invention. The first wall 1 comprises the C/C composite material 2 prepared in Example 1, which is vacuum-soldered to a substrate 3. Namely, the composite material 2 is bonded to a stainless steel substrate 3 having a thickness of 10 mm by a copper solder with interposition of a Ti plate 7 having a thickness of 1 mm and a solder portion 4. The substrate 3 is provided with cooling tubes 5 to improve the cooling effects. In this figure, the plasma-facing side 6 is disposed to face the plasma of the nuclear fusion reactor. In this manner, the composite material of the present invention is used as the first wall. Figure 2 illustrates a second embodiment of the first wall of the present invention. In this embodiment, the C/C composite material 2 obtained in Example 1 was used, and a thin metal plate 7' (pure Ti plate) was bonded to one side which is substantially perpendicular to the thickness direction. Namely, on a pure Ti plate (melting point: 1,675°C) 7 having a thickness of about 1 mm, a copper solder was placed, and the above C/C composite material was placed thereon. A weight of about 1 kg was placed thereon. In a vacuum furnace, the temperature was raised over a period of 1 hour and maintained at about 1,050°C for 5 hours to obtain a C/C composite material having Ti bonded thereto with the solder portion 4 interposed therebetween. Then, the thin metal plate 7 of this C/C composite material was mechanically connected to the substrate 3 by e.g. a bolt. Reference numeral 8 indicates a fixing plate, and numeral 9 indicates a connector. The substrate 3 may be provided with cooling tubes as in the first embodiment. Reference numeral 6 indicates the plasma-facing side. Figure 3 illustrates a third embodiment of the first wall of the present invention. In this embodiment, the C/C composite materials 2 and 2' are bonded around a cooling tube 5 with a solder portion 4 interposed therebetween. The plasma-facing side is disposed to face the plasma of the nuclear fusion reactor. Thus, the composite materials of the present invention are used as the first wall. The C/C composite material of the present invention has large heat conductivity and electrical conductivity in the thickness direction and thus is useful for application where heat or electrical conductivity in one direction is required. For example, it is useful as a material for a heat exchanger whereby heat removal or heat conduction is carried out by contacting one flat surface with a cooling jacket, or as a material for a switch. Further, the first wall made of the C/C composite material of the present invention has particularly excellent properties as the first wall, since the surface temperature, the vapor pressure and the sublimation loss are maintained at low levels even when exposed to a high heat load for a long period of time.

Claims: A carbon fiber-reinforced carbon composite material in the form of a plate-shaped part having a thickness direction and a longitudinal direction perpendicular to said thickness direction, said composite material having a density of at least 1.6 g/cm³ and a fiber-reinforcement consisting of carbon fibers comprising pitch-type carbon fibers, characterized in that said fibers are oriented in the thickness direction, the ratio of the thermal conductivity in the thickness direction to the thermal conductivity in the direction perpendicular to the thickness direction is at least 2 and the thermal conductivity in the thickness direction is at least 3 W/cm°C. The carbon fiber-reinforced carbon composite material as defined in claim 1 having a metal bonded thereto, wherein the metal is bonded to one side of the carbon composite material which is perpendicular to the thickness direction of the carbon composite material. A process for producing a carbon fiber-reinforced carbon composite material as defined in claim 1, which comprises impregnating long pitch-type carbon fibers with a thermosetting resin, followed by heating to obtain a fiber/resin composite, cutting the fiber/resin composite into pieces having a length longer than the thickness of the desired composite material, aligning the composite pieces in one direction substantially in parallel to one another, exerting a pressure to the aligned composite pieces in a direction perpendicular to the longitudinal direction of the fibers, molding them to cure the resin, followed by carbonization, and impregnating the carbonized product with pitch or with a thermosetting resin, followed by carbonization and, if necessary, by graphitization. A first wall of a nuclear fusion reactor to be disposed to face a plasma of the nuclear fusion reactor, which is composed essentially of a carbon fiber-reinforced carbon composite material as defined in claim 1, and which is to be disposed so that one side which is substantially perpendicular to the thickness direction, faces the plasma. The first wall according to claim 4, wherein a metal is metallurgically bonded to one side of the carbon composite material which is perpendicular to the thickness direction of the carbon composite material and which does not face the plasma.

Inventors: IOKI KIMIHIRO, NIWA KAZUO, OKUYAMA KOHEI, SEKI HITOSHI, TAKAHASHI TAKANORI, IOKI, KIMIHIRO, NIWA, KAZUO, OKUYAMA, KOHEI, SEKI, HITOSHI, TAKAHASHI, TAKANORI
Applicants: MITSUBISHI ATOMIC POWER IND, MITSUBISHI CHEM CORP, MITSUBISHI HEAVY IND LTD, MITSUBISHI CHEMICAL CORPORATION, MITSUBISHI GENSHIRYOKU KOGYO KABUSHIKI KAISHA, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: C04B35
IPC codes: F16D69, G21B1, C04B35