Abstract:
A C/C composite including a carbon matrix and carbon fibers and exhibiting a crystal structure of the carbon matrix differing between (a) the portion at and in the vicinity of the interface between the carbon fibers and the carbon matrix and (b) the other portion of the carbon matrix and excelling in strength and thermal conductivity is produced by a method which includes intercalating a carbonizing catalyst in carbon fibers, causing said carbon fibers having said carbonizing catalyst intercalated therein to be impregnated with a precursor of a carbon matrix, and then firing the product of impregnation.

Description:
This is a division of application of Ser. No. 07/664,664, filed on Mar. 5, 1991 now U.S. Pat. No. 5,215,689. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a high-performance C/C (carbon/carbon fiber) composite produced by means of intercalation and to a method for the production of the C/C composite. 
     In view of its super-heat resistant characteristics, the C/C composite is already being used in airplane brakes and fuel cells, for example, and other promising future fields of application include aerospace (e.g. rockkets) fusion reactors, and nuclear reactors. 
     The C/C composite is produced by mixing carbon fibers as a reinforcing agent with pitch or resin as a precursor of a carbon matrix and firing the resultant mixture. It, therefore, comprises a carbon matrix produced by firing the precursor and carbon fibers used as a reinforcing agent. 
     2. Description of the Prior Art 
     C/C composites have attracted attention as materials with high utility in extremely high temperature applications and various efforts have been made toward improving their performance. The factors which determine whether a C/C composite can be effectively used at extremely high temperatures are the kind of carbon fibers, the kind of carbon of the composite matrix, the condition of the carbon fibers in the matrix, and the temperature at which the precursor of the matrix carbon is fired for carbonization. 
     It has been demonstrated that properties of the C/C composite such as mechanical strength and thermal conductivity heavily hinge not only on the kinds of raw materials and the method of production but also on the condition of the interface between the carbon fibers and the carbon matrix. With existing techniques, however, it is difficult to control the condition of this interface. 
     In the carbonization of a low molecular compound such as pitch, when a Lewis acid such as aluminum chloride is added to the reaction system, it functions as a catalyst for the cationic polymerization reaction and enhances the carbonization yield and ensures production of carbon of a high capacity for graphitization. On the other hand, when potassium is added to the reaction system instead of a Lewis acid, a reaction having an anion radical as an intermediate proceeds prior to the carbonization reaction. The result is a carbon that has low capability for graphitization. 
     It is well known that an interlayer compound can be formed between layers of graphite by intercalating a guest substance such as aluminum chloride, potassium or other such chemical species. The intercalated chemical species generates an interaction in the ionic bond between the host and the guest. When this interlayer compound is heated to between about 100° C. and about 800° C., the guest chemical species is liberated deintercalated from between the layers of graphite. This temperature range approximately coincides with the temperature range in which the low molecular compound begins to undergo carbonization. 
     The present inventors focused their attention on the interrelation between the aforementioned reaction of catalytic carbonization and the nature of the graphite interlayer compound. They consequently found that the properties of the C/C composite such as strength and thermal conductivity can be improved by using carbon fibers obtained by intercalating therein a carbonizing catalyst as a guest substance and further that the interface between the carbon fibers and the carbon matrix in the C/C composite can be easily controlled by controlling the amount of the guest substance intercalated. This invention has been accomplished on the basis of this finding. 
     SUMMARY OF THE INVENTION 
     This invention is directed to a method for the production of a C/C composite, which method comprises intercalating a carbonizing catalyst in carbon fibers, causing the carbon fibers now having the carbonizing catalyst intercalated therein to be impregnated with a precursor of a carbon matrix, and firing the result to obtain a C/C composite comprising a carbon matrix formed by the firing of the precursor of the carbon matrix and carbon fibers having undergone deintercalation of the carbonizing catalyst, the crystal structure of the carbon matrix differing between (a) the portion at and in the vicinity of the interface between the carbon fibers and the carbon matrix and (b) the remaining portion of the carbon matrix, and to a C/C composite to be produced by the method described above. 
     The above and other objects and features of the invention will become apparent from the following detailed description with reference to the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view showing the crystal structure of the carbon matrix of C/C composite of this invention. 
     FIG. 2 is a graph showing the results of a three-point bending test for a C/C composite in each of Example and Comparative Experiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the method of this invention, the carbonizing catalyst intercalated in the carbon fibers is deintercalated during the aforementioned firing. The condition of crystallization of the carbon matrix at and in the vicinity of the interface between the carbon fibers and the carbon matrix can be controlled by varying the amount and the kind of the carbonizing catalyst intercalated. 
     Properties of the C/C composite such as strength and thermal conductivity can thus be controlled to meet specific requirements. 
     The method of this invention begins with the intercalation of at least one carbonizing catalyst in carbon fibers. Then, the carbon fibers which have the carbonizing catalyst intercalated therein are impregnated with a precursor of a carbon matrix. 
     The carbon fibers for use in this invention are not particularly restricted but may be selected from among carbon fibers heretofore used in the field. Carbon fibers which can be effectively used include coal tar pitch type carbon fibers, oil tar pitch type carbon fibers, and polyacrylonitrile type carbon fibers. 
     The carbonizing catalyst used is a chemical species for catalyzing the carbonization reaction. Specific examples are aluminum chloride and potassium. 
     Though the amount of the carbonizing catalyst intercalated is not particularly restricted, it is generally desired to be in the range of from about 0.5% by weight to about 10% by weight based on the carbon fibers. The carbonizing catalyst fails to manifest a discernible effect if the amount is less than 0.5% by weight. The finally produced C/C composite is deficient in strength if the amount exceeds 10% by weight. Particularly desirably, this amount is in the range of from 2% by weight to about 5% by weight. 
     The intercalation of the carbonizing catalyst to the carbon fibers can be carried out by any of the conventional methods such as the vapor-phase method and the liquid-phase method, for example. This operation is performed under conditions suitable for the quality which the C/C composite is required to exhibit. To be more specific, a C/C composite which satisfied predetermined requirements is obtained by controlling the amount of the carbonizing catalyst to be intercalated and consequently suitably controlling the condition of the interface between the carbon fibers and the carbon matrix in the C/C composite. 
     The strength of carbon fibers is generally impaired when a chemical species is intercalated therein. In the present invention, however, since the chemical species to be intercalated functions as a catalyst, the amount of this chemical species intercalated is so small as not to have an adverse effect on the strength of the carbon fibers. 
     Substances which can be effectively used as the precursor of the carbon matrix include pitches such as coal tar pitch and petroleum pitch, hydrocarbon gases such as methane and propane, and thermosetting resins such as phenol resin and furan resin. 
     The firing treatment given to the carbon fibers impregnated with the precursor of carbon matrix is generally carried out at a temperature in the range of from 500° C. to 3,300° C. When a pitch or a thermosetting resin is used as the precursor, the carbon fibers may be uniformly mixed with the pitch or resin as is commonly practised, whereafter the resultant mixture is subjected to the firing treatment. When a hydrocarbon gas is used as the precursor, the carbon fibers may be exposed to the gas and coated with a deposited layer obtained from the gas. 
     The C/C composite fails to acquire the required quality if the temperature of the firing treatment falls short of the lower limit, 500° C. The matrix of the composite is converted into graphite if the temperature exceeds 1,500° C. and the highest practical temperature is 3,300° C. 
     Desirably, the amount of the carbon fibers used is approximately in the range of 5 to 60% by volume, preferably 40 to 60% by volume, based on the total amount of the carbon fibers and the precursor. 
     The quality of the C/C composite becomes too poor for practical use if this amount falls short of the lower limit, 5% by volume. 
     The carbon fibers cannot be impregnated with the precursor if this amount exceeds 60% by volume. 
     When the carbon fibers impregnated with the precursor are subjected to the firing treatment described above, the following phenomenon ensues. 
     The carbonizing catalyst intercalated in the carbon fibers is liberated (deintercalated). In the meantime, the precursor is carbonized to form the matrix of the C/C composite. In consequence of the deintercalation of the carbonizing catalyst from the carbon fibers, there arises a difference in crystal structure of the carbon matrix differing between (a) the portion at and in the vicinity of the interface between the carbon fibers and the carbon matrix and (b) the remaining portion of the carbon matrix. 
     One example of this difference in the crystal structure of the carbon matrix is shown in FIG. 1, wherein reference numeral 1 designates a carbon fiber, 2 a carbon matrix and 3 a carbon layer having a more graphitized crystal structure than the carbon matrix 2. 
     The differing crystal structure of carbon matrix at and closely adjoining the interface between the carbon fibers and the carbon matrix varies with the kind and amount carbonizing catalyst intercalated in the carbon fibers. 
     By adjusting the various conditions mentioned above, therefore, the structure of the interface aimed at can be obtained. The relation between the structure of the interface and the conditions mentioned above can be easily determined by an experiment. 
     The C/C composite of this invention enjoys highly satisfactory adhesiveness of the matrix to the carbon fibers in the interface and, therefore, is greatly superior to conventional C/C composites in such properties as mechanical strength and thermal conductivity. 
     Now, this invention will be described more specifically below with reference to a working example, which is intended to be merely illustrative of and not in any sense limitative of this invention. 
     EXAMPLE 
     Separate portions of a refractory glass tube were charged with two grams of pitch type carbon fibers and one gram of aluminum chloride and, after displacement of the remaining gaseous phase with nitrogen, sealed by fusion. The reaction tube was placed in an electric furnace and the carbon fiber portion and the aluminum chloride portion thereof were maintained respectively at 120° C. and 100° C. for two hours so as to maintain the chlorine pressure in the reaction tube at 100 Torrs. As a result, 2.05 grams of carbon fibers having AlCl 3  intercalated therein (AlCl 3  content about 2.5% by weight) were obtained. 
     The carbon fibers were packed in a glass tube, impregnated under a vacuum with coal tar pitch to a volume content of about 50% and heat-treated in an autoclave (at 800° C. under 100 atmospheres), to produce a C/C composite of the present invention. A test piece cut from this C/C composite was subjected to a three-point bending test. As a result, the relationship between the load and the distance was as shown by curve A in FIG. 2. This test piece was further subjected to a physical property test and was consequently found to exhibit a specific gravity of 1.53 kg/cm 3 , Young&#39;s modulus of 3.54 kg/cm 2  and bending strength of 34.1 kg/cm 2 . 
     COMPARATIVE EXPERIMENT 
     A C/C composite was produced by following the procedure of Example 1, except that two grams of the same pitch type carbon fibers as used in Example 1 were used without intercalation of AlCl 3 . A test piece cut from this C/C composite was subjected to a three-point bending test and to a physical property test. The results of the three-point bending test were as shown by curve B in FIG. 1. As a result of the physical property test, the test piece was found to exhibit a specific gravity of 1.14kg/cm 3 , Young&#39;s modulus of 1.95 kg/cm 2  and bending strength of 20.1 kg/cm 2 . 
     It is clear from the test results that the C/C composites of this invention were superior to the C/C composite of the Comparative Experiment in three-point flexural strength and thermal conductivity. It will also be noted that C/C composites possessing different magnitudes of strength and thermal conductivity were obtained by using different kinds of carbonizing catalyst. 
     Thus, the method of this invention enables control of the condition of the interface between carbon fibers and carbon matrix.