Process for producing graphite blocks

Abstract: Graphite blocks are produced by a process wherein one or plural polymer films having a thickness of from 1 to 400 µm and selected from aromatic polyimides, aromatic polyamides and polyoxadiazoles are heat treated to obtain carbonaceous films. A plurality of the carbonaceous films are hot pressed under certain conditions to obtain thick graphite blocks. The blocks have a remarkably improved rocking characteristic and are useful as radiation optical elements such as X-ray or neutron ray monochromators or filters. The blocks may be curved to obtain bent-type graphite articles.

Description: This invention relates to a process for producing graphite blocks which have utility as X-ray and neutron ray monochromators, neutron ray filters and the like. Graphite holds an important position as industrial materials because of its outstanding heat and chemical resistances, and high electric conductivity, and has been widely used as electrodes, heating elements and structural materials. Especially, single crystal graphite has good spectral and reflective characteristics and has been widely used as X-ray or neutron ray monochromators, filters or spectral crystal articles. Natural graphite may be used for such purposes. Natural graphite with high quality occurs in an extremely limited amount and is intractable because of its powder or block form. Therefore, efforts of producing artificial graphite having such characteristics as natural single crystal graphite have been made. The production of such artificial graphite can be broadly classified into the following two processes. In the first process, graphite is produced by separation from the melts of Fe, N/C system, decomposition of carbides of Si, Al and the like, or cooling of carbon melts under high temperature and high pressure. The graphite obtained by the process is called Kish graphite, and has the same properties as those of natural graphite. According to this process, however, only fine flakes of graphite are obtained. Therefore, together with the complexity of the manufacturing process and the expensive cost, this process has not been used in the industrial production. The second process is one in which graphite is produced by pyrogenic deposition in a gas phase and hot working of gaseous hydrocarbons wherein re-annealing is effected at a temperature of 3400°C for a long time under pressure. Graphite thus obtained is called highly oriented pyrographite (HOPG) and has almost the same properties as those of natural graphite. This process enables one to produce graphite of very large sizes, unlike Kish graphite. This process has, however, the disadvantage that the manufacturing process is complicated with a low yield and the cost is very high. In order to solve the problems involved in the above two processes and to produce graphite easily and inexpensively, graphitization of various organic matters or carbonaceous materials by heating at temperature not lower than 3000°C has been attempted. In this process, graphite having the same characteristics as those of natural graphite or Kish graphite cannot be obtained. Most of the polymeric materials cannot rather be used for this purpose. For example, the thermal treatment has been attempted to graphitize polymers such as phenolformaldehyde resins, polyparaphenylenes, polyparaphenylene oxides, polyvinyl chloride and the like. Since all of these polymers belong to a class of non-graphitizable materials, any product having a high degree of graphitization has not yet been obtained. For example, natural graphite and Kish graphite have an electric conductivity, which is the most typical property of graphite, of from 1 x 10⁴ S/cm to 2.5 x 10⁴ S/cm. In contrast, only the product having 1 x 10³ S/cm to 2 x 10³ S/cm can generally be obtained by this process. This indicates that graphitization does not well proceed in this process. Several studies were made to solve the problems of the manufacturing process of graphite from such polymers as set out above wherein a number of polymers were used for graphitization. As a result, it was found that films of polymers such as an aromatic polyamide (PA), a polyoxadiazole (POD), an aromatic polyimide (PI), three polybenzobisthiazoles (PBBT), a polybenzooxazole (PBO), a polybenzobisoxazole (PBBO), a polythiazole and the like could be easily graphitized. Based on this finding a graphitization process was proposed in European Patent Application No. 0 205 970. According to this process, graphite having a high degree of graphitization can be obtained easily within a short time by heating the polymers indicated above to temperatures not lower than 1800°C, preferably not lower than 2500°C. The degree of graphitization is often expressed by X-ray diffraction parameters such as a lattice constant and a crystallite size in the direction of c axis, or by a rate of graphitization calculated therefrom, along with electric conductivity. The lattice constant is calculated from the (002) refraction line of X-ray, and a value nearer to the lattice constant of natural single crystal graphite of 6.708 angstroms indicates a more developed structure of graphite. The crystallite size in the direction of c axis is calculated from the half-width value of the (002) refraction line. A larger crystallite size indicates a more development of the plain structure of graphite. The crystallite size of natural single crystal graphite is 1000 angstroms or over. The degree or rate of graphitization is calculated from face spacing (d₀₂₂) (Les Carbons Vol. 1, p. 129, 1965). As a matter of course, the degree of graphitization of natural single crystal graphite is 100%. The electric conductivity is a value determined along the direction of ab plane of graphite wherein a larger electric conductivity shows a more resemblance to the graphite structure. Natural single crystal graphite has an electric conductivity of 1 x 10⁴ to 2.5 x 10⁴ S/cm. Another X-ray diffraction parameter used to evaluate the graphite structure is a rocking characteristic showing the manner of superposition of the ab planes. This is called a diffraction intensity curve which is obtained by rotating the crystal when monochromatic parallel X-ray fluxes are passed and measuring by fixing the value of 2ϑ at the angle where the (00ℓ) diffraction line appears and rotating the angle, ϑ. This value is evaluated by the half-width value of the absorption and expressed by the angle of rotation (°). A smaller value shows more clearly superposed ab planes. The process of producing graphite from afore-described polymer films of the specific type is a very good process since it is simple and inexpensive. However, further studies revealed that this process had several drawbacks. The first problem is that thick graphite blocks cannot be produced according to the process. The graphitization reaction may be considered to be apparently irrespective of the thickness of the starting film. In fact, this reaction depends strongly on the thickness of starting film. This has not been known but the above fact was experimentally confirmed. For instance, Table 1 shows the results of a test where four films of a polyoxadiazole with different thicknesses were graphitized to determine the lattice constant, degree of graphitization and electric conductivity along the ab planes of the resultant graphite. Thickness of Starting POD Film (µm) Treating Temperature (°C) Lattice Constant (angstroms) Degree of Polymerization (%) Electric Conductivity (S/cm) 5 2600 6.710 99 9800 25 2600 6.713 97 7800 100 2600 6.720 94 6100 450 2600 6.731 87 4900 The above results demonstrate that the thickness of the POD film apparently influences how the graphitization reaction proceeds. For example, the degree of the graphitization varies from 99 to 87%, depending on the thickness of the film. This reveals that a thin film of graphite is obtained, but a thick graphite block is difficult to obtain. The second problem of the prior art process is that mere heating of polymer materials does not improve the rocking characteristic. The rocking characteristic is an important characteristic when graphite crystals are used as an X-ray optical element although the thickness and rocking characteristic of graphite crystal depend on the type of radiation optical element as will be described hereinafter. With the POD graphite indicated in Table 1, the rocking characteristic is 6.7° for the starting film thickness of 5 µm, 10.5° for the starting film thickness of 25 µm, 12° for the starting film thickness of 100 µm, and 17° for the thickness of 400 µm. Thus, the rocking characteristic is far from satisfactory. Thick radiation optical elements having a good rocking characteristic cannot be obtained by mere heating of polymer films for graphitization. It is considered that the cause for the poor rocking characteristic resides in a difficulty in orientation of the ab planes because a greater film thickness permit gases to generate in larger amount from the inside of the film undergoing the thermal treatment. The older, but post-published EP-A 0 305 197 describes a method for making a graphite sheet or block, said method comprising the following steps: pressing at least two films made of a polymer selected from the group of polyoxadiazoles (POD), aromatic polyamides (PA) and aromatic polyimides (PI) at a pressure of not lower than 0.4 MPa (4 kg/cm²) and a temperature of not lower than 2200°C; in addition the at least two films may be thermally treated before the hot pressing step. It is an object of the invention to provide a process for producing graphite blocks from polymer films which solves the problems of the prior art processes. It is another object of the invention to provide a process for producing thick graphite blocks which are particularly suitable for use as X-ray and neutron ray monochromators and neutron ray filters. The above objects are achieved according to the invention by a process which comprises heat treating a 1 to 400 µm thick film of a polymer selected from the group consisting of aromatic polyimides, aromatic polyamides and polyoxadiazoles at a temperature sufficient to convert the film to a carbonaceous film, superposing a plurality of the carbonaceous films, and subjecting the plurality of carbonaceous films to hot pressing at a pressure of not lower than 800°C in such a way that the pressure is increased from zero to a level not higher than 200 kg/cm² and decreased to zero or a level lower than the first-mentioned level, and the pressure increasing and decreasing procedure is repeated several to several tens times whereby the resultant graphite block is wrinkle-free. Accordingly the pressure is thus intermittently, not continuously, applied to the films. The invention is advantageously developed by the measures mentioned in the subclaim. For appropriate application as radiation optical elements, the graphite block should preferable have a certain thickness and a certain rocking characteristic depending on the type of optical element. When used as an X-ray monochromator, the graphite block should preferably have a thickness of not smaller than 0.1 mm and a rocking characteristic of not larger than 1°. For application as a neutron ray monochromator, the thickness should preferably be not smaller than 2 mm with a rocking characteristic of not larger than 1.5°. and for application as a neutron ray filter, the thickness should preferably be not smaller than 1 cm with a rocking characteristic of not larger than 4°. The graphite block having such a great thickness as indicated above with a good rocking characteristic can be obtained according to the process of the invention. Figs. 1a to 1c are, respectively, schematic sectional views of structures of carbonaceous films for comparison; Figs. 1a to 1c show the inner structure of a carbonaceous film 10 obtained after the heat treatment. Fig. 1a shows a thick polymer film which has been treated, in which molecules 11 are in disorder, as shown, with the gas 12 being left without removal. Fig. 1b shows the result of a heat treatment of a plurality (13) of films which are merely stacked causing the films 11 to be deformed by shrinkage as shown with the tendency towards the disorder of the inner structure. If the thermal treatment is effected under pressure, there is the possibility that the carbonaceous films are broken as shown in Fig. 1c. optical element made of a double-bent graphite article. The first step of the process according to the present invention is first described wherein a film of a certain polymer is thermally treated to provide a carbonaceous film, and a plurality of the carbonaceous films are superposed. In the practice of the invention, starting films are made of a polymer selected from aromatic polyimides, aromatic polyamides and polyoxadiazoles. Polyoxadiazoles useful in the present invention include, for example, poly(p-phenylene-1,3,4-oxadiazole) having recurring units of the following formula Polymers of other oxadiazole isomers are also usable including poly(m-phenylene-1,3,4-oxadiazole), poly(p-phenylene-1,2,4-oxadiazole), poly(m-phenylene-1,2,4-oxadiazole), poly(o-phenylene-1,3,4-oxadiazole), poly(o-phenylene-1,2,4-oxadiazole), and copolymers of these oxadiazole isomers. The aromatic polyimides are polymers which are obtained by reaction between dicarboxylic acid anhydrides and aromatic primary diamines and are represented by the following general formula Poly[N,N'-(p,p'-oxydiphenylene)pyromellitimide is typical of the aromatic polyamide of the above formula wherein R₁ is and R₂ is and is commercially available from Du Pont De Nemours under the designation of Kapton. In the above formula, R₁ may be and the like, and R₂ may be The aromatic polyamides used in the present invention are ones which have recurring units of the following formula wherein R₃ represents and R₄ represents The film of these polymers have a thickness of from 1 to 400 µm, preferably from 1 to 200 µm. If the film thickness is over 400 µm, gases are produced in the films during the course of carbonization and graphitization, and the resultant carbon precursor does not become homogeneous in the inner structure and is thus non-graphitizable carbon. If this precursor is subsequently hot pressed, graphite of good quality cannot be obtained. The lower limit of the film thickness is not critical, but if the thickness is less than 1 µm, a greater number of carbonaceous films have to be produced for the production of graphite blocks for the same thickness, thus being poor in economy. The thermal treatment of the polymer film or films is carried out at a temperature sufficient to provide a carbonaceous film. The temperature is usually in the range of from 450 to 2000°C. Although the thermal temperature may be performed at temperatures over 2000°C, better results are obtained when hot pressing the thermally treated films in the temperature range defined above. The thermal treatment is a preliminary step prior to the hot pressing. In the thermal treatment the polymer films are treated separately. After the carbonaceous films have been produced through such a preliminary thermal treatment as described above, a plurality of the carbonaceous films are superposed and subjected to hot pressing which is a real heating and pressing procedure, so that graphitization of the carbonaceous films is caused to proceed thereby obtaining a graphite block. In the hot pressing step, the manner of application of the pressure and temperature control are important. More particularly, the hot pressing should be effected by pressing the films while removing wrinkles from the carbonaceous films which have been produced during the thermal treatment. The time of the hot pressing is not critical and is sufficient to allow satisfactory press bonding. The time is, for example, in the range of from several tens minutes to several hours. It will be noted that when polymer films are thermally treated and hot pressed, the thickness is reduced to about half the initial thickness. The thermal treatment and hot pressing are usually carried out in an inert gas atmosphere such as N₂, Ar or the like. Through the above production process, there is obtained a thick graphite block which has a remarkably improved rocking characteristic. It will be noted that when hot pressed, the thickness of the starting films is reduced to about 1/2. Examples of the invention are described. In the examples, the extent of graphitization was evaluated by measuring the lattice constant, rate of graphitization, electric conductivity and rocking characteristic according to the following procedures. (1) Lattice Constant (C o ) The X-ray diffraction line of sample is measured by the CuKα ray using a X-ray diffractometer known in the art. The value of C o is calculated from the (002) diffraction line which appears in the neighbourhood of 2ϑ equal to 26 to 27° by the use of the following Bragg's equation: nλ = 2dsinϑ where 2d is equal to C o , n is equal to 2, and λ is the wavelength of X-ray. (2) Degree of Graphitization (%) Degree of graphitization is calculated from the value of face spacing (d) according to the following equation: d₀₀₂ = 3.354g + 3.44(1-g) where g represents the degree of graphitization, from the perfect graphite when g = 1 to the amorphous graphite where 9 = 0. (3) Electric Conductivity (S/cm) The sample is fitted with four-terminal electrodes by using silver paste and golden wires. The electric conductivity is determined by applying the electric current to the outer electrodes and measuring the voltage drop at the inner electrodes, and further determining the width, length and thickness of the sample through a microscope. (4) Rocking Characteristic (°) The rocking characteristic at the peak position of the (002) diffraction line of graphite is measured by the use of an common X-ray diffraction apparatus. The rocking characteristic is determined as the half-value width of the obtained absorption. As described before, the wrinkles or internal strains are produced in the carbonaceous films during the heat treatment. In the hot pressing step, it is important how to remove these defects. The intermittent application of pressure in accordance with the present invention is very effective in the removal. The intermittent application of pressure is particularly described in examples. Better results are obtained when the maximum pressure is varied depending on the temperature. The maximum value of the pressure is suppressed to a relatively low level in a low temperature range and is increased in a high temperature range. More particularly, when the temperature is 2000°C or below, the pressure up to 50 kg/cm², preferably not higher than 20 kg/cm², is applied to the carbonaceous films at which the films are kept for a given time, e.g. several minutes, and then the pressure is returned to zero or a given low level, e.g. 10 kg/cm², followed by repeating the above cycle several to several tens times. Over 2000°C, the pressure is intermittently applied from 20 kg/cm² to a maximum level of not higher than 200 kg/cm² in the same manner as set out above wherein the maximum pressure level is reduced to a level lower than 20 kg/cm² or from 0 to 20 kg/cm and then the pressure application and reduction cycle is repeated a desired number of times. Example 1 Each of 25 µm thick films of polyoxadiazole, aromatic polyamide and aromatic polyimide was placed between graphite plates and heated at a heating rate of 20°C/minute up to 1000°C, at which it was maintained for 1 hour and was thus heat treated, thereby obtaining a carbonaceous film. Twenty carbonaceous films of the respective polymers were superposed and hot pressed by the use of a ultra-high temperature hot press, thereby obtaining graphite blocks. The hot pressing was effected in such a way that while the films were heated at a rate of 10°C/minute in the temperature range of from 1000 to 2800°C, a pressure was applied to the films gradually from 0 to 20 kg/cm² at which the films were kept for a given time, and the pressure was reduced to zero, followed by repeating the pressure application and the reduction 12 times intermittently in the above temperature range. More specifically, a pressure was gradually increased from 0 to the maximum level of 20 kg/cm² in about 2 minutes, at which the films were kept for about 2 minutes, followed by returning to a pressure-free condition in about 1 minute. While keeping the pressure-free condition for 3 minute, the above procedure was repeated 12 times in total. Thereafter, the temperature was raised to a level of higher than 2800°C and the maximum pressure was elevated to 50 kg/cm². The intermittent pressure application procedure as stated above was repeated until the maximum temperature reached 3000°C. At 3000°C, the pressing for 1 hour under a pressure of 50 kg/cm² was effected. The physical properties of the resultant graphite blocks are shown in Table 2. Film Material Lattice Constant C o [0.1 nm] (angstroms) Degree of Graphitization (%) Electric Conductivity (S/cm) Rocking Characteristic (°) polyoxadiazole 6.708 100 19100 0.62 aromatic polyamide 6.708 100 19200 0.60 aromatic polyimide 6.708 100 20000 0.51 The above results demonstrate that the rocking characteristic of the graphite blocks is in the range of from 0.51 to 0.62° and is thus very excellent.

Claims: A process for producing a graphite block which comprises heat treating a 1 to 400 µm thick film of a polymer selected from the group consisting of aromatic polyimides, aromatic polyamides and polyoxadiazoles at a temperature sufficient to convert the film to a carbonaceous film, superposing a plurality of the carbonaceous films, and subjecting the plurality of the superposed carbonaceous films to hot pressing at a temperature of not lower than 800°C in such a way that the pressure is increased from zero to a level not higher than 200 kg/cm² and decreased to zero or a level lower than the first-mentioned level, and the pressure increasing and decreasing procedure is repeated several to several ten times, whereby the resulting graphite block is wrinkle-free. A process according to claim 1, wherein the hot pressing is effected at a temperature of 2000°C or below in such a way that a pressure up to 20 kg/cm² is applied to the carbonaceous films at which the films are kept for a given time and then the pressure is returned to zero or a given low level, and this pressure application process is repeated several to several ten times, and then at a temperature higher than 2000°C in such a way that a pressure higher than 20 kg/cm² is applied to the films at which the films are kept for a given time and then the pressure is returned to a level not higher than 20 kg/cm², and the above procedure is repeated several to several ten times, thereby obtaining a graphite block.

Inventors: MURAKAMI MATSUAKI, NAKAMURA KATSUYUKI, NISHIKI NAOMI, WATANABE KAZUHIRO, YOSHIMURA SUSUMU, MURAKAMI, MATSUAKI, NAKAMURA, KATSUYUKI, NISHIKI, NAOMI, WATANABE, KAZUHIRO, YOSHIMURA, SUSUMU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1989
Main classification: C04B35
IPC codes: C04B35