Optical elements for radiation comprising graphite films

Abstract: This invention relates to optical elements for radiation which comprises a graphite film obtained from a film of a polymer such as polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene isophthalamide, polyphenylene benzoimidazole, polyphenylene benzobisimidazole, polythiazole or poly-p-phenylene-vinylene The graphite film is obtained by pyrolysis of the polymer film at 2800°C or higher at a pressure of not lower than 4 kg/cm² in vacuum or in an inert gas. Alternatively, the element may be made of a graphite film obtained from the polymers mentioned above and intercalated with a metal halide. The intercalated films are readily bonded by pressing in the form of a thick sheet or block. Still alternatively, the graphite films and the intercalated films may be superposed alternately and bonded by pressing, or the intercalated films may be sandwiched between the graphite films and bonded together to form a thick sheet or block. The optical element has application as a lens, monochromater, filter or analyzer for radiations

Description: This invention relates to optical elements for radiation which are particularly useful for X-ray spectroscopy, neutron spectroscopy and the like spectroscopic analyses and which are made of artificial graphite. As is known in the art, optical elements which are usually used in optical instruments for X-rays such as X-ray spectroscopes, X-ray microscopes and the like make use of the Bragg reflection of crystals although, in a specific case, total reflection of X-rays passing very closely to a reflection surface may be utilized. The crystals used for the above purpose should have a complete crystal structure and should be obtained in a desired size. Moreover, such crystals should have a small absorption coefficient for X-rays and should have appropriate flexibility when applied to curved spectroscopes. One of crystals which satisfy the above requirements is graphite. Graphite has a small absorption coefficient against X-rays and has been often employed as an optical element for X-rays. However, the single crystal of natural graphite with a large area cannot be obtained. Accordingly, it is usual to artificially obtain graphite crystals by hot processing of a pyrolytic deposit of hydrocarbon. For instance, there is known as artificial graphite compressionannealed pyrographite (CAPC) or high-oriented pyrographite (HOPG) commercially sold from Union Carbide Inc. These graphite products are produced by pyrolyzing a gaseous hydrocarbon at a temperature of about 1000°C to obtain graphite crystals and annealing the crystals at 3600°C over a long time of, say, several weeks under pressure. EP-A-0 223 047 discloses a graphite film which is produced by intercalating a donor type, acceptor type or covalent type material between the layers of a graphite film obtained by the thermal treatment of polyphenylene oxadiazole. EP-A-0 203 581 describes the preparation of graphite by heat treating various polymers such as polyphenylene oxadiazole and polybenzothiazoles. EP-A-0 205 970 discloses the preparation of a graphite film or fibre by heat treating a film or fibre of a polymer such as polyphenylene oxadiazole under tension at a temperature of 400°C to 700°C, and then graphitising the film or fibre by heat treating it at 1600°C or above. As is known, the Bragg equation is expressed as 2dsin ϑ = λ where d is the distance between the successive lattice planes, ϑ is a reflection angle, and λ is a wavelength of the reflected X-ray. It is stated that with the graphite of Union Carbide Inc., where a monochromatic X-ray, e.g. a K α line of Cu radiation (λ = 1.5418 x 10⁻¹⁰m (1.5418 Å)), reflects on the (002) plane, the distance between the single crystals of the graphite, i.e. d = 3.354 x 10⁻¹⁰m (3.354 Å). The half-value width of the reflection line, Δ δ 002, is stated to be about 0.7°. However, these artificial graphite products involve the problem that annealing under very high temperature and long time conditions at a high pressure is required as mentioned above, thus the production process being complicated with high production costs. For focussing X-rays, it is usual to appropriately bend a single crystal plate of silicon or to form a curved lens of graphite by machining. These are also complicated in the fabrication with high production costs. For the purpose of providing artificial graphite sheets with a large area which are simply fabricated without resorting to complicated procedures such as compression annealing and are thus inexpensive but which have complete crystallinity and good flexibility, we proposed in United States Patent No. 919,117 (corresponding to EP-A-0 219 345) graphitization of a polyphenylene oxadiazole (POD) by treatment at 2800°C or higher. The graphitized sheet was flexible and was found to be suitable as a radiation optical element such as for X-rays. The graphitized POD obtained by treating starting POD at a normal pressure at temperatures not lower than 2800°C has the following physical properties. (1) Reflection lines against CuK α (1.5418 x 10⁻¹⁰m, 1.5418 Å) are those corresponding only to (002), (004) and (006) planes. (2) The reflection angle (2ϑ) at the (002) plane is 26,576° and the distance between the lattice planes, d, is 3.354 x 10⁻¹⁰m (3,354 Å), which were in coincidence with those of the single crystal of graphite. (3) The half-value widths of the reflection line (having a center at 2 ϑ =26.576°) at the (002) plane were, respectively, 2.0° and 0.14° for thermal treatment at temperatures of 2800°C and 3000°C. (4) The graphitized POD had flexibility and the area or size of the product could be increased as desired depending upon the area or size of a starting POD sheet and the size of a thermal treatment furnace. The radiation optical element using the graphitized POD sheet exhibit good characteristics when applied as an X-ray lens, an X-ray monochromater or an optical element for neutron spectroscopy. However, the graphitized POD has the problem that a rocking characteristic which is the most important characteristic when it is applied as a radiation optical element is not satisfactory. For instance, the rocking characteristic of a graphitized POD product treated at 3000°C is 6.9°, which is unsatisfactory for use as a radiation optical element although such a characteristic may be further improved by hot pressing. Moreover, the graphitized POD is also unsatisfactory with respect to the reflectivity of radiation. The radiation reflectivity has the relation with the crystallinity along the c axis of graphite. It is known that if the crystallinity along the c axis is too good, an X-ray once passing into the crystal will suffer internal reflection, causing a total reflectivity to be lowered. In order to realize a good reflection efficiency, the crystallinity along the c axis of graphite should not be too good or too bad but is required to have an appropriate value. The graphitized POD is good in crystallinity along the c axis, which adversely influences the reflection efficiency on radiation. In recent years, X-rays or so-called soft X-rays having a wavelength of from approximately 9 x 10⁻¹⁰m or 10⁻⁹m (9 or 10 Å) to the order of 10⁻⁸m (several hundreds angstroms) are being utilized in the field of lithography of semiconductor, on which industrial importance is placed. Among soft X-ray elements dealing with an X-ray having about 10⁻⁹m (10 angstroms) to the order of 10⁻⁸m (several hundreds angstroms), those elements using diffraction should have a lattice distance, contributing to the diffraction, which corresponds to an intended wavelength, say approximately 10⁻⁹m (10 angstroms) to the order of 10⁻⁸m (several hundreds angstroms). With graphite currently used as the X-ray element, the lattice distance contributing to the diffraction is 3.354 x 10⁻¹⁰m (3.354 Å). Accordingly, such graphite cannot be used for the purpose of reflection of soft X-rays. It is therefore an object of the invention to provide an optical element for radiation which can solve the drawbacks of the prior art elements and can be simply manufactured with low production costs. It is another object of the invention to provide a radiation optical element which has better rocking and reflectivity characteristics than known elements using graphite from polyphenylene oxadiazole set forth before. It is a further object of the invention to provide an optical element for soft X-rays whose wavelength is in the vicinity of 10⁻⁹m (10 angstroms) as well as hard X-rays. It is a still further object of the invention to provide a radiation optical element which comprises a graphite sheet having a desired thickness capable of dealing with not only X-rays, but also neutron rays. It is another object of the invention to provide a radiation optical element made of a graphitized product of a polymer in the form of a sheet which may be used in combination with a suitable substrate. The optical element for radiation of the present invention is adapted for use in a radiation optical system which includes a radiation source capable of emitting a radiation and a means for receiving the radiation generated from the radiation source through the optical element. The optical element may be used as a convergent lens, a monochromater, an analyzer, a filter or the like means for radiations. The radiations may include X-rays including so-called soft and hard X-rays, neutron rays and the like. Accordingly, the invention provides an optical element which is adapted for use in a radiation optical system having a radiation source and a means for receiving the radiation generated from the radiation source and having passed through said optical element, said optical element comprising at least one graphite film, characterized in that said optical element comprises at least one further graphite film intercalated with a metal halide in such a way that said metal halide is present in individual spaces between adjacent lattice layers of said graphite film, preferably being present in every third space between lattice layers of said graphite film, the films having been bonded together by pressing. The invention further provides a process of producing an optical element as defined above, which process comprises pyrolysing at a temperature not lower than 2800°C and at a pressure not lower than 392.3 kPa (4 kg/cm²) in vacuum or in an inert gas at least one film of a polymer selected from polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene-isophthalamide, polyphenylene-benzoimidazole, polyphenylene-benzobisimidazole, polythiazole and poly-p-phenylene-vinylene to obtain at least one graphite film; separately pyrolysing at a temperature not lower than 2800°C at a pressure not lower than 392.3 kPa (4 kg/cm²) in vacuum or in an inert gas at least one film of a polymer selected from polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene-isophthalamide, polyphenylene-benzoimidazole, polyphenylene-benzoisimidazole, polythiazole and poly-p-phenylene-vinylene to obtain at least one graphite film and intercalating the thus obtained at least one graphite film with a metal halide so that said metal halide is present in individual spaces between any adjacent lattice layers of said graphite film, preferably being present in every third space between lattice layers of said graphite film, thereby obtaining at least one graphite film intercalated with a metal halide; and press bonding the said at least one graphite film and said at least one graphite film intercalated with a metal halide to obtain the optical element. The graphite film obtained in this process is significantly improved in rocking characteristic and reflection efficiency. The polyphenylene oxadiazole is included as the starting polymer. A graphite sheet or block having a particular desired thickness can be obtained by alternately superposing a number of graphite films and a number of the intercalated graphite films, or by sandwiching a number of the intercalated graphite films between two graphite films with or without further superposition of a plurality of the sandwiched sheets thereby forming a block of a desired thickness in the order of millimetres. In one preferred embodiment of the optical element of the invention, a plurality n + 1 of the graphite films and a plurality n of the graphite films intercalated with a metal halide are alternately superposed and have been bonded together by pressing to form a thick sheet so that graphite films provide both outermost layers. In another preferred embodiment, a plurality of the graphite films intercalated with a metal halide are sandwiched between two graphite films and have been bonded together by pressing to form a thick sheet. In the optical element of the invention the bonded films are typically formed on a flat substrate or on a cylindrically curved substrate. The formation of an intercalated graphite compound film on a substrate is favourable so as to impart mechanical strength thereto. In the accompanying drawings: Fig. 1 is an X-ray diffraction pattern of a known radiation optical element of graphite for K α line of Cu radiation; Fig. 2 is a schematic side view of a radiation optical element according to an embodiment of the invention; Fig. 3 is a schematic side view of a radiation optical element according to a further embodiment of the invention; Fig. 4 shows schematically an optical apparatus using a simple optical element (outside the scope of the invention); and Fig. 5 shows schematically an optical apparatus using another optical element (outside the scope of the invention). It is known that when polymers are pyrolyzed, they are carbonized in their original form. This is a good procedure of fabricating a carbonaceous material with a large area and good flexibility. However, the carbonaceous material obtained by the above procedure has, in most cases, a structure which is completely different from that of graphite. Polyphenylene oxadiazole which has been set forth before is a material which is exceptionally converted into graphite of good properties. We have made studies on a number of polymer materials including POD so as to obtain graphite films which are suitable for use in an optical element and which have better rocking and reflectively characteristics than the known graphite film obtained from POD. The starting polymer materials useful for the graphite films employed in the optical element of the present invention include, as indicated above, polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene-isophthalamide, polyphenylene-benzoimidazole, polyphenylene-bisimidazole, polythiazole and poly-p-phenylene-vinylene. In view of the characteristic properties and the ease in graphitization, polyphenylene oxadiazole and polypyromellitimide are preferred. These polymers are now commercially available and include polymers of isomers of the respective monomers. For instance, polythiazole used herein includes polymers of 1,2 and/or 1,3-isomer. In the process of the invention these polymers are subjected to the thermal treatment or pyrolysis in the form of a film or sheet preferably having a thickness of from 1 to 400 micrometers, preferably from 4 to 200 micrometers in order to facilitate graphitization efficiently although the film having a thickness outside the above range may be used. The thermal treatment or pyrolysis is effected at a temperature of not lower than 2800°C under a pressure of not lower than 392.3 kPa (4 kg/cm). Higher temperatures and high pressures are conducive to better rocking and reflectivity characteristics. In view of economy and ease in handling, the upper limit of the temperature is preferably 3600°C and the upper limit of the pressure is preferably about 49033 kPa (500 kg/cm²). Depending upon the type of polymer and the thickness of polymer film, the time required to complete graphitization of the film is usually from 10 to 180 minutes. A longer time may be used but further merits cannot be expected. The thermal treatment should be effected in vacuum or in a gas inert to the polymer used or the graphitization reaction. Such an inert gas may be argon, helium, nitrogen and the like. Graphite has a lattice distance of 3.354 angstroms and cannot thus be used as an optical element for soft X-rays, as stated earlier. As a diffraction grating for a wavelength of soft X-rays, there are known thin films of organic compounds such as EDDT, ADP or KAP or multi-layered thin films which are obtained by alternately depositing carbon and tungsten, or silicon and nickel. However, with the organic compounds, thermal stability and reflection intensity are not satisfactory. On the other hand, multi-layered films are very complicated in the fabrication procedure, coupled with another disadvantage that they cannot be applied to X-rays having a wavelength of not higher than 20 angstroms. More particularly, there is not known any film serving as a diffraction grating which exhibits good characteristics and are capable of being applied for soft X-rays having a wavelength of from about 10 to 20 angstroms. In the optical element of the invention, the above problem is solved by the provision of at least one graphite film which is interacted with a metal halide to form a film of a metal halide-intercalated graphite compound where the metal halide is intercalated inbetween layer lattices of the graphite. The intercalation of a metal halide into graphite involves two types including a first stage intercalation wherein the metal halide is included in an individual space between any adjacent lattice layers of graphite. Another type includes a second stage intercalation wherein the metal halide is intercalated in every third space between lattice layers of graphite. The intercalation used herein means both types of intercalations. The starting graphite film may be obtained by pyrolysing or thermally treating a film of a polymer, selected from those mentioned earlier, in vacuum or an inert gas at a temperature not lower than 2800°C and a pressure not lower than 392.3 kPa (4 kg/cm²). More particularly, a film or sheet of a polymer including polyphenylene oxadizole may be subjected to thermal treatment at a temperature of now lower than 2800°C either at a normal pressure or at a pressure of now lower than 392.3 kPa (4 kg/cm²) in vacuum or an inert gas. The intercalated graphite compound film using a metal halide is generally obtained by a gas phase method, a solvent method or an electrolytic method. In the practice of the invention, the intercalated film is obtained either by a procedure wherein a graphite film and a metal halide are heated in a stream of a halogen such as Cl, Br, I or F at a temperature of from 200 to 500°C for a time of from 5 to 10 days for the first stage intercalation and for a time of from 1 to 4 days for the second stage intercalation. or by a procedure wherein a graphite film and a metal halide are placed in a sealed glass and heated in vacuum under conditions indicated above. If the halogen stream is used, the halogen used should be a halogen of the metal halide used. For the first stage intercalation, a longer time is necessary. The metal halides used to react with the graphite include, for example, BCl₃, MgCl₂, AlCl₃, ScCl₃, TiCl₄, CrCl₃, MnCl₂, FeCl₃, CoCl₂, NiCl₂, CuCl₂, ZnCl₂, GaCl₃, YCl₃, NbCl₅, MoCl₅, RhCl₃, PdCl₂, CdCl₂, RuCl₃, ZrCl₄, InCl₃, HfCl₄, TaCl₅, WCl₆, ReCl₄, OsCl₄, PtCl₄, AuCl₃, HgCl₂, TlCl₃, BiCl₄, ICl, IBr, FeCl₂, BF₃, AlBr₃, SiF₄, TiF₄, FeBr₃, CuBr₂, PF₆, GaBr₃, NbF₅, MoF₆, CdBr₂, TaF₆, WF₆, OsF₃, AuBr₃, TlBr₃, and mixtures thereof. Of these, NiCl₂ and CuCl₂ are preferred. Aside from the metal halides, alkali metals such as Na or K may be used to form an intercalated graphite compound. However, these intercalated compounds are unstable and are not suitable for the purposes of the invention. The film of the intercalated graphite compound obtained in this manner has a high reflection efficiency for soft X-rays, a lattice distance larger than that of the graphite film, and high resistances to heat and moisture. In addition, the intercalated compound film is so high in flexibility that it can be bonded to a curved or flat support or substrate without breakage. For this purpose, an adhesive such as epoxide compounds, pitches and the like may be used. Films of the intercalated graphite compound have self-bonding properties. These films of the intercalated graphite compound are readily self-bonded by pressing to form a block or sheet of a desired thickness. The self-bonding properties of the intercalated graphite compound film are utilized in combination with graphite films in the optical element of the invention to make a thick film whose properties are predominant of those of the graphite films. It is difficult to make a thick film from the starting graphite film alone because of the generation of gases during the thermal treatment of the polymer film. If a thick polymer film is pyrolyzed in order to obtain a thick graphite film, gases generated in the vicinity of or near the film surface may readily escape to outside. However, the gases generated around the inner portion of the film are difficult to escape and may be included in the inside. As the pyrolysis proceeds further, the gases increase in amount, which may eventually lead to cracking of the entirety of the film. Further, formation of a block or sheet of graphite films by pressing requires relatively severe thermal treating conditions as set out before. To obtain a thick sheet or block having graphite properties, the adhesiveness of the films of the intercalated graphite compound can therefore conveniently be used. This is described with reference to Figs. 2 and 3. In Fig. 2, there is shown an optical element E which includes graphite films 10 and intercalated graphite compound films 12 which are superposed or stacked alternately and pressed. Fig. 3 shows an optical element E which includes graphite films 10′ and intercalated graphite compound films 12′ sandwiched between the graphite films 10′ as shown. If a desired number of graphite films and intercalated graphite compound films in number sufficient to be alternately superposed with the graphite films so that the graphite films are formed as an outermost layer on opposite sides, or if a desired number of intercalated graphite compound films are sandwiched between graphite films, a desired thickness can be readily obtained. The superposed or sandwiched films are appropriately bonded by pressing generally at a pressure of not less than 392.3 kPa (4 kg/cm²). In view of the ease in fabrication, the starting graphite and intercalated graphite compound films are conveniently both in the thickness of from 4 to 200 micrometers. The superposed or sandwiched block or sheet may be used as it is or after deposition on a curved or non-curved support through an adhesive. If deposition is made on a curved support, it is convenient to subject the block and the support to press molding using a mold of a desired form. By this, a radiation optical element having a desired radius of curvature in conformity with the radius of the curved support is obtained. As will become apparent from the Examples, the reflectivity of the block or sheet is predominantly influenced by the reflectivity of the graphite films, not of the intercalated graphite compound films. In addition, such a sheet or block has a plane distance, d, of 3.353 x 10⁻¹⁰m (3.353 angstroms) and is coincident with that of a graphite single crystal. Typical examples of applications of the optical element of the invention are described schematically in Figs. 4 and 5 wherein like reference numerals indicate like parts. Fig. 4 schematically shows an application of a simple radiation optical element (not according to the invention) to a convergent lens system for X-rays. In the figure, the system S includes a convergent lens 20 having a curved substrate 22 and a graphite film 24 bonded to the inner surface of the curved substrate 22. Indicated by 26 is a plate made, for example, of molybdenum having a hole 28 with a diameter of about 1mm through which a radiation 30 such as a CuK α line is passed. The radiation is reflected at the convergent lens 20 and focussed on a photographic plate 32. In this manner, a fine X-ray pattern is obtained. Instead of the optical element, shown, an optical element of the invention comprising a combination of the graphite and intercalated graphite compound films may be likewise used. Fig. 5 schematically shows an X-ray monochromater system using a film or sheet which is not according to the invention. In the figure, the monochromater is indicated as M and includes a convergent lens 20 having a curved substrate 22 and a graphite film 24 and a monochromater 34 for X-rays having a flat glass substrate 36 and a graphite film 38 formed on one side of the substrate 36. Reference numerals 26, 26′ are respectively, plates having small holes 28, 28′ and reference numeral 40 indicates a counter. In operation, when an X-ray 42 is incident on the monochromater, it reflects and passes through the hole 28 of the plate 26 on the convergent lens 24. The converged ray is passed through the small hole 28′ of the plate 26′ and focussed into the counter 10. It will be noted that when the angle of incidence of the X-ray 42 passed to the X-ray monochromater 34 is changed, the wavelength of the X-ray passing through the small hole 28 changes. The present invention is more particularly described by way of examples. In examples, the lattice constant, rate of graphitization, electric conductivity and rocking characteristic were measured by the following methods. (1) Lattice Constant (Co) A sample was subjected to measurement of an X-ray diffraction pattern using an X-ray diffractometer, PW-1051, by Philips and an Cuk α line. The constant was calculated from the equation of λ =2dsin ϑ (2d=Co) using a diffraction line at (002) in the vicinity of 2ϑ =26 - 27°. In the equation n=2 and λ is a wavelength of the X-ray used. (2) Rate of Graphitization The rate was calculated from the following equation using a plane distance, d, d₀₀₂ = 3.354g + 3.44(1-g) where g=1 means complete graphite and g=0 means amorphous carbon. (3) Electric Conductivity (S/cm) A sample was provided with four-terminal electrodes using a silver paste and a gold wire wherein a constant current was passed from the outer electrodes to detect a voltage drop at the inner electrodes. The width, length and thickness of the sample were determined through microscopic observation, from which the electric conductivity was calculated. (4) Rocking Characteristic An X-ray diffractometer TU-200B made by Rigaku Denki Co., Ltd. was used to measure a rocking characteristic at a peak of the graphite (002) line. The rocking characteristic was indicated as a half-value width of the resultant absorption. Reference Example 1 A film of a polybenzothiazole was pyrolyzed at a temperature of 2800°C under a pressure of 1961 kPa (20 kg/cm²) for 1 hour in argon obtain a graphite film. The resultant film had a size of 5 cm x 10 cm and a thickness of 30 micrometers. This film was bonded to a curved substrate made of Cu through an epoxide adhesive to make a convergent lens. This lens was used in a X-ray lens system as shown in Fig. 4 in which the plate 26 was made of molybdenum with the hole 28 having a diameter of 1 mm. When a CuK α line 30 was passed through the hole 28, it was reflected at the film 24 of the convergent lens 20 and converged toward a photographic plate 32 located at a focussing point. An image formed on the plate 32 was a single line with a length of 1 mm and a width of about 13 micrometers, thus being good in convergence. When passing twice through the convergent lens 20, a fine X-ray pattern with a width of 1 micrometer or below could be obtained. Reference Example 2 Two films of polypyromellitimide each having a thickness of 25 micrometers were pyrolyzed at 3000°C at a normal pressure for 1 hour to obtain 15 micrometer thick graphite films each with a size of 5 cm x 5 cm. One of the films was bonded to a smooth flat glass substrate through an epoxide adhesive. Another film was used to make a convergent lens as in Reference Example 2. These optical elements were used to assemble an X-ray monochromater system as shown in Fig. 5 wherein the plates 26, 26′ were each made of molybdenum. When an X-ray 42 from a Cu target was passed, a characteristic X-ray of CuK α was observed intensely at 13.288°. The line width was found to be 0.18°. When compared with a similar system using a natural graphite single crystal, the line width was reduced form 0.3° to 0.18°, thus revealing the high performance of the graphite of the invention. Reference Example 3 The general procedure of Reference Example 2 was repeated except that an X-ray monochromater was made as follows. One hundred and fifty CuCl₂-graphite intercalation compound films were superposed and pressed at a pressure of 1961 kPa (20 kg/cm²) to obtain an about 3 mm thick block with a size of 5 cm x 5 cm and this block was bonded to a glass substrate. The graphite used was obtained from polypyromellitimide. When an X-ray from a Cu target was passed, a characteristic X-ray of CuK α was intensely observed at ϑ = 4.704°. The line width was 0.2°. As will be apparent from the above, when the graphite intercalation compound was used as a radiation optical element, a plane distance of about 10⁻⁹m (10 Å) can be attained which would be difficult when using known multi-layered films. This means that there can be obtained a radiation optical element for soft X-rays with a wavelength of about 10⁻⁹m (10 angstroms). Example 1 50 polyphenylene oxadiazole films were pyrolyzed at a temperature of 3000°C for 30 minutes to obtain graphite films each having a thickness of 20 micrometers. On the other hand, 40 micrometer thick graphite films were obtained from 49 polyphenylene oxadiazole films under conditions as used above. These graphite films were intercalated with CuCl₂ to obtain forty nine first-stage CuCl₂-graphite intercalation compound films. The intercalated graphite films were, one by one, inserted inbetween the graphite films for alternate superposition, followed by pressing at a pressure of 4903 kPa (50 kg/cm²), thereby obtaining a superposed graphite block having a thickness of about 3 mm. This block was subjected to measurement of reflection line positions for CuK α (1.5418 x 10⁻¹⁰m, 1.5418 angstroms), relative intensities to a reflection intensity at plane (002) of the graphite plate taken as 100, and plane indexes. The results are shown in Table 1 below. The reflection lines are those corresponding to planes (001), (002) and (003) of the graphite intercalation compound and those corresponding to planes (002), (004) and (006) of the graphite. The reflection intensity is predominantly intense for the line corresponding to plane (002) of the graphite. The reflection angle (2 ϑ) of the graphite was 26.576° with a plane distance, d, of 3.354 x 10⁻¹⁰m (3.354 Å), coinciding with those of a graphite single crystal. the half-value width was 0.2° and thus, the graphite plate has satisfactory characteristics as an X-ray optical element. Example 2 Example 1 was repeated thereby making a graphite block. This block was press molded using a cylindrical tool having such a curvature as a curved substrate. The molded block was bonded to the curved substrate to make a converging lens. This lens was applied to a system as shown in Fig. 4. When a Cuk α line 30 was passed through the hole 28 of the molybdenum plate 26 and focussed through the lens 20 on the photographic plate 32. A single line image was obtained with a length of 1 mm and a width of about 15 micrometers. Thus, the lens had good convergence. When the Cuk α line 30 was passed twice through the lens 20, a fine pattern with a width of 1 micrometer or below could be obtained. Example 3 In the same manner as in Example 2 an about 3 mm thick superposed block was made and attached to a flat glass substrate to make an X-ray monochromater. This monochromater was assembled in a system as shown in Fig. 5. When the angle ϑ of the incidence of the X-ray 42 was changed, the wavelength of the X-ray 42 passing through the hole 28 of the Mo plate could be changed. After the passage though the hole 28, the X-ray was directed toward a convergent lens as used in Reference Example 3 at which it was changed in direction to pass through the hole 28′ and focussed into the counter 40. When an X-ray from a Cu target was passed, a characteristic X-ray of CuK α was intensely observed at ϑ = 13.288°. The line width was 0.2°. When comparing with the case where a natural graphite single crystal, the line width was reduced from 0.3° to 0.2°. Thus, the superposed graphite plate was confirmed to have good properties. Example 4 Polyphenylene oxadiazole films were thermally treated at 3000°C for 30 minutes in vacuum to obtain 20 micrometers thick graphite films. Part of the graphite films was used to make a first-stage CuCl₂-graphite intercalation compound films each with a thickness of 40 micrometers. The graphite films and the intercalation compound films were superposed as shown in Fig. 3 in such a way that 25 intercalation compound films were placed on one graphite film. Three layers of the above combination were superposed so that upper and lower films were the graphite films and pressed at a pressure of 4903 kPa (50 kg/cm²), thereby obtaining an about 3 mm thick graphite block serving as a radiation optical element. This block was subjected to measurements of reflection line positions, lattice distance, reflectivity and plane index. The results are shown in Table 2 below. The reflection lines are those corresponding to planes (001), (002) and (003) of the graphite intercalation compound and those corresponding to planes (002), (004) and (006) corresponding to the graphite. The reflectivity at the line corresponding to plane (002) of the graphite films and the reflectivity at the line corresponding to plane (002) of the intercalation compound films are substantially the same. The reflection angle (2 ϑ) at plane (002) of the graphite films is 26.576° with a plane distance, d, of 3.354 x 10⁻¹⁰m (3.354 Å), coinciding with those of a graphite single crystal. The plane distance calculated from the lines of the intercalation compound is substantially the same as that of a single film. The half-value width was 0.2° for the (002) line of the graphite, and 0.3° for the (001) line and 0.27° for the (002) line of the intercalation compound. Thus, the block obtained in this example was satisfactory in the characteristics for use as an X-ray optical element. In the above examples 1 to 4, graphite films obtained from polyphenylene oxadiazole are described. It should be noted when graphite films obtained from other polymer films and graphite intercalation compound films made of graphite films made from polyimide and metal halides are used, similar results are obtained. Moreover, application of the elements to X-rays has been described, the elements are also applicable as a monochromater, analyzer, lens or film for neutron spectroscopy.

Claims: An optical element (E) which is adapted for use in a radiation optical system having a radiation source and a means for receiving the radiation generated from the radiation source and having passed through said optical element, said optical element comprising at least one graphite film (10;10′), characterized in that said optical element comprises at least one further graphite film intercalated with a metal halide (12;12′) in such a way that said metal halide is present in individual spaces between adjacent lattice layers of said graphite film, preferably being present in every third space between lattice layers of said graphite film, the films having been bonded together by pressing. An optical element according to claim 1, wherein a number n + 1 of the graphite films (10) and a number n of the graphite films intercalated with a metal halide (12) are alternately superposed and have been bonded together by pressing to form a thick sheet so that graphite films provide both outermost layers. An optical element according to claim 1, wherein a plurality of the graphite films intercalated with a metal halide (12′) are sandwiched between two graphite films (10′) and have been bonded together by pressing to form a thick sheet. An optical element according to claim 1, 2 or 3, wherein said metal halide is at least one of BCl₃, MgCl₂, AlCl₃, ScCl₃, TiCl₄, CrCl₃, MnCl₂, FeCl₃, CoCl₂, NiCl₂, CuCl₂, ZnCl₂, GaCl₃, YCl₃, NbCl₅, MoCl₅, RhCl₃, PdCl₂, CdCl₂, InCl₃, HfCl₄, TaCl₅, WCl₆, ReCl₄, OsCl₄, PtCl₄, AuCl₃, HgCl₂, TlCl₃ and BiCl₄. An optical element according to any one of claims 1 to 4, wherein the bonded films are formed on a flat substrate or on a cylindrically curved substrate. A process of fabricating an optical element (E) as claimed in claim 1 which comprises: pyrolyzing at a temperature not lower than 2800°C and at a pressure not lower than 392.3 kPa (4 kg/cm²) in vacuum or in an inert gas at least one film of a polymer selected from polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene-isophthalamide, polyphenylene-benzoimidazole, polyphenylene-benzobisimidazole, polythiazole and poly-p-phenylene-vinylene to obtain at least one graphite film; separately pyrolyzing at a temperature not lower than 2800°C at a pressure not lower than 392.3kPa (4 kg/cm²) in vacuum or in an inert gas at least one film of a polymer selected from polyphenylene oxadiazole, polybenzothiazole, polybenzobisthiazole, polybenzooxazole, polybenzobisoxazole, polypyromellitimide, polyphenylene-isophthalamide, polyphenylene-benzoimidazole, polyphenylene-benzobisimidazole, polythiazole and poly-p-phenylene-vinylene to obtain at least one graphite film and intercalating the thus obtained at least one graphite film with a metal halide so that said metal halide is present in individual spaces between any adjacent lattice layers of said graphite film, preferably being present in every third space between lattice layers of said graphite film, thereby obtaining at least one graphite film intercalated with a metal halide; and press bonding the said at least one graphite film and said at least one graphite film intercalated with a metal halide to obtain the optical element. A process according to claim 6, wherein a plurality of graphite films and a plurality of graphite films intercalated with a metal halide are alternately put one on another and pressed bonded to obtain a block.

Inventors: MURAKAMI MUTSUAKI, WATANABE KAZUHIRO, YOSHIMURA SUSUMU, MURAKAMI, MUTSUAKI, WATANABE, KAZUHIRO, YOSHIMURA, SUSUMU
Applicants: JAPAN RES DEV CORP, MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD., RESEARCH DEVELOPMENT CORPORATION OF JAPAN
Application year: 1989
Main classification: G02B1
IPC codes: G21K1