FR2664389A1 - RADIATION - RESISTANT OPTICAL ARTICLES MADE OF A HIGH ISOTOPIC PURITY MONOCRYSTALLINE DIAMOND. - Google Patents

Abstract

The invention relates to a monocrystalline diamond having an isotopic enrichment of carbon 12 or carbon 13; this diamond, which has a very high degree of radiation resistance, typically more than a factor of more than 10 to that of a type IIa natural diamond, is useful for the manufacture of optical articles, such as windows, lenses, arrays, mirrors and lasers.

Description

The invention relates to optical articles resistant to radiation and

  more particularly articles made with a

  specific kind of monocrystalline diamond.

  There has recently been considerable interest in the development of materials resistant to incident radiation damage. These materials are useful in the manufacture of optical articles that can be used with lasers or laser elements. is particularly the case of the free electron lasers whose beams are extremely difficult to transmit,

  focus or reflect, as a result of the intensity of the radiation.

  Currently, the mirrors of these lasers must be placed at distances of several thousand meters, because a greater proximity causes an irreversible deterioration of the mirror under

the effect of radiation.

  In the current US patent application, serial NO 07/536 371, monocrystalline diamond compositions having the highest thermal conductivity of all currently known materials are described. These compositions are characterized by their isotopic purity which is minus 99.2% by weight Various applications of such a diamond are mentioned, including as

  thermal conductors, abrasives and optical filters.

  The present invention is based on the discovery that a monocrystalline diamond of high isotopic purity is characterized by an extremely high resistance to radiation. More particularly, a monocrystalline diamond comprising 99.9% of carbon 12 has been found to have a threshold of deterioration by radiation more than ten times that of a monocrystalline diamond of normal isotopic purity (ie 98.9% carbon 12 and 1.17% carbon 13). The invention thus relates to optical articles resistant to radiation deterioration, these articles comprising a monocrystalline diamond consisting of at least 99.2% by weight of

carbon 12 or carbon 13.

  An essential feature of the articles of the invention is the use, for their manufacture, of a monocrystalline diamond which has been enriched in carbon 12 or carbon 13. As previously explained, it has been discovered that the increase in the resistance to radiation , which results from the use of carbon of chemical and isotopic purity, is much higher than would be expected from theoretical considerations The isotopic distribution of diamond must be at least 99.2% by weight of carbon 12 or carbon 13 , carbon 12 being preferred. Thus, the other isotope must be present in a proportion of at most 8 parts per 1000.

  isotopic distributions of at least 99.9% by weight.

  Various methods can be used to prepare an isotopically enriched monocrystalline diamond. In general, they include all of the following steps: (A) preparing a diamond of carbon 12 or carbon 13 with isotopic enrichment; and (B) converting this diamond into a monocrystalline diamond by diffusion under high pressure through a catalyst-solvent

  metal to a region containing a diamond seed crystal.

  In step A, it is possible to separate a gaseous compound from carbon, such as carbon monoxide, into carbon species 12 and carbon species 13 by differences in diffusivity, and to transform the carbon fraction 12 into solid carbon. according to techniques known in the art, such as combustion in a reducing flame, in the case of carbon monoxide The carbon thus formed can then be converted into diamond under conventional conditions including high temperature and pressure conditions or

  chemical vapor deposition conditions.

  Alternatively, other methods including shock formation and chemical vapor deposition processes under conditions producing a mixture of diamond and graphite may be used. In processes of the latter type, the carbon species 13 is concentrated in the diamond phase and the carbon species 12 in the graphite phase. Other diamond precursors, which can be used in an enriched form, include pyrolytic graphite, amorphous or vitreous carbon, hydrocarbons

liquids and polymers.

  It is generally found that conventional methods of diamond formation by chemical vapor deposition are particularly suitable for the preparation of diamond of isotopic purity. In such processes, a layer of diamond is deposited on at least one substrate. Any substrate may be used. suitable for diamond deposition: examples of such materials are boron, boron nitride, platinum, graphite, molybdenum, copper, aluminum nitride, silver, iron, nickel, silicon , alumina and silica and their combinations Metallic molybdenum substrates are particularly suitable

  in many conditions and are often preferred.

  The method of chemical vapor deposition of diamond on a substrate is known and it is unnecessary to present the details here In summary, it requires a high energy activation of a mixture of hydrogen and a hydrocarbon, typically methane, for the hydrogen gas to be converted into atomic hydrogen which reacts with the hydrocarbon to form elemental carbon This carbon is deposited on the substrate in the form of diamond Activation can be effected by a conventional means providing activation to high energy producing atomic hydrogen from molecular hydrogen; such means include thermal means typically employing heated filaments, flames, DC discharges and

  microwave or radio frequency or similar.

  Thermal processes, and in particular those using a filament, which utilize one or more resistance heating elements having heated wires or filaments, are often preferred for the purposes of the invention. In these methods, the filaments are typically made of tungsten, tantalum, molybdenum and rhenium metal; Tungsten is often preferred because it is relatively inexpensive and is particularly suitable. The filaments typically have diameters of about 0.2 to 1.0 mm and often preferably about 0.8 mm. The distances between the filaments and the substrate (s) are

  generally of the order of 5 to 10 mm.

  Typically, these filaments are heated to temperatures of at least 2 000 C and the optimum temperature of the substrate is in the range of 900 to 1 000 C. The pressure in the deposition chamber is maintained at a maximum value of about 1 010 mbar, typically of the order of 13 mbar The hydrogen-hydrocarbon mixture generally contains a hydrocarbon in a proportion of at most about

  2% by volume in relation to total gases For a description of

  illustrative process for diamond preparation by chemical vapor deposition, reference is made to US patent applications Serial Nos. 07 / 389,210 and 07 / 389,212 of the same proprietor. Hydrocarbon used in the chemical phase deposition process Isotopic enrichment To avoid contamination, it is essential to use equipment that does not contain natural carbon as an impurity For this the chemical vapor deposition chamber must be made of materials essentially incapable of dissolving carbon.

  typical of this type are quartz and copper.

  The thickness of the diamond layer formed by chemical vapor deposition on the substrate is not critical. In general, it is convenient to deposit at least as much diamond as is required to produce a single crystal having the size. Of course, it is also possible to produce a larger quantity of diamond formed by chemical vapor deposition to use it for

prepare several crystals.

  It is possible to convert the product of the chemical vapor deposition process directly into a diamond of high thermal conductivity by means of high pressure means, as described hereinafter, using it as a block, However, the process of the invention is particularly effective when the diamond enrichment

  isotopic is first divided into small fragments.

  The division into small fragments can be achieved by means known in the art, such as grinding and spraying. The particle size is not critical as long as a sufficient degree of splitting into small fragments is achieved; the known form in

art as "égrisé" is suitable.

  Stage B, the production of a monocrystalline diamond, is conventional, except that the isotopically enriched diamond produced in step A is used as raw material. The use of diamond instead of graphite or another allotropic variety of carbon as a raw material has two effects: it is possible to use an isotopic enrichment material that is easy to obtain, and the contraction of volume, observed during the conversion of graphite or other varieties allotropic diamond, is avoided, which allows to produce a single crystal of

  regular structure and high quality.

  The process for producing a monocrystalline diamond under

  high pressure is also known in the art and its description

  It seems pointless to refer to, for example, Encvcloredia of Physical Science & Technology, Vol. 6, pp. 492-506 (Academic Press, Inc., 1987); Strong, The Physics Teacher, January 1975, pp

  7-13, and US Patents 4,073,380 and 4,082,185, for a description

  general process It generally comprises the diffusion of carbon used as raw material through a liquid bath made of a metal catalyst-solvent at pressures of the order of 50 000 to 000 mbar and at temperatures in the range of about 1,300 to 1,500 ° C. A negative temperature gradient, typically about 500 ° C, is preferably maintained between the material to be processed and the deposition region which contains a diamond seed.

  on which crystalline growth can begin.

  The catalyst-solvents used in step B are known in the art. They include, for example, iron; mixtures thereof with nickel, aluminum, nickel and cobalt, nickel and aluminum, and nickel, cobalt and aluminum; and mixtures of nickel and aluminum Iron-aluminum mixtures are often preferred for the production of a monocrystalline diamond, a material consisting of 95% (by weight) of iron and% of aluminum being particularly preferred for the invention. . After the preparation of the monocrystalline diamond, it is often preferred to remove by polishing the portion attributable to the seed crystal. The preparation of an isotopically enriched monocrystalline diamond is illustrated by an example in which a diamond layer is first deposited by chemical vapor deposition on a molybdenum substrate in a chamber made of quartz and copper which neither of them dissolve significant quantities of carbon. The substrate is arranged vertically in a plane parallel to the plane of a tungsten filament of about 0.8 mm in diameter and 8-9 mm in diameter. This plane is created in the vessel a vacuum corresponding to a pressure of about 13 mbar, the filament is heated to about 2000 C by passing an electric current and is passed through the container a mixture of 98.5% ( in volume) of hydrogen and 1.5% of methane The methane used is substantially free of impurities and contains 99.9% carbon isotope 12 After removal and analysis by mass spectroscopy of the diamond thus obtained, it is found that 99.91% of the carbon that comp dare are

consisting of carbon 12.

  The diamond obtained by isotopically enriched chemical vapor deposition is milled and sprayed and used as a carbon source for the growth of a monocrystalline diamond under conditions of high pressure and temperature. More particularly, a conventional apparatus is used. at a belt at 52,000 bar and at 1 4000 ° C. using a catalyst-solvent mixture made of 95% (by weight) of iron and 5% of aluminum. A small seed of monocrystalline diamond (0.005) is used to initiate the growth. carat) normal isotopic distribution, and maintains a negative temperature gradient of about 500 C between the diamond formed by chemical vapor deposition and the seed crystal The operation is continued until a monocrystal of 0, 95 carat was produced The analysis shows that 99.93% of the carbon that forms it consists of the isotope 12 C The diamond is polished with a

  standard polisher for diamond to eliminate the crystalline germ.

  An extremely high resistance to the radiation of the diamond prepared as above is observed when an attempt is made to measure its thermal conductivity by detecting the mirage of thermal waves produced by the impact on its surface of a modulated argon ion laser beam. It is first necessary to deposit a laser-absorbing film on the surface of the diamond. In the case of a natural diamond of type I Ia, an argon-fluorine excimer laser operating at a wavelength of 193 nm, which graphitizes the surface with formation of a thick graphite layer of about 60 nm. It has been determined that the threshold of deterioration by a laser of the natural diamond is 300 m J / cm 2 Similar attempts to graphitize the surface Isotope-enriched diamond failed even when the laser fluence was increased by a factor of 10 So the threshold of deterioration by a laser beam

  isotopically enriched diamond is greater than 3000 m J / cm 2.

  Theoretical considerations for diamond indicate that high thresholds of laser beam damage of the articles of the invention are observed in the range of about 150 to 220 nm. According to the types of interactions between the excited electrons and the phonons in the diamond article, it is also possible that an elevation of the threshold is observed at the wavelengths

less than 115 nm.

  The optical articles of the invention comprise windows, lenses, gratings and mirrors adapted to the incidence of radiation, in particular of light radiation and more particularly of lasers. They are particularly useful in the case of free electron lasers. that can be focused or reflected by setting up the optical article at a distance of

  the order of 5 meters, or less, from the laser source.

  The use of diamond as an active ingredient in a laser is also known. Thus, lasers comprising an isotopically enriched diamond as the active ingredient constitute another

aspect of the invention.

  The articles of the invention are of conventional structure, with the exception of the diamond which is used there.

  according to methods known in the art.

Claims (4)

  1 Optical article resistant to damage by radiation, characterized in that it comprises a monocrystalline diamond consisting of at least 99.2% by weight of carbon 12 or carbon 13. 2 Article optic consists of at least 3 Article according to laser. 4 Article according to laser. Article according to laser. 6 Article according to laser. 7 Article according to active laser.   8 Item according to at least 99.9 9 Article according to laser. Article according to laser. 11 Article according to laser.   12 Article according to laser. 13 Article according to active laser.   te according to claim 1, including the diamond 99.2% carbon 12.   claim 2, which is claim 2, claim 2, claim 2, claim 2, claim 2, of carbon 12.   claim 8, claim 8, claim 8, claim 8, claim 8, who is who is who is whose who is who is who is who is a window of a mirror of a lens of a network of a diamond material is a window of a mirror of a lens of a network of a material