JPH04234001A - Radiation resisting optical product comprising single crystal diamond having high isotope purity - Google Patents

Abstract

PURPOSE: To obtain optical products exhibiting a resistance characteristic to radiation damage by composing the optical products of single crystal diamond consisting of at least 99.2wt.%<12> C or<13> C. CONSTITUTION: The optical products are composed of the single crystal diamond consisting of at least 99.2wt.%<12> C or<13> C. Namely, the high-purity diamond consisting of the<12> C or<13> C is deposited on a substrate. This deposited diamond is formed by growing the high-purity diamond obtd. by another method to the single crystal under an ultra-high pressure. Windows for lasers, lenses, diffraction gratings, mirrors and active materials for lasers are manufactured from this single crystal. The radiation resistance characteristic is improved by using the carbon having such high chemical purity and isotope purity.

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to radiation resistant optical articles, and more particularly to optical articles made from a unique type of single crystal diamond material.

[0001]Recently, much interest has been directed to the development of materials that are resistant to damage by incident X-rays. Such materials are useful in the manufacture of optical products that can be used with or as part of lasers. This is particularly the case with free electron lasers, whose radiation is so powerful that it is extremely difficult to transmit, focus or reflect it. Currently, mirrors for such lasers must be placed at distances of several thousand meters. because,
If placed closer than that, the mirror would suffer irreversible radiation damage.

Co-pending US Patent Application No. 07/53637
No. 1 discloses a single crystal diamond composition that has the highest thermal conductivity of any currently known material. These compositions are characterized by having an isotopic purity of at least 99.2% by weight. A variety of uses have been disclosed for such compositions, including thermal conductors, abrasives, and optical filters.

The present invention is based on the discovery that single crystal diamond of high isotopic purity has an extremely high degree of radiation resistance. To be more specific, 99.9% by weight
A single-crystal diamond consisting of 12C of It was found that the radiation damage threshold was more than twice as high.

According to the invention, therefore, at least 99.
An optical article resistant to radiation damage is provided, characterized in that it is constructed by single crystal diamond consisting of 2% by weight of 12C or 13C.

One essential feature of the optical product of the present invention
One is that high-purity 12C or 13
The purpose is to use single-crystal diamond made of C. As explained below, the improvement in radiation resistance obtained by using carbon with high chemical and isotopic purity was found to be significantly greater than expected based on theoretical considerations. Such diamonds must consist of at least 99.2% by weight of 12C or 13C, preferably 12C. That is, the other isotope must be present in an amount of at most 0.8% by weight. Note that an isotopic purity of at least 99.9% by weight is preferred.

Various methods can be used to produce single crystal diamond with high isotopic purity. Generally speaking, such a method involves (A) producing highly purified 12C;
Or prepare a diamond consisting of 13C and then (
B) converting the diamond into single crystal diamond by diffusing the diamond under high pressure through a metal catalyst/solvent material into a region containing diamond seeds.

In step A, for example, a gaseous carbon compound such as carbon monoxide becomes 12C due to the difference in diffusivity.
The isotopes and 13C isotopes are separated and the 12C fraction is then converted to solid carbon by means known in the art (eg, combustion of carbon monoxide in a reducing flame). The carbon thus obtained may be converted to diamond under normal conditions (eg, high temperature and high pressure conditions or CVD conditions).

Alternatively, bombardment and CVD under conditions to produce a mixture of diamond and graphite.
Methods such as the law may also be used. In such methods, 13C isotopes are concentrated in the diamond phase and 12C isotopes are concentrated in the graphite phase. Other diamond precursors that can be used in concentrated form include pyrolytic graphite, amorphous or glassy carbon, liquid hydrocarbons, and polymers.

[0009] Generally, the simplest method for preparing diamond with high isotopic purity is the ordinary CVD method. In such a method, a diamond layer is deposited on at least one substrate. Any substrate material suitable for depositing diamond can then be used. Examples of such materials include boron,
Included are boron nitride, platinum, graphite, molybdenum, copper, aluminum nitride, silver, iron, nickel, silicon, alumina and silica, and combinations thereof. Among these, substrates made of metallic molybdenum are particularly suitable for use under various conditions and are therefore preferred in many cases.

Chemical vapor deposition methods for depositing diamond on substrates are well known and there is no need to repeat the details here. Briefly, chemical vapor deposition requires activating a mixture of hydrogen and a hydrocarbon (usually methane) with high energy. As a result, hydrogen gas is converted to atomic hydrogen, which then reacts with hydrocarbons to produce elemental carbon. Such carbon is then deposited as diamond on the substrate. The above activation can be accomplished by conventional means providing high energy activation to generate atomic hydrogen from molecular hydrogen. Such means include thermal means, usually consisting of heated filaments, flame means, direct current discharge means, and electromagnetic wave means, including microwaves, radio frequencies, and the like.

For purposes of the present invention, thermal methods are often preferred, particularly those using one or more resistive heating elements including a heating filament (ie, filament methods). In such methods, the filament typically consists of metal tungsten, tantalum, molybdenum or rhenium. Among these, tungsten is often preferred because it is relatively cheap and has excellent suitability as a filament. The filament diameter typically ranges from about 0.2 to 1.0 mm, although a diameter of about 0.8 mm is often preferred. The distance between the filament and the substrate is generally about 5 to 10 mm.

Typically, the filament described above is heated to a temperature of at least about 2000°C, and the optimum temperature of the substrate is 90°C.
It is within the range of 00 to 1000°C. The pressure within the reaction chamber is maintained at a value of up to about 760 Torr (usually around 10 Torr). The hydrogen-hydrocarbon mixture generally contains hydrocarbons in an amount up to about 2% by volume, based on its total volume. For illustrations of CVD methods for diamond production, see co-pending U.S. Patent Application Nos. 07/389,210 and 07/3, owned by the owner of the present invention.
See the specification of No. 89212.

[0013] When employing such a CVD method, a hydrocarbon having high isotopic purity is used. In order to prevent its contamination, it is essential to use equipment that does not contain natural carbon as an impurity. For such purposes, it is necessary that the reaction chamber be made of a material that is substantially incapable of dissolving carbon. Typical examples of such materials are quartz and copper.

The thickness of the diamond layer deposited on the substrate according to the CVD method is not particularly critical. Generally speaking, it is advantageous to deposit at least as much diamond as is necessary to produce a single crystal of the desired size. Of course, a larger amount of diamond may be prepared for use in producing multiple crystals.

[0015] It is also possible to directly convert diamond in the form of flat plates, thin plates or fragments obtained according to the CVD method into diamond with high thermal conductivity by high-pressure means as described below. However, to most efficiently carry out the method of the present invention, it is preferable to first grind diamond with high isotopic purity.

Such comminution can be accomplished by means known in the art, such as crushing or pulverization. The particle size of the diamond is not particularly important as long as a sufficient degree of comminution is achieved. Note that a form known in the art as "diamond grit" is suitable.

Step B for producing single crystal diamond is conventional except that the diamond with high isotopic purity obtained in Step A is used as a raw material. There are two advantages to using diamond as a raw material rather than graphite or other carbon allotropes. This means that readily available isotopically pure materials can be used, and the volume shrinkage seen when converting graphite and other carbon allotropes to diamond is avoided, resulting in a uniform texture. This makes it possible to produce high-quality single-crystal diamond.

[0018] Processes for producing single-crystal diamond under high pressure are also well known in the art, so a detailed explanation thereof does not seem necessary. A general description of such methods can be found, for example, in the Encyclopedia of Physical Sciences, and Technology.
Clopedia of Physical Science
nce & Technology), Volume 6 (Academic Press, 1987), pp. 492-506, Strong, “The Physics
The Physics Teach
r)'' (January 1975), pages 7-13, and the specifications of U.S. Pat. Nos. 4,073,380 and 4,082,185. In such methods, generally 50
Pressure of about 000~60000 atm and about 1300~
At temperatures in the range of 1500°C, the carbon used as feedstock is diffused through a liquid bath of metal catalyst-solvent material. In this case, it is preferred to maintain a negative temperature gradient, typically of about 50° C., between the material to be converted and the precipitation region (which contains diamond seeds for initiating crystal growth).

Catalyst/solvent materials useful in Step B are known in the art. Examples include iron, mixtures of iron with nickel, aluminum, nickel-cobalt, nickel-aluminum or nickel-cobalt-aluminum, and mixtures of nickel and aluminum. Iron-aluminum mixtures are often preferred for producing single crystal diamond. Among them, for the purpose of the present invention, 95% by weight of iron and 5% by weight are preferred.
Mixtures with % by weight aluminum are particularly preferred.

[0020] After producing the single crystal diamond, it is often preferable to remove the portion originating from the seed crystal by polishing.

[0021] In order to explain in more detail the method for producing single-crystal diamond with high isotopic purity, examples are shown below. First, a diamond layer was deposited on a molybdenum substrate according to a CVD method in a reaction chamber made of quartz and copper, which do not dissolve substantial amounts of carbon. The substrate was placed perpendicularly in a plane parallel to a tungsten filament of approximately 0.8 mm diameter at a distance of 8-9 mm from the filament. The reaction chamber is heated to about 10 Torr.
After evacuation to a pressure of r and heating the filament to approximately 2000° C. by supplying an electric current, a mixture of 98.5% by volume hydrogen and 1.5% by volume methane was flowed into the reaction chamber. The methane used was virtually free of impurities and contained 99.9% of the 12C isotope. When the diamond thus obtained was taken out and subjected to mass spectrometry, it was found that 99.91% of the carbon contained therein was 12C.

The CVD diamond with high isotopic purity as described above was crushed into powder and used as a carbon source for growing single crystal diamond under high temperature and high pressure conditions. Specifically, a conventional belt apparatus was used at a pressure of 52000 atmospheres and a temperature of 1400°C. A mixture of 95% by weight iron and 5% by weight aluminum was used as catalyst/solvent material. A small (0.00
A 5 carat) single crystal diamond seed crystal was used and a negative temperature gradient of approximately 50°C was maintained between the CVD diamond and the seed crystal. This process was continued until a 0.95 carat single crystal diamond was obtained. Analysis revealed that 99.93% of the carbon contained in it was the 12C isotope. The seeds were removed by polishing the diamond on a standard diamond polisher.

The extremely high radiation resistance of the single-crystal diamond produced as described above has been demonstrated by detecting the heat waves generated as a mirage when its surface is bombarded with a modulated argon ion laser beam. was discovered when measuring the thermal conductivity of For such measurements, it was first necessary to place a laser-absorbing film on the surface of the diamond. natural II
In the case of a-type diamond, this was achieved by the action of an argon-fluorine excimer laser operating at a wavelength of 193 nm. That is, by graphitizing the surface using such a laser, a graphite layer with a thickness of about 60 nm was formed. The laser damage threshold for such natural diamonds was found to be 300 millijoules/cm2. Similar attempts to graphitize the surface of isotopically pure diamond were unsuccessful even when the laser fluence was increased tenfold. Thus, the laser damage threshold for diamond with high isotopic purity is 3000
It was larger than millijoules/cm2.

According to theoretical considerations regarding diamond, the high laser damage threshold of the optical article of the present invention is approximately 150 ~
It can be seen that it is observed within the range of 220 nm. Note that depending on the type of interaction between excited electrons and phonons in the diamond product, an improvement in the threshold value may be observed at wavelengths lower than 150 nm.

Optical articles of the present invention include windows, lenses, diffraction gratings, and mirrors used to admit radiation (particularly light emitted from lasers). They are particularly useful in the case of free electron lasers, where the light beam may be focused or reflected by optical products located at a distance of about 5 meters or less from the laser source.

It is also known to use diamond as an active material for lasers. Accordingly, lasers that use isotopically pure diamond as the active material form another aspect of the invention.

The optical article of the present invention has a conventional construction except for the use of diamond material. Therefore, they can be manufactured by methods known in the art.

Claims (13)

[Claims] 1. An optical article resistant to radiation damage, characterized in that it is composed of single crystal diamond consisting of at least 99.2% by weight of 12C or 13C. 2. The optical article of claim 1, wherein said single crystal diamond comprises at least 99.2% by weight 12C. 3. The optical product according to claim 2, which is a window for a laser. 4. The optical product according to claim 2, which is a mirror for a laser. 5. The optical product according to claim 2, which is a lens for laser. 6. The optical product according to claim 2, which is a diffraction grating for a laser. 7. The optical article of claim 2, which is an active material for a laser. 8. The optical article of claim 2, wherein said single crystal diamond comprises at least 99.9% by weight 12C. 9. The optical product according to claim 8, which is a window for a laser. 10. The optical product according to claim 8, which is a mirror for a laser. 11. The optical product according to claim 8, which is a lens for a laser. Claim 12: Claim 8, which is a diffraction grating for a laser.
Optical products listed. 13. Claim 8, which is an active material for a laser.
Optical products listed.