IE911469A1 - Radiation-Hard Optical Articles From Single-Crystal Diamond¹Of High Isotopic Purity - Google Patents

Abstract

RD-20479 RADIATION-HARD OPTICAL ARTICLES FROM SINGLE-CRYSTAL DIAMOND OF HIGH ISOTOPIC Single-crystal diamond consisting of isotopically enriched carbon-12 has been found to have a very high degree of radiation hardness, typically greater by a factor of more than 10 than that of natural type IIa diamond. Therefore, it is possible to fabricate optical articles therefrom, such as laser windows, lenses, gratings and mirrors and active materials for lasers.

Description

This invention relates to radiation-hard optical 5 articles, and more particularly to articles manufactured from a unique genus of single-crystal diamond materials.

There has recently been considerable interest in the development of materials resistant to damage from impinging radiation. Such materials are of value in the fabrication of optical articles which may be employed with or as part of lasers. This is particularly true in the case of free electron lasers, which are extremely difficult to transmit, focus or reflect because of the intense radiation produced thereby. At present, mirrors for such lasers must be placed at distances of thousands of meters because closer placement results in irreversible radiation damage to the mirror .

In copending application Serial No. 07/536,371, there are disclosed single-crystal diamond compositions having the highest thermal conductivity of any material presently known. These compositions are characterized by their isotopic purity which is at least 99.2% by weight. Various utilities for such diamond are disclosed, including thermal conductors, abrasives and light-filtering articles.

The present invention is based on the discovery that single-crystal diamond of high isotopic purity is characterized by extremely high resistance to radiation.

More specifically, single-crystal diamond comprising 99.9% carbon-12 has been found to have a radiation damage threshold more than 10 times the value formed for single-crystal diamond of normal isotopic purity (i.e., 98.9% carbon-12 and 1.17% carbon-13).

RD-20479 Accordingly, the invention includes optical articles resistant to radiation damage, said articles comprising single-crystal diamond consisting of at least 99.2% by weight carbon-12 or carbon -13.

An essential feature of the articles of this invention is the employment in their manufacture of singlecrystal diamond wich has been enriched in carbon-12 or carbon-13. As explained hereinafter, it has been found that the increase in radiation hardness resulting from the employment of chemically and isotopically pure carbon is vastly greater than would be expected based on theoretical considerations. The isotope distribution of the diamond should be at least 99.2% by weight carbon-12 or carbon-13, with carbon-12 being preferred. That is, the other isotope should be present in a maximum amount of 8 parts per 1000.

An isotope distribution of at least 99.9% by weight is preferred.

Various methods may be employed for the preparation of isotopically enriched single-crystal diamond. In general, they all involve the following steps: (A) preparing diamond consisting of isotopically enriched carbon-12 or carbon-13; and (B) converting said diamond to single-crystal diamond by diffusion under high pressure through a metallic catalyst-solvent material to a region containing a diamond seed crystal.

In step A, a gaseous carbon compound such as carbon monoxide may be separated into carbon-12 and carbon-13 species via differences in diffusivity and the carbon-12 fraction converted to solid carbon by art-recognized means, such as combustion in a reducing flame in the case of carbon monoxide. The carbon thus formed may then be converted to diamond under conventional conditions, including high temperature and high pressure conditions or CVD conditions.

RD-20479 Alternatively, other methods may be employed including shock formation and CVD processes under conditions which produce a mixture of diamond and graphite. In processes of the latter type, the carbon-13 species will concentrate in the diamond phase and the carbon-12 species in the graphite phase. Other diamond precursors which may be employed in enriched form include pyrolytic graphite, amorphous or glassy carbon, liquid hydrocarbons and polymers.

It is usually found that conventional methods of 10 CVD diamond formation are most convenient for the preparation of isotopically pure diamond. In such methods, a layer of diamond is deposited on at least one substrate. Any substrate material suitable for diamond deposition thereon may be employed; examples of such materials are boron, boron nitride, platinum, graphite, molybdenum, copper, aluminum nitride, silver, iron, nickel, silicon, alumina and silica, as well as combinations thereof. Metallic molybdenum substrates are particularly suitable under many conditions and are often preferred.

The method of chemical vapor deposition of diamond on a substrate is known, and the details need not be repeated herein. In brief, it requires high-energy activation of a mixture of hydrogen and a hydrocarbon, typically methane, whereupon the hydrogen gas is converted to atomic hydrogen which reacts with the hydrocarbon to form elemental carbon. Said carbon then deposits on the substrate in the form of diamond. Activation may be achieved by conventional means involving high-energy activation which produces atomic hydrogen from molecular hydrogen; such means include thermal means typically involving heated filaments, flame means, D.C. discharge means and radiation means involving microwave or radio-frequency radiation or the like.

Thermal and especially filament methods, employing one or more resistance heating units including heated wires RD-20479 or filaments, are often preferred for the purposes of this invention. In such methods, the filaments are typically of metallic tungsten, tantalum, molybdenum and rhenium; because of its relatively low cost and particular suitability, tungsten is often preferred. Filament diameters of about 0.2-1.0 mm. are typical, with about 0.8 mm. frequently being preferred. Distances from filaments to substrate(s) are generally on the order of 5-10 mm.

Said filaments are typically heated at temperatures 10 of at least 2000'C and the optimum substrate temperature is in the range of 900-1000'C. The pressure in the deposition vessel is maintained up to about 760 torr, typically on the order of 10 torr. The hydrogen-hydrocarbon mixture generally contains hydrocarbon in an amount up to about 2% by volume based on total gases. For a description of illustrative CVD methods of diamond preparation, reference is made to copending, commonly owned applications Serial Nos. 07/389,210 and 07/389,212.

Isotopically enriched hydrocarbon is employed in the CVD method, when used. In order to avoid contamination thereof, it is essential to employ equipment which does not contain natural carbon as an impurity. For this purpose, the CVD chamber should be constructed of materials substantially incapable of dissolving carbon. Typical materials of this type are quartz and copper.

The thickness of the CVD diamond layer deposited on the substrate is not critical. In general, it is convenient to deposit at least as much diamond as will be needed to produce a single crystal of the desired size. Of course, the production of a larger amount of CVD diamond for use to make several crystals is also contemplated.

It is possible to convert the product of the CVD process directly to diamond of high thermal conductivity by high pressure means, as described hereinafter, employing the RD-20479 same in the form of a slab, sheet or broken pieces thereof. However, the method of this invention is most efficiently conducted if the isotopically enriched diamond is first comminuted.

Comminution may be achieved by art-recognized means such as crushing and powdering. The particle size thereof is not critical so long as a sufficient degree of comminution is attained; the form known in the art as grit diamond is suitable.

Step B, the production of single crystal diamond, is conventional except that the isotopically enriched diamond produced in step A is the raw material employed. Two things are achieved by using diamond rather than graphite or some other allotrope of carbon as the raw material: an easily obtained isotopically enriched material may be employed, and the contraction in volume encountered in the conversion of graphite and other allotropes to diamond is avoided, permitting production of a single crystal of regular structure and high quality.

The process for producing single-crystal diamond under high pressure is also known in the art, and a detailed description thereof is not deemed necessary. Reference is made, for example, to Encyclopedia of Physical Science_& Technology, vol. 6, pp. 492-506 (Academic Press, Inc., 1987); Strong, The Physics Teacher. January 1975, pp. 7-13; and U.S. Patents 4,073,380 and 4,082,185, for general descriptions of the process. It generally involves diffusion of the carbon employed as a source material through a liquid bath of a metallic catalyst-solvent material, at pressures on the order of 50,000-60,000 atmospheres and temperatures in the range of about 1300-1500*C. A negative temperature gradient, typically of about 50’C, is preferably maintained between the material being converted and the deposition region, which contains a diamond seed on which crystal growth can begin.

RD-20479 Catalyst-solvent materials useful 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 frequently preferred for the production of single-crystal diamond, with a material consisting of 95% (by weight) iron and 5% aluminum being particularly preferred for the purposes of the invention .

Following preparation of the single-crystal diamond, it is often preferred to remove the portion attributable to the seed crystal by polishing.

The preparation of isotopically enriched singlecrystal diamond is illustrated by an example in which a layer of CVD diamond was first deposited on a molybdenum substrate in a chamber constructed of quartz and copper, neither of which dissolves substantial amounts of carbon. The substrate was vertically disposed in a plane parallel to and 8-9 mm. distant from the plane of a tungsten filament about 0.8 mm. in diameter. The vessel was evacuated to a pressure of about 10 torr, the filament was heated to about 2000‘C by passage of an electric current and a mixture of 98.5% (by volume) hydrogen and 1.5% methane was passed into the vessel. The methane employed was substantially impurity-free and 99.9% thereof contained the carbon-12 isotope. Upon removal and mass spectroscopic analysis of the diamond thus obtained, it was found that 99.91% of the carbon therein was carbon-12.

The isotopically enriched CVD diamond was crushed and powdered, and was used as a source of carbon for the growth of a single-crystal diamond under high pressure and high temperature conditions. Specifically, a conventional belt apparatus was employed at 52,000 atmospheres and 1400'C, employing a catalyst-solvent mixture of 95% (by weight) iron and 5% aluminum. A small (0.005 carat) single-crystal RD-20479 diamond seed of normal isotopic distribution was used to initiate growth, and a negative temperature gradient of about 50’C was maintained between the CVD diamond and the seed crystal. The process was continued until a single crystal of 0.95 carat had been produced. It was shown by analysis that 99.93% of the carbon therein was the C-12 isotope. The diamond was polished on a standard diamond scaife to remove the seed crystal.

The extremely high radiation hardness of the 10 diamond prepared as described above was discovered during an attempt to measure its thermal conductivity by mirage detection of thermal waves generated by impingement on the surface of a modulated argon-ion laser beam. It was first necessary to deposit a laser-absorbing film on the surface of the diamond. In the case of natural type Ila diamond, this was done by the action of an argon-fluorine excimer laser operated at a wavelength of 193 nm., which graphitized the surface with the formation of a graphite layer approximately 60 nm. thick. The laser damage threshold of the natural diamond was determined to be 300 millijoules/cm.2. Similar attempts to graphitize the surface of the isotopically enriched diamond were unsuccessful, even when the fluence of the laser was increased by a factor of 10. Thus, the laser damage threshold of the isotopically enriched diamond was greater than 3000 millijoules/cm.2.

Theoretical considerations relating to diamond indicate that the high laser damage thresholds of the articles of this invention will be encountered in the range of about 150-220 nm. Depending on the types of interactions between excited electrons and phonons in the diamond article, it is also possible that the enhanced threshold will be seen at wavelengths lower than 115 nm.

The optical articles of this invention include windows, lenses, gratings and mirrors adapted for the RD-20479 impingement of radiation, particularly light radiation and more particularly lasers. They are of particular value in the case of free electron lasers, which may be focused or reflected by the placement of the optical article at a distance on the order of 5 meters, or even less, from the laser source.

The use of diamond as the active material in a laser is also known. Accordingly, lasers comprising isotopically enriched diamond as an active material are another aspect of the invention.

The articles of this invention are of conventional construction other than in the diamond material used therein. Accordingly, they may be produced by methods known in the art.

Claims (13)

1. An optical article resistant to radiation damage, said article comprising single-crystal diamond consisting of at least 99.2% by weight carbon-12 or carbon13 .

2. An optical article according to claim 1 wherein the diamond consists of at least 99.2% carbon-12.

3. 3. An article according to claim 2 which is a laser window.

4 . An article according to claim 2 which is a laser mirror.

5. An article according to claim 2 which is a laser lens .

6. An article according to claim 2 which is a laser grating

7 . An article according to claim 2 which is an active laser i

8. material. An article according to claim 2 wherein the diamond consists

9. An of at least 99.9% article according carbon-12 . to claim 8 which is a laser window.

10 . An article according to claim 8 which is a laser mirror.

11. An article according to claim 8 which is a laser lens.

12. An article according to claim which is < i laser grating.

13. An article according to claim 8 which is an active laser material.