Note: Descriptions are shown in the official language in which they were submitted.
- 1 - 2~2269
RD-20479
R~nT~ ~ OPTTC.~T. A~.TTCT.F.~; F~OM
S~NGT.F-r~VST~ DT~MONI-) OF L~G~ T.COTOPTC
PuRTTv
This invent-on relates to radiation-hard optical
articles, and more particularly to articles manufactured from
a unique genus of single-crystal diamond materials.
There has recently been considerable lnterest in
the development of materials resistant to damage f-om
impinging radiation. Such materials are of value in the
iO fabrication of optical articles which may be employed with or
as part of lasers. ~his is particularly true in the case of
free electron lasers, which are extremely difficult to
transmit, focus or -eflect because of the intense radiation
produced ~hereby. ~t present, mirrors for such lasers must
lS be placed at distances of thousands of meters because closer
pLacement results in irreversible radiation damage to the
. mlrror.
In copending application Serial No. 07/536,371,
there are disclosed single-crystal diamond compositions
having the hiqhest 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).
- 2 - 20~1226~
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 art cles of this
invention is the employment in their manufacture of single-
crystal 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.
:
~ 3 ~ 204226~
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
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
: 15 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
20422~
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-l.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
of at least 2000 C and the optimum substrate temperature is
in the range of 900-lOOO 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. 07t389,210
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
- 5 - 20~2269
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.
S 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 commlnution 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 E~yclopedia of Phys i ca l Scie~ce h
Tech~ology, vol. 6, pp. 492-506 (Academic Press, Inc., 198?);
25 Strong, .The physics Teacher, January 1975, pp. 7-13; and U.S.
Patents 4,073,380 and 4,082,18S, 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
30 of 50,000-60,000 atmospheres and temperatures in the range of
about 1300-1500 C. A negative temperature gradient,
typically of about SO C, is preferably maintained between the
material being converted and the deposition region, which
contains a diamond seed on which crystal growth can begin.
- 6 - 2~22~3
RD-20479
Catalyst-solvent materials useful ln step 8 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
nic~el and aluminum. Iron-aluminum mixtures are frequently
preferred for the production of single-crystal diamond, with
a material consisting of 95% (by weight) lron 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 single-
crystal 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 ~uartz 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 d~amond 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
~ 7 ~ 20~22~
RD-20479
diamond seed of normal isotopic distribution was used to
initiate growth, and a negative temperature gradient of abou~
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
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 IIa 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
25 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 lS0-220 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
- 8 ~ 2~22~
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.
