Note: Descriptions are shown in the official language in which they were submitted.
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INGLE-~RY5TA~ DIAMON~ OF ~Y HIGH
'rHER~aL ~NDUCTIVI~Y
This application is a continuation-in-part of
copending application Serial No. 07/448,469.
This invention relates to the preparation of single
crystal diamond of extremely high thermal conductivity.
Diamond substances with high thermal conductivity
are known in the art. For example, type IIA natural diamond,
characterized by a very high degree of purity, has a thermal
conductivity at 25 C ~298-K) on the order of 21 watts/cm.-K,
the highest of any material known prior to the present
invention. Its electrical conductivity, on the other hand,
is so low as to be negligible.
These properties make diamond an excellent material
for conducting heat away from heat-generating objects or
units. It may be used as a heat sink or as a
conductor/spreader to conduct heat from the heat-generating
objec~ or unit to a heat sink of some other material.
Various areas exist in which heat conductors of
very high thermal conductivity are necessary. One example is
in repeating stations for fiber optic networks. Signals are
transmitted by laser light over the fibers of such networks
for very great distances. 5ince these signals decrease
substantially in intensity over several kilometers, it is
necessary to construct "repeating stations" periodically
along the network, for the purpose of increasing the
intensity of the light transmitted along the network. In a
typical repeating station of this type, a photodetector is
employed to convert the weakened signal transmitted by fiber
optics to an electrical signal, which is then magnified,
reconverted to a light signal by a light-emitting diode, and
transmitted in turn along the next segment of the network.
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In order to minimize the requisite number of
repeating stations, optimum magnification of the signal in
any station is desirable. However, the amount of radiant
energy of any kind generated electrically is proportional to
the fourth power of the current employed. While a portion of
such radiant energy is in the form of light, the remainder
thereof is lost as heat. In any individual station,
therefore, very large amounts of heat are generated,
requiring efficient heat conductors to maintain the
operativeness of the repeating station.
The use of natural type IIA diamond as a conductor
of heat in these areas has not been foreclosed despite its
very high cost since the operative heat-conducting units are
very small, typically about 1 mm. on a side. When larger
heat-conducting elements are required, expense considerations
become important and the use of natural diamond may
accordingly be foreclosed.
Conventionally produced high-pressure synthetic
diamond of gem quality is lower in cost than natural diamond.
However, synthetic diamond of this type having high thermal
conductivity cannot effectively be produced directly from
graphite because a profound contraction in volume occurs in
the conversion and introduces imperfections into the
crystalline structure. For the most part, diamond prepared
by low-pressure chemical vapor deposition (hereinafter
sometimes "CVD") processes are not single crystal diamond and
have substantially lower thermal conductivities, typically on
the order of 12 watts/cm.-K at about 300-K (hereinafter
sometimes "room temperature conductivity").
In U.S. Patent 3,895,313, there are disclosed
various diamond materials which allegedly have very high
thermal conductivities, and which allegedly are useful as
optical elements for very high-power laser beams. In
particular, it is stated that synthetic diamond grown from
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isotopically pure carbon-12 or carbon-13 would be useful in
this way, with room temperature conductivity values in the
range of 10-20 watts/cm.-K being mentioned. This is, of
course, the order of magnitude of natural type IIA diamond
and also of diamond produced by chemical vapor deposition.
It is further suggested that at a temperature of 70 K
(-203 C), below the temperature of liquid nitrogen, a thermal
conductivity exceeding 200 watts/cm.-K may be obtained,
perhaps in diamond which has high isotopic purity and
"relevant properties which closely approach the limits
predicted by the theory of the perfect solid state" (i.e.,
are in single-crystal form). However, no methods for the
preparation of such diamond are suggested, and the state of
the art has hitherto been inadequate to put them in the
possession of the public.
The present invention provides a method for
preparing single-crystal diamond of very high chemical and
isotopic purity. The raw material for such diamond is itself
diamond, which eliminates the volume contraction encountered
in the high-pressure conversion of graphite to diamond. The
thermal conductivity of the diamond so produced has been
found to be higher than that of any substance presently known
including natural type IIA diamond, and also higher than the
values in the aforementioned U.S. Patent 3,895,313.
Accordingly, it is eminently suitable for use as a conductor
of heat and also in numerous other areas.
In one of its aspects, the invention is a method
-or preparing single-crystal diamond of high isotopic purity
which comprises the steps of:
(A) preparing diamond consisting of isotopically
pure carbon-12 or carbon-13; and
(B) converting said diamond to single-crystal
diamond by diffusion under high pressure through a metallic
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catalyst-solvent material to a region containlng a diamond
seed crystal.
An essential feature of the method of this
invention is the employment of isotopically pure carbon-12 or
carbon-13. As explained hereinafter, it has been found that
the increase in thermal conductivity resulting from the
employment of chemically and isotopically pure carbon is
vastly greater than would be expected based on theoretical
considerations. In general, the isotopic purity of the
hydrocarbon should be at least 99.2% by weight; that is, the
other isotope should be present in a maximum amount of 8
parts per 1000. An isotopic purity of at least 99.9% by
weight is preferred. The hydrocarbon should also have a high
degree of chemical purity.
lS Various methods may be employed in step A for the
preparation of diamond in isotopically pure form. For
example, a gaseous carbon compound such as carbon monoxide
may be separated into carbon-12 and carbon-13 species via
differences in diffusivity and then 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.
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 pxecursors which may be
employed in enriched form include pyrolytic graphite,
amorphous or glassy carbon, liquid hydrocarbons and polymers.
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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
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 activa~ion 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
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.
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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. 07/389,210
and 07/389,212.
Isotopically pure 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
CV3 chamber should be constructed of materials substantially
incapable of dissolving carbon. Typical materials of this
type are quartz and copper.
As between carbon-12 and carbon-13, the former is
usually preferred for various reasons. In the first place,
carbon-12 is present in nature in much higher proportions
than carbon-13, the latter typically occurring in amounts no
higher than about 1~ by weight; therefore, the use of carbon-
12 involves minimum expense. In the second place, thermal
conductivity is inversely proportional to the square of the
mass number of the isotope, and diamond prepared from carbon-
12 can therefore be expected to have a thermal conductivity
about 17~ greater than those prepared from carbon-13. In
some applications, however, carbon-13 is preferred and its
preparation and use are part of the invention.
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
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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
S high pressure means, as described hereinafter, employing the
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 pure 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 pure 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 pure 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 Fncyclo~edia of Physical Science &
Technology, vol. 6, pp. 492-506 ~Academic Press, Inc., 1987);
30 Strong, The Physics Teache~, 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 liguid bath of a
metallic catalyst-solvent material, at pressures on the order
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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.
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
by the method of this invention, it is often preferred to
remove the portion attributable to the seed crystal by
polishing. This is particularly true if the seed crystal is
not isotopically pure.
Studies on single-crystal diamond prepared
according to the invention and having various degrees of
isotopic purity have shown that at carbon-12 purity levels of
99.2%, 99.5% and 99.9% by weight, the room temperature
conductivities are, respectively, 10~, 25% and 40% greater
than that of natural type IIA diamond. At lower
temperatures, even higher differences in thermal conductivity
are to be expected. Such thermal conductivities are higher
than those of any material previously known. Single-crystal
diamond of this type is another aspect of the invention, as
is diamond prepared by the method described herein.
The reasons for the extremely high thermal
conductivity of the single-crystal diamond of this invention
are not fully understood. It is assumed, however, that the
phenomenon is principally a function of the mean free path of
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the phonons (i.e., lattice vihratlon modes) in the diamond
crystals. Thermal conductivity is directly proportional to
specific heat, sound velocity in and phonon mean free path of
the crystal, and isotope effects on specific heat and sound
velocity are negligible.
In a simplified calculation, the reciprocal of the
phonon mean free path can be considered as equal to the sums
of the reciprocals of the mean free paths attributable to
phonon-phonon scattering and to isotope effects. It has been
calculated that the isotope-related mean free path is 34,000
Angstroms and the phonon scattering-related path is 1900
Angstroms; consequently, the isotope effect would account for
a decrease in mean free path on the order of only about 5.2%.
One possible reason for the substantially greater observed
isotope effect on thermal conductivity is that contrary to
theory, the isotopic constitution of the diamond has a direct
effect on the mean free path attributable to phonon-phonon
scattering. This effect has apparently not been previously
recognized.
By reason of the aforementioned high thermal
conductivity value and essentially non-existent electrical
conductivity, the isotopically pure single crystal diamond of
the present invention is singularly useful as a conductor of
heat from electronic devices and similar heat-generating
sources. Articles comprising such a source in contact with
said diamond as a thermal conductor are anothex aspect of the
invention.
A further aspect is abrasive articles comprising
said diamond. Such articles can be expected to have
extraordinarily long life by reason of their ability to
dissipate frictional heat generated during their use.
Typical areas of application include abrasive grit, diamond
compacts, wire drawing dies, saw blades, scribing tools,
drills, tool sharpeners, and polishing tools for optical
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items, stone and precious stones including diamonds and
articles made therefrom.
Still another aspect is light-filtering articles
comprising said diamond with a pinhole aperture therein.
They are useful, for example, as spatial filters for laser
beams and the like. Articles of this type fabricated from
natural diamond are subject to radiation-induced damage,
probably thermal in nature. The substantially higher thermal
conductivity of the diamond of this invention may be expected
to greatly minimize damage of this kind.
The invention 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.
Thermal conductivity of the isotopically pure CVD
diamond was measured by the mirage detection of thermal waves
generated by a modulated argon-ion beam impinging on the
diamond crystals, in accordance with a conventional method.
The room temperature conductivity was shown to be about 12
watts/cm.-K. A control sample of similar CVD diamond
prepared from methane having the naturally occurring isotope
distribution (98.96% C-12, 1.04% C-13) had essentially the
same conductivity.
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The isotopically pure 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
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, and its room temperature
conductivity was compared with those of several other
materials, including a control single-crystal diamond
prepared from the CVD diamond with normal isotope
distribution. The results were as follows, all values being
in watts/cm.-K:
Iqotopically pure C-12 diamond ~thiq invention) 31.5
Control 21.18
Naturally occurring type IIA diamond 21.2
CVD diamond 12.0
Cubic boron nitride 7.6
Silicon carbide 4.9
Copper 4.0
8eryllium oxide 3.7
80ron phoqphide 3.6
Aluminum nitride 3.2
Silicon 1.6
Aluminum oxide 0.2
Thus, the room temper~ture conductivity of the
diamond of this invention is 48.7% greater than that of the
control. It is also much higher than the room temperature
conductivity of any other diamond or non-diamond material
which has been measured.
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At 70-K, below the boiling point of liquid
nitrogen, the diamond of the present invention is
theoretically predicted to have a conductivity of about 2675
watts/cm.-K, more than 13 times the minimum value predicted
S in the aforementioned U.S. Patent 3,895,313.
