CA2033138A1 - Superhard carbon metal - Google Patents

Abstract

Abstract of the Disclosure A superhard carbon composition, having a crystal structure cell consisting of (i) a unit cell of six carbon atoms with crystallographic hexagonal symmetry, (ii) all carbon atoms in flat three-fold coordinated configurations (sp2 bonding), and (iii) carbon atoms in layers of chains which zig-zag in a direction normal to the layers with each layer being rotated 60° with respect to its adjacent layer. The composition has a density of about 3.2 g/cm3, a bulk modulus and a hardness exceeding diamond (bulk modulus is 6.9 Mbar), and a bonding length of 1.45-1.47 angstroms.

Description

SUPERHARD CARBON METAL

Backqround of_~he Invention Technical Field This invention relates to the field of superhard materials and allotropic forms of carbon, and more particularly to a new phase o1E carbon embodying a new crystal structure comprised oE three-fold coordinated carbon only.

Discussion of the Prior Art Superhard materials refers to solids of hardness comparable to or in e~cess of that of diamond. Examples are diamond itself, cubic boron nitride (C-BN, tràdename:
borazon), boron subo~ides (B220), and a new (possibly hypothetical) compound C3N4. The extreme hardness of such materials is exploited in high-speed cutting and grinding tools and low-friction, long-wearing bearing surfaces. Recent advances in chemical vapor deposition (CVD) of such materials in thin film form has broadened the application o~ superhard materials as surface coatings. Howe~er, materials even harder than diamond have been avidly sought with unsatisfactory results.
This invention has discovered a new crystalline form of carbon which is harder than diamond, less dense than diamond, is metallic, and characterized by three-fold atomic coordination (sp2 bonding) only.
The only m~ntion in the prior art of which applicants are aware that a carbon phase harder than diamond has been made appears in articles by ~.M~
Matyusenko and V.E. Strel~nitskii, in Journal Qf Experimental and Theoretical Physics hçtter, Vol. 30, page 199 (1979); and by A.~. 8akai and V.E.
Strel'nitskii, in Annals of Technical Physics, Vol. 51, , ~, ''`~.
. - : : . :

.

- 2 - 2 ~ 3 3 ~L 3 ~

paqe 2414 (1981). Such articles purport to describe the process of ormation and properties of a "super-dense"
form of carbon which is both harder and denser than carbon. The existence of this new carbon phase, called C-8 or ~suparcubane~ is a sub~ect of dispute in the technical community because of its estreme difficulty to make and isolate. C-8 carbon consists of four-fold coordinated (sp3 bonded~ carbon in a highly distorted configuration and, disadvantageously~ will require high pressure procPssing.
Other investigators have speculated that other allotropic forms of carbon may exist, but this invention has established and teaches how to make a new allotropic form of carbon which is consists of sp2 coordinated carbon only, is metallic, is harder than diamond, but less dense, and has a higher bulk modulus than diamond.

Sum~ary of the Invention This invention has discovered a new crystalline phase of carbon having a crystal structure consisting of (i) a repeatiny unit cell of six ~arbon atoms with crystallographic he~a~onal symmetry, [ii) all carbon atoms in flat, three-fold coordinated configurations, and (iii) carbon atoms in layers of chains which zig-zag in a direction normal to the layers are joined to adjacent layers by vertical bonds and rotate by 60 from layer to layer. The new crystalline form of carbon is further characterized by a density of about 3~2 g/cm3 and is intrinsically hzrder than diamond. Because of the he~agonal s~mmetry and si~ atoms per cell, the structure shall hereinafter be referred to as ~the H-6 structure"
and the embodiment of the new crystal structure in carbon shall be referred to as "H-6 carbonU. H-6 carbon is fundame~tally different from diamond, in which each atom is four-fold coordinated (sp3 bonded); eYery carbon , : ~ , ~33~3~

atom in H-6 carbon is three-fold coordinated (sp2 bonded) and the bonds are shorter and stronger.
The H-6 carbon struct:ure is topologically related to diamond in the sense that it can be smoothly distorted from H-6 to diamond without breaking or crossing any existing bonds. Because sp2 carbon-carbon bonds are appreciably shorter (1.42-1.47 angstroms) and therefore stronger than sp3 C--C bonds ~1.54 angstroms), the bulk modulus of H-6 carborl, which is a measure of intrinsic hardness, is as much as 50% larger than that of diamond (6.9 Mbar as opposed to 4.4 Mbar for diamond).
Also, unlike diamond, H-6 carbon is metallic in that it e~hibits significant electrical conductivity at room temperature that is not eliminated by cooling to low temperature.

Summary of the Drawings Figure 1 is a schematic illustration of the H-6 structure showing the bonds between neighboring atoms as double lines;
Figure 2 is a drawing of the H-6 unit cell with its si~ nonequivalent atomic sites numbered as 1-6;
Figure 3 is a plan view (looking down along the ~vertical" a~is) of the H-6 unit cell shown in Figure 2, the dashed lines indicate the atomic motions associated with the conversion to the diamond structure;
Figure 4 is a schematic illustration of the apparatus useful in carrying out the chemical vapor deposition of H-6 carbon;
Figures 5-6 are stylized ~ball-and-stick"
drawings of the lattice structures of diamond (Fig. 5) and a related all-sp3 form of carbon (lonsdaleite, Fig. 6) which has properties very similar to those of diamond; and Figures 7-8 are stylized drawings of the - 4 - ~ 3 ~ ~ 3 ~

lattices of two known forms oE graphite: he~agonal (Fig.
7) and rhombohedral (Fig. 8).

De~iled Des~ription and Best Mo~e The great hardness of H-6 carbon can be understood by comparison to graphite. .Graphite consists of flat layers ~sheets) 10, 11, 12 of he~agonal rings of sp2 coordinated carbon, similar in appearance to chicken wire (see Figures 7-8). Graphite is in a sense a two-dimensional material in that there are no strong covalent bonds between the sheets, the sheets being held together only by the much weaker Van der Waals forces 13 associated with the fourth unused bonding electron (the so-called ~ electron3 at each carbon atom; the bonding length between sheets is long, about 3-4 angstroms. In the plane of the layers, graphite is actually comparable in hardness to diamond--the elastic modulus is comparable and the optical phonon (a lattice vibration~ freguency is significantly higher. This hardness is due to the greater amount of electronic charge associated with the sp2 bonds 14 relative to the sp3 bonds 15 of diamond (ses Figure 5), which in turn results in a much shorter bond length (1.42 angstroms versus 1.55 angstroms in diamond).
The H-6 structure (as shown in Figure 1) is comprised of layers 16, 17, lB, which in turn has carbon chains 19; the layers are joined by bonds 20 along the c (vertical) a~is. Each layer e~tends generally along an . a-b plane and the chains 19 are substantially all aligned 30 within a layer (such as chains l9a, l9b, l9c in layer 16, and chains l9d, l9e, l9f in layer 17~. The atoms of a chain in a layer zig-zag, such as atoms 28, 29, 30 in Figure ~. All carbon atoms are bonded to three other ca bon atoms (such as 21 bonded to 22, 23, 24 in the flat sp coordinated configuration 25. The alignment of - 5 - ~333~

chain structures comprising each layer rotates by 60 about the c a~is from layer to layer, resulting in hexagonal symmetry. With an ;n-chain repeat interval 25 of two atoms, and a vertical repeat interval 26 of three 5 layers, the H-6 structure has a unit cell consisting of six atoms tsee numbering of al:oms in Figures 1 and 2).
The H-6 structure is topologically related to diamond in that the lattice can be continuously deformed to that of diamond without breaking or crossing any existing bonds.
Figure 3 shows a top view of the H-6 unit cell and indicates (the dashed lines) the atomic motions associated with the conversion to diamond (numbering corresponds to Figure 23.
Differences in crystal structure between H-6 carbon and other forms of carbon can be seen by comparing Figure 1 to Figures 5 and 6 for diamond and related lonsdaleite, and Figures 7 and 8 for known forms of graphite. Diamond has a cubic lattice built up from sp3 bonded carbon atoms arranged in tetrahedra; the lattice layers follow the sequence ABCABC so that every third layer is identical tsee Figure 5). Lonsdaleite is a hexagonal modification of diamond also with tetrahedral coordination, but with layers stacked ABAB (see Figure 6). Lonsdaleite has bsen found in very small quantities in meteorites and shock-quenched diamond and has properties much like that of diamond.
Graphite consists of layers of sp2 bonded si~-fold rings of carbon (resembling chicken wire, see Figures 7 and 8). In both he~agonal and rhombohedral graphite the layers are stacked such that half of the atoms are directly above atoms in the adjacent layers, and half are centered over the open rings of the adjacent layers. There are two possible ~tacking sequences.
Hexagonal graphite has stacking ABAB and is shown in Figure 7, while rhombohedral graphite has stacking ABCABC
~see Figure 8).

- 6 - ~3 H 6 carbon also embodies an ABC stacking sequence and consists of sp2 bondad carbon only, but is distinct from graphite in that it contains no closed 5 rings.

Physical PrQPerties The H-6 structure is a topologically rigid, three-dimensional network of short and stiff sp2 carbon-carbon bonds ~it cannot be collapsed through bond rotations). The excess bond charge (over that of diamond) will be delocalized along the chains. Such electron delocalization renders a material metallic. The hydrocarbon polymer polyacetylene, analogous to a single chain in the H-6 structure to which H is added, is a poor metal due t-o the tendency of the electrons to gather on alternating carbon-carbon bonds. When such bond alternation is overcome, polyacetylene is an excellent electrical conductor. This bond alternation does not occur in H-6 carbon. Diamond is not metallic and graphite is semi-metallic (a poor metal~.
H-6 carbon has a density of about 3.2 g/cm3, less than that of diamond (3.5 g/cm3), but much ~reater than that of both forms of graphite (2.25 g/cm3).
Given that the covalent sp~ bond of the H-6 carbon are shorter and stronger than the sp3 bonds of diamond, reliable quantum mechanical calculations show that H-6 carbon is significantly harder than diamond--H-6 carbon has a bulk modulus of approximately 6~9 Mbar, as opposed to a 4.4 Mbar for diamond.
The H-6 structure has an intra-chain bond length of 1.47 ang.stroms and an inter-chain bond length ~along the c axis) of 1.45 angstroms. The cohesive energy of H-6 carPon is 6.94 eV~atom, 0.43 eV/atom smaller than that of graphite. While this difference is much larger ~ ~ "

., 7 ~ 3~ ~~3 than the .025- eV~atom difference between diamond and graphite, the H-6 structure is metastable against spontaneous conversion to diamond. The calculated energy barrier to diamond along the high symmetry path in Figure 3 is sufficient to establish the metastable condition.

Forminq H-6 carbon is metastable with respect to conversion to diamond or graphite. Because of its topological relationship with diamond, H-6 carbon must convert to diamond befor2 conversio~ to graphite. Thus, only stability with respect to conversion to diamond is at issue. Because ~-6 carbon is less dense than diamond, it cannot be formed by high pressure processes by which diamond is formed both in nature and in the laboratory;
the product of such high pressure processes must always be diamond or a denser material. Thus, H-6 carbon cannot occur in nature and, with certainty, 's a new composition of matter.
This invention utilizes very low pressures accompanied by the technigue of chemical vapor deposition (CVD) to generate and deposit H-6 carbon on a specially conditioned substrate under critical deposition conditions of atmosphere and temperature under which the H-6 carbon material is thermodynamically stable as a surface phase and subsurface conversion to the more stable diamond phase is inhibitedD
Low pressure CVD of H-6 carbon is carried out by energi~ing a dilute mixture of hydrocarbon gas (e.g., methane or acetylene) in hydrogen either by thermal ~e.g., a hot filament) or electromagnetic ~DC, radio frequency or microwave plasma) means to sufficient chemical activity that a significant partial pressure of atomic hydrogen ana a high density of reactive , ~ ,. .
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- 8 - ~33~3~

hydrocarbon radicals is obtained. In the case of thermal e~citation, the e~cited gas misture must be quickly cooled, usually by thermal expansion to achieve a metastable gas atmosphere in which the gas ~inetic temperature is lower than the temperature that would be assigned to the chemical composition were it at equillbrium (the so-called Uchemical temperature~). In the case of plasma excitation, the condition of gas metastability may be achieved in the region of excitation due to the inevitable discrepancy between the electron temperature, which is very high and so controls the gas chemistry, and the ion temperature which dominates the gas temperature and is quite low because the electromagnetic e~citation couples mainly to the electrons. Deposition from plasma e~cited gas may be -performed within the plasma region (immersion deposition) or, if needed, the plasma e~cited gas can be extra~ted and made to flow across a ubstrate outside the region of excitation (downstream deposition).
The substrate may be selected from the group of materials which is not affected by the high temperature and the reducing atmosphere of H-6 carbon CVD. A
suitable substrate can be the <111> crystallographic surface of diamond itself, the surface of which is topologicall~ identical to one layer of H-6 carbonO The substrate temperature should be kept quite low, 300-400C, which is low in comparison to standards of diamond deposition (600-1000C). The low substrate temperature is necessary to promote formation of sp2 structures and suppress collapse into the undesirable diamond structure. The temperature must be just high enough to permit etching of any true graphite which might form, but not so high as to allow etching of H-6 carbon.
. The allowed range of substrate temperatures in which the above conditions are met is in turn determined - . :

_ 9 _ 2 ~ J L 3 ~3 by the pressure, temperature, and composition of the excited hydrocarbon/hydrogen gas atmosphere. The overall pressure must be quite low, 0.1 to 1.0 Torr, relative to the standards for diamond CVD (10-100 Torr) to enhance to lifetime and therefore the population of atomic hydrogen which is essential in stabilizing the H-6 carbon surface, etching of graphitic structure~s, and promotion of the deposition chemistry. The eed gas must be relatively rich in hydrocarbons tin the range of 5-50% methane in hydrogen) to promote usefully rapid formation of sp2 structures on the substrate. The rate of formation and purity of the H-6 carbon deposit will depend in detail on the chemical composition of the metastable gas atmosphere. Addition to the feed gas of 02ygen-containing species ~water vapor, methanol, acetone, oxygen, carbon mono~ide or carbon dio~ide) can have significant effects on the rate and character of the deposition.
Excitation of the gas mixture can be achieved by a variety of means. Each alternative has advantages and drawbacks associated therewith. A hot ~ilament may e~pose the sub~trate to excessive radiant energy and thereby overheat it. Immersion in an overly energetic plasma may do the same. The best means are those in ~5 which the ~rowth substrate is not directly e~posed to the escitation region. This is difficult to achieve with a hot filament because shielding the radiative load requires intervening surfaces which interfere with gas flow and provide surfaces for the recombination of atomic hydroqen. Remote excitation with downstream deposition is more readily achieved by plasma excitation in which the hydrogenihydrocarbon mi~ture is e~cited in a prechamber from which it flows across the substrate.
. Deposition of H-6 carbon can be carried out by use of an apparatus similar to that shown in Figure 4 , .~ :

- 10 - 2e~3~3~

wherein a microwave plasma discharge apparatus 50 is shown. A quartz tube 51 is set across a microwave guide tube 52 which serves as the plasma excitation chamber 53. The feed yases described above are supplied from tanks 54 and 55 and held in proper proportions by an automatic flow controller 56 to provide a predetermined gas mi~ture 58 which is introduced at the top of the reactor ~cham~er 53). The pressure control system 64 has a pressure control valve 65 and a vacuum pump 67 to achieve chamber pressures of .1-1.0 Torr. The gas mi~ture is controlled to a total flow rate through the chamber of about 100-1000 sccm tstandard cubic centimet~rs per minute) during active deposition.
Microwave plasma is generated to heat the gas misture in the chamber and a substrate is placed therein. The gas passes through the region of microwave discharge, the eficiency of which is enhanced by the imposition of a magnetic f ield such that the electron cyclotron resonance (EC~) condition is met within the plasma. For a microwave frequency of about 2.45 GHz ~FCC standard), the required field is approsimately 850 Gauss. The plasma should be confi~ed to the center of the quartz tube. The substrate i~ attached to a holder 61 made of a suitable heat and chemical resistant material which can heat or cool the substrate as necessary to maintain the desired sur~ace temperature~ The substrate i5 usually held within the plasma for the CVD of diamond, but should be kept outside (downstream~ for H-6 carbon deposition. The desired distance from the plasma is determined mainly by the gas flow velocity, but is also af~ected by the gas composition and power density of the microwave excitation. The total gas pressure in the reactor is held constant by a pressure controller on the vacuum pump at the .gas egit from the chamber.
H-6 carbon will appear on the surface of diamond ~ 33 during conventional CVD growth of diamond, and convert to diamond beneath the growing surface (thus the above-menti~ned modifications in CVD to suppress such conversions). H-6 carbon will appear as very thin surface phases, stable under t:he conditions of conventional CVD growth of diamond. H-6 carbon is an intermediate structure necessary in the formation of diamond in the CVD process.

While particular embodiments of the invention have been illustrated and described, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the invention, and it is intended to cover in the appended claims all such modifications and equivalents as fall within the true spirit and scope of this invention.

Claims (11)

1. A superhard carbon composition, having:
(a) a crystal structure cell consisting of (i) a unit cell of six carbon atoms with crystallographic hexagonal symmetry, (ii) all carbon atoms in flat three-fold coordinated configurations, and (iii) carbon atoms in layers of chains which zig-zag in a direction normal to said layers with each layer being rotated 60°
with respect to its adjacent layer;
(b) a density of about 3.2 g/cm3;
(c) a bulk modulus and hardness exceeding diamond; and (d) metallic electronic properties.

2. The composition as in claim 1, in which said crystallographic hexagonal symmetry is P6222.

3. The composition as in claim 1, in which its bulk modulus is about 6.9 Mbar.

4. The composition as in claim 1, in which all carbon atoms are bonded to exactly three other carbon atoms as sp2 bonding.

5. The composition as in claim 1, in which said hardness exceeds at least 8000 kg/mm2 at 25°C.

6. The composition as in claim 4, in which the bonding length of each of said sp2 bonds is 1.45-1.47 angstroms.

7. The composition as in claim i, in which said layers of carbon atoms are joined to adjacent layers by vertical bonds (normal to the plane of the layers) and are stacked in an ABCABC sequence with an absence of closed rings.

8. A superhard carbon metal, characterized by a crystal lattice having layers of carbon atoms arranged in three chains which zig-zag in a direction normal to sp2 rings, each layer being rotated 60° with respect to its adjacent layer.

9. The composition as in claim 1, which exists as an intermediate form in CVD of diamond.

10. A method of making superhard carbon metal, comprised of the following steps:
depositing onto a temperature-resistant substrate by excitation of a gas mixture comprised of 5-50% methane in hydrogen, said gas mixture being maintained in the pressure range of 0.1-1.0 Torr, said substrate being maintained in the temperature range of 300-400°C.

11. The method as in claim 10, in which said substrate is a diamond <111> face.