WO2000015548A2

WO2000015548A2 - Fullerene based sintered carbon materials

Info

Publication number: WO2000015548A2
Application number: PCT/US1999/021174
Authority: WO
Inventors: Oleg A. Voronov; Gary S. Tompa
Original assignee: Diamond Materials Inc.
Priority date: 1998-09-14
Filing date: 1999-09-13
Publication date: 2000-03-23
Also published as: WO2000015548A3; JP2002524376A

Abstract

A new class of carbon materials and their synthesis. The new carbon materials are formed by high pressure and high temperature processing of fullerene based carbon powder. The new carbon materials are harder than graphite and can be harder than steel (when the starting fullerenes are single wall nanotubes) or almost as hard as diamond (when the starting fullerened are C60 buckyballs). The physical attributes of the materials can also be controlled by the pressing and heating parameters. These new carbon materials are conductive like graphite and unlike diamond which is an insulator. The materials can be formed by powder metallurgy techniques into any shape (cylinders, balls, tubes, rods, cones, foils, fibers or others). The new materials can also be readily doped, converted to diamond, formed within a porous composite or converted to diamond within the porous composite.

Description

FULLERENE BASED SINTERED CARBON MATERIALS

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority of US provisional application S.N. 60/100,078

filed Sept 14, 1998.

STATEMENT OF GOVERNMENT SUPPORT OF THE INVENTION

The work resulting in this invention was supported by the Defense Advanced Projects Agency (DARPA). Defense Small Business Innovation Research Program

ARPA order No. D611 , Amdt 27 issued by U.S.. Army Aviation and Missile

Commander under Contract DAAH01-98CR008.

BACKGROUND AND SUMMARY OF THE INVENTION

The present application is directed to a new class of carbon materials and their synthesis.

The conventional carbon materials are graphite, graphite-like ceramics or

diamond and diamond-like ceramics. These materials are produced from carbon-

containing compounds: oil, gases, coal, wood, coke, soot, graphite powder, diamond powder, hydrocarbons, polymers or mined as natural minerals. The lattice of

graphite consist of planar layers of hexagons, where the carbon atoms have sp²-

hybridization of the electron shells. The lattice of diamond consists of tetragons, where the carbon atoms have sp³-hybridization of the electron shells. Graphite is a

very soft and weak material, with a hardness of 1 on the Mohs scale (1-2 for some

graphite-like ceramics), it is conductive, sometimes referred to as a semimetal. Diamond is an extremely hard and tough material with a Mohs hardness of 10, it is

non-conductive, but may be made semiconductive with doping. _

Recently, much study has been made of ordered carbon molecules having

distinct geometries, such as spheres (known as "buckyballs") and tubular shapes

("nanotubes"), these geometric shapes are generally comprised of relatively large

numbers of carbon atoms such as C₆₀, C₇₀, C₈₀ etc. These ordered carbon molecules are often referred to as "Fullerenes" after the architect Buckminster

Fuller, whose geometric designs the molecules resemble. The new carbon materials

are synthesized (sintered) using powder metallurgy and/or ceramic pressing

techniques from relatively pure amounts of ordered carbon nanoparticles such as

Cgo buckyballs (which have a spherical icosahedral symmetry) and nanotubes

(which can be thought of as a tubular micro-crystal of graphite or a much elongated

buckyball with open or closed ends) at high pressures and high temperatures (HPHT).

The new carbon materials are formed by pressing and heating of powder in the form of specially prepared fullerenes. These carbon materials are much harder

than graphite and graphite like ceramics and are almost as hard as diamond, depending on the starting fullerenes, the pressing and heating parameters. These

new carbon materials are conductive like graphite. The material can be formed by

powder metallurgy techniques into any shape (cylinders, balls, tubes, rods, cones,

foils, fibers or others). The pressure of compacting is from 1.0-10.0 GPa, the temperature is 300-1000°C and the period of time is from 1-10000 second The

special carbon soots are (a) nanotube like, (b) buckyball like, or (c) mixtures of the

same with similar diameters (one dimension size) of particles of 0 7-7 0 nm The

particles are pure carbon of 99% or more preferably 99 9+ % (or specially doped by

other elements), separated by a narrow range of diameters, for example 0 7-1 0 nm, The physical properties of the new carbon materials that are produced

depend on the type and purity of the starting fullerenes A strong carbon conductive

material is formed by HPHT processing when the starting fullerenes comprise

purified single wall nanotubes (or a mixture of single walled nanotubes and

buckyballs) which has a hardness (7-9 ! on the Mohs Scale) greater than that of steel but less than that of silicon carbide (SiC) When the starting fullerenes comprise purified C₆₀ buckyballs of uniform size an extremely hard (9 %-10 on the

Mohs Scale) conductive amorphous carbon material is formed under HPHT

processing which has a hardness greater than that of silicon carbide and which is

only slightly less hard than non-conductive diamond or cubic boron nitride The new

carbon materials may be formed within porous ceramic composite "sponges" to form

other useful engineering materials by first impregnating the porous ceramic with the

appropriate carbon compound and then converting them directly into the new carbon materials

In particular, the two new carbon materials 1 ) nanotube based sintered carbon material and 2) buckyball based sintered carbon material, exhibit hardnesses better than stainless steel (for nanotube based sintered carbon

material) and near that of diamond (for buckyball based sintered carbon material)

These materials are near isotropic "polymeric" materials, not poly or single crystalline materials. The polymeric isotropicity is what sets these materials

apart-they are extremely tough, greatly resisting fracturing in comparison to c-BN or

diamond or other crystals.

Synthesis of the new carbon materials has been demonstrated for millimeter

sized pellets, which allow characterization of mechanical, electrical and other

properties that are pertinent to military, industrial and scientific applications. The

new carbon materials, with their high strength and toughness, may well fulfill the

need for lightweight engineering materials for military, aerospace, automotive, and

other industries. Since the materials are conductive, they may also be

superconductive or be made semiconductive, in either case especially with proper

dopants. It is theorized that the new carbon material is a semimetal and that the new

carbon material based ceramics may have the metallic and semiconductive type of conductivity depending on dopants and parameters of synthesis.

Traditional graphite may be transformed into diamond at pressure of 15 GPa

and temperature of 4000°C. Graphite mixed with metals Ni, Fe, Co or alloys or

hydrocarbons may be transformed into diamond in the P,T-region of the thermodynamical stability of diamond, for example at pressure of 5.5 GPa and

temperature of 1500°C. Graphite may be transformed into diamond in presence of

atomic hydrogen and a diamond substrate in the P,T region of the thermodynamical stability of graphite, for example at low pressure of 0.104 Mpa and a graphite

substrate temperature of 2000°C (if the temperature of the diamond substrate is 600-1000°C). Conversely, diamond may be transformed into graphite at pressure of

0.1 Mpa and temperature of 2000°C. Diamond mixed with metals Ni, Fe, Co or alloys

may be transformed into graphite in P,T-region of the thermodynamical stability of graphite, for example at pressure of 0 1 Mpa and temperature of 1000°C in inert gas

It has also been found that the buckyball based sintered carbon material can

be transformed into polycrystalline or monocrystal ne diamond at temperatures and

pressures less that than needed for graphite. Furthermore, the buckyball based

sintered carbon material may be transformed into monocrystalhne diamond in the_

presence of alloys that do not catalyze the transformation of graphite into diamond

The new buckyball based sintered carbon material can be used to provide

ceramic composite materials It was found that the smallest particles of carbon soot (buckyball C^) have the property of superplasticity in the temperature range of 200- 400°C at pressures of 0.01 to at least 1.0 GPa. Graphite, diamond, B₄C, WC/Co, Cu,

Ti, TiC, SiC, Be, W, B, Fe and other porous sponges were prepared by various

standard methods and impregnated with carbon soot at a pressure of 1.0 GPa and a

temperature of 300°C. The sample then was cooled, the pressure was thereafter

increased to 2.5 GPa and the temperature increased to 400°C and held for 1000

sec. The particles of soot were sintered together inside the pores by HPHT

treatment to produce composites with a new carbon material matrix which was found

to be harder than silicon carbide (30 Gpa).

DESCRIPTION OF THE PREFERRED EMBODIMENTS For optimum consolidation of the new fullerene based sintered carbon

materials it has been important to utilize high purity levels in the starting buckyball and nanotube powders. Generation of fullerenes by the conventional graphite

electric arc method yields Cgo, a number of higher fullerenes, and other

carbonaceous materials such as nanotubes, nanoparticles and insoluble residue (as a whole, known as soot or carbon black)

Production of fullerene based sintered carbon material generally requires. 1 )

purification of the starting material into at least 99%, and preferably >99.9%, pure carbon material of either buckyball C60 or single walled nanotubes 2) agglomeration

(compaction) of the purified fullerene powder into a relatively dense material and 3)

HPHT processing (sintering) of the purified, compacted fullerenes to produce the

fullerene based sintered carbon materials.

Purification of nanotubes from nanotube/nanoparticle mixtures has been tried

with standard techniques such as filtration, chromatography, centnfugation of sonicated solution of raw material Recently the oxidation of nanoparticles at higher temperatures has proved to yield high purity nanotubes . It has been reported that

carbon nanotubes are more resistant to oxidation in air than other fullerene derivatives, for example nanotubes oxidize completely at ~ 800° C, whereas

buckyballs require =515°C for complete oxidation. Alternatively, an iterative process

of sublimation and solvent rinses has been used to purify buckyballs and nanotubes

Previous experience in synthesis of microscopic quantities of buckyball

based sintered carbon material from C^ buckyballs has demonstrated the typical

need for long-term treatment of the buckyball powder samples at 160°C-400°C to

remove absorbed gases and contaminants of organic compounds. It has been found that sublimed and recondensed agglomerates make the best compaction material

because they overcome discrete particle adsorbate contamination and static charging. Of course, if a supplier can achieve sufficient levels of purity and agglomeration, the need to perform these steps is minimized. Sample preparation and compaction procedures

The following procedures for sample preparation and compactionwere used:

The samples were placed in graphite heating elements. As produced C^ buckyball powder contains a lot of admixtures of organic compounds. To purify the raw C^, -

the material was sublimated in a gradient quartz tube inserted into the furnace with

one cold end. The tube was connected to a vacuum pump and a helium cylinder. C_∞

was evaporated at the hot zone of the tube and deposited at the cold end. Organic

compounds were mostly removed into the vacuum system in gaseous form. The

growth of fullerene grains was observed through the quartz glass. When the size of the crystals was large enough, the cooled part of the tube was disconnected and the grains were poured into a crucible. The grains were then separated by size, (for

example 60/40 microns) using sieves and a shaking device, coarse grains up to

1000/800 microns were also used. Such pure fullerene C^ buckyball powder is

easily agglomerated by cold pressing in a die at a pressure of 0.05-0.50 Gpa. An

agglomerated material with density of p=l.6 g/cm^3, was achieved, which is greater

than the density of some types of graphite ceramics (p=1.5 g/cm³) Utilizing

precleaned buckyball and nanotube high purity agglomerates, powders were cold pressed into discs at pressures of 0.05 to 0.5 GPa

The same procedure was tried with multi-wall nano-tubes, however they could not be agglomerated by cold pressing. It is impossible to cold compact this

type of carbon black in the examined pressure range, as is also the case for

acethylene soot or any other tube or soot, where the particles are not separated

properly. The poured density of soot is about 0.1 g/cm³' which is only 2.5% of that of solid carbon. The density of agglomerated soot is 0.30-0.35 g/cm³. Agglomeration of

multi-wall nanotubes also gives a density of 0.35-0.40 g/cm³. It is possible to

achieve such density by agglomerating nanograin diamond-like explosive soot.

However, in the phase space examined, such density is not high enough for further sintering under high pressure, and HPHT treatment is not effective. It appears that

density of the sintered bulk material depends on the density of the"green body".

Thus, the initial powder density is a critical parameter. However, single-wall nano¬

tubes are easily agglomerated by the same method as buckyballs. Cold pressing

inside the die at a pressure of 0.05 to 0.5 GPa gives pellets with p=1.4-1.5 g/cm³

HPHT Consolidation of the Agglomerated Fullerene Powders

The synthesis of man made diamonds has taken place for many years, this

synthesis uses high pressure and temperature processing of carbon materials,

usually in the presence of metal alloys which act as catalysts. Thus machinery

capable of such HPHT processing has also been known for some time, as the

temperatures and pressures required for carrying out the steps to produce the

fullerene based sintered carbon materials of the present invention are either similar to, or less than, those for man made diamond production, the equipment used in

those processes may be used herein. A suitable HPHT apparatus is shown in U.S. Patent No. 3,746,484 to Vereshagin et al entitled "Apparatus for Developing High

Pressure and High Temperature" which issued on July 17, 1973; the disclosure of

which is hereby incorporated by reference as if fully set forth herein. Other suitable

apparatus is shown in U.S. Patent No. 2,941 ,242 to Hall which issued in June 1960. It should be noted for processes requiring pressures of less than 5.0 Gpa simpler

apparatus can be effectively used.

The HPHT equipment of the above noted Vereshagin et al patent includes a

graphite crucible for holding the powders to be processed. An electric current is

passed through the crucible which provides the necessary heating. The crucible is-

held between contoured anvils which are acted upon by a hydraulic press to provide

the necessary pressure. In the case at hand the buckyball material was purified and

then agglomerated to preferably form 50-100 micron grain size powder, although larger or smaller sizes are also useable. The powder was further agglomerated into

pellets by cold pressing. The samples were put into the graphite crucible, which also serves as the heater, and placed in the HPHT apparatus .

A number of samples were prepared from buckyball carbon soot processed

as described above, and thereafter subjected to HPHT sintering. In table 1 to follow

the sample number is shown in column 1 , the pressure used in the HPHT processing is shown in the first column, the pressure is shown in the second

column, the temperature is shown in the third column, the time to which the sample

is subjected to the temperature and pressure is shown in the fourth column, the

hardness of the processed sample is shown in the fifth column and the resistivity of the sample is shown in the sixth column:

Table 1

It is also seen that the hardness of the buckyball based sintered carbon

material can be controlled by the selection of pressure, temperature and processing

time. In summary, the results are: sample hardness grows with pressure,

temperature and holding time up to the temperature 800° C, then the hardness decreases. The samples sintered at 200-350°C are usually still soft; samples

sintered at 400-800°C are usually hard. The pressure of 2.5 GPa, temperature of

500°C, holding of 1000 sec. are high enough parameters to obtain the samples that scratch monocrystalline SiC, as well as all other materials, excluding cubic boron nitride (c-BN) and diamond. It was found that the conductivity of the samples

increases as the hardness increases, the soft samples were good insulators with the hardest samples having a resistivity of approximately 10² ohms /cm at ambient

temperature and pressure For single wall nanotube based sintered carbon material

the process parameters set forth in Table 1 will produce material of similar properties but with somewhat less hardness, see Table 2 to follow. It may also be

possible to utilize pressures less than 1.0 Gpa or greater than 10 Gpa if the other,

parameters are adjusted to compensate therefore.

Table 2 below compares certain physical properties of the fullerene based

sintered carbon materials synthesized herein with other carbon based materials-

graphite, diamond and ceramics based thereon It is seen that the nanotube based sintered carbon material is harder, denser and stronger than graphite and graphite based ceramics while still being conductive It is seen that the buckyball based sintered carbon material has hardness, density and strength properties which

closely approach that of diamond, yet the material is very conductive while diamond

is an insulator

Table 2

G-graphite, D-diamond,

Bu-buckyball based sintered carbon matenal (as sintered at P= 1.0-10.0 GPa) Nt-nanotube based sintered carbon matenal (as sintered at P= 1.0-10.0 GPa) *typιcal properties as shown by sample 5 Theoretical evaluation shows that the compressive strength and density of

the buckyball based sintered carbon materials should approach that of diamond

which has unsurpassed compressive strength and compressive strength to density

ratio. Diamond Creation -

Buckyball based sintered carbon material may be transformed into

polycrystalline diamond more readily than graphite ceramics at pressures of 7.0-9.0

Gpa. Generally, buckyball based sintered carbon material transforms into

polycrystalline diamond at lower temperature and for a shorter time in the presence

of Ni-Mn and Ni-Cr alloys than graphite. The temperature of transformation into diamond was 800-1300°C with a holding time of 0.1-100 sec. Transformation

usually occurs in 3-4 seconds, some samples were obtained in 1 second, perhaps

less. In addition to Ni based alloys, other suitable alloys for creation of

polycrystalline diamond are Fe and Co based alloys (Ni-Fe-Co, Ni-Cr, Ni-Fe-Co-Cr

and the like). A mixture with pure Ni transforms with a detonative reaction, so the

speed of transformation is higher than the speed of sound in the solid state.

A surprising result is that buckyball based sintered carbon material may be

transformed into monocrystalhne diamond in the presence of Al-Mg-Ca alloys and other alloys that do not catalyze the transformation of graphite into diamond.

Transformation at P=2.5-9.0 Gpa, T=400-1300°C and t=10-1000 seconds was

studied. The samples were white or white-grey, or black-grey nanograined powders

or an amorphous material. White transparent unshaped or cubical ly shaped with

mirror facets, white with black inclusions or black monocrystals of diamond may be

easily removed from this powder by tweezers. The size of crystals is 0.1-1 mm at a holding time of 100 seconds, electron beam diffraction analysis of these samples,

demonstrates that they are single crystals of diamond. X-ray analysis of these

samples demonstrates that they are pure carbon. They have a hardness of 10 on

Mohs scale. Composite Material -

The new buckyball based sintered carbon material can be used to provide ceramic composite materials. It was found that the smallest fullerene particles of

carbon soot (buckyball 0^) have the property of superplasticity in the temperature

range of 200-400°C at pressures of 0.01-1.0 GPa. Graphite, diamond, B, C,

B₄C,SiC, TiC, WC/Co, Cu, Ti, Fe, Be, W and other ceramic and/or metal porous composite "sponges" were prepared by various standard methods and impregnated

with buckyball based carbon soot at a pressure of 1.0 GPa and a temperature of 300°C. The sample then was cooled, the pressure was thereafter increased to 2.5

GPa and the temperature increased to 400°C and held for 1000 sec. The buckyball

particles were sintered together inside the pores after the HPHT treatment to

produce composites with a new carbon material matrix which was found to be harder than silicon carbide (30 Gpa). When processed into prototype cutting/drilling tool

bits, these composites have shown cutting rates comparable to or exceeding those

measured for commercially available polycrystalline diamond composites. Further HPHT processing at the parameters and alloys described above for diamond

creation can convert the buckyball based sintered carbon material into diamond within the porous matrix. Doped fullerene based sintered carbon material

The electronic properties of the fullerene based sintered carbon materials

can be altered by doping with hydrogen, boron, nitrogen, oxygen, sulphur, fluorine,

chlorine, and other elements. Such doping can result in tough new polymers and-

new organic compounds as well as providing semiconductivity or superconductivity

to the fullerene based sintered carbon materials. As the amount of dopant required

is typically 0.0001 to 1.0% by weight. The doping can be achieved by mixing the >99% fullerene powder(either buckyballs or nanotubes) with powders containing a predetermined quantity of the dopants, such as hydrocarbons (for example naphthalene) or carboranes (for example o-carborane). The fullerene carbon

powder and the dopant containing powder are then sintered together.

In summary, a new class of carbon materials is formed by the methodology of

the present application. The new carbon materials are formed by high pressure and

high temperature processing of fullerene based carbon powder. The new carbon

materials are harder than graphite and either almost as hard as diamond or harder

than steel, depending on the starting fullerenes (C^ buckyballs or single wall

nanotubes, respectively) as well as the pressing and heating parameters. The new

carbon materials are either completely amorphous and isotropic (when formed from

buckyballs) or almost completely amorphous and isotropic (when formed from single wall nanotubes). These new carbon materials are conductive like graphite and unlike diamond which is an insulator. The materials can be shaped by powder

metallurgy techniques into any configuration. The new materials can also be readily converted to diamond or formed within a porous composite. The invention has been described with respect to preferred embodiments. However, as those skilled in the art will recognize, modifications and variations in the specific details which have been described and illustrated may be resorted to without departing from the spirit and scope of the invention as defined in the

appended claims.

Claims

What is Claimed is:

1. A carbon material formed by the process of:

a) providing a fullerene based carbon powder, b) subjecting said fullerene based carbon powder to a pressure of 1.0 to 1QV0

Gpa, a temperature of from 300-1000°C for a period of time from 1 to 10000

seconds.

2. The carbon material as claimed in claim 1 , wherein the fullerene based powder comprises at least 99% buckyballs.

3. The carbon material as claimed in claim 1 , wherein the fullerene based powder

comprises at least 99% single walled nanotubes.

4. The carbon material as claimed in claim 1 , wherein the fullerene based powder

comprises at least 99.9% fullerenes.

5. The carbon material as claimed in claim 1 , wherein the pressure is at least 2.5 GPa, the temperature is at least 500°C, and the period of time is at least 1000 seconds.

6. The carbon material as claimed in claim 1 , wherein the fullerene based powder comprises 0.0001 to 1.0% of a dopant.

7. The carbon material as claimed in claim 6, wherein the dopant is selected from

the group consisting of hydrogen, boron, nitrogen, oxygen, sulphur, fluorine, and

chlorine.

8 A process for forming a hard sintered conductive carbon material, comprising the

steps of. a) providing an fullerene based carbon powder having at least 99% fullerenes,

b) agglomerating said fullerene based carbon powder,

c) subjecting said fullerene based carbon powder to pressure of 1.0 to 10.0 Gpa, a

temperature of from 300-1000°C for a period of time of from 1 to 10000 seconds.

9 The process as claimed in claim 8, wherein the fullerene based powder

comprises at least 99.9% by weight of single walled nanotubes.

10. The process as claimed in claim 8, wherein the fullerene based powder

comprises at least 99.9% by weight of buckyballs

11 The process as claimed in claim 10, further including the steps of d) providing an alloy used to convert carbon materials to diamond and e) subjecting said sintered carbon material to a pressure of 7.0 to 9.0 Gpa, a temperature of from 800-1300°C

for a period of time from 0.1 to 100 seconds to convert the sintered carbon material to polycrystalline diamond.

12. The process as claimed in claim 11 , wherein the alloys are based on at least one of Ni, Fe and Co

13. The process as claimed in claim 10, further including the steps of d) providing a

metal alloy selected form the group comprising aluminum, magnesium and calcium

alloys and e) subjecting said sintered carbon material to a pressure of 2.5 to 9.0-

Gpa, a temperature of from 400-1300°C for a period of time from 10 to 1000

seconds to convert the sintered carbon material to monocrystalhne diamond.

14. The process as claimed in claim 8, further including the steps of infiltrating said fullerenes by superplastic flow under temperature and pressure into a porous

composite material and said subjecting step takes place after said fullerene based

carbon powder has been infiltrated into the porous material.

15. The process as claimed in claim 14, wherein the superplastic flow takes place at

temperatures of 200-400°C at pressures of 0.1-1.0 Gpa.

16. The process as claimed in claim 8, wherein the fullerene based carbon powder comprises 0 0001 to 1 0% of a dopant.

17. A conductive carbon material comprising fullerenes subjected to heat, temperature and pressure sufficient to provide a hardness to the material of at least 1.0 Gpa and a resistivity of less than 10 ohms-cm

18. The process as claimed in claim 17, wherein the fullerenes comprise at least

99.9% by weight of single walled nanotubes.

19. The process as claimed in claim 17, wherein the fullerenes comprise at least

99.9% by weight of buckyballs.