Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1999-06-21
2001-06-12
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S44500R, C428S408000
Reexamination Certificate
active
06245312
ABSTRACT:
The present invention is concerned with materials technology, and in particular with the synthesis of superhard materials, especially superhard carbon material. The present invention is also concerned with a method for producing such superhard carbon material and articles made therefrom.
The superhard carbon material of the present invention can be used as instrumental, construction and semiconductor material, as devices and articles made of it as well as in metal working, the working of natural stone, of any other hard and superhard materials as well as in electronics.
Diamond, a natural polymer consisting of carbon, is known as the hardest material. Artificial superhard materials based on carbon, like diamond and lonsdalite (hexagonal diamond) have a hardness comparable with that of natural diamond (U.S. Pat. No. 3.488.153, Int. Cl. C01b 31/06, 1970). Superhard materials based on boron and nitrogen are also known, e.g. the cubic and wurtzite modification of boron nitride with a hardness approaching the hardness of diamond (A. V. Kurdyumov, A. N. Pilyankevich, “Fazovye prevrashcheniya v uglerode i nitride bora” (Phase Transformations in Carbon and Boron Nitride), Kiev: Naukova Dumka, 1979).
The structure of diamond and lonsdalite, as well as that of cubic (BNk) and wurtzite boron nitride (BNb) is based on coordination tetrahedrons, the apices of which in the diamond structures are occupied by carbon atoms and in the aforesaid modifications of boron nitride with boron and nitrogen. In an ideal crystal lattice of diamond the tetrahedrons are regular, the volume is 1.87 Å
3
, all four bonds are equal in length and the angles between the bonds are 109.47°, each. In the cubic structural modifications of these materials the bulk-polymerized tetrahedrons form layers in which they are in the state of identical orientation. In hexagonal structural modifications (lonsdalite, wurtzite boron nitride) the tetrahedrons of each subsequent layer are turned by 60° with respect to the tetrahedrons of the underlying layer. The tetrahedrons are interconnected, thereby forming a three-dimensional polymeric structure of carbon atoms. In tetrahedrons of diamond and lonsdalite the lengths of all four bonds are equal to 1.54 Å. The parameter of the unit cell of diamond is a=3.56 Å and the cell contains 8 carbon atoms.
Being the hardest material in comparison with other known materials (10 units according the Mohs' scale), diamond has a low electric conductivity (the width of the forbidden zone in pure crystals is 5.6 eV and the specific conductivity sigma<10
−9
ohm
−1
×cm
−1
). Diamond is chemically stable in aggressive media. An increase in electric conductivity by several orders of magnitude may be obtained only by applying special doping methods.
The main trends in obtaining diamonds and lonsdalite are:
(1) extraction from natural sources;
(2) synthesis from carbonic materials by various kinds of processings.
There are methods to synthesize diamond and lonsdalite by direct transformation of carbonic substances, mainly various kinds of graphite, at high pressure and temperature without catalytic solvents (U.S. Pat. No. 3,488,153). Diamond is also obtained by “Catalytic synthesis”, i.e. by addition of special substances to carbonic materials. Besides, lonsdalite excavated from natural sources and lonsdalite obtained by synthesis without catalysts exists in the samples mainly in strong combination with diamond and it is not possible to separate it.
To carry out the known methods of producing diamond by direct transformation a static pressure of not lower than 13 GPa and a temperature of not lower than 1600° C. is required. Thus, these methods are inefficient because the devices for this range of pressures have a small volume and a low stability. In catalytic synthesis diamonds are obtained mainly as powders and monocrystals which in the course of their growth entrap atoms of catalytic solvents, thereby deteriorating the properties. During the synthesis the catalytic solvents bind a part of the initial or starting carbonic material resulting in by-products which decrease the output of diamond and requires labor-consuming operations for their extraction and removal. A method for producing compact polycrystalline diamond using diamond powder without binding agents is of little use for the applied purposes, because diamond powder poorly cakes and the quality of the final product does not meet the requirements. The solids obtained with binding agents and catalysts have lower thermostability and the field of their application is thus reduced.
Currently a new allotropic form of carbon is described—fulleren—that is used as a starting material in diamond production (“The Fullerens”, edited by H. W. Kroto, J. E. Fischer, D. E. Cox, Oxford, New York, Seoul, Tokyo: Pergamon Press, 1993).
Fulleren is a molecule in which carbon atoms (60-240 and more) are bound in such a way that they form a hollow subspherical body. Thus, e.g. the molecule of fulleren C
60
is similar to a football. It is formed of 20 hexagons and 12 pentagons. The interatomic distances in the fulleren C
60
molecule are as short as in graphite, and the molecule diameter is about 0.65 nm.
Fullerite is a material based on fulleren molecules. The structure of the initial fullerite C
60
may be represented as a recurrent pattern of tetrahedrons containing molecules of fullerite C
60
in their apices. The volume of the tetrahedrons of the initial fullerite C
60
is about 119 Å
3
.
It is known that various kinds of fullerites are used in the manufacture of artificial diamonds at high nonhydrostatic pressure using “anvil” chambers (FR 2,684,090 F1, C01D 31/06, B01J 3/06, 1991). The powder of the initial fullerite—a mixture of C
60
-C
70
fullerite—is introduced into the central hole of a gasket of pyrophyllite that is placed on an “anvil” of the apparatus. Being exposed to a pressure of 20±5 GPa at a rate of 1 GPa/minute and at room temperature and as a result of a high shearing strain the initial fullerite decomposes at the molecular level yielding free amorphous carbon. Under the same conditions the formed amorphous carbon is transformed into a shiny transparent mass. X-ray diffraction has demonstrated that the formed final product is a polycrystalline diamond. Thus, this document describes the transformation of fullerites under high pressure into a known product—diamond, the properties of which are described above. This method is characterized by parameters that are typical of direct methods of transformation of carbonic substances into diamond. In this document it is said that the value of the lower level of the range may be 15 GPa, however under such conditions the output of the target product is low.
At present, in various fields of technology and industry materials are required having a hardness surpassing the hardness of diamond, e.g., for efficient working of hard, churlish alloys, and of diamond itself and instruments based on diamond, lonsdalite, cubic and wurtzite boron nitride. There is also a need for materials combining a high hardness with a high electric conductivity or semiconductor properties and possessing a considerable chemical inertness that is necessary, e.g., for construction of elements of electronic apparatuses and devices.
The present invention claims the creation of a superhard carbon material having such properties and a method for its production under conditions yielding a new superhard carbon material having a hardness at a level that is comparable with the hardness of known superhard materials and is even surpassing this level, possessing improved electric conductivity, thus permitting to produce articles with a hardness of about 170 GPa and an electric conductivity of about 1 ohm
−1
cm
−1
.
The new carbon material, the structure of which includes bulk-polymerized structural elements in the form of tetrahedrons solves this task. According to the present invention, the aforesaid structural elements in the form of tetrahedrons contain gro
Blank Vladimir Davidovich
Buga Serjei Gennadievich
Dubitsky Gennady Aleksandrovich
Popov Mikhail Yurievich
Serebryanaya Nadejda Ruvimovna
Hendrickson Stuart L.
Ministry for Sciene & Technology of Russia: Center f Superhard M
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