Chemistry of inorganic compounds – Nitrogen or compound thereof – Carbon containing
Reexamination Certificate
2000-07-27
2002-08-06
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Nitrogen or compound thereof
Carbon containing
Reexamination Certificate
active
06428762
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to carbon nitride and more particularly to a method for making C
3
N
4
. Still more particularly, the present invention relates to a method for solid-state synthesis of carbo-nitride powders in bulk quantities using halogenated triazines and alkaline metal nitrides as reagents.
BACKGROUND OF THE INVENTION
In many materials, the manner in which the atoms forming the material are arranged determines many of the physical properties of that material. For example, when carbon atoms are merely assembled randomly, the result is graphite, which is opaque to visible light and relatively soft. The carbon atoms in graphite are not arranged in any ordered fashion, so graphite is unstructured, or “amorphous.” When the same carbon atoms are subjected to extremely high pressures and high temperatures, however, they align themselves in the crystal structure commonly referred to as diamond. As is well known, diamond is clear to visible light, is very hard, and has a tight, cubic crystal structure. A difference in crystal structure between two materials having identical chemical compositions, such as the difference between graphite and diamond, is referred to as polymorphism.
Various carbon-nitrogen compositions are known in the art. In particular, the preparation of binary, ternary, and quaternary carbo-nitride materials, such as C
3
N
4
, B—C—N, Al—C—N, Al—B—C—N, Si—B—C—N, etc., is an area of current interest. Crystalline forms of these non-oxide lightweight materials are expected to possess a combination of extreme hardness, oxidation resistance and chemical inertness.
1-10
The most exciting material in this family is the crystalline form of a particular carbon nitride, C
3
N
4
, for which a hardness challenging that of diamond is predicted.
1-3
To date, this material has only been postulated and has never been successfully created in commercially meaningful quantities.
Based on its predicted crystal structure, this crystalline form of C
3
N
4
is commonly designated &bgr;-C
3
N
4
. The &bgr; designation is derived from the crystal structure for &bgr;-Si
3
N
4
, which is known and is analogous to the expected structure of the desired superhard form of C
3
N
4
.
In addition to this potentially superhard &bgr;-phase, the existence of &agr;-, cubic, pseudocubic and graphitic (amorphous) polymorph phases of carbon nitride have been recently suggested on basis of calculations
11,12
and experiments.
13,14
In fact, numerous experimental attempts to synthesize the &bgr;-form of carbon nitride using various chemical and physical thin film deposition techniques
15
have produced predominantly amorphous materials that lack the desired hardness and have nitrogen contents that are significantly different than the 57 at. % that would be expected for C
3
N
4
. For example, several methods of making C
2
N, C
2
N
2
, and C
4
N
5
materials are known in the art. Because of their different carbon:nitrogen ratios, these materials have different crystal structures and thus different mechanical properties from the predicted behavior for &bgr;-C
3
N
4
.
One possible exception to the foregoing characterization lies in the thin films prepared from single-source precursors by J. Kouvetakis et al.
16
and others. Although these films are asserted to have the desired C
3
N
4
stoichiometry,
13
C NMR analysis does not confirm the suggested a triazine-based, &bgr;-structure for this material. The observation of small &agr;-C
3
N
4
and &bgr;-C
3
N
4
or cubic C
3
N
4
crystallites embedded in an amorphous carbon nitride film has also been reported.
15,17-21
However, the true nature of these crystallites will remain uncertain until large crystals of this carbon nitride are synthesized and precisely characterized, and their mechanical properties tested. On the basis of the large amount of experimental results available so far, it has been suggested that the physical deposition methods that are known in the art do not yield the desired phases, and certainly do not yield the desired phase in bulk or in commercially meaningful amounts (“gram amounts”).
20,22
Hence, a method for producing C
3
N
4
in bulk is desired.
SUMMARY OF THE INVENTION
The present invention provides an effective method for producing bulk amounts of a carbon nitride having the composition C
3
N
4
and thus a carbon:nitrogen ratio of 3:4. According to the preferred embodiment, carbo-nitride powders having the desired composition can be produced in bulk quantities using a halogenated triazines and an alkaline metal nitrides as reagents. More specifically, the reagents comprise C
3
N
3
X
3
where X is a halogen, and M
3
N where M is an alkaline metal. It has been found that combining the two reagents in dry powdered form, heating them to a temperature above the boiling point of the triazine (C
3
N
3
X
3
) and holding them at an elevated temperature for a predetermined period of time produces bulk quantities of a compound having a C:N ratio of 3:4. As used herein, the designation a-C
3
N
4
refers to a composition having C
3
N
4
stoichiometry and an amorphous (“a-”) structure.
The present approach, which is based on fast solid state reactions, is particularly attractive since: (i) it uses the relatively cheap reagents and does not require synthesis of single-source precursors, as in the previously reported preparation of carbo-nitride; (ii) it produces powders with a higher nitrogen content than, for example, the carbon nitride powders of approximately C
4
N
5
stoichiometry described in German Patent DE 197 06 028.5, 1997
45
, and (iii) it may allow the design of reaction routes leading to production of not only binary, but also ternary and quaternary carbo-nitride materials with controlled stoichiometry, morphology, mechanical and electric properties. Ternary and quaternary carbo-nitride materials are those in which one or two, respectively, additional elements are present in the material in addition to carbon and nitrogen.
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patent: 5110679 (1992-05-01), Haller et al.
patent: 19706028 (1997-02-01), None
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Williams, D., Kouvetakis, J., O'Keeffe, M., “Synthesis of Nanoporous Cubic In(CN)3 and In1-xGax(CN)3 and Corresponding Inclusion Compounds”, Inorg. Chem.,37, 4617-4620, (1998). (No month).
Brousseau, L.C., Williams, D., Kouvetakis, J., O'Keefe, M., Synthetic Routes to Ga(CN)3 and MGa(CN)4 (M═Li, Cu), J. Am. Chem. Soc., 119, 6292-6296, (1997). (No month).
Teter, D.M., Hemley, R.J., “Low-Compressibility Carbon Nitrides”, Science, vol. 271, 53-55, (1996).
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Khabashesku Valery N.
Margrave John L.
Zimmerman John L.
Conley & Rose & Tayon P.C.
Langel Wayne A.
William Marsh Rice University
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