Claims
- 1. A process for forming a high purity product consisting essentially of silicon, hydrogen, nitrogen, and carbon which comprises reacting elemental silicon with an amine reactant at a low temperature.
- 2. The process of claim 1 wherein said low temperature comprises a temperature, equivalent at atmospheric temperature, of from about 0.degree. C. to about 300.degree. C.
- 3. The process of claim 2 wherein said amine reactant reacted with said elemental silicon is in a liquid state at the reaction temperature.
- 4. The process of claim 3 wherein said amine reactant consists of an amine dissolved in a liquid solvent to form a liquid solution at the reaction temperature.
- 5. The process of claim 3 wherein said amine reactant is heated to a molten state during said reaction.
- 6. The process of claim 2 wherein said amine reactant is reacted with said elemental silicon by refluxing the mixture at the boiling point of the amine reactant at the pressure employed in the reaction.
- 7. The process of claim 2 wherein the pressure of the reaction varies from atmospheric pressure up to about 20 atmospheres.
- 8. The process of claim 7 wherein the pressure of the reaction varies from atmospheric pressure up to about 10 atmospheres.
- 9. The process of claim 7 wherein said elemental silicon is reacted with said amine reactant for a period of from about less than 1 hour up to about 100 hours.
- 10. The process of claim 9 wherein said reaction time is from about 1 hour up to about 50 hours.
- 11. The process of claim 7 wherein said process is run on a continuous basis and said reactants have a contact time together of from less than 1 hour up to about 100 hours.
- 12. The process of claim 7 wherein said amine reactant is present in stoichiometric excess with respect to said elemental silicon reactant.
- 13. The process of claim 7 wherein said elemental silicon has a purity of 99.5 wt.% or greater.
- 14. The process of claim 7 wherein said elemental silicon used in said reaction has a particle size range of from about 0.01 microns to about 100 mesh (Tyler).
- 15. The process of claim 14 wherein said particulate elemental silicon used in said reaction has a particle size range of from about 0.01 microns to about 150 microns.
- 16. The process of claim 7 including the step of removing surface coatings on said particulate elemental silicon to promote reaction between said silicon and said amine reactant.
- 17. The process of claim 7 including the step of removing surface coatings on said particulate elemental silicon during the reaction of said particulate silicon with said amine reactant to promote reaction between said silicon and said amine reactant.
- 18. The process of claim 17 wherein said step of removing surface coatings on said particulate elemental silicon during the reaction of said particulate silicon with said amine reactant further comprises milling said silicon reactant to promote reaction between said silicon and said amine reactant.
- 19. The process of claim 7 wherein said amine reactant is selected from the class consisting of:
- (a) amines having the formula R.sup.1 R.sup.2 R.sup.3 (N) wherein R.sup.1 is a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; R.sup.2 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; and R.sup.3 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group;
- (b) a diamine, triamine, or tetramine; and
- (c) organic nitrogen compounds having a direct N--N bond, including cyclic organic nitrogen compounds.
- 20. The process of claim 7 wherein said amine reactant is selected from the class consisting of amines having the formula R.sup.1 R.sup.2 R.sup.3 (N), wherein R.sup.1 is a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; R.sup.2 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; and R.sup.3 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group.
- 21. A process for producing, a high purity reaction product consisting essentially of silicon, hydrogen, nitrogen, and carbon which comprises: reacting together, at a pressure of from about atmospheric pressure up to about 20 atmospheres, and a temperature, equivalent at atmospheric temperature, of from about 0.degree. C. to about 300.degree. C., elemental silicon having a particle size range of from about 0.01 to 100 mesh (Tyler) and a purity of at least about 99.5 wt.% with a reactive amine reactant in liquid form for a period of from about less than 1 hour up to about 100 hours,
- 22. The process of claim 21 including the s removing surface coatings on said particulate elemental silicon to promote reaction between said silicon and said amine reactant.
- 23. The process of claim 22 including the further step of milling said elemental silicon during the reaction of said silicon with said amine reactant to reduce the particle size, to accelerate the reaction time by exposing fresh surfaces of said silicon particulate for reaction with said amine reactant, and to remove surface coating on said particulate elemental silicon.
- 24. The process of claim 22 wherein said coating on said elemental silicon particulate is removed by chemical treatment of said silicon with a reagent capable of removing said coating.
- 25. The process of claim 22 wherein said coating on said elemental silicon particulate is removed by heating said particulate to a temperature of at least 1300.C in a reducing atmosphere.
- 26. The process of claim 21 wherein said elemental silicon has a purity of at least 99.999 wt.%.
- 27. The process of claim 21 including the further step of heating said reaction product to a temperature of from about 1000-1700.degree. C. in a nonreactive atmosphere to convert said reaction product to a crystalline material consisting essentially of silicon carbide and silicon nitride.
- 28. The process of claim 21 including the further step of heating to a temperature of from about 800 to about 1300.degree. C. so as to produce an amorphous intermediate, capable of subsequent conversion to a crystalline product.
- 29. A process for continuously producing, from particulate metallic silicon and a reactive amine liquid, an intermediate product consisting essentially of silicon, hydrogen, nitrogen, and carbon which is capable, upon subsequent heating to a temperature of at least 800.degree. C., of being converted into a high purity silicon carbide/silicon nitride ceramic which process comprises:
- (a) continuously reacting elemental silicon particles having a purity of at least 99.5 wt.%, at a temperature, equivalent at atmospheric pressure of from about 0.degree. C. up to about 300.degree. C. with a reactive amine selected from the class consisting of:
- (1) amines having the formula R.sup.1 R.sup.2 R.sup.3 (N) wherein R.sup.1 is a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; R.sup.2 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group; and R.sup.3 is hydrogen, a 1-18 carbon alkyl group, an aryl group, an aralkyl group, or an alkaryl group;
- (2) a diamine, triamine, or tetramine; and
- (3) organic nitrogen compounds having a direct N--N bond, including cyclic organic nitrogen compounds; and
- (b) continuously removing from said reaction vessel an intermediate product consisting essentially of silicon, hydrogen, nitrogen, and carbon.
- 30. A process for producing, at a low temperature, a high purity mixture of silicon carbide and silicon nitride which comprises:
- (a) reacting together, at a temperature, equivalent at atmospheric pressure, of from about 0.degree. C. up to about 300.degree. C., a particulate elemental silicon having a particle size range of from about 0.01 to 150 microns and a purity of at least about 99.5 wt.% and a reactive amine reactant in a liquid state to form an intermediate reaction product; and
- (b) heating said intermediate reaction product to a temperature of from about 1200-1700.degree. C. in a nonreactive atmosphere to convert said intermediate reaction product to a crystalline material consisting essentially of a mixture of silicon carbide and silicon nitride.
BACKGROUND OF THE INVENTION
The invention described herein arose in the course of, or under, Contract No. DE-FG03-85ER45221 between the U.S. Department of Energy and Rockwell International Corporation.
This invention relates to a process for the production of silicon, nitrogen organic compounds consisting essentially of silicon, hydrogen, nitrogen, and carbon which may be employed as intermediates or end products in a wide variety of chemical synthetic preparations including the formation of ceramic silicon carbide and silicon nitride materials, including films, fibers, or coatings. More particularly, this invention relates to the reaction of elemental silicon with organic amines at a low temperature to produce a high purity material consisting essentially of silicon, hydrogen, nitrogen, and carbon and further characterized by the substantial absence of halides or of silicon bonded to oxygen.
The formation of organosilicon compounds by reaction between elemental silicon and an alkyl or aryl halide was first described by Eugene G. Rochow in a paper entitled "The Direct Synthesis of Organosilicon Compounds", published in Volume 67 of the Journal of the American Chemical Society in June, 1945, at pp. 963-965. The Rochow process or reaction of methylene chloride with silicon is also described by J. J. Zuckerman on pp. 384-393 in a chapter entitled "Direct Synthesis of Organosilicon Compounds", in a book edited by H. J. Emeleus and A. G. Sharp entitled "Advances in Inorganic Chemistry and Radiochemistry" published by Academic Press, Volume 6, N.Y., 1964.
Silicon carbide is an important material having a diamond-like structure and a hardness nearly that of diamond. It is widely used as an abrasive for grinding and cutting metals.
Silicon carbide may be formed by the high temperature reaction between silica and a carbon source, e.g., coke, rice hulls, etc., at a temperatures of from 2000.degree. C. up to as high as 3000.degree. C. The resulting silicon carbide product may, however, have silica or carbon preset therein as impurities.
It is also known to form silicon carbide at lower temperatures from reactants which include organic materials. Gaul U.S. Pat. No. 4,404,153 teaches the formation of a silazane polymer by reacting together, at a temperature of 125 to 300.degree. C., a chlorine-containing disilane having the formula (C1.sub.a R.sub.b Si)2 with a disilazane having the general formula (R'.sub.3 Si)NH wherein R is vinyl, an alkyl group of 1-3 carbons, or the phenyl group; R' is vinyl, hydrogen, an alkyl group of 1-3 carbons, or the phenyl group; a has a value of 0.5-3; b has a value of 0-2.5; and the sum of a+b is equal to 3. The resulting silazane polymer can be converted to silicon carbide by heating it in an inert atmosphere or vacuum to 750.degree. C.
Baney et al U.S. Pat. No. 4,314,956 discloses aminated methyl polysilanes which are useful in the preparation of silicon carbide ceramics. The polysilane is said to have the average formula: ((CH.sub.3).sub.2 Si)--(CH.sub.3 Si) wherein there is also bonded to the silicon atoms other silicon atoms and radicals having the formula --NHR wherein R is hydrogen, an alkyl radical of 1 to 4 carbon atoms, or phenyl wherein essentially all of the remaining bonds on silicon are attached to chlorine or bromine atoms. As understood, the polysilane is formed by reacting a polyhalosilane starting material with ammonia or substituted or unsubstituted organic amines having the general formula NHR.sub.2.
Seyferth et al U.S. Pat. No. 4,650,837 describes a method for forming preceramic polymers useful for making silicon carbide and silicon nitride/silicon carbide ceramics. The preceramic polymer is formed by reacting together a polycarbosilane with a metal silylamide. The polycarbosilane reactant has repeating units with the formula [RSi(H)--(CH.sub.2).sub.q ] wherein R is H, a 1-6 carbon alkyl group, a 3-6 carbon cycloalkyl group, or a 6-10 carbon substituted or unsubstituted lower aryl group; and q is an integer of 1 or greater. The metal silylamide reactant has the formula [(R.sup.1 SiHNH).sub.a (R.sup.1 SiN).sub.b (R.sup.1 SiHNM).sub.3 ].sub.m wherein a+b+c=1; R.sup.1 is a 1-6 carbon alkyl group, a 2-6 carbon substituted or unsubstituted alkenyl group, a 6-10 carbon substituted or unsubstituted aryl group, a tri(lower)alkyl- or di(lower)alkylsilyl group, or a di(lower)alkylamino group; M is an alkali metal or one half equivalent of an alkaline earth metal; and m is an integer greater than 1. The polymer is said to be converted to a ceramic by heating it to 1000.degree. C. at 10.degree. C./min. Chlorine retention problems (in the form of MC1 salt residues) are, however, inherent in this process for making preceramic polymers thereby causing this process to be uneconomical.
While the processes described in these patents do produce silicon carbide or silicon nitride from organic derivatives at lower temperatures than the aforesaid reaction at 2200-3000.degree. C. between silica and a carbon source, the processes which use organic sources of carbon are usually contaminated with traces of halides from the reactants used to form the organic precursors.
The original investigation of low and high temperature direct reactions between elemental silicon and nitrogen compounds was conducted by E. Vigouroux, as quoted by J. W. Mellor in A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. VI, N.Y.: Wiley, 1961, p. 163. He discovered that ammonia reacts with silicon at bright red heat, forming the nitride with liberation of hydrogen. High temperature nitridation of silicon is also detailed by Mangels U.S. Pat. No. 4,235,857 and is otherwise well known. However, ultra-high-purity silicon is difficult to nitride at high temperature due to formation of protective nitride layers (exactly of the type used on semiconductors for passivation). According to S. S. Lin in an article entitled "Mass Spectrometric Studies on High Temperature Reaction Between Hydrogen Chloride and Silica/Silicon" in the Journal Electrochem. Society, Vol. 123, 1976, pp. 512-514 and another article entitled "Comparative Studies of Metal Additives on the Nitridation of Silicon" in the Journal Am. Ceram. Soc., Vol. 60 (1-2), 1977, pp. 78-81; halide, iron, or other cation catalysts are required in such nitriding processes. D. Campos-Loriz et al, in an article entitled "The Effects of Hydrogen on the Nitridation of Silicon" in the Journal Mat. Sci., Vol. 14, 1979, pp. 1007-1008, and H. Dervisbegovic et al in an article entitled "The Role of Hydrogen in the Nitridation of Silicon Powder Compacts" in the Journal Mat. Sci, Vol. 16, 1979, pp. 1945-55, further explored the catalytic effects of hydrogen and water vapor on nitridation of silicon with a view to overcome the sluggishness and high expense of the process.
It would be desirable to have a process wherein elemental silicon, which is available as a high purity starting material, could be reacted at a temperature, equivalent at atmospheric pressure to from about 0.degree. C. up to about 300.degree. C. with an organic amine reactant in its liquid state to form a high purity organic product consisting essentially of silicon, hydrogen, nitrogen, and carbon, without requiring a halide or other anion to be bonded to silicon prior to reaction with a given amine.
It is, therefore, an object of this invention to provide a process for forming a high purity product consisting essentially of silicon, hydrogen, nitrogen, and carbon by reaction of elemental silicon and a reactive amine at a low temperature.
It is another object of this invention to provide a process for forming a high purity product consisting essentially of silicon, hydrogen, nitrogen, and carbon from an initial reaction of elemental silicon and a reactive amine at a temperature of from about 0.degree. C. up to about 300.degree. C.
It is a further object of the invention to provide a process for forming silicon carbide or silicon nitride ceramic type materials, or mixtures or compounds of same, characterized by the substantial absence of halide or silicon oxide, from an intermediate reaction product consisting essentially of silicon, hydrogen, nitrogen, and carbon which, in turn, is produced at a temperature of from about 0.degree. C. up to about 300.degree. C. from a reaction between high purity elemental particulate silicon and a high purity amine reactant in a liquid state.
These and other objects of the invention will be apparent from the following description and accompanying flow sheet.
US Referenced Citations (33)
Non-Patent Literature Citations (8)
| Entry |
| J. W. Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. VI, (Wiley, New York, 1961), p. 163. |
| Rahaman et al., "Surface Characterization of Silicon Nitride and Silicon Carbide Powders", Am. Ceram. Soc. Bull., vol. 65(8), 1945, pp. 1171-1176. |
| Rochow, "The Direct Synthesis of Organosilicon Compounds", J. Am. Chem. Soc., Jun. 1945, pp. 963-965. |
| Zuckerman, "Direct Synthesis of Organosilicon Compounds", Advances in Inorganic Chemistry and Radio Chemistry, vol. 6, 1964. |
| D. Campos-Loriz et al., "The Effects of Hydrogen on the Nitridation of Silicon", J. Mat. Sci., vol. 14, 1979, pp. 1007-1008. |
| Dervisbegovic et al., "The Role of Hydrogen in the Nitridation of Silicon Powder Compacts", J. Mat. Sci., vol. 16, 1979, pp. 1945-1955. |
| Lin, "Comparative Studies of Metal Additives on the Nitridation of Silicon", J. Am. Cerm. Soc., vol. 60(1-2), 1977, pp. 78-81. |
| Lin, "Mass Spectrometric Studies on High Temperature Reaction Between Hydrogen Chloride and Silica/Silicon", J. Electrochem. Soc., vol. 123, 1976, pp. 512-514. |
Patent Grant
4914063
