Metal treatment – Process of modifying or maintaining internal physical... – Treating loose metal powder – particle or flake
Patent
1991-06-28
1994-08-23
Walsh, Donald P.
Metal treatment
Process of modifying or maintaining internal physical...
Treating loose metal powder, particle or flake
423346, 501 88, C01B 3136
Patent
active
053404173
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to the field of ceramic powders. More particularly, it relates to a process for producing silicon carbide ceramic powders.
BACKGROUND OF THE INVENTION
Silicon carbide is a ceramic material valued mainly for its high resistance to thermal stress and shock and its exceptional corrosion resistance in high temperature oxidizing environments. It has also found extensive use in the abrasives industry because of its hardness and wear resistance.
In general, silicon carbide exists in both an alpha and a beta form. The alpha phase is characterized as hexagonal, but exhibits many modifications or polytypes based upon stacking sequences in the layered structure. The beta phase, in contrast, is cubic. In both of these structures every atom is tetrahedrally surrounded by four atoms of the other species, forming strong near-covalent bonds. Alpha silicon carbide is assumed to be the stable high temperature phase, and cubic beta silicon carbide transforms slowly to the alpha phase above about 1650.degree. C. Various processes produce predominantly one or the other of these silicon carbide morphologies.
A number of methods of manufacturing silicon carbide have been developed. The most widely used, particularly in large-scale manufacturing, is the so-called Acheson process, in which mixtures of silica and carbon, along with a small amount of sawdust and common salt, are heated in large trough-type electric furnaces. A centrally mounted core of graphite and coke through which a large current can pass serves as a heater element. Maximum temperatures reached in this process approach 2700.degree. C.
Many other methods of manufacturing silicon carbide are disclosed in the literature, for example, in M. Yamamoto's survey article, "Present Situation of SiC Powder," Ceramics, Vol. 22, No. 1, p. 46 (1987). These methods include, for example: (1) the carbothermal reduction of silica and carbon in an inert atmosphere in a vertical furnace; (2) a direct reaction of silicon powder and fine carbon powder at around 1400.degree. C. in an inert atmosphere; (3) a sol-gel silica/carbon reduction process; and (4) a two-stage synthetic silica/carbon reduction process, which is carried out as a gas phase reaction. The two-stage synthetic silica/carbon reduction process involves synthesis of a homogeneous, high-purity mixture of silica and carbon by a gas phase reaction, followed by synthesis of beta-type silicon carbide by a solid state reaction. This method is described as producing spherical, high-purity products having a narrow particle size distribution without aftertreatments.
The methods involving carbothermal reduction of silica at high temperatures are based on a reaction approximating the following stoichiometric equation: through the synthesis and subsequent reaction of gaseous silicon monoxide according to the following sequence: silicon carbide. However, a number of problems must first be overcome to produce silicon carbide powder having desirable properties via the above chemistry.
One problem is that, at reaction temperatures above about 1150.degree. C., silicon monoxide is synthesized according to equation (2) above. The rate of synthesis becomes rapid above about 1600.degree. C. This silicon monoxide tends to condense at cool surfaces near the inlet. Thus, any continuous process must overcome silicon monoxide condensation problems associated with the continuous flow of a silica-containing feed precursor into a hot reaction vessel maintained at a reaction temperature above the generation temperature of the silicon monoxide.
Another problem is that, in addition to the silicon monoxide generation noted above, carbon monoxide is also generated in the reaction sequence of equations (2) and (3) above. Removing the carbon monoxide helps to promote the reaction. However, the gaseous silicon monoxide formed together with the carbon monoxide has a high vapor pressure and tends to be swept away and lost from the reaction chamber unless reacted with carbon. Silicon monoxide loss result
REFERENCES:
patent: 3271109 (1966-09-01), Mezey et al.
patent: 3709981 (1973-01-01), Lee et al.
patent: 3836356 (1974-09-01), Irani
patent: 3920446 (1975-11-01), Irani
patent: 4162167 (1979-07-01), Enomoto et al.
patent: 4217335 (1980-08-01), Sasaki et al.
patent: 4226841 (1981-10-01), Komeya et al.
patent: 4283375 (1981-08-01), Horne, Jr. et al.
patent: 4292276 (1981-09-01), Enomoto et al.
patent: 4390504 (1983-06-01), Enomoto et al.
patent: 4410502 (1983-10-01), Yamaguchi et al.
patent: 4467042 (1984-08-01), Hatta et al.
patent: 4529575 (1985-07-01), Enomoto et al.
patent: 4543240 (1985-09-01), Goldberger
patent: 4690811 (1987-09-01), Kida et al.
patent: 4702900 (1987-10-01), Kurachi et al.
patent: 4818511 (1989-04-01), Nishi et al.
patent: 4839150 (1989-06-01), Coyle et al.
patent: 4869886 (1989-09-01), Saiki et al.
patent: 5108461 (1992-04-01), Ruthner
patent: 5190737 (1993-03-01), Weimer et al.
Acta Chemica Scandinavica A 35, Oxidation of Silicon Carbide in Oxygen and in Water Vapour at 1500.degree. C., 1981, p. 247-254.
Advances in Ceramics, Production of Fine, High-Purity, Beta SiC Powder, 1987, pp. 257-263.
Lee et al., Formation of Silicon Carbide from Rice Hulls, 1975, pp. 195-198.
Communications of the American Ceramic Society, Beta SiC Powders Produced by Carbothermic Reduction of Silica in a High-Temperature Rotary Furnace, 1983, pp. C-111-113.
Derwent Publications, Nc., SiC Manufactured From Silica and Reactive Carbon, GB-031583.
Derwent Publications, Inc., SiC Manfr. Composed of Beta-type crystals, JA-100348 Chemical Abstracts, vol. 99, No. 424969t.
Moore William G.
Rafaniello William
Roach Raymond P.
Weimer Alan W.
Chi Anthony R.
Howard D. R.
The Dow Chemical Company
Tribble B. J.
Walsh Donald P.
LandOfFree
Process for preparing silicon carbide by carbothermal reduction does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for preparing silicon carbide by carbothermal reduction, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing silicon carbide by carbothermal reduction will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-500120