Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Reexamination Certificate
1999-05-07
2001-07-24
Nguyen, Ngoc-Yen (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C588S253000
Reexamination Certificate
active
06264908
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is in the field of silicon nitride synthesis. More particularly the present invention is in the field of methods and systems for low temperature synthesis of silicon nitride from silica, carbon and nitrogen gas using a catalyzed carbothermal reaction.
2. Review of Relevant Technology
There has been an increasing interest in the production and use of silicon nitride (Si
3
N
4
) and related compounds, such as silicon oxynitride (Si
2
N
2
O). Silicon nitride is a highly chemically resistant material that can have a variety of uses. It can be used as a high temperature, chemically resistant ceramic material. It can also be used as a protective coating.
There are a variety of different ways to manufacture silicon nitride. One method disclosed in U.S. Pat. No. 4,387,079 to Kasai et al. involves heat treating a nitrogen-containing silane, such as tetra-amide-monosilane or silicon imide, with ammonia at a temperature above 400° C. for a period of at least two hours to obtain silicon nitride. Preliminary, the nitrogen-containing silane is prepared by continuously reacting gaseous silicon tetra-chloride with gaseous ammonia at a temperature of −30° C. to 70° C. However, the drawback of the Kasai et al. process is that the reagents and methods disclosed therein are expensive and involve materials which can be harmful if handled improperly.
Another synthetic route for making silicon nitride is disclosed in U.S. Pat. No. 4,859,443 to Marosi. In Marosi, silicon nitride powders are prepared in a gas-phase reaction by reacting silicon tetrachloride with ammonia at above 500° C. in a fluidized bed of silicon nitride particles. This method requires the use of a fluidized bed of silicon nitride and only results in the formation of silicon nitride deposits on the pre-existing silicon nitride particles within the fluidized bed.
U.S. Pat. No. 5,662,875 to Bachelard et al. discloses a process in which silica, carbon and a seed crystal of silicon nitride are reacted, in a nitrogen countercurrent, in the presence of a volatile metal selected from the group consisting of beryllium, magnesium, calcium, strontium, germanium, tin, titanium, hafnium, sodium, and barium in a reaction zone possessing a temperature gradient. The Bachelard et al. reaction requires specialized heating zones as well as elevated temperatures of up to 1500° C.
Others have disclosed a carbothermal reaction between silica (SiO
2
) and carbon in the presence of pure nitrogen (N
2
) gas in an attempt to reduce the cost of producing silicon nitride as well as increasing its quality. Such attempts are reported in Durham et al., “Carbothermal Synthesis of Silicon Nitride: Effective Reaction Conditions,”
J. Am. Ceram. Soc.,
Vol. 74 (1), pp. 31-37 (1991). Durham et al. report that oxygen must carefully be removed from the reaction chamber, an excess of carbon relative to silica must be employed, and formation of silicon nitride only occurs within a very narrow temperature range of about 1350° C. to 1550° C.
A variety of other synthetic routes to silicon nitride are disclosed in U.S. Pat. Nos. 4,935,214 to Puger et al.; 5,232,677 to Fukuoku et al.; 5,258,169 to Wannagat et al; and 5,332,697 to Smith et al.
In view of the foregoing, it would be a significant advancement in the art to provide methods and systems for the synthesis of silicon nitride that avoided the use of dangerous and expensive chemical precursors.
It would be a her advancement in the art to provide methods and systems for manufacturing silicon nitride that could be carried out without dangerous and expensive chemicals while operating at a temperature far lower than presently required using conventional carbothermal methods.
It would be an additional advancement in the art if such methods and systems for low temperature synthesis of silicon nitride were able to employ catalytic means in order to carry out a modified carbothermal reaction sequence for silicon nitride at lower temperatures.
It would be a considerable advancement in the art to provide methods and systems for manufacturing silicon nitride that could utilize commonly found materials which were inexpensive or even considered to be waste products.
Such methods and systems for manufacturing silicon nitride are disclosed and claimed herein.
SUMMARY AND OBJECTS OF THE INVENTION
The present invention relates to improved methods and systems for manufacturing silicon nitride using fluidized bed technology. More particularly, the present invention employs fluidized beds that utilize silica sand and other forms of silica as the fluidizable particulate media. When finally divided carbon, such as soot produced by diesel engines, industrial burners, or by burning other carbon-containing fuels is introduced into a fluidized bed of silica, silicon nitride (Si
3
N
4
), silicon oxynitride (Si
2
N
2
O), or mixtures thereof are formed, typically on the surface of a metallic substrate In addition, virtually any inexpensive or waste carbonaceous material can be introduced into a fluidized bed in order to yield silicon nitride so long as the proper conditions are maintained.
The formation of silicon nitride, silicon oxynitride, or combinations thereof has been observed at temperatures as low as 150° C. In general, increasing the temperature above about 200° C. increases the rate of formation of silicon nitride and silicon oxynitride. Because others have reported that carbothermal production of silicon nitride by directly reacting carbon and silica in the presence of nitrogen gas only yield silicon nitride at temperatures exceeding 1350° C., and because the systems and methods of the present invention yield silicon nitride at much lower temperatures, it is believed that some form of catalysis is occurring which reduces the energy barriers inherent in forming silicon nitride using conventional carbothermal methods.
In general terms, the manufacture of silicon nitride and/or silicon oxynitride using the methods and systems disclosed herein requires the following: particulate silica, such as silica sand or silica gel, a source of carbon, nitrogen gas and the proper conditions necessary to generate catalytic conditions in which carbon is catalytically oxidized in the presence of silica to form carbon dioxide. Under such conditions a byproduct of the catalytic oxidation of carbon to carbon dioxide is silicon nitride and/or silicon oxynitride, depending on the conditions. In most cases silicon nitride and silicon oxynitride have been observed in the form of a film deposited on metallic substrates, such as stainless steel conduits normally associated with the venting of exhaust gases generated in a fluidized bed as well as screws, tools and other metallic substrates placed directly within the fluidized bed of silica particles. Thus, although the reaction mechanism is not entirely understood it appears that nitrogen normally found within air reacts in the presence of carbon and catalytically reactive silica to form silicon nitride and silicon oxynitride, which preferably form as a deposited film on the surface of metallic substrates.
The preferred temperature for forming silicon nitride and silicon oxynitride using the systems and methods disclosed herein is in a range from about 150° C. to about 500° C., more preferably in a range from about 200° C. to about 450° C., and most preferably in a range from about 250° C. to about 375° C. Nevertheless, it would appear that the formation of silicon nitride at any temperature below about 1000° C. using a fluidized bed of silica particles, carbon and nitrogen gas would appear to involve some kind of catalysis and is within the scope of the invention.
Although it is not entirely understood, it is believed that when silica is heated to a temperature in a range from about 150° C. to about 500° C. that there is considerable surface chemistry involving the release and reformation of hydroxyl groups with intermediate strained rings of silicon and oxygen being formed which quickly hydrolyze back to hydroxyl groups in the pre
Harrington Alan L.
Maganas Thomas C.
Nguyen Ngoc-Yen
Workman & Nydegger & Seeley
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