Compositions: ceramic – Ceramic compositions – Carbide or oxycarbide containing
Patent
1988-11-01
1991-01-22
Dixon, Jr., William R.
Compositions: ceramic
Ceramic compositions
Carbide or oxycarbide containing
501 95, 501 97, 501 98, C04B 3556, C04B 3558
Patent
active
049871048
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to ceramic products and is particularly concerned with a method of forming a ceramic product by converting silica to silicon nitride in a carbothermal reduction process in the presence of nitrogen, fabricating the silicon nitride into the desired shape of the ceramic product and sintering the fabricated silicon nitride.
BACKGROUND OF THE INVENTION
It is widely accepted that silicon nitride has excellent intrinsic properties such as strength, hardness, oxidation, creep and thermal shock resistance. However, because of the highly covalent nature of the chemical bonding in this material and corresponding low diffusion rates, traditional ceramic fabrication techniques do not lead to dense bodies. In order to achieve low porosity materials, densification aids are necessary which compromise the properties of fabricated specimens especially at elevated temperatures.
A development was the discovery that aluminium could be incorporated into the lattice structure of silicon nitride. The material so formed was believed to be a single phase solid solution of alumina in silicon nitride. Early workers found this material was easily fabricated using well known ceramic techniques However, it was subsequently reported that this material is multi-phased and that the original formula was incorrect. Thus, the solid solution was not as previously proposed but existed between silicon nitride and aluminium oxynitride Al.sub.3 ON.sub.3. The properties of the early materials were inferior to those of silicon nitride as a result of a residual grain boundary glass which also explained the greater ease of densification. The correct formula for aluminium substituted silicon nitride (designated .beta.-sialon) is
In order to sinter these materials to high density, sintering aids are necessary.
Another non-oxide ceramic with excellent intrinsic properties is silicon oxynitride. This material has been used as a bonding phase for silicon carbide refractories, and it has been reported that limited replacement of silicon and nitrogen by aluminium and oxygen can also occur in silicon oxynitride (O-sialon).
It has long been appreciated that rice hulls can be utilized as a raw material in the manufacture if silicon nitride and silicon carbide powders both in whisker and particulate forms. At present rice hulls pose a considerable waste disposal problem. With Australian rice production levels as high as 830 000 tonnes (1982), up to 160 000 tonnes of rice hulls are being produced annually. In the Australian context, if all the rice hull waste produced by the milling of rice was used to make silicon nitride, up to 20 000 tonnes could be produced per annum based on recent figures for the production of rice.
Rice hulls contain approximately 20 weight percent silica with the bulk being organic material. Pyrolysis of the rice hulls leads to a 60 percent weight loss. The resultant material consists of approximately 55 weight percent silica and 45 weight percent of carbon Both are in a finely divided state and intimately mixed. Upon ashing, a material containing in excess of 95 weight percent silica can be obtained Elements such as iron, aluminium, sodium, potassium, calcium and magnesium may also be present.
The formation of silicon nitride and silicon carbide from rice hulls involves a number of steps. The rice hulls are pyrolysed to decompose the organic component to carbon. This material is then heated at temperatures between 1000.degree. to 2000.degree. C. to form the silicon carbide or nitride. Early work on the formation of silicon carbide and nitride from rice hulls is the subject of United States parent specifications Nos. 3754076 and 3855395.
A problem with the conventional method for the manufacture of dense ceramic materials from the products of the carbothermal reduction of silica is that, after the process, unreacted silica can be present and must be separated from the product. This usually involves a leaching process. Additions of catalyst can be made to increase the reaction rate, and a c
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Commonwealth Scientific & Industrial Research Organization
Dixon Jr. William R.
Gallo Chris
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