Method of making yttrium silicon oxynitrides

Chemistry of inorganic compounds – Rare earth compound

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501 97, 501 98, C04B 3558, C01F 1700

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045017239

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

The invention relates to the art of making silicon oxynitrides and, more particularly, yttrium silicon oxynitrides.


BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT

Until recently, the prior art had not made yttrium silicon oxynitrides independently so that they could be used as a powder additive for a variety of subsequent chemical processes. Oxynitrides, if formed at all, were formed as a comingled byproduct of heating, such as by hot pressing a ternary system of silicon nitride, and two oxides used as hot pressing aids (see U.S. Pat. Nos. 4,102,698; 4,341,874; and 4,350,771). In each of these patents, a compact of the ternary powder system is sintered to a relatively high density during which secondary phases form, which may include oxynitrides. Of course, the process was not aimed at producing an oxynitride that could be extracted for other uses. The high density of the resulting product made it very difficult to separate out any oxynitrides that had been formed and even more difficult to convert the high density sintered product into a reground powder. But, more importantly, the purity and type of second phases produced as a result of such sintering could not be accurately controlled, and the chemistry constraints imposed by the presence of excess silicon nitride inhibited the formation of desired or selected yttrium silicon oxynitrides.
A recent effort (see U.S. Pat. No. 4,331,771) has been made to produce bonded silicon oxynitrides for utility as a structural material. In this effort, silicon powder and oxides are reacted in the presence of an oxygen
itrogen atmosphere. The method is designed to provide for a strong and dense oxynitride body required by the utilization of the body, which strength can be obtained only by the presence of impure substances in the oxynitride body.
What is needed is a method by which a low density, pure form of the oxynitride can be made economically for other uses with minimal regrinding. There is a need for a method by which stoichiometric amounts of the ingredients needed for a specific oxynitride can be chemically reacted to form the specific compound without byproducts and contaminating chemical reactions.


SUMMARY OF THE INVENTION

The invention is a method of making yttrium silicon oxynitrides of the formula Y.sub.a Si.sub.b O.sub.c N.sub.d, by the steps comprising: (a) mixing together, in intimate reactive contact, stoichiometric amounts of Y.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3 N.sub.4 to form a desired yttrium silicon oxynitride where a, b, c, and d represent the required element parts of the compound in equilibrium with the mixture element parts (when necessary, this requires taking into account the amount of SiO.sub.2 that is present as an oxide coating on the Si.sub.3 N.sub.4); (b) bearing the mixture in an inert atmosphere to a temperature level and for a time sufficient to convert the stoichiometric amounts of the mixture to the desired yttrium silicon oxynitride; and (c) grinding said heat agglomerated mixture to a powder.
It is preferable to heat the mixture to the temperature range of 1500.degree.-1550.degree. C. for at least 6 hours to produce low density Y.sub.10 Si.sub.6 O.sub.24 N.sub.2 (hereafter Y.sub.10 phase), and to heat the mixture to the temperature range of 1600.degree.-1650.degree. C. for at least 6 hours to produce low density YSiO.sub.2 N phase (hereafter Y.sub.1 phase). The lower temperatures and longer times of heating result in a low density (preferably no greater than 2.1 gm/cm.sup.3) for the heated powder. AS heating progresses in the nitrogen atmosphere, reaction gases may form (such as SiO) and form part of the atmosphere. Excess SiO.sub.2 may be needed in the initial charge to account for the SiO volatilization.


BEST MODE FOR CARRYING OUT THE INVENTION

A preferred method for carrying out the subject invention is as follows.
Yttrium silicon oxynitride, according to the formula Y.sub.a Si.sub.b O.sub.c N.sub.d, is made by mixing together stiochiometric amounts of Y.sub.2 O.sub.3, SiO.sub.2, and Si.sub.3

REFERENCES:
patent: 4102698 (1978-07-01), Lange et al.
patent: 4331771 (1982-05-01), Washburn
patent: 4341874 (1982-07-01), Nishada et al.
patent: 4350771 (1982-09-01), Smith

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