Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
1999-10-01
2002-05-28
Mullis, Jeffrey (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S329100, C525S331900, C525S333200, C528S004000, C528S009000
Reexamination Certificate
active
06395840
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to: precursor polymers which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure refractory metal carbides and/or refractory metal borides.
BACKGROUND OF THE INVENTION
Considerable effort has been devoted over the past 15 years to the development of effective methods for manufacturing ceramic matrix composites (CMC's). Several approaches with potential for industrial use have been identified. The development of CMC's with high temperature stability theoretically is possible; however, CMC's have not yet been developed for use in extremely high temperature applications, such as multistage nozzles for rocket motors. Such nozzles must be capable of exhibiting high strength even after repeatedly withstanding temperatures of 1600° C. and even higher.
Currently, multistage nozzles are made from tungsten and graphite, which have relatively high melting/sublimation points—a 3410° C. melting point for tungsten, and a 3650° C. sublimation point for graphite. The high temperature strength of a material is proportional to the melting point of that material. If CMC's could be made using materials with higher melting/sublimation points than tungsten and graphite, then the resulting CMC's should be effective alternative materials for making high temperature components, such as multistage nozzles.
Certain metal carbides and metal borides have melting temperatures even higher than the melting/sublimation points of tungsten and graphite. For example, hafnium carbide has a melting temperature of 3890° C. and tantalum carbide has a melting temperature of 3880° C. Metal carbides also exhibit desirable brittle to ductile transition temperatures in the range of 1725-1980° C.
A CMC having a matrix of a refractory metal carbide and/or metal boride and comprising between about 20-30% particulate silicon carbide theoretically would be an ideal alternative for tungsten and graphite in multistage nozzles. Such metal carbides and/or metal borides also might be useful as high temperature coatings for other surfaces which are exposed to high temperatures during operation. In fact, the United States Air Force has recently initiated a new program—Integrated High Pay-Off Rocket Propulsion Technology (IHPRPT)—to incorporate such advanced materials into rocket and space propulsion systems.
Unfortunately, the most widely used method for making CMC's—chemical vapor infiltration (CVI)—is slow, complex, and has many inherent difficulties. One major difficulty for high temperature applications is that CVI produces a CMC with substantial residual porosity (15-25%). The greater the porosity, the lower the strength of the CMC.
Polymer infiltration/pyrolysis (PIP) can produce a less porous CMC. However, PIP can only be used to make metal carbide/metal boride CMC's if precursor polymers are developed which will decompose upon pyrolysis or other energy treatment to yield substantially pure metal carbides and metal borides.
SUMMARY OF THE INVENTION
The invention provides a method for producing precursor polymers comprising: mixing a transition metal compound with an organic compound under conditions effective to form organo-transition metal complexes. The organic compound is a borane, a carborane, or an organometallic compound comprising a second metal which is displaceable by the transition metal. The second metal is bonded to at least one polymerizable organic component comprising at least one unsaturated carbon-carbon bond and an organic backbone polymer comprising a plurality of unsaturated carbon-carbon bonds. When the organic compound is a borane or a carborane, the transition metal compound comprises an organo-transition metal halide. When the organic compound an organometallic compound, a borane, or a carborane, the organo-transition metal complexes are subjected to conditions effective to polymerize the organo-transition metal complexes and to form the precursor polymer. Upon exposure to decomposition conditions, the precursor polymer decomposes to a refractory metal carbide or a refractory metal boride comprising about 3 wt% impurities or less.
DETAILED DESCRIPTION OF THE INVENTION
Synthetic inorganic and organometallic chemistry has been used to produce a variety of metal-containing polymer species which, upon pyrolysis or other energetic treatment, decompose to yield substantially pure metal carbides and/or metal borides. Two different approaches were used to obtain such organometallic precursor polymers.
Polymerization of Unsaturated Precursors
In a first embodiment, a transition metal salt is mixed with one or more organometallic(s) containing at least one unsaturated carbon-carbon bond to form organo-transition metal complexes, which are polymerized to form the precursor polymer. This embodiment has the advantage of guaranteeing that each unit of monomer will contain a metal atom. One disadvantage of this embodiment is that it does not produce high molecular weight precursor polymers.
High molecular weight precursor polymers are advantageous for use in a PIP process because high molecular weight precursor polymers tend to produce higher ceramic yields. Unfortunately, the viscosity of a polymer also increases with the molecular weight of the polymer. Precursor polymers with lower viscosity are preferred for an ideal PIP process. This inherent conflict may be resolved by using high molecular weight precursor polymers having relatively low viscosity, preferably a viscosity similar to a warm honey-like consistency. In order to produce such precursor polymers, the organo-transition metal complexes described above are polymerized with other comonomers which have low tendency to increase viscosity, as described in more detail below.
Preferred organometallics for use in this first embodiment include, but are not necessarily limited to metal coordinated substituted and unsubstituted allyl and vinyl organometallics comprising in the range of from about 2 to about 8 carbon atoms, preferably in the range of from about 2 to about 4 carbon atoms. Suitable allyl organometallics include, but are not necessarily limited to 1-methyl-2-propenyl magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, 2-methyl-1-propenyl magnesium chloride, 1-methyl-2-propenyl-magnesium bromide, allyl magnesium chloride, allyl magnesium bromide. Suitable vinyl organometallics include, but are not necessarily limited to substituted and unsubstituted: vinyl lithium chlorides; vinyl magnesium chlorides; vinyl magnesium bromides; and similar compounds. Such compounds are available from Aldrich Chemical Co. A preferred organometallic is allyl magnesium bromide.
The organometallic should be reacted with a salt of a transition metal, defined herein as a transition metal selected from the group consisting of hafnium, tantalum, zirconium, titanium, vanadium, niobium, chromium, molybdenum, and tungsten. Preferred transition metals are selected from the group consisting of tantalum, hafnium, and zirconium. Such salts include but are not necessarily limited to metal halides, metal nitrates, metal sulfates, and metal acetates, with preferred salts being hafnium and tantalum chloride. Hafnium and tantalum chloride, and other metal halides, are available from a number of chemical sources. For example, hafnium chloride is available from Advance Research Chemicals, Inc., Catoosa, Okla., and Teledyne Wah Cheng, Albany, Oreg. Hafnium boride is available from Noah Chemical, Div. Noah Technologies Corp., San Antonio, Tex. Hafnium bromide and tantalum bromide are available from Wilshire Chemical Co., Inc., Gardena, Calif. Tantalum chloride is available from several sources, including Aithaca Chemical Corp., Uniondale, N.Y. and Trinitech International, Inc., Twinsberg, Ohio.
In a preferred embodiment, the precursor polymer is formed by suspending hafnium or tantalum chloride in a suitable organic solvent, preferably dry ether, and chilling to a temperature in the range of from about −70° C. to about −90° C., preferably about −78° C
Paul Partha P.
Schwab Stuart T.
Mullis Jeffrey
Paula D. Morris & Associates P.C.
Southwest Research Institute
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