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High temperature amorphous composition based on aluminum phosphate

High temperature amorphous composition based on aluminum phosphate description/claims

This application is a continuation of and claims priority benefit from application Ser. No. 10/362,869 filed Jul. 15, 2003, which in turn claims priority from U.S. national application filed under 35 U.S.C. § 371 of international application no. PCT/US01/41790 filed on Aug. 20, 2001, which is a continuation-in-part of and claims priority benefit from application Ser. No. 09/644,495 filed Aug. 23, 2000, now issued as U.S. Pat. No. 6,461,415, each of which is incorporated herein by reference in its entirety.

The United States government has certain rights to this invention pursuant to Air Force Office of Scientific Research contract no. F49620-00-C-0022 and Department of Energy contract no. DOE DE-AC05-84OR21400-1DX-SY067V, both with Applied Thin Films, Inc.

This invention relates to synthetic inorganic compositions which remain metastable and possess other desired properties at mid and high temperature, for example, from 800° C. to 1400° C. and greater.

It is known to use metal oxide coatings for high temperature protection of substrates or other surfaces. Up to the present time, however, there are no known synthetic oxides which can remain amorphous and metastable at temperatures up to 1400° C. or greater. Silica, for example, is known to devitrify/crystallize at temperatures slightly greater than 850° C. Other non-oxide materials, such as silicon oxy-carbide and silicon oxy-nitride rapidly oxidize and form crystalline phases at high temperatures in air.

Aluminum phosphate is a well known inorganic material that has found many uses in applications including catalysts, refractories, composites, phosphate bonded ceramics, and many others. Aluminum phosphate has a low density (d=2.56 g/cm3). It is chemically inert and stable at high temperatures, as well as being chemically compatible with many metals and with most widely used ceramic materials including silicon carbide, alumina, and silica over a moderate range of temperatures.

Aluminum phosphate, however, is unsuitable for use as a high temperature ceramic material because it undergoes polymorphic transformations (quartz-type, tridymite and cristobalite) with corresponding large molar volume changes. Thus, it would be desirable to provide a synthetic form of aluminum phosphate which is metastable and remains substantially amorphous at increasing temperatures, or during heating and cooling cycles. Another desirable property would be to provide an aluminum phosphate composition having a low oxygen diffusivity at high temperatures or in harsh environments, in order to provide oxidation protection and corrosion resistance to substrates such as metals and ceramics.

The present invention relates generally to substantially amorphous aluminum phosphate materials and/or compositions exhibiting metastability and various other related properties under high temperature conditions. In part, metastability can be evidenced by retention of amorphous characteristics under oxidizing conditions, such morphology and the extent thereof due at least in part to the aluminum content of such materials/compositions, with those having an overall stoichiometric excess of aluminum exhibiting enhanced amorphous character and associated stability as compared to their stoichiometric counterparts. Such properties and related high-temperature attributes can be effected in the preparation of such materials/compositions, primarily by admixture of the aluminum precursor to the corresponding phosphate precursor to initiate various structural and/or compositional modifications which manifest themselves in the high temperature performance of the resulting aluminum phosphate material/composition. In particular, and as illustrated more clearly in the following examples, aluminum can be identified upon addition to a phosphate precursor and shown as contributing to the amorphous character and associated metastability of the resulting aluminum phosphate material/composition. Addition of a stoichiometric excess of aluminum precursor enhances the resulting amorphous character and metastability.

In part, the present invention is a metastable material including an aluminum phosphate composition, such a composition as can be represented having the formula Al1+xPO4+3x/2 wherein x is about 0 to about 1.5. The composition of such materials can be characterized by structural/compositional components absorbing radiation in the infra red spectrum at about 795 cm−1 to about 850 cm−1, and can be further characterized by their presence at temperatures of at least about 1000° C. Without regard to any particular material or compositional phase, in various preferred embodiments of this invention, as discussed more fully below, x is 0 or about 0. In various other preferred embodiments, depending upon desired performance properties of the metastable material and/or end use application, x is about 0.1 to about 1.0. Generally, such materials are substantially amorphous, the degree to which in part dependent upon the value of x and the aluminum content of the entire composition. Depending upon such content and morphology, such materials are metastable at temperatures at least about 1200° C. As illustrated below, in the following examples, such materials can also include crystalline particles including but not limited to CaWO4, Al2O3 and ErPO4, such inclusions as can result from temperature treatment and/or incorporation of suitable precursor components. Such inclusions can be provided, as desired, to affect various material physical characteristics and/or performance properties, including, but not limited to, modification of the material thermal expansion coefficient for a particular end use or composite fabrication.

In light of the above and in conjunction with the following examples and detailed descriptions, the present invention can also be a method of using the aluminum content of an aluminum phosphate composition to affect and/or control the metastability thereof. Such a method includes providing an aluminum phosphate composition from or using an aluminum salt precursor compound. The resulting aluminum phosphate composition has an aluminum content corresponding to that of the precursor compound, such aluminum content sufficient to provide a desired and/or predetermined compositional metastability. As illustrated below, and as would be understood to those skilled in the art, metastability of such a material can be evidenced spectroscopically showing an amorphous and/or non-crystalline aspect of the material. The material can have a certain metastability with an aluminum content stoichoimetric with respect to phosphate. Generally, the metastability of such a material can be enhanced with a stoichoimetric excess of aluminum. Aluminum content and resulting stability can be effected upon preparation of the corresponding precursor and admixture with a suitable phosphate precursor, as illustrated elsewhere herein. Metastability and various other optical, chemical and/or physical properties can benefit by inclusion of other metal components including but not limited to silicon, lanthanum and zirconium upon choice of and/or modification of a suitable precursor.

Accordingly, the present invention also includes, in part, a method for preparing a precursor solution for the formation of a metal phosphate composition, preferably an aluminum phosphate composition. Such a method includes preparation of a first solution of a metal and/or aluminum salt; preparing a phosphorus component; and admixing the solution and component. Typically, the phosphorus component is an alcoholic solution of phosphorus pentoxide, but other phosphorus components/phosphate precursors can be used with comparable effect as described elsewhere herein. Likewise and without limitation, the metal/aluminum component is provided in alcoholic solution, with choice of solvent dependent upon metal/aluminum solubility and compatibility with the corresponding phosphorus/phosphate component. Among the various departures from the prior art, this aspect of the present invention contemplates use of a stoichiometric excess of the corresponding metal and/or aluminum component in the preparation of such a precursor and use thereof in the subsequent formation of the desired metal and/or aluminum phosphate material/composition. As described more fully elsewhere herein, preferred embodiments of this invention are directed toward suitable aluminum salt components, precursors and resulting materials/compositions, but various other metal components can be incorporated into the precursor solution to provide the resulting material/composition associated thermal, optical, chemical and/or physical properties.

In part, the present invention also includes a method of using an aluminum phosphate composition to enhance the oxidation resistance of an associated substrate. The method includes (1) providing an aluminum phosphate composition of this invention, preferably one having the formula A11+xPO4+3x/2, wherein x is about 0 to about 1.5; and (2) applying the composition to a suitable substrate. In various preferred embodiments, depending upon end use application and/or fabrication technique, the composition can be annealed either prior or subsequent to substrate application. Regardless, as illustrated below, such a composition can be dip-coated to provide a film on the substrate. Alternatively, without limitation, the composition can be prepared as a powder then either plasma—or aerosol—sprayed onto a substrate.

In part, the present invention can also include an aluminum phosphate product with aluminum-oxygen-aluminum structural moieties, absorbing radiation in the infra red spectrum at about 795 cm−1 to about 850 cm−1. Such a product is obtainable and/or can be produced by (1) mixing an alcoholic solution of phosphorus pentoxide with a solution of an aluminum salt, the salt either stoichiometric or in stoichiometric excess with respect to the phosphorus precursor; and (2) heating the resulting admixture. Such a product is substantially amorphous, but can also provide for crystalline particulate inclusions, as discussed above. As a representative embodiment, such particles are crystalline erbium phosphate inclusions prepared by incorporating an erbium salt with the aforementioned aluminum salt solution. Alternatively, illustrating another aspect of this invention, the product can include Group III A and/or Group III B-VI B metal oxide particles in amounts sufficient to modify the thermal expansion coefficient of the resulting product.

With regard to one or more aspects of the preceding discussion, the present invention can include a new class of phosphate compounds formulated to contain an excess amount of metal species in the composition; that is, with reference to a preferred embodiment, the aluminum atoms exceed the number of phosphorus atoms found in stoichiometric aluminum phosphate. The excess can be more than one percent and preferably greater than five percent.

Whether compositions of this invention are stoichiometric or reflect an excess metal component methods for their preparation include those disclosed in U.S. Pat. No. 6,036,762, the entirety of which is incorporated herein by reference. In accordance therewith, a precursor solution is formed from two liquid components. The first component is a metal salt dissolved in alcohol. The second component of phosphorus pentoxide is dissolved in alcohol. The two components are then mixed together in the desired molar proportions to provide a stable precursor solution, with the phosphate portion at least partially esterfying to form a polymer-like structure which uniformly entraps the metal ion.

The solution, as such, may be heated directly to remove the alcohol portion and other species and provide a pure metal phosphate. Preferably, however, the solution is applied as a coating to a non-porous or porous substrate using any suitable method, and the coated substrate is heated, typically to a temperature of less than 600° C., to obtain a uniform and pure coating of the metal phosphate on the substrate.

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