Thin Films of Metal Phosphates and the Method of Their Formation

The invention relates to a method of providing a coat or film of metal phosphate on a substrate. It particularly relates to the decomposition of metal carboxylates in the presence of phosphorous resinates or metal alkoxides.

Coatings of metal phosphates generally have been formed from finely divided glass powders, pastes and cements. Formation of metal phosphate coatings by these methods requires first the formation of a powder, then a blending, coating and firing step to achieve the coat or film layer on a substrate.

Metal phosphate coatings are desirable for use in a variety of structures. The coats are useful both in the amorphous and crystalline form. In their crystalline form they are useful as molecular sieves, electro optic materials, ion exchangers, non-linear optical materials, solid electrolyte material, catalytic substrates, as well as catalysts. In their amorphous form they are useful as wear resistant surfaces.

U.S. Pat. No. 4,701,314--David and U.S. Pat. No. 4,622,310 --Iacobucci disclose methods of making metal phosphate powders by reacting a metal alkoxide in an organic solvent with a phosphoric acid solution. These materials are reacted to form the metal phosphate and then fired to drive off the solvent and recover the powder. These materials are not suitable to form a coating, rather than powders, as there will be phase separation after reaction of the components.

In an article by Freche et al in ANN. CHIM. FR., 1985, 10 pp. 549-559 the reaction of calcium acetate with ammonium phosphate is disclosed as a method of producing the calcium phosphate. However, the process of Freche et al is limited to water as a solvent.

Rothon in an article in Thin Solid Films, 77 (1981) pp. 149-153 discloses solution deposited metal phosphate coatings by reaction exchange of an inorganic aluminum salt and phosphoric acid. This method of formation of phosphate coatings has the disadvantage that it cannot easily be extended to metals other than the aluminum disclosed therein. Further, it involves the utilization of hazardous materials and the process can only produce polycrystalline films.

Hattori et al in an article In Advanced Ceramics, Vol. 3, No. 4, (1988) pp. 426-428 discloses a hydrothermal process in which the metal phosphate is formed at high pressure. The disadvantage of this process is the use of high pressure, as well as the inability of the process to form anything other than grains of the metal phosphate.

Therefore, there remains a need for an easy to perform process of producing films of metal phosphates on a substrate. There is particular need for a method of forming films by casting or dipping such that irregular shapes may be coated. Further, &here is a need for Processes that do not require first formation of metal phosphate powders prior to the formulation of these powders to form coatings of metal phosphates on a substrate.

An object of this invention is to overcome disadvantages of prior methods of forming metal phosphates on a substrate.

Another object of the invention is to form improved amorphous coating films and improved crystalline coating films of metal phosphates.

These and other objects of the invention are generally accomplished by mixing non-phosphorous containing metal resinates and phosphorous resinates, forming a coating of the mixture on a substrate and heating the mixture to recover a thin film coating of metal phosphate. The metal resinates and phosphorous resinates are defined as metal-ligand compounds where the ligand is thermally separable. The preferred ligands are carboxylates, alcoholates, and acetylacetonates. The heating decomposes the metal phosphate precursor coating materials to yield a metal phosphate. The phosphorous resinate may comprise an alkyl phosphate, arylphosphate, or a carboxylate substituted alkyl or aryl phosphate. The substituting carboxylic acids may be pure, such as 2-ethylhexanoic acid, mixtures of acids, such as neodecanoic acid, and naturally occurring acids, such as rosin (abietic acid). The metal resinate may be a metal carboxylate, a carboxylate substituted alkoxide, or carboxylate substituted acetylacetonate. Typical metals include but are not limited to alkali metals, alkaline earths, titanium, zirconium, and aluminum.

The invention has numerous advantages over prior processes of forming metal phosphates as films or coatings on a substrate. The materials may be formed either in the amorphous or crystalline phase based on the thermal treatment. Further, the process does not require first the formation of a metal phosphate powder and then of casting and firing. The process further does not require control of the atmosphere or high pressure. The process allows formation of metal phosphate coatings on irregular shapes not possible to coat by vapor deposition. The process also allows formation of uniform multimetal phosphates and blends of metal phosphates that cannot be easily formed by the vapor deposition techniques. By depositing multiple layers it is possible to adjust the thickness of the films and to produce layers of varying composition. These and other advantages will be apparent from the detailed description below.

The invention is generally performed by dissolving the non-phosphorous containing metal resinate in a solvent and adding a phosphorous resinate to the solution. Metal resinates are defined as metal ligand compounds where the ligand is thermally separable. Phosphorous resinates are defined as phosphorous-oxygen compounds with ligands which volatilizes upon thermal treatment. The pyrolysis products of the phosphorous resinate interact with non phosphorous containing metal resinate compounds to form metal phosphates and mixed metal phosphates when they interact with mixed precursor metal phosphate compounds. After mixing to obtain a homogeneous solution the material is coated onto a substrate. The coating is then heated to evaporate solvents, and to decompose the resinate and yield the metal phosphate. The resulting layer may be either amorphous or crystalline depending upon &he thermal treatment. Crystalline materials form at the higher temperatures for most materials. A typical heating temperature utilized to yield a crystalline coating film layer in this process is about 800.degree. C. for aluminum phosphate. The coating methods utilized in the invention may be any conventional method such as spin coating, spray coating, or dip coating. The substrate may be any material in which a phosphate coating is desired and which has the ability to survive the temperatures required for decomposition of the resinates. Typical of such substrate materials are fused quartz, silicon, aluminum oxide, and magnesium oxide.

The process may be performed with any non-phosphorous containing metal resina&e that results in formation of metal phosphate when decomposed after being mixed with a phosphorous resinate. Typical of such metal resinates materials are carboxylates of the transition elements, alkali metals, alkaline earths, and lanthanides. Preferred metal resinates are carboxylates of the Group 1 metals lithium, sodium and potassium, and the Group 2 metals magnesium, calcium, strontium, and barium. The nature of the product after heating is determined by their free energy of formation. Thus metal phosphates form when their free energy of formation is higher than the free energy of formation of the corresponding oxide.

The process may be performed with any phosphorous resinate that when combined with the metal resinate and solvent will result in a metal phosphate coating after heating. Suitable for use in the process are the alkyl and aryl phosphates, and carboxylate substituted alkyl and aryl aliphatic phosphorous compounds. Preferred for the process are cresyl phosphate and tri-ethyl phosphate. These materials are readily soluble in conventional solvents and result in homogeneous solutions and coatings that after heating form uniform metal phosphate layers.

The addition of fluorinated carboxylic acid, such as heptafluorobutyric acid (C.sub.4 F.sub.7 O.sub.2 H), or other fluorinating agent, such as fluorinated alcohol or fluorinated acetylacetonate with the non-phosphorous containing metal resinate and the phosphorous resinate will result in the formation of metal fluorophosphates if they have a favorable energy of formation compared with the metal phosphate.

The heating of the substrate onto which the metal phosphate precursor layer has been formed may be to any temperature that results in the decomposition of the precursor layer to result in the pure metal phosphate. Heating temperature typically is between about 550.degree. C. and 800.degree. C. for crystallization and may be at any rate that does not cause disruption of the layer as decomposition takes place. A preferred heating rate is about 50.degree. C./min. The temperature range for the preferred combination of chelated aluminum ethoxide and cresyl phosphate is to about 500.degree. C. for an amorphous layer and to about 800.degree. C. for formation of a crystalline layer.

The solvent, to dissolve the metal carboxylate or other resinate, may be any solvent that does not react, in a disruptive manner such as forming a precipitate or a gel with the metal carboxylate or the phosphorus containing agent. Typical of such solvents are benzene, toluene, xylene, and butanol. A preferred solvent is toluene as it is low in cost, low health hazard, and offers desirable coating advantages due to its surface tension and viscosity of casting liquids formed. The solvent utilized must be able to dissolve the metal resinates, such as 2-ethylhexanoates, neodecanoates, and carboxylate substituted alkoxides.

The coating technique utilized to form a layer of the casting liquid may be anything that will give a thin coat on a particular substrate. These include spin coating, spraying, doctor blade coating, and curtain coating. In spin coating a liquid is applied to a substrate which is then spun at a high rate of rpms such as 6 K. In dip coating the substrate is dipped into liquid and allowed to drain prior to heating. Spin coating results in very uniform thin film coatings.

The substrate onto which the casting solution is placed may be any substrate on which a metal phosphate coat would be useful. The material must be able to withstand the decomposition temperatures, such as 500.degree. C., that are used in forming the metal phosphates of the invention. Among suitable substrates are aluminum oxide, quartz, magnesium oxides, and silicon. The coatings are between about 500 to over 20,000 angstroms thick depending on the number of coatings.

The following examples are intended to be illustrative and not exhaustive of techniques in accordance with the invention. Parts and percentages are by weight unless otherwise indicated.

The preparation of resinate generally is carried out by one of the following processes:

In this type of reaction a metal oxide, hydroxide, carbonate, or salt reacts with a carboxylic acid to form a metal carboxylate.

where RCOOH is a carboxylic acid, and MO is a divalent metal oxide.

In this type of reaction one exchanges either completely or partially a ligand in a material such as a metal alkoxide (or alcoholate) or a .beta.-diketonate by a carboxylic group for example:

Following preparation the precursors are separated, concentrated, and assayed.

Listed below is the preparation of some non-phosphorous containing metal ligand compounds, and a description of the phosphorous containing compounds and their derivatives:

Combine 1 part by molar ratio of titanium tetrabutoxide, 4 parts neodecanoic acid. Heating to about 100.degree. C. with mixing is carried out with collection of butyl alcohol driven off until close to 3-moles of alcohol are removed. Thermogravimetric analysis (TGA) indicates the residue is 8.91% TiO.sub.2.

32.7 g neodecanoic acid

10.0 g KOH 87%

25.0 g toluene

5.0 g xylenes

All the above ingredients are mixed with the KOH slurry in toluene. Heating with stirring was carried out to Just before the reflux point. The reaction is exothermic and is characterized by bubbling. When this is completed, molecular sieves are added to remove water and heating is continued with stirring to just below the reflux point for an additional one-half hour. The resulting potassium concentration after filtering is 7.32% K.

7.4 g Ca(OH).sub.2

30.0 g 2-ethylhexanoic acid

Toluene

Mix 20 ml toluene and acid heat with stirring to point just below boiling. To this mixture add a slurry made of the calcium hydroxide and 20 ml toluene. The slurry is added slowly to permit the gradual formation and evaporation of water vapor. TGA results show a composition of 3.27% Ca.

2.16 g aluminum t-butoxide

35 g large excess ethylacetoacetate

Toluene

Mix the above materials and reflux for 2 hours. Temperature is defined by reflux condition. The temperature is increased during the last five minutes of heating until slight coloring occurs. Particulate matter is settled and filtered through a Buchner funnel. A brownish clear liquid is obtained and concentrated by distillation .about.100.degree. C. under reduced pressure. TGA results indicate 4.29% Al.sub.2 O.sub.3.

10.5 g Zr isopropoxide

22.5 g neodecanoic

Toluene is added as needed (.about.50 ml) and the solution is refluxed for about 2 hours in order to exchange isopropoxide groups and to remove them by evaporation. The resulting compound is filtered while hot. TGA shows 3.52% Zr.

(1)

Phosphorus Resinates (Engelhard 1-38241)

The resinate composition is a phenyl phosphate.

(2)

5.98 g cresyl phosphate (Eastman Chemicals No. T4420)

5.12 g rosin

8.5 g toluene

Combine ingredients and warm up gently until rosin is dissolved.

(3)

Triethyl Phosphate (Eastman Chemicals No. 4662)

(4)

4.65 g triethylphosphate 4662

7.9 g rosin

7 g xylenes

Combine the ingredients and warm up until rosin is dissolved.

In the examples below, unless otherwise stated, amorphous thin films were produced by dripping about 1/2 ml of the mixture over a substrate, typically fused quartz, and spin coating at 6KRPM for about 30-60 seconds. This was followed by drying the substrate and wet film and decomposition on a hot stage.

When crystalline films were desired, the substrate and amorphous thin films were thermally treated until crystallization was effected.

1.49 g titanium resinate (Engelhard No. 9428) Lot M 11573

2.10 g phosphorus resinate (Engelhard No. 15) Lot F-33241 (#1)

An aliquot of the sample was decomposed in a crucible on a hot plate until no further decomposition was evident. Following this treatment the resulting powder was thermally treated in a furnace held at 1000.degree. C. The residue is identified as TiP.sub.2 O.sub.7 by X-ray diffraction.

5.03 g Ti-resinate (Engelhard No. 9428). Composition is 7.2% titanium.

5.44 g tricresylphosphate

The procedure of Example 1 is repeated substituting the above ingredients. TiP.sub.2 O.sub.7 is obtained in the crystalline state after treatment at 1000.degree. C. for 16 hours.

2.61 g Zr-isopropoxide (E)

1.85 g tricresyl phosphate

2.00 g toluene

The procedure of Example 1 is repeated substituting the above ingredients. After decomposing the mixture, ZrP.sub.2 O.sub.2 is formed in the crystalline state at 1100.degree. C.

0.94 g potassium resinate (B)

1.47 g phosphorus resinate (#4)

The procedure of Example 1 is repeated substituting the above ingredients. After decomposition and thermal treatment to 900.degree. C. for two hours, potassium phosphate is identified by X-ray diffraction.

3.48 g Ca-resinate (Engelhard 772786)

0.93 g triethyl phosphate (#3)

0.89 g neodecanoic

2.0 g toluene

The procedure of Example 1 is repeated substituting the above ingredients. Powder film obtained is thermally treated to temperatures of about 800.degree. C. for one hour. The resulting powder is identified as calcium phosphate.

4.47 g calcium resinate (C) composition 3.27%

1.63 g cresyl phosphate excess (#2)

Three coatings were deposited onto a fused quartz substrate, with hot stage drying and decomposition after each coat was applied. This was followed by treatment in a furnace at 900.degree. C. for one hour in order to obtain a polycrystalline film.

2.25 g K-neodecanoate resinate (B) composition 7.32% K

1.63 g cresyl phosphate excess (#2)

The procedure of Example 1 is repeated substituting the above ingredients. After decomposition and thermal treatment to 900.degree. C., potassium phosphate is identified by X-ra diffraction.

1.09 g K resinate (B) 7.32% K

1.80 g Ti resinate 5.35% Ti (A)

0.80 g Cresyl phosphate (#2)

The procedure of Example 1 is repeated substituting the above ingredients. A portion of the thoroughly mixed liquid prior to spin coating is decomposed in a crucible and the powder obtained is treated at 1000.degree. C. for 3 1/2 hours. The x-ray spectrum identifies the powder as potassium-titanium-phosphate powder.

1.20 g Al resinate (D) 4.29% Al.sub.2 O.sub.3

0.30 g excess cresyl phosphate (#2)

The procedure of Example 1 is repeated substituting the above ingredients. The film was treated at 1000.degree. C. where crystallization occurred to form polycrystalline aluminum phosphate film.

0.85 g Ca resinate Engelhard composition 7.1% calcium lot#36011

0.33 g tricresyl phosphate

0.02 g heptafluorobutyric acid

1.1 g xylenes

The above materials are combined in a beaker. After slight heating, the mixture is stirred vigorously. Decomposition of a portion of it on a hot plate and treatment for 1/2 hour at 900.degree. C. gave a powder later identified by X-ray diffraction as fluoroapatite.

The invention has been described in detail with particular reference to preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.