Thin films of metal phosphates and the method of their formation

The invention is 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 are the alkali metals, alkaline earths, titanium, zirconium, and aluminum.

FIELD OF THE INVENTION 
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. 
PRIOR ART 
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. 
THE INVENTION 
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.

MODES OF PERFORMING THE INVENTION 
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. 
METAL RESINATE FORMATION 
The preparation of resinate generally is carried out by one of the 
following processes: 
1) Fusion 
In this type of reaction a metal oxide, hydroxide, carbonate, or salt 
reacts with a carboxylic acid to form a metal carboxylate. 
EQU MO+2RCOOH.fwdarw.M(OOCR).sub.2 +H.sub.2 O 
where RCOOH is a carboxylic acid, and MO is a divalent metal oxide. 
2) Metathesis 
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: 
EQU M(OR').sub.2 +xRCOOH.fwdarw.M(OR').sub.2-x (OOCR).sub.x +R'OH 
EQU M(AcAc).sub.2 +xRCOOH.fwdarw.M(AaAc).sub.2-x (OOCR).sub.x +xAcAcH 
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: 
Titanium Resinate (A) 
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. 
Potassium-Resinate (B) 
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. 
Calcium Resinate (C) 
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. 
Aluminum Resinate (D) 
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. 
Zirconium Resinate (E) 
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. 
Phosphorus Resinate 
(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. 
EXAMPLES 
Example 1--Titanium Phosphate 
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. 
EXAMPLE 2--Titanium Phosphate 
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. 
Example 3--Zirconium Phosphate 
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. 
Example 4--Potassium Phosphate 
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. 
Example 5--Calcium Phosphate 
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. 
Example 6--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. 
Example 7--Potassium Phosphate 
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. 
Example 8--Polassium Titanium Phosphate 
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. 
Example 9--Aluminum Phosphate 
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. 
Example 10--Calcium Fluorophosphate 
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.