A process for depositing a layer of zinc oxide onto a substrate

Abstract: Chromium acetyl acetonate is added to a zinc acetate-containing reactant mixture, to improve the chemical durability of a zinc oxide layer deposited on a glass substrate by the spray pyrolysis process.

Description: This invention relates generally to improving the chemical durability of zinc oxide films deposited onto substrates. Specifically the chemical durability of a zinc oxide layer deposited by spraying a solution of pyrolyzable zinc acetate onto a hot glass ribbon is improved by adding a quantity of chromium acetyl acetonate to the pyrolyzable solution. Zinc oxide is a well known component of coatings for automotive and architectural glazings. Such coatings are useful for providing enhanced performance characteristics, e.g., anti-reflectivity, electrical resistivity, infrared energy rejection, etc. A zinc oxide layer may conveniently be deposited onto a transparent substrate, either singularly or in combination with other metal and dielectric layers, by conventional methods such as, for example, spray pyrolysis, sputtering, vacuum evaporation, or chemical vapour deposition. A zinc oxide layer which is particularly useful for preparing high performance automotive and architectural glazings is formed by the spray pyrolysis process, in which a reactant mixture containing a soluble zinc salt is sprayed onto a hot glass ribbon being produced by the float glass process. The zinc-containing compound pyrolyzes at the surface of the glass and, in the presence of oxygen, produces a layer of zinc oxide thereon. Several factors are important in determining the thickness, uniformity, and chemical durability of the resultant zinc oxide layer. The temperature of the substrate must be high enough to effect the pyrolysis reaction, generally at least about 482°C (900°F) for the deposition of zinc oxide. The reactant mixture may be diluted with organic solvents or water for ease of control. However, the degree of dilution is limited by the need to attain a useful film deposition rate, and also by the chemistry of the pyrolysis reaction itself since the solubility limit must not be approached under the solution in-flight conditions. The spray pattern and velocity distribution of the droplets, as well as the turbulence and lateral vapour movement at the surface of the substrate, likewise affect the quality of the deposited layer. Finally, the zinc oxide layer, which otherwise would readily dissolve in acid and alkaline solutions, can be made more durable by codepositing other metal oxides. A particularly useful and well-known zinc-containing pyrolytic precursor is zinc acetate. See, for example, Major et al., "Highly Transparent and Conducting Indium-Doped Zinc Oxide Films by Spray Pyrolysis," Thin Solid Films, 108 (1983) 333-340. Spray pyrolysis utilising zinc acetate results in a high quality film having a uniform appearance, high optical transmittance, and good electrical resistivity. Zinc oxide layers produced utilising zinc acetate comprise columnar crystallites with average diameters of about one Angstrom and heights equal to the film thickness. U.S. Patent No. 3,850,665 to Plumat et al. discloses a process for forming a metal oxide coating on a substrate, wherein an acetyl acetonate coprecipitate of two or more metals such as, for example, zinc and chromium is dissolved in a solvent and sprayed onto a hot glass ribbon. The resultant coating is a metal oxide containing zinc chromite (ZnCr₂O₄). The patent does not teach the addition of an acetyl acetonate to a zinc acetate-containing solution for spray pyrolysis, for increasing the chemical durability of a zinc oxide layer. U.S. Patent No. 4,292,347 to Donley discloses a pyrolytic coating reactant for producing metal oxide films, comprising a metal carboxylate such as, for example, zinc 2-ethyl hexonate and a diketonate such as, for example, chromium acetyl acetonate. The patent states that the combination of these pyrolytic coating reactants improves the durability of the produced coating. Finally, U.S. Patent No. 4,239,816 to Breininger et al. discloses that a mixture of iron, chromium, and cobaltous acetyl acetonates, when pyrolytically applied to a glass surface, results in a more durable metal oxide than if any of the acetyl acetonates were to be pyrolytically deposited alone. The patent does not disclose the use of acetyl acetonates in combination with zinc acetate, to produce a layer of zinc oxide. In summary, the prior art does not suggest that spray solution adjuvants may be useful to increase the chemical durability for zinc oxide layers deposited by a spray pyrolysis process utilising zinc acetate as the zinc-containing pyrolysis reactant mixture precursor. It would be desirable to increase the chemical durability of a zinc oxide layer deposited onto a hot glass ribbon utilising a spray pyrolysis process, wherein zinc acetate is the zinc-containing pyrolytic precursor. Zinc oxide layers produced utilizing zinc acetate as the pyrolytic precursor have superior optical and physical properties, but suffer poor chemical durability when exposed to acid and alkaline solutions. Accordant with the present invention, a process for depositing a layer of zinc oxide onto a glass substrate, which process provides a zinc oxide layer having improved chemical durability, has surprisingly been discovered. The novel process is practiced by first preparing a pyrolysis reactant mixture comprising zinc acetate, solvent, and a zinc oxide chemical durability improving quantity of chromium acetyl acetonate. The glass substrate is then heated to a temperature sufficient to thermally decompose the reactant mixture. Finally, the reactant mixture is sprayed onto a surface of the glass substrate, to form a layer of zinc oxide on the surface thereof. It has been determined that the addition of about 0.1% to about 10% by weight, preferably about 1% to about 3% by weight, of chromium acetyl acetonate is effective to increase the chemical durability of a zinc oxide layer deposited by a spray pyrolysis process utilising a zinc acetate-containing reactant mixture. The process of the present invention is particularly suitable for preparing automotive or architectural glazings having one or more layers of zinc oxide incorporated into a high performance coating. The chemical durability of a zinc oxide layer deposited onto a hot glass ribbon by a spray pyrolysis process, wherein zinc acetate is utilised as the zinc-containing pyrolysis reactant, is improved by the addition of a quantity of chromium acetyl acetonate to the spray reactant mixture. The glass upon which the zinc oxide layer is deposited may be in the form of individual glass sheets, or in the form of a continuous glass ribbon produced by the well-known float glass process. Suitable glass substrates for use according to the present invention may include any of the conventional glass compositions known in the art as useful for preparing automotive or architectural glazings. The various chemical compositions which produce different glass substrates, e.g., borosilicate glass or soda-lime-silica glass, generally do not structurally nor chemically affect the deposited layer of zinc oxide. A preferred glass is commonly known in the art as soda-lime-silica glass, and may be of any thickness generally known as useful for providing support for a layer of zinc oxide. Where the glass to be coated with zinc oxide is being produced by the float glass process, the temperature of the glass is already sufficiently high to pyrolyze the coating reactants. Pyrolysis of the zinc acetate-containing reactant mixture generally is carried out at a temperature from about 482°C (900°F) to about 649°C (1,200°F), in the presence of an oxygen-containing atmosphere. Individual glass sheets which are to receive a coating of zinc oxide must be heated to approximately the specified temperature range to effect the pyrolysis reaction. The reactant mixture according to the present invention comprises zinc acetate, a quantity of chromium acetyl acetonate, and a solvent. The chromium acetyl acetonate is present in an amount effective to improve the chemical durability of the zinc oxide layer produced by spraying the reactant mixture against the hot surface of the glass. Zinc acetate pyrolyzes at the surface of the glass to form a layer of zinc oxide containing a small quantity of chromium dioxide commensurate with the amount of chromium acetyl acetonate contained in the reactant mixture. Zinc acetate is a well-known commercially available chemical reagent, generally produced by the reaction between acetic acid and zinc oxide. Chromium acetyl acetonate conveniently may be prepared by reacting together chromium chloride, acetylacetone, and sodium carbonate. Chromium acetyl acetonate precipitates as a metal chelate with the carbonyl oxygens of the organic portion of the molecule. Details concerning the preparation of acetyl acetonates are more fully set forth in Morgan and Moss, "Journal of the American Chemical Society," v. 105 (1914) 189-201. Solvents useful for preparing the reactant mixture of the present invention include a wide variety of organic materials such as, for example, dimethylformamide, benzene, toluene, xylene, hexane, heptane, methanol, ethanol, methylene chloride, perchloroethylene, chloroform, carbon tetrachloride, dimethylacetamide, acetonitrile, nitrobenzene, acetic acid, ethylene diamine, and the like, as well as mixtures thereof. A preferred solvent is dimethylformamide. The ingredients may be admixed in any conventional mixing device, and in any order. Generally, the solvent comprises from about 60% to about 95% by weight of the mixture. Preferably, the solvent is from about 80% to about 90% by weight of the mixture. The chromium acetyl acetonate is present in an amount which will improve the chemical durability of the zinc oxide layer formed when the reactant mixture is sprayed against a glass surface maintained at a pyrolyzing temperature. Generally, the chromium acetyl acetonate is from about 0.1% to about 10% by weight of the reactant mixture. Preferably, the chromium acetyl acetonate comprises from about 1% to about 3% by weight of the reactant mixture. The resultant zinc oxide layer may be deposited so as to form a layer having virtually any thickness, depending upon reaction time, spray conditions, glass temperature, etc. Generally, film thicknesses from about 50 Angstroms to about 5,000 Angstroms have been found to be useful for various automotive and architectural glazing applications, depending on the intended function of the zinc oxide coating. Other reagents may optionally be added to the reactant mixture, to alter the properties of the resultant zinc oxide layer. For example, a small amount of a fluorine-containing compound may be added to the reactant mixture to increase the electrical conductivity or the infrared radiation reflectivity of the formed zinc oxide layer. Suitable fluorine-containing compounds according to the present invention are, for example, trifluoroacetic acid, 1,1-difluoroethane, trifluoroethanol, methyl trifluoroacetate, ammonium fluoride, and the like. The inventive process is not only useful for forming a zinc oxide layer directly on a glass surface, but also for forming a zinc oxide layer on an already formed film which is strongly adherent to the glass surface. Thus, the present invention may be used to apply a zinc oxide layer over a previously applied dielectric or metal layer. Accordingly, the term "glass substrate" as used herein is intended to include glass having one or more of the aforementioned dielectric or metal layers. Examples of dielectric layers upon which a zinc oxide layer may be deposited according to the present invention include, but are not necessarily limited to, ZnO, SnO₂, MgF₂, Al₂O₃, TiN, SiO₂, MgO, TiO₂, and the like, as well as mixtures thereof. Contemplated metal layers having the same operability and utility upon which a zinc oxide layer may be deposited include, but are not necessarily limited to, Ag, Au, Cu, Ti, Al, Sn, etc., as well as alloys thereof. It is well known, for example, to manufacture a high performance automotive or architectural glazing having infrared rejection and/or electrical resistance heatability properties, comprising a glass substrate having multiple coatings of silver and zinc oxide thereon. The process of the present invention is particularly useful for manufacturing coated glazings, wherein zinc oxide is the outer layer of the coating exposed to the atmosphere. Zinc oxide coatings are normally susceptible to chemical attack by acidic or alkaline solutions. However, the process for forming a zinc oxide layer utilising a zinc acetate-containing pyrolysis reactant mixture additionally containing chromium acetyl acetonate results in a film which resists attack by acidic and/or alkaline solutions, in contrast to zinc oxide layers produced by the pyrolysis of a reactant mixute containing zinc acetate alone. EXAMPLE Chromium acetyl acetonate, zinc acetate, and dimethylformamide are admixed in the quantities specified in Table 1. A comparison solution containing no chromium acetyl acetonate is also prepared. The resulting solutions are sprayed onto clear soda-lime-silica glass sheets maintained at a temperature of about 502°C (935°F). The zinc oxide layer chemical durabilities are determined by etching in a weakly alkaline solution (pH about 8.75) followed by x-ray fluorescence analysis. It is observed that the zinc oxide layers formed by the process of the present invention are etched at a rate about 25% less than the rate for zinc oxide layers formed utilising a zinc acetate-reactant mixture containing no chromium acetyl acetonate. Pyrolysis Spray Solutions Component Example Comparison Zinc Acetate (wt. %) 14.8 14.8 Chromium Acetyl Acetonate (wt. %) 1.5 -0- Dimethylformamide (wt. %) 83.7 85.2 This example may be repeated with similar success by substituting the generically or specifically described reactants and/or reaction conditions recited herein for those actually used in the preceding example.

Claims: A process for depositing a layer of zinc oxide onto a glass substrate, comprising the steps of: A) preparing a pyrolysis reactant mixture, comprising: i) zinc acetate; ii) solvent; and iii) chromium acetyl acetonate; B) heating the glass substrate to thermally decompose the reactant mixture; and C) spraying the reactant mixture onto a surface of the glass substrate, to form a layer of zinc oxide on the surface thereof. A process for depositing a layer of zinc oxide onto a glass substrate according to Claim 1, wherein the solvent comprises from about 80% to about 90% by weight of the reactant mixture. A process for depositing a layer of zinc oxide onto a glass substrate according to Claim 1 or 2, wherein the chromium acetyl acetonate comprises from 1% to 3% by weight of the reactant mixture. A process for depositing a layer of zinc oxide onto a glass substrate according to any one of claims 1 to 3, wherein the glass substrate is heated to a temperature from 482°C to 649°C (from 900°F to 1,200°F) A process for depositing a layer of zinc oxide onto a glass substrate according to any one of the preceding claims, , wherein the reactant mixture includes a fluorine-containing compound. A process for depositing a layer of zinc oxide onto a glass substrate according to Claim 1, wherein the chromium acetyl acetonate comprises from 0.1% to 10% by weight of the reactant mixture.

Inventors: PLATTS DENNIS R, PLATTS, DENNIS R.
Applicants: FORD FRANCE, FORD MOTOR CO, FORD WERKE AG, FORD FRANCE S. A., FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1991
Main classification: C03C17
IPC codes: C03C17