CHEMICAL VAPOR DEPOSITION OF BISMUTH OXIDE

Abstract: Novel organometallic coating compositions comprising organo bismuth compounds are disclosed, along with a method for chemical vapor deposition to form bismuth oxide films on the surface of a substrate such as glass.

Description: Background of the Invention The present invention relates generally to the art of infrared-reflecting coated glass products, and more particularly to non-iridescent, high transmittance, low emissivity, infrared-reflecting coated glass products. Transparent infrared-reflecting films such as tin oxide may be deposited on a substrate such as glass by a variety of methods, including the application of thermally decomposable compounds to a heated surface. Useful methods for forming transparent infrared-reflecting tin oxide films are taught in U.S.-A-3 107 177, 3 677 814 4,263,335. Tin oxide films are especially effective infrared reflectors at thicknesses of about 100 to 800 nm (1000 to 8000 Angstroms). However, at such thicknesses the films tend to display interference effects, i.e., multiple visible colors commonly referred to as iridescence. These interference effects render the coated glass aesthetically unacceptable for most architectural applications. Iridescence is not observed in thinner films; however, these films have insufficient infrared reflectance to be practically useful. Likewise, iridescence is not observed in thicker films; however, these films tend to be hazy and difficult to deposit uniformly. Therefore, various methods to mask interference effects have been developed. U S-A-3 710 074 discloses an electrically heated multiple glazed window unit having an electroconductive coating on an enclosed surface and a selective reflecting film having an absolute infrared reflectance of at least 0.7 to improve the heat insulating character of the unit and reduce the visible iridescence of the conductive film. U S-A-4 069 630 discloses a heat reflecting multiple glazed window comprising a colored, heat absorbing exterior glass sheet having a heat reflecting tin oxide film on its interior surface, and an interior glass sheet which may be either clear glass or colored. The tin oxide film typically has an interference color from first order red to fourth order red, the visual effect of which is attenuated by the colored heat absorbing glass. U S-A-4 187 336; 4 206 252 and 4,308,316 disclose transparent glass window structures comprising a glass sheet bearing a first coating of infrared reflective material, wherein the observance of iridescence resulting from the first coating is reduced by a second coating of particular refractive index and thickness providing at least two interfaces forming means to reflect and refract light to interfere with the observance of iridescence. U S-A-4 377 613 discloses transparent window structures comprising a glass sheet bearing a coating of infrared reflective material wherein the observance of iridescence is reduced by provision of a very thin coating system beneath the infrared reflective coating which reflects and refracts light to interfere with the observation of iridescence. U S-A-4 971 843 discloses an alternative method for masking the visible interference effects of an infrared reflecting film in a window unit, which involves masking the visible interference effects of an infrared reflecting film by means of a second film having a uniform reflectance in the visible wavelength range combined with a luminous reflectance which is significantly higher than that of the infrared reflecting film. To produce a high transmittance, low emissivity unit, the thickness of the infrared reflecting film is preferably chosen to correspond with the first minimum in the reflectance curve. In GB-A-2177722 a method is disclosed for manufacturing a bismuth-containing oxide single crystal, in which vapour of bismuth or alkylated bismuth and vapour of an organic compound of other metal ion components of a crystal are introduced into a heated reaction chamber and reacted with oxygen gas, so that the desired bismuth-containg oxide single crystal is deposited on a substrate prepared in the reaction chamber. A B₁₀SiO₂₀ single oxide crystal can be produced by the use of bismuth or alkylated bismuth and alkylated silicon or silicon alkoxide or by the use of bismuth or alkylated bismuth and alkylated germanium or germanium alkoxide. A (Y 1x B x )₃Fe₅O₁₂ single crystal can be produced by the use of bismuth or alkylated bismuth yttrium alkoxide and carbonyl iron. JP-A-62-67175 describes a method for forming a ferroelectric film onto a substrate by contacting a reaction mixture comprising alkyl lead or alkyl bismuth compound and alcoholates of Ti, Zr, Si, Ge and Nb with the heated substrate in oxidizing atmosphere. Due to their low air-stability the compounds are conducted by an inert carrier gas into a mixer and oxygen of the equivalent necessary for oxidation is added. It is accordingly the object of the present invention to provide a process less complicated. This object has been attained by a method of forming a bismuth oxide coating by chemical vapor deposition comprising contacting a surface of a substrate in an oxidizing atmosphere with an organo bismuth compound at a temperature sufficient to thermally react said organo bismuth compound to deposit a bismuth oxide film on said surface characterized in that the organo bismuth compound selected from arylbismuthines, aralkylbismuthines, alkoxyarylbismuthines, and alkenylbismuthines is combined with air as carrier gas, and said substrate is glass. Description of the Preferred Embodiments Infrared reflecting films exhibiting visible interference effects may be useful on a monolithic sheet. Preferred articles in accordance with the present invention are multiple glazed window units comprising at least two panes, preferably both glass. Conventional glass compositions are useful, especially typical soda-lime-silica glass produced on a float line. Heat absorbing tinted glasses may be employed; but for high transmittance applications, clear glass is preferred. Of the various infrared reflecting films which may be useful for solar energy control, tin oxide is preferred. Tin oxide films may be deposited on glass surfaces by a variety of methods such as pyrolytic deposition, powder coating, chemical vapor deposition and cathode sputtering. Suitable methods include pyrolysis of alkyltin fluorides as taught in U S-A-3 677 814 and 4 263 335; chemical vapor deposition as taught in U S-A-3,850,679; powder coating as taught in U S-A-4,325,988 and 4 344 986; and cathode sputtering as taught in U S-A-3 477 936 and 3 506 556. Preferred tin oxide infrared reflecting films have a resistivity less than about 50 ohms per square, more preferably in the range of 20 to 30 ohms per square, and a low emissivity, preferably less than 0.4. The thickness of the film is chosen to correspond with a minimum in the luminous reflectance curve. Preferably, the film thickness corresponds to the first minimum since this point represents the lowest visible reflectance obtainable for a tin oxide film. This point corresponds with the second order blue interference effect at a thickness of about 140 nm (1400 Angstroms). Coating process parameters are adjusted to yield the minimum resistivity for the given thickness to provide maximum infrared reflectance and minimum emissivity. If lower resistivity is desired for higher solar energy performance, a thicker infrared reflecting tin oxide film may be formed, preferably at a thickness near the second minimum in the luminous reflectance curve, most preferably at the thickness corresponding with the third order blue interference effect, about 275 nm (2750 Angstroms). In the preferred embodiment wherein the thickness of the tin oxide infrared reflecting film corresponds with the first minimum in the spectral reflectance curve, the film typically appears blue by interference effects, the visible reflectance is about 10 percent, and the resistivity is generally about 45 to 50 ohms per square. While tin oxide has achieved widespread use because of its durability as well as its infrared reflecting properties, in some optical applications a higher refractive index coating, i.e. a refractive index greater than 2.0, is desired. Two well known high refractive index coatings are zinc sulfide and titanium oxide. However, each has a serious drawback. Zinc sulfide is not sufficiently durable for many purposes, while titanium oxide has a characteristic absorption which produces a blue color. Bismuth oxide, which has a refractive index of 2.4, is colorless, but when produced by sputtering is not durable. Bismuth oxide produced by chemical vapor deposition in accordance with the present invention is durable and colorless, and has a high refractive index of 2.4. Preferred organobismuth compounds for the chemical vapor deposition of bismuth oxide in accordance with the present invention are aryl bismuthines, preferably triarylbismuthines, most preferably triphenylbismuthine. However, other organobismuthines, such as alkenyl, alkoxy, aralkyl and fluoro derivatives thereof, which have suitable thermal stability and pyrolyzability, may be used as well. Preferred organobismuth compounds include air and water stable, colorless, nontoxic and relatively odorless crystalline compounds. Most trialkenylbismuthines, while sensitive to air, are clear liquids with suitable vapor pressure and sufficient stability for chemical vapor deposition, as are alkoxyarylbismuthines which resemble the triarylbismuthines. The colorless liquid diphenylethylbismuthine may also be suitable. The present invention will be further understood from specific examples which follow. EXAMPLE I Coated glass samples are made using a bench scale kinetic coater, a modified cold-wall reactor. The glass samples are placed on a metal block which is heated by induction to approximately 510°C (950°F). An air carrier stream, with approximately 0.01 mole % of triphenylbismuthine, is passed over the sample at a flow rate of 10 cm/s for about 8 minutes. Bismuth oxide films approximately 15 nm (150 Angstroms) thick are deposited. EXAMPLE II A glass substrate is placed in a ceramic tube approximately one inch in diameter and eight inches long. A high temperature heating tape is placed around the area where the substrate resides. A Teflon stopper with a gas inlet line is placed on one end of the ceramic tube. A gas outlet, along with a thermocouple, is placed on the other end of the ceramic tube. The substrate is heated to about 649°C (1200°F). An air carrier stream, with about 0.1 mole % of triphenylbismuthine, is passed through the ceramic tube at a flow rate of about 5 cm/s. Five samples are produced using this method ranging in film thickness from 10 to 120 nm (100 to 1200 Angstroms).

Claims: A method of forming a bismuth oxide coating by chemical vapor deposition comprising contacting a surface of a substrate in an oxidizing atmosphere with an organo bismuth compound at a temperature sufficient to thermally react said organo bismuth compound to deposit a bismuth oxide film on said surface characterized in that the organo bismuth compound selected from arylbismuthines, aralkylbismuthines, alkoxyarylbismuthines, and alkenylbismuthines is combined with air as carrier gas, and said substrate is glass. A method according to claim 1, wherein said organo bismuth compound is selected from triarylbismuthines and trialkenylbismuthines. A method according to claim 2, wherein said triarylbismuthine is selected from triphenyl bismuthine and tri-2,4-xylyl bismuthine. A method according to claim 3, wherein said triaryl bismuth compound is triphenyl bismuthine. A method according to claim 4, wherein said substrate is contacted with triphenyl bismuthine at a temperature of at least 350°C. A method according to claim 5, wherein said substrate is contacted with triphenyl bismuthine at a temperature in the range of 500 to 650°C.

Inventors: BLOSS, KARL HEINRICH, NEUMAN, GEORGE ANDREW
Applicants: PPG INDUSTRIES, INC.
Application year: 1989
Main classification: C23C16
IPC codes: C03C17, C01G29, C23C16