GB2114965A

GB2114965A - Vanadium oxide coatings

Info

Publication number: GB2114965A
Application number: GB08302654A
Authority: GB
Inventors: Charles Bernard Greenberg; James Kevin Scanlon
Original assignee: PPG Industries Inc
Current assignee: PPG Industries Inc
Priority date: 1982-02-01
Filing date: 1983-02-01
Publication date: 1983-09-01
Also published as: SE8803445L; AU559271B2; AU546405B2; FR2520727A1; DE3303154C2; GB2114965B; DE3347918A1; GB8302654D0; ES519443A0; DE3303154A1; SE8300497D0; AU1085983A; IT1160718B; DE3347918C2; AU4481785A; SE462801B; IT8319371A0; SE461980B; ES8504643A1; SE8300497L

Abstract

A method is disclosed for the deposition of vanadium oxide films on glass substrates, as well as a thermochromic VO2 film for the variable transmittance of solar energy and a conductive V2O3 film, both formed by chemical vapour deposition employing liquid vanadium compounds. Doping of thermochromic vanadium oxide films to lower the transition temperature is also disclosed.

Description

SPECIFICATION Vanadium oxide coatings The present invention relates generally to the art of metal oxide coated glass, and more particularly to the art of vanadium oxide coated glass. The expression "vanadium oxide" refers to any vanadium oxide, including mixtures of oxidation states, unless otherwise indicated explicitly or by contact.

U.S. Patent No. 3,483,110 to Rozgonyi discloses a method of making thin films of VO2 that possess the essential metal-semiconductor phase transition exhibited by the single crystal form, and that do not suffer deterioration under repeated cycling through the transition. In one embodiment, the process involves the steps of sputtering a V205 cathode in an inert atmosphere in the presence of a desired substrate to produce an amorphous film of VOx, where x is greater than 1.5 but less than 2, and then either weakly oxidizing the film to VO2, or strongly oxidizing the film to V205 and then reducing the V205 to V203.

Alternatively, a vanadium cathode may be sputtered in an inert atmosphere in a similar manner to produce a polycrystalline vanadium film, which is first oxidized to V205 and then reduced to V2 0.

The present invention involves the chemical vapour deposition of vanadium oxide films from a liquid organovanadium compound such as, for example, vanadium propylate. Vanadium oxide films containing VO2 exhibit both electrical and optical switching at a nominal transition temperature of about 680C. Glass coated with a vanadium oxide film containing VO2 in accordance with the present invention is particularly useful for passive solar energy control since it has significantly lower infrared transmittance in the metallic phase compared with the infrared transmittance of the semiconducting phase.Vanadium oxide films containing V203 are electroconductive at ambient temperature, exhibiting a resistivity less than about 1 ,000 ohms per square at a film thickness with luminous transmittance of about 24 to 35 percent on six millimetre thick clear float glass.

Vanadium oxide films containing V205 may also be prepared by chemical vapour deposition in accordance with the present invention. These films may subsequently be reduced to form thermochromic films containing VO2.

The present invention also involves the deposition of thermochromic films containing VO2 which have a lower switching temperature range than the normal transition temperature of about 690C. The depressed switching temperature range results from doping the VO2 film with other materials such as, for example, niobium, molybdenum, iridium, tantalum or tungsten compounds. Glass coated with a vanadium oxide film in accordance with the present invention is particularly useful for passive solar energy control since it has significantly lower infrared transmittance in the conductive phase compared with the infrared transmittance of the semiconducting phase, and has a sufficiently low transition temperature range to be useful in a wide variety of climatic conditions.

Figure 1 illustrates the optical switching of a vanadium oxide (VO2) film formed directly by chemical vapour deposition.

Figure 2 illustrates the electroresistive switching which accompanies optical switching as illustrated in Figure 1.

Figure 3 illustrates the optical switching of a doped vanadium oxide (primarily VO2) film in accordance with the present invention by comparing the solar energy transmittance of a coated glass sample at ambient temperature with the transmittance of the sample heated above its transition temperature.

Figure 4 illustrates the transition temperature range of a doped vanadium oxide (primarily VO2) film in accordance with the present invention by showing the resistance as a function of temperature.

Numerous metal and/or metal oxide coatings are shown to be useful for solar energy control.

Such coatings typically reflect a high proportion of incident solar energy to minimize heat gain inside a structure, while allowing sufficient transmission of the visible portion of the spectrum for interior lighting. A particularly desirable architectural window for passive solar energy control would be a variable transmittance window that would minimize transmittance in the summer when the temperature is high and incident solar energy is greatest, but transmit solar energy when the temperature is low. Variabie transmittance in a glass window is achievable by photochromism, which involves darkening in response to solar ultraviolet radiation, typically employing silver halides. However, absorption by the glass of solar radiation over the full spectral range results in heating and bleaching which deteriorate the photochromic properties of the glass.The present invention achieves variable transmittance by means of a thermochromic response, the result of an optical switching when a vanadium oxide film is heated by absorbed solar energy.

Vanadium oxide (VO2) undergoes a phase transition from the monoclinic crystallographic class to the tetragonal at a nominal temperature of 680C. This phase transition is accompanied by a rapid switch in electrical resistivity from semiconducting behaviour to metallic, a resistivity change of about 103 to 1 05-fold in a single crystal. In addition to electroconductive switching, a vanadium oxide (VO2) film also undergoes a substantial optical switching in the infrared spectral region as shown in Figures 1 and 2, along with a small amount of switching in the spectral range of visible light.

Vanadium oxide films are prepared in accordance with the present invention by chemical vapour deposition from organovanadium compounds such as, for example, liquid vanadium i-propylate or vanadium n-propylate. To be useful as a thermochromic window for passive solar energy control, the vanadium oxide coating should provide large optical switching in the solar infrared spectral range, a temperature range for switching that correlates with the actual temperatures attained by a window exposed to solar radiation, and adequate switching properties at a film thickness thin enough to avoid iridescence. Preferably, film thicknesses range from about 100 to about 1,500 Angstroms. These properties may be provided by vanadium oxide films prepared in accordance with the present invention.

Thin films of vanadium oxide can be prepared on glass substrates by chemical vapour deposition using a variety of organovanadium compounds, preferably those which are in liquid form at standard temperature and pressure. Sodalime-silica float glass and borosilicate glass are useful as substrates. The glass substrates are preheated, typically to a temperature of at least about 3500C, for example, in a conventional tube furnace open at both ends for ingress and egress of the substrates. An air driven pusher arm may be employed to feed a substrate into and out of the heating zone and onto a conveyor belt which carries the substrate to a CVD (chemical vapour deposition) coating chamber located below an exhaust hood.The CVD coating chamber contains a vanadium compound, such as, for example, liquid vanadium i-propylate or vandium npropylate, which is heated to a sufficiently high temperature to vapourize the vanadium compound. The vanadium compound vapours are carried in a gas stream to the heated substrate, whereupon the vanadium compound pyrolyzes to form vanadium oxide.

In a preferred embodiment of the present invention, vanadium i-propylate is vapourized and carried in a stream of non-oxidizing gas such as, for example, nitrogen or forming gas to a heated glass substrate. A vanadium oxide coating is formed on the glass which then travels within a tunnel flushed with forming gas to an annealing furnace wherein the coated glass is cooled to ambient temperature. When VO2 is formed from vanadium i-propylate, the resultant vanadium oxide coated oxide coated glass is semiconducting at ambient temperatures with a solar infrared transmittance typically above 30 percent at wavelengths of from 0.8 to 2.2 microns, while above the transition temperature, nominally 680C, the VO2 containing film is characteristically conductive and has a total solar infrared transmittance less than about 1 5 percent.

To enhance the optical response to a vanadium oxide film, it may be useful to prime the glass surface prior to chemical vapor deposition of the vanadium oxide coating. Optimum priming may be obtained with a tin oxide coating, typically 700 to 800 Angstroms thick. The tin oxide primer coating is preferably prepared in pyrolytic deposition of an organotin compound. Silicon and titanium dioxide films are also useful as primers. The use of such primer films, especially SnO2, appears to enhance the crystallinity and formation of VO2 rather than other vanadium oxides, thereby resulting in a VO2 rich film which has very good optical switching properties.

The optical switching properties of the vanadium oxide coating are determined by scanning in transmittance mode with a Cary 14 spectrophotometer (comparable spectrophotometers now available from Varian Associates) across the spectral range of 0.8 to 2.2 microns. The vanadium oxide coated glass sample is held in an insulated holder with a beam pass opening. Two cylindrical 25 watt heaters in contact with the glass edges just outside the beam pass opening are used to heat the vanadium oxide coated glass sample through the switching temperature range. A spectral scan is run both before and after heating without moving the sample. Typical results are shown in Figures 1 and 2.

The temperature range of the optical switching is determined in a separate experiment, which also provides a measure of the thermoresistive switching. The flat-head probe of an Omega Amprobes Fastemp temperature measuring device (available from Omega Engineering, Inc., Stanford, Connecticut) is clipped flush onto a narrow strip of the vanadium oxide film surface. In close proximity on either side of the probe are alligator clips connected to an ohmmeter for measurement or resistance. The resistance is measured as a function of temperature as the coated sample is heated through the transition temperature range, a sample measurement is illustrated in Figure 3.

In general, it appears that of the 103 to 103- fold thermoresistive switching capability of vanadium oxide (VO2), a thermoresistive switching on the order of about two-fold is sufficient to provide optical switching of the required magnitude for passive solar energy control in the spectral range of 0.8 to 2.2 microns.

The temperature range for optical switching, around the nominal 680C known for relatively pure single crystals of vanadium oxide (VO2), is near the range of about 45 to 600C actually attained in windows in summertime southern exposure. Also, it appears that optical switching properties are attainable with vanadium oxide films sufficiently thin to avoid visible iridescence.

VO2 films having depressed switching temperatures may be prepared in accordance with the present invention by doping with a compound of a metal, the cation of which has a larger ionic radium than the ionic radius of vanadium, for example, niobium, molybdenum, iridium, tantalum or tungsten oxides. In one embodiment of the present invention, glass substrates are immersed in a solution comprising one part by volume of vanadium 1-propylate, three parts by volume of 2-propanol and about 2.5 to 4 grams per litre of WOCI4 at room temperature. Film formation proceeds by hydrolysis in ambient air.The coated glass is then heated, preferably in a reducing atmosphere, more preferably in an atmosphere of forming gas containing an aromatic hydrocarbon, to a sufficient temperature for a sufficient time to form a thermochromic VO2 film which switches over a temperature range above room temperature but below the characteristic 680C. The VO2 film is semiconducting at ambient temperatures, with total solar infrared transmittance as shown in Figure 3, while above the transition temperature range, the VO2 containing film is characteristically conductive and has a solar infrared transmittance less than about 10 percent as shown in Figure 4.

Higher magnitude thermoresistive switching may be obtained, for example, by utilizing liquid vanadium i-propylate carried in nitrogen gas to form highly oxidized vanadium oxide (V205) on a glass surface heated to at least about 3000C. The vanadium oxide coated glass travels within a tunnel flushed with air to an annealing furnace also flushed with air, wherein the coated glass is cooled to ambient temperature. The resultant vanadium oxide coating is primarily V2Os. To obtain the thermochromic VO2, the vanadium oxide coating is reduced in a reducing atmosphere, preferably forming gas containing a small proportion, e.g. less than 10 percent, of aromatic hydrocarbon, at a temperature of about 325 to about 4750C.The thermochromic VO2 film formed in this manner is semiconducting at ambient temperature with a solar infrared transmittance as illustrated in Figure 2, while above the transition temperature, the To, film is characteristically conductive with a total solar infrared transmittance less than about 10 percent.

The thermoresistive switching to about 1 ,000 fold as shown in Figure 3.

Conductive thin films of vanadium oxide containing V203 can be prepared for example, by chemical vapour deposition preferably utilizing vanadium n-propylate. Glass substrates are preheated, typically to a temperature of at least about 4500 C, for example in a conventional tube furnace open at both ends for ingress and egress of the substrate. Liquid vanadium n-propylate is vapourized and carried in a stream of nitrogen gas to the heated substrate, whereupon the organovanadium compound pyrolyzes to form vanadium oxide. The vanadium oxide coated glass travels through a tunnel flushed with forming gas to an annealing furnace also flushed with forming gas, wherein the coated glass is cooled to ambient temperature.The resulting conductive V203 film is characteristically grey in transmission, compared with yellow to brown for VO2, and typically has a resistance measuring about 200 to about 300 ohms per square. Preferred film thicknesses range from about 200 to about 1,500 Angstroms.

The present invention will be further understood from the descriptions of specific Examples which follow.

Example I A glass substrate is heated to a temperature of about 6350C and contacted with a solution comprising two parts by volume of dibutyltin diacetate and one part by volume of methanol. A tin oxide coating about 700 to 800 Angstroms thick is formed on the glass surface. The tin oxide primed glass passes through a chemical vapour deposition chamber containing liquid vanadium i- propylate which is heated to 1 27 or to vapourize the organovanadium compound. The organovanadium vapours are carried in nitrogen to the moving glass substrate which is at a temperature of about 5300C. The vanadium oxide film formed on the glass substrate is yellow by transmission in fluorescent lighting. The vanadium oxide exhibits a transition over the temperature range of about 55 to 750C.The electrical switching of this film is from about 13,000 ohms at ambient temperature to about 5,000 ohms above the transition temperature range. The accompanying optical switching is shown in Figure 1.

Example II Vanadium i-propylate is heated to 12700 in a chemical vapour deposition chamber. The organovanadium vapours are carried in nitrogen to a moving glass substrate preheated to a temperature of about 400"C to deposit a vanadium oxide film on the unprimed glass substrate. The coated glass is cooled to ambient temperature in air, resulting in a vanadium oxide composition comprising V2O5. The V205 film is reduced to thermochromic VO2 by heating at a temperature of about 450 to about 4630C for about 23 minutes in a forming gas atmosphere containing an aromatic hydrocarbon (obtained by heating at 1 600C a bath of Califlux TT, a process oil available from Witco Chemical Corp., Los Angeles, California).The VO2 film exhibits an electrical switching from greater than 105 ohms at ambient temperature to about 500 to about 800 ohms above 680C, with an optical switching to less than 10 percent transmittance of solar radiation between 0.8 and 2.2 microns, as shown in Figure 2.

Example Ill Vanadium n-propylate is heated to 12700 to form vapours which are carried in nitrogen to a moving glass substrate preheated to a temperature of about 5300C. A conductive vanadium oxide (V203) film is formed on the glass substrate. The coated glass is cooled in a forming gas atmosphere. The vanadium oxide film is grey by transmission in fluorescent lighting and exhibits a resistance at ambient temperature of about 1 50 to about 190 ohms per square at a thickness which allows luminous transmittance of about 24 percent.

Example IV A clear sheet of float glass is immersed in a solution comprising one part by volume of vanadium i-propylate, three parts by volume of 2propanol and about 4 grams per litre of WOCI4 at ambient temperature. The glass is withdrawn and hydrolysis proceeds in the ambient air to form a clear film, yellow by transmission. The coated glass is then heated to 3750C in a forming gas. For a period of one minute, the coated glass is exposed to hydrocarbon vapours carried in a stream of forming gas. The hydrocarbon vapours are obtained by heating at 1600C a bath of Califlux TT (a process oil available from Whitco Chemical Corp., Los Angeles, California). The resultant vanadium oxide (VO2) coated glass exhibits optical switching as shown in Figure 3, and has a transition temperature as shown in Figure 4.

The above Examples are offered to illustrate the present invention. Various other substrates, such as for example, borosilicate glass, may also be used in the production of vanadium oxide films in accordance with the present invention. Other organovanadium compounds, such as, for example, vanadium ethylate, or butylate or vanadium, may be used as an inorganic vanadium compound, e.g. vanadium oxychloride, VOCI3.

Post deposition reduction of V205 may be avoided by incorporating a reducing agent, such as, for example, an aromatic hydrocarbon, in the atmosphere of the chamber during deposition.

Other dopants such as, for example, niobium, molybdenum, iridium or tantalum compounds, as well as other tungsten compounds, may also be used. The chemical vapour deposition method of the present invention is particularly useful for coating a moving ribbon of glass, such as, for example, in a continuous float glass process.

Useful methods and apparatus for chemical vapour deposition are described in U.S. Patents Nos. 3,850,679 and 3,951,100, the disclosures of which are incorporated herein by reference.

Various atmospheres may be utilized; oxidizing atmospheres such as for example, air, inert atmospheres such as, for example, nitrogen or argon, and reducing atmospheres such as for example, forming gas or other mixtures of inert gas and reducing agents. Other coating methods, such as for example, vacuum deposition, may be employed. The thermochromic VO2 films are preferably employed in multiple glazed window units for solar energy control by variable transmittance of infrared radiation. A preferred multiple glazed unit configuration is described in U.S. Patent No. 3,919,023, the disclosure of which is incorporated herein by reference. The scope of the present invention is defined by the following claims.

Claims

1. A method for the chemical vapour deposition of vanadium oxide films comprising the steps of: a. heating a glass substrate to a sufficient temperature to convert a vanadium compound to vanadium oxide; b. vapourizing a liquid vanadium compound; c. contacting a surface of the heated glass substrate with the vapour of the vanadium compound to deposit a vanadium oxide film on the glas surface.

2. A method according to claim 1 wherein the vanadium compound is vanadium i-propylate or vanadium n-propylate.

3. A method according to claim 1 or 2 wherein the glass substrate is coated with tin oxide, silicon, or titanium oxide as a primer layer prior to the chemical vapour deposition of the vanadium oxide.

4. A method according to claim 1,2 or 3, wherein the vapourized vanadium compound contacts the glass surface in a non-oxidizing atmosphere and a thermochromic film containing VO2 is formed.

5. A method according to claim 1,2 or 3 wherein the vanadium compound and the glass substrate are exposed to an oxidizing atmosphere, and a film containing V205 is formed.

6. A method according to claim 5 wherein the V2Os coated glass is further exposed to a reducing atmosphere to reduce the V2Os to VO2.

7. A method according to claim 1,2 or 3 wherein the vanadium oxide coated glass is exposed to a non-oxidizing atmosphere and an electrically conductive film containing V203 is formed.

8. A method according to any of claims 1 to 7 wherein the vanadium oxide film is deposited to a thickness of about 100 to about 1,500 Angstroms.

9. A glass substrate coated with vanadium oxide prepared according to a method as claimed in any of claims 1 to 8.

10. A method for depositing a vanadium oxide film comprising the steps of: a. heating a glass substrate to a temperature of at least about 35000; b. vapourizing vanadium i-propylate; c. conveying the vapourized vanadium 1propylate in a stream of non-oxidizing gas to the substrate; d. contacting a surface of the heated glass substrate with the vapourized vanadium i- propylate in a non-oxidizing atmosphere to deposit thereupon a vanadium oxide film; and e. cooling the vanadium oxide coated glass in a reducing gas atmosphere to obtain a thermochromic film containing VO2.

11. A glass substrate coated with vanadium oxide prepared according to the method claimed in claim 10.

12. A method for depositing a vanadium oxide film comprising the steps of: a. heating a glass substrate to a temperature of at least about 3000C; b. vapourizing vanadium i-propylate; c. conveying the vapourized vanadium i- propylate in a stream of inert gas to the substrate; d. contacting a surface of the substrate with the vapourized vanadium 1-propylate to form a vanadium oxide film; and e. cooling the vanadium oxide coated glass in air to obtain a film containing V2Os.

13. A method according to claim 12 wherein the V2Os is reduced to VO2 in a reducing atmosphere.

14. A method according to claim 1 3 wherein the reducing atmosphere comprises forming gas.

1 5. A method according to claim 14 wherein the reducing atmosphere further comprises an aromatic hydrocarbon.

1 6. A glass substrate coated with vanadium oxide prepared according to the method claimed in any of claims 12 to 15.

1 7. A method of making a conductive coated glass article comprising the steps of: a. heating a glass substrate to a temperature of at least about 40000; b. vapourizing vanadium n-propylate; c. conveying the vapourized vanadium npropylate to the substrate in a stream of nitrogen; d. contacting a surface of the heated glass substrate with the vapourized vanadium npropylate to deposit thereupon a film containing V2O3; and e. cooling the coated glass article in an atmosphere wherein oxidation state V203 is maintained.

1 8. A method according to claim 1 7 wherein the V203 coated glass is cooled in forming gas.

1 9. A method according to claim 17 or 18 wherein the film containing V2O3is deposited to a thickness of about 200 to about 1,500 Angstroms and has a resistivity less than 1,000 ohms per square.

20. A conductive coated glass article prepared according to a method as claimed in any of claims 17to 19.

21. An article for the variable transmittance of solar radiation comprising; a. a glass substrate; and b. a coating containing VO2 characterized by an optical switching within the temperature range of 25 to 750C whereby the total solar energy transmittance in the infrared region decreases by a factor of at least two when the coating is heated through its transition temperature range.

22. An article according to claim 21 wherein the coating containing vanadium oxide is from 100 to 1,500 Angstroms thick.

23. An article according to claim 21 or 22 wherein the total solar energy transmittance at ambient temperature in the spectral region from 0.8 to 2.2 microns to above 30 percent while the total solar energy transmittance of the article at a temperature above the transition temperature is less than 1 5 percent.

24. An article according to claim 23 wherein the VO2 coating is present on an interior surface of a glass substrate in a multiple glazed window unit configuration.

25. A method for making a thermochromic window comprising the steps of: a. heating a glass substrate to a sufficient temperature to convert vanadium n-propylate to vanadium oxide; b. vapourizing vanadium n-propylate; c. conveying the vapourized vanadium npropylate to the substrate in an atmosphere sufficiently oxidizing to form a thermochromic film comprising VO2 but insufficiently oxidizing to form a film which is not thermochromic as a result of the proportion of V205; d. contacting a surface of the heated glass substrate with the vapour of vanadium npropylate in the oxidizing atmosphere to deposit thereupon a film containing VO2.

26. A method according to claim 25 wherein the glass substrate is heated to a temperature of at least 4000C and the atmosphere comprises a mixture of inert gas and oxygen.

27. A method according to claim 25 or 26 wherein the glass substrate is heated to a temperature of at least 5000C and the atmosphere comprises a mixture of nitrogen and oxygen.

28. A method according to claim 27 wherein the atmosphere comprises 90 percent by volume nitrogen and 10 percent by volume oxygen.

29. A method according to any of claims 25 to 28 wherein the film containing VO2 is deposited to a thickness of about 100 to about 1,500 Angstroms.

30. A thermochromic window prepared according to a method as claimed in any of claims 25 to 29.

31. A method for making a thermochromic window comprising the steps of contacting a surface of a glass substrate with a vanadium compound and heating the glass substrate with a vanadium compound and heating the glass substrate to a sufficient temperature to obtain a vanadium oxide film comprising VO2, the film being doped with a compound which depresses the switching temperature of VO2.

32. A method according to claim 31 wherein the compound which depresses the switching temperature of VO2 is a compound of a metal the ionic radius of which is larger than the ionic radius of vanadium.

33. A method according to claim 31 or 32 wherein the compound which depresses the switching temperature of V02is a compound of niobium, tantalum, molybdenum, iridium or tungsten.

34. A method according to claim 31, 32 or 33 wherein the vanadium compound is vanadium npropylate, vanadium ethylate, vanadium butylate, vanadium oxychloride or a mixture thereof.

35. A method according to claim 34 wherein the glass is contacted at ambient temperature with a composition comprising vanadium i- propylate and tungsten oxychloride to form a film which is substantially heated po obtain a tungsten-doped thermochromic film comprising V02.

36. A thermochromic window prepared according to a method as claimed in any of claims 31 to 35.

37. A method of coating a substrate with vanadium oxide substantially as herein described

38. Substrates coated with vanadium oxide substantially as herein described.