GB2051878A - Heat-reflecting titanium dioxide coated glass panel. - Google Patents

Abstract

A heat reflecting pane is formed by vacuum deposition and subsequent oxidation of a layer of titanium on a glass substrate to form an inner anatase layer and an outer rutile layer. The evaporation of the titanium is commenced at a pressure above 1 x 10<-4> Torr, with an oxygen pressure of 1 x 10<-4> and is conducted at such a rate that the pressure is reduced to about 10<-5> Torr. Hence the first deposited portion of the layer is grown under conditions which in subsequent oxidation give an anatase structure, and the later deposited portion of the layer develops a rutile structure. Deposition of the titanium layer is continuous and the resulting anatase layer has a continuous density increase in the direction away from the substrate. A second layer of titanium may be deposited over the initial layer, under similar conditions, so that a second pair of anatase and rutile layers are formed on oxidation. The pane may also be subjected to thermal prestressing simultaneously with or subsequent to the oxidation heating.

Description

SPECIFICAtON A heat-reflecting TiO2 coated panel and a method of manufacture therefor This invention relates to a heat-reflecting panel and to a method of producing such a panel.

Heat-reflecting panels which are coated by evaporating a titanium layer in vacuo, which is then oxidised at elevated temperature in air to produce a heat-reflecting TiO2 film, are known and described e.g. in a publication by G. I glass entitled "Preparation, Properties and Optical Applications of Thin Films of Titanium Dioxide" (G. Hass, Vacuum, Vol. li, No.4, pages 331-345(1952)).

During the subsequent oxidation of the Ti film in air, two TiO2 structures are produced, depending on the conditions under which the Ti film is deposited by evaporation in vacuo. If the titanium is deposited in a good vacuum, i.e. 1 0' Torr or better, the futile structure develops, whereas if titanium is deposited relatively slowly in a lower vacuum, e.y. about 1 0M Torr, the anatase structure is produced. TiO2 films prepared in this manner have been used for various optical purposes in coating glass panes, e.g. as light dividers or light-reflecting coatings. To obtain maximum reflection, the film is given the thickness of a quarter-wavelength interference film, relative to that spectral region in which it is desired to modify the reflecting properties of the substrate.

A particular application of heat-reflecting panels of the aforementioned kind, is to use them in facade units or parapet slabs. It is desirable that the aforementioned parapet slabs should contain TiO2-coated glass panes having a high neutralcolour reflection in the visible spectral region, with a slight blue or green shade if required. In such parapet slabs, the TiO2 interference film is usually disposed at the outside of the building, the back of the glass pane being coated with opaque enamel or varnish, to screen those parts of the building behind the parapet slab.

TiO2 films having a rutile structure are very advantageous, particularly for the last-mentioned application, because rutile TiG2 films have a higher index af refraction than anatase films and thus may have a higher reflection factor, which is greatly desired in the case of facade components or parapet slabs. It has also been shown that rutile films are much harder and more wear-resistant than anatase films. Consequently panels having a rutile TiO2 interference film on the outside of the building may be exposed directly to the atmosphere without damage for a long period. In addition the panels or parapet slabs may be cleaned by the substances normally used for exterrial glass surfaces.

In various applications of panels coated with TiO2 in the initially-mentioned manner, more particularly for use as facade units or parapet slabs, it may be necessary to prestress the glass in order to conform to the safety regulations.

Prestressing is necessary when parapet slabs coated with rutile are enamelled on the back, because the enamel film is impervious to radiation and the glass may therefore become so heated by solar radiation that faults may occur if the glass substrate is not prestressed. The glass may be prestressed in known manner by heating it above the transformation temperature to the incipient softening temperature and then subjeeiting it to abrupt cooling. In the case of soda-lime silicate glass having the same chemical composition as conventional sheet glass, the heating temperatures required are about 570 to 6200 C.

Basically, there are two methods of prestressing this type of heat-reflecting panels during their manufacture. Firstly, the prestressing process may advantageously be combined with oxidation of the Ti coating deposited in vacuo.

Alternatively, the deposited Ti film may first be oxidised at an adequate temperature of, for example 400 to 5000 C, after which the glass pane coated with TiO2 csn be cooled arid then, in an additional step in another furnace, heated to the prestressing ternpei aturei.e. about 570 to 6200 C in the case of soda--lime silicate glass.

Theoretically, it is desirable to oxidize the Ti film to rutile TiO2 in the shortest time, and it is known (G. Hass, Vacuum, Volume II, No. 4, page 335, Fig.

3) that the oxidation speed is proportional to the oxidation temperature. If, however, in the known method, the oxidation temperature is increased to the value required for rapid o iidation, i.e. above about 5500 C, changes occur in the futile film; the films become opaque and cloudy1 and scatter so much transmitted and reflected light that the resulting panels are unsuitable For the aforementioned appIicetio.s such as parapet slabs or facade units. Surprisingl/, these changes occur only in rutile films. If the vacuum deposition conditions are changed, more particularly if the vacuum is lower and/or the deposition is slower, so that oxidation leads to anatase TiO2 films, the panels may be heated to elevated temperatures such as 5500 C or more without the aforementioned changes occurring. Of course, the aforementioned difficulties, i.e. the changes in rutile films always occur if the panes have to be thermally prestressed, since, as explained, the temperatures required are above abort 550 C, and preferably about 570 to 62.C0 C in the case of soda-lime silicate glass.These disadvantageous changes in the Tilm occur at the temperatures required for heating the panels for thermal prestressing, irrespective of whether the oxidation of the Ti films to TiO2 films and the heating to the prestressing tempereture occur in a single step or whether the Ti films are first oxidiscd at a relatively low temperature helow 5500 C and the panels subsequently heated, after further treatment is required, to the temperatures required for thermal prestressing.

In order to solve the problern oF producing heatreflecting panels of the afoiemftntioned kind, and to devise a method of producing then in which the TiG2 films have at least a predominantly rutile structure and i!l which there is effective elimination of the interfering changes in films on heating to temperatures above 550 C, as required for rapid oxidation of the Ti film and particularly required for thermal prestressing, Specification 1,534,122 proposes disposing an evaporated silicon oxide film, which does not cause interference, between the glass pane and the TiO2 film; the method according to Specification 1,534,122 suggests depositing a silicon oxide film which does not cause interference on the glass panel before the Ti film is deposited, and oxidising the thus-coated glass pane by heating it in air.

Alternatively, in Application 48384/78 (published as 2,029,861) teaches that the disadvantageous changes in the rutile TiO2 film on heating to temperatures above 5500 C, may be avoided if an intermediate film of anatase TiO2 is disposed between the glass pane and the rutile film. In that Application, the two films are produced as follows: A first Ti film is applied to the glass pane relatively slowly and in a relatively low vacuum, after which a second Ti film is applied relatively quickly at a relatively high vacuum, and subsequent oxidation by heating in air converts the first Ti film on the glass pane into the anatase intermediate film and the second Ti film into the rutile film.Other advantages of the heat-reflecting pane of the kind in question and the method of producing it of the kind in question, and its relation to the prior art, may be obtained from Application 48384/78, to which reference should be made to avoid repetition.

Basically, good results have been obtained by the heat-reflecting panel and method according to Application 48384/78 and the heat-reflecting panel and method according to Specification 1,534,122. In both cases however, particularly the latter minor but still disturbing problems occur and the resulting heat reflecting panels may have considerable, optically disturbing, deformation after the prestressing process. Apparently, even in the heat-reflecting panel of the kind in question, the stresses between the glass pane and the rutile film are so strong that considerable deformation of the pane may occur at the temperature required for thermal prestressing, when soda-lime silicate glass begins to soften. When the panes are prestressed, they deviate from flatness by about 3 to 5 mm per linear metre edge length.Deviations of this order are not usually acceptable for parapet slabs or facade units, which are nearly always required in large sizes, since the distortions in reflection, e.g. of buildings, are excessive. In this application, the maximum tolerated deviation from flatness is about 1 mm per linear metre edge length.

The invention, therefore, is directed towards improving the heat-reflecting panel and the method of the kind in question so as to improve the suitability of the resulting heat-reflecting panels, particularly for use as parapet slabs or facade units, by reducing the deviations from flatness occurring during rapid oxidation or prestressing.

Accordingly the invention provides a heatreflecting panel comprising a glass substrate which carries a film of anatase titanium dioxide and a film of rutile titanium dioxide and in which the film of anatase titanium dioxide is disposed intermediate the glass substrate and the rutile titanium dioxide and has a density which increases in the direction from the substrate.

The invention also provides a method of producing a heat reflecting panel, the method comprising depositing a layer of titanium by vacuum evaporation on to a glass substrate under conditions which will lead to formation of both an anatase structure and a rutile structure during subsequent oxidation, heating substrate and layer in air to oxidise the titanium layer to respective anatase and rutile films of titanium dioxide, the conditions for the deposition of the layer of the titanium being such that the subsequent oxidation produces the anatase film intermediate the substrate and the rutile film and the density of the anatase film increases in the direction away from the substrate.

In a preferred embodiment the entire Ti coating may be deposited continuously, using the same titanium-filled evaporators if required, and a nonuniform film structure is obtained by suitably controlling the vacuum conditions.

During production of a heat-reflecting panel the rate of oxygen supply may be maintained and the titanium evaporated at a speed such that the total pressure during the deposition of the titanium film falls to the 10- 5 Torr region, owing to the gettering effect during evaporation. The conditions are thus chosen so that the film begins to form under vacuum conditions which will result in the anatase structure film during the subsequent oxidation of the titanium film in air, and ends under vacuum conditions which subsequently gives rise to a rutile structure film.

The invention is based on the surprising discovery that, as in Application 48384/78, it is possible to avoid the disadvantageous changes in the rutile TiO2 film on heating to temperatures above 5500 C, if the conditions during the deposition of the titanium coating are altered so that the structure of the TiO film formed on oxidation in air, starting from the glass pane, changes from the anatase to the rutile structure. In the present invention the deposition conditions may be continuously altered so that the portions of the TiO2 film adjacent the glass pane have an anatase structure but a density which continuously increases from the glass pane, whereas the parts adjacent the side of the TiO2 film remote from the glass pane have a rutile structure.The main development from this, is that even the deviations from flatness occurring on heating to above 5500 C may be appreciably reduced, compared with heat-reflecting panels produced according to the teaching of Application 48384/78. For example, the density of the intermediate film, which is converted to the anatase structure by subsequent oxidation, the density increasing from the glass pane towards the rutile film, may be determined by corresponding measurements of the index of refraction and/or the hardness of the film. It is note-worthy that the continuous increase in density, of the anatase intermediate film may extend frorn the glass pane right into the rutile film and even over a considerable part of the total thickness of the rutile film.Glass panes having a non-uniform film structure of the last-mentioned kind should be understood to be embraced by the invention.

The non-uniform films, comprising the rutile film and the intermediate film continuously increasing in density in the aforementioned direction, have substantially the same surface hardness as films prepared according to the teaching of Specification 1,534,122 and Application 48384/78, provided the topmost layers of film, which have an exclusively rutile structure, are at least 80 A thick. The TiO2 film systems prepared according to the invention do not show any cracks if soda-lime silicate glass coated in the aforementioned manner is heated to temperatures above 5500 C, as required for rapidly oxidising the titanium film or films or for prestressing the glass.

An advantage of the method according to the present invention is that the TiO2 film is directly adjacent the glass surface, thus giving particularly good adhesion. Likewise the titanium film before oxidation adheres very firmly to glass, which simplifies manipulation of the Ti-coated panels before the Ti films are oxidised. Furthermore only one kind of evaporator is needed, and also titanium is available in wire form which is simpler to weigh out and load into the evaporator than the silicon monoxide coating material, as required by Specification 1,534,122 which is supplied in granular form.

The critical advantage, however, is the substantial reduction in deviations from flatness during prestressing of the panels. These deviations generally increase with the thickness of the TiO2 film. In the panels produced according to the invention, they are below a millimetre per linear metre of edge length in the case of films up to about 400 A thick. At a thickness above 400 A and up to about 600 A, which is the value of most interest for the present application, the deviations in some cases are greater than up to about 1.5 mm per linear metre of edge length.

In the last-mentioned case, i.e. when films about 600 A thick are required, a further improvement may be made by a modification of the deposition process constituting a special ambodiment of the invention. As before, the deviations from flatness can be reproducibly made less than 1 mm per linear metre of edge length.

The modification is as follows : Titanium is evaporated in two steps. After about half the material has been evaporated, the evaporation process is stopped, and when the gettering effect has stopped, the pressure returns to the initial value at the beginning of the first partial evaporation. When this value has been reached, the second partial coating system is evaporated in the corresponding manner. The resulting non uniform "double films" appear unexpectedly to have lower tension, relative to the glass pane, then a non-uniform single film according to the invention having the same total thickness. Thus, the deviations from flatness can be reproducibly kept below 1 mm per linear metre of edge length when the glass is prestressed, even in the case of relatively thick TiO2 films.

The skilled addressee could not have anticipated the improvement in flatness of heat reflecting panels produced according to the invention during the prestressing process. One possible explanation may be as follows. It is known that in the pressure range above about 1 x 10-4 Torr, the hardness and density of vapour deposited films decreases with increasing pressure in the coating chamber. It is thought that this is because the energy per atorn of evaporating material striking the substrate to be vacuum coated decreases as a result of the higher probability of collisions with the residual gas molecules, thus inhibiting short-range processes during film formation.The result, in the case of the procedure according to the invention, when the vacuum continuously improves during the deposition process, particularly in the case of the intermediate film having an anatase structure after oxidation of the aforementioned Ti coating, is a continuous transition from soft to harder regions of film, in the direction from the glass pane to the rutile film. This non-uniform film structure may be the reason why the tensions between the glass pane and the TiO2 film are considerably lower and thus why the deformation during prestressing is considerably less. This reduction in tension is probably the reason for another advantage of the heat-reflecting panels produced according to the invention compared with panels produced according to Specification 1,534,122 or Application 48384/78.It has been found that in the last-mentioned two cases the optical clouding of the TiO2 film when the panels are prestressed or when the titanium-coated panels are heated to a high temperature for rapid oxidation can be prevented only if the glass surface is carefully treated before coating and if fresh glass is used.

The last-mentioned requirement is particularly difficult in practice. When the method according to the invention is used, on the other hand, the sensitivity limit with regard to the purity and age of the glass panes is considerably higher, which results in considerable reduction in the cost of large-scale industrial manufacturing processes.

Presumably the decrease in tension between the TiO2 film and the glass pane, which is probably the critical factor in the improved quality of heatreflecting panels produced according to the invention during prestressing, similarly reduces the cloudiness, which is the result of cracks in the TiO2 film.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure 1 shows a sectional view of the structure of a first embodiment of a heat-reflecting panel according to the invention, and Figure 2 shows a sectional view of a second embodiment of a heat-reflecting panel according to the invention.

In Figure 1 a heat-reflecting panel comprising a glass pane 10, made for example of soda-lime silicate glass associated with a system of TiO2 films comprising an intermediate film 12 of anatase TiO2 adjacent pane 10 and a film 14 of rutile TiO2 on the side of the anatase TiO2 film 12 remote from pane 10. The density of the intermediate film 12, which can be determined e.g. by measuring the index of refraction and/or the hardness of the film, increases substantially cclntsinuously from pane 10 in the direction towards the rutile film 14.If required, the increase in density can continue in the rutile film. it can easily bn assumed that there is a "continuous" transition between the two films, in that the proportion of anatase crystallites in the transition region decreases continuously from the glass pane in the direction towards the rutile film 14, whereas the proportion of rutile crystallites increases, i.e.

the two modifications can exist side by side in the transition region. The important feature, however, is that an anatase film is always adjacent pane 10 and a purely rutile film is adjacent the side of the Ti coating remote from pane 10.

Figure 2 shows an embodiment differing from that in Figure 1 in that a "double-film system" is provided, i.e. a first intermediate film 1 2 and a first rutile film are followed by a second intermediate film 1 8 and a second rutile film 22 having a structure corresponding to films 12 and 14.

The heat-reflecting panel according to the embodiment shown in Figure 1 is preferably constructed by the method described hereinafter in example 1, whereas a heat-reflecting panel according to the embodiment shown in Figure 2 is obtained by the procedure in Example 2.

EXAMPLE 1 A float-glass pane 8 mm thick, of external dimensions 210 x 140 cm, was first cleaned in conventional manner by a flow discharge at a pressure of 3 x 10 2Torr in a vacuum deposition device. The pressure was reduced to 5 x 10 Torr by further pumping. Oxygen was then admitted through a needle valve until the pressure, in equilibrium with the inflowing oxygen had stabilized at 3 x 10-4 Torr. Deposition of the titanium film then began, the film being vapour deposited on the float-glass pane at the rate of 4 A per second. During the deposition process the pressure in the evaporator fell continuously to 4 x 10-5 Torr. The coated panel was then horizontally suspended in air from one of its two long edges in order to oxidise the film and prestress the glass.It was heated to 615" C in a prestressing oven and then cooled and thus thermally prestressed by cold air in a blast chamber. The resulting TiO2 film had a thickness of 320 A and, after the panels had cooled, was completely non-opaque. The panel was slightly biuish on the coated side. The reflectivity of the resulting panel, relative to the sensitivity of the human eye to brightness, was 32.5%. The deviation of the panels from flatness along all edges and also along the diagonals was less than 1 mm per linear metre edge length or linear metre of diagonals.

in a comparative test under otherwise similar conditions, Si2O3 film 130 A thick was first deposited at a pressure of 1.3 x 10-4 Torr by reactive evaporation of silicon monoxide by the method disclosed in Specification 1,534,122, after which a titanium film having the same thickness as in the example according to the invention was vapour-deposited at a pressure of 4 x 10-5 Torr. The panel was then heated in the same manner in the prestressing oven and then chilled.

The resulting panel, produced for comparison by the prior art, was slightly bluish on the coated side. The visual reflectivity was 34%. The rutile film was completely free from opacity, but the deviations from flatness was 3 mm per linear metre along a short edge and likewise 3 mm per linear metre along one of the two diagonals.

EXAMPLE 2 A float-glass pane having the same thickness and external dimensions as in Example 1 was used. After its surface had been cleaned by a glow discharge, the pressure in the evaporator was, as before, first reduced to 5 x 10-5 Torr and then adjusted to 3 x 10-4 Torr by admitting oxygen.

Next, in contrast to Example 1, a first titanium film was vapour-deposited, whereupon the pressure dropped continuously to 5 x 10--5 Torr as a result of the gettering effect of the evaporating titanium.

The rate of deposition was 5 A per second. After the gettering effect has disappeared and the pressure had returned to 3 x 10-9 Torr, a second titanium film having the same thickness was deposited in similar manner.

After being oxidised and prestressed as in Example 1, the panel had a silvery appearance on the coated side, with a visual reflectivity of 37% The TiO2film, which was 520 A thick, was completely free from opacity. The deviations of the panel from flatness were not more than 1 mm per linear metre at the edges and along the diagonals.

By contrast, a panel prepared in a comparative test under otherwise similar conditions to Example 1 , with an Si203 intermediate film and a 520 A thick rutile film, deviated from flatness by 3 mm per linear metre along one of the two short edges and 4.5 mm along one of the two diagonals. The visual reflectivity was 41%.

Another panel was produced in a comparative test and coated with a two-film system, likewise 520 A thick, consisting of a rutile film and an anatase intermediate film as taught in Application 48384/78. The deviations from flatness per linear metre were 2.0 mm along one of the two short edges and 3.0 mm along one of the two diagonals.

The visual reflectivity was 40%.

The preceding Examples show that, in the case of the method according to the invention where the TiO2 film has a "non-uniform" structure, the visual reflectivity is only slightly below the value obtained with a pure rutile film having the same thickness. The proportion of rutile in the film (rutile has a higher refractive index than anatase) can consequently be made fairly high, by suitably choosing the coating conditions, without cracking or disturbing optical deformation during the prestressing process. For example, tests on the aforementioned films showed that the ratio of rutile to anatase was about 3 1. Of course these tests on non-uniform films of the aforementioned kind can determine the two phases only approximately. However, the high reflection factors of the non-uniform films in conjunction with the aforementioned tests, show that, in the case of a film system according to the invention having a non-uniform structure, the proportion of rutile can be appreciably higher than the proportion of anatase without causing critical stresses between the film and the glass pane, sufficient to cause cracks and/or disturbingly reduce the flatness during the prestressing process.

Claims (23)

1. A heat reflecting panel comprising a glass substrate which carries a film of anatase titanium dioxide and a film of rutile titanium dioxide and in which the film of anatase titanium dioxide is disposed intermediate the glass substrate and the rutile titanium dioxide and has a density which increases in the direction from the substrate.

2. A heat reflecting panel according to claim 1, in which a further film of rutile titanium oxide and a further film of anatase titanium dioxide are disposed on the side of said film of titanium dioxide remote from the substrate, the further film of anatase titanium dioxide being disposed between the films of rutile titanium dioxide and having a density which increases in the direction from said film of rutile titanium dioxide to said further film of rutile titanium dioxide.

3. A heat reflecting panel according to claim 1 or claim 2 in which the density of at least one of the films of anatase titanium dioxide varies substantially continuously, and the films of titanium dioxide are formed by vacuum deposition and subsequent oxidation of a layer of titanium.

4. A heat-reflecting pane according to any one of the preceding claims in which claim in that the total thickness of the TiO2 coating is of the order of 30 to about 60 mm.

5. A heat-reflecting pane according to any one of the preceding claims, in which the rutile film, or at least the rutile film furthest from the glass pane, has a thickness of at least 80 A.

6. A heat-reflecting panel according to any one of the preceding claims, in which the glass substrate is thermally prestressed by heating to a temperature of at least 5500 C, and subsequent chilling.

7. A heat-reflecting panel according to any one of the preceding claims in which the glass substrate is thermally prestressed by heating to a temperature of between 570 and 6200 C, and then chilled.

8. A heat-reflecting panel according to any one of the preceding claims in which the glass substrate comprises soda-lime silicate glass.

9. A method of producing a heat-reflecting panel, the method comprising depositing a layer of titanium by vacuum evaporation on to a glass substrate under conditions which will lead to formation of both an anatase structure and a rutile structure during subsequent oxidation, heating substrate and layer in air to oxidise the titanium layer to respective anatase and rutile films of titanium dioxide, the conditions for the deposition of the layer of the titanium being such that the subsequent oxidation produces the anatase film intermediate the substrate and the rutile film and the density of the anatase film increases in the direction away frorn the substrate.

10. A method according to claim 9 in which the evaporation of the titanium is continuous and is commenced in an atmosphere at a total pressure greater than 1 x 10--4 Torr with an oxygen partial pressure of at least 1 x 1 0-4 Torr, and the titanium is evaporated sufficiently rapidly for the pressure to fall at least to the 10- Torr range.

11. A method according to claim 9 or claim 10 in which, before the beginning of reactive deposition by evaporation, the oxygen partial pressure is set at a value of at least 1 x 10-4 Torr by supplying oxygen through an inlet and the adjusted oxygen rate is maintained during the entire deposition process.

12. A method according to any one of claims 9 to 11 in which initially the total pressure is reduced to about 5 x 10-5 Torr and then oxygen is supplied such that the total pressure rises to about 3 x 10--4 Torr, after which the Ti coating is deposited at a rate of about 5 A/sec.

13. A method according to any of claims 9 to 12, in which a further layer of titanium is deposited after the initial layer and under similar conditions.

14. A method according to claim 13 in which the further layer of titanium is oxidised simultaneously with the initial layer.

15. A method according to any of claims 9 to 14, in which the substrate and titanium layer or layers are heated to a temperature of at least 5500 C to oxidise the Ti coating and/or for thermal prestressing in air by heating and subsequent chilling.

16. A method according to claim 15, in which the glass pane is thermally prestressed by chilling after heating to the oxidation temperature.

1 7. A method according to claim 1 5 in which the substrate and titanium layer or layers are first heated to a temperature adequate to oxidise the Ti, then further processed if required in a subsequent heating step.

18. A method according to any one of claims 9 to 1 7 in which the substrate and layers are heated to a temperature between 570 and 6200 C.

19. A method according to any one of claims 9 to 1 8 in which the glass substrate comprises soda-lime silicate glass.

20. A method substantially as hereinbefore described with reference to either of the accompanying drawings.

21. A heat-reflecting panel made according to the method of any of claims 9 to 20.

22. A heat-reflecting panel, substantially as hereinbefore described with reference to either of the accompanying drawings.

23. Any novel feature or combination of features described herein.