IE48228B1 - Process for manufacturing coated transparent substrates - Google Patents

Abstract

This invention relates to transparent glass window structures of the type bearing a first coating 6 of infra-red reflective material, which is advantageously less than about 0.85 microns in thickness, wherein the observance of iridescence resulting from such a first coating is markedly reduced by the provision of a layer 4 of continuously varying refractive index between the glass 2 and the coating 6, such that the refractive index increases continuously from the glass to the first coating, thereby preventing the observation of iridescence. The coatings are applied by passing the substrate through a reaction zone in parallel with a reactive gas mixture which contains components for forming the two coatings, wherein the reaction to form the first coating proceeds more rapidly. A particular advantage of the invention is its efficacy with clear and lightly tinted glasses wherein the problem of iridescent color has had its greatest commercial impact.

Description

The present invention relates to the coating of a transparent sheet or substrate with a film the properties of which film vary continuously through its thickness.

An important field of the present invention relates to 5 glass structures bearing a thin, functional, inorganic coating (e.g. a coating of tin oxide forming means to promote reflecti vity of infra-red light) which structures have improved appearance as a consequence of reduced iridescence historically associated with said thin coatings, and methods for achieving the aforesaid structures.

Glass and other transparent material can be coated with transparent semiconductor films such as tin oxide, indium oxide or cadmium stannate, in order to reflect infrared radiation. Such materials are useful in providing IS windows with enhanced insulating value (lower heat transport), e.g. for use in ovens, architectural windows, etc.. Coatings of these same materials also conduct electricity, and are employed as resistance heaters to heat windows in vehicles in order to remove fog or ice.

One objectionable feature of these coated windows is .-that they show interference colors (iridescence) in reflected light, and, to a lesser extent, in transmitted light. This iridescence has been a serious barrier to widespread use of these coated windows (see, for example, American Institute of Physics Conference Proceeding No. 25, New York, 1975, - 3 page 288).

In some circumstances, i.e. when the glass is quite dark in tone (say, having a light transmittance of less than about 25%) this iridescence is muted and can be tolerated. However, in most architectural wall and window applications, the iridescent effect normally associated with coatings of less than about 0.75 microns is aesthetically unacceptable to many people (see, for example, U.S. Patent 3,710,074 to Stewart).

Iridescent colors are quite a general phenomenon in transparent films in the thickness range of about 0.1 to 1 micron, especially at thicknesses below about 0.85 micron. Unfortunately, it is precisely this range of thickness which is of practical importance in most commercial applications. Semiconductor coatings thinner than about 0.1 micron do not show interference colors, but such thin coatings have a markedly inferior reflectance of infra-red light, and a markedly reduced capacity to conduct electricity.

Coatings thicker than about 1 micron also do not show visible iridesence in daylight illumination, but such thick coatings are much more expensive to make, since larger amounts of coating materials are required, and the time necessary to deposit the coating is correspondingly longer. Furthermore, films thicker than 1 micron have a tendency to show haze, which arises from light scattering from surface irregularities, which are larger on such films. Also, such films show a greater tendency to crack, under thermal stress, because of differential thermal expansion.

As a result of these technical and economical constraints, almost all present commercial production of such . 48 3 - 4 coated glass articles comprise films in the thickness range of about 0.1 to 0.3 microns, which display pronounced iridescent colors. Almost no architectural use of this coated glass is made at present, despite the fact that it would be cost5 effective in conserving energy to do so. For example, heat loss by infra-red radiation through the glass areas of a heated building can approximate about one-half of the heat loss through uncoated windows. The presence of iridescent colors on these coated glass products is a major reason for the failure to employ these coatings.

The present invention is concerned primarily, though not exclusively with a process for forming a gradient-type anti-iridescent layer.

Accordingly, from a first aspect, the present invention consists in a process for treating a transparent sheet, said process comprising the steps of flowing a gas mixture containing reactive components through a flow path defined by a reaction zone for said reactive components, said reaction cone being contiguous to and bounded by a surface of the 3'' transparent sheet to be coated, moving said surface through said reaction zone in a direction parallel to said flowing gas mixture, depositing, preferentially, a reaction product .err ti from more reactive components of said mixture on said surface of said sheet exposed earlier to said gas mix25 ture, thereby preferentially depleting said mixture of said more reactive components, and depositing preferentially, a reaction oroduct derived from less reactive components of said zixiire on said surface of said sheet exposed later to said depicted gas mixture, thus providing the sheet with a transparent film, the refractive index of which varies continuously through the thickness of said film, and depositing an upper coating zone on said film having a refractive index - 5 substantially the same as that of the most adjacent portion of said film.

From a second aspect, the invention consists in a process for treating a transparent glass sheet, said process comprising introducing into a first end of a reaction chamber and out the other end of said chamber a mixture of a first reaction gas from which a first transparent coating compound can be formed, a second reaction gas from which a second transparent coating compound can be formed, and a third gas which forms means to react with each of said reactant gases to form the first and second coating components, wherein said first reactant gas reacts at a substantially different rate with said third gas, than does said second reactant gas, the different rate of reaction with said third gas forming means to provide a difference in relative concentration of said reactant gases from one end of said chamber to the other and to provide different quantities of said coating compounds from one end to another, and continuously passing the transparent sheet to be coated through said reaction chamber from said first end of the chamber to said other end of the chamber, whereby said sheet is coated with a film of a transparent composition having a frequently changing refractive index as it moves through said chamber, said composition being formed by the deposition of said coating compounds, the changes in said composition being indicative of the relative reactivity and concentration of said reacting gases along said chamber, and depositing an upper coating zone on the film formed by said coating compositions having a refractive index substantially the same as that of the most adjacent portion of said film. - 6 Where the above defined process is used in the formation of a transparent film between the glass sheet and semiconductor film, the material may have variable refractive indices which lie between those of the glass and the semi5 conductor film. With a suitable choice of the changing composition of the transparent film, it has been discovered that the iridescent colors previously mentioned can be made too faint for most human observers to detect, and certainly too faint to interfere with widespread commercial use even in architectural applications.

In the preferred form of the invention, the deposited film forms a graded layer of material in which the refractive index varies, preferably in a smooth transition, as one moves through the film away from the glass toward the semi-conduc11 tor coating, from a value at the glass surface Hatching the index of the glass, to a refractive index value matching that of the overlying semi-conductor film, at a point proximate to that overlying film.

From a third aspect, the invention consists in a transit parent glass product substantially free of iridescent appearance, caving a glass substrate bearing a coating which is substantially uniform across the surface area thereof, said eating consisting of a lower coating zone comprising a material formed of at least two components which is charac21 terised by a gradual change from a first composition proximate to said substrate having a relatively high proportion of a first component to a second composition more remote from said substrate having a relatively large proportion of a second component, and an upper coating zone having a re11 tractive index substantially the same as that of said second component, whenever produced by the process of the invention. - 7 Apparatus used to carry out the process of the invention may comprise means for supporting said transparent sheet, for maintaining it at an elevated temperature and for moving it along a continuous processing path, a reaction zone having at one end thereof port means at a first station to introduce and distribute the gas mixture comprising reactive components which form the products that deposit on said sheet at different rates, a flow path forming the reaction zone wherein said mixture flows along said sheet surface, and port means at a second station to remove residual gas mixture, said second station being relatively positioned along said processing path with respect to said first station that said heated sheet forms means to provide sufficient energy to said gas mixture to achieve a substantial difference in composition between the gas mixture composition between said first station and said second station.

It is believed desirable, because of the subjective nature of color perception, to provide a discussion of the methods and assumptions which have been used to evaluate the invention disclosed herein. It should be realised that the application of much of the theory discussed below is retrospective in nature because the information necessarily is being provided in hindsight, i.e. by one having a knowledge of the invention disclosed herein.

In order to make a suitable quantitative evaluation of various possible constructions which suppress iridescent colors, the intensities of such colors were calculated using optical data and color perception data. In this discussion, the deposited film ia assumed to be in layers which are assumed to be planar, with uniform thickness and uniform refractive index within each layer. The refractive index - 8 changes are taken to be abrupt at the planar interfaces between adjacent film layers. A continuously varying refractive index may be modelled as a seguence of a very large number of very thin layers with closely spaced refrac5 tive indices. Real refractive indices are used, corresponding to negligible absorption losses within the layers. The reflection coefficients are evaluated for normally incident plane waves of unpolarized light.

Using the above assumptions, the amplitudes for re10 flection and transmission from each interface are calculated from Fresnel's formulae. Then these amplitudes are summed, taking into account the phase differences produced by propaga tion through the relevant layers. These results have been found to be equivalent to the Airy formulae (see, for example Optics of Thin Films, by F. Knittl, Wiley and Sons, New York, 1976) for multiple reflection and interference in thin films, when those formulae were applied to the same cases.

The calculated intensity of reflected light has been observed to vary with wavelength, and thus is enhanced in certain colors more than in others. To calculate the reflected color seen by an observer, it is desirable first to specify the spectral distribution of the incident light. or tne purpose, one may use the International Commission on Illumination Standard Illuminant C, which approximates normal daylight illumination. The spectral distribution of the reflected light is the product of the calculated reflection coefficient and the spectrum of illuminant C.

The color hue and color saturation as seen in reflection by a human observer, are then calculated from this reflected spectrum, using tbs uniform color scales such as those known to the art. One useful scale is that disclosed by Hunter in - 9 Food Technology, Vol. 21, pages 100—105, 1967. This scale has been UBed in deriving the relationship now to be disclosed.

The results of calculations, for each combination of refractive indices and thicknesses of the layers, are a pair of numbers, i.e. a and b, a represents red (if positive) or green (if negative) color hue, while b describes a yellow (if positive) or blue (if negative) hue. These color hue results are useful in checking the calculations against the observable colors of samples including those of the invention. A single number, c“, represents the color saturation: 2 1/2 c=(a +b ) . This color saturation index, c, is directly related to the ability of the eye to detect the troublesome iridescent color hues. When the saturation index is below a certain value, one is not able to see any color in the reflected light. The numerical value of this threshold saturation of observability depends on the particular uniform color scale used, and on the viewing conditions and level of illumination (see, for example, R. S. Hunter, The Measurement of Appearance, Wiley and Sons, New York, 1975, for a review of numerical color scales).

In order to establish a basis for comparison of structures a first series of calculations was carried out to simulate a single semi-conductor layer on glass. The refractive index of the semi-conductor layer was taken at 2.0, which is a value approximating tin oxide, indium oxide, or cadmium stannate films. The value 1.52 was used for the glass substrate; this is a value typical of commercial window glass. The calculated color saturation values are plotted in Figure 1 as a function of the semi-conductor film - 10 thickness. The color saturation is found to be high for reflections from films in the thickness range 0.1 to 0.5 microns. For films thicker than 0.5 micron, the color saturation decreases with increasing thickness. These re5 suits are in accord with qualitative observations of actual films. The pronounced oscillations are due to the varying sensitivity of the eye to different spectral wavelengths.

Each of the peaks corresponds to a particular color, as marked on the curve (R=Red, Y=Yellow, G=Green, B=Blue). lc Using these results, the minimum observable value of color saturation was established by the following experiment: Tin oxide films with continuously varying thickness, up to about 1.5 microns, were deposited on glass plates, by the oxidation of tetramethyltin vapor. The thickness profile was established by a temperature variation from about 450°C to 500°C across the glass surface. The thickness profile was then measured by observing the interference fringes under monochromatic light. When observed under diffused daylight, the films showed interference colors at the 2? correct positions shown in Figure 1. The portions of the films with thicknesses greater than 0.85 micron showed no observable interference colors in diffused daylight. The green peak calculated to lie at a thickness of 0.88 micron could not be seen. Therefore, the threshold of observability is about 8 of these color units. Likewise, the calculated blue peak at 0.03 micron could not be seen, so the threshold is above 11 color units, the calculated value for this peak. However, a faint red peak at 0.81 micron could be seen under good viewing conditions, e.g. using a black velvet back50 ground and no colored objects in the field of view being reflected, so the threshold is below the 13 color units - 11 calculated for this color. We conclude from these studies that the threshold for observation of reflected color is between 11 and 13 color units on this scale, and therefore we have adopted a value of 12 units to represent the threshold for observability of reflected color under daylight viewing conditions. In other words, a color saturation of more than 12 units appears as a visibly colored iridescence, while a color saturation of less than 12 units is seen as neutral.

It is believed that there will be little objection to commercialisation of products having color saturation values of 13 or below. However, it is much preferred that the value be 12 or below and, as will appear in more detail hereinafter, there appears to be no practical reason why the most advantageous products according to the invention, e.g. those characterised by wholly color-free surfaces, i.e. below about 8, cannot be made economically.

A value of 12 or less is indicative of a reflection which does not distort the color of a reflected image in an observable way. This threshold value of 12 units is taken to be a quantitative standard with which one can evaluate the success or failure of various multilayer designs, in suppressing the iridescence colors.

Coatings with a thickness of 0.85 micron or greater have color saturation values less than this threshold of 12. Experiments confirm that these thicker coatings do not show objectionable iridescence colors in daylight illumination.

In order that the present invention may be more readily understood reference will now be made by way of example to the accompanying drawings, invhich:4 8 2 2 8 - 12 Figure 1 is a graph illustrating the variation of calculated color intensity of various colors with semiconductor film thickness.

Figure 2 illustrates, schematically and in section, a nor.-iridescent coated glass substrate 5 constructed according to the invention, with an anti-iridescent interlayer 4 of contintausly-varying eanposition according to the invention. A tin oxide film 6 overlies layer 4 Figure 3 is a graph indicative of a typical gradient of refractive indices, idealised, and representing the gradual transition from 100% SiO2to Sn°2* Figure 4 is a section, somewhat simplified to facilitate the description thereof, of a novel apparatus for carrying out the process of the invention.

Figure 5 illustrates the experimental measurement of the gradient in chemical composition of a silica-tin oxide gradient zone prepared according to the invention.

Figure 6 shows an observed variation of the refractive index of the initial deposit of SiO.,—SnO., at the glass surface.. a; a function of gas composition. it has been discovered that a film intermediate atween the glass substrate and a semi-conductor layer can „e built up of a graded composition, e.g. gradually changing from a silica film to a tin oxide film. Such a film may be pictured as one comprising a vary large number of intermediate layers. Calculations have been made of reflected color saturation for a variety of refractive index profiles between class of refractive index n=1.52 and semi-conductor costings of refractive index n=2.0. For transition layers thicker than about 0.15 micron, the calculated color - 13 saturation index is usually below 12, i.e. neutral to the eye, and, for transitions, more than about 0.3 microns the color is always undetectable. The exact shape of the refractive index profile has very little effect on these results, provided only that the change is gradual through the graded layer.

A wide range of transparent materials are among those which can be selected to make products meeting the aforesaid criteria by forming anti-iridescent undercoat layers. Various metal oxides and nitrides,and their mixtures have the correct optical properties of transparency and refractive index.

Table A lists some mixtures which have the correct refractive index range between glass and a tin oxide or indium oxide film. The weight percents necessary can be taken from measured refractive index versus composition curves, or calculated from the usual Lorentz-Lorenz law for refractive indices of mixtures (Z. Knittl, Optics of Thin Filins, Wiley and Sons, New York, 1976, page 473), using measured refractive indices for the pure films. This mixing law generally gives sufficiently accurate interpolations for optical work, although the calculated refractive indices are sometimes slightly lower than the measured values. Film refractive indices also vary somewhat with deposition method and conditions employed.

Figure 3 gives a typical curve of refractive index versus composition for the important case of silicon dioxidetin dioxide mixtures.

Table A Some combinations of compounds yielding transparent mixtures whose refractive indices span the range from 1.5 - 14 48328 to 2.0.

SiO2 SnO2 Sio2Si3N4 Si°2 Ti°2 SiC) In„0_ 2 2 3 SiO„ Cd„SnO Other combinations of components suitable for use in the present invention include mixtures of silicon dioxide •with at least two of the group of compounds listed in the second column of Table A.

Films can be formed by simultaneous vacuum evaporation cf the appropriate materials of an appropriate mixture. For coating of large areas, such as window glass, chemical vapor deposition (CVD) at normal atmospheric pressure is more convenient and less expensive. However, the CVD method requires suitable volatile compounds for forming each material. Silicon dioxide can be deposited by CVD from gases such as silane, SiK., dimethylsilane (CH ) SiH„, etc.. Xn order to provide 4 2 2 2 the necessary gaseous reactants liquids which are sufficiently -.-oiacile at room temperature are a lacs t as convenient as gases: tetramethyltin is such a source for CVD of tin compounds, while (C^I^) ,,δίΗ^ and SiCl^ are volatile liquid sources for silicon bearing gaseous compounds.

A continuously graded layer of mixed silicon-tin oxide ray he built up during a continuous CVD coating process on a continuous ribbon of glass by the following novel procedure. A gas mixture is caused to flow in a direction parallel oo the glass flow, under (or over) the ribbon of het glass, as shown, for example, in Figure 4. The gas mixture contains an oxidizable silicon compound, an oxidiz48228 - 15 able tin compound, an oxygen or other oxidizing gas. The compounds are chosen so that the silicon compound is somewhat more quickly oxidized than is the tin compound, so that the oxide deposited on the glass where the gas mixture first strikes the hot glass surface, is mainly composed of silicon dioxide, with only a small percentage of tin oxide. The proportions of silicon and tin compounds in the vapor phase are adjusted so that this initially deposited material has a refractive index which closely matches that of the glass itself. Then, as the gas continues in contact with the glass surface, the proportion of tin oxide in the deposited film increases, until at the exhaust end of the deposition region, the silicon compound has been nearly completely depleted in the gas mixture, and the deposit formed there is nearly pure tin oxide. Since the glass is also continually advancing from the relatively silicon-rich (initial) deposition region to a relatively tin-rich (final) region, the glass receives a coating with a graded refractive index varying continuously through the coating thickness, starting at the glass surface with a value matching that of glass, and ending at its outer surface, with a value matching that of tin oxide. Subsequent deposition regions can then be used to build up further layers of pure tin oxide, or layers of tin oxide doped, for example with fluorine. These tin oxide layers may have a thickness from approximately 0.1 to 1.0 micron. The graded layer or coating may be in the range from 0.15 to 0.3 micron. Preferably, the total thickness of the graded and tin oxide layers is less than 1.0 micron. 8 2 2 8 - 16 A suitable gas mixture for this purpose, preferably includes the oxidizable silicon compounds, 1,1,2,2-tetramethy ldisi lane (HMe.jSiSiMe2H) ; 1,1,2-trimethyldisilane (H^MeSiSiMe^H), and/or 1,2-dimethyldisilane (H^MeSiSiMeHj) along with the organotin compound tetramethyltin (i-ie^Sn). It has been found that the initially deposited film is siliconrich, and has a refractive index close to that of glass, while the latter part of the deposit is almost pure tin oxide.

The Si-H bonds in the above-disclosed silicon corn12 pounds are highly useful in the process, since compounds without Si . H bonds, such as tetramethylsilane Me^Si, or hexamethyldisilane are oxidized more slowly than is tetramethyltin, and the initial deposit is mainly tin oxide, and the latter part of the deposit is mainly silicon dioxide. In such a case, i.e. when one is using compounds such as Me^Si, one may flow the gas and glass ir. opposite directions in order to achieve the desired gradation of refractive index, provided the gas flow is faster than the glass flow. However, the preferred embodi2? ment is to use the more easily oxidizable silicon compounds, and concurrent gas and glass flow directions.

It is also desirable, in forming coatings wherein ...e composition varies monotonieally '.-.ith distance from the substrate, that the silicon compounds have a Si-Si bond as well as a Si-H bond. For example, a compound containing Si-H out not SiSi bonds, dimethylsilane Me2SiH2, along with tetramethyltin, produces an initial deposit of nearly pure tin oxide, which then becomes silicon-rich at an intermediate time and finally becomes tin-rich still later in the deposi30 tion. Although Applicant does not wish to be bound by the - 17 theory, it is believed that the Si-Si-H arrangement facilitates rapid oxidation by an initial thermally induced decomposition in which the hydrogen migrates to the neighboring silicon HMe^Si-SiMe^H ~►Me^SiH^+Me^Si. The reactive dimethylsilylene Ms2Si species is then rapidly oxidized, releasing free radicals such as hydroxyl (OH), which then rapidly abstract hydrogen from the Si-H bonds, thus creating more reactive silylene radicals, forming a chain reaction. The tetramethyltin is less reactive to these radicals, and thus mainly enters into the later stages of the oxidation. The Me2SiH2 lacks the rapid initial decomposition step, and thus, cannot begin oxidation until after some tetramethyltin has decomposed to form radicals (CHy OH, 0 etc.) which then preferentially attack the Me^SiH^, at intermediate times, until the Me2SiH2 is consumed, after which stage the oxidation of tetramethyltin becomes dominant again.

It is preferred to have at least two methyl groups in the disilane compound, since the disilanes with one or no methyl substituents are spontaneously flammable in air, and thus must be pre-mixed with an inert gas such as nitrogen.

Other hydrocarbon radicals, such as ethyl, propyl, etc. may replace methyl in the above compounds, but the methyl ones are more volatile and are preferred.

Higher partially alkylated polysilanes, such as polyalkyl-substituted trisilanes or tetrasilanes, function in a way similar to the disilanes. However, the higher polysilanes are harder to synthesise, and less volatile than the disilanes, which are therefore preferred.

When the initial deposition or coating of the silica48228 - 18 tin oxide films contain less than about 40% of tin oxide at the interface with the glass substrate, and thus at least 60% silicon oxide, there will be little or no haze created at this interface. If, for some reason, it is desired to start the gradient above about 30% of tin oxide, it is preferable to have the glass coated, at the interface, with a haze-inhibiting layer, i.e. silicon dioxide. Such a haze-inhibiting layer may be very thin, e.g. in the nature of 25 to 100 angstroms. The gradient coating preferably has at least 95% tin oxide at the top thereof.

Figure 4 illustrates a section of a lehr in a float glass line. The structure of the lehr itself is not shown for purposes of clarity. The hot glass 10, e.g. about 500— 600°C, is carried on rollers 12, 14 and 16 through the lehr.

Between rollers 12 and 14 is positioned gas duct assembly 18 which comprises a gas inlet duct 20 and a gas outlet duct 22. Between ducts 22 and 20 and separated therefrom by heat exchanging wall members is a duct 25 forming means to carry a heat exchange fluid, ι-.'hich, in turn forms means lo to cool gas exhaust from duct 22 and to heat gas flowing through duct 20. The temperature of the heat exchange fluid is maintained at a sufficiently low temperature so that coating does not take place on the surface of the inlet duct.

Gas entering inlet 20 travels through a slit-like opening 28, thence along a reaction zone formed by the top surface 30 cf duct assembly 18 and the lower surface of glass sheet 10. Upon reaching a second slit-like opening 32, the remaining gas is exhausted through duct 22. It is during the passage of the gas along the lower surface of - 19 glass sheet 10 that a gradient coating is formed by the selective depletion of one of the reactants at different points along the length of the deposition zone between rollers 12 and 14. Means may be provided for moving the reaction zone horizontally and vertically to facilitate positioning of said reaction zone, and removal of said reaction zone from said lehr.

In the apparatus of Figure 4 a second gas duct assembly 38 is used to complete the deposition of a coating, e.g. by adding a fluoride-doped tin oxide coating to the pre-deposited gradient coating. Again, it is convenient to have gas enter the upstream port 28a, and leave the downstream port 32a,.

The ducting is suitably formed of corrosion resistant steel alloys and comprises a jacket 50 of thermal insulation.

In this application and accompanying drawings there is shown and described a preferred embodiment of the invention and suggested various alternatives and modifications thereof, but it is to be understood that these are not intended to be exhaustive and that other changes and modifications can be made within the scope of the invention. These suggestions herein are selected and included for purposes of illustration in order that others skilled in the art will more fully understand the invention and the principles thereof and will be able to modify it and embody it in a variety of forms, each as may be best suited in the condition of a particular case. - 20 Example 1 ο Glass heated to about 580 C is moved at a rate of 10 cm/sec across the apparatus shown in Figure 4. The temperature of the gas inlet duct is maintained at a temperature of about 300°C. by blowing appropriately heated or cooled air through the temperature control duct. The first deposition region reached by the glass is supplied with a gas mixture of the following composition (in mole percent): 1,1,2,2-tetramethyldisilane 0.7% tetramethyltin 1.4% bromotrifluoromethane 2.0% dry air balance The second deposition region is supplied with a gas mixture of the following composition (in mole percent): tetramethyltin 1.6% bromotrifluoromethane 3.0% dry air balance The flow rates of these gas mixtures are adjusted so that the average duration of contact between a given element of the gas mixture and the glass surface is about 0.2 seconds.

The resulting coated glass is color-neutral in appearance, in reflected daylight. It has a visible reflec25 tivity of 15%, and no visible haze. The infra-red relfectivity is 90% at a 10 micron wavelength. The electrical resitance is measured to be 5 ohms per square. The coating is about 0.5 microns thick. - 21 Example 2 The deposition described in Example 1 is repeated, the only difference being the composition of the gas mixture supplied to the first deposition region; 1,2-dimethyldisilane 0.4% 1.1.2- trimethyldisilane 0.3% 1.1.2.2- tetramethyldisilane about 0.02% tetramethyltin 1.5% bromotrifluoromethane 2.0% IO dry air balance The properties of the resulting product are indistinguishable from those of Example 1.

Samples of these coated glasses have been subject to Auger chemical analysis of the coating composition along with ion sputter-etching to reveal their chemical composition versus thickness. Figure 5 shows the resulting chemical composition profile of the deposit over the region in which it varies. Near the glass surface the deposit is mainly silicon dioxide, with about one silicon atom out of eight being replaced by tin. As the distance away from the glass surface increases, the tin concentration increases and the silicon concentration decreases, so that by distances greater than 0.18 micron from the glass surface, the deposit becomes tin oxide, with about 1.5 percent of the oxygen replaced by the fluorine. Using Figure 3, the silicon-tin composition profile is converted to a refractive index versus distance profile, which is also plotted in Figure 5. These results confirm the ability of the disclosed process to produce the desired variation of refractive index through the thickness of the deposited film. - 22 The iridescence visibility of different thicknesses of tin oxide coating placed on a glass substrate (the glass substrate is first coated with an ultra-thin film of silicon dioxide to provide an amorphous, haze-inhibiting surface) is shown in the following table.

Thickness of tin oxide Iridescence visibility LO 0. 3 micron 0.6 micron 0. 9 micron strong distinct, but weaker barely detectable except in 1.3 micron fluorescent light weak, even in fluorescent light The latter two materials are not aesthetically objects tionable for architectural use, confirming the visual color saturation scale used to evaluate the designs.

In order to provide the most effective suppression of iridescent color, it is desirable that the refractive index of the initial deposit match closely that of the glass subid strate, preferably to within + .04, or more preferably to within +.02 refractive index units. In order to achieve '?his match, one varies the parameters of the deposition, particularly the ratio of tin to silicon atoms in the inlet gas. As an example of such variation. Figure 6 shows the variation of refractive index in the initial deposit from tetramethyltin plus 1,1,2,2-tetramethyldisilane gas mixtures, as a function of gas composition. The other parameters for these depositions were fixed as in Example 1. Figure 6 shows, for example, that an initial deposit of refractive index 1.52 (appropriate to match usual window glass refractive indices) - 23 is produced by a gas composition of equal numbers of silicon and tin atoms. Matching to 1.52 +.02 is achieved when the gas composition is kept between 47 and 52 atomic per cent of tin. While these exact numbers may differ somewhat in other conditions of deposition such as other temperatures or other compounds, it is a matter of routine experimentation to establish calibration curves such as Figure 6, in order to produce a suitable match of refractive indices between the substrate and the initially deposited coating composition.

It is to be noted that the reflection of light from the surface of the coated products in the preceding table is about 16 to 17%, i.e. about 10% higher than that from the coated glass in Examples 1 and 2 which do have a graded undercoat according to the invention. - 24 15

Claims (15)

CLAIMS :1. A process for treating a transparent sheet, said process comprising the steps of flowing a gas mixture containing reactive components through a flow path defined hy a reaction zone for said reactive components, said reaction zone being contiguous to and bounded by a surface of the transparent sheet to be coated, moving said surface through said reaction zone in a direction parallel to said flowing gas mixture, depositing, preferentially, a reaction product derived from more reactive components of said mixture on said surface of said sheet exposed earlier to said gas mixture, thereby preferentially depleting said mixture of said more reactive components, and depositing preferentially, a reaction product derived from less reactive components of said mixture on said surface of said sheet exposed later to said depleted gas mixture, thus providing the sheet with a transparent film, the refractive index of which varies continuously through the thickness of said film, and depositing an upper coating or: said film having a refractive Index substantially the same as that of the most adjacent portion cf said film.

1. ,1,2-trimethyldisilane, 1,2 dimethyldisilane or mixtures thereof.

2. A process as claimed in claim 1, in which said transarer.t sheet is glass.

3. A process as claimed in claim 1 or 2, in which said If reaction products are produced by reaction of said gases induced by heat from said transparent sheet.

4. A process as claimed in any one of claims 1 to 3, in which said gas mixture includes oxidisable gaseous silicon and tin compounds and an oxidising gas. 30 5. A process as claimed in any one of claims 1 to 4, in - 25 which said gas mixture includes at least one partially alkylated polysilane, an organotin vapor, and an oxidising gas.

5. In the float glass line and beneath a glass sheet carried on said rolls. 31. Apparatus as claimed in any one of claims 26 to 30, including a second reaction zone mounted in series with the first reaction zone and forming means to provide an addi10 tional coating to said coating of pregressively-changing composition. 32. Apparatus as claimed in claim 26, and for forming a continuous coating of progressively changing composition on heated glass, constructed and adapted to operate substantially 5 oxide adjacent the interface between said glass substrate and said lower coating zone and wherein there is a hazeinhibiting layer of 25 to 100 angstroms of silicon dioxide at the interface of said glass substrate and said lower coating zone. 5 as it moves through said chamber, said composition being formed by the deposition of said coating compounds, the changes in said composition being indicative of the relative reactivity and concentration of said reactant gases along said chamber, and depositing an upper coating zone on the

6. A process as claimed in any one of claims 1 to 5, in which said gas mixture contains at least one methyldisilane and also tetramethyltin.

7. A process as claimed in any one of claims 1 to 6, in which said gas mixture contains 1,1,2,2 tetramethyldisilane, 8. And

8. A process as claimed in claim 4, in which the proportions of said reactive components are so selected to achieve a coating composition proximate to said sheet of at least 60% SiO^ and a coating composition most remote from said sheet of at least 95% tin oxide. 9. In which said reaction products are deposited at If such a rate that said change in the coating composition is monotonic resulting in a gradual increase of the refractive index of said coating as the thickness of said coating on sa-d substrate increases.

9. A process for treating a transparent glass sheet, said process comprising introducing into a first end of a reaction chamber and out the other end of said chamber a mixture of a first reaction gas from which a first transparent coating compound can be formed, a second reaction gas from which a second transparent coating compound can be formed, and a third gas which forms means to react with each of said reactant gases to form the first and second coating compounds, wherein said first reactant gas reacts at a substantially different rate with said third gas, than does said second reactant gas, the different rate of reaction with said third gas forming means to provide a difference in relative concentration of said reactant gases from one end of said chamber to the other and to provide different quantities of said coating compounds from one end to another, and continuously passing the transparent - 26 sheet to be coated through said reaction chamber from said first end of the chamber to said other end of the chamber, whereby said sheet is coated with a film of a transparent composition having a frequently changing refractive index 10. 22. A product as claimed in any one of claims 13 to 21, wherein the visible reflectivity of said product is about 15%, the color of said product is neutral , and said product is free of visible haze. 23. A product as claimed in claim 18 or 19, or any one of 15 claims 20 to 22, when dependent on claim 18 or 19, wherein the refractive index of the glass substrate is within +0.04 refractive index units of refractive index of the coating material of said lower zone. 24. A product as claimed in any one of claims 13 to 18, 20 wherein the thickness of said lower coating zone is between 0.15 micron and 0.30 micron. 25. A transparent glass product substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings. 25 26. Apparatus when used to carry out the formation of a film of continuously-varying refractive index as defined in the process of claim 1, comprising means for supporting said transparent sheet, for maintaining it at an elevated temperature and for moving it along a continuous processing path, a reaction zone having at one end thereof port means at a first station to - 29 introduce and distribute the gas mixture comprising reactive components which form the products that deposit on said sheet at different rates, a flow path forming the reaction zone wherein said mixture flows along said sheet surface, and port means at a second station to remove residual gas mixture, said second station being relatively positioned along said processing path with respect to said first station that said heated sheet forms means to provide sufficient energy to said gas mixture to achieve a substantial difference in composition between the gas mixture composition between said first station and said second station. 27. Apparatus as claimed in claim 26, in which said port means are mounted below said sheet, and said sheet forms the upper bondary of said processing path. 28. Apparatus as claimed in claim 26 or 27, in which said port means are slit-like openings arranged parallel to one another and substantially normal to the flow path of said sheet. 29. Apparatus as claimed in any one of claims 26 to 28, in which said reaction zone is adapted to fit between adjacent support rolls of a lehr or a float-glass manufacturing line, which support rolls form said sheet supporting and moving means, and wherein means are provided for moving said reaction zone vertically and horizontally to facilitate positioning said reaction zone, and the removal of said reaction zone from the lehr. 30. Apparatus as claimed in claim 29, in which said port means communicate respectively with inlet and outlet ducts, each of which share a common temperature control duct, adapted to carry a heat-transfer medium which stabilizes and - 30 controls the temperature of the apparatus, and which forms a cooling means for gas in the outlet duct and heating means for gas in the inlet duct, all said ducts forming an integral unit adapted to fit between rolls of the lehr

10. A process as claimed in any one of claims 1 to 3, 7, 10 film formed by said coating compositions having a refractive rnde-x substantially the same as that of the most adjacent portion of said film.

11. A process as claimed in any one of claims 1 to 10, wher·.-;- the upper coating zone is an infra-red reflective over’’/ ?.f tin oxide and wherein the total coating thickness is fr m about 0.1 to 1.0 micron thick.

12. A process for coating a substrate, substantially as hereinbefore described with reference to the accompanying 25 drawings.

13. A transparent glass product substantially free of iridescent appearance, having a glass substrate bearing a coating which is substantially uniform across the surface area thereof, said coating consisting of a lower coating 50 zone comprising a material formed of at least two components - 27 whioh is characterised by a gradual change from a first composition proximate to said substrate having a relatively high proportion of a first component to a second composition more remote from said substrate having a relatively large proportion of a second component, and an upper coating zone having a refractive index substantially the same as that of said second component, uhen produced ty the process according to any of claims 1 to 12.

14. A product as claimed in claim 13, wherein said lower coating zone is substantially linear with respect to its molecular proportion of said first component, as a function of distance from the glass substrate. 15. A product as claimed in claim 13 or 14, wherein said second component is selected from the group consisting of SnO„, Si„N.. TiO_, In_O . Cd.SnO. or a mixture of at least 2 34 2 23 24 two of this group. 16. A product as claimed in claim 13 or 14, wherein said first component is SiO^» said second component is SnO^, and said upper coating zone is fluorine-doped tin oxide. 17. A product as claimed in any one of claims 13 to 16, wherein the coating consisting of said lower and upper coating zones is less than one micron thick. 18. A product as claimed in any one of claims 13 to 17, wherein said substrate is a glass sheet and wherein said lower coating zone is at least 0.15 micron thick. 19. A product as claimed in any one of claims 13 to 16, wherein said substrate is a glass sheet and wherein said lower coating zone is at least 0.3 micron thick. 20. A product as claimed in any one of claims 13 to 19, wherein said material forming said lower coating zone con48228 - 28 tains less than 40% of tin oxide at the interface between said glass substrate and said lower coating zone. 21. A product as claimed in any one of claims 13 to 20, wherein the lower coating zone contains at least 30% of tin

15. As hereinbefore described with reference to Figure 4 of the accompanying drawings.