CH638762A5 - PROCESS FOR CONTINUOUSLY COATING A SUBSTRATE, PRODUCT RESULTING FROM THIS PROCESS AND APPARATUS FOR ITS IMPLEMENTATION. - Google Patents

Description

The present invention relates to glass structures having a thin inorganic coating (for example a coating of tin oxide intended to increase the reflection of infrared rays), structures which have a better appearance due to the reduction in iridescence which , as is known, are associated with these thin coatings, as well as with methods for producing such structures.

Glass and other transparent materials can be covered with transparent semiconductor films, for example tin oxide, indium oxide or cadmium stannate, in order to reflect infrared rays. These films are useful for producing windows with a better insulation coefficient (less good conductors of heat), for example for ovens,

for building windows, etc. Coatings made of these materials also have the property of conducting electricity and are used as heating resistors to heat the windows of vehicles in order to eliminate fogging or frost.

One of the drawbacks of these coated windows is that they exhibit interference colors (iridescences) by reflection and, to a lesser extent, by transparency. This iridescence is a serious obstacle to the widespread use of these coated windows (see for example “American Institute of Physics Conference Proceeding” No. 25. New York, 1975, page 288).

In some cases, for example when the glass is relatively dark (having, for example, a transmittance or a transmission coefficient of less than 25%), this iridescence is masked and can be tolerated. On the other hand, in most architectural applications for walls and windows, the iridescence which normally is associated with coatings whose thickness is less than about 0.75 | i is unacceptable from an esthetic point of view for most people (see, for example, US Patent No. 3710074 to Stewart).

Iridescence, that is to say iridescent coloring, is a general phenomenon Hey with transparent films whose thickness is between approximately 0.1 and 1 | i and, in particular, those whose thickness is less than about 0.85 | i. Unfortunately, it is precisely this range of thicknesses that is of great practical importance in most industrial applications. Semiconductor coatings, the thickness of which is less than about 1 | i, do not exhibit interference coloring, but such thin coatings reflect infrared rays much less well and are also significantly less good conductors of electricity.

Coatings with a thickness greater than about 1 n also do not exhibit iridescence or iridescence visible in daylight, but they are much more expensive to produce since they require large quantities of coating materials, and that the time required to deposit them is also longer. In addition, films with a thickness greater than 1 µi tend to produce a haze which results from the scattering of light by surface irregularities, which are larger on such films. In addition, these films have a greater tendency to crack under the action of thermal stresses due to the differences in thermal expansion coefficients.

The result of these technical and economic constraints is that almost all of the current industrial production of these coated glasses comprises films of which the thickness is between approximately 0.1 and 0.3 (i and which exhibits pronounced iridescent coloring. these coated glasses are almost not used in architecture, although this would result in savings due to the conservation of energy. For example, heat losses by infrared radiation passing through glazed areas of a heated building can account for approximately half of all heat loss in the case of uncoated glasses, however the presence of iridescent color on these coated glasses is the main reason why they are not used.

American patent application No. 784542 describes means for reducing this iridescence or these iridescence, to the point that they become practically invisible, by means of one or more additional layers superimposed on the main layer, including a gradient coating. The main object of the present invention is to provide improved means for forming such a gradient anti-iridescence layer.

Consequently, one of the aims of the present invention is to provide means for eliminating the visible iridescence of thin semiconductor films applied to a transparent substrate, while retaining the desirable properties of transparency in visible light, of reflection of infrared rays and electrical conductivity of this substrate.

Another object of the present invention is to achieve the above objectives:

without significantly increasing production costs compared to the use of ordinary iridescent films;

by a process which is continuous and perfectly compatible with modern manufacturing procedures in the glass industry;

by products that are extremely durable and resistant to light, mechanical abrasion and the action of chemical agents;

using materials which are sufficiently abundant and readily available to allow wide dissemination.

The invention also proposes to provide means for reducing the total amount of light reflected by the coated surface of the substrate, thereby increasing the total transparency of the latter.

To this end, the invention is defined as it is said in claims 1, 22 and 27.

In a preferred embodiment of the invention, a film is used which is composed of layers which merge continuously into one another to form a degraded layer, the refractive index of which varies preferably by a smooth transition, as the layer is passed away from the substrate towards the semiconductor coating, from a certain value on the surface of the substrate adapted to the refractive index thereof, to another value adapted to the index of the overlying semiconductor film, at a point close to this film.

It seems useful, due to the subjective nature of color perception, to set out the methods and hypotheses which were used to evaluate the invention. It can be seen that the application of a large part of the theory discussed below is retrospec5

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tif, because the necessary information was produced or provided retrospectively, that is to say after knowing the invention. The following remarks apply to the use of glasses as a substrate.

To make an appropriate quantitative assessment of the various possible structures for suppressing iridescence, the intensities of these colors were calculated using optical data and data relating to color perception. In the following description, it is assumed that the layers of the films are planar, that they have a uniform thickness and that each layer has a uniform refractive index. Changes in refractive indices are considered to be abrupt at the plane interfaces between the films or adjacent layers. A continuously varying refractive index can be thought of as a succession of a very large number of very thin layers having closely related refractive indices. Real refraction indices were used, corresponding to negligible absorption losses in the layers. The reflection coefficients were evaluated for plane waves with normal incidence of non-polarized light.

Using the above assumptions, the reflection and transmission amplitudes of each interface were calculated using Fresnel formulas. Then, these amplitudes were added taking into account the phase differences produced by the propagation of light through the layers considered. The results were found to be equivalent to Airy's formula (see for example "Optics of Thin Films", by Knittl, Wiley and Sons, New York 1976) relating to multiple reflections and interference in thin films, when this formula has been applied to the same cases.

It has been observed that the calculated intensity of the reflected light varies with the wavelength and that it is stronger for certain colors than for others. To calculate the reflected light seen by an observer, it is advisable beforehand to specify the spectral distribution of the incident light. For this purpose, it is possible to use the lighting standard of the International Lighting Commission C, which corresponds approximately to normal lighting by daylight. The spectral distribution of the reflected light is the product of the calculated reflection coefficient and the lighting spectrum C. We then calculate the hue and saturation of the color, as seen by reflection by a human observer from of this reflected spectrum, using uniform color scales which are known in the art. A useful scale for this purpose is that described by Hunter in "Food Technology", volume 21, pages 100-105, 1967. This scale has been used to develop the relationship which will now be described.

Results and calculations, for each combination of refractive index and layer thickness, are two numbers a and b. a represents a red (if positive) or green (if negative) hue, while b describes a yellow (if positive) or blue (if negative) hue. These results are useful for checking the calculations with respect to observable colors of samples, including those of the invention. A single number c represents the color saturation. c = (a2 + b3) '/ 2. This color saturation index c is directly related to the ability of the eye to detect annoying iridescent colors. When the saturation index is below a certain threshold, we are unable to see a color in the reflected light. The numerical value of this observability threshold depends on the particular uniform color scale used, as well as on the conditions of vision and the level of lighting (see, for example, RS Hunter, "The Measure-ment of Appearance", Wiley and Sons, New York 1975, for a census of digital color scales).

In order to establish a basis of comparison for the structures, a first series of calculations was performed based on a single semiconductor layer applied to glass. For the refractive index of the semiconductor layer, we took 2, which corresponds approximately to the films of tin oxide, indium oxide or cadmium stannate. 1.52 was used for the glass substrate; this is a typical value of industrial window glass. The calculated color saturation indices have been plotted in FIG. 1 depending on the thickness of the semiconductor film. Color saturation has been found to be high in the light reflected from films having a thickness of between 0.1 and 0.5 µm. For films greater than 0.5 µm in thickness, color saturation decreases with thickness. These results are in agreement with the qualitative observations on real films. The pronounced oscillations are due to variations in the sensitivity of the eye for the different spectral wavelengths. Each of the points corresponds to a particular color, marked on the curve (R = red, Y = yellow, G = green, B = blue).

Using these results, the minimum observable color saturation was established by the following experiment: tin oxide films were deposited with continuously variable thickness, thickness up to about 1.5 dd, on glass plates, by oxidation of a tetramethyltin vapor. The thickness profile was established by varying the temperature from about 450 ° C to 500 ° C along the surface of the glass. The thickness profile was then measured by observing the interference fringes under monochromatic light. Observed under diffused daylight, the films presented interference colors at the correct positions indicated in fig. 1. The parts of the films, the thickness of which was greater than 0.85%, did not show any observable interference colors in the scattered daylight. The green tip, which the calculation locates at a thickness of 0.88 n, was not visible. Consequently, the observability threshold is above 8 of these color units. Likewise, the blue tip that the calculation locates at 0.03 was not visible, so the threshold is above 11 color units, which is the value calculated for this tip. However, a faint red tip was visible at 0.81 µi under good observation conditions, for example using a black velvet background and in the absence of colored objects reflected in the field of vision, so that the threshold is located below the 13 color units calculated for this color. These studies conclude that the observation threshold for reflected colors is between 11 and 13 units of color on this scale and that is why the licensee adopted a value of 12 units to represent the threshold of observability of the colors reflected by operating in daylight. In other words, a color saturation greater than 12 units appears as a visibly colored iridescence, while a color saturation less than 12 units appears as a neutral coloration.

It is assumed that there are practically no objections to marketing products whose color saturation values are equal to or less than 13. However, it is much preferable that this value is equal to or less than 12 and, as it is explained in more detail below, it does not seem that there is a practical reason supposing that the most advantageous products according to the invention, for example those characterized by completely colorless surfaces, ie ie below about 8, cannot be produced under economic conditions.

A value of 12 or less indicates a reflection that does not distort the colors of an observably reflected image. This threshold value of 12 units has been taken as a quantitative standard with which the success or failure of various kinds of multilayers can be assessed in order to suppress iridescence.

Coatings with a thickness of 0.85 (i or more) have color saturation values below this threshold of 12, as shown in Fig. 1. Experiments confirm that these relatively thick coatings do no annoying iridescence in daylight.

It has been discovered that between the glass substrate and the semiconductor layer can be formed an intermediate film having a degraded composition, i.e. varying gradually from a silica film to an oxide film. tin. Such a film can be considered to be composed of a very large number of intermediate layers. The saturation of the reflected colors has been calculated for a large number of refractive index profiles between the refractive index of glass n = 1.52 and that of re5

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semiconductor clothing n = 2. For transition layers, the thickness of which is greater than approximately 0.15 (i, the calculated color saturation index is generally less than 12, that is to say neutral for the eye and for transitions greater than approximately 0.3 n, the color is always undetectable. The exact shape of the profile of the refractive indices has very little effect on these results, provided that the variation is gradual along the degraded layer.

A wide range of transparent materials can be chosen to manufacture products meeting the above criteria by forming anti-iridescence undercoats. Thus, various metal oxides and nitrides and their mixtures have the correct optical properties of transparency and the desired refractive indices. Table A below lists some mixtures which have the desired refractive indices between those of glass and those of a tin or indium oxide film. The necessary weight percentages can be obtained from the refractive indices,

measured according to the composition curves, or can be calculated from the usual Lorentz-Lorentz law relating to the refractive indices of mixtures (Z. Knittl, "Optics of Thin Films", Wiley and Sons, New York, 1976 , page 473), using the measured refractive indices of pure dandruff. This mixing law generally gives sufficiently precise interpolations for optical work, although the refractive indices thus calculated are sometimes slightly lower than the measured values. The refractive indices of the films also vary slightly with the deposition methods and the operating conditions.

Fig. 3 shows a typical curve of variations in the refractive indices as a function of the composition for the important case of mixtures of silicon dioxide and tin dioxide.

Table A This table indicates some combinations of compounds giving transparent mixtures, the refractive indices of which are between 1.5 and 2.

Si02 Sn02

Si02 Si3N4

Si02 Ti02

SiOz ln203

Si02 Cd2Sn04

The films can be formed by simultaneous evaporation under vacuum of the appropriate substances making up a suitable mixture. To cover large areas, for example a window pane, the deposition of CVD chemical vapor at normal atmospheric pressure is a more practical and less expensive process. However, the CVD process requires suitable volatile components to form each material. The silicon dioxide can be deposited by the CVD process from certain gases such as silane, SiH4, dimethylsilane (CH3) 2SiH2, etc. Some liquids that are sufficiently volatile at room temperature are almost as good as gases; Thus in the CVD process, tetramethyltin constitutes a source for the tin compounds, while (C2H5) 2SiH2 and SiCl4 are volatile liquids constituting sources for silicon.

A continuously graded layer, composed of a mixture of silicon and tin oxide, can be formed during a continuous CVD coating process on a continuous glass ribbon by the following new procedure: a gas mixture in a direction parallel to the direction of translation of the glass, under (or above) the strip of hot glass, as shown, for example, in FIG. 4. The gas mixture contains an oxidizable silicon compound, an oxidizable tin component and oxygen or another oxidizing gas. The compounds are chosen so that the silicon compound is oxidized a little faster than the tin compound, so that the oxide which is deposited on the glass at the place where the gaseous mixture initially strikes the hot surface of the glass is mainly composed of silicon dioxide with only a small percentage of tin dioxide. The proportions of the silicon and tin compounds, constituting the vapor phase, are adjusted so that the material initially deposited has a refractive index which is closely similar to that of the glass itself. Then, while the contact with the glass surface continues, the proportion of tin oxide increases in the deposited film until, at the end of the region where the deposit is made, the silicon compound is almost completely exhausted in the gas mixture, so that the deposit formed there is composed of almost pure tin oxide. Since the glass also advances continuously from the (initial) deposit region relatively rich in silicon to a (final) region relatively rich in tin, it is clear that the glass receives a coating having a degraded refractive index which varies in continuous along the thickness of the coating, starting from the surface of the glass with an index adapted to that of glass and ending, at its outer surface, with an index adapted to that of tin oxide. The following deposition regions, shown in fig. 3, can then be used to form other layers of pure tin oxide or layers of tin oxide doped, for example, with fluorine.

A gas mixture suitable for this purpose preferably comprises the following oxidizable silicon compounds: 1,1,2,2-tetra-dimethylsilane (HMe2SiSiMe2H); 1,1,2-trimethylsilane H2MeSi-SiMe2H, and / or 1,2-dimethylsilane (H2MeSiSiMeH2) as well as tetramethyltin (Me4Sn). It was found that the film initially deposited is rich in silicon and has a refractive index close to that of glass, while the last part of the deposit consists of almost pure tin oxide.

The Si-H bonds of the silicon compounds described above are extremely useful in the process since compounds without such bonds, such as tetramethylsilane Me4Si or hexamethylsilane Me3SiSiMe3, oxidize more slowly than tetramethyltin, so that the initial deposit is mainly composed of tin oxide while the last part of the deposit is mainly formed of silicon dioxide. In such a case, that is to say when using compounds such as Me4Si, it is possible to circulate the gas and the glass in opposite directions to obtain the desired gradation of the refractive indices, provided that the gas circulates faster than glass. However, the preferred embodiment is to use more easily oxidizable silicon compounds and to circulate them in the same direction as glass.

It is also useful, when forming coatings whose composition varies monotonically with the distance from the substrate, that the silicon compounds have an Si-Si bond at the same time as the Si-H bond. Thus, for example, a compound containing Si-H bonds, but having no Si-Si bonds such as dimethylsilane Me2SiH2, associated with tetramethyltin, initially produces a deposit of tin oxide almost pure, which becomes rich in silicon at an intermediate stage and ultimately becomes rich in tin at an even later stage in the deposition procedure. Although not wishing to be bound by any theory, the licensee assumes that the combination of Si-Si-H bonds facilitates rapid oxidation by an initial thermal decomposition in which migration of hydrogen to neighboring silicon occurs, according to the formula HMe2Si-SiMe2H-> Me2SiH2 + Me2Si. The reactive dimethylsilene Me2Si is then rapidly oxidized, releasing free radicals such as hydroxyls (OH), which then quickly subtracts hydrogen from the Si-H bonds, thus creating more reactive silene radicals, forming a chain reaction . Tetramethyltin is less reactive for these radicals and, therefore, occurs only in the later stages of oxidation. Me2SiH2 is lacking in the initial rapid decomposition stage and, therefore, cannot begin oxidation before a certain amount of tetramethyltin has decomposed to form radicals (CH3, OH, O, etc.) which , then, preferentially attack Me2SiH2 at intermediate stages, until the Me2SiH2 is exhausted, after which the oxidation of tetramethyltin becomes dominant again.

It is preferable that the disilane compound contains at least two methyl groups, since disilanes which have only one or no methyl groups are spontaneously flammable in air and, therefore, must be premixed with an inert gas such as nitrogen.

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

Partially alkylated higher polysilanes, such as tri-silanes or substituted polyalkyl tetrasilanes, operate in a manner analogous to disilanes. However, higher polysilanes are more difficult to synthesize and are less volatile than disilanes, which are therefore preferable.

When the initial deposit of the silica and tin oxide films contains less than about 40% of tin oxide, little or no haze is formed at the interface between the glass substrate and the coating it covers. If, for any reason, it is desired that the gradient starts above about 30% tin oxide, it is preferable that the glass is covered with an anti-veil layer, for example silicon dioxide. This anti-veil layer can be very thin, for example, of the order of 25 to 100 A.

The characteristics and advantages of the invention will emerge from the description which follows, given solely by way of example with reference to the appended drawing, in which:

- fig. 1 is a graph showing the variation of the calculated color intensity of various colors as a function of the thickness of the semiconductor film;

- fig. 2 schematically shows in section a glass comprising a non-iridescent layer, in accordance with the invention, with an intermediate, anti-iridescence layer having a composition varying continuously in accordance with the invention;

- fig. 3 is a graph representing a typical idealized variation curve of the refractive indices and which shows the gradual transition between a layer composed of 100% of Si02 and a layer formed of 100% of Sn02;

- fig. 4 is a schematic sectional view of a new device for implementing the method of the invention;

- fig. 5 is a diagram representing the experimental measurements of the chemical composition of a degraded silica-tin oxide zone prepared in accordance with the invention, and

- fig. 6 is a graph representing an observed variation in the refractive index of the initial deposit of Si02 - Sn02 formed on the surface of a glass as a function of the composition of the gas.

Fig. 4 shows a section of a machine for producing a float glass. The structure of the machine itself has not been shown for clarity. The hot glass 10, the temperature of which is about 500-600 C, is driven into the machine on rollers 12, 14 and 16. Between the rollers 12 and 14 is arranged an assembly 18 comprising a gas inlet duct 20 and a gas outlet or outlet conduit 22. Between the conduits 22 and 20, from which it is separated by a heat exchange wall 24, extends a conduit 25 which constitutes a means for transporting a fluid heat exchange which, for its part, constitutes a means for cooling the gases of the outlet duct 22 and for heating the gases circulating in the duct 20. The temperature of the heat exchange fluid is kept low enough so that it does not no coating is formed on the surface of the inlet duct.

The gas which circulates in the inlet conduit 20 passes through a slot 28 to direct itself along a reaction zone formed by the surface 30 of the assembly 18 and by the lower surface of the glass sheet 10. Upon reaching a second slot 32, the rest of the gas is evacuated through the conduit 22. During the passage of the gas along the lower surface of the glass sheet 10, a degraded coating is formed as a result of the selective exhaustion of one of reactants at the different points located along the deposition zone between the rollers 12 and 14.

In the apparatus shown in fig. 4, a second set of conduits 38 is used to complete the formation of the coating, for example by superimposing a coating of tin oxide doped with a fluoride on the first degraded coating. Here too, it is practical for the gas to enter through an upstream opening 28a and to exit through a downstream opening 32a.

The conduits are preferably formed from a corrosion resistant steel alloy and include a heat insulating jacket.

The following examples, which of course have no limiting character, will make the particular features of the invention better understood.

Example I

A glass heated to about 580 ° C is entrained at the speed of 10 cm / s in the apparatus shown in fig. 4. The temperature of the gas inlet pipe is maintained at around 300 ° C. by circulating air suitably heated or cooled in the temperature control pipe. In the first deposition region reached by the glass, a gas mixture having the following composition (in mole%) is introduced:

1,1,2,2-Tetramethyldisilane 0.7%

tetramethyl tin 1.4%

bromotrifluoromethane 2%

dry air the rest The second deposition region is supplied with a gas mixture having the following composition (in mole%):

Tetramethyltin 1.6%

bromotrifluoromethane 3%

dry air the rest

The flow rates of the gas mixtures are adjusted so as to obtain an average contact time between a given element of the mixture and the glass surface of approximately 0.2 s.

A coated glass is thus obtained which, by reflecting the light of day, has a neutral coloring. It has a visible reflectivity of 15% and has no haze. The reflectivity in infrared is 90% at a wavelength of 10 jx. The resulting electrical resistance is 5 £ 22. The coating has a thickness of about 0.5 µm.

Example 2

The deposition procedure of Example 1 is repeated, the only difference being that the gaseous mixture supplying the first deposition region has the following composition:

1,2-Dimethyldisilane 0.4%

1,1,2-trimethyIdisilane 0.3%

1,1,2,2-tetramethyldisilane about 0.02%

tetramethyl tin 1.5%

bromotrifluoromethane 2%

dry air the rest

The properties of the resulting product do not differ from those of Example 1.

Samples of these coated glasses are subjected to an Auger chemical analysis to determine the composition of the coating,

as well as an ion projection pickling to reveal its chemical composition as a function of thickness. Fig. 5 shows the profile of the chemical composition resulting from the deposition in the region in which it varies. It can be seen that, near the surface of the glass, the deposit consists mainly of silicon dioxide, about one in eight silicon atoms being replaced by tin. As one moves away from the surface of the glass, the concentration of tin increases and that of silicon decreases, so that, at distances greater than 18 (i from the surface of the glass, the deposit mainly consists of silicon dioxide, about one in eight silicon atoms being replaced by tin. As one moves away from the glass surface, the concentration of tin increases and that of silicon decreases, so that , at distances greater than 18 n from the glass surface, the deposit consists of tin oxide with approximately 1.5% of oxygen replaced by fluorine. Using Fig. 3, the profile is converted of silicon-tin composition in a profile of the refractive index as a function of the distance, which has also been plotted in Fig. 5. These results confirm the ability of the process described to produce the desired variation in the index of refraction along the thickness of the deposited film.

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Example 3

Tin oxide coatings of different thicknesses are applied to a glass substrate (a first coating, consisting of an ultra-thin film of silicon dioxide having previously been applied to the substrate in order to produce an amorphous anti-fog surface ).

Thickness

oxide

Visibility of tin iridescence

0.3 n strong

0.6 n distinct, but lower

0.9 n simply noticeable except in fluorescent light

1.3 | x weak, even under fluorescent light

The last two materials meet with no objection from an aesthetic point of view to be used in architecture, thus confirming the accuracy of the color saturation scale used to assess the quality of the products.

To obtain maximum efficiency in the removal of iridescence, it is useful that the refractive index of the initial deposit is close to that of the glass substrate, preferably at

± 0.04 or, better still, ± 0.02 refractive index unit. To obtain such an adaptation, the parameters of the deposition procedure are varied, in particular the ratio of the number of tin atoms to those of silicon in the inlet gas. An example of such a variation, shown in FIG. 6, shows the resulting variation in the refractive index of an initial deposit of tetramethyl tin and mixtures of 1,1,2,2-tetramethyldisilane, as a function of the composition of the gases. The other parameters of these deposits were the same as in Example 1. FIG. 6 shows, for example, that an initial deposit, having a refractive index of 1.52 (adapted to the refractive indices of standard window glasses), is produced by a gas containing equal numbers of silicon atoms and of tin. An adaptation to an index of 1.52 ± 0.02 is obtained when the composition of the gas is maintained between 47 and 52 atomic% of tin. Although these numbers may be slightly different under other deposition conditions, for example for other temperatures or other compounds, it is a routine matter to establish, by experiments, calibration curves such as those in fig. 6 to obtain a suitable adaptation of the refractive indices of the substrate and of the coating composition initially deposited.

It should be noted that the reflection of light by the surface of the coated products of Example 3 is approximately 16 to 17%, that is to say approximately 10% greater than that of the coated glasses of the examples. 1 and 2 which does not have a degraded undercoat according to the invention.

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Claims (32)

638,762 2 1. Process for continuously coating a substrate with a film formed from the reactive components of a gaseous mixture, and the properties of which vary continuously in the direction of the thickness, process characterized in that: a) the gas mixture is circulated in a reaction zone defined by a path of the reactive components contiguous to, parallel to, and limited by a surface of the substrate; b) a reaction product from a more reactive component of the mixture is deposited on a part of the surface; c) depositing a reaction product from a less reactive component of the mixture on another part of the surface; and d) the surface of the substrate in the reaction zone is moved in a direction parallel to the circulation path in order to obtain a variation in the composition of the film along its thickness when the substrate emerges from the reaction zone. 2. Method according to claim 1, characterized in that the reaction product from the most reactive component of the mixture is deposited on the part of the surface of the substrate which comes into contact with the gas mixture as soon as possible, and in that that the reaction product from the least reactive component of the mixture is deposited on the part of the surface of the substrate which comes later in contact with the gas mixture. 3. Method according to one of claims 1 or 2, characterized in that the substrate consists of glass. 4. Method according to claim 1, characterized in that the reaction products are produced by a reaction of the reactive components of the gas mixture, caused by the heat of the substrate. 5. Method according to claim 1, characterized in that the refractive index varies continuously between the lower face and the upper face of the film. 6. Method according to claim 5, characterized in that the gas mixture comprises volatile silicon and tin compounds, and an oxidizing gas. 7. Method according to claim 6, characterized in that the gas mixture comprises at least one partially aikylated polysilane. organotin vapor and an oxidizing gas. 8. Method according to claim 7, characterized in that the gas mixture contains at least one methyldisilane and tetra-methyltin. 9. Method according to claim 8, characterized in that the gas mixture contains 1,1,2,2-tetramethyldisilane; 1,1,2-trimethyldisilane; 1,2-dimethyldisilane or mixtures thereof. 10. Method according to one of claims 2 or 3, characterized in that the refractive index varies continuously between the lower face and the upper face of the film. 11. Method according to one of claims 2 or 3, characterized in that the gas mixture comprises volatile silicon and tin compounds, and an oxidizing gas. 12. Method according to one of claims 2 or 3, characterized in that the gas mixture contains at least one partially aikylated polysilane, an organotin vapor and an oxidizing gas. 13. Method according to one of claims 2 or 3, characterized in that the gas mixture contains at least one methyldisilane and tetramethyltin. 14. Method according to one of claims 2 or 3, characterized in that the gas mixture contains 1,1,2,2-tetramethyldisilane HMe2SiSiMe2H; 1,1,2-trimethyldisilane H2MeSiSiMe2H; 1,2-dimethyldisilane H2MeSiSiMeH2 or mixtures thereof. 15. Priced according to one of claims 2 or 3, characterized in that the reaction products are produced by a reaction due to the heat of the substrate. 16. Method according to claim 4, characterized in that the refractive index varies continuously between the lower face and the upper face of the film. 17. The method of claim 11, characterized in that the proportions of the reactive components are chosen so as to obtain a film which, near the substrate, comprises at least 60% of Si02 and the part of which is furthest from the substrate comprises at least 95% tin oxide. 18. The method of claim 1, characterized in that the gas mixture comprises a first gaseous reactive component, a second gaseous reactive component and a gas capable of reacting with these two components; one of these reactive components having a reaction rate different from that of the other reactive component on said gas. 19. Method according to one of claims 1, 2, 3, 4,13, 14, 17 or 18, characterized in that the reaction products are deposited at a speed such that a regular variation in the composition is obtained of the film, which results in a gradual increase in the refractive index of the film as its thickness increases on the substrate. 20. Method according to one of claims 1 to 18, characterized in that the coating operation ends with an outer layer of tin oxide reflecting infrared rays, the total thickness of the coating being between 0 , 1 and 1 dd .. 21. Method according to one of claims 1 to 18, characterized in that the thickness of the film is such that it forms a means for removing the iridescence visible on the surface of a product of the process, which is defined by a maximum color index of around 12. 22. Transparent product free of iridescence resulting from the method according to claim 1, characterized in that it comprises a glass substrate carrying a film consisting of a material formed of at least two components, which is characterized by a gradual variation or progressive from a first composition near the substrate comprising a relatively high proportion of a first component to a second composition further from the substrate comprising a relatively large proportion of the second component. 23. Product according to claim 22, characterized in that the film carries an upper layer having a refractive index practically equal to that of the second component. 24. Product according to one of claims 22 or 23, characterized in that in the film, the molecular proportion of the first component varies linearly as a function of the distance from the glass substrate. 25. Product according to claim 23, characterized in that the first component is Si02, the second component is Sn02, and the upper layer consists of tin oxide doped with fluorine. 26. Product according to claim 25, characterized in that the coating constituted by the film and the upper layer is less than 1 | i thick. 27. Apparatus for implementing the method according to claim 1, comprising means for supporting the glass, for maintaining it at a high temperature and for driving it along a continuous treatment trajectory, characterized in that it further comprises a reaction zone having, at one of its ends, openings at a first station for introducing and for distributing a gaseous reaction mixture comprising reactants forming products which are deposited on said glass at different speeds , a circulation path forming a reaction zone in which said mixture flows along said glass surface, openings at a second station for discharging the rest of the gaseous reaction mixture, this second station being placed, with respect to the first station, along said treatment path so that the heated glass forms a means for supplying sufficient energy to said gas reaction mixture between the first station and the second post. 28. Apparatus according to claim 27, characterized in that the openings are arranged under the glass and in that the latter forms the upper limit of the treatment path. 29. Apparatus according to claim 27, characterized in that the openings have the form of slots arranged parallel to one 5 10 15 20 25 30 35 40 45 50 55 60 65 3 638,762 the other and which are practically perpendicular to the path of circulation of the glass. 30. Apparatus according to claim 28, characterized in that it is adapted to be installed between the adjacent support rollers of a float glass production line, and in that it comprises means for driving said apparatus vertically and horizontally for easy positioning and removal. 31. Apparatus according to claim 27, characterized in that the inlet and outlet openings communicate with inlet and outlet ducts, each of which shares a common temperature control duct, suitable for transporting a transfer agent from heat which stabilizes and regulates the temperature of the apparatus and which forms a means of cooling for the gas circulating in the outlet duct and a means of heating for the gas of the inlet duct, all these conduits forming a unitary assembly adapted to be installed between the rollers of a float glass production line and under a glass substrate carried by said rollers. 32. Apparatus according to claim 27, characterized in that it further comprises a second coating apparatus mounted in series on the glass production line, this second apparatus forming a means for producing an additional coating on the film having a composition gradually variable.