LU85252A1 - LOW EMISSIVITY GLAZING - Google Patents

Description

I * 1

The present invention relates to a glazing comprising a light transmitting sheet carrying on one of its faces a coating of electrically conductive and light transmitting metallic oxide, which reduces the emissivity of infrared radiation from this face of the glazing .

Such glazing is widely used when it is desired to take advantage of solar radiation as much as possible and to reduce the heat loss by emission of infrared radiation of medium and long wavelength. For example, for a glazed solar collector, it is very important to obtain a high efficiency, that the glazing transmits as high a proportion of solar radiation as possible while emitting as low a proportion of radiation at wavelengths corresponding to the temperature of the collector itself. Ideally for this reason, such glazing should transmit all of the radiation in the solar spectrum while emitting no radiation at wavelengths greater than, "say, 2,000 nm or 3,000 nm.

Such glazing is also used in horticultural constructions, for example greenhouses, frame frames and bells where it is advantageous to reduce the heat loss while allowing the maximum exposure of the plants to visible light, so as to encourage its growth.

Another use of such glazing resides in residential buildings, again with the main objective of reducing heat losses, and here a slightly different consideration appears, in the sense that the quality of the transmitted visible light can be as important as its quantity. In particular, it is desirable that the glazing allow a clear vision, that is to say that it transmits light in a non-diffuse manner.

One of the main objects of the present invention is to promote the light transmission factor of glazing comprising a coating of low emissivity in the infrared.

The present invention provides a glazing unit comprising a sheet transmitting light carrying on one of its faces a coating of metallic oxide electrically conductive and transmitting I light, which reduces the emissivity of the infrared radiation of this 2 side of the glazing, characterized in that the compositions and thicknesses of the light-transmitting sheet and of the electrically conductive coating are such that only they have a light transmittance of at least 70%, and in that the electrically conductive coating 5 is surmounted by a dielectric coating whose thickness is less than 160 nm and which increases the light transmission factor of the glazing.

As used in the present description the term "light transmission factor" corresponds to the ratio of the amount of visible light transmitted to the amount of incident visible light, these amounts being corrected integrations of the amounts of transmitted and incident light over the entire spectrum of visible light i, the integrations being corrected to take account of the spectral distribution of the radiation source and the spectral sensitivity characteristics of the human eye. The measurements are carried out with the light directed towards the face of the glazing carrying the coating, by means of a spectrophotometer and the light source considered at the spectral composition of the illuminant C as defined by the International Lighting Commission. This illuminant can be considered as representing average daylight. The correction factor for the sensitivity of the eye that is applied, is also standardized by the International Commission on Lighting.

The invention offers the advantage of increasing the light transmission factor through the glazing. It is well known in the art that the reduction in emissivity due to a conductive coating is linked to the presence of free charge carriers on the surface of the glazing. We have found, however, that even if such a dielectric coating increases the emissivity of the surface carrying the conductive coating, provided that the thickness of the dielectric coating is less than 160 nm, the increase in emissivity may be perfectly acceptable and the glazing according to the invention presents a favorable compromise between its transmission factor of visible light and its infrared emission properties.

It has in fact been found that the emissivity of the glazing according to the invention in the infrared increases substantially proportional to the fact that the dielectric coating has a thickness of less than 140 nm and preferably not exceeding 110 nm.

In the preferred embodiments of the invention, the dielectric coating has an optical transmission thickness of at least 60 and preferably at least 70 nm. The optical thickness in transmission of a coating is defined as its geometric thickness multiplied by its refractive index. By using this characteristic of the invention, considerable advantages can be obtained from the point of view of the light transmittance factor, by interference extinction of the reflected visible light. It was thought that, in order to obtain the maximum benefit from these interference effects, the coating should have an optical thickness equal to a quarter of the wavelength of the light with the highest transmission. In fact, it has been found that this is not true in the case where coatings of two different materials are applied according to the invention.

The optimum thickness of the dielectric coating intended to increase the light transmission factor is also a function of the thickness of the underlying conductive coating. The variation in thickness op-. timale, which is substantially sinusoidal, is explained by the table below which gives the optimum geometric thickness of a dielectric coating of silica deposited on doped tin oxide coatings, of different geometric thicknesses, deposited on a soda-lime glass substrate. The composition of the silica coating is SiOx where x is between about 1.95 and 2.0, but for ease of reference it will be referred to as a coating of silicon dioxide (S1O2). Similarly, doped tin oxide coatings will be mentioned under the formula stannic oxide (SnC ^).

SnO ^ 350 nm 385 nm 420 nm 455 nm 490 nm

SiC> 2 100 ωη 55 nm 100 nm 55 nm 100 nm.

The SiO £ coatings have a refractive index of about 1.41, so the corresponding values for optical thickness are 141 nm and 77.5 nm.

Preferably, the refractive index of the dielectric coating is equivalent, to the nearest ten percent, to the square root of the refractive index of the electrically conductive coating. The adoption of this preferred feature of the present invention reduces

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'4 the total amount of light reflected at the interfaces between the dielectric coating and, on the one hand air, and on the other hand the conductive coating. For example, the SnO2 coatings cited above have a refractive index of about 2.0. The refractive index of the coatings of SiO 4 »1.41, is substantially equal to the square root of 2.

It is surprising that compliance with this condition also gives significant advantages by reducing the proportion of transmitted light which is transmitted diffuse.

. In some preferred embodiments of the invention, the electrically conductive coating comprises tin oxide. The use of infrared low emissivity coatings. red consisting of tin oxide is well known per se and such coatings can be formed which have good light transmission and good infrared emissivity. In addition, such coatings can be very resistant to abrasion.

In other preferred embodiments of the invention, the electrically conductive coating comprises indium oxide. Indium oxide coatings are also well known per se and these coatings may exhibit better properties than tin oxide, with respect to the high light transmittance and low infrared emissivity, but have the disadvantage of being relatively easily damaged by abrasion.

For this reason, indium oxide coatings are not widely used in circumstances where they are exposed. Since, according to the present invention, the electrically conductive coating, for example of indium oxide, is surmounted by another coating, such a relative lack of abrasion resistance must no longer have disadvantages.

In preferred embodiments of the invention, the dielectric coating has higher abrasion resistance than the electrically conductive coating. The term "abrasion resistance" is used herein to denote abrasion resistance as measured according to the American National Standard 35 ^ no. Z 26. 1-1977 Test No. 18 for safety glass. Preferably, the dielectric coating is made of silica. Silica can form highly transparent coatings which have excellent abrasion resistance and good chemical stability. In addition, silica coatings with a refractive index of about 1.41 can be easily made. This is especially useful, as mentioned above, when the refractive index of the underlying electrically conductive coating is approximately equal to (1.41) ^, as is case for tin oxide.

Such a silica coating can be formed in various ways. In one of these methods, a transparent sheet of glass carrying an electrically conductive coating is immersed in a solution of an organosilicate compound such as a solution of tetramethylorthosilicate SiiOCH ^) ^ in methanol. When exposed to air, SiiOCHg) ^ is converted to Si (OH) ^ and this in turn is converted to SiC> 2 by air heating, preferably at 500-600 ° C. The desired thickness j of the coating can be achieved by adjusting the speed at which the sheet is removed from the solution. In another process particularly suitable for the formation of a continuous coating on glass. hot carrying a conductive coating, for example a continuous ribbon of freshly formed glass carrying this coating, the dielectric coating is formed by deposition from vapor phase reagent, for example by contacting hot glass coated with vapor silicon hydride in the presence of atmospheric oxygen.

Coatings having very smooth surfaces can be formed in these two ways. The roughness of the surface is one of the many factors having an effect on the proportion of diffuse / non-diffuse light transmission. When using fairly thick conductive coatings, for example with a thickness close to 1,000 nm, it is difficult to remove any roughness from the surface 30 of the coating. This roughness favors the diffusion of the transmitted light. Applying an additional coating with a smoother surface can improve this phenomenon, so that a higher proportion of the transmitted light is non-diffuse. It should be noted that this phenomenon does not depend on the refractive index of the dielectric 35 I coating.

J Among other materials that can be used for f- ". "6 forming the dielectric coating are the calcium and magnesium fluoride. It should be noted, however, that these materials are not very resistant to abrasion, although they can advantageously be used for the formation of a coating which must be deposited on an inner face of multiple glazing.

In certain preferred embodiments of the invention, the dielectric coating is a coating of polymeric material. Such coatings, for example a silicone or a polyurethane, can be formed by immersing a glass sheet 10 coated with conductive material in a solution of the desired dielectric material, while the uncoated side of the glass sheet is protected. Such coatings are preferably used on an internal face of multiple glazing so as to be protected against abrasion.

Preferred embodiments of the present invention will now be described by way of example with reference to the accompanying schematic drawings in which:

Figure 1 is a sectional view of a detail of a glazing unit according to the invention and Figure 2 is a sectional view of a detail of a hollow panel comprising a glazing unit according to the invention.

In FIG. 1, a light-transmitting sheet 1, for example of glass, has a coating 2 on one of its faces. This coating 2 is an electrically conductive metal oxide coating which reduces the emissivity of the surface carrying the coating with respect to infrared radiation, in particular infrared radiation having lengths. wave greater than 3.000 nm. The conductive coating 2 is surmounted by a dielectric coating 3 whose thickness does not exceed 160 nm and which increases the light transmission factor of the glazing.

In FIG. 2, a glazing sheet 4 transmitting light carries on one of its faces a conductive coating 5 of metal oxide transmitting light which reduces the emissivity of the glazing with respect to infrared radiation, which is in turn on-35 "mounted with a dielectric coating 6 whose thickness is less than 7"

The sheet of vitreous material 4 is secured via a peripheral insert 7 to a second glazing sheet 8 which may or may not have a coating. The coverings 5, 6 are arranged inside the panel so as to be protected from abrasion.

The following table 1 gives the sheet properties 1 of 6 mm thick float glass bearing coatings 2 of fluorine-doped tin oxide which are surmounted by dielectric coatings 3 of silica.

The silica coatings are chemically stable and resistant to abrasion, so that they can be exposed to atmospheric conditions. Such glazing can be incorporated in a hollow panel for example as described with reference to FIG. 2 with the face carrying the coatings facing inward or outward from the panel. The refractive index of tin oxide coatings is about 2.0 and that of silica coatings is about 1.41.

In Table 1, the various columns relate to the following properties: I Actual thickness of the coating of SnC 2 (nm) 20 II Infrared emissivity of the glass carrying the coating of SnO 2 (%) III Light transmittance factor of the glass carrying the coating of SnC ^ (%) IV Actual, optimal thickness of the coating of SiC ^ (nm) V Infrared emissivity of the glass coated with Sn02 and SiC ^ (%) 25 VI Light transmittance of the glass coated with Sn02 and Si02 (% ) TABLE 1

1 II m IV V VI

350 29 83 100 35 90 30 420 21 85 100 25 90.5 490 16 82 100 20 90.3 840 12 75 100 17 90.3

The deposition of tin oxide coatings as mentioned above is well known and does not need to be detailed here.

Specific examples of suitable methods of depositing such coatings can be found in British patent application "8 published under No. 2, 119. 360 A.

Following are examples of two methods of depositing silica coating, one being a batch process, the other a continuous process.

5 Batch process for silica deposition

A solution is produced in the following proportions: tetramethylorthosilicate SiiOCH ^) ^ 0. 55 mole

Water 0.75 mL

HCl 1.0 mL

10 Methanol ad. 1.0 L

The glass carrying the metal oxide coating is immersed in such a solution and is smoothly removed therefrom. The . coated glass is then heated in air to as high a temperature as possible (for example between 500 ° C and 15,600 ° C) to convert SiiOCH ^) ^ first to Si (OH ) ^ and finally in

Si ° 2.

The thickness of the resulting SiO 3 coating is controlled by varying the rate at which the sheet is removed from the solution.

Such a process produces a sheet of glass carrying a layer of metal oxide and the two faces of which carry a coating of silica, unless of course the uncoated side has been protected during immersion or cleaned after immersion and before heating. The second silica coating has a negligible effect on the properties of the glass sheet.

Continuous silica deposition process

Sheets or continuous ribbon of hot glass coated with metal oxide is / are contacted with silicon hydride in vapor phase. If the glass is hot enough, for example at a temperature above 500 ° C, the silicon hydride pyrolyses in the presence of oxygen to form an adherent coating of silica.

Example 6 of British Patent No. 1,524,326 gives a method of forming a silicon coating on a glass ribbon by exposing the ribbon to silicon hydride vapor in an oxygen-free atmosphere. This process can easily be changed / simply by performing the air coating, to form

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9 a coating of silica (SiO 4).

The influence of the variation in the thickness of a dielectric SiO 3 coating deposited on a SnO coating, 840 nm thick on the light transmittance and the infrared emissivity is shown in the following table 2 (6 mm thick float glass).

TABLE 2

Thickness SiC ^ Transmission factor Infrared light emissivity (nm) (%) 10 0 75 12 50 86.7 14.5 75 89-6 15.8 100 90. 3 17 150 85.4 19.5 15 Note that the infrared emissivity of the glazing increases linearly with the thickness of the dielectric coating while the light transmission factor reaches a maximum when the dielectric coating has a thickness of 100 nm (optical thickness 141 nm). This is due to interference effects.

Instead of forming a conductive coating of tin oxide, it is possible to use another metal oxide, for example indium oxide. Coatings of doped indium oxide can be formed in the same manner as coatings of tin oxide using a solution of indium chloride. In fact, the indium oxide coatings as thin as 100 nm have excellent low infrared emissivity. The indium oxide coatings themselves do not have a high abrasion resistance, but they can be protected by a hard surface coating, for example of silica which has better abrasion resistance . Otherwise, they are preferably used as shown in Figure 2, inside multiple glazing.

In the case where the dielectric coating is not exposed, such as the coating 6 inside the hollow glazing shown in FIG. 2, the coating can be made of a fragile material.

Examples of such relatively fragile materials are magnesium and calcium fluorides which can be deposited as thin coatings having a refractive index of 1.38 and 1.43 respectively.

Alternatively, the dielectric coating can be made of polymeric material. Many such materials have a refractive index of about 1.5. Such materials can be applied to glass sheets carrying a metal oxide coating by immersion in a solution of the desired polymeric material.

Dielectric coatings of silicone or polyurethane can be applied in this way.

It should be noted, however, that such coatings should not have a thickness greater than 160 nm in order not to lose the benefit of applying the conductive coating with low emissivity. For example, if an 840 nm SnO2 coating is surmounted by a 1,000 nm thick polymeric dielectric layer, the infrared emissivity of the glazing will be approximately 90%, substantially the same as that uncoated glass.

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Claims (10)

1. Glazing comprising a sheet transmitting light carrying on one of its faces a coating of metallic oxide conductive of electricity and transmitting light, which reduces the emissivity of the infrared radiation of this face of the glazing, characterized 5 that the compositions and thicknesses of the light transmitting sheet and of the electrically conductive coating are such that only they have a light transmittance of at least 70%, and in that the electrically conductive coating is surmounted by a dielectric coating whose thickness is less than 160 nm and which increases the light transmission factor of the glazing. 2. Glazing according to claim 1, characterized in 1 that the dielectric coating has a thickness less than 140 nm and preferably not exceeding 110 nm. 3. Glazing according to one of claims 1 or 2, ca-15 characterized in that the dielectric coating has an optical thickness in transmission of at least 60 and preferably at least 70 nm. 4. Glazing according to one of claims 1 to 3, characterized in that the refractive index of the dielectric coating is equivalent to ten percent, to the square root of the refractive index of the conductive coating of 1. 'electricity. 5. Glazing according to one of claims 1 to 4, characterized in that the electrically conductive coating comprises tin oxide. 6. Glazing according to one of claims 1 to 4, ca-25 characterized in that the electrically conductive coating comprises. indium oxide. 7. Glazing according to one of claims 1 to 6, characterized in that the dielectric coating has a higher abrasion resistance than the electrically conductive coating. 8. Glazing according to one of claims 1 to 7, ca acterized in that the dielectric coating consists of silica. 9. Glazing according to one of claims 1 to 6, characterized in that the dielectric coating is a coating of polymeric material. A a. 9 12 10. Glazing according to one of claims 1 to 9, characterized in that the light transmitting sheet is a glass sheet. ; 4 · Λ 1 X