INFRARED RADIATION REFLECTING GLASS

TECHNICAL FIELD

The invention relates to material technology and optics. The invention concerns specifically a coated glass and a method for producing same.

BACKGROUND OF THE INVENTION

Heat loss through windows of a building is one of the most important factors contributing to the energy consumption of the building. Traditionally, heat loss of a glass has been depicted by a U-value that represents the power loss through a material. Typically, the U-value of an uncoated glass having a thickness of 4mm is about 6W/m ² K and that of a window made from such glass is about 5W/m ² K.

The U-value in windows should be considerably lower than the above-mentioned value. According to the Finnish building regulations, the U-value of a new window must be 1.4W/m ² K at the maximum. Such a U-value is achieved by using multi-glazed windows in which the space or spaces between the window glasses are filled with gas having poor thermal conductivity, or by coating the glasses used in the windows with an infrared radiation reflecting material.

The loss of heat through the windows is reduced by coating the window with a reflecting layer. Ideally, this type of film transmits the solar radiation energy into the room, which energy is mostly con- centrated at the shortwave portion of the radiation spectrum, at wavelengths below 2μm. At the same time, the film should reflect the heat radiation of the room at a wavelength of about 5 - 50μm back into the room.

The reflective film is normally either a hard coating or a soft coating. The hard coating is pro-

duced pyrolytically in conjunction with the float glass production process. It is normally tin oxide doped with fluorine or antimony, but may also be for example doped or undoped indium oxide or zinc oxide. The emissivity of the film is typically in the range of 0.14 - 0.2, i.e. it reflects 80 - 86% of the heat radiation back into the room. The soft coating is typically a multi-layer structure with a number of metal, typically silver, layers with insulating layers therebetween. The emissivity of soft-coated glasses is typically in the range of 0.04 - 0.08, i.e. they reflect 92 - 96% of the heat radiation back into the room. Soft coatings are produced in an off-line process, typically by sputtering the coating layers onto the surface of the glass in a vacuum.

Description of the known prior art is disclosed e.g. in the following publications: Report on the Coatings on Glass, Technology Roadmap Workshop held in Livermore, California, Jan. 18-19, 2000, Re- port prepared by Sandia National Laboratories, Liver- more, CA, USA; Helena Bulow-Hube, A breakthrough for coated glazing in Sweden. Will double-pane windows take over the market?, Energi och Miljό, N:o 2, 2002; M.Arbab, L . J. Shelestak and C.S.Harris, Value-Added Flat-Glass Products for the Building, Transportation Markets, Part 2, American Ceramic Society Bulletin, Vol.84, No.4, 2005, pp. 34-38.

The coatings according to the known technology, and specifically the superlow-e coatings provided by sputtering, also reflect and/or absorb solar radiation to a considerable extent. Therefore, solar energy cannot be optimally utilized in lighting and/or as heat energy in buildings. In the energy rating for windows, the "Window-Energy-Rating (WER) " system is being introduced globally, rating windows from A to G according to their energy-efficiency. In determining

the energy rating, the capacity of the window to utilize solar radiation energy and the air transmission of the window are considered in addition to the U-value. The formula for calculating the WER rating varies ac- cording to climatic zones. In Finland, the window energy rating is calculated as:

Where E is the E-value calculated for the window and determining the rating, g is the total transmission of solar energy (a value between 0 and 1) and L is the air transmission (m ³ /m ² h) .

The U-value cannot be considerably improved with new coatings, because the reflectivity is already close to the theoretically accessible maximum value. Therefore, to gain competitive strength, it is essential for the glass producer to be able to affect the g-value . Thus far, the only solution of the prior art for increasing the g-value has been to improve the basic glass. The improvement is achieved by reducing the absorption of solar radiation into the glass, which is provided by reducing the amount of impurities, spe- cifically of iron, in the glass. However, the problem of the absorption of the solar radiation into the reflective layer still persists. This problem is more significant with silver coatings than with a doped tin oxide coating. Another problem of the prior art is that the films deposited on the surface of the glass produce tinted interference rings in the glass, so that the films must be provided with multi-layer structures in order to remove these rings. The multi-layer struc- tures further increase the absorption of the solar radiation.

OBJECTIVE OF THE INVENTION

The objective of the invention is to eliminate the above-mentioned problems of the prior art and to disclose a new type of infrared radiation reflecting glass structure and a method for producing said structure. The structure according to the invention can be used e.g. in low energy windows.

SUMMARY OF THE INVENTION

The product according to the invention is characterized by what has been presented in claim 1.

The use according to the invention is characterized by what has been presented in claim 18. The infrared radiation reflecting glass according to the invention comprises a glass sheet and a surface layer arranged on the surface of the glass sheet, the surface layer being formed of an electrically conductive IR-radiation reflecting material as a structure that is perforated in a netlike manner and comprises a set of adjacent holes free from said material and distributed over the area of the glass sheet. The characteristic diameter of a hole is not more than 10 micrometres. According to the invention, the glass according to the invention is used as a window glass.

Electromagnetic radiation is reflected from the electrically conductive surface layer on the surface of the glass according to the invention, because the electromagnetic radiation that hits the reflective surface layer provides an alternating electric and magnetic field in the surface layer. The surface layer may be e.g. an electrically conductive film, coating or a similar planar electrically conductive layer. It is an important observation from the standpoint of the invention that in order to provide re-

flection, the surface layer does not need to be a uniform film. The electromagnetic radiation that hits the electrically conductive surface layer tends to produce a circulating current in the layer, which is a prereq- uisite for the reflection of the electromagnetic radiation from the layer. The formation of the circulating current (and therefore the reflection of the radiation from the surface layer) requires that the current is able to circulate in the film along a uniform closed framework. The interior of the framework may be of a non-conducting material, for example an uncoated glass having good transmission of IR-radiation and visible light.

The glass according to the invention, in which the surface layer made from the IR-radiation reflecting material is arranged into a netlike structure, operates as a high-pass filter. The glass transmits most of the electromagnetic radiation in which the frequency passes a specific threshold (or corre- spondingly, the wavelength is shorter than a specific threshold) . This threshold is determined by the size of the holes in the surface layer. Electromagnetic radiation that has longer wavelength than the threshold is reflected from the surface layer of the glass. An estimate for the wavelength threshold can be calculated by having the characteristic diameter of the uncoated area, i.e. a hole, inside the above- mentioned closed conductive framework about 35% of the wavelength threshold. It is obvious to a person skilled in the art that the "conductive framework" refers herein to a free-formed framework comprised of a conductive material and defining a non-conductive free-formed area (hole) inside it, such as a glass surface. The word "diameter" refers to the diameter of a circular area and "characteristic diameter" refers to a diameter that a free-formed area would have if

its surface area was the same as the surface area of a circular area.

In one embodiment of the invention, the characteristic diameter of a hole is not more than 3.5 mi- crometres.

Further, in one embodiment of the invention, the characteristic diameter of a hole is not more than 1.5 micrometres.

In one embodiment of the invention, the sur- face layer is a film attached to the surface of the glass sheet.

In one embodiment of the invention, the surface layer is a coating deposited on the surface of the glass sheet. In one embodiment of the invention, at least

10 holes are surrounded by a galvanically interconnected electrically conductive coating.

Galvanic contact between a number of conductive frameworks surrounding the holes enhances the in- duction of the circulating currents to the conductive parts on the surface layer and therefore promotes the operation of the glass according to the invention as an IR-filter.

In one embodiment of the invention, the pro- portion of the combined surface area of the holes in the total surface area of the surface layer is not more than 80%.

Further, in one embodiment of the invention, the proportion of the combined surface area of the holes in the total surface area of the surface layer is not more than 50%.

Further, in one embodiment of the invention, the proportion of the combined surface area of the holes in the total surface area of the surface layer is not more than 20%.

Further, in one embodiment of the invention, the proportion of the combined surface area of the holes in the total surface area of the surface layer is at least 10%. If it is desirable to reflect most of the IR- radiation from the surface layer, the characteristic diameters of the holes are small, e.g. about 1.5 micrometres (μm) . In this case, the proportion of the area coated with the electrically conductive material is large, e.g. about 80% of the total surface area of the surface layer. If it is desirable to improve the g-value of the glass, e.g. a window, the diameters of the holes in the surface layer can be selected to be larger, so that some of the short-wave IR-radiation penetrates the window. In this case, the proportion of the area coated with the electrically conductive material may be e.g. about 50% of the total surface area of the surface layer. If it is desirable to improve the g-value of the window further, the diameter of the holes in the surface layer can be further enlarged. In this case, the diameters of the holes may be e.g. about 10 μm, so that the proportion of the area coated with the electrically conductive material may be e.g. about 20% of the total surface area of the surface layer.

The width of the conductive framework may preferably be about lμm, but it is obvious that the width may vary considerably within the limits of the production technique, and the line width does not need to be the same in all areas of the coating.

In one embodiment of the invention, the surface layer contains mainly silver.

In one embodiment of the invention, the surface layer contains mainly doped or undoped tin oxide.

In one embodiment of the invention, the surface layer contains mainly doped or undoped zinc oxide .

The transmission of the glass at the visible light wavelength range can be improved by making the electrically conductive part of the surface layer of the glass from a material that transmits visible light. Such materials include e.g. tin oxide and zinc oxide. Transmission of the visible light is important when the glass according to the invention is used e.g. in windows. Due to the netlike structure, the glass according to the invention may also provide reduction of the interference of the visible light, which is why e.g. a window glass provides less distortion of the visible light from outside.

In one embodiment of the invention, the diameter of the holes varies in different areas of the glass .

In one embodiment of the invention, the thickness of the surface layer varies in different areas of the glass.

By modifying the characteristic diameter of the holes and the thickness of the surface layer in different areas of the glass, the reflection and/or transmission spectrum of the glass can be adjusted selectively as a function of the location. This is useful particularly when using large glass surfaces and/or in situations where the electromagnetic radiation that hits the glass is known to have e.g. a non- homogenous spectrum as a function of the location.

In one embodiment of the invention, the perforated surface layer is formed by pulsed laser deposition (PLD) or by perforating a uniform film and setting it on the surface of the glass as a surface layer or by forming the holes hydrometallurgically into a

uniform surface layer on the surface of the glass by etching .

In one embodiment of the invention, the perforated surface layer is formed by depositing on the surface of the glass a surface layer containing silver salts, selectively exposing the surface layer to light, reducing the surface layer to silver and developing the surface layer in order to form holes in the surface layer. The perforated surface layer of the glass according to the invention can be produced in various ways, applying e.g. thermal or plasma-assisted thin film production methods such as ALD, CVD and PECVD. Other possible methods of depositing the conductive film on the surface of the glass sheet include e.g. flame spraying and sputtering. When the glass sheet is coated with the conductive layer, the holes can be formed into the surface layer e.g. chemically by etching or by laser patterning. The perforated film may also be produced as a separate element which is added as a surface layer to the glass sheet after the perforation .

An efficient manner of producing large surface layers is e.g. pulsed laser deposition (PLD), in which the perforated surface layer may be deposited selectively directly onto the glass sheet without a separate patterning step that would require etching. A separate etching step for producing the holes is also unnecessary in a process in which the surface layer is produced selectively by exposing to light and reducing to silver a film that contains mainly silver salts and has been deposited onto the glass sheet.

DETAILED DESCRIPTION OF THE INVENTION

In the following section, the invention will be described in an exemplifying manner with reference to the accompanying drawings, in which

Fig. 1 schematically represents one embodi- ment of the glass according to the invention,

Fig. 2 schematically represents one embodiment of the glass according to the invention,

Fig. 3 schematically represents one embodiment of the glass according to the invention, Fig. 4 schematically represents one apparatus for producing one embodiment of the glass according to the invention and

Fig. 5 represents as a flow chart one method for producing one embodiment of the glass according to the invention.

For simplifying purposes, the repeating elements of the invention are referred to by the same numerals in the following examples.

Embodiments of the infrared radiation (IR- radiation) reflecting glass 1 according to the invention presented in Fig. 1 - 3 comprise a glass sheet 4, an electrically conductive surface layer 2 and un- coated areas, i.e. holes 3, in the surface layer 2. In the glass 1 presented in Fig. 1 the shape of the holes 3 is square. In the glass 1 presented in Fig. 2 the shape of the holes 3 is circular. In the glass 1 presented in Fig. 3 the diameter of the holes 3 varies in different parts of the glass 1.

The surface layer 2 of the structure accord- ing to Fig. 1 may be comprised for example of holes 3 of a rectangular or square shape having a characteristic diameter of about 5μm and being surrounded by a silver net having a line width of e.g. about lμm. The reflecting structure of Fig. 1 is realized on the sur- face of the glass sheet 4 in such manner that a silver net, in which the width of the electrically conductive

net lines associated with the surface layer 2 and defining the holes 3 is about lμm, is deposited on the surface of a flat glass. The size of the area between the net lines, i.e. of a hole 3, may be e.g. about 5μm x 5μm.

Infrared radiation at a wavelength of about 14 - 50μm is reflected back from the structure according to the example of Fig. 1, but the shorter-waved solar radiation energy at a wavelength of about less than 2μm penetrates the surface layer 2 and, depending on the absorption of the glass sheet 4, also the entire glass structure. Therefore, the wavelength threshold for this glass 1 is about 14μm. The surface area of the surface layer 2 that is not coated with silver, for example in the structure according to Fig. 1, may constitute e.g. about 70% of the total surface area of the surface layer 2, so most of the radiation that hits the structure does not fall on the silver net. Therefore, most of the electromagnetic radiation at a clearly shorter wavelength than the characteristic diameter of the holes 3 penetrates the glass structure and is not reflected/absorbed from/into the net lines. Transmission of the electromagnetic radiation that is shorter-waved than the threshold can be improved further through the glass 1 by producing the conductive part of the surface layer 2 surrounding the holes 3 of a transparent conductor material, such as tin oxide or alloys thereof (e.g. indium tin oxide, ITO) or zinc oxide or alloys thereof. The heat radiation spectrum of a room typically settles mainly in the range of about 5 - 50μm, the intensity peak setting at a wavelength of about 30μm. Therefore, when using the glass 1 according to the example of Fig. 1 e.g. as a window glass, heat losses through the windows of a building are considerably reduced, because the heat radiation at a wave-

length of about 14 - 50μm is not able to escape through the glass 1. At the same time, the shorter- waved solar radiation from outside at a wavelength of less than 2μm penetrates the glass 1 in the areas which do not have the conductive film.

Fig. 2 and Fig. 3 show a glass structure having the same operational principle as that of Fig. 1. In the glass 1 of Fig. 2, the holes 3 are circular, so the conductive film defining the holes 3 does not have a constant line width. This type of hole geometry in the surface layer 2 may intensify the induction of the circulating currents into the surface layer 2, which may sharpen the radiation spectrum of the structure, so that the wavelength threshold may be adjusted more precisely to a specific wavelength, depending on the size of the holes 3.

In the glass 1 of Fig. 3, the size of the holes 3 varies in different parts of the glass 1. This type of hole geometry can be used to accommodate to the possibly different radiation spectra in different parts of the glass 1. The varying hole size as in Fig. 3 is applicable for example when using large glass surfaces .

If it is desirable to reflect all IR- radiation of the room from the surface layer 2, the maximum of the characteristic diameter of a hole 3 is set, according to the radiation at a wavelength of 5μm, to about 1.5μm. In this case, the proportion of the area coated with the electrically conductive mate- rial may be large, e.g. about 80% of the total surface area of the surface layer 2. If it is desirable to improve the g-value of the glass, e.g. a window, the diameters of the holes 3 in the surface layer 2 can be so selected that the wavelength threshold is set at an IR-radiation of lOμm, so that some of the short-wave IR-radiation penetrates the window. However, because

the intensity of the short-wave radiation in the total IR-radiation is small, there is no substantial loss of heat energy. In this case, the diameters of the holes are about 3.5μm, so that the proportion of the area coated with the electrically conductive material may be e.g. about 50% of the total surface area of the surface layer 2. If it is desirable to improve the g- value of the window further, the diameters of the holes in the surface layer can be so selected that the wavelength threshold is set at an IR-radiation of 30μm. In this case, the diameters of the holes are about 10μm, so that the proportion of the area coated with the electrically conductive material may be e.g. about 20% of the total surface area of the surface layer 2.

When the glass is used as a window glass, the goal may be to achieve a g-value that is as high as possible and a U-value that is as low as possible. In this case, the size of the holes 3 in the surface layer 2 should be dimensioned to be so small that the IR-radiation of the room is reflected as largely as possible from the surface layer 2. At the same time, however, the combined surface area of the holes 3 in the total surface area of the surface layer 2 must be maintained as large as possible so that the g-value of the glass would not be reduced. Consequently, it is logical to make small holes 3 close to each other in the surface layer 2, in which case the line width of the conductive areas surrounding the holes 3 may re- main small. Too small a line width may in turn impede the production of the circulating currents, which should be taken into consideration when designing the surface layer 2.

An efficient manner of producing large sur- face layers 2 is e.g. pulsed laser deposition (PLD) in which the perforated surface layer may be deposited

selectively directly onto the glass sheet without a separate patterning step requiring etching. Fig. 4 shows the operating principle of a method based on PLD for producing one embodiment of the infrared radiation reflecting glass 1 according to the invention. Fig. 4 shows the principle of one embodiment of the method. A glass sheet 4 is placed in a sputtering device or a corresponding vacuum device in which the glass sheet 4 advances on conveyor rolls or the like. For the pur- poses of clarity, these rolls or the sputtering device have not been drawn in the figure. Under the glass sheet 4 there is a silver-coated sheet 5 which may be for example a silver-coated strip moving from one roll to another. A pulsed laser beam 6 is directed through the glass sheet 4 to the silver-coated sheet 5 using a laser source 7 and a rotating prism 8. Correspondingly, a second pulsed laser beam 9 is directed from a second laser source 10 through the glass sheet 4 to the silver-coated sheet 5. The laser beams cause the silver to peel off from the surface of the silver- coated sheet 5, and the silver evaporates and adheres to the surface of the glass sheet 4 as a silver strip 11, 12. The rectangular or square areas defined by the vertical 11 and horizontal 12 silver strips and not being coated with silver remain as holes 3 on the surface layer 2 of the glass 1.

A separate etching step for producing the holes 3 is also unnecessary in a process (presented in Fig. 5) in which the surface layer 2 is produced by reducing to silver a film that contains mainly silver salts and is deposited onto the glass sheet 4 (step Sl) . In this method, the surface layer 2 is perforated by selectively exposing to light (step S2) the film containing the silver salts, e.g. through a suitable mask, so that the film is reduced to metallic silver in the exposed areas, while the composition in the un-

exposed areas remains substantially the same. The selectively exposed film may then be developed (step S3) with a suitable chemical in such manner that only the areas reduced to silver remain on the surface of the glass sheet 4 as part of the surface layer 2. The light-sensitive silver salt applicable in the method is e.g. silver bromide. The developing step S3 of the method resembles in all essential respects the process of developing a photograph. It is obvious to a person skilled in the art that the embodiments presented in the examples are not the only ways of providing the perforated conductive surface layer 2. This type of surface may also be provided e.g. by masking the surface prior to coating it with silver or by perforating a uniform silver film that is positioned onto the glass sheet 4 after perforation. What is substantial from the standpoint of the invention is that the electrically conductive part of the surface layer 2 is not uniform but comprises holes 3.

The invention is not limited merely to the examples referred to above; instead, many variations are possible within the scope of the claims.