1. Field of the Invention
The invention relates to transparent and, in particular, glass substrates covered with a stack of thin layers incorporating at least one metal layer able to reflect solar radiation and/or infrared radiation of considerable wavelength.
The invention also relates to the use of such substrates for the manufacture of solar protection or control and/or thermal insulation glazings. These glazings are used both for equipping buildings and vehicles, with a particular aim of reducing air conditioning requirements and/or reducing excessive overheating resulting from the ever-increasing size of the glazed surfaces in car bodies.
2. Discussion of the Background
A known layer stack type for giving solar protective properties to substrates is constituted by at least one metal layer, such as a silver layer, which is placed between two dielectric material layers of the metal oxide type. This stack is generally obtained by a succession of deposits carried out by a method using a vacuum such as magnetic field-assisted cathodic sputtering.
Thus, patent application WO 90/02653 discloses a laminated glazing intended for cars and whose outermost glass substrate with respect to the vehicle body is provided with a stack of five layers on its inner face in contact with the thermoplastic material interlayer. This stack consists of two silver layers intercalated with three zinc oxide layers, the silver layer closest to the outer substrate carrying the stack having a thickness slightly exceeding that of the second silver layer.
The laminated glazings according to said application are used as windscreens, which explains why they have very high light transmission values TL of approximately 75%, in order to meet the safety standards in force and therefore have a relatively high solar factor value SF. (It is pointed out that the solar factor of a glazing is the ratio between the total energy entering the room through said glazing and the incident solar energy).
The object of the invention is to develop a transparent substrate carrying a stack of thin layers having two layers which reflect radiation in the infrared and which are more particularly of a metallic type, so that they have a high selectivity, i.e. the highest possible TL/SF ratio for a given value of TL, while ensuring that said substrate has an aesthetically satisfactory visual appearance in reflection.
Accordingly one object of the invention is to provide a transparent substrate, particularly of glass and having multiple thin layers, on which are successively deposited a first dielectric material layer, a first layer having infrared reflection properties and in particular based on metal, a second dielectric material layer, a second layer having infrared reflection properties, particularly based on metal, and finally a third dielectric material layer. According to the invention, the thickness of the first layer having infrared reflection properties, i.e. that closest to the carrying substrate, corresponds to about 50-80%, particularly 55 to 75% and preferably 60-70% of the thickness of the second layer having infrared reflection properties. An advantageous example corresponds to a thickness of the first layer corresponding to about 65% of the second.
This great asymmetry in the thicknesses of the layers having infrared reflection properties makes it possible to advantageously modify the values of TL and SF so as to obtain glazings having a good selectivity, i.e. a good compromise between the need for transparency and that of providing an optimum protection against solar heat rays.
Moreover, the choice of such an asymmetry leads to another advantageous consequence. Not only does it make it possible to obtain glazings having an attractive visual appearance, particularly in reflection, i.e. having a neutral xe2x80x9cwhitewashedxe2x80x9d coloring, but the visual appearance remains virtually unchanged regardless of the angle of incidence with which the glazing is observed. This means that an external viewer of the facade of a building, entirely equipped with such glazings, does not have a visual impression of a change of shade as a function of the location on the facade at which he is looking. This characteristic of appearance in uniformity is very interesting, because it is presently highly desired by building architects.
Moreover, the visual appearance of the glazing, both in reflection and transmission, can also be refined and controlled by an adequate selection of the materials and the relative thicknesses of the three dielectric material layers.
Thus, according to a non-limiting first embodiment of the invention, the optical thickness of the first dielectric material layer is chosen to be about equal to that of the third dielectric material layer. About equal optical thicknesses is to mean within 10%, preferably within 5%, more preferably within 2.5% of the thickness of the other layer. The optical thickness of the second dielectric material layer is then advantageously chosen above or equal to 110% of the sum of the optical thicknesses of the two other dielectric material layers (i.e. the first and third layers) and preferably corresponding to about 110 to 120% of said sum.
In a second embodiment relating to the relative thicknesses of the dielectric material layers, which is advantageous, consists of choosing an optical thickness of the first dielectric material layer which exceeds the optical thickness of the third dielectric material layer. Thus, the optical thickness of the first dielectric material layer can correspond to at least 110% of the optical thickness of the third dielectric material layer, particularly at least 110 to 140%, especially 115 to 135% and preferably about 125% of the optical thickness of the latter. In the case of the drawing, it is recommended that the optical thickness of the second dielectric material layer be chosen to be about equal to the sum of the optical thicknesses of the two other dielectric material layers. About equal optical thicknesses is to mean within 10%, preferably within 5%, more preferably within 2.5% of the thickness of the other layer.
In the first and second embodiments cases, such relative proportions between the optical thicknesses of the dielectric material layers makes it possible to obtain colors in reflection and even also in transmission, which are aesthetically appreciated and in particular blue and green.
However, the second embodiment has an additional advantage compared with the first embodiment, to the extent that it optimizes the xe2x80x9cnon-sensitivityxe2x80x9d of the complete stack to thickness variations of the different dielectric material layers forming it. This means that slight thickness variations of one of the dielectric material layers in the stack does not lead to flagrant appearance deficiencies between individual glazings or on the surface of the same glazing. This is very important from the industrial standpoint, where manufacture takes place of glazings having a considerable size and/or in large numbers with the aim of retaining appearances and performance characteristics which are as uniform as possible between individual glazing batches, particularly within individual zones of the same glazing.