Glass substrates coated with a stack of thin layers having reflective properties for infrared and/or solar radiation.

0001The invention relates to transparent substrates, in particular glass, coated with a stack of thin layers comprising at least one metallic layer capable of acting on solar radiation and on infrared radiation of long wavelength.

0002The invention also relates to the use of such substrates for manufacturing glazing for thermal insulation and / or sun protection. These are intended both to equip buildings and vehicles, with a view in particular to reducing the air conditioning effort and / or reducing excessive overheating caused by the ever increasing importance of the glazed surfaces in the passenger compartment.

0003A known type of stack of thin layers intended to give the substrate properties, in particular low-emissivity, consists of at least one metallic layer such as a silver layer placed between two layers of dielectric material such as layers of metal oxide. This stack is generally obtained by a succession of deposits made by a technique using vacuum, such as sputtering possibly assisted by magnetic field.

0004If the silver layer essentially determines the thermal performance, in particular in terms of emissivity of the coated glazing, the layers of dielectric material fulfill several roles since they act first of all on the optical appearance of the substrate in an interference manner. They also make it possible to protect the silver layer from chemical and / or mechanical aggressions.

0005It is thus known from European patent application EP-A-0 226 993 a stack comprising a silver layer interposed between two layers of a dielectric material consisting of a mixed oxide of tin and zinc. This stack also has very thin so-called metal bonding layers, in particular copper, arranged at the interface between the silver layer and each of the oxide layers in order to increase mutual adhesion.

0006Furthermore, French patent FR-B-2 641 271 describes a stack in which the silver layer is interposed between two coatings of dielectric material, each of these coatings being made up of a plurality of metal oxide layers. The coating underlying the silver layer consists of three superimposed oxide layers including a tin oxide layer, the one adjacent to the silver layer being zinc oxide and having according to this document a protective effect of silver, in particular by making it less vulnerable to attack by oxygen. The thickness of the zinc layer must remain low, because, according to this patent, zinc oxide, which is not very resistant moreover, would risk, if too thick, to weaken the entire stack.

0007However, it is not only a question of increasing the durability of the stack by increasing its resistance to corrosion of an oxidative, chemical or mechanical nature. The essential objective of this type of stack remains an optimization of its thermal performance. However, until then, the simplest solution to lower the emissivity consisted in increasing consequently the thickness of the layer of functional silver, solution presenting two disadvantages on the optical and esthetic level: on the one hand, an increase thickness causes a reduction in the light transmission of the carrier substrate, therefore a reduction in its transparency. On the other hand, it is accompanied by a much more marked coloring of the latter, in particular in reflection. It was therefore necessary to establish a compromise, by privileging according to the needs either rather the thermal properties, or rather the optical properties of the stack.

0008The object of the invention is then to develop a new type of stack with functional layer (s) which has both improved thermal and optical properties, and very particularly both low emissivity and a very high light transmission.

0009The solution according to the invention consists of a transparent substrate, in particular made of glass, provided with a stack of thin layers comprising at least one layer with properties in the field of solar radiation and / or in the field of infrared, in particular metallic and low-emissive, which is placed between two coatings based on dielectric material. Said stack is designed, according to the invention, so that the coating underlying this metal layer comprises at least one "wetting" coating comprising a layer based on zinc oxide deposited on an oxide layer metallic with formula M_xO_y with y / x ≧ 2.5, and which may in particular be based on niobium oxide or tantalum oxide. It can also be antimony oxide Sb₂O₅ or tungsten oxide WO₃. Said layer based on zinc oxide is also optionally surmounted by a thin layer based on metal such as zinc, titanium, tantalum, tin or a metal alloy such as nickel-chromium.

0010The metallic layer which has properties in the field of solar radiation and / or in the infrared is within the framework of the invention preferably low-emissive and based on silver. It is hereinafter referred to as the "functional layer".

0011Quite surprisingly, if we compare the performance, in terms of optical and thermal properties, of a stack as defined in this way with those of a stack comprising the same functional layer of the same thickness but devoid of the sequence consisting of the oxide-based layer M_xO_y with y / x ≧ 2.5 surmounted by the zinc oxide layer under the functional layer, it can be seen that the stack according to the invention gives its carrier substrate both a very low emissivity and a transmission very high light. The authors of the invention have thus discovered that the choice of the nature of the coatings of dielectric material, and very particularly of the coating underlying the functional layer, had an influence not only on the chemical and / or physical durability of the stack protecting the functional layer, but also on its intrinsic properties, such as its emissivity value or its light transmission.

0012To explain this phenomenon, we can put forward the hypothesis that the superposition of a metal oxide with a high oxygen content, such as niobium or tantalum oxide and a zinc oxide, would create particularly favorable conditions for a good wetting, good nucleation of the functional metal layer. Said layer would then be more homogeneous, more continuous, which would explain the exceptional improvement in its thermal and optical properties. What is quite surprising is that it has thus been shown that it is a superposition of specific and different oxide layers which allows such a result. This would mean that the growth of the functional layer on the directly underlying oxide layer would not take place independently of the layer which is placed below this oxide layer, and that there would be synergy, cooperation between the two superimposed layers to provide the optimal deposition surface for the functional layer. On the contrary, one might have thought, at first sight, that only the layer intended to be brought directly into contact with the silver layer could have any role of "wetting". However, it has been verified that a functional layer of the same performance cannot be obtained if it is deposited on a single oxide layer directly deposited on the substrate, whether it is made of niobium, tantalum or zinc oxide.

0013The invention thus allows obtaining lower emissivity values ​​with identical functional layer thickness, or even an unchanged emissivity level for a reduced thickness.

0014The great advantage of the invention lies in the fact that this optimization of the emissivity does not operate at the expense of optical performance, in particular of light transmission, and of the neutrality of the coloring, very particularly in reflection. However, this is a crucial point both in the building sector and in the automobile industry, for example. Previously, as already mentioned, lower emissivity values ​​were obtained only at the cost of a reduction in light transmission and more intense coloring, in parallel.

0015The invention now offers the possibility of making the two characteristics evolve favorably at the same time, by lowering the emissivity and by increasing the light transmission of the stack at a given functional layer thickness.

0016Preferably, a layer based on metal of the niobium, titanium, tantalum type or on the basis of a metal alloy such as nickel-chromium, tantalum-chromium or niobium-chromium is also deposited on the functional layer. It is said to be "sacrificial" because its role is to protect the functional layer from oxidation during the subsequent deposition of the dielectric material above it, deposition usually carried out by reactive sputtering, that is to say in the presence of oxygen. In the final product, this sacrificial layer is therefore, depending on its thickness and the deposition conditions, partially or completely oxidized.

0017The coating of dielectric material which is located above the functional layer advantageously comprises at least one layer based on tin oxide, tantalum oxide, zinc oxide, niobium oxide or titanium oxide, and / or a layer of silicon nitride. It is possible to select a layer of an oxide or even to superimpose two layers of oxides according to different criteria, which can be, for example, criteria for the rate of deposition of the oxides (the tin oxide is deposited very quickly by sputtering) or specific hardness criteria (niobium oxide and tantalum oxide as well as silicon nitride are known to have a higher resistance, in particular mechanical, than other oxides).

0018Regarding the thicknesses of the layers and coatings of the stack according to the invention, it is preferred to choose a functional layer thickness of between 7 and 13 nanometers, preferably between 9 and 12 nm. However, if we aim less at a very high T glazing_L and very low-emissivity (rather for cold countries), but rather a glazing perhaps a little less transparent but more anti-solar (rather for hot countries), one can of course increase this thickness of functional layer up to 20 to 25 nm.

0019The overall thickness of the underlying coating is advantageously between 20 and 55 nanometers, preferably between 30 and 45 nm.

0020The overall coating thickness above the functional layer is also advantageously between 20 and 55 nanometers.

0021Advantageously, the layer based on zinc oxide has a thickness of between 3 and 40 nanometers, preferably between 10 and 35 nanometers, and in particular greater than 15 nanometers.

0022At the same time, a thickness of between 3 and 40 nanometers, in particular between 10 and 35 nanometers and more particularly between 5 and 25 nm, for example at least 10, is chosen for the layer based on niobium or tantalum oxide. nanometers.

0023Furthermore, the thickness of the sacrificial layer is preferably chosen to be between 0.5 and 6 nanometers, in particular between 1 and 3 nanometers. For the optional metallic layer which can overcome the zinc oxide layer, a range of thicknesses preferably ranging from 0.2 to 2 nanometers is chosen.

0024The invention also relates to multiple glazing of the insulating, low-emitting type incorporating at least one substrate coated with such a stack. If it is a double glazing, the stack of layers is preferably arranged on face 3 and / or on face 2, while if it is a triple glazing, it is possible to envisage to have a stack on face 3, on face 5 or on these two faces. (It should be remembered that, conventionally, the faces are numbered from the outside, once the glazing has been fitted).

0025The invention also relates to laminated glazings incorporating at least one substrate coated with such a stack of layers. They can then be advantageously used as anti-solar glazing, and are very particularly useful for equipping automobiles, given their high light transmission. They can also be used as heated glazing, providing current leads for the layers with properties in the infrared of the silver type. Indeed, such layers, with low emissivity, also have a high electrical conductivity.

0026The use of the substrates according to the invention is thus very varied, since they can be used both in the manufacture of multiple insulating glazing units and in that of laminated glazing units.

0027Double glazing can thus be produced, the emissivity of which is less than or equal to 0.05, in particular less than 0.045, and the light transmission of which nevertheless remains very high: the latter may indeed be greater than or equal to 80%. We can also emphasize that, in double-glazing, we obtain in reflection saturation values ​​c * in the colorimetry system (L, a *, b *) of at most 5, which results in a color in reflection very neutral, very aesthetic.

0028It goes without saying that the invention can also advantageously be applied to substrates having not one but several functional layers, for example two functional layers interposed between three coatings based on dielectric material. At least one of the functional layers is then provided with the wetting coating according to the invention. One can thus have a stack of the type Nb₂O₅ / ZnO / Ag / Nb / ZnO / Nb₂O₅ / ZnO / Ag / Nb / SnO₂. The thicknesses of the silver layers and of the overall thicknesses of the dielectric coatings on either side of them can advantageously be chosen from the ranges recommended in European patent application EP-0 638 528.

0029The details and advantageous characteristics of the invention will now emerge from the following nonlimiting examples, with the aid of FIG. 1.

0030It is specified that, in these examples, the successive deposits of thin layers are made by a sputtering technique assisted by magnetic field, but could also be carried out by any technique using vacuum and allowing good control of the layer thicknesses obtained.

0031The substrates on which the stacks are deposited are substrates of soda-lime-silica glass 4 millimeters thick.

0032In FIG. 1, the glass substrate 1 is surmounted by a layer of zinc oxide 3 which is placed on a layer of niobium oxide 2. Then there is the functional layer based on silver 4, surmounted a sacrificial layer 5 based on niobium, above which is placed a layer of tin oxide 6. This figure is very schematic, and, for clarity, does not respect the proportions as to the thicknesses of the various materials represented.

0033The deposition installation comprises at least one spraying chamber provided with cathodes equipped with targets made of suitable materials under which the substrate 1 passes successively. These deposition conditions by each of the layers are as follows:<ul id="ul0001" list-style="dash"><li>the silver-based layer 4 is deposited using a silver target, under a pressure of 8.10⁻³ mbar (0.8 Pa) in an argon atmosphere,</li><li>layer 6 based on SnO₂ is deposited by reactive sputtering using a tin target, under a pressure of 3.10⁻³ mbar (0.3 Pa) and in an argon / oxygen atmosphere of which 60% by volume d 'oxygen,</li><li>layers 2 and 3 based on Nb₂O₅ and ZnO are deposited by reactive spraying using respectively a niobium target and a zinc target, under a pressure of 3.10⁻³ mbar (0.3 Pa) and in an argon / oxygen atmosphere of which approximately 60% by volume of oxygen, for the deposition of ZnO and 25% for the deposition of Nb₂O₅.</li><li>layer 5 based on Nb is deposited using a niobium target, under a pressure of 8.10⁻³ mbar (0.8 Pa) and in an argon atmosphere.</li></ul>

0034The power densities and the running speeds of the substrate are adjusted in a known manner to obtain the desired layer thicknesses.

0035Examples 1 to 4 are produced in accordance with the invention, with stacks as illustrated in FIG. 1. The following examples 5 to 8 are comparative examples, they lack the sequence Nb₂O₅ / ZnO under the layer d money and will highlight the advantages of the invention.

0036Table 1 below indicates, in nanometers, the thicknesses of the layers involved in the stacks according to Examples 1 to 6:<tables id="tabl0001" num="0001"><img file="EP0678484A2_D0001.tif" /></tables>

0037The last comparative examples 7 and 8 differ from the previous ones by the type of the sequence of layers interposed between the glass 1 and the silver layer 4:

EXAMPLE 7

0038The sequence of layers is as follows:<dl id="dl0001"><dt>glass (1)</dt><dd>: 4 mm</dd><dt>SnO₂</dt><dd>: 38 nm</dd><dt>Ag (4)</dt><dd>: 10.5 nm</dd><dt>Nb (5)</dt><dd>: 1.2 nm</dd><dt>SnO₂ (6)</dt><dd>: 38 nm.</dd></dl>

EXAMPLE 8

0039He recommends a layer of ZnO (3) under the silver layer, but deposited on an oxide different from the niobium or tantalum oxide recommended in the invention.

0040The sequence according to this example is as follows:<dl id="dl0002"><dt>glass (1)</dt><dd>: 4 mm</dd><dt>SnO₂</dt><dd>: 28 nm</dd><dt>ZnO (3)</dt><dd>: 10nm</dd><dt>Ag (4)</dt><dd>: 10.5 nm</dd><dt>Nb (5)</dt><dd>: 1.2 nm</dd><dt>SnO₂ (6)</dt><dd>: 38 nm.</dd></dl>

0041Each of the 8 substrates thus coated is then mounted in double glazing using another similar glass substrate (soda-lime-silica 4 mm thick) separated from the first by a gas slide, here of l 'argon, 15 millimeters thick.

0042Table 2 below indicates, for examples 1 to 4, the light transmission value T_L in percentage, (measured according to the illuminant D₆₅) the value of solar factor FS (without unit, and which corresponds to the ratio of the total energy passing through the glazing on the incident solar energy), the value of surface thermal transmission called factor K and expressed in W / m².K, the emissivity value ε without unit, and the values ​​a * and b * of the coloring in reflection according to the colorimetry system (L, a *, b *). <tables id="tabl0002" num="0002"><table frame="all"><title>TABLE 2</title><tgroup cols="7" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="22.50mm" /><colspec colnum="2" colname="col2" colwidth="22.50mm" /><colspec colnum="3" colname="col3" colwidth="22.50mm" /><colspec colnum="4" colname="col4" colwidth="22.50mm" /><colspec colnum="5" colname="col5" colwidth="22.50mm" /><colspec colnum="6" colname="col6" colwidth="22.50mm" /><colspec colnum="7" colname="col7" colwidth="22.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center"><b>Example</b></entry><entry namest="col2" nameend="col2" align="center"><b>T</b>_<b>L</b></entry><entry namest="col3" nameend="col3" align="center"><b>FS</b></entry><entry namest="col4" nameend="col4" align="center"><b>K</b></entry><entry namest="col5" nameend="col5" align="center">ε</entry><entry namest="col6" nameend="col6" align="center"><b>at*</b></entry><entry namest="col7" nameend="col7" align="center"><b>b *</b></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">1</entry><entry namest="col2" nameend="col2" align="center">81.1</entry><entry namest="col3" nameend="col3" align="center">0.62</entry><entry namest="col4" nameend="col4" align="center">1.13</entry><entry namest="col5" nameend="col5" align="center">0.043</entry><entry namest="col6" nameend="col6" align="center">1.8</entry><entry namest="col7" nameend="col7" align="center">-3.8</entry></row><row><entry namest="col1" nameend="col1" align="center">2</entry><entry namest="col2" nameend="col2" align="center">80.3</entry><entry namest="col3" nameend="col3" align="center">-</entry><entry namest="col4" nameend="col4" align="center">1.14</entry><entry namest="col5" nameend="col5" align="center">0.045</entry><entry namest="col6" nameend="col6" align="center">-0.25</entry><entry namest="col7" nameend="col7" align="center">-2.4</entry></row><row><entry namest="col1" nameend="col1" align="center">3</entry><entry namest="col2" nameend="col2" align="center">80.3</entry><entry namest="col3" nameend="col3" align="center">-</entry><entry namest="col4" nameend="col4" align="center">1.14</entry><entry namest="col5" nameend="col5" align="center">0.046</entry><entry namest="col6" nameend="col6" align="center">-0.6</entry><entry namest="col7" nameend="col7" align="center">-2.4</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">4</entry><entry namest="col2" nameend="col2" align="center">80.1</entry><entry namest="col3" nameend="col3" align="center">-</entry><entry namest="col4" nameend="col4" align="center">1.13</entry><entry namest="col5" nameend="col5" align="center">0.042</entry><entry namest="col6" nameend="col6" align="center">-0.4</entry><entry namest="col7" nameend="col7" align="center">-4.9</entry></row></tbody></tgroup></table></tables>

0043Table 3 below now groups the light transmission values ​​T_L and of emissivity ε of the double glazing units according to all the comparative examples 5 to 8, as well as for example 1 according to the invention, for the record: <tables id="tabl0003" num="0003"><table frame="all"><title>TABLE 3</title><tgroup cols="3" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="52.50mm" /><colspec colnum="2" colname="col2" colwidth="52.50mm" /><colspec colnum="3" colname="col3" colwidth="52.50mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center"><b>EXAMPLES</b></entry><entry namest="col2" nameend="col2" align="center"><b>TL</b></entry><entry namest="col3" nameend="col3" align="center">ε</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">1</entry><entry namest="col2" nameend="col2" align="center">81.1</entry><entry namest="col3" nameend="col3" align="center">0.043</entry></row><row><entry namest="col1" nameend="col1" align="center">5</entry><entry namest="col2" nameend="col2" align="center">78.5</entry><entry namest="col3" nameend="col3" align="center">0.062</entry></row><row><entry namest="col1" nameend="col1" align="center">6</entry><entry namest="col2" nameend="col2" align="center">79.2</entry><entry namest="col3" nameend="col3" align="center">0.060</entry></row><row><entry namest="col1" nameend="col1" align="center">7</entry><entry namest="col2" nameend="col2" align="center">77.5</entry><entry namest="col3" nameend="col3" align="center">0.077</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">8</entry><entry namest="col2" nameend="col2" align="center">79.7</entry><entry namest="col3" nameend="col3" align="center">0.053</entry></row></tbody></tgroup></table></tables>

EXAMPLE 9

0044This example is in accordance with the invention. It is close to examples 1 to 3 above, with the difference that it adds an additional layer of SnO₂ between the glass (1) and the Nb₂O₅ (2) / ZnO (3) sequence of the invention. Table 4 below indicates the thicknesses of the layers of the stack in nm:<tables id="tabl0004" num="0004"><table frame="all"><title>TABLE 4</title><tgroup cols="2" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="78.75mm" /><colspec colnum="2" colname="col2" colwidth="78.75mm" /><thead valign="top"><row><entry namest="col1" nameend="col1" align="center"><b>GLASS (1)</b></entry><entry namest="col2" nameend="col2" align="center"><b>EXAMPLE 9</b></entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="left">SnO₂</entry><entry namest="col2" nameend="col2" align="center">20</entry></row><row><entry namest="col1" nameend="col1" align="left">Nb₂O₅ (2)</entry><entry namest="col2" nameend="col2" align="center">8</entry></row><row><entry namest="col1" nameend="col1" align="left">ZnO (3)</entry><entry namest="col2" nameend="col2" align="center">12</entry></row><row><entry namest="col1" nameend="col1" align="left">Ag (4)</entry><entry namest="col2" nameend="col2" align="center">10</entry></row><row><entry namest="col1" nameend="col1" align="left">Nb (5)</entry><entry namest="col2" nameend="col2" align="center">1</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="left">SnO₂</entry><entry namest="col2" nameend="col2" align="center">40</entry></row></tbody></tgroup></table></tables>

0045Table 5 below groups together for this substrate, mounted in double glazing like the substrates of the previous examples, the values ​​of T_L, FS, K, ε, a * and b * explained above:<tables id="tabl0005" num="0005"><img file="EP0678484A2_D0002.tif" /></tables>

0046Chemical resistance tests, in particular resistance to acid attack, showed that all of Examples 1 to 4 → and 10 according to the invention exhibited satisfactory durability.

0047The surface condition of the silver layers in some of these examples was analyzed by atomic force microscopy (by reproducing the stacks a second time but only "up to" the silver layer, without continuing the deposition of the layers. following). We were thus able to assess their roughness, measured by the value of the mean square deviation (known as “Root Mean Square” or RMS in English), on a surface of one micrometer and expressed in angstroms:<ul id="ul0002" list-style="none"><li>The silver layer according to Example 2, that is to say deposited on the sequence of Nb₂O₅ / ZnO oxides is of an RMS roughness of 7 angstroms,</li><li>The silver layer according to Example 8, that is to say deposited on the SnO₂ / ZnO oxide sequence, has an RMS roughness of 25 angstroms,</li><li>The silver layer according to Example 7, that is to say deposited on a single layer of SnO₂ has an RMS roughness of 30 angstroms,</li><li>The silver layer according to Example 5, that is to say deposited on a single layer of Nb₂O₅ has an RMS roughness of 20 angstroms,</li><li>· The silver layer according to Example 6, that is to say deposited on a single layer of ZnO has an RMS roughness of 27 angstroms.</li></ul>

0048An analysis by X-ray spectroscopy was also carried out: the analysis on the silver layer according to Example 2 has two very characteristic peaks, the first at 34.2 degrees corresponding to the crystallization peak of ZnO, the second at 38.25 degrees, significantly higher and corresponding to the peak of crystallization of silver.

0049The same analysis made on the silver layers of examples 5 and 6 gives a very different result: the ZnO of example 6 is almost amorphous (not really a peak), and the silver layer has a pseudo-peak 20 times lower than in the case of Example 2. For Example 6, the "peak" of silver is completely nonexistent, indistinguishable from "background noise".

0050Different remarks can be made in view of these results: Tables 1 and 2, it can be seen that all of the examples 1 to 4 and 9 according to the invention allow the production of functional double glazing crossing the threshold of 80% of T_L while keeping the emissivity very low, at a value less than 0.050. In addition, the saturation c * (equal to the square root of the sum of the squares of a * and b *) remains confined to a value of at most 5. Such a combination of optical and thermal performance is very advantageous and surprising, knowing that the silver layer intrinsically has a certain absorbent character tending to decrease the light transmission.

0051From Table 2, one can also emphasize the great flexibility of implementation of the invention: since, according to the invention, the coating is divided between glass 1 and silver layer 4 into two oxide layers 2, 3, it is possible to adjust / modify the geometric thickness of each of them, even if it it is desirable to keep overall for all of these two layers a similar optical thickness (the optical thickness is the product of the geometric thickness of a layer or of a set of layers by the refractive indices of the constituent materials of these). It is noted that the variations in thickness of each of the layers 2, 3 do not cause very large variations in the performance of the stack, which is an asset.

0052Tables 4 and 5 verify that the advantageous effects of the sequence of oxides according to the invention are preserved under the silver layer, even if their thicknesses are slightly reduced and by adding, to compensate, an additional layer of SnO₂ on the substrate, which has the advantage of a high deposition rate and a moderate cost.

0053Can be taken into account the relative deposition rates of the oxides of layers 2, 3: if we favor high production yields, we can especially prefer to deposit a layer 3 of ZnO a little thicker and a layer 2 of Nb₂O₅ a bit finer. (Zinc oxide deposits faster than niobium oxide).

0054On the other hand, niobium oxide acts favorably on the colorimetry, in particular, of the glazing, because of its high refractive index. Everything is therefore a question of choice of criteria and the invention allows very fine optimizations.

0055From Tables 2 and 3, it can be verified that only the Nb₂O₅ (or Ta₂O₅) / ZnO sequence allows optimal deposition of the silver layer, thereby improving its properties. Thus, Examples 5 and 6 which recommend a single layer respectively in Nb₂O₅ or in ZnO between the glass 1 and the silver layer 4 lead to significantly higher emissivity values ​​and values ​​of T_L significantly lower, even if they are a little better than that of Example 7 inserting only SnO₂ between glass and silver.

0056There is therefore indeed an additional technical wetting effect which a single oxide layer does not seem to be able to achieve, whatever the nature of the oxide.

0057Example 8 is close to the teaching of the aforementioned patent FR-B-2,641,271, using a thin layer of zinc oxide below the silver layer. It can be seen that it also fails to match the performances of Examples 1 to 4 according to the invention. It may be noted in this connection that the stacks according to the invention use zinc oxide layers of significant thickness, without however posing any particular problems of durability, which again reinforces the idea of ​​a synergy between the two layers of oxides 2, 3, which would perhaps increase their mutual adhesion, which would have the consequence of slowing down the deterioration or delamination of the zinc oxide.

0058Analysis results by atomic force microscopy and by X-ray spectroscopy confirm that only the sequence of oxides according to the invention under the silver layer makes it possible to improve the quality thereof, and this in two ways: first by smoothing it, making it much less rough, then promoting its crystallization in an extremely clear manner, which seems in fact caused by the better crystallization of the zinc oxide layer on which it grows.

0059Furthermore, the choice of a specific sacrificial layer 4 is not indifferent from the optical point of view, and from the point of view of the durability of the stack. Thus, the Nb layer has proved to be interesting, insofar as it is a little less absorbent, for example, than an alloy of the nickel-chromium type. It therefore hardly penalizes stacking in terms of light transmission.

0060It is also possible to deposit a thin metal-based layer under the silver layer, possibly to further improve its growth, but above all to further increase the durability of the stack. It is not essential, as shown by the good results of the stacks according to Examples 1 to 4 and 9 which are devoid of it. However, it can indeed have a beneficial effect, in particular on the corrosion resistance of the stack.