Deposition of a titanium oxide layer on a vitreous surface to provide an antireflective coating for e.g. glass roofs

PHOTOCATALYTIC COATING SUBSTRATE AND ITS MANUFACTURING PROCESS

The invention relates to generally transparent or semi-transparent substrates, in particular made of glass, plastic material, glass-ceramic, and which are provided with a coating with photocatalytic properties to give them an anti-fouling or, more precisely, self-cleaning function.

An important application of these substrates relates to glazing, which can have very diverse applications, from utility glazing to glazing used in household appliances, from glazing for vehicles to glazing for buildings.

It also applies to reflective glazing of the mirror type (mirror for homes or vehicle rear-view mirror) and to opacified glazing of the spandrel type.

The invention also applies, similarly, to non-transparent substrates, such as ceramic substrates or any other substrate which can in particular be used as an architectural material (metal, tiles, etc.). It is preferably applied, whatever the nature of the substrate, to substantially planar or slightly curved substrates.

Photocatalytic coatings have already been studied, in particular those based on titanium oxide crystallized in the anatase form. Their ability to degrade soiling of organic origin or microorganisms under the effect of UV radiation is very interesting. They also often have a hydrophilic character, which allows the evacuation of mineral dirt by splashing water or, for exterior glazing, by rain.

This type of coating with anti-fouling, bactericidal and algicidal properties has already been described, in particular in patent WO97 / 10186, which describes several methods of obtaining it.

The object of the invention is therefore to improve the techniques for depositing this type of coating, in particular with a view to simplifying them. At the same time, it also aims to improve the appearance of the coating, more particularly to improve the optical properties of the substrate which is provided with it.

The subject of the invention is first of all a method for depositing by cathodic sputtering a coating with photocatalytic properties comprising titanium oxide at least partially crystallized in anatase form on a transparent or semi-transparent carrier substrate. The characteristic of the invention consists in carrying out the spraying on the substrate heated to a temperature of at least 100 ° C.

In fact, as is known from the aforementioned patent WO97 / 10186, this type of coating can be deposited by cathodic sputtering. It is a vacuum technique which makes it possible, in particular, to very finely adjust the thicknesses and the stoichiometry of the deposited layers. It is generally assisted by a magnetic field for more efficiency. It can be reactive: we then start with an essentially metallic target, here based on titanium (possibly alloyed with another metal or with silicon), and the sputtering is carried out in an oxidizing atmosphere, generally an Ar / O mixture.₂. It can also be non-reactive; we then start with a target which is already in the oxidized form of titanium (possibly alloyed).

However, the layers obtained by this type of technique are generally amorphous, whereas the functionality of the coating according to the invention is directly linked to the fact that it must be significantly crystallized. This is the reason why, as recommended in the aforementioned patent, there is a need to crystallize (or increase the degree of crystallization) of the coating by subjecting it to a heat treatment, for example of the order of 30 min to several hours at at least 400 “C.

However, it has been shown in the context of the present invention that this post-deposition treatment step could be avoided, or at least made optional, by spraying the layer onto the hot substrate, and not at room temperature.

This solution has at least five advantages:

an energy saving during manufacture, * -► the possibility of using substrates which could not withstand heat treatments at temperatures of at least 400 or 500 ° C without degradation, in the event that annealing required interposing between substrate and photocatalytic coating a barrier layer to the diffusion of elements of the substrate (of the alkaline type when it comes to glass), the possibility of using a thinner barrier layer, or even completely removing the barrier layer, since the heat treatment according to the invention is much less aggressive than annealing, a much shorter manufacturing cycle (since the heat treatment of the substrate is much shorter and at a much lower temperature ), the elimination of the storage of “semi-finished” products to be annealed.

However, levels of photocatalytic activity are obtained for the coatings which are quite similar to those for the coatings deposited and then annealed.

However, this was not a gamble won in advance, insofar as one could expect that prolonged annealing would be essential in order to progressively grow the crystallized seeds within the amorphous oxide matrix. This was not the case: a hot deposition promotes the deposition of the directly crystallized layer, at least in part.

It was also not obvious that the coating thus deposited “hot” would preferentially crystallize in anatase form rather than in rutile form (the anatase form is much more photocatalytic than the rutile or broockite form of titanium oxide).

To carry out the invention, there are different variants, in particular depending on the type of spray device available. The substrate can thus be heated prior to the actual deposition, outside the vacuum chamber. The substrate can also be heated during deposition, when the deposition chamber is equipped with ad hoc heating means. The heating of the substrate can therefore take place before and / or during the spraying of the coating.

Advantageously, the substrate is, during the spraying of the layer, at a temperature between 150 and 350 ° C, preferably at least 200 ° C and in particular between 210 and 280 ° C. Surprisingly, it has therefore been possible to obtain sufficiently crystallized layers without having to heat the substrate to the temperatures generally used for annealing, of at least 400 ° C to 500 ° C. Generally, when the coating is essentially based on titanium oxide (TiCh), and when it is deposited by sputtering ("hot" or at room temperature), it has a fairly high refractive index, greater than 2 or more. at 2.2 and generally between 2.35 and 2.50 (it can be slightly substoichiometric), in particular between 2.40 and 2.45. This is a fairly specific characteristic of this type of deposition, because coatings of the same nature deposited by other techniques, for example by sol-gel, tend to be much more porous and to have significantly lower refractive indices ( below 2 and even below 1.8 or 1.7).

It has been observed that the crystallographic structure of the coatings is influenced by whether they are cold deposited and then annealed or hot deposited. Thus, quite unexpectedly, the coatings deposited “hot”, in accordance with the invention, generally have an average TiOz crystallite size generally less than or equal to 30 nm, in particular between 15 and 30 nm. (whereas the coatings deposited "cold" then annealed tend to include crystallites of larger size, of at least 30 nm, generally between 30 and 50 nm).

As seen above, coatings based on photocatalytic T1O2 have a high refractive index. This means that they are reflective and that they give their carrier substrate a reflective appearance which is often considered unattractive. Further, the color in reflection, apart from this glossiness, may be undesirable. It is not easy to improve this aspect in reflection, because the photocatalytic functionality presents constraints: The coating must generally be in contact with the external atmosphere in order to receive UV rays and degrade external soiling. It cannot therefore be overcome with a low index layer. It must also have a given minimum thickness to be sufficiently effective.

Another aspect of the present invention therefore consisted in improving the reflection aspect of the substrate, without disturbing the photocatalytic activity of the coating, in particular by lowering its light reflection as much as possible and / or by giving it a color in reflection which is the same. more neutral as possible.

A subject of the invention is therefore also the transparent or semi-transparent substrate defined above and which is provided on at least part of at least one of its faces with a photocatalytic coating comprising titanium oxide at least partially crystallized anatase , this coating having a high refractive index of at least 2 or 2.2. According to the invention, this coating is considered to be part of a stack of thin anti-reflection layers, the coating being the last layer (that is to say the layer furthest from the carrier substrate). The anti-reflection stack is composed of an alternation of high and low index layers, and therefore ends in the present case with the high photocatalytic index layer.

For the purposes of the invention, the term “layer” is understood to mean a single layer or a superposition of layers. If it is a superposition of layers, it is considered that its overall thickness is the sum of the thicknesses of each of the layers and that its global index is the average of all the refractive indices of said layers. This also applies to the photocatalytic coating.

For the purposes of the invention, the term “antireflection” is understood to mean the function which makes it possible to lower the value of light reflection of the coated substrate, and / or to attenuate its color in reflection, in particular to make it the lightest and most. neutral, as aesthetic as possible (we also speak of an effect - anti-color ").

This is a fairly loose and unexpected adaptation of conventional anti-reflective stacks. In fact, in a known manner, these stacks alternate high and low index layers and end with low index layers (as close as possible to the refractive index, equal to 1, of air) and which are generally SiO-based layers₂, MgF₂... But here, the stacking ends with a high index layer, which is quite paradoxical. However, by appropriately selecting the characteristics of the different layers, this particular anti-reflective stack succeeds in significantly reducing the reflective character intrinsic to TiO.₂ high index, and to give the substrate an acceptable color in reflection (neutral, in pale shades avoiding reds and other hot colors deemed unsightly in favor of gray, blue, or green in particular).

Advantageously, the photocatalytic coating has a refractive index greater than or equal to 2.30, in particular between 2.35 and 2.50, or between 2.40 and 2.45. It is preferably deposited by sputtering. Its optical thickness is advantageously selected, together with those of the other layers of the stack, in order to lower the light reflection of the substrate. It has been shown that an optimal optical thickness is preferably around λ / 2 with λ around 580 nm. This corresponds to an optical thickness of between 250 and 350 nm, in particular between 270 and 310 nm; and at a geometric thickness of between 80 and 120 nm, in particular between 90 and 110 nm. This range of geometric thickness has been found to be sufficient to obtain, in parallel, a photocatalytic activity considered sufficient (the photocatalytic activity in fact depends on many parameters, including the thickness but also the surface roughness, the crystalline morphology of the layer, its porosity, ...).

Depending on whether the coating is spray deposited "hot" or at room temperature cold and annealed, it contains crystallites of variable size as seen above (generally less than 30 nm "hot", and range of 30 to 50 nm or more at room temperature).

The antireflection stack of the invention, in its simplest embodiment, comprises three layers, of which, successively, a high index layer, a low index layer, then the high index photocatalytic coating.

The high index layer (s) of the stack, apart from the photocatalytic coating, generally has an index of at least 1.9, in particular between 1.9 and 2.3 or between 1.9 and 2.2. It can be zinc oxide, tin, zirconium, aluminum nitride or silicon nitride. It can also be a mixture of at least two of these compounds.

The optical thickness of these high index layers is selected. Their optimum optical thickness is preferably around λ / 10 with λ around 580 nm. This corresponds to an optical thickness of between 48 and 68 nm, in particular between 53 and 63 nm, and to a geometric thickness of between 20 and 40 nm, in particular between 25 and 35 nm.

The low index layer (s) generally have an index of between 1.4 and 1.75, in particular between 1.45 and 1.65. They may for example be based on silicon oxide, aluminum oxide or a mixture of the two. The optical thickness of these low index layers is selected: their optimum optical thickness is preferably around λ / 20 with /. around 580 nm. This corresponds to an optical thickness of between 19 and 39 nm, in particular between 25 and 35 nm, and to a geometric thickness of between 15 and 30 nm, in particular between 20 and 28 nm.

According to another variant, in the stack with three layers mentioned above, it is possible to replace the high index layer / low index layer sequence by a layer with an “intermediate” refractive index, that is to say, of preferably greater than 1.65 and less than 1.9. The preferred index range is between 1.75 and 1.85. It can be based on silicon and / or aluminum oxynitride. It can also be based on a mixture of a low index oxide such as SiO₂ and at least one oxide with a higher index such as SnO₂, ZnO, ZrO₂, TiO₂. (the relative proportion between the oxides makes it possible to adjust the index).

This intermediate layer can also be used to replace the first high index layer / low index layer sequence of a stack containing not three but five or seven layers, for example.

The optical thickness of this intermediate index layer is selected. The optimum optical thickness is around λ / 4 with λ around 580 nm. This corresponds to an optical thickness of between 120 and 150 nm, in particular between 125 and 135 nm, and to a geometric thickness of between 65 and 80 nm, in particular between 68 and 76 nm.

As mentioned above, these different selections of optical thicknesses take into account the whole aspect in reflection of the substrate: an effort is made not only to lower the value of light reflection Rl but also to give it a color considered to be aesthetic. nowadays (that is to say more in cold colors than towards yellow or red) and as less intense as possible. The best compromise must therefore be found so that, as a whole, the appearance in reflection of the substrate is better. Depending on the applications, may be preferred rather the lowering of the value of Rl or rather the selection of a particular colorimetry in reflection (for example quantified by the values ​​of a * and b * of the colorimetry system L, a *, b * or by the dominant wavelength value associated with the color purity).

Advantageously, all of the layers of the anti-reflection stack can be deposited by cathodic sputtering, one after the other, on the same production line.

According to an optional variant of the invention, it is possible to insert between the substrate and the antireflection stack a barrier layer to the species liable to diffuse from the substrate. These are in particular alkalis when the substrate is made of glass. It is, for example, based on silicon oxide (or oxycarbide): the S1O2 can be deposited by cathode sputtering and the SiOC, in a known manner, by gas phase pyrolysis (CVD). It preferably has a thickness of at least 50 nm, for example between 80 and 200 nm. Chosen from this type of material, with a relatively low index (around 1.45 to 1.55), it is in fact generally largely “neutral” on the optical level. The silicon oxide may contain minority elements, in particular chosen from Al, C, N.

The subject of the invention is also glazing, in particular single glazing (a rigid substrate), laminated glazing, multiple glazing of the double glazing type and which comprises at least one substrate coated in the manner described above.

Said glazing preferably has, thanks to the anti-reflection effect of the invention, a light reflection Rl (layer side) which remains at most 20%, in particular at most 18%. Preferably, this light reflection has a pleasant shade in blues or greens, with negative values ​​of a * and b * in the colorimetry system (L, a *, b *) and in particular less than 3 or 2.5 in absolute values. The color is thus a color that is both pleasing to the eye and pale, not very intense.

The invention will be described below in more detail, with non-limiting exemplary embodiments.

Comparative Examples 1 and 1 relate to the hot deposition of photocatalytic T1O2 layers by cathodic sputtering.

EXAMPLE 1

A first layer of S1O2 by CVD, 80 nm, then a second layer of TiO was deposited on a clear silico-soda-lime glass, 4 mm thick.₂ 90 nm photocatalytic.

The T1O2 layer was deposited by magnetic field assisted sputtering. This is a reactive sputtering, in the presence of oxygen from a titanium target. The glass is preheated to a temperature of about 220 ° C to 250 ° C. This temperature is kept constant within 5 during the spraying of the layer, with the aid of a heating device placed opposite the target.

The resulting T1O2 layer has a refractive index of 2.44. It is crystallized in the anatase form (it can also include amorphous zones), with an average crystallite size of less than 25 nm.

Its photocatalytic activity was quantified using a test using palmitic acid: This involves depositing a given thickness of palmitic acid on a photocatalytic coating, to expose it to UV radiation centered on 365 nm with a power per unit area of ​​approximately 50 W / m2 throughout the duration of the test, then to measure the speed of disappearance of palmitic acid according to the following relationship:

V (nm.h '¹) = [palmitic acid thickness (nm)] / [2 x ti / 2 disappearance (h)]

With the layer according to the invention, a photocatalytic activity of the order of 10 to 30 nm.h 'is obtained by this calculation.¹, depending on the choice of pressure type deposition parameters.

The glass thus coated with the two layers has, according to the illuminant D65, a light reflection R_L of 23%, with values ​​of a * and b * in reflection according to the colorimetry system (L, a *, b *) of the order of 17 and 28 respectively.

The photocatalytic activity of the layer is therefore interesting, but its optical aspect still clearly reflecting, with a color that is too intense.

It should be noted that it is possible to increase the photocatalytic activity of the layer by subjecting it, after deposition, to conventional annealing (for one or more hours at at least 400 ° C.). Note also that the photocatalytic activity of the layer remains substantially unchanged if an SiO sublayer is used.₂ thinner, 50 nm instead of 80 nm.

COMPARATIVE EXAMPLE 1

Example 1 is repeated, but this time the T1O2 layer is deposited on an unheated substrate, then treated for four hours at approximately 500 to 550 C. In addition, the S1O2 sublayer is thickened to 150 nm. The morphology of the layer is a little different, with an average crystallite size rather greater than 30 nm.

Its photocatalytic activity is similar to that of the layer of Example 1 without annealing, but it is lower if a lesser thickness of the SiO 2 sublayer is chosen. This therefore confirms that the "hot" deposition according to the invention, allowing " to save »an often long annealing operation, is not obtained to the detriment of the performance of the layer. This also confirms a subsidiary advantage of the invention: by hot depositing, avoiding annealing, it is possible to use, with identical photocatalytic performance, a thinner barrier sublayer (hence, here again, a manufacturing time and cost. reduced product).

Examples 2 et seq. Relate to the incorporation of a high index T1O2 photocatalytic layer, in particular deposited by cathodic sputtering, in antireflection stacks in order to improve their optical properties.

EXAMPLE 2 - (ACHIEVED)

The following stack of layers is deposited on a silico-soda-lime float glass 4 mm thick:

Glass / Si₃NOT₄⁽¹⁾ / S1O2 ⁽²⁾ / TiO₂⁽³⁾ nm 22 nm 104 nm (geometric thicknesses)

The Si layer (1)₃NOT₄ is deposited by reactive sputtering in the presence of nitrogen from an Al-doped Si target.

The S1O2 layer (2) is deposited by reactive sputtering in the presence of oxygen from an Al-doped Si target.

The T1O2 layer (3) is photocatalytic and was hot deposited as described in Example 1.

Optionally, an additional layer can be inserted between the glass and the Si layer₃NOT₄, a layer of SiO₂ of about 100 nm obtained as the other layer of SiC> 2 (2) described above. It has virtually no influence on the optical properties of the substrate and can serve as an alkali barrier layer with respect to the glass. It is optional, especially since the layers of the antireflection coating under the photocatalytic layer, namely the layers (1) and (2) themselves constitute completely satisfactory barrier layers, in addition to their optical properties: these two layers already form a 100 nm barrier to species liable to diffuse out of the glass.

Alternatively, one can use a layer of TiO₂ cold deposited and then annealed as described in comparative example 1.

In reflection on the layers side, the result for such a stack is as follows:

Rl (according to illuminant D₆₅): 17.3% a * (R_l) = -2 b * (Rl) = -2.8

Xd (nm) = 494 nm (dominant wavelength of light reflection) pe (%) = 2.5% (purity of color in reflection).

We see, compared to Example 1, a significant drop in the value of Rl, we obtain here a color in the blue-green, rather pale. Overall, therefore, there is an aesthetically improved appearance in reflection.

EXAMPLE 3

It is very close to example 2, only the thickness of the TiO layer changes a little.₂.

Here we have:

Glass / Si₃NOT₄⁽¹⁾ / SiO2 ⁽²⁾ / TiO2 ⁽³⁾ nm 22 nm 99 nm (geometric thicknesses)

The result in light reflection is as follows (with the same conventions as for example 2):

R_L = 17.9% a * = -0.8 b * = -0.7

AT_d(nm) = 494 nm pe (%) = 0.8%

So here we have a slightly different compromise, with a value of R_Lslightly higher but values ​​of a * and b * lower in absolute values.

EXAMPLE 4 - (MODELING)

It is very close to example 2, only the thickness of the first Si layer changes₃NOT₄ :

Glass / Si₃NOT₄¹¹¹ / SiO2 ^,2) / TiO2 ⁽³⁾ nm 22 nm 104 nm (geometric thicknesses)

The result in light reflection is as follows (always with the same conventions):

Rl = 15.8% a * = 0 b * = -9%_d(nm) = 475 nm pe (%) = 4.9%

Here we have greatly reduced the value of R_L, but the color in reflection has changed hue.

EXAMPLE 5 - (MODELING / COMPARATIVE)

Here, compared to Example 2, all the thicknesses change.

We have :

Glass / Si₃NOT₄⁽¹⁾ / SiO2 ⁽²⁾ / TiO2 ⁽³⁾ nm 30 nm 75 nm (geometric thicknesses)

The result in light reflection is as follows:

R_l = 25.8% a * = -0.3 b * = -0.7

Àd (nm) = 492 nm pe (%) = 0.5%

If the substrate has a satisfactory color in reflection, on the other hand it has a value of R_l well beyond 20% which is too high: the thicknesses chosen are not optimal.

EXAMPLE 6 - (MODELING / COMPARATIVE)

Here we move further away from the layer thicknesses recommended by the invention, with the following stack:

Glass / Si₃NOT₄^,1) / SiO2 ^{(Z |} / TiO2 ⁽³ⁱ nm 20 nm 60 nm (geometric thicknesses)

The result in light reflection is as follows:

Rl = 30% a * = 2.3 b * = 7.2

Àd (nm) = 587 nm pe (%) = 14%

It has at the same time a very high Rl value, a color in reflection which is little sought after and in addition to being more intense. Its appearance in reflection is therefore not satisfactory.

EXAMPLE 7 - (ACHIEVED)

The stacking is this time as follows:

Glass / Si₃NOT₄⁽¹⁾ / SiO2 ⁽²⁾ / TiO2 ⁽³⁾ nm 27 nm 105 nm (geometric thicknesses)

We therefore replaced the S13N4 with SnO₂, deposited by reactive sputtering in the presence of oxygen from a tin target.

The result in light reflection is as follows:

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17,4%

-2,8

-2,7

Àd (nm) = 496 nm pe (%) = 2.8%

The aspect in reflection is close to that obtained in Example 2.

EXAMPLE 8 - (MODEL)

Here, one substitutes for the first two layers a single layer of index

1.84 in silicon oxynitride SiON.

So we have the stack:

Glass / SiON / / TiO₂ nm 101 nm (geometric thicknesses)

The result in light reflection is as follows:

R_l= 17.4% a * = 0 b * = -1.08

/._d(nm) = 480 nm pe (%) = 1%

The aspect in reflection is therefore satisfactory.

EXAMPLE 9 - (MODEL)

It replicates Example 8, but with an index of 1.86 for the layer of

If we.

The aspect in reflection is a little modified:

R_L = 17.8% a * = -1.1 b * = -1.5

AT_d(nm) = 494 nm pe (%) = 1.3%

In conclusion, the invention has developed a new method of vacuum deposition of layers comprising photocatalytic TiO2. It has also developed a new type of antireflection stack ending in a high index layer, a stack that is easy to produce industrially and notably attenuates the reflective aspect of T1O2 without degrading its photocatalytic properties. It makes it possible to obtain glazings in blues or in pale greens in reflection, while retaining substantial photocatalytic layer thicknesses, of the order of around a hundred nanometers.

The invention in its two aspects (product and process) can be applied in the same way to photocatalytic coatings which do not contain only TiC