EP2947179A1

EP2947179A1 - Method of fabricating a coated substrate

Info

Publication number: EP2947179A1
Application number: EP14305750.3A
Authority: EP
Inventors: Guillaume De Combarieu; David Grosso; Xavier PAQUEZ; Cédric BOISSIERE
Original assignee: Centre National de la Recherche Scientifique CNRS; Universite Pierre et Marie Curie Paris 6; Areva Renouvelables SAS
Current assignee: Centre National de la Recherche Scientifique CNRS; Universite Pierre et Marie Curie Paris 6; Adwen France SAS
Priority date: 2014-05-21
Filing date: 2014-05-21
Publication date: 2015-11-25

Abstract

The method of coating a substrate by sol-gel deposition comprises:
- a step of depositing a layer of a first sol-gel composition on the substrate, the first sol-gel composition being intended to form a first film, the first sol-gel composition comprising at least one ruthenium dioxide (RuO₂) precursor and at least one inorganic matrix precursor; and
- a step of firing the layer of first sol-gel composition up to a first temperature; and then
- a step of depositing a layer of a second sol-gel composition on the layer of first sol-gel composition, the second sol-gel composition being intended to form an inorganic second film, ruthenium being substantially absent of the second sol-gel composition; and
- a step of firing the layer of first sol-gel composition and the layer of second sol-gel composition up to a second temperature strictly higher than the first temperature, such as to obtain the first film comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and the second film deposited on the first film, ruthenium being substantially absent of the second film.

Description

The present invention relates to a method of fabricating a material comprising a substrate and a coating, namely for use in a solar receiver.
Solar thermal power plants use solar radiations to heat a thermal fluid. A solar thermal power plant generally comprises a solar receiver having at least one receiver tube for circulation of the thermal fluid, the receiver tube being subjected to solar radiations, namely concentrated solar radiations, in view of heating the thermal fluid. It is desirable that the receiver tube exhibits high absorbance, low emissivity and long term stability under high working temperatures, to promote efficiency and durability of the receiver tube.
One of the aims of the invention is to propose a method of fabricating a coated substrate exhibiting high absorbance, low emissivity and long term stability under high working temperatures.
To this end, the invention proposes a method of coating a substrate by sol-gel deposition, the method comprising:

a step of depositing a layer of a first sol-gel composition on the substrate, the first sol-gel composition being intended to form a first film, the first sol-gel composition comprising at least one ruthenium dioxide precursor and at least one inorganic matrix precursor; and
a step of firing the layer of first sol-gel composition up to a first temperature; and then
a step of depositing a layer of a second sol-gel composition on the layer of first sol-gel composition, the second sol-gel composition being intended to form an inorganic second film, ruthenium being substantially absent of the second sol-gel composition; and
a step of firing the layer of first sol-gel composition and the layer of second sol-gel composition up to a second temperature strictly higher than the first temperature, such as to obtain the first film comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and the second film deposited on the first film, ruthenium being substantially absent of the second film.

In specific embodiments, the method may optionally comprise one or several of the following features:

the first temperature is comprised between 150 and 400 °C, namely between 250 and 300 °C, preferably around 300 °C;
the duration of the step of firing up to the first temperature is comprised between 10 seconds and 30 minutes;
the second temperature is superior to 350°C, preferably superior to 500°C, namely between 550°C and 700°C;
the duration of the step of firing up to the second temperature is at least 20 seconds;
the first sol-gel composition comprises as matrix precursor at least one silicon alkoxide, at least one metal alkoxide and/or at least one metal chloride;
the first sol-gel composition comprises TEOS, CeCl₃, ZrCl₄, TiCl₄ and/or AlCl₃ as matrix precursor;
the second sol-gel composition comprises as matrix precursor at least one silicon alkoxide, at least one metal alkoxide and/or at least one metal chloride;
the second sol-gel composition comprises TEOS, CeCl₃, ZrCl₄, TiCl₄ and/or AlCl₃ as matrix precursor;
the coating comprises RuO₂ nanoparticles having a size comprised between 2 and 40 nm, preferably between 2 and 30 nm, more preferably between 2 and 20nm, namely between 2 and 10 nm;
the method comprises forming the first film with several layers, by repeating for each layer the steps of depositing a layer of the first sol-gel composition on the substrate to form a first film layer and the step of firing the layer up to the first temperature;
the first film has a thickness which is at least 10 nm;
the second film has a thickness which is at least 5 nm.

The invention also relates to a coated substrate comprising a first film deposited on the substrate, the first film comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and a second film deposited on the first film, ruthenium being substantially absent of the second film.
The invention further relates a solar receiver element made of a coated substrate as defined above.
The invention and its advantages will be better understood upon reading the following description given solely by way of example and with reference to the attached drawings, in which:

Figure 1 is a cross-sectional view of coated substrate;
Figure 2 is a cross-sectional view of a solar collector comprising receiver tubes;
Figures 3 - 7 are graphics illustrating reflectance measured for example coatings.

The substrate 4 illustrated on Figure 1 coated with a multilayer solar selective coating 6. The substrate 4 has opposed first face 4A and second face 4B. The coating 6 covers the first face 4A.
The substrate 4 has any shaped required for the desired application. The substrate 4 is for example a sheet or a tube.
The substrate 4 is made of any suitable material. In a preferred embodiment, the substrate is made of metal, namely of stainless steel. Alternatively, the substrate 4 is made of a ceramic material, for instance Fe oxide, Cr oxide, Ni oxide, Al oxide, Si oxide, or mixed oxides thereof, glass, steel oxide, silicon, germanium, carbide, fluoride, silicide or boride.
The coating 6 comprises a first film 8 deposited on the substrate 4, said first film 8 comprising ruthenium dioxide (RuO₂) nanoparticles 10 dispersed in an inorganic matrix 12, and a second film 14 deposited on the first film 8, ruthenium being substantially absent from the second film 14. The first film 8 is sandwiched between the substrate 4 and the second film 14.
The first film 8 and the second film 14 are deposited by a sol-gel process.
The term "sol-gel process" denotes a process of known type wherein a medium comprising at least one mineral precursor (for example a mineral alkoxide of formula M(OR)n or a metallic salt of formula MXn, wherein M denotes Si or a metal such as Al, Zr, Ce, Ti, Y, Nb Hf or B; n denotes the valence of the element M; O denotes oxygen; and the n groups OR or X are identical or different organic groups or anionic counter anions such as X = Cl-, Br-, NO₃-, or R = H, respectively) is hydrolyzed. As it hydrolyses, the precursor is converted into hydroxylated species (typically of formula M(OH)n wherein M and n have the meanings given hereinbefore) which condense together to form mineral oxide by a process which is comparable to a polymerization of the mineral precursor. Typically, those hydrolysis and condensation reactions first yield a sol (suspension of oxide entities of various degree of polymerization from monomers to dense particles) which gradually becomes a gel through percolation of the said entities; hence the generic expression "sol-gel" given to processes of this type.
A coating obtained by a sol-gel process is obtained by depositing the sol on the substrate or, preferably, by depositing on the substrate the sol-gel composition comprising the mineral precursor in the partially hydrolyzed-condensed state only (for example in the sol state) and then allowing gelling of the composition to take place only in the deposit so made. The gelling is usually triggered and considerably sped up by the evaporation of the non-volatile components (mainly solvent) after deposition of the sol layer onto the substrate.
In order to strengthen the coating obtained after drying of the gel, it can be of interest to subject the coating to a heat treatment, in a so-called "annealing" step.
Coatings produced by the sol-gel technique are in the form of mineral oxide matrices wherein different species can be trapped, which can be used to modify the physicochemical properties of the coating. In order to trap such species within the coating, the species must simply be introduced into the composition for forming the sol, the species thereby ultimately being trapped in the gel and then in the solid matrix obtained after drying of the gel.
In the present case, the first film 8 and the second film 14 are deposited on the substrate 4 by operating the following steps:

a first deposition step of depositing a layer of a first sol-gel composition on the substrate, the first sol-gel composition being intended to form the first film 8, the first sol-gel composition comprising at least one ruthenium dioxide (RuO₂) precursor and at least one inorganic matrix precursor; and
a first firing step of firing the layer of first sol-gel composition up to a first temperature; and then
a second deposition step of depositing a layer of a second sol-gel composition on the layer of first sol-gel composition, the second sol-gel composition being intended to form the second film 14, ruthenium being substantially absent of the second sol-gel composition; and
a second firing step of firing the layer of first sol-gel composition and the layer of second sol-gel composition up to a second temperature strictly higher than the first temperature, such as to obtain the first film 8 comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and the second film 14 deposited on the first film 8, ruthenium being substantially absent of the second film.

The first sol-gel composition comprises at least one matrix precursor.
The first sol-gel composition advantageously comprises, as the main matrix precursor, at least one silicon alkoxide. A preferred silicon alkoxide is tetraethyl orthosilicate or TEOS (Si-(OC₂H₅)₄). Hydrolysis and condensation of TEOS result in polymeric gel of SiO₂.
The first sol-gel composition advantageously comprises, as matrix precursor, at least one metal chloride. Preferred metal chloride are: AlCl₃, ZrCl₄, CeCl₃, TiCl₄, BCl₃. Hydrolyze and condensation of these metal chloride results in particles of aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), cerium dioxide (CeO₂), titanium dioxide (TiO₂) and boron oxide (B₂O₃). These must be combined with Si02 to form the amorphous matrix as a solid state solution at temperatures below the temperature of crystallization.
In a preferred embodiment, the first sol-gel composition comprises as matrix precursor essentially TEOS. The resulting matrix is thus formed essentially of a solid amorphous network of SiO₂.
In a variant the first sol-gel composition comprises a mixture of matrix precursors.
In a preferred embodiment, the first sol-gel composition comprises, as matrix precursor, a mixture of TEOS and at least one metal chloride, in particular AlCl₃. The resulting matrix is thus formed of a solid network of amorphous aluminosilicate network.. The volume ratio of TEOS over AlCl₃ in the first sol-gel composition is comprised between 1:0 to 1:1, preferably comprised for example between 2:1 and 4:1.
The first sol-gel composition comprises at least one nanoparticle precursor. Preferably, the first sol-gel composition comprises at least one RuO₂ nanoparticle precursor. A preferred RuO₂ precursor is ruthenium trichloride (RuCl₃), but one can use nitrate, acetate, amines salts of Ru or even preformed RuO₂, for instance.
The volume ratio of matrix precursor(s) over nanoparticles precursor(s) in the sol-gel composition is preferably comprised between 20:1 and 1:1.
The first sol-gel composition comprises a solvent. In one embodiment, the solvent comprises one or several alcohols, such as methanol, ethanol, propanol, butanol and isomers thereof. A preferred solvent is ethanol or EtOH (C₂H₆O). Optionally, water is added as a co-solvent and as reactive to allow hydrolysis.
Optionally, the first sol-gel composition comprises one or more acidic components to control the pH of the solution and control hydrolysis rate. In a preferred embodiment, the first sol-gel composition comprises hydrogen chloride (HCl) and/or trihydrogen phosphate (H₃PO₄).
The first sol-gel composition is prepared by dissolving the matrix precursor(s) and the nanoparticle precursor(s) into the media (solvent, acid, water).
Preferably, the first sol-gel composition is prepared by dissolving the nanoparticle precursor into the solvent and then dissolving the matrix precursor or mixture of matrix precursors into the solvent, and then optionally adding water and one or more acidic components.
The second sol-gel composition is similar to the first sol-gel composition except that ruthenium is substantially absent of the second sol-gel composition. The second sol-gel composition is deprived of RuO₂ nanoparticles precursor.
The second sol-gel composition comprises one matrix precursor or a mixture of matrix precursors.
The second sol-gel composition advantageously comprises, as matrix precursor, at least one silicon alkoxide. A preferred silicon alkoxide is TEOS.
The second sol-gel composition advantageously comprises, as matrix precursor, at least one metal chloride. Preferred metal chloride are: AlCl₃, ZrCl₄, CeCl₃, TiCl₄, B(OH)₃.
In a preferred embodiment, the second sol-gel composition comprises as matrix precursor essentially TEOS.
In another preferred embodiment, the second sol-gel composition comprises as matrix precursor a mixture of TEOS and at least one metal chloride, in particular AlCl₃. The resulting second film is thus formed of a solid network of SiO₂ and Al₂O₃ particles. The volume ratio of TEOS over AlCl₃ in the first sol-gel composition is preferably comprised for example between 2:1 and 4:1.
The second sol-gel composition comprises a solvent. In one embodiment, the solvent comprises one or several alcohols, such as methanol, ethanol, propanol, butanol and isomers thereof. A preferred solvent is ethanol or EtOH (C₂H₆O). Optionally, water is added as a co-solvent to promote hydrolysis.
Optionally, the second sol-gel composition comprises one or more acidic components. In a preferred embodiment, the first sol-gel composition comprises HCl and/or H₃PO₄.
In a preferred embodiment, the second sol-gel composition is identical to the first sol-gel composition except that the nanoparticles precursor is omitted.
Once the first sol-gel composition is prepared, the first sol-gel composition is allowed to age during a certain period of time to promote hydrolysis and partial condensation of before deposition.
Similarly, once the second sol-gel composition is prepared, the second sol-gel composition is allowed to age during a certain period of time to promote hydrolysis and partial condensation of the before deposition.
A layer of the first sol-gel composition is deposited over the substrate 4. The first sol-gel composition is deposited over the substrate for example by dip coating or spray coating or any other processes able to deposit a layer of uniform characteristics such as thickness, pattern, to obtain a film of optical quality.
The first step of firing is operated under conditions of time and temperature adapted to rigidify sufficiently the SiO₂ network. These conditions are however low enough to prevent too much diffusion that would lead to the full crystallisation of RuO₂ and above all the growth of the particles by diffusive sintering. The time and temperature conditions are thus critical and chosen with great care.
The first temperature is comprised between 150°C and 400°C, preferably between 250°C and 350°C. The first firing temperature is preferably around 300°C.
The firing up to the first temperature is operated during a time period comprised between 10 seconds and 30 minutes. The time period is preferably around 1 to 5 minutes.
A layer of the second sol-gel composition is then deposited over the first film, for example by dip coating or spray coating or any other process able to deposit a film of uniform characteristics such as thickness, pattern, to obtain a film of optical quality.
The second step of firing is operated under temperature conditions allowing completing the conversion of the nanoparticles precursor into nanoparticles.
The second temperature is superior to 350°C, preferably superior to 500°C, namely between 550 and 700°C.
The firing up to the second temperature is operated during a time period of at least 20 seconds.
After firing up to the second temperature, the first film and the second film are formed.
The substrate 4 is thus coated with the first film 8 deposited on the substrate 4, the first film comprising RuO₂ nanoparticles dispersed in the inorganic matrix, and the second film 14 deposited over the first film 8, ruthenium being substantially absent of the second film 14.
The matrix 12 is advantageously formed of a solid structure comprising mainly amorphous silicon dioxide (SiO₂) network, optionally reinforced with aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), cerium dioxide (CeO₂), titanium dioxide (TiO₂) and/or boron trioxide (B₂O₃).
The second film 14 is formed of a solid inorganic matrix. The matrix is advantageously formed of a solid structure comprising mainly an amorphous silicon dioxide (SiO₂) network, optionally reinforced with aluminum oxide (Al₂O₃), zirconium dioxide (ZrO₂), cerium dioxide (CeO₂), titanium dioxide (TiO₂) and/or boron trioxide (B₂O₃).
Owing to the invention, the first film embedding RuO₂ nanoparticles, is obtained in a controlled manner. Indeed, it practically proves difficult to fire a sol-gel composition containing RuO₂ precursors without avoiding growth of RuO₂ particles from nanometric sizes to micrometric sizes, which will depart the silica network and will be found laying on the silica surface. RuO₂ microparticles do not have the same optical properties as RuO₂ nanoparticles because of the wave length of the considered light. Without being bonded to any theory, this may be due to a migration of Ru-containing intermediates towards the free surface and the said migration being activated by heat during firing. However, the second film obtained from a second sol-gel composition deprived of ruthenium coated over the first sol-gel composition before completing firing of the first film, forms a barrier against Ru diffusion towards the free surface. As a result, stable RuO₂ nanoparticles dispersed in an inorganic matrix are obtained in a controlled manner. They are stabilized against growth by diffusive sintering.
Besides, the steps of firing stabilize and strengthen the first film 8 and the second film 14 by drying the sol-gel composition and promoting condensation of the films.
The RuO₂ nanoparticles of the first film have nanometric sizes. Nanometric sizes here refer to a size that is inferior to 100nm. In a preferred embodiment, the RuO₂ nanoparticles have a size comprises between 2 and 40 nm, preferably between 2 and 30 nm, more preferably between 2 and 20nm, namely between 2 and 10 nm. The size of the RuO₂ nanoparticles is the diameter of the nanoparticles as measured by X-Ray diffraction (XRD).
The first film is formed with a thickness of at least 10 nm
In a variant, the first film is formed of one single layer.
In another variant, the first film is formed of several layers by repeating for each layer the sequence of depositing a layer of the first sol-gel composition on the substrate and then firing the deposited layer (along with previously deposited layer(s)) up to the first temperature. Layers are deposited on the substrate and fired up to the first temperature until the desired thickness is reached for the first film, and then the second film is deposited over the layers of first sol-gel composition and subsequently the layers of first sol-gel composition and the layer of second sol-gel composition are fired to the second temperature.
The second film is formed with a thickness of at least 5 nm, namely at least 10 nm.
The second film may be formed with on single layer or alternatively with several superimposed layers by repeating the steps of depositing a layer of the second sol-gel composition and firing up to the second temperature (along with previously deposited layer(s) of first sol-gel composition and layer(s) of second sol-gel composition). The final total thickness of the second film is related to the minimal thickness of the layer stated above, which then can be stacked successively to obtain any value of total thickness.
The substrate 4 is for example made of suitable materials. In a preferred embodiment, the substrate 4 is made of metal.
The RuO₂ nanoparticles are particularly suited to absorb light radiations. The stability of the RuO₂ nanoparticles is compatible with use of the coated substrate for example in a solar receiver tube, to enhance heat absorption in the receiver tube.
Figure 2 is a cross-sectional view of a solar linear Fresnel receiver (LFR) comprising a Fresnel reflector assembly 20 comprising several reflector 22 for reflecting incident solar radiations towards a solar receiver 24 comprising several receiver tubes 26 for circulation of a heat transfer fluid. Each receiver tube 26 is made of a coated substrate as described above, the substrate being coated on its external surface with solar selective coating comprising a first film doped with RuO₂ nanoparticles and a second film substantially deprived of ruthenium.

Example 1

A coating is deposited on a substrate as follows:

Example 1	TEOS (g)	AlCl₃ (g)	RuCl₃ (g)	EtOH (g)	H₂O (g)	HCl (g)	H₃PO₄ (g)
first composition	1,67	-	1	21,1	1,04	0,173	0,03
second composition	2,44	-	-	21,51	1,05	0,15	-

Example 1	Deposition method	Withdrawal speed (mm/s)	Curing temperature (°C)	Curing time (S)	Thickness (nm)
first composition	dip coating		4	300	60	60-70
second composition	dip coating	1	600	300	50-55

The mean size of the RuO₂ nanoparticles in the coating is around 4 nm.
Figure 3 illustrates four curves of measures of the reflectance as a function of the wavelength, each curve corresponding to a respective temperature as follows (27,4 °C, 299,6°C, 449,0°C and 599,4 °C).
Figure 3 shows no significant variation of optical properties of the coating of example 1 in the range of temperature from approximately ambient temperature to 600°C.

Example 2:

A coating is deposited on a substrate as follows:

Example 2	TEOS (g)	AlCl3 (g)	RuCl3 (g)	EtOH (g)	H2O (g)	HCl (g)	H3P04 (g)
first composition	1,67	-	1,33	21,1	1,04	0,17	0,03
second composition	2,44	-	-	21,51	1,05	0,15	-

Example 2	Deposition method	Withdrawal speed (mm/s)	Curing temperature (°C)	Curing time (s)	Thickness (nm)
first composition	dip coating	3	300	60	50-60
second composition	dip coating		3,8	600	300	80-90

The mean size of the RuO₂ nanoparticles in the coating is around 7 nm.
Figure 4 illustrates six curves of measures of the reflectance of the coating of example 2 as a function of the wavelength after ageing at 600°C under air, each curve corresponding to a respective ageing time (ageing of 0 hours; 20 hours; 70 hours; 130 hours; 156 hours; 180 hours).
Figure 4 shows no significant deterioration in optical properties in the visible light wavelength range.

Example 3

A coating is deposited on a substrate as follows:

Example 3	TEOS (g)	AlCl3 (g)	RuCl3 (g)	EtOH (g)	H2O (g)	HCl (g)	H3P04 (g)
first composition	1,67	-	1,33	21,1	1,04	0,17	0,03
second composition	2,44	-	-	21,51	1,05	0,15	-

Example 3	Deposition method	Withdrawal speed (mm/s)	Curing temperature (°C)	Curing time (s)	Thickness (nm)
first composition	dip coating	3	250	60	50-60
second composition	dip coating	1	600	300	50-55

The mean size of the RuO₂ nanoparticles in the coating is around 5 nm.
Figure 5 illustrates fives curves of measures of the reflectance of the coating of example 3 as a function of the wavelength after ageing at 600°C under air, each curve corresponding to a respective ageing time (ageing of 0 hours; 50 hours; 90 hours; 110 hours; 170 hours). It will be noticed that two curves are superimposed, whereby only four curves are distinguishable on Figure 5.
Figure 5 shows no significant deterioration in optical properties in visible light wavelength range.

Example 4

Example 4 is a comparative example in which the coating is formed of a first film with the first composition as in example 1 and is deposited as in example 2 without provided a second film and using a curing temperature of 500°C.
Figure 6 shows a curve of measure of the reflectance of the coating of comparative example 4 as a function of the wavelength at ambient temperature.
Reflectance is significantly lower and the optical properties obtained are not good enough for the application to a solar reflector of a solar power plant.

Example 5:

A coating is deposited on a substrate as follows:

Example 5	TEOS (g)	AlCl₃ (g)	RuCl3 (g)	EtOH (g)	H2O (g)	HCl (g)	H3PO4 (g)
first composition	1,67	-	1,33	21,1	1,04	0,17	-
second composition	2,44	-	-	21,51	1,05	0,15	-

Example 5	Deposition method	Withdrawal speed (mm/s)	Curing temperature (°C)	Curing time (s)	Thickness (nm)
first composition	dip coating	3	300	60	50-60
second composition	1st layer by dip coating	1	600	300	50-55
	2^nd layer by dip coating	1	600	300	50-55

Figure 7 shows a curve illustrating the measure of the reflectance as a function of wavelength after ageing at 600°C under air during 225 hours. As illustrated on Figure 7, reflectance is good. This example shows the possibility to coat several layers of the second sol-gel composition with obtaining good optical properties for the coating.

Claims

Method of coating a substrate by sol-gel deposition, the method comprising:
- a step of depositing a layer of a first sol-gel composition on the substrate, the first sol-gel composition being intended to form a first film, the first sol-gel composition comprising at least one ruthenium dioxide (RuO₂) precursor and at least one inorganic matrix precursor; and

- a step of firing the layer of first sol-gel composition up to a first temperature; and then

- a step of depositing a layer of a second sol-gel composition on the layer of first sol-gel composition, the second sol-gel composition being intended to form an inorganic second film, ruthenium being substantially absent of the second sol-gel composition; and

- a step of firing the layer of first sol-gel composition and the layer of second sol-gel composition up to a second temperature strictly higher than the first temperature, such as to obtain the first film comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and the second film deposited on the first film, ruthenium being substantially absent of the second film.
Method according to claim 1, wherein the first temperature is comprised between 150 and 400 °C, namely between 250 and 300 °C, preferably around 300 °C.
Method according to claim 1 or 2, wherein the duration of the step of firing to the first temperature is comprised between 10 seconds and 30 minutes.
Method according to claim any one of the preceding claims, wherein the second temperature is superior to 350°C, preferably superior to 500°C, namely between 550°C and 700°C
Method according to claim any one of the preceding claims, wherein the duration of the step of firing up to the second temperature is at least 20 seconds.
Method according to any one of the preceding claims, wherein the first sol-gel composition comprises as matrix precursor at least one silicon alkoxide, at least one metal alkoxide and/or at least one metal chloride.
Method according to any one of the preceding claims, wherein the first sol-gel composition comprises TEOS, CeCl₃, ZrCl₄, TiCl₄ and/or AlCl₃ as matrix precursor.
Method according to any one of the preceding claims, wherein the second sol-gel composition comprises as matrix precursor at least one silicon alkoxide, at least one metal alkoxide and/or at least one metal chloride.
Method according to any one of the preceding claims, wherein the second sol-gel composition comprises TEOS, CeCl₃, ZrCl₄, TiCl₄ and/or AlCl₃ as matrix precursor.
Method according to any one of the preceding claims, wherein the coating comprises RuO₂ nanoparticles having a size comprised between 2 and 40 nm, preferably between 2 and 30 nm, more preferably between 2 and 20nm, namely between 2 and 10 nm.
Method according to any one of the preceding claims, comprising forming the first film with several layers, by repeating for each layer the steps of depositing a layer of the first sol-gel composition on the substrate and the step of firing the layer of first sol-gel composition up to the first temperature.
Method according to anyone of the preceding claims, wherein the first film has a thickness which is at least 10 nm.
Method according to anyone of the preceding claims, wherein the second film has a thickness which is at least 5 nm.
Coated substrate comprising a first film deposited on the substrate, the first film comprising nanoparticles containing ruthenium dioxide (RuO₂) dispersed in an inorganic matrix, and a second film deposited on the first film, ruthenium being substantially absent of the second film.
Solar receiver element made of a coated substrate as in claim 14.