Design, properties and degradation mechanisms of Pt-Al2O3 multilayer coating for high temperature solar thermal applications

    Design, properties and degradation mechanisms of Pt-AL2O3 Multilayer coating for High Temperature Solar Thermal applications Carine Gremion, Christian Seassal, Emmanuel Drouard, Arnaud Gerthoffer, Nathalie Pelissier, Cedric Ducros PII: DOI: Reference:

S0257-8972(15)00528-9 doi: 10.1016/j.surfcoat.2015.08.076 SCT 20551

To appear in:

Surface & Coatings Technology

Received date: Revised date: Accepted date:

17 April 2015 12 August 2015 12 August 2015

Please cite this article as: Carine Gremion, Christian Seassal, Emmanuel Drouard, Arnaud Gerthoffer, Nathalie Pelissier, Cedric Ducros, Design, properties and degradation mechanisms of Pt-AL2O3 Multilayer coating for High Temperature Solar Thermal applications, Surface & Coatings Technology (2015), doi: 10.1016/j.surfcoat.2015.08.076

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ACCEPTED MANUSCRIPT Design, properties and degradation mechanisms of Pt-AL2O3 Multilayer coating for High Temperature Solar Thermal applications Carine GREMION1*, Christian SEASSAL2, Emmanuel DROUARD2, Arnaud

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Univ. Grenoble Alpes, F-38000 Grenoble France, CEA, LITEN, F-38054 Grenoble, France

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GERTHOFFER1, Nathalie PELISSIER1, Cedric DUCROS1.

Université de Lyon, Institut des Nanotechnologies de Lyon (INL), UMR CNRS 5270 École

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Centrale de Lyon, 36 avenue Guy de Collongue, F-69134 Écully Cedex, France *corresponding author: [email protected] ; phone: +33438780728 ; address: CEA

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Grenoble, 17 rue des Martyrs 38054 Grenoble cedex 9

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Abstract:

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Thin films composite materials, especially alumina-platinum (Al2O3-Pt) multilayer coatings, are promising materials for Concentrated Solar Power (CSP) applications, because of their

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good resistance to heat and oxidation. In this paper, we will present our work on the design and realization of such coatings. Our absorbers are composed of a substrate (Si, stainless steel and Inconel), a metallic infrared (IR) reflector and an alternation of thin Al2O3 and Pt layers. The absorber structure was optimized by optical simulations and then we used magnetron sputtering to deposit these coatings on different substrates. Then we made optical characterization, Transmission electron microscopy (TEM) and chemical characterization to study these coating as deposited and after thermal ageing at 650°C in air. By using different kind of IR reflector (molybdenum (Mo) reflector, Pt reflector or no reflector) we demonstrate that the choice of this layer is of great importance for the stability of the whole absorber. We show that Mo reflector is not suitable for applications at high temperature in air. Best results were obtained with a 7 layer stack, comprising a Pt reflector. A solar absorption of α = 0.93 1

ACCEPTED MANUSCRIPT and a thermal emissivity of ε = 0.43 (calculated for a temperature of 650°C) were achieved

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after ageing at a constant temperature of 650°C in air during 100 h.

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Key words: Solar thermal absorbers, Multilayer coating, Selective absorber, Platinum,

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Alumina, Thermal ageing, Ageing tests

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1 Introduction

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Solar energy harvesting is a promising way to provide carbon free energy in the future. One

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way to harvest such energy can be solar thermal power systems, which unlike photovoltaic

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cells, convert solar energy into heat and don’t use the photoelectrical effect. The produced heat can be used in industrial processes implying heating or in thermal power stations.

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To convert efficiently solar energy to heat, a good optical selectivity is required, i.e. an absorption as close as 100 % in the visible range and an emissivity as close as 0 % in the

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infrared (IR) range [1]. It can also be expressed in terms of reflectivity with a reflectivity close to 0 % in the visible range and close to 100 % in the IR range. The transition between

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these two wavelength ranges depends on the working temperature of the absorber, as it

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defined the intrinsic emissivity of the material, and is usually comprised between 1.5 and 2µm

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in wavelength. Very few materials have good intrinsic selectivity, which means we need to engineer them to achieve the desired optical properties. The most commonly used absorbers are black paints, but they can’t withstand temperatures higher than 200°C. Recent studies

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focused on cermet (ceramic metal composites) or multilayer structures because they can benefit from the thermal stability and transparency of the ceramic and the high absorption of metals [2].

Attaining high temperature is also a key issue of thermal solar power. Nowadays, commercial solutions can operate around 300 to 500°C and need a vacuum environment, because of the high oxidation or diffusion rate of the used materials at higher temperature [3]. For example, it’s the case of molybdenum (Mo) - silica (SiO2) metal-ceramic composite (or cermet), where the Mo particles imbedded in the SiO2 matrix tend to oxidize rapidly in air. But, to be cost effective, solar thermal power needs to operate at temperature higher than 600°C, to enhance the conversion yield of the thermal energy into electricity via Carnot heat engine. It must also 3

ACCEPTED MANUSCRIPT be able to work in air to avoid the cost of the vacuum tube. This requires working with materials which can withstand such high temperature and are chemically stable for more than 20 years.

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Noble metals are a good choice for this application, because of their good resistance to

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oxidation and their metallic optical behavior, especially platinum (Pt) due to its high melting temperature (2041,3 K) [4]. Many absorbers were made with cermet composed of an alumina (Al2O3) matrix containing metal particles, because of Al2O3 resistance to oxidation and

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thermal stability. For example, Thornton et al. [5] deposited Pt-Al2O3 cermet on different IR reflector materials and studied resistance to ageing at temperatures ranging from 300°C to

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600°C for 150 h. If the absorption varied of more than 2 %, it was considered that the absorber had failed, but the authors have not analyzed their samples to understand the

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degradation mechanisms. More recently, Nuru et al. [6] studied the thermal stability of a three

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layer structure made of a 7 nm Pt layer between to Al2O3 layers on a copper (Cu) substrate.

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They showed that, for temperatures of 600°C and higher, the Cu from the substrate diffused in the absorber to the surface and degraded the optical properties of the absorber. The Cu diffusion effect was so strong that it overshadowed every others possible mechanisms. These

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studies highlight the need for more precise understanding of the degradation mechanisms taking place in a Pt-Al2O3 based absorber, to be able to avoid this degradation or, at least, reduce it. In this study, we optimized and deposited a solar selective absorber, based on a Pt-Al2O3 multilayer structure. We used optical simulation to optimize our absorbers absorption and emissivity. Pt-Al2O3 multilayers were realized by magnetron sputtering. We used Transmission electron microscopy (TEM) imaging to study the impact of the Pt layer thickness on the morphology of this layer and Glow Discharge Spectroscopy (GDS) to analyze the chemical composition of our structure. Reflectance spectra were measured to 4

ACCEPTED MANUSCRIPT study the optical properties of our coatings. We also investigated the influence of the IR reflector material on the optical properties of the coating, by realizing absorbers with IR reflector materials such as molybdenum (Mo), Pt or no IR reflector layer. The impact of the

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evolution of the optical properties during ageing at 650°C.

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IR reflector material on the thermal stability of the absorber was studied by measuring the

2 Materials and Methods

First of all, we used an optical optimization program, called Optilayer, to determine the best

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multilayer configuration.

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Figure 1

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The Figure 1 represents the two main designs of structures, optimized and realized in our study. The three layer structure presented on Figure 1 a) was used to study the morphology of

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the Pt layers depending on their thickness and the chemical composition of the layers. Figure 1 b) represents the complete absorber structure we designed. We used a needle optimization

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software program, called Optilayer, to determine the best number of layers and the best layer thickness, for our needs. We fixed a target reflectance value of 0 % of reflectance between 300 nm and 1700 nm to the program, because the absorption is more critical than the emissivity in term of overall performance of the system [7]. We also implemented the basic structure and materials we wanted to use and Optilayer gave us the best number of layers and the best thickness for each layer to fit the reflectance target. We started with a structure composed of a Si substrate, an IR reflector layer, an Al2O3 layer, a Pt layer and another Al2O3 layer and we defined thickness ranges for each layer. Then the program calculated the best thickness for each layer to fit the specified target the most accurately. The simulation program repeated the same operation for a 2 Pt layers structure, a 3 Pt layers structures and so on, until 5

ACCEPTED MANUSCRIPT we stop the calculation. It must be taken into account that our simulations are done with the hypothesis that our layers are continuous. Then, the optimized structures were deposited on 25x25 mm² Si (100) substrates and on

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25x25 mm² glass substrates for the three layers structure and the Pt IR reflector structure and

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additionally on 45x45 mm² 316 stainless steel (SS) substrates for the structure with no reflector or Mo reflector. Before deposition, the substrates were washed using a cleaning equipment. The Mo reflective layer was deposited by direct current (DC) magnetron

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sputtering of a Mo target. The Pt layer deposition was made by DC-pulsed sputtering of a Pt target in an Ar atmosphere and the Al2O3 layer deposition was realized by reactive sputtering

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of an Al target in a mixt O2-Ar atmosphere (11 % O2, 89 % Ar). We measure the poisoning of the Al target to select our condition of deposition. The multilayer structure was obtained by

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sliding the substrate holder in front of the Pt or Al target alternatively. The deposition rate

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was approximately 14.9 nm/min for the Pt and 8.7 nm/min for the Al2O3.The pressure in the

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chamber was 5,4.10-3 mbar for the Pt and 4,5.10-3 mbar for the Al2O3. We used a power of 200 W on the Pt target and 600W on the Al2O3 target.

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The optical measurements were realized in the UV-visible range (250-2500 nm) with a Perkin-Elmer Lambda 950 spectrophotometer and in the IR range (2500-20000 nm) with a Bruker Equinox 55 spectrophotometer. In both cases, we made our measurements with an integrating sphere. After these measurements, we calculated the absorption and emissivity of each sample according to the following equations, given by Arancibia-Bulnes et al. [8]: α = (∫_300^2000 (1-R(λ)) * Φ(λ) * dλ) / (∫_300^2000 Φ(λ) * dλ) (1) ε= (∫_300^20000 (1-R(λ)) * ε_0 (T,λ) * dλ) / (∫_300^20000 ε_0 (T,λ) * dλ) (2) where α is the absorption, ε is the emissivity, R the reflectivity, λ the wavelength, Φ the solar energy as a function of the wavelength and ε0 the black body radiation as a function of the 6

ACCEPTED MANUSCRIPT wavelength for a chosen temperature. In this study, we chose a temperature of 650°C, which was the temperature used for our ageing tests. TEM images were realized with a JEOL 2000FX and a JEOL 3010 for the high resolution

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images. Energy Dispersive X-ray Spectroscopy (EDS) was also performed during TEM

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imaging. The samples for the top view images were deposited on copper grids. Those for the cross-section were deposited on Si substrates, and then pieces were cleaved and bonded face to face. The obtained structure was thinned in the middle by Focus Ion Beam until it was thin

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enough for TEM imaging.

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Glow Discharge Spectroscopy (GDS) analysis were performed with an Horiba GD Profiler 2. The ageing tests were realized in a resistive heating furnace, in ambient atmosphere. The

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samples were heated at a rate of 200°C/h until they reached the temperature of 650°C. After

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the desired ageing time, the furnace was left to cool without any applied cooling rate, for 30

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hours.

3 Results and discussion

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3.1 Simulation results

Needle optimization enables us to rapidly test several configurations with different number of layers. In order to do this, we implement the basic structure and materials we want to use and Optilayer gives us the best number of layers and the best thickness for each layer to fit the reflectance target. The results are summarized in Table I and they indicate that adding more than 3 Pt layers doesn’t improve the absorption much, but rather increases the emissivity. It also increases the complexity of the structure and the deposition time, so we choose this 3 Pt layers structure.

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ACCEPTED MANUSCRIPT Table I

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As we studied different materials as IR reflector, we optimized the layers’ thickness for each

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of the different materials (Mo, Pt or no reflector) while keeping the same number of layers.

thickness varies between 2.5 and 8 nm.

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3.2 Material and optical characterizations

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The Al2O3 layers thickness was comprised between 60 and 80 nm, while the Pt layers

We began our study with three layer samples, to analyze the basic material properties. On Si

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(100) and glass substrate, we deposited a simplified structure composed of a Pt layer comprised between two 50 nm thick layers of Al2O3. The Pt layer thickness was 2, 5 or 10

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nm. TEM images showed us that for thickness less than 10 nm, the Pt layer wasn’t

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continuous, but formed a percolated lattice of Pt nanocrystals, as can be seen on Figure 2.

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Figure 2 a and c show the top view of a 2 nm thick Pt layer sample, in bright field (a) and EDS (c). Figure 2 b and d show the same type of sample, but with a 5 nm Pt layer, in bright

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field (b) and EDS (d). For the sample with 2 nm of Pt, the Pt layer is semi continuous and has the form of a percolated lattice. For the 5 nm sample, the Pt layer seems nearly continuous but still has holes in it, as it can be seen on Figure 2 d. Other characterization to be performed, like small angle X-ray scattering (SAXS) and grazing angle X-ray reflectometry (GAXR), can bring complementary information on the structure of the Pt layer, as reported in [9].

Figure 2

Figure 3 8

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Figure 3 displays TEM cross-section pictures of a 10nm Pt layer. It shows that the Pt layer is crystalline and continuous. It is composed of small Pt grains, presenting a totally random

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orientation with one another. Figure 2 and Figure 3 demonstrate that the Pt layers aren’t as

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continuous and homogenous as the ones we used in our simulations. It implies that our simulations aren’t really accurate and that some optical phenomena can have been ignored. We also performed GDS profiles of this kind of sample, before and after ageing at 650°C

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during 10h, to have chemical analysis. The results are shown in Figure 4, where a) is the

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sample as deposited and b), the sample after ageing at 650°C for 10h. On both graphs, we can see a clear Pt peak, which implies that the Pt doesn’t diffuse in the

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Al2O3 matrix during the ageing, as it was the case in the cermet studied by Maaza et al. [10].

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On the Figure 4 a), the difference in Al amount between the two Al2O3 layers and the high Al

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peak near the Pt layer are certainly due to a process artifact we will correct on following samples. As we used a reactive deposition process, the oxidation of the Al target had changed after the first deposition and it can affect the deposition process. This kind of peak at the

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interface was evidenced by Sella et al. [11] on Ti/Ni multilayers by Secondary Ion Masse Spectroscopy (SIMS) analysis. They observed a Ti peak at the interface between Ni and Ti and explained it by the fact that the Ti is more prone to oxidation than Ni and it affects the deposition process. In our case, this difference disappears after ageing, probably because of a reorganization of the top Al2O3 layer at higher temperature. But the more striking feature on these profiles is the diffusion of Si in the bottom Al2O3 layer during the ageing process. Before ageing, on Figure 4 a), we have a straight transition between the bottom Al2O3 layer and the Si substrate. But on Figure 4 b), Si was detected just after the Pt peak, which means that Si has diffused all the way through the Al2O3 layer to the Pt layer. No Si is detected in the

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ACCEPTED MANUSCRIPT top Al2O3 layer, so it seems the Pt layer acts as a diffusion barrier for Si. Longer ageing tests

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are needed to confirm this hypothesis.

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Figure 4

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Based on our simulations, we deposited three absorbers: one with no IR reflector (No Refl), one with a Mo IR reflector (Mo Refl) and one with a Pt IR reflector (Pt Refl). Figure 5 shows

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the reflectance spectra of these three absorbers. The calculated absorption and emissivity can

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be found in Table II

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Figure 5

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It can be seen, from the curves of Figure 5 and from the values of Table II, that the Pt Refl

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sample had the poorest absorption of all, even though the difference is small, but it has the best emissivity and the sharpest transition between low reflectance and high reflectance. The No Refl absorber has the highest emissivity, which was predictable, as it doesn’t have an IR

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reflective layer. The Mo Refl has the best absorption value but an emissivity close to the one of the No Refl sample. At ambient temperature, the Mo Refl offers the better absorption but, as it must resist to high temperature, ageing tests are needed to discriminate the three absorbers. 3.3 Ageing tests After acquiring these preliminary results, we began the ageing tests on our absorbers. Figure 6 and Figure 7 represent the reflectance spectra of the No Refl samples on Si (Figure 6 a) and on SS substrate (Figure 6 b,) and Mo Refl samples on Si (Figure 7 a) and on SS substrate (Figure 7 b) after different ageing time. Table II shows the corresponding values of absorption 10

ACCEPTED MANUSCRIPT and emissivity, calculated from the measured reflectivity spectra and for a working

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temperature of 650°C.

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Figure 6

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For the No Refl sample on Si, we can see that the emissivity is high (41.11 %) and the transition isn’t sharp, even though the absorption value is good (93.73 %). It can be explained

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by the fact that Si is transparent in the IR region, reducing the overall reflectivity. After 5min of ageing at 650°C, the absorption remains quite the same, but the reflectivity in the IR range

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drops by nearly 20 %. After 10h, the reflectivity drops even more, while the absorption remains constant.

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On Figure 6 b) we can see the reflectance spectra of No Refl structure on a SS substrate. For

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the as deposited sample, the reflectance is very similar to the one on a Si substrate. The main

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difference is the better emissivity for the SS substrate, which has a higher reflectivity in the IR range than the Si substrate. On the contrary, the reflectance evolution with ageing time is

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really different between the two samples. On the Si substrate, the IR reflectance decreases with ageing time, whereas with the SS substrate, the maximum reflectance doesn’t change, but the transition between low and high reflectance is shifted to the higher wavelength, increasing the total emissivity.

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The ageing behavior of the Mo Refl sample on a Si substrate is quite different from the one of the No Refl sample. The 5 min ageing creates a shift in the oscillation in the visible range, but 11

ACCEPTED MANUSCRIPT the absorption value remains the same. The optical reflection in the IR range doesn’t change either. On the contrary, after 10h of ageing at 650°C, degradations appear. The absorber structure has a poor adhesion to the substrate and tends to delaminate. For this reason, we

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were barely able to measure the visible reflection spectra, before the absorber layer totally

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broke. It shows us that reflection increases a lot in the visible range, causing a decrease of the

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absorption. For the Mo Refl on SS substrate, on Figure 7 b), the reflectance versus ageing time is similar to that on Si substrate, but appears to degrade faster. The mechanical resistance

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and adhesion seem better with the SS substrate, so we were able to measure the IR reflectance

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of the Mo Refl on SS substrate.

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Table II

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Figure 8 represents the reflectance spectra of the Pt Refl sample on Si after different ageing times. The black curve, without ageing, presents a strong reflection peak around 600 nm and a

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sharp transition between the visible and IR range. It implies that the absorption isn’t so good, but the emissivity is quite low, considering it was calculated for a working temperature of 650°C. After 5min of ageing, the oscillations in the visible range are less pronounced than before and are shifted to the lower wavelength. The slop of the transition between low reflectance and high reflectance is less sharp and the IR reflectance is lower. After 40 h of ageing, the oscillations in the visible range drift again to the shorter wavelength, but the total absorption remains the same. The IR reflectivity doesn’t change either. After 100 h, the visible part of the spectra remains exactly the same, but the IR reflectance drops again.

Figure 8 12

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As shown by Table II, the optical ageing behavior of the different absorbers is strongly affected by the choice of the IR reflector material. The degradation of the optical properties is

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radically different between the three samples. In the case with no reflector, the reflectance in

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the IR region drops drastically after each ageing test whereas, in the case of the Mo reflector, it is the absorption that is more strongly affected. The Pt reflector also shows a drop of its reflectance, but much lower than that of the No reflector case.

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In the Mo Refl case, the delamination and degradation of the optical properties are likely due

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to the oxidation of the Mo layer in contact with ambient air at high temperature [12]. The Mo oxide doesn’t have a metallic behavior, so it changes radically the structure’s optical

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properties. The oxidation of Mo also causes the degradation of the mechanical properties of

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the layer.

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In view of the observed Si unidirectional diffusion into the first Al2O3 and the Pt antidiffusion barrier nature (Figure 4), one could conclude that the degradation mechanism for the No Refl absorber is mainly driven by such a diffusion phenomenon. Indeed, there is

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nothing to stop the diffusion of the Si atoms in the active layers and the Si impurities must change the optical index of the Al2O3, decreasing the reflectance in the IR range. On the contrary, in the case of the Pt Refl absorber, the thick Pt reflector layer may act as a diffusion barrier or, at least, slows the diffusion of the Si atoms in the Al2O3 layer. It can explain the fact that the degradation of the optical properties is slower and less pronounced than with the other reflector materials. X-ray reflectivity analyses may bring better understanding of this phenomena, as it was done by Gibaud et al. in [13]. In the case of absorbers on SS substrates, the evolution of the reflectance spectra with ageing time is quite different from the one of samples on Si substrates, so we may infer that this 13

ACCEPTED MANUSCRIPT degradation is more related to the evolution of the protective oxide layer on top of the SS substrate and to diffusion of elements from this layer in the absorber. New experiments and chemical analyses are ongoing to understand what happens on metallic substrates.

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4 Conclusions

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Pt-Al2O3 multilayer absorber coatings were simulated and deposited on Si and stainless steel substrates by pulsed sputtering, with different IR reflector materials (Mo, Pt or no reflector). TEM images revealed that, unlike the hypothesis made in simulation, our Pt layers weren’t

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continuous for thickness less than 10 nm. This fact must have an impact on the optical

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properties that isn’t taken into account in the simulations. Ageing tests were performed on these absorbers at 650°C for several hours. These ageing tests highlight different ageing

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behavior depending on the IR reflector used. Samples with a Mo reflector or no reflector on Si

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substrate show an important drop of their optical properties after 10 h of ageing. On the contrary, the optical properties of the sample with a Pt IR reflector remain high even after 100

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h of annealing at 650°C. These different behaviors can be explained by different mechanisms taking place in each case. For the Mo reflector, it seems the Mo layer oxidized in air at our

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working temperature and loose its reflectance property. In the No Refl case, Si atoms diffused from the substrate into the Al2O3 layers, causing a decrease in IR reflectance. The Pt IR reflector layer may act as a diffusion barrier, slowing the diffusion of the Si atoms and so, the degradation of the optical properties of the absorber. In the case of Mo reflector and no reflector on stainless steel substrates, it seems the degradations are due to the evolution of the protective oxide layer and maybe the diffusion of elements form this layer in the absorber. As we have seen that atoms from the substrate can diffuse and cause great damage to the absorber, we must now focus our study on the ageing of Pt Refl absorber on different substrates, like stainless steel or super alloys. Acknowledgement 14

ACCEPTED MANUSCRIPT We would like to thank the Horiba Company in Paris for letting us use their GDS equipment and for their help with the measurements.

C. Trease, H. Hadavinia, P. Barrington, Solar Selective Coatings: Industrial Sate-of-

C. Kennedy, Review of mid-to high-temperature solar selective absorber materials, NREL Tech. Rep. (2002) 1–58.

N. Selvakumar, H. Barshilia, Review of physical vapor deposited (PVD) spectrally

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[3]

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the-Art, Recent Patents Mater. Sci. 6 (2013) 1–19.

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C. Sella, M. Maaza, B. Pardo, F. Dunsteter, J.. Martin, M.. Sainte Catherine, et al., Structural investigation of Pt-Al2O3 co-sputtered nano-cermet films studied by small angle X-ray scattering, grazing angle X-ray reflectometry, TEM and AFM, Surf.

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effects on the structure and magnetic properties of Ni/Ti multilayers, J. Magn. Magn.

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Absorption (%)

Emissivity (%)

1

80.75

27.95

2

93

28.88

3

95.13

36.24

4

95.48

5

95.48

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Number of Pt layers

41.34 41.4

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Table I Simulated absorption and emissivity, calculated for a working temperature of

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650°C, as a function of the number of Pt layers, with a Pt IR reflector layer

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Absorption

Emissivity

Absorption Emissivity on

on Si (%)

on Si (%)

on SS (%)

No Refl

93.88

45.88

93.70

39.65

No Refl 5 min

94.56

65.34

93.92

46.23

No Refl 10 h

93.57

85.75

67.77

Refl Mo

95.32

47.69

95.13

46.78

Refl Mo 5 min

95.61

39.06

93.61

56.03

91.78

28.55

93.63

34.31

Refl Pt 40 h

93.16

35.21

Refl Pt 100 h

92.73

40.56

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Refl Pt 5 min

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SS (%)

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65.03

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Refl Pt

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Sample

Table II Calculated absorption and emissivity for the different reflector and different ageing time. ε has been calculated for a working temperature of 650°C

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Figures’ captions:

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Figure 1: Diagram of the different structures used in this study: a) three-layer structure and b)

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multilayer structure.

Figure 2: TEM surface images of samples with a 2 nm Pt layer (a and c) and a 5 nm Pt layer

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(b and d), in bright field (a and b) and EDS (c and d).

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Figure 3: TEM images of the three-layer structure with a low (a) and high (b) magnification. Figure 4: GDS profiles of a three layers sample before and after ageing at 650°C for 10 h.

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Figure 5: Reflectance spectra of an absorber with no IR reflector layer (black curve), an absorber with a Mo IR reflector layer (dashed curve) and an absorber with a Pt IR reflector

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layer (grey curve) on Si substrate.

Figure 6: Reflectance spectra of an absorber with no IR reflector layer on Si substrate (a) and

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an absorber with no IR reflector layer on SS substrate (b) after different ageing time: no ageing (black curve), 5 min at 650°C (dashed curve) and 10 h at 650°C (grey curve). Figure 7: Reflectance spectra of an absorber with a Mo IR reflector layer on Si substrate (a) and an absorber with a Mo IR reflector layer on SS substrate (b) after different ageing time: no ageing (black curve), 5 min at 650°C (dashed curve) and 10 h at 650°C (grey curve). Figure 8: Reflectance spectra of an absorber with a Pt IR reflector layer on Si substrate, after different time of ageing: no ageing (black curve), 5 min at 650°C (dashed black curve), 40 h at 650°C (grey curve) and 100 h (dashed grey curve).

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ACCEPTED MANUSCRIPT Highlights  Solar thermal power needs new thermoresistant materials  Pt-Al2O3 multilayer coatings were developed to meet this need

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 Different IR reflector materials were used and aging tests performed

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 Pt IR reflector showed the best results after aging at 650°C

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