Thermal stability of electron beam evaporated AlxOy/Pt/AlxOy

multilayer solar absorber coatings

Z.Y. Nuru a,b,d,n, C.J. Arendse b, T.F. Muller b, S. Khamlich a,d, M. Maaza a,c,d

a Nanosciences African Network (NANOAFNET), iThemba LABS-National Research Foundation, Old Faure road, 7129 Somerset West, South Africab Department of Physics, University of Western Cape, Private Bag X 17, Bellville 7535, South Africac Council for Scientific and Industrial Research, PO Box 395, Pretoria 0001, South Africad UNESCO-UNISA Africa Chair in Nanosciences/Nanotechnology, College of Graduate Studies, University of South Africa (UNISA), Muckleneuk ridge,PO Box 392, Pretoria, South Africa

a r t i c l e i n f o

Article history:Received 2 November 2012Received in revised form2 April 2013Accepted 18 September 2013

Keywords:SelectivityAnnealingOptical propertiesThermal stability

a b s t r a c t

This paper reports the effect of thermal annealing on AlxOy/Pt/AlxOy multilayer solar absorber coatingsdeposited by an electron beam (e-beam) vacuum evaporator onto copper substrate at room temperature.The samples were annealed at different temperatures for different duration in air. The samples werecharacterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-rayspectroscopy (EDS), atomic force microscopy (AFM), Raman spectroscopy, UV–vis spectrophotometer andemissometer. The results showed changes in morphology, structure, composition, and optical propertiesdepend on both temperature and duration of annealing. The XRD pattern showed the intensity of Ptdecreased with increasing annealing temperature and therefore, disappeared at high temperature. Withincreasing annealing temperature, an increase in the size of Pt particles was observed from SEM. TheAlxOy/Pt/AlxOy multilayer solar absorber coatings deposited onto Cu substrate were found to be thermallystable up to 500 1C in air for 2 h with good spectral selectivity (α/ε) of 0.951/0.09. At 600 1C and 700 1C,the spectral selectivity decreased to 0.92/0.10 and 0.846/0.11 respectively, which is attributed to thediffusion of Cu and formation of CuO and Cu2O phases. The formation of CuO and Cu2O phases wereconfirmed by XRD, Raman spectroscopy and optical modelling. Long term thermal stability study showedthe coatings were thermally stable in air up to 450 1C for 24 h.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Currently the world relies on fossil fuels to produce 86% of itsenergy. However, the combustion of fossil fuels generates green-house gas, which is the major factor responsible for globalwarming [1]. To overcome the environmental problems and tofulfil the ever increasing demand of energy, mankind should userenewable energy sources. The sun, one of renewable sources, isthe vast majority of energy used by people on the earth. The sun'senergy is environmental friendly and can be converted to heat andelectricity. Hence to harness this inexhaustible energy, solar-thermal systems are the most important candidates. The criticalpart of these systems is the absorber surface, which attractsconsiderable attention for photo-thermal conversion [2–7]. Toincrease the photo-thermal conversion efficiency, the solar absor-ber surface should capture the maximum solar energy in the high

intensity visible and near infrared spectral regions and shouldhave minimum emittance for thermal IR radiation [8]. Moreover, itshould be thermally stable at high temperatures (4400 1C) in airfor concentrating solar power systems [9]. Therefore, to enhanceits operating efficiency and withstand degradation performance,the thermal stability of the optical properties and microstructureof the absorber surface are critical.

Some selective solar absorber coatings for high temperatureapplications have been reported [10–12]. Okuyama et al. [10]developed graded Ni–MgO and Ni–SiO2 cermets onto quartz usingvacuum evaporation and was thermally stable in vacuum at 420 1Cand 515 1C, respectively. Double cermet Mo–AlN deposited bysputtering on copper substrate was thermally stable in vacuumup to 500 1C [11]. Graded W–Al2O3 was deposited onto stainlesssteel using sputtering and found to be thermally stable up to580 1C in vacuum [12]. These coatings are stable in vacuum forhigh temperature solar-thermal applications. However, coatingsneed to be stable in air in case vacuum is breached [13]. Mean-while, there are some recently reported solar selective materials,which are thermally stable in air at high temperature [14–18].Brashilia et al. [14] deposited a multilayer of AlxOy/Al/AlxOy onto

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n Corresponding author at: Nanosciences African Network (NANOAFNET),iThemba LABS-National Research Foundation, Old Faure road, 7129 Somerset West,South Africa. Tel.: þ27 734935948; fax: þ27 218433543.

E-mail addresses: zebibnate@yahoo.com, zebib@tlabs.ac.za (Z.Y. Nuru).

Please cite this article as: Z.Y. Nuru, et al., Thermal stability of electron beam evaporated AlxOy/Pt/AlxOy multilayer solarabsorber coatings, Solar Energy Materials and Solar Cells (2013), http://dx.doi.org/10.1016/j.solmat.2013.09.027i

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copper substrates, and found to be stable up to 400 1C. Selvakumaret al. [15] reported that multilayer of HfOx/Mo/HfO2 depositedonto copper substrates exhibited thermal stability up to 400 1C.Al2O3/Mo/Al2O3 deposited onto stainless steel substrates exhibitedthermal stability up to 550 1C [16]. TiAl/TiAlN/TiAlON/TiAlO wasdeveloped onto copper substrates and exhibited thermal stabilityat 650 1C [17]. Lei et al. developed double cermet TixAl1�x/TiN/(TiN–AlN)/AlN onto stainless steel and found to be thermallystable up to 400 1C [18].

The main reasons resulting in degradation of the selective solarabsorber coatings at high temperature are oxidation and inter-facial diffusion of the layers and the substrate [19]. Aluminiumoxide (Al2O3) coatings have high hardness, excellent dielectricproperties, refractoriness and good thermal properties and Pt hasthe capacity to resist oxidation even at high temperature [20]. Dueto these properties, the composite of Pt and Al2O3 coatings wasfound the most promising solar absorber materials for hightemperature concentrating solar-thermal applications. It has beenfound to be stable up to 600 1C in air [21–25]. Hence, due to thisreason, alternative layer of vacuum evaporated Al2O3 and Pt havebetter thermal stability in air at high temperature than thereported multilayer solar absorber coatings even though most ofthe reported materials, which have good thermal stability, areeither graded or double cermet coatings. Vacuum evaporated Co–Al2O3, Au–Al2O3, Ag–Al2O3, and Cr–Al2O3 have also been devel-oped for evacuated tube collectors [26].

A number of materials such as TiAlN/TiAlON/Si3N4 [27], TiAlN/AlON [28], NbAlN/CrAlON/Si3N4 [29], HfMoN/HfON/Al2O3 [30], andTiAlN/TiAlON/SiO2 [31] have been developed recently for hightemperature solar-thermal applications using sputtering and putforward to meet the basic characteristics of solar absorber materi-als. These studies still need to focus on the thermal stability of themicrostructure, component distribution, interlayer diffusion andtheir mechanism under high temperature [32].

Recently, we developed a new spectrally selective AlxOy/Pt/AlxOy multilayer solar absorber using e-beam evaporation. Thestructural and optical properties of the multilayers have beenreported elsewhere [20,33]. In this paper, we present the effect ofannealing on the microstructural, morphology, composition, opti-cal properties and thermal stability of AlxOy/Pt/AlxOy multilayerabsorber deposited by e-beam evaporation, which was heated inair at different temperatures and durations.

2. Experimental details

AlxOy/Pt/AlxOy multilayer stacks of 900 Å AlxOy top layer, 70 ÅPt middle layer and 400 Å AlxOy base layer were deposited ontocopper (3�3 cm2) substrates using a 3 kW high vacuum e-beamevaporation system at room temperature. The experimentaldetails have been described in detail elsewhere [20,33].

To study the thermal stability of the multilayer coatings, theywere heated in air in an ultra-Furnace of Wirsam scientific typeUF12/40/200 in the range of 300–700 1C at an increment of 100 1Cfor 2 h. The temperature was increasing from room temperature tothe desired temperature at a rate of 5 1C/min and was cooled downat a rate of 2 1C/min. The accuracy of the set temperature was77 1C. Similarly, we also tested long term thermal stability of themultilayer absorbers in air at 350 1C, 450 1C and 550 1C for 24 h.Thereafter the required analytical techniques were used to char-acterize the coatings.

The microstructure of the as-grown and annealed coatings wasrecorded by using a X-ray diffraction (XRD) Model Bruker AXS D8advance of radiation Cu (Kα¼1.5406 Å) and a DILOR–JOBIN-YVON–SPEX integrated Raman spectroscopy. The morphology of the filmswas acquired using a Leo-Stero Scan 440 Scanning electron

microscope (SEM). The chemical composition was studied usingenergy-dispersive X-ray spectroscopy (EDS) using an acceleratingvoltage of 15 kV. Heavy Ion Elastic Recoil Detection analysis (HI-ERDA) was used as compliment to the EDS results. The surfaceroughness was studied using Veeco nanoman Atomic force micro-scopy (AFM) V operated in tapping mode.

The Heavy Ion ERDA technique consists in the detection of recoilions knocked off the surface region of a target sample by a (heavy)projectile beam coming in at a grazing incidence angle to the samplesurface. The result is that of 2-D scatter plots showing different curves,each corresponding to a particular ion mass, from which elementalenergy spectra and hence depth profiles can be extracted [34]. Thecurrent beam transport settings at our iThemba LABS Gauteng tandemaccelerator limit the heaviest incident beam available for ERD analysesto 26MeV 63Cu7þ , and this is the beam that was used, at 151 grazingincidence angle. The detector system, a mass dispersive ToF-energyspectrometer, was mounted at 301 to the incident beam direction. Tocircumvent the problem of interference of Cu signals due to thescattered beam ions with those due to the recoiled Cu substrate atomswe used the (1:9) ratio of the recoil to scattering cross section toextract the recoil portion from the total Cu spectrum. A quickcalculation using SIMNRA [35] shows that this ratio is constant overa wide energy range of the incident beam and so would still hold forrecoils ejected from deep ( up to �500 nm) within the sample.

Spectral reflectance was measured with a Cary 5000 UV–vis–NIR spectrophotometer of Varian, Inc. model internal DRA-2500 inthe wavelength range of 0.3–2.5 μm. The solar absorptance wascalculated from the measured reflectance data and weighted bysolar irradiance using standard AM1.5 solar spectrum in the abovewavelength range. Thermal emittance spectra were acquired by anemissometer model AE1, which has an accuracy of 70.01 emit-tance units. The solar absorptance and thermal emittance values ofbare copper substrate were 0.203 and 0.03 respectively. In order tounderstand the degradation mechanism, an optical spectrumsimulation program called SCOUT [36] was also used.

3. Results and discussions

3.1. Structural properties

Fig. 1 shows the XRD patterns of the as-deposited and annealedAlxOy/Pt/AlxOy multilayer solar absorber coating deposited ontocopper substrate. The prominent diffraction peaks of the as-deposited films at 2θ¼40.011, 43.31, 50.441 and 74.081 correspond

Fig. 1. XRD pattern of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown(a) and annealed at different temperatures in air for 2 h: 300 1C (b), 400 1C (c),500 1C (d), 600 1C (e), and 700 1C (f).

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to (111) plane of FCC Pt (JSPDS no. 00-001-1190) and (111), (200)and (220) plane of highly crystalline FCC Cu (JSPDS no. 00-004-0836) substrate, respectively. It is observed that annealing in airfrom 300 to 600 1C for 2 h shows a decrease in the intensity of thePt (111) peak, which is due to the decrease in thickness of Pt layers,as probed by the HI-ERDA and optical modelling. Further increaseof annealing temperature to 700 1C in air leads to a completedisappearance of Pt diffraction peak, and to the formation of weakdiffraction intensities of CuO and Cu2O phases. The Pt peakposition shifts from 40.011 to higher angles and reaches 40.631after annealing at 600 1C in air. This is mainly due to the fact thatheating decreases the stress, which was induced during thedeposition process. No diffraction peaks corresponding to AlxOy

were observed, which shows AlxOy is X-ray amorphous.The Raman spectrum of the as-deposited and annealed up to

700 1C, which will be discussed later, showed two broad bandscentred at approximately 190 and 833 cm�1. These bands do notcorrespond to any Al2O3 phases rather correspond to the vibrationof aluminium and oxygen atoms, respectively [37]. Al2O3 exists inseveral crystalline phases such as γ-Al2O3 with Fd 3m space groupand α-Al2O3 with R 3c space group. The former does not exhibitany active Raman modes [38] but the latter one exhibits sevenRaman active phonon modes: 2A1g (418tcm�1 and 6481cm�1) and5Eg (332, 431, 450, 526 and 745 cm�1) [39]. Hence, the Ramandata confirms the XRD result.

3.2. Surface morphology

Fig. 2(a)–(f) represents the surface morphology of the AlxOy/Pt/AlxOy multilayer solar absorber deposited onto Cu substrates in theas-deposited and annealed at 300 1C, 400 1C, 500 1C, 600 1C and700 1C in air for 2 h respectively. The as-deposited multilayer solar

absorber surface depicts dense spherical Pt particles consisting ofsome larger Pt particles with sizes 100–250 nm, and some small Ptparticles with size less than 100 nm. After annealing at 300 1C,400 1C and 500 1C as shown in Fig. 2(b)–(d) respectively, an increasein the size of Pt particles has been observed. At 600 1C the Ptparticles become irregular and agglomerate into larger particles(see Fig. 2(e)). Further increase of annealing temperature to 700 1C(Fig. 2(f)) leads to the formation of only CuO on the surface. Itshould be noted that the formation of larger Pt particles plays asignificant role in atomic diffusion by acting as a path for Cu atomicmovement towards the surface, which has been oxidized. No crackswere observed on the surface of the AlxOy/Pt/AlxOy multilayer solarabsorber, even after being annealed at 700 1C.

Fig. 3(a)–(f) depicts the AFM images of AlxOy/Pt/AlxOy multi-layer solar absorber deposited onto Cu substrates in the as-deposited state and annealed at 300 1C, 400 1C, 500 1C, 600 1Cand 700 1C in air for 2 h respectively. The images show columnargrowth, as would be expected from evaporated coatings grownfrom vapour phase deposition at low substrate temperature [40].The particle sizes and lengths in the annealed samples are largerthan those of the as-deposited. With increasing annealing tem-perature, the average diameter as well as the average surfaceroughness increased, as summarised in Table 1. The averageroughness of the AlxOy/Pt/AlxOy multilayer solar absorber isrelatively constant up to 500 1C, after which a noticeable increaseis observed at 600 1C and 700 1C, which is attributed to thenoticeable increase in the particles size.

3.3. Compositional properties

Fig. 4(a)–(f) shows the EDS analysis of AlxOy/Pt/AlxOy multilayersolar absorber deposited onto Cu substrates in the as-deposited

Fig. 2. SEM images of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a), and annealed at different temperatures in air for 2 h: 300 1C (b), 400 1C (c), 500 1C (d),600 1C (e), and 700 1C (f).

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state and annealed at 300 1C, 400 1C, 500 1C, 600 1C and 700 1C in air for2 h respectively. EDS spectra showed only sharp peaks from platinum,aluminium, oxygen and from the copper substrate, confirming the purenature of the prepared AlxOy/Pt/AlxOymultilayer solar absorber coatings.It is observed that annealing of the films leads to a change incomposition. The intensity of the platinum and aluminium peaksdecreased slightly up to 500 1C and diminishes gradually at 600 1C,and finally disappears at 700 1C. On the other hand, the signal of copperand oxygen increased as annealing temperature increased. This was alsoconfirmed by HI-ERDA analysis. It showed clearly an outward diffusionof copper from the substrate towards the surface and a decrease in theconstituents of the multilayer solar absorber coatings. The averageatomic fraction ratio of aluminium-to-oxygen at the surface (thicknessof 90 nm) of AlxOy/Pt/AlxOy multilayer solar absorber coatings heated upto 500 1C was roughly 0.38:0.6. Thereafter, a reduction of the relativeatomic fraction with depth was observed. The thickness of the Pt layer

decreased at 600 1C and completely disappeared at 700 1C. Theobserved composition matches with the result obtained from EDS,XRD and Raman analysis.

3.4. Thermal stability in air

It is known that thermal stability is critical for solar absorbermaterials as it affects the two basic characteristics; i.e. solarabsorptance and thermal emittance of the solar absorber material.Fig. 5 shows the reflectance spectra of AlxOy/Pt/AlxOy multilayersolar absorber in the as-deposited state and annealed at 300 1C,400 1C, 500 1C, 600 1C and 700 1C in air each for 2 h. The corre-sponding absorptance and emittance values of the multilayercoating are given in Table 2. To achieve high solar selectivity, thecoatings should exhibit low reflectivity in the visible near infrared(vis–NIR) region but high in the infrared region.

The reflectance of the as-deposited multilayer solar absorberwas about 5% in the visible near infrared region and increasedsharply from 1100rnm and reflected 80% at around 2500 nm. Thereflectance of the coating annealed at 300 1C showed absorptance(Δα¼0) and emittance (Δε¼þ0.01). With the increase of heattreatment temperature up to 500 1C, the spectral reflectanceexhibited no significant change in the vis–NIR wavelengths witha change in absorptance (Δα¼�0.001) and emittance(Δε¼þ0.01) values. This indicates that AlxOy/Pt/AlxOy multilayersolar absorber have good thermal stability up to 500 1C in air,which is attributed to the high temperature thermal stability ofAl2O3 that acts as a diffusion barrier for oxygen and to theunreactive Pt metal, which resist oxidation and chemical reaction[32]. According to Barshilia [14], Al2O3 based solar selective

Fig. 3. AFM images of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a), and annealed at different temperatures in air for 2 h: 300 1C (b), 400 1C (c), 500 1C (d),600 1C (e), and 700 1C (f).

Table 1Average Pt particle size and surface roughness of AlxOy–Pt–AlxOy multilayer solarabsorber as-deposited and annealed at different temperatures in air.

Annealing temperature(1C) in air

Average diameter(nm)

Average surfaceroughness (nm)

As-deposited 220 6.5770.12300 221.3 6.6170.01400 224 6.6770.13500 227 6.6970.01600 287.8 18.670.04700 358 3570.12

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coatings deposited onto quartz, stainless steel, superalloy andmolybdenum substrates are thermally stable in air up to 500 1C.Hence it is worth to mention, AlxOy/Pt/AlxOy multilayer solarabsorber coatings deposited onto copper substrates appeared tobe thermally stable in air up to 500 1C.

The reflectance increase below 1000rnm and the cut off shiftedtowards the higher wavelength at 600 1C. The absorptance valuedecreased (Δα¼�0.032) and the emittance value increased(Δε¼þ0.02). At 700 1C, it was found a total change in the shapeand values of the reflectance. Consequently, the absorptance(Δα¼�0.104) and emittance (Δε¼þ0.03) values of the multilayercoatings changed significantly, indicating the degradation of themultilayer absorber. Hence in order to elucidate the mechanismbehind the degradation of the coatings above 500 1C, the experi-mental reflectance spectra were fitted with an optical simulationprogram called SCOUT by using the model presented [20]. From thefit to the experimental reflectance curves the thickness and the

Fig. 4. EDS spectra of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a), and annealed at different temperatures in air for 2 h: 300 1C (b), 400 1C (c), 500 1C (d),600 1C (e), and 700 1C (f).

Fig. 5. Reflectance spectra of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown and heat treated in air for 2 h.

Table 2Effect of annealing in air for 2 h on solar absorptance and thermal emittance valuesof the AlxOy–Pt–AlxOy multilayer solar absorber deposited onto Cu substrate.

Temperature(1C)

Absorptance (α) Emittance (82 1C)

As-deposited Annealed Δα As-deposited Annealed Δε

Roomtemperature

0.952 – – 0.08 – –

300 0.952 0.952 – 0.08 0.09 0.01400 0.952 0.951 �0.001 0.08 0.09 0.01500 0.952 0.951 �0.001 0.08 0.09 0.01600 0.952 0.92 �0.032 0.08 0.1 0.02700 0.95 0.846 �0.104 0.08 0.11 0.03

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constituents of the AlxOy/Pt/AlxOy multilayer solar absorber coatings as-deposited and annealed up to 500 1C did not show significant change.The coatings were AlxOy �90showPt�8t �/AlxOy �39 nm/Cu sub-strate. However for the multilayer solar absorbers annealed at 600 1Cand 700 1C, the thickness and the constituents of the layers weredifferent from the other samples. The sample annealed at 600 1C wascomprised of 37% Cu:63% AlxOy top layer �79 nm, 26% Cu:84% Pt mid-dle layer �3 nm and 66% Cu:34% AlxOy base layer �28 nm and samegoes to the sample annealed at 700 1C. It was comprised of 49% Cu:51%O top layer �18 nm thick, 83% Cu:17% AlxOy middle layer �60 nm, andbase 92% Cu:8% AlxOy base layer �28 nm. Hence one can observe thatthe main degradation mechanism of the AlxOy/Pt/AlxOy multilayerabsorber is attributed to the diffusion of copper towards the surface.

The microstructural stability of the AlxOy/Pt/AlxOy multilayersolar absorber coatings was studied using Raman spectroscopy.The Raman spectra of the as-deposited and annealed up to 700 1C inair are shown in Fig. 6. The as-deposited multilayer absorberexhibited two broad bands centred at 190 and 833 cm�1, whichcorrespond to the vibration of aluminium and oxygen atoms,respectively. The shape of Raman spectra did not show significantchange even after annealed the sample up to 600 1C, indicating thestable structure of the multilayer solar absorber coatings. However,at 700 1C, the two bands disappeared and three peaks centred ataround 211, 296 and 621 cm�1 appeared, which correspond to theformation of CuO and Cu2O [41]. These phases of copper appeareddue to the outward diffusion of copper in to the coating.

3.5. Long term thermal stability in air

Long term thermal stability is also the most important criteriafor use the solar absorber material for concentrating solar-thermalapplications. The AlxOy/Pt/AlxOy multilayer solar absorber coatingswere heat treated in air at 350 1C, 450 1C and 550 1C for 24 h. Fig. 7shows the reflectance spectra of the multilayer coatings in the as-deposited state and annealed for 24 h. The absorptance andemittance values are shown in Table 3. It is evident from the tablethat the change in absorptance (Δα¼0.000, þ0.001) and emit-tance (Δε¼þ0.00, þ0.01) values of the coating at 350 1C and450 1C, respectively was not substantial. However, at 550 1C theabsorptance decreased (Δα¼�0.157) and the emittance increased(Δε¼þ0.02) drastically. The XRD pattern showed formation ofCuO and Cu2O phases at 550 1C, which is shown in Fig. 8. The AFMimages of AlxOy–Pt–AlxOy multilayer solar absorber coating in theas-deposited state and heat treated at 350 1C, 450 1C, and 550 1C inair for 24 h showed a slight increase in average roughness value as

shown in Fig. 9. The representative Raman spectra (Fig. 10) alsoconfirmed that there was no significant change in microstructureof the multilayer absorber at 450 1C but at 55 1C, CuO and Cu2Owere appeared. Therefore, diffusion of Cu and formation of itsoxide phases, are responsible for the degradation of AlxOy/Pt/AlxOy

multilayer solar absorber coatings.In general, it is observed that the major degradation of AlxOy/Pt/

AlxOy multilayer solar absorber coatings was diffusion of coppertowards the surface. Hence, it is worth to mention that thismultilayer absorber could further improve its thermal stability ifa diffusion barrier layer is deposited between the base AlxOy layerand copper substrate.

Fig. 6. Raman spectra of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a), and annealed at different temperatures in air for 2 h: 300 1C (b), 400 1C(c), 500 1C (d), 600 1C (e), and 700 1C (f).

Fig. 7. Reflectance spectra of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown and heat treated in air for 24 h.

Table 3Effect of annealing in air for 24 h on solar absorptance and thermal emittancevalues of the AlxOy–Pt–AlxOy multilayer solar absorber deposited onto Cu substrate.

Temperature(1C)

Absorptance (α) Emittance (82 1C)

As-deposited Annealed Δα As-deposited Annealed Δε

Roomtemperature

0.96 – – 0.07 – –

350 0.96 0.96 – 0.07 0.07 –

450 0.962 0.963 0.001 0.07 0.08 0.01550 0.96 0.803 �0.157 0.07 0.09 0.02

Fig. 8. XRD pattern of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown(a) and annealed at different temperatures in air for 24 h: 350 1C (b), 450 1C (c) and550 1C (d).

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4. Conclusion

AlxOy/Pt/AlxOy multilayer solar absorber films have been depos-ited onto a copper substrate at room temperature using the e-beam vacuum evaporator. The multilayer absorber was annealedin air for different temperatures and durations. XRD analysisshowed a decrease in the intensity of Pt grain for the multilayersheated from 300 to 600 1C in air for 2 h, and completely dis-appeared and formed CuO and Cu2O phases at 700 1C. This wasalso confirmed by Raman spectroscopy. A slight increase in themean size of Pt particles was observed from the samples heatedfrom 300 to 500 1C in air for 2 h. The average surface roughness of

the multilayers was found to increase slightly with annealingtemperature. EDS, HI-ERDA and optical modelling spectra con-firmed formation of oxidized copper and the disappearance of Pt.The optical properties of AlxOy/Pt/AlxOy in air for 2ih was found tobe stable up to 500 1C, with solar selectivity (α/ε) of 0.951/0.09. At600 1C and 700 1C, its selectivity decreased significantly to 0.92/0.10 and 0.846/0.11, respectively, which is attributed to the diffu-sion of copper and formation of CuO and Cu2O phases. Long termannealing showed that the multilayer coating was stable up to450 1C in air for 24 h and showed spectral selectivity of 0.963/0.08.

Acknowledgements

We would like to thank for the finance support from theOrganization of Women Scientists for Developing World “OWSDW”,Swedish International Development Cooperation Agency “Sida”iThemba labs-National Research Foundation of South Africa, theUNESCO UNISA Africa Chair in Nanosciences & Nanotechnology andthe Abdus Salam ICTP via the NANOAFNET.

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Fig. 9. AFM images of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a) and annealed at different temperatures in air for 24 h: 350 1C (b), 450 1C (c) and550 1C (d).

Fig. 10. Raman spectra of AlxOy–Pt–AlxOy multilayer solar absorber coatings as-grown (a) and annealed at different temperatures in air for 24 h: 350 1C (b), 450 1C(c) and 550 1C (d).

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