CERAMICSAvailable online at www.sciencedirect.com

Ceramics International 40 (201

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reaction (EOR), FischerTropsch synthesis and for thermo-electric power generation material [1217]. Numerous studieshave been conducted to establish the physicochemical, mag-

nCorresponding author. Tel.: 62 761 566937.nnCorresponding author. Tel.: 61 8 9360 2867.netic, conductivity, electrochemical and thermal properties ofcoppercobalt oxides [14,15,1820]. On the other hand,

http://dx.doi.org/10.1016/j.ceramint.2014.08.0120272-8842/& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

E-mail addresses: amun_amri@unri.ac.id (A. Amri),z.jiang@murdoch.edu.au (Z.-T. Jiang), c.yin@tees.ac.uk (C.-Y. Yin).1. Introduction

Solar thermal collectors such as the ubiquitous solar hot waterpanels are designed to collect solar radiation and convert it intouseful heat energy for various industrial and domestic applica-tions. A signicant component that affects the efciency of asolar thermal collector system is the solar selective absorber(SSA) coating [1] which, ideally, should absorb the incomingsolar radiation (high solar absorptance) as much as possible withconcurrent low thermal emittance. The most frequently usedindustrial SSAs in recent years are the metal particles in ceramic(cermet) structures which can be synthesized via electroplating/

electrochemical or sputtering/vacuum deposition techniques[2,3]. Though these techniques are effective, they are, none-theless, not environment-friendly [46] and sputtering/vacuumdeposition processes are technically complicated and not cost-effective [3,711]. Concerted efforts by materials scientists arecurrently underway in seeking alternative SSA materials synth-esis processes which exhibit benets such as simplicity, cost-effectiveness and environment-friendly aspect.Copper cobalt oxides (CuxCoyOz) are versatile metal oxides

which have applications in a variety of important catalyticreactions such as conversion of syngas to higher alcohols,oxidation of carbon monoxide (CO) by O2, oxygen evolutionsCopper cobalt oxide (Cu2CoO3) thin lm coatings integrated with silica (SiO2) antireection (AR) layer have been deposited on the top ofaluminum substrates using a simple solgel dip-coating method. Reectance spectra of the coatings were generated using spectroscopic methodswhile the coatings were subjected to an accelerated thermal durability test. The addition of silica changed the reectance spectra of coatingswithin the wavelength range of 0.315.4 mm. The absorptance decreased with the increase of the withdrawal rate in range of 1040 mm/min,while the emittance increased with the increase of the withdrawal rate. The optimum optical parameters for this study were absorptance,S84.96%; emittance, T5.63% corresponding to the coating with a silica AR layer at withdrawal rate of 10 mm/min. The coatings with thesilica AR layer were shown to be thermally durable in which no discernible cracking phenomenon was observed. The degradation of thecoatings with the silica AR layer was predominantly governed by temperature changes rather than exposure time.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: Coppercobalt oxide; Silica antireection layer; Solgel dip-coating; Accelerated thermal durability testAbstractOptical properties and thermal durabcoatings with integrated

Amun Amria,n, Zhong-Tao Jiangb,nn, Nick WTrevor Pryorb, M.

aDepartment of Chemical Engineering, UnbSchool of Engineering and Information Technolog

cSchool of Science and Engineering, Teesside University

Received 26 February 2014; received in revisAvailable onlinINTERNATIONAL

4) 1656916575

ity of copper cobalt oxide thin lmilica antireection layer

attb, Chun-Yang Yinc, Nicholas Mondinosb,ahbubur Rahmanb

sity of Riau, Pekanbaru 28293, Indonesiaurdoch University, Murdoch, 6150 WA, Australiarough Road, Middlesbrough, TS1 3BA, United Kingdom

form 25 June 2014; accepted 5 August 2014August 2014

www.elsevier.com/locate/ceramint

natsolar-based optical properties of the coppercobalt oxides thinlm coating are comparatively less well-studied [14].In our previous studies, we prepared copper cobalt oxide

(Cu2CoO3) thin lm coatings deposited on highly reectingaluminum substrate via a simple, cost-effective andenvironment-friendly solgel dip-coating route [2124]. Ourcoatings (before the addition of antireection layer) exhibitedpromising absorptance properties as compared to other selec-tive absorber materials synthesized from more complicatedsolgel process routes [25,26]. The light absorption selectivitybehavior is due to the combination of factors of the intrinsicproperties of coating, the lm thickness and reectivityproperties of substrate [24]. However, to maximize theabsorptance in UVvis range and to protect the coatings fromany degradation due to external factors, an antireection (AR)layer on the top of the absorber layer is, therefore, needed[27,28]. The aim of the present work is to investigate theoptical properties and the thermal durability of copper cobaltoxide thin lm coatings with a silica anti reection layer(Cu2CoO3SiO2). A material which will be used as antire-ective layer should have a refractive index value between thevalue of the material underneath and the value of air (n=1) [2].For the case of Cu2CoO3, to the best our knowledge, noprevious data on refractive index has been published. Sincemost copper cobalt oxide family and spinel minerals haverefractive indices of around 1.75 [2], we utilized such value forour compound. Therefore, as SiO2 refractive index is widelyknown to be about 1.5 [2,29], we surmise that SiO2 should bea good choice as antireective material for our absorber layer.Besides that SiO2 is well-known to be a very resilient withfavorable mechanical characteristics. The durability dataobtained from this study are also useful in establishing thephysical (wear and thermal) resistance of the coatings againstextreme weather and external conditions.

2. Experimental

2.1. Preparation of thin lm coatings with silica antireectionlayer

The coppercobalt oxides thin lm coatings were preparedfrom their respective chemical; precursors (0.25 M copperacetate and 0.25 M cobalt chloride) using a solgel dip-coatingmethod as specied in our previous studies [21,22]. The dip-coating withdrawal rate was xed at 120 mm/min. Finalannealing was conducted at 500 1C for 1 h. The heating rateof the annealing process was 50 1C/min, while cooling wasperformed inside the furnace for 10 min before allowed to coolto room temperature. The coppercobalt oxides (Cu2CoO3)thin lm coatings were obtained using this route.To prepare the silica antireection layer, tetraethyl orthosi-

licate/TEOS (C8H20O4Si, Alfa Aesar, 499%), absolute etha-nol (Merck), Milli-Q water, hydrochloric acid (32% w/w) wereused as received. Commercial aluminum (2 4 cm2) was usedas substrate. This AR layer was synthesized using a solgel

A. Amri et al. / Ceramics Inter16570method customized from Bostrom et al. [28]. Firstly, TEOSwas mixed with ethanol while 0.06 wt% HCl solution wasgradually added to the TEOS-ethanol solution. The molarratios of ethanol and water to the TEOS were 5 and 4,respectively. To ensure complete hydrolysis process, theresulting mixture was stirred for 24 h in a closed container.The obtained solution with pH of 2.1 was used for the ARlayer deposition by dip-coater with withdrawal rates rangingfrom 10 to 40 mm/min. The wet AR layer was subsequentlystored in a desiccator before nal annealing to 400 1C for30 min in an oven furnace and nally allowed to cool to roomtemperature overnight inside the furnace. All samples werekept in vacuum desiccator with silica gel and the samples werefurther dried in laboratory vacuum drying oven at 60 1Covernight prior to any characterisation analysis.

2.2. Characterizations

The optical performance of the coatings with silica the ARlayer on reective aluminum substrates (opaque surfaces) wascalculated based on the absorptance (S) and emittance (T)values. These values were obtained from the measurements ofmonochromatic reectance in the wavelength area from 0.3 to2.7 mm by using an Ultra violetvisible-near infrared (UVvisNIR) spectrometer and wavelength area more than 2.7 mm byusing a FTIR spectrometer.The solar absorptance and emittance values of the samples

were calculated based on the reectance data as elucidated byDufe and Beckman [29]. A template of reectance dataagainst spectral distribution (using Air Mass standard ofAM1.5 for absorptance and blackbody radiation standard of100 1C for emittance) in equal energy increment was createdusing an Excel worksheet. The near-normal hemisphericalsolar reectance spectrum was recorded from 0.3 to 2.7 mmusing a UVvisNIR Jasco V-670 double beam spectro-photometer with 60 mm integrating sphere. Deuterium andhalogen lamp were used as the light source. Infrared reec-tance spectra in the wavelength area from 2.7 to 15.4 mm wereobtained using a reected-off type of Perkin Elmer Spectrum100 FTIR spectrometer with integrating sphere within therange from 500 to 4000 cm1. The coating surface wascontacted on the diamond surface area and a pressure armwas positioned and locked at force of 80 N to maintainhomogenous attachment onto the surface. The reectancespectrum was obtained after four scans with resolution of2 cm1. Background correction was performed before thecollection of each spectrum.The accelerated thermal durability test was conducted using

an oven furnace based on the PC (performance criterion) valueof IEA SHC Task 27 [30] while the adhesion effectivenessbetween the lm absorber and substrate was evaluated by thephysical/cracking inspection before and after thermal test. ThePC value is dened as the following equation:

PC S0:5100 1where S is the difference of absorptance values before andafter the thermal test while 100 is the difference of emittance

ional 40 (2014) 1656916575values (where the emittance values were measured at theblackbody radiation standard of 100 1C) before and after the

samples. The increase of absorptance after the addition ofsilica is attributed to the enhanced solar absorption by the silicanetwork which reduces the reective properties of coppercobalt oxide coating surface where the variation of reectiveproperties here are related to interference phenomena governedby lm thickness and refractive index ratios. Further increasesof withdrawal rates, however, seem to have marginal effect onthe absorptance values which suggests that the AR layerthickness does not signicantly inuence absorptance (asopposed to reectance spectra prole, especially in the NIRarea (40.8 mm)). This may be elucidated based on thecharacteristic of the Dufe and Beckman method [29] incounting the absorptance value where the denser countoccurred in the UVvis wavelengths area compared to theNIR area due to the denser spectral distribution of solarirradiance in the UVvis area [32,33], therefore the reectancespectra curve prole below 0.8 mm becomes crucial to note.

3.2. Emittance and selectivity

The reectance spectra of copper cobalt oxide thin lmcoatings with and without a silica AR layer within the mid-far

natthermal test [30]. In this test, the initial S and T values of thecoatings were determined before the thermal test, and theybecame the basis to establish the temperature (T1) applied inthe thermal test. The PC values were evaluated after t18, 36,75, 150, 300 and 600 h of thermal testing. The time t1 wasdened as the last testing time where the PC value was stillless than 0.05. If the PC value was similar to or less than 0.01after t1600 h and there was no cracking, then the absorbercoating was deemed to have passed the accelerated thermaltest. If PC value was more than 0.05 at t1300 h or the PCvalue was higher than 0.01 after t1600 h, then an additionalthermal test at a higher temperature (T3) for t3 hours would berequired (t3 corresponded to the previously determined t1).After this additional test, if PC (T3, t3)ZPC (T1, t1) and thecoatings did not crack, the coating had passed the acceleratedthermal test. Lastly, if the PC value was greater 0.05 att1r150 h, an additional test using a lower temperature (T2) fort2 hours would be required (where t2 corresponded to thepreviously determined t1). If PC (T2, t2)rPC (T1, t1) and therewas no cracking, the absorber had passed the acceleratedthermal test.The surface morphologies of the thin lm samples were

analyzed using FESEM (Zeiss Neon 40EsB). Prior to theanalysis, the sample was mounted on the substrate holder usingcarbon tape and sputter-coated with platinum to reduce anycharging effects. InLens detectors were used at magnicationsof 1 mm with an aperture size of 30 mm while the extra hightension voltage eld emission gun was set at 5 kV.

3. Results and discussion

3.1. Reectance spectra and solar absorptance

Fig. 1 shows the reectance spectra (wavelength range of0.32.7 mm) of the copper cobalt oxide thin lm coatings withthe silica AR layer synthesized at different withdrawal rates.The spectrum of copper cobalt oxide coating without a silicaAR layer which has the thickness of around 320 nm is alsopresented for comparison purposes. Interestingly, it can beclearly seen that the addition of a silica AR layer at increasingdip-coating speeds has a substantial effect on the reectancecurves prole. The relatively signicant changes occur whenthe AR layer was deposited at withdrawal rates of 20 and40 mm/min where their interference peaks (labeled n) andthe absorption edges (labeled #) positions shift to the lowerwavelengths area compared to the coating with silica synthe-sized using withdrawal rate 10 mm/min or coating withoutsilica (see the lines labeled 2 and 3 versus lines labeled 0and 1 in Fig. 1). Likewise, the distance between theinterference peak and absorption edge (amplitude) of thesetwo former silica coatings decreases 60% compared to theamplitude of coating with silica synthesized using the with-drawal rate of 10 mm/min or the coating without silica.Theoretically, the increase of the withdrawal rate wouldincrease the thickness of the silica AR layer [31]. However,

A. Amri et al. / Ceramics Intereven though the thicknesses of silica layer were not measureddirectly and the thickness of copper cobalt oxide layer waskept similar, from the reection curve of the silica coatedsamples, it may be expected that the physical thickness for thewithdrawal speed of 20 mm/min is equivalent to that for thespeed of 40 mm/min. These observations evidently imply thatthe reectance spectra prole relatively alters with the increasein the thickness of the silica AR layer.Low spectral reectance in the solar wavelength area

indicates high absorptance and vice versa. The increases ofabsorptance values for the three coatings with silica AR layersare ca 1.01.5% while the AR layer synthesized with dip-speedof 10 mm/min exhibits the highest absorptance among all the

Fig. 1. Reectance spectra of copper cobalt oxide thin lm coatings with andwithout the silica AR layer within a wavelength range of 0.32.7 mm withcorresponding solar absorptance (S) values.

ional 40 (2014) 1656916575 16571infrared wavelength range are presented in Fig. 2. Similarly,the addition of silica AR layer at increasing dip-coating speeds

Table 1Accelerated thermal durability parameter values obtained in the thermal test.

Parameters Testing times (t1) at 265 1C

18 h 36 h 75 h 150 h

S 0.005 0.002519 0.000628 0.001132T 0.0283 0.08412 0.10895 0.14197PC 0.01915 0.039541 0.053847 0.069853

Thermal test at 235 1C for 179 hS 0.0019T 0.060247PC 0.032024

nathas a substantial effect on the reectance curves prole. Withinthe wavelength range of around 38 mm, the addition of an ARlayer increases the reectance. However, within the wave-length range of around 810 mm, the AR layer absorbs toomuch infrared light, thus increasing the thermal emittance (T).Within this range, the thicker the AR layer, the higher theinfrared absorption. This phenomenon is due to the strongphonon absorption of the SiO stretching modes as alsoreported by other researchers [26,34]. The relatively weakerphonon absorption in the wavelength range of around 15 mm isalso observed and can be attributed to the phonon absorptiontypically exhibited by the copper cobalt oxide family [26].The value is dened as a weighted fraction between

emitted radiation and the Planck black body distribution. Itmay be determined based on the reectance spectrum data

Fig. 2. Reectance spectra of copper cobalt oxide thin lm coatings with andwithout silica AR layer within wavelength range of 3.015.4 mm withcorresponding solar emittance (T) values.

A. Amri et al. / Ceramics Inter16572[29]. This parameter is generally used to explain the perfor-mance of a solar selective absorber in the mid-far infraredwavelength range. High spectral reectance within this rangeindicates low thermal emittance and vice versa. From Fig. 2,the overall thermal emittance of copper cobalt oxide thin lmcoatings with a silica AR layer increases with the increase ofdip-speed. The signicant increase of emittance at ca 50%occurs when the dip-speed is increased from 20 to 40 mm/minwhich implies increased heat loss from the coating surface anddecreased efciency. The marginal increase of emittance valueis shown by the coating with the AR layer synthesized usingdip-speed 10 mm/min.Fig. 3 shows the optimal reectance curve of copper cobalt

oxide thin lm coating with silica AR layer (dip-speed of 10 mm/min) within the wavelength range of 0.315.4 mm corresponding tothe optimum absorptance value of S84.96% and emittancevalue of T5.63% (selectivity, s15.1). An emittance valuebelow 10% can be categorized as a good emittance performancefor a selective absorber material [3,4,35]. The dashed line in Fig. 3is the extrapolation line created in place of noisy spectrumat the end of the spectrum measurement range generated by theequipments used (UVvisNIR and FTIR).Fig. 3. Reectance spectrum of copper cobalt oxide thin lm coatings with ARlayer (dip-speed of 10 mm/min) within wavelength range of 3.015.4 mm.

ional 40 (2014) 16569165753.3. Accelerated thermal durability test

The accelerated thermal durability test was conducted todetermine the estimated service lifetime of a selective absorbersurface based on its thermal behavior at a high temperaturerange. This is because the real application of a selectiveabsorber is strictly insulated under a transparent glass cover;therefore the thermal behavior became the essential factordetermining the quality of the absorber lm. The InternationalEnergy Agency (IEA) developed an accelerated thermaldurability test to assess the thermal collector performancecalled performance criterion (PC) through the IEA SHC Task27 [30]. This test procedure assumes that the activation energyof a certain degradation process is sufcient to ensure absorberdurability under natural working conditions of a at thermalcollector [36].Table 1 shows the PC values of the coatings with a silica

AR layer based on the thermal test of the IAEA SHC Task 27.Based on this procedure, it was selected the temperature of265 1C as applied in the thermal test (T1) for several testingtimes (t1) as our coating showed the optimum absorptance

value of around 8485% and emittance value of around 56%.From Table 1, it can be seen that the variations of emittancebefore and after the thermal test (T) are much higher than thevariations of absorptance (S). The change of emittance(T) values increases with the increase of testing time, whilethe change of absorptance (S) values is relatively small andnegligible. As such, the PC values absorber coatings are moreinuenced by the changes in emittance. The signicant changeof spectral reectance in the mid-IR area (Z2.5 mm) after thethermal test is due to the thermal load that causes the thermo-chemical redox reactions experienced by absorber coatingmaterials which increase the IR absorption [30,37,38]. ThePC values increase with the increase in the testing times whilefor the testing times of 75 and 150 h, the recommended PCvalue (PC=0.05) for a qualied absorber was exceeded.As such, longer testing times were not required. Instead, anadditional test was carried out using a lower temperature

(T2=235 1C) for t2=179 h. It is observed that the PC (235 1C;179 h) is less than PC (265 1C; 36 h). Based on the PC valuecriteria, the coatings with the silica AR layer pass theaccelerated thermal durability test. Fig. 4 shows the reectancespectra of copper cobalt oxide thin lm coatings with an ARlayer within the wavelength range of 0.315.4 mm before andafter the thermal test at 265 1C for 36 h and 235 1C for 179 h.The possibility of residual water content that may inuence

the optical performance of coatings can be dismissed since allcoatings were synthesized at temperatures above 400 1C andthe samples were treated as described in Section 2.1. None ofthe samples showed observable visual changes and cracksbefore or after the thermal test, see Fig. 5. The thermal testseven improve the appearance of lm coating (i.e. smoother)with integrated silica antireection layer (Fig. 5b). Thisindicates the high thermal endurance of coatings and the goodadhesion between the coating and the substrate. Based on thephysical and cracking inspections, the coatings with AR layersfulll the performance criteria as good material for solarselective absorber. Nonetheless, the results in Table 1 revealthat the degradation of the coatings with the silica AR layer ismore governed by the temperature regime than the exposuretime. This may be detected by comparing the PC valuesbetween temperatures 235 1C and 265 1C where the PC valuecan be retained remain low (o0.05) by decreasing thetemperature test even though the test was carried out at alonger of exposure time. As such, the coatings are qualied for

A. Amri et al. / Ceramics International 40 (2014) 1656916575 16573Fig. 4. Reectance spectra of copper cobalt oxide thin lm coatings with AR

layer before and after accelerated thermal durability test at: (a) 265 1C for 36 hand, (b) 235 1C for 179 h.Fig. 5. Surface morphology of coatings for (a) before and (b) after thermaldurability test.

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4. Conclusions

The copper cobalt oxide thin lm coatings with the silicaantireection (AR) layer have been successfully deposited onreective aluminum substrates using the solgel dip-coatingmethod. The addition of silica changed the reectance spectraof coatings within the wavelength range of 0.315.4 mm. ThePC values results and physical inspections showed that thecoatings with silica AR layer passed the accelerated thermaldurability test without any cracking detected. The degradationsof the copper cobalt oxide thin lm coating with silica ARlayer is governed more by the temperature changes regimethan the exposure time indicating that the coating is qualiedfor long periods of uses in low temperature applications suchas for domestic solar water heater systems or for faade coatingapplications.

Acknowledgements

We would like to acknowledge Mr. Ken Seymour for histechnical assistance. M. Mahbubur Rahman is highly gratefulto Murdoch University for providing the PhD scholarships.

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