optimisation of metal sputtered and electroplated substrates for solar selective coatings

ARTICLE IN PRESS

0960-1481/$ - se

doi:10.1016/j.re

�CorrespondE-mail addr

isee_2000@hotm

Renewable Energy 33 (2008) 1275–1285

www.elsevier.com/locate/renene

Optimisation of metal sputtered and electroplated substratesfor solar selective coatings

M. Farooqa, Iftikhar A. Rajab,�

aPakistan National Accreditation Council, Evacuee Trust Complex, F-5/1, Islamabad, PakistanbCOMSATS Institute of Information Technology, University Road, Abbottabad, Pakistan

Received 23 November 2006; accepted 19 June 2007

Available online 27 August 2007

Abstract

This study describes the performance of selective coatings to have maximum solar absorptance and minimum thermal emittance, in

relation to substrate preparation. Aluminium and copper substrates, covered with sputtered or electroplated metal base layer, have been

used to see the influence of different types of substrates for solar selective coatings. The effect of the base layer material, thickness,

deposition process and deposition condition, on the optical performance of selective coatings has been analysed. Nickel was electroplated

and nickel and vanadium were sputtered as a base layer on the Al and Cu substrates. A comparison of plated and sputtered nickel

substrates for Ni:SiO2 and V:Al2O3 composite solar selective coatings is presented. Theoretical results using computer simulation for

solar selective composites on various substrates, and the effect of the base layer thickness on these substrates are compared with

experimental results. The effects of the base layer thickness for cobalt and tungsten are also included. The sputtered base layers selective

coatings produced higher absorptance along with higher emittance and electroplated base layer coatings resulted in comparatively lower

absorptance and lower emittance. Hundred nanometre metal sputtered base layer is optimised for solar selective coatings.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Reflective substrates; Selective coating; Sputtering; Electroplating

1. Introduction

In photothermal solar energy conversion, the mostimportant and critical part of the solar collector is theabsorber surface, which should absorb maximum solarradiation and convert it into heat, which may be transferredto the heat transfer medium with minimum loss due tore-radiation in the thermal infrared range. A surface that isminimum reflective in the solar spectrum (high absorp-tance) and highly reflective in the thermal spectrum (lowemittance) can achieve the objective. In order to do so,a metal–dielectric composite film, which should absorbmaximum solar radiation, is deposited on a metal substrate,which is highly reflective in the thermal spectrum. At hightemperatures, metal substrate diffuses into the compositefilm, destroying the physical properties of the coating.

e front matter r 2007 Elsevier Ltd. All rights reserved.

nene.2007.06.025

ing author. Tel.: +92992 383591; fax: +92 992 383441.

esses: [email protected] (M. Farooq),

ail.com, [email protected] (I.A. Raja).

Substrate diffusion into the film causes oxidation, which hasbeen observed on heat treatment [1]. For solar thermalapplications, the film material and substrate should notchange their physical and chemical properties. This mayhave adverse affect on the solar absorptance and thermalemittance.Thin films of metal are deposited onto metal substrates

to avoid the diffusion into the composite film duringaccelerated ageing tests, which decreases the opticalperformance of selective coatings. Substrate preparationconditions affect coating optical properties. However, baselayers prepared by different techniques behave optically indifferent ways. Even a minor change in the depositionconditions affects the base layer performance significantly.Selection of coating material and coating process

combination for a specific substrate requirement can becomplex. There are great numbers of possible combina-tions, not all of which lead to satisfactory solutions.Coating material selection is the key to find the acceptablesolution to overcome the interdiffusion and higher

www.elsevier.com/locate/renene

dx.doi.org/10.1016/j.renene.2007.06.025

mailto:[email protected]

mailto:[email protected],

mailto:[email protected]

ARTICLE IN PRESSM. Farooq, I.A. Raja / Renewable Energy 33 (2008) 1275–12851276

emittance problems in solar selective coatings. It beginswith the examination of previously studied materials toreveal the nature of diffusion and emittance problem,which can indicate the coated surface properties requiredto provide the satisfactory remedy. With this information,both the candidate coating material and coating processcan be selected. Confusion can occur in selecting coatingmaterial due to the combination of properties that makedifficult to optimise simultaneously such as diffusion andemittance.

We have studied and prepared base layers of electro-plated nickel, sputtered nickel and vanadium. The opticalperformances resulting from the two preparation techni-ques are quite different. Bulk, polished metals and sputteredmaterials have different optical behaviours, especially in thethermal infrared region due to their structural differences.We will see that sputtered metals substrates have lowerreflectance in the thermal than the electroplated substrates.The reasons for using sputtered metal on a metal substrateare to avoid the diffusion of the metal substrate into the filmand to reduce the number of processes for fabricating theselective absorbers.

On the other hand, electroplated nickel has more grainboundaries than sputtered materials. Water particles andgases, which might be present, enable rapid diffusion pathsof manifold order of magnitudes higher than the latticediffusivity [2]. The diffusion of the aluminium layer on asilicon substrate at 400 1C has been reported, for 30minheat treatment [3]. The thickness of the diffusion barrier isvariable, as it depends on the operating temperatures.The 300-nm thick diffusion barrier does work at 550 1C,but 150 and 80 nm thick films fail at 500 1C [4]. There areconsiderable data on the thermal stability of selectivecoatings, which show a decrease in solar selective coatingperformance, due to diffusion and substrate oxidation[5–7]. Copper substrates are good thermal conductors, buthave the disadvantage of reaction with the deposited film[8]. High vacuum sputtered thin films are comparativelyimpurity free and there are less chances of interdiffusionthan the electroplated substrates. In this study, we willoptimise the base layer that should have acceptable thermalemittance and minimum diffusion into the compositecoating.

2. Experimental

High vacuum sputtering system, a simplified electroplat-ing apparatus and a self-designed computer simulationwere used to investigate the effect of base layer for solarselective coatings. Copper and aluminium substrates(50 cm2) were used to deposit the base layer and metal–dielectric composite film. The substrates were cleaned firstwith 20% dilute HNO3 and then 10M NaOH at roomtemperature.

For electroplating, potentiostat was used as a powersource and a temperature-controlled bath was employed tostudy the effect of temperature on plating. The nickel plate

was used as an electrode to deposit a uniform film onto thesubstrates. The composition of the solution was as follows:

NiSO4. 7H2O
124 g/l
H3BO3
30 g/l CoSO4
. 7H2O
15 g/l NH4Cl 37 g/l
Nickel sulphate, being a low cost, commercially availablematerial, was used as a source of nickel ions for deposition.For improving the brightness, which should give a highreflectance in the far infrared spectrum, cobalt sulphatewas used. When a potential difference is applied across thetwo electrodes, a current flows owing to the movement ofions towards the electrodes of opposite charge. Cleansubstrates improve adhesion and produce smooth surfaces.A weak hydrochloric acid solution (5%) was used toremove the top oxide layer from the cleaned metallicsubstrates. The deposition efficiency was between 80% and95% dependent on deposition conditions.A Nordiko NS-3750 series magnetron sputtering system

was used to deposit base layer and fabricate thin film solarselective absorbers. The system contains a 1.25 kW RF anda 6 kW DC generators with 400 � 1200 electrodes, 10 cm awayfrom the target. The substrate carrier is octagonal in shapeand can be adjusted to spin at various speeds for requiredco-sputtering deposition. The chamber was evacuated tobelow 10�6 Torr before entering argon gas for sputtering.The sputtering pressure was maintained at 3.0–3.5mTorr.A computer simulation tool has been developed to

investigate the effect of base layer for selective absorbers.The design tool has been validated by experimental workand has been used further to investigate systematically keycoating design parameters for various refractive indexmaterials with the objective of optimising the base layer ofthe selective absorber coatings.Optical properties of the coating in the solar spectrum

from 0.3 to 2.5 mm were measured using a double beamratio recording, Beckman 5240 integrating sphere spectro-photometer. The sphere (0.15m diameter) is coated withbarium sulphate (BaSO4). The references are pressedBaSO4 powder and aluminium mirror to measure diffuseand specular reflectance, respectively. Basically this instru-ment is divided into two spectral ranges from 0.3 to 0.8 and0.8 to 2.5 mm with the source of deuterium and tungstenlamps. The standard instrumental error of the instrument is70.01.Near-normal hemispherical total reflectance was also

determined with a Fourier Transform single beam ratiorecording spectrophotometer, Bruker IFS66, equippedwith a gold integrating sphere and mercury cadmiumtelluride (MCT) infrared detector, controlled by computeroperated OPUS/IR software [9]. Spectral emittance valueswere derived for wavelengths up to 20 mm. The standardinstrumental error of the instrument is 70.006.Integrated values of solar absorptance at AM-2 [10] and

thermal emittance at 300K [11] were calculated.

ARTICLE IN PRESS

0.00

0.02

0.04

0.06

0.08

0.10

0.12

εAl

Cu

Calculated

V:Al2O3

Experimental

Ni:SiO2

Experimental

V:Al2O3

Fig. 3. Thermal emittances of 200 nm thick V:Al2O3 composites on

copper and aluminium substrates.

0.940

0.945

0.950

0.955

0.960

0.965

0.970

Calculated

V:Al2O3

Experimental

Ni:SiO2

Experimental

V:Al2O3

α

Al

Cu

Fig. 2. Solar absorptances of 200 nm thick V:Al2O3 composites on copper

and aluminium substrates.

M. Farooq, I.A. Raja / Renewable Energy 33 (2008) 1275–1285 1277

3. Results and discussion

3.1. Optical properties of aluminium and copper substrates

The optical performance of cleaned aluminium andcopper substrates has been investigated. Aluminium sub-strates exhibit higher IR reflectance and reduced emittance.A cleaned aluminium substrate is 1% more reflective thanthe cleaned copper [12]. The calculated reflectance spectraof the V:Al2O3 composites on copper and aluminiumsubstrates are shown in Fig. 1. Two hundred nanometerthick, V:Al2O3 and Ni:SiO2 coatings were fabricatedwith a 50 nm sputtered base layer of vanadium and nickel,respectively, on cleaned copper and aluminium substrates.The optical properties regarding solar absorptance andthermal emittance of fabricated coatings were measuredand compared to calculated results obtained using thedesign tool. The results are presented in Figs. 2 and 3. Theeffect of the change of substrate on the performance of thecoating is minimal (1–2%), owing to the high concentra-tion of metal in the composite film. Moreover, a 50 nmthick base layer reduces the effect of different substrates.There is a small decrease in solar absorptance for the filmscoated on aluminium substrates, as aluminium is morereflective in the visible part of the solar spectrum and theemittance has been reduced due to the higher reflectance ofAl in the infrared. The cleaning difference for Al and Cusubstrates may have produced different surface structureon the substrate as well.

Calculated and the experimental results show the sametrend for both the substrates. The difference in the infraredreflectance is more significant than the reflectance changein the solar spectrum. The addition of a base layerdecreases dependence on the substrate material, especiallyin the solar range.

0.5 1.0 1.5 4 14121086

0.0

0.2

0.4

0.6

0.8

1.0

Al substrate

Cu substrateRe

fle

cta

nce

Wavelength (μm)

Fig. 1. Calculated reflectances of 200 nm thick V:Al2O3 composites on copper and aluminium substrates.

ARTICLE IN PRESS

0 1 3 4 52

Em

itta

nce (

ε)

Current Dendity (A/dm2)

0.00

0.02

0.04

0.06

0.08

0.10

0.12

Fig. 5. Effect of current density on the thermal emittance of nickel-plated

copper.

M. Farooq, I.A. Raja / Renewable Energy 33 (2008) 1275–12851278

3.2. Effect of electroplated nickel substrates

3.2.1. Optics and plating current density

The problem of substrate diffusion into the film canbe reduced through the use of a metallic base layer [13].A series of nickel electroplating experiments weremade to achieve maximum reflectance in the infrared(3–20 mm) in order to decrease the thermal emittance.The electroplating current density is one of the mostimportant parameters to be investigated. Though a lotof work has been performed on nickel-plating, littleemphasis has been placed on the hemispherical reflectanceof the material. The influence of the current densitieson particle size and structure is prominent. The resultsindicate that particle size at low current densities isbigger and at higher current densities is smaller [13].Optically, the greater the current density, the higheris the reflectance in the thermal spectrum. Fig. 4 showsthe optical behaviour of nickel plated onto coppersubstrates at different current densities. The biggercrystals formed at the lower current densities have lowreflectance, possibly due to porosity and coherent struc-ture. With an increase in the current density, the filmsshow higher reflectance owing to a lower porosity ofthe film. We are interested in having low reflectance inthermal spectrum resulting in low thermal emittancesubstrates to fabricate the selective absorbers onto. Theeffect of nickel-plating current density on e is shown inFig. 5. The film of 5.0A/dm2 current density plated nickelsubstrate is smoother and exhibits a lower emittance in thethermal range, but also a lower solar absorptance isobserved.

The hemispherical reflectance of 200 nm thick V:Al2O3

composites exhibits an increase in solar absorptance andemittance by 2.7% and 6.8%, respectively, for 1.0A/dm2

over a 2.0A/dm2 current density plated copper substrate.

2 4 6 8

0.8

0.6

0.4

0.2

0.0

1.0

Reflecta

nce

Wavele

Fig. 4. Effect of current density on the spec

The effect of the nickel-plated current density on theperformance of the coatings is shown in Fig. 6.It is concluded that the preparation conditions of the

substrate are more critical than the substrate material,especially for nickel-plated substrates.

3.2.2. Significance of thickness

The effect of thickness on the optical performanceof nickel-plated copper is significant at low currentdensities and almost negligible at high current densities.As the electrodeposited film becomes thicker, its porosityreduces and the surface becomes smoother at lowercurrent densities [14]. For 5A/dm2 current density nickel-plated copper, there seems to be no significant effect ofthickness on the thermal emittance. By increasing thenickel film thickness from 70 to 215 nm, e is reducedfrom 0.012 to 0.011. The deposited particles seem to bevery small in size and closely packed. After one layer is

10 12 14 16 18

0.5 A

0.25 A

1.0 A

2.5 A

ngth (μm)

tral reflectance of nickel plated copper.

ARTICLE IN PRESS

2 A Ni plated Cu1 A Ni plated Cu

α

0.12

0.08

0.04

0.00

0.02

0.06

0.10

0.14

ε

0.96

0.94

0.92

0.90

0.88

0.86

α (Left Y-axis) ε (Right Y-axis)

800 n

m thic

k

800 n

m thic

k

Fig. 6. Optical performance of 200 nm thick, V:Al2O3 composites on different thicknesses and current densities of Ni plated copper substrates.

2 4 6 8 18161412100.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

200 nm thickness

950 nm thickness

Reflecta

nce

Wavelength (μm)

Fig. 7. Effect of thickness on the optical performance of the 2A/dm2 plated nickel on copper. Emittances for 200 nm thick and 950 nm thick nickel film

plated onto copper are 0.058 and 0.040, respectively.


plated onto copper, further plated layers of nickelwould merely add to the thickness without improving theoptical reflectance but may be helpful to act as a diffusionbarrier.

However, lower current densities change the opticalproperties of the plated nickel copper with the thickness.As the film thickness increases, the porosity decreases andhigh reflectance is obtained in the infrared spectrum. Fig. 7shows the reflectance of two different film thicknessesplated at 2A/dm2 current density. It was observed that afilm thickness of up to 1000 nm reduces the film porosity tosignificant levels. A further increase in thickness does notdecrease the emittance in the thermal spectrum as shown inFig. 8 where the thickness of nickel plated onto coppersubstrates at 2A/dm2 current density is varied from 250to 1631 nm.

3.2.3. Influence of temperature

Most of the electroplating solutions have an optimumtemperature range, therefore, for obtaining satisfactorydeposits certain physical conditions have to be controlled.Low temperature nickel-plating solution has produced ahigh deposition rate, also resulted in a bright deposit withgood adhesion [15]. As discussed earlier, large grain size isa factor for low thermal reflectance. The required nickel-plating material should be of low emittance. Hence, low-temperature plating is preferred for solar application as itproduces higher IR reflectance substrates. The hemisphe-rical reflectance of plated nickel at various temperatures isshown in Fig. 9. The best results were obtained at 25 1C(room temperature). The increase in temperature resultedin a higher emittance base layer. The emittance dependencyon the plating temperature is shown in Fig. 10.

ARTICLE IN PRESSM. Farooq, I.A. Raja / Renewable Energy 33 (2008) 1275–12851280

3.3. Sputtered base layers

3.3.1. Nickel as a base layer

Nickel base layer coatings have been prepared byelectroplating and sputtering on copper substrates. Elec-troplated nickel on copper is optically very reflective(dependent on the deposition parameters), but copperdiffuses into the electroplated nickel at 350 1C, even at10 mm thickness [16]. Impurities in the plating bath andwater particles contribute to the changes in structure ofthe film. An almost pure thin film can be fabricated in ahigh vacuum chamber. DC sputtering was utilised tosputter metals as base layers on metallic substrates. Unlikeelectroplated nickel, sputtered nickel as a base layer isthickness dependent and affects the optical performance ofthe substrate. The infrared reflectance decreases with anincrease in thickness of sputtered nickel on copper andaluminium substrates. The coating emittance is higherwhen the composite film is fabricated on the thickersputtered nickel base layer.

200 400 600 800 1000 1200 1400 1600 1800

0.044

0.045

0.046

0.047

0.048

0.049

0.050

Em

itta

nce (

ε)

Thickness (°A)

Fig. 8. Effect of thickness on the thermal emittance for 2A/dm2 current

density nickel-plated copper.

2 4 6 8

0.0

0.2

0.4

0.6

0.8

1.0

Re

fle

cta

nce

Wavele

Fig. 9. Comparison of the hemispherical spectral reflectances of nickel

Fig. 11 represents the performance of 50 and 100 nmthick sputtered nickel base layers on copper substratesunderneath a 200 nm thick Ni:SiO2 composite film. Theabsorptance and emittance has increased with the baselayer thickness; the solar absorptance increases slowly, andthe thermal emittance increases rapidly. By increasing thebase layer thickness from 50 to 100 nm, the absorptancehas increased by 0.2% and the emittance by 1.2%.The base layer thickness was further increased and

the optical performance did not change the pattern ofincreasing emittance with the thickness when nickel is thebase layer. The choice of dielectric e.g., SiO2 or Al2O3 forcomposite films changes the solar absorptance and thermalemittance of the coatings, but the effect of the base layerthickness remains the same, as shown in Fig. 12. The solarabsorptance increased by 2.2% and emittance increasedby 6.5%, when the base layer thickness was increasedby 113 nm.It is observed that with the increase of the base layer

thickness up to 100 nm, the solar absorptance increases

1816141210

35 C

45 C

25 C

ngth (μm)

base layers plated onto copper substrates at different temperatures.

25 30 35 40 45

0.04

0.05

0.06

0.07

0.08

Em

itta

nce

(ε)

Plating Bath Temperature (°C)

Fig. 10. Thermal emittances of Ni base layers plated onto Cu substrates at

different temperatures.

ARTICLE IN PRESS

0

0.950

0.955

0.960

0.965

Base Layer Thickness (nm)

α

α (Left Y-axis)

0.09

0.10

0.11

0.12

0.13

10050

ε

ε (Right Y-axis)

Fig. 11. Optical performance of 200 nm thick 70% metallic Ni:SiO2 composite on 50 and 100 nm thicknesses of sputtered Ni on copper substrates.

20 40 60 80 100 120 140 160

0.98

0.96

0.94

0.92

0.90


α

α (Left Y-axis)

0.04

0.06

0.08

0.10

0.12

0.14

ε

ε (Right Y-axis)

Fig. 12. Optical performance of 200 nm thick 70% metallic Ni:Al2O3 composite on different thicknesses of sputtered Ni on copper substrates.


considerably. Further increase in base layer thickness doesnot effectively increase the absorptance, but the thermalemittance is raised. It has also been observed that the nickelsputtered base layer does not show the rapid cut-off regionbetween the solar and thermal ranges, which shows lowinfrared reflectance.

3.3.2. Vanadium as a base layer

A metal, which could rectify the drawbacks of sputterednickel on copper and aluminium substrates, after beingsputtered, would be of importance. Such a metal should bethermally stable for medium range temperature (4150 1C)applications. Vanadium satisfies this condition and is stablein air up to 660 1C.

Sputtered vanadium, used as a base layer for 80%metallic composites of V:Al2O3, shows high solar absorp-tance and low thermal emittance on a bulk copper

substrate, contrary to sputtered nickel on copper whichhas a higher emittance. The experimental results withand without a 50 nm thick base layer are shown in Fig. 13.The solar absorptance and thermal emittance differencesare due to changes in the base layer. Similar resultswere observed for 70% metallic films. Three base layerthicknesses were studied: 0, 25 and 50 nm. The differencein absorptance with and without a 25 nm thick base layerfilm is significant (6.4%). A further 25 nm increase in thebase layer thickness has increased a by 2.4%, as shown inFig. 14.The base layer thickness was further increased to 300 nm

for a different design of composite film. The design of thefilm also changes the overall performance of the film, butthe influence of the base layer thickness observed to be thesame. For thicker base layers, the cut-off point is movedtowards longer wavelengths resulting the increase of solar

ARTICLE IN PRESS

500

α

0.08

0.07

0.06

0.05

0.04

ε

0.96

0.95

0.94

0.93

0.92

0.91

0.90

α (Left Y-axis)

ε (Right Y-axis)

Fig. 13. Effect of the base layer thickness on the performance of 80% metallic 200 nm thick V:Al2O3 composites.

0 5040302010

0.86

0.88

0.90

0.92

0.94

0.96


α

α (Left Y-axis)0.07

0.06

0.05

0.04

0.03

0.08

ε

ε (Right Y-axis)



absorptance and thermal emittance. The solar absorp-tances and thermal emittances of these coatings are shownin Fig. 15.

The sputtered base layer creates a rougher surface on thesubstrate, which increases the absorptance. The base layerthickness becomes a part of the active film and, ignoringthe other factors, the thicker films are more absorbing.On the other hand, the emittance increases linearlywith the increase in base layer thickness. The presence ofthe sputtered base layer always increases the solarabsorptance, due to the change in the surface geometryof the substrate. Since the base layer increases the coatingemittance, the base layer thickness was kept between 50and 100 nm. The effect of base layer thickness for nickeland vanadium has been compared and shown in Fig. 16.Nickel sputtered base layer films are some 3% moreemissive than the vanadium.

3.4. Theoretical results of base layer thickness

The effect of the base layer thickness was studiedtheoretically for various composite designs and materials.All behave in the same way, with increases in thicknessof the base material. It was assumed that the base layeris a thin film, whose metallic volume fraction is unity.It is noted that the effective medium theory has beenapplied to the metallic films that considers the smallparticle sizes of the base film. The base material changed itsoptical characteristics by considering it as a part of thecomposite film. The cut-off wavelength increases withincrease in base layer thickness, as observed experimen-tally. The reflectance spectra of Fig. 17 show shifts of thecut-off wavelength with the base layer thickness, forV:Al2O3 composites. The solar reflectance is lower for thethicker base layer coatings. The values of a and e for

ARTICLE IN PRESS

50 100 150 200 250 300

0.98

0.96

0.94

0.92

0.90


α

α (Left Y-axis)0.30

0.35

0.20

0.25

0.10

0.15

0.00

0.05

ε

ε (Right Y-axis)


0 20 40 60 80 100 120 140 1600.04

0.05

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.13

V:Al2O3

Ni:SiO2

ε


Fig. 16. Effect of the base layer thickness on the thermal emittance of nickel and vanadium based coatings.


different thickness base layer for V:Al2O3 composites areshown in Fig. 18.

It is concluded that the base layer acts as a part of thecomposite film. The shifts of the cut-off peaks with the baselayer thickness indicate that the base layer is part of thecoating and is not simply a metallic reflector.

3.5. Effect of base layer thickness for other materials

Calculations were obtained for other material combina-tions and the studied metal was used as a base material. Inall cases, the absorptance increases with the increase in thebase layer thickness. For further increases in the base layerthickness, the absorptance either increases or remainsconstant, as shown in Fig. 19, similarly observed experi-mentally. As the base layer is part of the active film, withunity volume fraction, so the interference effect will shift

with increase in thickness of the composite film or baselayer. For a 200 nm thick composite film, a 50–100 nm baselayer is suggested, as the absorptance has almost reached amaximum.The emittance increases linearly with the increase in

the base layer thickness for all materials, as confirmed inFig. 20. Though the emittance is different for differentmaterials, increasing emittance for all materials shows thatthe base layer is part of the active film.

4. Conclusion

The choice and preparation of the substrate affects theoptical performance of the composite selective absorbers.Bulk and nickel electroplated substrates have relativelyhigh solar and thermal reflectance, which produceslow absorptance and emittance. A sputtered base layer

ARTICLE IN PRESS

0.94

0.95

0.96

0.97

0.98

150100500

α

α (Left Y-axis)

0.025

0.024

0.023

0.022

0.021

0.020

0.026


ε

ε (Right Y-axis)

Fig. 18. Theoretical results of the base layer thickness on the performance of 70% metallic 200 nm thick V:Al2O3 composites.

0 50 100 150 200 250

0.93

0.92

0.91

0.90

0.89

0.88

0.87

0.86

W:Al2O3

Co:SiO2

Ni:SiO2

V:Al2O3

ε


Fig. 19. Theoretical effect of the base layer thickness on the solar absorptance of 70% metallic 200 nm thick single layer composites of different materials.

0.5 1.0 1.5 4 141210860.0

0.2

0.4

0.6

0.8

1.0

without base layer

50 nm base layer

100 nm base layer

150 nm base layer

Re

fle

cta

nce

Wavelength (μm)

Fig. 17. Theoretical effect of the base layer thickness on the hemispherical reflectance of 70% metallic 200 nm thick V:Al2O3 composites.


ARTICLE IN PRESS

0 50 100 150 200 250

0.034

0.032

0.030

0.028

0.026

0.024

0.022

0.020

W:Al2O3

Co:SiO2

Ni:SiO2

V:Al2O3

ε


Fig. 20. Theoretical effect of the base layer thickness on the thermal emittance of 70% metallic 200 nm thick single layer composites of different materials.


enhances the solar absorptance due to the surface rough-ness. The thickness of the sputtered base layer affects theemittance of the coating significantly. If the thickness ismore than 200 nm, the absorptance remains constant butthe emittance increases linearly. The optimised base layerthickness was found to be in the range 50–100 nm. Therespective emittances of sputtered nickel and vanadium aresignificantly different, and nickel is more emissive. Othermaterials such as Co:SiO2 and W:Al2O3 also show similarbehaviour.

Acknowledgements

We are thankful to the Islamic Development Bank,Jeddah, Saudi Arabia for financial support, without whichit would have been impossible to pursue this research.Special thanks to Professor M. Hutchins for his consistentguidance and Mr. A. Wahid Baig for assistance inconfiguration of this paper.

References

[1] Smith GB, Hillery P, Teytz K. Ranking solar-absorber structural

parameters in terms of impact on performance and stability. Thin

Solid Films 1983;108(3):239–45.

[2] Yousif KM, Smith BE, Jeynes C. Study of durability of (molybde-

num–copper)-black coatings in relation to their use as solar selective

absorbers. Renew Energy 1994;5(1–4):324–9.

[3] Smith GB, Teytz K, Hillery P. The substrate impact on the structure

and thermal stability of solar selective black chrome. Sol Energy

Mater 1983;9:21–41.

[4] Sharma SK, Mehra NC. Spectrally selective black nickel coating

prepared by a conversion process. Thin Solid Films 1992;213(1):80–5.

[5] Henager Jr. CH. Reflectance modification through controlled surface

texturing by sputter etching. Sol Energy Mater 1981;4:403–11.

[6] Sebastian PJ, Quintana J, Avila F. Retention of the optical

absorptance in thermally aged black chrome on variably sensitized

copper. Sol Energy Mater Sol Cells 1997;45(1):65–74.

[7] Kumar SN, Malhotra LK, Chopra KL. Low cost electrolyses nickel

black coating for photothermal conversion. Sol Energy Mater 1980;

3:519–32.

[8] Sella C, Kaba A, Berthier S, Lafait J. Low cost selective absorber

based on a Fe–Al2O3 cermet film. In: SPIE, vol. 653: optical materials

technology for energy efficiency and solar energy conversion V, 1986.

[9] Ageorges P. Transfer report from MPhil to Dphil. SEMRL Oxford

Brookes University, 1994.

[10] Moon P. Proposed standard solar radiation curves for engineering

use. J Franklin Inst 1940;230:583–617.

[11] Sargent SI. A complete table black body radiation function. Bull Am

Meteor Soc 1972;53:360.

[12] Farooq M, Green AA, Hutchins MG. High performance sputtered

Ni:SiO2 composite solar absorber surfaces. Sol Energy Mater Sol

Cells 1998;54(1–4):67–73.

[13] Dennis JK, Such TE. Nickel and chromium plating. Butterworth &

Co (Publishers) Ltd, 1986. p. 16–24.

[14] Lowenheim FA. Modern electroplating. London: Wiley; 1963.

p. 40–50.

[15] Chen K, Chen Y. A new low-temperature electroless nickel plating

process. Plat Surf Finish 1997;84(9):80–2.

[16] Silman H, Isserlis G, Averill AF. Protective and decorative coating

for metals. Finishing Publishing Ltd; 1978. p. 14.

optimisation of metal sputtered and electroplated substrates for solar selective coatings

Documents