Influence of Spray Pyrolysis Deposition Parameters on the Optical Properties of Porous Alumina Films

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ScienceDirect Energy Procedia 48 (2014) 97 – 104

SHC 2013, International Conference on Solar Heating and Cooling for Buildings and Industry September 23-25, 2013, Freiburg, Germany

Influence of spray pyrolysis deposition parameters on the optical properties of porous alumina films Elena Ieneia, Andreea C. Mileaa, Anca Dutaa,* a

RTD Centre Renewable Energy Systems and Recycling, Transilvania University of Brasov, Romania, Eroilor 29, 500036 Brasov, Romania

Abstract Cermets, ceramic-metallic composites, have been extensively investigated for application as solar selective coatings due to their excellent optical properties. They consist of fine metal particles embedded in a ceramic matrix. The ceramic matrix main role is to increase the absorption coefficient (Dsol>0.9) while the metallic particles decrease the thermal emittance (HT<0.1). Due to its good optical properties, and thermal and chemical stability, alumina is a promising candidate for a cermet matrix. The matrix morphology has an important role in the design of the cermet structures with optimum spectral selectivity. There are several methods to produce solar-thermal absorbers, and one low cost and upscale alternative is spray pyrolysis deposition (SPD). In this study it is shown that SPD parameters can be an important tool in tailoring pours alumina as matrix that can be infiltrated with metal and/or metal oxide particles for preparing solar selective coating. The results show that the matrix porosity can be significantly influenced by the thermal treatment, the addition of morphology controlling agents into the precursor solution and less by the number of spraying sequences. © 2014The TheAuthors. Authors. Published by Elsevier © 2014 Published by Elsevier Ltd. Ltd. SHCunder 2013responsibility under responsibility Selection andpeer peerreview review scientific conference committee Selection and by by thethe scientific conference committee of SHCof2013 of PSE AGof PSE AG. Keywords: Alumina Matrix; Spray Pyrolysis Seposition (SPD); Solar Thermal Coatings

1. Introduction One of the most important components of flat plate solar-thermal collectors is the spectral selective absorber plate. Therefore, it is important to develop coatings which efficiently absorb energy in the UV-Vis and near-IR

* Corresponding author. Tel.: +40- 0723-561-089; fax: +40-268-474-098. E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd.

Selection and peer review by the scientific conference committee of SHC 2013 under responsibility of PSE AG doi:10.1016/j.egypro.2014.02.012

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regions (high solar absorbance, αS>0.9 ) and have low thermal emissivity (εT<0.1) in the infrared region. Solar absorbers based on cermet coatings, using alumina matrix have been reported by various authors as Pt– Al2O3 [1], Ni–Al2O3 [2,3], Mo–Al2O3 [4,5], Ag-Al2O3 [6], Co-Al2O3 [7] composites, with good optical properties and thermal stability. Now-a-days, electrochemical deposition and magnetron sputtering are the two main techniques used to prepare porous Al2O3 matrix for solar–thermal energy conversion applications. Due to the limitation of equipment cost, environment pollution and absorber quality, the development of alternative, low cost and environmentally friendly routs is of interest. Spray pyrolysis (SPD) is a widely used method for obtaining films, for a broad range of applications, having thus industrial viability. Previous research, [8] allow obtaining Al/Al2O3/NiO absorber coatings with spectral selectivity above 19, using an alumina thin layer deposited at 400oC by SPD, followed by annealing. Although competitive, the energy consumed in obtaining this coating is high therefore further research was devoted to developing novel alternatives using lower temperatures. One possible route combines the synthesis at room temperature of highly dispersed alumina sols, followed by their spraying at lower temperature, for the fast and uniform removal of the solvent and of the by-products. Thus, in the current study, alumina matrix with porous structure was synthesized by alumisol spray pyrolysis. The crystalline structure, optical properties, and morphology of the as prepared alumina films were characterized, and the effect of the precursor concentration, annealing temperature and number of spraying sequence were investigated. Nomenclature SPD αS εT Psun PB nsp PEG400 CPEG T Sa

Spray Pyrolysis Deposition solar absorbance thermal emissivity normal solar spectral irradiation spectral radiance of a black body at a temperature T spraying sequences number Poly ethylene glycol with average molecular weight ~400 g/mol concentration of PEG, [mol/L] heat treatment temperature, [°C] Average roughness, [nm]

2. Experimental details 2.1. Films deposition In this study, aluminum oxide thin layers were deposited on aluminum substrate using SPD and an alumisol was used as the precursor system. Three different Al2O3 sols were prepared starting from aluminum chloride hexahydrate, AlCl3·6H2O (98%, Scharlau Chemie), as previously reported [9], using different amounts of PEG400 (synthesis grade, Scharlau Chemie). The original alumisol contains 0.6 mol/L PEG400 (noted as SGI). The molar concentration of PEG400 for the other sols was decreased at 0.3 mol/L (noted as SGII ), respectively at 0.1 mol/L (noted as SGIII). The sols were sprayed onto the aluminum substrates on samples with the active surface of 2.5×2.5 cm2 and 0.7 mm thickness, cut from commercial aluminum plate (99.5%, Beofon), chemically cleaned with alkaline solutions (10-15 g/L NaOH, 30-50 g/L Na2CO3, 30-50 g/L Na3PO4) and anodized in nitric acid and sulfuric acid solution during 10 min with a fixed anodization current of 3A [10]. The deposition was performed in open atmosphere using air as carrier gas (p = 2 bar), at a spraying angle of 45° and the nozzle-substrate distance 15 cm. The substrate temperature was kept constant during the deposition (T = 150°C) and the number of spraying sequences was varied at 5, 10, 15 to 30, with a break between two pulses of 30 s.

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Annealing at 200°C, 350°C, and 500°C was tested for further stabilization of the films and for removing the surfactant template and the by-products. 2.2. Characterization techniques The alumina thin films structure was investigated using an X-Ray diffractometer (Brucker-AXS-D8) with CuKα radiation source (λ = 1.5406 × 10-10 m), in the range 2θ = 10-70°. Surface morphology, roughness and voids distribution were examined using Atomic Force Microscopy (AFM, NT-MDT model NTEGRA Prima EC), in semicontact mode with “GOLDEN” silicon cantilever (NCSG10, force constant 0.15 N/m, tip radius 10 nm). Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) were performed by an S3400N field emission SEM (Hitachi, Japan) at an accelerating voltage of 20 kV. The solar absorptance (αS) was determined through reflectance spectra R (λ) recorded in the wavelength range 280-2500 nm, using a Perkin Elmer UV-VIS-NIR spectrophotometer (Lambda 950 equipped with integrating sphere to 150 mm). The thermal emittance (εT) was calculated based on the FTIR spectra (Bruker Vertex 70, equipped with an integrating sphere coated with gold) obtained in reflectance mode, in the range from 2.5 to 25 μm, after 32 scans. The values of these coefficients can be calculated using the equations (1) and (2) [11]: 2.5 Pm

³ >1  R(O ) P

sun

DS

(O )dO @

0.3 Pm

(1)

2.5 Pm

³P

sun

(O )dO

0.3 Pm

25Pm

³ [1  R(O )]P (O )dO B

HT

2.5 Pm

(2)

25Pm

³P P (O )dO B

2.5 m

were: Psun(λ) is the normal solar spectral irradiance defined by the ISO standard 9845-1 (1992) for air mass (AM) 1.5 and PB(λ) is the spectral radiance of a black-body given by Plank’s law (Eq. (3)), at a given temperature (here room temperature).

PB

C

O [e 5

1 C2 / OT

 1]

C1=3.743 X 10-16 Wm2

(3)

C2=1.4387 X 10-2 mK 3. Results and discussion The optical parameters and the results on films roughness are presented in Table 1. By increasing the number of deposition sequences, from 10 to 15, the roughness strongly increases showing that, even PEG400 has a templating effect in the alumisol, this effect is not preserved when the film is formed on the heating plate. The sample obtained in 30 deposition sequences has a much lower roughness but this is more the effect of the significantly longer duration of the deposition process that allows the film to restructure during the growth stages. As the optical parameters did not significantly improve by increasing the number of spraying

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sequences (and the thickness), further investigations were done using only 10 deposition sequences, enough to obtain a homogeneous matrix film, suitable for further infiltration. For the as deposited films (at 150oC), by decreasing the PEG400 content a slight decrease in the solar absorptance is registered along with a much more significant decrease in the thermal emmittance and this is not accompanied by a similar trend in morphology variation, thus one may conclude that is the result of possible trapped by-products (particularly fine carbon grains, more likely to be trapped at small dimensions, thus at lower PEG400 concentration). In the sol-gel process, organic polymers are often applied to improve the strength of the produced alumina coatings [12]. According to Liu et al. [13], PEG provides increased density to the alumina thin coatings without agglomeration of Al2O3 nanoparticles and result in an increase of the thermal stability [14,15]. Still, as the result in Table 1 show, the amount of PEG400 significantly influences the particle aggregation, with a maximum in the roughness and in the voids diameter registered for the composition containing 0.3 mol/L templating agent and annealed at 200oC and above. Since the samples containing lower (0.1 mol/L) and higher (0.6 mol/L) amounts of PEG are similar, and quite different from the samples SGII it may be concluded that there is a critical concentration where PEG may act as a flocculant, randomly and loosely binding the alumina particles. Annealing allows the template (partial) removal, leaving large pores. Table 1. The influence of spray paralysis deposition parameters to the morphological and optical properties Sample

CPEG [mol/L]

nsp

T [°C]

αS

εT

Sa [nm]

Pore sizes [nm]

SGI_5_150

0.6

5

150

0.45

0.44

162

448

SGI_5_200

0.6

5

200

0.47

0.43

127

326

SGI_5_350

0.6

5

350

0.44

0.27

68

187

SGI_5_500

0.6

5

500

0.34

0.23

100

273

SGI_10_500

0.6

10

500

0.42

0.29

194

411

SGI_15_500

0.6

15

500

0.44

0.32

414

1197

SGI_30_500

0.6

30

500

0.46

0.35

108

290

SGII_5_150

0.3

5

150

0.44

0.28

98

264

SGII_5_200

0.3

5

200

0.50

0.42

519

1447

SGII_5_350

0.3

5

350

0.28

0.33

365

1098

SGII_5_500

0.3

5

500

0.26

0.19

243

744

SGIII_5_150

0.1

5

150

0.37

0.26

180

433

SGIII_5_200

0.1

5

200

0.49

0.50

104

301

SGIII_5_350

0.1

5

350

0.37

0.31

240

633

SGIII_5_500

0.1

5

500

0.27

0.12

121

378

Fig. 1. FTIR spectra of Al2O3 films obtained from sol I after 5 spraying sequences at different treatment temperatures

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These assumptions are confirmed by the FTIR spectra in Fig. 1. The band observed at 3450 cm -1 is attributed to –OH stretching of hydrogen bonded aluminum oxy-hydroxide (AlOOH) and the band at 1647 cm-1 is assigned to the –OH bending vibration of a weakly bounded water molecule [16]. The peaks obtained at 2893 cm-1 and 1354 cm-1 indicate the C-H bond while the band at 1472 cm-1 is assigned to C-C bonding vibrations coming from the carbonaceous residues. The band at 1105 cm-1 is assigned to the Al-OH bending vibration of Al-OH-Al groups [16]. The formation of Al-O bond is confirmed by the broad band observed between 500 cm-1 and 800 cm-1 obtained due to the stretching and vibration modes of AlO4 and AlO6 groups. By increasing the annealing temperature, the amount of carbon by-products is decreasing, but their specific signals are not completely removed. These data show that the composition of the alumina films may actually contain more than Al 2O3. Therefore, initially the XRD spectra were recorded, Fig. 2. The first peaks corresponding to crystalline structures are registered, as expected, after annealing at 500oC confirming that at lower temperatures, the alumina films were amorphous. The crystalline samples contain hexagonal Al2O3 (ICCD 12-0539 and ICCD 26-0031). The XRD patterns also display strong peaks corresponding to aluminum substrate (cubic Al, ICCD 04-0787), as very thin alumina films were obtained.

Fig. 2. X-ray diffraction patterns of Al2O3 films obtained from different sols at the temperature of 500°C and 5 spraying sequences

Complementarily, EDX surface analyses were done, to further elucidate the films composition and the results are presented in Table 2. The data show significant amounts of residual carbon (compounds) along with chlorine. Chlorine compounds have to be avoided as they can further promote corrosion, especially in humid atmosphere and at higher temperature as the working conditions in the solar-thermal collectors are. Therefore, annealing at temperatures higher than 200oC is recommended. The Al/O atomic ratio corresponds to the Al2O3 formula (2:3) only for the SGI_5_200 sample, having large oxygen excess for SGI_5_150 sample, confirming possible oxy-hydrated compounds as AlOOH or AlOCl, as previously reported, [17]. For the samples annealed at higher temperatures (350o, 500oC) this ratio is about 1 showing an aluminum excess, but this can be the result of the substrate signal that becomes obvious as result of the films densification with a corresponding decrease in thickness. These very thin films have the lowest thermal emittance.

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Elena Ienei et al. / Energy Procedia 48 (2014) 97 – 104 Table 2. EDX data concerning the surface concentrations of the elements. Sample

C atom%

O atom%

Al atom%

Cl atom%

O/Al atomic ratio

SGI_5_150

12.29

55.53

26.86

5.32

2.07

SGI_5_200

-

58.85

38.98

2.17

1.51

SGI_5_350

-

53.94

46.06

-

1.17

SGI_5_500

-

47.17

52.83

-

0.89

SGII_5_500

-

51.90

48.10

-

1.08

SGIII_5_500

-

52.25

47.75

-

1.09

The optical properties are depending on the composition and crystallinity of the films but the most important factor affecting these values is the surface morphology and aspect. The surface topology should be tuned to allow multiple reflections that enhance the absorptance and reduces emittance. As the optimized alumina thin films represent the matrix in a composite spectral selective coating, they are expected to be able to further accommodate the pigment and metal nanoparticles. The AFM images are presented in Fig. 3. The results confirm the possibility of tuning the morphology by varying the PEG 400 content; large agglomerates of 300…1000nm are structured to form open pores after the sample are annealed at 500 oC. The as deposited samples have a much more compact structure, explaining thus the rather poor emittance (lack of multiple reflections).

Fig.3. 2D AFM images. inset: pore size distribution of samples: a) SGI_5_500. b) SGII_5_500. c) SGIII_5_500. d) SGIII_5_150. e) SGI_15_500

Elena Ienei et al. / Energy Procedia 48 (2014) 97 – 104

The SEM micrographs give the overall picture of the assemblies, Fig. 4.

Fig. 4. SEM images of alumina films. after 5 spraying sequences with variation of PEG400 concentration and temperature

As the optical properties and the roughness data already outlined, the low PEG 400 concentration allows obtaining more homogeneous films, with fewer cracks, even before annealing. The cracks are formed during deposition and are growing during annealing and their amount decreases by lowering the template’s concentration. This effect is increased by increasing the number of deposition sequences, as Fig. 5 presents for the as deposited and for the annealed samples obtained with 15 deposition sequences.

Fig.5. SEM images of alumina films obtained from sol I after 15 spraying sequences at different temperatures

Considering that the SGIII_5_500 has the best combination of optical properties (lowest emittance and highest spectral selectivity) we may conclude that films homogeneity and lack of cracks are most important. On the other hand, as initially stated, emmittance is expected to decrease when infiltrating the matrix and developing the cermet; in this view, the samples SGII_200 and SGIII_200, annealed at a rather low temperature (200 oC) may also represent good matrixes candidates. 4. Conclusion The SPD synthesis of alumina thin films was investigate starting from a stable sol of Al2O3 obtained using poly ethylene oxide as templating agent. The results show that the thermal emittance of alumina matrix can be reduced by annealing and by using small amounts (0.1 mol/L) of PEG400 as additive. Optimal size of the pores (~200-300 nm) leads to improved absorptance due to multiple reflections that may occur inside the pores, even at lower annealing temperatures, as 150…200oC.

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Our approach offers a flexible and economic alternative to the current industrial deposition processes for obtaining porous alumina matrixes that can be infiltrated with metal and/or metal oxide particles aiming at developing spectral selective coatings for flat plate solar thermal collectors. Acknowledgements This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD). ID76945 financed from the European Social Fund and by the Romanian Government and by the project EST IN URBA 28/2012 PN-II-PT-PCCA-2011-3.2-1235, developed within the program PNII – Partnership in priority domain, with the support of ANCS, CNDI-UEFISCDI. References [1] Vien TK, Sella C, Lafait J, Berthier S. Pt–Al2O3 selective cermet coatings on super alloy substrates for photo thermal conversion. Thin Solid Films. 1985; 126:17–22. [2] Othonos A, Nestoros M, Palmerio D, Christofodes C, Bes RS. Traverse JP. Photothermal radiometry on nickel (pigmented aluminium oxide) selective solar absorbing surface coatings. Sol. Energy Mater. Sol. Cells. 1998; 5:171-9. [3] Sathiaraj TS, Thangaraj J, Al Sharbaty H, Agnihotri OP. Optical properties of selectively absorbing R.F.sputtered Ni–Al2O3 composite films. Thin Solid Films. 1991; 195:33–42. [4] Du X, Wang C, Wang T, Zhou L, Chen B, Ru N. Microstructure and spectral selectivity of Mo–Al2O3 solar selective absorbing coatings after annealing. Thin Solid Films. 2008; 516:3971–7. [5] Zhang Q-C, Yin Y, Mills DR. High efficiency Mo–Al2O3 cermet selective surfaces for high-temperature application. Sol. Energy Mater. Sol. Cells. 1996; 40: 43–53. [6] Barshilia HC, Kumar P, Rajam KS, Biswas A. Structure and optical properties of Ag–Al2O3 nanocermet solar selective coatings prepared using unbalanced magnetron sputtering. Solar Energy Materials & Solar Cells. 2011; 95:1707–15. [7] Niklasson GA, Granqvist CG. Optical properties and solar selectivity of coevaporated Co–Al2O3 composite films. Journal of Applied Physics. 1984; 55:3382–410. [8] Ienei E, Isac L, Cazan C, Duta A. Characterization Of Al/Al2O3/NiOx Solar Absorber Obtained By Spray Pyrolysis. Solid State Sciences. 2010; 12:1894-7. [9] Milea CA, Ienei E, Bogatu C, Duta A. Sol-Gel Al2O3 powders-matrix in solar thermal absorbers. J.Sol-Gel Sci. Technol. 2013; 67:112-20. [10] Anicai L, Trifu C, Dima L. Anodic oxidation and coloring of aluminum powders. Metal Finishing. 2000; 98: 20-5. [11] Xiaoa X, Miaoa L, Xu G, Lu L, Su Z, Wang N, Tanemura S. A facile process to prepare copper oxide thin films as solar selective absorbers. Appl. Surf. Sci. 2011; 257: 10729-36. [12] Jia J, Zhou J, Zhang J, Yuan Z, Wang S. The influence of preparative parameters on the adhesion of alumina washcoats deposited on metallic supports. Appl. Surf. Sci. 2007; 253: 9099-104. [13] Liu X, Peng TY. Yao JC. Synthesis and textural evolution of alumina particles with mesoporous structures. Journal of Solide State Chemistry. 2010; 13: 1-25. [14] Radic N, Grbic B, Rozic L, Novakovic T, Petrovic S, Stoychev D, Stefanov P. Effects of organic additives on alumina coatings on stainless steel obtained by spray pyrolysis. J. Non-Cryst. Solids. 2011; 357: 3592-7. [15] Novakovic T, Radic N, Grbic B, Dondur V, Mitric M, Randjelovic D, Stoychev D, Stefanov P. The thermal stability of porous alumina/stainless steel catalyst support obtained by spray pyrolysis. Appl. Surf. Sci. 2008; 255: 3049-55. [16] Teoh GI, Liew KY, Mahmood WAK. Synthesis and characterization of sol-gel alumina nanofibers. J. Sol-Gel. Particulate Science and Technology. 2007; 44: 177-86. [17] Ienei E, Isac L, Duta A. Synthesis Of Alumina Thin Films By Spray Pyrolysis. Revue Roumaine de Chimie. 2010; 55:161-5.