Copper Cermets used as Selective Coatings for Flat Plate Solar Collectors 1

The aim of the present work is to offer a practical mode to obtain thin films of Cu/CuO x cermets by two different methods: electrodeposition and spray pyrolysis deposition (SPD). The cooper precursor in both cases was copper acetate. The films morphology for the SPD samples was optimized by adding copolymers of maleic anhydride with controlled hydrophobia (vinyl acetate/methyl methacrylate) in the precursor’s solution. The samples were characterized from the point of view of: topography (AFM/STM), composition (XRD, using Cu K α 1 ), and optical properties (UV-VIS and FT-IR spectrometry). These cermet materials are extensively studied due to their applications as selective coatings for solar collectors.

A flat plate solar collector is one of the key components of the active solar heating systems.They are the most common solar collectors for use in solar water-heating and in solar space heating applications.A flat plate collector basically consists of [1]: -an absorber: with the role to absorb and transform the incidence radiation into heat; -absorber's channels: in which the thermal agent is transporting the produced heat; -insulation: to minimise the heat losses; -glazing (monolayer or double layer); -a weather tight container which encloses the above components.
In order to obtain optimum values for α s and ε T , the metal absorber must be coated with a selective surface [4 -8].Cermets (ceramic/metal composite materials) are mainly used as absorber coatings due to their excellent optical properties, as the ceramic matrix increases α s and the metallic particles decrease ε T. Several technologies for obtaining these materials are established, like physical vapour deposition [9], DC reactive magnetron sputtering, RF sputtering [10]; although they have ver y good reproducibility and high thin film quality, the main drawback is the cost of the deposition technique.
Copper compounds are widely used in different applications such as solar absorbers for solar collectors or photovoltaic cells [11][12][13][14][15]. Copper based cermet with copper oxides matrix is a promising material to be used as selective coating due to its good optical properties, good adhesion to metal surfaces, and thermal stability.Various deposition techniques have been reported for CuO x thin films preparation (e.g. chemical vapour deposition [16], electrochemical deposition [17], cathodic arc deposition [18]).SPD is an attractive method due to its low cost compared with the techniques mentioned above and the possibility of depositing large area of thin films.
The paper presents the synthesis and characterization of Cu/CuO x cermets obtained by two different methods: electrodeposition and spray pyrolysis deposition and also * m.voinea@unitbv.ro;Tel.: +40 (0) 721781410 discusses the influence of the deposition conditions on the output parameters, α s and ε T .

Experimental part
Thin films of copper oxides were deposited by SPD onto microscopic glass (G) for reference and copper substrate (C, Cu) from aqueous and aqueous-ethanolic solutions (W:Et = 1:1) of (CH 3 COOH) 2 Cu 0.1 M (Merck).Copolymers of maleic anhydride (synthesised at Petru Poni Institute), 50 ppm, were added in the spraying solutions.
The glass substrates (1.5 .3 cm area of glass for microscopy, Heinz Herenz) were cleaned before each SPD deposition in ultrasonic bath with ethanol, while the copper substrate (1.5 .3 cm area flat pieces of Cu 99.9% -Beofon) was either chemically cleaned with concentrated HNO 3 solution or mechanically polished (sand paper No 800).The substrate temperature was varied from 150 to 250 o C. Air was used as carrier gas (p = 1.4 bar).The distance from the nozzle to the heated substrate was 20 cm and the spraying sequences number was 30 or 60, with a break between two sequences of 30 and 60 s, respectively.The spraying angle was fixed during all the depositions (45 o ).
For the electrodeposited films, the substrate -1.5 .3 cm flat pieces of Cu 99.9% -was chemically cleaned with concentrated HNO 3 solution.A multichannel potentiostat, PAR BioLogic VSP (Mecrosistem) with three electrode system (working electrode: sample, counter electrode: platinum plate, reference electrode: Ag/AgCl/KCl sat (SAE), E Ag = 0.197 V) was used.All the potentials were recorded versus SAE.The precursor concentration ((CH 3 COO) 2 Cu) was varied from 0.01 M to 0.1 M, while the pH ranged from 2.8 to 6.6.The influence of CH 3 COONa addition (0.01 -0.4 M) in the electrolytic bath and various deposition periods (20-60 minutes) was studied.

Results and discussions
The efficiency of a solar absorber coating depends on the solar absorptance (α s ) and on the emittance (ε T ).The values of these coefficients can be calculated using the relations ( 1) and ( 2) [19]. where: -α s : normal solar absorptance; that is the weighted fraction between absorbed radiation and incoming solar radiation; -R (λ): the material's reflectance; -I sol (λ): the normal solar irradiance for air mass 1.5 (the ISO standard 9845-1 (1992)) [20].
ε T : thermal emissivity; that is the weighted fraction between the absorbed energy and the blackbody energy, at 100 o C; -I p (λ): the blackbody energy, at 100 o C. The structural and optical properties of the thin layers, deposited via spray pyrolysis technique can be tailored by modifying precursor solution composition (precursor nature, type of solvent, complexing agent) and deposition parameters (substrate temperature, spraying sequences number), as presented in table 1.
The solar absorption coefficient increases at moderate deposition temperatures (150 -200 o C) and higher precursor solution concentration due to the thick(er) films formation.Further research is necessary to determine the maximum film thickness in order to have optimal values for λ s .For elevated substrate temperatures a part of the precursor solution reacts above the substrate, resulting in lower values of the film thickness, non-adherent films or in copper oxides powder formation.
The annealing process (3 h, at 400 o C, in air) increases the λ s value of the Cu/CuO x layers obtained using copper substrate.The copper oxidation to CuO during this treatment may explain the phenomen, as the presence of CuO has a benefic effect on the solar absorptance [21].
The emittance has lower values for the samples obtained on copper substrate, the values being favourable influenced by the metal presence in the cermet structure.
The XRD spectra recorded for the thin layers deposited onto microglass substrate (fig.1) shows crystalline CuO as predominant phase.The samples obtained on copper substrate (fig.2) contain a mixture of three crystalline phases: Cu, Cu 2 O, and CuO.The cermet is formed by the oxides matrix CuO x (obtained by SPD and thermal oxidation) and by the metal; i.e the copper from the substrate.The chemical structure (the crystalline phases) is not influenced by the complexing agents, a fact confirmed by the XRD spectra (fig.2). (1) (2) Depending on the deposition parameters, different morphologies were obtained (figs.3 and 4).Thin layers of CuO x were deposited in relatively uniform thin films with different porosity.Long deposition duration along with higher deposition temperatures results in porous structures.The explanation is linked with higher growth rates, in the primary deposition steps, which enable the fast formation of larger crystallite size.The porosity is also controlled by adding copolymers of maleic anhydride as complexing agents into the spraying solution.These copolymers are forming, in the first step, copper complex compounds which are degraded in the next step, leading to a porous nanostructure (fig.4).The mean roughness of the sample deposited from a precursor solution containing copolymers of maleic anhydride (C2) is 92.2 nm, while the roughness of sample C1, obtained in the same deposition conditions, but without complexing agent, is 230 nm.Thus, it may be concluded that the complexing agent favours the nucleation rate, leading to smooth films.The hydrophilic form could not be used due to precipitation in the solution and segregation on the substrate.

Table 1 THE CORRELATION BETWEEN THE DEPOSITION PARAMETERS AND THE OPTICAL PROPERTIES FOR THE SAMPLES OBTAINED ON COPPER AND GLASS SUBSTRATE, BY SPD
Electrochemical deposition is, usually, a controllable technique, so that parallel experiments were performed in order to set-up a reference for the less -energy intensive technique, which is SPD.The optimisation of the electrolytic bath composition (copper acetate, sodium acetate concentration, and pH) was performed, aiming to lower the energy consumption.
Figure 5 shows the voltammetric curves obtained for a solution based on (CH 3 COO) 2 Cu 0.01 M and different sodium acetate concentrations, at constant temperature and pH (25 o C and 5.5, respectively).
Three cathodic peaks can be observed for the solution containing CH 3 COONa 0.1 M. The first peak, positioned at 0.11 V, corresponds to the formation of soluble copper (I) compounds according to the following reaction: The deposition of Cu over the Cu 2 O film at -0.8 V is described by the reaction: (5) Similar cathodic peaks are mentioned in the literature on nanocrystalline TiO 2 substrate [22] or on Ti substrate [17].
For higher concentration of sodium acetate (fig.5 c), only the peak associated with the reaction (1) was observed, while for lower sodium acetate concentration the voltammogram shows a large peak corresponding to the slowly deposition of Cu 2 O film (fig.1a).This peak is about 0.2 V anodically shifted, compared with case (b).Thus, the sodium acetate has a double role: increases the ionic strength of the solution, therefore the growth rate is increasing and, simultaneously, insures a slowly release of the copper ions from the complexes into the aqueous environment, allowing a uniform growth of Cu 2 O and Cu film.
The influence of the pH on the electrolytic bath containing (CH 3 COO) 2 Cu 0.01 M and CH 3 COONa 0.1 M was investigated.The voltammogramms corresponding to the three different pHs are plotted in figure 6.The pH was adjusted by adding acetic acid.This addition prevents the copper hydroxide precipitation and, consecutively decreases the electrolyte ohmic resistance, thus increasing the growth rate.
The experimental tests lead to the conclusion that strong acid environment favours the copper (I) soluble compounds formation and/or the deposition of a single phase: copper, while a slightly acidic pH (5.5) enables the copper and copper oxides formation.For the natural pH solution (6.5), the peaks are broader and they are   anodically shifted with 0.25 V, allowing the simultaneously deposition of copper and copper oxides.Unfortunately, the deposition current decreases substantially, resulting in a low deposition rate.Thus, a slightly acidic pH (pH=5.5) is favourable to the co-deposition of copper and copper oxides.No significant variation of the electrolyte pH was recorded during the deposition process, confirming that only copper is involved in the cathodic reactions.After optimising the CH 3 COONa concentration and the pH, the influence of different (CH 3 COO) 2 Cu concentration were tested.
The three peaks corresponding to the equations (3-5) are showed in figure 7, corresponding to the following deposition parameters: large range of potential from 0 to -0.8 V, (CH 3 COO) 2 Cu 0.01 M, CH 3 COONa 0.1 M. By increasing the copper acetate concentration (fig.7b and 7c ), only a larger peak anodically shifted about -0.35 V, is showed.This fact results in the possible co-deposition of copper and copper (I) oxide on the copper substrate, at intermediate electrode potential.
A set of samples were electrodeposited at various electrode potentials (-0.1 to -0.8V), from a solution containing (CH 3 COO) 2 Cu 0.05 M and CH 3 COONa 0.1 M at pH=5.The deposition period was varied from 20 to 60 min.
Table 2 gives the values of the thermal emittance (ε T ) for the electrodeposited films, calculated from the FTIR reflectance spectra [5].This was a criterion for choosing the optimum deposition conditions.
The best emittance was recorded for the sample Cu7.This low value is due to the copper metallic particles embedded in the Cu 2 O which accumulates and stores heat, therefore reducing the ε T values.The co-deposition of Cu and Cu 2 O in the range of electrode potential -0.4…-0.8V is demonstrated by the XRD pattern of the Cu7 sample (fig.8) and it is in accordance with the peaks showed in the voltammograms (fig.7b).
The structural test showed the formation of a single crystalline phase: Cu, according to the reduction equation ( 5), for samples deposited at E deposition < -0.65 V (fig.9.).Moreover, the thermal emittance decreases with decreasing the deposition potential, indicating the Cu formation.Higher values of the thermal emittance, calculated for samples Cu1, Cu4 (table 2) are associated with the deposition of a thin film mainly consisting of Cu 2 O.
The deposition duration which influences the optical properties of the film was optimized at 20 min.Longer deposition periods do not improve substantially the optical properties of the electrodeposited thin layers.
The samples morphology was analysed by AFM measurements.The films are relatively uniform and they present a mean roughness of 110.1 nm.

Conclusions
The work discusses the optimization of the Cu/CuO x cermets optical properties, for the use as solar thermal coatings.
The solar absorption coefficient increases at moderate temperatures and higher precursors' solution concentration for the SPD deposited layers.The annealing process increases the values of the α s , while ε T presents good values for all the samples obtained on copper substrate (<0.1).
Structural analyses show that the layers obtained by SPD contain mainly crystalline CuO using aqueous solutions of copper acetate or a mixture of crystalline Cu/CuO x using ethanolic aqueous solution.The thin films obtained via electrodeposition consist in crystalline Cu/Cu 2 O.
The layers morphology can be tailored by adding complexing agents as copolymers of maleic anhydride.The chemical crystalline structure is not influenced by the complexing agents, these observation being also confirmed by the XRD spectra.
In electrodeposition, slightly acidic medium (pH=5.5) is favourable for copper and copper oxides co-deposition, in the range of potential -0.16...-0.5 V. Voltammetry studies prove that the role of sodium acetate is to increase the ionic strength and the deposition rate.
Further studies will focus on the protective layer deposition and the adhesion properties between the corrosion -protecting barrier and the copper cermet substrate.

Fig. 1 .
Fig. 1.XRD patterns for samples obtained on glass substrate via SPD

( 3 )
The second peak is positioned at -0.41 V and indicates the deposition of Cu 2 O:(4)

Table 2
THERMAL EMITTANCE FOR THE ELECTRODEPOSITED FILMS