M. Sanchez, D. Leinen, J. R. Ramos-Barrado, F. Martin.

Laboratorio de Materiales y Superficie. Departamentos de Ffsica Aplicada & Ingenieri’a
Qui’mica. Universidad de Malaga. 29071 Malaga. Spain

Introduction

The radiation emitted from the sun peaks from 300 to 2000 nm. A warm object emits most energy in the infrared region. Selective surfaces make use of this spectral separation to maximise the energy absorbed in the solar spectral range (300-2000 nm) and minimise the energy emitted in the infrared spectral range (beyond 2000 nm). The ideal characteristic of a photothermal converter can be approximated by an absorber-reflector tandem. A practicable solar selective absorber is obtained when a metal of high infrared reflectance is coated with a thin film of high solar absorptance. The thin film must be highly absorbing over the solar spectrum and transparent in the infrared, to allow the metallic reflector to transmit through this region and to determine the thermal emittance. The solar absorptance (as) represents the proportion of absorbed radiation in the solar region, and the thermal emittance (et) represents the proportion of heat radiation emitted in the infrared. Therefore, as should be as close to 1.0 as possible, whereas et should be as close to zero as possible. To enhance the solar absorptance of the coated metal, the solar spectral region should have the lowest possible reflectance and, to suppress the thermal emittance, the infrared spectral region should have the highest possible reflectance.

Black paints have been often applied as absorbing films, with high solar absorptances but also with high emittances. On the other hand, various single or multi-component transition metal oxides exhibiting a black colour, have been studied with the aim of obtaining solar absorber coatings like Co2O3, CuO, MnO2 [1-4]. Within this context, we have tested the possibility to obtain molibdenum and molybdenum/tin oxide films by chemical spray pyrolysis on copper sheets, with the purpose to use them as selective surfaces. Molybdenum oxide (MoO3), is an important material for electrochromic devices. In addition to electrochromic properties, this material also demonstrates photochromic and thermochromic properties, due to the formation of colour centres by light irradiation and temperatures effects, respectively [5]. Tin oxide has been used as infrared mirror, usually doped with F, in smart windows [6]. Also, we have tested the possibility to use thin films of copper and zinc sulphide. ZnS is a high refractive-index material with low absorption from 400 to 14000nm [7-8], and copper sulphide have been used in air-glass tubular solar collectors as absorber coating, in photodetector and photovoltaic applications. First, we have used copper sulphide with the intention to grow a buffer layer between copper and the oxide absorbent film.

In spray pyrolysis, we use aqueous solutions of the metal precursor which are sprayed by an air stream onto the heated substrate surface, where pyrolysis take place and the oxide film grows. A problem is the need to heat the copper sheet in order to allow the precursor to decompose and to form the metallic oxide film, and at the same time, to avoid the oxidation of the copper substrate. We assume that at our conditions for spray pyrolysis, the presence of some amount of copper oxide at the interface is practically unavoidable. We have tried to minimize this problem using low temperatures of deposition.

Experimental

Compressed air was used to atomise the solution through a spray nozzle over the heated substrate. Air is directly compressed from the atmosphere, using filters to remove water and oil waste in order to avoid contamination. Aqueous solutions of (NH4)6Mo7O24- 4H2O (10’2 M) and SnCl2- H2O (5- 10’3 M), have been used as precursors of Mo and Sn respectively. The solutions were pumped into the air stream by means of a syringe pump at a rate of 50 cm3/h. A stream of 20 l/min of air, measured under atmospheric conditions, was used to atomise the solution. A 5 cm x 5 cm of copper substrate, was uniformly coated. The time of deposition was changed to obtain different film thickness. Thermal treatments were carried out with a 800 W halogen lamp.

Aqueous solutions of CuCl2, Zn chloride or Zinc acetate, and thiourea in a molar ratio from 1:2 to 1:4 were used as precursor solutions to obtain the copper sulphide and zinc sulphide films. The concentrations of the copper and zinc precursor solutions were 10-2 M. The substrate temperature was in the range of 225-250 °C, and the time of deposition of 3 to 7 min.

Measurements of the near-normal hemispherical reflectance were performed on a Perkin-Elmer lambda 19 UV-Vis-NIR spectrometer equipped with an integrating sphere coated with BaSO4 (0.3-2.5 pm range), and with a Bruker IF66/S spectrometer equipped with a diffuse gold coated sphere for the MIR — FIR range.

The solar absorptance as was calculated by weighted integration of the spectral reflectance with the hemispherical solar spectrum AM1.5. The thermal emittance et was calculated by weighted integration of the spectral reflectance with the Planck Black Body radiation distribution at 373 K.

The XRD spectrums were recorded with a SIEMENS D-501 diffractometer with CuKa radiation. Scanning electron microscopy (SEM) pictures were obtained with a JEOL JSM 5300 apparatus. Surface and in-depth composition, films were studied by X-ray Photoelectron Spectroscopy (XPS) using a PHI 5700 spectrometer with Mg Ka (1253.6 eV) and Al Ka radiation as excitation sources. The energy scale of the spectrometer has been calibrated using Cu 2p3/2, Ag 3d5/2 and Au 4f7/2 photoelectron lines at 932.7 eV, 368.3 eV and 84.0 eV respectively. PHI ACCESS ESCA-V6.0 F software package was used for data acquisition and analysis. Atomic concentrations were determined from the photoelectron peak areas using Shirley background subtraction and sensitivity factors provided by the instrument manufacturer. Spectra were referenced to the C1s line of the adventitious carbon at 284.8 eV.