C. Nunes1 , M. J. Brites1, J. F.Mendes1, M. Lopes Prates1, J. Alexandre1, M. C. Ferreira1,
A. J.Martins2, V. Teixeira2, J. Carneiro2, M. J. Carvalho1

1INETI — Estrada do Pago do Lumiar, 1649-038 Lisboa, Portugal 2Universidade do Minho — Centro de F^sica, Campus de Gualtar, 4710-057 BRAGA, Portugal * Corresponding Author, clarisse. nunes@ineti. pt

Abstract

This paper presents results of application of natural and synthetic organic pigments in selective solar absorber coatings. Some of these photo excite pigments were tested like solar radiation absorber pigments in paint coatings and others as a form to increase the absorption of solar radiation in titanium oxide monolayer selective coatings produced by reactive magnetron sputtering. Morphologic aspects and optical properties are discussed as a function of deposition parameters and of formulation parameters. The main goal is to prepare solar absorber surfaces as a tandem of TiOx thin films, organic pigment using a conductive substrate or of paint with organic and/or inorganic pigments and conductive substrate, all reaching great photo thermal conversion efficiency, high durability and low costs to the ending product and stability at medium range temperatures (120-150°C). The best optical properties for titanium oxide sputtered films were 88% for solar absorption, with 7% of emissivity for deposition parameters of: pulsed frequency 200kHz, reverse time of 0.4ps, discharge current of 0.7A, argon flow rate of 50ml/min and oxygen flow rate changing from 0 to 2.5ml/min. The results obtained with paints were not satisfactory. The best couple values for solar absorption and emissivity were respectively 94%, and 74%.

1. Introduction

In this paper we present some results of organic pigment application in selective solar absorber coatings developed within a Portuguese research project [1], obtained as a tandem of TiOx thin films, organic pigment and conductive substrate, described elsewhere [2, 3]. This idea is a spin off of the work done by members of the research team in the field of TiOx thin films deposition by reactive magnetron sputtering in DC or pulsing DC mode over conductive substrate [4], and of synthesis and characterization of [60] and [70] fullerene-coumarin dyads [5, 6], in a parallel way as that done to develop dye-sensitized nanocrystalline TiO2 thin films for PV applications [7], and to develop paints with inorganic or organic pigments [8, 9, 10].

Although many and good work has been done in the area of selective solar absorber surfaces, still subsists interest to develop selective solar absorber surfaces, with high durability, low costs and thermal and environmental stability, to apply in solar thermal conversion, mainly for higher values of medium range temperatures (120-150°C). Our idea is that titanium oxide films could be a good stake, because of high stability, no toxicity and possibility of wide range of refraction index variation [11-12].

It’s known that optical properties of materials are dependent of factors like the composition (electrons distribution in atomic external energy levels, minimum energy necessary to change for places of higher energy, type of link between atoms and molecules, minimum energy to cause inter or intra-molecular vibrations), the structure (that determine the strength of link between atoms and molecules and consequently the movement of more external electrons), the morphology (interfaces

between columns or grains could be scattering places of radiation, and shape, size and orientation relatively to incident radiation are also determinants in scattering effects and radiation interference phenomena’s, the same happens with defects, displacements, discontinuities, hols and them shape and size), the surface topography (adequate roughness could be motive of multiple reflections of incident radiation, with some absorption at each incidence and also forcing radiation to come into material) and the presence of different layers with their interfaces (which could cause interference phenomena’s motivated by adequate conjugation of thickness with composition layer and level of package, determinant in values of refraction indices, being optical thickness equal to physical thickness multiplied by refraction index, etc [13]).

Having in attention described topics, titanium oxide films obtained by magnetron sputtering seems to be adequate option for selective solar absorption coatings once that refraction index can be changed with adequate control of deposition parameters, between certain limits, to control composition, package, thickness, number of different layers, structure, morphology and topography of coatings [13-17].

2. Experimental details