Ruthenium Oxide Films For Selective Coatings

ARTICLE IN PRESS

Solar Energy Materials & Solar Cells 90 (2006) 832–840 www.elsevier.com/locate/solmat

Ruthenium oxide films for selective coatings
Ulises Morales-Ortiza, Alejandro Avila-Garcı´ ab,Ã, V. Hugo Lara C.a
b a ´ ´ ´xico, D.F., Mexico Depto. Quımica, Area Electroquımica, U.A.M. Iztapalapa, A. P. 55-534, 09340 Me ´ ´ctrica, Seccio de Electro ´n ´nica del Estado So ´lido, CINVESTAVdel I.P.N., Departamento de Ingenierıa Ele ´xico 07360, D.F., Mexico Ap. Postal 14-740, Me

Available online 23 May 2005

Abstract Although commercial selective surfaces are already available, investigation on different deposition methods and materials still goes on at many laboratories. In this work, ruthenium oxide films upon metallic substrates are assessed for this usage. Deposition of thefilms was made at room temperature by either spraying or dipping method in a ruthenium chloride alcoholic solution. After deposited on titanium substrates, the films were heat-treated at temperatures between 450 and 500 1C. When deposited on no-polished substrates, such films not only exhibit a high solar absorptance (a0:98), but also a high infrared emittance (0:8), which yield a low selectivity(S ¼ a= ¼ 1:2). By deposition of similar films on polished substrates, absorptance decreases (a0:74), but emittance significantly decreases as well (0:12), resulting in a net selectivity increase (S6). On the other hand, evaporating a thin (20 nm) gold film upon the surface of a coating on a no-polished substrate also improves noticeably its emittance value (0:16) and a lower decrease inabsorptance is achieved (a0:91), resulting in a selectivity increase (S5:7). These preliminary promising results indicate the high potential for using these films as solar selective coatings, but in order to optimize such selectivity values, further work to establish a close control on the deposition parameters and the substrate roughness value, should be done. r 2005 Elsevier B.V. All rightsreserved.
Keywords: Ruthenium oxide; Selective surfaces; Optical properties

ÃCorresponding author. Tel.: +52 50613800x6259; fax: +52 57477114.

E-mail address: aavila@gasparin.solar.cinvestav.mx (A. Avila-Garcı´ a). 0927-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2005.04.019

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U. Morales-Ortiz et al. / Solar Energy Materials & SolarCells 90 (2006) 832–840 833

1. Introduction Several applications exist for ruthenium oxide films. Among them, its commercial use as the active material in resistances for cryogenic temperature measurements has become quite important. Also, their electrochromic properties have been studied [1]. Several works can be found about their use as catalysts [2,3], conductive electrodes [4–6] andhydrocarbon sensors [7]. Nevertheless, none can be found regarding its use based on their optical properties. Considering that the expectations will become good for solar thermal systems in the next years [8], finding alternative materials for solar radiation selective coatings is certainly important for the future development of more efficient solar power thermal systems. Two relevant characteristicsfavoring RuO2 for selective coating applications are its high chemical and thermal stability [9], which are necessary for high temperature applications. The aim of this work is to assess the possibility of using simple methods to fabricate RuO2 films on metallic substrates, to use them as selective solar absorbers in high temperature (400 1C) systems. The first stage, which is carried out in this work,consists of doing a preliminary characterization of the material obtained and measurement of the important parameters for this task; namely, the absorptance in the solar spectrum range and the emittance in the medium infrared range. Then, two usual methods to improve such parameters are applied: deposition of the films on polished substrates and addition of a highly conducting upper layer.

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