Oxidation of Small Metal Particles. R. Gatta, G.A. Nikiassona, Maiken Heimb d C.G. GranqvistC a Physics Department, b Dept. of Inorganic Chemistry Chalmers University of Technology and University of Gothenburg S-41296 Gothenburg, Sweden. C Dept. of Technology, Uppsala University P.O. Box 534 S-75121 Uppsala, Sweden ABSTRACT Selective absorbers utilize in many cases the enhanced solar absorptance of small metal particles embedded in a layered ceramic medium. A crucial factor in determining the performance of such an absorber is the degradation rate of this layer. The oxidation of ultrafme gas evaporated metal particles is examined here. Several metals develop a stable oxide film around the surface of small particles. It is shown that there is a linear dependence between the size of the metal core and the particle radius for Al, Mg, Be and Sn in room temperature and for Si at high temperatures. Charge quantization is proposed as a possible mechanism responsible for this phenomenon. Thermogravimethc measurements on Ni particles show a rapid oxidation for intermediate temperatures. i• INTRODUCTION The degradation of selectively solar absorbing coatings was studied by several investigato&' Several commercial coatings were studied as a part of Task X of an International Energy Agency (lEA) project.3A In conjunction with the same project, some studies of the physical processes responsible for the degradation of nickel pigmented aluminum oxide5 and black chrome coatings6 were carried out. The results have been summarized in a review.7 A study of the degradation process of a spuuered Mo/A1203 cermet selective coating was published recently.8 In the latter case, it was shown that the degradation at low pressure and high temperature could be described as a propagation of an oxidation front through the cermet layer. The rate limiting factor there was the penetration of oxygen into the cermet layer. In the more porous nickel pigmented alumina coating, the rate limiting factor was found to be the oxidation kinetics of the metal particles. A remarkable result was that those ultrafine particles seemed to display a much slower oxidation kinetics than that of a pure metal sheet.57 The study of this phenomenon in bare metal particles, and a suggestion for the physical origin of it, is the subject of this investigation. Ulirafine nickel particles were produced by gas evaporation. The method has been described extensively in the past.9'1° We used a bell-jar system with a diffusion pump having a base pressure of i06 Torr. A resistive evaporation source was used in an argon atmosphere in the pressure range of 1-10 Torr. Particle diameters were estimated from Transmission electron micrographs to be between 50 and 200A, depending on the Ar pressure. The particles were then oxidized in dry air during Thenno Gravimetry Analysis (TGA), and the mass gain was monitored as function of temperature and time. We used a Mettler Thermo Analyzer for that purpose. In addition, TGA was performed on micron size commercial nickel powder (CERAC), ultrafine commercial particles (ULVAC, Japan), and bulk samples. A comparison of the results to previously published work on ultrafine particles1 1,12 yielded new information. 2. BASIC THEORY The theory of metal oxidation, first presented by Mott and Cabrera,13 was further developed by many investigators,14 19d was extended and summarized in the two volumes by Fromhold20'21. Fromhold treated specifically the growth of an oxide layer on the surface of a spherical metal particle22'23. The steady state condition for the current density J5, V.J=O (1) 338 ISPIE Vol. 2017 0-8194-1266-X/93/$6.00 Downloaded from SPIE Digital Library on 22 Feb 2010 to 130.238.21.142. Terms of Use: http://spiedl.org/terms