Síntesis de óxidos nanoestructurados basados en aluminio, silicio y cobalto, para su empleo en superficies selectivas de colectores solares

Abstract

During the last years, the family of the 9-12 percent Cr steel has been object of different studies that cover from their properties to their behavior in different environments and in different metallurgical conditions. One aspect that has attracted interest is the formation of different types of carbides and/or carbonitrides when these steels are subjected to thermal treatments, and the influence of these particles on the properties such as creep resistance or the interaction with hydrogen. In this thesis, the influence of the microstructure on the behavior against hydrogen in two 9 percent commercial steels has been studied. The steels were in different metallurgical conditions that ranged from normalized, with a microstructure of lath martensite, to annealed, whose microstructure is a ferritic matrix with precipitated carbides. The materials in the different metallurgical conditions were characterized by using to different techniques, and were subjected to hydrogen permeation tests with gas phase charging and electrochemical detection. The experimental results showed a great influence of the metallurgical condition on hydrogen trapping. Hydrogen trapping is mainly related to the density and nature of the different precipitated particles. In contrast, the role of the dislocations on hydrogen trapping seems secondary in this type of steels. The techniques mentioned above were also applied to a Fe-9 percent alloy prepared in the laboratory. In this way, it could be established that the matrix delays the diffusion and contributes to hydrogen trapping; the effect of the precipitated particles and dislocations mentioned above is superposed to this matrix effect. The experimental results of the Fe-9 percent alloy are supported by theoretical calculations based on ab-initio methods which were performed using the SIESTA code which is based on the density functional theory [DFT]. These calculations predict that, in the vicinity of a substitutional Cr atom, a particular site appears where H is stabilized; besides, in this region the migration energy barrier for H increases. The migration barriers were calculated using the Monomer method adapted to SIESTA code.Tesis para optar al título de Doctor en Ciencia y Tecnología, Mención Materiales.