Mo coordination and Thermal Stability of the Mo-Si 3 N 4 Absorbers for Solar Selective Coatings
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Mo coordination and Thermal Stability of the Mo–Si3N4 Absorbers for Solar Selective Coatings C. Prieto1, E. Céspedes1, D. Hernández-Pinilla1, A. Rodríguez-Palomo1, O. Sánchez1, F. Jiménez-Villacorta1, E. Salas-Colera1,2 1
Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Cantoblanco, 28049 - Madrid, Spain 2
Spanish CRG beamline at the European Synchrotron Radiation Facility, 71 Avenue des Martyrs, 38000 Grenoble, France.
ABSTRACT The study of the chemical stability of solar selective coatings (SSC) for concentrated solar power (CSP) becomes essential for their use at high temperatures. In this paper, the short range order around Mo in Mo-Si3N4 cermets is studied for the first time by X-ray absorption spectroscopy. The information obtained by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies gives new insights of the origin of the optical behavior of the cermets cermets after vacuum and air annealing treatments. The established optical and structural correlation becomes of great importance for the design and optimization of SSC for practical applications.
INTRODUCTION Efficiency in concentrated solar power (CSP) systems is based on solar selective coating (SSC) to allow high solar absorbance and low thermal emittance. Conventional parabolic trough collector (PTC) works in vacuum at high temperatures. Nowadays, target temperatures are in the 600 650 ºC range. This condition involves significant development of materials forming the SSC [1], which should be stable at the desire working temperature. Even if PTC tubes work in vacuum (or inert gas), air stability at high temperature is required for practical applications due to occasionally undesired losses of inert conditions. On the other hand, receivers of high concentration solar tower systems operate in air at and above 800 ºC, but no coatings with suitable parameters of selectivity are currently available. Therefore, air stable SSCs are nowadays highly desirable for CSP technology. Good examples of efficient SSCs consist of multilayer films prepared by sputtering with precise control of thickness and composition to achieve the required optical properties [2]. The required abrupt transition from high absorption at the UV-Vis to high reflectance at the IR region is obtained by multi-reflection interference in the multilayer system. Common SSC multilayer architecture includes a metallic infrared reflector (IR-mirror), an absorber material and an antireflective (AR) layer on top. Absorber materials with excellent properties are formed by a hybrid material that allows gradual modulation of the extinction coefficient (from the antireflective layer to the IR-mirror) easily to obtain the required interference effect.
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