Thermochromic LaCoO3 selective layer for self-regulated thermal solar collectors

Abstract

Thermochromic rare-earth perovskite LaCoO3 is a promising functional material for the new generation of selective layers and for passive regulation in thermal solar collectors. Indeed, under stagnation conditions, the solar collector overheat and this inconvenience can be skirt using a selective layer with tunable emissivity. Materials capable of reversibly switching their infrared emissivity (ε) at a certain temperature efficiently avoid overheating in such systems. For LaCoO3, a distinguishable thermochromic effect is noticed starting at 77 °C. However, due to structural and chemical complexity, its design is challenging and requires a deep understanding of crystallization mechanisms. In this work, we investigate the influence of the co-deposition process of La and Co metallic targets on perovskite crystallization and optical properties of the material. All films were deposited on aluminum substrates by magnetron sputtering in the elemental or the compound mode (Ar or Ar/O2 atmosphere) using a semi-industrial reactor with back-and-forth scrolling of the substrate. The samples were post annealed in air for 2, 5, and 10 min at temperatures ranging between 500 and 600 °C and characterized using XRD, SIMS, TEM, and infrared camera. The measurements of infrared emissivity variation show an increase of the emissivity when the temperature raises confirming the great prospects of LaCoO3 usage for the considered application. A prototype of a solar collector equipped with LaCoO3 deposited in the elemental mode was built in order to demonstrate the potentiality of this perovskite as a thermoregulated selective layer.