Influence of as-deposited non-uniform stoichiometry on thermochromic properties of LaCoO3 selective layers

Abstract

This work is dedicated to the elaboration and structural and optical characterization of a LaCoO3 selective layer for solar thermal collectors. LaCoO3 is a thermochromic material capable (similar to VO2 that is currently used for this application) of preventing overheating of the collector. Samples were prepared using two types of sputtering reactors - a rotary confocal sputtering reactor and an in-line semi-industrial reactor, allowing us to produce samples with areas of 1-2 cm2 and 21.0 × 29.7 cm2, respectively. The samples possess overall stoichiometry, with the La/Co ratio being equal to 1. However, TEM cross-sectional evaluations reveal that the structure of as-deposited films issued from the semi-industrial reactor corresponds to La/Co multilayers. Deposited at room temperature, metallic LaCo films should be annealed in air at 600 °C in order to crystallize and/or oxidize the perovskite structure. All films were characterized using, among other techniques, XRD, HRTEM, FTIR, and an IR camera. A strong correlation between the distribution of the elements (i.e., the presence or absence of the La/Co multilayers) and optical properties and a strong dependence of the latter on thickness and annealing conditions were ascertained. Nevertheless, for both types of reactors, good values of emissivity change, reaching as high as Δϵ = 64%, were obtained.