Influence of annealing on structure and thermochromic property of spark plasma sintered La_1−xSr_xMnO₃ compounds

Abstract

Sr-doped lanthanum manganite (La_1−xSr_xMnO₃) compounds as potential thermal control materials have been prepared by spark plasma sintering followed by annealing at different temperatures. The structure, transport behavior and thermochromic property of the samples were then studied as a function of annealing. With the increasing of annealing temperature, the relative density was increased, and well-crystallized La_1−xSr_xMnO₃ compounds with a dense texture (relative density > 98 %) were obtained after annealing at 1,273–1,373 K. The measurement of transport property showed an improvement in electrical conductivity due to the increasing of Mn⁴⁺/Mn³⁺ratio after annealing, which as a result enhanced the Mn³⁺–O–Mn⁴⁺ double exchange interaction. The conduction mechanisms above the metal–insulator transition temperature (T_P) was further discussed, and we found that the small polaron hopping model could be applied in this regime. Moreover, the annealed samples tended to reduce the localization of the charge carriers and thereby a decreasing in hopping energy was observed. The infrared emissivity (ε) of the La_1−xSr_xMnO₃ compounds changed with temperature and an shape change near T_P was observed, which was considered to be associated with the metal–insulator transition. The thermochromic properties were also affected by annealing process, where a larger variability in emissivity (Δε = 0.38) was obtained for the annealed samples.