Spark Plasma Sintering And Properties Of La_1-xSr_xMnO₃Ceramics

The emissivity of rare-earth doped lanthanum manganites (LaMnO3) canchange accordingly with the environment temperature, which makes them ideal smartthermal control materials used in aeronautics field for the protection of microsatellite.In order to solve the problems emerging in the hot-pressed LaMnO3-based ceramicssuch as high sintering temperature, low relative density and nonuniform grain size,Spark Plasma Sintering (SPS) was employed in the present study to prepare LaMnO3ceramics doped with Sr (La1xSrxMnO3), followed by post-annealing to improve theelectrical property and emissivity.Firstly, La1xSrxMnO3(x=0~0.4) powders were synthesized by solid-statereaction at different temperatures (900~1200C). The effects of synthesistemperature as well as Sr doping content on the phase composition, morphology,chemical and infrared structure were studied. The results showed that the rawmaterials began to react completely with the increasing of synthesis temperature, andthe crystallinity of the as-synthesized powders was improved. La1xSrxMnO3(x=0~0.4) powders in single perovskite structure were obtained at1200C, whose grainsize was uniform. With increasing Sr doping content, the lattice structure of La1xSrxMnO3powders changed from orthorhombic to cubic, and more Mn3+becameMn4+which caused the blue shift of Mn-O peaks in the infrared absorption spectra.Then, the as-synthesized La1-xSrxMnO3powders were densified by SPS, and theeffects of sintering temperature as well as Sr doping content on phase composition,relative density, microstructure, electrical property and emissivity were investigated.It was found that the relative density of the ceramics was increased with sinteringtemperature, and single-phased La1-xSrxMnO3ceramics with a high relative density(97.4%) and uniform grain size were obtained at1100C. The concentration of Mn4+increased gradually with increasing Sr doping content, which promoted the doubleexchange effect of Mn3+-O2--Mn4+and as a result led to the enhancement in electricalconductivity and emissivity.Finally, the SPS-sintered La1-xSrxMnO3ceramics were post-annealed toimprove the electrical property and emissivity. The results suggested that the ceramics annealed at1000oC was in single-phase, whose relative density increased to about98%and the concentration of Mn4+was increased. After annealing, the maximumemissivity of La1-xSrxMnO3ceramics was decreased slightly, but the variability ofemissivity was enlarged (for La0.9Sr0.1MnO3, the value is0.38), indicating bettervariable-emittance property.