Research On Composition, Structure And Infrared Emissivity Of Lanthanum Manganites

With the development of the aviation industry, the research of active thermal control material in the spacecraft becomes more and more important. The lanthanum manganites doped with alkali-earth metal was paid more and more attention as active thermal control material.In this paper, the Sr doped lanthanum manganites with different compositions and structures were prepared by solid state reaction method, sol-gel method,co-precipitation method, respectively. The crystal structure, microstructure, particle size and infrared absorption were invested. Then the emissivity of the samples with different compositions and structures was tested. The influence of the variation of the property of the samples on their emissivity was discussed. The results show that:1. Among the Sr doped lanthanum manganites prepared by solid state reaction method, wtith the increase of Sr doped level, the growth trend of the crystal along with the?104? crystal plane becomes strong gradually. This may be one of the reasons which influence the emissivity of the samples. The microstructures of the samples change from irregular bulk to sphere and slice with the increase of Sr doped samples.The range of the emissivity of the samples with different microstructures is bulk>sphere>slice. The main reason for variation of emissivity may be the microstructures and infrared absorption strength of the samples. The particle shapes ofLa_1-xSr_xMnO₃ with x=0.2 change from irregular to spherical with the extension of calcining time. The infrared absorption strength of the sample with 15 h calcining time is stronger than the other two samples. Maybe this is the main reason for that this sample shows large emissivity.2. Among the Sr doped lanthanum manganites prepared by sol-gel method, The crystal cell volume ofLa_1-xSr_xMnO₃ with x=0.2 is smallest. But the particle size is the biggest and that increases the scatter of infrared ray. So that it influences the absorption of infrared and then the sample shows little emissivity. There are micro pore existing in theLa_1-xSr_xMnO₃ with x=0.3. The structure of the particle is loose and porous. This can increase the absorption of infrared ray, and then the sample shows large emissivity. The strength of infrared absorption peak is stronger than the other two samples, but the emissivity is not largest. This shows that the influence of the absorption of the chemical bonds is weaker than that of microstructure andparticle size. With the increase of calcining time, the crystal structures ofLa_1-xSr_xMnO₃ with x=0.2 change from orthorhombic to rhombohedral and its microstructure changes from porous particle to slice particle. This can increase reflection of infrared ray and decrease emissivity as a result.3. Among the Sr doped lanthanum manganites prepared by co-precipitation method, Infrared absorption ofLa_1-xSr_xMnO₃ with x=0 is strongest and its emissivity is largest. The crystal structure ofLa_1-xSr_xMnO₃ with x=0.2 belongs to monoclinic and its crystal volume is smallest. With increasing doped level, the emissivity of lanthanum manganites decreases, gradually. The crystal distortion caused by doping may have effect on decreasing emissivity. The particle size ofLa_1-xSr_xMnO₃ with x=0.3 is biggest and this can increase the infrared ray scatter of the particles effectively. With the extension of the calcining time, the crystals ofLa_1-xSr_xMnO₃ with x=0.2 become intact and the crystal size becomes more homogenic. The emissivity of the samples increases gradually.4. The crystal structure, microstructure, particle size and infrared absorption can influence the emissivity of lanthanum manganites. The microstructure of samples makes main influence on the emissivity according to the comparison with other factors.