Fabrication, Microstructure And Properties Of Perovskite-like Manganites

Perovskites-like manganites are widely used in many areas due to their complex electromagnetic character when doped with some other elements.They show advantages as following:tunable Curie temperature,cheap price,stable chemical properties and high resistivity when they are used as magnetic refrigerants;some kinds of perovskites-like manganites were found to have magnetocaloric effect larger then Gd when treated with the same condition.If we can resolve the problem that magnetic entropy will not greatly reduce along with the Curie temperature increasing,then they will have a wide application prospect. Impacts of sintering temperature and alkaline earth metals Sr and Ca on microstructure, crystal structure and properties of are studied in this paper.Hence,the specimens with their Curie temperature near room temperature and magnetic entropy change as high as possible treated with low field were prepared.The larger⊿S_M perovskites-like manganites speciments were prepared by adjusting composition and sintering temperature.After sintering at 1723K,the La_0.825Sr_0.15Ca_0.025MnO₃ has perovskites-like type,low porosity,even grain size,280K of Tc,3.29 J·kg^-1·K^-1 of⊿S_M when treated with magnetic field of 1.5T,and the La_0.8Ca_0.025Sr_0.175MnO₃ 305K of Tc,2.48 J·kg^-1·K^-1 of⊿S_M.Adding of alkaline earth metals Sr and Ca has a considerable effect on the structure, microstructure and properties of materials.Adding of Sr slows the sintering process,the porosity of specimen increases and grain size reduces as the amount of Sr increases; bond-angle of Mn-O-Mn expands,bond-length of Mn-O get shorter,distortion of crystal lattice becomes less,both of Tc and⊿S_M rise.Adding of Ca promotes the sintering process for the grain size get larger.When Ca is added to substitute La,bond-angle of Mn-O-Mn expands,bond-length of Mn-O get longer,both of Tc and⊿S_M increase;When Ca is added to substitute Sr,bond-angle of Mn-O-Mn decreases,bond-length of Mn-O get shorter,both of Tc and⊿S_M decrease;When Ca is added to substitute both of La and St,bond-angle of Mn-O-Mn increases,bond-length of Mn-O get shorter,Tc increases and⊿S_M decreases.Sintering temperature is an important parameter in fabrication of materials.Research results show that impacts of sintering temperature on crystal structure,microstructure and properties are not in a monotony trend.Specimens with different composition have different rules.The factor of sintering temperature and composition interact with each other,they have impacts on the phase composite,crystal structure,microstructure,Tc and⊿S_M together.