Techneque Preparation For YBa₂Fe₃O_8+w And Researches On The Doping Effect

At the present time, scholars from the domestic and the international have done much reseach about superconductor YBa2Cu3O7-δ (YBCO), while little reseach about YBa2Fe3O8+w (YBFO) which is much similar with YBCO in structure. Especially, there are nearly none study on YBFO at home. YBFO is in all probability not only very good buffer material for YBCO coated conductor, but also perfect functional material with weak antiferromagnetic characteristic. Therefore, the research on YBFO is much more significative in theory and application.It supplies valuable references to further recognize YBFO with123system the triple-perovskite structure by exploring the technology of preparation for YBFO and the doping effects about this system. Therefore, the dissertation launches the researches according to a few following aspects:the technology of preparation for YBFO, substitution of alkaline-earth metals Ca, Sr for Ba, rare earth metals La, Pr, Gd for Y, transition metals Co, Ni and Mn for Fe in YBFO, etc.. Concretely, there are some aspects as follows:As for the preparation for YBFO samples, we synthesize many kinds of YBFO samples under different experimental conditions by the basic ideas of solid-phase and liquid-phase reaction. By optimizing the conditions of preparation, the more complete preparation technology for YBFO is obtained in the laboratory, which supplies valuable references to the large-scale production for YBFO. The dissertation pays attention to adopt a comparative method in the process of synthesizing samples, and summarizes the experimental conditions of various samples, viz., firstly, solid-phase reaction is adopted to probe into the experimental conditions, then, liquid-phase reaction are used to synthesize many kinds of high purity YBFO samples by those conditions. After obtaining pure YBFO, the dissertation gives out some measurement and study about the crystal structure, electric character and magnetic resistance effect etc. properties about YBFO. The experiment in which liquid-phase reaction is used to synthesize pure YBFO has also verified research considerations in the dissertation are feasible.In the researches on doping effect of Ca and Sr in YBFO, considering two kinds of possible circumstances during doping Sr into YBFO, firstly, the dissertation attempts to synthesize (Y1-xCax)Ba2Fe3O8+w (YCBFO), YBa2-xSrxFe3O8+w (YBSFO) and (Y1-xSrx)Ba2Fe3O8+w (YSBFO) samples with few impurities, then, the structure characteristics of YCBFO and YBSFO are analysed by XRD, their electric and magnetic character are studied and discussed, and the differences and similarities of doping two kinds of alkaline-earth metals into YBFO are compared.With respect to study on doping the rare earth elements La, Pr and Gd into YBFO, it is discussed that the synthesizing of the object product which are doped with rare earth into YBFO and the doping effect of La, Pr and Gd, respectively. The physical properties of REBa2Fe3O8+w (RE=Y, La, Pr, Gd) have been synthetically analyzed and compared. The inherent mechanism of the new phases which form in the process of doping in YBFO have been studied.With respect to studies on doping the transition metal Co into YBFO, the dissertation adopts the formula YBa2(Fe1-xCox)3O8+w (YBFCO), we find the solid-solution limit of Co in YBFO x is about0.5, the Co doping leads to crystal parameters decreasing of YBFO, Co may substitute for Fe site in YBFO. By the analysis, we find that Co accomodates environments of square-pyramidal and octahedral configurations. We have also studied the electric character and magnetic resistance effect etc. properties about YBFCO.With respect to studies on doping the transition metals Ni and Mn into YBFO, the dissertation adopts the formula YBa2Fe3-xTRxO8+w (TR=Ni, Mn). We observe that the solid-solution limits of both Ni and Mn in YBFO x are about0.3. Both Ni and Mn enter the Fe sublattice in YBFO, which lead to its crystal parameters decreasing. By the comparation of the resistivity-temperature curves among Co, Ni and Mn, the dissertation explains the varying situations about carriers concentration and mobility after YBFO are doped with these transition metals. The dissertation discusses magnetic resistance effect etc. character of Ni and Mn doping into YBFO system.