Preparation And Superconductivity Of Bi_xO_yS_z Superconductors

A kind of novel BiS2-based superconducting material Bi4O4S3 appeared in 2012. However, due to the complexity of the BiOS compounds systems, the samples prepared in the experiment are usually a mixture of various phases. Adjusting the initial reaction, changing the reaction conditions and increasing the number of reaction will affect the final product to a certain extent. This paper aims to explore preparation BixOySz superconductors with different nominal compositions. Polycrystalline BixOySz samples have been synthesized using solid state reaction method. The effects of increasing the number of reactions, changing reaction conditions and F doping on the superconducting properties of the systems were studied. The nominal composition of Bi4O4S3 sample is prepared by molten salt method.In chapter 1, the background of this paper is introduced by the discovery of superconductivity, basic properties, microscopic theory, superconducting material status, application. The main research content of this thesis is presented in order to study the BiS2-based layered superconductor.In chapter 2, the characterization of the structure and electromagnetic properties of the samples and the experimental design of this thesis are described. The solid state reaction method of BixOySz compounds and the molten salt method of Bi4O4S3 sample in the experimental procedure are introduced in detail, including the components of raw materials, the heat treatment process and the experimental instrument. Furthermore, the BixOySz compounds of crystal structure, microstructure, electrical transport properties and magnetic properties are introduced in detail.In chapter 3, polycrystalline BixOySz samples with nominal composition of Bi4O4S3, Bi3O2S3 and Bi2OS2 have been synthesized using solid state reaction method. The experiment results show that the superconducting transition temperature of the three heat treatments is lower than that of the superconducting transition temperature of a heat treatment. So, repeate heat treatment is bad for the superconductivity of BixOySz compounds. Besides, these same nominal compositions of Bi4O4S3, Bi3O2S3 and Bi2OS2 samples are explored by quenching treatment. The results show that the superconducting transition temperature of the samples are higher than the the annealing treatment of the samples, the resistivity and the antimagnetic of the samples are lower than that of the samples prepared by annealing treatment.In chapter 4, polycrystalline BixOy-nFnSz samples have been synthesized using solid state reaction method. The transport and magnetization properties of these compounds have been illustrated. The experimental results show that the change of F content affects the superconductivity of Bi4O4-nFnS3 compound. Small amount of F doping would suppress the superconductivity. However, for the sample with nominal composition of Bi4O3.04F0.96S3, the superconducting transition temperature rises up to 5.05 K. But the superconducting transition temperature of the samples with nominal composition of Bi3O2S3 and Bi2OS2 gradually decreased with the increase of F doping content. The resistivity of the sample with nominal composition of Bi3O2S3 decreases with the increase of F doping content.In chapter 5, the nominal composition Bi4O4S3 sample is prepared by molten salt method. The experimental results show that the structural characteristic of the sample prepared by molten salt method is different from the same nominal composition samples prepared by solid state reaction. Furthermore, there is no superconductivity in the sample. Further detail investigations are still needed to prepare Bi4O4S3 sample by molten salt method.