Molten Salt Techneque Preparation Of Bismuthate Superconductors And Researches On The Doping Effect

In this dissertation, it aims at exploring new techniques of preparing superconducting bismuthate with high stability and the doping effet on BaPb_1-xBi_xO₃ (BPBO) and Ba_1-xK_xBiO₃ (BKBO) systems. These reseaches will provide for the further exploration of new superconductors. The main contents involved are presented as follows: molten salt method preparation of BPBO with controllable microstructures and BKBO with high stabibility, Insulator-Metal transition model in bismuthate, La or Pr doping effect on the superconductivity of Ba_0.6K_0.4BiO₃, Hg doping effect on BaPb_0.75Bi_0.25O₃ and MgB₂.In Chapter 3, a new molten salt technique is developed in preparing BPBO. In this process, the crystal configuration and size can be controlled by accommodating the reaction conditions which lead to the controllable growth of crystals. At the last part, the molten salt mechanism of synthesizing BPBO is studied which have gave predominance to the advantages of molten salt method in preparing BPBO samples with high quality. This can be recognized to be the first innovative point. On the other hand, BaPb_0.75Bi_0.25O₃ superconductors have been synthesized by solid state reaction, sol-gel and high temperature high pressure methods. Compared with the samples synthesized by above methods, it possesses obvious higher superconducting transition temperature for the sample prepared by molten salt method. The successful application of molten salt method not only provides a new route for BPBO preparation, but also refers to the development of new controllable high temperature superconductors.In the process of preparing BKBO, a new molten techniqure is also developed. We have optimized the traditional molten salt method by accommodating the raw materials, molten salts, sintering temperature and time to investigate the effects on the components and superconductivity of BKBO. Finally succeed in synthesizing single phase BKBO samples with high stability. This can be recognized to be the second innovative point. This research has successfully sovled the problems of low stability in BKBO and the difficulties in preparing single phase BKBO samples which become to the groundwork for industrial production of BKBO superconductors. The molten salt method is then successfully popularized into other superconductors, such as YBa₂Cu₃O₇å’ŒNaCoO₂. A newly preparation route has been presented in such oxide superconductors. This can be recognized to be the third innovative point. Considerable merits of molten salt method can be summarized as follows: accomplish at one step, easy to operate, synthesize at low temperatures and low facility request. These are beneficial to commercial production which will promote the practical applications of both BKBO and more superconductors. Besides, High quality single crystals can be offered to theoretical researches if this molten salt technique would be further modified.In chapter 5 and 6, the doping effects on crystal structures and superconductivity of BKBO and BPBO samples are studied. La or Pr doping suppress the superconductivity of Ba_0.6K_0.4BiO₃ differently. Hg doping leads to the phenomenon of superconductivity reentrant in BaPb_0.75Bi_0.25O₃ samples. That means the superconductivity is first suppressed to vanish and then recovered with the doping level increased which lead to the appearance of two superconducing areas in the phase diagram. Explarations of this phenomenon have been illustrated with the electronic structure model. This can be recognized to be the forth innovative point.For BKBO and BPBO, analyses on the Insulator-Metal transition mechanism of these two systems are also carried out from the energy band point of view. Qualitative analysis suggests that the superconductivity of bismuthate and the distortion of structures are closely correlated which might be exists in the compounds with simple provskite structure. This can be recognized to be the fifth innovative point. This theory on superconductivity might guide the exploration of new bismuthate superconductors.The last part of this dissertation focuses on the doping effect of Hg in MgB₂. At high temperature and high pressure, the critical current density of the samples is definitely enhanced. HgO reacts with the raw materials in MgB₂ which lead to the formation of MgHg alloy which presented to be liquid at high temperature and high pressure. In this case, MgHg alloy will restrain the growth of MgB₂ crystals and make large amount impurity particles homogeneously dispersed in the MgB₂ matrix which would act as new normal point defects and enhance the critical current density effectively. For the mechanism of critical current enhancement, there are intrinsic differences between Hg doping and other element doping. Hg doped MgB₂ samples synthesized at high pressure could not only enhance properties in application but also carry more impurities which would reduce the requirements of high purities in the synthesizing process which is favorable in commercial production. This can be recognized to be the last innovative point. Further accomodation of the mercury addition level might optimize the performance of MgB₂ which would turn into a potential technique to prepare MgB₂ bulks and wires with high quality on an industrial scale.