【作者】 崔雅静；

【导师】 赵勇；

【作者基本信息】 西南交通大学 ， 材料学， 2009， 博士

【摘要】 本论文旨在探索制备高稳定性铋酸盐超导材料的新技术以及研究铋酸盐超导体的化学掺杂效应,为在该体系继续探索超导新材料提供依据。论文涉及主要研究内容如下:微观形貌可控的BaPb_1-xBi_xO₃（BPBO）样品的熔盐制备技术,性能高度稳定的Ba_1-xK_xBiO₃（BKBO）样品的熔盐制备技术,BKBO和BPBO超导体系中的绝缘体-金属转变机理研究,La和Pr掺杂对Ba_0.6K_0.4BiO₃样品超导电性的影响,Hg掺杂对BaPb_0.75Bi_0.25O₃样品超导电性的影响,以及Hg掺杂对MgB₂超导性能的影响。论文第三章开发出一种BPBO熔盐制备新技术。通过调整制备工艺,可控制样品的晶粒形貌和尺寸等,实现BPBO超导体的可控生长。本章最后,对制备BPBO超导体熔盐反应机理进行分析研究,突出体现了熔盐法制备BPBO样品的优越性。此为论文创新点之一。另一方面,利用固相反应法,溶胶凝胶法和高温高压法制备BaPb_0.75Bi_0.25O₃超导体,并与熔盐法制备样品相比较,发现熔盐法制备样品的超导转变温度明显高于其它样品。熔盐法的成功应用为BPBO超导体的制备提供了一个新的途径,也为可控新型高温超导材料的研究提供了有价值的参考。在BKBO的制备过程中,开发出了一种新的熔盐制备技术。通过改良传统熔盐法,并研究反应原料配比,熔盐量,反应温度和反应时间对样品组分和超导电性的影响,成功制备出性能高度稳定的系列单相BKBO超导体。此为论文创新点之二。此研究解决了BKBO化学稳定性差和单相BKBO样品制备困难的问题,为大规模制备BKBO超导体奠定了基础。此后,将熔盐法应用于YBa₂Cu₃O₇和Na_xCoO₂超导前驱体的制备,为高效低成本地制备这类氧化物超导材料提供了一个新的途径。此为论文创新点之三。熔盐法只需单步反应即可完成,操作简单方便,制备温度低,所需设备少且对设备要求较低,适合应用于大规模生产,推动BKBO及更多超导材料在实际生产中的应用进程。进一步改进熔盐法可用于制备单晶材料,为理论研究提供高品质样品。第五章和第六章分别研究掺杂对BKBO和BPBO样品晶体结构及超导电性的影响。其中La和Pr的掺杂在不同程度上抑制了Ba_0.6K_0.4BiO₃的超导电性。Hg掺杂则导致BaPb_0.75Bi_0.25O₃样品超导电性的“再入”现象,即随着掺杂浓度的增加,体系的超导转变经历以下过程:超导电性被抑制到完全消失然后再次进入超导相,使得超导相图中出现两个超导区。文中通过电子结构模型的构建,对这一现象的机理进行解释。此为论文创新点之四。针对BKBO和BPBO系统,分别从能带理论出发分析其绝缘体—金属转变机理,并定性分析其超导电性与宏观晶格畸变的关系,建立简单钙钛矿结构化合物中的超导模型。此为论文创新点之五。此理论的提出为在该体系中探索新超导材料提供了有价值的参考。论文最后一部分研究Hg掺杂对MgB₂样品超导电性及性能的影响。高压条件下合成的Mg_1.05-x（HgO）_xB₂样品临界电流密度显著提高。HgO与反应原料中的镁结合形成MgHg合金相,高温高压条件下,MgHg合金呈液态,均匀分布在MgB₂基体中,一方面抑制晶粒长大,另一方面使氧化物杂质在MgB₂样品中均匀分布,有效提高样品的临界电流密度,其机理与其它氧化物掺杂提高临界电流密度机制有本质的区别。与其它掺杂样品相比较,利用高温高压技术合成的Hg掺杂MgB₂样品,杂相承载能力增强并有效提高了样品的应用性能,降低了制备过程中对反应原料纯度相当高的要求,有利于工业生产应用。此为论文创新点之六。若进一步对反应初始原料配比及反应条件进行优化,可能得到性能更为优越的产品,从而为Hg掺杂MgB₂超导体提供更为有利的应用前景。更多还原

【Abstract】 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 YBa₂Cu₃O₇和NaCoO₂. 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.更多还原