超导材料的临界性能、交流损耗以及力学特性的理论研究

发布时间：2018-04-28 06:58

本文选题：第II类超导体 + 金兹堡-朗道理论　；参考：《兰州大学》2016年博士论文

【摘要】：超导体一般处于强磁场、大电流等复杂物理环境中,其临界性能会受到外加磁场、电流和机械荷载等因素的影响。超导材料在传输电流时会产生交流损耗,在电磁体力作用下会发生力学变形甚至破坏。研究超导材料在多物理场下的临界性能、交流损耗以及力学特性对于其实际应用非常重要。本学位论文从宏观角度研究了非理想第II类超导体的断裂行为、磁弹性变形以及交流损耗等问题,从微观角度分析了理想第II类超导体的临界性能以及磁通流动不稳定特性。首先,基于临界态模型和超导薄带磁通穿透理论,数值研究了含有一条中心裂纹和两条共线裂纹的超导薄带断裂行为。采用虚拟裂纹闭合法,求得最大应力强度因子出现在磁场下降阶段。运用场相关的临界态模型计算得到的最大应力强度因子大于Bean模型得到的最大值。由于电流密度跟磁场的相关性,采用场相关的临界态模型得到的结果比Bean模型的结果更加准确。考虑铁磁基底对超导薄带电磁响应的影响,理论研究了超导/铁磁复合双层带在运输电流和垂直磁场下的磁弹性变形,并且与数值计算结果吻合。其次,对于二代高温超导电缆的交流损耗,采用保角变换的方法,研究了弧形超导/铁磁双层带以及其圆周排列形成的超导电缆的交流损耗规律,并且在小电流或小磁场下给出了交流损耗的解析式。重点讨论了外场幅值、弧形超导带宽度以及圆柱上超导带的数量对超导电缆交流损耗的影响。最后,基于变形超导体的金兹堡-朗道理论,研究了应变对第II类超导体上临界磁场和下临界磁场的影响。上临界磁场随着应变的增加而减小,这与现有的实验结果基本一致。进一步从理论上预测下临界磁场随应变的增加而增加,并分析了耦合参数和金兹堡-朗道参量?对下临界磁场的作用。结合二流体模型,给出了应变与超导临界温度的关系表达式。临界温度随着应变的增加而降低,并且拉伸应变和压缩应变对超导临界温度的影响是非对称的,这与小应变情形下的实验结果一致。根据时间相关的金兹堡-朗道理论研究含有裂缝缺陷的细观超导体的涡旋动力学和电场引起的磁通流动不稳定性。数值模拟了相滑移线的出现过程及其对超导体内电流-电压特征曲线的影响。探讨了裂缝倾斜角度、长度以及裂缝间距对涡旋动力学过程的影响,从而通过调整缺陷尺寸和位置以及外场大小来引导相滑移线并出现各种涡旋动力学场景。
[Abstract]:Superconductors are generally in complex physical environments such as strong magnetic field and high current. The critical properties of superconductors are affected by external magnetic field, current and mechanical load. The superconducting material will produce AC loss when transmitting current, mechanical deformation and even destruction will occur under the action of electromagnetic force. It is very important to study the critical performance, AC loss and mechanical properties of superconducting materials in multi-physical fields. In this dissertation, the fracture behavior, magnetoelastic deformation and AC loss of non-ideal type II superconductors are studied from a macroscopic perspective. The critical properties and flux instability of ideal type II superconductors are analyzed from a microscopic point of view. Firstly, based on the critical state model and the flux penetration theory, the fracture behavior of the superconducting strip with one central crack and two collinear cracks is numerically studied. By using the virtual crack closure method, the maximum stress intensity factor is obtained at the magnetic field decreasing stage. The maximum stress intensity factor calculated by using the field-dependent critical state model is larger than that obtained by the Bean model. Because of the correlation between the current density and the magnetic field, the results obtained by using the field-dependent critical state model are more accurate than those obtained by the Bean model. Considering the effect of ferromagnetic substrate on the electromagnetic response of superconducting strip, the magnetoelastic deformation of superconducting / ferromagnetic composite bilayer strip under transport current and vertical magnetic field is studied theoretically, and the results are in good agreement with the numerical results. Secondly, for the AC loss of the second generation HTS cable, the AC loss law of the arc superconducting / ferromagnetic bilayer tape and the superconducting cable formed by its circumferential arrangement is studied by conformal transformation method. At the same time, the analytical formula of AC loss is given at low current or small magnetic field. The effects of the amplitude of the external field, the width of the arc superconducting tape and the number of the superconducting tape on the cylinder on the AC loss of the superconducting cable are discussed. Finally, the effect of strain on the upper and lower critical magnetic fields of class II superconductors is studied based on the Goldsbauer-Landau theory of deformed superconductors. The upper critical magnetic field decreases with the increase of strain, which is consistent with the experimental results. It is further predicted that the critical magnetic field increases with the increase of strain, and the coupling parameters and the Kingzbau-Landau parameter are analyzed. The effect on the lower critical magnetic field. Based on the two-fluid model, the expression of the relationship between strain and superconducting critical temperature is given. The critical temperature decreases with the increase of strain, and the effect of tensile strain and compression strain on superconducting critical temperature is asymmetrical, which is consistent with the experimental results in the case of small strain. The vortex dynamics of mesoscopic superconductors with crack defects and the flux flow instability caused by electric field are studied according to the time-dependent Kingsbauer-Landau theory. The appearance process of phase slip line and its influence on the current-voltage characteristic curve in superconductor are numerically simulated. The effects of slanting angle length and spacing of cracks on the vortex dynamics are discussed. The phase slip line is guided by adjusting the size and position of the defect and the size of the external field and all kinds of vortex dynamics scenes appear.
【学位授予单位】：兰州大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TM26

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