超導磁浮及高溫超導線材磁性的研究

Abstract

本論文中，利用電磁學及超導倫敦理論，我們理論上地探究磁力顯微鏡 (MFM) 可能應用於準靜態超導磁浮系統的研究以及討論銀被覆超導線材Bi-2223、T1-1223和 T1-1234 的磁性質。於超導磁浮部份，著重於點偶極矩（或MFM探針）與第二類超導體在邁斯納及混合態時的靜磁作用。在超導線材方面，這些線材是以粉末填充法製成，磁性的測量是經由超導量子干涉儀 (SQUID) 所量測的。 首先，我們計算靜磁作用能及作用在點偶極的磁力，此點偶極位於超導薄膜的上方且薄膜內有一根渦漩。若極薄的超導膜內有一個圓形缺陷，由缺陷所捕捉的渦旋所產生的向量位勢與沒有缺陷釘扎效應情況下比較，相差了 (1-b/Λ) 的因子，其中 b 是缺陷的半徑及Λ是有效穿透深度，只要與被捕捉的渦漩有關的作用，此因子就會出現。在磁偶矩場下，對於薄膜有及無圓形缺陷釘扎效應兩條件下，我們分析渦漩產生在超導膜內的條件及點偶極與渦漩間磁力的改變。 於有限厚度的超導膜中，我們計算與厚度相依的磁力及渦漩穿透。在超導體內部的電流密度分佈以及兩渦漩線間的作用位勢也被研究計算，至於塊材樣品的研究，可容易藉由有限厚膜的推展而得。此外，我們也研究第二類超導體在一懸浮磁場中磁力的磁滯行為，此方法是基於臨界態及磁化強度模型。 這些不同成份的高溫超導線材的磁性質是經由量測不可逆線、磁滯曲線及磁豫曲線而來探討的。線材樣品的不可逆線與其它高溫氧化物超導體作比較。T1-1223和T1-l234線材的磁通釘扎能力比Bi-2223線材強，其理由亦有討論，這些線材的錢滯曲線圖經由磁通蠕動模型的分析，可看出熱助磁通運動在這些線材內是扮演重要的角色。最後，我們研究超導電性雜質在第二類超導體內作為一個釘扎中心的效應，此研究可對晶粒間與晶粒在磁通釘扎中角色的瞭解提供另一途徑。

In this dissertation, we theoretically explored the possible application of magnetic force microscope (MFM) in the study of superconducting magnetic levitation system in the quasi static state by using electromagnetic couple with the London theory of superconductivity and discussed the magnetic properties of Ag-sheathed Bi-2223, T1-1223 and T1-1234 superconducting tapes. In the superconducting levitation topic, emphasis is on the magnetostatic interaction between a point dipole tip (or the MFM tip) and a type-Ⅱ superconductor in the Meissner and the mixed state. In the superconducting tapes topic, these tapes were fabricated by powder-in-tube method, and the magnetic measurements were performed by a SQUID. First, we calculate the magnetostatic interaction energy and the magnetic force acting on a point dipole placed above a superconducting thin film with a single vortex. If there is a circular defect in the sufficiently thin superconducting film. The vector potential due to the source of a trapped vortex by the defect is reduced by a factor (1-b/Λ) as compared with that free of defect pinning, where b is the radius of the defect and Λ is the effective penetration depth. This factor appears as long as the interaction with relation to the trapped vortex. The analysis of the possibility of the creation of a vortex under the field of a magnetic dipole without pinning effect of a circular defect. The conditions of the vortex creation in the thin film and the change in the magnetic force acting between the point dipole and the vortex were examined. The thickness dependence of the magnetic force and vortex creation are calculated for the superconducting thin film with finite thickness. The current density distributions within the superconductor and the interaction potential between two vortices are also calculated. The study of bulk samples can be easily obtained by extending the results of films of finite thickness. In addition, we studied the hysteric force behavior of the type-Ⅱ superconductor in a levitated magnetic field. This approach is based upon the critical-state model and the magnetization model. The magnetic properties of these high-Tc superconducting tapes of different compositions were investigated by measuring the magnetic irreversibility lines, the magnetic hysteresis curves and the magnetic relaxation curves. The irreversibility lines of our samples were compared with that of other high-Tc oxide superconductors. The flux pinning of T1-1223 and T1-1234 tapes stronger than in Bi-2223 tapes, and the reason was discussed. The analysis of the magnetic hysteresis curves on the basis of the flux creep model leads to conclude that the thermally activated flux motion plays an essential role in these tapes. Finally, we examined the effectiveness of a superconducting impurity in a type-Ⅱ superconductor as a pinning center. This investigation may be served as another approach to understand the role of intergrain and intragrain in flux pinning.