Study of the Magnetic Flux Density between Bulk Superconductors

• Main Authors: Ming-Hsueh Yu, 游明學
• Other Authors: In-Gann Chen
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/82220805703520487446

碩士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 97 === The melt-textured high-temperature supercondutor (HTSC) bulk can trap multi-tesla field under high magnetic field at low temperature. Three phenomena were studied when the applied-field is removed: 1. The Lorentz force generated by the interaction of trapped field and the induced persistent superconducting current results in the outward electromagnetic stress, which may be strong enough to cause the fracture of the bulk. 2. The flux motion causes localized heating and the raising temperature, which will lower the critical current density(Jc), and subsequently degrades the ability of field trapping, or even flux-jump. 3. Mini-magnets consist of two Y-Ba-Cu-O bulks exhibited larger field than that of single disk. However,the distance between the two disks will affect the distribution and strength of magnetic flux density. In order to improve the fracture strength (σf) and toughness (K1c) of Y-Ba-Cu-O superconductor bulk, commercial resin and carbon fiber impregnation have been utilized. However, different samples exhibited different fracture strength after the same process of impregnation. The SEM observation showed that the samples with lower fracture strength consists of much larger pores (a=1.5mm) than those of stronger ones (a≤ 0.1mm). This phenomenon can be explained as follows: The Lorentz force (i.e. stressσf) could be derived via the intensity of trapped-field of about 13 MPa. Based on the fracture formula:K1c=Y(α)σf(πa)^0.5 , when the stress (Lorentz force) with pore (a > 1.5 mm)xY(α)(πa)^0.5>K1c, the stress would larger than K1c and resulted in fracture. Therefore, samples without large pores were chosen to ensure higher fracture strength in the follow-up trapped-field experiments. The flux motion caused localized heating in the samples. The raising temperature will lower the critical current density, and subsequently degrades the ability of field trapping. It was measured that the Y-Ba-Cu-O bulk samples could trap higher magnetic field with decreasing temperatures when removing applied field from 9T(0.1T/min) under temperatures of 25K, 45K and 65K, respectively. The experiment also revealed that the flux-jump at 25K induced the elevated temperature of surface. The relationship between trapped-field and current density was investigated by monitoring the trapped-field profiles under different distance of gaps between bulk samples. It showed adequate agreement between Js(surface current density)+Jv(volume current density) model and experimental results. The larger gap between the bulks decreased the trapped-field which led to increasing of Jv and decreasing of Js. A same order of magnitude of Jc (measured by SQUID) can be obtained when fitted Js+Jv values were converted to an effective Jc.