高溫超導塊材在高磁場環境下冷卻至低溫後具有擄獲磁場的能力，本研究探討在移除外加磁場過程時三種可能遭遇之現象：1.塊材內部的感應超導電流與擄獲磁場產生之羅倫茲力(Lorentz Force)，此應力過大而導致塊材破裂。2.磁束移動產生的熱在塊材無法排除的情況下，造成臨界電流密度(Jc)與擄磁能力降低，甚至發生磁束跳躍。3.雙顆塊材之夾層空間可有效減少磁力線發散而有較高擄獲磁通，但塊材的間距會改變磁力線的分佈而影響超導體作為磁場源的應用性。
為了改善超導塊材破裂應力(K1c )與破壞韌性(σf)，曾使用碳纖維布與環氧樹脂環型披覆在塊材外圍以增強其強度。發現經過相同披覆處理的塊材樣品，具有不同的破壞強度。藉由SEM觀察具有破壞強度較低的樣品存在孔洞(a=1.5mm)，此孔洞尺寸遠大於其它破壞強度較高的樣品(a≤0.1mm)。其原因如下，在退場過程中從擄獲磁場強度推算樣品的羅倫茲力可達13MPa。根據破壞理論K1c=Y(α)σf(πa)^0.5，當內應力(σf=羅倫茲力)xY(α)(πa)^0.5>K1c，在樣品孔洞(a >1.5mm)時，試片內應力大於其K1c而造成破裂。所以進行後續超導擄磁實驗前，篩選無巨大孔洞的樣品，以確保其有較高的破壞度。
磁束移動造成熱的產生，溫度升高導致Jc下降或磁束跳躍，降低超導塊材的擄磁能力。實驗結果發現YBCO超導塊材分別在溫度25K、45K、65K，以0.1T/min速率移除場冷磁場9T，溫度愈低擄獲磁場愈高，並在25K發現磁束跳躍現象，導致塊材表面溫度上升。
變化雙顆塊材間距，觀察塊材間擄獲磁場形貌，計算擄獲磁場與電流密度的變化。使用雙電流(Js+Jv)模型與實驗值有較佳的逼近。當塊材間距愈大，擄獲磁場值愈低，且塊材間距愈小，體電流密度Jv 降低，而面電流密度Js增加。如將雙電流(Js+Jv)轉換為Jc，在溫度25K、45K下，與SQUID測量之Jc實驗值具有相同數量級。

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.

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