本研究主要在探討YBCO超導體在鑽洞後，分析並歸納已鑽洞塊材擄磁能力大於未鑽洞塊材的原因。本實驗除了準備一般未鑽洞塊材，也將塊材鑽以直徑2mm 4、8、16個洞以及直徑1mm 8、16、32個洞，並根據氧氣通道的增加、釘扎中心（pinning center）的變化以及影響塊材半徑的因子進行歸納與分析。
本研究於溫度77K，外加磁場1T下，進行塊材擄獲磁場能力的量測，發現鑽以直徑2mm孔洞的塊材中以4個洞的擄磁能力最大，在退火400小時後，其擄磁能力可高達2300 高斯（Gauss）。當退火時間由200小時增加至400小時，塊材擄磁能力的增幅會隨著鑽洞數增加而增加，以16個孔洞的擄磁增幅最大，有91%。而在直徑1mm孔洞的塊材中，則以8個洞之擄磁能力最大，塊材僅退火200小時，更可高達3000高斯的擄磁能力。經各項數據比較後，推測擄磁能力與塊材表面積以及體積有關。
鑽洞後塊材增加氧氣通道，有益於充氧退火度的有效增加，因此本研究首先進行重量增加量的量測，由重量增加量的結果中可推測已鑽洞塊材皆已達到充氧退火飽和，而未鑽洞塊材重量增加量則是持續上升。理論計算氧氣於溫度300℃，時間200小時的條件下，擴散深度僅有3μm，但在觀察已鑽洞塊材的雙晶面（twin plane）時，雙晶分佈距離卻高達500μm，然而未鑽洞塊材僅可部分區域觀察到退火相的分佈，由此推測出已鑽洞塊材會較未鑽洞塊材快速達到退火飽和。
由於影響擄磁能力的因子有塊材半徑、厚度以及電流密度，其中半徑因子是成三次方倍數影響，而電流密度的影響則是來自非超導項的比表面積，因此本研究接著進行孔隙率、211覆蓋率、面積收縮率以及微裂縫覆蓋率判斷半徑因子的改變，以Y211比表面積的計算判斷電流密度的影響。結果顯示，已鑽洞塊材之半徑因子與Y211比表面積的量值皆大於未鑽洞塊材，經計算值與實際值的比較後，雖有8.79%的差異，而其原因可能來自於計算過程假設兩塊材皆達退火飽和，以及其他未定量但可形成釘扎中心（Pinning center）的缺陷。
本研究將鑽直徑2mm 4個洞塊材進行溫度30K、外加磁場2T下，塊材擄磁能力的量測與釘扎能（pinning energy）的計算，結果顯示塊材最終可擄獲1.843T的磁場，並且有1.23eV之釘扎能。

SUMMARY

This study mainly discusses the mechanism of the trapped field enhancement of a YBCO single grain bulk, with the artificial holes, includes the diffusion pathways of oxygen, the volume of YBCO superconductor phase, and the pinning centers.

The perforated bulks have more weight increment than the plain bulks. The weight increment result shows the perforated bulks can be annealed more efficiently due to the great bulk surface area and high density of the microcracks. In the OM images, the perforated bulks show more twins and orthorhombic phases, which were produced by the oxygen insertion and the structure transformation.

According to the formula of a magnetic moment, a trapped field value relates to the volume of the superconductor phase, and electric current density which affected by the volume fraction and diameter of non-superconductor phase. As a result, with observation of the porosity, Y211 coverage ratio, crack distribution density, and the bulk shrinkage, the trapped field of a sample can be evaluated. It shows that the perforated bulks have more superconductor volume and larger surface ratio, which probably lead the perforated bulks to larger trapped field (3100 Gauss) than the plain bulks (1100 Gauss).

Key words: YBCO, artificial hole, large trapped field, efficient annealing

INTRODUCTION

In the Top-Seeded Melt-texture Growth (TSMG) process, Y123 will decompose into Y211, liquid phase, and oxygen in the high temperature (~1000℃). Oxygen near the surface of a bulk (~2.5 mm) can escape out, but the oxygen at the inner part of a bulk will be trapped and form as voids. These voids not only decrease the volume of the YBCO superconductive phase, but also cause the stress concentration of Lorentz force. Drilling c-oriented artificial holes on the bulk can make oxygen easily evacuate in the high temperature and reduce the porosity of the bulk. Some reports indicate that drilling holes will not only decrease the amount of voids, but also improve the trapped field of the YBCO bulk, by increasing the pathways of oxygen during annealing process.

This study discusses the factors of the trapped field of YBCO bulks, by measuring three major sections: the oxygen pathways, volume of superconductor phase, and pinning center. According to the results of all sections, it can conclude that the trapped field of a perforated bulk is larger than that of a plain bulk.

EXPERIMENTAL PROCEDURE

The starting material is a mixture of homemade powders with a ratio of 85 wt.% of YBa2Cu3Ox, 15 wt.% of Y2BaCuO5 and 0.15 wt.% in excess of CeO2. Cylindrical pellets, typically 25 mm in diameter and 10 mm in height, are press under a 25~35 kgf/mm2 uniaxial load and sintered at 850 ℃ for 8 hours. Artificial holes were drilled in the preforms, in the direction parallel to the c-axis of the samples. Samples were prepared with 4, 8, 16 and 32 holes respectively, with diameters of 1 mm and 2 mm. The single domain is then grown on the machined pellet from a SmBa2Cu3Ox seed. Once the growth is finished, the sample is annealing at 300℃ for 200 and 400 hours under flowing oxygen. The microstructure is observed under OM and SEM, and the values of porosity, Y211 coverage et al. are calculated by the software Image J. The spatial distribution of the magnetic field above the sample is measured with a miniature probe, fixed to a motor-driven xy micro-positioning stage scanning over the sample surface. The active area of the Hall probe is 1×1 mm2. The gap between the Hall probe and the sample surface is maintained at 1 mm. The trapped field in the drilled samples is measured after a field-cooling activation in a uniform magnetic flux density of 1 T generated by a large copper coil.

RESULT AND DISCUSSION

On a macroscale, the single domain still grows continuously in the section behind the artificial holes (away from the seed place) and does not influenced by the artificial holes. Under 77 K and external magnetic field 1 T by field-cooling, the perforated bulks can trap 3100 Gauss. However, the plain bulks can only trap 1070 Gauss. As the annealing time increase from 200 hours to 400 hours, the perforated bulk can trap more increment of magnetic lines than the plain bulks. But if the bulk has so many artificial holes, it will also lose a lot of volume and its superconductivity will decrease largely.

There are three kinds of analyses in this study, and they are oxygen pathways, volume of bulk, and pinning centers. In the analysis of oxygen pathways, the perforated bulks have more weight increment than the plain bulks after 200 hours annealing. But after 400 hours annealing, weight increment of the perforated bulks begins to decrease, but the plain bulks still increases. It is speculated that the perforated bulks reach the saturation. The microcracks also provide oxygen pathway to diffuse into the YBCO bulks, and the perforated bulks have more microcracks. The oxygen diffusion will transform YBCO bulk from tetragonal structure into orthorhombic structure, and at the same time, the twins will also be produced due to the stress release. In the OM images, the perforated bulks show more twins and orthorhombic phases.

According to the formula of magnetic moment, a trapped field value relates to the volume of the superconductor phase, and electric current density which affected by the volume fraction and diameter of non-superconductor phase. The perforated bulks not only have porosity of 1.03%, which is smaller than that of 6.69% of the plain bulks, but also has Y211 coverage of 15.11%, which is less than that of 28.25% of the plain bulks. After the perforated bulks shows less bulks shrinkage, it is speculated that its loss of liquid phase in the high temperature is less than the plain bulks. The perforated bulks have larger Y211 surface ratio than bulks without artificial holes. To compare the real trapped field with the simulation value (volume of bulk and surface ratio), there is a small error 8.79% which may come from the basis of annealing saturation of two kinds of samples and the pinning centers that is not calculated. The bulk with 4 holes of 2 mm diameter is also measured under 30 K and external magnetic field of 2T, and it can trap 1.83 T. After calculation, it has a pinning energy of 1.23 eV.

CONCLUSION

Bulks with artificial holes have larger trapped fields than those without artificial holes. Among of the perforated bulks, the trapped field of the bulk with 8 holes of 1 mm diameter is largest, 3100 Gauss. Weight increment reveals that the perforated bulks reach the annealing saturation faster. The OM images also show of the perforated bulks have microcracks, orthorhombic phase and twins. After the calculation of porosity, Y211 coverage, bulk shrinkage, microcrack coverage and Y211 surface ratio, bulks with artificial holes have large volume and Y211 surface ratio. It can conclude that the trapped field of a perforated bulk is larger than that of a plain bulk although there is still a 8.79% error.

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