臺灣博碩士論文加值系統

為了進一步探索氧化物超導體之超導機制及超導成因，高品質且大尺寸單晶的取得日益重要。而對Bi2Sr2CaCu2Oy(2212)系統，因為由液相析出2212晶體的過程為包晶反應，需以偏離劑量比組成的溶液來成長，因此需要深入了解相圖中平衡溶液區間及固溶相區間的相對關係，才能得到符合需求的單晶。
本論文採用傳輸溶液懸浮熔區法（Travelling Solvent Floating Zone method，簡稱TSFZ）來成長晶體，此法能有效控制成長氣氛及長晶速度等長晶參數，以及能避免高溫溶液與坩堝反應的問題。基於之前對2212相之起始結晶區間的了解，我們選擇鉍多銅少的組成作為起始溶液來成長不同Sr/Ca比(2.0, 1.8, 1.5, 1.0)的晶體，而長晶實驗都在低氧壓的氣氛下進行以避免2212相的分解。
為了增加長成晶體的尺寸，我們調整上下晶棒的轉速以得到微凸的長晶界面，而且觀 察轉速對擴散邊界層厚度的影響發現，強制對流為熔區內最主要的對流狀態。我們得到的最佳成長條件為：長速0.2mm/h及轉速30rpm。由此我們成功成長得到不同Sr/Ca比的單晶（最低到1.3），藉由對單晶以及淬冷熔區的組成分析，得到平衡的固溶相區間和溶液組成區間，以及彼此間結線的關係。
另外為了探討添加Pr進入Ca位置而破壞超導性質的特性，我們分別成長了10%，20%及100%的取代量，在全取代的實驗，我們得到尺寸為5×3mm2，結晶性非常好而且完全未夾雜2201相的Bi2.029Sr2.048Pr1.025Cu1.897Oy的單晶。

For further exploration of the mechanism and the origin of superconductivity in oxide superconductors, it's important to have high quality and large size superconductor single crystals. Owing to the peritectic by nature, it's helpful to grow Bi2Sr2CaCu2Oy(2212) from high-temperature solution using an off-stoichiometric flux. Thus far, the equilibrium relations between the solution composition range with primary crystallization field (the liquidus) and the solid solution range (the solidus) must be fully understood.
We employed Travelling Solvent Floating Zone (TSFZ) method to proceed the growth of Bi-2212 crystals. In this technique we have better control in various growth conditions, eq. growth atmosphere and growth rate. Furthermore, the reation between the high-temperature corrosive liquid and crucible were avoided. Based on our eralier study on the primary crystallization field of Bi2Sr2CaCu2Oy, we adopt Bismuth rich and copper deficient composition as a solvent to grow crystals with different Sr/Ca ratio (2.0, 1.8, 1.5, 1.0). The growth experiments were conducted in a low-oxygen atmosphere to prevent the decomposition of 2212 phase.
In the hope of increasing the dimension of single crystal grains, slightly convex growth interface was achieved by adjusting the rotation rate of the feed rod and the seed rod. Comparing the effect of the rotation rate on the diffusion boundary layer thickness, we found that forced convection is dominate within the molten zone. Our optimized growth conditions were: 0.2mm/h growth rate and 30rpm counter-rotation. Single crystals of different Sr/Ca ratios (down to 1.3) were successfully obtained in this study. From our compositional analysis of the single crystals and the quenched zones of the TSFZ experiments, the tie-line relation between the 2212 phase and the liquidus were realized.
Additionally, in order to investigate the breakdown behavior of the superconductivity when Pr was substituted into the Calcium site, we conducted growth experiments of Bi-2212 crystals with 10%, 20% and 100% Pr substitution. In the Pr total substitution experiment we obtained Bi2.029Sr2.048Pr1.025Cu1.897Oy single crystals with excellent crystallinity and no intergrowth of 2201 phae was found. These crystals were approximately 5×3mm2 in area.