鉍鍶鈣銅氧系列超導單晶晶體生長與晶體性質之研究

Crystal Growth and Charaterization of Bismuth-based Family Superconductors

葉國偉(Yeh Kuo Wei)

指導教授 ： 甘炯耀

國立清華大學/工學院/材料科學工程學系/博士(2007年)

https://doi.org/10.6843/NTHU.2007.00150

摘要

多晶樣品的均勻性不佳且不容易除去雜相攙雜等問題，而且多晶樣品是無法對鉍系層狀結構的異向性質做量測，另外晶界的存在亦影響到電子傳輸性質的量測；甚至一些重要實驗如比熱、中子繞射等實驗需要大尺寸單晶才能測得足夠訊號才能對原子結構作分析。因此本研究使用傳輸溶液懸浮熔區法(Traveling Solvent Floating Zone Method，TSFZ )進行Bi2(SrxCa3-x)Cu2Oy(x=1.5~2.0)、Bi2Sr2Ca1-xPrxCu2Oy(x=0~1)、Bi2Sr2GdCu2Oy和Bi2+xSr2-yCuO6+δ (Bi-2201)的晶體生長實驗，此法能有效控制成長氣氛和成長速度等長晶參數，以及能避免高溫溶液與坩堝反應的問題。
我們成功的成長了一系列Bi2Sr2(PrxCa1-x)Cu2Oy的晶體，Pr添加量x從0.2到0.7的晶體一般約為5mm´3mm大小，而Pr添加量1.0的晶體可達10mm´3mm，這些晶體皆為單相、結晶性佳的高品質單晶。我們一方面探討在添加稀土元素Pr之後，相圖中固溶相及液相區間組成之間的平衡關係；此平衡溶液區間隨著Pr添加量的增加而偏移到CuO較高的區域，而單晶組成則往CuO含量偏低的趨勢。同時由此不同Pr添加量的2212單晶，進行比熱量測以及利用磁場分別加在ab面和c軸方向量測異向磁化率，而獲取不同Pr添加量單晶的物理特性。
同時我們也成功的成長了稀土元素Gd添加的Bi2Sr2GdCu2Oy晶體，晶體的尺寸約為6mm´2mm，而也得到單晶的比熱以及異向磁化率特性。值得一提的是相較於Bi2(SrxCa3-x)Cu2Oy系統，此Gd添加的平衡溶液區間落在BiO1.5以及CuO較高的區域。因此針對這三個Gd、Pr以及Ca-2212系統，我們也由分凝現象(segregation behavior)來討論平衡溶液區間在相圖中的偏移與最大穩定生長速率之間的關係。
最後本研究也報導了Bi2+xSr2-yCuO6+δ (Bi-2201)的晶體生長結果，晶體尺寸約為20mm´4mm，針對這些高品質單晶，我們由不同的退火條件得到在晶體內較佳的氧比例，同時也獲得了晶體異向性的傳輸性質結果。

關鍵字

超導體 ； 晶體生長 ； 單晶

並列摘要

Bulk single crystals are indispensable in the understanding of the physical properties of high-Tc superconductors having a common feature of a layered perovskite structure with CuO planes. For the study of physics and chemistry of condensed phases, it is crucial to obtain good quality single crystals of the material concerned, because in polycrystalline materials, the properties of the grain boundaries are often stronger than that of the material itself. Thus, at this current stage of research, significant efforts were invested in the growth of high-quality large single crystals to further improve our understanding of high-Tc superconductivity.
This thesis presents the crystal growths of Bi2(SrxCa3-x)Cu2Oy, Bi2Sr2Ca1-xPrxCu2Oy and Bi2Sr2GdCu2Oy by using the traveling solvent floating zone method within an infrared radiation furnace. Crystals of Bi2(SrxCa3-x)Cu2Oy of various Sr/Ca ratios were obtained with a lowest value of approximately 1.32. A rotation rate of 30 rpm and a growth rate of 0.2 mm/h were established. Compositional analysis of the steady-state solvent zone indicated that a primary crystallization field (PCF) was present at the bismuth-rich and copper-deficient region relative to the 2212 stoichiometry, and that this compositional range approximately equal to that of Bi2.425Sr1.911Ca0.807Cu1.857Oy. At the same time, this PCF includes a corresponding tie-line relationship with a slightly copper-deficient 2212 single-phase region.
A systematic study of Bi2Sr2Ca1-xPrxCu2Oy, where x = 0–1.0, was undertaken to determine growth parameters, crystal properties as well as the phase compositions. The typical dimension of single crystals with actual doping ratios x from 0.2 to 0.7 was 5×3×0.2 mm3. Moreover, large single crystals of Bi2Sr2PrCu2Oy (x = 1.0), with a typical size of 10×3´0.2 mm3, were successfully grown for the first time. These crystals contained no impurities and showed a clean X-ray diffraction pattern of the pure Bi-2212 phase. As the Pr doping level increased, the liquidus composition shifted pronouncedly to the corner of CuO as shown in the pseudo-ternary BiO1.5-(Sr,Ca,Pr)O-CuO phase diagram. As a result, the solidus composition, Bi1.965Sr2.046Pr1.096Cu1.894Oy, and liquidus composition, Bi2.17Sr1.92Pr0.31Cu2.60Oy, were obtained. Some physical measurements, such as determining the structural modulation, molar magnetic susceptibility and heat capacity on Pr-doped 2212 crystal were also performed.
Crystals of Bi2Sr2GdCu2Oy were grown with the dimensions of approximately 6×2×0.2 mm3. As compared to the Bi-2212 solid solution region, the PCF of Bi2Sr2GdCu2Oy was approximately located at the BiO1.5-rich and CuO-rich side, as demonstrated in the pseudo-ternary BiO1.5-(Sr, Pr, Gd)O-CuO phase diagram. Such variation in solvent compositions may significantly influence the segregation behavior of each constituent so as to influence the maximum stable growth rates of each compound.
The ratio of the temperature gradient G over the growth rate R, i.e. G/R, is in direct relation to the occurrence of constitutional supercooling. In our experiments, a planar interface was observed, indicating that a stable growth condition was established before zone quenching. The minimum critical values of G/R are 5.39×1011 K-s/m2 of Bi2Sr2GdCu2Oy, 4.5×1011 K-s/m2 of Bi2Sr2CaCu2Oy, and 2.94×1011 K-s/m2 of Bi2Sr2PrCu2Oy. These data obtained from fundamental estimations are consistent with experimental results, indicating a decreasing behavior of growth difficulty.
Finally, Bi2+xSr2-yCuO6+δ (Bi-2201) crystals which compositions include Bi2.22Sr1.78CuOy and Bi2.3Sr1.7CuOy were grown. The typical size of the crystals is as large as 20×4 mm2. The transport properties (in-plane resistivity, out-of-plane resistivity and Hall coefficient) of some as-grown and post-annealed Bi-2201 crystals were reported.

並列關鍵字

superconductor ； crystal growth ； TSFZ ； single crystal ； Bi-2212 ； Bi-2201

參考文獻

References for Chapter 1

[5] H. Maeda, T. Tanaka, M. Fukutomi, T. Asano, Jpn. J. Appl. Phys., 27 (1988) L209.

[8] R. P. Gupta, M. Gupta, Phys. Rev. B, 49 (1994) 13154.

[15] Y. Kimishima, H. Kittaka, Phys. C, 160 (1989) 136.

[36] R.O. Suzuki, S. Kambara, H. Tsuchida, K. Shimizu, K. Ono, “Advances in Superconductivity Π”, eds. T. Ishiguro, K. Kajimura (Springer, Tokyo, 1989) 235.