摘要
单畴RE-Ba-Cu-O(简称REBCO,其中RE为稀土元素Nd、Sm、Gd、Eu、Y、 Yb等)高温超导块材具有较高的临界温度和临界电流密度、较强的捕获磁通和较大的磁悬浮力性能,以及良好的自稳定磁悬浮特性,在微型高场永磁体和超导磁悬浮等高新技术方面有着广泛的应用前景。GdBCO超导体便是高温超导家族中性能最好并最有可能早日投入实际应用的材料之一。
本文对单畴GdBCO超导块材的制备、特性及应用进行了比较系统的研究,主要包括以下三方面的内容：(1)采用顶部籽晶熔渗生长(TSIG)方法制备单畴GdBCO超导块材,并对影响样品单畴生长的关键工艺参数(慢冷温区、慢冷时间)进行了详细的调试和优化；(2)通过改变熔渗生长方法使用的液相源成分及前驱坯块的装配方式,对传统的TSIG工艺进行改进研究,以简化实验步骤,缩短实验周期,提高TSIG方法的制备效率和稳定性；(3)对制备的高温超导块材进行应用研究探索。介绍了本实验室设计发明的超导磁悬浮推进系统模型、超导磁悬浮列车模型等实验和演示装置。主要结论如下：
(1)用等温生长与淬火工艺相结合的方法研究了GdBCO样品在不同温度下的生长情况。实验结果表明,当样品在1040℃保温20h后没有出现籽晶诱导成核及生长的现象。在1040℃以下,样品开始在籽晶处成核并外延生长,且随着温度的降低(过冷度的增加),生长速率越来越大。在1020℃保温时,样品边缘开始出现随机成核。样品的微观结构研究表明,随着过冷度的增加,样品内捕获了越来越多的小尺寸Gd2BaCu05(Gd-211)粒子,使得块材中Gd-211第二相粒子所占的体积分数逐渐增大,粒子的平均尺寸逐渐减小,这种现象有利于样品J。的提高。综合以上实验现象并参考样品在慢冷生长过程中的实例,最终选定适合GdBCO单畴块材生长的慢冷温区为1035-1015℃。
(2)研究了不同慢冷时间(20h～90h)对GdBCO块材形貌及性能的影响。结果表明,当样品在1035-1015℃的温区内慢冷生长时,随着慢冷时间的延长,样品的单畴区生长得越来越大,样品边缘的随机成核可被有效抑制,样品的单畴生长越来越安全和稳定。样品的侧面形貌和剖面形貌均表明当慢冷时间达到70h-80h时,籽晶引导的单畴区扩展到样品底部,整个块材生长成为完整单畴体,在此情况下样品的磁悬浮力性能达到饱和值。样品的微观结构研究表明,随着慢冷时间的延长,样品内捕获的Gd-211粒子数目越来越少,且粒子在液相中的Ostwald粗化现象越来越严重。
(3)介绍了本实验室设计发明的高温摄像装置。利用该装置可以清楚观察到REBCO样品在高温下的晶体生长细节,实现对样品生长过程的实时观测。根据样品在不同温度下的生长形貌及随时间的变化情况,我们可以在短时间内制定出针对不同尺寸样品的最优化热处理程序,提高了工作效率,在探索研究大尺寸超导块材的制备技术时可发挥重大的作用。
(4)用两种新液相源LS2(Gd-211+3Ba3Cu508)和LS3(Gd2O3+10BaCuO2+6CuO)熔渗生长单畴GdBCO超导块材。结果表明,和传统熔渗生长方法使用的液相源LS1(Gd-123+Ba3Cu508)一样,使用新成分的液相源也可成功制备出单畴GdBCO超导块材,这是因为虽然三种液相源的物相成分不同,但却具有相同的Gd、Ba、Cu原子组分比,因此可起到相同的作用。此外,使用这两种新型液相源,整个熔渗生长工艺仅需Gd-211和BaCuO2两种前驱粉,这简化了实验步骤,缩短了实验周期,提高了制备效率,对单畴GdBCO超导块材的批量化制备有重要的意义。
(5)用一种新型Y2O3基液相源LS4(Y2O3+10BaCuO2+6CuO)成功熔渗生长出了单畴GdBCO超导块材。研究表明,样品内部捕获了较小尺寸的Gd-211粒子,表现出更高的磁悬浮力性能(28N)。此外,由于本文采用的GdBCO样品生长的慢冷温区(1035-1015℃)始终高于Y-123相的包晶分解温度(约1010℃),所以在GdBCO块材的生长过程中,Y2O3基液相源会一直保持Y-211相和液相的半熔融状态,不会出现Y-123相的生成及固化,因此液相源块可以最大程度地向上面的坯块供应液相,从而提高了液相源粉的利用率。
(6)比较了三种RE2O3基液相源LS3(Gd2O3+10BaCuO2+6CuO), LS4(Y2O3+10BaCuO2+6CuO)和LS5(Yb2O3+10BaCuO2+6CuO)熔渗生长的GdBCO单畴超导块材。结果表明,用这三种RE2O3基液相源均可成功制备出直径30mm的单畴GdBCO超导块材。其中由Y2O3基液相源LS4制备的样品中捕获的Gd-211粒子平均尺寸最小,样品的磁悬浮力最大(67.8N),因此我们认为Y2O3基液相源LS4是熔渗生长GdBCO超导块材的最佳选择。
(7)采用新型的前驱坯块装配方式熔渗生长单畴GdBCO超导块材。在该装配方式中,我们使用较大直径(30mm)的液相源块支撑并熔渗生长较小直径(20mm)的GdBCO样品。结果表明,该方法可以提高液相源块的支撑能力,有效避免样品出现倾斜或倒塌的现象,从而提高了熔渗生长方法的稳定性。此外,由于液相源块的直径始终大于其上的GdBCO样品,可以保证在生长过程中样品的整个底部始终与液相源直接接触,因此可为样品的完整生长提供充足的液相条件。超导性能的研究表明,由新型装配方式制备的GdBCO样品表现出比较高的超导转变温度(约91K)和自场临界电流密度(4.7×104A/cm2)。
(8)利用超导磁悬浮原理及直线电机驱动技术,设计制作了一种超导磁悬浮推进系统模型。该模型由装有YBCO或GdBCO超导块材的小车,永磁轨道,直线电机,光电开关等组成,可以演示小车在悬浮状态下,直线电机的驱动力与超导小车爬升高度之间的关系。该模型综合了直线电机驱动,超导磁悬浮,液氮冷却,轨道导向及光电自动控制技术,具有良好的演示效果。
(9)在超导磁悬浮推进系统模型的基础上,我们实验室设计发明了一种新型的高温超导磁悬浮列车演示装置,其结构简单、体积小、演示直观。该装置主要有两部分组成,一是直线电机,用于给小车提供动力,克服小车运动过程中因空气阻力等引起的能量损耗。二是环形闭合永磁轨道,可实现小车周而复始的自动悬浮运行。该演示装置涉及到力学、磁学、超导、电子电工、自动控制、低温等多方面的知识,不仅可获得良好的演示效果,还能增加人们对现代科技的认识并激发其对高新技术的兴趣。
Because of the high critical temperature and high critical current density, large levitation force and trapped field, also the property of self-stabilized levitation, the single-domain RE-Ba-Cu-O (REBCO, where RE is a rare earth element such as Nd, Sm, Gd, Eu, Y or Yb) high-temperature bulk superconductors have broad application prospects in the high and new technology areas such as high-magnetic-field permanent magnets and superconducting magnetic levitation systems. In the family of high-temperature superconductors, GdBCO superconductor is one of that which has the best performance and may be used earlier in practical applications.
In this paper, the fabrication, property and application of single-domain GdBCO bulk superconductors have been investigated in detail. The main work consists of three parts:(1) the single-domain GdBCO bulk superconductors have been fabricated by top seeded infiltration and growth (TSIG) technique, and the key technique parameters (slow-cooling temperature window and slow-cooling time) which affect the single-domain growth of the bulk have been optimized.(2) The conventional TSIG technique has been modified using liquid sources with new compositons and employing a novel configuration of the precursor pellets, which can simplify the fabrication process, save the experimental time, and improve the working efficiency and stability of the TSIG method.(3) Some application devices based on the produced bulk superconductors have been designed and constructed in our laboratory, including a superconducting maglev propeller system model and a superconducting maglev car model.
The main results are summarized as follow:
(1) The growth character of the GdBCO sample at different temperatures was investigated using a combined isothermal growth and quench technique. Experimental results indicated that, when the sample was held at1040℃for20h, the nucleation and epitaxial growth of GdBCO crystal from the seed did not occur. At temperatures just below1040℃, the nucleation and epitaxial growth started, and the growth rate of GdBCO crystal increased with the decreasing of undercooling temperature. When the sample was held at1020℃, random nucleation at the edge of the bulk appeared. Microstructural observation shows that, the sample can trap more and more small (Gd-211) particles with the increase of the undercooling. This phenomenon leads to an increase in volume fraction of Gd-211particles, as well as a decrease in the mean particle size, which are both beneficial to the improvement of Jc. According to the above results, the optimal temperature window for GdBCO bulk growth was selected as1035-1015℃.
(2) The effect of slow-cooling time on the morphology and performance of the GdBCO bulk was investigated. The results indicated that, when the sample was grown in the temperature window of1035-1015℃, the growth region of the GdBCO single domain increases with the increasing cooling time. The random nucleation of GdBCO grains can be effectively suppressed with prolonging the cooling time, thus the single-domain bulk grows more steadily. Both of the side and cross-section morphology view indicated that, the single domain expanded to the whole sample when the cooling time reaches to70h～80h, and then the levitation force of the sample nearly reached to a saturated value. Micro structural result showed that, the number of Gd-211particles entrapped in the bulk decreased with the increasing cooling time, and the Ostwald ripening phenomenon became more serious.
(3) A high temperature video camera device has been developed in our laboratory, and the growth details of REBCO sample at high temperatures can be observed in real time. According to the obtained information, we can establish the optimum heat treatment process for the crystal growth of different-sized samples in a rather short time, which can improve the work efficiency and will play an important role in the exploration and development of the fabrication technology for large bulks.
(4) Two new liquid sources LS2(Gd-211+3Ba3Cu5O8) and LS3(Gd2O3+10BaCuO2+6CuO) were employed for the infiltration and growth of single-domain GdBCO bulk superconductor. The results indicate that, just as the regular liquid source LS1(Gd-123+Ba3Cu5O8) used in the conventional IG process, the new luquid sources can also be used to fabricate single-domain GdBCO sample, because that they have the same atomic ratio of Gd, Ba and Cu. Furthermore, after employing the new liquid sources, only two precursor powders (Gd-211and BaCuO2) are needed to prepare for the whole TSIG process flow, which can simplify the experimental procedure, shorten the experimental period, and improve the fabrication efficiency.
(5) Single-domain GdBCO bulk superconductors have been fabricated successfully using a new kind of Y2O3-based liquid source. It was shown that much more small Gd-211particles were trapped in the samples and enhanced the sample to a higher levitation force (28N). Moreover, the lower temperature of slow-cooling temperature window for GdBCO growth used in this study (1015℃), is always higher than the peritectic decomposition temperature of Y-123phase (1010℃), thus the liquid source can be always kept in a partial melting state composed of Y-211and liquid. This can make the liquid source pellet supply liquid as much as possible, and the utilization rate of the liquid source powder is enhanced.
(6) Three RE2O3-based liquid sources LS3(Gd2O3+10BaCuO2+6CuO), LS4(Y2O3+10BaCuO2+6CuO) and LS5(Yb2O3+10BaCuO2+6CuO) have been used to fabricate large GdBCO single-domain bulks of30mm in diameter by the TSIG technique. More refined Gd-211particle distribution was observed in the sample fabricated using LS4, which also exhibited largest levitation force (67.8N). Thus the Y2O3-based liquid source LS4was considered as the best choice for TSIG process of GdBCO bulk.
(7) A novel configuration was employed for TSIG process of single-domain GdBCO bulk. In this configuration, we used liquid source pellet of largle diameter to support and grow small GdBCO sample, which could increase the support ability of the liquid source pellet and make a sufficient condition for the growth of the bulk in all dimensions. Superconductive measurements revealed that, the sample exhibited high superconducting transition temperature (about91K) and large self-field critical current density (4.7×104A/cm2).
(8) A superconducting maglev propeller system model was designed and constructed in our laboratory using the fabricated REBCO products, based on the principle of Superconducting magnetic levitation and the driving technology of linear motor. This system can demonstrate the levitated running of a small car model, and the climbing along the track drived by the linear motor, which can give a nice demonstration of maglev propeller system.
(9) On the basis of the maglev propeller system model, a superconducting maglev car demonstration device has been designed and constructed in our laboratory, which consists of two parts:i) a pair of linear motors, which can drive the car model to climb to a higher position; ii) a closed magnet track loop, to accomplish the automic and circular levitated-motion of the car model. This demonstration device has many advantages such as simple structure, small volume, and the intuitionistic demonstration effects.
引文
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