【作者】 周军；

【导师】 周又和；

【作者基本信息】 兰州大学 ， 固体力学， 2010， 博士

【摘要】 高温超导材料由于其具有较高的上临界磁场、较大的临界电流密度以及高于液氮沸点的临界温度,因此有着广阔的应用前景。利用高温超导块材的抗磁性和磁通钉扎效应设计的高温超导悬浮系统具有相当高的承载能力和独特的自稳定性,是未来高温超导应用的重要分支。由单块永磁体和单块高温超导体构成的悬浮系统的电磁相互作用力特性研究是超导悬浮系统安全运行设计的基础,并作为基础性研究课题受到广泛关注。本博士学位论文主要针对由单块圆柱永磁体和单块圆柱YBCO超导体构成的高温超导悬浮系统在低于液氮沸点温度下的不同温度循环过程中静态电磁力特性以及在液氮沸点温度永磁体垂直于悬浮系统轴向的水平运动对电磁力特性影响开展实验研究,发现了一些新的现象及特性；本文还改进了磁通冻结镜像模型模拟了永磁体水平运动对电磁力特性的影响。首先,测试了悬浮系统中高温超导体在低于临界温度的不同冷却温度下零场冷悬浮力-距离磁滞回线特性。实验表明：在低磁场情形及低于一定温度下,悬浮力-距离磁滞回线存在交叉现象,即在小于一定垂向位移时靠近过程悬浮力小于远离过程悬浮力。尽管悬浮力-距离磁滞回线的磁滞随着温度的降低而减小悬浮力-距离磁滞回线的最大悬浮力随温度的降低而增大并趋于饱和,交叉面积却随温度的降低而增大,交叉点的距离坐标也随温度的降低而增大。其次,研究了高温超导体在弛豫过程中经历温度循环及在非弛豫时变温对悬浮力的影响。实验表明在弛豫时从恒定温度持续降温将使得原本经过长时间弛豫已经趋于稳定的悬浮力继续快速减小,减小的幅度随初始恒温和降温最终到达温度之差的减小而减小；弛豫时在从超导体所经历过的最高温度处持续升温时,悬浮力随温度的升高而减小,减小的幅度与升温速度无关；弛豫时先在一定温度恒温一段时间后持续降温到另一温度稳定后,再次升温且不超过初始恒温时悬浮力随温度的升高而增加,最终增加的幅度不超过从初次恒温降温到另一温度时悬浮力减小的幅度,由此可以提出一个升温稳定预处理过程,即先在一定温度弛豫一段时间后持续降温到另一温度进行悬浮系统长时间运行,在此温度下运行时若出现升温且不超过初始恒温的温度不稳定情形时悬浮力不随温度的升高而减小、仅小幅度增加；恒温弛豫结束后,零磁场直接升温至另一温度后再弛豫、在再次弛豫结束后零磁场直接降温至初始恒温后再弛豫,该温度循环过程中不同初始恒温情形在恢复初始恒温后悬浮力减小幅度与升温后悬浮力减小幅度之比在升温达到的温度固定时差别很小最后,研究了初始冷却高度以及永磁体的水平运动速度对水平位移下悬浮力和导向力的影响。实验表明在由单块永磁和单块超导组成的轴对称悬浮系统中,发生垂直于轴对称方向的水平相对位移时,水平移动速度对悬浮力-水平位移磁滞回线有较大影响,随着水平速度的增大磁滞也增大,经过一次从中心轴对称位置到最大水平位移再回到中心位置的往复运动,相比于初始运动时的悬浮力,在冷却高度比较大时返回后的悬浮力下降,在冷却高度比较小时返回后的悬浮力上升。水平移动速度对导向力-水平位移磁滞的影响不明显。本文还改进了磁通冻结镜像模型使之能反映水平速度对悬浮力-水平位移磁滞回线的影响。更多还原

【Abstract】 Owing to its higher upper critical magnetic field, higher critical current density and transition temperature above the temperature of liquid nitrogen, high temperature superconductor (HTS) shows some very important features in applications. The diamagnetic and flux pinning characteristics make high temperature superconducting levitation system has excellent loading capacity and novel passive stable characteristic. Thus the high temperature superconducting levitation system become an important branch of applications of superconductivity. The electromagnetic force characteristic researches on the typical superconducting levitation style, a superconductor and a permanent magnet (PM) levitation system, have become the basis of the superconducting levitating system operation design, and a subject of considerable interest, which have been extensively investigated. This Ph. D. dissertation presents some experimental investigations of the static and dynamic electromagnetic force characteristics in the high temperature superconductor levitation system under some different temperature cyclings and lateral moving velocity conditions, some new phenomena and characteristics are found, and the results are explained; The frozen-image model is also modified to fit the experimental results of influences of lateral moving of permanent magnet on the electromagnetic forces.Firstly, the influences of different cooling temperature on the hysteretic curves of levitation force versus gap between the bulk HTS and the PM under zero-field cooling (ZFC) are described. It is found that with the decreasing of cooling temperature, there exists a crossing phenomenon on the hysteretic curves of levitation force. Even though the hysteresis loop area decreases with the decreasing of cooling temperature and the maximum levitation force of each hysteretic curves of different cooling temperature increase and saturate under lower cooling temperature condition, the crossing area and the gap of crossing point increase with the decreasing of cooling temperature.Secondly, the influences of some temperature cycling on the levitation force relaxation is described. It is found that the levitation force continue to decrease when the temperature of superconductor is decreased continually from the initial cooling temperature during the relaxation, and it will increase with the increasing of temperature after the continual decreasing. According to these results, a temperature stabilization pretreatment method is presented. Another temperature cycling that increase the temperature from the initial cooling temperature under zero magnetic field condition and then go back to the initial cooling temperature also under zero magnetic field condition will show a fixed ratio between the decreased levitation force ranges of increasing and decreasing processes if the target value of increasing process for different initial cooling temperature cases are the same.Finally, the effects of the initial cooling height (CH) and the lateral moving speed on the levitation force and the lateral force in the levitation system are described. The measurement results show that for some cooling height after the symmetrical movement (from the CH to levitation height) the effects of lateral movement and lateral moving velocity on the levitation force are remarkable. The levitation force will decrease for higher CH and increase for lower CH after the lateral movement. The hysteresis loop area of levitation force increase with the increasing of lateral moving velocity for higher CH, and the hysteresis loop area have nearly no change for lower CH. The hysteresis loop area of lateral force for all CH have nearly no change with the increasing of lateral moving velocity. The frozen-image method has also be modified to simulate the results of above, that the lateral moving speed has significant effect on levitation force-lateral displacement hysteresis loop curves.更多还原