| 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. |
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