Materials Science Forum Vols. 745-746

Abstract: The strengthening abilities of NbTi for 6-filamentary MgB_1.9(TiC)_0.08/Nb/Cu wires and the electrical properties have been investigated. The NbTi reinforcing core was suitable for manufacturing MgB₂ wires. It was observed that with the sintering temperature increasing, the yield strength decreased. The plasticity improved while the strength changed little by introduction of intermediate annealing during the cold processing of the wires. The transport properties of the wire degraded to some degree due to intermediate annealing. The mechanical properties have already met the application requirements of superconducting magnets in low field with the critical engineering current I_c above 90 A (J_ce exceeding 7.9×10³ A/cm²) at 35 K in self-field.

Abstract: Different from other applications, this paper explores how to use the stable high-Tc superconducting (HTS) magnetic levitation (maglev) to lift the sculpture art to a high levitation position for satisfying the artists requirement for the light artistic concept. With a combination of the permanent magnet (PM) levitation and HTS maglev technologies, a sculpture model was built to sustain a stable levitation with a levitation height of over 80 mm. In this hybrid HTS maglev sculpture model, three PM rings are arranged as upper, middle, and bottom as a double-layer axial levitation structure; six rectangular YBaCuO bulks are fixed along the side of the PM rings upper-layer as a stabilizer component along the radial direction. When the bottom PM ring was driven up by the lifting bolt, the upper PM ring, which was fixed with the art, can be lifted much higher. Then the required artistic effect will be enhanced. After the model was designed and optimized by finite element method, it was experimentally validated by the HTS maglev measurement system. This levitation design is helpful to the realization of a real-scale levitation sculpture art, and can also provide references to superconducting radial magnetic bearing systems.

Abstract: The potential application of bulk high temperature superconductor (HTS) magnets has attracted much attention because of the potential high trapped flux in HTS magnets. This paper focuses on the magnetization method design of bulk multi-seeded HTS magnets for obtaining their better flux-trapping performance. Firstly, three different magnetization methods were carried out based on the current experimental setup to find a better way of magnetizing a bulk melt-texture three-seeded YBaCuO superconductor. The experimental results indicated that when the three domains of this three-seeded YBaCuO bulk were magnetized in order, the maximum trapped flux was higher than that when only one domain was magnetized. However, this method costs about three times of the magnetization time than the other two methods and the increasing ratio was only about 11.11%. It has been found that another method of magnetizing only the middle domain could also get a good result such as the uniformity of trapped flux is good. In order to improve the current experimental magnetization conditions for further improvement, two sheets of iron were designed to attach two poles of the electromagnet (Lakeshore, Model EM4-CV) for increasing the magnetizing area, and that all domains of a bulk multi-seeded HTS can be magnetized in one time. Firstly, the appropriate size and thickness of the iron sheets was simulated and optimized by Comsol Multiphysics. It has been found that the magnetic field between two poles was highest when the thickness of iron was 2 mm and the length was 68 mm. Then, the simulating and optimization results had been verified by the following experiments. According to the comparison experiments, it is proved that to choose the magnetization method that only magnetizing the middle domain with the improved setup is helpful to obtain larger and more homogeneous magnetic flux for the bulk multi-seeded HTS magnet due to the added iron sheets.

Abstract: As YBCO coated conductors are usually prepared on nickel or nickel alloy substrates, additional ferromagnetic losses are generated in the substrates when the conductors are exposed to AC magnetic fields or carrying alternative currents in practical application. In this work, AC transport loss factors of YBCO tapes with Ni-W alloy substrates were studied in DC magnetic fields. The influence of ferromagnetic substrate on AC loss and the repressive effect of DC background magnetic field on ferromagnetic loss in the substrate were presented and analyzed. The results showed that the AC transport loss factor of YBCO tape with Ni-W substrate decreased gradually with the increasing background field, and the minimum value of AC loss factor appeared as the field reach to about 18 mT and 45 mT for parallel and perpendicular applied field respectively. Based on the analysis, the method to reduce AC transport current loss in coated conductor with nickel alloy substrate is proposed.

Abstract: t was theoretically and experimentally proved that High Temperature Superconducting (HTS) Maglev had huge potential employment in rail transportation and high speed launch system. This had attracted great research interests in practical engineering. The optimization design was one of the most important works in the application of the HTS Maglev. As the NdFeB permanent magnet and HTS materials prices increased constantly, the design optimization of the permanent guideway (PMG) of HTS maglev became one of the indispensable works to decrease the cost of the application. This paper first reviewed four types of PMGs used by the HTS Maglev, then disucssed their structures and magnetic fields. Finally, the optimization methods of these four PMGs were compared. It was suggested that with better optimization methods, the levitation performance within a limit cost got better. That would be helpful to the future numerical optimization of the PMG of the HTS maglev.

Abstract: Niobium with the transition temperature of 9 K is the usual material for TESLA superconducting accelerate cavity. The cavity must work at the temperature range of of 2~1.8 K, which would consume large energy. In addition, the low upper critical field is one of reasons causing the problem of accelerate cavities Q-slop. Comparing with Nb material, MgB₂ has several advantages in this application, such as higher Tc of ~39 K, higher upper critical field and the possible lower microwave loss, which would help increasing accelerating gradient and saving operation cost. Because of these benefit, more and more focus is put on this style of accelerating cavity, MgB₂-thick-film/metal-cavity-body. Using Hybrid Physical-Chemical Vapor Deposition method, the micronmeter-thick MgB₂ film on Nb substrate was prepared in the present investigation. The experimental results showed that Tc(0) reached 38.5 K and the Hc₂ was about 20 T. In order to test the films fracture toughness the film as the inner surface was bended. It was found that at small bending angle the influence on the superconducting properties was little. When the bend radius increased to 5 mm, some cracks smaller than 1 micron occured on the film surface. However, the film still attached to the Nb substrate and the Tc was as high as ~33.5 K, showing that the MgB₂ film fabricated by HPCVD had a good compact state with the Nb substrate and good mechanical toughness. These results indicate that the clean MgB₂ thick film has a better feature in the application of superconducting accelerate cavity.

Abstract: A series of MgB₂ bulks with different porosity have been successfully prepared by in-situ solid-state reaction at ambient pressure with a toluene-treatment boron powder. The mass density reaches 1.86 g cm^-3 at the most compact MgB₂ bulk, which has an imporous microstructure, excellent grains coupling, clean grain boundaries and nanosized grains (100~200 nm). Our results reveal that the porosity of MgB₂ can be significantly depressed by tuning the residual toluene content and heat treatment temperature in this way. Due to the degradation of porosity and thus the enhancement on grains connectivity and grains boundary pinning, the critical current density, J_c and irreversible fields Birr of MgB₂ are significantly improved. For the best sample, at 20K, 4T and 10K, 6T, the J_c reach 2400 and 3700 A cm^-2, which is higher than normal porous sample by a factor of 20 and 8, respectively; and the Birr at 20 K reaches 5.10 T.

Abstract: In this paper, we have prepared a series of Fe sheathed MgB₂ tapes with SiC, Malic acid and C₉H₁₁NO mono-and co-doping by in-situ powder-in-tube (PIT) method, and studied the effects of various C sources co-doping on transport critical current density and flux pinning of MgB₂/Fe tapes. The results suggest by various C sources co-doping, the amount of C substitution for boron largely increase, comparing to SiC, Malic acid or C₉H₁₁NO mono-doped sample. For MgB₂/Fe tapes with 5 mol% SiC+10 wt% Malic acid and 4 wt% C₉H₁₁NO+10 wt% Malic acid co-doping, the transport J_c at 4.2 K and 10 T are 14.8 and 13.5 kA/cm², respectively, which are clearly higher than the J_c of these dopants mono-doped samples. Furthermore, it is found that at the 4 wt.% C₉H₁₁NO doped MgB₂, the poor in-field J_c should be attributing to the cracks at grains boundary, which results in the bad grains-connectivity.

Abstract: Critical stress/strain is an important parameter to describe the transport property of superconductors under mechanical stress. The critical stress/strain of specified superconductor should be gotten before the superconductor is applied to manufacture devices. Determination of the critical stress/strain of specified superconductor is related to the chosen reduction percentage of the critical current under monotonic mechanical stress. However, in practical applications, superconductor also withstands repetitive mechanical stresses. Therefore, the influence of repetitive mechanical stress on the critical current or N value should be considered when the critical stress/strain of the superconductor is determined. Therefore, in this paper, the electrical transport property of copper reinforced Bi-2223/Ag tapes under both monotonic and repetitive tensile stress was studied theoretically and experimentally. Based on analysis of the change of critical current and N value under monotonic and different repetitive tensile stresses, the critical stress and strain of this superconductor tape were proposed.

Abstract: on beam assisted deposition (IBAD) is an important technique to fabricate the second generation high temperature superconducting (2G HTS) wires. Among the fabrication routes of 2G HTS long wires, IBAD route achieved the best performance in recent years. IBAD was adopted in this field in 1991 to obtain biaxially textured buffer layers, which helped to deposit high quality YBCO superconducting films on metallic substrates for the first time. Series of experimental and industrial researches on IBAD were carried out by many groups worldwide. And in the researches lasting for over two decades, the focused material for IBAD was changed from Yttria-Stabilized Zirconia (YSZ), Gd₂Zr₂O₇ (GZO) to MgO. In this paper, the research progresses and the main achievements were briefly reviewed.