| MgB2, with a relatively high-superconducting transition temperature (Te) of 39 K discovered in 2001, reaching the limite of BCS theory, has created a new superconducting transition temperature record in metallic compounds. Compared with the cuprate-related perovskite type structure high temperature superconductors, MgB2 has many advantages: relatively simple crystal structure, low cost of raw material and preparation, small anisotropy, long coherent length, almost no weak-link behavior of grain connectivity, and so forth, therefore is able to carry high current and considered to be superior in MRI applications in-20 K without liquid He coolant. Despite of all of these advantages, there are three existing problems limiting its practical use:First, the critical current density value Jc of the MgB2 reduces dramatically with increasing the magnetic field. Second, the barrier-precursor diffusion in in-situ PIT fabrication could result in the decline of the transport superconducting properties. Third, the mechanical performances of MgB2 are not high enough either, greatly limiting its use in magnets, for it is a kind of britte material. So it's necessary to improve the superconducting performance of MgB2 in magnetic fields, reduce or slow down the diffusion between the barrier and the precursor powder, and strengthen the MgB2 wire or tape to meet practical needs. Thus, the work in this thesis mainly focuses on the following three points:First, this thesis studied the doping effects of nano-sized C powder on the crystal structures and the superconducting properties of MgB2 wires. A series of MgB2-xCx (x=0,0.05, 0.10,0.15) wire samples were fabricated by standard in-situ PIT method and sintered in closed Ar atmosphere in tubular furnace. The experimental results show that the characteristic (110) peak shifts to high angle while (002) peak almost stays unchanged. This suggests that the dopant C substitute the B site and have little effects in Mg site, thus, resulting in the decline in a parameter and c parameter remains the same. Meanwhile, the dopant C suppresses the transition temperature Tc, Tc reduces with the C doping content increasing. From the experiments, the doping level of x=0.1 can effectively improve the superconducting performances of MgB2-xCx wires. And the carbon can be supplied enough energy to break into the MgB2 crystal lattice structure and substitute the B site at high temperature, indicating successful introduction of flux pinning centers.Second, the Nb-B diffusion tape couples were first made in the same manner as the mono-core MgB2 tapes to investigate the Nb-B diffusion behaviors obviously during the heat treatments. It is found that the borided niobium layer did not exist in the Nb-B couples sintered at low temperature 650 ?. The Nb-B interlayer came into shape at 750? and gradually grew thicker with further increasing the temperature. The main compositions of the Nb-B interface were NbB2 with a few amount of NbB, both of which could intercept the transport current in MgB2 tapes. High temperature heat treatments at 850? or above could develop Nb-B diffusion interlayer thick enough to isolate the transport current from MgB2 filaments completely. According to the experimental results of Nb-B diffusion tape couples, we modify the ratios of Mg to B slightly since vapor pressure of the Mg is very high in order to slow down the Nb-B reaction rate and reduce the thickness of the interlayer. The results suggest that a suitable quantity of excess Mg addition could prevent the Nb-B diffusion behavior at some degree in high temperature sintering, and hence had a positive role in maintaining the superconducting properties of the MgB2/Nb samples. And the suitable amount of Mg addition differed with the sintering conditions. Depending on the specific heat treatments, the suitable Mg:B ratios should be near 1.03:2 and 1.05:2 for 750? and 850, 950? treatments respectively.Third, we prepared a series of MgB2 wires with various structutes (different superconducting filament numbers) to investigate the mechanical properties. It has been found that the 6-filament MgB2 wire has the maximum axial strength with superior bending properties to the other wire samples in this thesis. Based on the mechanical properties of MgB2 wires with different filament numbers, three samples of 6-filament MgB2 wires enhanced by Cu, Cu-Nb and NbTi were also made to study the impacts of reinforcing cores on the wire strength improvements. SEM images and the room-temperature tensile tests show that the Cu-Nb has the right stiffness for the manufacturing of the 6-filament wires. By contrast, the Cu was too soft to strengthen the wire. The NbTi was too hard for cold drawing, meanwhile the intermediate annealing had little improvements for drawing. The wires reinforced by the Cu-Nb sintered at 750? had the maximum yield strength with the critical current value Ic exceeding 190 A at 20 K,1.5 T before and after 70 MPa axial load. And it also exhibited the lowest resistivity within the temperature range from 10 to 35 K. We calculated the hoop stress of the wires in the magnetic field on some assumptions for simplicity and found that the mechanical properties of the wires could reach the application requirements in field lower than 1 T.Next, according to the former experiments and the work-hardening characteristics of multi-filament Cu-Nb microcomposite, appropriate intermediate annealing treatments were introduced in the fabrications of three different Cu-Nb reinforced MgB2 wires (91-,853-and 8532 (853×853)-core Cu-Nb reinforced MgB2 round wires) as the Nb filament size exerts a big influence on Cu-Nb mechanical properties, the finer the stronger. In this manner, MgB2 wires could inherit strong strengths from the Cu-Nb reinforcing cores, and at the same time, the superconducting properties were also maitained. Just as we have expected, the tensile tests indicate multi-filament Cu-Nb cores (fine Nb filaments) play positive roles in the mechanical performances of the MgB2 wires. While, the superconducting properties of the enhanced wires declined to different extents due to the intermediate annealings. After the analysis of transport electrical results, we thought that 8532-core Cu-Nb reinforcing core was the limite for the fabrications of 6-filament MgB2 wire by in-situ PIT method. |
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