Materials Science Forum Vols. 546-549

Abstract: The powder-in-tube MgB2 wires were prepared by high energy milling of Mg and B powder. The powder was not mechanically alloyed for 2h short milling time. However, the MgB2 grains in wires were very small (20~100nm) and resemble the dimple after post-heat treatment. The clear evidence for transcrystlline fracture was observed. It indicated that the grain connection was greatly improved and the fluxing pinning was significantly enhanced. Another point to view was no intermediate annealing during the whole rolling process. The influence of the post-heat treatment on the transport current density was studied. Despite the lower Tc of about 35K, the transport current density reached to 3×104A/cm2 at 15K and 3.5T for wires sintered at 700°C.

Abstract: Bulk samples with the stoichiometry of Mg:B=1:2 and 1:4 were prepared by solid state reaction method. The microstructure and constituent of the samples were investigated by using the scanning electron microscope (SEM) and the x-ray diffraction (XRD). XRD results showed that the MgB2 phase was first formed in all the samples with different stoichiometry of Mg and B, which indicated that the MgB2 was the most thermodynamically stable phase in the Mg-B binary system. For the samples of Mg:B=1:2, the MgB2 single phase was formed very well when these samples were sintered at 650°C~700°C. For the samples with later added Mg reaching to the stoichiometry of Mg:B=1:2, a longer reaction time or higher reaction temperature was required for the formation of the MgB2 single phase. The SEM results showed that the samples with later added Mg had dense microstructures, suggesting that the later addition of Mg could reduce the porosity of the sample. A small increase of the superconducting transition temperature, Tc, in the Mg addition sample resulted from the dense microstructures.

Abstract: Fe-sheathed MgB2 tapes were prepared by the in situ powder-in-tube (PIT) technique using nanoscale Si/N/C and SiC whisker as doping materials, respectively. It is found that the doped tapes exhibited superior field performance and higher critical current (JC) values than the undoped tapes in the magnetic field up to 12 T. Moreover, the improvement of JC for the SiC whisker doped samples was more significantly than that in Si/N/C doped tapes, while flux pinning ability enhancements were similar. It is proposed that the difference of impurity content in MgB2 matrix, which affects the grain connectivity, is responsible for JC value difference between Si/N/C and SiC whisker doped samples.

Abstract: Superconducting MgB2 films have been fabricated on sapphire substrate by the method of hybrid physical-chemical vapor deposition (HPCVD). The film thickness ranges from 75 nm to 3 μm. The investigation on the physical properties, such as the transition temperature, TC, the transition width,
T, and the critical current density, JC, has indicated that the optimum film thickness is about 1 μm. At this thickness, Tc(onset) = 40.3 K and 0Tc = 0.15 K. At T = 5 K without external field, JC has been estimated as 5.6×105 A/cm2according to the Bean model. The film quality begins to degrade as the thickness exceeds 1 μm. This work has demonstrated that HPCVD is a simple but effective technique for the application in the production of MgB2 wires.

Abstract: Poly-crystalline MgB2 films have been grown on niobium substrate by using the hybrid physical-chemical vapor deposition (HPCVD) with a thickness about 1.0 μm, which have good transport performance with Tc(0) ranged from 38.5 K to 38.9 K, ,T about 0.1-0.2 K. Films are composed of regular plate-shape hexagonal MgB2 crystallites and have lattice constants approaching the bulk values. For this sample, the bending test has been also performed, and it can be found that the film adhered on the substrate firmly after bending to 120º.

Abstract: In-situ MgB2/Fe tapes were fabricated by the powder in tube (PIT) technology using Mg and B powders. The tapes were sintered by both an advanced spark plasma sintering (SPS) and a conventional vacuum furnace, respectively. The effects of these two sintering routes on the properties of the MgB2 tapes were investigated by the analysis of XRD, SEM and the magnetic Jc measurements. It was shown that at 10K, 0T, the magnetic Jc value of the tape sintered using SPS machine at 800°C for 15min under 30MPa were close to 106A/cm2, which was higher than that of the tapes annealed at 700°C for 2h under pressureless vacuum condition. It was found that the density and the homogeneity of the MgB2 core were strongly improved using SPS sintering process, which explained why the MgB2 tapes sintered by SPS method have a higher Jc value. Further enhancement of Jc in the tapes is expected for the understanding of the SPS sintering mechanism.

Abstract: Pulsed magnetic field was first employed in the study on MgB2 superconductors. Superconductivity properties and microstructures of Zn and SiC doped MgB2 were discussed in this paper. The superconductors showed different superconductivities after magnetic sintering. Critical current density and flux pinning force were relevant to the dopants properties in MgB2. Grains in the pulsed magnetic processed MgB2 matrix were smaller than those in the normal processed one. The refinement microstructures have caused the critical current density (Jc) anisotropy in both the Zn and SiC doped MgB2.

Abstract: We have successfully synthesized MgB2 thick films on 4H-SiC substrate by hybrid physical-chemical deposition (HPCVD). They have TC above 40 K, X-ray diffraction (XRD) shows the polycrystal bulk like structure. And JC estimated using magnetic hysteresis loops (MHLs) and Bean model is 7.4 MA/cm2 in self field, at 5 K.

Abstract: In order to exploit new strategies for fabricating high-temperature superconductors with special morphologies and investigate the effects on their superconductivity, we successfully utilized a facile polyol process for fabricating a large amount of La2CuO4 (LCO) microspheres using La(CH3COO)3·1.5H2O and Cu(CH3COO)2·H2O as raw materials and ethylene glycol as both complex agent and solvent. The well crystallized LCO microspheres can be obtained by annealing the La-Cu-complex precipitation formed after refluxing process at 750 oC for 10 hours. Field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) results indicated that the as-prepared sample had an average diameter of about 800 nm. The powder X-ray diffraction (XRD) patterns showed that the LCO microspheres were of pure phase. The magnetization measurement of the sample annealed at 900 oC in pure oxygen for 20 hours showed that the superconducting transition temperature (Tc) of the LCO microspheres was about 38 K, which in agreement with that of bulk counterparts.