Papers by Author: Y. Feng

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: 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: YBCO bulks with submicron Y2BaCuO5 particles have been prepared under oxygen partial pressures of 0.21atm and 1atm, respectively, by the powder melting process (PMP). The microstructures, oxygen contents and critical current densities of the samples have been investigated. The results showed that the sample grown under P(O2) = 1 atm contained Y2BaCuO5 particles with a little bigger average size and had a slightly higher density of stacking faults than the one prepared under P(O2) = 0.21 atm. Oxygen content measurements indicated that the as-grown sample under P(O2) = 1 atm had a higher oxygen content. After post-annealing in flowing oxygen, the oxygen content of the P(O2) = 1 atm sample was still higher than that of the P(O2) = 0.21 atm sample. They were examined to be 6.94 and 6.88 with X-ray diffraction for the two oxygenated samples, respectively. Magnetization measurements have been performed on the two types of oxygenated samples and critical current densities (Jc) have been calculated based on the extended Bean critical state model. The P(O2) = 0.21 atm sample exhibits Jc values of 5.47×105 A/cm2 and 2.22×104 A/cm2 in 1 T magnetic field at 20K and 77K, respectively. The P(O2) = 1 atm sample, meanwhile, has Jc values of 9.46×105 A/cm2 and 1.47×104 A/cm2 in 1 T magnetic field at 20K and 77K, respectively. Moreover, at 20K, the Jc of the P(O2) = 1 atm sample is higher than that of the P(O2) = 0.21 atm sample in an applied magnetic field up to 5 T. At 77K, however, the Jc of the P(O2) = 1 atm sample drops more quickly than that of the P(O2) = 0.21 atm sample. The mechanisms of the different microstructures, oxygen contents and magnetic field dependence behaviors of Jc for the two kinds of samples have been discussed.