Papers by Author: G. Yan

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.