Phase formation of polycrystalline MgB_2 at low temperature using nanometer Mg powder

摘要

The MgB_2 superconductor synthesized in a flowing argon atmosphere using nanometer magnesium powder as the raw materials, denoted as Nano-MgB_2, has been studied by the technique of in situ high temperature resistance measurement (HT-RT measurement). The MgB_2 phase is identified to form within the temperature range of 430-490 ℃, which is much lower than that with the MgB_2 sample fabricated in the same gas environment using the micron size magnesium powder, denoted as Micro-MgB_2 and reported previously. The sample density of the Nano-MgB_2 reaches 1.7 g/cm~3 with a crystal porosity structure less than a μm, as determined by the scanning electron microscope (SEM) images, while the Micro-MgB_2 has a much more porous structure with corresponding density of 1.0 g/cm~3. This indicates that the Mg raw particle size, beside the sintering temperature, is a crucial factor for the formation of high density MgB_2 sample, even at the temperature much lower than that of the Mg melting, ～ 650 ?. The X-ray diffraction (XRD) pattern shows a good MgB_2 phase with small amount of MgO and Mg and the transition temperature, T_C, of the Nano-MgB_2 was determined to be 39 K by temperature dependent magnetization measurements (M-T), indicating good superconducting properties.