Study On Reaction Process And Mechanism Analysis Of MgB₂ Superconductor

The discovery of superconductivity in magnesium diboride (MgB₂) at around 39K, which is the highest one in the known intermetallic compound superconductors, has drawn much attention of researchers around the world because of its wide advantages as small anisotropic, large coherence lengths and easy formation. By far, the researches of MgB₂ are mainly about the preparation of wires and tapes, but there is a lack of uniform standpoint about formation process and growth mechanism of polycrystalline MgB₂. Here, in this work, the systematic researches on formation condition and growth mechanism of MgB₂ superconductors are performed by Differential Thermal Analysis (DTA) during sintering process, which are desired to give an important theoretical basis for preparing high quality MgB₂ superconductors.The DTA curves showed that the reaction between Mg and B powders started at about 530℃and two exothermic peaks, which appeared before and after melting point of pure Mg, are corresponding to two different reaction mechanisms. Hence, different peak points and transition points, which were corresponding to different stage, were chosen to identify the phase formation process. Combined with SEM and XRD analysis, it was found that the nucleration rates of MgB₂ phase increased with increasement of temperature at the solid state reaction stage before 650℃, and then the nucleus of MgB₂ gown up to regular hexagon structure gradually at the solid-liquid reaction stage after 650℃.Generally, preparation of high quality polycrystalline MgB₂ is performed at low temperature to avoid voliatilization and oxidation of Mg. In this work, five low temperatures of 510℃, 530℃, 600℃, 650℃and 750℃, are selected as isothermal points to explore the effect of holding times on crystal morphology of formed MgB₂ phase. It was found that increasing isothermal time is also favorable for nucleation at the solid state reaction stage before 650℃, but the growth of MgB₂ grain is restricted by pinning action of pore. Additionally, solid-liquid sintering is benefit to decrease porosity of polycrystalline MgB₂ samples and obtain regular hexagon crystal morphology MgB₂ phase, which is the determinative factor of Tc . The values of Tc are closer to ideal one with the increase of temperature and holding time at given temperature.MgB₂ crystal whisker forms during solid-liquid sintering process: Firstly, the planar crystal nucleus formed in advance as the media supersaturation; Secondly, the planar crystal nucleus are prior to grow along the crystal plane of (101) adherent to existed sidesteps; Thirdly, other elementaries deposited at the sidesteps to realize secondary thickening; Then, the MgB2 crystal whiskers will form rapidly by iterative process of the three steps metioned above. The superconducting transition temperature ( Tc ) of the sample is about 38.5K showed that it has high density and obvious crystal structure, which provides a new visual angle for the discovery of MgB2 crystal whisker.