Formation Mechanism And Controlling Of Magnesium Diboride Phase

The researches on the formation mechanism and controlling of magnesium diboride (MgB2) have become more and more important for guiding the preparation of high quality MgB2 superconductors by optimzing the prepration process. In the present paper, polycrystalline MgB2 superconductor and Mg-Cu-B ternay alloy were prepared by traditional solid-state sintering method and high frequency induction metling method respectively. Moreover, the evolution mechanisms of MgB2 phase during the in-situ sintering process and in the undercooed Mg-Cu-B alloy melt were systematically explored by means of metallographic analysis, diffrerential thermal analysis, powders sintering theory, thermodynamics and kinetics ananlysis. Additionally, the evolutions of ex-situ and in-situ formed nano-dopants during in-situ sintering process, as well as, the effects on the formation process and superconductivity of polycrystalline MgB2 were clarified. More details were given as follows.A high-resolution Differerntial Thermal Analysis (DTA) apparatus was used to sinter the mixture of Mg and B powder and monitor the reaction process. Based on the classical sintering theory and multiple scan kinetic analysis method, the formation process and mechanism of polycrystalline MgB2 during in-situ sintering process were systemticlly investigated and the optimal sintering process was determined. It indicates that the reaction between Mg and B powders starts before Mg melting and not complete until the sintering temperature exceeds to 1023K, which is the best temperature to obtain MgB2 superconductor exhibiting the highest critical transition temperature (Tc). Solid-solid reaction stage (before Mg melting), the solution activated regions formed ahead at the contact areas between Mg and B particles by diffusion of atoms, then MgB2 phase precipitated from these regions. Furthermore, multiple scan kinetic analysis shows that the most probable mechanism function of this solid-solid reaction is Avrami–Erofeev equation, which represents that the formation of MgB2 phase is controlled by a mode of andom nucleation followed by instantaneous growth of nuclei. Therefore, the grains are in nano-scale at this stage. At the solid-liquid reaction stage (after Mg melting), the molten Mg promotes the reaction between Mg and residual B powders under the infilteration and enwrapping effects. Finally, the formed MgB2 grains grow up to regular hexagon morphology rapidly by Ostwald ripening mechanism.Based on the above observations, the doping effect of nano-Al2O3 powders on the formation of polycrystlline MgB2 and superconductivites were investigated. The results shows that nano-Al2O3 reacts with Mg to form nan-MgO and Al, which changes the formation sequence of MgB2, during the process of reaction bwteen Mg and B powder. The precipitated MgO particles restrict the growth of MgB2 grains and therefore increase the pinning effect of grain boundary. The MgO particles can also act as pinning centers to improve critical current density of MgB2, however, the pinning effect of MgO particles and superconductivity of MgB2 superconductor are destroyed following the increasement, local agglomeration and growth of MgO particles. Addtionally, Al atoms are partially substituted in the lattice of MgB2 at Mg sites to change the electronic structure of MgB2 and accordingly depress the critical transition temperature.The precipitation of MgO as scondary phase is inevitable during in-situ sintering process for preparation of polycrystalline MgB2 superconductor. The formation, evolution and effect on superconductivity of MgO phase were explored. It was found that the precipitation of nano-scale MgO particles can be controlled by adjusting the holding time during the solid-solid reaction stage. These nano MgO particles remain till the solid-liquid stage and act as effective pinning centers on the surfaces and boundaries of large MgB2 grains to improve its superconductivity.Eutectic Mg58.0Cu42.0 and Cu86.7B13.3 were adopted as precursors of Mg and B to prepare a ternary Mg15Cu75B10 alloy at a low temperature (1100℃) under ambient atmosphere by means of induce melting. Then, hyper-undercooling Mg15Cu75B10 alloys with different primary undercooling degrees were prepared by using cyclic overheating purification method. Based on the classical nucleation theory and transient nucleation theory, the thermodynamics and the kinetics for the competitive nucleation between MgB2 and MgB4 phase were calculated, and hence a critical condition for the formation of the MgB2 phase in hyper-undercooling Mg15Cu75B10 melt was proposed. The results indicate that MgB2 phase has smaller critical nucleation energy, higher nucleation rate and shorter incubation time comparing with MgB4 phase as the undercooling degree excesses to a critical value at a low undercooling range. Hence, the MgB2 phase nucleates primarily from the melt and grows up. However, in the high undercoolings (more than 223 K) range, both MgB2 and MgB4 phase have low nucleation rate and large incubation periods, so it is difficult for them to nucleate from undercooled melt.