The Research On Synthesis And Mechanism Of Combustion Synthesis Of Superconductor MgB₂

Combustion synthesis method shows its unique advantage in preparation for MgB₂ superconductor However, the knowledge of the reaction process and mechanism during combustion is extremely limited because of the unique Mg-B system and the rapid reaction process and high synthesis temperature. So the research on the mechanism of self-propagating combustion syhthesis MgB₂ has its important significance in both the perfection of the combustion method itself and extensive application of MgB₂.In this paper, a relatively general research on the weak exothermic reaction system of MgB₂ has been made from thermodynamics and structure dynamics. The relationships among the preheat temperature and adiabatic temperature are definite through the calculation of adiabatic temperature under ideal state.The pure intermetallic MgB₂ are syhthesis by self-propagating combustion mode and thermal explosion mode respectively, between which a comparison of the reaction process has been made, and the different technical demands are definite. The effect of different technical parameters in preparation for MgB₂ during thermal explosion reaction mode on the phase compose and morphology of product has been studied, these parameters include preheat temperature, preheating velocity, temperature holding time, pressure, Mg granularity and diluent. On this basis, an optimal technical parameters scheme of combustion synthesis MgB₂ has been summed up. The mechanism of thermal explosion synthesis in MgB₂ could be described with a melt and dissolution-precipitation mold. The reaction between Mg and B particles started with the melting Mg.A further experiment of Combustion Front Quenching (CFQ) was brought to success in cylindrical steel mould by adding some Al-Ti-C powder, which is accepted high exothermic system on the top of Mg-B samples. The physical and chemical changes of all regions in the quenched samples during the combustion synthesis were followed by scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron probe microscopy analysis (EPMA). The results show that the synthesis of MgB₂ was directly rather than through other intermediates and the process of synthesis MgB₂ can be expressed as: (i) solid combustion in the lower temperature;(ii)Mg melt in the preheat phase;(iii)liquid combustion in the combustion process;(iv)cooling and crystal of the product after the reaction;(v)uniformity of the composition in the heat preservation process. Themechanism of self-prapagating high-temperature synthesis in MgB2 could be described with a solid-state diffusion and melt and dissolution-precipitation mold. The SHS between Mg and B particles started with the solid-state diffusion reaction firstly, which resulted in the partical melt of Mg and then proceeded by a dissolution-precipitation mechanism. Namely the solid-state diffusion reaction between Mg and B particles at lower temperature led to the partical melt of Mg particles on the Mg/B boundary, which resulted in the dissolving of B particle into the Mg liquid, at the same time the eutectic reaction occurred, as the B content increased, the MgB2 grains then precipitated when B content in the Mg-MgB2 entectic was saturated.