Separation Of The MgB₂ Superconducting Phase From The Undercooled Mg-Cu-B Alloy Melt

The discovery of superconductivity in magnesium diboride (MgB₂) at around 39 K, which is the highest one in the known superconductors of intermetallic compound, has drawn much attention of researchers around the world because of its wide advantages as small anisotropic, large coherence lengths and easy formation process. In order to solve the problems of high vapor pressure and oxidation of Mg, and overcome disadvantages of strict requirements of experimental conditions and operation parameters in the traditional preparation of the single-crystal MgB₂, a new method of using two kinds of eutectic Mg-Cu and Cu-B alloys to synthesize ternary Mg₂₅Cu₆₅B₁₀ alloy by inductive melting technology was present in this work. The results showed that the MgB₂ phase with a maximum dimension about 40μm separated out from the Mg-Cu-B alloy melt directly, and the formed MgB₂ phase growed to a maximum dimension of about 110μm after heat treatment. It has been proved thermodynamically that the MgB₂ phase nucleates and grows from the melt as primary phase.Rapid solidification leads to a different structure in the solidified alloys, whereas the primary undercooling of alloy melt plays a crucial role on the phase evolution of solidified structure. Undercooled experiments would strongly adjust on the separation sequences of the MgB₂ phase from the alloy melt. Here different undercooling degrees of the explored Mg₂₅Cu₆₅B₁₀ alloy were acquired by cyclic inductive superheating, the corresponding phase evolution in the solidied structures were systematically explored. It is found that the solidified structures with low undercoolings were composed of MgB4 phase, Cu2Mg phase and Mg2Cu phase. When the undercooling increases to a critical level, the MgB₂ phase nucleates directly from the melt instead of MgB4 phase. When the primary undercooling was increased to a higher level, no Mg-B phase form from the undercooled alloy melt.Based on the calculation of the classic nucleation theory and transient nucleation theory, it indicates that when the undercooling of Mg₂₅Cu₆₅B₁₀ melt is higher than a critical value in the low undercoolings range, both the critical nucleation energy and the incubation time of the MgB₂ phase are less than that of MgB4 phase, the nucleation rate of the MgB₂ is higher than that of the MgB4 phase. So the MgB₂ phase nucleates directly from the melt. In the meanwhile, in the high undercoolings (larger 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. The above calculation results fits well with the experimental observations.