Research On The MgB₂ Reaction Phase Transition And The Growth Process Of The Crystal Nuclei

Since the discovery of the superconductivity with a critical temperature(J_c)39 K in 2001,MgB₂ has been believed to be a promising candidate for engineering applications in the temperature range of 20 K～30 K.However, compared with traditional low temperature superconductors,first, the critical current density of MgB₂ is lower, and MgB₂ often shows a rapid decrease in the critical current density with the increase of external magnetic field(H).Second, the microscopic mechanism of MgB₂ phase formation is still unclear, the latter is basic. Therefore, the lower critical current density and unintelligible phase formation process restrict the application of MgB₂ superconductor. Thus, in this paper, the reaction thermodynamics of the Mg-B system has been analyzed based on thermodynamic theory and the experiment of the SHS method prepareing MgB₂ in macroscopic aspect, and phase formation process has been analyzed by using the basic properties of magnesium and boron, the hybrid orbital theory, the frontier orbital theory and the theory of powder reaction in microscopic aspect. Aiming at to understand further superconductor MgB₂ .First, according to the experiment data of the SHS methode prepareing MgB₂ , the reaction thermodynamics of the Mg-B system has been analyzed by using thermodynamic theory, the results show that the change of the Gibbs free energy of the formation reaction of MgB₂, MgB4 is negative in the temperature range of 298K to 2000K, and the larger negative value, the easier formation reaction of MgB₂ occurs. Therefore we must control the synthesis condition to ensure adequate content of Mg in order to avoid the formation of non-superconductor MgB4. In the Mg-B system , the adiabatic temperature Tad =1614K <1800K, so the light needs to warm up before the SHS reaction, and preheating temperature can not be less than 501K. The relationship between the adiabatic temperature and preheat temperature show that the adiabatic temperature rises with the preheat temperature increasing.Second, The formation of the crystal nuclei of MgB₂ superconductor and the growth process of that are mainly analyzed based on the basic properties of magnesium and boron, the hybrid orbital theory, the frontier molecular orbital theory and the theory of powder reaction, which can be divided into three steps. First, two kinds of powder particle contact together and vibrate with opposite phase to generate the region where the crystal nucleus form and grow. Magnesium vapor contacts boron powder particle to react and generate MgB₂, magnesium vapor comes into the MgB₂ phase formation region to participate the reaction. The distribution of the two kinds of atom in the phase formation region is more favorable to the reaction. Second, two boron atoms react to generate B2 with sp² hybrid obits, the two valence electrons of magnesium atom fill theÏ€orbit of B₂ to formÏ€bond, as a result, the initial crystal nuclei of MgB₂ superconductor has been formed. Simple skeleton of the initial nuclei: there is aσbond between the two boron atoms, four exposed half full hybrid orbits lie on both sides of B₂ respectively, magnesium ion lies above the filledÏ€bond. The electromagnetic properties of the initial nuclei: Having a paramagnetism because of the unpaired electron in the four exposed half full hybrid orbits. There is a distribution of positive electric field near the space above the magnesium ion and a distribution of negative electric field near the space below theσbond. So, the electric field distribution of the initial crystal nuclei is not asymmetric. Third, in determined direction of the hybrid orbit plane, the initial crystal nucleus contact and react constantly each other along a-axis and c-axis respectively to make the growth of the nucleus along these three axes,the electrons in the four exposed half full hybrid orbits are paired and paramagnetism disappears, theÏ€bond evolves intoÏ€63, and magnesium ion is located the center of hexagonal boron by the neighbor electrons, crystal grains are formed finally. Solid-liquid interface may be more conducive to the formation of larger crystal grains. Larger grains may present and MgB4 and MgB7 may be formed by the initial crystal nuclei, at higher temperature.This phase formation process may further be tested based on the electromagnetic properties of the initial nuclei.