Magnesium diboride is the inorganic compound of magnesium and boron with the formula MgB2. It is a dark gray, water-insoluble solid. The compound becomes superconducting at 39 K (−234 °C), which has attracted attention. In terms of its composition, MgB2 differs strikingly from most low-temperature superconductors, which feature mainly transition metals. Its superconducting mechanism is primarily described by BCS theory.
Superconductivity Magnesium diboride's superconducting properties were discovered in 2001. Its critical temperature (Tc) of 39 K (−234 °C; −389 °F) is the highest amongst conventional superconductors. Among conventional (phonon-mediated) superconductors, it is unusual. Its electronic structure is such that there exist two types of electrons at the Fermi level with widely differing behaviours, one of them (sigma-bonding) being much more strongly superconducting than the other (pi-bonding). This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner. Theoretical understanding of the properties of MgB2 has nearly been achieved by modelling two energy gaps. In 2001 it was regarded as behaving more like a metallic than a cuprate superconductor.
Semi-Meissner state Using BCS theory and the known energy gaps of the pi and sigma bands of electrons (2.2 and 7.1 meV, respectively), the pi and sigma bands of electrons have been found to have two different coherence lengths (51 nm and 13 nm, respectively). The corresponding London penetration depths are 33.6 nm and 47.8 nm. This implies that the Ginzburg-Landau parameters are 0.66±0.02 and 3.68, respectively. The first is less than 1/√2 and the second is greater, therefore the first seems to indicate marginal type I superconductivity and the second type II superconductivity. It has been predicted that when two different bands of electrons yield two quasiparticles, one of which has a coherence length that would indicate type I superconductivity and one of which would indicate type II, then in certain cases, vortices attract at long distances and repel at short distances. In particular, the potential energy between vortices is minimized at a critical distance. As a consequence there is a conjectured new phase called the semi-Meissner state, in which vortices are separated by the critical distance. When the applied flux is too small for the entire superconductor to be filled with a lattice of vortices separated by the critical distance, then there are large regions of type I superconductivity, a Meissner state, separating these domains. Experimental confirmation for this conjecture has arrived recently in MgB2 experiments at 4.2 Kelvin. The authors found that there are indeed regimes with a much greater density of vortices. Whereas the typical variation in the spacing between Abrikosov vortices in a type II superconductor is of order 1%, they found a variation of order 50%, in line with the idea that vortices assemble into domains where they may be separated by the critical distance. The term type-1.5 superconductivity was coined for this state.
Synthesis Magnesium diboride was synthesized and its structure confirmed in 1953. The simplest synthesis involves high temperature reaction between boron and magnesium powders. Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction may involve diffusion of magnesium vapor across boron grain boundaries. At conventional reaction temperatures, sintering is minimal, although grain recrystallization is sufficient for Josephson quantum tunnelling between grains. Superconducting magnesium diboride wire can be produced through the powder-in-tube (PIT) ex situ and in situ processes. In the in situ variant, a mixture of boron and magnesium is reduced in diameter by conventional wire drawing. The wire is then heated to the reaction temperature to form MgB2. In the ex situ variant, the tube is filled with MgB2 powder, reduced in diameter, and sintered at 800 to 1000 °C. In both cases, later hot isostatic pressing at approximately 950 °C further improves the properties. An alternative technique, disclosed in 2003, employs reactive liquid infiltration of magnesium inside a granular preform of boron powders and was called Mg-RLI technique. The method allowed the manufacture of both high density (more than 90% of the theoretical density for MgB2) bulk materials and special hollow fibers. This method is equivalent to similar melt growth based methods such as the Infiltration and Growth Processing method used to fabricate bulk YBCO superconductors where the non-superconducting Y2BaCuO5 is used as granular preform inside which YBCO based liquid phases are infiltrated to make superconductive YBCO bulk. This method has been copied and adapted for MgB2 and rebranded as Reactive Mg Liquid Infiltration. The process of Reactive Mg Liquid Infiltration in a boron preform to obtain MgB2 has been a subject of patent applications by the Italian company Edison S.p.A. Hybrid physical–chemical vapor deposition (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB2) thin films. The surfaces of MgB2 films deposited by other technologies are usually rough and non-stoichiometric. In contrast, the HPCVD system can grow high-quality in situ pure MgB2 films with smooth surfaces, which are required to make reproducible uniform Josephson junctions, the fundamental element of superconducting circuits.
Electromagnetic properties Properties depend greatly on composition and fabrication process. Many properties are anisotropic due to the layered structure. 'Dirty' samples, e.g., with oxides at the crystal boundaries, are different from 'clean' samples.
The highest superconducting transition temperature Tc is 39 K. MgB2 is a type-II superconductor, i.e. increasing magnetic field gradually penetrates into it. Maximum critical current (Jc) is: 105 A/m2 at 20 T, 106 A/m2 at 18 T, 107 A/m2 at 15 T, 108 A/m2 at 10 T, 109 A/m2 at 5 T. As of 2008 : Upper critical field (Hc2): (parallel to ab planes) is ~14 T, (perpendicular to ab planes) ~3 T, in thin films up to 74 T, in fibers up to 55 T.
… excerpt ends here. Continue reading the full article.
