Magnesium diboride (MgB2) is an inorganic compound made of two elements, boron, and magnesium, that are abundant in the Earth’s crust. It is a water-insoluble dark grey solid. Over the past few years, MgB2 has progressed from a remarkable discovery to a promising, applied superconductor.
Magnesium Diboride: An Unusual Semiconductor
Magnesium diboride is a superconductor with a transition temperature of 39 K (−234 °C), one of the highest recorded transition temperatures (Tc). Although the compound was known for decades, its superconducting properties were discovered in 2001, primarily described by BCS theory.
The electronic structure of MgB2 is such that there are two types of electrons at the Fermi level with very different behavioral traits, one of which (sigma-bonding) is much more strongly superconducting than the other (pi-bonding).
This contradicts conventional theories of phonon-mediated superconductivity, which believe that all electrons act in the same way. Theoretical understanding of MgB2 properties has been almost attained by modeling two energy gaps. In 2001, it was thought to behave very much like a metallic than a cuprate superconductor.
Magnesium diboride has both pi and sigma electron bands. Using the BCS theory and known energy gaps, it has been found that the coherence lengths of these two bands are different, with the pi band having a length of 51 nm and the sigma band having a length of 13 nm. This difference in coherence length corresponds to a difference in London penetration depth, with the pi band having a depth of 33.6 nm and the sigma band having a depth of 47.8 nm.
The Ginzburg-Landau parameters are thus 0.66±0.02 and 3.68, respectively. Because the first is less than 1/√2, and the second is greater, the first clearly indicates marginal type-I superconductivity, and the second strongly suggests type-II superconductivity.
It’s been predicted that when two very different electron bands produce two quasiparticles, one with a coherence length indicating type I superconductivity and the other with a coherence length indicating type II superconductivity, vortices will attract at large distances and repel at small distances. At a critical distance, the potential energy among vortices is minimized.
As a result, a new phase called the semi-Meissner state has been proposed, in which the critical distance isolates vortices. When the applied flux is insufficient to fill the entire superconductor with vortices lattice isolated by the critical distance, large areas of type I superconductivity, a Meissner state, separate these domains.
This hypothesis has recently received experimental confirmation in MgB2 experiments at 4.2 Kelvin. The authors discovered that there are regimes with a much higher density of vortices.
While the typical spacing variation between Abrikosov vortices in a type II superconductor is 1%, they discovered a variation of the order of 50%, consistent with the idea that vortices assemble into domains isolated by the critical distance. For this state, the term type-1.5 superconductivity has been coined.
Preparation of Magnesium Diboride
Magnesium diboride is a simple binary compound whose structure was confirmed in 1953. It is synthesized via high-temperature reactions between magnesium and boron powders. However, because magnesium metal melts at 652 °C, the reaction may involve the diffusion of magnesium vapor across boron grain boundaries. Sintering is minimal at conventional reaction temperatures, but grain recrystallization is enough for Josephson quantum tunneling among grains.
Superconducting magnesium diboride wires are made through the powder-in-tube (PIT) process, which involves filling a tube with magnesium and boron powder, reducing the diameter of the tube, and then heating it to the reaction temperature.
This process can be carried out in situ (while the magnesium and boron remain in the tube) or ex-situ (after the magnesium and boron have been formed into a wire). Hot isostatic pressing at nearly 950 °C enhances the properties of the MgB2 wire in both cases.
Electromagnetic Properties of Magnesium Diboride
- Magnesium diboride has a very high critical temperature of 39K, meaning that it can remain superconducting even at extremely high temperatures.
- MgB2 is also a type-II superconductor, meaning that it can withstand increasing magnetic fields without losing its superconductivity.
- MgB2 has an extremely high critical current, 105 A/m2 at 20 T, 106 A/m2 at 18 T, 107 A/m2 at 15 T, 108 A/m2 at 10 T, and 109 A/m2 at 5 T.
- Magnesium diboride has a relatively high upper critical field. In thin films and fibers, it exhibits superconductivity up to 74 T and 55 T, respectively.
Thermal Conductivity of Magnesium Diboride
MgB2 is a multi-band superconductor, which means the superconducting energy gap varies depending on the Fermi surface. The sigma bond of boron in MgB2 is strong, and it induces a large s-wave superconducting gap, whereas the pi bond is weak and only induces a small s-wave gap.
The quasiparticle states of the large gap vortices are tightly confined to the vortex core. The quasiparticle states of the small gap, on the other hand, are connected to the vortex core and can easily delocalize and overlap. This delocalization significantly contributes to MgB2’s thermal conductivity.
Other Applications for Magnesium Diboride
- Magnesium diboride (MgB2) is a promising material for fuel in ramjets and as an ingredient in blast-enhanced explosives and propellants. This is due to its ability to burn completely when ignited in oxygen or mixtures with oxidizers.
- Most recently, it has been demonstrated that decoy flares containing magnesium diboride/Teflon/Viton exhibit 30–60% higher spectral efficiency, Eλ (J g−1sr−1), compared to Magnesium/Teflon/Viton (MTV) payloads.
- Magnesium diboride has shown promise as a potential fuel for hybrid rocket propulsion. When mixed with paraffin wax, it can improve the fuel grain’s combustion characteristics and mechanical properties.
- The MRI superconducting magnet system was built in 2006 using 18 km of MgB2 wires. The MRI used a closed-loop cryocooler, which did not require cryogenic liquids to be supplied externally. The system was created for medical imaging applications.
- MgB2 is a superconducting material with significant potential in power applications and electronic devices. MgB2-based power cables, microwave devices, and commercial MRI machines have all emerged in the last 15 years. Superconducting radio frequency (SRF) cavities are the next frontier for MgB2. SRF cavities are essential for high-energy physics research and are used in particle accelerators.
- MgB2 is used in superconducting low to medium field magnets, electric motors and generators, fault current limiters, and current leads due to the low cost of its constituent elements.