Magnesium diboride (MgB
2) is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.
Its superconductivity was announced in the journal
NatureNature is a prominent British scientific journal, first published on 4 November 1869. Most scientific journals are now highly specialized, and Nature is among the few journals that still publish original research articles across a wide range of scientific...
in March 2001. Its critical temperature (
Tc) of is the highest amongst
conventional superconductorConventional superconductors are materials that display superconductivity as described by BCS theory or its extensions.Critical temperatures of some simple metals:...
s. This material was first synthesized and its structure confirmed in 1953, but its superconducting properties were not discovered until 2001. The discovery caused great excitement.
Though generally believed to be a conventional (
phonon-mediatedBCS theory is the first microscopic theory of superconductivity, proposed by Bardeen, Cooper, and Schrieffer in 1957 since the discovery of superconductivity in 1911. It describes superconductivity as a microscopic effect caused by a condensation of pairs of electrons into a boson-like...
) superconductor, it is a rather unusual one. Its electronic structure is such that there exist two types of electrons at the
Fermi levelThe Fermi level is an energy that pertains to electrons in a semiconductor. It is the chemical potential that appears in the electrons' Fermi-Dirac distribution function,...
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 MgB
2 has almost been achieved with two energy gaps. In 2001 it was regarded as behaving more like a low-
Tc metallic than a high-
Tc cuprateCuprates are chemical compounds containing copper oxide. From 1986 to 2008 almost all known high temperature superconductors were cuprate superconductors....
superconductor.
Synthesis
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between
boronBoron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite....
and
magnesiumMagnesium is a chemical element with the symbol Mg, atomic number 12 and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the earth's crust by mass, although ninth in the Universe as a whole...
powders. Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction mechanism is considered to be moderated by magnesium vapor
diffusionMolecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material...
across boron grain boundaries. At conventional reaction temperatures,
sinteringSintering is a method for making objects from powder, by heating the material in a sintering furnace below its melting point until its particles adhere to each other...
is minimal, although enough grain recrystallization occurs to permit Josephson
quantum tunnellingWave-mechanical tunneling is an evanescent wave coupling effect that occurs in the context of quantum mechanics because the behaviour of particles is governed by Schrödinger's wave-equation. All wave equations exhibit evanescent wave coupling effects if the conditions are right...
between grains.
Superconducting magnesium diboride wire can be produced through the
powder-in-tubePowder-in-tube is a process used for making electrical conductors from brittle superconducting materials such as Niobium-tin,BSCCO, or magnesium diboride.PIT is sometimes called oxide powder in tube ....
(PIT) process. In the
in situ variant, a mixture of boron and magnesium is poured into a metal tube, which is reduced in diameter by conventional
wire drawingWire drawing is a metalworking process used to reduce the diameter of a wire by pulling the wire through a single, or series of, drawing die. There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for...
. The wire is then heated to the reaction temperature to form MgB
2 inside. In the
ex situ variant, the tube is filled with MgB
2 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.
Hybrid Physical-Chemical Vapor DepositionHybrid physical-chemical vapor deposition is a thin-film deposition technique that combines physical vapor deposition with chemical vapor deposition...
(HPCVD) has been the most effective technique for depositing magnesium diboride (MgB
2) thin films. The surfaces of MgB
2 films deposited by other technologies are usually rough and non-stoichiometric. Instead, the HPCVD system can grow high-quality
in situ pure MgB
2 films with smooth surfaces, which are required to make reproducible uniform
Josephson junctionsThe Josephson effect is the phenomenon of current flow across two weakly coupled superconductors, separated by a very thin insulating barrier. This arrangement—two superconductors linked by a non-conducting barrier—is known as a Josephson junction; the current that crosses the barrier is the...
, 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
A Type-II superconductor is a superconductor characterised by its gradual transition from the superconducting to the normal state within an increasing magnetic field. Typically they superconduct at higher temperatures and magnetic fields than Type-I superconductors...
, i.e. increasing magnetic fields 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
-Upper critical field:The magnetic field which completely suppresses superconductivity in a Type II superconductor at 0K ....
(Hc2): (parallel to ab planes) is ~14.8 T, (perpendicular to ab planes) ~3.3 T, in thin films up to 74 T, in fibres up to 55 T .
Semi-Meissner state
Using the BCS theory and the known energy gaps of the pi and sigma bands of electrons, which are 2.2 and 7.1 meV, the pi and sigma bands of electrons have been found to have two different coherence lengths, 51 nm and 13 nm . The corresponding London penetration depths are 33.6 nm and 47.8 nm. This implies that the Ginzburg-Landau constants are 0.66±0.02 and 3.68 respectively. The first is less than 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 short distances and repel at long 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 MgB
2 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 superconductivityThe term type-1.5 superconductor refers to a superconductor which has magnetic propertiesdistinct from type-I and type-II.Traditionally superconductors were divided into two classes: type-I and type-II according to their behavior in magnetic field....
was coined for this state.
Improvement by doping
Various means of doping MgB
2 with carbon (e.g. using 10% malic acid) can improve the upper critical field and the maximum current density
(with
polyvinyl acetatePolyvinyl acetate is a rubbery synthetic polymer with the formula n.-Preparation:Polyvinyl acetate is a rubbery synthetic polymer....
).
5% doping with carbon can raise H
c2 from 16 T to 36 T whilst lowering
Tc only from 39 K to 34 K. The maximum critical current (
Jc) is reduced, but doping with TiB
2 can reduce the decrease. (Doping MgB
2 with Ti is patented .)
The maximum critical current (
Jc) in magnetic field is enhanced greatly by doping with ZrB
2 .
Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected.
Thermal conductivity
MgB
2 is a multi-band superconductor, that is each Fermi surface has different superconducting energy gap. For MgB
2, sigma bond of boron is strong, and it induces large s-wave superconducting gap, and pi bond is weak and induces small s-wave gap.
The quasiparticle states of the vortices of large gap are highly confined to the vortex core.
On the other hand, the quasiparticle states of small gap are loosely bound to the vortex core. Thus they can be delocalized and overlap easily between adjacent vortices.
Such delocalization can strongly contribute to the thermal conductivity, which shows abrupt increase above H
c1Applications
Superconducting properties and cheapness make magnesium diboride useful for a variety of applications.
[http://www.iop.org/EJ/article/0953-2048/20/1/R01/sust7_1_R01.pdf Prospects for MgB2 superconductors for magnet application. Vinod et al. 2006. 13pp]
For those applications, MgB
2 powder is compressed with silver metal into tape via the
PITPowder-in-tube is a process used for making electrical conductors from brittle superconducting materials such as Niobium-tin,BSCCO, or magnesium diboride.PIT is sometimes called oxide powder in tube ....
process
In 2006 a 0.5 tesla open
MRIMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
superconducting magnetA superconducting magnet is an electromagnet that is built using coils of superconducting wire. They must be cooled to cryogenic temperatures during operation...
system was built using 18 km of MgB
2 wires. This MRI used a closed-loop
cryocoolerCryocoolers are the devices used to reach cryogenic temperatures by cycling certain gases.A cryostat is likely to be used to reach and/or maintain similar conditions or keep some environment in cryogenic stasis.-See also:* Cryogenic processor...
, without requiring externally supplied cryogenic liquids for cooling.
"...the next generation MRI instruments must be made of MgB
2 coils instead of NbTi coils, operating in the 20–25 K range without liquid helium for cooling. ...
Besides the magnet applications MgB
2 conductors have potential uses in superconducting transformers, rotors and transmission cables at temperatures of around 25 K, at fields of 1 T."
Thin coatings can be used in superconducting radio frequency cavities to minimize energy loss and reduce the inefficiency of liquid helium cooled niobium cavities.
Due to the low cost of its constituent elements, MgB
2 has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads.
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