Magnesium diboride
Encyclopedia
Magnesium diboride is a simple ionic
binary compound
that has proven to be an inexpensive and useful superconducting material.
Its superconductivity was announced in the journal Nature
in March 2001. Its critical temperature (Tc) of 39 K is the highest amongst conventional superconductor
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-mediated
) superconductor, it is a rather unusual one. 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 almost been achieved with two energy gaps. In 2001 it was regarded as behaving more like a metallic than a cuprate
superconductor.
and magnesium
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 diffusion
across boron grain boundaries. At conventional reaction temperatures, sintering
is minimal, although enough grain recrystallization occurs to permit Josephson quantum tunnelling
between grains.
Superconducting magnesium diboride wire can be produced through the 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 drawing
. The wire is then heated to the reaction temperature to form MgB2 inside. 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.
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.
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 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.
) can improve the upper critical field
and the maximum current density
(also with polyvinyl acetate
).
5% doping with carbon can raise Hc2 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 TiB2 can reduce the decrease. (Doping MgB2 with Ti is patented.)
The maximum critical current (Jc) in magnetic field is enhanced greatly by doping with ZrB2.
Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected.
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 Hc1.
In 2006 a 0.5 tesla open MRI
superconducting magnet
system was built using 18 km of MgB2 wires. This MRI used a closed-loop cryocooler
, without requiring externally supplied cryogenic liquids for cooling.
"...the next generation MRI instruments must be made of MgB2 coils instead of NbTi coils, operating in the 20–25 K range without liquid helium for cooling. ...
Besides the magnet applications MgB2 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.
Because of the low cost of its constituent elements, MgB2 has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads.
Ionic compound
In chemistry, an ionic compound is a chemical compound in which ions are held together in a lattice structure by ionic bonds. Usually, the positively charged portion consists of metal cations and the negatively charged portion is an anion or polyatomic ion. Ions in ionic compounds are held together...
binary compound
Binary compound
A binary compound is a chemical compound that contains exactly two different elements. Examples of binary ionic compounds include calcium chloride , sodium fluoride , and magnesium oxide , whilst examples of binary covalent compounds include water , carbon monoxide , and sulfur hexafluoride...
that has proven to be an inexpensive and useful superconducting material.
Its superconductivity was announced in the journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
in March 2001. Its critical temperature (Tc) of 39 K is the highest amongst conventional superconductor
Conventional superconductor
Conventional superconductors are materials that display superconductivity as described by BCS theory or its extensions.Critical temperatures of some simple metals:ElementTc Al1.20Hg4.15Mo0.92Nb9.26Pb7.19...
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-mediated
BCS theory
BCS theory — proposed by Bardeen, Cooper, and Schrieffer in 1957 — is the first microscopic theory of superconductivity since its discovery in 1911. The theory describes superconductivity as a microscopic effect caused by a "condensation" of pairs of electrons into a boson-like state...
) superconductor, it is a rather unusual one. Its electronic structure is such that there exist two types of electrons at the Fermi level
Fermi level
The Fermi level is a hypothetical level of potential energy for an electron inside a crystalline solid. Occupying such a level would give an electron a potential energy \epsilon equal to its chemical potential \mu as they both appear in the Fermi-Dirac distribution function,which...
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 almost been achieved with two energy gaps. In 2001 it was regarded as behaving more like a metallic than a cuprate
Cuprate
Cuprates are chemical compounds containing copper anion. Cuprates have been known for centuries and are widely used in inorganic and organic chemistry...
superconductor.
Synthesis
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between boronBoron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
and magnesium
Magnesium
Magnesium 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 and ninth in the known 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 diffusion
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
across boron grain boundaries. At conventional reaction temperatures, sintering
Sintering
Sintering is a method used to create objects from powders. It is based on atomic diffusion. Diffusion occurs in any material above absolute zero, but it occurs much faster at higher temperatures. In most sintering processes, the powdered material is held in a mold and then heated to a temperature...
is minimal, although enough grain recrystallization occurs to permit Josephson quantum tunnelling
Quantum tunnelling
Quantum tunnelling refers to the quantum mechanical phenomenon where a particle tunnels through a barrier that it classically could not surmount. This plays an essential role in several physical phenomena, such as the nuclear fusion that occurs in main sequence stars like the sun, and has important...
between grains.
Superconducting magnesium diboride wire can be produced through the 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 drawing
Wire drawing
Wire drawing is a metalworking process used to reduce the cross-section 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...
. The wire is then heated to the reaction temperature to form MgB2 inside. 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.
Hybrid physical-chemical vapor deposition
Hybrid Physical-Chemical Vapor Deposition
Hybrid 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 (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
Josephson effect
The Josephson effect is the phenomenon of supercurrent across two superconductors coupled by a weak link...
, 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 superconductorType-II superconductorA Type-II superconductor is a superconductor characterized by the formation of vortex lattices in magnetic field. It has a continuous second order phase transition from the superconducting to the normal state within an increasing magnetic field....
, 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 fieldUpper critical field-Upper critical field:The upper critical field is 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 fibers 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 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 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 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.
Improvement by doping
Various means of doping MgB2 with carbon (e.g. using 10% malic acidMalic acid
Malic acid is an organic compound with the formula HO2CCH2CHOHCO2H. It is a dicarboxylic acid which is made by all living organisms, contributes to the pleasantly sour taste of fruits, and is used as a food additive. Malic acid has two stereoisomeric forms , though only the L-isomer exists...
) can improve the upper critical field
Upper critical field
-Upper critical field:The upper critical field is the magnetic field which completely suppresses superconductivity in a Type II superconductor at 0K ....
and the maximum current density
(also with polyvinyl acetate
Polyvinyl acetate
Polyvinyl acetate, PVA, PVAc, poly, is a rubbery synthetic polymer with the formula n. It belongs to the polyvinyl esters family with the general formula -[RCOOCHCH2]-...
).
5% doping with carbon can raise Hc2 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 TiB2 can reduce the decrease. (Doping MgB2 with Ti is patented.)
The maximum critical current (Jc) in magnetic field is enhanced greatly by doping with ZrB2.
Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected.
Thermal conductivity
MgB2 is a multi-band superconductor, that is each Fermi surface has different superconducting energy gap. For MgB2, 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 Hc1.
Possible applications
Superconducting properties and cheapness make magnesium diboride attractive for a variety of applications. For those applications, MgB2 powder is compressed with silver metal into tape via the PIT process.In 2006 a 0.5 tesla open MRI
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
superconducting magnet
Superconducting magnet
A superconducting magnet is an electromagnet made from coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. In its superconducting state the wire can conduct much larger electric currents than ordinary wire, creating intense magnetic fields...
system was built using 18 km of MgB2 wires. This MRI used a closed-loop cryocooler
Cryocooler
Cryocoolers 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...
, without requiring externally supplied cryogenic liquids for cooling.
"...the next generation MRI instruments must be made of MgB2 coils instead of NbTi coils, operating in the 20–25 K range without liquid helium for cooling. ...
Besides the magnet applications MgB2 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.
Because of the low cost of its constituent elements, MgB2 has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads.
External links
- Essential Science Indicators on MgB2 (1992 – May 2002)
- Old material makes a new debut, US Department of Energy Research News, 2001