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| Magnesium diboride | |
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| Identifiers | |
| CAS number | [12007-25-9] |
| Properties | |
| Molecular formula | MgB2 |
| Molar mass | 45.93 g/mol |
| Density | 2.6 g/cm3 |
| Melting point |
1300 °C (decomp.) |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox references |
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Magnesium diboride (MgB2) is an inexpensive and simple superconductor. Its superconductivity was announced in the journal Nature in March 2001.[1] Its critical temperature (Tc) of 39 K is the highest amongst conventional superconductors. This material was first synthesized and its structure confirmed in 1953 [2] but its superconducting properties were not discovered until 2001 when they 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 being much more strongly superconducting than the other. This is at odds with usual theories of phonon-mediated superconductivity which assume that all electrons behave in the same manner. For this reason, theoretical understanding of the properties of MgB2 has not yet been achieved, particularly so in the presence of a magnetic field. In 2001 it was regarded as behaving more like a low-Tc metallic than a high-Tc cuprate superconductor [3] .
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Synthesis
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between boron and magnesium powders [3]. 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.[4] The surfaces of MgB2 films deposited by other technologies are usually rough and non-stoichiometric. Instead, 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, eg with oxides at the crystal boundaries, are different than 'clean' samples [5].
Tc up to 39 K.
It is a Type-II superconductor in that increasing magnetic fields gradually penetrate.
Max critical current (Jc) in magnetic fields : 10^5 A/m2 at 20T, 10^6 A/m2 at 18T, 10^7 A/m2 at 15T, 10^8 A/m2 at 10T, 10^9 A/m2 at 5T [5].
As of 2008 : Upper critical field (Hc2): (parallel to ab planes) approx 14.8 tesla, (perpendicular to ab planes) approx 3.3 tesla, thin films up to 74 tesla, in fibres up to 55 tesla [5].
Improvement by doping
Various means of doping MgB2 with carbon (eg using 10% malic acid) can improve the upper critical field and the maximum current density [6] [7] (with PVA (Poly Vinyl Acetate))[8].
5% doping with carbon can raise Hc2 from 16T to 36T whilst only lowering Tc from 39K to 34K [9]. Sadly the max critical current (Jc) is reduced but doping with TiB2 can reduce the decrease (see prev ref). (Doping MgB2 with Ti is patented [10].)
The max critical current (Jc) in magnetic fields are enhanced greatly by doping with ZrB2 [11].
Applications
Its superconducting properties and cheapness make magnesium diboride useful for a variety of applications [12].
In 2006 a 0.5 tesla open MRI superconducting magnet system was built using 18 km of MgB2 conductors. This MRI used a closed-loop cryocooler, without requiring externally supplied cryogenic liquids for cooling.[13][14]
"...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"[12].
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, MgB2 has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leadscitation needed.
References
- ^ Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani and Jun Akimitsu (1 Mar 2001). "Superconductivity at 39 K in magnesium diboride" (letter). Nature 410: 63–64. doi:.
- ^ Morton E. Jones and Richard E. Marsh (5 Mar 1954). "The Preparation and Structure of Magnesium Boride, MgB2". Journal of the American Chemical Society 76: 1434–1436. doi:.
- ^ a b http://arxiv.org/abs/cond-mat/0102216 [cond-mat/0102216] Strongly linked current flow in polycrystalline forms of the new superconductor MgB2
- ^ X.X. Xi et al, (14 February 2007). "MgB2 thin films by hybrid physical-chemical vapor deposition". Physica C 456: 22–37. doi:.
- ^ a b c http://www.iop.org/EJ/article/0953-2048/20/12/R01/sust7_12_R01.pdf Magnetic properties and critical currents of MgB2. M Eisterer 2007 27pp 1.5MB
- ^ Significant enhancement of Hc2 and Hirr in MgB2+C4H6O5 bulks at a low sintering temperature of 600 °C. M S A Hossain, J H Kim, X Xu, X L Wang, M Rindfleisch, M Tomic, M D Sumption, E W Collings and S X Dou. Supercond. Sci. Technol. 20 No 8 (August 2007) L51-L54 http://www.iop.org/EJ/article/0953-2048/20/8/L03/sust7_8_L03.pdf (18 refs)
- ^ http://www.iop.org/EJ/article/0953-2048/20/6/L02/sust7_6_L02.pdf The excellent superconducting properties of in situ powder-in-tube processed MgB2 tapes with both ethyltoluene and SiC powder added. H Yamada, N Uchiyama, A Matsumoto, H Kitaguchi and H Kumakura. Supercond. Sci. Technol. 20 No 6 (June 2007) L30-L33 (16 refs)
- ^ http://arxiv.org/abs/0708.3885 Carbon doping MgB2 with PVA
- ^ http://www.azom.com/details.asp?articleID=2542 MgB2 Properties Enhanced by Doping with Carbon Atoms
- ^ http://www.freepatentsonline.com/6953770.html US Patent 6953770
- ^ http://scholar.ilib.cn/Abstract.aspx?A=kxtb-e200621018 Doping effects of ZrC and ZrB2 in the powder-in-tube processed MgB2 tapes
- ^ a b 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
- ^ "First MRI system based on the new Magnesium Diboride superconductor". Columbus Superconductors. Retrieved on 2008-09-22.
- ^ Braccini, Valeria (2007). "Development of ex situ processed MgB2 wires and their applications to magnets". Physica C: Superconductivity 456. doi:.
External links
- Essential Science Indicators on MgB2
- Old material makes a new debut, US Department of Energy Research News
