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{{Cleanup|date=January 2009}}
{{chembox {{chembox
| Verifiedfields = changed
| verifiedrevid = 411783274
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| Name = Magnesium diboride
| verifiedrevid = 440108596
| ImageFile = MgB2powder.jpg
| ImageFile2 = Magnesium-diboride-3D-balls.png | Name = Magnesium diboride
| ImageFile = MgB2powder.jpg
<!--|ImageSize = 250px -->
| ImageName = Ball-and-stick model of the part of the crystal structure of magnesium diboride | ImageFile2 = Magnesium-diboride-3D-balls.png
| ImageName = Ball-and-stick model of the part of the crystal structure of magnesium diboride
| Section1 = {{Chembox Identifiers | Section1 = {{Chembox Identifiers
| Abbreviations =
| CASNo = 12007-25-9 | CASNo = 12007-25-9
| CASNo_Comment =
| CASNo_Ref = {{cascite}}
| CASNo_Ref = {{cascite|correct|CAS}}
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| PubChem = 15987061
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| RTECS =
| SMILES = =.
| SMILES1 = ......1=2==3=4==5=6==17=23=45=67......
| InChI =
| StdInChI = 1S/2B.Mg
| StdInChI_Ref = {{stdinchicite|changed|chemspider}}
| StdInChIKey = PZKRHHZKOQZHIO-UHFFFAOYSA-N
| StdInChIKey_Ref = {{stdinchicite|changed|chemspider}}
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| Section2 = {{Chembox Properties | Section2 = {{Chembox Properties
| Formula = MgB<sub>2</sub> | Formula = MgB<sub>2</sub>
| MolarMass = 45.93 g/mol | MolarMass = 45.93 g/mol
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| Solvent = other solvents | Solvent = other solvents
| SolubleOther = | SolubleOther =
| MeltingPt = 830 °C (decomp) | MeltingPtC = 830
| MeltingPt_notes = (decomposes)
| BoilingPt = | BoilingPt =
}} }}
| Section3 = {{Chembox Structure | Section3 = {{Chembox Structure
| CrystalStruct = Hexagonal, ] | CrystalStruct = Hexagonal, ]
| SpaceGroup = P6/mmm, No. 191 | SpaceGroup = P6/mmm, No. 191
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}} }}
'''Magnesium diboride''' is the ] of ] and ] with the formula MgB<sub>2</sub>. It is a dark gray, water-insoluble solid. The compound has attracted attention because it becomes ] at 39&nbsp;K (−234&nbsp;°C). In terms of its composition, MgB<sub>2</sub> differs strikingly from most low-temperature superconductors, which feature mainly transition metals. Its superconducting mechanism is primarily described by ].
'''Magnesium diboride''' (MgB<sub>2</sub>) is a simple ] ] that has proven to be an inexpensive and useful ].


==Superconductivity==
Its superconductivity was announced in the journal '']'' in March 2001.<ref>{{cite journal|title = Superconductivity at 39 K in magnesium diboride|url=http://rtn.elektronika.lt/mi/0102/nature410063.pdf|author = Jun Nagamatsu, Norimasa Nakagawa, Takahiro Muranaka, Yuji Zenitani and Jun Akimitsu|journal = ]|volume = 410|page = 63| date = 1 March 2001|doi = 10.1038/35065039|pmid = 11242039|issue = 6824|bibcode = 2001Natur.410...63N }}</ref> Its ] (''T''<sub>c</sub>) of {{convert|39|K|0}} is the highest amongst ]s. This material was first synthesized and its structure confirmed in 1953,<ref>{{cite journal |title = The Preparation and Structure of Magnesium Boride, MgB<sub>2</sub>|author = Morton E. Jones and Richard E. Marsh
Magnesium diboride's superconducting properties were discovered in 2001.<ref>{{cite journal|doi=10.1038/35065039|year=2001|last1=Nagamatsu|first1=Jun|last2=Nakagawa|first2=Norimasa|last3=Muranaka|first3=Takahiro|last4=Zenitani|first4=Yuji|last5=Akimitsu|first5=Jun|journal=Nature|volume=410|issue=6824|pages=63–4|pmid=11242039|title=Superconductivity at 39 K in magnesium diboride|bibcode = 2001Natur.410...63N |s2cid=4388025}}</ref> Its ] (''T''<sub>c</sub>) of {{convert|39|K|0}} is the highest amongst ]s. Among conventional (]) superconductors, it is unusual. Its electronic structure is such that there exist two types of ] at the ] 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. Theoretical understanding of the properties of MgB<sub>2</sub> has nearly been achieved by modelling two energy gaps. In 2001 it was regarded as behaving more like a metallic than a ].<ref name=autogenerated1>{{cite journal |doi=10.1038/35065559|arxiv=cond-mat/0102216 |year=2001 |last1=Larbalestier |first1=D. C. |last2=Cooley |first2=L. D. |last3=Rikel |first3=M. O. |last4=Polyanskii |first4=A. A. |last5=Jiang |first5=J. |last6=Patnaik |first6=S. |last7=Cai |first7=X. Y. |last8=Feldmann |first8=D. M. |last9=Gurevich |first9=A. |last10=Squitieri |first10=A. A. |last11=Naus |first11=M. T. |last12=Eom |first12=C. B. |last13=Hellstrom |first13=E. E. |last14=Cava |first14=R. J. |last15=Regan |first15=K. A. |last16=Rogado |first16=N. |last17=Hayward |first17=M. A. |last18=He |first18=T. |last19=Slusky |first19=J. S. |last20=Khalifah |first20=P. |last21=Inumaru |first21=K. |last22=Haas |first22=M. |journal=Nature |volume=410 |issue=6825 |pages=186–189 |pmid=11242073 |title=Strongly linked current flow in polycrystalline forms of the superconductor MgB2|bibcode = 2001Natur.410..186L |s2cid=4424264 |display-authors=8 }}</ref>
|journal = Journal of the American Chemical Society|volume = 76 |issue = 5| page = 1434| year = 1954|doi = 10.1021/ja01634a089}}</ref> but its superconducting properties were not discovered until 2001. The discovery caused great excitement.<ref> in the ] database</ref>


===Semi-Meissner state===
Though generally believed to be a conventional (]) superconductor, it is a rather unusual one. Its electronic structure is such that there exist two types of electrons at the ] 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<sub>2</sub> has almost been achieved with two energy gaps. In 2001 it was regarded as behaving more like a metallic than a ] superconductor.<ref name=autogenerated1>{{cite journal |doi=10.1038/35065559|arxiv=cond-mat/0102216 |year=2001 |last1=Larbalestier |first1=D. C. |last2=Cooley |first2=L. D. |last3=Rikel |first3=M. O. |last4=Polyanskii |first4=A. A. |last5=Jiang |first5=J. |last6=Patnaik |first6=S. |last7=Cai |first7=X. Y. |last8=Feldmann |first8=D. M. |last9=Gurevich |first9=A. |journal=Nature |volume=410 |issue=6825 |pages=186–189 |pmid=11242073 |title=Strongly linked current flow in polycrystalline forms of the superconductor MgB2.|bibcode = 2001Natur.410..186L }}</ref>
Using ] 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 ]s (51&nbsp;nm and 13&nbsp;nm, respectively).<ref name=Moshchalkov>{{cite journal| author = Moshchalkov, V. V.|title = Type-1.5 Superconductors| journal = Physical Review Letters| volume = 102| issue = 11| page = 117001|year =2009| doi =10.1103/PhysRevLett.102.117001| pmid=19392228| bibcode=2009PhRvL.102k7001M|arxiv = 0902.0997 |last2 = Menghini|first2 = Mariela|last3 = Nishio|first3 = T.|last4 = Chen|first4 = Q.|last5 = Silhanek|first5 = A.|last6 = Dao|first6 = V.|last7 = Chibotaru|first7 = L.|last8 = Zhigadlo|first8 = N.|last9 = Karpinski|first9 = J.|s2cid = 10831135|display-authors=etal}}</ref> The corresponding ]s are 33.6&nbsp;nm and 47.8&nbsp;nm. This implies that the ]s are 0.66±0.02 and 3.68, respectively. The first is less than 1/{{radic|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.<ref>{{cite journal|author1=Babaev, Egor |author-link1=Egor Babaev |author2=Speight, Martin |name-list-style=amp |doi=10.1103/PhysRevB.72.180502|arxiv=cond-mat/0411681|title= Semi-Meissner state and neither type-I nor type-II superconductivity in multicomponent systems|year=2005|journal=Physical Review B|volume=72|issue=18|pages=180502|bibcode = 2005PhRvB..72r0502B |s2cid=118146361 }}</ref> In particular, the potential energy between ] is minimized at a critical distance. As a consequence there is a conjectured new phase called the ], 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<sub>2</sub> 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 ] was coined for this state.


==Synthesis== ==Synthesis==
Magnesium diboride was synthesized and its structure confirmed in 1953.<ref>{{cite journal |title = The Preparation and Structure of Magnesium Boride, MgB<sub>2</sub>|author1=Jones, Morton E. |author2=Marsh, Richard E.
Magnesium diboride can be synthesized by several routes. The simplest is by high temperature reaction between ] and ] powders.<ref name=autogenerated1 /> Formation begins at 650 °C; however, since magnesium metal melts at 652 °C, the reaction mechanism is considered to be moderated by magnesium vapor ] across boron grain boundaries. At conventional reaction temperatures, ] is minimal, although enough grain recrystallization occurs to permit Josephson ] between grains.
|name-list-style=amp | journal = Journal of the American Chemical Society|volume = 76 |issue = 5| page = 1434| year = 1954|doi = 10.1021/ja01634a089}}</ref> The simplest synthesis involves high temperature reaction between ] and ] powders.<ref name=autogenerated1 /> Formation begins at 650&nbsp;°C; however, since magnesium metal melts at 652&nbsp;°C, the reaction may involve diffusion of magnesium vapor across boron grain boundaries. At conventional reaction temperatures, ] is minimal, although grain recrystallization is sufficient for Josephson ] between grains.{{Citation needed|date=January 2019}}

Superconducting magnesium diboride wire can be produced through the ] (PIT) ''ex situ'' and ''in situ'' processes.<ref>B.A.Glowacki, M.Majoros, M.Vickers, J.E.Evetts, Y.Shi and I.McDougall, Superconductivity of powder-in-tube MgB2 wires, Superconductor Science and Technology, 14 (4) 193 (April 2001) | DOI: 10.1088/0953-2048/14/4/304</ref> In the ''in situ'' variant, a mixture of boron and magnesium is reduced in diameter by conventional ]. The wire is then heated to the reaction temperature to form MgB<sub>2</sub>. In the ''ex situ'' variant, the tube is filled with MgB<sub>2</sub> powder, reduced in diameter, and sintered at 800 to 1000&nbsp;°C. In both cases, later hot isostatic pressing at approximately 950&nbsp;°C further improves the properties.{{Citation needed|date=January 2019}}


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.<ref>{{cite journal|title = MgB<sub>2</sub> reactive sintering from elements|author = Giunchi, G.| journal = IEEE Transactions on Applied Superconductivity|volume = 13|pages = 3060–3063|date = 6 August 2002|doi = 10.1109/TASC.2003.812090|issue = 2 |last2 = Ceresara|first2 = S.|last3 = Ripamonti|first3 = G.|last4 = Chiarelli|first4 = S.|last5 = Spadoni|first5 = M.|display-authors=etal|bibcode = 2003ITAS...13.3060G}}</ref> The method allowed the manufacture of both high density (more than 90% of the theoretical density for MgB<sub>2</sub>) bulk materials and special hollow fibers. This method is equivalent to similar melt growth based methods such as the ] used to fabricate bulk ] superconductors where the non-superconducting Y<sub>2</sub>BaCuO<sub>5</sub> 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 MgB<sub>2</sub> and rebranded as ]. The process of Reactive Mg Liquid Infiltration in a boron preform to obtain MgB<sub>2</sub> has been a subject of patent applications by the Italian company ]{{Citation needed|date=January 2019}}
Superconducting magnesium diboride wire can be produced through the ] (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 ]. The wire is then heated to the reaction temperature to form MgB<sub>2</sub> inside. In the ''ex situ'' variant, the tube is filled with MgB<sub>2</sub> 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.


] (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB<sub>2</sub>) thin films.<ref>{{cite journal|title = MgB<sub>2</sub> thin films by hybrid physical-chemical vapor deposition|author = X.X. Xi et al.| journal = Physica C|volume = 456|pages = 22–37|date = 14 February 2007|doi = 10.1016/j.physc.2007.01.029|bibcode = 2007PhyC..456...22X }}</ref> The surfaces of MgB<sub>2</sub> films deposited by other technologies are usually rough and ]. In contrast, the ] system can grow high-quality ''in situ'' pure MgB<sub>2</sub> films with smooth surfaces, which are required to make reproducible uniform ], the fundamental element of superconducting circuits. ] (HPCVD) has been the most effective technique for depositing magnesium diboride (MgB<sub>2</sub>) thin films.<ref>{{cite journal|title = MgB<sub>2</sub> thin films by hybrid physical-chemical vapor deposition|author = Xi, X.X.| journal = Physica C|volume = 456|pages = 22–37|date = 14 February 2007|doi = 10.1016/j.physc.2007.01.029|bibcode = 2007PhyC..456...22X |last2 = Pogrebnyakov|first2 = A.V.|last3 = Xu|first3 = S.Y.|last4 = Chen|first4 = K.|last5 = Cui|first5 = Y.|last6 = Maertz|first6 = E.C.|last7 = Zhuang|first7 = C.G.|last8 = Li|first8 = Qi|last9 = Lamborn|first9 = D.R.|last10 = Redwing|first10 = J.M.|last11 = Liu|first11 = Z.K.|last12 = Soukiassian|first12 = A.|last13 = Schlom|first13 = D.G.|last14 = Weng|first14 = X.J.|last15 = Dickey|first15 = E.C.|last16 = Chen|first16 = Y.B.|last17 = Tian|first17 = W.|last18 = Pan|first18 = X.Q.|last19 = Cybart|first19 = S.A.|last20 = Dynes|first20 = R.C.|issue = 1–2|display-authors=etal}}</ref> The surfaces of MgB<sub>2</sub> films deposited by other technologies are usually rough and ]. In contrast, the ] system can grow high-quality ''in situ'' pure MgB<sub>2</sub> films with smooth surfaces, which are required to make reproducible uniform ], the fundamental element of superconducting circuits.


==Electromagnetic properties== ==Electromagnetic properties==
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*The highest superconducting transition temperature ''T''<sub>c</sub> is 39 K. *The highest superconducting transition temperature ''T''<sub>c</sub> is 39 K.
*MgB<sub>2</sub> is a ], i.e. increasing magnetic fields gradually penetrates into it. *MgB<sub>2</sub> is a ], i.e. increasing magnetic field gradually penetrates into it.
*Maximum critical current (''J''<sub>c</sub>) is: 10<sup>5</sup> A/m<sup>2</sup> at 20 T, 10<sup>6</sup> A/m<sup>2</sup> at 18 T, 10<sup>7</sup> A/m<sup>2</sup> at 15 T, 10<sup>8</sup> A/m<sup>2</sup> at 10 T, 10<sup>9</sup> A/m<sup>2</sup> at 5 T.<ref name="Eisterer">{{cite journal|doi=10.1088/0953-2048/20/12/R01|title=Magnetic properties and critical currents of MgB<sub>2</sub>|year=2007|author=Eisterer, M|journal=Superconductor Science and Technology|volume=20|issue=12|pages=R47|bibcode = 2007SuScT..20R..47E }}</ref> *Maximum critical current (''J''<sub>c</sub>) is: 10<sup>5</sup> A/m<sup>2</sup> at 20 T, 10<sup>6</sup> A/m<sup>2</sup> at 18 T, 10<sup>7</sup> A/m<sup>2</sup> at 15 T, 10<sup>8</sup> A/m<sup>2</sup> at 10 T, 10<sup>9</sup> A/m<sup>2</sup> at 5 T.<ref name="Eisterer">{{cite journal|doi=10.1088/0953-2048/20/12/R01|title=Magnetic properties and critical currents of MgB<sub>2</sub>|year=2007|author=Eisterer, M|journal=Superconductor Science and Technology|volume=20|issue=12|pages=R47–R73|bibcode = 2007SuScT..20R..47E |s2cid=123577523 }}</ref>
*As of 2008 : ] (H<sub>c2</sub>): (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.<ref name="Eisterer"/> *As of 2008 : ] (H<sub>c2</sub>): (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.<ref name="Eisterer"/>

===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&nbsp;nm and 13&nbsp;nm.<ref name=Moshchalkov>{{cite journal| author = V. V. Moshchalkov, M. Menghini, T. Nishio, Q.H. Chen, A.V. Silhanek, V.H. Dao, L.F. Chibotaru, N. D. Zhigadlo, J. Karpinsky|title = Type-1.5 Superconductors| journal = Physical Review Letters| volume = 102| issue = 11| page = 117001|year =2009| doi =10.1103/PhysRevLett.102.117001| pmid=19392228| bibcode=2009PhRvL.102k7001M}}</ref> The corresponding London penetration depths are 33.6&nbsp;nm and 47.8&nbsp;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.<ref>Egor Babaev and Martin Speight, ""</ref> In particular, the potential energy between vortices is minimized at a critical distance. As a consequence there is a conjectured new phase called the ], 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<sub>2</sub> 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 ] was coined for this state.<ref name=Moshchalkov/>


===Improvement by doping=== ===Improvement by doping===
Various means of doping MgB<sub>2</sub> with carbon (e.g. using 10% ]) can improve the ] and the maximum current density<ref>{{cite journal|author=M S A Hossain et al. |title=Significant enhancement of H<sub>c2</sub> and Hirr in MgB<sub>2</sub>+C<sub>4</sub>H<sub>6</sub>O<sub>5</sub> bulks at a low sintering temperature of 600 °C|doi=10.1088/0953-2048/20/8/L03|year=2007|journal=Superconductor Science and Technology|volume=20|issue=8|pages=L51|bibcode = 2007SuScT..20L..51H }}</ref><ref>{{cite journal|title=The excellent superconducting properties of in situ powder-in-tube processed MgB<sub>2</sub> tapes with both ethyltoluene and SiC powder added|doi=10.1088/0953-2048/20/6/L02|year=2007|author=Yamada, H|journal=Superconductor Science and Technology|volume=20|issue=6|pages=L30|last2=Uchiyama|first2=N|last3=Matsumoto|first3=A|last4=Kitaguchi|first4=H|last5=Kumakura|first5=H|bibcode = 2007SuScT..20L..30Y }}</ref> Various means of doping MgB<sub>2</sub> with carbon (e.g. using 10% ]) can improve the ] and the maximum current density<ref>{{cite journal|author=Hossain, M S A|title=Significant enhancement of H<sub>c2</sub> and Hirr in MgB<sub>2</sub>+C<sub>4</sub>H<sub>6</sub>O<sub>5</sub> bulks at a low sintering temperature of 600&nbsp;°C|doi=10.1088/0953-2048/20/8/L03|year=2007|journal=Superconductor Science and Technology|volume=20|issue=8|pages=L51–L54|bibcode = 2007SuScT..20L..51H |s2cid=118204074 |display-authors=etal}}</ref><ref>{{cite journal|title=The excellent superconducting properties of in situ powder-in-tube processed MgB<sub>2</sub> tapes with both ethyltoluene and SiC powder added|doi=10.1088/0953-2048/20/6/L02|year=2007|author=Yamada, H|journal=Superconductor Science and Technology|volume=20|issue=6|pages=L30|last2=Uchiyama|first2=N|last3=Matsumoto|first3=A|last4=Kitaguchi|first4=H|last5=Kumakura|first5=H|bibcode = 2007SuScT..20L..30Y |s2cid=95092135 }}</ref>
(also with ]<ref>{{cite journal|doi=10.1016/j.physc.2007.05.046|arxiv=0708.3885|title= Effect of PVA doping on flux pinning in Bulk MgB<sub>2</sub>|year=2007|last1=Vajpayee|first1=A|last2=Awana|first2=V|last3=Balamurugan|first3=S|last4=Takayamamuromachi|first4=E|last5=Kishan|first5=H|last6=Bhalla|first6=G|journal=Physica C: Superconductivity|volume=466|pages=46–50|bibcode = 2007PhyC..466...46V }}</ref>). (also with ]<ref>{{cite journal|doi=10.1016/j.physc.2007.05.046|arxiv=0708.3885|title= Effect of PVA doping on flux pinning in Bulk MgB<sub>2</sub>|year=2007|last1=Vajpayee|first1=A|last2=Awana|first2=V|last3=Balamurugan|first3=S|last4=Takayamamuromachi|first4=E|last5=Kishan|first5=H|last6=Bhalla|first6=G|journal=Physica C: Superconductivity|volume=466|issue=1–2|pages=46–50|bibcode = 2007PhyC..466...46V |s2cid=118348153}}</ref>).


5% doping with carbon can raise H<sub>c2</sub> from 16 T to 36 T whilst lowering ''T''<sub>c</sub> only from 39 K to 34 K. The maximum critical current (''J''<sub>c</sub>) is reduced, but doping with TiB<sub>2</sub> can reduce the decrease.<ref>{{cite web|url=http://www.azom.com/details.asp?articleID=2542 |title=MgB<sub>2</sub> Properties Enhanced by Doping with Carbon Atoms}}</ref> (Doping MgB<sub>2</sub> with Ti is patented.<ref>Yong Zhao et al "MgB2—based superconductor with high critical current density, and method for manufacturing the same" {{US patent|6953770}}, Issue date: Oct 11, 2005</ref>) 5% doping with carbon can raise H<sub>c2</sub> from 16 to 36 T while lowering ''T''<sub>c</sub> only from 39 K to 34 K. The maximum critical current (''J''<sub>c</sub>) is reduced, but doping with TiB<sub>2</sub> can reduce the decrease.<ref>{{cite web|url=http://www.azom.com/details.asp?articleID=2542 |title=MgB<sub>2</sub> Properties Enhanced by Doping with Carbon Atoms|work=Azom.com|date=June 28, 2004}}</ref> (Doping MgB<sub>2</sub> with Ti is patented.<ref>Zhao, Yong ''et al.'' "MgB2—based superconductor with high critical current density, and method for manufacturing the same" {{US patent|6953770}}, Issue date: Oct 11, 2005</ref>)


The maximum critical current (''J''<sub>c</sub>) in magnetic field is enhanced greatly by doping with ZrB<sub>2</sub>.<ref>{{cite journal|url=http://scholar.ilib.cn/Abstract.aspx?A=kxtb-e200621018 |title=Doping effects of ZrC and ZrB<sub>2</sub> in the powder-in-tube processed MgB<sub>2</sub> tapes| author=Ma, Y.|journal=Chinese Science Bulletin|year=2006|volume=51|issue=21|pages =2669–2672|doi=10.1007/s11434-006-2155-4 }}</ref> The maximum critical current (''J''<sub>c</sub>) in magnetic field is enhanced greatly (approx double at 4.2 K) by doping with ZrB<sub>2</sub>.<ref>{{cite journal|url=http://scholar.ilib.cn/Abstract.aspx?A=kxtb-e200621018|archive-url=https://archive.today/20120215030242/http://scholar.ilib.cn/Abstract.aspx?A=kxtb-e200621018|url-status=dead|archive-date=2012-02-15|title=Doping effects of ZrC and ZrB<sub>2</sub> in the powder-in-tube processed MgB<sub>2</sub> tapes|author=Ma, Y.|journal=Chinese Science Bulletin|year=2006|volume=51|issue=21|pages=2669–2672|doi=10.1007/s11434-006-2155-4|bibcode=2006ChSBu..51.2669M| s2cid=198141335 }}</ref>


Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected. Even small amounts of doping lead both bands into the type II regime and so no semi-Meissner state may be expected.


==Thermal conductivity== ==Thermal conductivity==
MgB<sub>2</sub> is a multi-band superconductor, that is each Fermi surface has different superconducting energy gap. For MgB<sub>2</sub>, 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.<ref Name="Band">{{cite journal|url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Acond-mat%2F0201517 |title=Thermal conductivity of single crystalline MgB<sub>2</sub>|doi=10.1103/PhysRevB.66.014504|year=2002|last1=Sologubenko|first1=A. V.|last2=Jun|first2=J.|last3=Kazakov|first3=S. M.|last4=Karpinski|first4=J.|last5=Ott|first5=H. R.|journal=Physical Review B|volume=66|arxiv = cond-mat/0201517 |bibcode = 2002PhRvB..66a4504S }}</ref> MgB<sub>2</sub> is a multi-band superconductor, that is each Fermi surface has different superconducting energy gap. For MgB<sub>2</sub>, 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.<ref Name="Band">{{cite journal|url=http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai%3AarXiv.org%3Acond-mat%2F0201517|title=Thermal conductivity of single crystalline MgB<sub>2</sub>|doi=10.1103/PhysRevB.66.014504|year=2002|last1=Sologubenko|first1=A. V.|last2=Jun|first2=J.|last3=Kazakov|first3=S. M.|last4=Karpinski|first4=J.|last5=Ott|first5=H. R.|journal=Physical Review B|volume=66|issue=1|pages=14504|arxiv=cond-mat/0201517|bibcode=2002PhRvB..66a4504S|s2cid=119539678|access-date=2008-12-18|archive-date=2012-02-14|archive-url=https://web.archive.org/web/20120214151451/http://www.citebase.org/fulltext?format=application%2Fpdf&identifier=oai:arXiv.org:cond-mat%2F0201517|url-status=dead}}</ref>
The quasiparticle states of the vortices of large gap are highly confined to the vortex core. 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.<ref Name="DOS">{{cite journal|doi=10.1143/JPSJ.71.23|title=Field Dependence of Electronic Specific Heat in Two-Band Superconductors|year=2002|author=Nakai, Noriyuki|journal=Journal of the Physical Society of Japan|volume=71|pages=23|last2=Ichioka|first2=Masanori|last3=MacHida|first3=Kazushige|arxiv = cond-mat/0111088 |bibcode = 2002JPSJ...71...23N }}</ref> Such delocalization can strongly contribute to the thermal conductivity, which shows abrupt increase above H<sub>c1</sub>.<ref Name="Band"/> 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.<ref Name="DOS">{{cite journal|doi=10.1143/JPSJ.71.23|title=Field Dependence of Electronic Specific Heat in Two-Band Superconductors|year=2002|author=Nakai, Noriyuki|journal=Journal of the Physical Society of Japan|volume=71|pages=23–26|last2=Ichioka|first2=Masanori|last3=MacHida|first3=Kazushige|issue=1|arxiv = cond-mat/0111088 |bibcode = 2002JPSJ...71...23N |s2cid=119418871}}</ref> Such delocalization can strongly contribute to the ], which shows abrupt increase above H<sub>c1</sub>.<ref Name="Band"/>


==Possible applications== ==Possible applications==

Superconducting properties and cheapness make magnesium diboride attractive for a variety of applications.<ref Name="Vinod">{{cite journal|title= Prospects for MgB<sub>2</sub> superconductors for magnet application|doi=10.1088/0953-2048/20/1/R01|year= 2007|last1= Vinod|first1= K|last2= Kumar|first2= R G Abhilash|last3= Syamaprasad|first3= U|journal= Superconductor Science and Technology|volume= 20|pages= R1–R13}}</ref> For those applications, MgB<sub>2</sub> powder is compressed with silver metal into tape via the ] process.
===Superconductors===
Superconducting properties and low cost make magnesium diboride attractive for a variety of applications.<ref>{{cite journal |doi=10.1109/TASC.2009.2019287|title=Superconductors in Applications; Some Practical Aspects|year=2009|last1=Bray|first1=J.W.|journal=IEEE Transactions on Applied Superconductivity|volume=19|issue=3|pages=2533–2539|bibcode=2009ITAS...19.2533B|s2cid=30296918}}</ref><ref Name="Vinod">{{cite journal|title= Prospects for MgB<sub>2</sub> superconductors for magnet application|doi=10.1088/0953-2048/20/1/R01|year= 2007|last1= Vinod|first1= K|last2= Kumar|first2= R G Abhilash|last3= Syamaprasad|first3= U|journal= Superconductor Science and Technology|volume= 20|pages= R1–R13|s2cid=122933298 }}</ref> For those applications, MgB<sub>2</sub> powder is compressed with silver metal (or 316 stainless steel) into wire and sometimes tape via the ] process.
<gallery widths="220px" heights="170px"> <gallery widths="220px" heights="170px">
File:MgB2powder2.jpg File:MgB2powder2.jpg
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In 2006 a 0.5 tesla open ] ] system was built using 18&nbsp;km of MgB<sub>2</sub> wires. This MRI used a closed-loop ], without requiring externally supplied cryogenic liquids for cooling.<ref>{{cite web|url = http://www.columbussuperconductors.com/Press%20release.pdf |format=PDF| title = First MRI system based on the new Magnesium Diboride superconductor|publisher = Columbus Superconductors|accessdate = 2008-09-22}}</ref><ref>{{cite journal|journal = Physica C: Superconductivity|volume = 456 |year = 2007|doi = 10.1016/j.physc.2007.01.030|title = Development of ex situ processed MgB<sub>2</sub> wires and their applications to magnets|first = Valeria|last = Braccini|coauthor = Nardelli, Davide; Penco, Roberto; Grasso Giovanni|pages = 209–217|issue = 1–2|bibcode = 2007PhyC..456..209B }}</ref> In 2006 a 0.5 tesla open ] ] system was built using 18&nbsp;km of MgB<sub>2</sub> wires. This MRI used a closed-loop ], without requiring externally supplied cryogenic liquids for cooling.<ref>{{cite web |url=http://www.columbussuperconductors.com/Press%20release.pdf |title=First MRI system based on the new Magnesium Diboride superconductor |publisher=Columbus Superconductors |access-date=2008-09-22 |url-status=dead |archive-url=https://web.archive.org/web/20070630110339/http://www.columbussuperconductors.com/Press%20release.pdf |archive-date=2007-06-30 }}</ref><ref>{{cite journal |journal=Physica C: Superconductivity |volume=456 |year=2007 |doi=10.1016/j.physc.2007.01.030 |title=Development of ''ex situ'' processed MgB<sub>2</sub> wires and their applications to magnets |first1=Valeria |last1=Braccini |last2=Nardelli |first2=Davide |last3=Penco |first3=Roberto |last4=Grasso |first4=Giovanni |pages=209–217 |issue=1–2 |bibcode=2007PhyC..456..209B}}</ref>


"...the next generation MRI instruments must be made of MgB<sub>2</sub> coils instead of NbTi coils, operating in the 20–25 K range without liquid helium for cooling. ... "...the next generation MRI instruments must be made of MgB<sub>2</sub> coils instead of ] coils, operating in the 20–25 K range without liquid helium for cooling. ...
Besides the magnet applications MgB<sub>2</sub> conductors have potential uses in superconducting transformers, rotors and transmission cables at temperatures of around 25 K, at fields of 1 T."<ref Name="Vinod"/> Besides the magnet applications MgB<sub>2</sub> conductors have potential uses in superconducting transformers, rotors and transmission cables at temperatures of around 25 K, at fields of 1 T."<ref Name="Vinod"/>

A project at ] to make MgB<sub>2</sub> cables has resulted in superconducting test cables able to carry 20,000 amperes for extremely high current distribution applications, such as the high luminosity upgrade of the ].<ref></ref>

The ] ] design was based on MgB<sub>2</sub> for its poloidal coils.<ref name=I-FS></ref>


Thin coatings can be used in superconducting radio frequency cavities to minimize energy loss and reduce the inefficiency of liquid helium cooled niobium cavities. 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, MgB<sub>2</sub> has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads.{{Citation needed|date=September 2008}} Because of the low cost of its constituent elements, MgB<sub>2</sub> has promise for use in superconducting low to medium field magnets, electric motors and generators, fault current limiters and current leads.{{Citation needed|date=September 2008}}

===Propellants, explosives, pyrotechnics===

Unlike elemental boron whose combustion is incomplete through the glassy oxide layered impeding oxygen diffusion, magnesium diboride burns completely when ignited in oxygen or in mixtures with oxidizers.<ref>Koch, E.-C.; Weiser, V. and Roth, E. (2011), Combustion behaviour of Binary Pyrolants based on Mg, MgH<sub>2</sub>, MgB<sub>2</sub>, Mg<sub>3</sub>N<sub>2</sub>, Mg<sub>2</sub>Si and Polytetrafluoroethylene, ''EUROPYRO 2011'', Reims, France</ref> Thus magnesium boride has been proposed as fuel in ]s.<ref>Ward, J. R. "MgH<sub>2</sub> and Sr(NO<sub>3</sub>)<sub>2</sub> pyrotechnic composition" {{US Patent|4302259}}, Issued: November 24, 1981.</ref> In addition the use of MgB<sub>2</sub> in blast-enhanced explosives<ref>Wood, L.L. ''et al.'' "Light metal explosives and propellants" {{US Patent|6875294}}, Issued: April 5, 2005</ref> and propellants has been proposed for the same reasons. ]s containing magnesium diboride/]/] display 30–60% increased spectral efficiency, E<sub>λ</sub> (J g<sup>−1</sup>sr<sup>−1</sup>), compared to classical ](MTV) payloads.<ref>{{cite journal|doi=10.1002/prep.201200044|title=Metal-Fluorocarbon Pyrolants. XIII: High Performance Infrared Decoy Flare Compositions Based on MgB<sub>2</sub> and Mg<sub>2</sub>Si and Polytetrafluoroethylene/Viton®|year=2012|last1=Koch|first1=Ernst-Christian|last2=Hahma|first2=Arno|last3=Weiser|first3=Volker|last4=Roth|first4=Evelin|last5=Knapp|first5=Sebastian|journal=Propellants, Explosives, Pyrotechnics|volume=37|issue=4|pages=432}}</ref>
An application of magnesium diboride to hybrid rocket propulsion has also been investigated, mixing the compound in paraffin wax fuel grains to improve mechanical properties and combustion characteristics.<ref>{{Cite conference |last1=Bertoldi |first1=A. E. M. |last2=Bouziane |first2=M. |last3=Hendrick |first3=P. |last4=Vandevelde |first4=C. |last5=Lefebvre |first5=M. |last6=Veras |first6=C. A. G. |date=28 May – 1 June 2018 |title=Development and Test of Magnesium-Based Additive for Hybrid Rockets Fuels |conference=15th International Conference on Space Operations |publisher=American Institute of Aeronautics and Astronautics |doi=10.2514/6.2018-2383 |isbn=978-1-62410-562-3 |doi-access=free}}</ref>


==References== ==References==
{{reflist|2}} {{reflist|30em}}


==External links== ==External links==
{{Commons category|Magnesium_diboride|Magnesium diboride}}
*
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*, US Department of Energy Research News, 2001
* {{Webarchive|url=https://web.archive.org/web/20210208144702/http://www.eurekalert.org/features/doe/2001-12/dl-omm060502.php |date=2021-02-08 }}, US Department of Energy Research News, 2001


{{Magnesium compounds}} {{Magnesium compounds}}
{{Borides}}
{{Authority control}}


{{DEFAULTSORT:Magnesium Diboride}} {{DEFAULTSORT:Magnesium Diboride}}
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