A15 phases

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Unit cell of an A15 structure, e.g. B. of Nb 3 Sn

The A15 phases form a group of substances that is characterized by a certain A15 structure shown in the picture . Two examples of A15 phases are Cr 3 Si and Nb 3 Sn .

An A15 phase is either an intermetallic phase of the A 3 B type or a modification of the element tungsten . In 2015, more than 60 A15 phases were known among the binary compounds , as well as mixtures of them.

All A15 phases have a certain crystal structure , the so-called β-W structure of tungsten , which belongs to the cubic crystal system and belongs to the space group Pm 3 n (No. 223) . It was discovered on electrolytically preserved tungsten - that is, on an element - whereby one unit cell in β-tungsten contains eight tungsten atoms. Typically, A15 phases have the composition A 3 B, where A is a transition metal (e.g. Ti , V , Cr ; Zr , Nb , Mo ; Ta , W ) and B is e.g. B. a main group element ( Al , Si , P ; Ga , Ge , As ; In , Sn , Sb ; Pb , Bi ). The most important compounds with A15 phase are technically important superconductors . They are used for applications that require large currents and magnetic fields, e.g. B. for the imaging process of nuclear magnetic resonance in medicine. In such applications they are clearly superior to the previously known high-temperature superconductors . Template: room group / 223

common elements of binary A15 phases
Group
IVb 		Group
Vb 		Group
VIb 		Group
IIIa 		Group
IVa 		Group
Va
3 Al Si P
4th Ti V Cr Ga Ge As
5 Zr Nb Mon In Sn Sb
6th Ta Pb Bi

Properties and use of the intermetallic phases

A15-phase enable the construction of a superconducting magnet with magnetic flux densities of more than 10 T . The highest known transition temperature for A15 phases is 23.2  K for Nb 3 Ge .

The technically most important phase is Nb 3 Sn, as it is the only A15 material for which inexpensive conductor manufacturing processes have been developed. The first manufacturing process used for many years used strips with Nb 3 Sn, which were obtained from Nb strips by heating in liquid tin.

The A15 phases are extremely brittle and therefore cannot be plastically deformed. Conventional methods of drawing wire cannot therefore be used, and methods of casting the molten phases are also not practical. It is possible to process Nb 3 Sn as powder in a tube - the Nb 3 Sn can also only be produced in the tube.

Schematic representation of the Cr 3 Si or A15 structure of A 3 B phases. The A atoms are arranged in mutually orthogonal rows. The "bars" shown serve to illustrate and clarify this arrangement, they are not intended to symbolize any special bonds.

A15 phases can be thermally quite stable: A melting point of 2060 ° C is given for Nb 3 Al, Mo 3 Si peritectically changes into solid Mo and melt at 2025 ° C. Nb 3 Sn, on the other hand, already transforms into a tetragonal structure at 43 K.

Description of the structure

Schematic representation of the Cr 3 Si or A15 structure of A 3 B phases. Each B atom is surrounded by 12 A atoms, each A atom by four B atoms. The “bars” shown serve to clarify the arrangement and are not intended to represent any special bonds.

In the A15-A 3 B structure, the B atoms occupy the corners and the center of the unit cell . This corresponds to a body-centered cubic arrangement with two B atoms per unit cell at positions a (0,0,0) and a (½, ½, ½), where a is the lattice parameter. There are six A atoms in the unit cell, which are arranged in pairs on the surfaces of the cubic cell, namely at positions a (½, ± ¼, 0), a (0, ½, ± ¼), and a (± ¼, 0, ½). The A atoms are thus arranged on straight lines that run parallel to the three mutually perpendicular axes. Each B atom is surrounded by 12 A atoms, i.e. H. the coordination number is 12, with the 12 A atoms forming a distorted icosahedron rather than regular. The coordination number of A is 14, because every A at a short distance ( ) with two A atoms, at an intermediate distance with four B atoms (four equal bond lengths, angles that are strongly distorted compared to a tetrahedron) and at a longer distance ( ) with further 8 A atoms is adjacent.

Phases of tungsten: α-W and the A15 phase β-W

While ordinary tungsten, called α-W, has a body centered cubic bcc crystal structure, β-W has the A15 structure. For a long time it was disputed whether the β-W is a real modification of the pure tungsten or whether it is e.g. B. should be regarded as tungsten oxide. More recent work shows that the purest, oxygen-free tungsten does not form an A15 phase at all, since the bcc phase is the thermodynamically stable phase. Accordingly, oxygen stabilizes the A15 phase and plays an important role in its formation.

While α-tungsten has a transition temperature to superconductivity of 0.1 K, that of β-W is 1–4 K.

Historical

The first known A15 phase was the β-W structure of tungsten discovered in 1931. In 1933, the first binary compound with an A15 structure was discovered, the chromium silicide Cr 3 Si. In a paper published in February 1953, it was first shown that A15 phases can be superconducting. From the point of view of that time, surprisingly high transition temperatures were measured: 17.0 K for V 3 Si; 6.0 K for V 3 Ge, 1.30 K for Mo 3 Si and 1.43 K for Mo 3 Ge.

Theodore Geballe and Bernd Matthias , who worked together in Bell Laboratories , made important contributions to researching superconductivity in the A15 phases . For example, in 1967 they published the description of a solid solution of Nb 3 Al and Nb 3 Ge , the first known superconductor with a transition temperature above 20 K. Over 32 years, from 1954 until the discovery of high-temperature superconductors in 1986, the A15 phases held the top position the substances with the highest transition temperatures for superconductivity.

Belonging to the Frank Kasper phases

Together with other intermetallic compounds such as B. the Laves phases , the A15 phases belong to the Frank-Kasper phases , which are characterized by coordination polyhedra with coordination numbers of 12, 13, 14, 15 and 16 and are characterized by their brittleness , which is often unfavorable for technical applications .

Individual evidence

  1. ↑ a b c Wolfgang Klose, Alfred Müller: Superconducting A15 phases . Ed .: O. Madelung (=  Advances in Solid State Physics: Solid body problems . Volume 12 ). Pergamon, Vieweg, Springer, Braunschweig / Berlin / Heidelberg 1972, ISBN 3-540-75329-X , p. 599–625 , doi : 10.1007 / BFb0107697 ( limited preview in Google Book Search [accessed April 27, 2016]).
  2. ^ A b c d Gregory R. Stewart: Superconductivity in the A15 structure . In: Physica C . tape 514 , 2015, p. 28-35 , doi : 10.1016 / j.physc.2015.02.013 , arxiv : 1505.06393 .
  3. ↑ Louis Richard Testardi: Structural instability and superconductivity in A-15 compounds . In: American Physical Society (Ed.): Reviews of Modern Physics . tape 47 , no. 3 , July 1, 1975, p. 637-648 , doi : 10.1103 / RevModPhys.47.637 .
  4. ↑ a b Hellmuth Hartmann, Fritz Ebert, Otto Bretschneider: Electrolyses in phosphate melts. I. The electrowinning of α- and β-tungsten. In: G. Tammann, W. Biltz (ed.): Journal for inorganic and general chemistry . tape 198 , no. 1 . Leopold Voss, Wiley-VCH, Leipzig, Weinheim June 1, 1931, p. 116-140 , doi : 10.1002 / zaac.19311980111 .
  5. ^ JE Hirsch, MB Maple F. Marsiglio: Superconducting materials classes: Introduction and overview . In: Physica C . Superconductivity and its Applications. tape 514 , July 15, 2015, p. 1–8 , doi : 10.1016 / j.physc.2015.03.002 , arxiv : 1504.03318 .
  6. ↑ Helmut Krauth: 100 years of development of superconductors for magnet and energy technology. (PDF) (No longer available online.) In: 6th Superconductor Seminar Braunschweig. Technical University of Braunschweig, May 11, 2011, archived from the original on April 30, 2016 ; accessed on April 30, 2016 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / elenia.rz.tu-bs.de
  7. ↑ Masaki Suenaga: Metallurgy of Continuous Filamentary A15 Superconductors . In: Simon Foner, Brian Schwartz (Eds.): Superconductor Materials Science: Metallurgy, Fabrication, and Applications . Proceedings of a NATO Advanced Study Institute. tape 68 . Plenum Press / Springer Science & Business Media, New York, London 1981, ISBN 978-1-4757-0039-8 , pp. 201–274 ( limited preview in Google Book Search [accessed April 29, 2016]).
  8. ↑ Patent US20100031493 : Powder-in-Tube Process and Method of Manufacture. Published on February 11, 2010 , inventor: Leszek Motowidlo. ‌
  9. ↑ Patent US7459030 : Manufacturing method of Nb 3 Sn superconductive wire using powder technique. Published on December 2, 2008 , Inventors: Takayoshi Miyazaki, Hiroyuki Kato, Kyoji Zaitsu, Kyoji Tachikawa. ‌
  10. ^ A. Misra, JJ Petrovic, TE Mitchell: Microstructures and Mechanical Properties of a Mo 3 Si-Mo 5 Si 3 Composite . In: Scripta Materialia . tape 40 , no. 2 . Elsevier Science Ltd, December 1998, pp. 191-196 , doi : 10.1016 / S1359-6462 (98) 00406-0 .
  11. ↑ Roman Gladyshevskii, Karin Cenzual: Handbook of Superconductivity . Ed .: Charles K. Poole, Horacio A. Farach, Richard J. Creswick. Academic Press, London 2000, ISBN 0-12-561460-8 , 06. Crystal Structures of Classical Superconductors, pp. 130–131 ( limited preview in Google Book Search [accessed May 4, 2016]).
  12. ↑ a b c Wang Rong-Yao, Luo Qi-Guang: Structure of A15 Compounds . In: physica status solidi (a) . tape 96 , no. 2 . Wiley-VCH, August 16, 1986, p. 397-406 , doi : 10.1002 / pssa.2210960204 .
  13. ↑ YG Shen, YW Mai: Structure and properties of stacking faulted A15 tungsten thin films . In: Journal of Materials Science . tape 36 , no. 1 . Kluwer Academic / Springer, January 2001, p. 93-98 , doi : 10.1023 / A: 1004847009613 .
  14. ↑ AE Lita, D. Rosenberg; S. Nam; AJ Miller; D. Balzar; LM Kaatz; RE Schwall: Tuning of tungsten thin film superconducting transition temperature for fabrication of photon number resolving detectors . In: The IEEE Council on Superconductivity (Ed.): IEEE Transactions on Applied Superconductivity . tape 15 , no. 2 , June 2005, p. 3528-3531 , doi : 10.1109 / TASC.2005.849033 .
  15. ↑ Bertil Borén: X-Ray Investigation of alloys of silicon with Chromium, Manganese, Cobalt and Nickel. In: Arkiv Kemi Mineral Geol.11A (10). 1933, 1–28 English summary in the Metallurgical Abstracts, see page 178 (PDF)
  16. ↑ George F. Hardy, John K. Hulm: Superconducting Silicides and Germanides . In: American Physical Society APS (Ed.): Physical Review . tape 89 , no. 4 , February 15, 1953, p. 884 , doi : 10.1103 / PhysRev.89.884 .
  17. ↑ Bernd Theodor Matthias, Theodore H. Geballe, LD Longinotti, E. Corenzwit, GW Hull, RH Willens and JP Maita: Superconductivity at 20 Degrees Kelvin . In: AAAS (Ed.): Science . tape 156 , no. 3775 , May 5, 1967, pp. 645-646 , doi : 10.1126 / science.156.3775.645 .
  18. ^ Stanislaw M. Dubiel: Special Issue "Frank-Kasper Phases". In: Applied Sciences. MDPI AG, February 20, 2014, accessed April 30, 2016 .