A15 phases
Encyclopedia
The A15 phases are series of intermetallic compounds with the chemical formula A3B (where A is a transition metal
and B can be any element
) and a specific structure. Many of these compounds have superconductivity
at around 20 K
(−424 °F), which is comparatively high, and remain superconductive in magnetic fields of tens of teslas
(hundreds of kilogauss
). This kind of superconductivity (Type-II superconductivity
) is an important area of study as it has several practical applications.
was examined. The discussion if the β–tungsten structure is an allotrope of tungsten or a the structure of a tungsten suboxide was long-standing and in 1998 still articles about the discussion were published. In the end it seems most likely that the material is a true allotrope of tungsten.
The first inter-metallic compound with the typical A3B composition was the chromium silicide Cr3Si, discovered in 1933. Several other compounds of the A15 structure were discovered in the following years. No large interest existed in the research on those compounds. This changed with the discovery that Vanadium silicide V3Si showed superconductivity
at around 17 K
in 1953. In the following years several other A3B superconductors were found. Niobium-germanium
held the record for the highest temperature of 23.2 K from 1971 till the discovery of the cuprate superconductors
in 1986. It took time before the method to produce wires from the very brittle A15 phase materials was established. This method is still complicated. Though some A15 phase materials can withstand higher magnetic field intensity
and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most of the applications due to the easier manufacturing.
Nb3Sn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers.
The Voronoi diagram
of the A15 phase is known to have the least surface area among point sets in three-dimensional Euclidean space. This partition, also known as the Weaire-Phelan structure
, is often present in clathrate hydrates.
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
and B can be any element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
) and a specific structure. Many of these compounds have superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
at around 20 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
(−424 °F), which is comparatively high, and remain superconductive in magnetic fields of tens of teslas
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
(hundreds of kilogauss
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
). This kind of superconductivity (Type-II superconductivity
Type-II superconductor
A 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....
) is an important area of study as it has several practical applications.
History
The first time the A15 structure was observed in 1931 when an electrolytically deposited layer of tungstenTungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
was examined. The discussion if the β–tungsten structure is an allotrope of tungsten or a the structure of a tungsten suboxide was long-standing and in 1998 still articles about the discussion were published. In the end it seems most likely that the material is a true allotrope of tungsten.
The first inter-metallic compound with the typical A3B composition was the chromium silicide Cr3Si, discovered in 1933. Several other compounds of the A15 structure were discovered in the following years. No large interest existed in the research on those compounds. This changed with the discovery that Vanadium silicide V3Si showed superconductivity
Superconductivity
Superconductivity is a phenomenon of exactly zero electrical resistance occurring in certain materials below a characteristic temperature. It was discovered by Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum...
at around 17 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
in 1953. In the following years several other A3B superconductors were found. Niobium-germanium
Niobium-germanium
Niobium-germanium is a metallic chemical compound of niobium and germanium . It has A15 phase structure.It is a superconductor with a critical temperature of 23.2 kelvin ....
held the record for the highest temperature of 23.2 K from 1971 till the discovery of the cuprate superconductors
Cuprate
Cuprates are chemical compounds containing copper anion. Cuprates have been known for centuries and are widely used in inorganic and organic chemistry...
in 1986. It took time before the method to produce wires from the very brittle A15 phase materials was established. This method is still complicated. Though some A15 phase materials can withstand higher magnetic field intensity
Field strength
In physics, the field strength of a field is the magnitude of its vector value.In theoretical physics, field strength is another name for the curvature form...
and have higher critical temperatures than the NbZr and NbTi alloys, NbTi is still used for most of the applications due to the easier manufacturing.
Nb3Sn is used for some high field applications, for example high-end MRI scanners and NMR spectrometers.
The Voronoi diagram
Voronoi diagram
In mathematics, a Voronoi diagram is a special kind of decomposition of a given space, e.g., a metric space, determined by distances to a specified family of objects in the space...
of the A15 phase is known to have the least surface area among point sets in three-dimensional Euclidean space. This partition, also known as the Weaire-Phelan structure
Weaire-Phelan structure
In geometry, the Weaire–Phelan structure is a complex 3-dimensional structure representing an idealised foam of equal-sized bubbles. In 1993, Trinity College Dublin physicist Denis Weaire and his student Robert Phelan found that in computer simulations of foam, this structure was a better...
, is often present in clathrate hydrates.
Examples
- Vanadium-silicon
- Vanadium-galliumVanadium-galliumVanadium-gallium a superconducting alloy of vanadium and gallium often used for the high field insert coils of superconducting electromagnets.Vanadium-gallium tape is used in superconducting magnets...
- Niobium-germaniumNiobium-germaniumNiobium-germanium is a metallic chemical compound of niobium and germanium . It has A15 phase structure.It is a superconductor with a critical temperature of 23.2 kelvin ....
- Niobium-tinNiobium-tinNiobium-tin or triniobium-tin is a metallic chemical compound of niobium and tin , used industrially as a type II superconductor. This intermetallic compoundis a A15 phases superconductor...