Niobium and vanadium are type-II superconductors, while most other superconducting elements are type-I materials. Almost all compound and alloy superconductors are type-II materials.
The most commonly used conventional superconductor in applications is a niobium-titanium alloy - this is a type-II superconductor with a Tc of 11 K. The highest critical temperature so far achieved in a conventional superconductor was 39 K (-234 °C) in magnesium diboride.
In conventionalsuperconductors, superconductivity is caused by a force of attraction between certain conduction electrons arising from the exchange of phonons, which causes the conduction electrons to exhibit a superfluid phase composed of correlated pairs of electrons.
Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines.
In a class of superconductors known as type II superconductors (including all known high-temperature superconductors), an extremely small amount of resistivity appears when an electrical current is applied in conjunction with a strong magnetic field (which may be caused by the electrical current).
When a superconductor is placed in a weak external magnetic field H, the field penetrates for only a short distance λ, called the penetration depth, after which it decays rapidly to zero.
These problems may soon be alleviated with the development of high temperature superconductors suitable for use in industry, as these can be cooled using liquid nitrogen rather than liquid helium (which is more expensive and difficult to handle.) Promising future applications include high-performance transformers, power storage devices, electric power transmission, electric motors (e.g.
Many stories attribute additional properties to their fictional superconductors, ranging from infinite heat conductivity in Niven's novels (real superconductors conduct heat poorly, though superfluid helium has immense but finite heat conductivity) to teleportation in the Stargate movie and TV series.
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