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A superconductive material is one that, below a certain very low temperature, does not have any electrical resistance. A superconducting wire, then, is a length of wire made from superconductive materials. While most wires are made from flexible metals, some of the best materials for superconductors are brittle, so they must be packed in powder form into a tube in order to be used. Superconducting wire is useful because it allows for a higher current density than traditional wires. Just like other superconductors, however, superconducting wire requires very low temperatures to function, and the refrigeration process can be quite expensive.
High current density is one of the major considerations in circuit design and is highly important in designing small integrated chips for portable electronic devices. Current density can be simply described as the density of electric charge flowing through a conducting material. Most types of conductors, because of electrical resistance, release a great deal of heat at particularly high current densities. Superconducting wire, on the other hand, does not have electrical resistance, so release of excess heat is not an issue. Superconducting wire can, however, generate levels of current density so high that they generate a magnetic field that can have a deleterious effect on the superconductivity.
One of the major challenges of successfully implementing superconductive wires is the need for extremely low temperatures. Superconductive materials lose their superconductivity and begin to have electrical resistance above a certain threshold temperature, so it is essential that the materials be kept below that temperature. Many types of superconducting wire must be kept at temperatures well below 30 Kelvin (-406°F), requiring the use of liquid helium or liquid nitrogen. Some types of superconductors, in a phenomenon known as "high-temperature superconductivity," can function at temperatures as high as 125 Kelvin (-235°F). Cooling such superconductive materials, though still not a simple task, is less costly than managing the extremely low temperatures needed by others.
It is, unfortunately, quite difficult to turn many forms of superconductors, particularly high-temperature superconductors, into superconducting wire. Many high-temperature superconducting materials are quite brittle, so developing them into typical flexible wires is not plausible. It is often necessary to fill a tube with a powder form of the superconducting material. In many cases, the powder is enclosed in some form of metal, heated, and compressed into a wide, flat wire. Such superconducting wire lacks the flexibility of traditional wire but can be quite useful nonetheless.