a metal in which the total admixture content does not exceed 1 x 10–3 percent by weight.
The principal stages in the production of ultrapure metals are the preparation of pure chemical compounds, the reduction of the compounds to the elementary state, and further purification. Pure compounds are obtained by sorption, extraction, distillation, rectification, ion exchange, and recrystallization from aqueous solutions. The reduction of compounds is carried out by chemical means, heat treatment, or electrodeposition. The additional purification is ensured by electrolytic refining (Cu, Ni, Pb, Al, Ga), distillation or rectification (Zn, Cd, Hg), vacuum smelting (Cu, Sn, Al, Ga), or electron-beam or plasma smelting (V, Nb, Ta, W, Mo, Ti). Higher purity and the production of single crystals are made possible through directional crystallization, the extraction of crystals from melts, and zone recrystallization.
Ultrapure metals exhibit increased ductility, corrosion resistance, electrical conductivity, and lower recrystallization temperature. Highly sensitive methods, such as the spectral method with enrichment and polarographic, luminescence, mass-spectral, and radioactivation methods, are used to analyze the admixtures in ultrapure metals. The overall purity of metals is estimated using the ratio of specific resistances at 293°K and 4.2°K (S293/S42), since the ratio increases with increasing metal purity.
Ultrapure metals, for example, W and Mo, are used as structural materials in devices and apparatus in aviation and nuclear technology. Superconducting microwave resonators are manufactured from ultrapure niobium, ultrapure metals from groups II (Zn, Cd, Hg), III (Al, Ga, In), IV (Pb, Sn), and V (Bi) of Mendeleev’s table are used for the synthesis of simple and complex semiconductor compounds and solid solutions based on these compounds.
Ultrapure metals are also important in solid-state physics research, where they are used as standards, as well as in power engineering, space technology, and semiconductor technology.