Considerable progress has been made in recent years to […]
Considerable progress has been made in recent years to explain the electronic nature of the noble metals, i.e., copper, silver, and gold. These elements exhibit high conductivity because their conduction electrons show relatively little resistance to movement under an electric field. Copper in particular is an excellent conductor because outermost electrons have a large mean free path (about 100 atomic spacings) between collisions. The electrical resistivity is inversely related to this mean free path.
Copper is the preferred and predominant choice in the electrical industry because of its high conductivity, both electrical and thermal. In order to obtain the required properties, unalloyed high purity copper is almost always used. This article discusses the rationale for this choice, and pays particular attention to the underlying metallurgical principles.
Several electrically conductive metals are lighter than copper, but since they would require larger cross-sections to carry the same current, are unacceptable if limited space is a major requirement (e.g., in small electric motors). Consequently, aluminum is used mainly when excessive weight could become a problem. Copper possesses the best characteristics for commercial applications, inasmuch as silver must be dismissed because of its prohibitively high cost.
Annealability of copper is a complex characteristic that is governed by large local inhomogeneities which can change with deformation and thermal history, metal purity, and oxygen content. Impurities play a much smaller role in affecting annealing behavior when they have precipitated, as opposed to being in solid solution.
A correlation exists between annealing temperature and the atomic size difference between solvent (copper in this case) and solute (the impurity). Valence of a solute element is also an important parameter affecting annealability. However, due to the complexities associated with the thermodynamic interactions of multiple species, annealability cannot be simply related to such plausible parameters as atomic volume or valence of the solute.
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