Conducting Materials

The choice of a conductor for a given application depends not only on its electrical conductivity, but also on various other parameters, such as corrosion resistance, mechanical strength and workability. Copper and aluminium are the most widely used high conductivity materials, of the two, copper is more widely used. Only recently is aluminium being used as a substitute, particularly in view of the increasing price of copper. Hard copper is used in bus bars, commutators segments, contact functions like switches and relays while soft copper is used for magnet wires, cable strands etc.

Copper is the most widely used material as it has (i) the highest conductivity next only to silver, (ii) can be drawn into wires, strips or rolled into sheets (iii) fairly resistant to corrosion and (iv) has good mechanical strength. Generally in electrical power utilities copper of high purity better than 99% is required, very often purity of the order of 99.9% or better is needed. The impurities present affect both electrical and mechanical properties. 0.2% of Fe or 0.3% of As brings down the conductivity to 50% as shown in Fig. 14. Here the oxygen content must be less than 0.1%. Copper with total impurity content of less than 0.5% and of oxygen less than 0.02% has excellent mechanical properties. Such copper is used to draw thin wires. The wire produced by a cold working process has light tensile strength. Annealing produces soft copper having good ductility and low hardness.

Aluminium is progressively replacing copper in many applications. The electrical resistivity of aluminium is about 1.6 times that of copper and it has about one-third the density of copper. Also, linear coefficient of expansion, specific heat and melting point are higher. In mechanical properties it is inferior to copper. For the same resistance and length of wire, aluminium has half the weight of copper. Impurities have a similar effect on the electrical resistivity as in copper. An oxide film formed on the surface of aluminium prevents further corrosion of the material.