194 11.Electrical Properties of Materials Some alloys,when in the ordered state,that is,when the solute atoms are periodically arranged in the matrix,have a distinctly smaller resistivity compared to the case when the atoms are ran- domly distributed.Slowly cooled Cu3Au or CuAu are common examples of ordered structures. Copper is frequently used for electrical wires because of its high conductivity(Figure 11.1).However,pure or annealed cop- per has a low strength(Chapter 3).Thus,work hardening(dur- ing wire drawing),or dispersion strengthening (by adding less than 1%Al203),or age hardening (Cu-Be),or solid solution strengthening(by adding small amounts of second constituents such as Zn)may be used for strengthening.The increase in strength occurs,however,at the expense of a reduced conduc- tivity.(The above mechanisms are arranged in decreasing order of conductivity of the copper-containing wire.)The resistance in- crease in copper inflicted by cold working can be restored to al- most its initial value by annealing copper at moderate tempera- tures (about 300C).This process,which was introduced in Chapter 6 by the terms stress relief anneal or recovery,causes the dislocations to rearrange to form a polygonized structure with- out substantially reducing their number.Thus,the strength of stress-relieved copper essentially is maintained while the con- ductivity is almost restored to its pre-work hardened state(about 98%). For other applications,a high resistivity is desired,such as for heating elements in furnaces which are made,for example,of nickel-chromium alloys.These alloys need to have a high melt- ing temperature and also a good resistance to oxidation,partic- ularly at high temperatures. 11.3.Superconductivity The resistivity in superconductors becomes immeasurably small or virtually zero below a critical temperature,T,as shown in Fig- ure 11.9.About 27 elements,numerous alloys,ceramic materials (containing copper oxide),and organic compounds(based,for ex- ample,on selenium or sulfur)have been found to possess this property (see Table 11.1).It is estimated that the conductivity of superconductors below Te is about 1020 1/0 cm (see Figure 11.1). The transition temperatures where superconductivity starts range from 0.01 K (for tungsten)up to about 125 K(for ceramic su- perconductors).Of particular interest are materials whose Te is above 77 K,that is,the boiling point of liquid nitrogen,which is more readily available than other coolants.Among the so-called194 11 • Electrical Properties of Materials Some alloys, when in the ordered state, that is, when the solute atoms are periodically arranged in the matrix, have a distinctly smaller resistivity compared to the case when the atoms are ran￾domly distributed. Slowly cooled Cu3Au or CuAu are common examples of ordered structures. Copper is frequently used for electrical wires because of its high conductivity (Figure 11.1). However, pure or annealed cop￾per has a low strength (Chapter 3). Thus, work hardening (dur￾ing wire drawing), or dispersion strengthening (by adding less than 1% Al2O3), or age hardening (Cu–Be), or solid solution strengthening (by adding small amounts of second constituents such as Zn) may be used for strengthening. The increase in strength occurs, however, at the expense of a reduced conduc￾tivity. (The above mechanisms are arranged in decreasing order of conductivity of the copper-containing wire.) The resistance in￾crease in copper inflicted by cold working can be restored to al￾most its initial value by annealing copper at moderate tempera￾tures (about 300°C). This process, which was introduced in Chapter 6 by the terms stress relief anneal or recovery, causes the dislocations to rearrange to form a polygonized structure with￾out substantially reducing their number. Thus, the strength of stress-relieved copper essentially is maintained while the con￾ductivity is almost restored to its pre-work hardened state (about 98%). For other applications, a high resistivity is desired, such as for heating elements in furnaces which are made, for example, of nickel–chromium alloys. These alloys need to have a high melt￾ing temperature and also a good resistance to oxidation, partic￾ularly at high temperatures. The resistivity in superconductors becomes immeasurably small or virtually zero below a critical temperature, Tc, as shown in Fig￾ure 11.9. About 27 elements, numerous alloys, ceramic materials (containing copper oxide), and organic compounds (based, for ex￾ample, on selenium or sulfur) have been found to possess this property (see Table 11.1). It is estimated that the conductivity of superconductors below Tc is about 1020 1/# cm (see Figure 11.1). The transition temperatures where superconductivity starts range from 0.01 K (for tungsten) up to about 125 K (for ceramic su￾perconductors). Of particular interest are materials whose Tc is above 77 K, that is, the boiling point of liquid nitrogen, which is more readily available than other coolants. Among the so-called 11.3 • Superconductivity