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Unformatted text preview: 17.4 Thermal Conductivity 250 400 Thermal conductivity (W/m-K) ● S-255 200 300 150 200 100 100 50 0 0 10 20 Composition (wt% Zn) 30 0 40 Alloying metals with impurities results in a reduction in the thermal conductivity, for the same reason that the electrical conductivity is diminished (Section 12.8); namely, the impurity atoms, especially if in solid solution, act as scattering centers, lowering the efficiency of electron motion. A plot of thermal conductivity versus composition for copper–zinc alloys (Figure 17.4) displays this effect. Also, stainless steels, which are highly alloyed, become relatively resistive to heat transport. CERAMICS Nonmetallic materials are thermal insulators inasmuch as they lack large numbers of free electrons. Thus the phonons are primarily responsible for thermal conduction: k e is much smaller than k l . Again, the phonons are not as effective as free electrons in the transport of heat energy as a result of the very efficient phonon scattering by lattice imperfections. T...
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