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Thermal properties of materials

The thermal conductivity of pure copper at room temperature is about 400 W/(m·K), but in brass with about 30% zinc in solid solution it drops to about 120 W/(m·K). What is the main reason?

  1. AZinc hardly conducts heat, so the paths for heat are reduced by its volume
  2. BSolid solution softens the lattice and greatly lowers the speed of the phonons that carry most of the heat
  3. CSolute atoms scatter conduction electrons and shorten their mean free path, so the thermal conductivity falls along with the electrical conductivity
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Answer: C. Solute atoms scatter conduction electrons and shorten their mean free path, so the thermal conductivity falls along with the electrical conductivity

Heat in metals is carried mainly by conduction electrons, so the thermal conductivity is nearly proportional to the electrical conductivity, the two being linked by the Wiedemann–Franz law. Zinc atoms in the copper lattice disturb the periodicity of the atomic arrangement and scatter electrons, shortening their mean free path, so the electrical conductivity of brass is only about 30% of that of pure copper. The thermal conductivity likewise falls from about 400 to about 120 W/(m·K). Zinc itself is a metal with a thermal conductivity of about 116 W/(m·K), not a substance that fails to conduct heat. In metals, phonons carry only part of the heat to begin with, so a small change in the lattice properties cannot explain such a large drop.

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  1. The lattice thermal conductivity of an insulating single crystal decreases roughly in proportion to 1/T as temperature rises, at temperatures comparable to or above the Debye temperature. What is the main reason?
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