Metals are often described as having a high lustre, being malleable and ductile, and having good electrical and thermal conductivity. Many metals do have all these properties, but many non-metallic substances can also exhibit one or more of them. The most characteristic property of a metal is its high electrical and thermal conductivity. Moreover, these two properties are proportional to one another at constant temperature (Wiedemann-Franz-Lorenz law) [Smallman (1970)].
Over eighty elements in the periodic table are metals, characterized by having electrons in the outer shell which can be easily removed. This contributes the component of their electrical and thermal conductivity; the second major component of heat transfer within metals is the energy carried by lattice vibrations, called phonons, Klemens (1969) and Edwards (1990).
The atoms in a solid metal are packed together in a highly regular manner. The arrangement follows a specific pattern, and the structure of the metal is characterized by the unit of the pattern, known as a cell. In the majority of metals the atoms are packed so that they occupy the minimum volume; these are the body-centered cubic (bcc), face-centered cubic (fcc) and close-packed hexagonal (cph) arrangements. Other packing arrangements are orthorhombic, rhombohedral, tetragonal, etc.
Emission and absorption of heat by radiation is governed by temperature and surface finish. The latter can significantly affect radiation to and from metals due to surface oxidation [Roberts and Miller (1961)].
Selected thermal data for commonly used metallic elements are presented in Table 1 [Lide (1991/92) and Smithells (1992)]. The defining equations for thermal conductivity, λ, and coefficient of linear expansion, α, are as follows:
where is the rate of heat flowing a distance δ through a body with cross-sectional area A in time t.
where lT1 and lT2 are lengths at temperatures T1 and T2, respectively.
Table 1. Thermal data for commonly used metallic elements
|Handbook of Chemistry and Physics, CRC Press.|
Edwards, P. P. (1990) Chapter 7, Advances in Physical Metallurgy J. A. Charles and G. C. Smith, Eds.. The Institute of Metals, London.
Klemens, P. G. (1969) Chapter I, Thermal Conductivity. R. P. Tye, Ed., Academic Press, London and New York.
Lide, D. R., Ed. (1991/92) Handbook of Chemistry and Physics, CRC Press, Boca Raton, PL.
Roberts, J. K. and Miller, A. R. (1961) Heat and Thermodynamics, Blackie, London.
Smallman, R. E. (1970) Modern Physical Metallurgy, Butterworth, London.
Smithells, C. J. (1992) Smithells Reference Handbook. E. A. Branches and G. B. Brook, Eds., Butterworth-Heinemann.