Thermal Conductivity

Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole. The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material. Thermal conductivity is a reasonably straightforward concept when you are discussing heat loss through the walls of your house, and you can find tables which characterize the building materials and allow you to make reasonable calculations.

More fundamental questions arise when you examine the reasons for wide variations in thermal conductivity. Gases transfer heat by direct collisions between molecules, and as would be expected, their thermal conductivity is low compared to most solids since they are dilute media. Non-metallic solids transfer heat by lattice vibrations so that there is no net motion of the media as the energy propagates through. Such heat transfer is often described in terms of "phonons", quanta of lattice vibrations. Metals are much better thermal conductors than non-metals because the same mobile electrons which participate in electrical conduction also take part in the transfer of heat.

Conceptually, the thermal conductivity can be thought of as the container for the medium-dependent properties which relate the rate of heat loss per unit area to the rate of change of temperature.

More formal treatment

For an ideal gas the heat transfer rate is proportional to the average molecular velocity, the mean free path, and the molar heat capacity of the gas.

For non-metallic solids, the heat transfer is view as being transferred via lattice vibrations, as atoms vibrating more energetically at one part of a solid transfer that energy to less energetic neighboring atoms. This can be enhanced by cooperative motion in the form of propagating lattice waves, which in the quantum limit are quantized as phonons. Practically, there is so much variability for non-metallic solids that we normally just characterize the substance with a measured thermal conductivity when doing ordinary calculations.

For metals, the thermal conductivity is quite high, and those metals which are the best electrical conductors are also the best thermal conductors. At a given temperature, the thermal and electrical conductivities of metals are proportional, but raising the temperature increases the thermal conductivity while decreasing the electrical conductivity. This behavior is quantified in the Wiedemann-Franz Law:

where the constant of proportionality L is called the Lorenz number. Qualitatively, this relationship is based upon the fact that the heat and electrical transport both involve the free electrons in the metal. The thermal conductivity increases with the average particle velocity since that increases the forward transport of energy. However, the electrical conductivity decreases with particle velocity increases because the collisions divert the electrons from forward transport of charge. This means that the ratio of thermal to electrical conductivity depends upon the average velocity squared, which is proportional to the kinetic temperature.

Thermal conductivity table
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The Wiedemann-Franz Law

The ratio of the thermal conductivity to the electrical conductivity of a metal is proportional to the temperature. Qualitatively, this relationship is based upon the fact that the heat and electrical transport both involve the free electrons in the metal. The thermal conductivity increases with the average particle velocity since that increases the forward transport of energy. However, the electrical conductivity decreases with particle velocity increases because the collisions divert the electrons from forward transport of charge. This means that the ratio of thermal to electrical conductivity depends upon the average velocity squared, which is proportional to the kinetic temperature. The molar heat capacity of a classical monoatomic gas is given by

Qualitatively, the Wiedemann-Franz Law can be understood by treating the electrons like a classical gas and comparing the resultant thermal conductivity to the electrical conductivity.The expressions for thermal and electrical conductivity become:

Using the expression for mean particle speed from kinetic theory

the ratio of these quantities can be expressed in terms of the temperature. The ratio of thermal to electrical conductivity illustrates the Wiedemann-Franz Law

While qualitatively agreeing with experiment, the value of the constant is in error in this classical treatment. When the quantum mechanical treatment is done, the value of the constant is found to be:

This is in good agreement with experiment, as can be seen from the values in the table. The fact that the ratio of thermal to electrical conductivity times the temperature is constant forms the essence of the Wiedemann-Franz Law. It is remarkable that it is also independent of the particle mass and the number density of the particles.

The data is from C. Kittel, Introduction to Solid State Physics, 5th Ed., New York:Wiley, 1976, p. 178.

Lorenz number in 10^-8 Watt ohm/K^2
Metal273K373K
Ag
2.31
2.37
Au
2.35
2.40
Cd
2.42
2.43
Cu
2.23
2.33
Ir
2.49
2.49
Mo
2.61
2.79
Pb
2.47
2.56
Pt
2.51
2.60
Sn
2.52
2.49
W
3.04
3.20
Zn
2.31
2.33
Thermal conductivity tableElectrical conductivity table
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Thermal Conductivity

Heat transfer by conduction involves transfer of energy within a material without any motion of the material as a whole. The rate of heat transfer depends upon the temperature gradient and the thermal conductivity of the material. Algebraic methods can be used for the calculation of conduction heat transfer across plane walls, but for most geometries the heat transfer must be expressed in terms of the thermal gradient.

Conceptually, the thermal conductivity can be thought of as the container for the medium-dependent properties which relate the rate of heat loss per unit area to the rate of change of temperature.

The mathematical gradient of a function is a directional derivative which points in the direction of the maximum rate of change of the function. The direction of heat transfer will be opposite to the temperature gradient since the net energy transfer will be from high temperature to low. This direction of maximum heat transfer will be perpendicular to the equal-temperature surfaces surrounding a source of heat.

Thermal conductivity table
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