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Coefficients of expansion
Coefficients of thermal expansion are normally described either as the increase in length (or volume, esp. for liquids) per unit length at a given temperature, known as the expansivity, α. = (l/L)(dL/dT), or as the mean expansion coefficient over a temperature range,
= (1/L0)(ΔL/ΔT), where L is the instantaneous length, L0 is an initial length, T is temperature, and ΔL and ΔT are changes in length referenced to a temperature at which L0 was measured. The latter form is more common than the former in engineering texts. Significant differences in numerical data can arise between the two methods. In this section, expansivity data only are given. The latter form can be determined from the former by integration over ΔT. To a first approximation, cubical or volume expansivities of solids are three times the linear expansivity.
Coefficients of cubical expansion of liquids
The following table gives values for the cubical (volume) expansivity (l/V) (dV/dT) at T = 293 K (20 °C). Generally, the expansivity increases with increasing temperature.
Liquid | α/10−5K−1 | Liquid | α/10−5K−1 |
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Acetic acid | 107 | Ethyl bromide | 141 |
Acetone | 143 | Ethylene glycol | 57 |
Alcohol, methyl | 118 | Glycerol (glycerine) | 49 |
Alcohol, ethyl | 109 | Mercury* | 18.2 |
Aniline | 85 | Methyl iodide | 120 |
Benzene | 121 | n-Pentane | 158 |
Bromine | 112 | Sulphuric acid (100%) | 56 |
Carbon disulphide | 119 | Toluene | 107 |
Carbon tetrachloride | 122 | Turpentine | 96 |
Chloroform | 127 | m-Xylene | 99 |
Ether | 163 | Water** | 21 |
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* See also section 2.2.1 (Density of mercury).
** See also section 2.2.1 (Density of water).
Coefficients of linear expansion of solids
The expansivities of the majority of solid materials increase with increasing temperature, and can be represented by an equation of the form
Many materials exhibit anisotropic thermal expansion behaviour. When single crystals are in common use, data in the respective principal directions are given. Otherwise a homogeneous isotropic polycrystal-line solid is assumed. High levels of anisotropy and/or phase changes can lead to microc****king and thermal expansion hysteresis. Complex multiphase materials possess thermal expansion characteristics which are related to the expansion coefficients and elastic moduli of the individual components. Only approximate ranges can be cited. Further, more detailed data can be obtained from Touloukian et al., (1971).
Elements | α/(10−6 K−1 | ||||||||
100 K | 200 K | 293 K | 500 K | 800 K | 1100 K | 1500 K | |||
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Aluminium | 12.2 | 20.3 | 23.1 | 26.4 | 34.0 | – | – | ||
Antimony* | 9.1 | 10.5 | 11.0 | 11.7 | 11.7 | – | – | ||
Beryllium* | 1.3 | 7.1 | 11.3 | 15.1 | 19.1 | 21.6 | 23.7 | ||
Bismuth* | 12.3 | 13.1 | 13.4 | 12.7 | – | – | – | ||
Boron | – | – | 4.7 | 5.4 | 6.2 | 6.8 | – | ||
Cadmium | 26.9 | 29.8 | 30.8 | 36.0 | – | – | – | ||
Carbon, vitreous | – | – | 3.1 | 3.3 | 3.6 | 4.0 | 4.6 | ||
Carbon, diamond | 0.05 |
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