Everything about Coefficient Of Thermal Expansion totally explained
During heat transfer, the energy that's stored in the intermolecular bonds between atoms changes. When the stored energy increases, so does the length of the molecular bond. As a result, solids typically expand in response to heating and contract on cooling*; this response to temperature change is expressed as its coefficient of thermal expansion:
The
coefficient of thermal expansion is used:
- in linear thermal expansion
- in area thermal expansion
- in volumetric thermal expansion
These characteristics are closely related. The volumetric thermal expansion coefficient can be measured for all substances of condensed matter (liquids and solid state). The linear thermal expansion can only be measured in the solid state and is common in engineering applications.
* Some substances have a negative expansion coefficient, and will expand when cooled (for example, freezing water).
Thermal expansion coefficient
The
thermal expansion coefficient is a
thermodynamic property of a substance.
It relates the change in temperature to the change in a material's linear dimensions. It is the fractional change in length per degree of temperature change.
» is vanishingly small, and is almost universally ignored.
Anisotropy
In
anisotropic materials the total volumetric expansion is distributed unequally among the three axes and if the symmetry is monoclinic or triclinic even the angles between these axes are subject to thermal changes. In such cases it's necessary to treat thermal expansion as a tensor that has up to six independent elements. A good way to determine the elements of the tensor is to study the expansion by
powder diffraction.
Thermal expansion coefficients for some common materials
The expansion and contraction of material must be considered when designing large structures, when using tape or chain to measure distances for land surveys, when designing molds for casting hot material, and in other engineering applications when large changes in dimension due to temperature are expected. The range for α is from 10
-7 for hard solids to 10
-3 for organic liquids. α varies with the temperature and some materials have a very high variation. Some values for common materials, given in parts per million per
Celsius degree: (NOTE: This can also be in
kelvins as the changes in temperature are a 1:1 ratio)
NOTE: Theoretically, coeffecient of linear expansion can be found the coeffecient of volumetric expansion(β=3α). However, for liquids, α is calculated through the experimental determination of β, hence it's more accurate to state β here than α.
(The formula β=3α is usually used for solids)
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