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.

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 Celsiusdegree: (NOTE: This can also be in kelvins as the changes in temperature are a 1:1 ratio)

coefficient of linear thermal expansion α
material	α in 10-6/K at 20 °C
Mercury	60
BCB	42
Lead	29
Aluminum	23
Brass	19
Stainless steel	17.3
Copper	17
Gold	14
Nickel	13
Concrete	12
Iron or Steel	11.1
Carbon steel	10.8
Platinum	9
Glass	8.5
GaAs	5.8
Indium Phosphide	4.6
Tungsten	4.5
Glass, Pyrex	3.3
Silicon	3
Invar	1.2
Diamond	1
Quartz, fused	0.59

Applications

For applications using the thermal expansion property, see bi-metal and mercury thermometer

Thermal expansion is also used in mechanical applications to fit parts over one another, e.g. a bushing can be fitted over a shaft by making its inner diameter slightly smaller than the diameter of the shaft, then heating it until it fits over the shaft, and allowing it to cool after it has been pushed over the shaft, thus achieving a 'shrink fit'

There exist some alloys with a very small CTE, used in applications that demand very small changes in physical dimension over a range of temperatures. One of these is Invar 36, with a coefficient in the 0.6x10^-6 range. These alloys are useful in aerospace applications where wide temperature swings may occur.