Origin of thermal expansion
The energy-displacement relationship for atoms in a solid is shown schematically below.
As the temperature is raised, the amplitude of vibration increases. The asymmetrical nature of the potential well means that this is accompanied by an increase in the average inter-atomic spacing for longitudinal vibrations. The Coefficient of Thermal Expansion (CTE), or thermal expansivity, α , is the relative change in linear dimensions, per unit of temperature change. In general, ceramics have low thermal expansivities, metals higher and polymers higher still.
Thermal expansivity values for some selected engineering materials, along with some other material data, are given in this properties table.