Thermal expansion

Thermal expansion is the increase in a material’s dimensions as its temperature rises. It results from changes in the average spacing of its particles.

Thermal vibration: The oscillatory motion of atoms and molecules around their average positions, which increases with temperature. Greater vibration in anharmonic bonds shifts average particle spacing outward as temperature rises.
Bimetallic strip: A bonded strip of two metals with different thermal expansion coefficients that bends as temperature changes. Unequal expansion turns temperature change into bending in thermostats and switches.
Phase transition: A change between distinct physical states of matter, such as melting or boiling. A phase change can alter volume abruptly, unlike ordinary expansion within one phase.
Temperature: A physical quantity that characterizes thermal state and determines the direction of heat flow. Temperature change is the input that drives thermal expansion.
Thermal expansion coefficient anisotropy: The dependence of a material’s thermal expansion coefficient on direction. Directional expansion can distort components even when heating is uniform.
Anharmonic oscillator: An oscillator whose restoring force is not proportional to displacement, producing an asymmetric potential energy curve. Its asymmetric potential explains why greater vibration increases average bond length.
Expansion joint: A gap or flexible connection that accommodates dimensional change in structures or pipelines. It prevents restrained expansion from causing buckling or damage.
Thermal diffusion: The transport of heat through a material in response to temperature differences. Heat flow changes temperature distribution; it is distinct from the resulting dimensional change.
Strain: A measure of deformation expressed as change in dimension divided by original dimension. Thermal expansion is commonly quantified as strain per temperature change.
Negative thermal expansion: The increase in a material’s volume as its temperature decreases over a specified range. Some materials contract when heated, reversing the usual dimensional response.

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