Thermal expansion
From Online Dictionary of Crystallography
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Thermal expansion
Other languages
Dilatation thermique (Fr). Wärmeausdehnung (Ge). Dilatación tèrmica (Sp). термическое расширение (Ru). Dilatazione termica(It). 熱膨張(Ja)
Coefficient of thermal expansion
The coefficient of thermal expansion relates the deformation that takes place when the temperature T of a solid is varied to the temperature variation Δ T.
- isotropic media
the linear coefficient of thermal expansion, α, relates the relative variation (Δℓ/ℓ) of the length ℓ of a bar to the temperature variation ΔT. In the first order approximation it is given by:
α = (Δ ℓ/ℓ) /Δ T
- anisotropic media
the deformation is described by the strain tensor u_{ij} and the coefficient of thermal expansion is represented by a rank 2 tensor, α_{ij}, given by:
α_{ij} = u_{ij} / Δ T.
Volume thermal expansion
The volume thermal expansion, β, relates the relative variation of volume Δ V/V to Δ T:
- isotropic media
β = Δ V/V Δ T = 3 α,
- anisotropic media
it is given by the trace of α _{ij}:
β = Δ V/V Δ T = α _{11} + α _{22} + α _{33}.
Grüneisen relation
The thermal expansion of a solid is a consequence of the anharmonicity of interatomic forces. The anharmonicity is most conveniently accounted for by means of the so-called `quasiharmonic approximation', assuming the lattice vibration frequencies to be independent of temperature but dependent on volume. This approach leads to the Grüneisen relation that relates the thermal expansion coefficients and the elastic constants:
- isotropic media
β = γ κ c^{V}/V
where γ is the average Grüneisen parameter, κ the isothermal compressibility, c^{V} the specific heat at constant volume.
- anisotropic media
γ_{ij} = c_{ijkl}^{T} α_{kl} V/c^{V}
where the Grüneisen parameter is now represented by a second rank tensor, κ_{ij}, and c_{ijkl}^{T} is the elastic stiffness tensor at constant temperature.
For details see Sections 1.4.2 and 2.1.2.8 of International Tables Volume D.
Measurement
The coefficient of thermal expansion can be measured using diffraction methods (for powder diffraction methods, see Section 2.3 of International Tables Volume C, for single crystal methods, see Section 5.3 of International Tables Volume C), optical methods (interferometry) or electrical methods (pushrod dilatometry methods or capacitance methods). For details see Section 1.4.3 of International Tables Volume D.
See also
Chapters 2.3 and 5.3, International Tables Volume C
Chapters 1.4 and 2.1, International Tables of Crystallography, Volume D