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The
thermal expansion of a material is related to the anharmonicity of the
interatomic potential of its constituent atoms. Increasing the temperature
of the sample increases the vibrational energy of the atoms. As shown in
the diagram, the mean interatomic separation increases as the energy associated
with the vibration of the atoms is increased. The greater the anharmonicity
of the potential, the larger the thermal expansion of the material.
The
macroscopic thermal expansion behavior can be described in terms of a linear
thermal expansion coefficient, a,
defined as: a
= (DL/L0)
DT
where DL
is
the change in length of the sample produced by a temperature change DT.
The volume coefficient of expansion, b,
is given by: b
= (DV/V0)DT
= 3a |
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