13 - Measurements of Thermal Expansion

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The study of the thermal expansion coefficient of solid materials over a wide temperature range is important both from a theoretical point of view and for practical applications. The temperature dependence of thermal expansion coefficients gives information on the intrinsic properties of materials (binding forces, lattice dynamics, band and crystal structure, degree of crystallinity, specific heat, and phase transitions). This chapter provides the knowledge of thermal expansion coefficients, which is indispensable in the project of mechanical structures and cryogenic engineering, where apparatuses are made up of parts with very different thermal and mechanical properties. Several methods of measurement of the thermal expansion are discussed, which have been developed as a function of the material, dimension and shape of the sample, temperature range, and requested accuracy. The measurement of the linear expansion coefficient of a sample is done by recording length change due to a temperature variation. The only way of measuring the linear expansion coefficient of materials for temperatures below 300 K with great accuracy is to adopt high-intrinsic sensitivity methods. Several techniques used to measure the linear expansion coefficient for temperatures less than 300 K include capacitance dilatometer, interferometric dilatometer, laser speckle photography, electronic speckle pattern interferometry, etc. The measurement of the thermal expansion coefficient below room temperature is particularly difficult for low-expansion materials.