Mathematical definitions of these coefficients are defined below for solids, liquids, and gases. For isotropic materials, the area and volumetric thermal expansion coefficient are, respectively, approximately twice and three times larger than the linear thermal expansion coefficient. Substances that expand at the same rate in every direction are called isotropic. In general, substances expand or contract when their temperature changes, with expansion or contraction occurring in all directions. The volumetric thermal expansion coefficient is the most basic thermal expansion coefficient, and the most relevant for fluids. For solids, one might only be concerned with the change along a length, or over some area. The choice of coefficient depends on the particular application and which dimensions are considered important. Several types of coefficients have been developed: volumetric, area, and linear. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure, such that lower coefficients describe lower propensity for change in size. The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. This plays a crucial role in convection of unevenly heated fluid masses, notably making thermal expansion partly responsible for wind and ocean currents.Ĭoefficient of thermal expansion Thermal expansion changes the space between particles of a substance, which changes the volume of the substance while negligibly changing its mass (the negligible amount comes from energy-mass equivalence), thus changing its density, which has an effect on any buoyant forces acting on it. Common plastics exposed to water can, in the long term, expand by many percent. Ībsorption or desorption of water (or other solvents) can change the size of many common materials many organic materials change size much more due to this effect than due to thermal expansion. An interesting "cooling-by-heating" effect occurs when a glass-forming liquid is heated from the outside, resulting in a temperature drop deep inside the liquid. These discontinuities allow detection of the glass transition temperature where a supercooled liquid transforms to a glass. At the glass transition temperature, rearrangements that occur in an amorphous material lead to characteristic discontinuities of coefficient of thermal expansion and specific heat. The thermal expansion of glasses is slightly higher compared to that of crystals. In general, liquids expand slightly more than solids. Thermal expansion generally decreases with increasing bond energy, which also has an effect on the melting point of solids, so, high melting point materials are more likely to have lower thermal expansion. Unlike gases or liquids, solid materials tend to keep their shape when undergoing thermal expansion. ALLVAR Alloy 30, a titanium alloy, exhibits an anisotropic negative thermal expansion across a wide range of temperatures Factors affecting thermal expansion Fairly pure silicon has a negative coefficient of thermal expansion for temperatures between about 18 and 120 kelvin. Other materials are also known to exhibit negative thermal expansion. For example, the coefficient of thermal expansion of water drops to zero as it is cooled to 3.983 ☌ and then becomes negative below this temperature this means that water has a maximum density at this temperature, and this leads to bodies of water maintaining this temperature at their lower depths during extended periods of sub-zero weather. If an equation of state is available, it can be used to predict the values of the thermal expansion at all the required temperatures and pressures, along with many other state functions.Ĭontraction effects (negative thermal expansion) Ī number of materials contract on heating within certain temperature ranges this is usually called negative thermal expansion, rather than "thermal contraction". 7 Thermal expansion coefficients for various materials.5.1 Apparent and absolute expansion of a liquid.2.1 General thermal expansion coefficient.1.3 Factors affecting thermal expansion.1.2 Contraction effects (negative thermal expansion).
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