The thermal expansion coefficient, or “coefficient of thermal expansion,” is a physical property of materials that expresses the amount of change in a material’s size or volume as a result of changing temperature. The coefficient of thermal expansion is linear per degree of temperature change, meaning it is a constant ratio and not dependent on the material’s size or mass. For example, if a solid metal bar is heated up 10°C, it will expand by a certain amount, and the same amount if it is heated up by another 10°C. Not all materials expand in this way, though. Certain materials will actually contract when heated, meaning their coefficient of thermal expansion is negative.
Generally, when referring to the coefficient of thermal expansion, we’re referring to its linear coefficient. This linear coefficient is limited to small temperature ranges and uniform heating of the material. When a material is heated slowly and the temperature is raised significantly, the thermal expansion may be complex and nonlinear. In this case, specific heat-related properties such as the “expansion coefficient” become important because they describe the expansion of material over a range of temperatures instead of at one specific temperature.
The thermal expansion coefficients of a material depend on the structure and composition of the material itself (its crystalline and amorphous components, for instance), as well as on its material state. Generally, solids have higher coefficients of thermal expansion than liquids, and liquids higher than gases. That’s why the small amount of atmospheric expansion due to temperature changes has a much larger impact on tall structures such as bridges compared to shorter ones. Granted, even the most non-expansive material in a long bridge structure amounts to a lot of expansion.
The coefficient of thermal expansion is a useful property for architects, engineers, and other material scientists to consider when crafting objects. When two materials with different thermal expansion coefficients are used side by side, they may heat and cool at different speeds, leading to unwanted warping or bending. Likewise, in a hallway or an entity that has been designed to be a certain length, the expansion and contraction of materials due to temperature changes must be taken into account. Depending on the material, sometimes expansion joint gaps need to be in place as thermal expansion occurs.
Although materials may expand, it is also possible to make materials that are designed to become more contractive as the temperature increases. For example, certain alloys are forged in such a way that they become more rigid as the temperature of the environment increases. It’s useful for any material that is used in places that experience extremely harsh or cold climate changes. Air craft for example are engineered so the wings will become more rigid in cold climates to prevent too much flexing of the materials over the long flight distance. It’s also useful in shock absorbers, and even in electronics that require high temperature stability.
In general, the coefficient of thermal expansion is an essential property that must be considered when creating any product that is made out of multiple materials with different thermal expansion coefficients. Considering such properties helps create products that are better for consumer use, as well as products that use materials safely and efficiently.
