Thermal Expansion

The overall increase in volume of a material as its temperature rises is known as thermal expansion. Generally a linear expansion coefficient is used to describe the expansion of a solid, but a volume expansion coefficient is more appropriate for a liquid or gas. The expansion of a crystalline solid will be homogeneous in all dimensions if it is isometric (has the same structural configuration throughout).

Thermal Expansion

Thermal expansion, which does not include phase transitions, is the tendency of matter to change its shape, area, volume, and density in response to a change in temperature.

The average molecular kinetic energy of a substance is a monotonic function of temperature. When a substance is heated, the molecules begin to vibrate and move more, resulting in a greater distance between them. It’s difficult to find substances that contract as the temperature rises, and they only happen in a few temperature ranges.

The material’s coefficient of linear thermal expansion is defined as the relative expansion (also known as strain) divided by the change in temperature, and it fluctuates with temperature. Particles move faster as their energy increases, reducing the intermolecular interactions between them and thereby expanding the substance.

Factors Affecting Thermal Expansion

Solid materials, unlike gases and liquids, maintain their shape during thermal expansion.

Even though thermal expansion decreases as bond energy increases, which also affects the melting point of solids, materials with a high melting point are more likely to have lower thermal expansion. Liquids expand slightly more than solids in general.

When compared to crystals, the thermal expansion of glasses is slightly higher.  Rearrangements in an amorphous material at the glass transition temperature cause characteristic discontinuities in coefficient of thermal expansion and specific heat. These discontinuities enable for the identification of the glass transition temperature, which is the temperature at which a super cooled liquid becomes a glass.

When a glass-forming liquid is heated from the outside, an unusual “cooling-by-heating” process occurs, resulting in a temperature drop deep inside the liquid.

Many common materials can change size due to absorption or desorption of water (or other solvents); many organic materials change size far more due to this phenomenon than due to thermal expansion. When common plastics are exposed to water, they can expand by many percent over time.

Effect on Density

Thermal expansion changes the space between a substance’s particles, increasing its volume while negligibly altering its mass (the negligible difference is due to energy-mass equivalence), and therefore altering its density, which affects any buoyant forces acting on it. This is important in the convection of unevenly heated fluid masses, and it’s why wind and ocean currents are partly caused by thermal expansion.

Coefficient of Thermal Expansion

The coefficient of thermal expansion describes how an object’s size varies as the temperature changes. It calculates the fractional change in size per degree change in temperature at constant pressure, with lower coefficients indicating a reduced propensity for size change. There are three different sorts of coefficients: volumetric, area, and linear.

The coefficient to use is determined by the application and whatever dimensions are regarded important. For solids, the change along a length or over a specific area may be all that matters.

The volumetric thermal expansion coefficient is the most fundamental and important thermal expansion coefficient for fluids. When the temperature of a substance changes, the substance expands or contracts in all directions.

Isotropic substances are those that expand at the same rate in all directions. The area and volumetric thermal expansion coefficients are approximately twice and three times bigger than the linear thermal expansion coefficient for isotropic materials, respectively.

General Thermal Expansion Coefficient

 In General case of Gas, liquid, or solid the coefficient of thermal Expansion is given by

α=v=1V∂V∂Tp

The derivative’s subscript “p” denotes that the pressure is maintained constant during the expansion, and the subscript V emphasises that the volumetric (rather than linear) expansion is used in this generic formulation. The fact that the pressure is kept constant is critical in the case of a gas, because the volume of a gas varies significantly with pressure and temperature. This can be observed in the ideal gas for a low density gas.

Temperature Dependence

Solid’s thermal expansion coefficients are normally unchanged by temperature (unless at extremely low temperatures), whereas liquids can expand at varying rates depending on the temperature. There are, however, a few well-known exceptions: For example, the thermal expansion coefficient of cubic boron nitride varies significantly over a wide temperature range.

Application and Materials

Metals are the most common materials that require thermal expansion coefficient considerations, as thermal expansion is insignificant within small temperature ranges where other materials would not be damaged. Only metals, on the other hand, can withstand higher temperatures. 

There are a variety of applications where thermal expansion must be properly considered. In some circumstances, a low CTE of the material used is desired (for example, in low-expansion alloys), whereas in others, it is essential to be as high as possible (such as in aluminium alloys).

Conclusion

In this article we have studied about Thermal expansion and its coefficient. We have studied its temperature dependence and at last its application. When the temperature interacts with the body, the length, width, height, and volume of the material changes. Thermal expansion is visible in solids because the atoms are densely packed. A fractional change in size of a substance in response to a change in temperature is referred to as thermal expansion.