Thermodynamics: Thermal Properties of Materials

Introduction to the Thermal Properties of Materials

Thermal properties refer to how materials respond and react to changes in temperature and heat.
These properties determine how a material will behave when subjected to specific thermal conditions or changes in temperature.

Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to changes in temperature. It is generally an increase in dimensions.
The coefficient of linear expansion is an intrinsic property for each substance, and it measures how much a material expands per degree change in temperature.
This property has important consequences in various applications, including construction, manufacturing, and the design of thermal systems.

Thermal conductivity is a measure of a substance’s ability to transfer heat through a material by conduction.
Conduction occurs when heat flows through a material without the material moving. This happens because when one part of a material is heated, particles vibrate more rapidly, colliding with nearby particles and passing the energy along.
The SI unit of thermal conductivity is watt per metre per kelvin (W/(m·K)).
Materials with high thermal conductivities are called thermal conductors while those with low thermal conductivities are referred to as thermal insulators.

The specific heat capacity of a substance is the quantity of heat energy necessary to raise the temperature of a given quantity of the substance by one degree Celsius (or Kelvin).
Specific heat capacity can be measured through the formula Q = mc∆T, where Q is the heat gained or lost, m is the mass of the substance, c is the specific heat capacity, and ∆T is the change in temperature.
Water has a particularly high specific heat capacity.

When thermal expansion is constrained, it can produce thermal stress within a substance.
This stress can be calculated by the equation σ = Eα∆T, where σ is the thermal stress, E is Young’s modulus of the material, α is the coefficient of linear expansion, and ∆T is the temperature change.
Thermal stress can lead to fracturing and failure in some materials, so this is an important consideration in many engineering contexts.

Substances also exhibit characteristic thermal behaviour during phase changes, when they transition from solid to liquid (melting) or liquid to gas (evaporation).
This occurs at constant temperature, despite the input or removal of heat, due to the energy being used to change the phase, not the temperature. This is described by latent heat.
Latent heat of fusion involves the phase change from solid to liquid, while latent heat of vaporisation involves the change from liquid to gas.