# Materials Engineering Essay

2658 Words11 Pages
II. THERMAL PROPERTIES A. Thermal conductivity Thermal conductivity, k, is the property of a material reflecting its ability to conduct heat. It appears primarily in Fourier's Law for heat conduction. Thermal conductivity is measured in watts per Kelvin per meter (W·K−1·m−1). Multiplied by a temperature difference (in Kelvin, K) and an area (in square metres, m2), and divided by a thickness (in meters, m), the thermal conductivity predicts the rate of energy loss (in watts, W) through a piece of material. The reciprocal of thermal conductivity is thermal resistivity. Usually measured in kelvin-metres per watt (K·m·W−1). When dealing with a known amount of material, its thermal conductance and the reciprocal property, thermal resistance, can be described. B. Thermal diffusity Thermal diffusivity (symbol: [pic], but note that the symbols κ, D, and k are all commonly used) is the thermal conductivity divided by the volumetric heat capacity. It has the SI unit of m²/s. [pic] where: ▪ k : thermal conductivity (SI units: W/(m·K)) ▪ ρ : density (kg/m³) ▪ cp : specific heat capacity (J/(kg·K)) Substances with high thermal diffusivity rapidly adjust their temperature to that of their surroundings, because they conduct heat quickly in comparison to their volumetric heat capacity or 'thermal bulk'. C. Thermal expansion Thermal expansion is the tendency of matter to change in volume in response to a change in temperature. All materials have this tendency. When a substance is heated, its particles begin moving and become active thus maintaining a greater average separation. The change in the linear and volumetric dimension can be estimated to be: αL = coefficient of linear expansion L = initial length ∆T= change in temperature