In thermodynamics, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It is a measure of the rate of heat transfer inside a material and has SI units of m2/s. It is an intensive property. Thermal diffusivity is usually denoted by lowercase alpha (), but , , (kappa), , , are also used.
The formula is
where is thermal conductivity (W/(m·K)), is specific heat capacity (J/(kg·K)), is density (kg/m3). Together, can be considered the volumetric heat capacity (J/(m3·K)).
Thermal diffusivity is a positive coefficient in the heat equation:
One way to view thermal diffusivity is as the ratio of the time derivative of temperature to its curvature, quantifying the rate at which temperature concavity is "smoothed out". In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its energy storage capacity or "thermal bulk".
Thermal diffusivity and thermal effusivity are related concepts and quantities used to simulate non-equilibrium thermodynamics. Diffusivity is the more fundamental concept and describes the stochastic process of heat spread throughout some local volume of a substance. Effusivity describes the corresponding transient process of heat flow through some local area of interest. Upon reaching a steady state, where the stored energy distribution stabilizes, the thermal conductivity () may be sufficient to describe heat transfers inside solid or rigid bodies by applying Fourier's law.
Thermal diffusivity is often measured with the flash method. It involves heating a strip or cylindrical sample with a short energy pulse at one end and analyzing the temperature change (reduction in amplitude and phase shift of the pulse) a short distance away.
Thermal diffusivity of selected materials and substances +Thermal diffusivity of selected materials and substances and cited in Material Thermal diffusivity(mm2/s)Refs. Pyrolytic graphite, parallel to layers 1220 Diamond 1060–1160 Carbon/carbon composite at 25 °C216.5Helium (300 K, 1 atm)190 cited in Silver, pure (99.9%) 165.63Hydrogen (300 K, 1 atm)160 Gold 127 Copper at 25 °C 111 Aluminium 97 Silicon88 Al-10Si-Mn-Mg (Silafont 36) at 20 °C 74.2 Aluminium 6061-T6 Alloy 64Molybdenum (99.95%) at 25 °C 54.3 Al-5Mg-2Si-Mn (Magsimal-59) at 20 °C 44.0 Tin40Water vapor (1 atm, 400 K)23.38 Iron 23Argon (300 K, 1 atm)22Nitrogen (300 K, 1 atm)22Air (300 K)19 Steel, AISI 1010 (0.1% carbon) 18.8 Aluminium oxide (polycrystalline)12.0 Steel, 1% carbon 11.72 Titanium 9.3 Si3N4 with CNTs 26 °C9.142Si3N4 without CNTs 26 °C8.605 Steel, stainless 304A at 27 °C 4.2 Pyrolytic graphite, normal to layers3.6 Steel, stainless 310 at 25 °C 3.352 Inconel 600 at 25 °C 3.428 Quartz 1.4 Sandstone1.15Ice at 0 °C1.02 Silicon dioxide (polycrystalline) 0.83 Brick, common0.52Glass, window0.34Brick, adobe0.27 PC (polycarbonate) at 25 °C 0.144 Water at 25 °C0.143PTFE (Polytetrafluorethylene) at 25 °C0.124 PP (polypropylene) at 25 °C 0.096 Nylon0.09Rubber0.089–0.13Wood (yellow pine)0.082 Paraffin at 25 °C 0.081 PVC (polyvinyl chloride) 0.08 Oil, engine (saturated liquid, 100 °C) 0.0738 Alcohol 0.07
See also Diffusion List of thermodynamic properties
References