Thermal diffusivity
In thermodynamics, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure.<ref>Template:CRC90</ref> 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 (Template:Mvar), but Template:Mvar, Template:Mvar, Template:Mvar (kappa),<ref>Template:Cite book</ref> Template:Mvar,<ref name = AJP>Template:Cite journal</ref> Template:Mvar, <math>D_T</math> are also used.
The formula is<ref>Template:Cite book</ref> <math display="block">
\alpha = \frac{k}{\rho c_p},
</math> where
- Template:Mvar is thermal conductivity (W/(m·K)),
- Template:Mvar is specific heat capacity (J/(kg·K)),
- Template:Mvar is density (kg/m3).
Together, Template:Mvar can be considered the volumetric heat capacity (J/(m3·K)).
Thermal diffusivity is a positive coefficient in the heat equation:<ref>Template:Cite book</ref> <math display="block">
\frac{\partial T}{\partial t} = \alpha \nabla^2 T.
</math>
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 (Template:Mvar) may be sufficient to describe heat transfers inside solid or rigid bodies by applying Fourier's law.<ref>Template:Cite book</ref><ref>Template:Cite book</ref>
Thermal diffusivity is often measured with the flash method.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Parker"> Template:Cite journal</ref> 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.<ref> Template:Cite journal</ref><ref>Template:Cite conference</ref>
Thermal diffusivity of selected materials and substancesEdit
| Material | Thermal diffusivity (mm2/s) |
Refs. |
|---|---|---|
| Pyrolytic graphite, parallel to layers | 1220 | |
| Diamond | 1060–1160 | |
| Carbon/carbon composite at 25 °C | 216.5 | <ref name="Casalegno2010" /> |
| Helium (300 K, 1 atm) | 190 | <ref name="baierlein">Template:Cite book cited in Template:Cite book</ref> |
| Silver, pure (99.9%) | 165.63 | |
| Hydrogen (300 K, 1 atm) | 160 | <ref name="baierlein" /> |
| Gold | 127 | <ref name="eleccool">Template:Cite journal</ref> |
| Copper at 25 °C | 111 | <ref name="Casalegno2010">Template:Cite journal</ref> |
| Aluminium | 97 | <ref name="eleccool"/> |
| Silicon | 88 | <ref name="eleccool" /> |
| Al-10Si-Mn-Mg (Silafont 36) at 20 °C | 74.2 | <ref>Template:Cite journal</ref> |
| Aluminium 6061-T6 Alloy | 64 | <ref name="eleccool"/> |
| Molybdenum (99.95%) at 25 °C | 54.3 | <ref>Template:Cite conference</ref> |
| Al-5Mg-2Si-Mn (Magsimal-59) at 20 °C | 44.0 | <ref>Template:Cite journal</ref> |
| Tin | 40 | <ref name="eleccool" /> |
| Water vapor (1 atm, 400 K) | 23.38 | |
| Iron | 23 | <ref name="eleccool" /> |
| Argon (300 K, 1 atm) | 22 | <ref name="baierlein" /> |
| Nitrogen (300 K, 1 atm) | 22 | <ref name="baierlein" /> |
| Air (300 K) | 19 | <ref name="eleccool" /> |
| Steel, AISI 1010 (0.1% carbon) | 18.8 | <ref>Template:Cite book</ref> |
| Aluminium oxide (polycrystalline) | 12.0 | |
| Steel, 1% carbon | 11.72 | |
| Si3N4 with CNTs 26 °C | 9.142 | <ref name="Koszor2009">Template:Cite journal</ref> |
| Si3N4 without CNTs 26 °C | 8.605 | <ref name="Koszor2009" /> |
| Steel, stainless 304A at 27 °C | 4.2 | <ref name="eleccool"/> |
| Pyrolytic graphite, normal to layers | 3.6 | |
| Steel, stainless 310 at 25 °C | 3.352 | <ref>Template:Cite journal</ref> |
| Inconel 600 at 25 °C | 3.428 | <ref>Template:Cite journal</ref> |
| Quartz | 1.4 | <ref name="eleccool"/> |
| Sandstone | 1.15 | |
| Ice at 0 °C | 1.02 | |
| Silicon dioxide (polycrystalline) | 0.83 | <ref name="eleccool"/> |
| Brick, common | 0.52 | |
| Glass, window | 0.34 | |
| Brick, adobe | 0.27 | |
| PC (polycarbonate) at 25 °C | 0.144 | <ref name="HTHP3536pp627">Template:Cite journal</ref> |
| Water at 25 °C | 0.143 | <ref name="HTHP3536pp627" /> |
| PTFE (Polytetrafluorethylene) at 25 °C | 0.124 | <ref>Template:Cite journal</ref> |
| PP (polypropylene) at 25 °C | 0.096 | <ref name="HTHP3536pp627"/> |
| Nylon | 0.09 | |
| Rubber | 0.089–0.13 | <ref name="AJP" /> |
| Wood (yellow pine) | 0.082 | |
| Paraffin at 25 °C | 0.081 | <ref name="HTHP3536pp627"/> |
| PVC (polyvinyl chloride) | 0.08 | <ref name="eleccool"/> |
| Oil, engine (saturated liquid, 100 °C) | 0.0738 | |
| Alcohol | 0.07 | <ref name="eleccool"/> |