In heat transfer analysis, thermal diffusivity (usually denoted α but a, κ, k, and D are also used) is the thermal conductivity divided by density and specific heat capacity at constant pressure. It has the SI unit of m²/s. The formula is:
where
- is thermal conductivity (W/(m·K))
- is density (kg/m³)
- is specific heat capacity (J/(kg·K))
The denominator can be considered the volumetric heat capacity (J/(m³·K)).
In a sense, thermal diffusivity is the measure of thermal inertia. In a substance with high thermal diffusivity, heat moves rapidly through because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'. The substance generally does not require much energy transfer to or from its surroundings to reach thermal equilibrium.
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.
| Material | Thermal diffusivity (m²/s) |
Thermal diffusivity (mm²/s) |
|---|---|---|
| Pyrolytic graphite, parallel to layers | 1.22 × 10−3 | 1220 |
| Silver, pure (99.9%) | 1.6563 × 10−4 | 165.63 |
| Gold | 1.27 × 10−4 | 127 |
| Copper at 25°C | Wrong: should be 115E-6-->255 × 10−6 | 255 |
| Aluminium | 8.418 × 10−5 | 84.18 |
| Al-10Si-Mn-Mg (Silafont 36) at 20°C | 74.2 × 10−6 | 74.2 |
| Aluminum 6061-T6 Alloy | 6.4 × 10−5 | 64 |
| Al-5Mg-2Si-Mn (Magsimal-59) at 20°C | 44.0 × 10−6 | 44.0 |
| Steel, 1% carbon | 1.172 × 10−5 | 11.72 |
| Steel, stainless 304A | 4.2 × 10−6 | 4.2 |
| Steel, stainless 310 at 25°C | 3.352 × 10−6 | 3.352 |
| Inconel 600 at 25°C | 3.428 × 10−6 | 3.428 |
| Molybdenum (99.95%) at 25°C | 54.3 × 10−6 | 54.3 |
| Iron | 2.3 × 10−5 | 23 |
| Silicon | 8.8 × 10−5 | 88 |
| Quartz | 1.4 × 10−6 | 1.4 |
| Carbon/carbon composite at 25°C | 216.5 × 10−6 | 216.5 |
| Aluminium oxide (polycrystalline) | 1.20 × 10−5 | 12.0 |
| Silicon Dioxide (Polycrystalline) | 8.3 × 10−7 | 0.83 |
| Si3 N4 with CNTs 26°C | 9.142 × 10−6 | 9.142 |
| Si3 N4 without CNTs 26°C | 8.605 × 10−6 | 8.605 |
| PC (Polycarbonate) at 25°C | 0.144 × 10−6 | 0.144 |
| PP (Polypropylene) at 25°C | 0.096 × 10−6 | 0.096 |
| Paraffin at 25°C | 0.081 × 10−6 | 0.081 |
| PVC (Polyvinyl Chloride) | 8 × 10−8 | 0.08 |
| PTFE (Polytetrafluorethylene) at 25°C | 0.124 × 10−6 | 0.124 |
| Water at 25°C | 0.143 × 10−6 | 0.143 |
| Alcohol | 7 × 10−8 | 0.07 |
| Water vapour (1 atm, 400 K) | 2.338 × 10−5 | 23.38 |
| Air (300 K) | 1.9 × 10−5 | 19 |
| Argon (300 K, 1 atm) | 2.2×10−5 | 22 |
| Helium (300 K, 1 atm) | 1.9×10−4 | 190 |
| Hydrogen (300 K, 1 atm) | 1.6×10−4 | 160 |
| Nitrogen (300 K, 1 atm) | 2.2×10−5 | 22 |
| Pyrolytic graphite, normal to layers | 3.6 × 10−6 | 3.6 |
| Sandstone | 1.12–1.19 × 10−6 | 1.15 |
| Tin | 4.0 × 10−5 | 40 |
| Brick, common | 5.2 × 10−7 | 0.52 |
| Glass, window | 3.4 × 10−7 | 0.34 |
| Rubber | 1.3 × 10−7 | 0.13 |
| Nylon | 9 × 10−8 | 0.09 |
| Wood (Yellow Pine) | 8.2 × 10−8 | 0.082 |
| Oil, engine (saturated liquid, 100 °C) | 7.38 × 10−8 | 0.0738 |