What is "thermal diffusivity"

In heat transfer analysis, thermal diffusivity is the thermal conductivity divided by density and specific heat capacity at constant pressure. It measures the ability of a material to conduct thermal energy relative to its ability to store thermal energy, and is approximately analogous to whether a material is "cold to the touch". It has the SI unit of m²/s. Thermal diffusivity is usually denoted α but a, κ, K, and D are also used. The formula is:

$$\alpha = {k \over {\rho c_p}}$$

where

• k is thermal conductivity (W/(m·K))
• ρ is density (kg/m³)
• c is specific heat capacity (J/(kg·K))

Together, ρc  can be considered the volumetric heat capacity (J/(m³·K)).

As seen in the heat equation,

$$\frac{\partial T}{\partial t} = \alpha \nabla^2 T$$
,

thermal diffusivity is the ratio of the time derivative of temperature to its curvature, quantifying the rate at which temperature concavity is "smoothed out". In a sense, thermal diffusivity is the measure of thermal inertia. In a substance with high thermal diffusivity, heat moves rapidly through it because the substance conducts heat quickly relative to its volumetric heat capacity or 'thermal bulk'.

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

Material

Thermal diffusivity
(m²/s)

Thermal diffusivity
(mm²/s)

Pyrolytic graphite, parallel to layers

1.22 × 10

1220

Silver, pure (99.9%)

1.6563 × 10

165.63

Gold

1.27 × 10

127

Copper at 25 °C

1.11 × 10

111

Aluminium

9.7 × 10

97

Al-10Si-Mn-Mg (Silafont 36) at 20 °C

74.2 × 10

74.2

Aluminium 6061-T6 Alloy

6.4 × 10

64

Al-5Mg-2Si-Mn (Magsimal-59) at 20 °C

44.0 × 10

44.0

Steel, AISI 1010 (0.1% carbon)

1.88 x 10

18.8

Steel, 1% carbon

1.172 × 10

11.72

Steel, stainless 304A at 27 °C

4.2 × 10

4.2

Steel, stainless 310 at 25 °C

3.352 × 10

3.352

Inconel 600 at 25 °C

3.428 × 10

3.428

Molybdenum (99.95%) at 25 °C

54.3 × 10

54.3

Iron

2.3 × 10

23

Silicon

8.8 × 10

88

Quartz

1.4 × 10

1.4

Carbon/carbon composite at 25 °C

2.165 × 10

216.5

Aluminium oxide (polycrystalline)

1.20 × 10

12.0

Silicon Dioxide (Polycrystalline)

8.3 × 10

0.83

Si N with CNTs 26 °C

9.142 × 10

9.142

Si N without CNTs 26 °C

8.605 × 10

8.605

PC (Polycarbonate) at 25 °C

0.144 × 10

0.144

PP (Polypropylene) at 25 °C

0.096 × 10

0.096

Paraffin at 25 °C

0.081 × 10

0.081

PVC (Polyvinyl Chloride)

8 × 10

0.08

PTFE (Polytetrafluorethylene) at 25 °C

0.124 × 10

0.124

Water at 25 °C

0.143 × 10

0.143

Alcohol

7 × 10

0.07

Water vapour (1 atm, 400 K)

2.338 × 10

23.38

Air (300 K)

1.9 × 10

19

Argon (300 K, 1 atm)

22

Helium (300 K, 1 atm)

190

Hydrogen (300 K, 1 atm)

160

Nitrogen (300 K, 1 atm)

22

Pyrolytic graphite, normal to layers

3.6 × 10

3.6

Sandstone

1.12–1.19 × 10

1.15

Tin

4.0 × 10

40

Brick, common

5.2 × 10

0.52

Brick, adobe

2.7 × 10

0.27

Glass, window

3.4 × 10

0.34

Rubber

0.89 - 1.3 × 10

0.089 - 0.13

Nylon

9 × 10

0.09

Wood (Yellow Pine)

8.2 × 10

0.082

Oil, engine (saturated liquid, 100 °C)

7.38 × 10

0.0738