Difference Between Thermal Conductivity and Diffusivity

The key difference between thermal conductivity and diffusivity is that thermal conductivity refers to the ability of a material to conduct heat whereas thermal diffusivity refers to the measurement of the rate of transfer of heat of a material from its hot end to the cold end.

Thermal conductivity and thermal diffusivity are two terms that describe the heat transfer through a particular material.

1. Overview and Key Difference
2. What is Thermal Conductivity
3. What is Thermal Diffusivity
4. Side by Side Comparison – Thermal Conductivity vs Diffusivity in Tabular Form
5. Summary

What is Thermal Conductivity?

Thermal conductivity is a term that describes the ability of a particular material to conduct heat through itself. There are three ways that we can denote this term: k, λ or κ. Typically, a material having a high thermal conductivity shows a high rate of heat transfer. For example, metals usually have high thermal conductivity and are very efficient in conducting heat. Similarly, insulating materials such as Styrofoam have low thermal conductivity and shows a low rate of heat transfer. Therefore, we can use materials with high thermal conductivity in heat sink applications while we can use materials with low thermal conductivity in thermal insulation applications. Moreover, “thermal resistivity” is the reciprocal of thermal conductivity.

Mathematically, we can express thermal conductivity as q = -k∇T, where q is the heat flux, k is the thermal conductivity and ∇T is the temperature gradient. We call this “the Fourier’s law of heat conduction”.

We can define thermal conduction as the transport of energy due to random molecular motion across a temperature gradient. Therefore, we can distinguish this term from energy transport through convection and molecular work because it does not involve any microscopic flows or internal stresses that are work-performing.

When considering the units of measurement for thermal conductivity, the SI units are “Watts per meter-Kelvin” or W/m.K. However, in imperial units, we can measure thermal conductivity in BTU/(h.ft.°F) where BTU is British thermal unit, h is time in hours, ft is the distance in foot, and F is the temperature in Fahrenheit. Furthermore, there are two major ways of measuring the thermal conductivity of a material: steady-state and transient methods.

What is Thermal Diffusivity?

Thermal diffusivity is the measure of the rate of heat transfer of a material from the hot end to the cold end. Therefore, it is the thermal conductivity of a material divided by density and specific heat capacity at a constant pressure. The unit of measurement for this parameter is m²/s. It is an SI derived unit. Usually, we can denote this term as α. But there are other symbols as well. The mathematical expression of thermal diffusivity is as follows:

α = k/ρc_p

Here, k is the thermal conductivity, c_pis specific heat capacity, and ρ si the density. However, ρc_p together is named as volumetric heat capacity.

Most often, thermal diffusivity is measured using the flash method, which involves heating of a strip or a cylindrical part of a material sample with a short energy pulse at one end and analyzing the temperature change a short distance away.

What is the Difference Between Thermal Conductivity and Diffusivity?

Thermal conductivity and thermal diffusivity are two terms that describe the heat transfer through a particular material. The key difference between thermal conductivity and diffusivity is that thermal conductivity refers to the ability of a material to conduct heat whereas thermal diffusivity refers to the measurement of the rate of transfer of heat of a material from its hot end to the cold end.

Below infographic tabulates the differences between thermal conductivity and diffusivity for side by side comparison.

Summary – Thermal Conductivity vs Diffusivity

Thermal conductivity and thermal diffusivity are two terms that describe heat transfer through a particular material. The key difference between thermal conductivity and diffusivity is that thermal conductivity refers to the ability of a material to conduct heat whereas thermal diffusivity refers to the measurement of the rate of transfer of heat of a material from its hot end to the cold end.