Thermal conductivity measurement
What is the thermal conductivity of a material?
Definition of thermal conductivity: a physical quantity that quantifies a material’s ability to conduct heat more or less well. Thermal conductivity coefficients are often referred to as “Lambda” and denoted λ.
When two surfaces of an object differ in temperature, heat flows between them. This flow enables the temperature of the latter to be equilibrated more or less rapidly. The amount of heat that passes from one side to the other per unit of time depends on two parameters in particular: the amplitude of the temperature difference and the distance in meters between the two surfaces.
A material is considered conductive if it has a high thermal conductivity. Conversely, insulating materials are those for which the λ is low.
What parameters influence the lambda of a material?
Thermal conductivity is a physical property that varies slightly with temperature.
On the other hand, humidity has a strong impact on lambda. Water is a fluid that conducts heat very well. You can feel it when you wear damp clothing, which insulates much less from the cold!
Thermal conductivity: which unit?
Its unit of measurement is W/m/K. It’s a heat flow (in watts) per unit of distance (in meters), and per unit of temperature (difference in Kelvin).
Why and for whom are thermal conductivity values useful?
Thermal conductivity values are often used to compare the ability of insulating materials to limit heat conduction by thermal transfer. This is particularly true in the construction and building sectors. The performance of materials for insulating the surfaces of buildings (walls, roofs, glazing) is compared using the lambda values in the thermal regulations. The use of more insulating materials helps limit heat loss in buildings. The result is greater energy savings and improved indoor comfort.
Polyurethane foam, for example, has a thermal conductivity well below that of wood, but comparable to that of glass wool or cellulose wadding.
In other fields, whenever heat is transferred, thermal conductivity is a key parameter. It is therefore used in all thermal modeling, whatever the material or application. For liquids, it plays a key role in heat exchanger design (see dedicated article).
Examples of values
The table below gives some examples of lambda values for commonly used materials:
|Thermal conductivity (W/m/K)
How to measure thermal conductivity?
In the laboratory, there are many types of conductivity meter used to measure the thermal conductivity of your samples. Here are some examples.
Guarded hot plate
The principle of the guarded hot plate method involves placing two identical blocks of material to be analyzed around an electrically insulated hot plate capable of emitting joule effect heat between the two blocks.
Measurements are performed under steady-state conditions by measuring temperature with one or more thermocouples at the guard. The measuring temperature is regulated by placing two plates regulated to the desired measuring temperature around the samples.
This is a simple adaptation of the hot-plate method, in which the samples are not plates but rod-shaped. Temperature readings are recorded by sensors at different heights, enabling the temperature gradient to be evaluated and thermal conductivity to be determined.
Transient hot wire (THW)
The hot-wire method requires a specific device operating in transient mode. A conductive wire probe is placed either within the fluid sample (liquid, paste, etc.) or between two solid and flat sample plates. A current is applied to the wire for a few instants (of the order of a second), heating up the sample. Temperature is recorded by measuring the resistance of the wire during measurement. This is achieved by precise measurement of the applied voltage. The temperature variation as a function of time can be mathematically linked to the measured thermal conductivity.
Flowmetry method (HFM)
The flowmeter method involves determining conductivity by placing a sample between two plates regulated at different temperatures. The top plate is at a lower temperature than the bottom plate. A flowmeter test involves measuring the heat flow through the sample to determine thermal conductivity using Fourier’s law. The method requires the use of a reference sample.
Hot Disk method
The Hot Disk method is based on the use of a specific probe placed between two plates of sample to be analyzed. As in the case of the hot wire and the guard, a current is imposed in the central zone of the probe as a source of heating between the two samples. The sample’s thermal conductivity will then dissipate this heating more or less rapidly.
This is a transient mode measurement in which the temperature rise is not measured by a thermocouple probe, but by measuring a very precise variation in the electrical resistance of the heating wire, which varies as it heats up! This is a highly sensitive measurement that can be applied to almost any type of sample.
This method does not measure thermal conductivity, but thermal diffusivity. Nevertheless, it is possible to calculate thermal conductivity from this physical quantity by knowing the heat capacity and density of the sample. This approach is particularly well suited to small specimens and complex geometries for which other methods are not feasible. A device containing a laser source heats one of the sample surfaces for a short time. A probe continuously measures the transient temperature variation on the back of the sample. Mathematical processing of the heating through the sample provides the thermal diffusivity and calculates the thermal conductivity.
How to choose between all these methods?
Numerous standards exist for each of these different methods, and can be proposed to you.
The choice of technique depends on the physical condition of your samples. Measurements can be carried out on liquids, pastes, foams, solids, powders, metals, etc. The expected conductivity range is also taken into account, as is the desired measurement temperature. The easiest way to do this is to contact the Calnesis team, who will guide you towards the best solution for your project.