Expressed in W.m-1.K-1, this quantity corresponds to a flow of heat passing through a thickness of material for a fixed temperature. In fact, the higher the thermal conductivity, the more the material conducts heat. On the contrary, the smaller it is, the more insulating the material is.
Families of materials and associated conductivities
It should be noted that this quantity only makes sense for homogeneous materials. In addition, it should be noted that the coefficient of thermal conductivity (λ) of a material varies as a function of the temperature and the humidity thereof.
Principle of conductivity measurement
The measurement of thermal conductivity consists in studying the evolution of a heat flux through a thickness of a sample of material.
To do so, a technician inserts the sample to be characterized between two plates of the same material whose thermal conductivity is known (we at Themacs Engineering use two plates of Pyrex). A heating resistor is then applied in contact with a Pyrex plate. The heating of the plate is done by sending an electrical current through this heating resistor.
Thanks to a temperature sensor placed between the resistance and the Pyrex plate, the initial temperature can be determined. A temperature sensor is installed between each thickness of material so that the temperature can be monitored as a function of the thickness of the material being passed through.
Knowing the inlet and outlet temperatures of the sample, it is possible to deduce the flux flowing through it. From this flow, the thermal resistance and then the thermal conductivity of the sample are deduced. By repeating these measurements for different temperatures, and by means of these values, an average thermal conductivity λmoy of the material is obtained. It is this value of thermal conductivity which will be preserved as being specific to the material.
Characteristics deduced from the measurement of thermal conductivity
From the measurement of the coefficient of thermal conductivity, one can deduce the capacity of a material to isolate the heat, or on the contrary to let it escape.
But beyond this capacity alone, knowledge of the thermal conductivity of a material gives access to its calorific capacity, its diffusivity and its emissivity.