Mesure de conductivité thermique à température cryogéniques

For storing and transporting liquid gas, it is useful to know the thermo-physical properties of materials. Liquid hydrogen is now an important subject of development. This study concerns the development of a device for characterizing thermal conductivity at low temperature. The device is based on a steady-state measurement of the thermal gradient on a bar of known section. The most commonly used method for measuring the thermal conductivity of metals is the guarded bar method. This is based on the heating of a metal bar in steady state and by measuring the temperature gradient. This method is unavoidable at high temperature where the radiation represents significant losses. At cryogenic temperature, the most significant losses are conductive and convective losses.

In this study, the choice is to remove the thermal guard. To avoid exchanges with the air, the solution is to put the device in a high vacuum (1E-5mBar). We heat the sample with an electrical resistor on one side of the bar and we cool on the other side by plunging the enclosure in liquid nitrogen. The measurements made it possible to highlight the differences in behaviour of certain metals after having undergone annealing cycles. By using a sinusoidal heating regime, it was possible to identify the volumetric heat capacity as a function of the temperature. For composite materials, we measure the conductivity in two direction of the sample according to the direction of the fibre and according to the perpendicular direction of the fibre. Next step: 20K to reach the temperature for storing liquid hydrogen.