Measurement of thermal conductivity and diffusivity at cryogenic temperatures
Jean Pierre MONCHAU 1 (presenting author), Laurent IBOS 5, Thomas PIERRE 6, Marie-Reine N'SOUGAN 2, Gwendal DENIEL 2, Jean Pierre THERMEAU 2, Christophe CHAILAN 2, Patxi DUTHIL 3, Remy THOER 3, Michel PIAT 4, Rodolphe PETIT 1
1 THEMACS Ingénierie, 2 bis rue Alfred Nobel, 77420 Champs sur Marne , , France; 2 Laboratoire APC, Université Paris Cité, CNRS, Astroparticule et Cosmologie, F-75013 Paris, , France; 3 Laboratoire IJCLab, Université Paris-Saclay, CNRS/IN2P3, 91400 Orsay, , France; 4 IN2P3 Université Paris Cité 45 Rue des Saints-Pères, 75006 Paris, , France; 5 Laboratoire CERTES-Optimisthe Université Paris-Est-Créteil (U-PEC) 61, avenue du Général de Gaulle 94010 Créteil Cedex, , France; 6 Univ. Bretagne Sud, UMR CNRS 6027, IRDL, F-56100 Lorient, , France
The use of cryogenics in research and industry is growing rapidly. This applies both to the storage of liquid hydrogen and to research fields such as space, quantum physics and fundamental research. In all these areas, a thorough understanding of thermo-physical properties is essential.
This study focuses on the development of a device for characterising thermal properties at low temperatures.
The device measures the steady-state thermal gradient on a bar of known cross-section. The most commonly used method for measuring the thermal conductivity of metals is the guarded bar method. It consists of heating a metal bar in steady state and measuring the temperature gradient. This method is essential at high temperatures, where radiation represents significant losses.
At cryogenic temperatures, the most significant losses are those due to conduction and convection. To avoid exchanges with the air, the solution is to place the device under high vacuum (1E-5mbar to 1E-6mbar). The sample is heated using an electrical resistance placed on one side of the bar and the other side is cooled using a cold head capable of reaching 10K.
Using a Dirac comb heating regime, it was possible to identify the volumetric heat capacity as a function of temperature.
The device was broken down into quadrupoles representing the different elements through which the heat flow passes. The Dirac comb was chosen to excite the significant frequencies as a function of the assumed thermal diffusivity of the sample and its geometry. The advantage of the Dirac comb is its harmonic richness and the possibility of performing a Fourier series to determine the transfer function between two temperature measurement points. Using an inverse method, the characteristic time of the bar can be found, thereby allowing the thermal diffusivity and volumetric heat capacity to be calculated.
This study is supplemented by a sensitivity study and an uncertainty calculation to evaluate the performance of the device.
Keywords
Thermal conductivity|Thermal diffusivity|metrology