A practical cryogenic resistive sensor for thermal conductivity measurements

The line heat-source method is one among many classes of transient methods for measuring thermal properties, such as thermal conductivity, diffusivity and specific heat, which are directly related to heat conduction. Belonging to this category is the transient plane source (TPS) technique for simultaneous measurements of thermal conductivity and diffusivity of solids. The technique uses a 'resistive element' made of nickel foil in the form of a bifilar spiral (TPS element/sensor) covered on both sides with an insulating layer of Kapton. For the first time, the calibration procedures of such a sensor have been extended down to 35 K. By fitting the calibration data to a fourth-order polynomial, a reliable equation for the measured resistance of the TPS element covering the whole temperature range of interest (35-300 K) was obtained. A moderate decrease in the sensitivity of this sensor was observed around 60 K; it is attributed to a nearly temperature-independent resistivity coefficient (TCR) behaviour in this temperature range. The sensitivity decline is linked to a design factor that is related to the resistance behaviour of the material used in the construction of the sensor. To improve the practicality of using the sensor in this temperature range, a comparison was made between the behaviour of the TCR values of nickel and silver. It is found that a sensor made of silver foil will improve the accuracy and extend the measurements to lower temperatures.