Measurement of the thermal conductivity of high-purity aluminum and OFHC copper for the CUPID pulse tube cryocoolers thermal links

We present a campaign of thermal conductivity measurements of Oxygen Free High Conductivity (OFHC) copper ('99.9% purity) and high-purity 5N and 6N aluminum. High-purity aluminum is devoted to replace the current OFHC copper thermal links of the Pulse Tube Cryocoolers (PTC) installed in the 3He-4He dilution cryostat of the CUORE (Cryogenic Underground Observatory for Rare Events) experiment, in view of the future commissioning of the CUPID (CUORE Upgrade with Particle IDentification) experiment, both dedicated to the search of neutrinoless double beta decay. We perform thermal conductivity measurements on high-purity aluminum (Al-5N and Al-6N) and OFHC copper samples across the temperature range from 12 K to 45 K, and then extrapolate them down to 3.5 K, namely the minimum temperature achieved by the second stage of the CUORE PTC. We estimated that the thermal conductivity of high-purity aluminum at 3.5 K is up to ≃17 times higher than that of OFHC copper. However, the measured thermal conductivity of high-purity aluminum has been found to be limited by the boundary thermal resistance ascribed to the oxidation of the aluminum surface. By quantifying and accounting for such contribution, we estimated the actual thermal conductivity of Al-6N to be ≃61 times higher value than that of OFHC copper. These findings suggest that high-purity aluminum thermal links offer a promising solution to meet the stringent requirements of the successor of CUORE, the CUPID experiment, in terms of cooling power enhancement and vibration suppression.