Thermal conductivity measurements of PTFE and Al${}_2$O${}_3$ ceramic at sub-Kelvin temperatures

The design of low temperature bolometric detectors for rare event searches necessitates careful selection and characterization of structural materials based on their thermal properties. We measure the thermal conductivities of polytetrafluoroethylene (PTFE) and Al${}_2$O${}_3$ ceramic (alumina) in the temperature ranges of 0.17-0.43 K and 0.1-1.3 K, respectively. For the former, we observe a quadratic temperature dependence across the entire measured range. For the latter, we see a cubic dependence on temperature above ~0.3 K, with a linear contribution below that temperature. This paper presents our measurement techniques, results, and theoretical discussions. Keywords: Thermal Conductivity, PTFE, Al${}_2$O${}_3$ ceramic, Bolometer