Millikelvin thermal conductance measurements of compact rigid thermal isolation joints using sapphire–sapphire contacts, and of copper and beryllium–copper demountable thermal contacts

We present thermal conductance measurements of different types of bolted joint at sub-Kelvin temperatures. Joints containing sapphire surfaces provided good thermal isolation; the mechanism appears to be the existence of a very small area over which the surfaces are actually in contact. Various configurations were measured at temperatures between 100 mK and 4 K. The best joint contained sapphire discs separated by diamond powder and had a conductance of 0.26 μW K−1 (T/1 K)2.9, where T is temperature. A mechanical support structure constructed from similar joints, but using alumina powder, had a measured heat leak of 2.57 μW between 80 mK and 1.1 K and was capable of supporting a mass of over 10 kg. Joints between metal surfaces provided good thermal conduction; a bolted joint between copper and a beryllium–copper alloy (C17510 TF00) had a measured conductance of 46 mW K−1 at 100 mK, increasing linearly with temperature. Measurements were also made on a copper–copper compression joint using differential thermal contraction to provide the clamping force. The performance was approximately an order of magnitude worse than for the bolted joint. These measurements were all made as part of the development programme for the SCUBA-2 astronomical instrument; the design requirements were achieved for both insulating and conducting joints.