Anisotropic Thermal Conductivity of Polycrystalline Niobium for Superconducting Cavities

This study investigates the thermal conductivity Κ of polycrystalline niobium. We recall that superconducting cavities for particle accelerators are made of niobium. And, the thermal conductivity Κ is one of the factors which play an important role in the cooling of these cavities by the surrounding superfluid helium bath. Our experiments revealed a rather unexpected discrepancy in the thermal conductivity when measured (i) across a 4 mm thick sheet of Nb and (ii) along a 40 mm Nb rod. We emphasise the fact that in both cases the samples were fabricated from the same lot, underwent annealing and therefore have similar grain structure. To examine this discrepancy, we adapted a model describing the thermal conductivity of nanoparticles. Our model takes into account not only the influence of the average grain size , the Kapitza resistance d K R at grain-grain boundaries, but also the number of grains over which the measurements are performed. n