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Grain boundary Kapitza resistance and grain-arrangement induced anisotropy in the thermal conductivity of polycrystalline niobium at low temperatures

We investigate the impact of phonon grain boundary scattering in our thermal conduction measurements carried out on polycrystalline niobium samples in the superconducting state and of two different sizes. A simple model is developed which takes into account the average grain size d, the Kapitza resistance RK at grain–grain boundaries and the number of grains n over which the measurements are performed. Our model predicts a strong spatial anisotropy in the thermal conductivity for which can account for the discrepancy in our experimental data. The analysis enables the determination of the Kapitza resistance RK at Nb grain–grain boundaries. We show that with increasing temperature the grain boundary resistance remains dominant relative to the bulk thermal resistance.