Fundamental R&D - Nb
thermal studies
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MOP040 Low Temperature Thermal Conductivity of Niobium and Materials for SRF Cavities 144
  • M. Fouaidy
    IPN, Orsay, France
  A test facility, allowing the test of 4 samples simultaneously during each run, was developed for measuring at low temperature (T= 1.5 K - 10 K) the thermal conductivity k(T) of niobium and other materials used for the fabrication of SRF cavities. The measurements are performed using steady-state axial heat flow method with a careful control of heat leaks to the surrounding. Several samples of different materials (industrial Nb sheets, Ti’) were either tested as received or/and subjected to various Heat Treatment (H.T) prior to the experiment then tested. The resulting experimental data are presented and compared to the experimental results previously reported by other groups. As expected, H.T @ 1200°C with Ti gettering improves the Nb RRR by a factor of 3 and consequently k(T). Finally, the correlation between the Niobium RRR and the thermal conductivity. at T=4.2 K is confirmed in good agreement with the Wiedemann-Franz law.  
DOI • reference for this paper ※  
About • paper received ※ 04 July 2019       paper accepted ※ 05 July 2019       issue date ※ 14 August 2019  
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MOP041 Comparison of the Lattice Thermal Conductivity of Superconducting Tantalum and Niobium 148
  • P. Xu, N.T. Wright
    MSU, East Lansing, Michigan, USA
  • T.R. Bieler
    Michigan State University, East Lansing, Michigan, USA
  Funding: This work is supported by the U.S. Department of Energy, Office of High Energy Physics through Grant No. DE-FG02-13ER41974.
The thermal conductivity k of superconducting Ta behaves similarly to that of superconducting Nb, albeit at colder temperatures. This shift is due to the superconducting transition temperature of Ta being 4.3 K, versus 9.25 K for Nb. For example, the temperature of the phonon peak of properly treated Ta is about 1 K as opposed to a phonon peak at about 2 K for Nb. The typical value of k of Ta is smaller than Nb with the value at the phonon peak for Ta being O(10) W/ m/ K. Like Nb, k is dominated by phonons at these temperatures. This lattice k can be modeled by the Boltzmann transport equation, solved here by a Monte Carlo method using the relaxation time approximation. Individual scattering mechanisms due to boundaries, dislocations, and residual normal electrons are examined, and the phonon dispersion relation is included. Differences in the thermal response of deformed Ta, as compared with Nb, may be attributed to differences in dislocation densities of the two metals following similar levels of deformation. Boundary scattering dominates at the coldest temperatures. The phonon peak decreases and shifts to warmer temperatures with deformation.
DOI • reference for this paper ※  
About • paper received ※ 19 June 2019       paper accepted ※ 30 June 2019       issue date ※ 14 August 2019  
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