Role of Metallurgy in the Thermal Conductivity of Superconducting Niobium
- Saravan Kumar Chandrasekaran, MSU FRIB
Thursday, May 30, 4:00 PM - Special Seminar
NSCL Seminar Room
Superconducting radio frequency (SRF) cavities for particle accelerators are fabricated using high purity niobium to allow for continuous high power operation. A hot-spot generated at a surface defect during accelerator operation can quench superconductivity and limit its performance. Increased heat conduction to the liquid helium bath dissipates hot spots, maintaining performance. Properly treated niobium may exhibit a local maximum (i.e., phonon peak) in thermal conductivity at T ≈ 1.8 K, the accelerator operating temperature. Conductivity with a phonon peak may be more than ten times that without the peak. Fabrication from ingot to cavity involves substantial processing, which in turn defines the metallurgical state of the niobium. Understanding the role of metallurgy and processing on conduction in the phonon regime can lead to improved accelerator performance.
This study reveals the role of metallurgy (e.g., grain size and orientation, imperfection density, metal purity) and processing (e.g., deformation, heat treatment) on the magnitude of the phonon peak. This study examines the most comprehensive set of conditions in a group of related uniaxial tensile specimens of mono- and bicrystal niobium with purity of 100 :S RRR :S 200. Measurements of the thermal conductivity of the niobium were made with a coldest bath temperature of 1.6 K. A novel parameter estimation technique used data from a range of temperatures to determine parameters in a theoretically based model of the thermal conductivity of superconducting metals, without resorting to intermediate approximations.
The grain orientations and imperfection densities of the specimens were examined using electron backscatter diffraction and high energy X-ray diffraction, respectively. The role of deformation was examined by uniaxially straining several specimens to 2-38%, typical of SRF cavity manufacturing. The kinetics of recovering the phonon peak post-deformation was revealed by heat treating specimens from 140◦C for 48 h to 1200◦C for 2 h in a high-vacuum furnace. Two specimens were saturated with hydrogen to examine its effect on thermal conductivity.
The as-received specimens displayed no phonon peak due to thermal strain-induced dislocations from ingot production. The recovery of a phonon peak depended sigmoidally on the heat treatment temperature, with a plateau after 1000◦C. The dislocation density of the specimens decreased with increasing heat treatment temperature, but the vacancy concentration increased dramatically. Phonon conductivity appears to be dependent on the vacancy concentration. In addition, smaller RRR yielded a smaller phonon peak.
Results revealed that the density of dislocations introduced during deformation depends on both grain orientation and strain level. The recovery of the phonon peak by post-deformation heat treatment at 1000◦C for 2 h was complete for specimens with ≤ 74% reduction in the phonon peak due to strain-induced dislocations. Although hydrogen infused during typical cavity processing steps left the phonon peak unchanged, specimens saturated with hydrogen displayed a 25% reduction. The concentration of hydrogen was deduced to be affected by vacancy concentration of the specimens.
These results should assist those designing and manufacturing SRF cavities by guiding them to the processes that may provide the greatest possible thermal conductivity for niobium cavities operating at T = 1:8 K.