Physical properties and structure of large grain/single crystal niobium for superconducting RF cavities

Abstract

The R&D program on superconducting cavities fabricated from electron beam melted large grain/single crystal (LG/SC) niobium discs explores it's potential for production of approximately 1000 cavities for the European XFEL. Thermal, electrical, mechanical properties, crystal orientation and structure are investigated with the aim to make the fabrication procedure more efficient. In opposite to fine grain niobium the thermal conductivity of LG/SC has a pronounced maximum at 2K. Calculation found a correlation between thermal conductivity enhancement and phonon scattering at the grain boundaries. Detected enhancement is very susceptible to plastic deformation that can cause the complete elimination of the low temperature peak. The final annealing at 800°C of cavities made from large grain niobium is necessary for hydrogen outgassing, as well as for the thermal conductivity enhancement due to stress relaxation and recovery of crystal defects introduced at the cavity fabrication. The effects of annealing temperature up to 1200°C, heating rate, and holding time on the structure recovery after rolling are also established. Total elongation at the uniaxial tensile tests for LG is very high (50-110%) and depends significantly on the load direction, because only very few grains are in the gage length. The elongation after fracture by bi-axial testing (bulging test) for LG is lower (<15%) yet sufficient for deep drawing of half-cells. Metallographic investigation of and electron beam welding tests on, niobium single crystals show that an appropriate disc enlargement and annealing can be done without destruction of the single crystal. These tests showed that a cavity can be produced without grain boundaries even in the welding area. On base of the results a fabrication method of single crystal cavities is proposed.

Development of large grain cavities

“…4 (see also [9]). The experimental results demonstrate a pronounced phonon peak on large grain samples of the W. C. HERAEUS company heat treated at 800 C, while no phonon peak is observed on fine grain samples independently on the RRR value.…”

Advances in Large Grain Resonators for the European XFEL

“…It can be seen (Fig.8) that the average value of Eacc for LG cavities is about 5-7 MV/m higher compared to the average Eacc of conventional cavities. Plausible reasons for such differences could be the difference in the thermal conductivity caused by phonon peak, or the mirror-like shiny internal surface of LG cavities [9]. Another interesting aspect of LG cavity behavior that has to be stressed is the rather high Qo even at high gradients after BCP treatment.…”

Intrinsic thermoelectric power of group VB metals

“…Nb 3 Ge and Nb 3 Sn) 9 and in electronic components account only for a small share of the production 10 that includes cavity of electron accelerator 11 and Superconducting Quantum Interferometer Filter. 10 Like other members, niobium having bcc structure (a=0.3300 nm) with space group Im-3m and jungle-gym type of Fermi surface (FS) has been proven to be an interesting transition metal (TM) for its superconducting and electrical properties. [12][13][14][15][16][17][18][19][20][21] However, its thermoelectric power (S) is relatively poorly understood.…”