Effect of high temperature heat treatments on the quality factor of a large-grain superconducting radio-frequency niobium cavity

Large-grain Nb has become a viable alternative to fine-grain Nb for the fabrication of superconducting radio-frequency cavities. In this contribution we report the results from a heat treatment study of a large-grain 1.5 GHz single-cell cavity made of ``medium purity'' Nb. The baseline surface preparation prior to heat treatment consisted of standard buffered chemical polishing. The heat treatment in the range $800\char21{}1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ was done in a newly designed vacuum induction furnace. ${Q}_{0}$ values of the order of $2\ifmmode\times\else\texttimes\fi{}{10}^{10}$ at 2.0 K and peak surface magnetic field (${B}_{p}$) of 90 mT were achieved reproducibly. A ${Q}_{0}$ value of $(5\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}{10}^{10}$ at 2.0 K and ${B}_{p}=90\text{ }\text{ }\mathrm{mT}$ was obtained after heat treatment at $1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. This is the highest value ever reported at this temperature, frequency, and field. Samples heat treated with the cavity at $1400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ were analyzed by secondary ion mass spectrometry, x-ray photoelectron spectroscopy, energy dispersive x ray, point-contact tunneling, and x-ray diffraction, and revealed a complex surface composition which includes titanium oxide, increased carbon, and nitrogen content but reduced hydrogen concentration compared to a non-heat-treated sample.