Magnetic properties and hydrides precipitation observation of nitrogen doping niobium used for accelerator applications

Abstract The results from DC magnetization measurements and direct observation of hydrides precipitation of rectangular bulk nitrogen doping niobium (Nb) samples used for the fabrication of superconducting radio frequency (SRF) cavities were presented. The surface treatments of the Nb samples consisted of heavy electropolishing (EP), nitrogen doping and the subsequent EP with different amounts of material removal. The critical fields and the superconducting parameters were calculated from the magnetization data and their dependence on the subsequent EP material removal was presented. Unexpected pinning behavior of the N-doped Nb samples with subsequent EP material removal was observed and the Nb hydrides were suspected to be the possible contributor to the unexpected pinning effects of the N-doped Nb samples. Direct observation of Nb hydrides precipitation on both the N-doped and the un-doped Nb samples has been carried out using Scanning Electron Microscope (SEM) with a cold stand at 80 K, providing direct evidence of the N H interaction that the amounts of Nb hydrides precipitation on the N-doped samples can be reduced to varying degrees with different amounts of material removal. Based on the study of magnetic properties and Nb hydrides precipitation observation, a possible physical image of N H interaction affecting the magnetic properties of the N-doped Nb samples was proposed. The effects of subsequent EP material removal dependence of magnetic properties and Nb hydrides precipitation reveal the important role of N H interaction in the understanding of the physical mechanism of nitrogen doping phenomenon.