Revealing the role of nitrogen on hydride nucleation and stability in pure niobium using first-principles calculations

Abstract

Niobium provides the basis for all superconducting radio frequency (SRF) cavities in use, however, hydrogen is readily absorbed by niobium during cavity fabrication and subsequent niobium hydride precipitation when cooled to cryogenic temperatures degrades its superconducting properties. In the last few years the addition of dopant elements such as nitrogen has been experimentally shown to significantly improve the quality factor of niobium SRF cavities. One of the contributors to Q degradation can be presence of hydrides; however, the underlying mechanisms associated with the kinetics of hydrogen and the thermodynamic stability of hydride precipitates in the presence of dopants are not well known.Using first principles calculations, the effects of nitrogen on the energetic preference for hydrogen to occupy interstitial sites and hydride stability are examined. In particular, the presence of nitrogen significantly increased the energy barrier for hydrogen diffusion from one tetrahedral site to another interstitial site. Furthermore, the beta niobium hydride precipitate became energetically unstable upon addition of nitrogen in the niobium matrix. Through electronic density of states and valence charge transfer calculations, nitrogen showed a strong tendency to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen atoms leading to a very unstable state for hydrogen and hydrides. These calculations show that the presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties.

Surface resistance effects of medium temperature baking of buffered chemical polished 1.3 GHz nine-cell large-grain cavities

“…In figure 11, the BCP baseline sample had bean-like hydride precipitates and the EP baseline sample had leaf-like hydride precipitates rather than the previously reported lens-like [51] or dot-like [52] hydride precipitation on the niobium surface. The baked samples, either with BCP or EP with high-T baking, med-T baking or improved med-T baking, all had muchreduced hydride precipitation to an extent beyond the resolution of the utilized facility.…”

Surface characterization of nitrogen-doped high purity niobium coupons compared with superconducting rf cavity performance

“…This procedure produces surface Nb hydrides during the cooling down cycle, in the temperature range of 70-140 K [14] in the presence of excess high surface hydrogen concentration in non-nitrogen treated samples. After this test for hydride formation we found no evidence by SEM imaging for strain scars left by hydrides on the surface of any of the samples [15].…”

Temperature-dependent near-surface interstitial segregation in niobium

“…A potential scenario is that when at low temperatures, the available oxygen and possible nitrogen atoms segregate towards the surface where an excessive hydrogen concentration is present. This effect can lead to the trapping of hydrogen atoms, eventually suppressing the formation of performance-degrading niobium hydrides [15,19,23,24,63].…”

“…As evidenced by the x-ray diffuse scattering measurements, the subsurface interstitial concentration dramatically increases after the thermal treatment (523 K in nitrogen atmosphere), in line with the complete consumption of Nb 2 O 5 observed by XRR. At this stage, it is possible that a situation where c oxygen + c nitrogen > c hydrogen is achieved, therefore disrupting the niobium hydrides and increasing the diffusion barrier of hydrogen as well as favoring its trapping, which avoids clustering and the eventual reprecipitation of the hydride phase [15,19,23,24,[63][64][65].…”