Direct observation of hydrides formation in cavity-grade niobium

Niobium is an important technological superconductor used to make radio frequency cavities for particle accelerators. Using laser confocal microscopy we have directly investigated hydride precipitates formation in cavity-grade niobium at 77 and 140 K. We have found that large hydrides were usually formed after chemical or mechanical treatments, which are known to lead to a strong degradation of the quality factor known as $Q$ disease. From our experiments we can conclude that hydrides causing $Q$ disease are islands with a characteristic thickness of $\gtrsim 100\mathrm{nm}$ and in-plane dimensions $1–10\mu \mathrm{m}$. Our results show that mechanical polishing uploads a lot of hydrogen into bulk niobium while electropolishing leads to a mild contamination. Vacuum treatments at $600–800°\mathrm{C}$ are demonstrated to preclude large hydride formation in line with the absence of $Q$ disease in similarly treated cavities.

Figure 1

Phase diagram of Nb-H system (from Ref. 15).

Figure 2

(a) Lenslike structures after 77 K hold on a fine grain mechanically polished sample; (b) zoom-in from the selected area, arrows indicate surface relief due to hydrides; (c) smaller structures observed at another location.

Figure 3

Lenslike structures after 77 K hold on mechanically polished (a) fine grain sample; (b) single grain sample.

Figure 4

Surface profile across the AB line from Fig. 3.

Figure 5

Patterns obtained after keeping a mechanically polished sample at 140 K for 1 h.

Figure 6

Patterns obtained after keeping electropolished sample at 140 K for (a) 1; (b) 2; and (c) 4 hours.