Juergen Halbritter
Forschungszentrum Karlsruhe, IHM
Postfach 3640
76021 KARLSRUHE
Germany
Corrosion, i.e. oxidation in air, of
metals is well known and cost billions of dollars per year, despite modern
corrosion protection.
What happens to Nb, where corrosion
protection has not been applied to SRF cavities, yet?
Based on elaborate surface studies
at Karlsruhe the following scenario has to be dealt with: The strong,
directional Nb-O bonding via d-electrons together with the open lattice of
Nb-metal causes a sequence of reactions:
- Nb sucks up O, which precipitates to metallic NbOx(x≲1), especially, to the NbOx surface layer.
- A dielectric Nb2O5-y coating forms by
Cabrera-Mott oxidation where nanocrystalline Nb2O5-y
grows consisting of crystalline blocks (CB) of size 1nm and barrier height
ΦB ≃ 1 eV separated by crystallographic shear planes (CS) with ΦS ≃ 0.1eV housing localized states nL(z)
≃ 1019/cm3 easing the
charge transfer across Nb2O5-y.
- In oxidation the factor three volume increase by CB strains the
Nb surface being released by nucleated injection of NbOx into Nb up
to depth between 0.1-50µm. Nb2O5-y does not dissolve in
most acids.
-Nb2O5-y hydroxilize and chemisorbes water
and hydrocarbons.
Consequences of the O dissolution
and of the crack corrosion on Nb rf cavity performance reach from the reduced
energy gap δΔ ≃ 10xΔ by Ox in the
BCS surface resistance RBCS(T,f), to rf residual losses Rres(T
≲Tc/2,ω) ∝ ω2 , to the RBCS(T, ≲ 15mT) minimum, to hysteresis losses Rhys∝ ωB, to heating δR(T,B) ∝ (B/Bc)², and to dielectric interface losses RE∝ exp(-c/E), which do depend not only on Nb quality but also on the
oxidation process, e.g., speed or chemical environment, as will be elucidated.
Further improvements by corrosion protection will mentioned.