Vacancy-Hydrogen Interaction in Niobium during Low-Temperature Baking

Abstract

A recently discovered modified low-temperature baking leads to reduced surface losses and an increase of the accelerating gradient of superconducting TESLA shape cavities. We will show that the dynamics of vacancy-hydrogen complexes at low-temperature baking lead to a suppression of lossy nanohydrides at 2 K and thus a significant enhancement of accelerator performance. Utilizing Doppler broadening Positron Annihilation Spectroscopy, Positron Annihilation Lifetime Spectroscopy and instrumented nanoindentation, samples made from European XFEL niobium sheets were investigated. We studied the evolution of vacancies in bulk samples and in the sub-surface region and their interaction with hydrogen at different temperature levels during in-situ and ex-situ annealing.

Ion Intercalation in Lanthanum Strontium Ferrite for Aqueous Electrochemical Energy Storage Devices

“…The PALS analysis shown below is based on dedicated studies on a related perovskite, SrTiO 3 (STO), for the identification of the defect type based on the positron lifetime. − Although PALS cannot directly measure positive defects such as oxygen vacancies or protons, the technique is sensitive to their association with negative defects (e.g., cationic vacancies) resulting in the formation of an associate. A higher V O •• content is expected to lead to an increase of defect lifetime owing to the formation of larger open clusters. , Conversely, the effect of proton uptake depends on the defect site: the association of protons with a negatively charged vacancy leads to a contraction of the open volume and to lifetime decrease for proton incorporation as interstitials, whereas protons bonded to oxygen may give rise to a displacement of oxygen from the equilibrium sites, resulting in a distorted structure and larger defect open volumes. , Figure a shows the average defect size positron lifetime (τ av ) obtained as a function of E p for the KOH-oxidized and reduced samples as well as the O 2 -oxidized and the N 2 -reduced LSF50 thin films. Here, τ av = Σ i τ i · I i , as obtained from deconvolution of the measured lifetime in two-lifetime components (cf.…”

“…A higher V O •• content is expected to lead to an increase of defect lifetime owing to the formation of larger open clusters. 59,60 Conversely, the effect of proton uptake depends on the defect site: the association of protons with a negatively charged vacancy leads to a contraction of the open volume and to lifetime decrease for proton incorporation as interstitials, 61 whereas protons bonded to oxygen may give rise to a displacement of oxygen from the equilibrium sites, resulting in a distorted structure and larger defect open volumes. 51,62 Figure 5a shows the average defect size positron lifetime (τ av ) obtained as a function of E p for the KOHoxidized and reduced samples as well as the O 2 -oxidized and the N 2 -reduced LSF50 thin films.…”

Identification of Point Defects in Multielement Compounds and Alloys with Positron Annihilation Spectroscopy: Challenges and Opportunities

“…[11] More recently, e.g., vacancy-helium and vacancy-interstitial interactions have been elucidated in irradiated W [12,13] as well as vacancy-hydrogen interactions in Nb. [14] In elemental semiconductors, the determination of migration and formation mechanisms of vacancy-multidonor complexes in Si [15,16] and Ge [17] DOI: 10.1002/pssr.202100177 Three topical materials systems are discussed from the point of view of point defect characterization with positron annihilation spectroscopy. The family of IIInitride semiconductors and device structures made thereof poses interesting challenges for data interpretation due to preferential localization and annihilation with various elements.…”

“…[ 11 ] More recently, e.g., vacancy‐helium and vacancy‐interstitial interactions have been elucidated in irradiated W [ 12,13 ] as well as vacancy‐hydrogen interactions in Nb. [ 14 ] In elemental semiconductors, the determination of migration and formation mechanisms of vacancy–multidonor complexes in Si [ 15,16 ] and Ge [ 17 ] can be highlighted, as well as the direct observation of monovacancy migration at low temperatures in both materials. [ 18,19 ] Positron annihilation methods have revealed important vacancy‐driven phenomena in the ageing of dilute metal alloys, shedding light on, e.g., Cu precipitates in Fe, [ 20 ] interactions of quenched‐in vacancies and solutes in Al alloys, [ 21–24 ] or plastic deformation in various steel grades.…”

Direct evidence of microstructure dependence of magnetic flux trapping in niobium

“…Techniques that have been shown to beneficially alter the Nb surface within the first few nanometers to microns include low temperature baking 7 , high-temperature heat treatments 8 , and diffusion of solutes titanium (Ti) 8 , nitrogen (N) 9 at high temperatures of 800 -1400 °C. Surface modifications change the nature of surface oxide 10 , hydrogen concentration 11,12 , and create a dirty superconducting layer [13][14][15] . Promising new results indicate high Q 0 at high E acc up to 40 MV/m is possible by low-temperature N treatments in the range of 120-200 °C 13,16,17 .…”