Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

Sakamoto, Kunio; Une, Katsumi; Aomi, Masaki; Otsuka, Teppei; Hashizume, Kazuyoshi

doi:10.1080/00223131.2015.1058196

Cited by 15 publications

(6 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…However, experimental work [23,24] has consistently suggested Nb in ZrO 2 in fact occupies a range of states between 0 and 5+, with one X-ray absorption near edge structure (XANES) study by Froideval et al [25] unable to identify any 5+ Nb at all, thus casting doubt on the previous assumptions regarding the method by which Nb improves corrosion resistance and HPUF. Nb and Sn demonstrate some solid solubility in α-Zr and so an even distribution throughout the alloy is expected; this is an important point as the current investigation considers only single and paired defects isolated in a bulk oxide and thus an even distribution of dopants in the oxide layer is assumed.…”

Section: ••mentioning

confidence: 90%

The influence of alloying elements on the corrosion of Zr alloys

et al. 2016

View full text Add to dashboard Cite

Density functional theory (DFT) and autoclave corrosion tests in 360°C water were used to investigate the influence of Sb, Sc, Nb and Sn on the corrosion and hydrogen pick-up (HPU) of Zr-alloys. Sc was shown to have a strongly detrimental effect on alloy corrosion resistance. The Nb-Sb-Zr ternary alloy exhibited significantly improved corrosion resistance over Zr-Nb and ZIRLO, and had little measurable HPU after 195 days. The ratio of Sb Zr /Sb• Zr was shown to transition smoothly with applied space charge, implying Sb can act as a buffer to charge imbalance in the oxide layer.

show abstract

Section: ••mentioning

confidence: 90%

The influence of alloying elements on the corrosion of Zr alloys

et al. 2016

View full text Add to dashboard Cite

show abstract

“…The assumption that Nb exists only in the 5+ charge state in the oxide layer is supported by previous DFT simulation work [11]. However, several independent x-ray absorption near-edge spectroscopy (XANES) studies have identified oxidation states ranging from 5+ to 2+ (and also metallic Nb) in the oxide layer of Zr-Nb alloys [12,13,14,15,16]. Recently there has been some debate regarding the location of these lower charge state Nb ions (i.e.…”

Section: Introductionmentioning

confidence: 94%

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

et al. 2017

View full text Add to dashboard Cite

Zr-Nb alloys are known to perform better in corrosion and hydrogen pick-up than other Zr alloys but the mechanism by which this happens is not well understood. Atomistic simulations using density functional theory of both tetragonal and monoclinic ZrO2 were performed, with intrinsic defects and Nb dopants. The overall defect populations with respect to oxygen partial pressure were calculated and presented in the form of Brouwer diagrams. Nb is found to favour 5 + in monoclinic ZrO2 at all partial pressures, but can exist in oxidation states ranging from 5 + to 3 + in the tetragonal phase. Nb5+ is charge balanced by Zr vacancies in both phases, suggesting that contrary to previous assumptions, Nb does not act as an n-type dopant in the oxide layer. Clusters containing oxygen vacancies were considered, Nb2+ was shown to exist in the tetragonal phase with a binding energy of 2.4 eV. This supports the proposed mechanism whereby low oxidation state Nb ions (2 + or 3+) charge balance the build-up of positive space-charge in the oxide layer, increasing oxygen vacancy and electron mobility, leading to near-parabolic corrosion kinetics and a reduced hydrogen pick-up. Previous experimental work has shown that tetragonal ZrO2 transforms to the monoclinic phase during transition, and that during transition a sharp drop in the instantaneous hydrogen pick-up fraction occurs. The oxidation of lower charge state Nb defects to Nb5+ during this phase change, and the consequent temporary n-doping of the oxide layer, is proposed as an explanation for the drop in hydrogen pick-up during transition

show abstract

“…We have therefore tested near-edge spectra of Nb 2 O 5 (Nb 5+ ), NbO 2 (Nb 4+ ) and NbO (Nb 2+ ) for the modelling of the data. The XANES spectra of those Nb oxides show substantial differences at the edge and in the post-edge region (see, for example, Sakamoto et al, 2015), thus sensitively allowing their discrimination. It appears that by using the XANES of NbO, the fit errors and the uncertainties for the compositions are substantially smaller compared with the other oxides.…”

Section: Heat Treatments In Air At Low Pressuresmentioning

confidence: 99%

High-temperature treatments of niobium under high vacuum, dilute air- and nitrogen-atmospheres as investigated by in situ X-ray absorption spectroscopy

Klaes

Rothweiler

Bornmann

et al. 2021

J Synchrotron Radiat

View full text Add to dashboard Cite

Niobium metal foils were heat-treated at 900°C under different conditions and in situ investigated with time-resolved X-ray absorption fine-structure (EXAFS and XANES) measurements. The present study aims to mimic the conditions usually applied for heat treatments of Nb materials used for superconducting radiofrequency cavities, in order to better understand the evolving processes during vacuum annealing as well as for heat treatments in controlled dilute gases. Annealing in vacuum in a commercially available cell showed a substantial amount of oxidation, so that a designated new cell was designed and realized, allowing treatments under clean high-vacuum conditions as well as under well controllable gas atmospheres. The experiments performed under vacuum demonstrated that the original structure of the Nb foils is preserved, while a detailed evaluation of the X-ray absorption fine-structure data acquired during treatments in dilute air atmospheres (10−5 mbar to 10−3 mbar) revealed a linear oxidation with the time of the treatment, and an oxidation rate proportional to the oxygen (air) pressure. The structure of the oxide appears to be very similar to that of polycrystalline NbO. The cell also permits controlled exposures to other reactive gases at elevated temperatures; here the Nb foils were exposed to dilute nitrogen atmospheres after a pre-conditioning of the studied Nb material for one hour under high-vacuum conditions, in order to imitate typical conditions used for nitrogen doping of cavity materials. Clear structural changes induced by the N2 exposure were found; however, no evidence for the formation of niobium nitride could be derived from the EXAFS and XANES experiments. The presented results establish the feasibility to study the structural changes of the Nb materials in situ during heat treatments in reactive gases with temporal resolution, which are important to better understand the underlaying mechanisms and the dynamics of phase formation during those heat treatments in more detail.

show abstract

Change of chemical states of niobium in the oxide layer of zirconium–niobium alloys with oxide growth

Cited by 15 publications

References 19 publications

The influence of alloying elements on the corrosion of Zr alloys

The influence of alloying elements on the corrosion of Zr alloys

The effect of Nb on the corrosion and hydrogen pick-up of Zr alloys

High-temperature treatments of niobium under high vacuum, dilute air- and nitrogen-atmospheres as investigated by in situ X-ray absorption spectroscopy

Contact Info

Product

Resources

About