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

Abstract

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.

Design of Cathode Coating Using Niobate and Phosphate Hybrid Material for Sulfide-Based Solid-State Battery

“…Figure shows the Nb K-edge and P K-edge XAS spectra of the coated NCM523. The first inflection point (absorption edge) of the white-line peak is a proportional function to the formal chemical valence number of Nb. , This was previously confirmed for reference materials of the Li-Nb-O system . There was no change in the absorption edge of each LiP x Nb 1– x O 3 coating (Figure a) ( x = 0, 0.1, and 0.5; x = 0.75 was excluded because the Nb content was too low for accurate analysis).…”

“…XPS (a) Nb 3d and (b) P 2p spectra of x = 0–1 in LiP x Nb 1– x O 3 -coated NCM523. The dotted lines are peak positions of reference materials of Nb 3d 5/2 and P 2p. ,− …”

Degradation Analysis by X-ray Absorption Spectroscopy for LiNbO3 Coating of Sulfide-Based All-Solid-State Battery Cathode

“…First, the XAS spectra of Nb K-edge were measured for the cathode samples before and after the floating tests at 4.25–4.55 V vs Li/Li + in order to investigate the valence number of Nb (Figure ). It is well-known that the first inflection point (absorption edge) of the white-line peak is a function of the formal chemical valence number of the Nb, , and the spectra of the reference materials also show the same tendency (Figure a,b). Almost no shift was observed in the cathode samples before and after floating tests, which means that the valence number of Nb in cathode samples had remained unchanged at 5+ throughout based on comparison with reference materials (Figure c).…”

One-dimensional van der Waals transition metal chalcogenide as an anode material for advanced lithium-ion batteries

“…The oxidation states of Nb 2 Se 9 electrodes were analyzed in detail by examining the first derivatives of the Nb K-edge, as illustrated in Fig. S3,† in light of prior research by Klaes et al 48 The edge positions of Nb were decided by the first inflection points of the spectra. Initially, Nb in pristine Nb 2 Se 9 exhibited an oxidation state of +2.88, which was subsequently reduced to +0.13 upon the first full discharge (FD) and eventually restored to +2.95 after full charge (FC), approximating its original state.…”