Niobium Oxide Phases Research Articles

Improved bioactivity and corrosion resistance of NbO containing plasma electrolytic oxidation coating on ZM21 Mg alloy

ABSTRACT The present study reports the effect of niobium oxide (NbO) phase on the microstructure, corrosion, and bioactivity of coating on ZM21 Mg alloy by plasma electrolytic oxidation (PEO) process. NbO containing coatings are obtained by adding Potassium Niobate (KNbO3) along with the base electrolyte system, while the base electrolyte system consisted of sodium phosphate (Na3PO4⋅12 H2O) and potassium hydroxide (KOH). X-Ray Diffraction (XRD) was used for phase analysis, and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) was employed to extract the microstructural features. Tafel curves were generated to study the corrosion behaviour, and bioactivity test was conducted in the simulated body fluid (SBF) for 14 days to study the bioactivity. XRD results revealed that the PEO coatings consisted of MgO phase, whereas the sample coated in the electrolyte containing KNbO3 showed NbO phase in addition to MgO phase. NbO containing coating showed higher corrosion resistance and excellent bioactivity.

Structural Distortion in the Wadsley-Roth Niobium Molybdenum Oxide Phase Triggering Extraordinarily Stable Battery Performance.

Wadsley-Roth niobium oxide phases have attracted extensive research interest recently as promising battery anodes. We have synthesized the niobium-molybdenum oxide shear phase (Nb, Mo)13 O33 with superior electrochemical Li-ion storage performance, including an ultralong cycling lifespan of at least 15000 cycles. During electrochemical cycling, a reversible single-phase solid-solution reaction with lithiated intermediate solid solutions is demonstrated using in situ X-ray diffraction, with the valence and short-range structural changes of the electrode probed by in situ Nb and Mo K-edge X-ray absorption spectroscopy. This work reveals that the superior stability of niobium molybdenum oxides is underpinned by changes in octahedral distortion during electrochemical reactions, and we report an in-depth understanding of how this stabilizes the oxide structure during cycling with implications for future long-life battery material design.

Ultrafast-laser powder bed fusion of oxygen-deficient Nb2O5 ceramics with highly improved electrical properties

In this work, Nb2O5 layers with highly improved electrical properties respect to pristine material have been produced by ultrafast-laser powder bed fusion process. The conditions required for producing uniform and compact layers of Nb2O5 from powder material have been studied and optimized. It has been established that ultrafast-laser irradiation, performed in air at room temperature, leads to the formation of dense Nb2O5 layers with the high temperature monoclinic crystal structure (H-Nb2O5) but oxygen deficient. The layers show a preferential crystal orientation with the short axis of the monoclinic structure lying in the structure plane. This preferential orientation can be controlled by the laser irradiation conditions. Anisotropic resistivity has been observed as a consequence of the induced microstructure, while the overall material resistivity is decreased by more than eight orders of magnitude due to the oxygen deficiency. These results indicate that it is feasible to use ultrafast laser processing to promote high-temperature non-stoichiometric niobium oxide phases in a few seconds and with low energy consumption. The highly improved electrical properties of the laser irradiated Nb2O5 layers are extremely interesting for different electronic and sensing applications.

Large and constant absorption coefficient in NbxTi1−xO2 thin films throughout the visible range

We investigated the optical absorption properties of NbxTi1−xO2 thin films, a solid solution combining the reduced titanium and niobium oxide phases TiO2 and NbO2. The optical absorption properties of NbxTi1−xO2 thin films prepared by pulsed laser deposition at 600 °C were shown to be large in magnitude at an almost constant value of ≈17 µm−1. Because this large absorption coefficient is nearly independent of incident photon energy in the visible range (400–700 nm), the NbxTi1−xO2 thin films appear optically black. Flat and homogenous, optically black coatings like these are desirable for color isolation in flat panel displays. The origin of flat wavelength dispersion in NbxTi1−xO2 is the coexistence of semiconducting absorption and metallic light absorption mechanisms. Localized Nb-Nb dimers in the metallic NbxTi1−xO2 phase open an optical band gap which gives rise to semiconducting behavior. We show here that reduction results in strong visible light sensitization in normally transparent Nb-Ti oxides.

Effect of niobium oxide phase on the furfuryl alcohol dehydration

Different structural phases (e.g., TT, T, and H) of niobium oxide were synthesized, characterized by XRD and Raman, and utilized for the furfuryl alcohol dehydration and condensation under mild conditions (100°C and ambient pressure). Furfuryl alcohol conversion was dependent on reaction time and niobium oxide phase. Niobic acid and T/H phase transitional niobium oxide showed higher catalytic activity in comparison to a single crystalline phase niobium oxide. While T/H phase transitional niobium oxide showed higher conversion than that of niobic acid and TT phase niobium oxide, higher C9-C15 products' selectivity (>60%) was obtained with the latter catalysts.

Combinatorial Investigations of High Temperature CuNb Oxide Phases for Photoelectrochemical Water Splitting.

High-throughput combinatorial methods have been useful in identifying new oxide semiconductors with the potential to be applied to solar water splitting. Most of these techniques have been limited to producing and screening oxide phases formed at temperatures below approximately 550 °C. We report the development of a combinatorial approach to discover and optimize high temperature phases for photoelectrochemical water splitting. As a demonstration material, we chose to produce thin films of high temperature CuNb oxide phases by inkjet printing on two different substrates: fluorine-doped tin oxide and crystalline Si, which required different sample pyrolysis procedures. The selection of pyrolysis parameters, such as temperature/time programs, and the use of oxidizing, nonreactive or reducing atmospheres determines the composition of the thin film materials and their photoelectrochemical performance. XPS, XRD, and SEM analyses were used to determine the composition and oxidation states within the copper niobium oxide phases and to then guide the production of target Cu(1+)Nb(5+)-oxide phases. The charge carrier dynamics of the thin films produced by the inkjet printing are compared with pure CuNbO3 microcrystalline material obtained from inorganic bulk synthesis.

Observation of the Microstructure of Grain Boundary Oxides in Superconducting RF-Quality Niobium With High-Resolution TEM (Transmission Electron Microscope)

All superconducting radio-frequency (RF) cavities are currently manufactured using high-purity Nb sheet, and although properties of the best cavities are excellent, there is still considerable variability, and we examine here the chemistry at the Nb grain boundaries (GBs), which may be a source of cavity performance degradation. Some oxidization at GBs in niobium cavities may be inevitable during the long cavity fabrication process; due to the formation of hydrosoluable niobium oxide phases during the surface removal procedure, GBs in SRF Nb can be easily attacked, resulting in the formation of uncontrolled complex niobium oxides at GBs. These GB oxide phases can significantly suppress local surface superconductivity during RF cavity operation. Thus, observation of the GB oxide structure with high-resolution transmission electron microscope (TEM) has been attempted to identify the microstructural details that affect SRF Nb cavity performance. In this paper, we developed a TEM sample preparation technique for producing damage-free SRF NB samples with good electron transparency using metallographic and focused ion beam (FIB) methods. These sample preparation techniques enabled us to successfully observe the microstructure of a variety of GBs with high-resolution TEM. TEM bright field (BF) images showed that optimized cavity surface treatment can generate impurity segregations, oxide phases, and dislocation pile-ups at the vicinity of the GBs in SRF Nb. These surface defects are predicted to be strong sources for localized degradation of surface superconductivity, and these results help explain previous magnetooptical (MO) measurements that showed early magnetic flux penetration along the GB of BCP'ed Nb bicrystal under a dc condition.

Functional DMFC Cathode Catalysts and Supports Based on Niobium Oxide Phase

Composite electrocatalysts consisting of platinum nanophase supported on carbon black decorated with niobium oxide phase were prepared by a two step precipitation/reduction procedure. The intrinsic catalytic activity of these materials was evaluated for the oxygen electroreduction reaction, measured in the absence and presence of methanol. It was found that the NbxOy-composite materials are more hydrophilic and thus initially have lower intrinsic catalytic activity for oxygen reduction. However, materials containing NbxOy offer better overall platinum mass utilization, achieved through much better dispersion of the platinum nanophase. Such advantageous dispersion is observed even for high metal loadings. Another advantage of these materials is minimal effect of poisoning with the intermediates formed during methanol oxidation that results in the enhanced methanol tolerance in direct methanol fuel cells (DMFC).

Structure Characterization of ORR Active Niobium Oxide Phases in Partially Oxidized NbCN Catalysts

not Available.

Effects of Acidity and Pore Size Constraints on Supported Niobium Oxide Catalysts for the Selective Formation of Glycerol Monolaurate

AbstractSupported niobium oxide catalysts with 5–30 wt. % Nb2O5 were prepared by grafting niobium ethoxide onto MCM‐41 and hydrous zirconia. The supported samples contain Brønsted‐acid sites, whereas bulk niobium oxide has predominantly Lewis‐acid sites. In the esterification of glycerol with lauric acid, good activity and a high glycerol monolaurate selectivity was achieved when the active niobium oxide phase was coated within the pore channels of an MCM‐41 support, which has a mean pore diameter of 2.2 nm. Phosphated Nb2O5/MCM‐41 showed an even higher activity without any penalty in selectivity, enabling monolaurate yields of 89–90 %. Glycerol monolaurate is an important ingredient in the food, pharmaceutical, and cosmetic industries. A key feature of these pore confined catalysts is the sustained high selectivity to monolaurate even at high conversions. In contrast, niobium oxide supported on wide‐pored hydrous zirconia showed lower selectivity. The catalysts are active in the solventless esterification of a number of alcohols and acids and can be easily recycled for subsequent batch reactions, making them attractive as green catalysts for sustainable processes.

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part I: Plasmon and Near-Edges Fine Structure

A comprehensive electron energy-loss spectroscopy study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. In this work (Part I), the plasmons and energy-loss near-edge structures (ELNES) of all relevant Nb edges (Nb-N2,3, Nb-M4,5, Nb-M2,3, Nb-M1, and Nb-L2,3) up to energy losses of about 2600 eV and the O-K edge are analyzed with respect to achieving characteristic fingerprints of Nb in different formal oxidation states (0 for metallic Nb, +2 for NbO, +4 for NbO2, and +5 for Nb2O5). Chemical shifts of the Nb-N2,3, Nb-M4,5, Nb-M2,3, and Nb-L2,3 edges are extracted from the spectra that amount to about 4 eV as the oxidation state increases from 0 for Nb to +5 for Nb2O5. Four different microscopes, including a 200 keV ZEISS Libra with monochromator, were used. The corresponding wide range of experimental parameters with respect to the primary electron energy, convergence, and collection semi-angles as well as energy resolution allows an assessment of the influence of the experimental setup on the ELNES of the different edges. Finally, the intensity of the Nb-L2,3 white-line edges is correlated with niobium 4d-state occupancy in the different reference materials.

EELS of Niobium and Stoichiometric Niobium-Oxide Phases—Part II: Quantification

A comprehensive electron energy-loss spectroscopy (EELS) study of niobium (Nb) and stable Nb-oxide phases (NbO, NbO2, Nb2O5) was carried out. Part II of this work is devoted to quantitative EELS by means of experimental k-factors derived from the intensity ratio of the O-K edge and the Nb-M4,5 or Nb-M2,3 edges for all three stable Nb-oxides. The precision and accuracy of the quantification are investigated with respect to the influence of intensity-measurement energy windows, background subtraction, and sample thickness. Integration-window widths allowing optimum accuracy are determined. Owing to background-subtraction errors, the Nb-M4,5 edges rather than Nb-M2,3 are preferred for quantification. Different approaches are applied to improve the precision with regard to thickness-related errors. Thus, a precision up to +/-1.5% is achieved by averaging spectra from all three reference oxides to determine a k-factor using Nb-M4,5. Using the experimental k-factor, the determination of atomic concentration ratios CNb/CO in the range of 0.4 (Nb2O5) to 1 (NbO) was found to be possible with an accuracy of 0.6% (relative deviation between measured and nominal composition), whereas ratios of calculated partial ionization cross sections lead to less accurate results.

Nitridation of niobium oxide films by rapid thermal processing

The possibility of forming niobium oxynitride through the nitridation of niobium oxide films in molecular nitrogen by rapid thermal processing (RTP) was investigated. Niobium films 200 and 500 nm thick were deposited via sputtering onto Si(100) wafers covered with a thermally grown SiO(2) layer 100 nm thick. These as-deposited films exhibited distinct texture effects. They were processed in two steps using an RTP system. The as-deposited niobium films were first oxidized under an oxygen atmosphere at 450 degrees C for various periods of time and subsequently nitridated under a nitrogen atmosphere at temperatures ranging from 600 to 1000 degrees C for 1 min. Investigations of the oxidized films showed that samples where the start of niobium pentoxide formation was detected at the surface and the film bulk still consisted of a substoichiometric NbO(x) phase exhibited distinctly lower surface roughness and microcrack densities than samples where complete oxidation of the film to Nb(2)O(5) had occurred. The niobium oxide phases formed at the Nb/substrate interface also showed distinct texture. Zones of niobium oxide phases like NbO and NbO(2), which did not exist in the initial oxidized films, were formed during the nitridation. This is attributed to a "snow-plough effect" produced by the diffusion of nitrogen into the film, which pushes the oxygen deeper into the film bulk. These oxide phases, in particular the NbO(2) zone, act as barriers to the in-diffusion of nitrogen and also inhibit the outdiffusion of oxygen from the SiO(2) substrate layer. Nitridation of the partially oxidized niobium films in molecular nitrogen leads to the formation of various niobium oxide and nitride phases, but no indication of niobium oxynitride formation was found.

TEM studies of Nb2O5 catalyst in ball-milled MgH2 for hydrogen storage

In this paper we report microstructural studies by transmission electron microscopy (TEM) of magnesium hydride nanocomposites produced by ball milling. The work employed high angle annular dark field (HAADF) imaging, high resolution TEM (HRTEM), and electron energy loss spectroscopy (EELS). MgH2 samples had been prepared by ball-milling with 17wt% of Nb2O5. Samples milled for a short time (30h) show a decrease in particle size in comparison to MgH2 milled alone for the same amount of time. After only 30h milling, finely dispersed Nb2O5 was detected within the MgH2 matrix. After milling for longer times (300h) the amount of Nb2O5 dispersed within the Mg matrix increases. Analysis of material that had been milled for 300h and then subjected to hydrogen desorption by heating showed that MgH2 and Nb2O5 interact during the thermal desorption forming both a partially reduced niobium oxide phase and a mixed Nb-Mg-O phase.

EELS Investigations of Different Niobium Oxide Phases

Electron energy loss spectra in conjunction with near-edge fine structures of purely stoichiometric niobium monoxide (NbO) and niobium pentoxide (Nb2O5) reference materials were recorded. The structures of the niobium oxide reference materials were checked by selected area electron diffraction to ensure a proper assignment of the fine structures. NbO and Nb2O5 show clearly different energy loss near-edge fine structures of the Nb-M4,5 and -M2,3 edges and of the O-K edge, reflecting the specific local environments of the ionized atoms. To distinguish the two oxides in a quantitative manner, the intensities under the Nb-M4,5 as well as Nb-M2,3 edges and the O-K edge were measured and their ratios calculated. k-factors were also derived from these measurements.

Alkoxide Route to Mixed Oxides of Rhenium, Niobium, and Tantalum. Preparation and X-ray Single-Crystal Study of a Novel Rhenium−Niobium Methoxo Complex, Nb2(OMe)8(ReO4)2

Thermal decomposition of complex oxomethoxo compounds Nb4O2(OMe)14(ReO4)2 (I) and Ta4O2(OMe)14(ReO4)2 (II) has been investigated. The decomposition of (II) at temperatures exceeding 100 °C results in the formation of MeOMe, MeOCH2OMe, MeOH, and H2O vapor and is accompanied by sublimation at temperatures over 135 °C. Plausible mechanisms of the processes studied are discussed. A new bimetallic methoxo compound, Nb2(OMe)8(ReO4)2 (III), was synthesized by interaction between Re2O7 and Nb2(OMe)10 in toluene solution and its structure was determined by means of an X-ray single-crystal study. The structure can be considered as a product of substitution for terminal methoxo groups with two perrhenate groups in dimeric Nb2(OMe)10 molecules. Decomposition of II in an inert atmosphere at 900 °C gave a Re−Ta mixed oxide phase, supposedly based on the L-Ta2O5 structure. Decomposition of I and III under similar conditions leads to the formation of rhenium−niobium oxide phases. The results obtained provide a basis for ...

Niobium oxide thin films formed by plasma immersion oxygen ion implantation

In analogy to conventional beam-line ion implantation, plasma immersion ion implantation can be combined with a deposition technique to an ion assisted coating process. The structure and composition of a coating and its interface to the substrate can be modified by ion implantation. By means of electron beam evaporation and oxygen plasma immersion ion implantation niobium oxide films were prepared at low substrate temperatures ( <200 °C). The film composition and thickness were determined by Rutherford backscattering spectrometry. The results show that oxygen plasma immersion ion implantation leads to incorporation of oxygen into niobium in several steps, corresponding to niobium oxide phases with different stoichiometries. By contrast to conventional beam-line ion implantation at low pressures, two channels for oxidation can be distinguished, ion implantation of high-energy species and radiation enhanced in-diffusion of low-energy species from the plasma. The latter is driven by thermodynamic forces.

Electronic structure and crystal chemistry of niobium oxide phases.

Results of local-density linear muffin-tin orbital (LMTO) electronic-structure calculations for both known and recently obtained niobium oxide phases with different crystal lattices and different coordination of Nb atoms are presented. Perovskite-like compounds with one- or two-dimensional Nb-O condensed clusters, as well as hexagonal layered structures with trigonal-prismatic coordination of Nb atoms, are used. For ${\mathrm{SrNbO}}_{3}$, ${\mathrm{BaNb}}_{5}$${\mathrm{O}}_{8}$, ${\mathrm{BaNb}}_{4}$${\mathrm{O}}_{6}$, and ${\mathrm{Sr}}_{2}$${\mathrm{Nb}}_{5}$${\mathrm{O}}_{9}$, the Fermi level is located at the shoulder of the Nb d conduction band with a low density of oxygen states. For hexagonal ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ (M=Li,Na) compounds, oxygen contributions definitely appear at ${\mathit{E}}_{\mathit{F}}$, but remain small compared with those of the high-${\mathit{T}}_{\mathit{c}}$ superconducting cuprates. Pair-interaction energies and superexchange-interaction parameters were calculated using the LMTO Green-function method. Typical for high-${\mathit{T}}_{\mathit{c}}$ superconductors, antiferromagnetic coupling between metal atoms in the Nb-O planes is shown to exist, but not the ferromagnetic exchange between metal d and oxygen p states. The calculations show that perovskite niobium oxide systems are not good candidates in the search for new high-${\mathit{T}}_{\mathit{c}}$ superconducting materials, but that hexagonal layered niobium oxide phases of the type considered in this paper might be of interest in studies of low ${\mathit{T}}_{\mathit{c}}$ superconducting oxides.

Electronic structure and electron-phonon interactions in layered LixNbO2 and NaxNbO2.

Results of full-potential linear-muffin-tin-orbital calculations for the new intercalated niobium oxide phases ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ and ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ are presented. It is shown that alkali atoms are ionized, and their valence electrons completely fill the hybridized Nb d--O 2p conduction band in ${\mathit{M}}_{1.0}$${\mathrm{NbO}}_{2}$ crystals making them semiconducting. In the nonstoichiometric ${\mathit{M}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ phases, the Fermi level is located well inside the conduction band of predominant Nb 4d nature with significant contributions from O 2p states. Estimates of electron-phonon interaction parameters and ${\mathit{T}}_{\mathit{c}}$ in the scope of the rigid-ion approximation result in ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}12 K for ${\mathrm{Li}}_{0.5}$${\mathrm{NbO}}_{2}$ and ${\mathit{T}}_{\mathit{c}}$\ensuremath{\sim}9 K for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ in reasonable agreement with the experimental data---and show that superconductivity observed in these materials is of the usual electron-phonon type, similar to that in cubic ${\mathrm{Li}}_{0.5}$${\mathrm{TiO}}_{2}$. It is shown that the density of states at the Fermi level and ${\mathit{T}}_{\mathit{c}}$ for ${\mathrm{Na}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$ should be more critically dependent on stoichiometry than for ${\mathrm{Li}}_{\mathit{x}}$${\mathrm{NbO}}_{2}$. Since the electronic-structure peculiarities obtained in the calculations are very different than those typical for superconducting cuprates, it does not appear likely that any high-${\mathit{T}}_{\mathit{c}}$ superconductors should be expected in this class of compounds.

Molecular structures of supported niobium oxide catalysts under ambient conditions

The supported niobium oxide catalysts were investigated by Raman spectroscopy and X-ray photoelectron spectroscopy in order to determine the molecular structure and monolayer coverage of the surface niobium oxide phase on oxide supports (MgO, Al 2O 3, TiO 2, ZrO 2, and SiO 2). The molecular structures of the surface niobium oxide phases present in the supported niobium oxide catalysts under ambient conditions, where adsorbed moisture is present, are controlled by the surface pH of the system. Basic surfaces result in the formation of highly distorted NbO 6 groups and acidic surfaces result in the formation of slightly distorted NbO 6, NbO 7 and NbO 8 groups. The surface niobium oxide overlayer is stable to high calcination temperatures due to the strong surface niobium oxide-support interaction. The monolayer coverage of supported niobium oxide catalysts is reached at ≈19 wt.% Nb 2O 5/Al 2O 3, ≈7 wt.% Nb 2O 5/TiO 2, ≈5 wt.% Nb 2O 5/ZrO 2 and ≈2 wt.% Nb 2O 5/SiO 2, but not for the Nb 2O 5/MgO system due to the incorporation of Nb +5 into the MgO support.