Nb Atoms Research Articles

X-ray diffraction and theoretical study of the transition 2H-3R polytypes in Nb1+xSe2 (0 < x < 0.1)

Single crystals of 2H and 3R niobium diselenide were grown by a chemical transport reaction. The current determinations by single crystals X-ray diffraction reveal a non-stoichiometric composition. The structures are built from Se—Nb—Se slabs with Nb in trigonal prismatic coordination whereas the extra or additional Nb atoms are located in the octahedral holes between the slabs giving rise to the formula 2H and 3R-Nb1+xSe2 with 0.07 < x < 0.118. In particular, vacancy and Nb-Nb interactions may play an important role on the non-stoichiometry and the stacking mode in NbSe2. By increasing the number of additional Nb atoms in the pure 2H-NbSe2, a transition 2H to 3R polytype should occur in order to minimize the Nblayer—Nbextra—Nblayer repulsions between these adjacent slabs. The theoretical study shows that both 2H and 3R-Nb1+xSe2 are thermodynamically stable in the range 0 < x < 0.1.

Theoretical investigation of novel half Heusler compounds MRhSb (M = Nb & Ta): For optoelectronic and thermoelectric applications

AbstractThe main objective of this work is to make a detailed study on a new class of half heuslers which possess a 19 valence electron and which are sought for perpetually because of their thermoelectric performances. The mechanical, electronic structures, optical, and electrical transport properties are studied using full potential linearized augmented plane wave (LAPW) + local orbitals (lo) scheme, in the framework of density functional theory (DFT) with generalized gradient approximation (GGA) for the purpose of exchange correlation energy functional. The electronic structure is treated by the TB‐mBJ exchange‐correlation potentials. The independent elastic constants and the related mechanical properties are investigated. From the energy bands and density of states it is observed that the 3d‐states of Nb, Ta, and Rh atoms contribute mainly to the conduction band, which results in increase in electrical and thermal conductivity of NbRhSb and TaRhSb . The optical constants as the dielectric function, refractive index, optical reflectivity, and absorption coefficient were calculated and discussed in detail. The dependence of Seebeck coefficient, electrical conductivity, and power factor on the Fermi level is investigated.

3d Transition-Metal-Mediated Columbite Nanocatalysts for Decentralized Electrosynthesis of Hydrogen Peroxide.

Decentralized electrosynthesis of hydrogen peroxide (H2 O2 ) via oxygen reduction reaction (ORR) can enable applications in disinfection control, pulping and textile bleaching, wastewater treatment, and renewable energy storage. Transition metal oxides are usually not efficient catalysts because they are more selective to produce H2 O. Here, it is shown that divalent 3d transition metal cations (Mn, Fe, Co, Ni, and Cu) can control the catalytic activity and selectivity of columbite nanoparticles. They are synthesized using polyoxoniobate (K7 HNb6 O19 ·13H2 O) and divalent metal cations by a hydrothermal method. The optimal NiNb2 O6 holds an H2 O2 selectivity of 96% with the corresponding H2 O2 Faradaic efficiency of 92% in a wide potential window from 0.2 to 0.6V in alkaline electrolyte, superior to other transition metal oxide catalysts. Ex situ X-ray photoelectron and operando Fourier-transformed infrared spectroscopic studies, together with density functional theory calculations, reveal that 3d transition metals shift the d-band center of catalytically active surface Nb atoms and change their interactions with ORR intermediates. In an application demonstration, NiNb2 O6 delivers H2 O2 productivity up to 1 molH2O2 gcat -1 h-1 in an H-shaped electrolyzer and can yield catholytes containing 300× 10-3 m H2 O2 to efficiently decomposing several organic dyes. The low-cost 3d transition-metal-mediated columbite catalysts show excellent application potentials.

Tuning the electronic and magnetic properties of CoZrxNb1−xFeSi alloys for spintronic and thermoelectric applications

AbstractThis ab‐initio investigation presents the effect of substitution of Zr by Nb atom on the structural, electronic, magnetic, and transport properties of the quaternary Heusler CoZrFeSi alloy. The ab‐initio calculations were performed using the exchange‐correlation of the modified Becke‐Johnson potential. Our results predict that the crystalline phase for each of CoZrFeSi and CoNbFeSi alloys is classified as Type‐I. The calculated formation energy value of the Nb‐doped alloys indicates the thermodynamic stability of the alloys and their possible creation. The bulk modulus value predicts that the rigidity increases with increasing the concentration of the Nb substitute atoms in the alloy. The electronic band structure and the density of states indicate a half metallicity of the alloys with metallic and semiconducting behaviors in the spin up and down, respectively. The studied alloys have shown ferromagnetic character with a high magnetic moment value when increasing the Nb content up to 75%. The calculated Seebeck coefficient and the charge carrier concentration as a function of temperature predict that the present alloys prefer the n‐type doping with the ability to switch between hole and electron for transporting the charges through the Fermi level of CoZr1−xNbxFeSi (x = 0, 0.25, 0.75, and 1) alloys. However, CoZr0.5Nb0.5FeSi alloy prefers electrons as a charge carrier. Due to their determined properties, beneficial applications are predicted for the present alloys, such as magnetic tunnel junctions and spintronic applications.

Synthesis of novel volatile niobium precursors containing carboxamide for Nb2O5 thin films

A series of novel Nb(V) tBu-imido/N-alkoxy carboxamide complexes, NbCl2(NtBu)(pyridine)(edpa) (1), NbCl2(NtBu)(pyridine)(mdpa) (2), and Nb(NtBu)(mdpa)3 (3), were successfully synthesized by metathesis reaction between Nb(NtBu)Cl3(py)2 and several equivalents of Na(edpa) (edpaH = Nethoxy-2,2-dimethylpropanamide) and Na(mdpa) (mdpaH = N-methoxy-2,2-dimethylpropanamide). These complexes could potentially serve as precursors for Nb-containing thin films. Compounds 1–3 were characterized by FT-IR and 1H and 13C NMR spectroscopies, elemental analysis, and thermogravimetric analysis (TGA). Single crystal X-ray diffraction analysis suggested that complex 3 has a distorted pentagonal bipyramidal geometry around the central Nb atom, with three bidentate mdpa ligands and one tBu imido group saturating the coordination. TGA analysis revealed that the thermal characteristics of 3 were superior to those of 1 and 2. All prepared complexes were able to be distilled or sublimed and expected potential precursors for the fabrication of thin films.

Ab initio molecular dynamics study on local structure and dynamic properties of liquid Ni62Nb38 alloy

In the temperature range between 1873 K and 1233 K, structural evolution and dynamic properties of liquid Ni62Nb38 alloy have been studied by ab initio molecular dynamics. The evolution of local structure was characterized by pair distribution functions, structure factors, bond angle distributions, coordination numbers and chemical short-range order parameters, and electronic structure is mainly reflected by the density of states. Self-diffusion coefficients were calculated out to represent dynamic properties. It is found that with the decrease of temperature, short-range order tends to increase and medium-range order can be discovered in the under-cooling system. Meanwhile, a series of complex local structure like tetrahedron and icosahedron gradually forms in the liquid. Moreover, the whole system is hetero-coordinating. During the whole process, both Ni and Nb diffusion coefficients show a decreasing trend, effectively prohibiting nucleation and crystal growth, and it is beneficial to the amorphous transformation of liquid Ni62Nb38 alloy. The formation of chemical bonds between Ni and Nb atoms is mainly because of the hybridization interaction between Ni and Nb d-states.

Influences of Vacancy Concentration and Al Substitution on Structural, Electronic, and Elastic Properties of Nb5Si3 from First‐Principles Calculations

The effects of vacancy concentration and Al substitution on the structural, electronic, and elastic properties of Nb5Si3 are studied using first‐principles calculations. The formation energy, elastic modulus, and electronic properties of three different Nb vacancies in Nb5Si3 are discussed in detail. With the increase in vacancy concentration, the obtained shear modulus, Young's modulus, and hardness of Nb vacancy decrease, which are lower than those of pristine Nb5Si3. The bulk modulus/shear modulus (B/G) ratio increases with the increase in Nb vacancy concentration. However, these vacancies result in the transition from brittleness to ductility, and Nb5Si3 with 5% vacancy concentration exhibits ductile behavior. The calculation of electronic structure shows that these Nb vacancies change the local hybridization between Nb and Nb atoms. As the concentration increases, the position of the peak moves toward lower energy. It is predicted that vacancies can improve the ductility behavior of Nb5Si3.

Effect of electron transfer on metal-atom penetration into SiO2 in electric field: first-principles study

The penetration behavior of Ta, Nb, V, and Ti atoms into SiO2 substrate in the electric field are studied by the first-principles calculation, using the metal/SiO2 models. We found that the ionization charges of these atoms are extended over surrounding Si and O atoms and change with increasing the electric field reflecting the electron transfer from metal atoms to metal electrodes. These features are quite different from those of Ag, Au, Al, and Pt atoms discussed in our previous works [Y. Asayama et al., Mater. Sci. Semicond. Process. 70, 78 (2017); R. Nagasawa et al., Jpn. J. Appl. Phys. 57, 04FB05 (2018)]. It is shown that the variation of potential barrier for the penetration with increasing the electric field is approximately explained by considering these features and using the condenser-type model.

Optimization of Welding Process Parameters for Activated Tungsten Inert Welding of Inconel 625 Using the Technique for Order Preference by Similarity to Ideal Solution Methodology

This investigation aims to recognize the advanced input parameters for Inconel 625 by activated tungsten inert gas (A-TIG) welding. Bead on trial experiments was conducted with desirable input parameters such as welding current, torch travel speed, and arc gap according to the Taguchi L25 orthogonal array. The aspect ratio of weld geometry and Brinell hardness (HB) were considered the output parameters. Technique for order preference by similarity to ideal solution (TOPSIS) is a multi-criteria decision-making approach to select an optimum combination of input responses. A step-by-step approach to the application of TOPSIS was followed in this study. Exploratory preliminaries revealed that welding current (300 Amps), torch travel speed (75 mm/min), and arc gap (1 mm) initiated optimum input parameters with optimum conditions of hardness (262 HB) and an aspect ratio of 0.421. Optimized weld zone microstructure and scanning electron microscope (SEM) with energy-dispersive X-ray spectroscopy (EDS) analysis were undertaken for a complete report on the migration of Nb atoms towards the weld zone. From the attained outcomes, it was inferred that a faster cooling rate in A-TIG welding leads to the suppression of the laves phase which could be due to an activated flux combination.

Influence of Nb on Ti diffusion in γ-TiAl intermetallics studied by mechanical spectroscopy

The development of intermetallic titanium aluminides has been driven by the aeronautic and aerospace industries because of the excellent mechanical properties and low density of γ-TiAl based alloys. Up to now, several generations of γ-TiAl based alloys were developed with increasing complexity of the alloy systems. Nb is one of the most important alloying elements in γ-TiAl alloys and although it is considered as a slow diffuser, its influence has not been fully quantified yet. In this work we demonstrate, through mechanical spectroscopy measurements conducted on several γ-TiAl based alloys with different Nb content, that Nb impedes the diffusion of Ti atoms in the α2-Ti3Al phase. Internal friction measurements show a relaxation peak P(α2), which is associated with short distance diffusion of Ti atoms in the α2 phase, involving stress-induced rotation of dipoles Al-VTi-Al, whose activation energy is dependent on the Nb content. The increase of the activation energy is quantified as ΔEa(Ti)= 0.037 eV × at% Nb, being attributed to the next-neighbor interaction of Nb atoms with the local configuration of Ti-VTi. This mechanism also produces a further broadening of the relaxation peak, which is attributed to the near-next-neighbor interactions for high Nb contents. Finally, an atomic model for the mechanism responsible for this relaxation is proposed allowing to explain the observed experimental behavior.

Localized Nb clusters in U-Nb liquid alloys: An ab initio molecular dynamics study

Abstract By means of ab initio molecular dynamics (AIMD), we have evidenced the formation of the localized Nb clusters in the U-Nb liquid alloys even with the very low Nb concentrations. These localized Nb clusters are characterized, qualitatively and quantitatively, in terms of the pair correlation function, structure factor, coordination number and chemical short-range order parameters. The existence of the first highest peaks of the partial Nb-Nb pairs in both their pair correlation functions and structure factors reveals that the Nb atoms prefer to accumulate themselves together to form various localized clusters with three typical Nb networks of Nb2 dimers, opening chains (Nb3 or Nb4 chains) and closed triangles (Nb3 triangles) in the U-Nb liquid alloys. These Nb-Nb affinitive facts are further supported by the derived negative Nb-Nb chemical short-range order parameters. In contrast, both U-U and U-Nb interactions in these liquid alloys are further revealed to have the nearly zero, or positive, chemical short-range order parameters, corresponding to a relatively weak U-U, or repulsive U-Nb, interactions in the U-Nb liquid alloys.

Stability Effect of Metallic Traces in the Degradation of Materials

AbstractThe electrical devices, where alkyd resin is used as insulation material must have an improved thermal stability. The added filler, inorganic oxide compounds, have various contributions on the resin durability because of their differences in the electronic distribution. The PbTiO3 doped with Mn, Ce, In, Gd, Nb, Pr, and Nd atoms possessing certain features of stabilization played the role of protector during material oxidation. The investigations by isothermal and nonisothermal chemiluminescence revealed the effect of filler in the alkyd resin matrices subjected to the accelerated degradation by γ‐irradiation. The influence of doping atoms is discussed and the stabilization consequences are evaluated.

Interfacial Structure Change and Selective Dissolution of Columbite–(Fe) Mineral during HF Acid Leaching

The goal of the paper is to study the charge transfer and reactions at the columbite-(Fe) (FeNb2O6) mineral surface during the HF leaching process. In this paper, X-ray photoelectron spectroscopy (XPS), leaching experiments, and density functional theory (DFT) calculations were used to study the surface element adsorption, charge distribution, chemical state, and energy changes of the mineral surface during the process of leaching columbite–(Fe) with different concentrations of hydrofluoric acid. The results showed that as the concentration of F atoms was increased during the acid leaching process, the Nb–O bond was more likely to be broken than the Fe–O bond; the amount of charge transferred from Nb atom to F atom (0.78 e–0.94 e/atom) was greater than that from Fe atom to the F atom (0.25 e–0.28 e/atom), so it was determined that compared to Fe atoms, it was easier for the Nb atoms to bind to F. The results of XPS analysis showed that the electron binding energies of Nb5+–O, Fe3+–O, and Fe2+–O bonds on the mineral surface increased sequentially, and the M–O bond broke during the acid leaching process, forming more stable M–F bonds. Therefore, the Nb5+–F bonds were easier to form a stable structure. Combined with the ICP results, it was found that in the filtrate after 5M HF and 10M HF acid leaching minerals, c(Nb)/c(Fe) were 2.69 and 2.95, respectively, and the concentration ratio of Nb to Fe element in the mineral was 2 which was lower than 2.69 and 2.95, confirming the result of DFT calculation and illustrating that Nb atoms in columbite-(Fe) mineral were more soluble than Fe atoms.

Effect of Nb or Ta Interlayer on Microstructure and Mechanical Properties of Graphite/Ti6Al4V Alloy Joints

Graphite/Ti6Al4V (TC4) joining has been conducted with Cu–Ti–Ni filler and refractory metal (Nb or Ta) interlayer. Microstructural characterizations reveal that defect‐free interfaces are obtained among the refractory metal interlayer, fillers, and substrates, suggesting favorable interfacial bonding in the joints. The diffusion and dissolution of Nb or Ta atoms into the molten fillers occur during joining, leading to the formation of (Ti, R) solid solution ([Ti, R]ss, R = Nb, or Ta) in the filler layers and thickness decrease in the interlayer. Two filler layers adjacent to the interlayer primarily consist of Ti–Cu, Ti–Cu–Ni intermetallics, and (Ti, R)ss. A Ti‐rich diffusion layer and a thin TiC‐containing layer exist near TC4 and graphite substrates, respectively. The joints brazed with Nb and Ta interlayers exhibit shear strength of 2 1.0 ± 2.7 MPa and 20.2 ± 3.1 MPa, respectively, both higher than those without interlayer, implying that the introduction of refractory metal interlayer may benefit the joint stress relaxation by its intermediate coefficient of thermal expansion and high ductility. The joint strength with Nb interlayer is slightly higher than that with Ta interlayer, mainly attributed to the relatively lower elastic modules and yield strength of Nb.

Disorder in Strontium Barium Niobate: Local Structure, Phonons, and Polarization Dynamics.

The main representative of the tetragonal tungsten bronze family, strontium barium niobate, has been recently studied by the scattering and spectroscopic techniques [diffuse scattering (DS), Raman, infrared, and high-frequency dielectric spectroscopies] to decipher the nature of its ferroelectric-relaxor transformation tuned by composition. DS revealed that mesoscopic range ordered correlations along the polar axis are presented in the crystal, being less organized in the relaxor compositions with higher Sr content. For these compositions, transverse polar correlations are found to be anisotropic. Pair distribution function experiments and first-principle calculations in several compositions showed a concomitant new peak around ~6.5 Å, which develops on cooling and cannot be explained by the current structural models with long-range incommensurate oxygen tilts but with a short-range ordered tilt arrangement. This relates with the development of the macroscopic polarization, tightly linked to the structure, and shows two different contributions related to the two different crystallographic sites for the ferroactive Nb atom, recalling a picture of two independent polarization networks showing disparate dynamics, temperature behavior, and correlation lengths. Cationic substitution in different channels affects mainly the relaxations. Above the MHz range, all members of the solid solutions show three main relaxations, whose temperature dependence is unalike, due to the distinct correlation lengths and nature of the processes. On increasing Sr content, the frequencies are higher, and the contribution to the permittivity of excitations at GHz-THz ranges gets stronger. A soft anharmonic central mode at THz range is the characteristic for all the compositions, together with the slowing down of a relaxation at GHz-MHz ranges, revealing the coexistence of displacive and order-disorder mechanisms of the ferroelectric phase transition in these solid solutions.

Study of the characteristics and properties of the SiO2/TiO2/Nb2O5 material obtained by the sol\u2013gel process

The SiO2/TiO2/Nb2O5 material was set by the sol–gel method and was characterized by several techniques through thermogravimetric, spectroscopic, and textural analyzes. For the two synthesized materials, the specific surface area was 350.0 and 494.0 m2 g−1 (SiTiNb-A and SiTiNb-B, respectively). An enhance of the crystalline order with the temperature increase of the thermal treatment was observed. Through X-ray Photoelectron Spectroscopy analysis, the binding energy values for the Ti 2p and Nb 3d levels showed the insertion of Ti and Nb atoms in the silica matrix. The Electron Dispersive Spectroscopy analyses also confirmed the high dispersion of the metals presented on the materials surface. The Thermogravimetric Analysis showed weight loss for the of 37.6% (SiTiNb-A) and 29.7% (SiTiNb-B). The presence of the crystalline phases TiO2-anatase and monoclinic-Nb2O5 in the materials was confirmed through the data obtained by association of powder X-ray Diffraction and FT-Raman. Values obtained from optical band-gap aimed the dependence of the oxides concentration and the calcination temperature. Finally, the pyridine adsorption studies have indicated the presence of Lewis and Brønsted acid sites.

Sintering mechanism of low‐temperature co‐fired alumina featuring superior thermal conductivity

AbstractDensely sintered alumina is produced with a 5 wt% addition of a CuO‐TiO2‐Nb2O5‐Ag2O sintering aid under a firing temperature of only 835°C with a prolonged holding time of 96 hours. The sintered material exhibits a thermal conductivity of 20 W/mK, which is significantly greater than that of conventional low‐temperature co‐fired ceramic (LTCC) materials (~2‐7 W/mK). Additionally, suitable dielectric characteristics are observed, such as a relative dielectric constant εr of 11.3, quality factor–resonant frequency product Q × f of 4700 GHz, and temperature coefficient of resonant frequency τf of −87 ppm/K. The low‐temperature densification is revealed to occur mainly in the solid state, before liquid phase formation. Lattice constant measurements, transmission electron microscopy, and energy dispersive spectroscopy reveal an increase in unit cell volume upon densification and the incorporation of Cu2+ and Ti4+ ions into the alumina lattice, which promotes densification. The diffusion speeds of Cu2+ and Ti4+ ions are indirectly affected by Nb and Ag atoms by lowering the additive melting temperature. Therefore, sintering additives with low melting points and elements that incorporate into the lattice of the base material are effective for low‐temperature sintering of aluminum‐based oxides.

An octacoordinated Nb atom in the NbAl8H8+ cluster.

The NbAl8H8+ cluster was formed in a molecular beam and characterized by mass spectrometry and infrared spectroscopy. Density functional theory calculations show the lowest-energy isomer is a high symmetry singlet with the Nb atom placed at the center of a distorted hexagonal Al ring and coordinated by two AlH moieties, therefore exhibiting octacoordination. The unprecedented high-symmetric geometry is attributed to the 20 valence electrons; the central Nb atom adheres to the 18-electron rule and two additional delocalized electrons stabilize the hexagonal ring.

Chirality of small niobium clusters

We report on time-dependent density functional theory calculations to study the chiroptical properties of small $$\hbox {Nb}_{{9}}$$ clusters and their corresponding van der Waals $$\hbox {Nb}_{{9}}\hbox {Ar}_{n}$$ complexes. The three most stable 9-atom structures considered in the literature are analyzed, and we obtain their lowest-energy atomic configurations, UV–Vis response, and for the first time, the electronic circular dichroism (ECD) spectra of the systems. Excitation energies, as well as oscillator and rotational strengths are calculated using the M06 exchange-correlation functional together with a mixed basis approach where all electron DGDZVP and Def2SV basis are employed for Nb and Ar atoms, respectively. In all cases, the UV–Vis spectra look very similar and are characterized by the formation of a main absorption maxima located at $$\sim 240\hbox { nm}$$ ; however, the distribution of simulated ECD signals strongly depends on the geometrical structure of the clusters. Our calculated ECD bands show precise features that cover the experimentally accessible 200–300 nm range and extend near the visible part of the spectra, a fact that allows us to distinguish between different niobium cluster geometries. However, weighted averages of calculated ECD spectra for different $$\hbox {Nb}_{{9}}$$ clusters reveal that isomerization effects might lead to a notable increase/reduction in the intensity of CD lines since the simulated bands could add or cancel each other.

Effect of irradiation-induced cascade mixing on spinodal decomposition in U-Nb and U-Zr alloys: a phase field study.

The spinodal decomposition of the γ-phase in U–Nb and U–Zr alloys under irradiation was investigated using the phase-field method coupled with micro-elasticity theory and rate dependent cascade mixing model. Microstructure evolutions of spinodal decomposition in U–Nb and U–Zr alloys were simulated by considering different initial compositions and dose rates. The volume fraction and composition distribution under different cascade mixing were presented. The simulation results show that the volume fractions and equilibrium composition of the (Nb,Zr)-rich γ2-phase and the rate of spinodal decomposition are influenced by the dose rate and initial alloy composition. The cascade mixing can drive Nb or Zr atoms back into solution until a new equilibrium state between local cascade mixing and spinodal decomposition is reached. The evolution analysis indicated that irradiation-induced cascade mixing acts in opposition to thermodynamically driven spontaneous spinodal decomposition, which can not only slow down the spinodal decomposition but also reduces the composition range of the miscibility gap.