Incorporation Of Nb Research Articles

Self-assembly of [PNbxW12−xO40]n−Keggin anions – a simple way to mixed Nb–W polyoxometalates

This paper summarizes preparations of mixed phosphoniobotungstates [PNbxW12−xO40](3+x)−(x= 1–3) based on two different ways: incorporation of Nb into pre-existing lacunary-type phosphotungstates and self-assembly reactions.

The Effect of Nb Incorporation on the Electronic Properties of Anodic HfO2

Hafnium oxide and Nb doped HfO2 were grown by anodizing sputtering-deposited Hf and Hf-4at.%Nb. Photoelectrochemical characterization was carried out in order to estimate solid state properties such as band gap, flat band potential and electrons internal photoemission threshold energy as a function of thickness and composition of anodic oxides. Optical transitions at energy lower than the band gap value of the investigated anodic films were evidenced, and they are attributed to optical transitions involving localized states inside the band gap. Such states were located at 3.6 eV and 3.9 eV below the conduction band edge for the Nb free and Nb containing hafnium oxide, respectively. Impedance measurements were performed in a wide range of electrode potentials under anodic and cathodic polarization with respect to the flat band potential, confirming the formation of insulating oxides with high dielectric constant. The latter increases from 20 to 41 due to incorporation of Nb into the oxide.

Effect of Nb5 + content on the high temperature properties of the mixed conductors system La1 − xBaxCo1 − yNbyO3 − δ with 0.6 ≤ x ≤ 1.0 and 0 ≤ y ≤ 0.4

The replacement of Co by Nb in the La1−xBaxCo1−yNbyO3−δ (0.6≤x≤1.0) system and its effects on phase stability and high temperature properties of the cubic perovskite phase were investigated by X-ray diffraction, dilatometry, electrical conductivity, oxygen permeation and electrochemical impedance spectroscopy. The incorporation of Nb in the B site stabilizes the cubic perovskite in the as prepared samples. The Nb solubility increases from y=0.1, for the sample with Ba content x=0.6, to y=0.4 for 0.8≤x≤1.0. However, after a heat treatment at 750°C during 10days small amounts of secondary phases were detected for samples with x=0.9 and 1.0. The linear expansion of the cubic phases decreases with the Nb content as well as with the Ba content. The minimum value, α=12.9×10−6K−1, was obtained for La0.2Ba0.8Co0.6Nb0.4O3−δ. The behavior of electrical conductivity and oxygen permeation data with Nb content is explained from variations in the oxygen vacancy concentration, which is controlled by Nb5+ and Ba2+ contents. The minimum polarization resistance value, Rp=0.08Ωcm2, was obtained at 600°C for La0.2Ba0.8Co0.9Nb0.1O3−δ. The low Rp value of this compound and the crystal structure stability of various compositions in the system La1−xBaxCo1−yNbyO3−δ with x≥0.7 and y≥0.1 make these materials of interest as mixed conductors.

A robust high performance cobalt-free oxygen electrode La 0.5 Sr 0.5 Fe 0.8 Cu 0.15 Nb 0.05 O 3−δ for reversible solid oxide electrochemical cell

Abstract A novel cobalt-free perovskite oxide La0.5Sr0.5Fe0.8Cu0.15Nb0.05O3−δ (LSFCN) has been synthesized and evaluated as oxygen electrode for reversible solid oxide electrochemical cells (RSOCs). The performance and stability of the LSFCN based RSOCs have been characterized in fuel cell and electrolysis modes, and the reversibility of the cells has been proven. In FC mode, the cell exhibits the maximum power density of 1.10 Wcm−2 at 800 °C, and a stable output under 0.7 V at 700 °C during 108 h. The performance and stability of the cell in electrolysis mode are also remarkable. An electrolysis current of 0.85 A cm−2 is achieved at 750 °C with an applied voltage of 1.3 V, and no degradation as well as delamination are observed for the cell after 50 h electrolysis under voltage of 1.60 V (∼1.27 A cm−2) at 800 °C. The high performance of the LSFCN at both cathodic and anodic conditions may be attributed to the inherent high electrochemical activity of copper-iron based perovskites; and the incorporation of Nb5+ cations into perovskite lattice is responsible for the stability of LSFCN, which leads to the more stable crystal structure, lower thermal expansion coefficient and the reduced Sr segregation at surface.

Niobian rutile in Cr-V-bearing rocks of the Sludyanka metamorphic complex (Southern Baikal area)

Cr-V-bearing rocks of the Sludyanka metamorphic complex in the Southern Baikal area contain accessory niobian rutile with Nb contents unusually high for metamorphic rocks (10-12 to 20-25 wt.% Nb2O5, or, occasionally, up to 36-37 wt.%). Incorporation of Nb5+ into the rutile structure is balanced by Cr3+ and V3+, which reach 16 wt.% Cr2O3 and 8 wt.% (or, rarely, up to 20 wt.%) V2O3. The distribution of Nb, Cr, and V in rutile is very uneven, down to the microscale. It does not depend on PT conditions, being rather controlled by crystallization kinetics of minerals competing with rutile in the paragenesis.

Structural, optical and electrical investigations on Nb doped TiO2 radio-frequency sputtered thin films from a powder target

Pure and Nb doped TiO2 (TNO) thin films were deposited onto glass substrates by RF magnetron sputtering technique using a Nb and TiO2 mixture powder target at room temperature to explore the possibility of producing sputtered TNO films by a low cost process. The effect of Nb doping on the structure, morphology, optical and electrical properties of the prepared films was studied by systematically varying the Nb content from 2 to 6 wt%. GXRD results show that the deposited films mainly possess rutile phase with the (110) orientation. Raman spectra confirm that the deposited films are predominantly rutile phase. Surface roughness increases with the increase of Nb doping concentration , which may be attributed to the structural changes in the film due to the incorporation of Nb into the TiO2 lattice. Optical transmittance in the visible range reaches 85 % for the undoped films then it decreases as the doping content increases. Doping by niobium resulted in a slight increase in the optical band gap energy of the films due to the Burstein–Moss effect. The resistivity measurement of TNO films reveals that the Nb doping improves the electrical conductivity of the deposited films compared to the undoped one. The best value was observed for films deposited at 4 wt% Nb.

Formation and characterization of Al–Ti–Nb alloys by electron-beam surface alloying

The combination of attractive mechanical properties, light weight and resistance to corrosion makes Ti-Al based alloys applicable in many industrial branches, like aircraft and automotive industries etc. It is known that the incorporation of Nb improves the high temperature performance and mechanical properties. In the present study on Al substrate Ti and Nb layers were deposited by DC (Direct Current) magnetron sputtering, followed by electron-beam alloying with scanning electron beam. It was chosen two speeds of the specimen motion during the alloying process: V1=0.5cm/s and V2=1cm/s. The alloying process was realized in circular sweep mode in order to maintain the melt pool further. The obtained results demonstrate a formation of (Ti,Nb)Al3 fractions randomly distributed in biphasic structure of intermetallic (Ti,Nb)Al3 particles, dispersed in α-Al solid solution. The evaluated (Ti,Nb)Al3 lattice parameters are independent of the speed of the specimen motion and therefore the alloying speed does not affect the lattice parameters and thus, does not form additional residual stresses, strains etc. It was found that lower velocity of the specimen motion during the alloying process develops more homogeneous structures. The metallographic analyses demonstrate a formation of surface alloys with very high hardness. Our results demonstrate maximal values of 775HV [kg/cm2] and average hardness of 673HV [kg/cm2].

Influences of bacteria-based self-healing agents on cementitious materials hydration kinetics and compressive strength

Bacterially induced calcium carbonate precipitation has been proposed as an alternative and environmental technique to develop self-healing cementitious materials system in recent years. This study investigated the influences of bacteria-based self-healing agents on the rheology, hydration kinetics and compressive strength of cementitious materials to further verify the feasibility of bacteria-based self-healing agents for crack repairing. The results showed that the rheology of cement mortar was significantly improved by the addition of bacteria-based self-healing agents. Incorporation of bacteria-based self-healing agents in cement greatly influenced the hydration kinetics. The self-healing agent RB could delay the hydration of cement resulting in final setting time increase, however the self-healing agents JB and NB accelerated the hydration resulting in initial and final setting time decrease. In addition, compressive strength test results showed that incorporation of RB in cement mortar resulted in early age compressive strength decrease, but the 28d compressive strength increased compared to control. Incorporation of JB in cement mortar enhanced compressive strength, however incorporation of NB in cement mortar could result in unwanted compressive strength loss. Furthermore, MIP test results indicated that the pore size distributions were different between cement paste samples with and without bacteria-based self-healing agents. The addition of bacteria-based self-healing agents increased the porosity between 100 and 1000nm. The porosity between 100 and 1000nm of the sample with NB increased the most compared to the samples with RB and JB. The incorporation of JB increased the porosity between 100 and 1000nm, but at the same time the small pores with a diameter under 10nm increased. These results could be used to explain the compressive strength development.

Metallic impurities induced electronic transport in WSe2: First-principle calculations

Using density functional theory calculations, we have systematically explored the effect of V, Nb and Ta impurities on the electronic transport properties of 2H-WSe2. The formation energies elucidate dopants are preferred to substitute W atoms, and the incorporation of Nb into WSe2 is most thermodynamically favorable. The crystal structures almost hold the pristine WSe2 structure-type in spite of with slightly bond relaxation. More importantly, a pronounced electronic transport behavior has realized in all doped systems, which is mainly triggered by metal impurities. Our calculation suggests chemical doping is an effective way to precisely modulate WSe2 performance for target technological applications.

On the Stability and Electronic Structure of Transition-Metal Dichalcogenide Monolayer Alloys Mo1−xXxS2−ySey with X = W, Nb

Layered transition-metal dichalcogenides have extraordinary electronic properties, which can be easily modified by various means. Here, we have investigated how the stability and electronic structure of MoS 2 monolayers is influenced by alloying, i.e., by substitution of the transition metal Mo by W and Nb and of the chalcogen S by Se. While W and Se incorporate into the MoS 2 matrix homogeneously, forming solid solutions, the incorporation of Nb is energetically unstable and results in phase separation. However, all three alloying atoms change the electronic band structure significantly. For example, a very small concentration of Nb atoms introduces localized metallic states, while Mo 1 - x W x S 2 and MoS 2 - y Se y alloys exhibit spin-splitting of the valence band of strength that is in between that of the pure materials. Moreover, small, but evident spin-splitting is introduced in the conduction band due to the symmetry breaking. Therefore, transition-metal dichalcogenide alloys are interesting candidates for optoelectronic and spintronic applications.

Anomalous behavior of B1g mode in highly transparent anatase nano-crystalline Nb-doped Titanium Dioxide (NTO) thin films

The effect of Niobium doping and size of crystallites on highly transparent nano-crystalline Niobium doped Titanium Dioxide (NTO) thin films with stable anatase phase are reported. The Nb doping concentration is varied within the solubility limit in TiO2 lattice. Films were annealed in controlled environment for improving the crystallinity and size of crystallites. Elemental and thickness analysis were carried out using Rutherford backscattering spectrometry and cross sectional field emission scanning electron microscopy. Structural characteristics reveal a substitutional incorporation of Nb+5 in the TiO2 lattice which inhibits the anatase crystallites growth with increasing the doping percentage. The micro-Raman (MR) spectra of films with small size crystallites shows stiffening of about 4 cm−1 for the Eg(1) mode and is ascribed to phonon confinement and non-stoichiometry. In contrast, B1g mode exhibits a large anomalous softening of 20 cm−1 with asymmetrical broadening; which was not reported for the case of pure TiO2 crystallites. This anomalous behaviour is explained by contraction of the apical Ti-O bonds at the surface upon substitutional Nb5+ doping induced reduction of Ti4+ ions also known as hetero-coordination effect. The proposed hypotheses is manifested through studying the electronic structure and phonon dynamics by performing the near edge x-ray absorption fine structure (NEXAFS) and temperature dependent MR down to liquid nitrogen temperature on pure and 2.5 at.% doped NTO films, respectively.

Impurity-induced ferromagnetism and metallicity of WS2 monolayer

We perform a first-principles study of electronic structure and magnetic properties of V, Nb and Ta-doped WS2 monolayers. Formation energy calculations reveal that all the doped systems are thermodynamically more preferred under S-rich condition than W-rich condition, and that under either condition the incorporation of 5d Ta dopant into monolayer is the most favorable, followed by the incorporation of 4d Nb and 3d V. Electronic structure analysis reveals that all the doped systems show p-type doping and metallic character, i.e. metallicity in the V-doped case, while half-metallicity in the Nb and Ta-doped cases. Furthermore, ferromagnetism is induced by the impurities, which mainly comes from the coupling between unpaired d orbitals of the dopants and the nearest neighbors of W 5d and S 3p states. These findings indicate that doping metals in WS2 monolayers may act as a useful way to tailor the electronic structure of two-dimensional materials for many device applications.

Promoted Hexagonal Tungsten Bronzes as Selective Catalysts in the Aerobic Transformation of Alcohols: Glycerol and Methanol

M-containing tungsten oxides bronzes (M = Ti, Nb or V) with hexagonal tungsten bronze (HTB) structure have been investigated as catalysts for the aerobic transformation of glycerol and methanol. The catalysts were prepared hydrothermally and characterized by several physico chemical techniques, i.e. N2-adsorption, XRD, Raman spectroscopy and temperature programmed desorption of ammonia. Interesting variations in the thermal stability of the HTB-framework were observed according to the element introduced into the oxide structure. In addition, the incorporation of Ti and Nb modified the acid features of the hexagonal tungsten oxides, whereas V introduced new redox sites. The catalytic results for the aerobic transformation of glycerol and methanol in terms of conversion and nature of reaction products are discussed on the basis of the physico-chemical characteristics of catalysts.

The role of niobia location on the acidic and catalytic functionalities of heteropoly tungstate

12-Tungstophosphoric acid (TPA) was modified systematically by incorporating niobium into its primary and secondary structure. TPA supported on niobia catalyst was also prepared by impregnation method. These catalysts were characterized by FT-infra red, X-ray diffraction, Laser Raman, X-ray photo electron spectroscopy, X-ray absorption spectroscopy, UV–vis diffuse reflectance spectroscopy, solid state 31P nuclear magnetic resonance spectroscopy, temperature programmed desorption of ammonia and pyridine adsorbed FT-IR spectroscopy. Characterization results suggest the presence of intact Keggin structure after modification of parent TPA. Incorporation of Nb into the secondary structure results in generation of Lewis acidic sites. Nb containing TPA catalysts exhibited high acidity compared to the TPA supported on niobia catalyst. The activity of the catalysts was evaluated for benzylation of anisole with benzyl alcohol. Catalyst activity depended on the location of niobium ion in the heteropoly tungstate.

Facile Styrene Epoxidation with H2O2 over Novel Niobium Containing Cage Type Mesoporous Silicate, Nb-KIT-5

Niobium has been incorporated into cage-like ordered mesoporous KIT-5, a cubic close-packed (Fm3m) structured silica, for the first time by a direct hydrothermal synthesis method. Small-angle X-ray scattering spectra and nitrogen physisorption results confirm the formation of the KIT-5 structure. The incorporation of Nb and its coordination were confirmed by techniques such as elemental analysis, diffuse reflectance UV–Vis, Raman, HR-SEM and NH3-TPD. When tested as epoxidation catalyst for styrene with H2O2, Nb-KIT-5 mainly yielded (1,2-dimethoxyethyl) benzene with methanol as solvent. It also showed better catalytic activity compared to other mesostructured catalysts such as Nb-KIT-6 and Nb-SBA-15.

Enhancing the adsorption capacity of Sr2+ and Cs+ onto hexagonal tungsten oxide by doped niobium

The adsorption capacity of Sr2+ and Cs+ onto hexagonal tungsten oxide was enhanced by the incorporation of Nb.

A novel heterogeneous hybrid by incorporation of Nb2O5 microspheres and reduced graphene oxide for photocatalytic H2 evolution under visible light irradiation

A novel heterogeneous hybrid composed of Nb2O5 microspheres and reduced graphene oxide as photocatalyst exhibits superior photocatalytic activity and stability.

Synthesis of Li7La3(Zr2−X, NbX)O12 (X = 0 – 1) at Low Temperatures Using a Sol-Gel Process

The ceramic solid-electrolyte Li7La3Zr2O12 (LLZO) with a garnet crystal structure has developed as a promising solid electrolyte material. Positive properties: high ionic conductivity (4x10-4 Scm-1at 25ºC) [1], electrochemical stability with Li metal, and thermal and chemical stability. However, the conductivity of this material remains about two orders of magnitude lower than that of a common liquid organic electrolyte. The main disadvantage is related with the loss of lithium produced during the sintering process up to 1200ºC to obtain the cubic structure, responsible of produce the high conductivity. From this point of view, using a solution process for the development of this kind of electrolyte materials is a very attractive alternative in order to obtain lower thermal treatment. In this work, we explore the use of sol-gel process to obtain Lithium garnet-type oxides Li7La3(Zr2−X, NbX)O12(LLZNbO, X=0–1) at low temperatures. The partial substitution of Nb in the typical LLZO improves the ion conductivity based in their dependence on the lattice parameter [2].The sol-gel synthesis was developed using lithium nitrate, lanthanum nitrate, zirconium butoxide and niobium ethoxide as precursors. Ethylacetoacetate was used as a stabilizing agent for the alkoxides and ethanol as solvent. Firstly, the lithium and lanthanum salts were dissolved in ethanol. Separately, the zirconium and niobium alkoxides were reacted with stabilizing agent. The molar relation between Zr/Nb and stabilizing agent was adjusted in order to prevent the fast reaction with the Li/La solution. After 1 hour, the solutions were mixed and then, the final solution was stirred at 25ºC during 1 hour until a white gel was obtained. The gel was dried at 80ºC for 24 hours. Further, the powder was ground and calcined at 700ºC for 5 hours. The calcined powders were attrition milled with 4 mm diameter ZrO2balls in a toluene media at 300 RPM for 6 hours. The powder were pressed into pellets and sintered at 900ºC for 10 hours.The powders obtained after calcination showed wide particle size distribution between few microns until ~10µm, and some agglomerates were observed with sizes of ~20µm. The incorporation of niobium in the pristine LLZO produces bigger particles with regular shape; the effect was more evident with higher concentration of niobium, as shown in Figure 1. Pellet using unmilled powder produce materials with low relative density, 45% and 55% for LLZO and LLZNbO, respectively. The reduction of the particle size using ZrO2 ball milling resulted in an increase of the relative density upto 75% for Li7La3(Zr1.75, Nb0.25)O12. Figure 2A shows the small and regular particle shape obtained for the LLZO after milling (compare Fig.1A). However, the individual particle observed after sintering is an evidence of lack of pellet densification. Large particles and open pores were observed in the surface view, Figure 3A. On the other hand, the incorporation of Nb in the garnet crystal structure produces the binding of the particles, Figure 2B-D. Figure 2B displays the good sintering of the sample Li7La3(Zr1.75, Nb0.25)O12 with low presence of pores. Sintering is also verified on the surface of the pellet, Figure 3B, where the presence of individual particles was minimum. Higher Nb doped concentration presented low relative density (~60%). XRD patterns of undoped LLZO displayed tetragonal phase, while all LLZNbO compositions showed cubic phase and a small peak assigned to La2Zr2O7 was observed in Li7La3(Zr1.75, Nb0.25)O12. The Lattice parameter varied from 12.9463Å to 12.9203Å with the increase of Nb concentration (12.9682Å for LLZO sintered at 1230ºC[1]). The conductivity obtained was in the order of 10-6 to 10 -7S/cm at 50ºC.[1] R.Murugan, V.Thangadurai, W.Weppner, Angew. Chem. 2007,119, 7925– 7928[2] H.Imagawa, S.Ohta, Y.Kihira, T.Asaoka, Solid.State.Ionics 2013, http://dx.doi.org/10.1016/j.ssi.2013.10.059

(Invited) Thermochromic Phase Transitions in VO2-Based Thin Films for Energy-Saving Applications

Materials that contribute to more sustainable technologies are in high demand, and the highest impact can be made in the structures that are most ubiquitous. Widely implemented, thermochromic ‘smart’ building windows have the potential to save energy by reducing the heating and cooling loads on HVAC systems.1,2 Exploiting a crystalline phase transition in VO2, these window coatings shift from infrared transparent on cold days when solar heating is desirable to infrared reflective as the ambient temperature increases. In pure VO2, this transition occurs at 68°C, but the incorporation of 1-5% cation impurities can depress the transition temperature to Earth ambient temperatures.In this talk we describe our results on infrared thermochromism in V1-xMxO2thin films, for M = Ta, Nb, W, Hf, and others. To efficiently screen the vast compositional space of transitional metal impurities in VO2, we employ a combinatorial materials technique for both material synthesis and characterization. Thin film libraries of continuously varying composition are prepared by pulsed laser deposition (PLD) from two or more unary metal oxide targets, e.g. V2O5 and Ta2O5. Taking advantage of the spatial non-uniformity of deposition rates from each target, we produce library samples of composition V1-xMxO2where x can vary from <0.01 to 0.3 across a single 3” diameter wafer. A special high-throughput spectroscopic reflectance measurement apparatus has been constructed for screening these libraries for thermochromic transitions. A thermoelectric heater/chiller with a 3” diameter plate maintains library temperature at a setpoint between 5°C and 85°C, while near infrared (NIR) reflectance measurements are made at many library locations, and hence many unique compositions. In Figure 1, for each composition represented on the V1-xTaxO2 sample library, the NIR reflectance switches from a low reflectance state to a high reflectance state, a consequence of the crystalline phase transition that occurs. The transition is reversible with a <10°C hysteresis, and the material returns to its low reflectance state as the material is cooled back to 20°C. As the Ta impurity content increases, the transition temperature decreases.Figure 2 shows depression of transition temperatures in VO2 due to the incorporation of Nb or Ta. Both are found to effect a temperature depression that is linear with concentration at -4 °C/atomic percent. Additionally, the higher valence state impurities, such as W, Nb, and Ta, depress the VO2transition temperature while Hf incorporation has little to no effect.‘Smart’ window coatings enabled by the metal-insulator phase transition in lightly-substituted VO2have significant energy-saving potential in terms of decreased heating and cooling needs for commercial and residential buildings. Critically, the NIR transmission/reflection switch occurs due to ambient temperature, and so the technology is passive, requiring no external controls or power systems.

Mechanical and corrosion-resistant properties of Ti–Nb–Si–N nanocomposite films prepared by a double glow discharge plasma technique

Two quaternary Ti–Nb–Si–N nanocomposite films, with differing Nb contents, were deposited onto Ti–6Al–4V substrates using a double glow discharge plasma technique. The chemical composition and microstructure of the films were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS). The Ti–Nb–Si–N films exhibit a typical nanocomposite structure, in which nanocrystalline (Ti,Nb)N grains, ~8nm in diameter, were embedded in an amorphous SiNx matrix. The hardness and elastic modulus of the Ti–Nb–Si–N nanocomposite films were measured by nanoindentation. The electrochemical behavior of the Ti–Nb–Si–N nanocomposite films was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in a 3.5wt% NaCl solution. For comparative purposes, these measurements were also performed on TiN and Ti–Si–N films. The results show that the Ti–Nb–Si–N nanocomposite films exhibit both higher hardness and corrosion resistance than the TiN and TiSiN films, which could be attributed to their nanocomposite structure together with the incorporation of both Nb and Si into the TiN films.