Amount Of Nb Doping Research Articles

Nb-doped VO2 thin films with enhanced thermal sensing performance for uncooled infrared detection

VO2 thin films are widely studied materials for uncooled infrared detection because of their high temperature coefficient of resistance. In this research, Nb-doped VO2 films were synthesized with the expectation that Nb doping would reduce the metal-insulator-transition temperature via electron doping and strain effects caused by the relatively large ionic radius of Nb, thus improving the resistance temperature coefficient of the doped films. The doped films displayed superior thermal sensing abilities compared with pure VO2 films. As the amount of Nb doping increased, the thermal hysteresis and resistivity of the doped films decreased, while the transition temperature range widened. When the Nb-doping content reached 10%, the temperature coefficient of resistance reached -3.2%/K, while the metal-insulator transition temperature dropped to 52.2 °C. The high performance of the doped films proved that the strategy of Nb doping in VO2 was promising for thermal-sensing applications.

Effects of B-site ion (Nb5+) substitution on the microstructure and ionic conductivity of Li0.5La0.5TiO3 solid electrolytes

Nb-doped lithium lanthanum titanate (LLTO) ceramic electrolytes were fabricated by the conventional solid-state reaction method. Effects of Nb5+ substitution on the microstructure and ionic conductivity of LLTO were investigated systematically. It is found that the 4 at.% Nb-doped Li0.5La0.5TiO3 sintered at 1350 °C for 12 h shows the minimum amount of pores in microstructure and the highest total conductivity, 1.04 × 10−4 S/cm, being 1.6 times of that without Nb addition. Therefore, it is proved experimentally that a proper amount of Nb doping could be an effective and facile approach to improve the LLTO’s microstructure and total conductivity, thus making Nb-doped LLTO a good solid electrolyte candidate for use in lithium-ion batteries.

Nb-doped TiO2 Thin Films Prepared through TiCl4 Treatment for Improvement of Their Carrier Transport Property

ABSTRACTNb-doped TiO2 thin-films were prepared on fluorine-doped tin oxide (FTO) coated glass directly with niobium ethoxide and TiCl4 in water under the acidic conditions with several concentrations of HCl at 70-90 °C for 45 minutes or 1 hour followed by rinsing with water and annealing at 100 °C for 1 hour. Thin films of 0-1% Nb-doped TiO2 with rutile phase on FTO were obtained, which were confirmed through X-ray diffraction analyses and measurements of energy dispersive X-ray spectroscopy (EDS). Scanning electron microscopy observations equipped with EDS revealed that higher growth temperature over 90 °C is required for doping of Nb. While higher concentration of HCl resulted in much amount of Nb-doping. Band gap of rutile TiO2 gradually reduced from 3.3 eV to 3.23 eV through Nb-doping from 0% to 1%, which were estimated from uv-vis absorption spectroscopic analyses. Hall effect measurements by taking van der Pauw method confirmed that 2.26 times increase of the carrier density and 1.78 times enhancement of the conductivity have been achieved in the case of 1% Nb-doping.

The effect of Nb doping on hydrogen gas sensing properties of capacitor-like Pt/Nb-TiO2/Pt hydrogen gas sensors

Hydrogen gas sensors with capacitor-like Pt/Nb-TiO2/Pt structure and various amount of Nb doping were successfully prepared by magnetron sputtering and lift-off photolithography. The Nb incorporation in the TiO2 layer has been confirmed by XPS and GIXRD measurements. It has been shown that the Nb doping delays the anatase to rutile phase transformation during annealing and according to the AFM measurements it also improves the homogeneity of the sensing films. However, unexpectedly the response of the sensors with Nb doped TiO2 film is lower compare to the undoped Pt/TiO2/Pt sensors, which exhibit highest response (2.08×104) and shorter response time (49.4 s) to 1000 ppm hydrogen at 100 °C. This can be partially explained by the intrinsic characteristics (lower surface roughness and porosity) of Nb doped TiO2 films, which resulting in smaller active surface and lower diffusion of the oxygen and hydrogen gases into the films. Although the Nb doping did not improve the sensing performance, this study can be also useful for the future research on the capacitor-like metal oxide gas sensor.

X-site aliovalent substitution decoupled charge and phonon transports in XYZ half-Heusler thermoelectrics

In this work, the effects of niobium (Nb) doping on thermoelectric properties of n-type half-Heusler (Hf0.25Zr0.25Ti0.5)1-xNbxNiSn (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) compounds were investigated. The samples were synthesized by arc-melting method and followed by annealing, mechanical grinding and spark plasma sintering (SPS) process. The electrical conductivity, the Seebeck coefficient, the thermal conductivity, and the Hall coefficient measurements were performed in the range of temperature from room temperature to 923 K. By changing Nb-doping amount, the electrical conductivity increased with increasing the doping level, while the Seebeck coefficient relatively less decreased at high temperatures, resulting in the highest power factor of 36 × 10−4 (W/K2m) at 773 K with x = 0.02 sample. Furthermore, a significant reduction in the lattice thermal conductivity is attributed to anharmonic phonon-phonon interactions by aliovalent substitution into multi-element site and higher degree of phase separation, which was achieved through the synthesis process of these compounds. The enhancement of the power factor combined to low thermal conductivity leads to ∼20% increase of the dimensionless figure of merit (ZT, ∼1.0 at 773 K) in comparison to the parent Hf0.25Zr0.25Ti0.5NiSn compound. It is believed that this is attributed to a decoupling between the electrical conductivity, thermopower, and phonon transport by Nb-doping. The Nb-doped compounds also showed high stability of the power factor up to 873 K.

X-Site Aliovalent Substitution Decoupled Charge and Phonon Transports in XYZ Half-Heusler Thermoelectrics

In this work, the effects of niobium (Nb) doping on thermoelectric properties of n-type half-Heusler (Hf0.25Zr0.25Ti0.5)1-xNbxNiSn (x = 0, 0.005, 0.01, 0.015, 0.02, and 0.025) compounds were investigated. The samples were synthesized by arc-melting method and followed by annealing, mechanical grinding and spark plasma sintering (SPS) process. The electrical conductivity, the Seebeck coefficient, the thermal conductivity, and the Hall coefficient measurements were performed in the range of temperature from room temperature to 923 K. By changing Nb doping amount, the electrical conductivity increased with increasing the doping level, while the Seebeck coefficient relatively less decreased at high temperatures, resulting in the highest power factor of 36 x 10-4 (W/K2m) at 773 K with x = 0.02 sample. Furthermore, a significant reduction in the lattice thermal conductivity is attributed to anharmonic phonon-phonon interactions by aliovalent substitution into multi-element site and higher degree of phase separation, which was achieved through the synthesis process of these compounds. The enhancement of the power factor combined to low thermal conductivity leads to ~ 20% increase of the dimensionless figure of merit (ZT, ~1.0 at 773 K) in comparison to the parent Hf0.25Zr0.25Ti0.5NiSn compound. The Nb-doped compounds also showed high stability of the power factor up to 873 K.

Enhancement of Nb-doping on the properties of LiFePO4/C prepared via a high-temperature ball milling–based method

Li1-xNbxFePO4/C has been prepared by wet mechanical stirring–assisted high-temperature ball milling route. Wet mechanical stirring was considered to be effective to promote the dispersion of the precursor in solution. Effect of Nb-doping amount was explored for enhancing the properties of LiFePO4/C. The experimental results showed that Li0.98Nb0.02FePO4/C displayed a well-crystallized structure of LiFePO4 and excellent electrochemical performance. It exhibited that the initial discharge capacities were 164.9 mAhg−1 at 0.1 C and 118.8 mAhg−1 at a rate of 10 C, respectively. Moreover, it achieved a specific capacity of 114.3 mAhg−1 at a 10 C rate after 200 cycles with a capacity fading rate of 3.8%. Thus, Nb-doping is recommended as a beneficial technique to improve the electrochemical properties of LiFePO4.

Characterization of B‐Site Niobium‐Doped Pr0.4Sr0.6 (Co0.3Fe0.6)1‐xNbxO3‐δ (x=0, 0.05, 0.1, 0.2) Perovskites as Cathode Materials for Solid Oxide Fuel Cells

AbstractPerovskite materials of Pr0.4Sr0.6(Co0.3Fe0.6)1‐xNbxO3‐δ (PSCFNx, x=0, 0.05, 0.1, 0.2) were firstly synthesized as the cathode materials for solid oxide fuel cells (SOFCs) by doping niobium(Nb) at the B site of Pr0.4Sr0.6Co0.3Fe0.6O3‐δ using a solid state reaction method. The effect of Nb doping amount on the lattice structure, oxygen desorption and electrochemical properties of PSCFNx at different calcination temperatures were investigated. It is found that this material family had perfectly cubic structure based on the Pm‐3 m space group whose lattice size increased with x. These materials were thermally stable after calcination and showed desirable chemical compatibility with electrolyte in the condition of calcination at 1250 °C for 8 h under an air atmosphere. The optimum x value of 0.1 was found for the PSCFNx cathode materials, which had the minimum polarization resistance value of 0.018 Ωcm2 at 900 °C. As a result, the single cell with the PSCFN0.1 cathode delivered a power density of 0.731 Wcm−2 at 900 °C with humidified H2 (∼3% H2O) as the fuel and ambient air as the oxidant. It indicates that PSCFN0.1 should be a promising cathode material for the SOFCs.

Simple microwave synthesis and improved electrochemical performance of Nb-doped MnO2/reduced graphene oxide composite as anode material for lithium-ion batteries

Niobium-doped MnO2/reduced graphene oxide (Nb-MnO2/RGO) composite has been successfully synthesized via a simple microwave radiation method. The samples were systematically studied by X-ray diffraction (XRD), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and electrochemical measurements. As the anode material for lithium-ion batteries, the Nb-MnO2/RGO (molar ratio of Mn/Nb = 50:1) (NMG50) showed an outstanding reversible discharge capacity of 556.6 mAh g−1 after 50 cycles with a capacity retention of 77% at a charge-discharge rate of 0.1 A g−1 and the reversible discharge capacity can still retain 223.3 mAh g−1 at a current of 1 A g−1, which is much higher than those for Nb-MnO2/RGO (molar ratio of Mn/Nb = 10:1) (NMG10) and undoped MnO2/RGO (MG). The improved electrochemical performance could be attributed to the proper amount of Nb doping, which could enhance both the conductivity and the structure stability of MnO2.

(Invited) Development of Non Platinum Group Metal Oxide Cathode without Carbon Support for PEFCs

Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. Many ENEFARMs (micro CHP) are operating and fuel cell vehicles are also commercially available in Japan. However, the estimated amount of Pt reserve is limited and its cost is high. The dissolution of Pt cathode might be the final problem to be solved related to the stability in the present PEFC system. Additionally, the instability of carbon support is also a big problem especially for fuel cell vehicles. Carbon including graphite is thermochemically unstable at room temperature in air or oxygen containing atmosphere. A stable non-precious metal oxide cathode with stable metal oxide support might be the final goal for the cathode of PEFC for fuel cell vehicles. We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (1-4). In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon. Recently, we published the results of precious-metal-free and carbon-free cathodes based on oxides and demonstrated the superior durability of oxide-based cathodes by preparing titanium-niobium oxides mixed with Ti4O7 (TixNbyOz + Ti4O7) (5). The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V with respect to reversible hydrogen electrode. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, we successfully demonstrated that the precious-metal and carbon-free oxide-based cathodes had superior durability under the cathode conditions of a polymer electrolyte fuel cell. In order to qualify the role of Nb oxide for ORR, we used TiO2-Nb (Nb; 0.5 or 5atm%) rods (TOSHIMA Manufacturing Co., Ltd) as working electrodes. In addition, TiO2-Nb (0.5atm%) rod was heat-treated at 800 oC in 4%H2/Ar to examine the effect of heat-treatment under reductive atmosphere (0.5atm%, 800oC reduction). All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30 oC with a 3-electrode cell. Chronoamperometry (CA) was performed from 0.2 to 1.2 V vs. RHE under O2 atmosphere to obtain ORR current. The ORR current density was normalized by the electric charge of the double layer capacitance under N2 atmosphere. From the results of the potential-ORR current curves from 0.2 to 1.0 V of TiO2-Nb(0.5 or 5atm%) and (0.5atm%, 800oC reduction) electrodes, the ORR activity of the TiO2-Nb(0.5atm%) electrode was higher than that of the TiO2-Nb(5atm%) electrode. Although Nb doping is necessary to have some electric conductivity, small amount of Nb doping might be enough to get high ORR activity. In addition, the ORR activity of the TiO2-Nb(0.5atm%_800oC reduction) was higher than that of the TiO2-Nb(0.5atm%). Therefore, we found that the heat-treatment under reductive atmosphere enhanced the ORR activity. The catalytic activity might be affected by many factors. Among them we are studying the effects of crystal size, surface area, and crystal lattice. The results will be reported at the symposium. Our final target of our project is to obtain the catalyst that has better ORR activity and better durability compared to the present Pt with carbon support. We are moving towards the target. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. REFERENCES 1) A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, Journal of Electrochemical Society, 155, 2008, B400-B406 (2008). 2) A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, Journal of Physical Chemistry, ser. C, 117, 18837-18844 (2013). 3) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota, Physical Chemistry Chemical Physics, 17, 7643-7647 (2015). 4) N Uehara, A. Ishihara, M Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochimica Acta, doi:10.1016/j.electacta.2015.03.125. 5) A. Ishihara, M. Hamazaki, M. Arao, M. Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, and K. Ota, Journal of Electrochemical Society, 163(7), F603-F609 (2016).

Efficient Charge Separation of In‐Situ Nb‐Doped TiO2 Nanowires for Photoelectrochemical Water–splitting

AbstractOne‐dimensional TiO2 nanowire is the top‐performing semiconductor among photoanodes for photoelectrochemical water splitting, but still limited to a fraction of the theoretically photocurrent because of modest charge transport and low charge separation properties. Here, we demonstrate high‐crystallized nanowires of Nb‐doped rutile TiO2 for significantly increasing electron carrier density through a convenient method. The optimized Nb‐doped TiO2 nanowires (0.5 mol% in the Nb/Ti ratio) yield a high value of charge separation efficiency (ηsep) of 97.39% at 1.23 VRHE, which is about 3 times as much as that of pristine sample (33.61% at 1.23 VRHE). Moreover, the photoanode of Nb‐doped TiO2 nanowires can achieve high charge transfer efficiency (ηtrans) (91.07% at 1.23 VRHE) without any oxygen evolution reaction catalysts. These results indicate that through appropriate amount of Nb doping can move the performance of TiO2 nanowires to their theoretical potential.

Advancement of Group 4 and 5 Metal Oxide Cathode with Oxide Support for PEFCs

Polymer electrolyte fuel cells are expected for the residential and transportable applications, due to their high power density and low operating temperature. The ENEFARMs (micro-co-generation system using 700 W PEFC system) are operating more than 150,000 units in Japan. Fuel cell vehicles are also commercially available in Japan.However, the estimated amount of Pt reserve is too small to supply for the huge number of fuel cell devices. Pt is not stable in acid solution although Pt is relatively stable among the platinum group metals. The dissolution of Pt cathode might be the final problem to be solved related to the stability in the present PEFC system. In order to commercialize PEFCs systems more widely, the development of non-precious metal cathode is strongly required. Additionally, the instability of carbon support is also a big problem especially for fuel cell vehicles. Carbon including graphite is thermochemically unstable at room temperature in air or oxygen containing atmosphere. A stable non-precious metal oxide cathode with stable metal oxide support without carbon might be the final goal for the cathode of PEFC for fuel cell vehicles.We have reported that partially oxidized group 4 and 5 metal carbonitrides and organometallic complexes are stable in an acid solution and have definite catalytic activity for the oxygen reduction reaction (ORR) (1-4). The stability of metal oxide were evaluated by the solubility of a metal oxide in the acid solution. In oxygen containing atmosphere most of metals except Au form metal oxide thermochemically. In this paper we will report our recent advancement of the group 4 and 5 metal oxide catalyst with metal oxide support without carbon.Recently, we published the results of precious-metal-free and carbon-free cathodes based on oxides and demonstrated the superior durability of oxide-based cathodes by preparing titanium-niobium oxides mixed with Ti4O7 (TixNbyOz + Ti4O7) (5). The ORR activity of the TixNbyOz + Ti4O7 is higher than that of the Ti4O7, indicating that the TixNbyOz might have active sites for the ORR. The highest onset potential of the TixNbyOz +Ti4O7 was over 1.1 V with respect to reversible hydrogen electrode. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. Therefore, we successfully demonstrated that the precious-metal and carbon-free oxide-based cathodes had superior durability under the cathode conditions of a polymer electrolyte fuel cell.In order to qualify the role of Nb oxide for ORR, we used TiO2-Nb (Nb; 0.5 or 5atm%) rods (TOSHIMA Manufacturing Co., Ltd) as working electrodes. In addition, TiO2-Nb (0.5atm%) rod was heat-treated at 800 oC in 4%H2/Ar to examine the effect of heat-treatment under reductive atmosphere (0.5atm%, 800oC reduction). All electrochemical measurements were performed in 0.1 mol dm-3 H2SO4 at 30 oC with a 3-electrode cell. Chronoamperometry (CA) was performed from 0.2 to 1.2 V vs. RHE under O2 atmosphere to obtain ORR current. The ORR current density was normalized by the electric charge of the double layer capacitance under N2 atmosphere.From the results of the potential-ORR current curves from 0.2 to 1.0 V of TiO2-Nb(0.5 or 5atm%) and (0.5atm%, 800oC reduction) electrodes, the ORR activity of the TiO2-Nb(0.5atm%) electrode was higher than that of the TiO2-Nb(5atm%) electrode. Although Nb doping is necessary to have some electric conductivity, small amount of Nb doping might be enough to get high ORR activity. In addition, the ORR activity of the TiO2-Nb(0.5atm%_800oC reduction) was higher than that of the TiO2-Nb(0.5atm%). Therefore, we found that the heat-treatment under reductive atmosphere enhanced the ORR activity. The authors wish to thank to the New Energy and Industrial Technology Development Organization (NEDO) for their financial support. REFERENCES1) A. Ishihara, Y. Shibata, S. Mitsushima, K. Ota, Journal of Electrochemical Society, 155, 2008, B400-B406 (2008).2) A. Ishihara, M. Tamura, Y. Ohgi, M. Matsumoto, K. Matsuzawa, S. Mitsushima, H. Imai, K. Ota, Journal of Physical Chemistry, ser. C, 117, 18837-18844 (2013). 3) A. Ishihara, M. Chisaka, Y. Ohgi, K. Matsuzawa, S. Mitsushima, K. Ota, Physical Chemistry Chemical Physics, 17, 7643-7647 (2015).4) N Uehara, A. Ishihara, M Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, K. Ota, Electrochimica Acta, doi:10.1016/j.electacta.2015.03.125.5) A. Ishihara, M. Hamazaki, M. Arao, M. Matsumoto, H. Imai, Y. Kohno, K. Matsuzawa, S. Mitsushima, and K. Ota, Journal of The Electrochemical Society, 163 (7) F603-F609 (2016)

Advancement of Group 4 and 5 Metal Oxide Cathode with Oxide Support for PEFCs

In order to develop precious-metal-free and carbon-free cathodes, the ORR activities of Group 4 and 5 metal oxide mixtures with Ti4O7 support have been studied. The ORR activity of the TixNbyOz + Ti4O7 is higher than other oxide mixtures, indicating that the TixNbyOz might have high active sites for the ORR. The highest onset potential of the TixNbyOz + Ti4O7 was over 1.1 V with respect to reversible hydrogen electrode. No degradation of the ORR performance of TixNbyOz + Ti4O7 was observed during both start-stop and load cycle tests. TiO2-Nb (0.5 atm%) single crystal was heat-treated at 800 oC in 4%H2/Ar to examine the effect of heat-treatment under reducive atmosphere. The ORR activity of the TiO2-Nb(0.5atm%) electrode was high. Small amount of Nb doping might be enough to get high ORR activity and high temperature treatment was effective to get high activity.

Effect of Nb Doping on the Dielectric and Strain Properties of Lead–free 0.94(Bi1/2Na1/2)TiO3–0.06BaTiO3 Ceramics

(Bi1/2Na1/2)0.94Ba0.06(Ti1−xNbx)O3 (BNBTxNb) ceramics were investigated in terms of the crystal structure as well as the ferroelectric, dielectric, and piezoelectric properties. While little change was observed in the microstructure except for a slight decrease in the average grain size, a significant change was noticed in the temperature dependence of dielectric and piezoelectric properties. It was shown that the property changes are closely related to the downward shift in the position of the ferroelectric-to-relaxor transition temperature with increasing amount of Nb doping. A special emphasis is put on the fact that Nb doping is so effective at decreasing the ferroelectric-to-relaxor transition temperature that even at no more than 2 at.% Nb addition, the transition temperature was already brought down slightly below room temperature, resulting in the birth of a large strain at 0.46 %, equivalent to Smax/Emax = 767 pm/V. Key words: Lead-free piezo ceramics, Relaxor ferroelectrics, Electromechanical strain

Cobalt-free niobium-doped barium ferrite as potential materials of dense ceramic membranes for oxygen separation

Cobalt-free perovskite-type oxides with the nominal composition of BaNbyFe1−yO3−δ (y=0.025–0.20) are synthesized and evaluated as materials used in ceramic membranes for oxygen separation. The effects of Nb-doping on the crystal structure, surface morphology, electrical conductivity, chemical bulk diffusion and surface exchange, and oxygen permeability of the oxides are systematically investigated using XRD, SEM, four-probe DC conductivity, electrical conductivity relaxation technique, and oxygen permeation studies. A small amount of Nb-doping induces a sharp increase in electrical conductivity. A further increase in the Nb-doping amount, however, lowers the electrical conductivity as a result of the blocking effect of Nb5+ on electronic conduction. A small amount of Nb-doping has less impact on the sintering capability. From the oxygen permeation test, it was found that Nb-doping could significantly enhance the oxygen permeability, especially below 750°C. Among all of the compositions, BaNb0.05Fe0.95O3−δ shows the highest oxygen permeation fluxes, reaching 1.35 and 0.61mLcm−2min−1 for a membrane with a thickness of 1.0mm at 900 and 700°C, respectively. Furthermore, the membrane is rate-controlled mainly by bulk diffusion, indicating the potential to further improve the oxygen permeation flux via a thinner membrane.

Nb-Doped Colloidal TiO2 Nanocrystals with Tunable Infrared Absorption

We report a new colloidal synthesis of niobium-doped TiO2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb5+ on Ti4+ sites generating free carriers inside the conduction band of the TiO2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to “peanutlike” rods.

High performance oxygen permeable membranes with Nb-doped BaBi0.05Co0.95O3−δ perovskite oxides

BaBi0.05Co0.95−xNbxO3−δ (x=0–0.2) and BaBi0.1Co0.8Nb0.1O3–δ were synthesized via solid state reaction method and fabricated into disk-shaped membranes. The surface morphology, phase structure, oxygen nonstoichiometry and oxygen permeability of the perovskite membranes were investigated by SEM, XRD, TGA techniques and oxygen permeation studies. The experimental result indicates Nb-doping can significantly enhance the oxygen permeability compared to the un-doped disk membrane. Among all compositions, BaBi0.05Co0.8Nb0.15O3−δ showed the highest oxygen flux, reaching 2.92mlcm−2min−1 with 1.3mm thickness membrane at 900°C, coupled with the lowest activation energy, which is 44.17kJmol−1 at 750–900°C and 74.59kJmol−1 at 600–750°C. However, further increasing Nb-doping amount leads to a decrease in the oxygen permeability. Interestingly, the order of tendency towards increasing of oxygen nonstoichiometry with increasing temperature exactly corresponds to the order of oxygen permeation flux of various samples.

Ab initio studies of Nb doping effect on the formation of oxygen vacancy in rutile TiO 2

In this paper, the effect of Nb doping on the formation of O vacancies in rutile TiO 2 is studied by the use of ab initio density-functional calculations. The formation energies are calculated at different doping concentrations ( C Nb ) and the corresponding electronic structures are analyzed. It is shown that the formation energy is mainly determined by the structural relaxation. As C Nb increases, the formation energy first increases and then decreases with the maximum in vicinity of C Nb =11.1 at%. The investigation of electronic structures shows that the distribution of excess electrons, which is affected by the doping concentration and the types of oxygen vacancy, determines the relaxation and formation energy of oxygen vacancy. Our calculated results indicate that the formation of oxygen vacancy in rutile TiO 2 can be most effectively suppressed by Nb doping in a large range of Nb doping concentration. This may be one of the reasons why the proper amount of Nb doping can improve the oxidation resistance of γ - TiAl .

Synthesis and electrochemical properties of Nb-doped Li3V2(PO4)3/C cathode materials for lithium-ion batteries

Abstract Li3V2−xNbx(PO4)3/C cathode materials were synthesized by a sol–gel method. X-ray diffraction patterns demonstrated that the appropriate addition of Nb did not destroy the lattice structure of Li3V2(PO4)3, and enlarged the unit cell volume, which could provide more space for lithium intercalation/de-intercalation. Transmission electron microscopy and energy dispersive X-ray spectroscopy analysis illustrated that Nb could not only be doped into the crystal lattice, but also form an amorphous (Nb, C, V, P and O) layer around the particles. As the cathode materials of Li-ion batteries, Li3V2−xNbx(PO4)3/C (x ≤ 0.15) exhibited higher discharge capacity and better cycle stability than the pure one. At a discharge rate of 0.5C, the initial discharge capacity of Li3V1.85Nb0.15(PO4)3/C was 162.4 mAh/g. The low charge-transfer resistances and large lithium ion diffusion coefficients confirmed that Li3V2−xNbx(PO4)3/C samples possessed better electronic conductivity and lithium ion mobility. These improved electrochemical performances can be attributed to the appropriate amount of Nb doping in Li3V2(PO4)3 system by enhancing structural stability and electrical conductivity.

Effects of niobium doping site and concentration on the phase structure and oxygen permeability of Nb-substituted SrCoO x oxides

Niobium doping effect on phase structure, phase stability and electrical conductivity of SrCoO x oxides and oxygen permeability of the corresponding membranes were systematically investigated. Niobium was successfully incorporated into A-site, B-site or simultaneously A and B double sites of SrCoO x oxide and stabilized the perovskite structure with a cubic symmetry in air down to room temperature at a proper doping amount. However, the A-site doping could not stabilize the cubic structure under a more reducing atmosphere of nitrogen, as to SrCo 1− y Nb y O 3− δ , y ≥ 0.1 is necessary to sustain the cubic perovskite structure. Simultaneous doping of Nb at A and B sites is the most effective way to stabilize the perovskite structure under nitrogen atmosphere. Irrespective of doping site, the electrical conductivity decreased monotonously with Nb-doping amount. Both Nb x Sr 1− x CoO 3− δ and Nb z Sr 1− z Co 1− z Nb z O 3− δ envisaged a decrease in oxygen permeation flux with Nb-doping amount while SrCo 1− y Nb y O 3− δ reached the maximum flux at y = 0.1. Among all the membranes, SrCo 0.9Nb 0.1O 3− δ and Nb 0.05Sr 0.95Co 0.95Nb 0.05O 3− δ show the highest oxygen fluxes of 3.5 and 2.7 ml cm −2 min −1 at 900 °C under an air/helium gradient, respectively.