The aim of this study of the spatial dispersion of tin, niobium and tantalum mineralization associated with the Mayo Darlé granitoids was to produce prospecting guides through predictive maps of Sn, Nb and Ta in ...The aim of this study of the spatial dispersion of tin, niobium and tantalum mineralization associated with the Mayo Darlé granitoids was to produce prospecting guides through predictive maps of Sn, Nb and Ta in the region. It was based on a database (in appendix) obtained after analysis of rock samples (greisens and quartz veins) collected in the field, using a portable X-ray fluorescence (XRF) spectrometer. Two approaches were used: 1) structural studies in the field using the directions of veins and fractures 2) the use of variographic maps, an essential element in geostatistics for determining directional anisotropies. A joint synthesis of the modelling results shows that tin, tantalum and niobium mineralization at Mayo Darlé is concentrated along strike intervals N315E to N320E, with mineralization also occurring along strike N35E for high-grade Sn, medium-grade Ta and low-grade Nb. In short, mineral concentrations disperse progressively in space: positively from east to west for tantalum and niobium, and inversely for tin.展开更多
Niobium pentoxide(Nb_(2)O_(5))anodes have gained increasing attentions for high-power lithium-ion batteries owing to the outstanding rate capability and high safety.However,Nb2O5 anode suffers poor cycle stability eve...Niobium pentoxide(Nb_(2)O_(5))anodes have gained increasing attentions for high-power lithium-ion batteries owing to the outstanding rate capability and high safety.However,Nb2O5 anode suffers poor cycle stability even after modified and the unrevealed mechanisms have restricted the practical applications.Herein,the over-reduction of Nb5+has been demonstrated to be the critical reason for the capacity loss for the first time.Besides,an effective competitive redox strategy has been developed to solve the rapid capacity decay of Nb_(2)O_(5),which can be achieved by the incorporation of vanadium to form a new rutile VNbO_(4)anode.The highly reversible V^(3+)/V^(2+)redox couple in VNbO_(4)can effectively inhibit the over-reduction of Nb^(5+).Besides,the electron migration from V^(3+)to Nb5+can greatly increase the intrinsic electronic conductivity for VNbO4.As a result,VNbO4 anode delivers a high capacity of 206.1 mAh g^(−1)at 0.1 A g^(−1),as well as remarkable cycle performance with a retention of 93.4%after 2000 cycles at 1.0 A g^(−1).In addition,the assembled lithium-ion capacitor demonstrates a high energy density of 44 Wh kg^(−1)at 5.8 kW kg^(−1).In summary,our work provides a new insight into the design of ultra-fast and durable anodes.展开更多
This work demonstrates the viability of the powder-mixed micro-electrochemical discharge machining(PMECDM) process to fabricate micro-holes on C103 niobium-based alloy for high temperature applications.Three processes...This work demonstrates the viability of the powder-mixed micro-electrochemical discharge machining(PMECDM) process to fabricate micro-holes on C103 niobium-based alloy for high temperature applications.Three processes are involved simultaneously i.e.spark erosion,chemical etching,and abrasive grinding for removal of material while the classical electrochemical discharge machining process involves double actions i.e.spark erosion,and chemical etching.The powder-mixed electrolyte process resulted in rapid material removal along with a better surface finish as compared to the classical microelectrochemical discharge machining(MECDM).Further,the results are optimized through a multiobjective optimization approach and study of the surface topography of the hole wall surface obtained at optimized parameters.In the selected range of experimental parameters,PMECDM shows a higher material removal rate(MRR) and lower surface roughness(R_(a))(MRR:2.8 mg/min and R_(a) of 0.61 μm) as compared to the MECDM process(MRR:2.01 mg/min and corresponding Raof 1.11 μm).A detailed analysis of the results is presented in this paper.展开更多
Niobium nanoparticles with high purity were prepared by a sodiothermic reduction process using Nb2O5 as the raw material, LiCl, NaCl, KCl and CaCl2 as the diluents and sodium as the reducing reagent. The effects of th...Niobium nanoparticles with high purity were prepared by a sodiothermic reduction process using Nb2O5 as the raw material, LiCl, NaCl, KCl and CaCl2 as the diluents and sodium as the reducing reagent. The effects of the different molten salt systems, CaCl2 content, reaction time, excessive sodium and reaction temperature on the characteristics of the obtained niobium powder were discussed. The as-prepared niobium nanoparticles under the optimum experimental conditions were obtained by sodiothermic reduction with 20% excessive sodium in the NaCl-52mol%CaCl2- 2mol%Nb2O5 molten salts at 650 ℃for 6 h, and the molar ratio of the oxygen element in Nb2O5 and the CaCl2 of the molten salts is less than 19.23% at 750 ℃. Moreover, the particle size of niobium nanoparticles is ranged about 40~240 nm with increasing of reaction temperature from 650 to 800 ℃.展开更多
Using thermomechanical simulation experiment, the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the cond...Using thermomechanical simulation experiment, the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation. The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism, wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation, whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs, wherein, the nucleation rate of ferrite in the initial stage of transformation is low, and in the second stage, the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete. Using carbon extraction replica TEM, niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite. The mechanism of the effect of niobium on the isothermal transformation was discussed.展开更多
Niobium, as the most effective second-phase forming element, was added in the Fe-Crl3-C-N hard- facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were st...Niobium, as the most effective second-phase forming element, was added in the Fe-Crl3-C-N hard- facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther- modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condi- tion, a large number of secondary carbonitrides can pre- cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter- granular chromium depletion.展开更多
Bayan Obo ore deposit is endowed with and lie hidden in the Proterozoic strata. The localities and occurrences of the minerals in the ore deposit were described. It is obviously that some minerals are strata minerals ...Bayan Obo ore deposit is endowed with and lie hidden in the Proterozoic strata. The localities and occurrences of the minerals in the ore deposit were described. It is obviously that some minerals are strata minerals and some are post strata minerals. The rare earths, niobium and tantalum minerals are exactly the post strata minerals. In these minerals the hydrothermal metasomatic phenomena distinctly reveal their metallogenic characteristics. According to tectonic movement, magma activity, mineral paragenesis, hydrothermal metasomatism, geological age and lasting time scale of metallogenesis, and some other factors, it is supposed that genesis of rare earths, niobium and tantalum minerals in Bayan Obo ore deposit are closely related with hydrothermal metallogenic solution which is differentiated from silica acid and carbonic acid magma and derived from deep seated source, and then intruded into Proterozoic strata and metasomatized. It is recognized that the metallogenesis of Bayan Obo ore deposit is undergoing a long geological period and many episodes.展开更多
Phosphoric acid treated niobic acid(NbP)was used for the dehydration of xylose to furfural in biphasic solvent system,which was found to exhibit the best performance among the tested catalysts.The excellent performanc...Phosphoric acid treated niobic acid(NbP)was used for the dehydration of xylose to furfural in biphasic solvent system,which was found to exhibit the best performance among the tested catalysts.The excellent performance of NbP could be explained by the better synergistic cooperation between Bro¨nsted and Lewis acid sites.Moreover,NbP showed good stability and no obvious deactivation or leaching of Nb could be observed after six continuous recycles.展开更多
This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 + γ (Ti3Al + TiAl) and orthorhombi...This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 + γ (Ti3Al + TiAl) and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.展开更多
Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation indu...Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.展开更多
Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PE...Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PEMFC environment was investigated. It was showed that the plating film was distributed on the surface of 316 stainless steel like isolated islands with height less than 50 nm. The XPS, XRD results showed that a smooth and strong chemical inert film of Nb O and Nb2O5 was formed on the surface of 316 stainless steel. In simulated cathodic condition, the corrosion potential of Nb coated stainless steel was improved by 244 m V, whilst in an anodic condition, it was improved by 105 m V. The current densities for the coated 316 stainless steel were decreased to 2.479 4 μA·cm-2 from 14.810 μA·cm-2 at-0.1 V and to 0.576 μA·cm-2 from 13.417 μA/·cm-2 at 0.6 V, respectively. It was implied that the niobium coating effectively decreased the corrosion rate. The results of the electrochemical tests indicated that the corrosion resistance of stainless steel was greatly improved after coated with niobium.展开更多
Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obt...Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obtained by using the W/Cu composite filler metals.To determine the reasons for cracking in the Nb/SS joint and the function of the W/Cu composite filler metals on the improvement of the cracking resistance of the Nb/W/Cu/SS joint,the microstructures of the joints were studied by optical microscopy,scanning electron microscopy and X-ray diffraction.The cracking susceptibilities of the joints were evaluated with microhardness test on the cross section of the Nb/W/Cu/SS joint.The results showed that the Nb/W/Cu/SS joint was characterized by various solid solution.The formation of solid solution reduced the cracking susceptibility of the joint.展开更多
In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 sta...In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 stainless steel were evaluated by electrochemical tests in a simulated proton exchange membrane fuel cell (PEMFC) environment. The results showed that niobium could be electrodeposited on the surface of 304 stainless steel from ionic liquids, and a smooth and strong chemical inert compound film was obtained on the surface of 304 stainless steel, which was mainly composed of NbO and Nb2Os. The thin composite film acted as a barrier and remarkably improved the corrosion resistance of 304 stainless steel in the PEMFC environment.展开更多
Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematica...Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.展开更多
Aqueous hybrid supercapacitors are attracting increasing attention due to their potential low cost,high safety and eco-friendliness.However,the narrow operating potential window of aqueous electrolyte and the lack of ...Aqueous hybrid supercapacitors are attracting increasing attention due to their potential low cost,high safety and eco-friendliness.However,the narrow operating potential window of aqueous electrolyte and the lack of suitable negative electrode materials seriously hinder its future applications.Here,we explore high concentrated lithium acetate with high ionic conductivity of 65.5 mS cm−1 as a green“water-in-salt”electrolyte,providing wide voltage window up to 2.8 V.It facilitates the reversible function of niobium tungsten oxide,Nb18W16O93,that otherwise only operations in organic electrolytes previously.The Nb18W16O93 with lithium-ion intercalation pseudocapacitive behavior exhibits excellent rate performance,high areal capacity,and ultra-long cycling stability.An aqueous lithium-ion hybrid capacitor is developed by using Nb18W16O93 as negative electrode combined with graphene as positive electrode in lithium acetate-based“water-in-salt”electrolyte,delivering a high energy density of 41.9 W kg−1,high power density of 20,000 W kg−1 and unexceptionable stability of 50,000 cycles.展开更多
The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder f...The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder form and have a 0.5 mm particle size at dfferent proportions of 1 wt.% and 3 wt.%. The addition was done during casting of the metal in the mold at the last cooling step of the melt cast iron. These additions have a significant impact on the phenomenon of solidifi cation as the metal powder deposited in the sand molds creates new centers of germination and absorbs a lot of heat. The cooling rate directly affects the microstructure and electrochemical behavior. This is confirmed by SEM observations and electrochemical tests. Furthermore, the addition of niobium transforms the microstructure of gray cast iron from cellular structure into totally dendritic structure. As a consequence, the niobium addition affected the shape and size of graphite, thus considerably reducing the corrosion current density by increasing the polarization resistance Rp.展开更多
Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, b...Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confi rmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and fl at interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.展开更多
A niobium-modified layer on pure titanium surface was obtained by means of double glow plasma surface alloying technique. The modified layer was uniform, continuous, compact and well adhered to the substrate. The niob...A niobium-modified layer on pure titanium surface was obtained by means of double glow plasma surface alloying technique. The modified layer was uniform, continuous, compact and well adhered to the substrate. The niobium composition in the modified layer decreased gradually from the surface to the substrate. The oxidation behavior of the niobium-modified layer was investigated and com- pared with the untreated surface at 900 ~C for 100 h. Characterization of the layers was performed using X-ray diffraction and scanning electron microscope, respectively. The test results show that the oxidation behavior of pure titanium was improved by niobium alloying process. Niobium has a positive influence on the oxidation resistance.展开更多
Cobalt-based catalysts were prepared by a wet impregnation method on carbon nanotubes (CNTs) support and promoted with niobium.Samples were characterized by nitrogen adsorption,TEM,XRD,TPR,TPO and H2-TPD.Addition of...Cobalt-based catalysts were prepared by a wet impregnation method on carbon nanotubes (CNTs) support and promoted with niobium.Samples were characterized by nitrogen adsorption,TEM,XRD,TPR,TPO and H2-TPD.Addition of niobium increased the dispersion of cobalt but decreased the catalysts reducibility.Fischer-Tropsch synthesis (FTS) was carried out in a fixed-bed microreactor at 543 K,1 atm and H2/CO=2 for 5 h.Addition of niobium enhanced the C5+ hydrocarbons selectivity by 39% and reduced methane selectivity by 59%.These effects were more pronounced for 0.04%Nb/Co/CNTs catalyst,compared with those observed for other niobium compositions.展开更多
Bimetallic Co /Fe catalysts supported on carbon nanotubes( CNTs) were prepared,and niobium( Nb) was added as promoter to the 70 Co ∶30Fe /CNT catalyst. The physicochemical properties of the catalysts were characteriz...Bimetallic Co /Fe catalysts supported on carbon nanotubes( CNTs) were prepared,and niobium( Nb) was added as promoter to the 70 Co ∶30Fe /CNT catalyst. The physicochemical properties of the catalysts were characterized,and the catalytic performances were analyzed at the same operation conditions( H_2 ∶CO( volume ratio) = 2 ∶1,p = 1 MPa,and t = 260 ℃) in a tubular fixed-bed microreactor system. The addition of Nb to the bimetallic catalyst decreases the average size of the oxide nanoparticles and improves the reducibility of the bimetallic catalyst. Evaluation of the catalyst performance in a Fischer-Tropsch reaction shows that the catalyst results in high selectivity to methane,and the selectivity to C_(5+) increased slightly in the bimetallic catalyst unlike that in the monometallic catalysts. The addition of 1% Nb to the bimetallic catalyst increases CO conversion and selectivity to C_(5+). Meanwhile,a decrease in methane selectivity is observed.展开更多
文摘The aim of this study of the spatial dispersion of tin, niobium and tantalum mineralization associated with the Mayo Darlé granitoids was to produce prospecting guides through predictive maps of Sn, Nb and Ta in the region. It was based on a database (in appendix) obtained after analysis of rock samples (greisens and quartz veins) collected in the field, using a portable X-ray fluorescence (XRF) spectrometer. Two approaches were used: 1) structural studies in the field using the directions of veins and fractures 2) the use of variographic maps, an essential element in geostatistics for determining directional anisotropies. A joint synthesis of the modelling results shows that tin, tantalum and niobium mineralization at Mayo Darlé is concentrated along strike intervals N315E to N320E, with mineralization also occurring along strike N35E for high-grade Sn, medium-grade Ta and low-grade Nb. In short, mineral concentrations disperse progressively in space: positively from east to west for tantalum and niobium, and inversely for tin.
基金support from National Natural Science Foundation of China(51874142)Tip-top Scientific and Technical Innovative Youth Talents of Guangdong Special Support Program(2019TQ05L903)Young Elite Scientists Sponsorship Program by CAST(2019QNRC001).
文摘Niobium pentoxide(Nb_(2)O_(5))anodes have gained increasing attentions for high-power lithium-ion batteries owing to the outstanding rate capability and high safety.However,Nb2O5 anode suffers poor cycle stability even after modified and the unrevealed mechanisms have restricted the practical applications.Herein,the over-reduction of Nb5+has been demonstrated to be the critical reason for the capacity loss for the first time.Besides,an effective competitive redox strategy has been developed to solve the rapid capacity decay of Nb_(2)O_(5),which can be achieved by the incorporation of vanadium to form a new rutile VNbO_(4)anode.The highly reversible V^(3+)/V^(2+)redox couple in VNbO_(4)can effectively inhibit the over-reduction of Nb^(5+).Besides,the electron migration from V^(3+)to Nb5+can greatly increase the intrinsic electronic conductivity for VNbO4.As a result,VNbO4 anode delivers a high capacity of 206.1 mAh g^(−1)at 0.1 A g^(−1),as well as remarkable cycle performance with a retention of 93.4%after 2000 cycles at 1.0 A g^(−1).In addition,the assembled lithium-ion capacitor demonstrates a high energy density of 44 Wh kg^(−1)at 5.8 kW kg^(−1).In summary,our work provides a new insight into the design of ultra-fast and durable anodes.
文摘This work demonstrates the viability of the powder-mixed micro-electrochemical discharge machining(PMECDM) process to fabricate micro-holes on C103 niobium-based alloy for high temperature applications.Three processes are involved simultaneously i.e.spark erosion,chemical etching,and abrasive grinding for removal of material while the classical electrochemical discharge machining process involves double actions i.e.spark erosion,and chemical etching.The powder-mixed electrolyte process resulted in rapid material removal along with a better surface finish as compared to the classical microelectrochemical discharge machining(MECDM).Further,the results are optimized through a multiobjective optimization approach and study of the surface topography of the hole wall surface obtained at optimized parameters.In the selected range of experimental parameters,PMECDM shows a higher material removal rate(MRR) and lower surface roughness(R_(a))(MRR:2.8 mg/min and R_(a) of 0.61 μm) as compared to the MECDM process(MRR:2.01 mg/min and corresponding Raof 1.11 μm).A detailed analysis of the results is presented in this paper.
基金financially supported by the National Science Foundation of China(Nos.51102015and21071014)China Postdoctoral Science Foundation(No.20110490009)+2 种基金Research Fund for the Doctoral Program of Higher Education of China(No.20110006120027)the Program for New Century Excellent Talents in University(No.NCET-11-0577)the Program for Changjiang Scholars and Innovative Research Team in University(No.IRT0708)
文摘Niobium nanoparticles with high purity were prepared by a sodiothermic reduction process using Nb2O5 as the raw material, LiCl, NaCl, KCl and CaCl2 as the diluents and sodium as the reducing reagent. The effects of the different molten salt systems, CaCl2 content, reaction time, excessive sodium and reaction temperature on the characteristics of the obtained niobium powder were discussed. The as-prepared niobium nanoparticles under the optimum experimental conditions were obtained by sodiothermic reduction with 20% excessive sodium in the NaCl-52mol%CaCl2- 2mol%Nb2O5 molten salts at 650 ℃for 6 h, and the molar ratio of the oxygen element in Nb2O5 and the CaCl2 of the molten salts is less than 19.23% at 750 ℃. Moreover, the particle size of niobium nanoparticles is ranged about 40~240 nm with increasing of reaction temperature from 650 to 800 ℃.
基金Item Sponsored by National High Technology Research and Development Program of China(2002AA302501)
文摘Using thermomechanical simulation experiment, the kinetics of the isothermal transformation of austenite to ferrite in two HSLA low-carbon steels containing different amounts of niobium was investigated under the conditions of both deformation and undeformation. The results of optical microstructure observation and quantitative metallography analysis showed that the kinetics of the isothermal transformation of austenite to ferrite in lower niobium steel with and without deformation suggests a stage mechanism, wherein there exists a linear relationship between the logarithms of holding time and ferrite volume fraction according to Avrami equation, whereas the isothermal transformation of austenite to ferrite in high niobium steel proceeds via a two stage mechanism according to micrographs, wherein, the nucleation rate of ferrite in the initial stage of transformation is low, and in the second stage, the rate of transformation is high and the transformation of residual austenite to ferrite is rapidly complete. Using carbon extraction replica TEM, niobium carbide precipitation for different holding time was investigated and the results suggested that NbC precipitation and the presence of solute niobium would influence the transformation of austenite to ferrite. The mechanism of the effect of niobium on the isothermal transformation was discussed.
基金financially supported by the National Natural Science Foundation of China(No.51101050)Natural Science Foundation of Jiangsu Province of China(No.BK2011257)
文摘Niobium, as the most effective second-phase forming element, was added in the Fe-Crl3-C-N hard- facing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The ther- modynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condi- tion, a large number of secondary carbonitrides can pre- cipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and inter- granular chromium depletion.
文摘Bayan Obo ore deposit is endowed with and lie hidden in the Proterozoic strata. The localities and occurrences of the minerals in the ore deposit were described. It is obviously that some minerals are strata minerals and some are post strata minerals. The rare earths, niobium and tantalum minerals are exactly the post strata minerals. In these minerals the hydrothermal metasomatic phenomena distinctly reveal their metallogenic characteristics. According to tectonic movement, magma activity, mineral paragenesis, hydrothermal metasomatism, geological age and lasting time scale of metallogenesis, and some other factors, it is supposed that genesis of rare earths, niobium and tantalum minerals in Bayan Obo ore deposit are closely related with hydrothermal metallogenic solution which is differentiated from silica acid and carbonic acid magma and derived from deep seated source, and then intruded into Proterozoic strata and metasomatized. It is recognized that the metallogenesis of Bayan Obo ore deposit is undergoing a long geological period and many episodes.
基金supported by the Natural Science Foundation of China (No.21106143)100-talent project of Dalian Institute of Chemical Physics (DICP)the Independent Innovation Foundation of State Key Laboratory of Catalysis (No.R201113)
文摘Phosphoric acid treated niobic acid(NbP)was used for the dehydration of xylose to furfural in biphasic solvent system,which was found to exhibit the best performance among the tested catalysts.The excellent performance of NbP could be explained by the better synergistic cooperation between Bro¨nsted and Lewis acid sites.Moreover,NbP showed good stability and no obvious deactivation or leaching of Nb could be observed after six continuous recycles.
基金funded by National Project PCCA contract No.65/2012POS-CCE O 2.2.1 project INFRANANOCHEM-No 19/January 3,2009 of the EU(ERDF)and Romanian Government
文摘This work addresses the alloying of titanium aluminides used in aircraft engine applications and automobiles. The oxidation resistance behavior of two titanium aluminides of α2 + γ (Ti3Al + TiAl) and orthorhombic Ti2NbAl, recognized as candidates for high-temperature applications, was investigated by exposure of the alloys for 100 h in air. Thus, oxidation resistance was expressed as the mass gain rate, whereas surface aspects were analyzed using scanning electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and the type of oxidation products was analyzed by X-ray diffraction and Raman spectroscopy. The orthorhombic Ti2NbAl alloy was embrittled, and pores and microcracks were formed as a result of oxygen diffusion through the external oxide layer formed during thermal oxidation for 100 h.
基金This work was financially supported by the National Natural Science Foundation of China(No.50527402).
文摘Microstructure consisting of ferrite, bainite and retained austenite can be obtained through intercritical annealing and isothermal treatment in bainite transformation region for low silicon TRIP (transformation induced plasticity) steel containing niobium. Effects of strain rate, Nb content and soaking temperature in bainite region on microstructure and mechanical properties of test steels were investigated. It is shown that as strain rate ranges from 10^-2 to 10^-4 s^-1, the volume fraction of transformed martensite from retained austenite, as well as tensile strength, elongation rate and strength-ductility product, increases. When Nb is added, the volume fraction of retained austenite decreases, but tensile strength and yield strength increase. While Nb content reaches 0.014%, the steel exhibits high elongation and combination of strength and ductility. Higher retained austenite volume fraction and good mechanical properties are obtained in the test steels when the soaking temperature in bainite region is 400℃. The maximum values of tensile strength, total elongation rate and strength-ductility product can reach 739 MPa, 38% and 28082 MPa%, respectively.
基金Funded by the National Natural Science Foundation of China(No.21276036)the Fundamental Research Funds for the Central Universities(No.3132014323)
文摘Niobium was electrodeposited on 316 stainless steel bipolar plates of a fuel cell in water and air-stable choline chloride based ionic liquids. The electrochemical corruption property of bipolar plates in simulated PEMFC environment was investigated. It was showed that the plating film was distributed on the surface of 316 stainless steel like isolated islands with height less than 50 nm. The XPS, XRD results showed that a smooth and strong chemical inert film of Nb O and Nb2O5 was formed on the surface of 316 stainless steel. In simulated cathodic condition, the corrosion potential of Nb coated stainless steel was improved by 244 m V, whilst in an anodic condition, it was improved by 105 m V. The current densities for the coated 316 stainless steel were decreased to 2.479 4 μA·cm-2 from 14.810 μA·cm-2 at-0.1 V and to 0.576 μA·cm-2 from 13.417 μA/·cm-2 at 0.6 V, respectively. It was implied that the niobium coating effectively decreased the corrosion rate. The results of the electrochemical tests indicated that the corrosion resistance of stainless steel was greatly improved after coated with niobium.
基金supported by the National Natural Science Foundation of China(Grant No.51605205)the Natural Science Foundation of the Jiangsu Higher Education Institutions of China(Grant No.15KJB460009)
文摘Cracking in a laser weld of niobium to stainless steel occurred due to the formation of brittle,continuously distributed Nb-Fe intermetallic compounds.A crack-free joint,which had a tensile strength of 147 MPa,was obtained by using the W/Cu composite filler metals.To determine the reasons for cracking in the Nb/SS joint and the function of the W/Cu composite filler metals on the improvement of the cracking resistance of the Nb/W/Cu/SS joint,the microstructures of the joints were studied by optical microscopy,scanning electron microscopy and X-ray diffraction.The cracking susceptibilities of the joints were evaluated with microhardness test on the cross section of the Nb/W/Cu/SS joint.The results showed that the Nb/W/Cu/SS joint was characterized by various solid solution.The formation of solid solution reduced the cracking susceptibility of the joint.
基金supported by the National High-Tech Research and Development Program of China (No. 2009AA05Z120)the Young Teachers Program of Dalian Maritime University (No. DLMU-ZL-200720)
文摘In order to enhance the corrosion resistance of 304 stainless steel, niobium was electrodeposited on its surface in air- and water-stable ionic liquids. The electrochemical behaviors of bare and niobium-coated 304 stainless steel were evaluated by electrochemical tests in a simulated proton exchange membrane fuel cell (PEMFC) environment. The results showed that niobium could be electrodeposited on the surface of 304 stainless steel from ionic liquids, and a smooth and strong chemical inert compound film was obtained on the surface of 304 stainless steel, which was mainly composed of NbO and Nb2Os. The thin composite film acted as a barrier and remarkably improved the corrosion resistance of 304 stainless steel in the PEMFC environment.
基金Item Sponsored by High Technology Development Program of China (863) (2001AA332020)National Natural Science Foundation of China (50271015)
文摘Using Gleeble-1500 system, the influence of holding time on bainite transformation in deformed niobium microalloyed steel during continuous cooling was analyzed, and the carbides in upper bainite were also systematically researched. The results show that the occurrence of the static recrystallization decreases the amount of bainite with an increase in the holding time and the emergence of retained austenite (RA) with the longer holding time. Two types of carbides were observed in upper bainite with regard to their precipitation sites. They either existed between the bainite ferrite laths or co-existed with RA. The formation mechanism of two kinds of carbides was analyzed by combining TEM micrographs with the model.
基金Shengyang Dong and Yi Wang contributed equally to this work.This work was supported by the National Natural Science Foundation of China(Nos.U1802256,51672128,51802154)the Key Research and Development Program in Jiangsu Province(BE2018122)+1 种基金Jiangsu Specially-Appointed Professors Program,the Fundamental Research Funds for the Central Universities(NE2016005)the Startup Foundation for Introducing Talent of NUIST(1441622001004).
文摘Aqueous hybrid supercapacitors are attracting increasing attention due to their potential low cost,high safety and eco-friendliness.However,the narrow operating potential window of aqueous electrolyte and the lack of suitable negative electrode materials seriously hinder its future applications.Here,we explore high concentrated lithium acetate with high ionic conductivity of 65.5 mS cm−1 as a green“water-in-salt”electrolyte,providing wide voltage window up to 2.8 V.It facilitates the reversible function of niobium tungsten oxide,Nb18W16O93,that otherwise only operations in organic electrolytes previously.The Nb18W16O93 with lithium-ion intercalation pseudocapacitive behavior exhibits excellent rate performance,high areal capacity,and ultra-long cycling stability.An aqueous lithium-ion hybrid capacitor is developed by using Nb18W16O93 as negative electrode combined with graphene as positive electrode in lithium acetate-based“water-in-salt”electrolyte,delivering a high energy density of 41.9 W kg−1,high power density of 20,000 W kg−1 and unexceptionable stability of 50,000 cycles.
文摘The purpose of this study was to determine the impact of niobium addition as an inoculation element on the microstructure and electrochemical properties of EN-FGL250 gray cast iron. Niobium additions are in a powder form and have a 0.5 mm particle size at dfferent proportions of 1 wt.% and 3 wt.%. The addition was done during casting of the metal in the mold at the last cooling step of the melt cast iron. These additions have a significant impact on the phenomenon of solidifi cation as the metal powder deposited in the sand molds creates new centers of germination and absorbs a lot of heat. The cooling rate directly affects the microstructure and electrochemical behavior. This is confirmed by SEM observations and electrochemical tests. Furthermore, the addition of niobium transforms the microstructure of gray cast iron from cellular structure into totally dendritic structure. As a consequence, the niobium addition affected the shape and size of graphite, thus considerably reducing the corrosion current density by increasing the polarization resistance Rp.
基金Funded by the National Natural Science Foundation of China(No.50675165)the National Key Technology R&D Program(No.2006BAF02A29)+1 种基金the Specialized Research Fund for the Doctoral Pro-gram of Higher Education of China(20131420120002)the Shanxi Prov-ince Science Foundation(No.2013011025-1)
文摘Niobium carbide coating was produced by thermal-reactive diffusion technique on AISI 52100 steel in salt bath at 1 123 K, 1 173 K, and 1 223 K for 1, 2, 4, and 6 hours. The salt consisted of borax, sodium fl uoride, boron carbide, and niobium pentoxide. The presence of NbC phase on the steel surface was confi rmed by X-ray diffraction analysis. Microscopic observation showed that niobium carbide coating formed on the substrate was smooth and compact. There was a distinct and fl at interface between the coating and substrate. The micro-hardness of niobium carbide coating was 2892±145HV. The thickness of coating ranged from 1.6 μm to 14μm. The forming kinetics of niobium carbide coating was revealed. Moreover, a contour diagram derived from experimental data was graphed for correct selection of process parameters. Some mathematical equations were built for predicting the coating thickness with predetermined processing temperature and time. The results showed that these mathematical equations are very practical as well as the kinetics equation.
文摘A niobium-modified layer on pure titanium surface was obtained by means of double glow plasma surface alloying technique. The modified layer was uniform, continuous, compact and well adhered to the substrate. The niobium composition in the modified layer decreased gradually from the surface to the substrate. The oxidation behavior of the niobium-modified layer was investigated and com- pared with the untreated surface at 900 ~C for 100 h. Characterization of the layers was performed using X-ray diffraction and scanning electron microscope, respectively. The test results show that the oxidation behavior of pure titanium was improved by niobium alloying process. Niobium has a positive influence on the oxidation resistance.
基金supported by the Ministry of Science,Technology and Innovation (E-Science Fund Project No: 03-02-02-SF0036)the Ministry of Higher Education Malaysia (FRGS/2/2010/SG/UTP/02/03)the Short Term Internal Fund Universiti Teknologi PETRONAS (31/09.10)
文摘Cobalt-based catalysts were prepared by a wet impregnation method on carbon nanotubes (CNTs) support and promoted with niobium.Samples were characterized by nitrogen adsorption,TEM,XRD,TPR,TPO and H2-TPD.Addition of niobium increased the dispersion of cobalt but decreased the catalysts reducibility.Fischer-Tropsch synthesis (FTS) was carried out in a fixed-bed microreactor at 543 K,1 atm and H2/CO=2 for 5 h.Addition of niobium enhanced the C5+ hydrocarbons selectivity by 39% and reduced methane selectivity by 59%.These effects were more pronounced for 0.04%Nb/Co/CNTs catalyst,compared with those observed for other niobium compositions.
基金supprted by Short Term Internal Research Fund Universiti Teknologi PETRONAS(0153AA-D06)the Ministry of Education(Higher Education Department)under MyRA Incentive Grant for CO2-Rich Natural Gas Value Chain Program
文摘Bimetallic Co /Fe catalysts supported on carbon nanotubes( CNTs) were prepared,and niobium( Nb) was added as promoter to the 70 Co ∶30Fe /CNT catalyst. The physicochemical properties of the catalysts were characterized,and the catalytic performances were analyzed at the same operation conditions( H_2 ∶CO( volume ratio) = 2 ∶1,p = 1 MPa,and t = 260 ℃) in a tubular fixed-bed microreactor system. The addition of Nb to the bimetallic catalyst decreases the average size of the oxide nanoparticles and improves the reducibility of the bimetallic catalyst. Evaluation of the catalyst performance in a Fischer-Tropsch reaction shows that the catalyst results in high selectivity to methane,and the selectivity to C_(5+) increased slightly in the bimetallic catalyst unlike that in the monometallic catalysts. The addition of 1% Nb to the bimetallic catalyst increases CO conversion and selectivity to C_(5+). Meanwhile,a decrease in methane selectivity is observed.