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In situ real time synchrotron radiation powder X-ray diffraction (SR-PXD) experiments are utilized to study changes in the crystalline compounds under dynamic hydrogenation and dehydrogenation reactions of MgH2 ball milled with 8 mol% Nb2O5. The ball milling conditions were systematically varied to prepare three samples with different reactivity. Up to eight full cycles of hydrogen release and uptake were investigated for each sample, which reveal that Nb2O5 reacts with Mg forming a ternary oxide, MgxNb1−xO. The PXD data for the ternary oxide is similar to that observed for the isostructural compounds MgO and NbO although shifted to lower Bragg diffraction angles revealing an expansion of the unit cell. Rietveld refinements suggest that MgxNb1−xO has a limiting composition of x ∼ 0.6 after eight cycles of hydrogen release and uptake. At elevated temperatures Nb(II) is reduced to metallic Nb(0) and extracted from the ternary oxide and forms in a reaction with Mg. This work suggests that a ternary solid solution MgxNb1−xO is the active material responsible for the prolific kinetic properties for the additive Nb2O5. Mg0.6Nb0.4O has a ∼4.6% larger unit cell volume as compared to the binary oxides, MgO and NbO, which may lead to formation of cracks and hydrogen diffusion pathways in dense magnesium oxide surface layers. 相似文献
64.
S.B. GabrielAuthor Vitae M.C. BrumAuthor VitaeK.C.G. CandiotoAuthor Vitae H.R.Z. SandimAuthor VitaeP.A. SuzukiAuthor Vitae C.A. NunesAuthor Vitae 《International Journal of Refractory Metals and Hard Materials》2012,30(1):38-41
High-purity niobium powders can be obtained from the well-known hydride-dehydride (HDH) process. The aim of this work was the investigation of the structural phase transition of the niobium hydride to niobium metal as function of temperature, heating rate and time. The niobium powder used in this work was obtained by high-temperature hydriding of niobium machining chips followed by conventional ball milling and sieving. X-ray diffraction measurements were carried out in vacuum using a high-temperature chamber coupled to an X-ray diffractometer. During the dehydriding process, it is possible to follow the phase transition from niobium hydride to niobium metal starting at about 380 °C for a heating rate of 20 °C/min. The heating rate was found to be an important parameter, since complete dehydriding was obtained at 490 °C for a heating rate of 20 °C/min. The higher dehydriding rate was found at 500 °C. Results contribute to a better understanding of the kinetics of thermal decomposition of niobium hydride to niobium metal. 相似文献
65.
Two-layer thin film specimens of Nb2O5 and TiO2 were deposited on optical-grade quartz and n-type single crystalline silicon substrates with (100) crystallographic orientation by a magnetron deposition source under high vacuum. All samples were subjected to 1-5 h of resistive heating at ultra high vacuum, and in situ X-ray diffraction measurements (XRD) were made in the temperature range of 300-1373 K. Analysis of the XRD data confirmed the growth of TiNbO4 during cooling of the two-layered specimens which had been previously heated to 1373 K. Optical measurements revealed a band gap value of 3.78 eV for the direct transition and 3.29 eV for the indirect one. The samples had a transmittance of 85% in the visible range. Electrophysical measurements in high vacuum established the electroresistivity vs. temperature dependence in the range of 300-773 K, from 7.3 ? 10− 1 to 3.9 ? 10− 2 Ω cm, respectively. X-ray photoelectron spectroscopy measurements were used to examine the chemical shift for Nb 3d, with a value of − 1.1 eV in comparison with Nb5+ and matched to Nb4+[1], while the Ti lines correspond to Ti4+[2]. 相似文献
66.
The corrosion behavior and interfacial contact resistance (ICR) of niobium implanted SS316L used as the bipolar plate in a polymer electrolyte membrane fuel cell (PEMFC) are investigated. The ICR values of the bare and niobium implanted SS316L are measured to evaluate the electrical conductivity. The effects of ion implantation on the corrosion behavior are investigated by potentiodynamic and potentiostatic tests in the simulated PEMFC anode and cathode environments. The solutions after the potentiostatic test are analyzed by inductively-coupled plasma atomic emission spectrometry (ICP-AES). The surface topography of the samples before and after the potentiostatic test is monitored by SEM in order to investigate the mechanism and degree of corrosion. The XPS results indicate that the composition on the surface is altered by ion implantation. The electrochemical results reveal that the passivation current density of the Nb implanted SS316L decreases and has higher chemical stability in the simulated PEMFC environment. However, the ion implantation fluence affects the current density. The ICP results are in agreement with those of the electrochemical test disclosing that the bare SS316L has the highest dissolution rate in both the cathode and anode environments and niobium implantation reduces the dissolution rate significantly. SEM shows that the bare SS316L undergoes serious corrosion whereas after Nb ion implantation, corrosion is greatly retarded. The XPS depth profiles indicate that a passive film with a new composition consisting mainly of niobium oxide is formed after the potentiostatic test. Our results suggest that niobium implantation with proper ion fluences can significantly improve the corrosion resistance and the electric conductivity of SS316L in the simulated PEMFC environments. 相似文献
67.
Shu-Shu Wang 《Surface & coatings technology》2010,205(7):1886-1891
Niobium nitride (NbN) and zirconium nitride (ZrN) were deposited on Ti substrates by direct-current (DC) reactive magnetron sputtering; the deposited NbN and ZrN films served as intermediate layers of a Ti and porcelain interface. X-ray diffraction (XRD) results proved that the deposited NbN and ZrN films were polycrystalline with a cubic microstructure. The Ti and porcelain bonding strength of the samples in Group Control (27.2 ± 0.75 MPa), Group NbN (43.1 ± 0.59 MPa), and Group ZrN (52.4 ± 0.80 MPa) were measured. The surface roughness in the case of Group Control (1.863 ± 0.10 μm), Group NbN (2.343 ± 0.07 μm), and Group ZrN (2.346 ± 0.10 μm) was also investigated. Statistical analysis showed that both films helped improve the Ti and porcelain bonding strength and increase the surface roughness. Scanning electron microscopy (SEM) results showed that no apparent oxide layer was formed at the Ti and porcelain interface in both Group NbN and Group ZrN. Energy-dispersive X-ray spectroscopy (EDS) results showed that ZrN was more effective in preventing Ti oxidation than was NbN. Overall, the experimental results showed that the deposition of both NbN and ZrN films helps improve the Ti and porcelain bonding strength and that ZrN films are more effective. 相似文献
68.
F. Moura Souza J. Nandenha B.L. Batista V.H.A. Oliveira V.S. Pinheiro L.S. Parreira A.O. Neto M.C. Santos 《International Journal of Hydrogen Energy》2018,43(9):4505-4516
PdxNby/C binary electrocatalysts supported on Vulcan carbon XC72 were prepared by the sol-gel method. The materials are characterized by transmission electron microscopy, X-ray diffraction analysis, inductively coupled plasma–mass spectrometry and contact angle measurements. The electrocatalytic activity for ethanol electrooxidation reaction was studied by cyclic voltammetry, chronoamperometry, Tafel slope and accelerated durability testing. The direct ethanol performance and the products after the experiments were studied by Fourier transform infrared spectroscopy. Pd1Nb1/C (50:50 wt%) shows superior activity for ethanol oxidation compared to the other electrocatalysts prepared in this work. All electrocatalysts containing Nb show the highest current exchange density. The Tafel slope results suggest that the Nb modified the Pd-electrocatalyst to obtain a reaction path with high selectivity with only a single determining step with low production of the intermediates for the ethanol oxidation reaction. The best performance is obtained using Pd1Nb1/C 18.11 mW cm?2. The Pd1Nb1/C electrocatalyst displays the highest production of CO2 and the lowest production of acetaldehyde. Pd1Nb1/C shows the highest peak current density during 1000 cycles of the experiment and the lowest mass loss of Pd after the cycling test. We find that the Nb modifies the Pd electrocatalysts from the bifunctional mechanism and reduces the loss of Pd during the accelerated durability test. 相似文献
69.
Xueli Yao Lijun Fan Tian Gan Nianjun Hou Ping Li Yicheng Zhao Yongdan Li 《International Journal of Hydrogen Energy》2018,43(28):12748-12755
NbOx is added in Ni-Ce0.8Sm0.2O1.9 by impregnation as an anode material for solid oxide fuel cells fed with methanol. Nb (IV) and Nb (V) exist in the reduced anode. The addition of Nb reduces the binding energy of Ni. The catalytic activity of the anode and the performance of the single cell both increase with the increase of Nb. At 700 °C, the cell with 5NbOx-Ni-Ce0.8Sm0.2O1.9 anode and Ce0.8Sm0.2O1.9-carbonate electrolyte shows a output power density of 687 mW cm?2. Meanwhile, water produced in the anode is absorbed by NbOx and forms surface hydroxyl groups, which facilitates the removal of carbon. The addition of NbOx decreases the amount of deposited carbon in the humidified methanol atmosphere significantly, and an improved stability of the single cell is achieved. 相似文献
70.