Papers by Keyword: Hydrogen Permeability

Abstract: The temperature dependence of hydrogen solubility and diffusivity of Pd–53mol%Cu alloy membrane with the B2–type crystal structure has been investigated. The hydrogen permeation tests are performed using ultra–pure hydrogen (more than 9N) purified by a Pd–Ag alloy membrane to avoid any influences of impurities. It is found that the hydrogen permeability decreases significantly at low temperatures, especially near room temperature. The time dependence of hydrogen flux is monitored and found that the hydrogen flux decreases gradually during about 4 ~ 5 days after rapid cooling down to room temperature from 623 K.The results of the temperature dependence of the hydrogen permeability are analyzed in view of the consistent description of hydrogen permeation based on hydrogen chemical potential, where the hydrogen flux is proportional to the product of the mobility for hydrogen diffusion, B, and the PCT factor, f_PCT. In this study, the pressure–composition–isotherms (PCT curves) for Pd–53Cu alloy with B2 structure are measured for the first time by the in–situ XRD–PCT method, and they are applied to estimate the PCT factors. Then, the temperature dependence of the PCT factor and the mobility for hydrogen diffusion is evaluated. It is revealed that the decrement in hydrogen permeability at low temperatures is mainly attributable to the decrement of the mobility for hydrogen diffusion.According to the positron annihilation experiments, the defects density is considered to be small in Pd–53Cu alloy with the B2 structure even at room temperature, suggesting that the excess Cu atoms in Pd–53Cu alloy occupy the positions of Pd sublattice, at which the Cu atoms form a local BCC–Cu unit. The diffusion of Cu atoms corresponds to the diffusion of BCC–Cu units in the B2 structure. Therefore the diffusion of Cu atoms and the configuration of BCC–Cu units in B2 structure could be a key to understand the gradual transition of hydrogen diffusivity at low temperatures.

Abstract: High-entropy alloys of NbTaVTiNi, NbTaVTiCo, NbTaVHfCo, NbTaVHfNi, and NbTaVZrNi were synthesized by vacuum arc melting. The crystal structure was identified by an X-ray diffractometer analysis. Microstructures and element composition of the resulting alloys were investigated using a scanning electron microscope equipped with an energy dispersive X-ray spectrometer. All alloys presented the dendritic structure which is mainly composed of body-centered cubic (NbTaV,X) (X=Ti, Hf, Zr) phases and eutectic-like phases such as TiNi, TiCo, HfCo, HfNi and ZrNi. (NbTaV,X) phases take the function of hydrogen permeability while eutectic-like phases bear the responsibility of resistance to hydrogen embrittlement. Taking the microstructure of alloys into account, NbTaV-based high entropy alloys is appropriate for improving hydrogen permeability.

Abstract: The concept for alloy design of Nbbased hydrogen permeable membrane is applied to NbWMo ternary system. The alloying effects of tungsten and molybdenum on the solubility of hydrogen, the resistance to hydrogen embrittlement, the hydrogen permeability and diffusivity are investigated in a fundamental manner. It is found that the addition of tungsten and molybdenum into niobium decreases the hydrogen solubility. As a result, the resistance to hydrogen embrittlement improves and higher hydrogen pressures can be applied to the NbWMo alloy membrane. It is shown that the designed Nb5mol%W5mol%Mo alloy membrane with single solid solution phase exhibits excellent hydrogen permeability together with strong resistance to hydrogen embrittlement. In addition, it is found that the alloying of tungsten and molybdenum with niobium enhances the hydrogen diffusivity. In fact, the activation energy for hydrogen diffusion decreases in the order, pure Nb > Nb5mol%W > Nb5mol%W5mol%Mo.

Abstract: NiO-doped Al₂O₃ was synthesized with 20 wt% of nickel nitrate using the sol-gel process. P123 as an organic additive added to increase surface area of powder. The phase transformation, thermal evolution and surface morphology of the powder were characterized by XRD, TG-DTA and FE-SEM. The 20 wt% NiO-doped Al₂O₃ /10 wt% Ni composite membrane was prepared by hot press sintering (HPS) following with a mechanical alloying process. Hydrogen permeation flux for 20 wt% NiO-doped Al₂O₃ /10 wt% Ni membrane was obtained as 0.1 mol/m² s at 673K.

Abstract: The palladium membranes were fabricated by mean of electroless plating method which provided thin and uniform film on a tubular alumina porous support. Subsequently, the annealing process which is one of the effective ways to enhance the permeability of membrane was performed in the temperature range of 600-800°C. The annealing temperature and time were found to have influence on membrane microstructure and its performance. Palladium annealed at 700°C for 8 hrs in nitrogen atmosphere had the highest hydrogen permeability and separation factor when operated at 600°C. The membrane morphologies observed by SEM showed that the sample treated at higher temperature had lower surface roughness. The annealed palladium composite membrane at 800°C revealed that some voids which occurred during electroless plating process were enlarged and caused the lower separation factor.

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. It is found that the hydrogen solubility decreases by the addition of ruthenium, tungsten or molybdenum into niobium. The mobility for hydrogen diffusion during hydrogen permeation is estimated from the linear relationship between the normalized hydrogen flux, , and the product of the hydrogen concentration and the difference of hydrogen chemical potential, . It is found that the mobility for hydrogen diffusion during hydrogen permeation is larger for Nb-based alloys than pure niobium, especially at low temperature. The activation energy of the mobility for hydrogen diffusion decreases by the addition of ruthenium, tungsten or molybdenum into niobium.

Abstract: The susceptibility to fish-scale formation of cold rolled Al-killed low carbon enamel grade steel sheets is mainly determined by the hydrogen permeability. The role of the grain orientation in the hydrogen permeation time was investigated using scanning electron microscope based electron backscatter diffraction measurements. The fragmentations of massive cementite phase have a significant influence not only on the hydrogen permeability but also on the evolution of texture during the cold rolling process. Results showed that {111}[uvw] texture act as trapping site for hydrogen.

Abstract: Nb/NiZr composite alloy membranes have been reported to have hydrogen permeabilities higher than that of pure Pd. Since the hydrogen permeation behaviour in these composite alloys is highly microstructure sensitive, hydrogen permeability is likely to depend on annealing conditions. This work has looked into the effect of annealing on the hydrogen permeability of as-cast Nb-Ni-Zr alloys with the goal of helping in the advancement of Nb-based alloy membranes as cost-effective alternatives to the Pd-based alloy membranes used for hydrogen purification. Nb-Ni-Zr alloy ingots of different compositions were prepared by argon arc-melting. The samples were vacuum sealed in quartz tubes and annealed isochronally for 1h between 500°C and 900°C. It was found that the samples annealed at 900°C exhibit higher hydrogen permeability than the as-cast samples. However, these samples were found to be less resistant to hydrogen embrittlement and the membrane exhibited cracks after the permeation test. The main mechanical failure mechanism was due to intragranular cracking for the alloys with high Nb content whilst the mechanism was observed to be intergranular cracking for alloys with lower Nb-content. The mode of failure did not change after annealing.

Abstract: The hydrogen solubility and the hydrogen permeability have been measured for Nb-based alloys in order to investigate the alloying effects on the hydrogen diffusivity during hydrogen permeation. The hydrogen diffusion coefficient during hydrogen permeation is estimated from a linear relationship between the normalized hydrogen flux, , and the difference of hydrogen concentration, C, between the inlet and the outlet sides of the membrane. It is found that the hydrogen diffusion coefficient during the hydrogen permeation is increased by alloying ruthenium or tungsten into niobium. On the other hand, the activation energy for hydrogen diffusion in pure niobium under the practical permeation condition is much higher than the reported values measured for dilute hydrogen solid solutions. It is interesting that the activation energy for hydrogen diffusion decreases by the addition of ruthenium or tungsten into niobium.

Abstract: The hydrogen diffusion coefficients are investigated during the hydrogen permeation through Nb-based hydrogen permeable membranes at high temperature. It is found that the hydrogen diffusion coefficient for pure niobium under practical conditions is much lower than the reported values measured for dilute hydrogen solid solutions. Surprisingly, the hydrogen diffusion is found to be faster in Pd-Ag alloy with fcc crystal structure than in pure niobium with bcc crystal structure at 773K during the hydrogen permeation. It is also found that the addition of Ru or W into niobium increases the hydrogen diffusion coefficient under the practical conditions.