Chikashi Nishimura

V–Ni alloy membranes for hydrogen purification

Abstract In order to establish technical fundamentals for practical application of V–15%Ni alloy developed for hydrogen purification membrane, deformation behavior and hydrogen permeation performance of Pd-plated cold-rolled and unrolled samples have been examined. Once the solidification microstructure was refined by plastic works at high temperatures, cold rolling could be performed without any difficulty to produce thin strips. Pd-plated membranes machined from a cold-rolled sheet of V–15Ni alloy exhibited slight trapping behavior of hydrogen, resulting in a decrease in apparent hydrogen diffusivity and steady-state hydrogen permeability. Nevertheless, the hydrogen flux was observed to increase inversely in proportion to the thickness of the membrane due to a much greater influence of thinning on the permeation flow. The durability tests showed that hydrogen permeability decreased by 5% in 2 weeks at 573 K and 30% in 1 week at 473 K. The deteriorated permeability was recovered successfully by a baking treatment.

Hydrogen permeation characteristics of vanadium–aluminium alloys

Abstract Hydrogen permeation characteristics of V–Al alloys (0.1–40 at.% Al) were investigated using the gas permeation technique in the temperature range of 523–673 K. The hydrogen permeability of V–Al alloys decreased considerably when the Al content exceeded 20 at.%.

Hydrogen permeation characteristics of V–Ni–Al alloys

Abstract We prepared a series of V–(15−x)Ni– x Al (0.09⩽x⩽4.5) alloys by arc melting and investigated their hydrogen permeation characteristics using palladium-coated disk samples. In x⩽0.9 alloys, precipitates of the sigma phase were observed in the matrix of the bcc phase. In alloys with higher Al content, a single bcc phase was observed. The hydrogen permeability increased in parallel with the Al content at temperatures higher than 473 K . The hydrogen permeability of the V–10.5Ni–4.5Al membrane was 6.29×10 −8 mol H 2 m −1 s −1 Pa −1/2 at 623 K , or about twice that of the V–15Ni membrane. The hydrogen diffusivity was only weakly dependent on the Al content, hence the increase in the hydrogen permeability in parallel with the Al content was concluded to be mostly due to the increase in hydrogen solubility. The hydrogen permeability of the V–10.5Ni–4.5Al membrane decreased to 59% of the initial value during a permeation test conducted for 102 h . However, a subsequent baking treatment with air introduced into the permeation system resulted in a complete recovery of the hydrogen permeability to the initial value.

Preparation of palladium-coated V and V–15Ni membranes for hydrogen purification by electroless plating technique

Abstract A simple electroless palladium plating treatment on V and V–15Ni alloy was attempted to prepare sandwich-type membranes for hydrogen purification. The obtained coating on the membranes was characterized using SEM–EDX and AFM, and hydrogen permeability and hydrogen diffusion coefficients for the plated membranes were determined. Sensitizing–activating treatment with SnCl2 and PdCl2 solutions before the electroless plating treatment enabled palladium to cover the membranes densely. The V membrane with electroless palladium plating, whose hydrogen permeability was 5.0×10 −7 mol H 2 m −1 s −1 Pa −1/2 at 623 K , showed cracking during the hydrogen permeation experiment at 473 K . Hydrogen permeability for the V–15Ni membrane with electroless palladium plating was 3.4×10 −8 mol H 2 m −1 s −1 Pa −1/2 at 623 K , which was comparable to that of the sample coated by the conventional methods including evaporation and electrochemical plating. The long-period permeation test showed that the V–15Ni membrane with electroless palladium plating possessed satisfactory durability to be used as hydrogen purification material.

Hydrogen permeation through magnesium

Abstract Hydrogen permeation characteristics have been investigated for pure magnesium membranes with a palladium overlayer using a gas permeation technique in the temperature range 473–493 K and in the hydrogen pressure range 0.1–10 kPa. For thicker specimens (thickness >2 mm), the permeation process was rate-controlled by the diffusion of hydrogen in bulk magnesium. The diffusion coefficient was determined to be D=1.54×10 −6 exp (−24,100/RT) m 2 s −1 . The steady-state hydrogen permeability was determined to be Φ ss =2.77×10 −7 exp (−35,700/RT) mol H 2 m −1 s −1 Pa −1/2 . D and Φ ss were independent of the applied hydrogen pressure, suggesting that D is independent of hydrogen content and the solubility of hydrogen obeys Sieverts’ law. The hydrogen solution constant K was determined to be K=1.8×10 −1 exp (−11,600/RT) mol H 2 m −3 Pa −1/2 .

Hydrogen permeation and transmission electron microscope observations of V-Al alloys

Abstract Hydrogen permeation and transmission electron microscope (TEM) observations were performed for V–Al alloys (10–40 mol% Al). Hydrogen permeability of V–Al alloys decreased with aluminum content, but not in a monotonous manner. Below 20 mol% of aluminum, hydrogen permeability of V–Al alloys decreased linearly with aluminum content. From 20 to 30 mol% of aluminum, hydrogen permeability decreased abruptly. A15 phase which is shown in the V–Al phase diagram was not observed in any samples, quenched from 1373 K or aged at 853 K for 100–350 h. Instead, some precipitates of 200 nm to 1 μm were observed at grain boundaries, sub-boundaries and inside the grains. The amount of the precipitates, however, was too small to explain the significant drop of hydrogen permeability observed in the alloys with more than 20 mol% Al. The long-time permeation test showed that V–10Al sample possessed satisfactory durability to be used as hydrogen purification materials.

Hydrogen permeation characteristics of V-15Ni membrane with Pd/Ag overlayer by sputtering

Abstract Pd–Ag alloy coated V-15Ni composite membranes were prepared by co-sputtering of separate pure Pd and Ag targets using a DC multi-target sputtering system where voltage for each target can be controlled independently. The film composition was controlled by changing the target voltage. Hydrogen permeation characteristics of as-prepared composite membranes were investigated using the gas permeation technique in the temperature range of 423–673 K. The hydrogen permeation of the composite membranes was mainly bulk-diffusion limited at a temperature higher than 473 K. At 473 K or lower, the hydrogen permeability of the composite membranes increased with the Ag content until 30 at%.

Small-Angle X-Ray Scattering and Proton Conductivity of Anhydrous Nafion–Benzimidazole Blend Membranes

The small-angle X-ray scattering (SAXS) and proton conductivity of Nafion-benzimidazole (na―bz) blend membranes were investigated at various annealing temperatures. The SAXS data showed that the bz base as a proton source was successfully incorporated in the Nafion nanostructure, and the bz in the nanostructure was stable under both wet and dry conditions at room temperature. The nanostructural stability of the na―bz blend membrane was also investigated at various temperatures and was compared to the X-ray diffraction and proton conductivity data. In the na―bz blend membrane, both the nanostructure and the bz were stable up to 150°C under anhydrous (nonhumidified) conditions. The proton conductivity was also stable over this temperature range. The na―bz blend membrane may be useful as an anhydrous membrane for high temperature polymer electrolyte fuel cells.

Modification of thermal conductivity and thermal boundary resistance of amorphous Si thin films by Al doping

We investigate the effects of Al doping on the thermal conductivity and thermal boundary resistance of a-Si thin films. Au/Al-doped a-Si/Si structures were prepared by depositing Al-doped amorphous Si films of different Al doping concentrations and thickness on Si substrates by magnetron sputtering. The thermal resistances of the structures were measured to calculate the thermal conductivities of the films. The thermal conductivities of the 150 nm-thick films were higher than those of 100 nm-thick films, and a sharp increase in thermal conductivity with increasing Al doping concentration was observed in the 150 nm-thick films but not in the 100 nm-thick films. Furthermore, the thermal boundary resistances at the two interfaces in the structures also increased with increasing Al doping concentration. Our findings could be used to tailor the thermal resistance of materials for thermal management in semiconductor devices as well as for development of thermal barrier coatings and thermoelectric materials with good performance.

Fast Oxidation of Porous Cu Induced by Nano-Twinning

The fcc lattice of porous Cu prepared by dealloying Al2Cu with HCl aqueous solution exhibits a high density of twinning defects with an average domain size of about 3 nm along the ⟨111⟩ directions. The high density of twinning was verified by X-ray diffraction and qualitatively interpreted by a structural model showing the 5% probability of twinning defect formation. Most of the twinning defects disappeared after annealing at 873 K for 24 h. Twinned Cu reveals much faster oxidation rate in comparison to that without (or with much fewer) twinning defects, as shown by X-ray diffraction and hydrogen differential scanning calorimetry. Using ab initio DFT calculations, we demonstrate that twinning defects in porous Cu are able to form nucleation centers for the growth of Cu2O. The geometry of the V-shaped edges on the twinned {211} surfaces is favorable for formation of the basic structural elements of Cu2O. The fast oxidation of porous Cu prepared by dealloying can thus be explained by the fast formation of the Cu2O nucleation centers and their high density.