Masao Komaki

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%.

Preparation of V-based Alloy Membranes using Chemical Transport Process

Novel preparation process of V-Ni alloys for hydrogen purification membrane using chemical transport was investigated. Vanadium, NH 4 Cl, and PtCl 2 as evaporating source were put in one side of a fused-silica tube, and Ni substrate was put in the other side. The fused-silica tube was sealed in vacuo, and set in a furnace with temperature gradients. Evaporating source temperature was 1173–1273 K. Substrate temperature was 3–100 K higher than the evaporating source temperature. This process consists of formation of HCl from NH 4 Cl and PtCl 2 , chemical transport of vanadium under temperature gradient via chlorides in the presence of HCl, and diffusion of the transported V into the Ni substrate. EDX line profile of cross section of the substrates after the process demonstrated that V diffused into the Ni substrates. Distribution of V concentration in the substrates was dependent on the substrate temperature and the temperature difference between the substrate and the evaporating source. Heating condition was optimized to obtain homogeneous V-Ni alloy. When the substrate temperature and evaporating-source temperature were 1228 K and 1223 K, respectively, V diffused homogeneously into the Ni substrate with thickness of 20 μm, and V concentration attained in the substrate was higher than 70at%.