Niobium Membrane Research Articles

Testing of PAV-ONE, a Permeator Against Vacuum mock-up with niobium membrane, in lithium-lead eutectic at 350 °C

Tritium extraction from lithium-lead eutectic alloy (LiPb) is one of the biggest challenges to be solved for the exploitation of the WCLL (Water Cooled Lithium-Lead) as the Breeding Blanket (BB) of EU DEMO reactor. The Permeator Against Vacuum (PAV) is one of the most promising technologies to reach this goal, but it needs a bigger amount of experimental data to demonstrate its viability and to support the scale-up towards DEMO. For this reason, an experimental campaign was performed at ENEA Brasimone in the TRIEX-II facility, where PAV-ONE, a PAV mock-up based on niobium U-tubes, was characterized in flowing LiPb. This paper describes the results obtained at a temperature of 350 °C with a hydrogen partial pressure in LiPb in the range 110–230 Pa, conditions that are relevant for the WCLL BB. Furthermore, the results at 350 °C are compared with those previously obtained at 450 °C on the same mock-up and with the same flow rate, showing a reduction in the extracted hydrogen flux of about one order of magnitude. Finally, an analytical model was set up to reproduce the experimental results of the PAV mock-up. The model results had a closer fit to experimental results when using lower bound properties. This suggests that mass transport through the PbLi and molecular recombination from the PAV surface were the rate-limiting processes.

Manufacturing of PAV-ONE, a Permeator against Vacuum Mock-Up with Niobium Membrane

The Permeator Against Vacuum (PAV) is one of the proposed technologies for the Tritium Extraction System of the WCLL BB (Water-Cooled Lithium-Lead Breeding Blanket) of the EU DEMO reactor. In this paper, the manufacturing of the first PAV mock-up with a niobium membrane with a cylindrical configuration is presented. This work aimed to demonstrate the possibility of manufacturing a relevant-size PAV to be later tested in the TRIEX-II facility. The adopted prototypical solutions are described in detail, starting with the methodology developed to join the Nb tubes with a 10CrMo9-10 (A182 F22) plate. Dedicated manufacturing and welding procedures, based on vacuum brazing with a nickel-based brazing alloy, were developed to solve the problem. This new kind of brazing was first analyzed to check the morphology of the joint and then tested to check its capability to withstand the TRIEX-II operative conditions. In parallel, the compatibility with a lithium-lead environment was analyzed by exposing samples of niobium and 10CrMo9-10 (A335 P22) to a flow of the eutectic alloy at 500 °C up to 4000 h. Finally, the PAV mock-up was installed in the TRIEX-II facility.

Engineering design of a Permeator Against Vacuum mock-up with niobium membrane

Permeator Against Vacuum (PAV) is one of the technologies proposed for the Tritium Extraction and Removal System (TERS) of the Water-Cooled Lithium Lead Breeding Blanket (WCLL BB). The paper presents the activity aimed at the engineering design of a PAV mock-up with a niobium membrane, in order to later assemble and qualify it. Experience gained in the engineering design of the mock-up, the heating system, the instrumentation, and the vacuum line is illustrated. This experience will be useful for the preliminary design, the manufacturing and the operation of the PAV with niobium membrane for DEMO. Niobium was selected as membrane material of this mock-up because of its high permeability and for its lower cost compared to vanadium, the other candidate material for membranes. Besides, niobium has a lower tendency to oxidation than vanadium. Oxidation would reduce the hydrogen isotopes permeation flux. In this paper, the solution adopted to manufacture the PAV mock-up, a complex component with niobium and P22 parts, is illustrated. The Nb/P22 welding issues are also presented, in particular related to the compatibility of the welded joints with LiPb. In the chosen design, the LiPb flows with two passages in 16 (8 + 8) niobium “U” shaped pipes installed in a vacuum chamber and welded to a P22 plate. The U-pipes configuration was selected to minimize the welding area, the volume of the component and the membrane thickness while trying to preserve the highest possible extraction efficiency.

The effect of surface oxides and grain sizes on the deuterium permeation behavior of niobium membranes

Group 5 transition metal niobium is considered as the promising hydrogen separation membrane material because of its theoretical highest atomic hydrogen permeability, good mechanical strength, low price and easy shaping. However, the permeability of hydrogen in the dense metal membrane is affected by surface oxides, gain sizes, defects and so on. In this paper, the effect of surface oxides and grain sizes on the permeation behavior of deuterium through niobium membranes is studied. The niobium membranes with micro- and nanocrystalline were first treated by mechanically or electro-polished to remove the surface oxides, and then the deuterium permeability tests were performed in the temperature range of 600–800 ℃ at the driving pressures of 50 kPa by gas-driven permeation method. Results showed that the niobium membrane with nanocrystalline and “clean” surface had the highest deuterium permeation than that with microcrystalline or unclean surface. More importantly, grain size and grain boundary had a greater influence on the deuterium permeation behavior of niobium membranes than the thin surface oxides. These results shed new light on the enhancement of hydrogen and its isotopes permeation in the niobium-based hydrogen separation membranes. Further work is to study the plasma-driven permeation of deuterium through nanocrystalline niobium membrane in order to obtain the super-permeation rate.

Consistent description of hydrogen permeability through metal membrane based on hydrogen chemical potential

A new description of the hydrogen permeation through metal membrane has been proposed according to the diffusion equation based on the hydrogen chemical potential. The diffusion of hydrogen atom through metal membrane is found to be linked with the shape of the pressure-composition-isotherm (PCT curve) of the material. Thus, the analysis of the PCT curves is very important to understand the hydrogen permeability of metal membranes. The validity of the new diffusion equation has been verified by a series of hydrogen permeation test with a pure niobium membrane. There is a linear relationship between the hydrogen flux and the PCT factor, fPCT, indicating that the hydrogen flux obeys the new diffusion equation. This new description of hydrogen permeability is very useful for alloy design and setting the permeation conditions to get high hydrogen flux through metal membrane.

Microstructural analysis of thermal degradation of palladium-coated niobium membrane

The microstructural change in the Pd-coated Nb hydrogen permeable membrane proceeding by hydrogen permeability degradation was observed by cross-sectional transmission electron microscopy (TEM). The columnar microstructure of the Pd layer in the as-deposited sample forms voids open to the surface in the initial stage of heating. These voids correspond to the surface pores observed by scanning electron microscopy (SEM). The microstructural and compositional analysis revealed the formation of the intermetallic compound Pd3Nb during extended heating, which is found to be responsible for the permeability degradation of Pd-coated Nb membrane.

Catalytic decomposition of H2S in a double-pipe packed bed membrane reactor: Numerical simulation studies

Abstract H2 recovery from large quantities of toxic H2S, the by-product of various minerals, coal and petroleum processing operations is an attractive proposition for efficient resource and energy utilization. This study has examined the analysis of a double-pipe membrane reactor internally packed with a Ru-Mo catalyst for the catalytic desulfurization of H2S and H2 generation. Due to the endothermic nature of the reaction, a non-isothermal model was employed. The effect of tube and shell-side residence times, sweep gas (argon) flow direction within the annular space between shell and inner membrane tube (relative to the H2S feed) and the shell-to-tube side pressure ratio on H2S conversion were investigated. The counter-current mode of reactor operation generally gave better H2S conversions than the co-current mode (40–60%). Irrespective of shell-to-tube side pressure ratio, conversion in both modes of operation exceeded the thermodynamically-imposed ceiling by between two- and five-fold increase. The niobium membrane proposed was internally-coated with platinum to improve resistance to H2S attack. The analysis showed that an optimum Pt thickness of about 1 μm is attainable although the possible formation of hairline cracks may necessitate the use of a higher coating. The model also accounted for the temperature-dependency of the membrane H2 permeability in order to have a more realistic assessment of the reactor performance.

Enhanced hydrogen embrittlement of Pd-coated niobium metal membrane detected by in situ small punch test under hydrogen permeation

The hydrogen embrittlement of pure niobium metal membrane was investigated under a hydrogen atmosphere by using a newly developed in situ small punch apparatus. The boundary for the ductile-to-brittle transition of the palladium-coated pure niobium was determined from a series of the in situ small punch tests. The measured boundary was found to be shifted greatly to the lower hydrogen content region as compared to that of palladium non-coated niobium membrane. The present result will provide us a clue to the design of niobium-based permeable membrane against the hydrogen embrittlement.

Tritium Superpermeability: Experimental Investigation and Simulation of Tritium Recirculation in “Prometheus” Setup

The superpermeability phenomenon was suggested to use in fusion machines to separate the fuel mixture from helium ashes, to arrange in-vessel fuel recirculation contours and effective evacuation of fuel mixture out of machine exhaust. To develop this technology and to simulate tritium recirculation in RFNC-VNIIEF at the “Prometheus“ setup the experiments on superpermeation of hydrogen isotopes through metal membrane were realized.The results of experiments on superpermeation of hydrogen isotopes through cylindrical niobium membrane are presented. As the experiment has shown, membrane pumping rate is inversely proportional to square root of isotope mass and amount to 2.5 l/cm2s for protium, 1.8 l/cm2s for deuterium and 1.5 l/cm2s for tritium. The possibility of effective pumping, separation of hydrogen isotopes from helium and residual gas, compression and recuperation of hydrogen isotopes by means of superpermeable membrane was demonstrated. It follows from results that the separation of D/T from He with employment of the techniques of superpermeable membranes might reduce the total amount of tritium in fuel cycle and substantially enhance the resource of cryogenic pumps evacuating helium.

Hydrogen superpermeable membrane operation under plasma conditions

The effect of ion bombardment on hydrogen plasma-driven permeation through a superpermeable niobium membrane was investigated. It was found that the increase of membrane temperature and the doping of membrane material with oxygen results in the decrease of ion bombardment effect and in permeability increase. It was demonstrated that membrane decarbonization leads to the formation of a membrane state resistant to sputtering. Possible applications of the membrane resistant to ion bombardment as plasma facing components are considered.

“Prometheus” Setup for Study of Tritium Superpermeation

ABSTRACTThe experimental setup is put in operation designed to study the phenomenon of the superpermeation of hydrogen isotopes, including tritium, through metals and to demonstrate the possibility of membrane pumping. The permeation of atomic hydrogen through the niobium membrane was shown to occur in the superpermeation regime. For the first time superpermeation of tritium through a metallic membrane was experimentally observed. The possibility of effective pumping, compression and recuperation of hydrogen isotopes by means of superpermeable membrane was demonstrated.

Anomalous isotope effect in the permeation, retention, and reemission at interaction of energetic hydrogen with niobium

A niobium membrane sample was placed in H or D plasma and electrically biased. Isotope effects for H vs. D in factors of 20, 40, and 40, respectively, were observed in plasma driven permeation, retention, and in the reemission, within a narrow range of bias voltages (40–80 V) at the lowest metal temperature investigated (910 K). The phenomenon occurred at the “superpermeation” of suprathermal hydrogen arising from an oxygen monolayer at the metal surface. The phenomenon is supposed to be caused by dynamics of the oxygen monolayer under the action of ion sputtering and surface segregation of dissolved O. Such and even much stronger isotope effects are also expected on other metals with a similar “real” surface. This isotope effect may be important for D/T-mixture recycling, retention and permeation at its interactions with plasma facing components of fusion reactors as well as for the applications of superpermeable membranes for pumping of hydrogen isotopes and their separation from He.

Low loss niobium flexure suspension systems

Niobium membrane flexure suspensions have been proved to be able to achieve high pendulum Q-factors, whereas niobium cantilever suspensions have been proved to give high internal Q-factors in sapphire test masses. Here we present the proposed sapphire test mass suspension systems based on the use of niobium flexures. This suspension system has the advantage of being robust while maintaining the high Q of the sapphire test mass. We show that it is also advantageous to use Nb flexures in future cryogenic detectors.

Correlation between Balmer α emission and hydrogen flux through a superpermeable niobium membrane in a low-pressure multicusp plasma source

The purpose of this paper is to correlate hydrogen or deuterium flux through superpermeable membranes with incident hydrogen or deuterium atom flux from the plasma. To this aim a hydrogen or deuterium plasma is created in a hybrid multicusp plasma source. We investigate Balmer α emission from the multicusp plasma and the output pressure behind a superpermeable niobium membrane immersed in the plasma.The output pressure is proportional to the flux of atoms and ions arriving on the membrane. We find that both output pressure and excited atoms emission satisfy plasma parameters relations. It is thus verified that plasma-driven superpermeation of hydrogen is due essentially to neutral atoms from the plasma incident to the membrane.

Effect of ion bombardment on plasma-driven superpermeation of hydrogen isotopes through a niobium membrane

Hydrogen plasma-driven permeation through the superpermeable niobium membrane was investigated under bombardment of the input membrane surface with hydrogen, deuterium and helium ions with energy 0–250 eV over the range of membrane temperature 910–1420 K. The membrane surface was covered with a nonmetal monolayer generating a potential barrier responsible for the superpermeability to suprathermal hydrogen particles. Both an increase of ion energy and an increase of mass of sputtering ions result in a significant decrease of permeability due to destruction of the nonmetal monolayer, when the ion energy is higher than the threshold energy of surface film sputtering. On the contrary, the increase of the membrane temperature results in the decrease of the ion bombardment effect and in the increase of the membrane permeability due to recovery of the surface barrier through segregation of impurities dissolved in the membrane bulk onto the membrane surface. To increase the membrane ability to recover the potential barrier, oxygen was dissolved in the membrane bulk up to a concentration of 2.5 at.%. This resulted in a significant decrease of the damaging effect of ion bombardment and in the extension of the range of the membrane temperature and ion energy over which plasma-driven superpermeability was observed.

Membrane bias effects on plasma-driven permeation of hydrogen through niobium membrane

Niobium membrane modeling a pumping system in the divertor region of a fusion plasma machine was immersed in hydrogen plasma in order to study interaction of the membrane with hydrogen ions in the energy range from a few eV to 200 eV, controlled by changing the membrane bias voltage. Bombardment by ions of an energy higher than 50 eV resulted in a decrease of the plasma-driven permeation/retention and in an enhancement of the boundary processes of absorption/reemission of thermal molecules. At ion energies below 50 eV, the effect of ion bombardment on plasma driven permeation and on the kinetic coefficients of boundary processes was non-monotonic in ion energy having a maximum at 10 eV. Both dissolution of oxygen in the membrane and increase of membrane temperature lead to diminution of the bias effect due to rise of recovering rate of surface impurity layer.

High-Q niobium membrane flexure pendulum

We report here the first observed high pendulum Q-factor with a niobium membrane flexure. The Q-factor is close to the predicted value, confirming the Q-enhancement theory and that the niobium Q-factor is not strongly stress dependent. Results were limited by residual gas.

Nb interaction with hydrogen plasma

A niobium membrane was immersed in hydrogen plasma and could be electrically biased to vary the energy of bombarding ions in the range of 1–200 eV. The fluxes of plasma driven absorption and permeation were almost entirely governed by incident suprathermal neutrals (mostly, thermal atoms), whose energy does not depend on membrane bias, but the ions of controllable energy do affect the neutral-induced permeation through modifying the membrane surface. At the zero bias a high temperature-independent plasma driven permeation (superpermeation) was observed alongside of an enhanced absorption. Bombardment by ions of an energy higher than 50 eV resulted in a sharp decrease of the plasma driven permeation/retention and in an acceleration of boundary processes of absorption/reemission of thermal molecules. At ion energies below 50 eV, the effect of ion bombardment on the plasma driven permeation and the kinetic coefficients of boundary processes were nonmonotonic in ion energy, having a maximum at ∼10 eV. Both an in situ doping with O of the bulk of Nb and a membrane temperature increase reduced the effects of ion bombardment to their complete disappearance. Responsible for that was the replenishment by means of surface segregation of an oxygen monolayer sputtered by ion bombardment.

Superpermeable membrane for particle control in divertor: the effect of impurity deposition

The effect of impurity (stainless steel (SS) components, carbon) deposition onto niobium membrane surface on the membrane permeability to hydrogen particles is investigated with a plasma device. The deposition of SS components onto the upstream surface of the membrane at the membrane temperature ( T M)<800°C decreases plasma driven permeation (PDP) rate but increases molecule gas driven permeation (GDP) rate. In the case of carbon deposition at T M<800°C, PDP rate is reduced while GDP rate remains almost unchanged. It is argued that deposited SS components layer enhances the absorption/release of hydrogen molecules, on the other hand, deposited carbon layer lets hydrogen molecules freely pass through but traps suprathermal hydrogen particles. The impurities deposition effects on PDP rate disappear at T M≥800°C. It appears to be due to the dissolution of the impurities deposited onto the upstream surface into the membrane bulk within the measurements.

Deuterium pumping experiment with superpermeable Nb membrane in JFT-2M tokamak

A new divertor pumping system with superpermeable membranes has been installed in the JFT-2M tokamak to investigate the applicability of membrane pumping to fusion devices. The membrane pumping is based on suprathermal (>1 eV) hydrogen permeation through a niobium membrane. Deuterium pumping is observed with divertor plasmas in tokamak environment. The pumping flux through the membrane increases by NBI heating and additional gas puffing and it is found that the pumping flux is directly proportional to the pressure in the divertor chamber where the pumping system is placed. The maximum permeation flux density is 7.3×10 19 D/m 2 s for a tokamak discharge with a dense and cold divertor plasma.