Amorphous Membranes Research Articles

Effects of Catalysts and Membranes on the Performance of Membrane Reactors in Steam Reforming of Ethanol at Moderate Temperature

Steam reforming of ethanol in the membrane reactor using the Pd77Ag23 membrane was evaluated in Ni/CeO2 and Co/CeO2 at atmospheric pressure. At 673 K, the H2 yield in the Pd77Ag23 membrane reactor over Co/CeO2 was found to be higher than that over Ni/CeO2, although the H2 yield over Ni/CeO2 exceeded that over Co/CeO2 at 773 K. This difference was owing to their reaction mechanism. At 773 K, the effect of H2 removal could be understood as the equilibrium shift. In contrast, the H2 removal kinetically inhibited the reverse methane steam reforming at low temperature. Thus, the low methane-forming reaction rate of Co/CeO2 was favorable at 673 K. The addition of a trace amount of Ru increased the H2 yield effectively in the membrane reactor, indicating that a reverse H2 spill over mechanism of Ru would enhance the kinetical effect of H2 separation. Finally, the effect of membrane performance on the reactor performance by using amorphous alloy membranes with different compositions was evaluated. The H2 yield was set in the order of H2 permeation flux regardless of the membrane composition.

Hydrogen permeability and mechanical properties of NiNb-M (M = Sn, Ti and Zr) amorphous metallic membranes

In this paper, we report on the hydrogen permeability at 673 K of 45-μm thick splat-quenched Ni60Nb35M5 (M = Sn, Ti and Zr) amorphous metallic membranes. Changes in mechanical properties, induced by hydrogen, were evaluated by nanoindentation. The effect of different elemental substitutions on both the hydrogen permeability and the thermal stability are discussed. Membrane crystalline structure was probed with X-ray diffraction and membrane thermal properties were analyzed with differential scanning calorimetry. The Young’s modulus and hardness changes of the membranes were measured by nanoindentation. All of these ternary Ni–Nb membranes maintain their amorphous structure after 24 h of hydrogen permeability testing at 673 K. The Ni60Nb35Zr5 alloy membranes exhibited a hydrogen permeability of 10−10 mol m−1 s−1 Pa−0.5, the maximum of the three compositions tested. The hydrogen permeability declined slightly with time during the testing. Overall the amorphous metallic membranes were thermally stable and maintained their amorphous structure. A decrease in free volume is hypothesized to be the reason for i) the increase of Young’s modulus and hardness; ii) hydrogen permeability decrease over time.

Influence of gallium ion beam acceleration voltage on the bend angle of amorphous silicon cantilevers

This paper describes a plastic reshaping technique for Si thin membranes by using focused ion beam (FIB) processing. FIB is used to locally pattern and implant Ga ions into the membranes. The combination of Ga ion doping and alkali wet etching enables us to fabricate nanometer-thick Ga-ion-doped amorphous Si membranes, which can be bent upward at arbitrary angle by controlling the FIB beam irradiation condition. The bending mechanism is discussed in the light of Ga ions implanted depth from the membrane surface. By using this technique, a micrometer-sized chute structure with several different angles is produced.

Amorphous, turbostratic and crystalline carbon membranes with hydrogen selectivity

Hydrogen production by catalytic steam reforming of renewable hydrocarbons like bio-methane or bio-ethanol has become an attractive goal of sustainable chemistry. Side reactions as in ethanol steam reforming decrease the hydrogen selectivity. A low-temperature catalytic membrane reactor with a hydrogen-selective membrane is expected to solve this problem. Three different carbon membranes are investigated with respect to their performance to extract hydrogen selectively from the binary and ternary reaction mixtures (H2/CO2), (H2/CO2/H2O), and (H2/ethanol) as model systems for bio-ethanol steam reforming. The three carbon membranes under study are (i) an amorphous carbon layer prepared by physical vapour deposition (PVD) of carbon on an porous alumina support using a carbon fibre yarn, (ii) a turbostratic carbon layer obtained by pyrolysis of a supported organic polymer blend as precursor, and (iii) a crystalline carbon prepared by pressing of graphite flakes into a self-supporting disc. For the equimolar binary feed mixture (H2/CO2) all carbon membranes were found to be hydrogen selective. For the ternary feed mixture (41vol.-% H2/41vol.-% CO2/18vol.-% H2O), in the case of the amorphous and crystalline carbon membrane, the hydrogen selectivity remains also in the presence of steam. The turbostratic carbon membrane separates preferentially steam (H2O) from the ternary feed mixture (H2/CO2/H2O).

Nanostructured carbon-based membranes: nitrogen doping effects on reverse osmosis performance

Ultrathin, flexible and highly water-permeable nanostructured carbon (NC)-based membranes are formed on porous polymer supports by plasma high-power impulse magnetron sputtering in order to fabricate carbon-based membranes for water desalination. The carbon membranes are produced at room temperature using mixtures of argon (Ar), nitrogen (N2) and methane (CH4) as precursors, and this procedure constitutes a simple solvent-free, waste-free scalable process. Structural characterization, molecular simulation, water permeation and salt rejection assessments are used to correlate the performance and membrane structure. Molecular simulations indicate that nitrogen doping on the carbon-based membranes drastically modifies the pore distribution and avoids the formation of clustered regions of high-density carbons. The optimum NC-based membrane has up to 96% salt rejection rate for 0.2 wt% NaCl saline water, with high water permeability ca. 25 l m−2 h−1 MPa−1. The NC-based membranes as active layers for desalination membranes exhibit attractive characteristics which render them a potential alternative to current polymeric technology used in reverse osmosis processes. An ultrathin and flexible carbon-based membrane for desalination can reject up to 96% of salt impurities with high flow rates. Amorphous carbon thin films have a mix of diamond-like and graphite-like chemical bonds that give rise to intriguing applications in semiconductors and designer coatings. Now, Morinobu Endo of Shinshu University in Japan and colleagues has developed a procedure to induce special nanostructure into amorphous carbon membranes for use in water treatment. The team formed sputtered carbon layers by using a mixture of argon, nitrogen and methane gases onto a porous ultrafiltration polymer film using a rapid, high-powered plasma technique. Desalination tests showed that this simple and scalable technique produced robust membranes with high salt rejection rates, particularly when nitrogen dopants were incorporated into the amorphous carbon structure. Nitrogen doping of nanostructured carbon-based membranes allows to produce ultrathin reverse osmosis membranes, which exhibit high robustness for water desalination applications. Structural and chemical characterization, water permeation and salt rejection tests, and computational modeling of these carbon-based membranes is discussed. Their salt rejection performance and degradation resistance reveal a strong dependency on the amount of nitrogen doping within the carbon structure. The properties shown by our nanostructured carbon membranes render them a potential alternative to current polymer-based membranes.

Developments in the Ni–Nb–Zr amorphous alloy membranes

Most of the global H2 production is derived from hydrocarbon-based fuels, and efficient H2/CO2 separation is necessary to deliver a high-purity H2 product. Hydrogen-selective alloy membranes are emerging as a viable alternative to traditional pressure swing adsorption processes as a means for H2/CO2 separation. These membranes can be formed from a wide range of alloys, and those based on Pd are the closest to commercial deployment. The high cost of Pd (USD *31,000 kg-1) is driving the development of less-expensive alternatives, including inexpensive amorphous (Ni60Nb40)100-xZrx alloys. Amorphous alloy membranes can be fabricated directly from the molten state into continuous ribbons via melt spinning and depending on the composition can exhibit relatively high hydrogen permeability between 473 and 673 K. Here we review recent developments in these low-cost membrane materials, especially with respect to permeation behavior, electrical transport properties, and understanding of local atomic order. To further understand the nature of these solids, atom probe tomography has been performed, revealing amorphous Nb-rich and Zr-rich clusters embedded in majority Ni matrix whose compositions deviated from the nominal overall composition of the membrane.

Effect of surface coating with palladium on hydrogen permeability of Pd33Ni52Si15 amorphous alloy membrane

Effect of surface coating with palladium on hydrogen permeability of Pd33Ni52Si15 amorphous alloy membrane

Molecular dynamic simulations of pressure-driven water transport through polyamide nanofiltration membranes at different membrane densities

The amorphous aromatic polyamide membranes with different membrane densities were modeled to study the porous structure of free-volume pores and the pressure-driven water transport by using molecular dynamics simulations.

Plasma-enhanced chemical vapor deposition of amorphous carbon molecular sieve membranes for gas separation

Amorphous carbon membranes were successfully synthesized onto a SiO2–ZrO2/α-Al2O3 nanoporous substrate via plasma-enhanced chemical vapor deposition (PECVD) at room temperature.

MD simulation and cluster analyses of the gas permeability of parylene AF4 membranes

In poly-p-xylylene (parylene) AF4 membranes, amorphous and crystalline areas were constructed by a novel computational technique: a combination of NVT + NPT-molecular dynamics (MD) and “gradually reduce the size” (GRS) methods. The corresponding free volume was determined via the homology cluster method. Crystalline parylene AF4 membranes were found to provide less free volume for the adsorption and transfer of gases than amorphous parylene AF4. The permeability results showed that amorphous and crystalline parylene AF4 membranes have different permeation properties, and the corresponding permeability coefficients approach those observed experimentally. This work determined the behaviours and directions of diffusing gases via a cluster analysis technique. Gas diffusion occurs only along the y-axis in the crystalline area and randomly in the amorphous area. The relative permeability was close to the experimental value.

Highly conductive solvent-free polymer electrolyte membrane for lithium-ion batteries: Effect of prepolymer molecular weight

The present article demonstrates the effects of molecular weight of network precursor on properties of solid-state polymer electrolyte membrane (PEM) comprised of poly (ethylene glycol) diacrylate (PEGDA) prepolymer, succinonitrile (SCN) plasticizer, and lithium bis(trifluoromethane sulphonyl) imide (LiTFSI) salt using two different PEGDA molecular weights. To provide guidance for crosslinking reaction, ternary phase diagrams of the PEM precursor mixtures were established. Upon photopolymerization within the wide isotropic region, completely amorphous membranes were obtained in several compositions. The PEM having a higher PEGDA molecular weight (i.e., 6000g/mol) revealed both improved impact strength with an elongation at break of ~80 % and higher ambient temperature ionic conductivity (~1.4×10−3Scm−1) relative to the low molecular weight counterpart (i.e., 700g/mol). Both PEMs exhibited excellent electrochemical (4.8V/Li/Li+) and thermal (140°C) stabilities. Specific discharge capacities of the PEM containing half-cells using LiFePO4 or Li4Ti5O12 electrodes against lithium electrode reached the level of ~140mAhg−1 at 0.2°C, indicating potential applications in all solid-state lithium-ion batteries.

Influence of synthesis conditions on the performance of chitosan–Heteropolyacid complexes as membranes for low temperature H2–O2 fuel cell

Flat, free-standing chitosan/phosphotungstic acid (PTA) polyelectrolyte membranes were prepared by in-situ ionotropic gelation process at room temperature on porous alumina support firstly impregnated by H3PW12O40. Scanning electron microscopy revealed the formation of compact and homogeneous membranes, whose thickness resulted to be dependent on chitosan concentration and reticulation time. X-ray diffraction and Fourier transform infrared spectroscopy (FTIR) evidenced the formation of almost amorphous membrane without appreciable concentration of not protonated NH2 groups and PTA3− ions with preserved Keggin structure. Membranes were tested as proton conductor in low temperature H2–O2 fuel cell allowing to get peak power densities up to 300 mW cm−2. Electrochemical impedance measurements allowed to estimate polyelectrolytes' conductivity up to 28 mS cm−1 at room temperature.

DNA Translocation in Nanometer Thick Silicon Nanopores.

Solid-state nanopores are single-molecule sensors that detect changes in ionic conductance (ΔG) when individual molecules pass through them. Producing high signal-to-noise ratio for the measurement of molecular structure in applications such as DNA sequencing requires low noise and large ΔG. The latter is achieved by reducing the nanopore diameter and membrane thickness. While the minimum diameter is limited by the molecule size, the membrane thickness is constrained by material properties. We use molecular dynamics simulations to determine the theoretical thickness limit of amorphous Si membranes to be ∼1 nm, and we designed an electron-irradiation-based thinning method to reach that limit and drill nanopores in the thinned regions. Double-stranded DNA translocations through these nanopores (down to 1.4 nm in thickness and 2.5 nm in diameter) provide the intrinsic ionic conductance detection limit in Si-based nanopores. In this regime, where the access resistance is comparable to the nanopore resistance, we observe the appearance of two conductance levels during molecule translocation. Considering the overall performance of Si-based nanopores, our work highlights their potential as a leading material for sequencing applications.

The hydrogen permeability of Cu–Zr binary amorphous metallic membranes and the importance of thermal stability

We synthesized three compositions of amorphous metallic Cu–Zr (Zr=37, 54, 60at%) membranes by splat quenching. We measured the thermal properties with differential scanning calorimetry (DSC). We tested hydrogen permeability of the membranes before and after Pd coatings. Experimentally, we found that the hydrogen permeability of the alloys is lower than that predicted by existing modeling results; we hypothesize this may be the result of oxides formed on the surface of our membranes. The formation of oxides is composition dependent. The permeability change during the testing time indicates that hydrogen has promoted structural change at a temperature lower than the glass transition temperature (Tg). Our original experimental results show that thermal stability is critical for hydrogen separation applications in amorphous metallic membranes.

Inorganic Membranes for the Recovery of Effluent from Municipal Wastewater Treatment Plants

Effluent from the secondary treatment of municipal wastewater was treated with membrane filtration to reduce its toxicity. Five different inorganic membranes were tested, namely, an α-alumina microfiltration membrane, an anatase titania ultrafiltration membrane, a γ-alumina nanofiltration membrane, an amorphous titania nanofiltration membrane, and an amorphous hybrid organo-silica membrane. The permeabilities and selectivities (color, UV254-absorbing components, conductivity and inorganic nitrogen compounds) of the membranes were determined, and the γ-alumina nanofiltration membrane was found to be the most promising membrane for the treatment of the effluent. The effluent flux was measured to be approximately 40 L m–2 h–1 for the γ-alumina nanofiltration membrane, and it removed nearly 75% of the UV254-absorbing components and 15% of the ions. It also removed 40% of the CuCl and 25% of the CuSO4 from the spiked effluent. The fouling resistance was less pronounced for the γ-alumina membrane compared with ...

Fabrication and Characterization of Single Phase α-Alumina Membranes with Tunable Pore Diameters.

Nanoporous and single phase α-alumina membranes with pore diameters tunable over a wide range of approximately 60–350 nm were successfully fabricated by optimizing the conditions for anodizing, subsequent detachment, and heat treatment. The pore diameter increased and the cell diameter shrunk upon crystallization to α-alumina by approximately 20% and 3%, respectively, in accordance with the 23% volume shrinkage resulting from the change in density associated with the transformation from the amorphous state to α-alumina. Nevertheless, flat α-alumina membranes, each with a diameter of 25 mm and a thickness of 50 μm, were obtained without thermal deformation. The α-alumina membranes exhibited high chemical resistance in various concentrated acidic and alkaline solutions as well as when exposed to high temperature steam under pressure. The Young’s modulus and hardness of the single phase α-alumina membranes formed by heat treatment at 1250 °C were notably decreased compared to the corresponding amorphous membranes, presumably because of the nodular crystallite structure of the cell walls and the substantial increase in porosity. Furthermore, when used for filtration, the α-alumina membrane exhibited a level of flux higher than that of the commercial ceramic membrane.

The process of solid-state dewetting of Au thin films studied by in situ scanning transmission electron microscopy

Solid-state dewetting describes the transformation of thin films into a set of particles or droplets at temperatures well below the melting temperature of the bulk. In this work in situ scanning transmission electron microscopy has been used to study the dewetting behavior of discontinuous Au thin films (15nm and 22nm thick) on amorphous silicon nitride membranes at temperatures ranging from 300°C to 600°C. The combination of microscopic and statistical information enabled not only the qualitative discussion of the observed processes but also the quantification of the kinetics as well as the development of a model of the underlying morphological mechanism. A model-free master curve approach to the temporal evolution of the covered area at different temperatures is used to determine the activation energy of dewetting (1.04±0.14eV for the 15nm thick film). A closer inspection reveals a multiple power law behavior, which is discussed in the frame of depercolation. Retraction of finger-like structures is found to be the dominant morphological mechanism based on the observed linear relationship between covered area and boundary length.

Hydrogen permeation properties of Pd-coated Pd33Ni52Si15 amorphous alloy membrane

Abstract The vast majority of experimental techniques used for the measurements of hydrogen permeability through metallic membranes whose one or both surfaces are covered with a thin Pd film is based on the assumption that a ratio of film-to-membrane thickness is small enough to cause hydrogen flow to be independent of the Pd film thickness. In an attempt to verify this assumption, we have measured the hydrogen flow through the Pd33Ni52Si15 amorphous membrane covered with Pd film of 10, 20, and 30 nm in thickness. Contrary to our expectations, we have found a dramatic decrease in hydrogen flow with the increase in Pd film thickness. Our findings are discussed in terms of potential barrier between the two different phases.

Amorphous Zr-based thin films fabricated by magnetron sputtering for potential application in hydrogen purification

Amorphous hydrogen separation membranes are under development because of their resistance to hydrogen embrittlement, improved mechanical properties, resistance to corrosion and most importantly lower intrinsic cost. The Closed Field Unbalanced Magnetron Sputter Ion Plating (CFUBMSIP) is a versatile technique for deposition of high quality thin-films of almost any composition, while enabling the control of film size, thickness and shape. In this work, it was demonstrated that thin-films (∼3–6μm) of amorphous Zr40.5Ni59.5, Zr54Cu46 and Zr30Cu57.5Y12.5 could be deposited onto glass substrates by the CFUBMSIP technique. XRD measurements only showed one broad peak for each alloy, with a peak centred between 36° and 42° 2θ, indicating that the films were amorphous. Surface analysis by SEM and confocal microscopy suggest deposition of continuous films. The thermal stability of the films appears to be mainly governed by the alloying elements and their compositions. However, the measured activation energies indicated that the nucleation and growth mechanism in the magnetron sputtered films may be different from that reported for melt-spun amorphous alloys with similar compositions.

Combined binary collision and continuum mechanics model applied to focused ion beam milling of a silicon membrane

Many experiments indicate the importance of stress and stress relaxation upon ion implantation. In this paper, a model is proposed that is capable of describing ballistic effects as well as stress relaxation by viscous flow. It combines atomistic binary collision simulation with continuum mechanics. The only parameters that enter the continuum model are the bulk modulus and the radiation-induced viscosity. The shear modulus can also be considered but shows only minor effects. A boundary-fitted grid is proposed that is usable both during the binary collision simulation and for the spatial discretization of the force balance equations. As an application, the milling of a slit into an amorphous silicon membrane with a 30keV focused Ga beam is studied, which demonstrates the relevance of the new model compared to a more heuristic approach used in previous work.