Permeation hysteresis in PdCu membranes

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H 2 at 823 K reveal retardation of the fcc(H) --> bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (1.3 +/- 0.2) mol.m (-2).s (-1) exp[(-11.1 +/- 0.6) kJ.mol (-1)/( RT)] and that in the fcc single phase regime above 873 K by J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].     

Real-time monitoring of metal deposition and segregation phenomena during preparation of PdCu membranes

The N₂ and H₂ evolution, respectively, were monitored during deposition of Pd and Cu from electroless plating baths to obtain in-process control of the composition during preparation of 3–7 μm thick PdCu membranes on tubular ceramic substrates. Compositions estimated by gas evolution compare favorably to those measured in post-mortem XRD and EDS analyses, mostly differing by not more than 1 at.%. This result suggests that use of gas evolution measurements to enable in-process control of composition to within 1 at.% is feasible. Annealing experiments in an H₂ atmosphere demonstrated that, at 893 K, only 48 h are needed to form a stoichiometrically homogeneous, 9.5 μm thick, face centered cubic (fcc) Pd₆₃Cu₃₇ membrane from sequentially deposited layers; at 723 K, the same transformation requires over 2 weeks. The appearance of transient body centered cubic (bcc) and fcc phases with lower Pd contents signaled compositional segregation in the initial stages of alloy formation at 723 and 773 K and could be a source of persistent stoichiometric heterogeneity particularly in bcc PdCu membranes. The H₂ fluxes of fcc Pd₅₈Cu₄₂ and Pd₇₀Cu₃₀ membranes were J_H2=(1.6±1.1) mol m⁻² s⁻¹ exp[(−24.8±0.4)kJ mol⁻¹/RT] and J_H2=(3.7±0.6) mol m⁻² s⁻¹ exp[(−21.3±1.0)kJ mol⁻¹/RT], respectively, at 100 kPa H₂ pressure difference.     

Cu6S4 cluster based twelve-connected face-centered cubic and Cu19I4S12 cluster based fourteen-connected body-centered cubic topological coordination polymers

Hydrothermal reaction of Cu(MeCO2)2, (4-pyridylthio)acetic acid and NH4SCN resulted in a twelve-connected face-centered cubic topological metal-organic framework [Cu3(pdt)2(CN)] (pdt = pyridinethiolate) in which Cu6S4 clusters act as twelve-connected nodes and pyridine rings and cyanides act as connectors. As an extension, an unprecedented fourteen-connected body-centered cubic coordination polymer [Cu19I4(pdt)12(SH)3] has been synthesized by three methods, in which nanosized chiral Cu19I4S12 clusters act as fourteen-connected nodes and triple pyridine rings and hydrosulfides act as connectors. The in situ S-C(sp(3)), S-C(sp(2)), and S-C(sp) cleavage reactions have been observed in the work.     

Validation of carrier-mediated transport of H(+) and Na(+) through mobile-site membranes

The response of membranes containing neutral ion carriers of either H(+) or Na(+) to an externally-applied potential step was investigated using previously developed techniques for the analysis of charge and mass transport in ion-selective membranes. The results from constant-resistance membranes, e.g. membranes with unperturbed negative site concentration profiles, showed that tridodecylamine behaved as a carrier for H(+), and there was no evidence for proton hopping from stationary carriers. In addition, the experimental outcome supported the assumption of a failure of the Donnan exclusion principle at very low pH levels in these membranes. The results from membranes containing the Na(+) carrier illustrated the significant concentration polarization of ionic species, which was related to significant changes in bulk membrane resistance.     

Electron energy distribution functions and rate and transport coefficients of H2/H/CH4 reactive plasmas for diamond film deposition

Electron energy distribution functions (eedf) and rate and transport coefficients for H₂/H/CH₄ mixtures have been calculated by solving a stationary Boltzmann equation as a function of reduced electric field E/N, of molar fraction, and of different concentrations of electronically excited states. Superelastic electronic collisions superimpose structures to eedf especially for E/N values < 40 Td.     

Dynamics of H2O and Na+ in nafion membranes

We investigate the transport properties of a model of a hydrated Na-Nafion membrane using molecular dynamics simulations. The system consists of several Nafion chains forming a pore with the water and ions inside. At low water content, the hydrophilic domain is not continuous and diffusion is very slow. The diffusion coefficient of both water and Na+ increases with increasing hydration (more strongly so for Na+). The simulations are in qualitative agreement with experimental results for similar systems. The diffusion coefficient is an average over the motion of ions or water molecules located in different environments. To better understand the role of the environment, we calculate the distribution of the residence times of the ion (or water) at different locations in the system. We discuss the transport mechanism in light of this information.     

Glass-like electronic and thermal transport in crystalline cubic germanium selenide

Thermoelectric properties of orthorhombic or rhombohedral GeSe have attracted great attention recently,with the rise of its structural analog Sn Se.However,the p-type cubic GeSe with higher symmetry and higher valence band degeneracy,which might exhibit higher thermoelectric performance,has never been synthesized.Here we report on the successful synthesis of p-type crystalline cubic GeSe by alloying with Sb2Te3 and the spontaneously formed Ge-vacancies.An unexpected glass-like temperature independent lattice thermal conductivity is observed in crystalline cubic GeSe,which results from strong phonon scattering by vacancy-induced disorders.Combining the multiple scattering theory and chemical bond analysis,we further reveal the existence of Anderson localization induced by Ge-vacancies.The Anderson localization results in a nearly constant Seebeck coefficient with increasing the carrier concentration.These results provide a general insight towards understanding and improving the thermoelectric properties of thermoelectric materials with vacancies and atomic-scale disorders.     

Electromodulated molecular transport in gold-nanotube membranes

We have developed a new class of synthetic membranes that contain monodisperse Au nanotubes with inside diameters of molecular dimensions (<1 nm). The Au nanotubes span the complete thickness of the membrane and can act as conduits for molecule and ion transport between solutions placed on either side of the membrane. We have recently become interested in the concept of electromodulating neutral molecule transport across these membranes. This communication describes a novel approach for accomplishing this objective. This approach makes use of an anionic surfactant which, when a positive potential is applied to the Au nanotube membrane, partitions into the nanotubes to charge the solution side of the electrical double layer at the tube walls. Because of the hydrophobic tail of the surfactant, this renders the nanotube interior hydrophobic, and the membrane now preferentially extracts and transports neutral hydrophobic molecules. Because the anionic surfactant can be expelled from the nanotubes by applying a negative potential, this provides a route for reversibly electromodulating neutral molecule transport in these membranes.     

Facilitated CO2 transport membranes utilizing positively polarized copper nanoparticles

We suggest copper nanoparticles as a CO(2) carrier by employing the ionic liquid BMIM(+)BF(4)(-) for facilitated transport membranes. The copper nanoparticles were prepared by dissociating the copper metal into the ionic liquid as a facile synthesis. The interactions between BF(4)(-) of the ionic liquid and the surface of copper nanoparticles caused the Cu surface to be partially positively charged, resulting in increased activity in copper-CO(2) complexation.     

H2 separation using defect-free, inorganic composite membranes

Defect-free, microporous Al(2)O(3)/SAPO-34 zeolite composite membranes were prepared by coating hydrothermally grown zeolite membranes with microporous alumina using molecular layer deposition. These inorganic composite membranes are highly efficient for H(2) separation: their highest H(2)/N(2) mixture selectivity was 1040, in contrast with selectivities of 8 for SAPO-34 membranes. The composite membranes were selective for H(2) for temperatures up to at least 473 K and feed pressures up to at least 1.5 MPa; at 473 K and 1.5 MPa, the H(2)/N(2) separation selectivity was 750. The H(2)/CO(2) separation selectivity was lower than the H(2)/N(2) selectivity and decreased slightly with increasing pressure; the selectivity was 20 at 473 K and 1.5 MPa. The high H(2) selectivity resulted either because most of the pores in the Al(2)O(3) layer were slightly smaller than 0.36 nm (the kinetic diameter of N(2)) or because the Al(2)O(3) layer slightly narrowed the SAPO-34 pore entrance. These composite membranes may represent a new class of inorganic membranes for gas separation.     

Oxygen dissolution and transport in cobaltporphyrin-bound organophosphazene membranes

Rubbery poly(organophosphazene)s were synthesized, and were combined with cobaltporphyrin (CoP) which binds molecular oxygen rapidly and reversibly. The apparent oxygen-binding equilibrium constant (K_app) is in proportion to the physical solubility coefficient of oxygen in the polymer, although the reduced equilibrium and thermodynamic parameters are not dependent on the polymer matrix species. Diffusivity of oxygen via the fixed CoP(D_C) is enhanced for poly(organophosphazene) membranes with a larger oxygen diffusion constant. Poly(organophosphazene) membranes with both a large K_app and D_C yield high oxygen permeability.     

Ion transport and clustering in nafion perfluorinated membranes

A theory of ion-clustering is presented for Nation resins. An expression for the cluster diameter is derived; it correctly describes the experimental variations in cluster diameter with cation form, equivalent weight, and water content of the polymers. The theory further suggests that short channels connecting adjacent clusters are thermodynamically stable in support of the cluster-network model proposed by one of us earlier. The percolative aspects of ion transport and an ionic insulator-to-conductor transition in Nation resins are discussed.     

Highly rectified ion transport through 2D WSe2/MoS2 bi-layered membranes

Through a two-step vacuum-filtration process, WSe₂ and MoS₂ nanosheets were sequentially deposited onto a polymeric nanoporous support, forming WSe₂/MoS₂ bi-layered heterostructure. Highly rectified ion transport phenomenon is observed through the heterogeneous 2D layered membranes.     

Heats of transport of some chlorides in H2O and D2O

Heats of transport of 0.01 m HCl, Me₄NCl, and Et₄NCl in H₂O and 0.01 m KCl, NaCl, and CsCl in D₂O were determined from the measurement of initial (homogeneous) and final (Soret steady state) thermoelectric powers of the thermocell (T)Ag–AgCl/Solution/AgCl–Ag(T+T) at a mean temperature of 25°C. The results are discussed in terms of ion-solvent interaction and properties of H₂O and D₂O.     

Amino acid ionic liquid-based facilitated transport membranes for CO2 separation

Supported liquid membranes incorporating amino acid ionic liquids remarkably facilitate CO(2) permeation under dry and low humid conditions.     

Ion transport in inhomogeneous ion exchange membranes

The effect on current efficiency produced by fluctuations in an ion-exchange membrane's fixed ion concentration (due to ion clustering) is determined by considering transport in several model membranes containing different fixed ion distributions. Electroneutrality is not assumed, so as to include nonlinear effects due to space charge in the clusters. The Nernst—Planck and Poisson equations are solved using perturbation theory, and the case of small fluctuations in fixed charge density is considered in order to obtain analytic solutions to the perturbation equations. Results show that an inhomogeneous distribution of fixed ions gives rise to increased current efficiency when the counterions and co-ions experience in the membrane unequal forces due to the potential produced by the fixed ions. The length scale over which the inhomogeneities occur is important in determining the current efficiency.     

Ion transport across biomembranes and model membranes

The milestones formerly achieved in the comprehension of ion transport across biological membranes on the basis of electrochemical concepts and/or instrumentation are briefly summarized. The various types of model membranes presently employed for the investigation of ion transport across biomembranes are reviewed and their requirements for the incorporation and functional investigation of membrane proteins are examined. The potential of model membranes for the elucidation of many problems in molecular membrane biology and for the realization of microarray sensors individually addressable to membrane proteins by electrochemical means is assessed.     

Sorption and transport mechanism of gases in polycarbonate membranes

The transport phenomena of oxygen and nitrogen across a pure polycarbonate (PC) and a cobalt(III) acetylactonate (Co(acac)₃) containing PC membrane was studied. Co(acac)₃ was added into a polycarbonate membrane to enhance its oxygen solubility. The oxygen sorption isotherms was measured. It was found that the oxygen solubility decreased sharply as pressure increased, especially at low pressure region. On the contrary, the oxygen permeability increased slightly with respect to pressure. Both the solution-diffusion model and traditional dual mobility model were unable to explain the inconsistent pressure dependency between solubility and permeability. Instead of adopting Langmuir-Henry sorption model, a modified dual mobility model which incorporates BET-type isotherm to describe oxygen sorption. The diffusivity of molecules moving at the first adsorbed layer was assumed to be different from those moving at higher layers. This modified dual mobility model satisfactorily described both the pressure dependency of oxygen solubility and permeability. It was also found that the increase of oxygen/nitrogen selectivity was not due to the elevation of oxygen to nitrogen solubility ratio but due to the mobility ratio of oxygen to nitrogen at the higher adsorption layers.