Diffusion Separation Research Articles

A highly selective sorption process in POSS-g-PDMS mixed matrix membranes for ethanol recovery via pervaporation

Incorporating nanofillers into polymer matrix to construct mixed matrix membranes (MMMs) is an effective approach to improve either sorption selectivity or diffusion selectivity or both. In this contribution, octa-vinyl polyhedral oligomeric silsesquioxanes (V-POSS) particles were chemically incorporated into polydimethylsiloxane (PDMS) matrix to fabricate POSS-g-PDMS MMMs. POSS-g-PDMS MMMs exhibited a homogeneous phase and achieved good integration between V-POSS and PDMS. Compared with pure PDMS membrane, POSS-g-PDMS MMMs possessed higher crosslinking density and gel content as well as enhanced ethanol affinity and hydrophobicity, which contributed to the significant improvement of ethanol sorption selectivity with a little sacrifice of diffusion selectivity. The chemical incorporation of V-POSS into PDMS matrix significantly enhanced both separation factor and permeation flux, which showed reversal trade-off effect. The best pervaporation performance of POSS-g-PDMS MMMs was achieved at 5 wt% POSS loading, with separation factor of 17.7 and permeation flux of 536 g/(m2 h), increased by 130% and 260% compared with pure PDMS membrane, respectively. An excessive POSS loading sharply depressed the fast transport of small molecules and finally led to the decline of diffusion selectivity and separation factor. The sorption and diffusion selectivity results suggested selective sorption was a major contributor to the high separation factor of MMMs at 5 wt% POSS loading, and diffusion was a rate-controlling process. The effect of variation of external driving forces and intrinsic membrane properties induced by increasing feed temperature and ethanol concentration on pervaporation performance was also investigated in detail. This work may provide useful insights of PDMS based mixed matrix membranes, especially those with covalent bonding between organic and inorganic phases for pervaporation recovery applications.

Interplay between Charge Carrier Mobility, Exciton Diffusion, Crystal Packing, and Charge Separation in Perylene Diimide-Based Heterojunctions.

Two of the key parameters that characterize the usefulness of organic semiconductors for organic or hybrid organic/inorganic solar cells are the mobility of charges and the diffusion length of excitons. Both parameters are strongly related to the supramolecular organization in the material. In this work we have investigated the relation between the solid-state molecular packing and the exciton diffusion length, charge carrier mobility, and charge carrier separation yield using two perylene diimide (PDI) derivatives which differ in their substitution. We have used the time-resolved microwave photoconductivity technique and measured charge carrier mobilities of 0.32 and 0.02 cm2/(Vs) and determined exciton diffusion lengths of 60 and 18 nm for octyl- and bulky hexylheptyl-imide substituted PDIs, respectively. This diffusion length is independent of substrate type and aggregate domain size. The differences in charge carrier mobility and exciton diffusion length clearly reflect the effect of solid-state packing of PDIs on their optoelectronic properties and show that significant improvements can be obtained by effectively controlling the solid-state packing.

3D printing of polylactic-co-glycolic acid fiber scaffolds using an antisolvent phase separation process

We present the results of experimental studies into the computer-controlled fabrication of polylactic-co-glycolic acid (PLGA) scaffolds under conditions of diffusion and antisolvent phase separation of PLGA/tetraglycol (TG) solutions in aqueous media. The effect of the initial concentrations of the polymer solutions and the 3D printing process parameters on the type of the fiber structures formed was studied. The resultant scaffolds were found to be produced in the form of either fractal-like or spongy polymeric microstructures. The specific type of the structures formed was shown to depend on the intensity of the diffusion processes in the ternary system PLGA–TG–antisolvent. This approach can be used for highly efficient 3D printing of both microporous and hollow fiber biocompatible scaffolds of various architectonics.

Specialized source-detector separations in near-infrared reflectance spectroscopy platform enable effective separation of diffusion and absorption for glucose sensing.

We present an approach for accurate glucose sensing in turbid media using a spectrally resolved reflectance setup. Our proposed reflectance setup uses specialized source-detector separations (SDSs) to enable an effective separation of diffusion and absorption signals. Additionally, we adjust the selected SDSs to their optimal values to acquire maximum sensitivity to glucose in the two signals. The separation can help to enhance the sensitivity to glucose both for the diffusion and absorption signals, as they always suppress each other by causing opposite effects on the reflected diffuse light intensity. Monte Carlo simulations and experiments for glucose sensing are used to test the method. The acquired optimal SDSs could provide a reference for noninvasive blood glucose sensing.

Focused ion beam-induced droplet formation and motion

Nanometer-scale droplet formation and motion under the effects of a focused ion beam is described using a continuum model that combines a thin-film fluid model for dynamics with a Cahn–Hilliard type model for chemical diffusion and phase separation. The focused ion beam (FIB) is represented as a time and space dependent mass and chemical species source. Drops form and move due to the combined effects of a concentration-dependent surface energy, a concentration gradient on the edges of the droplet, and a beam angle-dependent net FIB incoming mass flux. The velocity is comparable to observations, and it has a similar dependence on the beam angle of incidence. Likewise, the ion dose at which droplets reach a width of 60 nm is in good agreement with experiments. The model also explains the biased motion of droplets under a raster-scan pattern focused ion beam.

Influence of Zn–Si donor–acceptor pair traps in invert InGaP/GaAs dual-junction solar cell by MOCVD

In In(Ga)P materials, Zn atoms diffuse into the adjacent Si doping layer at high temperatures, forming Zn–Si donor–acceptor pairs which function as non-radiation carrier recombination centers. The traps lead to decreased carrier diffusion length and separation efficiency in the space charge region of InGaP subcell, resulting in degradation of photoelectric conversion efficiency. In this paper, by adopting an InGaP p–i–n junction structure, the invert growth thin film cell was able to reach the same external quantum efficiency performance as the upright InGaP/GaAs dual-junction solar cell. The Zn doping peak was not observed at the Si doping interface layer in the electrochemical capacitance–voltage measurement. A 675 nm peak was observed at 5 and 30 K and absent in 77 K, as was resolved by low-temperature PL measurement. The peak showed a strong correlation with the Zn diffusion into adjacent Si doping InGaP layer. We speculate that the 675 nm peak represented the energy of Zn–Si donor–acceptor pair traps.

Efficient triplet pair separation from intramolecular singlet fission in dibenzopentalene derivatives

In intramolecular SF (iSF), the strong coupling nature and confinement of diffusional separation of 1(TT) limits the extraction and harnessing of triplet energy. In order to investigate the possible 1(TT) separation and the role of molecular parameters on it, a series of iSF-capable dibenzopentalene derivatives (DBPs) have been synthesized and their photoinduced dynamics are monitored. iSF takes place in DBPs, accompanied by consecutive 1(TT) separation in polycrystalline film with almost 100% yield. It is suggested the strong intermolecular coupling provided by the closely packing configuration in the film facilitates the disentanglement of correlated 1(TT). Highly efficient triplet pair separation to yield free triplets makes one step forward for utilizing triplet energy from iSF materials for further optoelectronic applications.

Barodiffusion in Slow Flows of a Gas Mixture

Barodiffusion in slow flows of a gas mixture is studied with an approximation using hydrodynamic equations of motion for the individual mixture components. It is shown that consideration of the viscous momentum transfer and the contribution of Knudsen layers for the mixture flowing in a channel has a considerable effect on the value of the barodiffusion factor. The relations are obtained for the mean diffusion fluxes of components and for the total flux of the mixture in a circular cylindrical capillary; these relations are valid for moderately small Knudsen numbers used for calculation of the diffusion baroeffect and separation effect when the gas mixture flows in a set of capillaries connecting two volumes. The modification of the relations for the barodiffusion factor (and for the diffusion slip coefficient cross-linked with it) allows interpreting the sign alteration of these effects observed experimentally for some gas mixtures at intermediate Knudsen numbers.

Isotope separation of low boiling-point substances by plasma centrifuge with circulation

ABSTRACT A new plasma centrifuge (PC) for isotope separation has been proposed. The physical and mathematical models of the processes taking place in the PC are presented. A partially ionized medium rotates by a high-frequency (HF) electromagnetic field. Simultaneously, axial circulation is excited by a traveling magnetic field(TMF) to provide an increase of the separation performance in an axial direction. By assuming that the plasma medium is weakly ionized, diffusion separation can be assumed to undergo processes that are similar to those in a neutral gas. Charged particles in the medium move due to an applied rotating HF field, transferring force action to the neutral gas. By including a heated chamber wall, the PC is capable of separating low-boiling substances, including the isotopes of alkaline and alkaline-earth elements.

Satellite-augmented diffuse solar radiation separation models

Separation models predict diffuse horizontal irradiance from other meteorological parameters such as the global horizontal irradiance or zenith angle. From a mathematical point of view, the separation modeling problem is a regression, where the regressors are observed or computed variables and the regressand is the unobserved diffuse fraction. The most successful minute-resolution separation model prior to 2016 was proposed by Engerer, which is constructed using a trend component (cloud enhancement) and a main effect (logistic function). Subsequently, the Starke model published in 2018 further improved the Engerer model. It is herein shown that the logistic-function type of model, and many other separation models, can be transformed into a (condition-based) multiple linear regression problem. Under this transformation framework, two new models are proposed, which strictly dominate the performance of the Engerer model and the Starke model, at all 7 test sites across the continental United States, making them probably the most accurate separation models to date. The new models are also tested at 5 European sites with unseen data (i.e., not involved during model parameter fitting); their performance again dominates all benchmarking models. The new separation models leverage half-hourly satellite-derived diffuse fraction. Since satellite data are available globally, the satellite-augmented separation models have universal applicability. However, despite their good performance, empirical separation models suffer from a series of issues. Hence, models driven by atmospheric physics are the “true gems” that one should pursue.

Conformation-Controlled Molecular Sieving Effects for Membrane-Based Propylene/Propane Separation.

Membrane-based separation is poised to reduce the operation cost of propylene/propane separation; however, identifying a suitable molecular sieve for membrane development is still an ongoing challenge. Here, the successful identification and use of a metal-organic framework (MOF) material as fillers, namely, the Zr-fum-fcu-MOF possessing an optimal contracted triangular pore-aperture driving the efficient diffusive separation of propylene from propane in mixed-matrix membranes are reported. It is demonstrated that the fabricated hybrid membranes display a high propylene/propane separation performance, far beyond the current trade-off limit of polymer membranes with excellent properties under industrial conditions. Most importantly, the mechanism behind the exceptional high propylene/propane selectivity is delineated by exploring theoretically the efficiency of sieving of different conformers of propane through the hypothesized triangular rigid pore-aperture of Zr-fum-fcu-MOF.

Porous Fe2O3 nanotubes with α-γ phase junction for enhanced charge separation and photocatalytic property produced by molecular layer deposition

Constructing nanotubular morphologies and heterojunctions are two effective strategies to enhance the charge separation and transport of α-Fe2O3 for improved photocatalytic performance, while the fabrication of porous α-Fe2O3 nanotubes with precisely tailored wall thickness, pore structure, crystallinity, and junctions still remains a big challenge. Herein, two novel molecular layer deposition (MLD) procedures are designed to prepare porous Fe2O3 nanotubes with tunable pore structure and phase junction. The organic fractions of the obtained Fe-hybrid MLD films not only act as soft templates to generate nanopores in nanotube walls but also play a key role in the formation of phase-junction. The porous structure and phase-junction significantly improve the mass diffusion and charge separation efficiency of Fe2O3 nanotubes, leading to a drastically increased photocatalytic activity for photo-Fenton reaction. Especially, the porous α-γ Fe2O3 nanotubes produced by two-step AB MLD from iron tert-butoxide and ethylene glycol exhibit the highest photocatalytic activity, which is more than a 6.5-fold and 20-fold improvement compared with the nonporous pure α-Fe2O3 nanotubes and commercial α-Fe2O3 nanoparticles, respectively. The MLD method provides a new bottom-up approach to develop efficient Fe2O3 based heterostructure porous photocatalysts for waste-water cleaning and water splitting.

Development and Analysis of an Innovative Flat-Metal Separator Integrating the GDL with Gas-Flow Channels as PEFC Components

The gas diffusion layer (GDL) plus separator in the polymer electrolyte fuel cell contributes more than 1/3 to the stack cost, even at the mass production stage. Therefore, we have taken on the challenge to develop a new, integrated concept for these crucial components, based on intelligent application of low-cost technologies and, most importantly, uniform transport of reactant and product over the whole catalyst layer via forced cross-flow between neighboring interdigitated channels formed in the GDL. This new design concept promises to significantly lower cost, as well as improving performance and durability. Specifically, the design incorporates a corrosion-resistant flat-metal separator (FMS) with a thin carbon/resin coating, and a low-cost GDL, consisting of short carbon fibers (CFs) bonded with resin, forming gas-flow channels whose manufacture should be compatible with mass-production. At the optimized FMS + GDL, we have achieved 5.7 mΩ/cm2 contact-resistance and complete immunity to corrosion over 4,000 h in 0.1 M H2SO4 solution at 90°C. By combining various CFs and resins, the gas-diffusivity, electrical resistance and mechanical strength have been balanced and are sufficient for GDL application. A limiting current of 3.3 A/cm2 in a single cell has been achieved under pure H2 and140 kPa air at 80°C.

О бародиффузии при медленных течениях газовой смеси

AbstractBarodiffusion in slow flows of a gas mixture is studied with an approximation using hydrodynamic equations of motion for the individual mixture components. It is shown that consideration of the viscous momentum transfer and the contribution of Knudsen layers for the mixture flowing in a channel has a considerable effect on the value of the barodiffusion factor. The relations are obtained for the mean diffusion fluxes of components and for the total flux of the mixture in a circular cylindrical capillary; these relations are valid for moderately small Knudsen numbers used for calculation of the diffusion baroeffect and separation effect when the gas mixture flows in a set of capillaries connecting two volumes. The modification of the relations for the barodiffusion factor (and for the diffusion slip coefficient cross-linked with it) allows interpreting the sign alteration of these effects observed experimentally for some gas mixtures at intermediate Knudsen numbers.

Controlling Molecular Packing and Orientation via Constructing a Ladder-Type Electron Acceptor with Asymmetric Substituents for Thick-Film Nonfullerene Solar Cells.

A nonfullerene acceptor, IDTT-OB, employing indacenodithieno[3,2- b]thiophene (IDTT) decorated with asymmetric substituents as the core, is designedly prepared. In comparison with the analogue IDT-OB, extending the five-heterocyclic indacenodithiophene (IDT) core to seven-heterocyclic fused ring endows IDTT-OB with more broad absorption and elevated highest occupied molecular orbital energy level. In addition, IDTT-OB shows a more intense molecular packing and a higher crystalline behavior with a strong face-on orientation in the neat film and the PBDB-T:IDTT-OB blend film. Furthermore, an ideal nanomorphology with a domain size of 19 nm can be obtained, which is in favor of exciton diffusion and charge separation. Accordingly, PBDB-T:IDTT-OB-based polymer solar cells demonstrate a maximum power conversion efficiency (PCEmax) of 11.19% with an impressive fill factor of 0.74, comparable to the state-of-the-art acceptors with similar molecular backbones. More importantly, IDTT-OB-based devices show good tolerance to the film thickness, which maintain a high PCE of 10.20% with a 250 nm thick active layer, demonstrating that the asymmetric acceptor is profound for fabricating high-efficiency thick-film nonfullerene solar cells.

Molecular Dynamics Simulations of Noble Gas Fractionation during Diffusion through Silica Nanopores

Ne retention and slight enrichment of Kr–Xe relative to atmospheric levels have been found in biogenic chert samples. Previous studies suggested that the incorporation of noble gas atoms is dependent upon the structure rather than the environment. In this study, we perform molecular dynamics simulations of dissolved noble gas atomic diffusion through 1–4 nm diameter silica pores. Bulk-liquid-like water does not exist in the 1 nm diameter nanopores, which hinder noble gas diffusion into or out of the pores. In ≥2 nm diameter nanopores, noble gas atoms transport with bulk-liquid-like water into the center of the pores but size-controlled diffusive separation occurs at the layer of surface water and in the interior of the silica structure. The motion of large atoms (Kr and Xe) in surface water is governed by significant adsorption. Relatively small Ne atoms are able to cross the surface water layer and diffuse into the crystal interior. As a result of its moderate size and the negligible interaction with the interfacial surface, Ar lies beyond the adsorption and silica structure diffusion regimes. Therefore, our simulation results indicate that noble gas entrapment is expected to occur in nanoscale fluid circulation during sediment-to-chert lithification.

From bulk to microphase separation in scalar active matter: a perturbative renormalization group analysis

We consider a dynamical field theory (Active Model B+) that minimally extends the equilibrium Model B for diffusive phase separation of a scalar field, by adding leading-order terms that break time-reversal symmetry. It was recently shown that such active terms can cause the bulk phase separation of Model B to be replaced by a steady state of microphase separation at a finite length scale. This phenomenon was understood at mean-field level as due to the activity-induced reversal of the Ostwald ripening mechanism, which provides the kinetic pathway to bulk phase separation in passive fluids. This reversal occurs only in certain ranges for the activity parameters. In this paper we go beyond such a mean-field analysis and develop a -loop renormalisation group (RG) approach. We first show that, in the parameter range where bulk phase separation is still present, the critical point belongs formally to the same (Wilson–Fisher) universality class as the passive Model B. In contrast, in a parameter range associated with microphase separation, we find that an unstable non-equilibrium fixed point of the RG flow arises for , colliding with the Wilson–Fisher point in and making it unstable in . At large activity, the flow in this region is towards strong coupling. We argue that the phase transition to microphase separation in active systems, in the physically relevant dimensions and , very probably belongs to a new non-equilibrium universality class. Because it is governed by the strong-coupling regime of the RG flow, our perturbative analysis leaves open the quantitative characterization of this new class.

The influence of synthesis conditions on the redox behaviour of LiFePO4 in aqueous solution

To contribute to the knowledge on the influence of synthesis procedure on the intercalation kinetics of lithium ions into phospho-olivines, LiFePO4/C composite samples (LFPC) were synthesized in two ways, the first one in a sol-gel procedure (SG), and the other in a solid-state reaction (SS). The X-ray diffractograms (XRD) of both samples overlapped with that of pure LiFePO4, taken from the crystallographic database. Scanning electron microscopy pictures indicated the high degree of interparticle sintering, which caused a considerable agglomerate growth. The results of potentiodynamic measurements in aqueous LiNO3 solution revealed that for SS sample, three times higher initial capacity from that of SG one, (amounting to 74 mAh g−1 at 5 mV s−1). However, capacity fade on rising scan rate is much more expressed for SS sample than for SG one. We suggest that a different degree of material utilization due to the incomplete coverage of olivine particles by carbon explains this difference. The technique of separation of diffusion and capacitance currents was applied in a kinetic analysis, but it was shown to be inappropriate. We suggest the inapplicability of classic CV theory to the intercalation system accompanied by phase transition. Instead, a model of ohmic resistance determination of process kinetics was considered. LFPC-SS sample delivers three times larger capacity in LiNO3, amounting to 74 mAh g−1 at 1 mV s−1.

Facile Fabrication of Large-Area Atomically Thin Membranes by Direct Synthesis of Graphene with Nanoscale Porosity.

Direct synthesis of graphene with well-defined nanoscale pores over large areas can transform the fabrication of nanoporous atomically thin membranes (NATMs) and greatly enhance their potential for practical applications. However, scalable bottom-up synthesis of continuous sheets of nanoporous graphene that maintain integrity over large areas has not been demonstrated. Here, it is shown that a simple reduction in temperature during chemical vapor deposition (CVD) on Cu induces in-situ formation of nanoscale defects (≤2-3 nm) in the graphene lattice, enabling direct and scalable synthesis of nanoporous monolayer graphene. By solution-casting of hierarchically porous polyether sulfone supports on the as-grown nanoporous CVD graphene, large-area (>5 cm2 ) NATMs for dialysis applications are demonstrated. The synthesized NATMs show size-selective diffusive transport and effective separation of small molecules and salts from a model protein, with ≈2-100× increase in permeance along with selectivity better than or comparable to state-of-the-art commercially available polymeric dialysis membranes. The membranes constitute the largest fully functional NATMs fabricated via bottom-up nanopore formation, and can be easily scaled up to larger sizes permitted by CVD synthesis. The results highlight synergistic benefits in blending traditional membrane casting with bottom-up pore creation during graphene CVD for advancing NATMs toward practical applications.

Experimental and numerical study on the performance of an axial fan with a Gurney flap

Gurney flap is a miniature lift-enhancement device installed at the airfoil trailing edge and has been successfully applied to fixed wing aircraft and low-speed horizontal wind turbines. In this article, Gurney flap is extended to increase pressure output of a diffusive cascade flow in rotating turbomachinery, which is complicated for its three dimensionalities and diffusive separation characteristics. Wind tunnel tests and computational fluid dynamic simulations were accomplished on an axial fan profiled with an NACA 65-(12)10 airfoil to investigate the effects of Gurney flap on the performance of a high solidity. We present the detailed flow features of the fan with and without Gurney flap after validating the simulation results with the experimental datum. The experimental results show positive Gurney flap effects on fan’s pressure rise and flow rate improvement. However, negative Gurney flap effects on fan’s efficiency are more evident than Gurney flap on isolated airfoils. Detailed flow field analysis from computational fluid dynamic computation reveals that the increased airfoil pressure loading along the fan blade chord strengthens the tip leakage flow, which induces more tip second flow losses than in the baseline fan. In addition to the positive lift enhancement, the net Gurney flap effect in diffusion cascade is influenced by the three-dimensional flow structure.