Permeability of anodic alumina membranes with branched channels

Influence of heat transfer and wetting angle on condensable fluid flow through nanoporous anodic alumina membranes

The flow of isobutane and of freon 142b (1-chloro-1,1-difluoro-ethane) through anodic alumina membranes with pore diameters between 18 and 60 nm in a capillary condensation regime is experimentally and theoretically explored. The capillary condensation effect increases the membrane permeance for condensable gases from 25 to 150 m³(STP) m^-2 bar^-1 h^-1 at certain conditions. To describe the experimental results, a model is suggested accounting for heat transfer from the condensing to the evaporating meniscus, different boundary conditions for the heat transfer between the environment and the membrane, and wettability of the pore wall. The proposed model indicates a large influence of heat supply from the environment to the membrane on the permeance in the capillary condensation regime and a moderate influence of condensate contact angle in the range of 0-60°. Measuring the temperature of the permeate side of the membrane allows to find a suitable boundary condition to describe heat transfer. The obtained boundary condition yields an excellent fit of experimental results of condensate flow through membranes with different pore diameters for the two utilized fluids. Also, confocal Raman spectroscopy gave evidence on the fraction of pores filled with condensate.

Oxidized Carbon-Based Spacers for Pressure-Resistant Graphene Oxide Membranes

In this study, we report the influence of carbon-based spacer-oxidized derivatives of fullerenes (fullerenols) C60(OH)26−32 and graphene oxide nanoribbons on the performance and pressure stability of graphene-oxide-based composite membranes. The impact of the intercalant shape and composition on the permeance of the selective layers for water vapors has been studied under pressure gradients. It is shown that the insertion of ball-shaped fullerenols between graphene oxide nanoflakes allows a suppression in irreversible permeance loss to 2−4.5% and reversible permeance loss to <25% (at 0.1 MPa), while retaining large H2O/N2 selectivities of up to ~30,000. The demonstrated approach opens avenues for the highly effective stabilization of GO membranes at elevated pressures for industrial-scale dehumidification.

Mass flow and momentum flux in nanoporous membranes in the transitional flow region

An experimental study of momentum transfer in nanoporous polymeric track-etched membranes with pore diameters ranging from 100 to 1300 nm and nanochannel lengths of 12-20 μm was performed using He, N₂, CO₂, and SF₆ propellants in a wide range of plenum and background pressures. Mass flux through the membranes was elaborated as a combination of Knudsen diffusion and viscous flow at Knudsen numbers above 0.1 and become choked at lower Knudsen numbers. The discharge coefficient for the membranes attained was 0.6, making the permeation rate similar to that of thin orifices. The effect is attributed to the mirror reflection of the molecules from the pore walls at low angles of incidence. The exhaust gas velocity is found to be dependent on the plenum to background pressure ratio and channel length-to-diameter ratio, reaching 0.9 of the velocity of the gas expanded to vacuum (up to 2 M). Close to an isothermal expansion occurs in nanochannels of all sizes. A general quantitative description for gas expansion in nanochannels is provided. The highest thrust is generated in the choked flow regime with the SF₆ propellant and a value of 4.5 N cm^-2 is attained at a propellant consumption of 0.165 kg (cm² s)^-1 for the membranes with 1300 nm nanochannels. The specific impulse of 138 s is reached for helium. The results show the prospects of the utilization of nanoporous membranes in cold gas propulsion systems.

Permeability of anodic alumina membranes grown on low-index aluminium surfaces

Porous anodic aluminium oxide (AAO) membranes have various practical applications in separation and purification technologies. Numerous approaches have been utilized to tailor the transport properties of porous AAO films, but all of them assume an isotropic nature of anodized aluminium. Here, the impact of aluminium crystallography on the permeability of AAO membranes is disclosed. A comparative study of AAO membranes formed on low-index aluminium surfaces by anodizing in a sulphuric acid electrolyte is presented. Small-angle x-ray scattering is used to quantify the out-of-plane pore arrangement. AAO grown on an Al(100) substrate possesses a porous structure with minimal point defects and pore tortuosity, providing the highest permeability of individual gases in a series of AAO membranes. These findings can also be applied for the fabrication of highly permeable AAO membranes on polycrystalline Al foils.

Anodic alumina membrane capacitive sensors for detection of vapors

Here we report membrane capacitive sensors based on anodic aluminum oxide (AAO) Au/AAO/Au structures fabricated by aluminum anodization, followed by gold electrodes sputtering on the countersides of porous ceramic membrane. Electrochemical impedance spectroscopy with AC amplitude 5-100 mV in the frequency range of 1-1000 Hz was utilized for sensor characterization in the presence of water and organic vapors in a full range of P/P₀. The sensors illustrate ultimate sensitivity to ambient environment with exponential-scale capacitance relation to vapors content resulting in typical 4-6 orders of magnitude response signal change for 15-85% P/P₀ range at a single AC frequency, and up to 7 orders of magnitude response range for 0-100% P/P₀ pressure range with using two different AC frequencies. In case of water vapors, the sensitivity increases from ~0.5 nF/RH% at ~20 RH% to over ~1.0 μF/RH% at ~80 RH%. The sensors are capable for highly accurate sensing of gas humidity as well as any dissociative vapors with pKa <30. They are also sensible to polar components with high enough dipole moment or polarizability. The capacitance is affected by any adsorbed molecules, including those having zero dipole moment. The data for sensor response to CH₃OH, C₂H₅OH, CH₂ClCHF₂, i-C₄H₁₀ depending on partial pressures is provided. Due to high porosity (10-30%) and gaseous permeance (up to 200 m³(STP) m^-2 bar^-1 h^-1) the sensors offer fast response rate and a possibility for flow-through measurements, providing also a mass-flow response option, which was tested with SF₆, CO₂, N₂ and He. The basic principles of dielectric loss sensor and the equivalent scheme were proposed for sensor operation in different environment, allowing estimating sensor response.

Optical and Electrochemical Characterization of Nanoporous Alumina Structures: Pore Size, Porosity, and Structure Effect

Three nanoporous alumina structures (NPASs) obtained by the two-step anodization method were optically and electrochemically characterized. Two of the structures were symmetric (NPAS-Sf and NPAS-Ph) and one was asymmetric (NPAS-And); pore size ranged from 10 nm to 100 nm and porosity was 12% in the case of the symmetrical NPAS and 23% and 30% for each surface of the asymmetric structure NPAS-And(A) and (B), respectively. Optical parameters of the studied samples (refraction index and extinction coefficient) were obtained from ellypsometric spectroscopy measurements carried out for wavelengths ranging between 250 nm and 1700 nm (visible and near infrared regions), with the total average refraction indices being 1.54, 1.52, 1.14, and 1.05 for NPAS-Sf, NPAS-Ph, NPAS-And(A), and NPAS-And(B), respectively, which indicates porosity control of refraction index values. Electrochemical characterizations (concentration potential and impedance spectroscopy measurements) were performed with NaCl solutions, and they allowed us to estimate samples of effective fixed charge concentration (1.22 × 10−2 M, 1.13 × 10−3 M, and 1.15 × 10−3 M), ion transport numbers, permselectivity (33.0%, 3.1%, and 9.6%), and the electrical resistance of each solution/sample system as well as the interfacial effects associated to solution concentration–polarization, which seems to be mainly controlled by pore size and sample symmetry.

Tuning the Optical Properties of Hyperbolic Metamaterials by Controlling the Volume Fraction of Metallic Nanorods

Porous films of anodic aluminum oxide are widely used as templates for the electrochemical preparation of functional nanocomposites containing ordered arrays of anisotropic nanostructures. In these structures, the volume fraction of the inclusion phase, which strongly determines the functional properties of the nanocomposite, is equal to the porosity of the initial template. For the range of systems, the most pronounced effects and the best functional properties are expected when the volume fraction of metal is less than 10%, whereas the porosity of anodic aluminum oxide typically exceeds this value. In the present work, the possibility of the application of anodic aluminum oxide for obtaining hyperbolic metamaterials in the form of nanocomposites with the metal volume fraction smaller than the template porosity is demonstrated for the first time. A decrease in the fraction of the pores accessible for electrodeposition is achieved by controlled blocking of the portion of pores during anodization when the template is formed. The effectiveness of the proposed approach has been shown in the example of obtaining nanocomposites containing Au nanorods arrays. The possibility for the control over the position of the resonance absorption band corresponding to the excitation of collective longitudinal oscillations of the electron gas in the nanorods in a wide range of wavelengths by controlled decreasing of the metal volume fraction, is shown.

Magneto-optical effects in hyperbolic metamaterials

Highly anisotropic metal-dielectric structures reveal unique dispersion properties providing new optical effects. Here we study experimentally linear optical and magneto-optical response of arrays of plasmonic gold nanorods and similar structures complemented by a thin nickel film. We show that both types of structures reveal distinct optical features expected for hyperbolic media and associated with the epsilon-near-zero (ENZ) and epsilon-near-pole (ENP) points. In the case of Ni-containing nanocomposites, we observe linear magneto-optical effects in transmission through the structure, increasing in the vicinity of these points. This observation reveals an important role of the local field enhancement in a hyperbolic medium associated with ENZ and ENP dispersion points in the appearance of magneto-optical activity of magnetic hyperbolic metamaterials.

Liquid permeation and chemical stability of anodic alumina membranes

A study on the chemical stability of anodic alumina membranes and their performance in long-term water and organic solvent permeation experiments is reported. Anodic alumina possesses high stability for both protonic and aprotonic organic solvents. However, serious degradation of the membrane occurs in pure water, leading to a drastic decrease of permeance (over 20% of the initial value after the passing of 0.250 m³/m² of pure water). The drying of the membrane induces further permeance drop-off. The rate of membrane degradation strongly depends on the pH of the penetrant solution and increases in basic media. According to ²⁷Al NMR and thermogravimetry results, the degradation of the membranes is associated with the dissolution of water-soluble [Al₁₃O₄(OH)₂₄(H₂O)₁₂]⁷⁺ polyhydroxocomplexes and their further redeposition in the form of [Al(OH)₄]^-, resulting in channels blocking. This process intensifies in basic pH due to the high positive charge of the anodic alumina surface. An approach for improving anodic aluminum oxide stability towards dissolution in water by carbon CVD coating of the membrane walls is suggested.

Gas permeation through nanoporous membranes in the transitional flow region

An experimental study on the permeability of anodic alumina (20-120 nm) and track-etched (30 nm) nanoporous membranes for different gases in the transitional flow regime is reported in the range of Knudsen numbers from 0.1 to 10. A significant variation (up to 30%) of the membrane permeance for different gases at the same Knudsen numbers is reported with certainty. It is established that this discrepancy relates to a molecule's effective collision area, which is poorly described in the frameworks of conventional gas permeation models. Two models are proposed for the description of the effect: self-diffusion of penetrate gases due to intermolecular collisions and enhancement of the slip flow contribution due to tangential momentum accommodation growth with the decrease of a molecule's effective collision area. The best fit parameters for the simultaneous fit of the experimental data with different models for 30 membrane-gas pairs are given.

Nanomechanical humidity detection through porous alumina cantilevers

We present here the behavior of the resonance frequency of porous anodic alumina cantilever arrays during water vapor adsorption and emphasize their possible use in the micromechanical sensing of humidity levels at least in the range of 10-22%. The sensitivity of porous anodic aluminium oxide cantilevers (Δf/Δm) and the humidity sensitivity equal about 56 Hz/pg and about 100 Hz/%, respectively. The approach presented here for the design of anodic alumina cantilever arrays by the combination of anodic oxidation and photolithography enables easy control over porosity, surface area, geometric and mechanical characteristics of the cantilever arrays for micromechanical sensing.

Controlled hydrothermal pore reduction in anodic alumina membranes

Porous anodic aluminium oxide nanostructures are popular templates for the fabrication of a wide range of nanomaterials. When open at both ends, they are now being used as model membranes, called anodic alumina membranes (AAM). In both cases, their appeal resides in the possibility of accurately controlling pore size via the anodization voltage, with a narrow size distribution. This characteristic, though, is maintained only in specific pore size ranges, reflecting specific ordering regimes in the material. Outside these domains, less ordered structures are obtained. Furthermore, the smallest pores currently achieved by anodization are about ∼10 nm in diameter, using sulphuric acid, which yields very thin and fragile nanostructured membranes. In this work we address these limitations by decoupling the control of pore size from the anodization stage. We achieve this by subjecting AAMs produced under a high order regime (40 V, 0.3 M oxalic acid) to a post-anodization hydrothermal treatment using steam. With this process we were able to decrease the pore size by 80% down to ∼10 nm. The membranes retain their integrity and are more robust than AAMs with the same pore structure produced via anodization in sulphuric acid.

Effect of Porosity and Concentration Polarization on Electrolyte Diffusive Transport Parameters through Ceramic Membranes with Similar Nanopore Size

Diffusive transport through nanoporous alumina membranes (NPAMs) produced by the two-step anodization method, with similar pore size but different porosity, is studied by analyzing membrane potential measured with NaCl solutions at different concentrations. Donnan exclusion of co-ions at the solution/membrane interface seem to exert a certain control on the diffusive transport of ions through NPAMs with low porosity, which might be reduced by coating the membrane surface with appropriated materials, as it is the case of SiO₂. Our results also show the effect of concentration polarization at the membrane surface on ionic transport numbers (or diffusion coefficients) for low-porosity and high electrolyte affinity membranes, which could mask values of those characteristic electrochemical parameters.

Longitudinal pore alignment in anodic alumina films grown on polycrystalline metal substrates

A quantitative analysis of longitudinal pore alignment in anodic alumina films grown on polycrystalline metal substrates was performed on the basis of small-angle X-ray diffraction mapping. The very high sensitivity of the diffraction pattern to the orientation of the anodic alumina film allowed the average pore alignment within the irradiated area to be determined, with an accuracy better than 0.1°. It is shown that pores deviate from the orientation orthogonal to the metal surface by a small angle that is constant within a single-crystal grain. Strong correlation between the longitudinal pore alignment within the anodic alumina film and the grain structure of the aluminium substrate indicates the important role of the crystallographic orientation of the metal in the pore growth process.

Comparative study of structure and permeability of porous oxide films on aluminum obtained by single- and two-step anodization

A comparative study of the structure and transport properties of porous aluminum oxide films obtained by single- and two-step anodization was carried out. It is shown that the oxidation regime significantly affect the number of dead-ended channels, which results in more than twice the variation in membrane permeability. The effect is explained by multiple branching of channels on the initial stages of organization of the porous structure. Branching also occurs on later stages governing mass transport properties of porous anodic alumina films. A model describing transport properties of anodic aluminum oxide membranes based on pore branching on domain boundaries was suggested to fit experimental results of permeance of membranes obtained by both single- and two-step anodization.