Adjacent Pores Research Articles

Novel approach to the characterization of the pore structure and surface chemistry of porous carbon with Ar, N2, H2O and CH3OH adsorption

Characterization of porous carbon for its pore size distribution is traditionally carried out with simple pore models of well-defined geometry (slit or cylinder) with both ends opened to the gas surroundings. These idealised models do not adequately describe the inherently complex structure of porous carbons. An improved model should be sufficiently simple, not only for characterization, but also for the design of separation and purification processes. The following factors should be included: (1) energetic variation along the pore axis due to constrictions or strong energy sites, (2) connectivity between adjacent pores and (3) the presence of functional groups. In this work, we report results from computer simulations, using argon and nitrogen as non–polar or weakly polar adsorbates and water and methanol as examples of associating fluids. The new model is able to produce all hysteresis loops classified by the IUPAC as well as loop types reported earlier by de Boer [1]. Experimental isotherms are commonly measured at 77 K and 87 K (nitrogen and argon boiling points, respectively), but in recent years cryostats have become increasingly available in characterization laboratories and our simulations show that by measuring isotherms at different temperatures new insights about the porous structure can be gained from the variation of the hysteresis loop with respect to temperature which may change, for example, from a single-loop to a double-loop via a fused-loop as the temperature is varied.

Pore-scale analysis of formation damage in Bentheimer sandstone with in-situ NMR and micro-computed tomography experiments

Abstract We investigated fines movement through sandstone in-situ at the micrometre pore scale and studied the associated pore-scale mechanisms leading to formation damage. We used two in-situ techniques to accomplish this, namely nuclear magnetic resonance T 2 relaxation time (NMR) measurements (of pore size distributions) and high resolution x-ray micro-computed tomography (μCT; at high resolutions of (0.89 μm) 3 and (3.4 μm) 3 ). The μCT images showed the precise 3D location of the fines particles in the plug and demonstrated that initially pore throats are plugged, followed by filling of adjacent pore bodies by solid particles. These measurements in combination with traditionally used (indirect) permeability and production curve measurements and ex-situ SEM imaging enabled us to propose a new mechanistic pore-scale plugging model; furthermore we demonstrated that the amount of fines trapped decayed rapidly with core depth. We conclude that it is feasible to analyse formation damage in-situ by a combination of NMR and μCT measurements.

高碱度烧结矿低温还原过程中粉化机理的研究

于显微镜下观察高碱度烧结矿低温还原20、40、60 min后显微结构的变化。研究发现烧结矿在低温还原时赤铁矿向磁铁矿的转变很少，烧结矿低温还原时显气孔率和体积几乎不变。进一步研究发现，低温还原时铁酸钙分解为赤铁矿生成新的小气孔，随反应的进行气孔变大导致相邻气孔不断合并，使烧结矿变为疏松大气孔结构，强度不断下降，这是造成烧结矿低温粉化的一个重要原因。 The change of microstructure of high-basicity sinter after undergoing low-temperature reduction for 20, 40 and 60 min was observed. The study found that hematite transformed rarely to magne-tite in low-temperature reduction process of sinter, and the sinter’s apparent porosity and volume were almost constant. Further study found that with calcium ferrite decomposing into hematite in low-temperature reduction process, new small pores were generated, and the pores became larger along with the reaction, which resulted in merging of the adjacent pores. After that, the sinter’s structure became loose with big pores, and its intensity decreased, which caused chalking of the sinter in low temperature.

Effects of hydrophilic/hydrophobic properties of gas flow channels on liquid water transport in a serpentine polymer electrolyte membrane fuel cell

The droplet dynamics in the serpentine flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a serpentine channel with the aim of effectively preventing flooding. Three-dimensional two-phase flow simulations employing the volume of fluid (VOF) method have been performed. Liquid droplets emerging from four adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the serpentine flow channel. The effects of contact angle variation of the channel walls on liquid water removal have been tested in terms of liquid water saturation and coverage of liquid water on the gas diffusion layer (GDL) surface. The numerical results show that the hybrid case, which consists of hydrophilic channel walls at the straight part and hydrophobic walls at the turning part of the serpentine flow channels, enhances water removal compared with two other cases in which the channel wall is homogeneously hydrophilic or hydrophobic. The three-dimensional visualization of liquid water droplets reveals that the hydrophobic wall at the turning part reduces the water saturation in the channel and the hydrophilic wall at the straight part prevents the liquid water from covering the GDL surface.

High-aspect-ratio vertically aligned GaAs nanowires fabricated by anodic etching

Large-scale self-aligned GaAs nanowire arrays were successfully fabricated by the anodic etching of an n-type GaAs (111)B substrate. Although pore generation occurred randomly at the early stage of anodic etching, homogeneous pore growth with a high pore density was accomplished spontaneously on the entire surface of the substrate by prolonged anodic etching under optimized conditions. The GaAs pore walls gradually dissolved during anodic etching and finally three adjacent pores were interconnected to yield a GaAs nanowire with a diameter of approximately 200 nm, a length of approximately 110 μm, and a high aspect ratio of over 500. Aggregates of GaAs nanowires exhibited a good electron emission property, a low turn-on electric field (2.5 V μm−1), and a stable field emission current. The field-emission characteristics were enhanced by increasing the spacing between emission sites through post-chemical etching.

Formation, morphology and control of high-performance biomedical polyurethane porous membranes by water micro-droplet induced phase inversion

Biomedical polyurethane (BPU) porous membranes with controlled morphology and excellent permeability and mechanical properties were prepared via a method involving a phase inversion induced by water micro-droplets, which were generated by an ultrasonic atomizer. The cross-section morphology, air permeability and mechanical properties of the porous membranes were investigated. The SEM images demonstrated that the adjacent pores were connected by a micro-hole, serving as a “backdoor” for the pore. An interconnected porous structure was obtained, improving the air permeability of the BPU membrane relative to the membrane produced by immersion precipitation. Our studies indicated that the diameter of the pores in the membrane depended on the solution viscosity, allowing porous membranes with a desired morphology to be obtained by adjusting the polymer concentration and solution viscosity. The application of micro-droplets of water during membrane preparation reduced the exchange rate between the solvent and nonsolvent, resulting in the microphase separation of polymer molecules and the formation of a uniform porous structure in the membrane, which improved the air permeability and mechanical properties of the BPU porous membranes. This is a simple and effective preparation method for high-performance porous membranes with potential applications in tissue engineering scaffolds, controlled-release drug delivery and vascular grafts.

Assessment of Cytocompatibility and Magnetic Properties of Nanostructured Silicon Loaded with Superparamagnetic Iron Oxide Nanoparticles

The aim of this work is to create a biocompatible superparamagnetic nanocomposite applicable as vehicle for magnetically guided drug delivery. Therefore Fe3O4-nanoparticles have been infiltrated or chemically grown within nanostructured silicon. Both materials, the nanostructured silicon as well as the iron oxide nanoparticles, offer low toxicity and investigations concerning the cell-viability have been carried out. The magnetic properties of the system have been optimized concerning the blocking temperature TB and the magnetic moment M which means that TB has to be far below room temperature and M should be as high as possible, whereas these both properties are counteracting. TB indicates the transition between superparamagnetic behavior and blocked state of the composite. Due to the fact that TB is not only dependent on the particle size (diameters between 4 and 10 nm have been investigated) but also on the magnetic interactions between the particles there are two main routes to fabricate such a composite with desired TB - first a modification of the pore-loading with a concomitant variation of the distance between the particles within one pore and second a variation of the porous silicon morphology influencing the distance between particles within adjacent pores. To ensure that there is no remanence after the external field has been switched off, magnetic coupling between the particles has to be kept sufficiently low. A particle-size/distance and template dependent assessment of the magnetic properties and the cytotoxicity of the nanocomposite will be presented. Furthermore the differences between infiltrated Fe3O4-nanoparticles and chemically grown particles inside the pores will be figured out.

Morphology Controlled Magnetic Interactions of Porous Silicon Embedded Nanostructures

In the frame of this work porous silicon templates with different morphologies, especially with respect to the roughness of the pore-walls are filled with diverse ferromagnetic metals to fabricate self-assembled three dimensional arrays of nanostructures (wires, particles) with distinct magnetic properties. The fabricated templates offer a quasi-regular self-assembled pore-arrangement with oriented pores. Also the pore-filling is self-assembled by electrochemical deposition and there are multiple possibilities to tailor the magnetic behavior of these systems such as material, morphology, inter particle distance or interfaces. One critical parameter for this purpose is the roughness of the pore walls and the surface of the metal structures, respectively. To vary the morphology of the templates also magnetic field assisted etching has been employed and a remarkable decrease of the dendritic pore growth has been achieved resulting in smoother pore walls. Within the pores of these templates ferromagnetic metals are deposited and the modification of the magnetic properties compared to metal structures embedded in conventional etched (without magnetic field) porous silicon templates has been figured out. The magnetic anisotropy between easy axis and hard axis magnetization could be increased significantly and furthermore the magnetic behavior becomes comparatively hard magnetic. These properties can be ascribed to less magnetic coupling between nanostructures of adjacent pores and modified stray fields due to less dendritic metal nanowires. The magnetic behavior of three dimensional metal nanowire/nanoparticle arrays with regard to morphological parameters as well as to the influence of the inner interfaces of the composites will be presented. The presented system is of interest due to the silicon base material and the broad tunability of the magnetic properties for integrated magnetic and magnetooptic devices.

Using stable Mg isotopes to distinguish dolomite formation mechanisms: A case study from the Peru Margin

The magnesium isotope composition of diagenetic dolomites and their adjacent pore fluids were studied in a 250m thick sedimentary section drilled into the Peru Margin during Ocean Drilling Program (ODP) Leg 201 (Site 1230) and Leg 112 (Site 685). Previous studies revealed the presence of two types of dolomite: type I dolomite forms at ~6m below seafloor (mbsf) due to an increase in alkalinity associated with anaerobic methane oxidation, and type II dolomite forms at focused sites below ~230mbsf due to episodic inflow of deep-sourced fluids into an intense methanogenesis zone. The pore fluid δ26Mg composition becomes progressively enriched in 26Mg with depth from values similar to seawater (i.e. −0.8‰, relative to DSM3 Mg reference material) in the top few meters below seafloor (mbsf) to 0.8±0.2‰ within the sediments located below 100mbsf. Type I dolomites have a δ26Mg of −3.5‰, and exhibit apparent dolomite-pore fluid fractionation factors of about −2.6‰ consistent with previous studies of dolomite precipitation from seawater. In contrast, type II dolomites have δ26Mg values ranging from −2.5 to −3.0‰ and are up to −3.6‰ lighter than the modern pore fluid Mg isotope composition. The enrichment of pore fluids in 26Mg and depletion in total Mg concentration below ~200mbsf is likely the result of Mg isotope fractionation during dolomite formation, The 26Mg enrichment of pore fluids in the upper ~200mbsf of the sediment sequence can be attributed to desorption of Mg from clay mineral surfaces. The obtained results indicate that Mg isotopes recorded in the diagenetic carbonate record can distinguish near surface versus deep formed dolomite demonstrating their usefulness as a paleo-diagenetic proxy.

Relationship between mineralogy and porosity in seals relevant to geologic CO2 sequestration

Porosity and permeability are key petrophysical variables that link the thermal, hydrological, geochemical, and geomechanical properties of subsurface formations. The size, shape, distribution, and connectivity of rock pores dictate how fluids migrate into and through micro- and nano-environments, then wet and react with accessible solids. Three representative samples of cap rock from the Eau Claire Formation, the prospective sealing unit that overlies the Mount Simon Sandstone, a potential CO2 storage formation, were interrogated with an array of complementary methods. neutron scattering, backscattered-electron imaging, energy-dispersive spectroscopy, and mercury porosimetry. Results are presented that detail variations between lithologic types in total and connected nano- to microporosity across more than five orders of magnitude. Pore types are identified and then characterized according to presence in each rock type, relative abundance, and surface area of adjacent minerals, pore and pore-throat diameters, and degree of connectivity. We observe a bimodal distribution of porosity as a function of both pore diameter and pore-throat diameter. The contribution of pores at the nano- and microscales to the total and the connected porosity is a distinguishing feature of each lithology observed. Pore:pore-throat ratios at each of these two scales diverge markedly, being almost unity at the nanoscale regime (dominated by illitic clay and micas), and varying by one and a half orders of magnitude at the microscale within a clastic mudstone. Individual minerals, primarily illite and glauconite, have unmistakable pore and pore-throat signatures and contribute disproportionately to connected reactive surface area. The pore types created or evolved during diagenesis mediate profound differences between bulk and pore-network-accessible mineral associations in the mudstones. Results of this study can ultimately be used to inform reactive-transport simulations of effective reactive surface area.

An Overview of Inverted Colloidal Crystal Systems for Tissue Engineering

Scaffolding is at the heart of tissue engineering but the number of techniques available for turning biomaterials into scaffolds displaying the features required for a tissue engineering application is somewhat limited. Inverted colloidal crystals (ICCs) are inverse replicas of an ordered array of monodisperse colloidal particles, which organize themselves in packed long-range crystals. The literature on ICC systems has grown enormously in the past 20 years, driven by the need to find organized macroporous structures. Although replicating the structure of packed colloidal crystals (CCs) into solid structures has produced a wide range of advanced materials (e.g., photonic crystals, catalysts, and membranes) only in recent years have ICCs been evaluated as devices for medical/pharmaceutical and tissue engineering applications. The geometry, size, pore density, and interconnectivity are features of the scaffold that strongly affect the cell environment with consequences on cell adhesion, proliferation, and differentiation. ICC scaffolds are highly geometrically ordered structures with increased porosity and connectivity, which enhances oxygen and nutrient diffusion, providing optimum cellular development. In comparison to other types of scaffolds, ICCs have three major unique features: the isotropic three-dimensional environment, comprising highly uniform and size-controllable pores, and the presence of windows connecting adjacent pores. Thus far, this is the only technique that guarantees these features with a long-range order, between a few nanometers and thousands of micrometers. In this review, we present the current development status of ICC scaffolds for tissue engineering applications.

Study on the Surface Morphologies of Nickel-Phosphorus Ultra-Black Films

Ni-P ultra-black films having conical pores with the diameter of ~ 10-30 μm and the depth of ~ 15-30 μm were prepared by chemical etching of electroless plated Ni-P films using 8 mol/L nitric acid at 40 °C for 60 s. The phase composition and microstructure of the film samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. The results show that the diameter and depth of the etching pores become larger and the flat top regions bounded by etching pores become smaller by the coalescence of adjacent pores with the increase of etching time. The surface morphologies of the etched Ni-P films are characterized by the distribution of conical pores.

Biocompatible iron oxide/porous silicon nanocomposite with tunable magnetic properties

AbstractIron oxide nanoparticles of 5 and 8 nm in size have been infiltrated into the pores of porous silicon. The aim is to create a superparamagnetic (SPM) nanocomposite system with maximized magnetic field induced moment. Therefore the particle size versus the superparamagnetic behaviour has been figured out. The blocking temperature TB which indicates the transition between superparamagnetic behaviour and blocked state is not only dependent on the particle size but also on the magnetic interactions between them which can be varied by the distance between the particles. The magnetic behavior of the nanocomposite has been examined with regard to the magnetic interactions between the particles within the pores. Dependent on the size but also the distance between the particles, which has been adjusted by the various concentrations, TB has been shifted. Furthermore the modification of the porous silicon template results in a change of the transition temperature due to the variation of the particle distance of adjacent pores. To fabricate distinct composite systems suitable in biomedicine the porous silicon template as well as the loading of the pores with the particles has to be adjusted. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of pore diameter and structure of mesoporous sieve supported catalysts on hydrodesulfurization performance

A series of mesoporous sieves with different pore diameters and structures have been controllably synthesized by adding variable amounts of swelling agent 1,3,5-trimethylbenzene in the preparation process for SBA-15. NiMo catalysts supported by mesoporous sieves are prepared by using the newly proposed Mo-based organic–inorganic hybrid nanocrystals as precursors to load Mo and subsequently using wetness impregnation to load Ni. The hydrodesulfurization (HDS) activity of these obtained catalysts is evaluated in a continuously flowing tubular fixed-bed microreactor using 1wt% dibenzothiophene (DBT) in heptane as a model compound. The results show that HDS ratio has gradually increased as the pore diameter of supports is enlarged from 7 to 11nm with the mesostructure keeping an ordered two-dimensional hexagonal symmetry. When the structure is in the intermediate stage of structural transition from ordered hexagonal mesostructure to mesocecullar structure, HDS ratio is decreased dramatically due to the disordered and collapsed structure to block the pore channels. After the mesocecullar structure with three-dimensional open pore channels is formed which is more convenient for the molecular diffusion, HDS ratio has increased again as a result. It is concluded that both the pore size and window size connecting the adjacent pores play an important role in the HDS activity but the window size is of more significant importance, which is the key point for the enhanced catalytic performance.

Electrodeposited Metal Nanotube/Nanowire Arrays in Mesoporous Silicon and Their Morphology Dependent Magnetic Properties

Porous silicon in the mesoporous regime is used as template for electrodeposition of ferromagnetic metals within the pores to fabricate self-assembled three dimensional arrays of nanotubes, nanowires or nanoparticles. Depending on the electrochemical deposition parameters the geometry of the metal deposits can be tuned and thus desired magnetic properties can be achieved. The morphology of the templates is modified due to different anodization processes. Additional to a conventional etching process magnetic field assisted etching is applied. Furthermore the magnetic properties in dependence on the morphological characteristics, especially the pore-growth of the porous silicon are investigated. An important point is the concomitant modification of the magnetic coupling of metal nanostructures of adjacent pores.

Numerical simulations of water droplet dynamics in hydrogen fuel cell gas channel

The droplet dynamics in the cathode gas flow channel of a hydrogen fuel cell has been numerically investigated to obtain ideas for designing a flow channel to effectively prevent flooding. Three-dimensional two-phase flow simulations employing the volume of fluid method have been performed. Liquid droplets emerging from two adjacent pores at the hydrophobic bottom wall are subjected to airflow in the bulk of the gas flow channel. The effects of various parameters (pore distance, locations, sidewall contact angle, and airflow rate) on the liquid water removal from the gas channel have been investigated in terms of liquid water saturation, coverage of liquid water on the gas diffusion layer (GDL) surface, and change in the pressure drop in the channel. The numerical results show that the coalescence of two adjacent droplets enhances the water removal as compared to two separate, small droplets. It is also observed that droplets generated near the hydrophilic sidewall can be attached to the upper corner of the channel walls, which prevents the liquid water from covering the GDL surface, whereas the hydrophobic sidewall may cause clogging of the gas channel with liquid water at a low airflow rate.

The importance of lattice flexibility for the migration of ethane in ZIF-8: Molecular dynamics simulations

Recent adsorption, diffusion and permeation studies give experimental evidence that ethane can easily adsorb and diffuse in the ZIF-8 framework. This finding is surprising since the pore window of ZIF-8 – derived from XRD data – is virtually too small for ethane migration. Therefore, the diffusion mechanism of ethane in ZIF-8 and the influence of the diffusing guest molecules on the flexible lattice are investigated in detail in this paper with focus on the diffusion limiting step, the passage of the ZIF-8 window between adjacent cavities. Contrary to zeolite research, well established interaction parameter sets are still not available for MOF simulations. Hence, a large part of this paper is devoted the comparison of several parameter sets for the molecular interactions. Also, the conservation not only of the unit box size in NPT simulations but also the conservation of the geometrical structure of the lattice (most important the “windows” between adjacent pores) has been examined. Employing a suitable parameter set, two interesting effects can be observed: (i) The window size of ZIF-8 is smaller at higher ethane loadings, while simultaneously (ii) at higher loadings the guest molecules within the cavity exert forces that press a given probe molecule toward the window. These two effects nearly compensate each other leading to a weak dependence of the self-diffusivity upon the concentration of guest molecules.However, no gate-opening mechanism by a torsional motion of the linker molecule methylimidazolate could be found in our MD simulations because of a too large activation energy for this motion.

Effects of organic and inorganic fertilization on soil aggregation in an Ultisol as characterized by synchrotron based X-ray micro-computed tomography

Long-term fertilization practices generally improve soil aggregation through associated increases in organic matter over time. However, the influence of organic versus inorganic fertilization on aggregate structures may be quite different. In this paper, we aimed to quantify the three-dimensional (3D) microstructure of soil aggregates as influenced by different long-term fertilization practices. Soil aggregates with diameters of approximately 5mm were collected from an Ultisol with a long-term fertilization trial established in 1986. The treatments were no fertilizer (CK), chemical fertilizer (NPK), and chemical fertilizer plus organic manure (NPK+OM). The aggregate microstructure was determined with synchrotron based X-ray micro-computed tomography (SR-μCT) and digital image analysis techniques. Mean corn yields and soil organic carbon were the highest in NPK+OM, followed by NPK and then by CK. Aggregate stability was highest in NPK+OM, and lowest in NPK. The number of pores, number of pore throats, and number of paths between adjacent nodal pores were all significantly decreased by the NPK+OM treatment relative to the NPK and CK treatments. However, microstructural pore properties were mostly not different between NPK and CK treatments. This study demonstrates that organic fertilization can improve soil aggregation, while inorganic fertilization is ineffective, even after 25years. The different mechanisms by which organic and inorganic fertilization practices influence soil aggregation deserve further investigation.

Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates

The magnetic function of a Ni-nanowire/silicon-template system has been explored in corporation with an advanced process. Arrays of nanopores with a mean diameter of 35 nm have been fabricated by anodization of silicon wafers under an external magnetic field (8 T) perpendicular to the substrate. Due to a guided supply of holes from the substrate during the anodization, well controlled straight nanopores have been formed with a high aspect ratio, and then isolated Ni nanowires have been grown along these nanopores by electrodeposition. The fabricated samples show a significantly enhanced magnetic anisotropy with little crosstalk between adjacent pores.

Porous silicon/Ni composites of high coercivity due to magnetic field-assisted etching

Ferromagnetic nanostructures have been electrodeposited within the pores of porous silicon templates with average pore diameters between 25 and 60 nm. In this diameter regime, the pore formation in general is accompanied by dendritic growth resulting in rough pore walls, which involves metal deposits also offering a branched structure. These side branches influence the magnetic properties of the composite system not only due to modified and peculiar stray fields but also because of a reduced interpore spacing by the approaching of adjacent side pores. To improve the morphology of the porous silicon structures, a magnetic field up to 8 T has been applied during the formation process. The magnetic field etching results in smaller pore diameters with less dendritic side pores. Deposition of a ferromagnetic metal within these templates leads to less branched nanostructures and, thus, to an enhancement of the coercivity of the system and also to a significantly increased magnetic anisotropy. So magnetic field-assisted etching is an appropriate tool to improve the structure of the template concerning the decrease of the dendritic pore growth and to advance the magnetic properties of the composite material.