Initial Pore Structure Research Articles

Effect of Meso- and Micro-Porosity in Carbon Electrodes on Atomic Layer Deposition of Pseudocapacitive V2O5 for High Performance Supercapacitors

Atomic layer deposition (ALD) of vanadium oxide is a viable means to add pseudocapacitive layers to porous carbon electrodes. Two commercial activated carbon materials with different surface areas and pore structures were acid treated and coated by V2O5 ALD using vanadium triisopropoxide and water at 150 °C. The V2O5 ALD process was characterized at various temperatures to confirm saturated ALD growth conditions. Capacitance and electrochemical impedance analysis of subsequently constructed electrochemical capacitors (ECs) showed improved charge storage for the ALD coated electrodes, but the extent of improvement depended on initial pore structure. The ALD of V2O5 onto mesoporous carbon increased the capacitance by up to 46% after 75 ALD cycles and obtained a maximum pseudocapacitance of 540 F/g(V2O5) after 25 ALD cycles, while maintaining low electrical resistance, high columbic efficiency, and a high cycle life. However, adding V2O5 ALD to microporous carbons with pore diameters of <11 A showed far less...

Initiated Chemical Vapor Deposition (iCVD) of Highly Cross-Linked Polymer Films for Advanced Lithium-Ion Battery Separators.

We report an initiated chemical vapor deposition (iCVD) process to coat polyethylene (PE) separators in Li-ion batteries with a highly cross-linked, mechanically strong polymer, namely, polyhexavinyldisiloxane (pHVDS). The highly cross-linked but ultrathin pHVDS films can only be obtained by a vapor-phase process, because the pHVDS is insoluble in most solvents and thus infeasible with conventional solution-based methods. Moreover, even after the pHVDS coating, the initial porous structure of the separator is well preserved owing to the conformal vapor-phase deposition. The coating thickness is delicately controlled by deposition time to the level that the pore size decreases to below 7% compared to the original dimension. The pHVDS-coated PE shows substantially improved thermal stability and electrolyte wettability. After incubation at 140 °C for 30 min, the pHVDS-coated PE causes only a 12% areal shrinkage (versus 90% of the pristine separator). The superior wettability results in increased electrolyte uptake and ionic conductivity, leading to significantly improved rate performance. The current approach is applicable to a wide range of porous polymeric separators that suffer from thermal shrinkage and poor electrolyte wetting.

Electromigration Phenomena in Sintered Nanoparticle Ag Systems Under High Current Density

Electromigration (EM) refers to the movement of atoms inside a conductor due to momentum exchange with the conduction electrons. In this work the EM effect in samples of porous Ag fabricated from nanoparticles of Ag in a pressure free sintering process is studied. Current densities of 2.5×104 − 1.7×105 A/cm2 were applied to the samples for periods ranging up to 500 h. In a typical EM setup with a non-porous conductor, void formation occurs at the cathode and hillock formation at the anode. In this study, voids were not directly observed, but cracks were formed after prolonged electromigration, presumably as a result of void accumulation and coalescence. When the samples were placed in 150 °C ambient no hillocks were observed, but at room temperature nanorods were formed with sizes ranging up to 20 μm in length, typically 25 nm in diameter and with aspect ratios ranging from 20 to 1000. It was found that interrupting and restarting the current resulted in growth of new nanorods rather than growth of existing ones, and that growth was limited by welding of individual nanorods when a critical number density was reached. While similar nanorods have been formed from Ag thin films using thermal stress , the location of nanorods was unusual in that while the number density was highest at the anode, significant numbers also appeared at central and cathode locations. Another unusual feature of the observed EM was that the initial porous structure became refined with coarse pores and grains transforming into a fine grained and fine pored structure with elongated and locally orientated pores and grains. Elemental composition studies provide tentative understanding of the nanorod number density, size distribution and growth mechanism. In the geometry utilized for this study, temperature gradients are known to strongly influence the divergence of the EM induced atomic flux and hence resistivity measurements and COMSOL Finite Element modelling was used to determine the temperature in the sample taking into account joule heating, convection and conduction processes.

Morphological analysis of high-impact polypropylene using X-ray microCT and AFM

Detailed visualization and understanding of high-impact polypropylene (hiPP) particle morphology and its evolution on the scale of entire particles is important for obtaining the required mechanical and optical properties of this material and for the quantitative description of mass transport in hiPP particles. Shape and size of hiPP particles as well as the spatial distribution of pores, PP homopolymer and ethylene-propylene rubber (EPR) phases in individual particles is visualized by micro-computed tomography (microCT) and atomic force microscopy (AFM) techniques. The morphology of each hiPP particle depends on many factors including catalyst system, the initial pore structure of porous catalyst and homopolymer particles, reactor conditions and particle history. These factors affect the required content, the constitution and the spatial distribution of EPR phase. To understand particle morphology and its evolution in a quantitative way we need a high-quality morphology data. The presented results include the AFM visualization of rubber layer on the surface of hiPP particles, the microCT visualization of not only porous homopolymer PP particles but also of large rubber domains in hiPP particles and the AFM mapping of the fine networked structure of EPR rubber and pores in hiPP particles.

Synthesis of Porous Carbon Supported Palladium Nanoparticle Catalysts by Atomic Layer Deposition: Application for Rechargeable Lithium–O2 Battery

In this study, atomic layer deposition (ALD) was used to deposit nanostructured palladium on porous carbon as the cathode material for Li-O2 cells. Scanning transmission electron microscopy showed discrete crystalline nanoparticles decorating the surface of the porous carbon support, where the size could be controlled in the range of 2-8 nm and depended on the number of Pd ALD cycles performed. X-ray absorption spectroscopy at the Pd K-edge revealed that the carbon supported Pd existed in a mixed phase of metallic palladium and palladium oxide. The conformality of ALD allowed us to uniformly disperse the Pd catalyst onto the carbon support while preserving the initial porous structure. As a result, the charging and discharging performance of the oxygen cathode in a Li-O2 cell was improved. Our results suggest that ALD is a promising technique for tailoring the surface composition and structure of nanoporous supports in energy storage devices.

Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO2 at 850–1050 °C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO2 gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.

Self-reaction of initial active groups in coal

For further understanding of self-heating of coal, we tested the reactions of seven different ranks of coal under inert atmosphere. In the test, 50-gram of coal sample ranged from 0.18mm to 0.38mm was put into a special designed copper reaction vessel and let pure nitrogen to flow into the coal sample from the bottom at a rate of 100mL/min. The programmed temperature enclosure was run at a programmed rate of 0.8°C/min. The concentration of the carbon oxides and the coal temperature were tested. The results show that the coal reactions under inert atmosphere can generate CO and CO2. The reactions under inert atmosphere are affected by coal ranks, initial pore structure of coal and sulfur content. For low ranks of coal, the productions of carbon oxides are piecewise. The coal temperature is lower than the surrounding temperature throughout the reactions under inert atmosphere, but it rises quickly and reaches a crossing point temperature in the later stage under dry-air atmosphere. Based on the analysis, it indicates the self-reaction of initial active groups exists in the self-heating of coal besides the reactions in the two parallel reactions model. Spontaneous combustion of coal is due to both the oxidation heat accumulation and the chain reaction. A new reaction model of self-heating of coal was proposed.

Elaboration of alumina nanowire field normal to a generic substrate

Nanoporous alumina film was realised on a silicon substrate. The structure investigated by scanning electron microscopy is presented and reveals homogeneous pores on the entire sample surface. The pore-cell is polygonal and has sometimes a hexagonal shape. The pore-cell is limited by six-edges, leading to alumina nanowires brushes normal to the substrate after an enlargement of the pores. Owing to the geometry of the initial pore structure, the resulting nanowires have an exponential shape with a foot larger than the top, leading to good mechanical behaviour. The alumina annealing has also been investigated by grazing incidence X-ray diffraction measurements and reveals the presence of the γ-Al2O3 phase for a temperature upper than 800°C.

An Adaptive Random Pore Model for Multimodal Pore Structure Evolution with Application to Char Gasification

An extension of the random pore model has been developed that allows different pore sizes to grow at different rates, depending on the instantaneous pore-scale reactant penetration at a given location within a reacting porous particle. This is accomplished by incorporating pore-scale effectiveness factors, consistent with the random pore geometry, into equations for the growth of each individual pore size. This framework allows the evolution of the char structure with local conversion to adapt to changes in boundary conditions (reactants, temperature) and the development of intraparticle gradients, rather than being predetermined by the initial pore structure (i.e., by the value of the random pore model structural parameter, ψ). This framework also facilitates the calculation of intrinsic kinetic parameters from experimental measurements by providing an estimation of the fraction of the total surface area participating in a given reaction. Without using any fitting parameters, the model has been found to ...

VPO catalysts synthesized on substrates with modified activated carbons

VPO catalysts were prepared on oxidized and unoxidized activated carbons differing in initial porous structure. Carbons were oxidized under relatively soft (30% H 2O 2, 200 °C) and hard (50% H 2O 2, 350 °C) conditions. Carbon modification was carried out hydrothermally in a traditional autoclave (HTT) or a microwave reactor (MWT). The synthesis was also carried out under hydrothermal (HTS or MWS) conditions. V 2O 5 and NH 4VO 3 were used as precursors. The samples are characterized by diversified porous structure at S BET = 732–1617 m 2/g and V por = 0.44–0.90 cm 3/g, as well as various degree of VPO crystallinity. Possibility of preparation of the VPO catalysts under ecologically appropriate conditions, i.e. in aqueous solutions, was shown.

Characterization of porous structure of coal char from a single devolatilized coal particle: Coal combustion in a fluidized bed

The combustion characteristics of coal char are highly dependent on initial pore structure of devolatilized char as well as on the structural evolution during the combustion of char. The development of pore structure also throws light on the mechanism of the combustion process. In the present work evolution of pore structure of partially burnt coal char of Indian origin has been investigated experimentally in a batch-fluidized bed and analyzed. The BET surface area, micropore surface area and porosity of char at various levels of carbon burn-off have been determined. Experimental specific surface area has been found to agree well with theoretical prediction using random pore model. Modified random pore model is used to determine the active surface area. Char combustion mechanism based on shrinking unreacted core and shrinking reacted core models are delineated during the course of reaction at various bed temperatures. This is substantiated with the proportional representation of ash and carbon matrix in scanning electron microscope images. It is also concluded that in the present investigation the mean pore size is much smaller and hence the Knudsen diffusion predominates. Analysis based on similar experimental observations and models for pore structure evolution to investigate char combustion reaction regime has not been reported in literature.

Microstructural evolution during pressureless infiltration of aluminium alloy parts fabricated by selective laser sintering

The infiltration pathway has been examined during the fabrication of aluminium components formed by a rapid prototyping technology. An AA 6061 alloy preform is first prepared by selective laser sintering. In a second operation, the aluminium precursor powders are transformed into a skeletal AlN structure, which is then pressureless infiltrated by AA 6061. The infiltration process was conducted under nitrogen, argon or a vacuum. The infiltration distance and infiltration pathway was determined by density measurements which were confirmed by metallographic examination and quantitative image analysis. Slug flow behaviour was not observed. Rather, the results suggest that the infiltrant first penetrates the entire sample length and then progressively fills the cross-section. Comparing the microstructures of the surface and the centre of the as-infiltrated samples indicates that the pathway into the porous perform is dependent on the infiltration atmosphere. Under vacuum, the infiltrant firstly fills the interior of the sample and then propagates to the surface. In contrast, under nitrogen or argon, the infiltrant firstly penetrates along the surface before filling the inside. In all cases, the infiltrant preferentially fills the pore regions with the highest curvature. This causes the highly convoluted initial pore structure to decompose into a large number of smaller, rounder pores, which eventually fill and disappear.

Prediction of the Conditions for Breathing of Metal Organic Framework Materials Using a Combination of X-ray Powder Diffraction, Microcalorimetry, and Molecular Simulation

The adsorption of C1 to C4 linear hydrocarbons in the flexible metal organic framework MIL-53(Cr) has been followed by adsorption manometry coupled with microcalorimetry and Synchrotron X-ray powder diffraction. This experimental investigation was completed by molecular modeling. In the case of methane, the solid remains rigid whatever the adsorbate amount. However for the C2-C4 series, an increasing flexibility of the structure is observed, which is ascribed first to a breathing of the material from a large pore to a narrow pore form followed by a further expansion at high pressure. The collected thermodynamic and structural information suggests that a minimum adsorption enthalpy of ca. 20 kJ mol (-1) in the initial large pore structure of MIL-53(Cr) is required to induce the structural transition "large to narrow pore". Further, the enthalpy of adsorption can be used to predict the pressure at which the structure reopens. Finally, the magnitude of the breathing can be related to the size of the probe molecule via the van der Waals volume. The above trends have been successfully verified in the case of water and carbon dioxide. This combined experimental and theoretical approach gives the first elements for the prediction of whether or not the MIL53 and similar flexible structures will respond to gas loading and what would be the pressure required and further the amplitude of the induced breathing.

Formation of compaction products in methane dry reforming on a Ni-containing catalyst

The formation of compaction products and their morphological features in the process of methane dry reforming on a Ni3Al catalyst were studied. It was shown that the initial porous structure of a sample was retained in the course of reaction, and the formation of carbon deposits did not decrease the catalytic activity of Ni3Al over a long period of time.

A biodegradable vascularizing membrane: A feasibility study

Regenerative medicine and in vivo biosensor applications require the formation of mature vascular networks for long-term success. This study investigated whether biodegradable porous membranes could induce the formation of a vascularized fibrous capsule and, if so, the effect of degradation kinetics on neovascularization. Poly( l-lactic acid) (PLLA) and poly( dl-lactic-co-glycolic) acid (PLGA) membranes were created by a solvent casting/salt leaching method. Specifically, PLLA, PLGA 75:25 and PLGA 50:50 polymers were used to vary degradation kinetics. The membranes were designed to have an average 60 μm pore diameter, as this pore size has been shown to be optimal for inducing blood vessel formation around nondegradable polymer materials. Membrane samples were imaged by scanning electron microscopy at several time points during in vitro degradation to assess any changes in pore structure. The in vivo performance of the membranes was assessed in Sprague–Dawley rats by measuring vascularization within the fibrous capsule that forms adjacent to implants. The vascular density within 100 μm of the membranes was compared with that seen in normal tissue, and to that surrounding the commercially available vascularizing membrane TheraCyte. The hemoglobin content of tissue containing the membranes was measured by four-dimensional elastic light scattering as a novel method to assess tissue perfusion. Results from this study show that slow-degrading membranes induce greater amounts of neovascularization and a thinner fibrous capsule relative to fast degrading membranes. These results may be due both to an initially increased number of macrophages surrounding the slower degrading membranes and to the maintenance of their initial pore structure.

Organized mesoporous silica films as templates for the elaboration of organizednanoparticle networks

Tremendous work achieved in the last 20 years on nanoparticle synthesis has allowed us tostudy many new physical properties that are found in the nanometre size range. Newdevelopments are now expected when considering assemblies of nanoparticles such as 2Dor 3D organized arrays. These systems are indeed expected to exhibit originalphysical properties resulting from particle–particle interactions. Studies in this fieldare clearly dependent on the elaboration of materials with controlled particlesize, organization and interparticle distance. This paper presents a strategy ofelaboration that is based on the use of organized mesoporous silica films as templates.These films are made by sol–gel polymerization around surfactant assemblies andfurther elimination of the surfactant. This provides porous matrices with a poreorganization that is the almost perfect replica of the initial micellar structure. The use ofsuch films for the elaboration of organized arrays of nanoparticles is detailed inthe case of CdS and Ag particles. The formation of particles inside the pores isachieved through impregnation with precursors that are allowed to diffuse inside thepores. This leads to particles with a size and a spatial arrangement that is directlyrelated to the initial pore structure of the films. This process opens a wide range ofinvestigations due to the relative ease of fabrication over large surfaces and the numerouspossibilities offered by the elaboration of porous films with different pore sizes andorganizations.

Structural evolution up to 1100 °C of xerogels prepared from TEOS sonohydrolysis and liquid phase exchanged by acetone

Silica xerogels were prepared from sonohydrolysis of tetraethoxysilane and exchange of the liquid phase of the wet gel by acetone. Monolithic xerogels were obtained by slow evaporation of acetone. The structural characteristics of the xerogels were studied as a function of temperature up to 1100°C by means of bulk and skeletal density measurements, linear shrinkage measurements and thermal analyses (DTA, TG and DL). The results were correlated with the evolution in the UV–Vis absorption. Particularly, the initial pore structure of the dried acetone-exchanged xerogel was studied by small-angle X-ray scattering and nitrogen adsorption. The acetone-exchanged xerogels exhibit greater porosity in the mesopore region presenting greater mean pore size (∼4nm) when compared to non-exchanged xerogels. The porosity of the xerogels is practically stable in the temperature range between 200°C and 800°C. Evolution in the structure of the solid particles (silica network) is the predominant process upon heating up to about 400°C and pore elimination is the predominant process above 900°C. At 1000°C the xerogels are still monolithic and retain about 5vol.% pores. The xerogels exhibited foaming phenomenon after hold for 10h at 1100°C. This temperature is even higher than that found for foaming of non-exchanged xerogels.

129Xe NMR study of Xe adsorption on multiwall carbon nanotubes

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Mechanical Resilience of Degraded Soil Amended with Organic Matter

Organic matter may help prevent the degradation of the soil pore structure by mechanical stresses, and consequently the impact of compaction and surface erosion. To obtain a mechanistic understanding of some of the processes involved, we amended an Ultisol with peat particles, at amendment rates of 0, 10, and 50 g kg−1, and subjected the peat‐soil mixtures to different wet/dry cycles (1, 4, or 10). Mechanical stresses were imposed to the soil as direct shear, surface shear, and virgin compression tests. The objective was to determine the effect of increasing organic matter on the resistance and resilience of the soil pore structure to mechanical stresses, after wet/dry cycles. Peat amendment increased soil porosity, mainly in the fine pores (&lt;6 μm), whereas an increased number of wet/dry cycles affected the coarse pore fraction (&gt;50 μm). Soil shear strength decreased with a greater peat‐amendment rate. For soil amended with 50 g kg−1 of peat, increasing the number of wet/dry cycles increased the compressibility index from 1.14 to 1.43 and the friction angle under shear by 6°. Although peat may decrease the resistance to compression, the initial soil pore structure and the resilience to this mechanical stress was greatly improved. Wetting and drying cycles improved the impact of peat on the mechanical resistance of the soil, but reduced resilience slightly. The results suggest that to amend with organic matter will improve recovery from vehicle traffic damage and improve water retention during dry periods, providing better conditions for plants and microbes.

Self-Peeling of Porous Nickel Foam from the Electrochemically Etched Porous Silicon

AbstractA low-doped p-type silicon wafer was wet-etched to form macropores in a high-aspect-ratio, straight and parallel manner along the Si (100) direction. It was then plated in aqueous alkaline solution containing Ni2+. Metallic Ni was rapidly deposited in the macropores on the sidewall surface without using a reducing agent or activation treatment at slightly elevated temperature. After being immersed for certain duration, the single crystalline Si of sidewalls was replaced by polycrystalline Ni while the initial porous structure was still maintained. When Ni became dominant in the entire porous regime, the porous film more than 200 μm thick was discovered to be able to peel off very easily from the Si substrate beneath. In this way, a piece of nickel foam with straight pores of very high aspect ratio is self-formed and self-peeled.