lGas diffusion layer structure is reconstructed based on the stochastic parameter method.lPorosity distributions and gradients are considered in the geometric model generation.l“V” shaped porosity ...distribution blocks water transport in the middle region.lHigher porosity gradient leads to fewer water penetration paths and smaller saturation.l“V” shaped porosity distribution with low gradient leads to high water flow rate.
Water management significantly influences the life and power output of the proton exchange membrane fuel cell (PEMFC). The gas diffusion layer (GDL) provides key transport paths of reactant gas and liquid water in the PEMFC. This study uses the volume of fluid (VOF) method to numerically investigate the liquid water transport in the GDL with different porosity distributions. The GDL geometry is reconstructed based on the stochastic parameter method. Five GDL structures with different porosity distributions are investigated on the liquid water transport. And different contact angles are defined on the carbon fiber surface and their effect is discussed. The results show that the porosity distribution and contact angle have dramatic influence on the water distribution and saturation in the GDL. The liquid water accumulation tends to occur in the high porosity GDL region. The water saturation is higher and flow rate is lower in the GDL with inverted “V” shaped porosity distribution than “V” shaped porosity distribution. The water saturation and flow rate is smaller when the porosity gradient is greater for both the “V” and inverted “V” shaped porosity, due to fewer water penetration paths. The water saturation in the hydrophilic GDL is higher than in the hydrophobic GDL for all the five porosity distributions.
Amorphous metallic foams are prospective materials due to unique combination of their mechanical and energy-absorption properties. In the present work, atomistic dynamics simulations are performed ...under isobaric conditions with the pressure p = 1.0 atm in order to study how cooling with extremely high rates (5 x 10.sup.13-5 x 10.sup.14 K/s) affects the formation of pores in amorphous titanium nickelide. For equilibrium liquid phase, vaporization temperature T.sub.b and the equation of states in the form of rho(T) are determined. It is found that the porosity of this amorphous solid does not depend on cooling at such high rates, whereas the pore morphology depends on the magnitude of the cooling rate. The obtained results will be in demand in study of mechanical properties of amorphous metallic foams with a nanoporous structure.
When impregnated with manganiferous precursors, γ-Alsub.2Osub.3 may be converted into α-Alsub.2Osub.3 under relatively mild and energy-saving conditions. In this work, a manganese assisted conversion ...to corundum at temperatures as low as 800 °C is investigated. To observe the alumina phase transition, XRD and solid-state sup.27Al-MAS-NMR are applied. By post-synthetical treatment in concentrated HCl, residual manganese is removed up to 3 wt.-%. Thereby, α-Alsub.2Osub.3 with a high specific surface area of 56 msup.2 gsup.−1 is obtained after complete conversion. Just as for transition alumina, thermal stability is an important issue for corundum. Long-term stability tests were performed at 750 °C for 7 days. Although highly porous corundum was synthesized, the porosity decreased with time at common process temperatures.
This study presents the correlation between process parameters and porosity formation in a TiAl6V4 alloy produced by selective laser melting. The porosity is investigated by 2D and 3D methods aiming ...to identify the mechanisms of void formation, their morphology as well as volume fraction as a function of the energy density. An evident minimum volume fraction is observed between process parameters with significant overheating and insufficient fusion. It is shown that these two marginal regions define two mechanisms of void formation. Two dominant types of voids morphology are identified and examined regarding pore orientation versus their elongation, which together with the curvature distribution analysis allow revealing critical defects.
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•The initial porosity of SLM Ti6Al4V was significantly reduced by applying a systematic processing strategy.•Marginal applied energy densities - excessive or insufficient - lead to two different mechanisms of pore formation.•The correlation between pore orientation, and morphology allowed revealing voids critical for mechanical performance.•Process optimization should consider pore formation mechanisms besides overall porosity to achieve low-void microstructures.
Designing organic components that can be used to construct porous materials enables the preparation of tailored functionalized materials. Research into porous materials has seen a resurgence in the ...past decade as a result of finding of self‐standing porous molecular crystals (PMCs). Particularly, a number of crystalline systems with permanent porosity that are formed by self‐assembly through hydrogen bonding (H‐bonding) have been developed. Such systems are called hydrogen‐bonded organic frameworks (HOFs). Herein we systematically describe H‐bonding patterns (supramolecular synthons) and molecular structures (tectons) that have been used to achieve thermal and chemical durability, a large surface area, and functions, such as selective gas sorption and separation, which can provide design principles for constructing HOFs with permanent porosity.
HOF the shelf: Hydrogen‐bonded organic frameworks (HOFs) are described systematically based on hydrogen‐bonding patterns (supramolecular synthons) and molecular structures (tectons). HOFs can show thermal and chemical durability, a large surface area, and permanent porosity.
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•The adsorptive removal of both organic and inorganic contaminants using ZIF-67 based composites has been reviewed.•Synthesis, modification, and functionalization of ZIF-67 have been ...summarized.•Possible mechanisms and parameters affecting the performances of adsorbents are explained.•The limitations and challenges that still arise during the synthesis are discussed.•Some key ideas for future research have also been proposed to expand the of ZIF-67-based composites.
Zeolitic imidazolate framework (ZIF) based composites are developed and utilized widely on account of their enormous surface area, uniform pore size, pore tunability, and rich topological diversity for the removal of several industrial wastes like dyes, heavy metals, pharmaceuticals, and agricultural wastes from wastewater via adsorption. ZIF-67, a subclass of ZIFs, and its derivatives have been developed over the past years by researchers for utilization in adsorption technology. ZIF-67 is fundamentally composed of tetrahedrally coordinated Co2+ with 2-methylimidazole, which demonstrates electrostatic attraction, π-π interaction, and a complex formation mechanism based on pollutants nature. In most of the cases, from the adsorption kinetics and isotherm studies, ZIF-67-based adsorbents exhibit pseudo-second order kinetic model and Langmuir isotherm model, respectively, where an endothermic process is revealed in most of the thermodynamic studies. In this review, the synthesis procedure of ZIF-67-based adsorbents with experimental findings, adsorption mechanism, factors affecting the adsorption process, analysis of efficiency of adsorbents along with their reusability, and finally, the challenges and future outlooks of ZIF-67-based adsorbents for liquid-phase adsorption has been presented.
This paper presents the results of comprehensive cement paste porosity and gas permeability tests. The tests conducted concerned ordinary Portland cement (OPC) cement pastes with varying water-cement ...ratios ranging from 0.3 to 0.6. The tests were conducted after the curing of cement paste for 90 days and two years under laboratory conditions. Open porosity was determined using three methods: helium pycnometry, mercury intrusion porosimetry, and water saturation. Permeability was determined using a modified RILEM-Cembureau method. The results obtained demonstrated that permeability does not change significantly over time despite the observed material shifts in open porosity characteristics caused both by further progress in hydration and by the carbonation process that occurs. The results of the tests conducted also permitted the quantitative determination of the impact of the water-cement ratio, age, and the progress of carbonation on open porosity measured using different methods and also on the gas permeability of the pastes.
The lithium–sulfur (Li–S) battery is a promising technology for large‐scale energy storage and vehicle electrification due to its high theoretical energy density and low cost. Reducing the sulfur ...cathode porosity has been identified recently as a viable strategy for improving the cell practical energy density and minimizing pore‐filling electrolytes to extend cell life at lean electrolyte conditions. Direct use of a low‐porosity cathode for Li–S battery results in poor electrode wetting, nonuniform electrode reactions, and thus early cell failure. To understand and mitigate the barriers associated with the use of low‐porosity electrodes, multiscale modeling is performed to predict electrode wetting, electrolyte diffusion, and their impacts on sulfur reactions in Li–S cells by explicitly considering the electrode wettability impacts and electrode morphologies. The study elucidates the critical impact of low tortuosity and large channel pore design for promoting electrode wetting and species diffusion. It is suggested that the secondary particle size should be comparable with the electrode thickness to effectively promote electrolyte wettability and sulfur reactivity. This study provides new insights into the low‐porosity electrode material and designs and is expected to accelerate the development of practical high‐energy Li–S batteries.
Multiscale modeling and experimental approaches are used to study the working principle of the low‐porosity cathode for lithium–sulfur batteries. Increasing cathode secondary particle size and reducing pore channel tortuosity can promote electrolyte wettability and sulfur reactivity, which makes reducing cathode porosity a viable strategy for improving the cell practical energy density and cycling life at lean electrolyte conditions.
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