High Volume Fraction Of Voids Research Articles

Development of a Lycopodium powder-based superhydrophobic nanofiber membrane suitable for desalination

A biobased, green, inexpensive additive, Lycopodium particles, which are spores of the “Lycopodium clavatum” plant, were incorporated in the poly(vinylidene fluoride) PVDF electrospun nanofiber membranes (ENMs) for desalination by direct contact membrane distillation (DCMD). Superhydrophobic ENMs were prepared using this additive (PVDF-ENMs-Lyc). Thanks to their morphological structure and their prominent surface superhydrophobicity (anti-wetting) character, the resulting PVDF-ENMs-Lyc exhibited an improved liquid entry pressure (LEP), a high void volume fraction (greater than 87.2%), a good salt rejection factor (greater than 99.93%) and a reasonably high permeate flux (greater than 51.76 kg·m−2·h−1) at 80 °C, which are of great practical importance for water desalination by DCMD. The optimum membrane prepared with 3 wt% Lycopodium in the dope solution demonstrated a stable permeate flux of 52.4 ± 0.6 kg·m−2·h−1 with an electrical conductivity around 4.76 ± 0.46 μS/cm (NaCl rejection factor of 99.998 ± 0.036%) during 25 h DCMD desalination experiment using 35 g/L NaCl aqueous solution (similar to seawater concentration). The presented results pave the way for superhydrophobic nanofibrous membrane engineering suitable for membrane contactors by electrospinning in a single step without surfactants, organic additives, or chemical post-treatments, just by the incorporation of a green additive like Lycopodium powder.

Disorder and cavity evolution in single-crystalline Ge during implantation of Sb ions monitored in-situ by spectroscopic ellipsometry

Ion implantation has been a key technology for the controlled surface modification of materials in microelectronics and generally, for tribology, biocompatibility, corrosion resistance and many more. To form shallow junctions in Ge is a challenging task. In this work the formation and accumulation of shallow damage profiles was studied by in-situ spectroscopic ellipsometry (SE) for the accurate tracking and evaluation of void and damage fractions in crystalline Ge during implantation of 200-keV Sb + ions with a total fluence up to 1016 cm−2 and an ion flux of 2.1 × 1012 cm−2s−1. The consecutive stages of damage accumulation were identified using optical multi-layer models with quantitative parameters of the thickness of modified layers as well as the volume fractions of amorphized material and voids. The effective size of damaged zones formed from ion tracks initiated by individual bombarding ions can be estimated by numerical simulation compared with the dynamics of damage profiles measured by ion beam analysis and ellipsometry. According to our observations, the formation of initial partial disorder was followed by complete amorphization and void formation occurring at the fluence of about 1 × 1015 cm−2, leading to a high volume fraction of voids and a modified layer thickness of ≈200 nm by the end of the irradiation process. This agrees with the results of numerical simulations and complementary scanning electron microscopy (SEM) measurements. In addition, we found a quasi-periodic time dependent behavior of amorphization and void formation represented by alternating accelerations and decelerations of different reorganization processes, respectively. For the understanding and prevention of adverse void formation and for controlled evolution of subsurface nanocavities or cellular surface texture the in-situ monitoring of the dynamics of structural damage accumulation by the developed SE method is essential.

Electrospun Nanofiber Membranes Incorporating PDMS-Aerogel Superhydrophobic Coating with Enhanced Flux and Improved Antiwettability in Membrane Distillation.

Electrospun nanofiber membranes (ENMs) have garnered increasing interest due to their controllable nanofiber structure and high void volume fraction properties in membrane distillation (MD). However, MD technology still faces limitations mainly due to low permeate flux and membrane wetting for feeds containing low surface tension compounds. Perfluorinated superhydrophobic membranes could be an alternative, but it has negative environmental impacts. Therefore, other low surface energy materials such as silica aerogel and polydimethylsiloxane (PDMS) have great relevancy in ENMs fabrication. Herein, we have reported the high flux and nonwettability of ENMs fabricated by electrospraying aerogel/polydimethylsiloxane (PDMS)/polyvinylidene fluoride (PVDF) over electrospinning polyvinylidene fluoride- co-hexafluoropropylene (PVDF-HFP) membrane (E-PH). Among various concentrations of aerogel, the 30% aerogel (E-M3-A30) dual layer membrane achieved highest superhydrophobicity (∼170° water contact angle), liquid entry pressure (LEP) of 129.5 ± 3.4 kPa, short water droplet bouncing performance (11.6 ms), low surface energy (4.18 ± 0.27 mN m-1) and high surface roughness ( Ra: 5.04 μm) with re-entrant structure. It also demonstrated nonwetting MD performance over a continuous 7 days operation of saline water (3.5% of NaCl), high antiwetting with harsh saline water containing 0.5 mM sodium dodecyl sulfate (SDS, 28.9 mN m-1), synthetic algal organic matter (AOM).

Supercritical CO2–EtOH expanded liquid processing to produce tailored PEEK-WC membranes

Supercritical CO2 expanded ethanol is used to induce a phase separation process for one-step preparation of porous skinless membranes from a modified poly(etheretherketone). The process takes the advantage of the flexibility of the procedure that is also timesaving and greener than traditional phase separation methods. Skin-less membranes with modular structures have been obtained, combining different processing parameters, including SC-expanded liquid composition, polymer concentration, temperature and pressure. Kinetics and thermodynamics that control the phase separation have been analyzed in relation to diffusivity and solvent power of the SC-expanded liquid. Combining the different processing parameters, open structures with high void volume fraction and high degree of interconnectivity have been obtained, as confirmed by a very low resistance to mass transfer. Nitrogen fluxes up to 350 L cm−2 min−1 have been measured through membranes prepared at 250 bar, 35 °C, and 10% w/w.

Mechanical characteristics and strengthening effectiveness of random-chopped FRP composites containing air voids

In random-chopped fiber-reinforced polymer (FRP) composites used as a retrofit material, a high volume fraction of voids is inevitable due to the manufacturing characteristics. In this paper, the mechanical characteristics and strengthening effectiveness of random-chopped FRP composites containing air porosity are investigated through experiments and numerical analysis. Coupon-shaped specimens with various material compositions were manufactured to examine the uniaxial tensile performance, and the air voids in the composites were measured by a microscope camera. In order to predict the overall performance of the composites, a micromechanical formulation that accounts for porosity was newly developed. The derived model was incorporated into a finite element (FE) code, and the model parameters were estimated by comparing uniaxial tensile test results for various systems of random-chopped FRP composites. In addition, concrete beams strengthened with the composites were produced to evaluate their load-carrying capacity. The FE predictions of the composite structures were then compared with experimental data to verify the predictive capability of the proposed numerical framework.

Computational characterization on mechanical behavior of polymer electrolyte membrane based on nonaffine molecular chain network model

This paper investigates the mechanical behavior of polymer electrolyte membrane based on a nonaffine molecular chain network model (nonaffine model), which may account for the change in the entanglement situation for the physical linkages of the molecular chain during the deformation process. After the identification of the parameters used in the nonaffine model with the experimental data of the fresh membrane, a computational model is proposed for the degraded membrane, in which the swelling parts of the degraded membrane are replaced with voids. Employing the extended 2D homogenization method, the effect of the change in the entanglement situation for the physical linkages of the molecular chain and that of the macroscopic loading conditions on the mechanical behavior of the membrane with different volume fractions of voids are discussed. The results indicate that the dramatic increase in the average number of segments in a single chain, which occurs inside the microscopic sharp shear band, leads to a great strain softening after the macroscopic yield. Moreover, such strain softening tends to appear at early deformation stage for the membrane with high volume fraction of voids under high macroscopic triaxiality loading condition.

Studies on Micro- to Macroscopic Mechanical Behavior of Porous Polymer under Compaction

The 2D extended homogenization model based on molecular chain network theory is employed to investigate the micro-to macroscopic mechanical behavior of polymer with randomly distributed voids under macroscopic compaction. A parametric study is performed to quantify the effect of the volume fraction of voids and the macroscopic stress triaxiality of loading conditions on the compaction behavior of porous polymer. The results suggest that the onset of localized shear band at the ligament between voids leads to the macroscopic yield of porous polymer. Furthermore, the microscopic localized shear deformation behavior is promoted in the polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, which results in the early appearance of the macroscopic yield. After the macroscopic yield, a remarkable strain hardening is shown in the macroscopic response of porous polymer under high macroscopic stress triaxiality loading condition, which is due to the onset and propagation of a large number of shear bands at the ligaments between voids. On the other hand, microscopic buckling develops at the narrowest ligament in the polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, which leads to the relative low macroscopic deformation resistance. Furthermore, to develop the macroscopic constitutive model of polymer with high-volume fraction of voids under high macroscopic stress triaxiality loading condition, a modified Gurson model is proposed which takes account of microscopic buckling and gives a good agreement with the unit cell computational results.

A study of kinetic effects due to using microfibrous entrapped zinc oxide sorbents for hydrogen sulfide removal

A modified Amundson model was proposed to characterize breakthrough curves during gas phase desulfurization using fixed bed reactors. It contains two characteristic variables: saturation time (τ) and shape factor (lumped K). The model has been verified experimentally at 400∘C. Combined with mass transfer correlations, the model was utilized to investigate kinetic influences due to using microfibrous entrapped sorbent (MFES) with high void volume fraction. Because of the utilization of nanosize ZnO grains supported on highly porous SiO2 particles (100–200μm), the intra-particle diffusion resistance of MFES was minimized resulting in a saturation capacity near the theoretical value. The microfibrous entrapment and the use of small sorbent particles enhanced the external mass transfer rate; the high void volume fraction reduced the capacity density resulting in increased overall lumped K. As a result, MFES demonstrated a longer breakthrough time, a sharper breakthrough curve, and higher ZnO utilization than ZnO extrudates tested at equivalent conditions. Moreover, MFES can be optimally used as a polishing layer at the downstream end of a packed bed to maximize the overall desulfurization performance.

Gold Nanoparticle/Polymer Nanocomposites: Dispersion of Nanoparticles as a Function of Capping Agent Molecular Weight and Grafting Density

The dispersion of polymer-covered gold nanoparticles in high molecular weight (MW) polymer matrixes is reported. Complete particle dispersion was achieved for PS125-Au in the polystyrene (PS) matrixes studied (up to and including Mn = 80 000 g/mol). PS19-Au, on the other hand, exhibits complete dispersion in a low MW PS matrix (Mn = 2000 g/mol) but only partial dispersion in higher MW matrixes (up to 80 000 g/mol). Similarly, PEO45-Au is fully dispersed in a low MW poly(ethylene oxide) (PEO) matrix (Mn = 1000 g/mol) but only partially in a higher MW PEO matrix (Mn = 15 000 g/mol). Wetting of the polymer-Au brushes by the polymer matrix is associated with dispersibility. Theory predicts that, for dense polymer brushes, wetting is achieved when the MW of the polymer brush equals (and is greater than) that of the polymer matrix. The observed partial dispersion of the PS19-Au and PEO45-Au nanoparticles in matrixes whose MW is greater than the brush MW is attributable to the existence of a high volume fraction of voids within the brush. These voids arise from the unique geometry of the nanoparticle surface arising from the juxtaposed facets of the gold nanoparticle. PS125-Au brushes are wetted by PS matrixes whose degree of polymerization is larger than 125, probably because of their lower grafting density on the gold core or the high fraction of void volumes caused by the facets on the gold cores. Dispersion thus occurs when the matrix MW is greater than that of the brush.

An investigation of shock-induced damage in oxygen-free high conductivity copper

Shock-induced damage in soft-recovered explosively formed projectiles (EFP) of oxygen-free high conductivity copper was investigated using transmission electron microscopy, scanning electron microscopy and optical microscopy. A high volume fraction of voids and extensive adiabatic shear bands were observed in the centre, around the longitudinal axis of the EFPs. The premature fracture of some projectiles was found to be caused by flow localization and the development of adiabatic shear bands in locations other than the centre of the projectiles. The fracture path followed the trace of a shear band.

Silver–magnesium fluoride cermet films 1: Preparation and microstructure

We have prepared and studied seventeen thin films ranging in composition from pure silver to pure magnesium fluoride. Using mass thicknesses based on quartz crystal oscillators and physical thicknesses obtained from a stylus profilometer, we have measured the void volume fraction of the films as a function of their composition. We have shown the Ag-MgF(2) system to be capable of incorporating a high volume fraction of voids. We report on the results of electron microscope studies of the films, which show the dependence of film microstructure on composition, thickness, and the nature of its substrate.

モンテカルロ法による延性破壊のシミュレーション解析

Ductile fracture analysis by Monte Carlo simulation is carried out for square model that contains randomly distributed voids under uniaxial tension. It is assumed that the coalescence of voids is the results of internal necking or shear crack between voids. Ductile fracture strain and surfaces are computed. The following results are obtained. (1) The coalescence between adjascent voids occurs by internal necking. The coalescent mechanism between comparatively remote voids is shear crack. Predicted fracture surfaces are coincident well with observed one. (2) At high volume fraction of voids, internal necking occurs predominantly. And decrease the volume fraction of voids, shear crack increases. (3) On the first stage of flow, internal necking tends to occur. On the latter stage of flow, shear crack occurs frequently. (4) Relation between ductile fracture strains and volume fraction of voids is in good agreement with the experimental results for copper dispersion alloys by Edelson and Baldwin.

Void Behavior as Influenced by Pressure and Plastic Deformation

Void behavior has a pronounced effect on the properties and soundness of most materials, and is strongly influenced by the magnitude of hydrostatic pressure during plastic deformation. Using the upper bound theorem approach with a model of idealized geometry to simulate a void-material composite, an analytical criterion for the minimum effective pressure necessary to initiate a permanent void volume change is developed. This pressure is called critical pressure, and its absolute value is the same whether for compression and void closure or tension and void opening. A deviation parameter is also defined, and it indicates that voids of a given geometry will start to open faster under tension than they will start to close under compression of the same magnitude. To compare the aforementioned analytical predictions with real material behavior, copper split billets with artificially introduced voids of predetermined geometries were deformed under different magnitudes of hydrostatic pressure: the process of wire drawing for low pressure deformation, and the fluid environment of hydrostatic extrusion for higher pressure deformation. Those characteristics found to lead to significant void volume change are: (a) high void volume fraction, (b) large relative void size, and (c) voids of unity or greater aspect ratio. Experimental data compared well with the analytical curves; thus, the analytical expressions should be useful in explaining and predicting the behavior of voids in some real materials.