Microporous Defects Research Articles

Effect of Process Parameters and Layer Thickness on the Quality and Performance of Ti-6Al-4V Fabricated by Selective Laser Melting

To improve the quality of thick powder bed and realize the matching of thick powder bed and thin powder bed in the later stage, the influence of process parameters for the single-track, multi-layer fabrication, relative density, surface quality, defect, remelting, and boundary optimization performance of different layer thicknesses of Ti-6Al-4V fabricated by selective laser melting were investigated. It is more conducive to the stable forming of single-track when the point distance is half the diameter of the laser beam, and the exposure time is appropriately extended. The thin powder bed needs the corresponding point distance and exposure time under the laser power of 280–380 W to obtain high-density specimens. The thick powder bed needs to be able to ensure the formation of high-quality specimens under the smaller point distance and longer exposure time under higher laser power of 380 W. Both thick powder bed and thin powder bed will cause un-melted defects between molten pools, spheroidization defects caused by splashing, and microporous defects. The remelting process can significantly improve the surface quality of the formed specimen, but the surface quality of the thick powder bed is worse than that of the thin powder bed. The boundary quality of thick powder bed is worse than that of thin powder bed, and the boundary shape has a greater influence on the quality of the SLM forming boundary. Different strategies should be adopted to form the boundary of different shapes. Increasing the boundary count and increasing the laser power are more conducive to the improvement of boundary quality.

Repairing of graphene oxide membranes based on SPEEK substrate for organic solvents nanofiltration through PEI needle thread method

Owing to the increasing need to mitigate excessive organic solvents waste, the efficient separation and purification of organic solvents have attracted more attention. Carbon nanomaterials have attracted great attention in separation and purification technology. However, fabrication of GO-based organic solvents nanofiltration (OSN) membranes with stable and high effective separation performance is still a challenge due to their tendency to swell and inevitable GO interlaminar defects. In this work, we presented stable and crosslinked GO membranes based on (sulfonated polyether ether ketone) SPEEK substrate via polyethyleneimine (PEI) repairing for the first time. Specifically, the hydrogen bonds between the SPEEK substrate and GO eliminated interface incompatibility. The mitigation of microporous defects via PEI endowed the membranes with good permselectivity. A facile glutaraldehyde (GA) crosslinking restricted the expansion of the GO layer to promise high selectivity and long-term stability in organic solvents. The elemental analysis and Nano IR were used to evaluate the appearance of PEI among the GO interlayer. The resulting membranes exhibited ultrahigh flux for pure solvents (30, 27.16, 13.88, 9.94, 7.42 L m−2h−1bar−1 for water, acetone, methanol, ethanol, and IPA) and good dyes molecular separation performance (over 99%). Meanwhile, the membranes could highly separate mixed dyes with different molecular weights. Most importantly, long-term filtration operation and ultrasonic processing further proved the stability of the membranes. Our technique opens opportunities for fabricating stable and high effective membranes with carbon nanomaterials in organic solvents.

Ultralight hard carbon nanotubes nanofiber foam/epoxy nanocomposites for comprehensive microwave absorption performance

AbstractIn this study, a novel three‐dimensional carbon nanotubes (CNTs) structure was constructed using a phenolic resin template method, which has a multilevel microporous structure and a woven CNTs network microstructure. The CNTs nanofiber foam/epoxy composite was prepared by a vacuum impregnation method. Due to the multiple reflections and interface effects produced by the special structure, the best microwave absorption effect is achieved with a lower CNTs content. When the CNTs content is 0.92 wt%, the lowest reflection loss is −42.13 dB (6.6 GHz, 2.9 mm). Under the premise of not adding magnetic loss materials, only CNTs are used as a reinforcing material, and a resin matrix composite with excellent electromagnetic wave absorption performance without micropore defects is obtained. This method has universal applicability and broadens the further application of resin matrix composite in the aerospace and electronic fields.

Laser Welding of Laser Powder Bed Fusion Manufactured Inconel 718: Microstructure and Mechanical Properties

This paper focuses on the laser weldability of additively manufactured (AM) Inconel 718. The experiments of this research were conducted on different series of AM Inconel 718 alloy, i.e. as­-built, heat­ treated (HT), and HT after welding, and comprehensively characterized using optical microscope and electron back scattering diffraction (EBSD). The weld morphology and microstruc­tural evolution of the fusion zone were recorded. The mechanical properties of the welded AM Inconel 718 were evaluated by tensile tests and hardness measurements. It was found that solidification crack and micropore defects are induced in the as­built AM Inconel 718. However, defect­free weld was promoted in the HT alloy. The changes in hardness profiles and tensile strength under the experimen­tal parameters were further reported. Homogenous hardness of 500 HV across the weld was obtained when HT was applied after the LW.

Graphene oxide membranes with stable porous structure for ultrafast water transport.

The robustness of carbon nanomaterials and their potential for ultrahigh permeability has drawn substantial interest for separation processes. However, graphene oxide membranes (GOms) have demonstrated limited viability due to instabilities in their microstructure that lead to failure under cross-flow conditions and applied hydraulic pressure. Here we present a highly stable and ultrapermeable zeolitic imidazolate framework-8 (ZIF-8)-nanocrystal-hybridized GOm that is prepared by ice templating and subsequent in situ crystallization of ZIF-8 at the nanosheet edges. The selective growth of ZIF-8 in the microporous defects enlarges the interlayer spacings while also imparting mechanical integrity to the laminate framework, thus producing a stable microstructure capable of maintaining a water permeability of 60 l m-2 h-1 bar-1 (30-fold higher than GOm) for 180 h. Furthermore, the mitigation of microporous defects via ZIF-8 growth increased the permselectivity of methyl blue molecules sixfold. Low-field nuclear magnetic resonance was employed to characterize the porous structure of our membranes and confirm the tailored growth of ZIF-8. Our technique for tuning the membrane microstructure opens opportunities for developing next-generation nanofiltration membranes.

Study on performance optimization of 316L stainless steel parts by High-Efficiency Selective Laser Melting

In this study, the layer thickness in the Selective Laser Melting (SLM) process was increased to 200 µm to solve the problem of the low build rate of SLM. And adopted the heat treatment process to improve the mechanical properties of specimens. The single-tracks and multi-layer fabrication experiments of the aerosolized 316L stainless steel powder were carried out by using 400 W laser power. The process parameters were optimized by analyzing the density, the microstructure, the tensile properties and the defect mechanism of the molded parts. The experiment results showed that when the exposure time was 120–160 μs and the overlap rate was 30–50%, the relative densities of the samples could be higher, up to 99.99%. The un-melted defects, the balling defects, and the micropore defects were observed in the SLM process. Un-melted defects can be avoided by adjusting the distance between the points and the hatch spacing. Balling included large balling and spatter balling. The large balling can be completely avoided by adjusting the process parameters. The spatter balling was divided into droplet spatter and powder spatter, which cannot be completely eradicated but can be inhibited by decreasing the exposure time. The build rate can be enhanced to 12.4 mm3/s. By the heat treatment process, the yield strength, the tensile strength and the elongation of the sample can reach 665 MPa, 718 MPa, and 35%, respectively.

Efficient three-dimensional characterization of C/C composite reinforced with densely distributed fibers via X-ray phase-contrast microtomography

Carbon fiber (CF)/pyrolytic graphite (PG) composites are promising structural materials for molten salt reactors because of their superior performance. Due to the minor density difference between CF and PG, existing methods are impractical for efficient three-dimensional characterization of CF/PG composites. Therefore, in this study, a method based on in-line phase-contrast X-ray microtomography was developed to solve the aforementioned problem. Experimental results demonstrate that the method is suitable for comprehensive characterization of CF/PG composites. The relationship between the microporous defects and fiber orientations of such composites was also elucidated. The findings can be useful for improving the manufacturing process of CF/PG composites.

Superhydrophobic ZIF-8-Based Dual-Layer Coating for Enhanced Corrosion Protection of Mg Alloy.

Magnesium (Mg) and its alloys are regarded as the most promising engineering materials because of their unique property. However, the Mg alloys were easily corroded in humid environments, which restricted their wider applications. Herein, the superhydrophobic ZIF-8/PVDF/LDH (SZPL) double-layered coating was fabricated on Mg alloys via electrodeposition and dip-coating methods, which consisted of the underlying layered double hydroxide (LDH) transition structure and top superhydrophobic zeolitic imidazolate framework-8 (ZIF-8) layer. Besides, the LDH transition structure not only worked as a protection shield but also strengthened the binding force between the substrate and the top superhydrophobic ZIF-8 layer. The top superhydrophobic ZIF-8 layer could serve as an armor on the LDH layer to further prevent the corrosive ions from infiltrating the microporous defects. In addition, the as-prepared SZPL double-layered coating showed robust superhydrophobic and self-cleaning properties, which could block the electrolyte invasion. Furthermore, the electrochemical tests demonstrated that the SZPL coating highly enhanced the corrosion protection ability of Mg alloys. Moreover, the superhydrophobic ZIF-8-based coating could still retain excellent anticorrosion property after immersion in 3.5 wt % NaCl solution for 7 days. The enhanced anticorrosion ability was ascribed to the fact that a synergistic effect of the underlying LDH transition layer hindered the transmission of aggressive ions and the top superhydrophobic ZIF-8-based coating decreased the contact area of the substrate with corrosive solution. Therefore, such coatings offer a new strategy for fabricating excellent anticorrosive coatings with robust superhydrophobicity and self-cleaning performance on metal substrates.

Relating Selectivity and Separation Performance of Lamellar Two-Dimensional Molybdenum Disulfide (MoS2) Membranes to Nanosheet Stacking Behavior.

Increased demand for highly selective and energy-efficient separations processes has stimulated substantial interest in emerging two-dimensional (2D) nanomaterials as a potential platform for next-generation membranes. However, persistently poor separation performance continues to hinder the viability of many novel 2D-nanosheet membranes in desalination applications. In this study, we examine the role of the lamellar structure of 2D membranes on their performance. Using self-fabricated molybdenum disulfide (MoS2) membranes as a platform, we show that the separation layer of 2D nanosheet frameworks not only fails to demonstrate water-salt selectivity but also exhibits low rejection toward dye molecules. Moreover, the MoS2 membranes possess a molecular weight cutoff comparable to its underlying porous support, implying negligible selectivity of the MoS2 layer. By tuning the nanochannel size through intercalation with amphiphilic molecules and analyzing mass transport in the lamellar structure using Monte Carlo simulations, we reveal that small imperfections in the stacking of MoS2 nanosheets result in the formation of catastrophic microporous defects. These defects lead to a precipitous reduction in the selectivity of the lamellar structure by negating the interlayer sieving mechanism that prevents the passage of large penetrants. Notably, the imperfect stacking of nanosheets in the MoS2 membrane was further verified using 2D X-ray diffraction measurements. We conclude that developing a well-controlled fabrication process, in which the lamellar structure can be carefully tuned, is critical to achieving defect-free and highly selective 2D desalination membranes.

Micropore-Boosted Layered Double Hydroxide Catalysts: EIS Analysis in Structure and Activity for Effective Oxygen Evolution Reactions.

Since the oxygen evolution catalysis process is vital yet arduous in energy conversion and storage devices, it is highly desirous but extremely challenging to engineer earth-abundant, noble-metal-free nanomaterials with superior electrocatalytic activity toward effective oxygen evolution reactions (OERs). Herein, we construct a prismlike cobalt-iron layered double hydroxide (Co-Fe LDH) with a Co/Fe ratio of 3:1 utilizing a facile self-templated strategy. Instead of carbon-species-coupled treatment, we focus on ameliorating the intrinsic properties of LDHs as OER electrocatalysts accompanied by the hierarchical nanoflake shell, well-defined interior cavity, and numerous microporous defects. In contrary to conventional LDHs synthesized via a one-pot method, Co-Fe LDHs fabricated in this work possess a huge specific surface area up to 294.1 m2 g-1, which not only provides abundant active sites but also expedites the kinetics of the OER process. The as-prepared Co-Fe LDH electrocatalysts exhibit advanced electrocatalytic performance and a dramatic stability of the OER in an alkaline environment. In particular, the contribution of micropore defects is clearly discussed according to the electrochemical impedance spectroscopy analysis, in which the time constant of the OER at the micropore defect is several orders of magnitude smaller than that at the exterior of Co-Fe LDHs, forcefully verifying the intrinsic catalytic activity enhancement derived from the micropore defects. This work provides a promising model to improve OER electrocatalyst activity via produce defects and research the contribution of micropore defects.

Single iron atoms stabilized by microporous defects of biomass-derived carbon aerogels as high-performance cathode electrocatalysts for aluminum–air batteries

Biomass-derived carbon aerogel with hierarchical porosity and FeN4 single atom sites outperforms platinum towards the oxygen reduction reaction in alkaline media and can be used as the cathode catalyst for aluminium–air batteries.

Development process of micropores partial discharge of silicon rubber in prefabricated cable joint

Abstract To clarify the relationship between microporous defects and the development of partial discharge(PD) in silicone rubber, meanwhile evaluate insulation state of the prefabricated power cable joints quantitatively, the microporous defect is made based on real power cable joint, and PD signals are measured by step test method with the development of the defect. The discharge number, total energy and average energy of PD is gained to calculate the curves changed over time, based on which the breakdown process is divided into 4 stages, then phased spectra and grayscale maps are calculated for reflection of PD repetition rate and average energy in phase domain. Phase width, skewness of spectrum and coefficient of variation of parameter are employed to indicate the spectra statistical shape feature in each stage. The study indicates that the shape features of spectra are changed during the development of defect, the 3 features parameters are monotonously changed, and the changing rate is significantly higher at the end of the breakdown process. Evaluation method for micropores PD of silicon rubber is proposed based on the analysis of the parameters and the trend of PDs development.

A thin-film nanocomposite nanofiltration membrane prepared on a support with in situ embedded zeolite nanoparticles

Abstract A novel approach to fabricating thin-film nanocomposite (TFN) nanofiltration membranes was reported in this study. It involved the preparation of a polysulfone support in situ embedded with zeolite nanoparticles followed by interfacial polymerization to form the polyamide layer. Compared with the TFN membranes prepared by the conventional method (TFN-C), the new TFN membrane (TFN-I) had higher loading and more uniform dispersion of nanoparticles in the polyamide layer. The nanoparticles incorporation resulted in an increase of surface roughness but no change of surface hydrophilicity. The TFN-I membrane doubled the water permeability compared with the control membrane (TFC). The TFN-I membrane had a similar rejection of MgSO4 (>93% at 150 psi) and negatively charged pharmaceuticals (PhACs) with TFC, but a reduced rejection of NaCl and a slightly lower rejection of neutral and positively charged PhACs of small molecular weights. The TFN-I membrane performed much better than the TFN-C membrane. The demonstrated performance of TFN-I could be due to the internal pores of zeolite nanoparticles, the increased membrane surface roughness and, though undesirable, the microporous defects between the nanoparticles and the polyamide matrix. The newly developed approach is highly promising for the fabrication of TFN nanofiltration and reverse osmosis membranes of improved performance by incorporating various nanoparticles.

Defect‐Minimized PEDOT:PSS/Planar‐Si Solar Cell with Very High Efficiency

Hybrid solar cells made of a p‐type conducting polymer, poly(3,4‐ethyl thiophene):polystyrenesulfonate (PEDOT:PSS), on Si have gained considerable interest in the fabrication of cost‐effective high‐efficiency devices. However, most of the high power conversion efficiency (PCE) performances have been obtained from solar cells fabricated on surface‐structured Si substrates. High‐performance planar single‐junction solar cells have considerable advantages in terms of processing and cost, because they do not require the complex surface texturing processes. The interface of single‐junction solar cells can critically influence the performance. Here, we demonstrate the effect of adding different surfactants in a co‐solvent‐optimized PEDOT:PSS polymer, which, in addition to acting as a p‐layer and as an anti‐reflective coating, also enhances the device performance of a hybrid planar‐Si solar cell. Using time‐of‐flight secondary ion mass spectrometry, we conduct three‐dimensional chemical imaging of the interface, which enables us to characterize the micropore defects found to limit the PCE. Upon minimizing these micropore defects with the addition of optimized amounts of fluorosurfactant and co‐solvent, we achieve a PEDOT:PSS/planar‐Si cell with a record high PCE of 13.3% for the first time. Our present approach of micropore defect reduction can also be used to improve the performance of other organic electronic devices based on PEDOT:PSS.

Reprint of: An experimental study of micropore defects in MFI membranes

In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data.

An experimental study of micropore defects in MFI membranes

In the present work, two ultra-thin MFI membranes, prepared using hydroxide and fluoride ions as mineralizing agents, respectively, were carefully examined by permporometry. The amount of micropore defects, as determined by permporometry, differed significantly between the two different membranes. For the first time, it was demonstrated that the micropore defects determined by permporometry were most likely open grain boundaries. The results were verified by direct observation of the open grain boundaries by a state-of-the-art XHR-scanning electron microscopy instrument. In addition, the permporometry data were also consistent with permeation data using 1,3,5-trimethylbenzene (TMB) as a probe molecule, separation data using an equimolar mixture of n-hexane and TMB, and nitrogen adsorption data.

Interfacial Micropore Defect Formation in PEDOT:PSS-Si Hybrid Solar Cells Probed by TOF-SIMS 3D Chemical Imaging

Conducting p-type polymer layers on n-type Si have been widely studied for the fabrication of cost-effective hybrid solar cells. In this work, time-of-flight secondary ion mass spectrometry (TOF-SIMS) is used to provide three-dimensional chemical imaging of the interface between poly(3,4-ethylene-dioxythiophene):polystyrenesulfonate (PEDOT:PSS) and SiOx/Si in a hybrid solar cell. To minimize structural damage to the polymer layer, an Ar cluster sputtering source is used for depth profiling. The present result shows the formation of micropore defects in the interface region of the PEDOT:PSS layer on the SiOx/Si substrate. This interfacial micropore defect formation becomes more prominent with increasing thickness of the native oxide layer, which is a key device parameter that greatly affects the hybrid solar cell performance. Three-dimensional chemical imaging coupled with Ar cluster ion sputtering has therefore been demonstrated as an emerging technique for probing the interface of this and other polymer-inorganic systems.

Alumina Hollow Fiber Supported ZIF-7 Membranes: Synthesis and Characterization

ZIF-7 membrane has been prepared on the outer surface of alumina hollow fibers by the solvothermal synthesis. The synthesis conditions, including reaction temperature, time, and solution concentration, are investigated. At the reaction temperature of 100 degrees C for 8 h, microsized ZIF-7 crystals are grown on the outer surface of the hollow fibers. A continuous and dense ZIF-7 membrane with a thickness of about 5 microm is obtained after twice crystallization at 100 degrees C for 8 h. The gas separation test indicates the as-prepared ZIF-7 membranes have intercrystalline defects as the H2/N2 ideal selectivity is less than the Knudsen diffusion. After post-modification with beta-cyclodextrin aqueous solution, the micropore defects of ZIF-7 membrane is significantly reduced and the membrane has H2/N2 ideal selectivity of 3.9. The gas permeances for H2 and N2 are almost constant from room temperature to 150 degrees C for the ZIF-7 hollow fiber membranes before and after beta-cyclodextrin modification.

Characterization of flow-through micropores in MFI membranes by permporometry

Abstract Permporometry was used for the first time to characterize flow-through micropore defects down to 0.7 nm in size in MFI zeolite membranes. Helium was used as the non-adsorbing gas and n-hexane or benzene was used as the adsorbate. The helium flow through zeolite pores was estimated using percolation theory and the remaining flow was assigned to flow-through defects. The area distribution of flow-through defects was estimated from the data using a simple model and similar results were obtained using both adsorbates. The total area of defects determined using n-hexane as the adsorbate was as low as about 0.7% of the membrane area and defects with a width below 1 nm constituted 97% of the total defect area for the best membrane. The permporometry results were supported by n-hexane/1,3,5-trimethylbenzene separation experiments. The permporometry data were also consistent with HR-SEM observations indicating the presence of narrow open grain boundaries, and absence of large cracks and pinholes.

Mordenite and ZSM-5 hydrophilic tubular membranes for the separation of gas phase mixtures

Water was selectively separated from water/He, water/HC and water/HC/He mixtures (HC being methane, propane or n-butane) using mordenite/ZSM-5/chabazite and ZSM-5 membranes. The membranes were prepared on both alumina and stainless steel tubular porous supports by in situ liquid phase hydrothermal synthesis. Water permeated faster because its preferential adsorption on the pores of the hydrophilic zeolite membranes; furthermore, capillary condensation in microporous defects between zeolite crystals is also possible under the experimental conditions used. The influence of the nature of the hydrocarbon in the mixtures and of the operating conditions (temperature and feed composition) on the separation performance of the zeolite membranes was analyzed.