Increase In Number Of Layers Research Articles

Unprecedented cell structure variation in multilayered alternating PS/PMMA foams

The objective of the present work was to study the cell structure variation in multilayered alternating polystyrene/poly(methyl methacrylate) (PS/PMMA) foams with the increase of the layer number. The multilayered PS/PMMA foams were prepared via the combination of the co-extrusion technique and the subsequent batch foaming process with carbon dioxide (CO2) as the foaming agent. Controlled with the layer-multiplying elements (LMEs) in the co-extrusion system, multilayered alternating PS/PMMA stacks with various layers (8, 16, 32, 64 and 128) were prepared. It is found that with the increase in layer number, three types of cell structures were observed: the alternating big-cell/small-cell layers, alternating foam/film layers, and the almost uniform cell structure. It is believed that the layer thickness is a crucial factor in governing the formation and hence the variation of the cell structure. For a higher layer thickness, the foaming of the adjacent PS and PMMA layers are independent on each other, thus generating the alternating big-cell/small-cell layers. For a relatively thin layer thickness, the probable gas transfer from the PMMA layers to PS layers during foaming would hinder the formation of the small cells in PMMA, thus bringing the alternating foam/film layers, i.e., bricklaying-like structure. For a very thin layer thickness, the heterogeneous cell nucleation induced by the incompatible interfaces would be favorable for formation of the uniform cell structure.

High Anisotropic Optoelectronics in Two Dimensional Layered PbSnX2 (X = S/Se).

We systematically study the giant anisotropic optoelectronics in layered PbSnX2 (X = S/Se). The highly anisotropic optoelectronics mainly originates from the asymmetric sublattices SnX, resulting in the anisotropy of photoelectronic properties with fascinating visible light absorption range in single-layer and bilayer PbSnX2. We employ uniaxial strain in both the x and y directions and find an indirect-to-direct band gap transition, while the quasiparticle indirect band gap presents excellent linear scaling with biaxial strain in monolayer PbSnX2. We also demonstrate ultrahigh anisotropic mobilities of electrons (μy > μx) and holes (μx > μy) in both single-layer and bilayer PbSnX2 (X = S/Se), and spin-orbit coupling effects and the increase of layer number significantly reduce exciton binding energies and band gaps. Finally, the strong layer dependence of the band structure is clearly seen when the film thickness is less than 4 layers. Our results provide a fundamental understanding of highly anisotropic PbSnX2 (X = S/Se) and show two potential candidates in photoelectric applications.

Layer effect on thermal expansion in blue phosphorene monolayer and few-layer

The thermal expansion properties of blue phosphorene monolayer, two stacking types of bilayers, and trilayer are investigated by first-principles calculations. It is found all the 2D materials possess negative thermal expansion properties at low temperatures. At high temperatures, only the monolayer retains negative thermal expansion, while the other three turn into positive thermal expansion. The increasing layer number weakens the negative thermal expansion for the 2D materials, resulting from the changes of Grüneisen parameter. The temperature dependent elastic properties are also investigated.

Precise Layer‐Dependent Electronic Structure of MBE‐Grown PtSe2

Abstract2D platinum diselenide (PtSe2) has received significant attention for 2D transistor applications due to its high carrier mobility. Here, using molecular beam epitaxy, the growth of 2D PtSe2 is investigated on highly oriented pyrolytic graphite (HOPG) and their electronic properties are unveiled via X‐ray photoelectron spectroscopy, Raman spectra, and scanning tunnelling microscopy/spectroscopy as well as density functional theory (DFT) calculations. PtSe2 adopts a layer‐by‐layer growth mode on HOPG and shows a decreasing bandgap with increasing layer number. For the layer numbers from one to four, PtSe2 has bandgaps of 2.0 ± 0.1, 1.1 ± 0.1, 0.6 ± 0.1, and 0.20 ± 0.1 eV, respectively, and becomes semimetal from the fifth layer. DFT calculations reproduce the layer‐dependent evolution of both the bandgap and band edges, suggest an indirect bandgap structure, and elucidate the underlying physics at the atomic level.

Interfacial Friction Anisotropy in Few-Layer Van der Waals Crystals.

Friction anisotropy is one of the important friction behaviors for two-dimensional (2D) van der Waals (vdW) crystals. The effects of normal pressure and thickness on the interfacial friction anisotropy in few-layer graphene, h-BN, and MoSe2 under constant normal force mode have been extensively investigated by first-principle calculations. The increase of normal pressure and layer number enhances the interfacial friction anisotropy for graphene and h-BN but weakens that for MoSe2. Such significant deviations in the interfacial friction anisotropy of few-layer graphene, h-BN and MoSe2 can be mainly attributed to the opposite contributions of electron kinetic energies and electrostatic energies to the sliding energy barriers and different interlayer charge exchanges. Our results deepen the understanding of the influence of external loading and thickness on the friction properties of 2D vdW crystals.

에폭시 수지가 적용된 유리섬유 복합재료의 극저온 환경 기계적 특성 분석을 통한 LNG CCS 적용성 평가

Consumption of Liquefied Natural Gas (LNG) has increased due to environmental pollution; therefore, the need for LNG carriers can efficiently transport large quantities of LNG, is increased. In various types of LNG Cargo Containment System (CCS), Membrane-type MARK-Ⅲ composed of composite materials is generally employed in the construction of an LNG carrier. Among composite materials in a Mark-Ⅲ system, glass-fiber composites act as a secondary barrier to prevent the inner hull structure from leakage of LNG when the primary barrier is damaged. Nevertheless, several cases of damage to the secondary barriers have been reported and if damage occurs, LNG can flow into the inner hull structure, causing a brittle fracture. To prevent those problems, this study conducted the applicability assessment of composite material manufactured by bonding glass-fiber and aluminum with epoxy resin and increasing layer from three-ply (triplex) to five-ply (pentaplex). Tensile tests were performed in five temperature points (25, -20, -70, -120, and -170°C) considering temperature gradient in CCS. Scanning Electron Microscopy (SEM) and Coefficient of Thermal Expansion (CTE) analyses were carried out to evaluate the microstructure and thermos-mechanical properties of the pentaplex. The results showed epoxy resin and increasing layer number contributed to improving the mechanical properties over the whole temperature range.

Layer-by-layer electrostatic self-assembled coatings based on flaxseed gum and chitosan for Mongolian cheese preservation

Mongolian cheese, a daily essential dairy product of Mongolians, is nutritious but susceptible to microbial contamination. Edible coatings consisted of casein, chitosan and flaxseed gum were developed to extend the shelf life of Mongolian cheese by self-assembly on its surface. Due to the amphoteric properties of casein, positive and negative charges were obtained as a template by adjusting the isoelectric points. With chitosan as the positive charge provider and flaxseed gum as the negative charge provider, edible coatings were formed on the surface of Mongolian cheese by layer-by-layer electrostatic self-assembly technology. Chitosan inhibited the proliferation of internal microorganisms (molds and yeasts) when it was the innermost layer, and resisted the invasion of foreign microorganisms (Escherichia coli and Staphylococcus aureus) while it was the outermost layer. And, this effect became stronger with the increase of layer numbers. The layer-by-layer electrostatic self-assembly technology of polysaccharide not only improved the quality of Mongolian cheese but extended its shelf life.

On the prospects of layeredness in tantalum pentoxide

First principles calculations were carried out at different levels of theory to study the structural and electronic properties of Ta2O5 in bulk and slab periodicities. The material behaves as 2D layered material in slab periodicities with the average intra-layer TaO bond length 1.7 –1.9 Å and average interlayer distance 2.35 Å. The investigations also involved periodic energy decomposition analysis (pEDA), potential energy surface (PES) and phonon structure calculations. The electronic properties reveals that conduction band of the material comprises of Ta-d whereas the valence band is of O-p states. The bulk and monolayer are direct band gap semiconductors having respective values of band gap as 0.20 eV and 1.6 eV whereas the material with increase in number of layers exhibited metallic nature due to Ta-d states at Fermi level. The material stabilized as layered with calculated value of exfoliation energy as 30 meV/atom.

Evolution of the interface microstructure of short-period Cr/Ti multilayers with increase in number of bi-layers

Evolution of interface microstructure with addition of more number of layers has been investigated on Cr/Ti multilayer which is important for application as a mirror or monochromator in the “water window” soft X-ray spectral regime. Two sets of Cr/Ti multilayers at moderate and low bi-layer thickness regime have been prepared for this purpose using Ion Beam Sputtering technique. In one set, Cr/Ti multilayer with 25, 50, 60 and 100 numbers of bilayers with moderate bi-layer thickness of 3.8 nm have been deposited. In another set, four samples each having low bilayer thickness of 2.4 nm with 25, 50, 75 and 100 number of bi-layers have been prepared and the multilayers have subsequently been characterized with Specular X-ray reflectivity and diffuse X-ray scattering measurements at grazing incidence with hard X-rays. It is observed that cumulative effect of interface imperfections became more pronounced with increasing number of bilayers resulting in continuous increase in the interface width in both the thickness regimes; however, its evolution is not exactly same for the two sets of samples. Diffuse X-ray scattering result reveals that for the first set of samples, interface imperfections are dominated by interface roughness with no change in the interface diffusion with increase in number of layers, while for the second set, both interface roughness as well as interface diffusion contribute towards the cumulative accumulation of interface imperfections which is also confirmed by cross-sectional Transmission Electron Microscopy. Finally, performance of the multilayers in water window regime has been checked by soft X-ray reflectivity measurements using synchrotron radiation. The above results have direct implication in utilisation of these multilayers over a wide bi-layer thickness regime required to cover the full ‘water-window’ spectrum.

Mechanical properties of graphene, defective graphene, multilayer graphene and SiC-graphene composites: A molecular dynamics study

In this article, using molecular dynamics (MD) simulation, several mechanical properties of graphene were calculated for pristine single layer graphene sheet (SLGS), defective SLGS, multilayer graphene sheets (MLGS) without defects and for composites of SiC and graphene. Firstly, the room temperature elastic moduli (Young's, shear and bulk), constants and Poison's ratios were simulated for the above structures and then their properties such as stress vs strain under tensile and compressive loading were simulated within the temperature range of 300K–2000K. The simulations were performed in both zigzag and armchair directions of graphene surface and differences were observed in the mechanical properties. Simulations revealed that the location of defects as well as their type influence the elastic moduli and constants at room temperature and that their tensile and compressive properties vary with temperature. For both MLGS, and SiC-graphene composites, the properties improved with increase in number of continuous graphene layers.

Exchange interaction, disorder, and stacking faults in rhombohedral graphene multilayers

We apply the mean-field Hartree Fock theory of gapped electronic states at charge neutrality in bilayer graphene to thin films of rhombohedral graphite with up to thirty layers. For the ground state, the order parameter (the separation of bands at the valley center) saturates to a constant non-zero value as the layer number increases, whereas the band gap decreases with layer number. We consider chiral symmetry breaking disorder in the form of random layer potentials and chiral preserving disorder in the form of random values of the interlayer coupling. The former reduces the magnitude of the mean band gap whereas the latter has a negligible effect, which is due to self-averaging within a film with a large number of layers. We determine the ground state in the presence of an individual stacking fault which results in two pairs of low-energy bands and we identify two separate order parameters. One of them determines the band gap at zero temperature, the other determines the critical temperature leading, overall, to a temperature dependence of the band gap that is distinct to that of pristine rhombohedral graphite. In the presence of stacking faults, each individual rhombohedral section with $m$ layers contributes a pair of low-energy flat bands producing a peak in the Berry curvature located at a characteristic $m$-dependent wave vector. The Chern number per spin-valley flavor for the filled valence bands in the ground state is equal in magnitude to the total number of layers divided by two, the same value as for pristine rhombohedral graphite.

Epoxy resin composite containing nanocarbon-coated glass fiber and cloth for electromagnetic interference shielding

Functional composite materials (FCMs) play an important role in many areas including the aerospace, and transportation industries as well as in domestic appliances. In this study, two types of FCMs were prepared using nanocarbon-coated glass fibers (NCGFs) and glass fiber cloths (NCGFCs) as reinforcements and epoxy resin as the matrix. Nanocarbon coatings of glass fiber cloths have good electrical conductivity, providing the FCMs with the electrical and thermal conductivity and electromagnetic shielding performance. Moreover, an increase in NCGF content and number of NCGFC layers improves the conductive, thermal and electromagnetic shielding properties of the FCMs. The electrical conductivity of the FCM containing 20 wt.% NCGFs reaches 6.68 × 10−6 S/m. The electromagnetic shielding properties of FCM containing both NCGF and NCGFC exceeds 20 dB. Overall, this study provides a new method for constructing fiber weave shielding composites and expands the application field of glass fibers reinforced composites.

Optical and Electronic Properties of Atomically Thin Organic-WSe2 Hybrid Structures

Two-dimensional (2D) transition metal dichalcogenides such as WSe2 have garnered much attention due to their extraordinary layer-number-dependent optoelectronic properties. To exploit the unique properties for novel optoelectronic applications, it is important to construct heterojunctions and understand their layer-number-dependent characteristics. In this work we have investigated heterojunctions made of 2D WSe2 flakes decorated with organic dye molecules elucidated the optical and electronic properties as a function of layer number. We introduce a facile one-step approach to deposit a uniform layer of eosin Y (EY) molecules on various number of WSe2 layers, forming hybrid heterojunctions. Photoconductive atomic force microscopy (PC-AFM), Kelvin probe force microscopy (KPFM), and photoluminescence (PL) spectroscopy probed the organic/inorganic heterojunctions. The hybrid structures show drastically enhanced photocurrents due to the lowered barrier height by the staggered band alignment. The barrier heights of the hybrid structures monotonically decrease as the layer number increases. The surface potential measurements reveal that the work functions of the hybrids are smaller than those of the pristine WSe2 samples. Moreover, the hybrid samples show spatially uniform current and photocurrent responses, indicating the high quality and uniform heterojunctions. Lastly, the hybrid samples show a drastic PL quenching due to p-doing on WSe2. These findings will open new opportunities for novel WSe2-based 2D heterostructured optoelectronics.

Effect of lithological rhythm of a confined hydrate reservoir on gas production performance using water flooding in five-spot vertical well system

The lithological features of sediments control the heterogeneity in permeability. This study numerically investigates the effect of lithological rhythm of a confined hydrate reservoir on gas production for warm water injection scenarios. The investigated lithological rhythm structures range from simple rhythms to complex composite rhythms. The depressurization in multiple wells causes significant gas production. However, the decrease in pressure gradient constrained the production of gas and water. The warm water injection enhances the production of gas and water with various lag times, though it results in a low energy efficiency. The positive and reverse rhythms have similar energy efficiencies. The gas and water production rate slightly decrease with the increase in the permeability contrast. The maximum gas production rate increases with the increase in number of composite rhythmic layers. However, the water production changes only slightly. The rhythm structures significantly affect the gas and water production rates. A high permeability region in the bottom part of the reservoir is conducive to warm water breakthrough. However, it is unfavorable for the dissociation of remnant hydrate in low permeability regions. Moreover, the results show that the rhythm structures also affect the gas productivity of each rhythmic layer. The results presented in this study help us understand the effect of complex and real rhythm structures on the gas production performance and the dynamic evolution of hydrate dissociation zone.

Experimental and Numerical Investigation on Undrained Behavior of Geogrid Reinforced Pond Ash

Geogrid reinforced pond ash is an effective composite material, which can be utilized as a construction material for various geotechnical engineering purposes. In this study, both experimental and numerical investigations have been carried out on shear strength characteristics of unreinforced and geogrid reinforced pond ash samples under undrained condition. Consolidated undrained (CU) triaxial tests have been performed on 100 mm x 200 mm cylindrical ash samples under static compressive loading. The investigations include the influence of confining pressure and number of geogrid layers on stress–strain characteristics of pond ash. Finite element package PLAXIS 2D was used for numerical simulation of unreinforced and geogrid reinforced pond ash models under undrained condition. Both experimental and numerical results show that the peak deviatoric stress increases as confining pressure increases for both unreinforced and reinforced ash samples. For a particular confining pressure, with increase in number of geogrid layers the value of peak deviatoric stress increases and its corresponding strain decreases. The increase in peak deviatoric stress is about 27%–93% for one-layer geogrid reinforced sample and 37%–117% for two-layer geogrid reinforced sample compared to unreinforced ash sample. Finite element analysis shows a good match in the stress–strain plot with the experimental result.

Designing high thermal conductivity of polydimethylsiloxane filled with hybrid h-BN/MoS2 via molecular dynamics simulation

With the rapid development of modern electronic technology, addressing the heat dissipation problem of devices has become more and more urgent. Herein, the effects of molybdenum disulfide (MoS2) and hybrid hexagonal boron nitride (h-BN)/MoS2 fillers on the thermal conductivity of polydimethylsiloxane (PDMS) were investigated by reverse non-equilibrium molecular dynamics (RNEMD) simulation with fully atomistic model. Firstly, we simulated the thermal conductivity of MoS2 and BN fillers. It was found that the thermal conductivity of the two fillers enhanced with the increase of their sizes. The intrinsic thermal conductivity of MoS2 and BN obtained via the extrapolation were 52 W/(m·K) and 606 W/(m·K) separately. For 1–5 layers, the number of layers has almost no effect on the thermal conductivity of MoS2, but as the number of layers increase, the thermal conductivity of BN first decreases and then reaches a plateau. Then, we studied the thermal conductivity of the PDMS-based composites. Due to the existence of holes in the PDMS/MoS2 system, the thermal conductivity of PDMS/MoS2 composites is not much improved compared with that of PDMS. Therefore, we then introduced a hybrid BN/MoS2 filler into the composite to solve the void problem in the system. The results proved that this method was effective. The introduction of the hybrid BN/MoS2 filler effectively promoted the dispersion of the two fillers in the PDMS matrix and greatly reduced the holes in the composites. Compared with the composites filled with only one kind of filler, PDMS/BN/MoS2 composites possess better thermal conductivity. The most surprising finding is that there exists an optimal loading ratio of the two fillers, and the thermal conductivity of the composite reached the maximum when the mass ratio of the two fillers is approximately 1:1, attributed to the change of the volume fraction of holes (Φ). In addition, with the increase of hybrid BN/MoS2 filler content, the thermal conductivity of the composites is also increasing. The oscillating shear can also promote the orientation of the filler along the shear direction and improve the thermal conductivity of composites along that direction. Therefore, composites based on hybrid BN/MoS2 nano-fillers can be used to improve the thermal conductivity of insulating polymer materials. The findings should shed some light on the design and preparation of polymer composites with excellent thermal conductivity and electrical insulation properties.

Grain selection and growth orientation of prior-β phase for Ti-6-4 during additive manufacturing: insights from a modeling perspective

The microstructure of Ti-6-4 components produced by additive manufacturing suffers from the coarse and elongated prior-β grain, which leads to a decrease of the tensile behavior and the occurrence of anisotropy. To understand and control the grain evolution, a multiscale simulation is applied to investigate the relationship between the grain selection, growth orientation, and the molten pool morphology with the different deposition layer numbers and processing parameters. The accuracy of the simulation is validated by experiments in both qualitative and quantitative ways. Results show that when the grain with unfavorable orientation loses the competitive growth with its neighbors, there will be a great chance that the blocked grain is eliminated in the following layer-and-layer deposition, which leads to the increase of the grain width. The size of the molten pool increases remarkably as the layer number increases, which lays a heavy burden on the stability of the molten pool. The analytical relationship between the molten pool morphology and the grain growth orientation is also deduced. The flat molten pool causes the grains with the direction close to the building direction to have greater survival potential. Besides, decreasing the line power energy shows little effect on the stability of the molten pool and the grain growth orientation, especially when the deposited layer number is large. The revealing mechanisms will help in understanding and further controlling the grain evolution.

The eruption of carbon chains in the oxidation of 2D Tin+1Cn (n = 1, 2, 3) MXenes

The oxidation of MXenes has arousedconsiderable interest andattention in recent years, which is closely related to the synthesisprocessand environmental stability. However, there still remain many issues unclear about the oxidation of MXenes including the exquisite observation of the atomic reactions, and the influences of oxidizing gas and layer numbers. By combining reactive force field (ReaxFF) molecular dynamics simulations and density functional theory (DFT) calculations, the oxidation characteristics and mechanism of MXenes have been investigated. Herein, the oxidation of MXenes preferentially begins with O2 adsorption on the surface Ti layers, inducing the lattice distortion of Ti layers and the fracture of Ti-C bonds. Without the constraint from Ti atoms, the free C atoms reassemble to generate carbon chains inside the sheet. Surprisingly, the chains could erupt out of the sheet although some obstruction would appear. As a result of the reformation of Ti-C bonds and the blocking of interstitial O atoms, a few chains would be stuck inside. Besides, the layer number has great effects on the oxidation kinetics. As the layer number increases, the adsorption of O2 becomes easier but the eruption of carbon chains becomes weaker. Our findings would be instructive in enhancing corrosion resistance of MXenes and the preparation of oxidized MXenes.

Effect of laser energy per unit powder feed on Hastelloy-X walls built by laser directed energy deposition based additive manufacturing

Laser Directed Energy Deposition (LDED), an additive manufacture process, has many inherent advantages and is attractive particularly for fabrication of thin walled components. The present work focuses on LDED of Hastelloy-X (Hast-X) walled structures built at three different values of Laser Energy per unit Powder Feed (LEPF), i.e., 8.2 kJ/g, 10.7 kJ/g and 13.2 kJ/g. The effect of LEPF on the built walls is examined for the geometry, microstructure and mechanical properties. The variations in built wall geometry are analyzed using statistical tools along the length, cross-section and build direction. The maximum deviation along wall height and width are found at LEPF of 13.2 kJ/g and 8.2 kJ/g, respectively. The examination along the build direction shows the maximum deviation in profile at lower layers with 8.2 kJ/g and at top layers with 13.2 kJ/g. The defect free deposits having dendritic growth at all conditions is observed. The dendrites size increases with increase in LEPF. A marginal reduction in mechanical properties, i.e., average microhardness (~8%), yield strength (~3.7%) and ultimate strength (~4%) are observed with increase in LEPF. Further, temperature distribution from finite element analysis based numerical simulation is used to understand the effect of LEPF on the wall structures. An increase in preheat temperature on previous layers with increase in number of layers and LEPF is observed. The study provides an understanding on the effect of process parameters on LDED built Hast-X wall structures.

Probing interlayer interaction via chiral phonons in layered honeycomb materials

van der Waals (vdW) interaction plays a significant role in controlling the physical properties of layered materials. Typically, the vdW interlayer interaction can be calculated by density functional theory or experimentally characterized by quantum capacitance measurement. Here, we report the probing of the interlayer interaction in layered honeycomb materials via chiral phonons. Through helicity resolved Raman measurements, we observed a reduced chirality of the Raman $G$ mode with increasing layer numbers. We introduced interlayer coupling terms into the traditional Raman $G$ mode tensor to simulate the reduced phonon chirality in Raman spectra. Our Raman tensor calculation results agree with the experiments well, suggesting that the interlayer interaction can significantly influence the lattice vibration. Our demonstration provides a perspective for characterizing the interlayer interactions in vdW layered materials with honeycomb lattice structure.