Study on contact stresses of un‐stepped rounded D‐ring under uniform squeeze rate and internal pressure by photoelastic experimental hybrid method

The stresses that occur in an un‐stepped rounded D‐ring loaded with various internal pressures and compressed to a 20% squeeze rate are analyzed using photoelastic experimental hybrid method. The analysis shows that the contact stresses on both the upper and front sides of the D‐ring increase as the applied internal pressure increases with the magnitudes of σ_X and σ_Y being considerably higher than those of τ_XY. At a pressure of zero and 20% compression, the isochromatic fringe patterns of the un‐stepped D‐ring are almost symmetrical. However, as the internal pressure applied to the D‐ring increases the isochromatic fringe patterns shift and curve smoothly towards the extrusion gap making the upper and front sides to be more stressed than the lower side. At a pressure of 1.96 MPa and above, the contact stresses on the upper side were found to be almost constant along the contact length. By supplying a fillet radius of 2.1 mm at the corners of the un‐stepped D‐ring the high stresses at the corners were reduced by up to 45%. The results from the study further demonstrated that the un‐stepped rounded D‐ring with a ratio of r/d = 0.3 to be can be an alternative choice to the O‐ring for sealing applications. Interestingly, the pressures that the un‐stepped D‐ring can withstand before extrusion are higher than those of an O‐ring.     

Universal Imaging of Full Strain Tensor in 2D Crystals with Third‐Harmonic Generation

Quantitatively mapping and monitoring the strain distribution in 2D materials is essential for their physical understanding and function engineering. Optical characterization methods are always appealing due to unique noninvasion and high‐throughput advantages. However, all currently available optical spectroscopic techniques have application limitation, e.g., photoluminescence spectroscopy is for direct‐bandgap semiconducting materials, Raman spectroscopy is for ones with Raman‐active and strain‐sensitive phonon modes, and second‐harmonic generation spectroscopy is only for noncentrosymmetric ones. Here, a universal methodology to measure the full strain tensor in any 2D crystalline material by polarization‐dependent third‐harmonic generation is reported. This technique utilizes the third‐order nonlinear optical response being a universal property in 2D crystals and the nonlinear susceptibility has a one‐to‐one correspondence to strain tensor via a photoelastic tensor. The photoelastic tensor of both a noncentrosymmetric D_3h WS₂ monolayer and a centrosymmetric D_3d WS₂ bilayer is successfully determined, and the strain tensor distribution in homogenously strained and randomly strained monolayer WS₂ is further mapped. In addition, an atlas of photoelastic tensors to monitor the strain distribution in 2D materials belonging to all 32 crystallographic point groups is provided. This universal characterization on strain tensor should facilitate new functionality designs and accelerate device applications in 2D‐materials‐based electronic, optoelectronic, and photovoltaic devices.     

Colloidal Single‐Layer Photocatalysts for Methanol‐Storable Solar H2 Fuel

Molecular surfactants are widely used to control low‐dimensional morphologies, including 2D nanomaterials in colloidal chemical synthesis, but it is still highly challenging to accurately control single‐layer growth for 2D materials. A scalable stacking‐hinderable strategy to not only enable exclusive single‐layer growth mode for transition metal dichalcogenides (TMDs) selectively sandwiched by surfactant molecules but also retain sandwiched single‐layer TMDs' photoredox activities is developed. The single‐layer growth mechanism is well explained by theoretical calculation. Three types of single‐layer TMDs, including MoS₂, WS₂, and ReS₂, are successfully synthesized and demonstrated in solar H₂ fuel production from hydrogen‐stored liquid carrier—methanol. Such H₂ fuel production from single‐layer MoS₂ nanosheets is CO_x‐free and reliably workable under room temperature and normal pressure with the generation rate reaching ≈617 µmole g^?1 h^?1 and excellent photoredox endurability. This strategy opens up the feasible avenue to develop methanol‐storable solar H₂ fuel with facile chemical rebonding actualized by 2D single‐layer photocatalysts.     

2D‐GaS as a Photocatalyst for Water Splitting to Produce H2

Using first‐principles local and hybrid density functional theoretical calculations, a thickness‐dependent electronic structure of layered GaS is determined, and it is shown that 2D GaS has an electronic structure with valence and conduction bands that straddle the redox potentials of hydrogen evolution reaction and oxygen evolution reaction up to a critical thickness (<5.5 nm). Here, simulations of adsorption of H₂O on nanoscale GaS reveal that localized electronic states at its edges appear in the gap and strengthen the interaction with H₂O, further activating the surface atomic sites. It is thus predicted that GaS synthesized with a controlled thickness and preferred edges may be an efficient catalyst for photocatalytic splitting of water. Experiments that verify some of the predictions in this study are presented, and it is shown that GaS is effective in absorption of light and evolution of H₂ (887 μmol h⁻¹ g⁻¹) in the presence of aqueous solution of hydrazine (1% v/v). This study should open up the use of nanoscale GaS in conversion of solar energy into environment‐friendly chemical energy in the form of hydrogen.     

Microstructure and Processing of 3D Printed Tungsten Microlattices and Infiltrated W–Cu Composites

Tungsten is of industrial relevance due its outstanding intrinsic properties (e.g., highest melting‐point of all elements) and therefore difficult to 3D‐print by conventional methods. Here, tungsten micro‐lattices are produced by room‐temperature extrusion‐based 3D‐printing of an ink comprising WO₃–0.5%NiO submicron powders, followed by H₂‐reduction and Ni‐activated sintering. The green bodies underwent isotropic linear shrinkage of ≈50% during the thermal treatment resulting in micro‐lattices, with overall 35–60% open‐porosity, consisting of 95–100% dense W–0.5%Ni struts having ≈80–300 μm diameter. Ball‐milling the powders and inks reduced the sintering temperature needed to achieve full densification from 1400 to 1200 °C and enabled the ink to be extruded through finer nozzles (200 μm). Partial sintering of the struts is achieved when NiO is omitted from the ink, with submicron interconnected‐porosity of ≈34%. Several tungsten micro‐lattices are infiltrated with molten copper at 1300 °C under vacuum, resulting in dense, anisotropic W–Cu composites with 40–65% tungsten volume fraction. Partially sintered struts (containing nickel) with submicron open porosity are also infiltrated with Cu, resulting in co‐continuous W–Cu composites with wide W struts/Cu channels at the lattice scale (hundreds of micrometers), and fine W–Cu interpenetrating network at the strut scale (hundreds of nanometers) allowing for the design of anisotropic mechanical and electrical properties.

     

Three‐dimensional mixed‐mode (I and II) crack‐front fields in ductile thin plates — effects of T‐stress

It has been well‐established that the non‐singular T‐stress provides a first‐order estimate of geometry and loading mode (e.g. tension versus bending) effects on elastic–plastic crack‐front field under mode I loading conditions. The objective of this paper is to exam the T‐stress effect on three‐dimensional (3D) crack‐front fields under mixed‐mode (modes I and II) loading. To this end, detailed 3D small strain, elastic–plastic simulations are carried out using a 3D boundary layer (small‐scale yielding) formulation. Characteristics of near crack‐front fields are investigated for a wide range of T‐stresses (T/σ₀ = ?0.8, ?0.4, 0.0, 0.4, 0.8). The plastic zones and thickness and angular and radial variations of the stresses are studied, corresponding to two values of the remote elastic mixity parameters M_e = 0.3 and 0.7, under both low and high levels of applied loads. It is found that different T‐stresses have a significant effect on the plastic zones size and shapes, regardless of the mode mixity and load level. The thickness, angular and radial distributions of stresses are also affected markedly by T‐stress. It is important to include these effects when investigating the mixed‐mode ductile fracture failure process in thin‐walled structural components.     

Highly Efficient 2D/3D Hybrid Perovskite Solar Cells via Low‐Pressure Vapor‐Assisted Solution Process

The fabrication of multidimensional organometallic halide perovskite via a low‐pressure vapor‐assisted solution process is demonstrated for the first time. Phenyl ethyl‐ammonium iodide (PEAI)‐doped lead iodide (PbI₂) is first spin‐coated onto the substrate and subsequently reacts with methyl‐ammonium iodide (MAI) vapor in a low‐pressure heating oven. The doping ratio of PEAI in MAI‐vapor‐treated perovskite has significant impact on the crystalline structure, surface morphology, grain size, UV–vis absorption and photoluminescence spectra, and the resultant device performance. Multiple photoluminescence spectra are observed in the perovskite film starting with high PEAI/PbI₂ ratio, which suggests the coexistence of low‐dimensional perovskite (PEA₂MA_n?1Pb_nI_3n+1) with various values of n after vapor reaction. The dimensionality of the as‐fabricated perovskite film reveals an evolution from 2D, hybrid 2D/3D to 3D structure when the doping level of PEAI/PbI₂ ratio varies from 2 to 0. Scanning electron microscopy images and Kelvin probe force microscopy mapping show that the PEAI‐containing perovskite grain is presumably formed around the MAPbI₃ perovskite grain to benefit MAPbI₃ grain growth. The device employing perovskite with PEAI/PbI₂ = 0.05 achieves a champion power conversion efficiency of 19.10% with an open‐circuit voltage of 1.08 V, a current density of 21.91 mA cm^?2, and a remarkable fill factor of 80.36%.     

Rapid and Low‐Temperature Salt‐Templated Production of 2D Metal Oxide/Oxychloride/Hydroxide

2D materials have played an important role in electronics, sensors, optics, electrocatalysis, and energy storage. Many methods for the preparation of 2D materials have been explored. It is crucial to develop a high‐yield, rapid, and low‐temperature method to synthesize 2D materials. A general, fast (5 min), and low‐temperature (≈100 °C) salt (CoCl₂·6H₂O)‐templated method is proposed to prepare series of 2D metal oxides/oxychlorides/hydroxides in large scale, such as MoO₃, SnO₂, SiO₂, BiOCl, Sb₄O₅Cl₂, Zn₂Co₃(OH)₁₀ 2H₂O, and ZnCo₂O₄. The as‐synthesized 2D materials possess an ultrathin feature (2–7 nm) and large aspect ratios. Additionally, these 2D metal oxides/oxychlorides/hydroxides exhibit good electrochemical properties in energy storage (lithium/sodium‐ion batteries) and electrocatalysis (hydrogen/oxygen evolution reaction).     

Metal‐Free 2D/2D Phosphorene/g‐C3N4 Van der Waals Heterojunction for Highly Enhanced Visible‐Light Photocatalytic H2 Production

The generation of green hydrogen (H₂) energy using sunlight is of great significance to solve the worldwide energy and environmental issues. Particularly, photocatalytic H₂ production is a highly promising strategy for solar‐to‐H₂ conversion. Recently, various heterostructured photocatalysts with high efficiency and good stability have been fabricated. Among them, 2D/2D van der Waals (VDW) heterojunctions have received tremendous attention, since this architecture can promote the interfacial charge separation and transfer and provide massive reactive centers. On the other hand, currently, most photocatalysts are composed of metal elements with high cost, limited reserves, and hazardous environmental impact. Hence, the development of metal‐free photocatalysts is desirable. Here, a novel 2D/2D VDW heterostructure of metal‐free phosphorene/graphitic carbon nitride (g‐C₃N₄) is fabricated. The phosphorene/g‐C₃N₄ nanocomposite shows an enhanced visible‐light photocatalytic H₂ production activity of 571 µmol h^?1 g^?1 in 18 v% lactic acid aqueous solution. This improved performance arises from the intimate electronic coupling at the 2D/2D interface, corroborated by the advanced characterizations techniques, e.g., synchrotron‐based X‐ray absorption near‐edge structure, and theoretical calculations. This work not only reports a new metal‐free phosphorene/g‐C₃N₄ photocatalyst but also sheds lights on the design and fabrication of 2D/2D VDW heterojunction for applications in catalysis, electronics, and optoelectronics.     

2D Perovskites with Giant Excitonic Optical Nonlinearities for High‐Performance Sub‐Bandgap Photodetection

Two‐dimensional (2D) perovskites have proved to be promising semiconductors for photovoltaics, photonics, and optoelectronics. Here, a strategy is presented toward the realization of highly efficient, sub‐bandgap photodetection by employing excitonic effects in 2D Ruddlesden–Popper‐type halide perovskites (RPPs). On near resonance with 2D excitons, layered RPPs exhibit degenerate two‐photon absorption (D‐2PA) coefficients as giant as 0.2–0.64 cm MW^?1. 2D RPP‐based sub‐bandgap photodetectors show excellent detection performance in the near‐infrared (NIR): a two‐photon‐generated current responsivity up to 1.2 × 10⁴ cm² W^?2 s^?1, two orders of magnitude greater than InAsSbP‐pin photodiodes; and a dark current as low as 2 pA at room temperature. More intriguingly, layered‐RPP detectors are highly sensitive to the light polarization of incoming photons, showing a considerable anisotropy in their D‐2PA coefficients (β_[001]/β_[011] = 2.4, 70% larger than the ratios reported for zinc‐blende semiconductors). By controlling the thickness of the inorganic quantum well, it is found that layered RPPs of (C₄H₉NH₃)₂(CH₃NH₃)Pb₂I₇ can be utilized for three‐photon photodetection in the NIR region.     

Three-dimensional effects on fatigue crack closure in the small-scale yielding regime – a finite element study

Plasticity induced closure often strongly influences the behaviour of fatigue cracks at engineering scales in metallic materials. Current predictive models generally adopt the effective stress‐intensity factor (ΔΚ_eff = Κ_max–Κ_op) in a Paris law type relationship to quantify crack growth rates. This work describes a 3D finite element study of mode I fatigue crack growth in the small‐scale yielding (SSY) regime under a constant amplitude cyclic loading with zero T‐stress and a ratio Κ_min/Κ_max = 0 . The material behaviour follows a purely kinematic hardening constitutive model with constant hardening modulus. Dimensional analysis suggests, and the computational results confirm, that the normalized remote opening load value, Κ_op/Κ_max, at each location along the crack front remains unchanged when the peak load (Κ_max), thickness (B) and material flow stress (σ₀) all vary to maintain a fixed value of . Through parametric computations at various K levels, the results illustrate the effects of normalized peak loads on the through‐thickness opening–closing behaviour and the effects of σ₀/E, where E denotes material elastic modulus. The examination of deformation fields along the fatigue crack front provides additional insight into the 3D closure process.     

High Mobility 2D Palladium Diselenide Field‐Effect Transistors with Tunable Ambipolar Characteristics

Due to the intriguing optical and electronic properties, 2D materials have attracted a lot of interest for the electronic and optoelectronic applications. Identifying new promising 2D materials will be rewarding toward the development of next generation 2D electronics. Here, palladium diselenide (PdSe₂), a noble‐transition metal dichalcogenide (TMDC), is introduced as a promising high mobility 2D material into the fast growing 2D community. Field‐effect transistors (FETs) based on ultrathin PdSe₂ show intrinsic ambipolar characteristic. The polarity of the FET can be tuned. After vacuum annealing, the authors find PdSe₂ to exhibit electron‐dominated transport with high mobility (µ _{e (max)} = 216 cm² V^?1 s^?1) and on/off ratio up to 10³. Hole‐dominated‐transport PdSe₂ can be obtained by molecular doping using F₄‐TCNQ. This pioneer work on PdSe₂ will spark interests in the less explored regime of noble‐TMDCs.     

Low‐Energy Facets on CdS Allomorph Junctions with Optimal Phase Ratio to Boost Charge Directional Transfer for Photocatalytic H2 Fuel Evolution

Low‐energy facets on CdS allomorph junctions with optimal phase ratio are designed to boost charge directional transfer for photocatalytic H₂ fuel evolution. Fermi energy level difference between low‐energy facets as driving force promotes electrons directional transfer to hexagonal CdS(102) facet and holes to cubic CdS(111) facet. The optimal allomorphs CdS presents superior photocatalytic H₂ evolution rate of 32.95 mmol g^?1 h^?1 with release in a large amount of visible H₂ bubbles, which is much higher than single‐phase CdS with high‐energy facets and even supports noble metal photocatalysts. This scientific perspective on low‐energy facets of allomorph junctions with optimal phase ratio breaks the long‐held view of pursuing high‐energy crystal surfaces, which will break the understanding on surface structure crystal facet engineering of photocatalytic materials.     

Carbon‐Intercalated 0D/2D Hybrid of Hematite Quantum Dots/Graphitic Carbon Nitride Nanosheets as Superior Catalyst for Advanced Oxidation

Efficient charge separation and sufficiently exposed active sites are important for light‐driving Fenton catalysts. 0D/2D hybrids, especially quantum dots (QDs)/nanosheets (NSs), offer a better opportunity for improving photo‐Fenton activity due to their high charge mobility and more catalytic sites, which is highly desirable but remains a great challenge. Herein, a 0D hematite quantum dots/2D ultrathin g‐C₃N₄ nanosheets hybrid (Fe₂O₃ QDs/g‐C₃N₄ NS) is developed via a facile chemical reaction and subsequent low‐temperature calcination. As expected, the specially designed 0D/2D structure shows remarkable catalytic performance toward the removal of p‐nitrophenol. By virtue of large surface area, adequate active sites, and strong interfacial coupling, the 0D Fe₂O₃ QDs/2D g‐C₃N₄ nanosheets establish efficient charge transport paths by local in‐plane carbon species, expediting the separation and transfer of electron/hole pairs. Simultaneously, highly efficient charge mobility can lead to continuous and fast Fe(III)/Fe(II) conversion, promoting a cooperative effect between the photocatalysis and chemical activation of H₂O₂. The developed carbon‐intercalated 0D/2D hybrid provides a new insight in developing heterogeneous catalysis for a large variety of photoelectronic applications, not limited in photo‐Fenton catalysis.     

In‐plane and out‐of‐plane constraint parameters along a three‐dimensional crack‐front stress field under creep loading

Full‐field three‐dimensional (3D) numerical analyses was performed to determine in‐plane and out‐of‐plane constraint effect on crack‐front stress fields under creep conditions of finite thickness boundary layer models and different specimen geometries. Several parameters are used to characterize constraint effects including the non‐singular T‐stresses, the local triaxiality parameter, the T_z ‐factor of the stress‐state in a 3D cracked body and the second‐order‐term amplitude factor. The constraint parameters are determined for centre‐cracked plate, three‐point bend specimen and compact tension specimen. Discrepancies in constraint parameter distribution on the line of crack extension and along crack front depending on the thickness of the specimens have been observed under different loading conditions of creeping power law hardening material for various configurations of specimens.     

Robust mixed H 2/H ∞ controller for uncertain neutral state‐input delayed systems via LMI optimization approach

Abstract

In this paper, the problem of designing a robust mixed H ₂/H _∞ controller for a class of uncertain neutral state‐input delays system is considered. Based on Lyapunov‐Krasovskii functional theory, a delay‐dependent criterion is derived for the existence of a desired mixed H ₂/H _∞ controller, which can be easily constructed by certain feasible linear matrix inequalities (LMIs). Furthermore, a convex optimization problem is formulated to solve for a robust mixed H ₂/H _∞ controller which achieves the minimization of an upper bound of the closed‐loop H ₂ perforance measure.     

Nonconfinement Structure Revealed in Dion–Jacobson Type Quasi‐2D Perovskite Expedites Interlayer Charge Transport

Dion–Jacobson (DJ) type 2D perovskites with a single organic cation layer exhibit a narrower distance between two adjacent inorganic layers compared to the corresponding Ruddlesden–Popper perovskites, which facilitates interlayer charge transport. However, the internal crystal structures in 2D DJ perovskites remain elusive. Herein, in a p‐xylylenediamine (PDMA)‐based DJ perovskite bearing bifunctional NH₃⁺ spacer, the compression from confinement structure (inorganic layer number, n = 1, 2) to nonconfinement structure (n > 3) with the decrease of PDMA molar ratio is unraveled. Remarkably, the nonconfined perovskite displays shorter spacing between 2D quantum wells, which results in a lower exciton binding energy and hence promotes exciton dissociation. The significantly diminishing quantum confinement promotes interlayer charge transport leading to a maximum photovoltaic efficiency of ≈11%. Additionally, the tighter interlayer packing arising from the squeezing of inorganic octahedra gives rise to enhanced ambient stability.     

Electrostatically Assembling 2D Nanosheets of MXene and MOF‐Derivatives into 3D Hollow Frameworks for Enhanced Lithium Storage

As an essential member of 2D materials, MXene (e.g., Ti₃C₂T_x) is highly preferred for energy storage owing to a high surface‐to‐volume ratio, shortened ion diffusion pathway, superior electronic conductivity, and neglectable volume change, which are beneficial for electrochemical kinetics. However, the low theoretical capacitance and restacking issues of MXene severely limit its practical application in lithium‐ion batteries (LIBs). Herein, a facile and controllable method is developed to engineer 2D nanosheets of negatively charged MXene and positively charged layered double hydroxides derived from ZIF‐67 polyhedrons into 3D hollow frameworks via electrostatic self‐assembling. After thermal annealing, transition metal oxides (TMOs)@MXene (CoO/Co₂Mo₃O₈@MXene) hollow frameworks are obtained and used as anode materials for LIBs. CoO/Co₂Mo₃O₈ nanosheets prevent MXene from aggregation and contribute remarkable lithium storage capacity, while MXene nanosheets provide a 3D conductive network and mechanical robustness to facilitate rapid charge transfer at the interface, and accommodate the volume expansion of the internal CoO/Co₂Mo₃O₈. Such hollow frameworks present a high reversible capacity of 947.4 mAh g^?1 at 0.1 A g^?1, an impressive rate behavior with 435.8 mAh g^?1 retained at 5 A g^?1, and good stability over 1200 cycles (545 mAh g^?1 at 2 A g^?1).     

T‐stress corrected notch stress intensity factors with application to welded lap joints

The definition, content and application of the notch stress intensity factors (NSIFs) characterizing the stress field at rounded slit tips (keyholes) is discussed. The same is done in respect of the T‐stress transferred from the corresponding pointed slit tips. A T‐stress based correction of the NSIF K_1,ρ is found to be necessary. The applicability of the T‐stress term supplemented by higher‐order terms in Williams’ solution to the slit tip stresses in tensile‐shear loaded lap joints is discussed in more detail. The role of the T‐stress in constituting the near‐field stresses of rounded slit tips is shown to cause a difference between internal and external slit tip notches. The notch stress equations for lap joints proposed by Radaj based on structural stress and by Lazzarin based on a finite element model of the rounded notch are reconsidered and amended based on the derivations above.     

The photoelastic constant and internal stress around micropipe defects of 6H-SiC single crystal

Hexagonal silicon carbide (6H-SiC) single crystals made by a modified Lely method contain a ‘micropipe’ which is one kind of serious defects degrading device performance. The micropipe accompanies strong internal stress around itself. We determined the photoelastic constant in the plane of (00·1) 6H-SiC and then estimated the magnitude of the internal stress around the micropipes. The photoelastic constant was 2.73 brewster at λ=546 nm. The internal stress around the micropipe was estimated to be 113∼166 MPa.