Distortion Of Interface Research Articles

Cover Feature: Defect Engineering of van der Waals Solids for Electrocatalytic Hydrogen Evolution (Chem. Asian J. 22/2020)

Defect engineering in van der Waals solids plays an important role in triggering the catalytic activity of their inert components for the hydrogen evolution reaction (HER). It could efficiently modulate the chemical components and band-gap states of van der Waals solids, thus impacting the reactants′ capture, activation and release process. In this Minireview, the state-of-the-art progress of defect engineering in van der Waals solids for HER, involving zero-dimensional defects (e. g., atomic vacancies and heteroatom doping), one-dimensional defects (e. g., under-coordinated edges and domain boundaries), two-dimensional defects (e. g., vertical heterostructures and interfacial distortions), and other typical defects is summarized. In particular, focus is laid on their advantages in material modification and corresponding catalytic mechanisms. In addition, the challenges and perspectives of these catalytic materials in terms of both experimental synthesis and fundamental understanding of the defect structures are also proposed. More information can be found in the Minireview by Ruitao Lv et al.

The Effect of Gas Nonideality on the Interface Reflectivity When Interacting With a Shock Wave

The phenomenon of shock wave refraction on a gaseous interface with plasma was analyzed for nonideal gas effects. The developed model was applied to nondissociating diatomic nitrogen gas at 3000 K. Comparatively to ideal gas, the interface reflectivity was found noticeably decreased resulting in 12% more intense shock transmission. The transition point determining the character of reflected wave was also found shifted with its coordinate being strongly dependent on incident shock strength and sort of gas. It was found that a delay in gas pressure drop and a sharp reduction of vorticity generated in the postshock flow are possible. The influence of real-gas effects on the shock instability locus, interface distortion, and wave drag reduction is discussed.

Effect of rapid cold stamping on fracture behavior of long strip S′ phase in Al−Cu−Mg alloy

Abstract High-angle annular dark-field scanning transmission electron microscopy and selected area electron diffraction techniques were used to study the mechanism that underlies the influence of rapid cold-stamping deformation on the fracture behavior of the elongated nanoprecipitated phase in extruded Al−Cu−Mg alloy. Results show that the interface between the long strip-shaped S′ phase and the aluminum matrix in the extruded Al−Cu−Mg alloy is flat and breaks during rapid cold-stamping deformation. The breaking mechanisms are distortion and brittle failure, redissolution, and necking. The breakage of the long strip S′ phase increases the contact surface between the S′ phase and the aluminum matrix and improves the interfacial distortion energy. This effect accounts for the higher free energy of the S′ phase than that of the matrix and creates conditions for the redissolution of solute atoms back into the aluminum matrix. The brittle S′ phase produces a resolved step during rapid cold-stamping deformation. This step further accelerates the diffusion of solute atoms and promotes the redissolution of the S′ phase. Thus, the S′ phase necks and separates, and the long strip-shaped S′ phase in the extruded Al−Cu−Mg alloy is broken into a short and thin S′ phase.

Modeling the Interface between Lithium Metal and Its Native Oxide.

Owing to their high theoretical capacities, batteries that employ lithium (Li) metal as the negative electrode are attractive technologies for next-generation energy storage. However, the successful implementation of lithium metal batteries is limited by several factors, many of which can be traced to an incomplete understanding of surface phenomena involving the Li anode. Here, first-principles calculations are used to characterize the native oxide layer on Li, including several properties associated with the Li/lithium oxide (Li2O) interface. Multiple interface models are examined; the models account for differing interface (chemical) terminations and degrees of atomic ordering (i.e., crystalline vs amorphous). The interfacial energy, formation energy, and strain energies are predicted for these models. The amorphous interface yields the lowest interfacial formation energy, suggesting that it is the most probable model under equilibrium conditions. The work of adhesion is evaluated for the crystalline interfaces, and it is found that the O-terminated interface exhibits a work of adhesion more than 30 times larger than that of the Li-terminated model, implying that Li will strongly wet an oxygen-rich Li2O surface. The electronic structure of the interfaces is characterized using Voronoi charge analysis and shifts in the Li 1s binding energies. The width of the Li/Li2O interface, as determined by deviations from bulklike charges and binding energies, extends beyond the region exhibiting interfacial structural distortions. Finally, the transport of Li ions through the amorphous oxide is quantified using ab initio molecular dynamics. Facile transport of Li+ through the native oxide is observed. Thus, increasing the percentage of amorphous Li2O in the solid electrolyte interphase may be beneficial for battery performance. In total, the phenomena quantified here will aid in the optimization of batteries that employ high-capacity Li metal anodes.

Interface Displacement and Dynamic Phasor Mapping Equivalence Based Hybrid Simulation for HVAC/DC Power Grids

In the electromagnetic transient (EMT) and transient stability (TS) hybrid simulation, the entire power system is artificially split into two sub-grids, and sub-grids interact with each other via an interface. Thus interface distortions emerge, including latency and errors. The influence of interface latency is quantitated based on a demo circuit containing delayed interaction. Moreover, the principles of improving hybrid simulation interface accuracy are concluded. Inspired by the principles, a novel interface displacement (ID) and dynamic phasor mapping equivalence (DP-ME) interface scheme is proposed. The scheme makes sub-grids at opposite sides of an interface loosely coupled and avoids interface variable form conversion by applying two techniques. 1) Displacement of the partition interface from the converter buses to the control loops and interior of the EMT sub-grid to utilize the built-in inertias. 2) Construction of the DP-ME model to calculate the injected power from EMT to TS sub-grid. Tests based on designed cases and an actual HVAC/DC power grid give credibility to the proposed ID & DP-ME scheme in comparison with full electromagnetic simulation.

Dynamics of nonisothermal two-thin-fluid-layer systems subjected to harmonic tangential forcing under Rayleigh–Taylor instability conditions

The stability of a nonisothermal system consisting of two superimposed fluid layers: a thin liquid film layer and a gas layer sandwiched between differentially heated horizontal solid plates in the gravity field, is investigated. The system is assumed to be subjected to the Rayleigh–Taylor instability (RTI) with the Marangoni effect that either enhances the RTI or opposes it and to the tangential harmonic vibration of the upper substrate. A set of reduced evolution equations is derived based on the weighted-residual integral boundary layer approach, and the investigation is carried out in the framework of this set. The base state of the system represents a time-periodic flow, and its linear stability analysis is carried out using the Floquet theory in the large-time limit. The nonlinear dynamics of the system is investigated numerically in the case of either a static or vibrating substrate. Among the possible outcomes of the nonlinear dynamics, there is the emergence of ruptured states of the liquid film with rupture taking place at either the upper or lower substrate and also the emergence of saturated continuous flows of the liquid film. We also find that the nonlinear dynamics of the system is consistent with the results of the linear stability analysis in terms of enhancement or attenuation of interfacial distortion.

Stabilizing platinum atoms on CeO2 oxygen vacancies by metal-support interaction induced interface distortion: Mechanism and application

Exploring thermally robust single atom catalysts (SACs) is of great significance. Here, we develop a universal strategy for stabilizing Pt atoms on the mono-oxygen vacancies of CeO2 with diverse exposed facets. The stabilization mechanism was proposed that the formed Pt-O-Ce interface will be taken into distortion spontaneously to keep thermodynamics stable through strong metal-support interactions. The highest degree of Pt-O-Ce distortion is achieved over Pt1-CeO2{100} material, which exhibits exceptional efficiency and thermal stability for oxygenated hydrocarbon removal. The enhanced adsorption capacity of O2 and methanol confirmed in the distortion interface is seen as another crucial reason for improving the stability of SACs. Methanol oxidation on Pt1-CeO2{100} obeys the L-H mechanism under relatively low temperature and then goes through to the MVK mechanism with temperature increasing. We believe that these results would bring new opportunities in the fabrication of SACs and applications of them in thermal reactions.

Electronic Structure and Trap States of Two-Dimensional Ruddlesden–Popper Perovskites with the Relaxed Goldschmidt Tolerance Factor

Two-dimensional Ruddlesden–Popper perovskites (2D RPPs) have been considered as promising building blocks for optoelectronic applications owing to optical properties comparable to the ones of 3D perovskites, together with superior stability. In addition, the more flexible structure adopted by such perovskites leads to a relaxation of the Goldschmidt tolerance factor (τ) requirement. Herein, we compare the crystalline and electronic structures, as well as the photophysics of two 2D perovskite single crystals (n-BA)2(MA)2Pb3I10 (BMAPI) and (n-BA)2(EA)2Pb3I10 (BEAPI) (n-BA = n-butylamine) containing small A-cations (MA, methylammonium) and large A-cations (EA, ethylammonium), respectively. The latter presents a relaxed τ (τEA > 1) compared with the requirement of a stable phase in 3D perovskites (τ < 1). Such relaxed τ is beneficial from the structural flexibility of the long organic cation bilayer and the pronounced lattice distortions in the 2D perovskite structures. We further elucidate how the greater lattice distortions concurrently modulate the electronic structure as well as trap densities in these 2D RPPs. The electronic band gap (Eg) of BEAPI (2.08 ± 0.03 eV) is ∼0.17 eV larger than the one of BMAPI (1.91 ± 0.03 eV). This is mainly because of a shift in the valence band maximum associated with the expansion of the Pb–I bond length in BEAPI. In addition, the overall trap state densities for BMAPI and BEAPI are calculated to be ∼2.18 × 1016 and ∼3.76 × 1016 cm–3, respectively, as extracted from the time-resolved photoluminescence studies. The larger trap density in BEAPI can be attributed to the stronger interfacial lattice distortion that sets in when large EA cations are contained into the inorganic crystal lattice. (Less)

Correlating polar distortions and interfacial charge at the polar/non-polar LaCrO3/SrTiO3 (001) interface

The relationship between the sheet carrier concentration, ns, of LaCrO3(LCO)/SrTiO3(STO) heterostructures and their structural properties has been investigated. Under low oxygen partial pressure, the STO substrate is reduced during growth as evidenced by a high ns of 1016 cm−2. By controlling the post-growth annealing conditions, heterostructures with ns of 1013–1016 cm−2 are achieved. The atomic-scale structure of the samples is obtained using high-resolution synchrotron x-ray diffraction measurements. For heterostructures with ns at or below 3 × 1013 cm−2, polar distortions are present within the LCO layers and increase in magnitude with a decrease in sheet carrier concentration. These distortions are absent for samples with ns on the order of 1015–1016 cm−2 where interfacial carriers play a role in alleviating the polar discontinuity at the LCO/STO interface. These results suggest that interfacial charge carriers and polar distortions can act as complementary mechanisms to alleviate the polar discontinuity at polar/non-polar complex oxide interfaces.

Atomic deformation mechanism and interface toughening in metastable high entropy alloy

Metastable high entropy alloy (HEA) with excellent properties have attracted extensive attentions recently. However, as a consequence of limited experiments of high-resolution transmission electron microscopy (HRTEM) and the difficulties of molecular dynamic (MD) simulations for the phase transformation process, the detailed atomic deformation mechanisms in the HEA is not well understood. We carry out the in situ HRTEM observation of the martensitic transformation process and find surprisingly wide phase interface between the parent and the martensite in a typical high strength and high elongation metastable HEA. One specific interatomic potential is developed for the metastable HEA and large-scale MD simulation is carried out to investigate the martensitic transformation process from body-centered cubic to hexagonal close packed structures. The whole processes of the stress-induced martensitic transformation (nucleation, incubation, bursting and propagating of the new phase) are well reproduced in the MD simulations, suggesting its good agreements with the HRTEM observations. The width of the phase interface mainly depends on the competition between interfacial energy and lattice distortion energy during the martensitic transformation process. This wide phase interface acts as a buffer to coordinate the martensitic transformation induced strain and as a buffer storage for dislocation gliding and pile-up. As a result, the metastable HEA achieves a high strength combined with a large tensile elongation. The revealed atomic-scale deformation and corresponding interatomic potential should be useful to guide the design in the new series of high-performance metastable alloy.

Drag force on a particle straddling a fluid interface: Influence of interfacial deformations.

We numerically investigate the influence of interfacial deformations on the drag force exerted on a particle straddling a fluid interface. We perform finite element simulations of the two-phase flow system in a bounded two-dimensional geometry. The fluid interface is modeled with a phase-field method which is coupled to the Navier-Stokes equations to solve for the flow dynamics. The interfacial deformations are caused by the buoyant weight of the particle, which results in curved menisci. We compute drag coefficients as a function of the three-phase contact angle, the viscosity ratio of the two fluids, and the particle density. Our results show that, for some parameter values, large drag forces are not necessarily correlated with large interfacial distortions and that a lower drag may actually be achieved with non-flat interfaces rather than with unperturbed ones.

Directed assembly and micro-manipulation of passive particles at fluid interfaces via capillarity using a magnetic micro-robot

Controlled assembly of microscale objects can be achieved by exploiting interactions that dominate at that length scale. Capillary interactions are an excellent candidate for this purpose; microparticles trapped at fluid interfaces disturb the interface shape, migrate, and assemble to minimize the interfacial area. These interactions are independent of microparticle material properties and so can be used to assemble objects of arbitrary materials. By using a magnetic robot as a mobile distortion source, additional control over assembly can be achieved. For example, millimeter-scale magnetic robots that are heavy enough to distort the interface have been used to generate long range capillary attractions and collect passive particles that are hundreds of micrometers in diameter. However, for smaller robots and particles, gravity is less important, and capillary interactions rely on interface distortions from undulated contact lines. We use a magnetic microrobot to manipulate passive microparticles at the water/hexadecane interface via an interplay of hydrodynamic and capillary interactions. Furthermore, we demonstrate preferred docking at corners of a square microrobot without the need for high resolution position control. We modulate the strength of docking interactions, allowing structure assembly and release. Finally, we design undulated docking stations with multiple stable sites for cargo delivery. The ability to dynamically manipulate microparticles and their structures at fluid interfaces creates new possibilities for manufacturing of complex microstructures.

Photodetector Based on Spontaneously Grown Strongly Coupled MAPbBr3/N-rGO Hybrids Showing Enhanced Performance.

Recently, metal-halide perovskites have emerged as a candidate for optoelectronic applications such as photodetectors. However, the poor device performance and instability have limited their future commercialization. Herein, we report the spontaneous growth of perovskite/N-rGO hybrid structures using a facile solution method and their applications for photodetectors. In the hybrid structures, perovskites were homogeneously wrapped by N-rGO sheets through strong hydrogen bonding. The strongly coupled N-rGOs facilitate the charge carrier transportation across the perovskite crystals but also distort the surface lattice of the perovskite creating a potential barrier for charge transfer. We optimize the addition of N-rGO in the hybrid structures to balance interfacial structural distortion and the intercrystal conductivity. High-performance photodetection up to 3 × 104 A/W, external quantum efficiency exceeding 105%, and detectivity up to 1012 Jones were achieved in the optimal device with the weight ratio between perovskites and N-rGO to be 8:1.5. The underlying mechanism behind the optimal N-rGO addition ratio in the hybrids has also been rationalized via time-resolved spectroscopic studies as a reference for future applications.

First-principles calculation on the structure stability, hydrogen trapping behaviour, and adhesion properties of the Zr(0001)[sbnd]ZrC(100) interface

The structure stability, hydrogen trapping behaviour, and adhesion properties of the Zr(0001)ZrC(100) interface are investigated by first-principle calculations. Periodically distributed distortion and stress-relaxed regions with lengths of 16.6 Å are formed to relax the lattice mismatch at the Zr(0001)ZrC(100) interface. Homogeneous covalent 3ZrC bonds are formed in the distortion region. The interfacial sinking effect exists only in the interface stress-relaxed region where the interstitial hydrogen has a considerably lower formation energy of −0.61 eV after the zero-point energy correction, than those in the interface distortion region, Zr, and ZrC bulk of 0.95, −0.10, and 1.30 eV, respectively. After the interstitial sites in the interface stress-relaxed region are fully occupied by hydrogen, the interface energy is significantly reduced from 2.34 to 1.52 J/m2. The work of separation Wsep along the interface is approximately 3.78–3.91 J/m2, while the brittle fracture of the Zr(0001)ZrC(100) interface always occurs at the Zr matrix (along the Zr1 layer) with the lowest Wsep of 3.18–2.95 J/m2. Hydrogen-induced electron localisation exists at the interface, which reduces the interface structure stability. In addition, a hydrogen passivation layer forms owing to the formation of ZrH and CH bonds at the interface.

Non‐destructive testing method of micro‐debonding defects in composite insulation based on high power ultrasonic

It is crucial to ensure the adhesive quality of the interface in composite insulation equipment for power grid security. Due to viscoelasticity of silicone rubber and roughness of interface, the thickness of the local air layer in the defective interface is <100 μm. This kind of defect is called micro-debonding defect in this paper, and it is significantly smaller than the millimetre-scale air gaps which could be defected by the x-ray, THz, and traditional ultrasonic method. It remains challenging to detect the micro-debonding in composite insulation by non-destructive testing methods. In this paper, the acoustic model and bi-linear stiffness model for micro-debonding in different stages are established. Accordingly, a conjecture is proposed that the bonding property of composite insulation is related to the acoustic impedance of interface and non-linear distortion of its constitutive relation. The result obtained from numerical simulation with and without defect is compared with the high-power ultrasonic experimental data to validate the correctness of the theoretical model. In addition, it can be concluded that the non-linear distortion of high-power ultrasonic wave can be effectively used to diagnose micro-debonding defect at the interface of composite insulation in its early stage (1–20 μm).

Confinement of magnetism in atomically thin La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 heterostructures

At crystalline interfaces where a valence-mismatch exists, electronic, and structural interactions may occur to relieve the polar mismatch, leading to the stabilization of non-bulk-like phases. We show that spontaneous reconstructions at polar La0.7Sr0.3MnO3 interfaces are correlated with suppressed ferromagnetism for film thicknesses on the order of a unit cell. We investigate the structural and magnetic properties of valence-matched La0.7Sr0.3CrO3/La0.7Sr0.3MnO3 interfaces using a combination of high-resolution electron microscopy, first principles theory, synchrotron X-ray scattering and magnetic spectroscopy and temperature-dependent magnetometry. A combination of an antiferromagnetic coupling between the La0.7Sr0.3CrO3 and La0.7Sr0.3MnO3 layers and a suppression of interfacial polar distortions are found to result in robust long-range ferromagnetic ordering for ultrathin La0.7Sr0.3MnO3. These results underscore the critical importance of interfacial structural and magnetic interactions in the design of devices based on two-dimensional oxide magnetic systems.

Molecular dynamics study of the translation and rotation of amphiphilic Janus nanoparticles at a vapor-liquid surface

We study the effects of heterogeneity on interfacial pinning and hydrodynamic drag using molecular dynamics (MD) simulations of Janus nanospheres at a liquid/vapor interface. We construct the free energy landscape for this system, both in the continuum approximation using surfaces tensions and the flat-interface approximation and atomistically using MD and thermodynamic integration. The results of the two methods differ in detail due to interfacial distortion and finite width, as well as thermal fluctuations, and only the MD landscape is consistent with simulations of a nanosphere approaching the interface from the liquid or vapor side. When dragged along an interface, these Janus particles exhibit a velocity-dependent tilt accompanied by a weak variation in drag force, but never an enhancement of the drag force beyond the value when fully immersed. This velocity dependence arises when the interface is pinned at heterogeneities and prevents the particle from rotating, and similar behavior is observed for homogeneous but non-spherical particles. The occurrence of different particle orientations having different drag coefficients may lead to an apparent violation of the Stokes-Einstein relation.

Interfacial structure and electronic properties of TiO2/ZnO/TiO2 for photocatalytic and photovoltaic applications: A theoretical study

Understanding interface properties of heterojunction is an important step towards controllable and tunable interfaces for photocatalytic and photovoltaic based devices. To this aim, we propose a thorough study of a double heterostructure system consisting of two semiconductors with large band gap, namely, wurtzite ZnO and anatase TiO2. We demonstrate via first-principle calculations two stable configurations of ZnO/TiO2 interfaces. Our structural analysis provides a key information on the nature of the complex interface and lattice distortions occurring when combining these materials. The study of the electronic properties of the sandwich nanostructure TiO2/ZnO/TiO2 reveals that conduction band arises mainly from Ti3d orbitals, while valence band is maintained by O2p of ZnO, and that the trapped states within the gap region frequent in single heterostructure are substantially reduced in the double interface system. Moreover, our work explains the origin of certain optical transitions observed in the experimental studies. Unexpectedly, as a consequence of different bond distortions, the results on the band alignments expect different electron accumulation between the two interfaces. Such behavior provides more choice for the sensitization and functionalization of TiO2 surfaces.

Depinning force of a receding droplet on pillared superhydrophobic surfaces: Analytical models

It was experimentally shown that a depinning force of a receding droplet on a micropillared superhydrophobic surface has an apparently linear correlation with the maximal three-phase contact line attainable along an actual droplet boundary. However, the experimental observation has not yet been supported by any theoretical basis or analysis. This study establishes an analytical model that theoretically supports the experimental observation on the basis of the dynamics of a contact line. The depinning force of a receding droplet was experimentally measured in evaporation on micropillared superhydrophobic surfaces with varying structure pitches but a fixed size. The analytical model was established by considering the energy dissipated by the displacement of a microscopic contact line on pillar tops and the energy consumed for the distortion of a liquid-gas interface between pillar tops. The model shows that the depinning force can theoretically be estimated by the physical dimensions of pillars and the intrinsic hydrophobicity of the surface, showing excellent agreement with the experimental results. Especially, the model indicates that the depinning force is fundamentally determined and practically controllable by the normalized maximal contact line at the droplet boundary, which can effectively be represented as the ratio of the pillar top perimeter to pitch.

Bubble interaction of annular flow in micro-channel boiling

The development of heat transfer models of micro-channel boiling relies on understanding of all individual flow regimes as well as interactions and transitions between them. The presented paper performs a detailed study on the interactions between annular flow and its trailing bubbles in a cylindrical micro-channel with a diameter of 0.4 mm. Inlet mass flux is 400 kg/(m2∙s) and constant wall heat flux is 80 and 160 kW/m2, respectively. The growth rate of the bubble and the flow regime transition are validated against published experimental data. Comparisons of the results between transition flow and single annular flow show that the transition processes enhance bubble evaporation. A new phenomenon is observed: extreme high local heat transfer rates are observed under certain conditions. A detailed investigation of the thin film region unveil that this enhancement is a consequence of the interface distortion caused by the presence of the trailing bubble. This bubble interface deformation depends primarily on a few factors including wall heat flux, surface tension, trailing bubble.