Effect Of Interfacial Energy Research Articles

Poly(vinyl alcohol)‐controlled synthesis of monodispersed crosslinked poly(methyl methacrylate) microparticles with significantly improved mechanical properties

AbstractA facile synthesis concept focusing on the poly(vinyl alcohol) (PVA) concentration induced interfacial energy effects in seed swelling polymerization was realized for the successful fabrication of a series of monodispersed poly(methyl methacrylate) (PMMA) microparticles with four crosslinking agents, 2,2‐bis[(methacryloyloxy)methyl]butyl methacrylate, 1,2‐ethanediyl bis(2‐methylacrylate), 1,3‐divinylbenzene and 1,2‐ethanediyl diacetate. We revealed a special role of PVA in regulating the diffusion of crosslinking agents as well as the swelling degree of latex seeds and, more noticeably, the consequent improved mechanical properties of the crosslinked PMMA microparticles formed. The highest recovery rate obtained from the PMMA microparticles with 0.6 wt% PVA approached 67%, which is higher than or comparable to reported values. This work not only developed a new facile synthesis approach for producing highly uniform tough crosslinked PMMA microparticles for more potential applications but also provided valuable insights into fundamental aspects of seed swelling polymerization. © 2019 Society of Chemical Industry

Strain‐Induced Dissolution and Precipitation of Secondary Phases and Synergetic Stengthening Mechanisms of Al–Zn–Mg–Cu Alloy during ECAP

The evolution of secondary phase of 7A60 aluminum alloy before and after equal channel angular pressing (ECAP) deformation and the resultant variation of mechanical properties are studied by means of X‐ray diffractometer (XRD), transmission electron microscopy (TEM), optical microscope (OM), and Instron testing machine. The results show that grains are refined to submicron level after ECAP (from 9.86 µm to about 1 µm). As the strain increases, the main secondary phase (MgZn2) dissolves into the aluminum matrix to form a supersaturated solid solution. Part of the solute atoms diffuses along the vacancies and dislocations under the combined effect of interface energy, strain energy, and grain boundary energy. In addition, grain boundary segregation occurs and further leads to relatively coarse secondary phase. With the increase of temperature, discontinuous precipitation (DP), which occurs from the grain boundary to the grain internal, dominates the formation process of secondary phases. The phase transformation brings about significant precipitation strengthening, resultantly, enhances strength, and ductility are achieved for 7A60 aluminum alloy via ECAP. Moreover, the precipitation strengthening effect, which is closely related with the dissolution and DP, first enhances and then attenuates with the increase of ECAP passes. Dislocation strengthening and Hall‐Petch strengthening aggravate monotonically with the ECAP passes but play less important roles in the overall stress responses.

Assembly structures and dynamics of active colloidal cells.

Many types of active matter are deformable, such as epithelial cells and bacteria. To mimic the feature of deformability, we built a model called an active colloidal cell (ACC), i.e. a vesicle enclosed with self-propelled particles (SPPs), which as a whole can move actively. Based on the model, we then study the role of deformability in the assembly structures and dynamics of ACCs by Langevin dynamics simulation. We find that deformability weakens the self-trapping effect and hence suppresses the clustering and phase separation of the deformable soft ACCs (sACCs). Instead of forming a large compact cluster like ordinary SPPs, sACCs pack into a loose network or porous structure in the phase-separation region. The condensed phase is liquid-like, in which sACCs are strongly compressed and deformed but still keep high motility. The interface between the gas and the condensed phases is blurry and unstable, and the effective interfacial energy is very low. Our work gives new insights into the role of deformability in the assembly of active matter and also provides a reference for further studies on different types of deformable active matter.

A numerical model for calculation of the restitution coefficient of elastic-perfectly plastic and adhesive bodies with rough surfaces

A contact mechanics model based on Boundary Element Method (BEM) is used to simulate the contact of rough spheres having elastic-perfectly plastic adhesive behaviour. The model has been verified for smooth surfaces with the classical Hertz contact behaviour in elastic regime, JKR theory in the adhesive regime and Thornton and Ning's analytical solution in the elasto-plastic adhesive regime. In this paper, the effect of yield stress, interface energy and surface roughness on the Coefficient of Restitution (COR) is investigated and reported. The results show that surface roughness dramatically affects the COR and higher roughness values lead to lower CORs in general. In addition, it is found that in the presence of surface roughness, the COR varies based on different maximum indentation depths in 3 different stages; the asperity dominant, bulk elastic dominant and bulk plastic dominant. Interestingly, it was shown that there is a critical indentation depth beyond which the effect of surface roughness will disappear and rough surfaces act like smooth ones. This critical indentation depth is proportional to the Root Mean Square roughness (Rq) value of the surface. Numerical results suggest that the yield stress influences the COR and higher yields stress results in higher COR for both rough and smooth surfaces. Furthermore, the effect of interface energy on the COR for rough surfaces is minimal and only significant at low indentations.

Atomic-scale analyses of Nb3Sn on Nb prepared by vapor diffusion for superconducting radiofrequency cavity applications: a correlative study

We report on atomic-scale analyses of the microstructure of an Nb3Sn coating on Nb, prepared by a vapor diffusion process for superconducting radiofrequency (SRF) cavity applications using transmission electron microscopy, electron backscatter diffraction and first-principles calculations. Epitaxial growth of Nb3Sn on a Nb substrate is found and four types of orientation relationships (ORs) at the Nb3Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (HR-STEM) analyses. Thin Nb3Sn grains are observed in regions with a low Sn flux and they have a specific OR: Nb3Sn //Nb and Nb3Sn //Nb The Nb3Sn/Nb interface of thin grains has a large lattice mismatch, 12.3%, between Nb and Nb3Sn (002) and a high density of misfit dislocations as observed by HR-STEM. Based on our microstructural analyses of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial migration with this particular OR. The Sn-deficient regions are seen to form initially at the Nb3Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb3Sn. The formation of Sn-deficient regions and the effects of interfacial energies on the formation of Sn-deficient regions at different interfaces are estimated by first-principles calculations. The finding of ORs at the Nb3Sn/Nb interface provides important information about the formation of imperfections in Nb3Sn coatings, such as large thin-regions and Sn-deficient regions, which are critical to the performance of Nb3Sn SRF cavities for accelerators.

Computational modelling of postbuckling behavior of composite T-stiffened panels with different bonding methods

This study concerns a computational model for the postbuckling failure mechanism of composite T-stiffened panels with different bonding methods. The model is then validated against the available experiments and excellent agreement is obtained. Further, the model is used to predict the postbuckling behavior of the stiffeners co-cured, co-bonded, and secondary bonded to the skin, where the new separation phenomenon including margin cohesive failure, local debonding, and global debonding are captured. Moreover, a parametric study is also carried out to investigate the effect of skin-stiffener interface energy and strength on the postbuckling response of composite panels. The numerical results show that the bonding method has little effect on the uniaxial compressive stiffness of the composite T-stiffened panel, but influences the collapse load and failure process by affecting the skin-stiffener interface behavior. Besides, the collapse load and the failure mode are influenced by the ratio of the interface strength to the interlaminar shear strength. The fiber failure location and shape of load-shortening curve are affected by the ratio of the interface energy to the interlaminar fracture energy.

Influences of interface properties on the wave propagation in the dipolar gradient elastic solid

The present work mainly focuses on the influence of interface material parameters, namely the interface mass density, the interface elastic rigidity, and the interface inertial interaction constants, on the reflection and transmission behavior of elastic wave propagating through dipolar gradient elastic solids. First, the interface kinetic energy density and interface deformation energy density are taken into account. By application of Hamilton’s variation principle, the governing equations and the boundary conditions of the dipolar gradient elastic solid are obtained. Due to the consideration of microstructure effects of the material, the interface conditions can be proposed in different forms. These interfacial conditions which include the microstructure effects and interface energy effects are then used to determine the amplitude ratio of reflection and transmission waves. A numerical example is provided for the generalized internal roller interface. The influence of the interface material parameters upon the reflection and transmission coefficients in terms of energy fluxes ratio is discussed based on the numerical results. It is revealed that the reflection and transmission behavior can be manipulated by the elaborated design of the interface at both the macroscale and the microscale.

Competitive growth of Al2O3/YAG/ZrO2 eutectic ceramics during directional solidification: Effect of interfacial energy

AbstractThe microstructure evolution and growth behavior of the Al2O3/Y3Al5O12(YAG)/ZrO2 ternary eutectic ceramics during directional solidification were well investigated. During directional solidification of the Al2O3/YAG/ZrO2 ternary eutectic ceramics, {} Al2O3 paralleled with {001}ZrO2 while they did not parallel with {001}YAG at the same time in the competitive growth stage. All of the interfaces parallel to each other finally. The area percentage of the Al2O3/ZrO2 and YAG/ZrO2 interfaces are 40.4 ± 0.2% and 30.8 ± 0.1%, respectively, higher than that of the Al2O3/YAG (28.8 ± 0.2%). The content of Al2O3 and YAG phases are 39.9% and 41.1%, respectively, almost double of that of ZrO2. The interfaces of Al2O3/ZrO2 and YAG/ZrO2 are shorter and more dispersed than that of the Al2O3/YAG. It was found that the interfacial energy of Al2O3/ZrO2 and YAG/ZrO2 interfaces are lower than that of Al2O3/YAG. It can be concluded that interfacial energy plays a decisive role in affecting the crystallographic orientation and interfaces distribution in the Al2O3/YAG/ZrO2 eutectic since the interfaces of Al2O3/ZrO2 and YAG/ZrO2 with lower interfacial energy can be formed more easily during directional solidification. Therefore, the contents of Al2O3/ZrO2 and YAG/ZrO2 interfaces are higher. This study can provide theoretical guidance for interface design of multi‐phase materials.

Structural Variation of Self-Organized Mg Hydride Nanoclusters in Immiscible Ti Matrix by Hydrogenation.

Hydrogenation of nonequilibrium alloys may form nanometer-sized metal hydride clusters, depending on the alloy compositions and hydrogenation conditions. Here in the Ti-rich compositions of the immiscible Mg-Ti system MgH2 clusters are embedded in a Ti-H matrix. Our previous works have indicated that the interface energy between the two metal hydrides reduces the stability of MgH2. The aim of our study is to obtain the structural information on the nanometer-sized clusters. Indeed, MgD2 clusters embedded in a face-centered-cubic (FCC) Ti-D matrix is found in Mg0.25Ti0.75D1.65 by means of 2H magic angle spinning nuclear magnetic resonance (MAS NMR). The atomic pair distribution function (PDF) analysis of neutron total scattering data suggests that the MgD2 clusters have an orthorhombic structure, which is different from a rutile-type body-centered-tetragonal (BCT) structure of α-MgD2 observed in the Mg-rich compositions. Our results suggest that we can tune the thermodynamics of hydrogen absorption and desorption in Mg-H using the interface energy effect and accompanying stress-induced structural change, which contributes to the substantial development of lightweight and inexpensive hydrogen storage materials.

The influence of particle interfacial energies and mixing energy on the mixture quality of the dry-coating process

We investigate the effect of particle interface energies and mixing energy input on the macroscopic behavior of the dry-coating process by using the discrete element method (DEM). It is observed that the quality of the coating process is governed by two dimensionless numbers: the Stokes number St (mixing energy/strength of agglomerates) and the reduced intermixing coefficient Λ (cohesion /adhesion strength). Three unfavorable and one favorable process regimes were identified, and represented in a regime map as a function of St and Λ. For low St and Λ, carriers are lumped and random mixing is fairly poor. For low St and high Λ, the agglomerates are merged together and remain intact. At high St, the fine-carrier adhesion breaks and creates abundance of debris. Between these regions process conditions are favorable as is supported by experimental evidences. Results of this study can be used to establish guidelines for efficient operation of the dry-coating process in a high-shear mixer.

Modeling of Precipitation Behavior of Complex Precipitates in Ferrite and Validation with Experiment

Based on solid solution theory, mass conservation law and classical nucleation, growth and coarsening theory, a thermo/kinetic model is developed to predict the precipitation behavior of complex precipitates in Nb‐V bearing steel. The equilibrium composition of matrix and precipitates is calculated by the thermodynamic model and is input in the kinetic model to calculate the driving force for precipitation at a given temperature. The diffusivity and concentration of alloying elements is considered in the entire precipitation process. The effect of interfacial energy of precipitates and activation energy on precipitation behavior are studied. Simulation results indicate that the interfacial energy plays a significant role in the entire process of precipitation and there is no overlap between nucleation and coarsening when the interfacial energy is in the range of 0.5–0.55 Jm−2. The mean particle diameter predicted by the proposed model is compared with the experimental results and a good agreement was obtained.

Spontaneous perpendicular exchange bias effect in L1-MnGa/FeMn bilayers grown by molecular-beam epitaxy

We report on the spontaneous perpendicular exchange bias effect in as-grown L10-MnGa/FeMn bilayers. An FeMn layer with different thicknesses is introduced as an antiferromagnetic layer to couple with single-crystalline ferromagnetic L10-MnGa, which is epitaxially grown on a GaAs (001) substrate by molecular-beam epitaxy. The perpendicular exchange bias shows a strong dependence on both the thickness of the FeMn layer and the measurement temperature. A large spontaneous perpendicular exchange bias up to 8.9 kOe is achieved in L10-MnGa/FeMn bilayers at 5 K without any external magnetic treatment. The corresponding effective interfacial exchange energy Jeff is estimated to be 1.4 mJ/m2. The spontaneous perpendicular exchange bias effect in the (001) textured L10-MnGa/FeMn bilayers paves the way for spintronic devices based on exchange biased perpendicularly magnetized materials.

Oxidation Protection with Amorphous Surface Oxides: Thermodynamic Insights from Ab Initio Simulations on Aluminum.

Native surface films play a key role in the oxidation and corrosion protection of functional and structural materials. Here, we present a fully ab initio approach for understanding the thermodynamic driving force behind the initial phase selection among amorphous and crystalline structures for a surface film growing on a crystalline substrate. We apply the approach to elucidate the competition among corundum (α), spinel (γ), and amorphous (am.) Al2O3 films growing on aluminum metal. We show that the amorphous Al2O3 film becomes thermodynamically the most stable form below around ∼1 nm, that is, the relative energetic stabilities of thin polymorphic Al2O films follow am. < γ < α. As the film thickness increases, the relative stability relation first changes to γ < α < am. and then to the bulk limit of α < γ < am. The nanoscale γ films distort substantially to form exclusively four- and fivefold-coordinated Al-O polyhedra, lose the close-packed O framework, and become "amorphous-like", that is, exhibit both short-range order and energetic characteristics that are commensurate with the amorphous form. Our results provide a quantitative, first-principles confirmation for the early hypotheses on the thermodynamic stability of amorphous surface films and provide insights for the critical role they play in oxidation protection. Handling the complexities associated with the initial film growth, including bulk, surface, interface, and strain energy effects in realistically complex ab initio simulations, we expect this approach to contribute to understanding of the mechanism behind effective passivation films for aluminum alloys and beyond.

Excellent selector performance in engineered Ag/ZrO2:Ag/Pt structure for high-density bipolar RRAM applications

A high-performance selector with bidirectional threshold switching (TS) characteristics of Ag/ZrO2/Pt structure was prepared by incorporating metallic Ag into the ZrO2 matrix. The bidirectional TS device exhibited excellent switching uniformity, forming-free behavior, ultra-low off current of &amp;lt;1 nA and adjustable selectivity (from 102 to 107). The experiment results confirmed that metallic Ag clusters were penetrated into the ZrO2 matrix during the annealing process, which would function as an effective active source responsible for the bidirectional TS. The volatile behavior could be explained by the self-dissolution of unstable filaments caused by minimization of the interfacial energy and thermal effect. Furthermore, a bipolar-type one selector-one resistor (1S-1R) memory device was successfully fabricated and exhibited significant suppression of the undesired sneak current, indicating the great potential as selector in a cross-point array.

Extraction of effective solid-liquid interfacial free energies for full 3D solid crystallites from equilibrium MD simulations.

Molecular dynamics simulations of an embedded atom copper system in the isobaric-isenthalpic ensemble are used to study the effective solid-liquid interfacial free energy of quasi-spherical solid crystals within a liquid. This is within the larger context of molecular dynamics simulations of this system undergoing solidification, where single individually prepared crystallites of different sizes grow until they reach a thermodynamically stable final state. The resulting equilibrium shapes possess the full structural details expected for solids with weakly anisotropic surface free energies (in these cases, ∼5% radial flattening and rounded [111] octahedral faces). The simplifying assumption of sphericity and perfect isotropy leads to an effective interfacial free energy as appearing in the Gibbs-Thomson equation, which we determine to be ∼177 erg/cm2, roughly independent of crystal size for radii in the 50-250 Å range. This quantity may be used in atomistically informed models of solidification kinetics for this system.

Heterogeneous Nucleation of Colloidal Crystals on a Glass Substrate with Depletion Attraction.

The heterogeneous nucleation of colloidal crystals with attractive interactions has been investigated via in situ observations. We have found two types of nucleation processes: a cluster that overcomes the critical size for nucleation with a monolayer, and a method that occurs with two layers. The Gibbs free energy changes (ΔG) for these two types of nucleation processes are evaluated by taking into account the effect of various interfacial energies. In contrast to homogeneous nucleation, the change in interfacial free energy, Δσ, is generated for colloidal nucleation on a foreign substrate such as a cover glass in the present study. The Δσ and step free energy of the first layer, γ1, are obtained experimentally based on the equation deduced from classical nucleation theory (CNT). It is concluded that the ΔG of q-2D nuclei is smaller than of monolayer nuclei, provided that the same number of particles are used, which explains the experimental result that the critical size in q-2D nuclei is smaller than that in monolayer nuclei.

Micromechanics of substrate-supported thin films

The mechanical properties of metallic thin films deposited on a substrate play a crucial role in the performance of micro/nano-electromechanical systems (MEMS/NEMS) and flexible electronics. This article reviews ongoing study on the mechanics of substrate-supported thin films, with emphasis on the experimental characterization techniques, such as the rule of mixture and X-ray tensile testing. In particular, the determination of interfacial adhesion energy, film deformation, elastic properties and Bauschinger effect are discussed.

Predicting the morphologies of γʹ precipitates in cobalt-based superalloys

Cobalt-based alloys with γ/γ′ microstructures have the potential to become the next generation of superalloys, but alloy compositions and processing steps must be optimized to improve coarsening, creep, and rafting behavior. While these behaviors are different than in nickel-based superalloys, alloy development can be accelerated by understanding the thermodynamic factors influencing microstructure evolution. In this work, we develop a phase field model informed by first-principles density functional theory and experimental data to predict the equilibrium shapes of Co-Al-W γ′ precipitates. Three-dimensional simulations of single and multiple precipitates are performed to understand the effect of elastic and interfacial energy on coarsened and rafted microstructures; the elastic energy is dependent on the elastic stiffnesses, misfit strain, precipitate size, applied stress, and precipitate spatial distribution. We observe characteristic microstructures dependent on the type of applied stress that have the same γ′ morphology and orientation seen in experiments, indicating that the elastic stresses arising from coherent γ/γ′ interfaces are important for morphological evolution during creep. The results also indicate that the narrow γ channels between γ′ precipitates are energetically favored, and provide an explanation for the experimentally observed directional coarsening that occurs without any applied stress.

Equivalent inclusions in micromechanics with interface energy effect

In order to apply classical micromechanics in predicting the effective prop-erties of nanocomposites incorporating interface energy, a concept of equivalent inclusion (EI) is usually adopted. The properties of EI are obtained by embedding a single inclusion with the interface into an infinite matrix. However, whether such an EI is universal for different micromechanics-based methods is rarely discussed in the literature. In this pa-per, the interface energy theory is used to study the applicability of the above mentioned EI. It is found that some elastic properties of the EI are related only to the properties of the inclusion and the interface, whereas others are also related to the properties of the matrix. The former properties of the EI can be applied to both the classical Mori-Tanaka method (MTM) and the generalized self-consistent method (GSCM). However, the latter can be applied only to the MTM. Two kinds of new EIs are proposed for the GSCM and used to estimate the effective mechanical properties of nanocomposites.

δ‐Doping Effects on Electronic and Energetic Properties of LaAlO3/SrTiO3 Heterostructure: First‐Principles Analysis of 23 Transition‐Metal Dopants

AbstractThe 2D electron gas (2DEG) formed at the interface between two insulating perovskite oxides such as LaAlO3 and SrTiO3 provides a playground for developing all‐oxide electronic devices, though improving the 2DEG mobility is still a great challenge. One possible way of improving the 2DEG mobility is via δ‐doping at the heterointerface. As a proof of concept, one recent experiment achieves an ultra‐high 2DEG mobility of 73 000 cm2 V−1 s−1 at the LaAlO3/SrTiO3 heterointerface via Mn δ‐doping. Here the electronic and energetic properties of δ‐doped LaAlO3/SrTiO3 are studied with 23 transition‐metal dopants from group 3 to group 10 using first‐principles calculations. A clear trend is found for the electron effective mass and interfacial energy change in the δ‐doped LaAlO3/SrTiO3 with various dopants, and there exists a trade‐off between achieving light effective mass bands and forming energetically favorable structures. It is found that the Fe, Co, Ni, Ru, Rh, Pd, Os, and Ir could also serve as promising candidate dopants to produce light effective mass bands and relatively high energetic stability, in addition to the experimentally confirmed Mn dopant. The findings of this study provide a wide avenue to increase the 2DEG mobility in the LaAlO3/SrTiO3 heterostructure via δ‐doping with transition metals.