arXiv subjects

Seunghwa Ryu

Publications and source records attributed to Seunghwa Ryu.

36 records · Page 2Linked to original sources

arXiv2022

Revealing 3-dimensional core-shell interface structures at the single-atom level

Nanomaterials with core-shell architectures are prominent examples of strain-engineered materials, where material properties can be designed by fine-tuning the misfit strain at the interface. Here, we elucidate the full 3D atomic structure of Pd@Pt core-shell nanoparticles at the single-atom level via atomic electron tomography. Full 3D displacement fields and strain profiles of core-shell nanoparticles were obtained, which revealed a direct correlation between the surface and interface strain. It also showed clear Poisson effects at the scale of the full nanoparticle as well as the local atomic bonds. The strain distributions show a strong shape-dependent anisotropy, whose nature was further corroborated by molecular statics simulations. From the observed surface strains, the surface oxygen reduction reaction activities were predicted. These findings give a deep understanding of structure-property relationships in strain-engineerable core-shell systems, which could pave a new way toward direct control over the resulting catalytic properties.

cond-mat.mtrl-sci↗

arXiv2021

A combined experimental/numerical study on the scaling of impact strength and toughness in composite laminates for ballistic applications

In this paper, the impact behaviour of composite laminates is investigated, and their potential for ballistic protection assessed, as a function of the reinforcing materials and structures for three representative fibre-reinforced epoxy systems involving carbon, glass, or para-aramid fibre reinforcements, respectively. A multiscale coupled experimental/numerical study on the composite material properties is performed, starting from single fibre, to fibre bundles (yarns), to single composite ply, and finally at laminate level. Uniaxial tensile tests on single fibres and fibre bundles are performed, and the results are used as input for non-linear Finite Element Method (FEM) models for tensile and impact simulation on the composite laminates. Mechanical properties and energy dissipation of the single ply and multilayer laminates under quasi-static loading are preliminarily assessed starting from the mechanical properties of the constituents and subsequently verified numerically. FEM simulations of ballistic impact on multilayer armours are then performed, assessing the three different composites, showing good agreement with experimental tests in terms of impact energy absorption capabilities and deformation/failure behaviour. As result, a generalized multiscale version of the well-known Cuniff criterion is provided as a scaling law, which allows to assess the ballistic performance of laminated composites, starting from the tensile mechanical properties of the fibres and fibre bundles and their volume fraction. The presented multiscale coupled experimental-numerical characterization confirms the reliability of the predictions for full-scale laminate properties starting from the individual constituents at the single fibre scale.

physics.app-ph↗

arXiv2020

Stress Induced Structural Transformations in Au Nanocrystals

Nanocrystals can exist in multiply twinned structures like the icosahedron, or single crystalline structures like the cuboctahedron or Wulff-polyhedron. Structural transformation between these polymorphic structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high temperature diffusion mediated processes. Thus, there is limited experimental evidence of displacive motion mediated structural transformations. Here, we report the high-pressure structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell that is driven by displacive motion. In-situ X-ray diffraction and transmission electron microscopy were used to detect the transformation of multiply twinned nanocrystals into single crystalline nanocrystals. High-pressure single crystalline nanocrystals were recovered after unloading, however, the nanocrystals quickly reverted back to multiply twinned state after redispersion in toluene solvent. The dynamics of recovery was captured using transmission electron microscopy which showed that the recovery was governed by surface recrystallization and rapid twin boundary motion. We show that this transformation is energetically favorable by calculating the pressure-induced change in strain energy. Molecular dynamics simulations showed that defects nucleated from a region of high stress region in the interior of the nanocrystal, which make twin boundaries unstable. Deviatoric stress driven Mackay transformation and dislocation/disclination mediated detwinning are hypothesized as possible mechanisms of high-pressure structural transformation.

cond-mat.mtrl-sci↗

arXiv2020

How and When the Cassie-Baxter Droplet Starts to Slide on the Textured Surfaces

The Cassie-Baxter state droplet has many local energy minima on the textured surface, while the amount of the energy barrier between them can be affected by the gravity. When the droplet cannot find any local energy minimum point on the surface, the droplet starts to slide. Based on the Laplace pressure equation, the shape of a two-dimensional Cassie-Baxter droplet on a textured surface is predicted. Then the stability of the droplet is examined by considering the interference between the liquid and the surface microstructure as well as analyzing the free energy change upon the de-pinning. Afterward, the theoretical analysis is validated against the line-tension based front tracking method simulation (LTM), that seamlessly captures the attachment and detachment between the liquid and the substrate. We answer to the open debates on the sliding research field: (i) Whether the sliding initiates with the front end slip or the rear end slip, and (ii) whether the advancing and receding contact angles measured on the horizontal surface are comparable with the front and rear contact angle of the droplet at the onset of sliding. Additionally, a new droplet translation mechanism promoted by cycle of condensation and evaporation is suggested.

cond-mat.soft↗

arXiv2019

Gravitational effect on the advancing and receding angle of a 2D Cassie-Baxter droplet on a textured surface

Advancing and receding angles are physical quantities frequently measured to characterize the wetting properties of a rough surface. Thermodynamically, the advancing and receding angles are often interpreted as the maximum and minimum contact angles that can be formed by a droplet without losing its stability. Despite intensive research on wetting of rough surfaces, the gravitational effect on these angles has been overlooked because most studies have considered droplets smaller than the capillary length. In this study, however, by combining theoretical and numerical modeling, we show that the shape of a droplet smaller than the capillary length can be substantially modified by gravity under advancing and receding conditions. First, based on the Laplace pressure equation, we predict the shape of a two-dimensional Cassie-Baxter droplet on a textured surface with gravity at each pinning point. Then the stability of the droplet is tested by examining the interference between the liquid surface and neighboring pillars as well as analyzing the free energy change upon depinning. Interestingly, it turns out that the apparent contact angles under advancing and receding conditions are not affected by gravity, while the overall shape of a droplet and the position of the pinning point is affected by gravity. In addition, the advancing and receding of the droplet with continuously increasing or decreasing volume is analyzed, and it is showed that the gravitational effect plays a key role on the movement of the droplet tip. Finally, the theoretical predictions were validated against line-tension based front tracking modeling that seamlessly captures the attachment and detachment between the liquid surface and the solid substrate.

cond-mat.soft↗

arXiv2019

Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure

As circuitry approaches single nanometer length scales, it is important to predict the stability of metals at these scales. The behavior of metals at larger scales can be predicted based on the behavior of dislocations, but it is unclear if dislocations can form and be sustained at single nanometer dimensions. Here, we report the formation of dislocations within individual 3.9 nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We used a combination of x-ray diffraction, optical absorbance spectroscopy, and molecular dynamics simulation to characterize the defects that are formed, which were found to be surface-nucleated partial dislocations. These results indicate that dislocations are still active at single nanometer length scales and can lead to permanent plasticity.

cond-mat.mtrl-sci↗

arXiv2018

Investigation of effective thermoelectric properties of composite with interfacial resistance using micromechanics-based homogenisation

We obtained the analytical expression for the effective thermoelectric properties and dimensionless figure of merit of a composite with interfacial electrical and thermal resistances using a micromechanics-based homogenisation. For the first time, we derived the Eshelby tensor for a spherical inclusion as a function of the interfacial resistances and obtained the solutions of the effective Seebeck coefficient and the electrical and thermal conductivities of a composite, which were validated against finite-element analysis (FEA). Our analytical predictions well match the effective properties obtained from FEA with an inclusion volume fraction up to 15%. Because the effective properties were derived with the assumption of a small temperature difference, we discuss a heuristic method for obtaining the effective properties in the case where a thermoelectric composite is subjected to a large temperature difference.

cond-mat.mtrl-sci↗

arXiv2018

Applicability of the interface spring model for micromechanical analyses with interfacial imperfections to predict the modified exterior Eshelby tensor and effective modulus

Closed-form solutions for the modified exterior Eshelby tensor, strain concentration tensor, and effective moduli of particle-reinforced composites are presented when the interfacial damage is modeled as a linear spring layer of vanishing thickness; the solutions are validated against finite element analyses. Based on the closed-form solutions, the applicability of the interface spring model is tested by calculating those quantities using finite element analysis (FEA) augmented with a matrix-inhomogeneity non-overlapping condition. The results indicate that the interface spring model reasonably captures the characteristics of the stress distribution and effective moduli of composites, despite its well-known problem of unphysical overlapping between the matrix and inhomogeneity.

cond-mat.other↗

arXiv2018

Systematic investigation of the deformation mechanisms of a γ-TiAl single crystal

We propose a theoretical framework to predict the deformation mechanism of the γ-TiAl single crystal without lattice defects by combining the generalized stacking fault energy and the Schmid factor. Our theory is validated against an excellent testbed, the single crystal nanowire, by correctly predicting four major deformation mechanisms, namely, ordinary slip, super slip, twinning, and mixed slip/fracture observed during the tensile and compressive tests along 10 different orientations using molecular dynamics simulations. Interestingly, although lattice defects are not taken into account, the theoretical predictions match well with existing experiments on bulk specimen with only a few exceptions; the exceptions are discussed based on the size-dependent deformation mechanism in the presence of preexisting dislocation sources. We expect that the method in this paper can be generalized to study various ductile intermetallic crystals where conventional Schmid law does not hold well.

cond-mat.mtrl-sci↗

arXiv2018

Modified Eshelby tensor for an anisotropic matrix with interfacial damage

We derive a simple tensor algebraic expression of the modified Eshelby tensor for a spherical inclusion embedded in an arbitrarily anisotropic matrix in terms of three tensor quantities (the 4th order identity tensor, the elastic stiffness tensor, and the Eshelby tensor) and two scalar quantities (the inclusion radius and interfacial spring constant), when the interfacial damage is modelled as a linear-spring layer of vanishing thickness. We validate the expression for a triclinic crystal involving 21 independent elastic constants against finite element analysis (FEA).

cond-mat.other↗

arXiv2018

Phase field modeling of crack propagation under combined shear and tensile loading with hybrid formulation

The crack phase field model has been well established and validated for a variety of complex crack propagation patterns within a homogeneous medium under either tensile or shear loading. However, relatively less attention has been paid to crack propagation under combined tensile and shear loading or crack propagation within composite materials made of two constituents with very different elastic moduli. In this work, we compare crack propagation under such circumstances modelled by two representative formulations, anisotropic and hybrid formulations, which have distinct stiffness degradation schemes upon crack propagation. We demonstrate that the hybrid formulation is more adequate for modeling crack propagation problems under combined loading because the residual stiffness of the damaged zone in the anisotropic formulation may lead to spurious crack growth and altered load-displacement response.

cond-mat.mtrl-sci↗

arXiv2018

Theoretical study of the effective modulus of a composite considering the orientation distribution of the fillers and the weakened interface

In the manufacturing process of a filler-reinforced composite, the fillers in the matrix are aligned due to the shear flow occurring during the drawing stage, and the interface between the matrix and the fillers form various imperfections that lead to debonding and slip under mechanical loading. Hence, there have been numerous micromechanics studies to predict effective moduli of the composites in the presence of partial alignment of fillers and interface imperfections. In this study, we present an improved theory that overcomes two limitations in the existing micromechanics based approaches. First, we find that the interface damage tensor, which has been developed to model the weakened interface between matrix and fillers, has singularities that cause non-physical predictions (such as infinite or negative effective moduli). We correct the mathematical mistakes to remove singularities and derive analytic expressions of the damage tensor for ellipsoidal inclusions. Second, we reveal that the previous theory on the effective moduli with axisymmetric filler orientation distribution fails because the longitudinal and transverse moduli do not converge in the limit of random orientation distribution. With appropriate corrections, we derive an analytic expression for the orientation average of arbitrary transversely isotropic 4th order tensor under general axisymmetric orientation distribution. We apply the improved method to compute the effective moduli of a representative composite with non-uniform filler orientation and interface damage.

cond-mat.mtrl-sci↗

arXiv2018

A micromechanics-based analytical solution for the effective thermal conductivity of composites with orthotropic matrices and interfacial thermal resistance

We obtained an analytical solution for the effective thermal conductivity of composites composed of orthotropic matrices and spherical inhomogeneities with interfacial thermal resistance using a micromechanics-based homogenization. We derived the closed form of a modified Eshelby tensor as a function of the interfacial thermal resistance. We then predicted the heat flux of a single inhomogeneity in the infinite media based on the modified Eshelby tensor, which was validated against the numerical results obtained from the finite element analysis. Based on the modified Eshelby tensor and the localization tensor accounting for the interfacial resistance, we derived an analytical expression for the effective thermal conductivity tensor for the composites by a mean-field approach called the Mori-Tanaka method. Our analytical prediction matched very well with the effective thermal conductivity obtained from finite element analysis with up to 10% inhomogeneity volume fraction.

cond-mat.mtrl-sci↗

arXiv2018

The effect of the misfit dislocation on the in-plane shear response of the ferrite/cementite interface

Although the pearlitic steel is one of the most extensively studied materials, there are still questions unanswered about the interface in the lamellar structure. In particular, to deepen the understanding of the mechanical behavior of pearlitic steel with fine lamellar structure, it is essential to reveal the structure-property relationship of the ferrite/cementite interface (FCI). In this study, we analyzed the in-plane shear deformation of the FCI using atomistic simulation combined with extended atomically informed Frank-Bilby (xAIFB) method and disregistry analyses. In the atomistic simulation, we applied in-plane shear stress along twelve different directions to the ferrite/cementite bilayer for Isaichev (IS), Near Bagaryatsky (Near BA) and Near Pitsch-Petch (Near PP) orientation relationship (OR), respectively. The simulation results reveal that IS and Near BA ORs show dislocation-mediated plasticity except two directions, while Near PP OR shows mode II (in-plane shear) fracture at the FCI along all directions. Based on the xAIFB and disregistry analysis results, we conclude that the in-plane shear behavior of the FCI is governed by the magnitude of Burgers vector and core-width of misfit dislocations.

cond-mat.mtrl-sci↗

arXiv2016

Wetting theory for small droplets on textured solid surfaces

Conventional wetting theories on rough surfaces with Wenzel, Cassie-Baxter, and Penetrate modes suggest the possibility of tuning the contact angle by adjusting the surface texture. Despite decades of intensive study, there are still many experimental results that are not well understood because conventional wetting theory, which assume an infinite droplet size, has been used to explain measurements of finite-sized droplets. In this study, we suggest a wetting theory that is applicable to any droplet size based on the free energy landscape analysis of various wetting modes of finite-sized droplets on a 2D textured surface. The key finding of our study is that there are many quantized wetting angles with local free energy minima; the implication of this is remarkable. We find that the conventional theories can predict the contact angle at the global free energy minimum if the droplet size is 40 times or larger than the characteristic scale of the surface roughness. Furthermore, we confirm that the pinning origin is the local free energy minima and obtain the energy barriers of pinning as a function of geometric factors. We validate our theory against experimental results on an anisotropic rough surface. In addition, we discuss wetting on a non-uniform rough surface with a rough central region and flat edge. Our findings clarify the extent to which the conventional wetting theory is valid and expand the physical understanding of wetting phenomena of small liquid drops on rough surfaces.

physics.flu-dyn↗

arXiv2016

Molecular dynamics simulation of nanoindentation on nanocomposite pearlite

We carry out molecular dynamics simulations of nanoindentation to investigate the effect of cementite size and temperature on the deformation behavior of nanocomposite pearlite composed of alternating ferrite and cementite layers. We find that, instead of the coherent transmission, dislocation propagates by forming a widespread plastic deformation in cementite layer. We also show that increasing temperature enhances the distribution of plastic strain in the ferrite layer, which reduces the stress acting on the cementite layer. Hence, thickening cementite layer or increasing temperature reduces the likelihood of dislocation propagation through the cementite layer. Our finding sheds a light on the mechanism of dislocation blocking by cementite layer in the pearlite.

cond-mat.mtrl-sci↗

arXiv2015

An analytical benchmark for MD codes: testing LAMMPS on the 2nd generation Brenner potential

An analytical benchmark is proposed for graphene and carbon nanotubes, that may serve to test whatsoever molecular dynamics code implemented with REBO potentials. By exploiting the benchmark, we checked results produced by LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator) when adopting the second generation Brenner potential, we made evident that the code in its current implementation produces results which are offset from those of the benchmark by a significant amount, and provide evidence of the reason.

cond-mat.mtrl-sci↗

arXiv2011

Entropic Effect on the Rate of Dislocation Nucleation

Dislocation nucleation is essential to our understanding of plastic deformation, ductility and mechanical strength of crystalline materials. Molecular dynamics simulation has played an important role in uncovering the fundamental mechanisms of dislocation nucleation, but its limited time scale remains a significant challenge for studying nucleation at experimentally relevant conditions. Here we show that dislocation nucleation rates can be accurately predicted over a wide range of conditions by determining the activation free energy from umbrella sampling. Our data reveal very large activation entropies, which contribute a multiplicative factor of many orders of magnitude to the nucleation rate. The activation entropy at constant strain is caused by thermal expansion, with negligible contribution from the vibrational entropy. The activation entropy at constant stress is significant larger than that at constant strain, as a result of thermal softening. The large activation entropies are caused by anharmonic effects, showing the limitations of the harmonic approximation widely used in solids. Similar behaviors are expected to occur in other nucleation processes in solids.

cond-mat.mtrl-sci↗