Seunghwa Yang

Observation of inhomogeneous domain nucleation in epitaxial Pb(Zr,Ti)O3 capacitors

We investigated domain nucleation process in epitaxial Pb(Zr,Ti)O3 capacitors under a modified piezoresponse force microscope. We obtained domain evolution images during polarization switching process and observed that domain nucleation occurs at particular sites. This inhomogeneous nucleation process should play an important role in an early stage of switching and under a high electric field. We found that the number of nuclei is linearly proportional to log(switching time), suggesting a broad distribution of activation energies for nucleation. The nucleation sites for a positive bias differ from those for a negative bias, indicating that most nucleation sites are located at the ferroelectric/electrode interfaces.

Scale bridging method to characterize mechanical properties of nanoparticle/polymer nanocomposites

Multiscale analysis to characterize the size effect of silica nanoparticles on the mechanical properties of nanoparticle/polymer nanocomposites is developed and verified through a molecular dynamics simulation and continuum micromechanics model. In the micromechanics model, the particle-matrix interface mechanical property is incorporated, and the thickness and elastic properties of the interface are extracted from the atomistic structures. The proposed multiscale micromechanics model accurately reflects the size effect of the nanoparticle. The prediction by the current multiscale model at various volume fractions is compared to the results of the molecular dynamics simulations in order to validate the present multiscale analysis model.

A scale-bridging method for nanoparticulate polymer nanocomposites and their nondilute concentration effect

Due to the enhanced interaction among constituents and the resultant effect of particle size on the elastic modulus, nanocomposites with volume fractions higher than 12% show critical nondilute concentrations. In order to describe nondilute concentrations and the effect of particle size, a multi-inclusion model was adopted and a method was developed to transfer nanoscale physical information to a continuum-based analytical model. The accuracy and applicability of the present scale-bridging method at dilute and nondilute concentrations were verified by comparing results of the present method with those of molecular dynamics and finite element analysis.

Multiscale modeling of cross-linked epoxy nanocomposites to characterize the effect of particle size on thermal conductivity

A sequential multiscale model to characterize the size effects of nanoparticles on the effective thermal conductivity of SiC/epoxy nanocomposites is developed through non-equilibrium molecular dynamics (NEMD) simulations and continuum micromechanics. Even at the fixed volume fraction condition of a spherical nanoparticle, a significant particle size effect on the thermal conductivity of SiC/epoxy nanocomposites has been demonstrated using NEMD simulations. The main contributions of the particle size dependency are Kapitza thermal resistance at the interface between the particle and matrix, and the formation of highly densified polymer sheathing (adsorption layer) near the particle. To account for these two effects in a continuum regime, both the Kapitza interface and the effective interphase are defined in a micromechanics model, and a four-phase multiscale bridging method is suggested. The thermal conductivity of the effective interphase is implicitly obtained from the four-phase micromechanics model. Th...

Sequential thermoelastic multiscale analysis of nanoparticulate composites

The thermoelastic properties of SiC/epoxy nanocomposites are investigated through a molecular dynamics (MDs) simulation and micromechanics bridging method. One major finding from the MD simulation is that not only the elastic modulus but also the thermal expansion coefficient of the nanocomposites exhibits particle-size dependency at fixed volume fractions. In order to describe such effects that are observed from atomistic simulations, a micromechanics-based scale bridging method is suggested that handles both the elastic and residual fields of the nanocomposites with the help of the effective interface concept and sequential information transfer.

Electric-field-controlled directional motion of ferroelectric domain walls in multiferroic BiFeO3 films

We describe the directional ferroelectric domain wall motion in a multiferroic BiFeO3 thin film, which was grown epitaxially on a vicinal (001) SrTiO3 substrate. A structural analysis of the film shows that a strain gradient is developed in our film, which creates a symmetry breaking in a ferroelectric double-well potential. The asymmetric double-well potential can cause ferroelectric domain walls to move sideways with preferred directionality under a vertical electric field. Our results suggest the possibility of controlling the direction of domain growth with an electric field by imposing constraints on ferroelectric films, such as a strain gradient.

Step bunching-induced vertical lattice mismatch and crystallographic tilt in vicinal BiFeO3(001) films

Epitaxial (001) BiFeO3 thin films grown on vicinal SrTiO3 substrates are under large anisotropic stress from the substrates. The variations of the crystallographic tilt angle and the c lattice constant, caused by the lattice mismatch, along the film thickness were analyzed quantitatively using the x-ray diffraction technique. By generalizing the Nagai model, we estimated how step bunching resulted in the vertical lattice mismatch between adjacent BiFeO3 layers, which induced the strain relaxation and crystallographic tilt. The step bunching was confirmed by the increased terrace width on the BiFeO3 surface.

Polarity-dependent kinetics of ferroelectric switching in epitaxial BiFeO3(111) capacitors

We report on the intriguing polarity-dependent kinetics of polarization switching in epitaxial BiFeO3(111) capacitors. Two seemingly incompatible switching kinetics were observed depending on the polarity of the applied switching bias. Under a negative switching bias, the polarization switching process occurs mainly through sideways domain wall motion, but under a positive switching bias, domain nucleation governs the polarization reversal. The modified piezoresponse force microscopy reveals these polarity-dependent ferroelectric domain evolutions. This polarity dependence of ferroelectric switching kinetics is attributed to defect-related local fields that have different distributions near film/electrode interfaces, probably due to structural relaxation in the BiFeO3(111) film.

Composition-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films

We investigated the time-dependent polarization switching behaviors of (111)-preferred polycrystalline Pb(ZrxTi1−x)O3 thin films with various Zr concentrations. We could explain all the polarization switching behaviors well by assuming Lorentzian distributions in the logarithmic polarization switching time [refer to J. Y. Jo et al., Phys. Rev. Lett. 99, 267602 (2007)]. Based on this analysis, we found that the Zr ion substitution for Ti ions would induce broad distributions in the local field due to defect dipoles, which makes the ferroelectric domain switching occur more easily.

Analysis of thermal conductivity of polymeric nanocomposites under mechanical loading

When the plastic deformation is applied to neat polymer, the polymer chains are aligned and the thermal conductivity of neat polymer increases linearly along the loading direction. However, the thermal conductivity change of nanocomposites consisting of polymer matrix and nanofillers during plastic deformation is not simple. The volume fraction and size of nanofillers scarcely affect the structural change of polymer chains during the plastic deformation. In this study, the structural change of polymeric materials according to the mechanical loading and its effect on the thermal transport properties are investigated through a molecular dynamics simulation. To investigate the effects of nanofiller, its volume fraction, and size on the thermal transport properties, the unit cells of neat amorphous nylon 6 and nanocomposites consisting of amorphous nylon 6 matrix and spherical silica particles are prepared. The molecular unit cells are uniaxially stretched by applying constant strain along the loading directi...