Shenyang Y. Hu

Mesoporous silicon sponge as an anti-pulverization structure for high-performance lithium-ion battery anodes

Nanostructured silicon is a promising anode material for high-performance lithium-ion batteries, yet scalable synthesis of such materials, and retaining good cycling stability in high loading electrode remain significant challenges. Here we combine in-situ transmission electron microscopy and continuum media mechanical calculations to demonstrate that large (>20 μm) mesoporous silicon sponge prepared by the anodization method can limit the particle volume expansion at full lithiation to ~30% and prevent pulverization in bulk silicon particles. The mesoporous silicon sponge can deliver a capacity of up to ~750 mAh g(-1) based on the total electrode weight with >80% capacity retention over 1,000 cycles. The first cycle irreversible capacity loss of pre-lithiated electrode is <5%. Bulk electrodes with an area-specific-capacity of ~1.5 mAh cm(-2) and ~92% capacity retention over 300 cycles are also demonstrated. The insight obtained from this work also provides guidance for the design of other materials that may experience large volume variation during operations.

Effect of substrate constraint on the stability and evolution of ferroelectric domain structures in thin films

The stability and evolution of ferroelectric domain structures in thin films are studied. Elastic solutions are derived for both elastically anisotropic and isotropic thin films with arbitrary domain structures, subject to the mixed stress-free and constraint boundary conditions. These solutions are employed in a three-dimensional phase-field model to investigate simultaneously the effect of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, surface topology, domain shapes, and their temporal evolution for a cubic-to-tetragonal ferroelectric phase transition. A specific example of a [001] orientated film heteroepitaxially grown on a [001] cubic substrate is considered. It is shown that the shapes of a-domains with tetragonal axes parallel to the film surface are significantly different from those of c-domains with tetragonal axes perpendicular to the film surface. For the substrate constraints and temperatures under which both a- and c-domains coexist, both types of a-domains are present with their tetragonal axes perpendicular to each other, and the domain wall orientations deviate from the 45 orientation generally assumed in thermodynamic analyses. It is demonstrated that a substrate constraint results in sequential nucleation and growth of different tetragonal domains during a ferroelectric phase transition.

A PHASE-FIELD MODEL FOR EVOLVING MICROSTRUCTURES WITH STRONG ELASTIC INHOMOGENEITY

An efficient phase-field model is proposed to study the coherent microstructure evolution in elasti- cally anisotropic systems with significant elastic modulus inhomogeneity. It combines an iterative approach for obtaining the elastic displacement fields and a semi-implicit Fourier-spectral method for solving the time- dependent Cahn-Hilliard equation. Each iteration in our iterative numerical simulation has a one-to-one correspondence to a given order of approximation in Khachatuyrans perturbation method. A unique feature of this approach is its ability to control the accuracy by choosing the appropriate order of approximation. We examine shape dependence of isolated particles as well as the morphological dependence of a phase- separated multi-particle system on the degree of elastic inhomogeneity in elastically anisotropic systems. It is shown that although prior calculations using first-order approximations correctly predicted the qualitative dependence of a two-phase morphology on elastic inhomogeneity, the local stress distributions and thus the driving force for microstructure evolution such as coarsening were in serious error quantitatively for systems with strong elastic inhomogeneity.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

Phase-field model of domain structures in ferroelectric thin films

A phase-field model for predicting the coherent microstructure evolution in constrained thin films is developed. It employs an analytical elastic solution derived for a constrained film with arbitrary eigenstrain distributions. The domain structure evolution during a cubic→tetragonal proper ferroelectric phase transition is studied. It is shown that the model is able to simultaneously predict the effects of substrate constraint and temperature on the volume fractions of domain variants, domain-wall orientations, domain shapes, and their temporal evolution.

Effect of electrical boundary conditions on ferroelectric domain structures in thin films

The domain structures in a ferroelectric thin film are studied using a phase-field model. A cubic-to-tetragonal ferroelectric phase transition in lead titanate thin film is considered. Both elastic interactions and electrostatic interactions are taken into account. The focus is on the effect of electrical boundary conditions on the domain morphologies and volume fractions. It is shown that different electric boundary conditions may have a significant effect on the domain structures.

SOLUTE SEGREGATION AND COHERENT NUCLEATION AND GROWTH NEAR A DISLOCATION—A PHASE-FIELD MODEL INTEGRATING DEFECT AND PHASE MICROSTRUCTURES

Abstract A diffuse-interface field model is proposed for describing diffusional processes in coherent systems with arbitrary microstructures and arbitrary spatial distribution of structural defects such as dislocations. It takes into account the effect of both the coherency elastic energy of a microstructure and the elastic coupling between the coherency strains and defect strains. In this model, any arbitrary spatial distribution of defects is described using the micromechanics concept of space-dependent “stress-free” or “eigen” strains. As examples, the solute segregation as well as the nucleation and diffusional growth of a coherent precipitate around an edge dislocation are considered. It is shown that coherent nucleation may become barrierless under the influence of the local elastic field of a dislocation.

Computer simulation of spinodal decomposition in constrained films

The morphological evolution during spinodal decomposition of a binary alloy thin film elastically constrained by a substrate is studied. Elastic solutions, derived for elastically anisotropic thin films subject to the mixed stress-free and constraint boundary conditions, are employed in a three-dimensional phase-field model. The Cahn–Hilliard diffusion equation for a thin film boundary condition is solved using a semi-implicit Fourier-spectral method. The effect of composition, coherency strain, film thickness and substrate constraint on the microstructure evolution was studied. Numerical simulations show that in the absence of coherency strain and substrate constraint, the morphology of decomposed phases depends on the film thickness and the composition. For a certain range of compositions, phase separation with coherency strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Similar to bulk systems, the negative elastic anisotropy in the cubic alloy results in the alignment of phases along 100 elastically soft directions.  2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Effect of interfacial dislocations on ferroelectric phase stability and domain morphology in a thin film—a phase-field model

A phase-field model was developed for predicting the domain structure evolution in a thin film with an arbitrary distribution of dislocations and subject to a substrate constraint. The effect of interfacial dislocations on the formation of tetragonal ferroelectric domains in a cubic paraelectric matrix was studied. It was found that the presence of interfacial dislocations locally modifies the ferroelectric transition temperature and leads to the preferential formation of ferroelectric domains around misfit dislocations. The types of tetragonal variants depend on the directions of the dislocation lines and their Burgers vectors.

Ferroelectric domain morphologies of (001) PbZr1−xTixO3 epitaxial thin films

Ferroelectric domain morphologies in (001) PbZr1−xTixO3 epitaxial thin films were studied using the phase-field approach. The film is assumed to have a stress-free top surface and is subject to a biaxial substrate constraint. Both the electrostatic open-circuit and short-circuit boundary conditions on the film surfaces were considered. The phase-field simulations indicated that in addition to the known tetragonal and rhombohedral phases, an orthorhombic phase becomes stable in films under large tensile constraints. The orthorhombic domain structure contains (100) and (010) 90° domain walls and (110) and (1–10) 180° domain walls. For the rhombohedral phase in a thin film, the domain walls are found to be along {101}, (100), and (010) of the prototypical cubic cell. It is shown that the short-circuit boundary condition and compressive substrate constraint enhance the out-of-plane polarization component while the open-circuit boundary condition and tensile substrate constraint suppress it. It is also shown t...

Effect of solutes on dislocation motion ¿a phase-field simulation

Abstract Based on recent advances in phase-field models for integrating phase and defect microstructures as well as dislocation dynamics, the interactions between diffusional solutes and moving dislocations under applied stresses are studied in three dimensions. A new functional form for describing the eigenstrains of dislocations is proposed, eliminating the dependence of the magnitude of the dislocation Burgers vector on the applied stress and providing correct stress fields of dislocations. A relationship between the velocity of the dislocation and the applied stress is obtained by theoretical analysis and numerical simulations. The operation of Frank–Read sources in the presence of diffusional solutes, the effect of chemical interactions in solid solution on the equilibrium distribution of Cottrell atmosphere, and the drag effect of Cottrell atmosphere on dislocation motion are examined. The results demonstrate that the phase-field model correctly describes the long-range elastic interactions and short-range chemical interactions that control dislocation motion.