Fuqian Yang

Crack Pattern Formation in Thin Film Lithium-Ion Battery Electrodes

Cracking of electrodes caused by large volume change and the associated lithium diffusion-induced stress during electrochemical cycling is one of the main reasons for the short cycle life of lithium-ion batteries using high capacity anode materials, such as Si and Sn. In this work, we study the fracture behavior and cracking patterns in amorphous Si thin film electrodes as a result of electrochemical cycling. A modified spring-block model is shown to capture the essential features of cracking patterns of electrode materials, including self-similarity. It is shown that cracks are straight in thick films, but show more wiggles in thin films. As the thickness of film decreases, the average size of islands separated by cracks decreases. A critical thickness bellow which material would not crack is found for amorphous Si films. The experimental and simulation results of this work provide guidelines for designing crack free thin-film lithium ion battery electrodes during cycling by patterning the electrode and reducing the film thickness.

Size-dependent effective modulus of elastic composite materials: Spherical nanocavities at dilute concentrations

The effect of surface energy on the effective elastic properties was analyzed for elastic composite materials containing spherical nanocavities at dilute concentration. Closed-form solutions of the effective shear modulus and bulk modulus were obtained, which turn out to be a function of the surface energy and size of the nanocavity. The dependence of the elastic response on size of the nanocavity in composite materials is different from the classic results obtained in the linear elasticity theory, suggesting the importance of the surface energy of the nanocavity in analyzing the deformation of nanoscale structures.

Size effect on the coalescence-induced self-propelled droplet

An analysis based on the energy conservation is presented for the self-propelled droplet during coalescence of two droplets of the same size over a superhydrophobic rough surface. The self-propelled behavior occurs only for the coalescence of droplets with a certain range of radius. An analytical relation is established among the coalescence-induced velocity, surface energy, viscous dissipation, and droplet size if gravity is negligible. The coalescence-induced velocity increases with increasing droplet size to a maximum and then decreases with the size, which is in good accord with the experimental observation reported in the literature

Fracture mechanics for a Mode I crack in piezoelectric materials

Using linear piezoelectricity theory, the effect of a Griffith crack on stress and electric fields in an infinite piezoelectric material under electric and tension loading has been studied by using appropriate boundary conditions. A closed-form solution to the Mode I fracture problem is obtained for external loading used to open the crack. By including electrostatic energy in the calculation of crack driving force, the energy release rate is found to be the third power function of the external loading if electric field inside the crack is not zero at the crack tip. The results may be used to explain some nonlinear phenomenon observed in the indentation of piezoelectric ceramics.

A Tensile Deformation Model for In-situ Dendrite/Metallic Glass Matrix Composites

In-situ dendrite/metallic glass matrix composites (MGMCs) with a composition of Ti46Zr20V12Cu5Be17 exhibit ultimate tensile strength of 1510 MPa and fracture strain of about 7.6%. A tensile deformation model is established, based on the five-stage classification: (1) elastic-elastic, (2) elastic-plastic, (3) plastic-plastic (yield platform), (4) plastic-plastic (work hardening), and (5) plastic-plastic (softening) stages, analogous to the tensile behavior of common carbon steels. The constitutive relations strongly elucidate the tensile deformation mechanism. In parallel, the simulation results by a finite-element method (FEM) are in good agreement with the experimental findings and theoretical calculations. The present study gives a mathematical model to clarify the work-hardening behavior of dendrites and softening of the amorphous matrix. Furthermore, the model can be employed to simulate the tensile behavior of in-situ dendrite/MGMCs.

Thickness effect on the indentation of an elastic layer

Sneddons solution has been used in indentation tests to analyze elastic behavior of materials, in which the result ignores the effect of specimen thickness. However, in the real situation, the specimen thickness especially for thin films may not be negligible when the ratio of contact radius to the specimen thickness is larger than one. An analytical solution of the load-displacement relationship was derived for the indentation problem of an elastic layer by a rigid flat-ended cylindrical indenter. To obtain the closed-form solution, frictionless condition was used at contact interfaces. The contact force is found to be inversely proportional to the film thickness and independent of the contact radius if the contact radius is much larger than the film thickness. Then the effect of adhesion between the elastic layer and the indenter was addressed and an analytical solution of the pull-off force was obtained. The pull-off force is a function the layer thickness and Poissons ratio, which increases with Poissons ration and the ratio of the contact radius to the layer thickness.

Nucleation-related defect-free GaP/Si(100) heteroepitaxy via metal-organic chemical vapor deposition

GaP/Si heterostructures were grown by metal-organic chemical vapor deposition in which the formation of all heterovalent nucleation-related defects (antiphase domains, stacking faults, and microtwins) were fully and simultaneously suppressed, as observed via transmission electron microscopy (TEM). This was achieved through a combination of intentional Si(100) substrate misorientation, Si homoepitaxy prior to GaP growth, and GaP nucleation by Ga-initiated atomic layer epitaxy. Unintentional (311) Si surface faceting due to biatomic step-bunching during Si homoepitaxy was observed by atomic force microscopy and TEM and was found to also yield defect-free GaP/Si interfaces.

Modeling and numerical simulation of bioheat transfer and biomechanics in soft tissue

A mathematical model describing the thermomechanical interactions in biological bodies at high temperature is proposed by treating the soft tissue in biological bodies as a thermoporoelastic media. The heat transfer and elastic deformation in soft tissues are examined based on the Pennes bioheat transfer equation and the modified Duhamel-Neuman equations. The three-dimensional governing equations based on the proposed model is discretized using a 19-point finite-difference scheme. The resulting large sparse linear system is solved by a preconditioned Krylov subspace method. Numerical simulations show that the proposed model is valid under our test conditions and the proposed numerical techniques are efficient.

Reduced leakage current, enhanced ferroelectric and dielectric properties in (Ce,Fe)-codoped Na0.5Bi0.5TiO3 film

Na0.5Bi0.5TiO3 (NBT), Ce-doped NBT (NBTCe), Fe-doped NBT (NBTFe), and (Ce,Fe)-codoped NBT (NBTCeFe) thin films were fabricated on LaNiO3(100)/Si substrates by metal organic decomposition. The leakage current density of NBTCeFe at 500 kV/cm is reduced by approximately two orders of magnitude by reducing the density of oxygen vacancies and forming the defect complexes, compared with NBT film. Enhanced ferroelectricity is achieved in NBTCeFe with a large remanent polarization of 24 μC/cm2 due to the reduced leakage current, extra A-site vacancies, and lattice distortion. The NBTCeFe also exhibits a dielectric constant of 585 and dielectric loss of 0.05 at 10 kHz.

Diffusion-induced beam bending in hydrogen sensors

The diffusion-induced bending of both single-layer and bilayer beam structure is analyzed by using linear elastic beam theory and the Moutier theorem. A closed form solution of the radius of curvature due to diffusion is obtained. For the single-layer beam structure, the radius of curvature is inversely proportional to the bending moment created by nonuniform concentration distribution. For the bilayer beam structure, the curvature is a linear function of the mismatch strain between the two layers and the bending moment introduced by diffusion. The mismatch strain depends on the concentration and the partial molar volume of the diffusing component in both layers. Application to microelectromechanical systems hydrogen sensors with a layer of Pd is shown.