Laurence Brassart

REACTIVE FLOW IN LARGE-DEFORMATION ELECTRODES OF LITHIUM-ION BATTERIES

An electrode in a lithium-ion battery may undergo inelastic processes of two types: flow and reaction. Flow changes the shape of the electrode, preserves its composition and volume, and is driven by the deviatoric stress — a process similar to the plastic flow of a metal. By contrast, reaction changes the composition and volume of the electrode, and is driven by a combination of the mean stress and the chemical potential of lithium in the environment. Both flow and reaction are mediated by breaking and forming atomic bonds. Here we formulate a continuum theory of large-deformation electrodes by placing flow and reaction on the same footing. We treat flow and reaction as concurrent nonequilibrium processes, formulate a thermodynamic inequality and a rheological model, and couple the two processes through a chemomechanical flow rule. Within this theory, the driving force for reaction — the mean stress and the chemical potential — can stimulate flow in an electrode too brittle to flow under a mechanical load...

Surface Coating Mediated Swelling and Fracture of Silicon Nanowires during Lithiation

Surface passivation of silicon anodes is an appealing design strategy for the development of reliable, high-capacity lithium-ion batteries. However, the structural stability of the coating layer and its influence on the lithiation process remain largely unclear. Herein, we show that surface coating mediates the swelling dynamics and the fracture pattern during initial lithiation of crystalline silicon nanopillars. We choose conformally nickel coated silicon architectures as a model system. Experimental findings are interpreted based on a chemomechanical model. Markedly different swelling and fracture regimes have been identified, depending on the coating thickness and silicon nanopillar diameter. Nanopillars with relatively thin coating display anisotropic swelling similar to pristine nanopillars, but with different preferred fracture sites. As the coating thickness increases, the mechanisms become isotropic, with one randomly oriented longitudinal crack that unzips the core-shell structure. The morphology of cracked pillars resembles that of a thin-film electrode on a substrate, which is more amenable to cyclic lithiation without fracture. The knowledge provided here helps clarify the cycling results of coated nanosilicon electrodes and further suggests design rules for better performance electrodes through proper control of the lithiation and fracture.

Mechanics of Supercooled Liquids

Pure substances can often be cooled below their melting points and still remain in the liquid state. For some supercooled liquids, a further cooling slows down viscous flow greatly, but does not slow down self-diffusion as much. We formulate a continuum theory that regards viscous flow and self-diffusion as concurrent, but distinct, processes. We generalize Newton’s law of viscosity to relate stress, rate of deformation, and chemical potential. The self-diffusion flux is taken to be proportional to the gradient of chemical potential. The relative rate of viscous flow and self-diffusion defines a length, which, for some supercooled liquids, is much larger than the molecular dimension. A thermodynamic consideration leads to boundary conditions for a surface of liquid under the influence of applied traction and surface energy. We apply the theory to a cavity in a supercooled liquid and identify a transition. A large cavity shrinks by viscous flow, and a small cavity shrinks by self-diffusion. [DOI: 10.1115/1.4028587]

Micromechanical modeling of short glass-fiber reinforced thermoplastics-Isotropic damage of pseudograins

A micromechanical damage modeling approach is presented to predict the overall elasto‐plastic behavior and damage evolution in short fiber reinforced composite materials. The practical use of the approach is for injection molded thermoplastic parts reinforced with short glass fibers. The modeling is proceeded as follows. The representative volume element is decomposed into a set of pseudograins, the damage of which affects progressively the overall stiffness and strength up to total failure. Each pseudograin is a two‐phase composite with aligned inclusions having same aspect ratio. A two‐step mean‐field homogenization procedure is adopted. In the first step, the pseudograins are homogenized individually according to the Mori‐Tanaka scheme. The second step consists in a self‐consistent homogenization of homogenized pseudograins. An isotropic damage model is applied at the pseudograin level. The model is implemented as a UMAT in the finite element code ABAQUS. Model is shown to reproduce the strength and th...

Effective transient behaviour of inclusions in diffusion problems

This paper is concerned with the effective transport properties of heterogeneous media in which there is a high contrast between the phase diffusivities. In this case the transient response of the slow phase induces a memory effect at the macroscopic scale, which needs to be included in a macroscopic continuum description. This paper focuses on the slow phase, which we take as a dispersion of inclusions of arbitrary shape. We revisit the linear diffusion problem in such inclusions in order to identify the structure of the effective (average) inclusion response to a chemical load applied on the inclusion boundary. We identify a chemical creep function (similar to the creep function of viscoelasticity), from which we construct estimates with a reduced number of relaxation modes. The proposed estimates admit an equivalent representation based on a finite number of internal variables. These estimates allow us to predict the average inclusion response under arbitrary time-varying boundary conditions at very low computational cost. A heuristic generalisation to concentration-dependent diffusion coefficient is also presented. The proposed estimates for the effective transient response of an inclusion can serve as a building block for the formulation of multi-inclusion homogenisation schemes.

Transient and asymptotic kinetics of mass transfer by coupled surface and grain boundary diffusion in sintering under strain rate control

An original procedure is developed for simulating pore surface evolution during sintering at high strain rate while distinguishing two types of diffusion fluxes: transient surface fluxes governed by short-range curvature gradients and coupled fluxes at surface and grain boundary governed by strain rate. The latter fluxes become dominant asymptotically, i.e. after damping-out of transient fluxes. The procedure aims at allowing the prediction of the strain rate dependence of macroscopic viscosity, a concept which is meaningful only during the asymptotic stage. The problem is addressed in two-dimension. It is shown that the asymptotic solution of the general partial differential equation of the problem can be obtained as the solution of an ordinary differential equation, of which the resolution lends itself to a semi-analytical procedure. An estimate is also proposed for the rate of convergence of the general solution towards the asymptotic solution. The accuracy of the mathematical procedure is validated by a comparison of the evolution of asymptotic profiles and exact profiles calculated fully numerically during densification or expansion of the system. A method is proposed for mapping the conditions of existence of an asymptotic stage. The method can account for the dependence of average grain coordination on relative density.