arXiv subjects

M. Z. Bazant

Publications and source records attributed to M. Z. Bazant.

22 records · Page 2Linked to original sources

arXiv2007

Anisotropic surface reaction limited phase transformation dynamics in LiFePO4

A general continuum theory is developed for ion intercalation dynamics in a single crystal of a rechargeable battery cathode. It is based on an existing phase-field formulation of the bulk free energy and incorporates two crucial effects: (i) anisotropic ionic mobility in the crystal and (ii) surface reactions governing the flux of ions across the electrode/electrolyte interface, depending on the local free energy difference. Although the phase boundary can form a classical diffusive "shrinking core" when the dynamics is bulk-transport-limited, the theory also predicts a new regime of surface-reaction-limited (SRL) dynamics, where the phase boundary extends from surface to surface along planes of fast ionic diffusion, consistent with recent experiments on LiFePO4. In the SRL regime, the theory produces a fundamentally new equation for phase transformation dynamics, which admits traveling-wave solutions. Rather than forming a shrinking core of untransformed material, the phase boundary advances by filling (or emptying) successive channels of fast diffusion in the crystal. By considering the random nucleation of SRL phase-transformation waves, the theory predicts a very different picture of charge/discharge dynamics from the classical diffusion-limited model, which could affect the interpretation of experimental data for LiFePO4.

cond-mat.mtrl-sci↗

arXiv2002

Thermodynamic Behavior of a Model Covalent Material Described by the Environment-Dependent Interatomic Potential

Using molecular dynamics simulations we study the thermodynamic behavior of a single-component covalent material described by the recently proposed Environment-Dependent Interatomic Potential (EDIP). The parameterization of EDIP for silicon exhibits a range of unusual properties typically found in more complex materials, such as the existence of two structurally distinct disordered phases, a density decrease upon melting of the low-temperature amorphous phase, and negative thermal expansion coefficients for both the crystal (at high temperatures) and the amorphous phase (at all temperatures). Structural differences between the two disordered phases also lead to a first-order transition between them, which suggests the existence of a second critical point, as is believed to exist for amorphous forms of frozen water. For EDIP-Si, however, the unusual behavior is associated not only with the open nature of tetrahedral bonding but also with a competition between four-fold (covalent) and five-fold (metallic) coordination. The unusual behavior of the model and its unique ability to simulation the liquid/amorphous transition on molecular-dynamics time scales make it a suitable prototype for fundamental studies of anomalous thermodynamics in disordeered systems.

cond-mat.mtrl-sci↗

arXiv2000

Asymptotic Analysis of Diffuse-Layer Effects on Time-Dependent Interfacial Kinetics

We investigate the subtle effects of diffuse charge on interfacial kinetics by solving the governing equations for ion transport (Nernst-Planck) with realistic boundary conditions representing reaction kinetics (Butler-Volmer) and compact-layer capacitance (Stern) in the asymptotic limit $ε= λ_D/L \to 0$, where $λ_D$ is the Debye screening length and $L$ is the distance between the working and counter electrodes. Using the methods of singular perturbation theory, we derive the leading-order steady-state response to a nonzero applied current in the case of the oxidation of a neutral species into cations, without any supporting electrolyte. In certain parameter regimes, the theory predicts a reaction-limited current smaller than the classical diffusion-limited current. We also analyze the impedance of the electrochemical cell when a small AC current modulation is added to an applied DC current. At sufficiently high AC frequencies, the Maxwell displacement current is found to exceed the Faradaic conduction current, and experimentally observed ``negative impedances'' (out of phase AC voltage responses) are predicted close to the reaction-limited current. Overall, we demonstrate that the dynamics of diffuse charge plays a fundamental role in nonequilibrium surface reactions when the transport of one of the reacting species is coupled to the total interfacial reponse of the compact and diffuse layers.

cond-mat.soft↗

arXiv1999

Front dynamics during diffusion-limited corrosion of ramified electrodeposits

Experiments on the diffusion-limited corrosion of porous copper clusters in thin gap cells containing cupric chloride are reported. By carefully comparing corrosion front velocities and concentration profiles obtained by phase-shift interferometry with theoretical predictions, it is demonstrated that this process is well-described by a one-dimensional mean-field model for the generic reaction A + B (static) -> C (inert) with only diffusing reactant (cupric chloride) and one static reactant (copper) reacting to produce an inert product (cuprous chloride). The interpretation of the experiments is aided by a mathematical analysis of the model equations which allows the reaction-order and the transference number of the diffusing species to be inferred. Physical arguments are given to explain the surprising relevance of the one-dimensional mean-field model in spite of the complex (fractal) structure of the copper clusters.

physics.chem-ph↗