Carl V. Thompson

All-carbon-nanofiber electrodes for high-energy rechargeable Li–O2 batteries

Hollow carbon fibers with diameters on the order of 30 nm were grown on a ceramic porous substrate, which was used as the oxygen electrode in lithium-oxygen (Li–O2) batteries. These all-carbon-fiber (binder-free) electrodes were found to yield high gravimetric energies (up to 2500 W h kgdischarged−1) in Li–O2cells, translating to an energy enhancement ∼4 times greater than the state-of-the-art lithium intercalation compounds such as LiCoO2 (∼600 W h kgelectrode−1). The high gravimetric energy achieved in this study can be attributed to low carbon packing in the grown carbon-fiber electrodes and highly efficient utilization of the available carbon mass and void volume for Li2O2 formation. The nanofiber structure allowed for the clear visualization of Li2O2 formation and morphological evolution during discharge and its disappearance upon charge, where Li2O2 particles grown on the sidewalls of the aligned carbon fibers were found to be toroids, having particle sizes increasing (up to ∼1 μm) with increasing depth-of-discharge. The visualization of Li2O2 morphologies upon discharge and disappearance upon charge represents a critical step toward understanding key processes that limit the rate capability and low round-trip efficiencies of Li–O2 batteries, which are not currently understood within the field.

Transient nucleation in condensed systems

Using classical nucleation theory we consider transient nucleation occurring in a one‐component, condensed system under isothermal conditions. We obtain an exact closed‐form expression for the time dependent cluster populations. In addition, a more versatile approach is developed: a numerical simulation technique which models directly the reactions by which clusters are produced. This simulation demonstrates the evolution of cluster populations and nucleation rate in the transient regime. Results from the simulation are verified by comparison with exact analytical solutions for the steady state. Experimental methods for measuring transient nucleation are assessed, and it is demonstrated that the observed behavior depends on the method used. The effect of preexisting cluster distributions is studied. Previous analytical and numerical treatments of transient nucleation are compared to the solutions obtained from the simulation. The simple expressions of Kashchiev are shown to give good descriptions of the n...

Stress and grain growth in thin films

Abstract The mechanical properties of polycrystalline thin films with thickness of 1 μm or less depend strongly on the grain geometry, the grain size, and the way in which the crystallographic orientations of the grains are distributed. Grain growth during film formation or during post-deposition annealing can play a dominant role in defining these microstructural characteristics, and therefore, the mechanical properties of films. Stress can suppress or promote grain growth. In the latter case, stress promotes texture evolution during grain growth. Grain growth can serve as a stress relief mechanism in both elastically isotropic and anisotropic materials, and can also promote plastic yielding.

On the approximation of the free energy change on crystallization

Abstract Approximations of the Gibbs free energy change for crystallization of an undercooled liquid. ΔG, are discussed and compared. When liquid heat capacity data are available, ΔG can be approximated to various degrees of accuracy depending on the completeness of the data. In the absence of these data, it is necessary to make further approximations. It is shown that Turnbulls [4] simple linear approximation for ΔG is generally applicable to pure metals. A new expression is proposed for use with easy glass forming alloys such as Au81.4Si18.6. An approximation due to Hoffman [1] is not appropriate for use with metals and alloys but is adequate for use with organic substances, such as ortho-terphenyl.

Secondary grain growth in thin films of semiconductors: Theoretical aspects

Secondary grain growth in thin films can lead to grain sizes much greater than the film thickness. Surface energy anisotropy often provides an important fraction of the driving force for secondary grain growth, especially in the early stages of growth. Surface‐energy‐driven secondary grain growth leads to the development of large grains with restricted crystallographic textures. A model is presented for growth of secondary grains into a uniform matrix of columnar normal grains. The model indicates that secondary grain growth rates should increase with grain boundary energy, surface energy anisotropy, grain boundary mobility, and temperature. While final secondary grain sizes will decrease with film thickness, their growth rates will increase. The final secondary grain sizes and orientations will be strongly affected by grain sizes and orientations in the initial film. The models presented here provide analytical tools for experimental study of secondary grain growth in thin films. They will be used in for...

Influence of Li2O2 morphology on oxygen reduction and evolution kinetics in Li–O2 batteries

Understanding the origins of high overpotentials required for Li2O2 oxidation in Li–O2 batteries is critical for developing practical devices with improved round-trip efficiency. While a number of studies have reported different Li2O2 morphologies formed during discharge, the influence of the morphology and structure of Li2O2 on the oxygen evolution reaction (OER) kinetics and pathways is not known. Here, we show that two characteristic Li2O2 morphologies are formed in carbon nanotube (CNT) electrodes in a 1,2-dimethoxyethane (DME) electrolyte: discs/toroids (50–200 nm) at low rates/overpotentials (10 mA gC−1 or E > 2.7 V vs. Li), or small particles (<20 nm) at higher rates/overpotentials. Upon galvanostatic charging, small particles exhibit a sloping profile with low overpotential (<4 V) while discs exhibit a two-stage process involving an initially sloping region followed by a voltage plateau. Potentiostatic intermittent titration technique (PITT) measurements reveal that charging in the sloping region corresponds to solid solution-like delithiation, whereas the voltage plateau (E = 3.4 V vs. Li) corresponds to two-phase oxidation. The marked differences in charging profiles are attributed to differences in surface structure, as supported by X-ray absorption near edge structure (XANES) data showing that oxygen anions on disc surfaces have LiO2-like electronic features while those on the particle surfaces are more bulk Li2O2-like with modified electronic structure compared to commercial Li2O2. Such an integrated structural, chemical, and morphological approach to understanding the OER kinetics provides new insights into the desirable discharge product structure for charging at lower overpotentials.

Matching Glass-Forming Ability with the Density of the Amorphous Phase

The density of the amorphous phase of metals is generally thought to be related to glass formation, but this correlation has not been demonstrated experimentally to date. In this work, systematic deflection measurements using microcantilevers and a combinatorial deposition method show a correlation between glass-forming ability and the density change upon crystallization over a broad compositional range in the copper-zirconium binary system. Distinct peaks in the density of the amorphous phase were found to correlate with specific maxima in the critical thickness for glass formation. Our findings provide quantitative data for the development of structural models of liquids that are readily quenched to the amorphous state. The experimental method developed in this work can facilitate the search for new glass-forming alloys.

The Dynamic Competition Between Stress Generation and Relaxation Mechanisms During Coalescence of Volmer-Weber Thin Films

Real-time measurements of stress evolution during the deposition of Volmer–Weber thin films reveal a complex interplay between mechanisms for stress generation and stress relaxation. We observed a generic stress evolution from compressive to tensile, then back to compressive stress as the film thickened, in amorphous and polycrystalline Ge and Si, as well as in polycrystalline Ag, Al, and Ti. Direct measurements of stress relaxation during growth interrupts demonstrate that the generic behavior occurs even in the absence of stress relaxation. When relaxation did occur, the mechanism depended sensitively on whether the film was continuous or discontinuous, on the process conditions, and on the film/substrate interfacial strength. For Ag films, interfacial shear dominated the early relaxation behavior, whereas this mechanism was negligible in Al films due to the much stronger bonding at the Al/SiO2 interface. For amorphous Ge, selective relaxation of tensile stress was observed only at elevated temperatures...

Texture development in polycrystalline thin films

The distribution of crystallographic orientations of the grains in a polycrystalline film can evolve through a number of kinetic processes. Orientation evolution can occur before, during and after coalescence of islands to form a continuous film, during thickening of a film, and during post-deposition annealing. The energetic constraints leading to texture selection include surface and interface energy minimization, as well as strain-energy minimization. The final texture of a film depends on which texture-selection mechanisms and driving forces dominate, and is different for different films, substrates, and deposition conditions.

The yield stress of polycrystalline thin films

In recent experiments it has been shown that the yield stress of polycrystalline thin films depends separately on the film thickness and the grain size. It was also shown that the grain size dependence varies as the reciprocal of the grain size. In this paper an analysis is presented which leads to these results and provides a more detailed understanding of the origins of the observed behavior.