Catherine M. Bishop

Microstructural Modeling and Design of Rechargeable Lithium-Ion Batteries

The properties of rechargeable lithium-ion batteries are determined by the electrochemical and kinetic properties of their constituent materials as well as by their underlying microstructure. In this paper a method is developed that uses microscopic information and constitutive material properties to calculate the response of rechargeable batteries. The method is implemented in OOF ,a public domain finite element code, so it can be applied to arbitrary two-dimensional microstructures with crystallographic anisotropy. This methodology can be used as a design tool for creating improved electrode microstructures. Several geometrical two-dimensional arrangements of particles of active material are explored to improve electrode utilization, power density, and reliability of the Li yC6uLixMn2O4 battery system. The analysis suggests battery performance could be improved by controlling the transport paths to the back of the positive porous electrode, maximizing the surface area for intercalating lithium ions, and

Relating atomistic grain boundary simulation results to the phase-field model

Abstract A coarse-graining method for mapping discrete data to a continuous structural order parameter is presented. This method is intended to provide a useful and consistent method of utilizing structural data from molecular simulations in continuum models, such as the phase field model. The method is based on a local averaging of the variation of a Voronoi tessellation of the atomic positions from the Voronoi tessellation of a perfect crystal (the Wigner–Seitz cell). The coarse-graining method is invariant to coordinate frame rotation. The method is illustrated with a simple two-dimensional example and then applied to a three-dimensional relaxation simulation using the silicon EDIP potential of a Σ5 grain boundary. Calculated results indicate that a continuous structural parameter is obtained that has grain boundary characteristics similar to phase-field models of grain boundaries. Comparisons to other coarse-graining measures of structure are discussed as well as applications to experimental data sets.

Crystallography and Morphology of MC Carbides in Niobium-Titanium Modified As-Cast HP Alloys

The microstructures of two as-cast heats of HP alloy stainless steels modified with niobium and titanium were examined with particular attention paid to the interdendritic niobium-titanium-rich carbides formed during solidification of these alloys. Generally, these precipitates obtain a blocky morphology in the as-cast condition. However, the (NbTi)C precipitates may obtain a nodular morphology. To provide further insight to the origin of the two different morphologies obtained by the (NbTi)C precipitates in the HP-NbTi alloy, the microstructure and crystallography of each have been studied in detail using scanning electron microscopy, transmission electron microscopy, various electron diffraction methods (EBSD, SAD, and CBED), and energy-dispersive X-ray spectroscopy.

Effect of charge separation on the stability of large wavelength fluctuations during spinodal decomposition

A stability analysis of phase separation of charged species by spinodal decomposition is presented. The charge effects introduce a short wave number cutoff for linear perturbations about the homogeneous, neutral solution. Phase field calculations using a semi-implicit spectral method support this conclusion. This suggests that coarsening is limited in ionic solid systems that are unstable with respect to charged-phase separation.

Microstructural Characterization and Image Analysis in Ex-Service HP Alloy Stainless Steel Tubes for Ethylene Pyrolysis

Carburization is the major degradation mechanism of the HP alloy stainless steel tubes used for ethylene pyrolysis. Due to the changes in the microstructure and magnetic properties of the heat-resistant HP grade (Fe-Cr-Ni) alloy tubes over their service life, the level of carburization can be detected non-destructively using eddy current methods. In the current project, the microstructure of a carburized ex-service Nb-modified HP alloy tube has been characterized via image analysis. The phase fractions and phase distributions have been measured in order to determine the microstructural features that contribute to the eddy current non-destructive testing response, to assist in improving the accuracy of remaining life estimates. Issues in differentiating the large number of phases were encountered when using electron backscattered images for image analysis, and thus, energy-dispersive x-ray spectroscopy maps have been used, in combination with the software programs ilastik, for automated image segmentation, and FIJI, for image processing. The chromium depletion of the austenite matrix of the tube, and thus the magnetic response, was observed to be predominantly influenced by the volume fraction of M7C3 and M23C6 chromium carbides, with potential contribution from the chromium-containing η-carbide intermetallic. Carbonitrides of the type Cr2(C,N) were seen to have negligible impact. A comparison between microstructures and magnetic response has determined that eddy current non-destructive testing can detect and measure carburization.

A simple model of fully-faceted grain growth and coarsening with non-linear growth laws

Abstract A numerical simulation of the growth and coarsening of completely faceted particles in a two-dimensional closed system is described. The particles grow from a supersaturated solution with driving forces including an anisotropic-Gibbs–Thompson effect. Linear and non-linear growth laws were incorporated. This allows comparison to data obtained from physical experiments of Si3N4 growing from a glass matrix with additions of various rare earths (La, Y, Lu) where particle growth is more or less anisotropic (depending on the particular rare earth dopant). Simulations explore the ranges of kinetic parameters for which particle shapes tend to be dictated by thermodynamic and/or kinetic anisotropy. Comparison to data taken from a series of experiments indicates that La probably has a non-linear growth effect whereas Lu is less so.

Microstructural Characterization and Image Analysis in Ex-Service Ethylene Pyrolysis Tubes

Ethylene is used extensively in the production of plastics, cabling, and automotive products. It is typically produced by the thermal cracking (pyrolysis) of more complex hydrocarbons, such as naphtha or ethane, at 950 to 1100°C inside HP alloy tubes (25%Cr-35%Ni) within an ethylene pyrolysis furnace. The main contributor to the failure of ethylene pyrolysis tubes is carburization of the tubes, which causes an increase in internal volume, a loss in weldability and a reduced ability to withstand thermal cycles [1-4]. Knowing the level of carburization of a tube in-situ can assist in remaining life estimates based on Finite Element Modeling, thermography, and fracture mechanics. Due to the changes in the microstructure and magnetic properties of the tubes over their service life, the level of carburization can be detected non-destructively using eddy current probes [3] and a tube crawler system [5]. However, the eddy current system requires calibration on ex-service tubes that have had their microstructure, mechanical properties and magnetic response characterized.

Equilibrium moisture content of a crosslinked epoxy network via molecular dynamics simulations

This study presents molecular dynamics (MD) simulation methods for determining the solubility limit of water in a crosslinked epoxy network. Procedures are first presented for dynamically crosslinking an epoxy network consisting of diglycidyl ether bisphenol a (DGEBA) and isophorone diamine (IPD). Water molecules are then introduced into the crosslinked DGEBA-IPD structure. The excess chemical potential for the absorbed water was determined through combining thermodynamic integration and Widoms test particle insertion methods. The limiting moisture uptake of the epoxy structure was determined through comparing the reduced chemical potential of the water held within the epoxy to that of pure water. The DGEBA-IPD epoxy system was found to have a moisture solubility of 3.50–3.75 wt.% when immersed in water at 300 K.

EBSD Characterization of Pilgered Alloy 800H After Heat Treatment

Austenitic stainless steel alloy 800H pipes were received in an as-pilgered condition. The pilgering process produces high levels of cold work through complex deformation processes. The pipes are subsequently solution annealed. To assess the influence of heat-treatment parameters on 800H, samples of the as-pilgered material were heat treated in the temperature range 600°C–1200°C for varying lengths of time. Using electron backscatter diffraction (EBSD), the recrystallization dynamics and grain size of all samples was measured. Additionally, the kernel average misorientation and grain orientation spread methods were employed to qualitatively analyze fine details in the strain levels in the matrix of both deformed and recrystallized grains. For 1-h anneal times, it was found that recrystallization of 800H initiates above heat-treatment temperatures of 800°C, and complete recrystallization of deformed material occurs above heat-treatment temperatures of 950°C. No significant increases in average grain size were observed until temperatures reached in excess of 1000°C. Increasing the heat-treatment time to more than 1 h had a negligible effect on the grain size for every heat-treatment temperature that was assessed. In high-temperature applications, pipes typically fail because of diffusion-based creep mechanisms. Therefore, accurate control of the grain size and microstructure during heat treatments is obligatory. This work provides a framework for controlling manufacturing processes to produce favorable microstructures in 800H for creep-based applications.

Formation of aluminium carbide by cast iron and liquid aluminium interaction

Abstract Aluminium carbide (Al4C3) has been identified in ex service and laboratory test specimens of cast irons exposed to liquid aluminium. In cast iron/aluminium couples, the formation of Al4C3 and κ-Fe3AlC was confirmed for the first time. The growth kinetics of Al4C3 were parabolic with an activation energy 145±23 kJ mol−1 (1023–1223 K), which is consistent with the reported activation energy for growth in graphite/liquid aluminium couples. The aim of this work is to shed light upon the origin and growth of Al4C3 in cast iron/liquid aluminium couples in the temperature range of 1023–1223 K.