Wilson K. S. Chiu

Nondestructive Reconstruction and Analysis of SOFC Anodes Using X-ray Computed Tomography at Sub-50 nm Resolution

A high-resolution, nondestructive X-ray computed tomography (XCT) technique is applied to image the three-dimensional (3D) microstructure of a solid oxide fuel cell (SOFC) composed of a solid yttria-stabilized zirconia (YSZ) electrolyte and a porous nickel YSZ (Ni-YSZ) anode. The X-ray microscope uses the 8 keV Cu Kα line from a laboratory X-ray source, with a reflective condenser optic lens providing a spatial resolution of 42.7 nm. The reconstructed volume data is visualized as 3D images and further postprocessed in binary-image format to obtain structural parameters. The porosity is calculated using a voxel counting method, and tortuosity is evaluated by solving the Laplace equation. A 3D representation of the microstructure is used to calculate true structural parameters and carry out a detailed study of the gas transport within an SOFC electrode at the pore scale. Simulation of multicomponent mass transport and electrochemical reactions in the anode microstructure using the XCT data as geometric input illustrate the impact of this technique on SOFC modeling.

Nondestructive Nanoscale 3D Elemental Mapping and Analysis of a Solid Oxide Fuel Cell Anode

Present solid oxide fuel cells (SOFCs) use complex materials to provide (i) sufficient stability and support, (ii) electronic, ionic, and mass transport, and (iii) electrocatalytic activity. However, there is a limited quantitative understanding of the effect of the SOFCs three dimensional (3D) nano/microstructure on electronic, ionic, and mass-transfer-related losses. Here, a nondestructive tomographic imaging technique at 38.5 nm spatial resolution is used along with numerical models to examine the phase and pore networks within an SOFC anode and to provide insight into the heterogeneous microstructures contributions to the origins of transport-related losses. The microstructure produces substantial localized structure-induced losses, with approximately 50% of those losses arising from phase cross-sectional diameters of 0.2 μm or less.

A Dusty Fluid Model for Predicting Hydroxyl Anion Conductivity in Alkaline Anion Exchange Membranes

Advances in metal-cation-free, quaternary ammonium, polymer alkaline anion exchange membranes (AAEMs) have provided a recent resurgence of interest in the alkaline fuel cell (AFC). The alkaline environment supported by the AAEM offers several potential advantages, including opportunities for the use of non-noble metal catalysts with high energy density and logistically favorable fuels and oxidants, such as methanol and air. However, recent experimental literature has shown that the AAEM derived AFCs have considerable resistive losses that can be attributed to the AAEM. This work describes a dusty fluid model used to predict AAEM conductivities as a function of relative humidity and membrane properties in an initial attempt at forming a framework for understanding the processes at work. A percolation model is used to account for the membrane structure. The model is validated using Nafion 115 conductivity data.

Extension of anisotropic effective medium theory to account for an arbitrary number of inclusion types

The purpose of this work is to extend, to multi-components, a previously reported theory for calculating the effective conductivity of a two component mixture. The previously reported theory involved preferentially oriented spheroidal inclusions contained in a continuous matrix, with inclusions oriented relative to a principle axis. This approach was based on Bruggemans unsymmetrical theory, and is extended to account for an arbitrary number of different inclusion types. The development begins from two well-known starting points; the Maxwell approach and the Maxwell-Garnett approach for dilute mixtures. It is shown that despite these two different starting points, the final Bruggeman type equation is the same. As a means of validating the developed expression, comparisons are made to several existing effective medium theories. It is shown that these existing theories coincide with the developed equations for the appropriate parameter set. Finally, a few example mixtures are considered to demonstrate the ...

Residual stress measurement in thin carbon films by Raman spectroscopy and nanoindentation

The reliability of hermetic carbon-coated optical fibers is affected by residual stresses in the coating created during the fiber draw process. Thermally induced residual stresses are caused by differences in the coefficient of thermal expansion (CTE) of the coating and the optical fiber. This mismatch creates shear stresses at the interface that can delaminate the film. This work presents and validates a surface residual-stress measurement technique using Raman spectroscopy. Since select Raman peaks for carbon films exhibit a wavenumber shift in proportion to the magnitude of residual stress, Raman spectra can be correlated to a theoretical model to obtain its residual stress. The model, validated with nanoindentation, shows an equi-biaxial stress field through the depth of the film. Nanoindentation also provides an accurate measure of residual stress in thin films with unknown material properties. The approach presented in this study is a non-destructive and non-intrusive method for measuring residual surface stress in thin films, and is ideal for small curved-surface specimens such as carbon-coated optical fibers.

Zone-doubled Fresnel zone plates for high-resolution hard X-ray full-field transmission microscopy

The use of zone-doubled Fresnel zone plates for sub-20 nm spatial resolution in full-field transmission X-ray microscopy and tomography at the hard X-ray regime (8–10 keV) is demonstrated.

Three-dimensional mapping of nickel oxidation states using full field x-ray absorption near edge structure nanotomography

The reduction-oxidation cycling of the nickel-based oxides in composite solid oxide fuel cells and battery electrodes is directly related to cell performance. A greater understanding of nickel redox mechanisms at the microstructural level can be achieved in part using transmission x-ray microscopy (TXM) to explore material oxidation states. X-ray nanotomography combined with x-ray absorption near edge structure (XANES) spectroscopy has been applied to study samples containing distinct regions of nickel and nickel oxide (NiO) compositions. Digitally processed images obtained using TXM demonstrate the three-dimensional chemical mapping and microstructural distribution capabilities of full-field XANES nanotomography.

Characterization of CVD carbon films for hermetic optical fiber coatings

Abstract Thin carbon films are being studied as hermetic coatings for optical fibers used in harsh environment applications. This work details an experimental study of pyrolytic carbon films grown in a cold walled chemical vapor deposition (CVD) reactor. The films are grown from methane, acetylene, propane and butane precursors on stationary 3-mm quartz rods. Results are presented at 15 and 600 torr, total pressure for substrate temperatures between 1050 and 1750 K. Hydrocarbon precursor gas concentrations are varied between 10 and 100% in N 2 dilutant. Auger electron spectroscopy (AES) shows mostly carbon composition, with trace levels of Cr and O 2 impurities. X-Ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS) reveal small amounts of SiC at the carbon–glass interface, but could not resolve a continuous transition layer. The thickness, microstructure and surface roughness are analyzed using environmental scanning electron microscopy (ESEM). ESEM reveals an increase in deposition rate with temperature and concentration for all hydrocarbon species and a noticeable change in surface morphology with deposition temperature and hydrocarbon species. Raman spectroscopy is used to relate film microstructure to deposition conditions, providing non-intrusive evaluation of crystalline size and hermetic properties.

Experimental and numerical study of conjugate heat transfer in a horizontal channel heated from below

Conjugate heat transfer has significant relevance to a number of thermal systems and techniques which demand stringent temperature control, such as electronic cooling and chemical vapor deposition. A detailed experimental and numerical study is carried out to investigate conjugate heat transfer in a common configuration consisting of a horizontal channel with a heated section. Experimental data obtained from this study provides physical insight into conjugate heat transfer effects and facilitates validation of numerical conjugate heat transfer models. The basic characteristics of the flow and the associated thermal transport are studied. The numerical model is used to carry out a parametric study of operating conditions and design variables, thus allowing for the characterization of the conjugate heat transfer effects

Lattice Boltzmann method for continuum, multi-component mass diffusion in complex 2D geometries

Multi-component gas diffusion in the continuum flow regime is often modelled using the Stefan–Maxwell (SM) equations. Recent advances in lattice Boltzmann (LB) mass diffusion models have made it possible to directly compare LB predictions with solutions to the SM equations. In this work, one-dimensional (1D) and two-dimensional (2D), equi-molar counter-diffusion of two gases in the presence of a third, inert gas is studied. The work is an extension and validation of a recently proposed binary LB model for components having dissimilar molecular weights. The treatment of inflow and outflow boundary conditions (for specifying species mole fractions or mole flux) is developed via the averaging of component velocities before and after collisions. Results for one and two spatial dimensions have been compared with analytic and numerical solutions to the SM equations and good agreement has been found for a wide range of parameters and for large variations in molecular weights. A novel molecular weight tuning strategy for increasing the accuracy has been demonstrated. The model developed can be used to model continuum, multi-component mass transfer in complex geometries such as porous media without empirical modification of diffusion coefficients based on porosity and tortuosity values. An envisioned application of this technique is to model gas diffusion in porous solid oxide fuel cell electrodes.