Several promising Li-ion battery technologies incorporate nanoarchitectured carbon networks, typically in the form of whisker/ particle blends bonded with thermoplastic binders to form the anodes. Degradation of these materials is currently a persistent problem, with damage presenting as blistering and/or delamination of the electrode. Both material composition and morphology play a role in these critical failure modes, and are explored in the present work as they affect conduction in practical battery materials. Lawrence Berkeley National Laboratories and the Institut de Recherche d'Hydro-Quebec supplied the materials studied in this work. Our present approach builds on our previous numerical work, incorporating real material morphology and careful selection of boundary conditions to reduce the numerical difficulties posed by singularities in the field solution, due to phase contrast, sharp corners, etc. In order to allow use of these models for various shapes of particles, we provide a few simple geometrical relations for calculation of total surface area for various morphologies of electrode materials. A four-point-probe technique was employed to obtain the experimental conductivities. Although the existence of contact resistance is well known, there is little literature regarding a technique to measure its value; here, we also present a method for quantifying it, assuming that the anode layer is comprised of two layers. Voltage functions for each layer are determined by enforcement of voltage continuity at the interfaces, current intensities at the inlet and outlet on both sides of interface, and assumption of zero voltage in the second layer as z → ϱ. The four-point-probe technique is suitable for the electrode materials tested, offering reasonable experimental precision in a simple setup. The results of this study offer some insight into the design of active materials. The model shows applicability to a wide variety of materials, including those comprised of fibers, particles, and flakes. Comparisons among simulation predictions and real material conductivities showed very good agreement. An obvious subject of future work is combined electrochemical, conduction, and mechanical modeling of these materials.

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A microscale model is presented in this study to simulate electrode kinetics of the oxygen electrode in a solid oxide electrolyzer cell (SOEC). Two mixed ionic/electronic conducting structures are examined for the oxygen producing electrode in this work: single layer porous lanthanum strontium cobalt ferrite (LSCF), and bilayer LSCF/SCT (strontium cobalt tantalum oxide) structures. A yttrium-stabilized zirconia (YSZ) electrolyte separates the hydrogen and oxygen electrodes, as well as a gadolinium doped-ceria (GDC) buffer layer on the oxygen electrode side. Electrochemical reactions occurring at the two-phase boundaries (2PBs) and three-phase boundaries (3PBs) of single-layer LSCF and bilayer LSCF/SCT oxygen electrodes are modeled under various SOEC voltages with lattice oxygen stoichiometry as the key output. The results reveal that there exists a competition in electrode kinetics between 2PBs and 3PBs, but 3PBs are the primary reactive sites for single-layer LSCF oxygen electrode under high voltages. These locations experience the greatest oxygen stoichiometry variations and are therefore the most likely locations for dimensional changes. By applying an active SCT layer over LSCF, the 2PBs become activated to compete with the 3PBs, thus alleviating oxygen stoichiometry variations and reducing the likelihood of dimensional change. This strategy could reduce lattice structural expansion, proving to be valuable for electrode-electrolyte delamination prevention and will be the focus of future work.

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Rapid and reliable assessment of volatile and semivolatile organic compounds in the environment using gas chromatography �GC � is often limited by cost of analysis, and time delays between sampling and analysis. Many environmental monitors incorporating GC systems are too large for portability, and lack sufficient sensitivity and/or selectivity to serve as practical environmental monitors. Frequently, a complete system redesign, due to nonlinear power scaling relative to component size, is required to reduce the mass and volume of power supplies, especially for the micro-systems of present interest. Here, we examined four strategies in reducing power demand by the largest consumer of power in a model micro GC, the preconcentrator. Our simulations included alterations in heater pad placement/size, reduction of thermal mass in the device, vacuum sealing, and incorporation of a gas dwell time during preconcentrator heating. Our numerical results were in general agreement with experimental findings in simpler systems, in terms of the benefits of vacuum sealing. The greatest reductions in power demand were achieved with vacuum sealing �51% � and reductions in thermal mass �15%�. Future work will address structural and materials issues involved in reduction of thermal mass, and also optimization of power supplies required to meet the multilevel power demands of these complex microelectromechanical systems. © 2005 American Vacuum Society. �DOI: 10.1116/1.1886821� I