AC impedance has provided a useful diagnostic tool in the Los Alamos polymer electrolyte fuel cell (PEFC) program. The author reviews the techniques he has used in ac impedance modeling. These techniques include equation implementation, model simplification and verification, least squares fitting, application of two-dimensional Laplace equation solvers handling complex interfacial boundary conditions, and interpretation of impedance features. The separate features of the complete electrode model are explained by analytic examples.

Using molecular dynamics (MD) simulation, the authors investigate the mechanical response of silicon to high dose ion-irradiation. The authors employ a realistic model to directly simulate ion beam induced amorphization. Structural properties of the amorphized sample are compared with experimental data and results of other simulation studies. The authors find the behavior of the irradiated material is related to the rate at which it can relax. Depending upon the ability to deform, the authors observe either the generation of a high compressive stress and subsequent expansion of the material, or generation of tensile stress and densification. The authors note that statistical material properties, such as radial distribution functions are not sufficient to differentiate between the different densities of the amorphous samples. For any reasonable deformation rate, the authors observe an expansion of the target upon amorphization in agreement with experimental observations. This is in contrast to simulations of quenching which usually result in a denser structure relative to crystalline Si. The authors conclude that although there is substantial agreement between experimental measurements and simulation results, the amorphous structures being investigated may have fundamental differences; the difference in density can be attributed to local defects within the amorphous network. Finally the …

Fermentation of glucose-xylose mixtures to ethanol was investigated in batch and continuous experiments using immobilized recombinant <i>Zymomonas mobilis</i> CP4(pZB5). This microorganism was immobilized by entrapment in k-carrageenan beads having a diameter of 1.5-2.5 mm. Batch experiments showed that the immobilized cells co-fermented glucose and xylose to ethanol and that the presence of glucose improved the xylose utilization rate. Batch fermentation of rice straw hydrolyzate containing 76 g/L glucose and 33.8 g/L xylose gave an ethanol concentration of 44.3 g/L after 24 hours, corresponding to a yeild of 0.46 g ethanol/g sugars. Comparable results were achieved with a synthetic sugar control. Continuous fermentation runs were performed in a laboratory scale fluidized-bed bioreactor (FBR). Glucose-xylose feed mixtures were run through the FBR at residence times of 2 to 4 hours. Glucose conversion to ethanol was maintained above 98% in all continuous runs. Xylose conversion to ethanol was highest at 91.5% for a feed containing 50 g/L glucose-13 g/L xylose at a dilution rate of 0.24 h^-1. The xylose conversion to ethanol decreased with increasing feed xylose concentration, dilution rate and age of the immobilized cells. Volumetric ethanol productivities in the range of 6.5 to 15.3 g/L-h were obtained.

The authors present a highly efficient molecular dynamics scheme for calculating dopant density profiles in group-IV alloy, and III-V zinc blende structure materials. Their scheme incorporates several necessary methods for reducing computational overhead, plus a rare event algorithm to give statistical accuracy over several orders of magnitude change in the dopant concentration. The code uses a molecular dynamics (MD) model to describe ion-target interactions. Atomic interactions are described by a combination of 'many-body' and pair specific screened Coulomb potentials. Accumulative damage is accounted for using a Kinchin-Pease type model, inelastic energy loss is represented by a Firsov expression, and electronic stopping is described by a modified Brandt-Kitagawa model which contains a single adjustable ion-target dependent parameter. Thus, the program is easily extensible beyond a given validation range, and is therefore truly predictive over a wide range of implant energies and angles. The scheme is especially suited for calculating profiles due to low energy and to situations where a predictive capability is required with the minimum of experimental validation. They give examples of using the code to calculate concentration profiles and 2D 'point response' profiles of dopants in crystalline silicon and gallium-arsenide. Here they can predict the experimental profile over five …

A lifetime-measurement instrument has been developed to characterize large silicon ingots prior to wafering and polishing. It uses the direct-current photoconductance decay method and localized probing and illumination to achieve the necessary sensitivity on low-resistivity, large samples. A 940-nm, 60-Wp, pulsed-laser diode beam (250-ms width, &lt;100-ns cut-off) lights the as-cropped silicon surface between two ohmic-contact probes. A user-friendly graphical interface supports data acquisition, lifetime calculation, and data storage. Pneumatic systems position the ingot and probes. Three-dimensional, finite-element analysis indicates that the detection depth of this technique is much better than the microwave or radio-frequency techniques. It also shows that the as-cropped surface finish is adequate for measuring bulk lifetimes on the order of 50 ms or less-a typical range for Czochralski ingots used in photovoltaic module production. Measurement repeatability and clear distinction among different grades of feedstock materials have been demonstrated.