Modeling the structural evolution of carbide-derived carbons using quenched molecular dynamics

We develop morphologically realistic models for amorphous carbon using quenched molecular dynamics. We show that as the thermal quench rate is decreased, the model structures become more highly ordered, forming large graphene-like fragments and regularly shaped porous features. The evolution of these changes is compared with a series of carbide-derived carbons synthesized from crystalline TiC using different chlorination temperatures. In general, we find that the structural changes in the models are similar to those seen in experiment and that these changes have a significant impact on pore size distributions, specific surface areas, and adsorption isotherms, which are used to empirically characterize microporous carbons.     

A MATHEMATICAL MODEL CHARACTERIZING THE DIFFUSION PROPERTIES OF MICROCAPSULES

Composite techniques are developed for numerical solutions of partial differential equations (PDE's) by the combinations of different solutions. The combination can be global or local depending on the usage of the composite. A global composite is one in which the composite solution is not utilized in any continued calculations of the different numerical solutions. Global composites are space by space combinations, i.e., the solutions are obtained over the entire space of independent variables. For local combinations, on the other hand, the composite is used in the continued calculations of the different solutions. Therefore, the local procedure is a line by line algorithm where the different solutions are combined and become the initial values for the continued calculations of the different solutions.

Composites considered are (i) global extrapolation, (ii) local extrapolation and local stabilization, (iii) alternating direction methods, and (iv) acceleration. By extrapolation the solution over the space of independent variables is calculated and combined with solutions for successively smaller grid spacings, characterized by h_m . Global extrapolation is applied to the finite difference solution of PDE's and shown to be effective for reducing the truncation error (TE) of fixed difference methods. On the basis of computation time the spacing sequence h₀ (m + 1) is shown to reduce the TE more efficiently than fco/2m for more than two extrapolations, where /i0 characterizes the largest spacing.     

Synthesis of bimetallic Pt-Pd core-shell nanocrystals and their high electrocatalytic activity modulated by Pd shell thickness

Bimetallic Pt-Pd core-shell nanocrystals (NCs) are synthesized through a two-step process with controlled Pd thickness from sub-monolayer to multiple atomic layers. The oxygen reduction reaction (ORR) catalytic activity and methanol oxidation reactivity of the core-shell NCs for fuel cell applications in alkaline solution are systematically studied and compared based on different Pd thickness. It is found that the Pd shell helps to reduce the over-potential of ORR by up to 50 mV when compared to commercial Pd black, while generating up to 3-fold higher kinetic current density. The carbon monoxide poisoning test shows that the bimetallic NCs are more resistant to the CO poisoning than Pt NCs and Pt black. It is also demonstrated that the bimetallic Pt-Pd core-shell NCs can enhance the current density of the methanol oxidation reaction, lowering the over-potential by 35 mV with respect to the Pt core NCs. Further investigation reveals that the Pd/Pt ratio of 1/3, which corresponds to nearly monolayer Pd deposition on Pt core NCs, gives the highest oxidation current density and lowest over-potential. This study shows for the first time the systematic investigation of effects of Pd atomic shells on Pt-Pd bimetallic nanocatalysts, providing valuable guidelines for designing high-performance catalysts for fuel cell applications.     

The reproductive cycle of the female American alligator (Alligator mississippiensis)

We examined ovarian and oviducal gross morphology and collected blood samples from wild female alligators in central Florida during most of the year. Females with vitellogenic follicles were observed throughout the year, although ovaries containing follicles larger than 15 mm were seen only during the spring and early summer (March-June). We detected a poor relationship between female body size and the number of vitellogenic follicles whereas body size was significantly correlated with clutch size. Plasma samples were analyzed for (1) estradiol-17beta (E2), testosterone (T), and corticosterone by specific radioimmunoassays; (2) vitellogenin by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis; and (3) total protein, phosphorus, and calcium by spectrophotometric assays. Reproductively active females showed elevated plasma concentrations of E2 during the fall (September-November) and spring (March-May) whereas non-reproductively active females exhibited basal levels with no apparent peaks. Vitellogenin was detected in the plasma during the same months that plasma E2 concentrations were elevated. Elevated plasma vitellogenin and E2 were not correlated with plasma total protein but were correlated with plasma calcium concentration. During late vitellogenesis, plasma T concentrations were elevated in reproductively active females coincident with a period of intense courtship and mating. Corticosterone plasma concentrations exhibit no significant monthly variation, nor apparent changes during various stages of reproductive activity although plasma concentrations were elevated during late gravidity. Our data suggest that female reproductive activity begins in the fall with an increase in plasma E2 concentration in September or October and vitellogenesis in October. Ovarian activity slows during winter and reactivates with the onset of spring.     

Real-time monitoring of diffusion in polymer films using fluorescent tracer

A method for real-time nondestructive monitoring of small molecules diffusion in polymeric films was developed. The method was based on detection of a fluorescent tracer eluting from the investigated polymer film into the solution in which this film was immersed. The kinetics of the tracer elution, monitored by the increase in solution fluorescence intensity, was used to deduce tracer diffusivity in polymer film. The data were treated using a straightforward mathematical model, describing diffusion from an infinite plane of a certain thickness immersed into a finite solvent bath. Fluorescent 7-diethylamino-4-methyl coumarin was used as a tracer. The diffusion of this tracer within plasticized poly(methyl methacrylate) and styrene-isoprene-styrene block copolymer matrices was monitored. The diffusion coefficients equal to 2 × 10^?9 cm²/s and 1 × 10^?9 cm²/s, respectively, were obtained. © 1995 John Wiley & Sons, Inc.     

Evidence for genetic correlation of hypnotic effects and cerebellar Purkinje neuron depression in response to ethanol in mice

In the present study, we compared phenotypic differences in behavioral and neurophysiological responses to acute ethanol administration among eight inbred strains of mice. Genetic variation for behavioral sedation, as measured by loss of the righting reflex (sleep time) after a hypnotic dose of ethanol, was shown to be present among the inbred strain population. In addition, a large genetic component of variation in the depressant action of ethanol on the spontaneous discharge of cerebellar Purkinje neurons was found. Results from an analysis of covariance of the behavioral and electrophysiological phenotypes, measured on each mouse among the inbred strains, provided strong evidence for a high genetic correlation between sleep time and inhibition of cerebellar Purkinje neuron discharge in response to acute ethanol administration. Taken together with our previously reported data on ethanol-induced electrophysiological changes in selectively bred lines, the results described here strongly support the hypothesis that the cerebellar Purkinje neuron is one important locus for the acute soporific effects of alcohol.