Plasma functionalization of silicon carbide crystalline nanoparticles in a novel low pressure powder reactor

In order to functionalize silicon carbide nanopowders with carboxylic groups, an r.f. (13.56 MHz) low pressure plasma reactor has been developed so that particles can be stirred during the processing to try to coat them on their whole surface. Coatings in an O₂/hexamethyldisilazane (HMDSN) mixture have first been optimized on flat substrates; X-ray Photoelectron Spectroscopy (XPS) analysis showed that the O₂/HMDSN gas mixture resulted in a coating evolving from a polymer-like structure to a more inorganic SiO_x-like structure as the oxygen ratio increased. For a large O₂/HMDSN value, carboxylic groups were detected on the sample surface. Silicon carbide nanoparticles have then been plasma processed in such a reactive atmosphere. XPS, Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Transmission Electron Microscopy (TEM) analyses evidenced the surface modification of the processed powder and confirmed the grafting of carboxylic groups.     

Localized high resolution electrochemistry and multifunctional imaging: scanning electrochemical cell microscopy

We describe highly localized electrochemical measurements and imaging using a simple, mobile theta pipet cell. Each channel (diameter <500 nm) of a tapered theta pipet is filled with electrolyte solution and a Ag/AgCl electrode, between which a bias is applied, resulting in a conductance current across a thin meniscus of solution at the end of the pipet, which is typically deployed in air or a controlled gaseous environment. When the position of the pipet normal to a surface of interest is oscillated, an oscillating component in the conductance current is generated when the meniscus at the end of the probe comes into contact with the surface and undergoes periodic (reversible) deformation, so as to modulate the solution resistance. This oscillating current component can be used to maintain gentle contact of the solution from the pipet cell with the surface and as a set point for high resolution topographical imaging with the pipet. Simultaneously, the mean conductance current that flows between the pipet channels can be measured and is sensitive to the local nature of the interface, informing one, for example, on wettability and ion flow into or out of the surface investigated. Furthermore, conductor or semiconductor surfaces can be connected as a working electrode, with one of the electrodes in the pipet serving as a quasi-reference electrode. This pipet cell then constitutes part of a dynamic electrochemical cell, with which direct voltammetric-amperometric imaging can be carried out simultaneously with conductance and topographical imaging. This provides multifunctional electrochemical maps of surfaces and interfaces at high spatial resolution. The prospects for the use of this new methodology widely are highlighted through exemplar studies and a brief discussion of future applications.     

A nanometre-sized porous phase in iron-carbon-boron system

A porous phase is detected in a Fe-0.28 wt.%C-0.1 wt.%B alloy. The porous phase is mainly located at the grain boundary region and the pore size ranges from about 10 nm to 500 nm. The chemical composition of the porous phase is very close to Fe₃(B_0.7C_0.3) with an orthorhombic lattice. The result shows an opportunity to produce bulk steel matrix composites with a porous second phase.     

Examining the neural correlates of choice behavior in a gambling task using steady state topography.

The present study investigated the behavioral and neuropsychological characteristics of decision-making behavior during a gambling task as well as how these characteristics may relate to the Somatic Marker Hypothesis and the Frequency of Gain model. The applicability to intertemporal choice was also discussed. Patterns of card selection during a computerized interpretation of the Iowa Gambling Task were assessed for 10 men and 10 women. Steady State Topography was employed to assess cortical processing throughout this task. Results supported the hypothesis that patterns of card selection were in line with both theories. As hypothesized, these 2 patterns of card selection were also associated with distinct patterns of cortical activity, suggesting that intertemporal choice may involve the recruitment of right dorsolateral prefrontal cortex for somatic labeling, left fusiform gyrus for object representations, and the left dorsolateral prefrontal cortex for an analysis of the associated frequency of gain or loss. It is suggested that processes contributing to intertemporal choice may include inhibition of negatively valenced options, guiding decisions away from those options, as well as computations favoring frequently rewarded options. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Sicher zündende,langzeitstabile Kaltkathode

Long term stable cold cathode with reliable ignition Cold cathodes gauges have been used in various areas of vacuum technology for decades. Their unique properties make them the method of choice even despite the residual magnetic field, poor ignition characteristics in the UHV‐region and long term stability. In other applications they do not significantly shorten the maintenance intervals. However, especially in the UHV or XHV region these properties lead to an extinguishing of cold cathode discharge. Considering the underlying processes and their correlation in this contribution an improved cold cathode will be presented. The measured data show a residual magnetic field of 1 mT in close vicinity to the gauge and an extended operating pressure range down to the 10^?12 mbar region. Furthermore the ignition characteristics were improved to < 60 s at 1×10^?9 mbar.     

Influence of potassium on the competition between methane and ethane in steam reforming over Pt supported on yttrium-stabilized zirconia

The effect of addition of potassium to Pt supported on yttrium-stabilized zirconia (PtYSZ) catalyst for steam reforming of methane, ethane and methane/ethane mixtures was explored. Addition of potassium has a positive effect on preferential steam reforming of methane in mixtures of methane and ethane over Pt/YSZ catalysts. The activity of potassium-modified catalysts increased with time-on-stream during steam reforming of mixtures of methane and ethane, while the ratio of reaction rates of methane and ethane remained constant. Most importantly, it was demonstrated that the presence of potassium prevents competition between methane and ethane during steam reforming. The reaction rate ratio in methane/ethane mixtures is changed from preferential ethane reforming on PtYSZ towards preferential methane conversion as a result of addition of potassium.     

Characterization of enzyme motions by solution NMR relaxation dispersion

In many enzymes, conformational changes that occur along the reaction coordinate can pose a bottleneck to the rate of conversion of substrates to products. Characterization of these rate-limiting protein motions is essential for obtaining a full understanding of enzyme-catalyzed reactions. Solution NMR experiments such as the Carr-Purcell-Meiboom-Gill (CPMG) spin-echo or off-resonance R 1rho pulse sequences enable quantitation of protein motions in the time range of microseconds to milliseconds. These experiments allow characterization of the conformational exchange rate constant, k ex, the equilibrium populations of the relevant conformations, and the chemical shift differences (Deltaomega) between the conformations. The CPMG experiments were applied to the backbone N-H positions of ribonuclease A (RNase A). To probe the role of dynamic processes in the catalytic cycle of RNase A, stable mimics of the apo enzyme (E), enzyme-substrate (ES) complex, and enzyme-product (EP) complex were formed. The results indicate that the ligand has relatively little influence on the kinetics of motion, which occurs at 1700 s (-1) and is the same as both k cat, and the product dissociation rate constant. Instead, the effect of ligand is to stabilize one of the pre-existing conformations. Thus, these NMR experiments indicate that the conformational change in RNase A is ligand-stabilized and does not appear to be ligand-induced. Further evidence for the coupling of motion and enzyme function comes from the similar solvent deuterium kinetic isotope effect on k ex derived from the NMR measurements and k cat from enzyme kinetic studies. This isotope effect of approximately 2 depends linearly on solvent deuterium content suggesting the involvement of a single proton in RNase A motion and function. Moreover, mutation of His48 to alanine eliminates motion in RNase A and decreases the catalytic turnover rate indicating the involvement of His48, which is far from the active site, in coupling motion and function. For the enzyme triosephosphate isomerase (TIM), the opening and closing motion of a highly conserved active site loop (loop 6) has been implicated in many studies to play an important role in the catalytic cycle of the enzyme. Off-resonance R 1rho experiments were performed on TIM, and results were obtained for amino acid residues in the N-terminal (Val167), and C-terminal (Lys174, Thr177) portions of loop 6. The results indicate that all three loop residues move between the open and closed conformation at about 10,000 s (-1), which is the same as the catalytic rate constant. The O (eta) atom of Tyr208 provides a hydrogen bond to stabilize the closed form of loop 6 by interacting with the amide nitrogen of Ala176; these atoms are outside of hydrogen bonding distance in the open form of the enzyme. Mutation of Tyr208 to phenylalanine results in significant loss of catalytic activity but does not appear to alter the kex value of the N-terminal part of loop 6. Instead, removal of this hydrogen bond appears to result in an increase in the equilibrium population of the open conformer of loop 6, thereby resulting in a loss of activity through a shift in the conformational equilibrium of loop 6. Solution NMR relaxation dispersion experiments are powerful experimental tools that can elucidate protein motions with atomic resolution and can provide insight into the role of these motions in biological function.     

Elucidating the thermal, chemical, and mechanical mechanisms of ultraviolet ablation in poly(methyl methacrylate) via molecular dynamics simulations

[Figure: see text]. Laser ablation harnesses photon energy to remove material from a surface. Although applications such as laser-assisted in situ keratomileusis (LASIK) surgery, lithography, and nanoscale device fabrication take advantage of this process, a better understanding the underlying mechanism of ablation in polymeric materials remains much sought after. Molecular simulation is a particularly attractive technique to study the basic aspects of ablation because it allows control over specific process parameters and enables observation of microscopic mechanistic details. This Account describes a hybrid molecular dynamics-Monte Carlo technique to simulate laser ablation in poly(methyl methacrylate) (PMMA). It also discusses the impact of thermal and chemical excitation on the ensuing ejection processes. We used molecular dynamics simulation to study the molecular interactions in a coarse-grained PMMA substrate following photon absorption. To ascertain the role of chemistry in initiating ablation, we embedded a Monte Carlo protocol within the simulation framework. These calculations permit chemical reactions to occur probabilistically during the molecular dynamics calculation using predetermined reaction pathways and Arrhenius rates. With this hybrid scheme, we can examine thermal and chemical pathways of decomposition separately. In the simulations, we observed distinct mechanisms of ablation for each type of photoexcitation pathway. Ablation via thermal processes is governed by a critical number of bond breaks following the deposition of energy. For the case in which an absorbed photon directly causes a bond scission, ablation occurs following the rapid chemical decomposition of material. A detailed analysis of the processes shows that a critical energy for ablation can describe this complex series of events. The simulations show a decrease in the critical energy with a greater amount of photochemistry. Additionally, the simulations demonstrate the effects of the energy deposition rate on the ejection mechanism. When the energy is deposited rapidly, not allowing for mechanical relaxation of the sample, the formation of a pressure wave and subsequent tensile wave dominates the ejection process. This study provides insight into the influence of thermal, chemical, and mechanical processes in PMMA and facilitates greater understanding of the complex nature of polymer ablation. These simulations complement experiments that have used chemical design to harness the photochemical properties of materials to enhance laser ablation. We successfully fit the results of the simulations to established analytical models of both photothermal and photochemical ablation and demonstrate their relevance. Although the simulations are for PMMA, the mechanistic concepts are applicable to a large range of systems and provide a conceptual foundation for interpretation of experimental data.     

Teen driver crash risk and associations with smoking and drowsy driving

Motor vehicle crashes are the leading cause of death for young people in the United States. The goal of this study was to identify risk factor profiles of teen and young adult drivers involved in crashes. General demographic and behavioral as well as driving-related factors were considered. Analysis of a nationally representative telephone survey of U.S. young drivers ages 14 to 22 (N = 900) conducted in 2005 was restricted to 506 licensed drivers (learners excluded). Statistically significant univariate associations between factors of interest and the primary outcome, crash involvement (ever) as a driver, were identified and included within a multivariate logistic regression model, controlling for potential demographic confounders. Aside from length of licensure, only driving alone while drowsy and being a current smoker were associated with having been in a crash. Gaining a better understanding of these behaviors could enhance the development of more customized interventions for new drivers.