Abiotic methylation of mercury in the aquatic environment

Methylation of inorganic mercury in the aquatic environment has been considered to be largely the result of biological processes, primarily involving sulfate-reducing bacteria. However, these processes cannot account for all of the methylmercury that is formed naturally. A growing body of evidence suggests that chemical reactions represent another possible pathway for mercury methylation in the aquatic environment. In order to assess the abiotic contribution to mercury methylation in the water column, and specifically the conditions under which this contribution may be significant, the current state of knowledge about environmentally significant methylation reactions is reviewed. Results of our laboratory-based investigations of aqueous mercury reactions with some potential methyl donors, including MeCo(dmg)(2)(H2O), a simple model for methylcobalamin, various methyltin compounds and methyl iodide, are presented. In each reaction, the yield of methylmercury and the rate of methylation depend strongly on environmental factors such as pH, temperature, and the presence of complexing agents, especially chloride.     

A kinetic study of nickel complexation in model systems by adsorptive cathodic stripping voltammetry

Adsorptive cathodic stripping voltammetry (AdCSV) in conjunction with the competing ligand-exchange method (CLEM) was investigated as a tool for measuring dissociation rate coefficients of nickel complexes in model systems. Dimethylglyoxime (DMG) was used as the competing ligand. Citric acid (CA) and a well-characterized fulvic acid (FA) were used as model ligands. The rate coefficients were calculated, and the consistency of equilibrium and kinetic data was discussed. The contributions of the disjunctive pathway (proceeding by the dissociation of the initial complex) and the adjunctive pathway (proceeding by the formation of an intermediate complex as a result of direct attack of the competing ligand on the initial complex) on the overall reactions were investigated. The reactions of Ni-CA or Ni-FA complexes with DMG were demonstrated to proceed by both disjunctive and adjunctive pathways. The predominant pathway for the overall reaction depends on the nickel-to-initial ligand and the DMG-to-initial ligand ratios. The reactions follow predominantly the disjunctive pathway for [DMG] > or = 3 mM and Ni-to-dissolved organic carbon (DOC) ratios greater than 10 nM Ni 2+/g of DOC. Since free nickel ion in freshwaters is reported to be toxic, its rate and pathway of formation are of environmental concern.     

Algorithmic Approach for Thermal Port Definition

An algorithmic technique is presented that allows estimation of maximum temperature rise in a thermal model with component-board thermal interaction. The technique is based on a generalized thermal port grouping to estimate the interface temperature profile derived from coarse port assignments. By using a very simple environment, it takes a library-type thermal component model with many thermal ports included and transforms it by grouping thermal ports based on a temperature profile, allowing a minimal number of port groups to provide the same accuracy as the original model in a shorter time. The usefulness of this technique is illustrated through simulation of two thermal models with extreme nonuniform temperature distribution on the interface surface. The prediction accuracy is evaluated by comparison with the final solutions to numerical simulation.     

Some results on Hg content in hair in different populations in Albania

The mercury content in hair, for exposed dental workers and unexposed groups of people living in Albania was studied. The influence of factors such as age, sex, body weight, fish consumption in the diet, number of dental amalgam fillings and time exposed to mercury, was determined. The mean value of mercury content in hair for the subjects under study (0.705 microg/g) was found to be lower than the value referred from the World Health Organization (WHO) for people who do not consume fish with a high methylmercury content (1-2 microg/g). A significant positive correlation of Hg content with the number of dental amalgam fillings, time under exposure and frequency of fish consumption in the diet was found. These correlations became stronger when exposed and unexposed groups of people were evaluated separately.     

A thermal design methodology for multifinger bipolar transistor structures

A technique is presented that allows the increase in maximum temperature rise due to thermal coupling in multifinger structures to be predicted for a wide range of finger lengths and spacings by reference to a single, normalized characteristic. Application of this approach to the design of thermal resistance in multifinger structures results in a fast and straightforward method for generating families of structures that meet given power and temperature criteria without thermal simulation-based optimization. The usefulness of this approach is illustrated through solution of a practical design problem, and the accuracy of the method is verified by comparison with the final solutions to numerical simulation.     

Passive parameterized time-domain macromodels for high-speed transmission-line networks

There is a significant need for efficient and accurate macromodels of components during the design of microwave circuits. Increased integration levels in microwave devices and higher signal speeds have produced the need to include effects previously neglected during circuit simulations. Accurate prediction of these effects involve solution of large systems of equations, the direct simulation of which is prohibitively CPU expensive. In this paper, an algorithm is proposed to form passive parametrized macromodels of large linear networks that match the characteristics of the original network in time, as well as other design parameters of the circuit. A novel feature of the algorithm is the ability to incorporate a set of design parameters within the reduced model. The size of the reduced models obtained using the proposed algorithm were less than 5% when compared to the original circuit. A speedup of an order of magnitude was observed for typical high-speed transmission-line networks. The algorithm is general and can be applied to other disciplines such as thermal analysis.     

Hierarchical thermal analysis of large IC modules

This paper presents a new approach to hierarchical thermal modeling using libraries of parametrized sub-models. It is demonstrated how to efficiently create thermal sub-models based on a parametrized model reduction technique. These sub-models are then used for fast simulation of complex parts using a hierarchical modeling building methodology that nests sub-models within sub-models. As an example of such a model parametrized thermal sub-models of a GaAs power cell, an integrated GaAs microwave power amplifier and an InP optical modulator are generated. A complete module is then built by attaching these sub-models to detailed models in a hierarchical manner, creating a thermal model of the entire system. This methodology allows a quick thermal analysis to be performed of very large systems. The thermal sub-models are small in size, boundary condition independent, have very short simulation times, and predict with high accuracy (better then 2% error) all internal temperatures. Finally, the optical modulator model is used as example of the computational efficiency of the methodology. Although an absolute speed-up is difficult to define two cases were provided with gains of around 30 to 40 times calculated. System memory requirements were also reduced by a factor of three.     

A general method for the connection of a component thermal model to a board

In this paper, a generalized method for the connection of a thermal component model in board- and system-level thermal simulations is presented. The method allows for the definition of uniform heat flow connections as well as the standard uniform temperature interface regions. The use of uniform heat flow ports will be shown to better handle cases where large temperature gradients are present in the base model. The two methods of connecting the component model will be evaluated using two different models. First, a simple example will be presented to illustrate the nonphysical behavior introduced by the use of uniform temperature connections. Second, a model of an electronic package will be used to evaluate the relative merits of the two connection methods with respect to board thermal conductivity and boundary conditions present on the board and the package. It will be shown that the results from use of uniform heat flow connections are generally better than from use of uniform temperature regions with respect to predicting junction and board temperatures.     

Fast Simulation of steady-state temperature distributions in electronic components using multidimensional model reduction

In this paper, a model reduction technique is applied to the thermal modeling of electronic components and devices with complex geometries. The reduced-order model is capable of predicting a complete detailed three-dimensional temperature distribution in the original model. The small size and the simplicity of the reduced model allows for the very quick simulation of the device under a wide range of input parameters, such as different boundary conditions and power distributions. Use of the reduced-order model in a thermal design cycle can have a significant effect on both prediction accuracy and simulation efficiency. In the paper, the usefulness of this technique is demonstrated through examples from different electronic devices and packages. Accuracy of the reduced-order model is validated by comparison with the solution to a detailed numerical model.