Impedance boundary method of moments with extended boundaryconditions

An improved impedance boundary method of moments is proposed to accurately and efficiently determine the propagation characteristics of multilayer planar optical waveguides whose refractive index profiles contain large step index changes or a graded-index region combined with step index changes. Extended impedance boundary conditions are used in the impedance boundary method of moments to replace regions in the waveguide structure which have refractive index discontinuities. Two methods for determining the extended boundary conditions include the standard transfer matrix method and a procedure utilizing a cascaded transmission line representation with successive calculation of input impedances. Simulation results show significantly improved convergence rates in the determination of propagation constants     

A comprehensive compact-modeling methodology for spiral inductors in silicon-based RFICs

A new comprehensive wide-band compact-modeling methodology for on-chip spiral inductors is presented. The new modeling methodology creates an equivalent-circuit model consisting of frequency-independent circuit elements. A fast automated extraction procedure is developed for determining the circuit element values from two-port S-parameter measurement data. The methodology is extremely flexible in allowing for accurate modeling of general classes of spiral inductors on high- or low-resistivity substrate and for large spirals exhibiting distributed trends. The new modeling methodology is applied to general classes of spirals with various sizes and substrate parameters. The extracted models show excellent agreement with the measured data sets over the frequency range of 0.1-10 GHz.     

A new circuit augmentation method for modeling of interconnects and passive components

This paper presents a new methodology for automated broadband model generation from S-parameter data for interconnects and passive components. The new methodology is based on augmenting an existing equivalent circuit model with a macromodel (black-box) network described by rational functions while simultaneously perturbing the equivalent circuit component values. The macromodel network is determined using standard least-squares or vector-fitting approaches. The perturbation of the equivalent circuit parameter values is achieved during the macromodel generation by means of global optimization based on intelligent search algorithms. The new approach is demonstrated on several two-port test example structures including a broadband probe tip structure and a CMOS spiral inductor.     

Negative differential resistance in a resonant quantum wirestructure

It is shown that the transmission characteristics of a semiconductor split-gate device with a double constriction exhibit resonant behavior similar to that in resonant-tunneling diodes (RTDs). The double constriction is modeled with cascaded quantum waveguides of different widths and analyzed by applying a mode-matching technique to the wave function at the discontinuities. At low temperature, the calculated current-voltage characteristics exhibit a region of negative differential resistance. Current-voltage characteristics are presented for different constriction widths and temperatures exhibiting peak-to-valley ratios of up to 4:1 which persist up to temperatures around 10 K     

Acrylamide in heated potato products – analytics and formation routes

Acrylamide is a highly polar, water‐soluble molecule with low, but significant volatility. Most common methods for determination of acrylamide are high performance liquid chromatography‐tandem mass spectrometry (LC‐MS‐MS), gas chromatography‐mass spectrometry (GC‐MS) after bromination, GC‐MS without and LC‐MS after derivatisation with mercaptobenzoic acid. The different clean‐up procedures used are presented and the advantages and disadvantages of the individual methods are discussed. Acrylamide is formed during food preparation if free asparagine and reducing sugars are present, water activity is low and the product temperature exceeds 100 °C. The influence of different parameters on acrylamide formation in model systems is discussed. In fried potatoes the greatest amount of acrylamide is formed from asparagine and reducing sugars. Alternative routes of formation, as for example via acrolein and acrylic acid are much less important.     

Microtubule-associated protein 2 and the organization of cellular microtubules

Microtubule-associated proteins (MAPs) are prominent components of the neuronal cytoskeleton that can promote microtubule formation and whose expression is under strong developmental regulation. They are thought to be involved in organizing the structure of microtubule fascicles in axons and dendrites, although whether they form active cross-links between microtubules or serve as strut-like spacer elements has yet to be resolved. In the experiments reported here we explored their influence on microtubules by expressing them in non-neuronal cells using DNA transfection techniques. We confirm earlier reports that microtubule-associated proteins of the MAP2/tau class can induce bundling of microtubules. In addition we find that MAP2 causes the rearrangement of microtubules in the cytoplasm in a manner that is dependent on the length of the microtubule bundles. Short bundles are straight and run across the cytoplasm whereas long bundles form a marginal band-like array at the periphery. We suggest that the latter arrangement is produced when microtubule bundles that are too long to fit inside the diameter of the cell bend under the restraining influence of the cortical cytoskeleton. In confirmation of this, we show that when the cortical actin network is depolymerized by cytochalasin B the MAP2-containing microtubule bundles push out cylindrical extensions from the cell surface. These results suggest that the induction of stiff microtubules bundles by MAP2, coupled with a breach in the cortical actin network, can confer two of the properties characteristic of neuronal processes; their cylindrical form and the presence of fasciculated microtubules.     

Investigation of the Stability of Quaternary Ammonium Methyl Carbonates

Quaternary ammonium compounds are used commercially for a variety of applications and some are of interest as ionic liquids. For many years dimethyl carbonate has been touted as a green reagent, including its use for methylation (quaternization) of tertiary amines. In addition, substitution of the methyl carbonate by other anions can be efficiently and cleanly accomplished by reaction with the corresponding acid. How stable are these methyl carbonate quaternary compounds? High field ¹³C NMR shows that in the presence of water, the methyl carbonate is converted to bicarbonate. Headspace GCMS indicates that the alkylammonium methyl carbonate salts are stable below 170–180 °C while the bicarbonate salts are stable to only about 140 °C. Thermal decomposition occurs by decarboxylation and by dealkylation.