bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of adsorptive and diffusive properties of carbon dioxide - methane - water in coal-like structures using molecular simulations
Authors: Salih Alaaeldin , Müller Erich
Published: December 2008

This work employs molecular modeling to simulate both the equilibrium and kinetic behavior pertaining adsorption/desorption and permeation of carbon dioxide within the enhanced coal bed methane (ECBM) process and the corresponding CO~2~ sequestration processes in deep coal mines. The coal structure is modeled on the basis of a carbon, nanotubes, slit pores and more realistic “virtual” carbons. (from bottom up in the figure) The grand canonical Monte Carlo method is used to simulate the adsorption isotherms and isosteric heats of adsorption of the methane-carbon dioxide-water ternary mixture into the carbon structure, while Molecular Dynamics is applied to the same systems to study the competitive diffusion properties of these fluids and to observe the preferential pathways associated with the non-equilibrium flow (kinetics) within these nanoporous media. Although this study focuses on using a rigid carbon matrix, approaches to introducing dynamic carbon structures that mimic the swelling behaviour of coals are discussed.
An Integrated IGCC-CSS Design Course for Graduate Students in Petroleum Engineering
Authors: Falcone Gioia , Jafarpour Behnam , Barrufet Maria
Published: December 2008

A new graduate course on CO2 Capture and Uses was offered for the first time at Texas A&M, Pet. Eng., in Fall 2008. A multidisciplinary team of instructors from the Pet. Eng. & Chem. Eng. departments was assembled to ensure the appropriate expertise.The objective of the course is to let the students understand the need for / potential of Carbon Capture and Storage (CCS) & Enhanced Oil Recovery (EOR).
Design of carbon dioxide storage
Authors: Blunt Martin
Published: December 2008

Carbon capture and storage (CCS) – the collection of carbon dioxide (CO~2~) from industrial sources and its injection underground – is an important technology to reduce CO~2~ emissions to the atmosphere, mitigating climate change. The North Sea, with mature hydrocarbon fields and saline aquifers offers an attractive storage location for CO~2~ produced by the UK’s gas and coal-fired power plants. The principal concern with CCS is to ensure that the CO~2~ does not leak into the oceans or atmosphere over hundreds or thousands of years. We propose a storage strategy where CO~2~ and brine are injected together followed by brine injection alone. We predict that using this technique around 95% of the CO~2~ can be rendered immobile in pore-scale (10s micron) droplets in the porous rock. Over thousands to billions of years the CO~2~ may dissolve or precipitate as carbonate, but it will not migrate upwards and so is effectively sequestered. This design is demonstrated through numerical simulation of field-scale flow through a North Sea aquifer coupled to an experimentally-based model of small-scale trapping. The CO~2~ is trapped during the decades-long lifetime of the injection phase, avoiding the need for extensive monitoring for centuries. The method does not rely on impermeable cap rock to contain the CO~2~; this is only a secondary containment for the small amount of remaining mobile gas. Furthermore, the reduced mobility ratio between injected and displaced fluids leads to a more uniform sweep of the aquifer leading a larger storage capacity than injecting CO~2~ alone.
Reactivation of Limestone-Derived Sorbents using Hydration: Preliminary Results From a Fluidised Bed
Authors: Blamey John , Fennell Paul , Dugwell Denis
Published: December 2008

A simple method of CO~2~ capture is by using the calcium looping cycle. The calcium looping cycle uses CaCO~3~ as a CO~2~ carrier, via the reversible reaction CaO(s) + CO~2~(g) = CaCO~3~(s), to extract CO2 from the exhaust stream and provide a pure stream of CO~2~ suitable for sequestration. A problem associated with the technology is that the capacity of the sorbent to absorb CO~2~ reduces significantly with the number of cycles of carbonation and calcination. The energy penalty of the cycle is considerably increased by cycling unreacted sorbent: hydration of unreactive sorbent has emerged as a promising strategy of reducing this penalty by regenerating the reactivity of exhausted sorbent.A small atmospheric pressure fluidised bed reactor has been built and tested, that allows repeated cycling between two temperatures up to 1000 °C. Work presented here focuses on the effects of variation of the calcination temperature before hydration. Hydration has been found to more than double the reactivity of a spent sorbent cycled under the mildest conditions studied (calcination temperature of 840 °C). However, as calcination temperature is increased the observed reactivation decreases until little reactivation is observed for the sorbent cycled at 950 °C. The primary reason for this appears to be a substantial increase in friability of particles, with reactivity normalised for mass losses appearing similar independent of cycling temperature.
Improving the worthiness of the Elder problem as a benchmark for buoyancy driven convection models
Authors: van Reeuwijk Maarten , Mathias Simon , Simmons Craig , Ward James
Published: December 2008

An important trapping mechanism associated with the geosequestration of CO~2~ is that of dissolution into the formation water. Although supercritical CO~2~ is significantly less dense than water, experimental data reported in the literature show that the density of an aqueous solution of CO~2~ could be slightly greater. Under normal situations, the transfer of gas to solution is largely controlled by the relatively slow process of molecular diffusion. However, the presence of variable densities can trigger off gravity instabilities leading to much larger-scale convection processes. Such processes can potentially enhance rates of dissolution by an order of magnitude. Consequently there is a need for future performance assessment models to incorporate buoyancy driven convection (BDC). A major issue associated with BDC models is that of grid convergence when benchmarking to the Elder problem. The Elder problem originates from a heat convection experiment whereby a rectangular Hele-Shaw cell was heated over the central half of its base. A quarter of the way through the experiment, Elder (1967) observed six plumes, with four narrow plumes in the center and two larger plumes at the edges. As the experiment progressed, only four plumes remained. The issue is that depending on the grid resolution used when seeking to model this problem, modelers have found that different schemes yield steady states with either one, two or three plumes. The aim of this paper is to clarify and circumvent the issue of multiple steady state solutions in the Elder problem using a pseudospectral method.
Pressure buildup during CO2 injection in brine aquifers using the Forchheimer equation
Authors: Mathias Simon , Hardisty Paul , Trudell Mark , Zimmerman Robert
Published: December 2008

If geo-sequestration of CO2 is to be employed as a key emissions reduction method in the global effort to mitigate climate change, simple yet robust screening of the risks of disposal in brine aquifers will be needed. There has been significant development of simple analytical and semi-analytical techniques to support screening analysis and performance assessment for potential carbon sequestration sites. These techniques have generally been used to estimate the size of CO2 plumes for the purpose of leakage rate estimation. A common assumption has been that both the fluids and the geological formation are incompressible. Consequently, calculation of pressure distribution requires the specification of an arbitrary radius of influence. In this talk, a new similarity solution is derived using the method of matched asymptotic expansions. By allowing for slight compressibility in the fluids and formation, the solution improves on previous work by not requiring the specification of an arbitrary radius of influence. A large-time approximation of the solution is then extended to account for non-Darcy inertial effects using the Forchheimer equation. Both solutions are verified by comparison with finite difference solutions. The results show that inertial losses will often be comparable, and sometimes greater than, the viscous Darcy-like losses associated with the brine displacement, although this is strongly dependent on formation porosity and permeability.
Screening of species of the endophytic fungus Phomopsis for the production of the anticancer drug taxol
Authors: Kumaran Rangarajulu Senthil , Hur Byung‐Ki
Published: December 2008

Three different strains of the endophytic fungus Phomopsis were isolated from the healthy leaves of Taxus cuspidata (Japanese yew), Ginkgo biloba (ginkgo or maidenhair tree) and Larix leptolepis (Japanese larch) and screened for the production of taxol on a modified liquid medium for the first time. The presence of taxol was confirmed by spectroscopic and chromatographic analyses. The amount of taxol produced by this fungus was quantified by HPLC. The maximum amount of fungal taxol production was recorded as 418 μg/litre in the strain BKH 27. The yield was increased to 8360‐fold that found for the fungus Taxomyces andreanae reported previously [Stierle, Strobel and Stierle (1993) Science 260, 214–216]. The fungal taxol extracted also showed a strong cytotoxicity towards the human cancer cells in an apoptosis assay. All the three isolates showed positive sign towards PCR for the conserved sequence of the taxadiene synthase gene. The results suggest that Phomopsis could be an excellent alternative source for taxol and may serve as a potential genetic‐engineered species for the enhanced production of taxol.
PIASy inhibits LRH-1-dependent CYP11A1 expression by competing for SRC-1 binding
Authors: Hsieh Hsiang-Tsan , Wang Chih-Hung , Wu Mei-Ling , Yang Feng-Ming , Tai Yu-Chen , Hu Meng-Chun
Published: December 2008

The orphan nuclear receptor LRH-1 (liver receptor homologue-1; NR5A2) plays a critical role in development, bile acid synthesis and cholesterol metabolism. LRH-1 is also expressed in the ovary where it is implicated in the regulation of steroidogenic genes for steroid hormone synthesis. In the present study, we investigated the molecular mechanisms of the transcriptional regulation of CYP11A1 by LRH-1 and found that LRH-1-mediated transactivation was markedly repressed by PIASy [protein inhibitor of activated STAT (signal transducer and activator of transcription) y], the shortest member of the PIAS family. The suppression of LRH-1 activity requires the N-terminal repression domain. Although PIAS proteins also function as E3 SUMO (small ubiquitin-related modifier) ligases and enhance SUMO conjugation, PIASy-mediated repression was independent of LRH-1 SUMOylation status. In addition, histone deacetylase activity was not involved in the inhibition of LRH-1 by PIASy. Immunoprecipitation and mammalian two-hybrid analyses indicated that PIASy interacted with LRH-1 through the C-terminal region, including the AF-2 (activation function-2) motif, which was also involved in the interaction between LRH-1 and the co-activator SRC-1 (steroid receptor co-activator-1). PIASy inhibited the binding of SRC-1 to LRH-1, although overexpression of SRC-1 partially overcame the PIASy inhibition of LRH-1 induction of the CYP11A1 promoter. The results of the present study suggest that competition with co-activators may be an important mechanism underlying the PIASy repression of LRH-1-mediated transactivation.
A rigorous analysis of the cavity equations for the minimum spanning tree
Authors: Bayati Mohsen , Braunstein Alfredo , Zecchina Riccardo
Published: December 2008

We analyze a new general representation for the minimum weight Steiner tree problem that translates the topological connectivity constraint into a set of local conditions, which can be analyzed by the so-called cavity equation techniques. For the limit case of the spanning tree, we prove that the fixed point of the algorithm arising from the cavity equations leads to the global optimum.
Infrared analysis of hole properties of Mg-doped p-type InN films
Authors: Fujiwara Masayuki , Ishitani Yoshihiro , Wang Xinqiang , Che Song-Bek , Yoshikawa Akihiko
Published: December 2008

Mg-doped InN films grown by plasma-assisted molecular beam epitaxy were characterized by infrared reflectance. Signatures of p-type conductivity in the spectra were obtained in the same doping density range where the existence of net acceptors was found by electrolyte capacitance-voltage measurements. Numerical spectrum analysis, which takes into account the large broadening factor of the normal mode energies of longitudinal optical phonon–plasmon coupling yielded high hole densities in the range of (0.1–1.2)×1019 cm−3 and optical mobilities in the range of 25–70 cm2/V s.
Temperature dependence of the dynamics of zone boundary phonons in ZnO:Li
Authors: Yadav Harish Kumar , Sreenivas K. , Gupta Vinay , Katiyar R. S.
Published: December 2008

Investigations of zone boundary phonons in ZnO:Li system (Li concentration: 10%) and their dynamics with temperature are reported. Additional modes at 127, 157, and 194 cm−1 are observed and assigned to zone boundary phonons at critical point M in the Brillouin zone [J. M. Calleja and M. Cardona, Phys. Rev. B 16, 3753 (1977)] due to breakdown of crystal translational symmetry with Li incorporation in ZnO. Anharmonicity in peak frequency and linewidth of the zone boundary phonons in a temperature range from 100 to 1000 K is also analyzed taking into account the decay of zone boundary phonons into three- and four-phonon modes (cubic and quadratic anharmonicities). The anharmonic behavior of peak frequency is found to be feebly dependent on three-phonon decay process but thermal expansion of lattice together with four-phonon decay process appropriately defines the temperature dependence. Linewidths, however, follow the simple four-phonon decay mechanism. E2(low) mode, on the other hand, shows a linear temperature dependency and therefore follows a three-phonon decay channel. The calculated values of phonon lifetimes at 100 K for the 127, 157, 194 cm−1, and E2(low) modes are 8.23, 6.54, 5.32, and 11.39 ps. Decay of the zone boundary phonon modes compared to E2(low) mode reveals that dopant induced disorder has a strong temperature dependency.
Using neural networks, optimized coordinates, and high-dimensional model representations to obtain a vinyl bromide potential surface
Authors: Manzhos Sergei , Carrington Tucker
Published: December 2008

We demonstrate that it is possible to obtain good potentials using high-dimensional model representations (HDMRs) fitted with neural networks (NNs) from data in 12 dimensions and 15 dimensions. The HDMR represents the potential as a sum of lower-dimensional functions and our NN-based approach makes it possible to obtain all of these functions from one set of fitting points. To reduce the number of terms in the HDMR, we use optimized redundant coordinates. By using exponential neurons, one obtains a potential in sum-of-products form, which greatly facilitates quantum dynamics calculations. A 12-dimensional (reference) potential surface for vinyl bromide is first refitted to show that it can be represented as a sum of two-dimensional functions. To fit 3d functions of the original coordinates, to improve the potential, a huge amount of data would be required. Redundant coordinates avoid this problem. They enable us to bypass the combinatorial explosion of the number of terms which plagues all HDMR and multimode-type methods. We also fit to a set of ∼70 000 ab initio points for vinyl bromide in 15 dimensions [M. Malshe et al., J. Chem. Phys. 127, 134105 (2007)] and show that it is possible to obtain a surface in sum-of-products form of quality similar to the quality of the full-dimensional fit. Although we obtain a full-dimensional surface, we limit the cost of the fitting by building it from fits of six-dimensional functions, each of which requires only a small NN.
Analysis of the flow structure inside the valveless standing wave pump
Authors: Nabavi Majid , Siddiqui Kamran , Dargahi Javad
Published: December 2008

The flow structure inside the valveless standing wave pump is investigated experimentally. The two-dimensional velocity fields inside the chamber of this novel pump at different phases of the excitation signal are measured using the synchronized particle image velocimetry technique. The variations in the pump flow rate, pressure loss coefficients, and rectification capability of the diffuser-nozzle element are analyzed. According to the results obtained in this paper, the net flow rate of the pump increases with an increase in the pressure (or Reynolds number). The interactions of three different flow fields inside the pump chamber (pumping flow, acoustic, and streaming velocities) are studied. It is found that, while the pumping flow has an effect on the acoustic velocity patterns only around the inlet and outlet orifices, the streaming velocity structures are drastically affected by the pumping flow.
Phase coexistence in the hard-sphere Yukawa chain fluid with chain length polydispersity: Dimer thermodynamic perturbation theory
Authors: Hlushak S. P. , Kalyuzhnyi Yu. V.
Published: December 2008

An extension of the dimer version of Wertheim’s thermodynamic perturbation theory is proposed and used to treat polydisperse mixture of the hard-sphere Yukawa chain fluid with chain length polydispersity. The structure and thermodynamic properties of the reference system, represented by multicomponent mixture of the Yukawa hard-sphere dimers, are described using polymer mean spherical approximation. Explicit analytical expressions for the Helmholtz free energy, chemical potential, and pressure in terms of the two chain length distribution function moments are derived. The theory is used to calculate the full liquid-gas phase diagram, including critical binodal, cloud and shadow curves, and distribution functions of the coexisting phases. Effects of fractionation in terms of the distribution function and its first and second moments are studied. Predictions of the theory for these effects are in qualitative agreement with the corresponding experimental predictions, obtained recently for the polydisperse mixture of polymers in a single solvent. In particular, both theory and experiment predict that longer chain polymers equilibrate to the liquid phase while shorter chain polymers are predominantly encountered in the gas phase.
Field emission from graphene based composite thin films
Authors: Eda Goki , Emrah Unalan H. , Rupesinghe Nalin , Amaratunga Gehan A. J. , Chhowalla Manish
Published: December 2008

Field emission from graphene is challenging because the existing deposition methods lead to sheets that lay flat on the substrate surface, which limits the field enhancement. Here we describe a simple and general solution based method for the deposition of field emitting graphene/polymer composite thin films. The graphene sheets are oriented at some angles with respect to the substrate surface leading to field emission at low threshold fields (∼4Vμm−1). Our method provides a route for the deposition of graphene based thin film field emitter on different substrates, opening up avenues for a variety of applications.
A statistical mechanical model for antiparallel β-sheet/coil equilibrium
Authors: Hong Liu
Published: December 2008

In this paper, a simple statistical mechanical model for the antiparallel β-sheet/coil equilibrium is constructed. It is based on the transfer matrix method, which has been widely used in the helix/coil transition theory. However, to correctly represent the structure of antiparallel β-sheet, we have to extend the former numerical matrix to an 11×11 operator one. All calculation rules of the operators are given, which can be automatically done by computers. In the end, the partition functions for homopolymers of varied lengths, applications to experimental data, and detailed antiparallel β-sheet/coil transition are shown.
Theoretical investigation on molecular rectification on the basis of asymmetric substitution and proton transfer reaction
Authors: Liu Hongmei , Li Peng , Zhao Jianwei , Yin Xing , Zhang Haoli
Published: December 2008

A series of linear conjugated molecular wires (diphenylacetylene connected by double or triple bonds) asymmetrically substituted by various functional groups was investigated by using density functional theory combined with nonequilibrium Green’s function method. The transportation behaviors of these models did not show obvious rectification, inferring that the simple asymmetric substitution of the conjugation chain was insufficient to improve the molecular rectification. We proposed that the molecular transportation can be modulated by proton transfer between the adjacent dissociable groups on the molecular wire. The theoretical calculations showed that the rectification ratio increased about six times at 1.0V after proton transfer. This behavior was interpreted by means of transmission spectra and spatial distribution of molecular orbitals; the alignment of molecular orbitals to the Fermi level promoted by proton transfer is also responsible for the rectification.
Epitaxial growth of quantum rods with high aspect ratio and compositional contrast
Authors: Li L. H. , Patriarche G. , Fiore A.
Published: December 2008

The epitaxial growth of quantum rods (QRs) on GaAs was investigated. It was found that GaAs thickness in the GaAs/InAs superlattice used for QR formation plays a key role in improving the QR structural properties. Increasing the GaAs thickness results in both an increased In compositional contrast between the QRs and surrounding layer, and an increased QR length. QRs with an aspect ratio of up to 10 were obtained, representing quasiquantum wires in a GaAs matrix. Due to modified confinement and strain potential, such nanostructure is promising for controlling gain polarization.
Polymeric electrochemical element for adaptive networks: Pulse mode
Authors: Smerieri Anteo , Berzina Tatiana , Erokhin Victor , Fontana M. P.
Published: December 2008

An electrochemically controlled polymeric heterojunction working as a memristor, i.e., having memory properties, was investigated in pulse mode, mimicking synaptic behavior of signal transmission in biological systems. Influence of parameters such as pulse duration, interval between pulses, and value of potential base level was analyzed. Learning capabilities were shown to be reversible and repeatable for both potentiation and inhibition of signal transmission. The adaptive behavior of the element was investigated and was shown to be more efficient than the dc mode.
Direct measurement of the complex refractive index in the extreme ultraviolet spectral region using diffraction from a nanosphere array
Authors: Mills B. , Chau C. F. , Rogers E. T. F. , Grant-Jacob J. , Stebbings S. L. , Praeger M. , de Paula A. M. , Froud C. A. , Chapman R. T. , Butcher T. J. , Baumberg J. J. , Brocklesby W. S. , Frey J. G.
Published: December 2008

Using extreme ultraviolet (XUV) radiation from a high harmonic source, we observe diffraction from a single-layer self-assembled hexagonal array of 196±1.2 nm diameter polystyrene spheres. The Mie solution is used to predict the correct form factor for a single sphere and hence model the intensities of the observed diffraction peaks for the first three orders. By measuring the diffraction intensities in this way, we demonstrate a technique for obtaining the complex refractive index of a material at multiple wavelengths in the XUV from a single measurement. We present experimental results for polystyrene in the range of 25–30 nm.
Nonlinear behavior in the time domain in argon atmospheric dielectric-barrier discharges
Authors: Shi Hong , Wang Yanhui , Wang Dezhen
Published: December 2008

A vast majority of nonlinear behavior in atmospheric pressure discharges has so far been studied in the space domain, and their time-domain characters are often believed to exact the periodicity of the externally applied voltage. In this paper, based on one-dimensional fluid mode, we study complex nonlinear behavior in the time domain in argon atmospheric dielectric-barrier discharges at very broad frequency range from kilohertz to megahertz. Under certain conditions, the discharge not only can be driven to chaos from time-periodic state through period-doubling bifurcation, but also can return stable periodic motion from chaotic state through an inverse period-doubling bifurcation sequence. Upon changing the parameter the discharge undergoes alternatively chaotic and periodic behavior. Some periodic windows embedded in chaos, as well as the secondary bifurcation occurring in the periodic windows can also be observed. The corresponding discharge characteristics are investigated.
Lattice and grain-boundary diffusion of As in Ni2Si
Authors: Blum I. , Portavoce A. , Mangelinck D. , Daineche R. , Hoummada K. , Lábár J. L. , Carron V. , Perrin C.
Published: December 2008

The diffusion coefficient of As in 260 nm thick polycrystalline Ni2Si layers has been measured both in grains and in grain boundaries (GBs). As was implanted in Ni2Si layers prepared via the reaction between a Si layer and a Ni layer deposited by magnetron sputtering on a (100) Si substrate covered with a SiO2 film. The As concentration profiles in the samples were measured using secondary ion mass spectroscopy before and after annealing (400–700 °C). The diffusion coefficients in the grains and the GBs have been determined using two-dimensional finite element simulations based on the Fisher model geometry. For short time annealing (1 h) and temperatures lower than 600 °C, lattice diffusion has not been observed. However, GB diffusion was evidenced for temperatures as low as 400 °C. For higher thermal budgets, As diffuses simultaneously in the volume of the grains and in the GBs. Lattice diffusion is characterized by a pre-exponential factor D0v∼1.5×10−1 cm2 s−1 and an activation energy Qv∼2.72±0.10 eV. In the case of GB diffusion, the triple product of the As segregation coefficient (s), the GB width (δ), and the diffusion coefficient (DGB) is found to be sδDGB=9.0×10−3 exp(−3.07±0.15 eV/kT) cm3 s−1. Various types of simulations were used in order to support the discussion of the results.
Strain-controlled anisotropic electronic transport in Bi0.4Ca0.6MnO3 films
Authors: Chen Y. Z. , Sun J. R. , Liang S. , Lu W. M. , Shen B. G.
Published: December 2008

Structural and resistive anisotropy has been studied for the Bi0.4Ca0.6MnO3 films grown on (011)-oriented SrTiO3 substrates. Strong anisotropic transport behaviors are observed when significant lattice strains exist. The ratio of the two resistivities along the a and c axes of the films can be tuned between ∼1 and ∼13 by adjusting the a/c ratio between ∼1.01 and ∼1.04, which can be conducted simply by decreasing film thickness from 100 to 10 nm. Considerable anisotropy emerges and develops when film thickness drops below ∼60 nm. With the decrease in film thickness, a change in preferred growth direction of the films is also observed. These features of the lattice effects could be useful for the design of artificial materials and devices.
Structure and migration of (112) step on (111) twin boundaries in nanocrystalline copper
Authors: Xu Luhua , Xu Di , Tu K. N. , Cai Yuan , Wang Ning , Dixit Pradeep , Pang John H. L. , Miao Jianmin
Published: December 2008

Pulse-electroplated copper that contains a high density of {111}/⟨112⟩ nanotwins has been found to greatly improve the yield strength while maintaining good electrical conductivity. The thermal stability of nanotwins is a concern and has been studied by in situ transmission electron microscopy (TEM) characterization from 200 to 350 °C in the present work. It was found that the (112¯) twin boundary in a junction of (111)/(112¯)/(111) twins migrates to eliminate the (111) twin boundaries. We propose that it is the dominant mechanism that reduces the twin density in the range of temperature studied. The driving force is provided by the elimination of the two (111) boundaries. The inverse migration of the (112¯) twin boundary driven by a high strain is possible if enough stress has been applied to the copper, e.g., the strain introduced during pulsed electroplating. On the other hand, the migration of (111) twin boundary in the direction normal to the twin plane was not found. However, we propose that it can happen if a (112¯) step migrates on the (111) surface, provided that there exists a driving force. The structure and mobility of (112) twin boundary has been characterized by high resolution TEM. We observed that the (112) step on (111) twin plane has a height of three atomic layers. It is a unique structure unit of the (112) twin.
Dielectric permittivity simulation of random irregularly shaped particle composites and approximation using modified dielectric mixing laws
Authors: Calame J. P.
Published: December 2008

Finite difference quasielectrostatic modeling is used to predict the dielectric permittivity of composites consisting of irregular particles in a background matrix. Representations of particles having undulating surfaces described by sums of harmonic functions are created on the computer and subsequently packed into a three-dimensional cellular model space. Composite dielectric permittivities as a function of volumetric filling fraction and particle undulation amplitude were simulated using constituent permittivities similar to the low-field behavior of barium titanium oxide (particles) and polyvinylidene fluoride-trifluoroethylene-chlorofluoroethylene (terpolymer matrix). An increase in particle roughness (undulation amplitude) causes a more rapid increase in composite permittivity than that predicted by random spherical particle simulations. The dielectric behavior of irregular particle composites is also simulated over a wide range of ratios of particle permittivity to matrix permittivity, where both permittivities are purely real. An empirical mixing law, which is a modification of the Hanai equation with an exponent 1/μ instead of 1/3, is investigated and found to be in excellent agreement with the simulations. Additional empirical expressions that provide approximate values of μ in terms of the particle undulation amplitude and the ratio of constituent permittivities are developed. Together, the empirical expressions are potentially useful as a predictive mixing law for irregular particle systems.
Ion thermal effects in oscillating multi-ion plasma sheath theory
Authors: Vranjes J. , Pandey B. P. , Tanaka M. Y. , Poedts S.
Published: December 2008

The effects of the ion temperature are discussed in a two-ion electron plasma and for a model applicable to the oscillating sheath theory that has recently been much of the focus in research. The differences between the fluid and kinetic models are pointed out, as well as the differences between the approximative kinetic description (which involves the expansion of the plasma dispersion function), and the exact kinetic description. It is shown that the approximative kinetic description, first, cannot describe the additional acoustic mode which naturally exists in the plasma with an additional ion population with a finite temperature, and, second, it yields an inaccurate Landau damping of the bulk ion acoustic mode. The reasons for these two failures of the model are described. In addition to this, a fluid model is presented that is capable of capturing both of these features that are missing in the approximative kinetic description, i.e., two (fast and slow) ion acoustic modes, and the corresponding Landau damping of both modes.
Growth behaviors of ultrathin ZnSe nanowires by Au-catalyzed molecular-beam epitaxy
Authors: Cai Y. , Wong T. L. , Chan S. K. , Sou I. K. , Su D. S. , Wang N.
Published: December 2008

Ultrathin ZnSe nanowires grown by Au-catalyzed molecular-beam epitaxy show an interesting growth behavior of diameter dependence of growth rates. The smaller the nanowire diameter, the faster is its growth rate. This growth behavior is totally different from that of the nanowires with diameters greater than 60nm and cannot be interpreted by the classical theories of the vapor-liquid-solid mechanism. For the Au-catalyzed nanowire growth at low temperatures, we found that the surface and interface incorporation and diffusion of the source atoms at the nanowire tips controlled the growth of ultrathin ZnSe nanowires.
Quantum cutting downconversion by cooperative energy transfer from Ce3+ to Yb3+ in borate glasses
Authors: Chen Daqin , Wang Yuansheng , Yu Yunlong , Huang Ping , Weng Fangyi
Published: December 2008

Quantum cutting downconversion (DC) involving the emission of two near-infrared (NIR) photons for each ultraviolet (UV) photon absorbed is realized in the Ce3+/Yb3+ codoped borated glasses. Upon excitation of Ce3+ ion with an UV photon at 330 nm, Yb3+ ions emit two NIR photons at 976 nm through an efficient excitation of Ce3+:5d and subsequent cooperative energy transfer (ET) from Ce3+:5d to Yb3+:F25/2. The maximum ET efficiency and the corresponding DC quantum efficiency were estimated to be 74% and 174%, respectively.
Generation of polychromatic radially and azimuthally polarized beams
Authors: Grosjean T. , Suarez M. , Sabac A.
Published: December 2008

We propose a fiber-based system aimed at generating radially and azimuthally polarized beams over spectral ranges of several tens of nanometers. The device is either adaptable to polychromatic sources such as supercontinuum fiber lasers or to tunable ones (optical parametric oscillator, laser diode, etc.). A preliminary version of the system has been engineered to work in the visible portion of the spectrum. The concept proposed here can be adapted to the infrared or blue-UV spectral domains. As a starting application, polychromatic Bessel beams of the first order have been produced.
Effects of artificially structured micrometer holes on the transport behavior of Al-doped ZnO layers
Authors: Elm M. T. , Henning T. , Klar P. J. , Szyszka B.
Published: December 2008

We study the transport properties of artificially structured n-type ZnO:Al thin films prepared by rf magnetron sputtering on glass substrates. The samples were patterned with an array of 4×4 and 8×8 μm2 holes. With decreasing hole size, the resistance of the samples increases. Filling the holes with Au or Al increases and decreases the resistance, respectively. All samples show a negative magnetoresistance, which becomes more pronounced with decreasing hole diameter. The filling of the holes with Au or Al reduces the effects of the artificial structuring on the magnetoresistance.
Analysis on the mechanism of photoluminescence quenching in π-conjugated polymers in photo-oxidation process with broadband transient grating
Authors: Wang Ying-hui , Peng Ya-jing , Mo Yue-qi , Yang Yan-qiang , Zheng Xian-xu
Published: December 2008

Photoinduced oxidation process of poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] is probed by using broadband transient grating technique. The results suggest that the damage of bonds in the side and main chains leads to the line shape variation and efficiency decreasing of luminescence in photoluminescence quenching, and their temporal evolution and damage rates are found to vary with molecular energy states.
Scatterometry for in situ measurement of pattern reflow in nanoimprinted polymers
Authors: Patrick Heather J. , Germer Thomas A. , Ding Yifu , Ro Hyun Wook , Richter Lee J. , Soles Christopher L.
Published: December 2008

We use optical scatterometry to extract the time evolution of the profile of nanoimprinted lines in low and high molecular mass polymer gratings during reflow at the glass transition temperature. The data are obtained continuously during the anneal using a spectroscopic ellipsometer and analyzed using a rigorous-coupled-wave model. We show excellent agreement of scatterometry results with ex situ measurements of line height by atomic force microscopy and specular x-ray reflectivity. The in situ scatterometry results reveal differences in the shape evolution of the grating lines indiscernible by other methods.
Terahertz generation in multiple laser-induced air plasmas
Authors: Chen Meng-Ku , Kim Jae Hun , Yang Chia-En , Yin Stuart Shizhuo , Hui Rongqing , Ruffin Paul
Published: December 2008

An investigation of the terahertz wave generation in multiple laser-induced air plasmas is presented. First, it is demonstrated that the intensity of the terahertz wave increases as the number of air plasmas increases. Second, the physical mechanism of this enhancement effect of the terahertz generation is studied by quantitatively measuring the intensity of the generated terahertz wave as a function of phase difference between adjacent air plasmas. It is found out that the superposition is the main mechanism to cause this enhancement. Thus, the results obtained in this paper not only provide a technique to generate stronger terahertz wave but also enable a better understanding of the mechanism of the terahertz generation in air plasma.
Retrieving effective parameters for quasiplanar chiral metamaterials
Authors: Menzel Christoph , Rockstuhl Carsten , Paul Thomas , Lederer Falk
Published: December 2008

We introduce and compare two approaches for retrieving quantitatively the effective chirality of metamaterials at normal incidence. The retrieval employs either the reflected and transmitted amplitude of a plane wave illuminating a finite three-dimensional (3D) metamaterial or the dispersion relation of the pertinent Bloch modes in an infinite metamaterial. Both approaches are applied to characterize a 3D metamaterial consisting of a stack of metallic gammadions separated by dielectric films. It will be shown that the results coincide if transitional effects between the metamaterial and the environment can be neglected.
The Effect of Face Eccentricity on the Perception of Gaze Direction
Authors: Todorović Dejan
Published: December 2008

The perception of a looker's gaze direction depends not only on iris eccentricity (the position of the looker's irises within the sclera) but also on the orientation of the lookers' head. One among several potential cues of head orientation is face eccentricity, the position of the inner features of the face (eyes, nose, mouth) within the head contour, as viewed by the observer. For natural faces this cue is confounded with many other head-orientation cues, but in schematic faces it can be studied in isolation. Salient novel illustrations of the effectiveness of face eccentricity are ‘Necker faces’, which involve equal iris eccentricities but multiple perceived gaze directions. In four experiments, iris and face eccentricity in schematic faces were manipulated, revealing strong and consistent effects of face eccentricity on perceived gaze direction, with different types of tasks. An additional experiment confirmed the ‘Mona Lisa’ effect with this type of stimuli. Face eccentricity most likely acted as a simple but robust cue of head turn. A simple computational account of combined effects of cues of eye and head turn on perceived gaze direction is presented, including a formal condition for the perception of direct gaze. An account of the ‘Mona Lisa’ effect is presented.
Similarity, Typicality, and Category-Level Matching of Morphed Outlines of Everyday Objects
Authors: Panis Sven , Vangeneugden Joris , Wagemans Johan
Published: December 2008

During visual object categorisation, a match must be found between the input image and stored information about basic-level categories. Graf [2002 Form, Space and Object (Berlin: Wissenschaftlicher Verlag Berlin)] suggested the involvement of analogue transformational, shape-changing processes in aligning the memory representation of the category with the perceptual representation of the current stimulus. Here we compare the predictions of alignment models with those of exemplar-based models, using morphing between four exemplar outlines within each of eleven categories. Overall, with increasing transformational distance between two exemplars of the same category, reaction times to decide whether they belong to the same category in a sequential matching paradigm increased, while rated similarity between the two exemplars decreased. However, in contrast to alignment accounts, exemplar-based accounts can correctly predict the observed dissociation between typicality and categorisation time, and allow the observed deviations from sequential additivity and nonlinear relations between transformational distance and rated similarity. Discussion of integrations of exemplar-based theories with neglected processes, such as information accumulation, response competition, response priming, and gain-modulation leads to a view of the recognition process from input to response, which increases the validity and scope of modern exemplar-based categorisation and recognition models.
A Psychophysical and Computational Analysis of the Spatio-Temporal Mechanisms Underlying the Flash-Lag Effect
Authors: Cravo André M , Baldo Marcus V C
Published: December 2008

Several accounts put forth to explain the flash-lag effect (FLE) rely mainly on either spatial or temporal mechanisms. Here we investigated the relationship between these mechanisms by psychophysical and theoretical approaches. In a first experiment we assessed the magnitudes of the FLE and temporal-order judgments performed under identical visual stimulation. The results were interpreted by means of simulations of an artificial neural network, that was also employed to make predictions concerning the FLE. The model predicted that a spatio-temporal mislocalisation would emerge from two, continuous and abrupt-onset, moving stimuli. Additionally, a straightforward prediction of the model revealed that the magnitude of this mislocalisation should be task-dependent, increasing when the use of the abrupt-onset moving stimulus switches from a temporal marker only to both temporal and spatial markers. Our findings confirmed the model's predictions and point to an indissoluble interplay between spatial facilitation and processing delays in the FLE.
Conformal geometry of the retinal nerve fiber layer
Authors: Airaksinen P. Juhani , Doro Stephen , Veijola Jukka
Published: December 2008

The nerve fiber layer of the human retina is made up of the retinal segments of ganglion cell axons. Its geometry can be described mathematically as a fibration of a 2D domain: a partition of a certain region into smooth curves. Here, we present a simple family of curves that closely models the observed geometry of the nerve fiber layer. For each retina, the pattern depends on 2 parameters,AandB: A computer program determinesAandBfor a given retina and the theory matches the retina with a standard deviation of ≈6–8°. These particular curves turn out to be the curves that would be generated if the growing ganglion cell axon tip moved down a gradient toward a source of diffusible neuroattractant at the disk and away from a weaker macular diffusible repellant. Thus, this model provides morphological evidence that diffusible substances provide positional information to the embryonic ganglion cell axons in finding their way to the optic nerve head.
Transduction of the cytoplasmic domain of CTLA-4 inhibits TcR-specific activation signals and prevents collagen-induced arthritis
Authors: Choi Je-Min , Kim Seung-Hyung , Shin Jae-Hoon , Gibson Thomas , Yoon Byoung-Seok , Lee Dong-Ho , Lee Seung-Kyou , Bothwell Alfred L. M. , Lim Jong-Soon , Lee Sang-Kyou
Published: December 2008

CTLA-4 (CD152) negatively regulates T cell activation signaling, and the cytoplasmic domain of CTLA-4 (ctCTLA-4) itself has the capacity to inhibit T cell activation
in vitro
and
in vivo
. In this study, the inhibitory mechanisms of the cell-permeable recombinant protein Hph-1-ctCTLA-4 on T cell activation and its ability to prevent collagen-induced arthritis were analyzed. Hph-1-ctCTLA-4 prevented human and mouse T cell activation and proliferation by inhibition of T cell receptor-proximal signaling and the arrest of the cell cycle. Furthermore, Hph-1-ctCTLA-4 protected human umbilical vein endothelial cells (HUVEC) from the human CTL allo-response. The incidence and severity of collagen-induced arthritis were significantly reduced and the erosion of cartilage and bone was effectively prevented by i.v. injection and transdermal administration of Hph-1-ctCTLA-4. Inflammatory cytokine production (IL-1β, IL-6, TNF-α, IL-17A) and collagen-specific antibody levels were significantly reduced, and the numbers of activated T cells and infiltrating granulocytes were substantially decreased. These results demonstrate that systemic or transdermal application of a cell-permeable form of the cytoplasmic domain of CTLA-4 offers an effective therapeutic approach for autoimmune diseases such as rheumatoid arthritis.
Quantifying errors in trace species transport modeling
Authors: Prather Michael J. , Zhu Xin , Strahan Susan E. , Steenrod Stephen D. , Rodriguez Jose M.
Published: December 2008

One expectation when computationally solving an Earth system model is that a correct answer exists, that with adequate physical approximations and numerical methods our solutions will converge to that single answer. With such hubris, we performed a controlled numerical test of the atmospheric transport of CO
2
using 2 models known for accurate transport of trace species. Resulting differences were unexpectedly large, indicating that in some cases, scientific conclusions may err because of lack of knowledge of the numerical errors in tracer transport models. By doubling the resolution, thereby reducing numerical error, both models show some convergence to the same answer. Now, under realistic conditions, we identify a practical approach for finding the correct answer and thus quantifying the advection error.
Structure of membrane-bound α-synuclein from site-directed spin labeling and computational refinement
Authors: Jao Christine C. , Hegde Balachandra G. , Chen Jeannie , Haworth Ian S. , Langen Ralf
Published: December 2008

α-Synuclein is known to play a causative role in Parkinson disease. Although its physiological functions are not fully understood, α-synuclein has been shown to interact with synaptic vesicles and modulate neurotransmitter release. However, the structure of its physiologically relevant membrane-bound state remains unknown. Here we developed a site-directed spin labeling and EPR-based approach for determining the structure of α-synuclein bound to a lipid bilayer. Continuous-wave EPR was used to assign local secondary structure and to determine the membrane immersion depth of lipid-exposed residues, whereas pulsed EPR was used to map long-range distances. The structure of α-synuclein was built and refined by using simulated annealing molecular dynamics restrained by the immersion depths and distances. We found that α-synuclein forms an extended, curved α-helical structure that is over 90 aa in length. The monomeric helix has a superhelical twist similar to that of right-handed coiled-coils which, like α-synuclein, contain 11-aa repeats, but which are soluble, oligomeric proteins (rmsd = 0.82 Å). The α-synuclein helix extends parallel to the curved membrane in a manner that allows conserved Lys and Glu residues to interact with the zwitterionic headgroups, while uncharged residues penetrate into the acyl chain region. This structural arrangement is significantly different from that of α-synuclein in the presence of the commonly used membrane-mimetic detergent SDS, which induces the formation of two antiparallel helices. Our structural analysis emphasizes the importance of studying membrane protein structure in a bilayer environment.