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Model Calculations (model + calculation)
Selected AbstractsChemical Vapor Transport of Ternary Oxides in the System Ni/Mo/O , Experiments and Model Calculations.CHEMINFORM, Issue 43 2006Udo Steiner Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] A rare landform: Yerköprü travertine bridges in the Taurids Karst Range, TurkeyEARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2002C. Serdar Bayari Abstract Two examples of travertine bridges are observed at 8 to 15 m above stream level in the Lower Zamanti Basin, Eastern Taurids, Turkey. Yerköprü-1 and Yerköprü-2 bridges are currently being deposited from cool karstic groundwaters with log PCO2 > 10,2 atm. The surface area and the total volume of travertine in Yerköprü-1 bridge are 4350 m2 and 40 000 m3, whereas the values for Yerköprü-2 are 2250 m2 and 20 000 m3, respectively. The interplay of hydrogeological structure, local topography, calcite-saturated hanging springs, algal activity and rapid downcutting in the streambed appear to have led to the formation of travertine bridges. Aeration through cascades and algal uptake causes efficient carbon dioxide evasion that enhances travertine formation. Algal curtains aid lateral development of travertine rims across the stream. Model calculations based on a hypothetical deposit in the form of a half-pyramid implied that lateral development should have occurred from both banks of the stream in the Yerköprü-1 bridge, whereas one-sided growth has been sufficient for Yerköprü-2. The height difference between travertine springs and the main stream appears to be a result of Pleistocene glaciation during which karstic base-level lowering was either stopped or slowed down while downcutting in the main stream continued. Copyright © 2002 John Wiley & Sons, Ltd. [source] Methyl- tert -hexyl ether and methyl- tert -octyl ether as gasoline oxygenates: Anticipating widespread risks to community water supply wells,ENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 11 2007Jeff Snelling Abstract The widespread contamination of groundwater resources associated with methyl- tert -butyl ether (MtBE) use has prompted a search for replacement oxygenates in gasoline. Among the alternatives currently under development are higher methyl- tert -alkyl ethers, notably methyl- tert -hexyl ether (MtHxE) and methyl- tert -octyl ether (MtOcE). As was the case with MtBE, the introduction of these ethers into fuel supplies guarantees their migration into groundwater resources. In the present study, a screening-level risk assessment compared predicted well water concentrations of these ethers to concentrations that might cause adverse effects. A physicochemical model which has been successfully applied to the prediction of MtBE concentrations in community water supply wells (CSWs) was used to predict well water concentrations of MtHxE and MtOcE. The results indicate that these ethers are likely to contaminate water supply wells at slightly lower levels than MtBE as a result of migrating from leaking underground fuel tanks to CSWs. Because very little data is available on the physicochemical and environmental properties of MtHxE and MtOcE, estimation methods were employed in conjunction with the model to predict well water concentrations. Model calculations indicated that MtHxE and MtOcE will be present in many CSWs at concentrations approaching the concentrations that have caused widespread public health concern for MtBE. Based on these results and the possibility that MtHxE and MtOcE are potential carcinogens, testing of the toxicological properties of these ethers is recommended before they are used to replace MtBE in gasoline. [source] Lake age and water level affect the turbidity of floodplain lakes along the lower RhineFRESHWATER BIOLOGY, Issue 3 2003F. C. J. M. Roozen SUMMARY 1. We sampled a set of 93 lakes situated in the floodplains of the lower River Rhine in search for morphometric and other factors that explain their variation in clarity. 2. Lakes with a drop in summer water level were less turbid at the time of sampling, mainly because of a lower concentration of inorganic suspended solids (ISS). 3. We also found that older lakes were more turbid than younger lakes and that this was largely because of an increase in phytoplankton. 4. Water clarity was positively related to lake depth and the presence of vegetation. 5. Model calculations indicated that the underwater light climate was strongly affected by chlorophyll and ISS, the latter being the dominant factor affecting Secchi depth. Dissolved organic carbon (DOC) was less important. 6. The high concentration of ISS suggests that intensive resuspension occurs in most of the lakes. Using a simple wave model, and assuming that vegetation protects sediments against resuspension, we could eliminate wind resuspension as an important process in 90% of the lakes, leaving resuspension by benthivorous fish as probably the most important factor determining transparency. 7. Chlorophyll a concentration showed a strong positive correlation to ISS concentration, suggesting that resuspension may also have a positive effect on phytoplankton biomass in these lakes. 8. In conclusion, in-lake processes, rather than river dynamics, seem to be driving the turbidity of floodplain lakes along the lower River Rhine. [source] Bacterial formation of phosphatic laminites off PeruGEOBIOLOGY, Issue 3 2009E. T. ARNING Authigenic phosphatic laminites enclosed in phosphorite crusts from the shelf off Peru (10°01, S and 10°24, S) consist of carbonate fluorapatite layers, which contain abundant sulfide minerals including pyrite (FeS2) and sphalerite (ZnS). Low ,34Spyrite values (average ,28.8,) agree with bacterial sulfate reduction and subsequent pyrite formation. Stable sulfur isotopic compositions of sulfate bound in carbonate fluorapatite are lower than that of sulfate from ambient sea water, suggesting bacterial reoxidation of sulfide by sulfide-oxidizing bacteria. The release of phosphorus and subsequent formation of the autochthonous phosphatic laminites are apparently caused by the activity of sulfate-reducing bacteria and associated sulfide-oxidizing bacteria. Following an extraction,phosphorite dissolution,extraction procedure, molecular fossils of sulfate-reducing bacteria (mono- O -alkyl glycerol ethers, di- O -alkyl glycerol ethers, as well as the short-chain branched fatty acids i/ai -C15:0, i/ai -C17:0 and 10MeC16:0) are found to be among the most abundant compounds. The fact that these molecular fossils of sulfate-reducing bacteria are distinctly more abundant after dissolution of the phosphatic laminite reveals that the lipids are tightly bound to the mineral lattice of carbonate fluorapatite. Moreover, compared with the autochthonous laminite, molecular fossils of sulfate-reducing bacteria are: (1) significantly less abundant and (2) not as tightly bound to the mineral lattice in the other, allochthonous facies of the Peruvian crusts consisting of phosphatic coated grains. These observations confirm the importance of sulfate-reducing bacteria in the formation of the phosphatic laminite. Model calculations highlight that organic matter degradation by sulfate-reducing bacteria has the potential to liberate sufficient phosphorus for phosphogenesis. [source] Toward a BSSE-free description of strongly interacting systemsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2002G.J. Halász Abstract The so-called "chemical Hamiltonian approach" (CHA) gives perfect a priori BSSE-free description of weak intermolecular interactions, but has been found inappropriate for describing strong interactions taking place within a molecule. Here, we propose a simple modification of the CHA/F BSSE-free SCF method, which retains all the good properties of the CHA/F method for the intermolecular case but can be used also to describe covalent and ionic interactions. This is vital for calculating whole potential surfaces of chemical reactions in a consistent manner, which was found impossible by using the a posteriori counterpoise correction method. Model calculations are presented for covalent and ionic chemical bonds and for a rare gas,proton system. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002 [source] Performance of fractionating reactors in the absence of rate limitationsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 9 2004Jeroen L den Hollander Abstract A fractionating reactor for equilibrium-limited reactions is studied theoretically. Reactant A is fed in the center of the countercurrent fractionating system. Product P is effectively transported with the auxiliary phase, while product Q is effectively transported with the main phase, in which the reaction takes place. Model calculations were based on partition and reaction equilibrium at all stages. These show that if the initial reactant concentration and the flow rates are properly selected, the extent of conversion will significantly exceed the corresponding batch conversion. To approach complete conversion in the fractionating reactor, and to recover both products in a pure form, net transport of reactant in either of the countercurrent directions should be prevented. However, irrespective of the number of equilibrium stages, this situation cannot be fully reached when the reactant feed stream is too large (compared with the main and auxiliary streams). Nonetheless, one of the two products may be recovered in a pure form even for such large feed streams. Copyright © 2004 Society of Chemical Industry [source] Overcoming merohedral twinning in crystals of bacteriorhodopsin grown in lipidic mesophaseACTA CRYSTALLOGRAPHICA SECTION D, Issue 1 2010Valentin Borshchevskiy Twinning is one of the most common crystal-growth defects in protein crystallography. There are neither efficient rational approaches for the growth of nontwinned protein crystals nor are there examples of systematic studies of the dependence of the twinning-ratio distribution on crystallization conditions. The description of the twinning phenomenon has been covered even less for membrane-protein crystals and is non-existent for crystals grown using lipidic phases (in meso). In the present work, possibilities for overcoming merohedral twinning are investigated for crystals of the membrane protein bacteriorhodopsin (bR) grown in meso. It is shown that traditional crystallization additives are not effective in the case of the in meso crystallization of bR. The twinning ratio was determined for 310 crystals grown under different crystallization conditions. A correlation of the twinning ratio with the growth rate of the crystals was observed. Slow growth indicated that crystals had a noticeable chance of avoiding twinning. Model calculations were performed in order to rationalize this observation. The calculations confirmed the experimental observation that most crystals consist of two twin domains and showed that under this condition small changes in the probability of twin-domain formation lead to dramatic changes in the number of nontwinned crystals, which explains why slow crystal growth results in a considerable number of nontwinned crystals. [source] Raman scattering of acoustical modes of silicon nanoparticles embedded in silica matrix,JOURNAL OF RAMAN SPECTROSCOPY, Issue 1-3 2006M. Ivanda Abstract The Raman scattering from acoustical phonons of silicon quantum dots in glass matrix was investigated. Two peaks that correspond to symmetric and quadrupolar spheroidal vibrations were found. A model calculation for in- and off-resonance scattering conditions was used, which considered the homogeneous broadening due to interaction with matrix and the inhomogeneous broadening due to particle size distribution. A strong dependence of the light-to-vibration coupling coefficient on the particles size was needed for fitting the Raman data. This result suggests that resonance with electronic transitions of the silicon nanoparticles is important for excitation at 514.5 nm. The size distribution obtained from the Raman data is in agreement with the results of high-resolution transmission electron microscopy. Copyright © 2006 John Wiley & Sons, Ltd. [source] Development of a differential pumping system for soft X-ray beamlines for windowless experiments under normal atmospheric conditionsJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2010Y. Tamenori A novel design for a differential pumping system has been investigated. This system allows windowless experiments in a soft X-ray beamline under normal atmospheric conditions. The new design consists of an aperture-based four-stage differential pumping system, based on a simple model calculation. A prototype system with a total length of 600,mm was constructed to confirm the validity of the design concept. Relatively short conductance-limiting components allow easy installation and alignment of the system on a synchrotron beamline. The fabricated system was installed on a beamline to test the transmission of soft X-rays through atmospheric helium. [source] Determination of pseudophakic accommodation with translation lenses using Purkinje image analysisOPHTHALMIC AND PHYSIOLOGICAL OPTICS, Issue 2 2005Achim Langenbucher Abstract Purpose:, To determine pseudophakic accommodation of an accommodating posterior chamber intraocular lens (translation lens) using Purkinje image analysis and linear matrix methods in the paraxial space. Methods:, A 2 × 2 system matrix was defined for each Purkinje image I to IV using refraction, translation and mirror matrices. Image size (m) and axial image position (z) was determined as an example for an off-axis object (a 0.2 m off-axis object located 0.5 m in front of the cornea.). First, our method was applied to the phakic relaxed (emmetropic) and accommodated (6.96 D) Le Grand eye. Secondly, for demonstration of the applicability of the calculation scheme to the pseudophakic eye, we provide a clinical example where we determine the accommodation amplitude of the translation lens (1 CU, HumanOptics, Erlangen, Germany) from photographed Purkinje images in the relaxed and accommodated state. From the biometric data: axial length 23.7 mm, corneal power 43.5, corneal thickness 550 microns, implanted intraocular lens (IOL) with a refractive power of 20.5 D (shape equi-biconvex, refractive index 1.46), and refractive indices of the cornea, aqueous and vitreous from the Le Grand model eye, we calculated the refractive state and the sizes of Purkinje images I and III initiated from two off-axis light sources. Results:, For the Le Grand model eye, Purkinje image II (z/m = 3.5850 mm/0.0064) is slightly smaller than and directly in front of image I (z/m = 3.8698 mm/0.0077). Purkinje image III (z/m = 10.6097 mm/0.0151) is nearly double the size of image I and during accommodation it moves from the vitreous into the crystalline lens. Purkinje IV (z/m = 4.3244 mm/,0.0059) is inverted, three quarters the size of image I, lies in the crystalline lens and moves slightly towards the retina. For the pseudophakic eye, pseudophakic accommodation of 1.10 D was calculated from the proportion of distances between both Purkinje images I and III in the relaxed (3.04) and accommodated (2.75) state, which is in contrast to the total subjective accommodation of 2.875 D evaluated with an accommodometer. Conclusions:, We present a straightforward mathematical strategy for calculation of the Purkinje images I,IV. Results of our model calculation resemble the values provided by Le Grand. In addition, this approach yields a simple en bloc scheme for determination of pseudophakic accommodation in pseudophakic eyes with accommodative lenses (translation lenses) using Purkinje image photography. [source] Effect of non-abrupt doping profiles on the carrier sheet density in one-side modulation-doped GaAs/AlGaAs quantum wellsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue S2 2004F. M. S. Lima Abstract A theoretical study on the influence of non-abrupt doping profiles on the electronic subband structure in one-side modulation-doped GaAs/AlGaAs quantum wells with graded interfaces is presented. Emphasis is given on the dependence of the carriers sheet density on both the spacer thickness and the donors diffusion length. A proper discretization method for solving the coupled Schrödinger and Poisson equations self-consistently over a nonuniform grid is proposed. This furnishes numerically stable results for the electron sheet density even when non-abrupt doping and heterointerface profiles are included in the model calculation. The depletion approximation is surpassed and a significant increase in the electron sheet density is obtained when non-abrupt doping profiles are taken into account. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] The Oxidation Kinetics of Mercury in Hg/O/H/Cl SystemASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 3-4 2005Y. Qiao The potential for regulation of mercury emission from coal-fired boilers is a concern for the electric utility industry. Field data show a wide variation in the fraction of mercury that is emitted as a vapor versus that retained in the solid products. The reason for this variation is not well understood. Near the end of the flue gas path, mercury exists as a combination of elemental vapor and HgCl2 vapor. The data show that HCl2 is more likely to be removed form the flue gas. The need to describe accurately mercury reaction products and their concentration-time correlation prompted investigation of mercury chemical kinetic mechanisms and their application to real combustion systems. This paper uses chemical equilibrium analysis to study the speciation of mercury in flue gases during coal combustion and gasification. The paper presents a simple kinetic model of mercury oxidation in the Hg/O/H/Cl system. The results porn the model calculation are in reasonable agreement with the Mamani-Paco and Heleble [1] experimental data [source] Stem cell separation: A bottleneck in stem cell therapyBIOTECHNOLOGY JOURNAL, Issue 1 2010Kornelia Schriebl Dr. Abstract The substantial progress in embryonic stem cell (ESC) research could lead to new possibilities in the treatment of various diseases. Currently, applications of ESC for cell therapy are impeded by the presence of potentially teratoma-forming undifferentiated ESC. Thus, a selective and quantitative removal of undifferentiated ESC from a pool of differentiated and undifferentiated cells is essential before cell therapy. We evaluated the highly selective magnetic activated cell sorting (MACS) method for the quantitative removal of undifferentiated ESC. We found that the clearance rates for undifferentiated ESC decreased with decreasing amount of undifferentiated ESC in the cell pool. Using a simplified model calculation we could predict that, assuming an initial purity of 60%, an estimated 31 steps are required to achieve less than 10,1 cell per 109 cells. Thus, a log clearance rate of 10, which would be necessary for a therapeutically application, is hard to achieve. Our work clearly indicates that the current MACS technology is insufficient to meet the purification needs for cell therapy. [source] Risk factors of neonatal respiratory distress following vaginal delivery and caesarean section in the German populationACTA PAEDIATRICA, Issue 1 2009Andrea Heinzmann Abstract Background: The incidence of caesarean section (CS) is steadily rising world-wide. In particular, CS on maternal demand is performed more frequently. In parts, this might be due to insufficient information of pregnant women about neonatal risks of CS. We sought to specify neonatal outcomes following different modes of delivery, i.e. vaginal delivery, primary CS and secondary CS and to define risk factors for respiratory morbidity and hospitalization. Methods: We analysed 2073 births (gestational age > 35 weeks) during a two-year period at a tertiary obstetric and neonatal centre in Germany. Statistical analyses were performed for single parameters by SPSS as well as by logistic regression to account for possible confounders. Furthermore, extensive model calculation was done. Results: Respiratory morbidity was increased following primary and secondary CS (p = 0.001). By multiple logistic regression, the strongest effect on respiratory symptoms was seen with gestational age, each week more in utero reducing the risk by an odds ratio (OR) of 0.69 (95% CI: [0.61; 0.79]; p = 1.9 × 10,8). Furthermore, a significant interaction between mode of delivery and gestational age was found for the risk of respiratory symptoms (p = 0.0035). Conclusion: For every eight newborns delivered by primary CS one more than expected with vaginal delivery is hospitalized. It is highly relevant to recognize that each week of gestational age reduces the risk of respiratory symptoms, especially if primary CS is performed. The higher rate of respiratory morbidity and neonatal admission following CS should be clearly recognized in counselling of pregnant women. [source] Managing Stock Option Expense: The Manipulation of Option-Pricing Model Assumptions,CONTEMPORARY ACCOUNTING RESEARCH, Issue 2 2006Derek Johnston Abstract This paper examines whether firms that voluntarily recognize stock option expense in their financial statements manage that expense downward more than firms that do not recognize the expense by adjusting option-pricing model assumptions. To examine this issue, I collect option-pricing model assumptions from fiscal year 2002 for both a sample of firms that voluntarily recognize stock option expense ("recognizing firms") and a sample of control firms that do not ("disclosing firms"). The empirical results suggest that recognizing firms manage the recognized stock-based compensation expense reported in their financial statements downward more than do firms that only disclose the expense. Additional analyses reveal that recognizing firms assume a lower level of volatility than disclosing firms in the option-pricing model calculations; however, I find no evidence that recognizing firms manage the dividend yield and risk-free interest rate assumptions more than disclosing firms. The Financial Accounting Standards Board (FASB) recently issued Statement of Financial Accounting Standards No. 123(R), which requires the expensing of the fair value of stock options, so these results may be of interest to capital-market participants and the FASB as they assess the reliability of stock option expense as determined by option-pricing models. [source] An attenuation model for distant earthquakesEARTHQUAKE ENGINEERING AND STRUCTURAL DYNAMICS, Issue 2 2004Adrian Chandler Abstract Large magnitude earthquakes generated at source,site distances exceeding 100km are typified by low-frequency (long-period) seismic waves. Such induced ground shaking can be disproportionately destructive due to its high displacement, and possibly high velocity, shaking characteristics. Distant earthquakes represent a potentially significant safety hazard in certain low and moderate seismic regions where seismic activity is governed by major distant sources as opposed to nearby (regional) background sources. Examples are parts of the Indian sub-continent, Eastern China and Indo-China. The majority of ground motion attenuation relationships currently available for applications in active seismic regions may not be suitable for handling long-distance attenuation, since the significance of distant earthquakes is mainly confined to certain low to moderate seismicity regions. Thus, the effects of distant earthquakes are often not accurately represented by conventional empirical models which were typically developed from curve-fitting earthquake strong-motion data from active seismic regions. Numerous well-known existing attenuation relationships are evaluated in this paper, to highlight their limitations in long-distance applications. In contrast, basic seismological parameters such as the Quality factor (Q -factor) could provide a far more accurate representation for the distant attenuation behaviour of a region, but such information is seldom used by engineers in any direct manner. The aim of this paper is to develop a set of relationships that provide a convenient link between the seismological Q -factor (amongst other factors) and response spectrum attenuation. The use of Q as an input parameter to the proposed model enables valuable local seismological information to be incorporated directly into response spectrum predictions. The application of this new modelling approach is demonstrated by examples based on the Chi-Chi earthquake (Taiwan and South China), Gujarat earthquake (Northwest India), Nisqually earthquake (region surrounding Seattle) and Sumatran-fault earthquake (recorded in Singapore). Field recordings have been obtained from these events for comparison with the proposed model. The accuracy of the stochastic simulations and the regression analysis have been confirmed by comparisons between the model calculations and the actual field observations. It is emphasized that obtaining representative estimates for Q for input into the model is equally important.Thus, this paper forms part of the long-term objective of the authors to develop more effective communications across the engineering and seismological disciplines. Copyright © 2003 John Wiley & Sons, Ltd. [source] Dose-related effects following oral exposure of 2,4-dinitrotoluene on the western fence lizard, Sceloporus occidentalisENVIRONMENTAL TOXICOLOGY & CHEMISTRY, Issue 2 2008Jamie G. Suski Abstract 2,4-dintitrotoluene (2,4-DNT) is an explosive frequently found in the soil of military installations. Because reptiles can be common on these sites, ecological risk assessments for compounds such as 2,4-DNT could be improved with toxicity data specific to reptiles. Western fence lizards, Sceloporus occidentalis, were used to develop a laboratory toxicity model for reptiles. A hierarchical approach was used; acute to subchronic studies were conducted to provide toxicity data relevant to short- and long-term exposures. First, a modified median lethal dose (LD50) study was conducted on male and female lizards using a stage-wise probit model. The LD50 was 577 mg/kg for female and 380 mg/kg for male lizards. Subsequently, a subacute experiment was conducted to further assess 2,4-DNT toxicity to male lizards and to define exposure levels for a longer term, subchronic study. The subchronic study was conducted for 60 consecutive days; male lizards were exposed to 0, 9, 15, 25, 42, 70 mg/kg/d. Dose-dependent mortality was observed in the three highest dose groups (25, 42, and 70 mg/kg/d); all other animals survived the study duration. Benchmark dose model calculations based on mortality indicated a 5% effect level of 15.8 mg/kg/d. At study termination, a gross necropsy was performed, organ weights were taken, and blood was collected for clinical and hematological analysis. Body weight, kidney weight, food consumption, postdose observations, and blood chemistries all were found to be significantly different from controls at doses above 9 mg/kg/d. Also, preliminary results suggest behavioral observations, and reduced food consumption may be a sensitive indicator of toxicity. The present study indicates Sceloporus occidentalis is suitable for evaluating toxicity of compounds to reptilian species. [source] A Theoretical Study of the Exchange Coupling in Hydroxo- and Alkoxo-Bridged Dinuclear Oxovanadium(IV) CompoundsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 1 2004Antonio Rodríguez-Fortea Abstract A density functional study of exchange coupling in hydroxo- and alkoxo-bridged dinuclear oxovanadium(IV) compounds is presented. Coupling constants calculated for full unmodeled structures are in good agreement with experimentally reported values, confirming the ability of the computational strategy used in this work to predict the exchange coupling in dinuclear VIV compounds. The influence of the configuration of the [VO(,-OR)2VO]2+ core in compounds with two edge-sharing octahedrally coordinated oxovanadium(IV) centers, of the nature of the terminal ligands and of structural distortions on the coupling constant have been analyzed through model calculations. The results indicate that the coupling constant is less affected by these factors than for hydroxo- and alkoxo-bridged CuII compounds. The calculations support the orbital models usually employed in qualitative interpretations of magneto-structural correlations, showing good correlations between the calculated coupling constants and the overlap between the two magnetic orbitals or the square of the orbital gap. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Synthesis of Core,Shell Inorganic NanotubesADVANCED FUNCTIONAL MATERIALS, Issue 15 2010Ronen Kreizman Abstract New materials and techniques pertaining to the synthesis of inorganic nanotubes have been ever increasing since the initiation of the field in 1992. Recently, WS2 nanotubes, which are produced now in large amounts, were filled with molten lead iodide salt by a capillary wetting process, resulting in PbI2@WS2 core,shell nanotubes. This work features progress in the synthesis of new core,shell nanotubes, including BiI3@WS2 nanotubes produced in a similar same manner. In addition, two new techniques for obtaining core,shell nanotubes are presented. The first is via electron-beam irradiation, i.e., in situ synthesis within a transmission electron microscope. This synthesis results in SbI3 nanotubes, observed either in a hollow core of WS2 ones (SbI3@WS2 nanotubes), or atop of them (WS2@SbI3 nanotubes). The second technique involves a gaseous phase reaction, where the layered product employs WS2 nanotubes as nucleation sites. In this case, the MoS2 layers most often cover the WS2 nanotube, resulting in WS2@MoS2 core,shell nanotubes. Notably, superstructures of the form MoS2@WS2@MoS2 are occasionally obtained. Using a semi-empirical model, it is shown that the PbI2 nanotubes become stable within the core of MoS2 nanotubes only above a critical core diameter of the host (>12,nm); below this diameter the PbI2 crystallizes as nanowires. These model calculations are in agreement with the current experimental observations, providing further support to the growth mechanism of such core,shell nanotubes. [source] Impact of Simulation Model Solver Performance on Ground Water Management ProblemsGROUND WATER, Issue 5 2008David P. Ahlfeld Ground water management models require the repeated solution of a simulation model to identify an optimal solution to the management problem. Limited precision in simulation model calculations can cause optimization algorithms to produce erroneous solutions. Experiments are conducted on a transient field application with a streamflow depletion control management formulation solved with a response matrix approach. The experiment consists of solving the management model with different levels of simulation model solution precision and comparing the differences in optimal solutions obtained. The precision of simulation model solutions is controlled by choice of solver and convergence parameter and is monitored by observing reported budget discrepancy. The difference in management model solutions results from errors in computation of response coefficients. Error in the largest response coefficients is found to have the most significant impact on the optimal solution. Methods for diagnosing the adequacy of precision when simulation models are used in a management model framework are proposed. [source] Quantifying contributions to storm runoff through end-member mixing analysis and hydrologic measurements at the Panola Mountain Research Watershed (Georgia, USA)HYDROLOGICAL PROCESSES, Issue 10 2001Douglas A. Burns Abstract The geographic sources and hydrologic flow paths of stormflow in small catchments are not well understood because of limitations in sampling methods and insufficient resolution of potential end members. To address these limitations, an extensive hydrologic dataset was collected at a 10 ha catchment at Panola Mountain Research Watershed near Atlanta, GA, to quantify the contribution of three geographic sources of stormflow. Samples of stream water, runoff from an outcrop, and hillslope subsurface stormflow were collected during two rainstorms in the winter of 1996, and an end-member mixing analysis model that included five solutes was developed. Runoff from the outcrop, which occupies about one-third of the catchment area, contributed 50,55% of the peak streamflow during the 2 February rainstorm, and 80,85% of the peak streamflow during the 6,7 March rainstorm; it also contributed about 50% to total streamflow during the dry winter conditions that preceded the 6,7 March storm. Riparian groundwater runoff was the largest component of stream runoff (80,100%) early during rising streamflow and throughout stream recession, and contributed about 50% to total stream runoff during the 2 February storm, which was preceded by wet winter conditions. Hillslope runoff contributed 25,30% to peak stream runoff and 15,18% to total stream runoff during both storms. The temporal response of the three runoff components showed general agreement with hydrologic measurements from the catchment during each storm. Estimates of recharge from the outcrop to the riparian aquifer that were independent of model calculations indicated that storage in the riparian aquifer could account for the volume of rain that fell on the outcrop but did not contribute to stream runoff. The results of this study generally indicate that improvements in the ability of mixing models to describe the hydrologic response accurately in forested catchments may depend on better identification, and detailed spatial and temporal characterization of the mobile waters from the principal hydrologic source areas that contribute to stream runoff. Copyright © 2001 John Wiley & Sons, Ltd. [source] The changes of magnetic properties of the perovskite-type titanium oxides by the structural distortions: In the case of LaTiO3 and YTiO3INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2007Taku Onishi Abstract In the perovskite-type titanium oxides, the changes of magnetic properties by GdFeO3 -type lattice distortion are observed. In this study, we have performed cluster model calculations based on the density functional theory method, and have obtained the effective exchange integral (Jab) in order to elucidate the magnetic change by the GdFeO3 -type lattice distortion. The components of deciding the orbital ordering and magnetic property have also been discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2007 [source] The dynamic behavior of magnetic colloids in suspensionJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2007Tina Autenrieth The dynamic behavior of magnetic colloidal particles in suspension is investigated. The particles of the core,shell colloid consist of a cobalt ferrite core embedded in a silica shell and are stabilized by surface charges. As the suspension is strongly opaque to visible light, it can not be probed by dynamic light scattering techniques as a result of strong multiple scattering as well as absorption effects. Therefore, the static and dynamic behavior is probed with small-angle X-ray scattering and X-ray photon correlation spectroscopy (XPCS), respectively. Using XPCS, we are able to study the diffusion coefficient of an opaque colloidal system as a function of the scattering vector. In this paper, we report on the behavior of the colloidal system in the absence of an external magnetic field, meaning that the magnetic moments of the particles are oriented randomly. We find no evidence for magnetic interactions in the static data, while the dynamic XPCS data deviate very significantly from the predictions of model calculations. [source] Theoretical characterizations of HAsXH (X = N, P, As, Sb, and Bi) isomers in the singlet and triplet statesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2008Chin-Hung Lai Abstract The lowest singlet and triplet potential energy surfaces for all group 15 HAsXH (X = N, P, As, Sb, and Bi) systems have been explored through ab initio calculations. The geometries of the various isomers were determined at the QCISD/LANL2DZdp level and confirmed to be minima by vibrational analysis. In the case of nitrogen, the global minimum is found to be a triplet H2NAs structure. For the phosphorus case, singlet trans -HAsPH is found to be global minima surrounded by large activation barriers, so that it should be observable. For arsenic, theoretical investigations demonstrate that the stability of HAsAsH isomers decreases in the order singlet trans -HAsAsH > triplet H2AsAs > singlet cis -HAsAsH > triplet HAsAsH > singlet H2AsAs. For antimony and bismuth, the theoretical findings suggest that the stability of HAsXH (X = Sb and Bi) systems decreases in the order triplet H2AsX , singlet trans -HAsXH > singlet cis -HAsXH > triplet HAsXH > triplet H2XAs > singlet H2AsX > singlet H2XAs. Our model calculations indicate that the relativistic effect on heavier group 15 elements should play an important role in determining the geometries as well as the stability of HAsXH molecules. The results obtained are in good agreement with the available experimental data and allow a number of predictions to be made. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008 [source] Effect of gas evolution on mixing and conversion in a flow-through electrochemical reactorAICHE JOURNAL, Issue 9 2009Matthew A. Petersen Abstract Flow-through electrolytic reactors (FTER) emplaced below the subsurface may be used to control the migration of groundwater contamination away from source zones. During prior studies with FTERs, water electrolysis and associated gas generation have occurred concurrently with contaminant degradation. Gas evolution-induced mixing within the electrode assembly has the potential to impact system performance. A mathematical model of the system was developed to capture the impact of mixing on transport processes in the system. Corresponding transient and steady-state tracer experiments using ferricyanide as a model contaminant were conducted to quantify mixing-dependent parameters and verify modeling results. Over a range of relevant groundwater flowrates, Peclet numbers were between 0.1 and 10, indicating that mixing was a important process under low-flow conditions. Comparison of experiments and model calculations demonstrated that incorporating gas evolution into the model was necessary for accurate performance prediction. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Theoretical analysis of the effects of asymmetric membrane structure on fouling during microfiltrationAICHE JOURNAL, Issue 6 2009Weiyi Li Abstract There is a growing interest in the use of both asymmetric and composite membranes for microfiltration and ultrafiltration processes. This includes particle removal applications in the semiconductor industry and virus clearance in biopharmaceutical applications. Filter fouling plays an important role in these processes. Although flux decline models have been developed for homogeneous membranes, the effects of asymmetric membrane structure on flux decline behavior remain poorly understood on a fundamental level. Here, we develop a theoretical model to describe the effects of asymmetric membrane structure on flux decline. The asymmetric structure was described by the spatial variation in Darcy permeability in the directions normal to and parallel to the membrane surface. The velocity profile and flux decline because of pore blockage were described using Darcy's law and a pore blockage and cake filtration model. Flux decline data were obtained using pseudocomposite membranes with highly interconnected polyvinylidene fluoride membranes (PVDF) and straight through pore polycarbonate track-etched membranes (PCTE). Model composite membranes were formed by layering PCTE or PVDF membranes with different pore sizes on top of each other. Flux decline data for the composite membrane were in good agreement with model calculations. The results provide important insights into the effects of asymmetric membrane pore structures on flux decline. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Tetrahedral intermediates in reactions of carboxylic acid derivatives with nucleophiles,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 3 2005Martin Adler Abstract Transacylation reactions of carboxylic acids, carboxylic acid esters, carboxylic acid amides and other carboxylic acid derivatives are among the most widespread and most important reactions in chemistry and biochemistry. Already in 1887, Claisen suggested a tetrahedral intermediate in transformations of carboxylic acid derivatives with nucleophiles. A historical overview gives insight into the studies to detect possible tetrahedral intermediates in such reactions. However, only in recent years has detailed information concerning the structures of such species become available. In this review, neutral, cationic and anionic tetrahedral intermediates are described which serve as models for transacylations under neutral, acid-catalysed or basic conditions. The characteristically different structures correspond nicely with experimental experience with reactions of carboxylic acid derivatives and with quantum chemical model calculations on tetrahedral intermediates. Finally, by means of model calculations, an explanation is given for the fast reactions of Weinreb amides, RC(O)N(CH3)OCH3, with organolithium and even with Grignard reagents: the reactions are determined by comparatively stable chelate transition states. Copyright © 2004 John Wiley & Sons, Ltd. [source] Image contrast in X-ray reflection interface microscopy: comparison of data with model calculations and simulationsJOURNAL OF SYNCHROTRON RADIATION, Issue 6 2008P. Fenter The contrast mechanism for imaging molecular-scale features on solid surfaces is described for X-ray reflection interface microscopy (XRIM) through comparison of experimental images with model calculations and simulated measurements. Images of elementary steps show that image contrast is controlled by changes in the incident angle of the X-ray beam with respect to the sample surface. Systematic changes in the magnitude and sign of image contrast are asymmetric for angular deviations of the sample from the specular reflection condition. No changes in image contrast are observed when defocusing the condenser or objective lenses. These data are explained with model structure-factor calculations that reproduce all of the qualitative features observed in the experimental data. These results provide new insights into the image contrast mechanism, including contrast reversal as a function of incident angle, the sensitivity of image contrast to step direction (i.e. up versus down), and the ability to maximize image contrast at almost any scattering condition defined by the vertical momentum transfer, Qz. The full surface topography can then, in principle, be recovered by a series of images as a function of incident angle at fixed momentum transfer. Inclusion of relevant experimental details shows that the image contrast magnitude is controlled by the intersection of the reciprocal-space resolution function (i.e. controlled by numerical aperture of the condenser and objective lenses) and the spatially resolved interfacial structure factor of the object being imaged. Together these factors reduce the nominal contrast for a step near the specular reflection condition to a value similar to that observed experimentally. This formalism demonstrates that the XRIM images derive from limited aperture contrast, and explains how non-zero image contrast can be obtained when imaging a pure phase object corresponding to the interfacial topography. [source] Kinetics and Molecular Weight Development of Dithiolactone-Mediated Radical Polymerization of StyreneMACROMOLECULAR REACTION ENGINEERING, Issue 4 2009Jesús Guillermo Soriano-Moro Abstract Calculations of polymerization kinetics and molecular weight development in the dithiolactone-mediated polymerization of styrene at 60,°C, using 2,2,-azobisisobutyronitrile (AIBN) as initiator and , -phenyl- , -butirodithiolactone (DTL1) as controller, are presented. The calculations were based on a polymerization mechanism based on the persistent radical effect, considering reverse addition only, implemented in the PREDICI® commercial software. Kinetic rate constants for the reverse addition step were estimated. The equilibrium constant (K,=,kadd/k -add) fell into the range of 105,106 L,·,mol,1. Fairly good agreement between model calculations and experimental data was obtained. [source] | |||||||||||||||||||||||||||||||||||||||||||