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Microscopic Model (microscopic + model)
Selected AbstractsAnalysis of a microcrack model and constitutive equations for time-dependent dilatancy of rocksGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2003Zuan Chen SUMMARY Based on experimental observations and theoretical analyses, the author introduces an ideal microcrack model in which an array of cracks with the same shape and initial size is distributed evenly in rocks. The mechanism of creep dilatancy for rocks is analysed theoretically. Initiation, propagation and linkage of pre-existing microcracks during creep are well described. Also, the relationship between the velocity of microcrack growth and the duration of the creep process is derived numerically. The relationship agrees well with the character of typical experimental creep curves, and includes three stages of creep. Then the damage constitutive equations and damage evolution equations, which describe the dilatant behaviour of rocks, are presented. Because the dilatant estimated value is taken as the damage variable, the relationship between the microscopic model and the macroscopic constitutive equations is established. In this way the mechanical behaviour of rocks can be predicted. [source] Modeling and simulation of vehicle projection arrival,discharge process in adaptive traffic signal controlsJOURNAL OF ADVANCED TRANSPORTATION, Issue 3 2010Fang Clara Fang Abstract Real-time signal control operates as a function of the vehicular arrival and discharge process to satisfy a pre-specified operational performance. This process is often predicted based on loop detectors placed upstream of the signal. In our newly developed signal control for diamond interchanges, a microscopic model is proposed to estimate traffic flows at the stop-line. The model considers the traffic dynamics of vehicular detection, arrivals, and departures, by taking into account varying speeds, length of queues, and signal control. As the signal control is optimized over a rolling horizon that is divided into intervals, the vehicular detection for and projection into the corresponding horizon intervals are also modeled. The signal control algorithm is based on dynamic programming and the optimization of signal policy is performed using a certain performance measure involving delays, queue lengths, and queue storage ratios. The arrival,discharge model is embedded in the optimization algorithm and both are programmed into AIMSUN, a microscopic stochastic simulation program. AIMSUN is then used to simulate the traffic flow and implement the optimal signal control by accessing internal data including detected traffic demand and vehicle speeds. Sensitivity analysis is conducted to study the effect of selecting different optimization criteria on the signal control performance. It is concluded that the queue length and queue storage ratio are the most appropriate performance measures in real-time signal control of interchanges. Copyright © 2010 John Wiley & Sons, Ltd. [source] The oxygen vacancy in Ga2O3: a double resonance investigation,MAGNETIC RESONANCE IN CHEMISTRY, Issue S1 2005H. J. Kümmerer Abstract When produced under reducing conditions, ,-Ga2O3 is transformed into an n -type semiconductor with delocalized conduction electrons that exhibit a very strong electron spin resonance (ESR) and a strong hyperfine coupling to the gallium nuclei of the host lattice. We apply the Overhauser-shift technique to investigate single crystals of this compound. With extension to the high magnetic field of a W-band spectrometer, we were able to resolve all spectral lines that were recorded and to assign them to their corresponding electronic and nuclear states. This separate analysis was the basis to access additional sample characteristics: the hyperfine coupling that is actually averaged out in the ESR signal, as well as the nuclear relaxation rates could be analyzed. Systematic measurements by varying the microwave power revealed the Overhauser shift in thermal equilibrium. The signal could be tracked to very small microwave saturation parameters, at which the deviation from the usual linear relation between power and shift becomes evident and the shift clearly approaches a constant value. This value in equilibrium was determined directly from a fit to a sequence of measurements, whereas standard X-band experiments only provided indirect conclusions. The probability densities of the electrons at the nuclei in the two nonequivalent crystallographic positions,the lattice sites with octahedral and tetrahedral coordination,could also be determined directly. The enhanced resolution revealed an otherwise hidden substructure in the nuclear resonance signals. On the basis of a microscopic model, this structure could be used to probe the environment of the oxygen vacancy more precisely and to determine the extension of the electronic wave function of the donor electrons. Copyright © 2005 John Wiley & Sons, Ltd. [source] Chemical bath deposition of CdSe and CdS nanocrystalline films: tailoring of morphology, optical properties and carrier dynamicsPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 10 2008Abstract We review the results of our research towards tailoring morphology and optical properties of films consisting of closely-spaced nanocrystals of CdSe and CdS whose optical band-gap can be tuned to cover the whole visible spectral range. On basis of the obtained results, in particular of photoexcited carrier dynamics, we have proposed a microscopic model that describes well the optical properties of the films. We have also showed that the spin relaxation of electrons in these nanocrystalline films is different than that in mutually isolated nanocrystals of the same size. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of NO2 molecule adsorption on free charge carriers and spin centers in porous siliconPHYSICA STATUS SOLIDI (A) APPLICATIONS AND MATERIALS SCIENCE, Issue 8 2005E. A. Konstantinova Abstract The effect of nitrogen dioxide (NO2) adsorption on free charge carriers and spin centers in porous silicon has been studied by FTIR and ESR spectroscopy. The silicon dangling bond (Pb1 -center) density rises with increasing NO2 pressure (P) while free charge carrier concentration depends on P nonmonotonically. The experimental results are explained by a microscopic model taking into account both the formation of P,(NO2), donor,acceptor pairs and NO2 -induced oxidation of Si nanocrystal surfaces. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Preface: phys. stat. sol. (b) 245/3PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 3 2008Christopher W. Smith This is the third Special Issue of physica status solidi (b) focusing on materials with a negative Poisson's ratio or other ,anomalous' physical properties. This issue contains selected papers from the First International Conference on Auxetics and Anomalous Systems held at the University of Exeter, UK, on 4,6 September 2006. Around 50 participants from all over the world as well as from a wide range of scientific and engineering disciplines contributed to what was a highly successful conference. This conference follows in the footsteps of two previous workshops held at the Mathematical Research and Conference Centre in B,dlewo near Pozna,, Poland, in 2004 and 2005 [1, 2]. The papers selected for this issue publish recent results obtained for ,anomalous systems' in experiment, theory and computer simulations. In the following we summarize very briefly their contents. Alderson and Coenen compare the performance of auxetic composites to similar systems with conventional positive Poisson's ratios. They find that there are indeed differences which appear to arise from the change of the overall Poisson's ratio of the composite, some beneficial like a rise in impact tolerance at low impact rates, and others deleterious such as the reduced tolerance at higher impact rates. This is one of the first investigations of possible applications for auxetic materials. The two papers by Gaspar and Koenders both examine the effects of disorder upon anomalous properties, especially negative Poisson's ratio. In the first one Gaspar demonstrates how a mean strain estimate fails to predict negative values of Poisson's ratio because of an inability to account for local fluctuations in elastic properties. For instance it is shown that the volume fraction of auxetic regions in an globally auxetic material (measured experimentally) are smaller than a mean strain homogenisation would require. Koenders and Gaspar explore the elastic properties, and especially Poisson's ratio, of a heterogeneous 2D network of bending beams. They predict auxetic behaviour arising from localised disorder in the packing, and therefore effective locally aggregated elastic properties of the beams. In the three articles by Gatt et al. and Grima et al. models based on simple geometry are used to explain the behaviour of seemingly disparate systems, i.e. 2D honeycombs systems and zeolite SiO2 networks. Two papers concerning honeycombs demonstrate relationships between elastic properties and structure and the bounds for auxetic behaviour. The paper concerning the zeolite Natrolite uses numerical force field based energy minimisation methods to simulate the response of this particular zeolite to applied forces and then simplifies the predicted properties even further by considering structural units as rigid 2D polyhedra linked by flexible hinges. In a similar vein, though using a different approach and concerning a very different form of matter, Heyes shows how the heterogeneity in an assembly of particles in a liquid can affect the elastic properties of a liquid and notably the infinite frequency Poisson's ratio. Heyes uses the Molecular Dynamics approach to simulate a Lennard,Jones fluid under various pressures, notably comparing behaviour under positive and negative pressures. In their first paper Jasiukiewicz and co-authors derive elastic constants of 2D crystals for all four classes of 2D crystalline solids: hexagonal (isotropic), quadratic, rectangular, and oblique systems. In their second paper they demonstrate conditions required for auxetic behaviour of 2D crystals. Auxetic solids are further divided into those with some negative Poisson's ratios (auxetic), all negative Poisson's ratios (completely auxetic) and no negative Poisson's ratios (non-auxetic). Lakes and Wojciechowski consider counterintuitive properties of matter, like negative compressibility, negative Poisson's ratio, negative thermal expansion, negative specific heat, and negative pressure. They present and interpret experimental observations of negative bulk modulus in pre-strained foams. They propose also a constrained microscopic model which exhibits negative compressibility. Finally, they solve a very simple thermodynamic model with negative thermal expansion. Martin et al. take a long stride toward a real world application of auxetic materials with a wide ranging study starting with numerical modelling of a wingbox section to experimental testing in a wind tunnel. They show that an auxetic core in a wing box section can allow a passive aero-elastic response which can be tailored by careful design of the core so that camber, and thus drag, is reduced with increasing airspeed but without sacrificing structural integrity. Miller et al. consider another anomalous physical property, negative thermal expansivity, and its application in the form of particulate composites for amelioration of stresses arising from thermal mismatch. They show via experiments that particles with a negative coefficient of thermal expansion may be used as a composite reinforcer to reduce overall thermal expansion and behave according to the standard volume fraction based models. Narojczyk and Wojciechowski examine the effects of disorder upon the bulk elastic properties of 3D fcc soft sphere systems in terms of particle size. Systems, such as colloids, can be thought of in such terms. The study shows that higher order moments of probability distribution do not influence the bulk elastic properties much, but that lower moments such as the standard deviation of particle size influence the elastic properties greatly. The "hardness" of the particle interaction potential is also important in this context. In general, it is shown that the effect of increasing polydispersity is to increase the Poisson's ratio, except the [110] [10] directions. Scarpa and Malischewsky in their paper on Rayleigh waves in auxetic materials show how the Rayleigh wave speed is affected by the Poisson's ratio. The behaviour is complex and depends upon the homogeneity within the material, for instance slowing with decreasing Poisson's ratio in isotropic solids, but showing the reverse trend and increased sensitivity to Poisson's ratio in laminate composites. Scarpa et al. explore the buckling behaviour of auxetic tubes via three types of model, a simple beam mechanics and Eulerian buckling model, a 3D linear elastic FE model and a bespoke non-linear continuum model. The more sophisticated models provide increasing insight into the buckling behaviour though the simple beam model predicts reasonably well in the pre-buckling linear region. Some unexpected and interesting behaviour is predicted by the continuum model as the Poisson's ratio approaches the isotropic limit of ,1, including increasing sensitivity to Poisson's ratio and rapid mode jumping between integer wave numbers. The paper by Shilko et al. presents an analysis of a particular kind of friction joint, a double lap joint, and explores the effects of altering the elastic properties of one component, in particular it's Poisson's ratio. The manuscript introduces the evolution of smart materials from monolithic materials, and the classification of composites exhibiting negative Poisson's ratios. The paper then presents the case of a double lap joint and performs a sensitivity type study, via a 2D FE model, of the effects of changing the elastic properties and degree of anisotropy of one section of the model on various parameters defining the limits of functionality of the joint. The main finding is that an enhanced shear modulus, via a negative Poisson's ratio, can endow such a friction joint with superior performance. Manufacturing of auxetic materials on a commercial scale has proved to be the largest obstacle to their fuller exploitation. The paper by Simkins et al. explores one route for post processing of auxetic polymers fibres produced by a conventional melt extrusion route. Simkins et al. showed that a post process thermal annealing treatment, with carefully optimised parameters, was able to even out otherwise inhomogenous auxetic properties, and moreover improve other elastic and fracture properties often sacrificed for auxetic behaviour. We gratefully acknowledge the support given by the sponsors of the conference, namely the EPSRC of the UK and Auxetic Technologies Ltd. (UK). We also thank the Scientific Committee, the Organising Committee, and all the participants of the conference. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Uniaxial stress study of the Cu,H complex in ZnOPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 12 2006E. V. Lavrov The cover picture of the current issue refers to the article by Lavrov and Weber which was selected as Editor's Choice [1]. The picture shows a microscopic model of the Cu,H complex investigated in the paper. The complex consists of a substitutional Cu atom at the Zn site with an H atom located between nearby O and Cu in the basal plane of the ZnO lattice. The atoms are shown in different colors: yellow for copper, red for hydrogen, cyan for oxygen, and grey for zinc. Hydrogen forms a strong bond with the O atom which gives rise to a local vibrational mode at 3192 cm,1 investigated in the paper under uniaxial stress. The c -axis is parallel to the Cu,O bond pointing to the top of the figure. The authors work at the Institute of Applied Physics/Semiconductor Physics, TU Dresden, Germany. [source] GaN growth on LiNbO3 (0001) , a first-principles simulationPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 7-8 2010Simone Sanna Abstract The growth of GaN on the LiNbO3 (0001) surface is simulated by means of first-principles total-energy calculations. Firstly the adsorption of single N and Ga monolayers is investigated and then the layer-by-layer growth of GaN on the polar substrate within different orientations is modeled. While adsorbing a N layer does not heavily affect the substrate morphology, the adsorption of a Ga layer causes a rearrangement of the atomic structure. Furthermore the N layer is more strongly bound to the substrate than the Ga layer. On the basis of our results, we propose a microscopic model for the GaN/LiNbO3 interface. The GaN and LiNbO3 (0001) planes are parallel, but rotated by 30° each other, with in-plane epitaxial relationship [100]GaN II [110]. In this way the (0001) calculated in-plane lattice mismatch between GaN and LiNbO3 is minimal and equal to 6.79% of the GaN lattice constant. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Influence of iodine molecule adsorption on electronic properties of porous silicon studied by FTIR and EPR spectroscopyPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 6 2007L. A. Osminkina Abstract The infrared absorption and electron paramagnetic resonance spectroscopy are used for studying the effect of adsorption of iodine (I2) molecules, strong acceptors of electrons, on the electronic properties of silicon nanocrystals in micro- and mesoporous silicon layers with different types of doping impurities. It is found that the concentration of free charge carries (holes) in silicon nanocrystals, obtained by electrochemical etching of p-type and n-type silicon wafers, sharply increases in the presence of I2 molecules. At the same time the decrease of the dangling silicon bonds (Pb -centers) concentration is observed. A microscopic model proposed for explaining this effects presumes the formation of donor-acceptor pairs (Pb1+ -I2,), and shallow acceptor states on the porous silicon surface which, together with the initial dopant and surface defect states, specify the charge carrier type and concentration in the silicon nanocrystals. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Lattice dynamics and the electromechanical properties of BaTiO3 in tetragonal phasePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2006Ali Khalal Abstract A lattice dynamical formalism using the shell model (R. A. Cowley, Phys. Rev. 134, A981 (1964) [1]) is applied to ferroelectric crystal BaTiO3, in the tetragonal phase. This microscopic model includes the short-range interactions of axially symmetric type and the long-range Coulomb interactions takes into account the electronic polarizability of constituent ions. The stability conditions of crystal are used to determine several first-derivatives potential for the crystals. Zone center phonons are used for fitting. In general, we obtained the best agreement between the calculated and observed frequencies. A calculation of the oblique phonons and electromechanical coupling are presented and compared with the available experimental data. It can be seen that, the electromechanical coupling is at a maximum when the acoustic wave is polarized parallel to the polar c -axis, and there is zero coupling when the polarization is perpendicular to this axis. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Modeling Na clusters in Ar matricesANNALEN DER PHYSIK, Issue 7 2005F. Fehrer Abstract We present a microscopic model for Na clusters embedded in raregas matrices. The valence electrons of the Na cluster are described by time-dependent density-functional theory at the level of the local-density approximation (LDA). Particular attention is paid to the semi-classical picture in terms of Vlasov-LDA. The Na+ ions and Ar atoms are handled as classical particles whereby the Ar atoms carry two degrees of freedom, position and dipole polarization. The interaction between Na+ ions and electrons is mediated through local pseudo-potentials. The coupling to the Ar atoms is described by (long-range) polarization potentials and (short-range) repulsive cores. The ingredients are taken from elsewhere developed standards. A final fine-tuning is performed using the NaAr molecule as benchmark. The model is then applied to embedded systems Na8ArN. By close comparison with quantum-mechanical results, we explore the capability of the Vlasov-LDA to describe such embedded clusters. We show that one can obtain a reasonable description by appropriate adjustments in the fine-tuning phase of the model. [source] | ||||||||||