Development Of Theoretical Methods Research Articles

Evaluation of the transformation kinetics of Ga 7.5Se 92.5 chalcogenide glass using the theoretical method developed and isoconversional analyses

The transformation kinetics from glass to crystalline for Ga 7.5Se 92.5 chalcogenide glass were studied using the differential scanning calorimetry (DSC) technique. The kinetic parameters of present chalcogenide glass under non-isothermal conditions are analyzed by the theoretical method developed (TMD) and the isoconversional (model-free) method for a heating rate range of 5–90 K/min. The average values of the effective activation energy for crystallization ( E eff. ) and the pre-exponential factor ( A) are 73.61 ± 2.6 kJ/mol and 7.44 × 10 9 min −1, respectively. The average value of the impingement exponent ( γi) is 1.9636 ± 0.28 and the dependence of the maximum transformed fraction ( α P ) on γi, indicates that the mode of impingement is due to “anisotropic growth”. From the average values of the kinetic exponents ( n and m), the transformation process of Ga 7.5Se 92.5 chalcogenide glass is volume nucleation with two- and one-dimensional growth, which are operating simultaneously during the glass–crystalline transformation. The average values of the separate activation energies for growth ( E G ) and nucleation ( E N ) are 78.02 ± 0.15 kJ/mol and 69.79 ± 0.74 kJ/mol, respectively. The results of the kinetic parameters of present chalcogenide glass using the isoconversional (model-free) method indicate that the effective activation energy of crystallization ( E eff. ( α)) is not constant but varies with the degree of transformation and hence with temperature. The reaction model that may describe transformation process is Avrami–Erofeev model ( g( α) = [−ln(1 − α)] 1/ n ) with n equal to 3 and 2 for the heating rate ranges of 5–35 K/min and 40–90 K/min, respectively. Generally, the obtained results of all transformation kinetic parameters of Ga 7.5Se 92.5 chalcogenide glass using theoretical method developed analysis are in good agreement with that obtained according to the isoconversional method.

3DSDSCAR—a three dimensional structural database for sialic acid-containing carbohydrates through molecular dynamics simulation

The inherent flexibility and lack of strong intramolecular interactions of oligosaccharides demand the use of theoretical methods for their structural elucidation. In spite of the developments of theoretical methods, not much research on glycoinformatics is done so far when compared to bioinformatics research on proteins and nucleic acids. We have developed three dimensional structural database for a sialic acid-containing carbohydrates (3DSDSCAR). This is an open-access database that provides 3D structural models of a given sialic acid-containing carbohydrate. At present, 3DSDSCAR contains 60 conformational models, belonging to 14 different sialic acid-containing carbohydrates, deduced through 10 ns molecular dynamics (MD) simulations. The database is available at the URL: http://www.3dsdscar.org.

Temperature gradient in wood during grinding

Grinding is a commonly used method for producing pulps for papermaking, but its rather poor energy efficiency is a drawback. This paper focuses on developing a model dealing with temperature rise in wood during grinding. The model paves the way for the development of theoretical methods which can be used for reducing the energy consumption of the process. In grinding, wood is loaded by grits, which cause stress waves in the wood matrix. The stress waves fatigue the wood and ultimately separate fibres from the matrix, but because of wood’s viscoelasticity, part of the mechanical energy of waves is converted into heat. In order to understand the wood temperature increase in this process, a mechanistic model is developed here. The model is based on three hypotheses: a flux of mechanical energy occurs through the wood, the magnitude of the flux can be derived from the contact mechanics of the grits, and the rise in wood temperature can be determined from the dissipation of the flux. A temperature distribution in the actual grinding process was simulated with the model. The simulated temperatures were compared with a measured temperature profile obtained from the literature. The modelled and measured temperatures matched quite well. The simulations show that an increase in grit size results higher temperatures, whereas an increase in the distance between grits gives lower temperatures. The main result of the study is that the Hertz theory of contact mechanics can be considered an adequate method for analysing the effect of grits in the grinding process. The result shows that the Hertz theory is applicable when fatigue models are developed; these models can then be used to reduce the energy consumption of the process.

Theoretical Processing in Understanding the Structures and Properties of Layered Double Hydroxides

We review the techniques,applications,characteristics,and insights gained from the use of theoretical calculations that were applied to the study of layer double hydroxides (LDHs) materials by using a series of typical case studies. The advantages and shortcomings of different theoretical calculation methods (quantum mechanics,molecular mechanics,geometric model,and electrostatic potential energy model) for the study of the properties of LDHs minerals are compared. Based on quantum mechanics calculations,we obtained information about template effects on the construction of layered double hydroxides,super molecular interactions in LDHs containing simple anions,electronic properties,and reaction pathways etc. Compared with quantum mechanics,molecular mechanics is quicker in obtaining information about the interlayer structure,arrangement,orientation,hydration,and the swelling trajectory as well as elastic constants etc of LDHs intercalated with various anions. The geometric model and electrostatic potential energy model offer a more intuitive and visual mathematical model of LDHs minerals. The calculations were done on the verge of full size LDHs,which may allow the prediction of the crystal structure. Along with the development of theoretical methods and computer techniques,computational simulation method has become an effective adjust to experimental techniques for obtaining the microscopic structures,electronic and dynamic properties of LDHs minerals.

On the glass-crystal transformation kinetics by using differential scanning calorimetry under non-isothermal regime: Application to the crystallization of the Ag 0.16As 0.46Se 0.38 semiconductor glass

A procedure has been developed to obtain an evolution equation with the temperature for the actual transformed volume fraction under non-isothermal regime, to calculate the kinetic parameters and to analyze the glass-crystal transformation mechanisms in solid systems where a large number of nuclei already exists and no other new nuclei are formed during the thermal treatment. In this case, it is assumed that the nuclei only grow, “site saturation”, during the thermal process. Once an extended volume of transformed material has been defined and spatially random transformed regions have been assumed, a general expression of the extended volume fraction has been obtained as a function of the temperature. Considering the mutual interference of regions which grow from separate nuclei (impingement effect) and from the quoted expression, the actual transformed volume fraction has been deduced. The kinetic parameters have been obtained, by assuming that the reaction rate constant is a time function through its Arrhenian temperature dependence. The developed theoretical method has been applied to the crystallization kinetics of the Ag 0.16As 0.46Se 0.38 glassy alloy as-quenched and previously reheated. In accordance with the corresponding results, it is possible to establish that in the considered alloy the nuclei were dominant before the thermal treatment, and because of it the reheating does not change in a considerable way the number of the pre-existing nuclei in the material, which is a case of “site saturation”. The comparison of the quoted results with the values obtained by means of Matusita method confirms the reliability of the theoretical method developed (TMD). Moreover, the obtained values for the kinetic parameters coincide in a satisfactory way with the results calculated by means of the Austin–Rickett (AR) equation under non-isothermal regime. Besides, the experimental curve of the transformed fraction shows a better agreement with the theoretical curves of the developed method and of the Austin–Rickett model than with the corresponding curve of the Avrami model. Accordingly, it seems appropriate to choose the Austin–Rickett equation in order to describe the crystallization mechanism of the above-mentioned glassy alloy.

Theoretical and computational chemistry in Italy: an overview

The development of new theoretical methods for the prediction and interpretation of physico-chemical observables has been accompanied—sometimes stimulated and sometimes dragged—by a wealth of new and sophisticatedcomputationaltoolsspecificallydevisedby theoretically oriented chemists for molecular systems in different environments. At the same time, the rapid increase of computer power and the relative decrease of economic costs has lead to a capillary diffusion of computational techniques in many areas of chemical research, which were previously hardly touched by the extensive use of in silico approaches. This indubitable progress has been made possible by the advances in the development of theoretical methods attuned to molecular problems, like quantum mechanical(QM)techniquesusinglocalizedbasissetsandrooted either on wave-function or density functional models. Further progress toward the treatment of complex systemsisbeingdone,thankstothedevelopmentoflinearly scaling methods, multiscale approaches combining QM, molecular mechanics (MM), mean field (MF) models, effective Born–Oppenheimer and extended Lagrangian dynamical algorithms, sophisticated statistical mechanicsapproache tosoftmaterials,andmanyothers.Finally, one should not underestimate the importance of transparent, WYSIWYG-like implementations of many of these techniques, which is one of the main incentives for the spreading of commercial programs which are nowadays present in many experimental chemistry laboratories.

Exploiting biological complexity for strain improvement through systems biology.

Cellular complexity makes it difficult to build a complete understanding of cellular function but also offers innumerable possibilities for modifying the cellular machinery to achieve a specific purpose. The exploitation of cellular complexity for strain improvement has been a challenging goal for applied biological research because it requires the coordinated understanding of multiple cellular processes. It is therefore pursued most efficiently in the framework of systems biology. Progress in strain improvement will depend not only on advances in technologies for high-throughput measurements but, more importantly, on the development of theoretical methods that increase the information content of these measurements and, as such, facilitate the elucidation of mechanisms and the identification of genetic targets for modification.

Channelized hotelling and human observer study of optimal smoothing in SPECT MAP reconstruction

We compared the performance of a channelized Hotelling observer (CHO) to that of human observers to determine an optimal smoothing parameter /spl beta/ for an SKE/BKE detection task in a SPECT MAP (maximum a posteriori) reconstruction. The study is motivated in part by the recent development of theoretical methods that can rapidly predict CHO signal-to-noise ratios (SNRs) for MAP reconstructions. We found that a CHO not adjusted for internal noise effects was less predictive of the optimal smoothing parameters than one that used human observer data to tune the CHO for internal noise. We used a three-channel, square profile, radially symmetric channel structure, and, for internal noise, a method that altered the diagonal elements of the channel covariance matrix. The human observer study for two different signals A and B showed that /spl beta/ in the range 0.5-10.0 produced high detectability as measured by high d/sub A//sup 2/, while the CHO without internal noise showed high SNR/sup 2/ for /spl beta/ in the wider range 0.01-10.0. The CHO at location A was modified by internal noise utilizing human data at A, so that the d/sub A//sup 2/ and SNR/sup 2/ overlapped well, but when these internal noise parameters from A were applied at B, the curves did not overlap well. Nevertheless, both modified CHOs predicted a /spl beta/ range in accord with human data. We conclude that CHOs may need some way of incorporating internal noise without having to conduct a human study in the first place to determine internal noise parameters.

Preface

Disciplines of science including computational chemistry have seen rapid advances in recent years. The use of theoretical methods such as quantum mechanics, molecular dynamics, and statistical mechanics have successfully characterized chemical systems and are used to design new materials, drugs, and chemicals. Jackson State University (JSU), in collaboration with the U.S. Army Engineer Research and Development Center (formerly Waterways Experiment Station), has organized the series Conferences on Current Trends in Computational Chemistry (CCTCC). Over the years, CCTCC has become a significant international event. The main purpose of the Conference is to bring together researchers working in the rapidly changing field of computational chemistry and to allow for the exchange of new scientific ideas. The 11th CCTCC was held in Jackson, Mississippi, at the Hilton Hotel on November 1–2, 2002. Eleven plenary lectures covered a wide range of problems that include applications of computational chemistry and recent developments of theoretical methods. The 11th Conference also featured a talk given in the after-dinner “Noble Lecture Series.” As suggested by Professor Pople, a new tradition for the second decade of the CCTCC conferences was initiated. Starting from the CCTCC-11 each meeting will feature a talk named after eminent computational chemists. The first presentation in this series was given by Professor David Buckingham from the University of Cambridge. Over 200 participants from almost 30 countries attended the 11th CCTCC meeting. Such diversity creates an environment which encourages a broad exchange of new scientific ideas. The poster displays and coffee breaks facilitated scientific discourse and cordial discussions as part of the CCTCC tradition. Three poster sessions consisting of approximately 150 presentations provided examples of research topics studied by current computational chemistry methods. A noteworthy number of attractive posters were presented by undergraduate and graduate students. Three best student posters were awarded by the Parallel Quantum Solutions. The articles published in this volume highlight the wide variety of the contributions to the conference. We would like to express our sincere gratitude to all authors for their contributions. We also wish to acknowledge the support of the 11th CCTCC by: The Army High Performance Computing Research Center, National Science Foundation (CREST Program), Parallel Quantum Solutions, U. S. Army Engineer Research and Development Center.

Interpretation of wastage mechanisms of materials exposed to elevated temperature erosion-corrosion using erosion—corrosion maps and computer graphics

One of the most significant advances in the study of erosion-oxidation and wear in recent years has been the development of theoretical methods to construct maps where the change in erosion or wear mechanism is given as a function of typically two parameters. This approach has important implications for optimization of process parameters in wear and in erosion-corrosion. The direct application of such work is in process engineering where guidelines on the control of parameters to minimize erosion-corrosion are practically non-existent. Although there have been a range of excellent approaches towards the development of erosion-oxidation maps there have been some areas which have not been addressed. Firstly, because erosion-corrosion involves a wide range of parameters, the maps which have been developed to date have only considered the effects of temperature and velocity. In addition, there has been no significant attempt to combine variables on the maps, or incorporate a materials selection parameter on the maps. Finally, there have been few attempts to demonstrate the physical significance of the regimes on the maps, in order to clarify the mechanism of damage on the surface. The object of this research is to address the above areas in the development work on the maps to date. It is shown how the maps can be used to show metal recession as opposed to erosion rates by modifying the regime definitions proposed to date. The relative advantages and limitations of the models developed to date will be discussed.

Computer analysis of DNA and protein sequences

Some recent trends in the development of theoretical methods for DNA and protein sequence analysis are reviewed, with particular emphasis on the design of new databases, motif searches, sequence alignment algorithms and applications of neural networks.

The Response of Helicopter Rotors to Vibratory Airloads

Abstract : Structural response data from flight or wind tunnel tests of eight full-scale rotors were examined and compared for high-speed flight conditions and in the absence of blade stall or maneuvers. Both similarities and differences in the behavior of the rotors were observed, and these findings are useful in determining appropriate tests for development of theoretical methods. Limited use is made of airload measurements and theoretical calculation in examining these data. Major similarities observed in the rotor behavior include: 1) 3/rev vibratory flap bending moments are remarkably similar among all the rotors at high speed; 2) the root oscillatory chord bending induced by lag dampers is similar for three of the articulated rotors despite differences in the damper type; and 3) torsion moment and pitch-link loads show same positive- negative loading over the advancing side of the disk caused by the unsteady pitching moments at the blade tip. Differences that were observed include: 1) the vibratory chord bending-moment behavior appears to be dependent on rotor stiffness in part, but differences seen are not easily explained; 2) the CH-53A root oscillatory chord bending-moment data do not show the damper-induced loads that are seen on the other articulated rotors with hydraulic lag dampers; and 3) the AH-1G torsion response is very different from that of the articulated rotors. Rotor blades; Rotary wings. (edc)

Total energy all-electron theory of surface structural, electronic, and magnetic properties

In the last decade, intense experimental efforts using advanced techniques for sample preparation and characterization have provided a vast quantity of data. These developments have challenged present theoretical understanding and have encouraged the development of theoretical methods interpreting the phenomena observed. Our total energy all-electron local density theoretical approach, implemented as the full potential linearized augmented plane wave (FLAPW) method, is described and shown to have high precision (to 10−9 in the total energy) and stability for describing the structural, electronic, and magnetic properties of (i) free surfaces [including surface relaxation and reconstruction, e.g., W(001)], (ii) interface phenomena in Au/Cr/Au(001) sandwiches [e.g., prediction of the enhanced magnetic moment on the interface Cr site], (iii) catalytic promotion and poisoning of molecular dissociation on surfaces [e.g., CO + K or S/Ni(001)]. The three examples just cited illustrate the present predictive power and sophistication of total energy all-electron calculations in elucidating properties of direct interest to experimentalists.

EXPERIMENTAL EVIDENCE OF S*‐WAVE*

ABSTRACTDuring development of theoretical methods to compute synthetic seismograms, a new type of wave called S*‐wave was discovered by Hron and Mikhailenko. This wave propagates with the shear‐wave velocity and can be interpreted as a non‐geometrical wave arrival with large amplitudes strongly depending on the depth of a pure P‐point source.In this first experimental verification of the existence of S*‐waves by means of two‐dimensional model‐seismics it is demonstrated that:1. the S*‐wave exists and depends on the source distance from the free surface; 2. the S*‐wave is generated as an ordinary shear wave on the free surface at the point located directly above the P‐source, as illustrated in the synthetic seismograms. The measured seismograms agree remarkably well with the computed ones.

Tensile testing of a wire rope strand

Experimental tests are reported on wire rope strands subjected to static axial loads. The wire rope strands are held with a polyester resin and silica filler in conical end grips which are capable of full end fixity, partial restraint, or zero torsional resistance (free ends). Strain gauge load cells monitor the tensile load and the associated twisting moment developed in a strand which is restrained at both ends. A new instrument (‘extrometer’) was designed to record simultaneously the extension and rotation over a predetermined gauge length. Strain gauges are used to measure the surface strains on the wires in the outer layer of the strand. Preliminary tests on seven-wire strands demonstrate that the extrometer instrument provides reliable results. The extension and rotation characteristics recorded on a seven-wire strand under tensile load agree reasonably well with the corresponding theoretical predictions, but the wire surface strains reveal an unequal load sharing between nominally identical helical wires. The experimental data presented here is more complete than previously published work on the tensile behaviour of strands and should assist in the development of theoretical methods and numerical schemes suitable for design purposes.

Photoemission from atoms and molecules

Photoemission measurements form stringent tests for theoretical models. The interplay between theory and photoemission experiments has contributed vonsiderably to our present understanding of relaxation and correlation phenomena in many-electron atoms and molecules. In fact, the continuing progress in the development of theoretical methods has gone hand in hand with the development of experimental techniques to study photoionization processes over a broad energy range. The experimental set-ups used for atomic and molecular photoemission will be described. This includes the presentation of light sources, monochromators, atomic and molecular beam sources, electron energy analysers specially designed and optimized for gas phase photoemission experiments using synchrotron radiation. The capabilities of the most advanced instruments will be evaluated. The impact of VUV photoemission experiments on atomic and molecular physics will be showcased by outstanding results.

Computer-aided design: aerodynamics

In recent years, there have been rapid advances in the development of theoretical methods in aerodynamics, particularly for transonic flow calculations. Many papers have been written on this topic. It is however appropriate that there should now be a paper that concentrates not so much on the methods themselves but on the use of the methods for computer-aided design. One could say that the most striking development in the UK in the past two-three years has been that the methods are no longer merely within the preserve of the research establishments; they are being actively used by the aircraft industry in the design of actual aircraft projects. Any air of scepticism about their value has been dispelled. The designers are now appreciating through their own experience that with the aid of a powerful computer and indeed in some cases, a computer of only modest capacity, they can specify a shape for the first wind tunnel model of a new project with much more confidence that the test results will show that the shape is near what is required to achieve their design objectives. It is already being claimed that in this way, the time-scale of the design cycle is being shortened considerably. Also, the computer and the theoretical methods are being used increasingly in the interpretation of the test data. The forecasting of the full-scale characteristics on the basis of the model test data is developing into more of a science and less of an art based on past experience.

A precision determination of the Lamb shift in hydrogen

For over 40 years, optical and microwave spectroscopists, and atomic, nuclear and elementary particle physicists have been engaged in measuring the 22S½-22P½energy level separation in atomic hydrogen (the Lamb shift) and attempting to predict the splitting theoretically. The discrepancies encountered have influenced the development of theoretical methods of calculation in the areas of atomic structure, quantum electrodynamics and elementary particle physics. In this paper we present the results of a precision microwave determination of the Lamb shift, using a fast atomic beam and a single microwave interaction region. The value obtained is in substantial agreement with the earlier determinations and with the recent calculation by Mohr but is in disagreement with the earlier calculation by Erickson. This disagreement is further accentuated if recent modifications to the size of the proton are included, whereas the agreement with Mohr’s calculation is not affected. The experimental method uses a 21 keV beam of metastable 2 s hydrogen atoms which are obtained by charge exchange of a proton beam extracted from a radio frequency (r.f.) ion source. The experiment is performed in essentially zero magnetic field and uses a precision transmission line interaction region to induce r.f. transitions at the Lamb shift frequency. The result for the 22S½F= 0 to 22P½F = 1 interval in zero field is 909.904 ± 0.020 MHz corresponding to a Lamb shift of 1057.862 ± 0.020 MHz. The paper discusses the method and the host of corrections for systematic effects which need to be applied to the line centre, many of which have not been sufficiently understood or controlled in previous experiments. The paper is introduced with a brief survey of significant landmarks in calculation and measurement of the Lamb shift and concludes with a comparison of the present theoretical and experimental positions.

Laser-excited resonant isotopic V→V energy transfer: H35Cl–H37Cl, H79Br–H81Br, D35Cl–D37Cl, and D79Br–D81Br

An isotopically selective transverse discharge chemical laser coupled with infrared fluorescence techniques is used to measure vibration-to-vibration energy transfer rates between v=1 levels of the isotopic species H 35Cl?H37Cl, H 79Br?H 81Br, D 35Cl?D 37Cl, and D 79Br?D 81Br. The measured rate constants are kHCl= (1.91±0.04) ×10−11, kHBr= (1.50±0.06) ×10−11, kDCl= (1.18±0.08) ×10−11, and kDBr= (8.34±0.17) ×10−12 cm3 molecule−1 sec−1. All of the processes are nearly resonant and the rates for the hydrogenated species are on the order of one-tenth gas-kinetic. The rates for DCl and DBr are found to be 1.6 and 1.8 times slower than the corresponding hydrogen halides. Direct comparison of the results with theoretical predictions for such resonant energy transfer processes shows disagreement, indicating that further development of theoretical methods for these systems is desirable.