Volume 20, Issue 6
September 1997, pages 737-908
pp 737-737 September 1997 India-Japan Seminar On New Materials
pp 739-744 September 1997 India-Japan Seminar On New Materials
In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc.
pp 745-754 September 1997 India-Japan Seminar On New Materials
We present results of ourab initio molecular dynamics simulations on the atomic and electronic structure of clusters of divalent metals, aluminum and antimony, which exhibit a range of bonding characteristics e.g. non-metal-metal transition, metallic and covalent respectively. Results of these studies have been used to develop icosahedral AI12X (X = C, Si and Ge) superatoms with 40 valence electrons which correspond to a filled electronic shell. It is found that the doping leads to a large gain in the binding energy as compared to Al13, suggesting this to be a novel way of developing species for cluster assembled materials. Further studies of adsorption of Li, Si and Cl atoms on Al7 and Al13 clusters show marked variation in the adsorption behaviour of clusters as a function of size and the adsorbate. Silicon reconstructs both the clusters and induces covalency in Al-Al bonds. We discuss the adsorption behaviour in terms of the superatom-atom interactions.
pp 755-767 September 1997 India-Japan Seminar On New Materials
The binding energies to copper, silver and gold (111) surfaces of self-atom clusters have been calculated. The activation energies of motion of these ad-atom clusters, vacancies and divacancies on copper, silver and gold (111) surface, and of the conversion of ad-atom clusters on (111) and (100) have been calculated by use ofn-body embedded atom potentials and molecular dynamics.
pp 769-776 September 1997 India-Japan Seminar On New Materials
Adsorption and desorption of CO on Ni(100) and Pt(111) surfaces are presented. At the thermodynamic equilibrium, the site occupation between the terminal and the bridged sites are described with the free energy of the system, including the vibrational entropy. Adsorption of CO onto a cold surface as 20 K has also been studied by infrared reflection absorption spectroscopy (IRAS). The occupation ratio of bridged CO to terminal CO species on Ni(100) at 20 K ranges from ∼ 2·8 to 0·7 at the total coverage from 0·003 to 0·15 ML. Such strong coverage dependence of the occupation ratio even at small coverages suggests that the interaction between CO molecules operates at relatively long range (> 10 Å). The isotope experiments suggest that there is substantial interaction between preadsorbed (accommodated) CO species and incoming (mobile) CO species. Desorption process is also affected by the interaction between the adsorbed CO and the incoming species. The effect of temporal bimolecular CO interaction on the desorption kinetics is also discussed.
pp 777-786 September 1997 India-Japan Seminar On New Materials
The surface morphology ofa-Si: H is strongly dependent on substrate material and temperature, especially in the initial stage of deposition. Furthermore, some surface treatments could induce a drastic change in the initial growth mode. For example, the hydrogen plasma treatment of highly oriented pyrolytic graphite (HOPG) surface prior toa-Si: H deposition changed the growth mode from inhomogeneous to homogeneous one. In order to elucidate this surface chemical process, molecular orbital calculations were performed and compared with the experimental observation of the surface by AFM and STM. The calculation verified hydrogen addition tosp2 carbons on HOPG to facilitate the bonding of SiH3 to neighbouring carbons, which correspond to the nucleation or pinning of precursors as the origin of homogeneous growth ofa-Si: H film.
pp 787-803 September 1997 India-Japan Seminar On New Materials
An extensive amount of experimental work has been reported in the literature on the ordering behaviour of Ni-Mo alloys containing 8–33 at% of Mo, which exhibit both short-range and long-range ordering phenomena and a competition among several fcc-based long-range ordered structures. We have used local-density-based tight binding linear muffin-tin orbital (TB-LMTO) method in conjunction with ‘augmented space recursion + orbital peeling’ (ASR + OP) for the determination of ground state energies of these superstructures in terms of effective pair interactions up to the fourth nearest neighbour pairs. The ordering behaviour of the four competing fcc-based superstructures has been studied using the mean-field-based ‘static concentration wave’ (SCW) model in terms of the free energy-order parameter plots (Landau plots) and the free energy-composition plots. The instability domains with respect to concentration fluctuations, both short wavelength (ordering) and long wavelength (clustering) have been identified from these calculations. This information has been used to predict the sequence of transformation events in the Ni-Mo alloys undergoing ordering and/or clustering and the results are compared with those obtained experimentally.
pp 805-815 September 1997 India-Japan Seminar On New Materials
A molecular orbital approach to alloy design has recently made great progress. This is applicable not only to structural alloys, but also to functional alloys. In this paper we have focussed on two materials as examples: high Cr ferritic steels and hydrogen storage alloys.
pp 817-822 September 1997 India-Japan Seminar On New Materials
Ultra water-repellent films for which contact angles of water drops exceed 150° were prepared by microwave plasma-enhanced CVD using two kinds of organosilicon compound and fluoro-alkyl silane (FAS) at low substrate temperature. Hexamethyldisilane (HMDS) and hexamethyldisiloxane (HMDSO) were used as starting materials. Molecular orbital (MO) calculation suggested that HMDS was more easily decomposable than HMDSO. The films prepared with HMDS and FAS had ultra water repellency. On the other hand, water repellency of the films prepared with HMDSO and FAS was similar to that of polytetrafluoroethylene.
pp 823-843 September 1997 India-Japan Seminar On New Materials
We study thin film growth using a lattice-gas, solid-on-solid model employing the Monte Carlo technique. The model is applied to chemical vapour deposition (CVD) by including the rate of arrival of the precursor molecules and their dissociation. We include several types of migration energies including the edge migration energy which allows the diffusive movement of the monomer along the interface of the growing film, as well as a migration energy which allows for motion transverse to the interface. Several well-known features of thin film growth are mimicked by this model, including some features of thin copper films growth by CVD. Other features reproduced are—compact clusters, fractal-like clusters, Frank-van der Merwe layer-by-layer growth and Volmer-Weber island growth. This method is applicable to film growth both by CVD and by physical vapour deposition (PVD).
pp 845-878 September 1997 India-Japan Seminar On New Materials
Geometry and energy of argon clusters confined in zeolite NaCaA are compared with those of free clusters. Results indicate the possible existence of magic numbers among the confined clusters. Spectra obtained from instantaneous normal mode analysis of free and confined clusters give a larger percentage of imaginary frequencies for the latter indicating that the confined cluster atoms populate the saddle points of the potential energy surface significantly. The variation of the percentage of imaginary frequencies with temperature during melting is akin to the variation of other properties. It is shown that confined clusters might exhibit inverse surface melting, unlike medium-to-large-sized free clusters that exhibit surface melting. Configurational-bias Monte Carlo (CBMC) simulations ofn-alkanes in zeolites Y and A are reported. CBMC method gives reliable estimates of the properties relating to the conformation of molecules. Changes in the conformational properties ofn-butane and other longern-alkanes such asn-hexane andn-heptane when they are confined in different zeolites are presented. The changes in the conformational properties ofn-butane andn-hexane with temperature and concentration is discussed. In general, in zeolite Y as well as A, there is significant enhancement of thegauche population as compared to the pure unconfined fluid.
pp 879-883 September 1997 India-Japan Seminar On New Materials
The valuable catalytic and adsorptive properties of heterogeneous catalysts and the challenging area of nanostructure materials provide ample reason for establishing a firm theoretical understanding of their structure and behaviour. Computer simulation studies can contribute significantly in achieving an understanding of structure property relationships by the synthesis of current understanding and data, and by their capacity in revealing critical conceptual issues whose resolution demands additional experimentation. In the present communication I emphasize on the activity of our group in the area of computer simulation. Our group activity involves generation of new codes, better compatibility to solve the problem as well as application of the available computation technique in solving the problems generated in industry and in academics. We are concentrating on basic research as well as application using integrated computational chemistry as a tool.
pp 885-900 September 1997 India-Japan Seminar On New Materials
The paper presents an overview of the analytical methods as well as finite element method employed by the author in a few earlier investigations pertaining to modelling and simulation of deformation at microscopic scale. The following case-studies are considered for illustrations: deformation of a set of powder particles during hot isostatic pressing; effective properties of a typical particulate metal matrix composite and porous material; constitutive behaviour of a material exhibiting transformation-induced plasticity; shear band formation in polycrystalline material.
The paper describes certain generalized techniques for constructing the microstructural geometries, assigning material properties and imposing boundary conditions. The concept of generating two-phase geometries using a master mesh and the generalized plane strain approach to handle two-dimensional approximations used in the above studies are also reviewed. In contrast to the commonly employed unit cell models based on certain regular geometries, the present method uses the actually observed microstructural geometries. This method accommodates more realistic and complex conditions compared to those supported by the well-explored analytical methods. Although, only homogeneous and isotropic systems have been discussed in this paper, this method can be easily extended to inhomogeneous and anisotropic cases as well. In general, the technique is emerging as a suitable numerical tool for designing materials for specific applications.
pp 901-908 September 1997 India-Japan Seminar On New Materials
We present a brief overview of computational studies of elastic stress effects during precipitation of coherent particles. These studies include those which employ a continuum model, an atomistic (microscopic) model, and a field model. Their relative merits and limitations are discussed. Also, elastic stress effects during precipitation are presented in order to highlight the capabilities of the different computational techniques used in these studies.
Volume 42 | Issue 5
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