• Volume 22, Issue 5

      August 1999,   pages  827-936

    • Foreword

      Masao Doyama P Rama Rao

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    • Computer modelling and simulation in materials science

      N Ramakrishnan P Ramarao

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      The paper presents a brief overview of different types of modelling and simulation along with the distinguishing features between the two. Spatial as well as temporal size scales with a special reference to multiscale modelling are explained with illustrations. The paper includes a discussion on numerical experiments and their validation based on the authors’ work on FEM simulation of crack-tip blunting during ductile fracture. Attention is drawn to the use of a 3P technique involving integrated simulation of deformation (property) at microstructural level, the process at macroscopic level and the performance at the product level.

    • Crystal growth study using combination of molecular dynamics and Monte Carlo methods

      Masao Doyama

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      Molecular dynamics method although provides details of energies of the system as a function of time, is not suited to simulate the processes involving activation processes. Therefore, we attempted to combine the molecular dynamics and Monte Carlo methods. Using molecular dynamics, the energies of the system were calculated which were subsequently combined with Monte Carlo method using random numbers, epitaxial growth of (111) plane of copper, silver, and gold. While surface adsorption and surface diffusion for copper, silver, and gold were simulated by use of molecular dynamics method, the relation between the growth rate of thin films and the packing density of atoms were obtained using Monte Carlo simulation. Thus, by combining the results of the molecular dynamics method and the Monte Carlo method the growth process of thin films at elevated temperatures were obtained, which is too tedious to be calculated by molecular dynamics alone.

    • Computer simulation studies of Ar clusters

      P Padma Kumar K J Rao

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      Results of the solid-liquid transition of Ar13 cluster in a spherically symmetric external potential have been presented. The transition temperature is observed to show an elevation with pressure. The broadening of the heat capacity peaks indicate the transition becoming more diffused with pressure. The icosahedral structure of the cluster remains unaltered under pressure. Ar55 cluster has also been studied by similar approach. A possible connection between glass transition phenomenon and melting of clusters under pressure has been examined.

    • Integrated computational chemistry system for catalysts design

      S Salai Cheettu Ammal Seiichi Takami Momoji Kubo Akira Miyamoto

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      The understanding of valuable catalytic and adsorptive properties of heterogeneous catalysts at atomic and electronic levels is essential for the design of novel catalysts. Computer simulation studies can significantly contribute to provide a rational interpretation of the observed experimental results and suggest modification of new catalysts. Our recent work on the application of integrated computer simulation methods to investigate the structure and catalytic properties of solid surfaces including zeolites, transition metals and their oxides have been reviewed in this paper. We have emphasized the effectivity and applicability of integrated computer simulation system to solve the problems in a variety of targets of industrial and academic importance.

    • The ubiquitous icosahedral B12 in boron chemistry

      E D Jemmis M M Balakrishnarajan

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      Though boranes exhibit a wide variety of polyhedral structures, all the three polymorphs of elemental boron essentially contain icosahedral B12 units as the predominant building block in their unit cell. Theoretical and experimental studies on boranes show that the icosahedral arrangement leads to most stable boranes and borane anions. This paper attempts to explain the phenomenal stability associated with the icosahedral B12 structure. Using fragment molecular orbital theory, the remarkable stability of B12H122− amongcloso boranes are explained. The preferential selection icosahedral B12 unit by elemental boron is explained by improvising a contrived B84 sub-unit of the β-rhombohedron, the most stable polymorph. This also leads to a novel covalent way of stuffing fullerenes with icosahedral symmetry.

    • A novela-Si : H solar cell designed by two-dimensional device simulation

      Kaori Miyazaki Hiroshi Fujioka Masaharu Oshima Hideomi Koinuma

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      The performance of a novel hydrogenated amorphous silicon (a-Si : H) solar cell that utilizes the field effect (field effect solar cell (FESC)) has been investigated theoretically. The analysis has been done for bothp- andn-channel FESCs, employing a two-dimensional device simulator which is based on current continuity and Poisson equations. The calculation predicted that bothn-channel andp-channel FESCs could offer improved conversion efficiency as compared with the conventional-dopedp-i-n solar cell with equal qualityi (intrinsic)-layer. The observed improvement can be mainly attributed to the quantum-efficiency increase for high-energy photons. Cell parameters that may affect the energy conversion efficiency of FESCs have also been evaluated with the simulator.

    • Calculation of friction coefficient of a solid-liquid interface via a non-equilibrium molecular dynamics simulation

      Sundaram Balasubramanian Christopher J Mundy

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      The problem of characterizing a fluid flow near a solid surface is considered. The interface parameters are the friction coefficient and the hydrodynamic location of the interface. A method to obtain quantitatively these parameters from a non equilibrium molecular dynamics (NEMD) simulation is presented. Results for the flow of a soft sphere fluid confined between two corrugated surfaces are presented.

    • Computational condensed matter physics

      B K Godwal

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      In the high pressure laboratory at BARC, we are pursuing a program to study the behaviour of materials under static and dynamic pressures. Theoretical component has been an integral part for guiding and interpreting new experiments. The initial phase of such efforts was devoted to the development of equation of state models at arbitrary temperatures and matter densities. With the advent of diamond anvil cell device and the simultaneous provision for laser heating of the compressed samples for static high pressure studies, and with the improvements of the diagnostic techniques in dynamic shock methods, the focus of our studies switched over to the predictions and interpretations of phase transitions. Often these efforts have led to intense experimental studies and sometimes helped in resolving the controversies in data. We adopted the first principles electronic structure calculations for high pressure studies. Our work on the electronic topological transition in zinc led to many experimental and theoretical investigations. The results of electronic structure changes in similar metal cadmium shall be compared with existing understanding in Zn under pressure. Our studies on Nb and other compounds like intermetallics and borocarbides have revealed interesting electronic structure changes under pressure. However, the electronic structure based investigations of structural stabilities at high pressures involve tedious trial and error effort, which is avoided in theab initio molecular dynamics simulations. The current status of our efforts in the use of this technique is illustrated with the example of quasicrystal based clusters.

    • Design of hydrogen storage alloys with the aid of molecular orbital method

      H Yukawa M Takagi M Morinaga

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      The electronic structures of hydrogen storage alloys are calculated by the DV-Xα molecular orbital method. The results revealed that hydrogen interacts more strongly with hydride-non-forming elements, B, (e.g. Ni, Mn, Fe) than hydride-forming elements, A, (e.g. La, Zr, Ti, Mg) in every hydrogen storage alloy, despite there being a larger affinity of A elements for hydrogen than B elements in the binary metal-hydrogen system. The presence of such a metal-hydrogen interaction is a characteristic of hydrogen storage alloys. Furthermore, the A/B compositional ratio of hydrogen storage alloys can be understood in terms of a simple parameter, 2Bo(A-B)/[Bo(A-A)+Bo(B-B)], where the Bo(A-B), Bo(A-A) and Bo(B-B) are the bond orders between atoms given in the parentheses.

    • Numerical simulations of fracture initiation in ductile solids under mode I, dynamic loading

      R Narasimhan S Basu

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      In this paper, an overview of some recent computational studies by the authors on ductile crack initiation under mode I, dynamic loading is presented. In these studies, a large deformation finite element procedure is employed along with the viscoplastic version of the Gurson constitutive model that accounts for the micro-mechanical processes of void nucleation, growth and coalescence. A three-point bend fracture specimen subjected to impact, and a single edge notched specimen loaded by a tensile stress pulse are analysed. Several loading rates are simulated by varying the impact speed or the rise time and magnitude of the stress pulse. A simple model involving a semi-circular notch with a pre-nucleated circular hole situated ahead of it is considered. The growth of the hole and its interaction with the notch tip, which leads to plastic strain and porosity localization in the ligament connecting them, is simulated. The role of strain-rate dependence on ductile crack initiation at high loading rates, and the specimen geometry effect on the variation of dynamic fracture toughness with loading rate are investigated.

    • Canab initio simulation really predict properties of materials prior to experimental

      Yoshiyuki Kawazoe

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      In this paper we have presented all-electron full-potentialab initio simulation method with introduction of mixed-basis, and have cited several typical examples which indicate that it is possible to predict properties of materials prior to experimental. Based on theab initio calculation of the total energy, cluster variation, and direct methods function, it is possible to bridge the limited scheme of theab initio treatment to real complex materials. Furthermore, to overcome the limited computer power, we have developed parallel processing codes and tested their efficiencies as well.

    • Chitin and chitosan fibres: A review

      M N V Ravi Kumar

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      Chitin is the most abundant natural amino polysaccharide and estimated to be produced annually almost as much as cellulose. It has become of great interest not only as an underutilized resource, but also as a new functional material of high potential in various fields and the recent progress in chitin chemistry is quite noteworthy. The purpose of this review is to take a closer look at fibres made of chitin and its derivatives. Based on the current research and existing products, some new and futuristic approaches, in the development of novel fibres and their applications have been thoroughly discussed.

    • Magnetization and initial permeability studies of Nd3+ substituted Zn-Mg ferrite system

      B P Ladgaonkar C B Kolekar A S Vaingankar

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      Polycrystalline ferrites, ZnxMg1−xFe2−yNdyO4 (x=0·00, 0·20, 0·40, 0·60, 0·80 and 1·00;y=0·00, 0·05 and 0·10), were prepared by standard ceramic method. The compositions, on characterization by X-ray diffraction, showed formation of single phase cubic spinels. Magnetic study of the compositions showed increase in magnetic moment,nβ, with Zn2+ concentration up tox=0·4 and a decrease thereafter. This was attributed to the existence of canted spin. The compositions forx=0·8 and 1·0 showed paramagnetic behaviour at and above room temperature. The substitution of Nd3+ ion caused reduction in the magnetic moment and Curie temperatures. Substituted Nd3+ ion showed its occupancy on octahedral (B) site. The dependence of the initial permeability was studied in the temperature range 298 K-700 K. This μi-T curve reflects the positive temperature coefficient of initial permeability for the compositionsx≤0·4 andy=0·00. On substitution of Nd3+ ion (i.e.y=0·05 and 0·10), the μi-T curves flatten, showing almost temperature independence of initial permeability. This is explained by positive contribution to the anisotropy constant,K1, by substituted rare-earth, Nd3+ ion.

    • Optical and electrical properties of zinc oxide films prepared by spray pyrolysis

      Benny Joseph K G Gopchandran P K Manoj Peter Koshy V K Vaidyan

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      Zinc oxide thin films were prepared on glass substrates from an aqueous solution of zinc acetate by spray pyrolysis. These films were characterized using X-ray diffraction, scanning electron microscopy and optical transmission. The films were highly transparent to the visible radiation and electrically conductive. Films deposited at optimum conditions exhibited a resistivity of 3·15×10−3 Ωm along with a transmittance of 98% at 550 nm.

    • Photoelectrochemical investigation on spray depositedn-CdIn2S4 thin films

      K Y Rajpure V L Mathe C H Bhosale

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      Polycrystalline thin films ofn-CdIn2S4 have been spray deposited onto amorphous and fluorinedoped tin oxide (FTO) coated glass substrates at the optimized substrate temperature of 380°C. The films were characterized by X-ray diffraction (XRD) and optical absorption studies. XRD studies revealed that the films were polycrystalline with spinel cubic structure. The optical absorption studies showed the band gap energy to be 2·14 eV. Photoelectrochemical (PEC) investigations were carried out using cell configurationn-CdIn2S4/1 M NaOH+1 M Na2S+1 M S/C. Using Butler model, the optical band gap and minority carrier diffusion length (LP) were found to be 2·22 eV and 0·07 µm, respectively. Gartner’s model was used to calculate the minority carrier diffusion length and the donor concentration (ND) for CdIn2S4 films at three different wavelengths.ND was found to be of the order of 1016 cm−3.

    • Fatigue crack growth behaviour of gamma base titanium aluminides under different loading conditions

      R Gnanamoorthy Y Mutoh

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      Static and cyclic fatigue crack growth behaviour of gamma base titanium aluminides with three different microstructures were investigated. Influence of cyclic test frequency on fatigue crack growth behaviour was also studied at room temperature under a controlled humidity condition. The crack growth behaviour both under static and cyclic loading was strongly influenced by the microstructure. The threshold stress intensity and crack growth behaviour under cyclic loading were much inferior than that under static loading indicating the ‘true-cyclic fatigue’ effect exhibited in gamma base titanium aluminides. No significant effect of test frequency on the crack growth behaviour was observed for the equiaxed and duplex microstructure materials.

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