Debye temperatures of uranium chalcogenides from their lattice dynamics

Phonon dispersion relations in uranium chalcogenides have been investigated using a modified three-body force shell model. From the phonon frequencies, their Debye temperatures are evaluated. Further, on the basis of the spin fluctuation in the heavy fermion uranium compounds, UPt₃ and UBe₁₃, the possible superconducting transition temperatures of chalcogenides are theoretically predicted. The Tc values are in the same range as of those in UPt₃ and UBe₁₃.     

Multicomponent polyolefin blends with ordered and disordered microstructures

A rich variety of microstructures have been obtained in multicomponent polyolefin blends comprising two incompatible homopolymers and a block copolymer. The stability of the microstructures is due to the surfactant-like character of the block copolymer. Due to the simplicity of their chemical structure (empirical formula CH₂ for all components), interactions between polyolefins are dispersive in nature. This simplicity enables the development of predictive theories for thermodynamics and microstructure formation. The agreement between recent experimental results and theory is encouraging. In many respects, these systems are polymeric analogs of oil/water/detergent systems. It is hoped that multicomponent polyolefin blends will serve as model materials for understanding the general principles of surfactancy and microstructure formation.     

Thermodynamic modelling of phase equilibria in Al-Ga-P-As system

A generalized thermodynamic expression of the liquid Al-Ga-P-As alloys is used in conjunction with the solid solution model in determining the solid-liquid equilibria at 1173 K and 1273 K. The liquid solution model contains thirtyseven parameters. Twentyfour of them pertain to those of the six constituent binaries, twelve refer to the specific ternary interactions. Additionally the liquid solution model also contains a specific quaternary interaction parameter. The latter has been evaluated here based on the experimental data available in the literature. The present research shows an excellent agreement between the derived and experimental values at 1173 K and 1273 K for the system. The article also presents a comparison between the evaluated values with those based on the regular solution model for the liquid alloys.     

Superconductivity and electrical resistivity in alkali metal doped fullerides: Phonon mechanism

We consider a two- peak model for the phonon density of states to investigate the nature of electron pairing mechanism for superconducting state in fullerides. We first study the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate phonon frequencies from the dynamical matrix for the intermolecular alkali- C₆₀ phonons. Electronic parameter as repulsive parameter and the attractive coupling strength are obtained within the random phase approximation. Transition temperature,T _c, is obtained in a situation when the free electrons in lowest molecular orbital are coupled with alkali-C₆₀ phonons as 5 K, which is much lower as compared to reportedT _c (≈ 20 K). The superconducting pairing is mainly driven by the high frequency intramolecular phonons and their effects enhance it to 22 K. To illustrate the usefulness of the above approach, the carbon isotope exponent and the pressure effect are also estimated. Temperature dependence of electrical resistivity is then analysed within the same model phonon spectrum. It is inferred from the two- peak model for phonon density of states that high frequency intramolecular phonon modes play a major role in pairing mechanism with possibly some contribution from alkali-C₆₀ phonon to describe most of the superconducting and normal state properties of doped fullerides.     

Double stripe reconstruction of the Pt(111) surface

We have studied the reconstruction of the Pt(111) surface theoretically, using a 2D generalization of the Frenkel-Kontorova model. The parameters in the model are obtained by performingab initio density functional theory calculations. The Pt(111) surface does not reconstruct under normal conditions but experiments have shown that there are two ways to induce the reconstruction: by increasing the temperature, or by depositing adatoms on the surface. The basic motif of this reconstruction is a ‘double stripers with an increased surface density and alternatinghcp andfcc domains, arranged to form a honeycomb pattern with a very large repeat distance of 100–300 Å. In this paper, we have studied the ‘double stripe’ reconstruction of the Pt(111) surface. In agreement with experiment, we find that it is favourable for the surface to reconstruct in the presence of adatoms, but not otherwise.     

Variations in first principles calculated defect energies in GaAs and their effect on practical predictions

There is an abundant literature on calculations of formation and ionization energies of point defects in GaAs. Since most of these energies, especially the formation energies, are difficult to measure, the calculations are primary means of obtaining their values. However, based on the assumptions of the calculations, the reported values differ greatly among the various calculations. In this paper we discuss the sources of errors and their impact on practical predictions valuable in GaAs device fabrication. In particular, we have compared a large set of computed energies and selected the most appropriate values. Then, in the context of GaAs material quality, we investigated the impact of errors in calculation of formation energies on the performance of the GaAs substrate for device fabrication. We find that in spite of the errors inherent in ab initio calculations, it is possible to correctly predict the behaviour of GaAs substrate.     

Rings, chains and planes: Variation ofT g with composition in chalcogenide glasses

We propose a microscopic, phenomenological model for the decrease in the viscosity observed at glass transition. Our model is primarily applicable to chalcogenide glasses. According to this model, the decrease in the viscosity at glass transition is mainly due to the breaking of the Van der Waals bonds in the chalcogenides. Using this model, we derive a relationship between the glass transition temperature,T _g ,and the molar volume V _m.The validity of this relation is checked using experimental data available in the literature for two binary systems (Ge-Se and As-S) and a pseudo-binary system (As ₄₀ Se _x Te _60-x .     

Effect of pressure on the phonon properties of europium chalcogenides

Lattice vibrational properties of europium chalcogenides have been investigated at high pressure by using a simple lattice dynamical model theory viz. the three-body force rigid ion model (TRIM) which includes long range three-body interaction arising due to charge transfer effects. The dispersion curves for the four Eu-chalcogenides agree reasonably well with the available experimental data. Variation of LO, TO, LA and TA phonons with pressure have also been studied at the symmetry points of the brillouin zone (BZ) for Euchalcogenides for the first time by using a lattice dynamical model theory. We have also calculated the one phonon density of states and compared them with the first order Raman scattering results. The calculation of one phonon density of states for Eu-chalcogenides has also been extended up to the phase transition pressure. We observed a pronounced shift in phonon spectrum as pressure is increased. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore and respectfully dedicated to Prof. R K Singh on his 60th birthday     

Microstructure and electrical conductivity of Al-SiCp composites produced by spray forming process

Al- SiC_p composites have been synthesized by spray forming process with variation in particle flow rate, size of reinforcement particles and their volume fraction. The microstructure of composites and their electrical conductivity have been investigated. The results showed a uniform dispersion of large size particulate phase in the matrix of the primary α- phase with its equiaxed grain morphology. However, clustering of small size particles was observed at the grain boundary and grain junctions. The grain size of the composite materials was observed to be lower than that of the base Al- alloy. The composite materials invariably indicated their lower electrical conductivity compared to that of the monolithic Al- alloy. The electrical conductivity of composites decreased with increase in the volume fraction and decrease in size of the reinforcement particles. A high flow rate of particles during spray deposition resulted in a decrease in its conductivity. These results are explained in the light of thermal mismatch between the matrix and the reinforcement phases resulting in generation of high dislocation density. The droplet- particle interaction and resulting microstructure evolution during the spray deposition of the composites are discussed.     

Application of genetic algorithms to hydrogenated silicon clusters

We discuss the application of biologically inspired genetic algorithms to determine the ground state structures of a number of Si-H clusters. The total energy of a given configuration of a cluster has been obtained by using a non-orthogonal tight-binding model and the energy minimization has been carried out by using genetic algorithms and their recent variant differential evolution. Our results for ground state structures and cohesive energies for Si-H clusters are in good agreement with the earlier work conducted using the simulated annealing technique. We find that the results obtained by genetic algorithms turn out to be comparable and often better than the results obtained by the simulated annealing technique.     

Anisotropic behaviour of semiconducting tin monosulphoselenide single crystals

Single crystals of ternary mixed compounds of group IV-VI in the form of a series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), have been grown using direct vapour transport technique. The grown crystals were characterized by the X-ray diffraction analysis for their structural parameter determination. All the grown crystals were found to be orthorhombic. The microstructure analysis of the grown crystals reveals their layered type growth mechanism. From the Hall effect measurements Hall mobility, Hall coefficient and carrier concentration were calculated with all crystals showingp-type nature. The d.c. electrical resistivity measurements perpendicular toc-axis (i.e. along the basal plane) in the temperature range 303–453 K were carried out for grown crystals using four-probe method. The d.c. electrical resistivity measurements parallel to c-axis (i.e. perpendicular to basal plane) in the temperature range 303–453 K were carried out for the same crystals. The electrical resistivity measurements showed an anisotropic behaviour of electrical resistivity for the grown crystals. The anisotropic behaviour and the effect of change in stoichiometric proportion of S and Se content on the electrical properties of single crystals of the series, SnS_xSe_1-x (wherex = 0, 0.25, 0.50, 0.75 and 1), is presented systematically.     

Multi-scale modeling strategies in materials science—The quasicontinuum method

The problem of prediction of finite temperature properties of materials poses great computational challenges. The computational treatment of the multitude of length and time scales involved in determining macroscopic properties has been attempted by several workers with varying degrees of success. This paper will review the recently developed quasicontinuum method which is an attempt to bridge the length scales in a single seamless model with the aid of the finite element method. Attempts to generalize this method to finite temperatures will be outlined.     

High purity materials as targets for radioisotope production: Needs and challenges

Radionuclides have become powerful and indispensable tools in many endeavours of human activities, most importantly in medicine, industry, biology and agriculture, apart from R&D activities. Ready availability of radionuclides in suitable radiochemical form, its facile detection and elegant tracer concepts are responsible for their unprecedented use. Application of radioisotopes in medicine has given birth to a new branch, viz. nuclear medicine, wherein radioisotopes are used extensively in the diagnosis and treatment of variety of diseases including cancer. Artificial transmutation of an element employing thermal neutrons in a reactor or high energy particle accelerators (cyclotrons) are the routes of radioisotope production world over. Availability of high purity target materials, natural or enriched, are crucial for any successful radioisotope programme. Selection of stable nuclides in suitable chemical form as targets with desired isotopic and chemical purity are among the important considerations in radioisotope production. Mostly the oxide, carbonate or the metal itself are the preferred target forms for neutron activation in a research reactor. Chemical impurities, particularly from the elements of the same group, put a limitation on the purity of the final radioisotope product. Whereas the isotopic impurities result in the production of undesirable radionuclidic impurities, which affect their effective utilization. Isotope Group, BARC, is in the forefront of radioisotope production and supply in the country, meeting demands for gamut of radioisotope applications indigenously for over four decades now. Radioisotopes such as¹³¹I,⁹⁹Mo,³²P,⁵¹Cr,¹⁵³Sm,⁸²Br,²⁰³Hg,¹⁹⁸Au etc are produced in TBq quantities every month and supplied to several users and to Board of Radiation and Isotope Technology (BRIT). Such a large production programme puts a huge demand on the reliable sources of availability of high purity target materials which are at present mostly met through import. Availability of suitable target materials, their purity considerations and our efforts in finding sources of raw materials for sustaining the radioisotope programme are discussed here.     

Spinodal decomposition in fine grained materials

We have used a phase field model to study spinodal decomposition in polycrystalline materials in which the grain size is of the same order of magnitude as the characteristic decomposition wavelength (Xsu). In the spirit of phase field models, each grain (i) in our model has an order parameter (η _i) associated with it;η _i has a value of unity inside the ith grain, decreases smoothly through the grain boundary region to zero outside the grain. For a symmetric alloy of composition,c = 0–5, our results show that microstructural evolution depends largely on the difference in the grain boundary energies, y_gb, of A-rich (a) and B-rich (β) phases. If Y _gb ^α is lower, we find that the decomposition process is initiated with an a layer being formed at the grain boundary. If the grain size is sufficiently small (about the same as λ_sd), the interior of the grain is filled with the β phase. If the grain size is large (say, about 10λ_SD or greater), the early stage microstructure exhibits an A-rich grain boundary layer followed by a B-rich layer; the grain interior exhibits a spinodally decomposed microstructure, evolving slowly. Further, grain growth is suppressed completely during the decomposition process.     

Studies on poly (hydroxy alkanoates)/(ethylcellulose) blends

Biodegradable polymers represent one of the most significant area of research today. Among these polymers, poly (β-hydroxy butyrate co β′-hydroxy valerate) i.e. PHBV have received special attention because of their unique combination of properties. They are perfectly biocompatible, biodegradable polymers and can be processed by any conventional technique. In the present study an attempt has been made to develop the biodegradable blends of PHBV by blending them with ethyl cellulose (EC). Ethyl cellulose has been selected to monitor the biodegradation rate of PHBV and also for making the blends cost effective. The blends are thoroughly characterized for their compatibility, by the measurement of viscosity of blends and through FT-IR. Various applications of PHBV/EC blend in agriculture and pharmaceutical industries are being explored. Paper presented at the 5th IUMRS ICA98, October 1998, Bangalore.     

Estimation of CE-CVM energy parameters from miscibility gap data

The powerful framework of cluster expansion- cluster variation methods (CE- CVM) expresses alloy free energy in terms of energy (model) parameters, macroscopic variables (composition and temperature) and microscopic variables (correlation functions). A simultaneous optimization of thermodynamic and phase equilibria data using CE- CVM is critically dependent on giving good initial values of energy parameters, macroscopic and microscopic variables, respectively. No standard method for obtaining the initial values of the energy parameters is available in literature. As a starting point, a method has been devised to estimate the values of energy parameters from consolute point (miscibility gap maximum) data. Empirical relations among energy parameters, temperature (T _c), composition (x _c) andd ² T/dx² at the consolute point, have been developed using CE- CVM free energy functions for bcc and fcc structures in the tetrahedron and tetrahedronoctahedron approximations, respectively. Thus from the observed data ofT _c,x _c andd ² T/dx² in the above relations, good initial values of energy parameters can be obtained. Further, a necessary modification to the classical NR method for solving simultaneous nonlinear/transcendental equations with a double root in one variable and a simple root in the other has been presented.     

Simple explanation for the reentrant magnetic phase transition in Pr<Subscript>0.5</Subscript>Sr<Subscript>0.41</Subscript>Ca<Subscript>0.09</Subscript>MnO<Subscript>3</Subscript> perovskite

The reentrant magnetic phase transition in Pr _0.5Sr_0.41Ca_0.09MnO₃ perovskite is explained using the Ising spin model on the square lattice with mixed ferromagnetic and antiferromagnetic exchange interactions. It is shown using numerical calculations that this effect is strongly affected by the external magnetic field and lattice disorder.     

Study of electrical properties of glassy Se100- xTexalloys

Temperature and frequency dependence of a.c. conductivity have been studied in glassy Se_100- x _Te x(x = 10, 20 and 30) over different range of temperatures and frequencies. An agreement between experimental and theoretical results suggests that the a.c. conductivity behaviour of selenium-tellurium system (Se_100- xTex)can be successfully explained by correlated barrier hopping (CBH) model. The density of defect states has been determined using this model for all the glassy alloys. The results show that bipolaron hopping dominates over single-polaron hopping in this glassy system. This is explained in terms of lower values of the maximum barrier height for single-polaron hopping. The values of density of charged defect states increase with increase in Te concentration. This is in agreement with our previous results obtained from SCLC measurements.     

Homogenization of zinc distribution in vertical Bridgman grown Cd0.96Zn0.04Te crystals

One of the most pressing issues in the growth of high quality single crystal Cd _0.96 Zn _0.04 Te material, is to achieve homogenization of the high axial variation of Zn concentration, caused by the larger than unity segregation coefficient of Zn in CdTe. This is achieved in our crystals (i) by thermal annealing of the CdZnTe crystal, which redistributes the as grown Zn distribution by solid state diffusion of Zn (this solid state diffusion of Zn occurs at three stages (a) during the growth when the solidified crystal is near to the melting point temperature, (b) during the post growth annealing of the crystal at a high temperature and (c) during the cooldown to room temperature) and (ii) by the reduction of Zn segregation during the growth stage by enhanced convective mixing of the melt, through a proper choice of ampoule and furnace dimensions. By adopting suitable growth parameters and sufficient post growth annealing it has been possible to grow Cd _0.96 Zn _0.04 Te crystals, which have nearly 75% of their fraction within 1% Zn concentration variation.