Strong shock approximation in the new theory of shock dynamics

This paper investigates the propagation of a strong shock into an inhomogeneous medium using the new theory of shock dynamics. The equations are simple to solve and involve no trial-and-error method commonly used in this case. The results compare favourably with earlier results obtained in the case of self-similar flows, which arise as a special case of this theory.

The nature of charge ordering in rare earth manganates and its strong dependence on the size of the A-site cations

Charge ordering in rare earth manganates of the type Ln(0.5)A(0.5)MnO(3) (Ln = rare earth, A = alkaline earth) is highly sensitive to the average radius of the A-site cations, [r(A)]. Tn the small [r(A)] regime (e.g., Y0.5Ca0.5MnO3), charge ordering occurs in the paramagnetic state, the transformation to an antiferromagnetic state occurring at still lower temperatures. At moderate [r(A)] values (e.g., Nd0.5Sr0.5MnO3), a ferromagnetic metallic state transforms to a charge-ordered antiferromagnetic state with cooling. These two distinct types of charge ordering and associated properties are explained in terms of the variation of the exchange couplings J(FM) and J(AFM) with [r(A)] and the invariance of the single-ion Jahn-Teller energy with [r(A)]. A qualitative temperature-[r(A)] phase diagram, consistent with the experimental observations, has been constructed to describe the properties of the manganates in the different [r(A)] regimes. (C) 1997 Academic Press.

Solutions of cubic equations in quadratic fields

Let K be any quadratic field with O-K its ring of integers. We study the solutions of cubic equations, which represent elliptic curves defined over Q, in quadratic fields and prove some interesting results regarding the solutions by using elementary tools. As an application we consider the Diophantine equation r + s + t = rst = 1 in O-K. This Diophantine equation gives an elliptic curve defined over Q with finite Mordell-Weil group. Using our study of the solutions of cubic equations in quadratic fields we present a simple proof of the fact that except for the ring of integers of Q(i) and Q(root 2), this Diophantine equation is not solvable in the ring of integers of any other quadratic fields, which is already proved in [4].

Studies on the optimisation of unbalanced magnetron sputtering cathodes

The optimisation is reported on the design of unbalanced magnetron (UBM) sputtering cathodes. For the study, a planar circular cathode backed by a double-coil electromagnet (compatible for a 100 mm diameter target) was developed. The variation of the structure and strength of the magnetic field in front of the target was investigated for different current combinations in the electromagnetic coils, and its effect on the sputtering process was analysed. The observations on the magnetic field geometry revealed some interesting features, such as the balancing point of the fields along the axis (null-point), and the zero axial region over the target surface (B-z = 0 ring). The positions of both could be controlled by adjusting the ratio of the electric current in the coils. The magnetic field null-point could be used as a reference for the region of homogeneous film growth. The B-z = 0 ring was the location where the glow discharge concentrated (or where the maximum target erosion occurred). The diameter of the ring determined the area covered by the discharge and thus the sputtering efficiency. The optimum substrate position can be fixed according to the position of the null-point and optimisation of sputtering can be achieved by adjusting the diameter of the B-z = 0 ring. The results of this study should be helpful in the designing of an ideal UBM using permanent magnets as well as electromagnets. (C) 1999 Elsevier Science Ltd. All rights reserved.

Dehydrophenylalanine zippers: strong helix-helix clamping through a network of weak interactions

A decapeptide Boc-L-Ala-(DeltaPhe)(4)-L-Ala-(DeltaPhe)(3)-Gly-OMe (Peptide I) was synthesized to study the preferred screw sense of consecutive alpha,beta-dehydrophenylalanine (DeltaPhe) residues. Crystallographic and CD studies suggest that, despite the presence of two L-Ala residues in the sequence, the decapeptide does not have a preferred screw sense. The peptide crystallizes with two conformers per asymmetric unit, one of them a slightly distorted right-handed 3(10)-helix (X) and the other a left-handed 3(10)-helix (Y) with X and Y being antiparallel to each other. An unanticipated and interesting observation is that in the solid state, the two shape-complement molecules self-assemble and interact with an extensive network of C-H...O hydrogen bonds and pi-pi interactions, directed laterally to the helix axis with amazing regularity. Here, we present an atomic resolution picture of the weak interaction mediated mutual recognition of two secondary structural elements and its possible implication in understanding the specific folding of the hydrophobic core of globular proteins and exploitation in future work on de novo design.

Strong ground motion at Bhuj city during the Kutch earthquake

In the absence of near field strong motion records, the level of ground motion during the devastating 26 January 2001 earthquake has to be found by indirect means. For the city of Bhuj, three broad band velocity time histories have been recorded by India Meteorological Department. In this paper these data are processed to obtain an estimate of strong ground motion at Bhuj. It is estimated that the peak ground acceleration at Bhuj was of the order of 0.38 g. Ground motion in the surrounding region is indirectly found using available spectral response recorder (SRR) data. These instrument-based results are compared with analytical results obtained from a half-space regional model.

An appraisal of electric automobile power sources

Road transportation, as an important requirement of modern society, is presently hindered by restrictions in emission legislations as well as the availability of petroleum fuels, and as a consequence, the fuel cost. For nearly 270 years, we burned our fossil cache and have come to within a generation of exhausting the liquid part of it. Besides, to reduce the greenhouse gases, and to obey the environmental laws of most countries, it would be necessary to replace a significant number of the petroleum-fueled internal-combustion-engine vehicles (ICEVs) with electric cars in the near future. In this article, we briefly describe the merits and demerits of various proposed electrochemical systems for electric cars, namely the storage batteries, fuel cells and electrochemical supercapacitors, and determine the power and energy requirements of a modern car. We conclude that a viable electric car could be operated with a 50 kW polymer-electrolyte fuel cell stack to provide power for cruising and climbing, coupled in parallel with a 30 kW supercapacitor and/or battery bank to deliver additional short-term burst-power during acceleration.

Electrical properties of La-graded heterostructure of Pb1?xLaxTiO3 thin films

La-graded heterostructure films were prepared by sol-gel technique on platinum substrates and electrical properties of these films were compared with those of conventional thin films of similar compositions. X-ray diffraction results indicate the pure perovskite polycrystalline structure of these films. Atomic Force Microscopy analysis revealed a finer grain size and relatively lower surface roughness. Relatively higher values of Pm and Pr (69 and 38 ?C cm?2, respectively) and excellent dielectric properties with lower loss (K=1900, tan ?=0.035 at 100 kHz) were observed for La-graded heterostructure films. Also lower leakage current density (not, vert, similar2.5 nA cm?2) and a higher onset field (not, vert, similar50 kV cm?1) of space charge conduction indicated higher breakdown strength and good leakage current characteristics. The ac electric field dependence of the permittivity at sub-switching fields was analyzed in the framework of the Rayleigh dynamics of domain walls. The estimated irreversible domain wall displacement contribution to the total dielectric permittivity was 17 and 9% for conventional 15 at.% La doped PbTiO3 and La-graded heterostructure films, respectively. The improved dielectric and polarization behavior of La-graded heterostructure films may be attributed to homogenous dopant distribution compared to the conventional 15 at.% La doped PbTiO3 films.

A comparative study of Pt/CeO2 catalysts for catalytic partial oxidation of methane to syngas for application in fuel cell electric vehicles

In the framework of a project aimed at developing a reliable hydrogen generator for mobile polymer electrolyte fuel cells (PEFCs), particular emphasis has been addressed to the analysis of catalysts able to assure high activity and stability in transient operations (frequent start-up and shut-down cycles). In this paper, the catalytic performance of 1 at.% Pt/ceria samples prepared by coprecipitation, impregnation and combustion, has been evaluated in the partial oxidation of methane. Methane conversion and hydrogen selectivity of 96 and 99%, respectively, associated with high stability during 100h of reaction under operative conditions (start-up and shut-down cycles), have been obtained. (C) 2002 Elsevier Science B.V. All rights reserved.

Motion of a charged particle in superposed Heliotron and biconical cusp fields

We have studied the behaviour of a charged particle in an axially symmetric magnetic field having a neutral point, so as to find a possibility of confining a charged particle in a thermonuclear device. In order to study the motion we have reduced a three-dimensional motion to a two-dimensional one by introducing a fictitious potential. Following Schmidt we have classified the motion, as an ‘off-axis motion’ and ‘encircling motion’ depending on the behaviour of this potential. We see that the particle performs a hybrid type of motion in the negative z-axis, i.e. at some instant it is in ‘off-axis motion’ while at another instant it is in ‘encircling motion’. We have also solved the equation of motion numerically and the graphs of the particle trajectory verify our analysis. We find that in most of the cases the particle is contained. The magnetic moment is found to be moderately adiabatic.

Electric Field Directed Growth of Molecular Wires of Charge Transfer Molecules on Prefabricated Metal Electrodes

Molecular wires of charge transfer molecules were formed by co-evaporating the 7 7 8 8-Tetracyanoquinodimethane [TCNQ] (acceptor) and Tetrathiafulvalene [TTF] (donor) molecules across prefabricated metal electrodes. Molecular wires of TTF TCNQ were also formed by evaporating single complex of TTF:TCNQ across prefabricated metal electrodes The prefabricated metal electrodes were made using electron beam lithography on SiO2 and glass cover slip substrates. Even though TTF: TCNQ wires grown from both co-evaporation and evaporation techniques show semiconductor like behavior in temperature dependence of resistance they show different activation energies due the difference in stoichiometry of TTF and TCNQ.

Vortex dynamics and upper critical fields in ultrathin Bi films

Current-voltage (I-V) characteristics of quench condensed, superconducting, ultrathin Bi films in a magnetic field are reported. These I-V's show hysteresis for all films, grown both with and without thin Ge underlayers. Films on Ge underlayers, close to superconductor-insulator transition, show a peak in the critical current, indicating a structural transformation of the vortex solid. These underlayers, used to make the films more homogeneous, are found to be more effective in pinning the vortices. The upper critical fields (B-c2) of these films are determined from the resistive transitions in perpendicular magnetic field. The temperature dependence of the upper critical field is found to differ significantly from Ginzburg-Landau theory, after modifications for disorder.

A Macroscopic Model for First Return Stroke of Lightning

A lightning return stroke model for a downward flash is proposed. The model includes underlying physical phenomena governing return stroke evolution, namely, electric field due to charge distributed along the leader and cloud, transient enhancement of series channel conductance at the bridging regime, and the nonlinear variation of channel conductance, which supports the return stroke current evolution. Thermal effects of free burning arc at the stroke wave front and its impact on channel conductance are studied. A first-order arc model for determining the dynamic channel conductance along with a field-dependent conductivity for corona sheath is used in the model. The model predicts consistent current propagation along the channel with regard to current amplitude and return stroke velocity. The model is also capable of predicting the remote electromagnetic fields that are consistent with the experimental observations.

Potential and Electric Field Profiles for Transmission line Insulators

Transmission of bulk power at high voltages over very long distances has become very imperative. At present, throughout the globe, this task has been mostly performed by overhead transmission lines. The dual task of mechanically supporting and electrically isolating the live phase conductors from the support tower is performed by string insulators. Whether in clean condition or under polluted conditions, the electrical stress distribution along the insulators governs the possible flashover, which is quite detrimental to the system. However, a reliable data on stress distribution in commonly employed string insulators are rather scarce. Considering this, the present work has made an attempt to study accurately, the field distribution in 220 kV strings for six different types of porcelain/ceramic insulators (Normal and Antifog discs) used for high voltage transmission. The surface charge simulation method is employed for the required field computation. Voltage and electric stress distribution is deduced and compared across different types of discs. A comparison on normalised surface resistance, which is an indicator for the stress concentration under polluted condition, is also attempted.