• Volume 17, Issue 3

      June 1994,   pages  201-306

    • Single-crystal neutron diffraction study of ferroelectric Ba0·92Ca0·08TiO3

      H Rajagopal P U M Sastry A Sequeira P Ramasamy

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      Crystal structure of BaTiO3 doped with 8% Ca2+ is refined using single-crystal neutron diffraction data and it is shown that the doped Ca2+ ion substitutes only at the Ba sites. The refined cell (P4 mm) parameters area=b=3·982(3) Å,c=4·003(3) Å with a finalR value of 0·02 (onF). Existence of multiple domains in the crystal is ruled out based on refinement with multidomain model.

    • X-ray data for the substitutional solid solution of the binary system ammonium dihydrogen phosphate with boric acid

      M L N Madhu Mohan C Haranadh

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      X-ray powder diffraction data for the binary system ammonium dihydrogen phosphate with boric acid is studied and it is inferred that the binary belongs to tetragonal system and forms a substitutional solid phase. On the basis of the space group the binary is predicted to exhibit electrooptic property along with piezoelectric and electrostriction properties. It is proposed that the phosphorus atom of ammonium dihydrogen phosphate is substituted by the boron atom of boric acid.

    • Heterogeneous doping effects in KCl-Al2O3 composites

      Ashok Kumar K Shahi

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      The composite solid electrolyte systems KCl-Al2O3 have been synthesized by conventional as well as by solution casting methods and have been characterized by means of complex impedance analysis, XRD and DTA techniques. The samples prepared by solution casting method show about an order of magnitude higher conductivity than those prepared by the conventional method. The enhanced conductivity is attributed to the excess cation vacancies generated in the space charge region of the matrix phase surrounding the Al2O3 particles as a consequence of stabilization of cations at the dispersoid surface due to internal adsorption. The XRD and DTA analyses show that no new phase is present. Macroscopically, the compositional and particle size dependence of the conductivity are adequately explained on the basis of random resistor network percolation model.

    • Thin films of boron nitride grown by CVD

      A Ratna Phani

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      For the first time, thin films of boron nitride were deposited by chemical vapour deposition on to polished silicon and other metal substrates using the inorganic compound H3BNH3 (aminodiborane) and ammonia as carrier gas. The substrate temperature was varied from 400 to 600°C. The films were chemically inert and adherent to the substrates. The FTIR spectrum of the film showed B-N-B absorption at 800 cm−1, B-N stretching at 1056 cm−1, and also a weak absorption at 1340cm−1 corresponding to B-N-B bending vibration. Deposited films also exhibited X-ray diffraction pattern with interplanar spacing with (002) plane of hexagonal boron nitride.

    • Calorimetric investigations on As-Te-Se and Ge-As-Se glasses

      K Nandakumar J Philip

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      Results of calorimetric investigations performed on two groups of glasses, viz. As-Te-Se and Ge-As-Se, are described. The glass transition temperatureTg and specific heat at constant pressureCp of these two families of glasses have been determined using a differential scanning calorimeter. The composition dependence of glass transition temperature and variation of heat capacity during transition are discussed.

    • Optical constants of CdS thin films prepared by spray pyrolysis

      Sunny Mathew K P Vijayakumar

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      CdS thin films were prepared by spray pyrolysis techniques. Variable angle spectroscopic ellipsometry was used for optical constant calculations. Multiple angle measurements were taken in the most sensitive angle of incidence region. The sensitive regions of angle of incidence were obtained theoretically using 3-dimensional graph ofδψ andδΔ. Real partn and imaginary partk of the complex refractive index of the samples were calculated in the wavelength range 470–650 nm, taking into account surface roughness. Bruggeman’s effective medium approximation is used for analysis of the surface rough layer of the thin films.

    • Electric and dielectric properties of solution-gas interface-grown PbS films

      P S Nikam K A Pathan

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      Dielectric properties andI–V characteristics of solution-gas interface-formed PbS thin-film capacitors (Al/PbS/Al) of various thicknesses have been studied in the frequency range 10-106 Hz at various temperatures (300–443 K). Current-voltage (I–V) characteristics show space-charge-limited conduction. Dielectric constant (ε) increases with increasing film thickness and temperature and decreases with increase of frequency. The loss factor (tanδ) peaks observed in tanδ vs frequency and tanδ vs temperature reveal relaxation effect from dipolar orientation. These maxima shift to higher-temperature region with increasing frequency. The large increase in capacitance (C) and dielectric constant (ε) towards low-frequency (f) region indicates the possibility of an interfacial polarization mechanism in this region.

    • Electrical behaviour of lanthanum- and cobalt-doped strontium stannate

      O M Parkash K D Mandal C C Christopher M S Sastry Devendra Kumar

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      Electrical properties of lanthanum- and cobalt-doped strontium stannate, SrSnO3, have been studied as a function of temperature. All the compositions investigated have cubic structure. Measurement of Seebeck coefficient in one of the cobalt-doped SrSnO3 sample shows that it exhibitsp-type conductivity. Results of AC conductivity measurements show that conduction occurs by hopping of charge carriers in lanthanum-doped samples and forx=0·01 in cobalt-doped sample. Inx=0·05 sample in cobalt-doped system, conduction seems to occur in extended states.

    • A numerical investigation of ductile fracture initiation in a high-strength low-alloy steel

      R Narasimhan S V Kamat

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      In this work, static and drop-weight impact experiments, which have been conducted using three-point bend fracture specimens of a high-strength low-alloy steel, are analysed by performing finite-element simulations. The Gurson constitutive model that accounts for the ductile failure mechanisms of microvoid nucleation, growth and coalescence is employed within the framework of a finite deformation plasticity theory. Two populations of second-phase particles are considered, including large inclusions which initiate voids at an early stage and small particles which require large strains to nucleate voids. The most important objective of the work is to assess quantitatively the effects of material inertia, strain rate sensitivity and local adiabatic temperature rise (due to conversion of plastic work into heat) on dynamic ductile crack initiation. This is accomplished by comparing the evolution histories of void volume fraction near the notch tip in the static analysis with the dynamic analyses. The results indicate that increased strain hardening caused by strain rate sensitivity, which becomes important under dynamic loading, plays a benign role in considerably slowing down the void growth rate near the notch tip. This is partially opposed by thermal softening caused by adiabatic heating near the notch tip.

    • Mesogenic β-diketones and metallonematogens

      Archana Ghode Uma Shivkumar B K Sadashiva

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      Several new liquid-crystalline materials have been synthesized and their properties investigated. These belong to a series ofβ-diketones containing different terminal substituents. Mesophases of the smectic C, smectic A and nematic types have been observed in theseβ-diketones. Using these ligands many copper(II) and palladium(II) complexes have also been synthesized and their mesogenic behaviour characterized. Microscopic observations and differential scanning calorimetric measurements show that these chelates exhibit the nematic phase. The DSC data also indicate that the nematic phase of the ligands and complexes is associated with very low clearing transition enthalpies. All the metallonematogens reported are enantiotropic in nature.

    • Characterization of coprecipitated ferroelectric Bi2 VO5·5

      K V R Prasad G N Subbanna K B R Varma

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      Fine powders of orthorhombic bismuth vanadate (Bi2 VO5·5) have been synthesized by coprecipitation method. Powder X-ray diffraction and electron microscopic techniques have been used to characterize these samples. The formation of the monophasic Bi2 VO5·5 was confirmed. The compacted powders sintered at 1070K have been characterized for their dielectric properties as a function of both temperature (300–900 K) and frequency (100 Hz–10 MHz) and found to be superior to those obtained by the conventional solid-state reaction route.

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