• Volume 16, Issue 2

      April 1993,   pages  87-158

    • Processing and microstructure of non-oxide ceramic fibres

      V S R Murthy

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      Research and development efforts on high-temperature, oxidation-resistant fibres have increased over the past decade due to the demand for light-weight, stiff and strong composite materials in aerospace applications. Varieties of ‘high-performance’, continuous, non-oxide fibres with low-density, high tensile strength and tensile modulus have been developed either from organic precursors or via chemical vapour deposition for fabrication of ceramic matrix composites. Fibres derived from polymer precursors (e.g. Nicalon, Tyranno, HPZ) are small in diameter (compared to CVD monofilaments) and are ideally suited for ceramic composites. Processing, microstructural stability and mechanical properties of these newly developed SiC and Si3N4 base fibres are briefly reviewed in this paper.

    • Effect of bismuth replacement at barium site in Y-Ba-Cu-O system

      Ram Bilas

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      X-ray diffraction, electron paramagnetic resonance, microwave absorption and resistivity measurements were carried out on YBa2−xBixCu3O7 (0≤x≤0·5) superconductors in order to study the effect of bismuth on the structural and superconducting properties. A decrease in sintering temperature increased the amount of impurity phase. Transition from an orthorhombic (superconducting) phase to tetragonal structure produced no significant change in EPR zero-field signal at liquid nitrogen temperature (LNT). The non-resonance signal height decreased on higher concentration of bismuth. Sintered YBa2−xBixCu3O7 superconductor had a strong EPR zero-field signal at LNT. We have evaluated particle size from XRD and EPR studies. The average particle size was about 0·4µm.

    • Composition dependence of transition temperature in some ceramic superconductors

      R K Singh Dinesh Varshney N K Gaur A C Sharma

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      The composition dependence of transition temperature in some ceramic superconductors (La2−x(Ba, Sr)xCuO4) was studied by modifying our earlier approach and developing a Fourier-transformed effective potential which involves the effect of two-dimensional (2D) acoustic plasmons. This potential was used to obtain the pairing (electron-electron attraction) parameter (λ), the averaged Coulomb repulsive parameter (μ*) and the cut-off 2D acoustic plasmon frequency (ωc) required to compute the superconducting transition temperature (Tc) from the strong coupling theory. The variations ofTc with compositions (x) obtained for La2−x(Ba, Sr)xCuO4 show reasonably good agreement with experimental data.

    • Spectroscopic, dark and photoconductive properties of some polyene-iodine charge-transfer complexes

      Dilip Ghosh Susmita Hazra Prabir Pal T N Misra

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      Spectroscopic evidence has been provided to confirm that zeaxanthin, lutein and fucoxanthin form molecular charge-transfer (CT) complexes with iodine in the solid state. The semiconductive and photoconductive properties of CT complexes have been investigated in polycrystals in a sandwich cell configuration. Both dark and photoconductivity increases by several orders of magnitude on complex formation. The identical values of thermal activation energies for dark and photoconduction have been obtained for the complexes and this has been attributed to spontaneous carrier generation by CT interaction and their migration by trapping and detrapping mechanisms. Photoconduction action spectra in pure polyene and in polyene-iodine complex suggest that photoinjection from the electrode and direct electron hole pair production are the two photocarrier generation mechanisms operative in these crystals. The second mechanism predominates in pure materials whereas the first one becomes important in the complexes.

    • Studies on structure and properties of CNSL novolac resins prepared with succinic acid catalyst

      K Sathiyalekshmi

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      Cashew nut shell liquid (CNSL) was used to prepare a glass fabric laminate. Two fast curing model resins were prepared using cardanol and formaldehyde in the presence of succinic acid catalyst. The resins possess ortho-ortho and ortho-para linkages. The curing kinetics of the resins showed that the resin prepared with mole ratio 0·8 possesses good curing characteristics. The IR spectra of cured resin showed responses for high ortho linkage. A CNSL high ortho novolac resin was prepared with mole ratio 0·8. The glass fabric laminates prepared with CNSL resin showed good mechanical properties and dimensional stability. It is suggested that the candidate laminate can be used as secondary load bearing structures or panels.

    • A simple reactor for growth of HgCdTe films using LPE technique

      H L Chhatwal R K Dewakar

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      A reactor for growth of Hg1−xCdx Te epilayers by liquid phase epitaxy has been designed and developed. The layers have been successfully grown using this liquid phase epitaxy (LPE) reactor.

    • Growth and habit modification of sodium chlorate crystals

      V Surender K Kishan Rao

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      Single crystals of sodium chlorate were grown in the presence of selective impurities. The morphology of these crystals changed completely from cubic to tetrahedral form. Experiments were performed in the presence of different concentrations of impurities to ascertain their role in modifying the habit of these crystals. The presence of impurity in the solution was found to affect the habit of the crystals, although it does not appear to enter into the lattice.

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