• S K Das

      Articles written in Bulletin of Materials Science

    • DC-sputtered Bi(Pb)SrCaCuO films on single (MgO) and polycrystalline (MgO, CuO) substrates

      S K Das R Suryanarayanan B Lecuyer O Gorochov

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      Bi(Pb)SrCaCuO films have been deposited on single (MgO) and polycrystalline (poly MgO, CuO) substrates by DC sputtering. All the films became superconducting after post-annealing them in air. The films on single crystal MgO showedTc onset at 120 K andTc at 92 K. However, the extrapolated zero resistivity is obtained at 106 K. The ac susceptibility showed an onset at 106 K and a sharp transition at 85 K. The films on poly MgO and poly CuO had aTc=72 K. The preparation, X-ray diffraction and morphology of these samples are presented.

    • Fractography as well as fatigue and fracture of 25 wt% silicon carbide whisker reinforced alumina ceramic composite

      A K Ray S Banerjee E R Fuller S K Das G Das

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      Fatigue cracked and fast fractured regions in four-point bend specimens prepared from 25 wt% silicon carbide whisker reinforced alumina composite were examined by scanning electron microscopy. This composite was found to be susceptible to a fatigue crack growth phenomenon similar to that in the case of metallic materials, but with a higher crack growth exponent. In the fatigue region, the alumina matrix failed mainly in a transgranular mode and the whiskers mainly failed with a flat fracture surface but without their pullout. On the other hand, in the fast fracture region, the whiskers failed predominantly by pullout and the alumina matrix failed in a mixed mode with about half in transgranular and the other half in intergranular mode. Thus, to improve the fracture toughness of the material, the grain boundary strength of alumina and the matrix whisker interfacial bonding should be improved. To increase the resistance to fatigue, the fracture strength of the alumina grains should be improved by using finer α-alumina particles and the fatigue strength of the whisker have to be increased by improving the uniformity in distribution of β-SiC whiskers during hot pressing.

    • Function space theory of dynamic nucleation during solidification at high cooling rates

      S K Das

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      An analytical theory is proposed to study the dynamic nucleation of crystals from melt at very high cooling rates (10−6 to 10−12 °K/sec). The mathematical framework is found to be isomorphic with the function space theory, wave and matrix mechanics, which enables application of various approximate methods of the latter disciplines. In principle, the mathematical apparatus and concepts of function space and wave mechanics can be utilized to study the time varying nucleation process. The Arrhenius law has been used to extrapolate the self-diffusion coefficient as a function of temperature above the melting point than those below. Since, applicability of Arrhenius equation at very high degrees of supercooling is not known and has to be substituted with appropriate constitutive relationship based on free volume theory of transport, the conclusion derived from the present analysis will not be unique with respect to the certainty of crystallization during the solidification process.

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