• B K Das

      Articles written in Bulletin of Materials Science

    • Kinetics of the oxidation of CuFeO2 in Cu-Fe-O system

      G C Jain B K Das R Avtar

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      The oxidation kinetics of CuFeO2 in Cu-Fe-O system have been studied between temperature range 500° C and 900° C in an ambient atmosphere containing constant oxygen partial pressure in the flowing nitrogen. Thermogravimetric analysis (TGA) technique was used as the main tool. The oxidation rate was found to decrease and the activation energy calculated from the oxidation rate constants for various composition increased from 8 to 45 k cal/mole on increasing the Fe2O3 content in the system. The value of exponentn in Avrami’s equation=1 − exp (−ktn) also increased from 1·3 to 2·3. The microstructural changes associated with the oxidation have been studied using optical microscopy and correlated with results obtained from thermogravimetric analysis.

    • On the sintering kinetics of titania doped ceramic lanthanum chromite

      K P Bansal S Kumari B K Das G C Jain

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      The sintering behaviour of lanthanum chromite with the addition of 1 to 3 wt% TiO2 was studied. Densification was examined as a function of sintering temperature and TiO2 concentration. The results showed that pure LaCrO3 could not be densified to >75% of the theoretical density, while the densities exceeding 90% of the theoretical were achieved with 3 wt% titania addition at 1600° C.

      Metallographic study reveals a normal grain growth following the rate equationD2D02=kt. Activation energies both for densification and grain growth have been estimated to be ≈ (80±5) k cal/mole in case of 3 wt% TiO2 addition.

    • Development of zinc oxide varistors

      H S Kalsi B K Das

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      Grain growth kinetics of ZnO ceramics containing 6% additives of Bi2O3, CoO and MnO in equal molar ratios has been studied in the temperature range 1000 to 1350°C. It has been observed that the grain growth data fits theD2=kt law and the activation energy computed from the rate constants is 65 kcal/mol.

    • Electrical properties of polycrystalline silicon and zinc oxide semiconductors

      S N Singh S Kumari B K Das

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      Polycrystalline silicon and zinc oxide ceramic are important electronic materials. The electrical properties which determine the applications of polycrystalline silicon in integrated circuits and solar cells and that of ZnO ceramic in varistors are due primarily to grain boundary effects in them. A large amount of information in this area has already been gathered in literature but the quantitative understanding of grain boundary effects in these materials is not yet complete. In this review the important aspects of grain boundaries and their effects on transport and photoelectric properties of polycrystalline silicon and on the I–V characteristic of ZnO varistors are discussed.

    • Preparation and characterisation ofβ″-Al2O3

      H S Kalsi R P Tandon Balbir Singh R C Goel B K Das

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      Beta alumina solid electrolyte is a potential candidate in the fabrication of Na-S batteries. In the present study, it has been prepared in the form of discs by uniaxially as well as isostatically pressing and sintering in the temperature range 1585–1630°C, the highest sintered density of 3·25 g/cm3 has been achieved in the samples isostatically pressed and sintered at 1630°C. X-ray analysis of the samples shows formation ofβ″-phase. Microstructure of the sintered samples reveals some darker regions which are attributed to low soda content. Resistivity at 300°C measured by the two-probe method at a frequency of 1 MHz on samples having vacuum-deposited silver electrodes is 15 ohm-cm, which is slightly higher than the value of 5–13 ohm-cm reported in literature using molten sodium electrodes. The activation energy of conductivity is 0·24 eV which is comparable to the literature value of 0·24–0·35 eV.

    • Electrical properties of polycrystalline silicon and zinc oxide semiconductors

      S N Singh S Kumari B K Das

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    • Synthesis of Y-Ba-Cu-O superconducting material by citrate chemical route—some observations

      R K Kotnala S M Khullar B S Khurana K Jain M Sharma S Singh R C Goel G K Padam R B Tripathi B K Das

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      We present an overview of our powder synthesis procedure and give some initial results concerning the superconducting properties of YBa2Cu3O7−x.

    • Flux dynamics of YBCO/Ag composite

      Indu Dhingra B K Das S C Kashyap

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      DC magnetization measurements have been carried out on bulk YBCO/Ag composites with silver content up to 20wt per cent. DC fields in the range 0·5 mT to 200 mT have been used to investigate the inter- and intragranular properties at 77K. The AC susceptibility as a function of temperature at different AC fields (0·026–0·30 mT) has also been studied. Under small DC fields (≈ 4 mT), depending on the Ag content andHmax, the M-H loop shows a complicated behaviour. This behaviour can be explained on the basis of effect of strong field dependence of transport critical current, grain size and intragrain critical current densityJcgm on low-field M-H loop. The estimation of intergranular critical current densityJcjm from these loops does not remain a simple function of ΔM/d. The AC susceptibility measurements show a small increase inJc(T) with silver content under low AC fields only, consistent with the transportJc data; beyond thatJc(T) decreases. This improvement inJc(T) and transportJc with silver can be ascribed to the improved coupling between grains but not to the pinning. Also at higher field (Hmax>20 mT) the addition of Ag decreases the intragrain critical current density. The upper critical field of intergranular regionHc2j and lower critical field of intragrain regionHc1g also decrease with silver content.

    • Magnetic study above the curie temperature of γ-Fe2O3 in determining the dispersion nature of Co2+ ions in Co-modified γ-Fe2O3 thin films

      B K Das Sandip Dhara A C Rastogi

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      Gamma (γ) iron oxide thin films containing 6 at% of cobalt atoms selectively dispersed at interstitial and octahedral locations have been prepared by a reactive chemical vapour deposition process. Such dispersion gives microscopic Co-trapped and Co-doped regions inγ-Fe2O3 matrix and introduces magnetocrystalline anisotropy leading to high coercivity values of 64–112 kA/m. Temperature dependence of coercivity and saturation magnetization forγ-Fe2O3 films confirm the dispersion model.

    • Medium resolution magneto-optical Faraday rotation measurement system

      Sandip Dhara V N Moorthy A C Rastogi B K Das

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      Medium resolution magneto-optic Faraday rotation measurement setup has been reported. Measuremental setup reported is made up of simple polarized laser source, beam expander, analyzer and a bisected cross polarizedp-n detector for single ended measurements. Result of CoO modified yttrium iron garnet (YIG) films showed a rotation of 3 deg/μm which is comparable to earlier reported Co-doped YIG films.

    • Effect of post deposition reduction treatment of YIG thin films on stabilizing cubic garnet phase

      A C Rastogi V N Moorthy Sandip Dhara H P Gupta B R Awasthy B K Das

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      Yttrium iron garnet thin films have been prepared by low pressure metallo-organic chemical vapour deposition method (MOCVD). Dipivaloyl methanates of yttrium and iron have been used as the precursors in the MOCVD growth of the garnet films. Post deposition O2 annealing at 900°C is required to form a garnet phase which also shows orthoferrite and component Y2O3 and α-Fe2O3 phases. We show that a partial H2 reduction treatment minimizes secondary phases and stabilizes the garnet phase. These treatments also enhance the magnetic properties considerably.

    • Modeling of diffusion and oxidation in two dimensions during silicon device processing

      P K Singh B K Das

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      A process simulator named “2D-DIFFUSE” has been developed where the coupled diffusion equation of dopant impurity and point defects: interstitials and vacancies, has been solved numerically in two-dimension. The interaction of point defects has been modeled assuming quasi (i.e. local) equilibrium,C1Cv=C1*C*V and constant vacancy,Cv=Cv*, conditions. Indeed, these two assumptions decouple the two point defects diffusion equations. The processes modeled in the present version of the simulator include pre-deposition, diffusion and oxidation. The simulator is quite successful at modeling each process individually as well as integrating various processes and models. The program has also been applied to the simulation of phenomena as the dopant diffusion under various ambients, oxidation enhanced and retarded diffusion, emitter push effect etc. Comparisons between simulation based on point defect parameters from various sources have been made.

    • Photocatalytic degradation of Malachite Green dye by modified ZnO nanomaterial

      S MEENA DIPTI VAYA B K DAS

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      In this article we report a chemical sol–gel approach to synthesize zinc oxide nanomaterials capped with ethylene diamine tetra acetic acid (EDTA), citric acid and oleic acid, and to study the effect of the surface modification on their photocatalytic activity and the kinetics for the degradation of Malachite Green (MG) dye. The structural, optical and chemical features were systematically characterized by X-ray powder diffraction, scanning electron microscopy, Fourier transform infrared and UV–vis absorption spectroscopy. The objective of using the capping agents was to confine the size and control the growth and morphology of the nanomaterial. The smallest crystallite size was recorded as 29 nm for EDTA-capped rod-shaped ZnO. A comparison study of the effect of thethree different capping surfactants on ZnO nanomaterial for photocatalytic degradation ofMGdye under solar light showed that EDTA with higher denticity coordinated efficiently with the surface of ZnO nanocrystalline catalystsand hence demonstrated better decolouration of the dye under solar light. The dye degradation followed the psuedofirst-order kinetics. EDTA proved to be the best capping agent among all the three for ZnO nanomaterial.

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