• SUNIL KUMAR

      Articles written in Bulletin of Materials Science

    • Shape-forming and densification of ceramic superconductors

      O M Prakash R M Anklekar N K Khosla Sunil Kumar B Chaudhuri C A Thomas A P Magadum M Ramesh B T Rao T R Rama Mohan

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      Processing studies on varied shape-forming and densification of bulk ceramic superconductor, YBa1·5Ca0·5Cu3O7\t-\gd, are reported in this paper. Polyvinyl butyral-polyethylene glycol-trichloroethylene has been found to be the best binder-plasticizer-solvent system in plastic shape-forming. The effect of initial particle morphology on final densification has been the most sensitive single parameter as compared to compaction pressure and final sintering durations at ∼930°C. 1-2-3 powders of mean particle size ∼ 1·94 µm have yielded sintered densities ∼92% T.D. albeit with lower oxygen intake O6·7.

    • Relaxor behaviour in BaBi4Ti4O15 ceramics fabricated using the powders obtained by mechanochemically assisted synthesis route

      Sunil Kumar K B R Varma

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      Mechanochemically activated reactants were found to facilitate the synthesis of fine powders comprising 200–400 nm range crystallites of BaBi4Ti4O15 at a significantly lower temperature (700 °C) than that of solid-state reaction route. Reactants (CaCO3, Bi2O3 and TiO2) in stoichiometric ratio were ball milled for 48 h to obtain homogeneous mixture. The evolution of the BaBi4Ti4O15 phase was systematically followed using X-ray powder diffraction (XRD) technique. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were employed to probe its structural and microstructural details. The electron diffraction studies established the presence of correlated octahedral rotations and associated long-range polar ordering. High-resolution TEM imaging nevertheless revealed structural inhomogeneities leading to intergrowth defects. Dense BaBi4Ti4O15 ceramics with an average grain size of 0.9 𝜇m were fabricated using mechanochemically assisted synthesized powders at relatively low temperature (1000 °C). The effect of grain size on the dielectric and relaxor behaviour of BaBi4Ti4O15 ceramics was investigated. Fine-grained ceramics (average grain size ∼ 0.9 𝜇m) showed higher diffusion in phase transition, lower temperature of phase transition, lower Vogel–Fulcher freezing temperature and higher activation energy for the polarization reversal than those for coarse-grained ceramics (average grain size ∼ 7 𝜇m) fabricated via the conventional solid-state reaction route.

    • Orientation-dependent crack-tip blunting and crack propagation in a single crystal BCC iron

      SURAJIT KUMAR PAUL SUNIL KUMAR S TARAFDER

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      Atomistic simulations of cracks with four different orientations in body-centered cubic single crystal iron are presented using molecular dynamics. Crystal orientation has considerable effect on the activation and evolution ofcrack propagation mechanisms. The results reveal that (a) crack-tip blunting depends on the crystallographic orientation, (b) continuous generation of dislocations form crack tip occurs for large crack-tip blunting, and (c) absence of deformation activities like dislocation generation, twin formation, etc. at the crack tip results in crack propagation in a brittle manner.

    • Thin film chemiresistive gas sensor on single-walled carbon nanotubes-functionalized with polyethylenimine (PEI) for NO$_2$ gas sensing

      SUNIL KUMAR VLADIMIR PAVELYEV PRABHASH MISHRA NISHANT TRIPATHI

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      Chemical detection of toxic gases, such as greenhouse gases is still very important as a research topic. To design gas sensor detectors based on single-walled carbon nanotubes (SWCNTs) with high sensitivity and selectivity for the toxic environment is a continuous process. The aim is to detect NO$_2$ gas with better sensitivity. In the present work, the thin-film sensor is fabricated on SiO$_2$ substrate and it is functionalized with polyethylenimine (PEI). It has been established that PEI functionalized SWCNTs (F-SWCNTs) show high sensitivity towards strong electron-withdrawing particles. It was found that at room temperature, SWCNTs-PEI functionalized gas sensor exhibited a higher sensitivity of 37.00% as compared with bare SWCNTs gas sensor. The gas sensor has shown the repeatable response for the entire concentration range studied.The sensing properties and the PEI functionalization duration effects on the behaviour of SWCNTs-based gas sensors were demonstrated.

    • On processing structure–conductivity relations in NASICON-type LiSn$_2$(PO$_4$)$_3$

      TANVI PAREEK SUSHMITA DWIVEDI MANISH BADOLE SUNIL KUMAR

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      The present investigation focuses on understanding the structure–electric conductivity correlation in NASICON-type LiSn$_2$(PO$_4$)$_3$ (LSP) powders prepared via solid-state reaction method. LSP powders synthesized at different temperatures were characterized for their structural and electrical properties using lab source powder X-ray diffraction (XRD), high-resolution synchrotron X-ray diffraction (SXRD) and complex impedance spectroscopy. LSP powders prepared in 900–1000°C temperature crystallize in triclinic structure (space group, P $\bar{1}$) along with the small amount of SnO$_2$ (P4$_2$/mnm) impurity phase. Samples prepared at temperatures in 1050–1250°C range showed a mixed rhombohedral (R $\bar{3}$c) and triclinic structure with the fraction of the triclinic phase decreasing with an increase in calcination temperature. On further increase in the calcination temperature to 1300°C, LSP transformed to the rhombohedral structure. Moreover, temperature-dependent SXRD confirmed that the LSP powder exhibits a martensitic behaviour, where a pure triclinic structure transforms into a pure rhombohedral phase at 170°C and retains a partial rhombohedral phase on cooling back to room temperature. The highest value of conductivity was found to be ${\sim}$1.06 ${\times}$ 10$^{–6}$ Scm$^{–1}$ for the LSP powder with triclinic structure calcined at 900°C, with an associated activation energy of ${\sim}$0.24 eV. Rhombohedral LSP calcined at 1300°C exhibits the lowest conductivity and highest activation energy at room temperature ${\sim}$1.12 ${\times}$ 10$^{–8}$ Scm$^{–1}$ and ${\sim}$0.39 eV, respectively. This decrease in conductivity for the supposedly highconducting rhombohedral phase is attributed to the drastic increase in the fraction of the SnO$_2$ impurity phase, as confirmed by the XRD analysis.

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      Posted on October 12, 2020

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      Chemical Sciences 2020

      Prof. Surajit Dhara — School of Physics, University of Hyderabad, Hyderabad
      Physical Sciences 2020

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