• Rajesh Kumar

      Articles written in Bulletin of Materials Science

    • Synthesis of nano-carbon (nanotubes, nanofibres, graphene) materials

      Kalpana Awasthi Rajesh Kumar Himanshu Raghubanshi Seema Awasthi Ratnesh Pandey Devinder Singh T P Yadav O N Srivastava

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      In the present study, we report the synthesis of carbon nanotubes (CNTs) using a new natural precursor: castor oil. The CNTs were synthesized by spray pyrolysis of castor oil–ferrocene solution at 850°C under an Ar atmosphere. We also report the synthesis of carbon nitrogen (C–N) nanotubes using castor oil–ferrocene–ammonia precursor. The as-grown CNTs and C–N nanotubes were characterized through scanning and transmission electron microscopic techniques. Graphitic nanofibres (GNFs) were synthesized by thermal decomposition of acetylene (C2H2) gas using Ni catalyst at 600°C. As-grown GNFs reveal both planar and helical morphology. We have investigated the structural and electrical properties of multi-walled CNTs (MWNTs)–polymer (polyacrylamide (PAM)) composites. The MWNTs–PAM composites were prepared using as purified, with ball milling and functionalized MWNTs by solution cast technique and characterized through SEM. A comparative study has been made on the electrical property of these MWNTs–PAM composites with different MWNTs loadings. It is shown that the ball milling and functionalization of MWNTs improves the dispersion of MWNTs into the polymer matrix. Enhanced electrical conductivity was observed for the MWNTs–PAM composites. Graphene samples were prepared by thermal exfoliation of graphite oxide. XRD analysis confirms the formation of graphene.

    • Mechanical and morphological investigation of virgin polyethylene and silver nanoparticle-loaded nanocomposites film: comprehensive analysis of kinetic models for non-isothermal crystallization


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      This research was accomplished to examine the mechanical, morphological and crystallization kinetics study of polyethylene/silver nanoparticles (Ag-NPs) nanocomposite films. In this research, low-density polyethylene (LDPE) nanocomposite films were prepared containing Ag-NPs using maleic-anhydride-grafted low-density polyethylene (LDPE-g-MAH) as a compatibilizer by the melt mixing process. From mechanical property evaluation, it is revealed that the LDPE/LDPE-g-MAH/Ag-NPs nanocomposite films showed decreased tensile strength as compared with virgin LDPE matrix. Thermal characteristics of the prepared virgin LDPE and its nanocomposite films were studied by differential scanning calorimetry (DSC). Comprehensive analysis of different kinetic modelssuch as the Avrami and Mo model on non-isothermal crystallization kinetics was performed in order to correlate the rate of crystallization and its various kinetic parameters. Further, the macrokinetic equation as proposed by Malkinhas been applied to describe the kinetics of crystallization in the light of the Avrami equation. Concerning virgin LDPE and Ag-NP-reinforced LDPE, the former shows primary crystallization, whereas the later exhibits both primaryand secondary crystallization with varying Avrami exponents. Kinetic parameters are recognized, and confirm the influence of Ag-NPs on crystallization kinetics. X-ray diffraction spectroscopy and transmission electron microscopicanalysis of the nanocomposite films were conducted to verify the dispersion of inorganic filler particles in the resulting hybrids.

    • On the structural and electrical properties of metal–ferroelectric–high k dielectric–silicon structure for non-volatile memory applications


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      In this article, we report the structural and electrical properties of metal–ferroelectric–high k dielectric–silicon (MFeIS) gate stack for non-volatile memory applications. Thin film of sputtered SrBi$_2$Nb$_2$O$_9$ (SBN) was used as ferroelectric material on 5–15 nm thick high-k dielectric (Al$_2$O$_3$) buffer layer deposited using plasma-enhanced atomic layer deposition (PEALD). The effect of annealing on structural and electrical properties of SBN and Al$_2$O$_3$ films was investigated in the temperature range of 350–1000$^{\circ}$C. X-ray diffraction results of the SBN and Al$_2$O$_3$ show multiple phase changes with an increase in the annealing temperature. Multiple angle ellipsometry data show the change in the refractive index ($n$) of SBNfilm from 2.0941 to 2.1804 for non-annealed to samples annealed at 600$^{\circ}C. For Al$_2$O$_3$ film, $n$ < 1.7 in the case of PEALDand $n$ > 1.7 for sputtered film was observed. The leakage current density in MFeIS structure was observed to two orders of magnitude lower than metal/ferroelectric/silicon (MFeS) structures. Capacitance–voltage (C–V) characteristics for the voltage sweep of $−$10 to 10 V in dual mode show the maximum memory window of 1.977 V in MFeS structure, 2.88 Vwith sputtered Al$_2$O$_3$ and 2.957 V with PEALD Al$_2$O$_3$ in the MFeIS structures at the annealing temperature of 500$^{\circ}$C.

    • Study of precursor-dependent CuS nanostructures: crystallographic, morphological, optical and photocatalytic activity


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      Diethylenetriamine (DETA)-assisted solvothermal method is explored for the synthesis of CuS nanostructures (NSs). Cu(ethylxanthate)$_2$, Cu(morpholine-4-dithiocarbamate)$_2$ and Cu(piperazine-1-dithiocarbamate)$_2$ are used as efficient single source precursors for the synthesis of CuS NSs. DETA acts as stabilizing as well as reducing agent. Diffraction and electron microscopy techniques are used for the analyses of crystallographic and morphological features of the synthesized NSs. Synthesized NSs crystalize in hexagonal crystal structure, having average crystallite size ${\sim}$28.95, 32.85 and 33.30 nm for CuS prepared from Cu(ethylxanthate)$_2$, Cu(morpholine-4-dithiocarbamate)$_2$ and Cu(piperazine-1-dithiocarbamate)$_2$ precursors, respectively. Transmission electron microscope analysis shows the formation of multifaceted NSs. Topographical studies carried out with the help of field-emission scanning electron microscope reveal the agglomeration of layered NSs. Photocatalytic activity of synthesized CuS NSs is assessed by evaluating the degradation of methylene-blue dye aqueous solution under visible light irradiation. Photocatalytic activity dependence on morphology is thoroughly studied.

    • High ion conducting and thixotropic nature of water-soluble xanthan gum-based electrolytes


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      This research reports the preparation and characterization of xanthan gum (XG)-based gel electrolytes (GEs) comprising sodium hydroxide salt (NaOH) in deionized water (DW). The three-dimensional gel network has been formed without using any synthetic polymer or cross-linking agents. Ionic conductivity of GEs was evaluated with different parameters, such as salt concentration, gum concentration, temperature and with the passage of time. The maximum ionicconductivity of 74.8 mS cm$^{–1}$ was observed at room temperature even after 55 days for XG-based GE containing 0.625 M NaOH. A small change in pH values for XG-based GEs have been observed with temperature in the range of 10–70°C and at different time span. Thixotropic behaviour of GEs under the application of stress has also been analysed by rheological studies. There was no discernible change in ionic conductivity with temperature and with the passage of time, which make it desirable for their use in different device applications.

    • Photocatalytic degradation efficiencies of ZnO nanoparticles and CeO$_2$ nanosheets synthesized via combustion method


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      In this study, a simple, inexpensive, scalable solution combustion process devoid of toxic chemicals is suggested for the synthesis of ZnO and CeO$_2$ nanostructures. Different techniques were used to characterize the morphologies, crystal phases, purity and composition of as-synthesized nanostructures. For ZnO nanoparticles, field emission scanning electron microscopy examination revealed spheroidal, elongated hexagonal rods, triangular and pentagonal morphologies, whereas for CeO$_2$, sheet-like morphologies with various thicknesses were observed. X-ray diffraction investigation indicated wurtzite hexagonal and cubic fluorite phases for ZnO nanoparticles and CeO$_2$ nanosheets with crystallite sizes of 49.50 and 11.04 nm, respectively. Energy dispersive X-ray spectrometry, electron mapping and elemental distribution images confirmed the purity of the synthesized nanostructures. The optical bandgap of ZnO nanoparticles and CeO$_2$ nanosheets were found to be 3.28 and 3.55 eV, respectively. ZnO nanoparticles and CeO$_2$ nanosheets demonstrated outstanding photocatalytic efficiencies for the degradation of model dyes like Congo red (CR), rhodamine (RhB) and methylene blue (MB). However, ZnO nanoparticles outperformed CeO$_2$ nanosheets in photodegradation. Under UV-light irradiation, the degradation rate of the dyes was reduced in the following sequence for both photocatalysts: CR>MB>RhB. The low degradation efficiency of CeO$_2$ nanosheets can be attributed to their higher bandgap energy as compared to ZnO nanoparticles. Further, photocatalytic degradation of different dyes followed Langmuir–Hinshelwood pseudo-first-order kinetic model. The exceptional dye-degrading abilities of the as-synthesized nanostructures, synthesized using the combustion process, show that they are suited for photocatalysis applications.

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    • Dr Shanti Swarup Bhatnagar for Science and Technology

      Posted on October 12, 2020

      Prof. Subi Jacob George — Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bengaluru
      Chemical Sciences 2020

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

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      Posted on July 25, 2019

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