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PublicationConference Paper Structural and hydrogenation studies of ZnO and Mg doped ZnO nanowires(2012) Jai Singh; M.S.L. Hudson; S.K. Pandey; R.S. Tiwari; O.N. SrivastavaIn this work, Mg doped zinc oxide (Mg xZn 1-xO, x = 5, 10 and 20 at. %) nanowires were successfully prepared by two step process. Initially, ZnO nanowires were grown by thermal evaporation of Zn powder under oxygen atmosphere. Mg powder was doped in as grown ZnO through solid state diffusion at low temperature. Energy dispersive x-ray spectroscopy (EDAX), transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-Visible absorption spectra analysis reveals that the Mg doping on ZnO nanowires induces lattice strain in ZnO. Rietveld analysis of XRD data confirms the wurtzite structure and a continuous compaction of the lattice (in particular, the c-axis parameter) as x increases. The hydrogenation properties of ZnO nanowires and Mg doped ZnO (Mg xZn 1-xO, x = 0, 5, 10 and 20 at. %) nanowires were studied. The hydrogenated samples were further investigated through XRD and Fourier transform infrared spectroscopy (FTIR). The hydrogen storage capacity of as grown ZnO nanowires has been estimated to be 0.57 wt. % H 2 at room temperature. However, the hydrogen storage capacity gets increased to ∼1 wt. % upon doping ZnO with 10 at. % Mg. Further increase in Mg concentration decreases the hydrogen storage capacity of ZnO nanowires. Thus for 20 at. % Mg doped ZnO; the hydrogen absorption capacity gets decreased from ∼1 wt. % to 0.74 wt. %. The mechanism of hydrogen storage in ZnO nanowires and Mg doped samples of ZnO has been discussed. © 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.PublicationArticle High yield synthesis and characterization of graphitic carbon nanofibers by spray pyrolysis(Science Press, 2008) Bipin-Kumar Gupta; O.N. SrivastavaGram-scale (>1 g) synthesis and characterization of graphitic/carbon nanofibers (GNFs) is described. The GNFs were grown by the spray pyrolysis of benzene and characterized by X-ray diffraction and transmission/scanning electron microscopy. The optimum growth conditions for a high yield of GNFs were found to be a spray nozzle diameter (inner diameter) of -0.52 mm, a flow rate of benzene solution of -5 mL/min, a reaction temperature of -750°C and a flow rate of hydrogen, as carrier gas, of -1 500 mL/min. The typical length and diameter were -60μm and -250 nm, respectively. Most of the GNFs exhibited coiled/helical-like fibers. The yield of GNFs in the spray pyrolysis growth mode was found to be of the order of 1.45 g per run.PublicationArticle Impact of Zn substitution on phase formation and superconductivity of Bi1.6Pb0.4Sr2Ca2Cu 3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12(2005) R.B. Saxena; Rajiv Giri; V.P.S. Awana; H.K. Singh; M.A. Ansari; B.V. Kumaraswamy; Anurag Gupta; Rashmi Nigam; K.P. Singh; H. Kishan; O.N. SrivastavaSamples of series Bi1.6Pb0.4Sr2Ca 2Cu3-xZnxO10-δ with x = 0.0, 0.015, 0.03, 0.06, 0.09 and 0.12 are synthesized by solid-state reaction route. All the samples crystallize in tetragonal structure with majority (> 90%) of Bi-2223 (Bi2Sr2Ca2Cu3O10) phase (c-lattice parameter ∼ 36 Å). The proportion of Bi-2223 phase decreases slightly with an increase in x. The lattice parameters a and c of main phase (Bi-2223) do not change significantly with increasing x. Superconducting critical transition temperature (Tc) decreases with x as evidenced by both resistivity [ρ(T)] and ac magnetic susceptibility [χ(T)] measurements. Interestingly the decrement of Tc is not monotonic and the same saturates at around 96 K for x > 0.06. In fact Tc decreases fast (∼ 10 K/at%) for x = 0.015 and 0.03 samples and later nearly saturates for higher x values. Present results of Zn doping in Bi-2223 system are compared with other Zn-doped HTSC (high temperature superconducting) systems, namely the RE-123 (REBa2Cu3O7) and La-214 ((La, Sr)2CuO4). © World Scientific Publishing Company.PublicationArticle Investigations on the synthesis and microstructural characteristics of Hg(Bi)-Ba-Ca-Cu-O HTSC in relation to critical current density(2004) Rajiv Giri; G.D. Verma; S.K. Malik; D. Kundaliya; R.S. Tiwari; O.N. SrivastavaIn the present study, we have investigated the effect of Bi doping at the Hg site in the oxygen deficient HgOδ layer of HgBa 2Ca2Cu3O8+δ cuprate superconductor in relation to the occurrence of substructures in the form of long period polytypoid (LPP), defect matrix substructures, stacking faults and their influence on superconducting properties, particularly the critical current density. Bulk polycrystalline samples have been prepared by solid state reaction process through precursor route. It has been observed that the as grown Hg1-xBixBa2Ca2Cu 3O8+δ variants (with x=0.05, 0.10, 0.15, 0.20, 0.25) correspond to the Hg(Bi):1223 phase. Transmission electron microscopic (TEM) observations reveal the presence of long period polytypoid-like structures, associated defect matrix substructure and stacking faults. A correlation between the details of defect substructures associated with LPP and the transport critical current density has been found to exist. It is suggested that this correlation arises due to defect matrix substructure and stacking faults providing effective flux pinning sites. The optimum transport critical current density Jct of ∼1.05×103A/cm 2 at 77K has been observed for the composition Hg 0.85Bi0.15Ba2Ca2Cu3O 8+δ. TEM studies reveal that the phase with this composition has the highest density of ordered LPP structures and associated defect matrix substructures. Such an observation bringing out a correlation between transport critical current density and LPP together with defect matrix substructure appears to be the first of its type. Evaluation of intra-grain critical current density through magnetic measurements for the optimum phase Hg 0.85Bi0.15Ba2Ca2Cu3O 8+δ revealed that this phase also has corresponding highest Jc (intra-grain) ∼4.7×104A/cm2 at 77K. © 2003 Elsevier B.V. All rights reserved.PublicationArticle Synthesis of nano-decagonal quasicrystalline material by mechanical alloying(American Scientific Publishers, 2014) T.P. Yadav; N.K. Mukhopadhyay; O.N. SrivastavaMechanical milling has been established as a viable route for the synthesis of a variety of advanced materials including nano-crystalline materials. In the present investigation, the nano-decagonal (nano-D)quasicrystalline phase in Al–Co–Ni has been synthesized by mechanical milling. The mechanical alloying of the Al70Co15Ni15 alloy composition was performed in a high-energy ball mill up to 60 hours under liquid hexane medium at the speed of 400 rpm with a ball to powder ratio of 40: 1. It was found that the evolution of nano-decagonal phase appears after 60 h of milling. The stability of the nano-D phase was investigated under vacuum (base pressure 10−6 mbar) annealing at 600 °C. It is interesting to observe the coarsening of the nano-D phase instead of any other phase transformation. The scanning and transmission electron microscopic investigation was carried out in details for the microstructural and structural characterization. © 2014 American Scientific Publishers.PublicationArticle High-yield production of graphitic nanofibers(2008) Bipin Kumar Gupta; O.N. SrivastavaThe primary aim of the present paper is focused on the gram-scale (>1 g) synthesis and characterization of graphitic/carbon nanofibers (GNF). The GNFs were grown by spray pyrolysis of benzene. The optimum growth conditions for the high-yield GNFs are as follows: spray nozzle diameter (inner diameter) ∼0.52 mm, flow rate of benzene solution ∼5 ml/min with reaction temperature ∼750 °C and flow rate of hydrogen as carrier gas ∼1500 cc/min. The structural and microstructural characterizations have been evaluated by X-ray diffraction and transmission as well as by Scanning electron microscopy. The typical length and diameter are ∼60 μm and ∼250 nm, respectively. Most of the GNF samples exhibit coiled/helical-like fibers. The yield of GNF in the spray pyrolysis growth mode has been found to be of the order of gram-scale (1.45 g) per run. The details of the experimental parameters for optimum growth conditions have been studied and will be described. © 2008 International Association for Hydrogen Energy.PublicationArticle Investigations on the structural, microstructural and dehydrogenation/rehydrogenation behavior of Ti doped sodium aluminum hydride materials(2007) Bipin Kumar Gupta; O.N. SrivastavaIn this paper, we have carried out investigations on Ti doping to increase the desorption kinetics of sodium aluminum hydride (NaAlH4) materials. Ti powder can be directly used for doping in the preparation of Ti doped NaAlH4 materials (Ti:NaAlH4). Out of various materials corresponding to NaAlH4- x mol % Ti (x = 1, 2, 3, 4, 5, 6), we have found that the material with x = 3 mol % is the optimum material. It shows the maximum storage capacity ∼ 5.23 wt % for the first two reactions at 150 and 210 °C, respectively. The first decomposition reaction of Ti doped NaAlH4 material was found to depend upon milling time. The optimum milling time for Ti doping was 10 min under argon atmosphere at 1500 rpm. The desorption kinetics improved nearly three times for Ti doped materials as compared to the procured materials. Rehydrogenation occurred at 110 °C under ∼ 10 MPa pressure in hydrogen environment within 8 h. The hydrogen storage capacity of this material was found to gradually decrease to 2.93 wt% after 15th cycles. The structural and microstructural characterizations have been evaluated by X-ray diffraction and scanning electron microscopy. The results of the present investigations will be described and discussed. © 2006 International Association for Hydrogen Energy.PublicationArticle Effect of cationic size in Hg(Tl/Bi)Ba2Ca2Cu3O8+δ on superconducting and microstructural characteristics(2001) Rajiv Giri; H.K. Singh; R.S. Tiwari; O.N. SrivastavaIn this paper we have reported investigations on the effect of simultaneous substitution of Bi and Tl at the Hg site in the oxygen deficient HgOδ layer of HgBa2Ca2Cu3O8+δ cuprate superconductor. Bulk polycrystalline samples have been prepared by the two-step solid state reaction process (precursor route). It has been observed that the as grown HgBi0.2-xTlxBa2Ca2 Cu3O8+δ (with x = 0.00, 0.05, 0.10, 0.15, 0.20) corresponds to the 1223 phase. It has been found that the Tc varies with the average cationic size of the dopant cations. The optimum Tc of ∼131 K has been found for the composition HgBi0.15Tl0.05Ba2Ca2Cu3 O8+δ. This composition leads to the average dopant cation size of ∼1.108 Å which is very close to the size of Hg2+ (∼1.11 Å). The microstructure for HgBi0.15Tl0.05Ba2Ca2Cu3 O8+δ has been found to be most dense and this phase exhibits the highest stability. The Jc of the optimum material HgBi0.15Tl0.05Ba2Ca2Cu3 O8+δ is found to be ∼1.29 × 103 A/cm2 at 77 K.PublicationArticle Ball-milled carbon and hydrogen storage(2002) Kalpana Awasthi; R. Kamalakaran; A.K. Singh; O.N. SrivastavaWe report the formation of carbon in different nanoparticle forms obtained by ball-milling of graphitic carbon. Ball-milling of graphite was carried out in Szegvari attritor at room temperature for varied times i.e. 24, 48 and 100 h in hexane medium. The characterization of ball-milled graphitic carbon (BMC) samples was done by X-ray diffractometry, scanning electron microscopy and transmission electron microscopy. The self-coagulated carbon agglomerates were obtained for the case of 24 and 100 h BMC samples. The formation of coiled nanotubes and nanofibres was observed in the BMC sample. The BMC samples with and without nickel (Ni) catalyst were subjected to hydrogenation cycling in a Sievert's type apparatus fabricated in our laboratory. It has been found that BMC sample can adsorb hydrogen. The hydrogen adsorption capacity has been found to be ∼ 0.6 wt%. © 2002 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved.PublicationArticle PLD deposited ZnO films on different substrates and oxygen pressure: A study of surface morphology and optical properties(2012) Jai Singh; P.K. Srivastava; P.K. Siwach; H.K. Singh; R.S. Tiwari; O.N. SrivastavaThin films of ZnO were prepared by pulsed laser ablation on single crystal strontium titanate (STO, 001) and yttrium stabilized zirconium oxide (YSZ, 001) substrates at temperature ~600 °C under varying oxygen pressure of ~50, 100 and 200 mTorr. These as deposited ZnO thin films have been subjected to structural, microstructural and optical characterization by employing X-ray diffraction (XRD), scanning electron microscopy (SEM), and PL-UV spectroscopy, respectively. XRD reveals that the films are epitaxial having single phase wurtzite structure with preferred orientation along (002) direction. The film quality increases with increase in oxygen pressure as evident by enhanced (00.2) peak intensity at higher oxygen pressure (~200 mTorr) on both substrates. SEM results show that the density and morphology of film improves at higher oxygen pressure because of increased particle size. Photoluminescence (PL) measurements indicate that all the thin films show intense near bandedge PL emissions around ~380 nm without the presence of any deep level emission at higher wavelength. All the UV peaks on YSZ substrate are located close to ~380 nm whereas on STO substrates the UV peaks shifts towards lower wavelength as oxygen pressure increases from 50 mTorr to 200 mTorr. The optical band gap on YSZ was found to be ~3.28 eV whereas on STO it varies in the range of ~3.28 to 3.30 eV with increasing oxygen pressure. The variation in band gap on STO has been attributed to the compressive strain on ZnO films due to lattice mismatch between ZnO and STO. The improved optical properties at higher oxygen pressure and on different substrate have been discussed in relation to the microstructural variation because of oxygen incorporation and lattice strain arising due to lattice mismatch of substrate (STO and YSZ) with ZnO. © 2012 by American Scientific Publishers.