2009
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PublicationArticle Synthesis and microstructural studies ofiron oxypnictide LaO 1-xFxFeAs superconductors(2009) Chandra Shekhar; Sonal Singh; P.K. Siwach; H.K. Singh; O.N. SrivastavaWe report on the synthesis and structural/microstructural studies of iron-based fluorine doped LaOFeAs superconductors. We have successfully synthesized fluorine doped superconducting LaO1-xFxFeAs materials by choosing lower temperature (∼1150 °C) and longer synthesis duration (∼60h) as compared to the standard values of these employed in the pioneering first contribution (Kamihara et al 2008 J.Am.Chem.Soc. 130 3296). A decrease of lattice parameters, as determined by x-ray diffraction, confirms the substitution of fluorine. The superconducting transition temperature is 27.5K which is observed at a doping level of x = 0.2. This superconducting material LaO1-xFxFeAs exhibits interesting microstructural characteristics. These relate to the existence of another structural phase, besides the standard phase, having c parameters of ∼12.67. This suggests the existence of a modulated structure, similar to the cuprates, in these new oxypnictides. This phase may have new impact on this new high-TC family. © 2009 IOP Publishing Ltd.PublicationArticle Effect of annealing below crystallization temperature on structural and mechanical properties of Co66Si16B12Fe 4Mo2 metallic glass(National Institute of Optoelectronics, 2009) P. Kumar; K. Singh; T.P. Yadav; O.N. SrivastavaThe influence of thermal treatment on the structural and mechanical properties of Co66Si16B12Fe4Mo 2 metallic glass has been investigated in present study. Co 66Si16B12Fe4Mo2 metallic glass has been subjected to isothermal annealing at 450°C for different time such as 1, 2, 5 and 8 h and studied by X-ray diffraction and scanning electron microscopy techniques. The Voight function analysis has been used for calculation of effective crystallite size and relative strain of all the annealed samples. The crystallite size has been found to be ̃ 40 - 60 nm after different h of annealing. The mechanical properties of above alloy before and after annealing has been studied by micro-hardness technique. Sample annealed for 8 h at 450°C becomes completely crystallized due to local structural rearrangements of the amorphous matrix by promoting the retention of nucleation during annealing and shows the maximum hardness among all crystalline and glassy samples.PublicationArticle Effect of growth temperature on bamboo-shaped carbon-nitrogen (C-N) nanotubes synthesized using ferrocene acetonitrile precursor(2009) Ram Manohar Yadav; Pramod Singh Dobal; T. Shripathi; R.S. Katiyar; O.N. SrivastavaThis investigation deals with the effect of growth temperature on the microstructure, nitrogen content, and crystallinity of C-N nanotubes. The X-ray photoelectron spectroscopic (XPS) study reveals that the atomic percentage of nitrogen content in nanotubes decreases with an increase in growth temperature. Transmission electron microscopic investigations indicate that the bamboo compartment distance increases with an increase in growth temperature. The diameter of the nanotubes also increases with increasing growth temperature. Raman modes sharpen while the normalized intensity of the defect mode decreases almost linearly with increasing growth temperature. These changes are attributed to the reduction of defect concentration due to an increase in crystal planar domain sizes in graphite sheets with increasing temperature. Both XPS and Raman spectral observations indicate that the C-N nanotubes grown at lower temperatures possess higher degree of disorder and higher N incorporation.PublicationArticle Spray pyrolysis deposited multiferroic BiFeO3 films(2009) P.K. Siwach; Jai Singh; H.K. Singh; G.D. Varma; O.N. SrivastavaBiFeO3 (BFO) films were prepared by nebulized spray pyrolysis technique on a single crystal LaAlO3 (001) substrate at ∼300 °C and annealing in oxygen at ∼550 °C for 1 h. In all films BFO is the majority phase having a rhombohedrally distorted structure with R3m symmetry and lattice parameters are a=3.96 Å and α=89.7°. Atomic force microscopy reveals smooth and dense surface morphology. Despite the G -type antiferromagnetic spin structure of BFO the present BFO show well-defined M-H loops and unexpected ferromagnetism as evidenced by large saturation magnetization, ∼125 emu/cc. The origin of anomalous ferromagnetism in the present films has been traced to the presence of nanometric Fe2 O3 embedded in the BFO matrix. © 2009 American Institute of Physics.PublicationArticle Effect of multielement doping on low-field magnetotransport in La0.7-xMmxCa0.3MnO3 (0.0≤x≤0.45) manganite(2009) P.K. Siwach; Pankaj Srivastava; H.K. Singh; A. Asthana; Y. Matsui; T. Shripathi; O.N. SrivastavaWe report the synthesis, structure and low-field magnetotransport properties of Mischmetal (Mm)-doped La0.7-xMmxCa0.3MnO3 (0≤x≤0.45) manganite. Mischmetal-Mm-is a natural mixture of rare earth elements La, Ce, Pr and Nd with ∼28%, 50%, 6% and 16% composition, respectively. All the samples crystallize in orthorhombic structure. Increasing x (Mm), corresponding to decreasing the La-site average ionic radii (〈rA〉) hence increasing the size mismatch (i.e. variance σ2), results in strong suppression of ferromagnetism (TC) and the associated metallicity (TIM). It may be pointed out that Mm (La, Ce, Pr and Nd) substitution has been done to create two effects. First, creation of multivalence of Mn (2+, 3+ and 4+) via Ce substitution and second to create higher degree of disorder due to size difference brought in not only by Ce but also by Pr and Nd. Evidences and arguments based on XPS analysis suggest that multivalent ions La, Mm and Ca, and the resulting presence of Mn2+, Mn3+ and Mn4+, causes the simultaneous operation of ferromagnetism-double exchange (Mn2+/Mn3+ and Mn3+/Mn4+) and antiferromagnetic-superexchange (Mn3+/Mn3+ and Mn2+/Mn2+) interaction. In addition, Mm doping also creates inhomogenities at La-as well as Mn-site due to size and valency difference. A curiously huge magnetoresistance as high as ∼63% for x=0.35, under a moderate magnetic field of ∼10 kOe has been observed and even at low magnetic field of ∼3 kOe MR is ∼30%. The competing double exchange and superexchange coupled with inhomogenities are the most likely cause for the occurrence of large ∼63% CMR in the Mm-doped LCMO. © 2008 Elsevier B.V. All rights reserved.PublicationArticle Phase formation in rapidly quenched Cu-based alloys(2009) Devinder Singh; T.P. Yadav; R.S. Tiwari; O.N. SrivastavaIn the present investigation, we report the formation of γ-brass type phase in the rapidly quenched Cu50Ga30Mg 5Ti15 and Cu50Al30Mg 5Ti15 alloys. Rapid solidification of Cu 50Ga30Mg5Ti15 alloy shows the formation of simple cubic γ-brass type phase (a = 0.863 nm), which on annealing at 1,023 K for 60 h transforms to disordered type γ-brass phase (a = 0.879 nm). It has been observed that intensity modulation of electron diffraction spots corresponding to simple cubic γ-brass phase is similar to the intensity modulation observed in the mirror orientation of icosahedral quasicrystalline phase. Contrary to the crystalline phase formation in Cu-Ga-Mg-Ti alloy, rapid solidification of Cu50Al30Mg 5Ti15 shows the formation of amorphous and nanocrystalline bcc γ-brass type phase (a = 0.870 nm), which on annealing transforms to ordered γ-brass type phase (a = 0.872 nm). The structural and microstructural characterization was done through X-ray diffraction and transmission electron microscopy. © 2009 Springer Science+Business Media, LLC.PublicationArticle Broad temperature range low field magnetoresistance in La0.7Ca0.3MnO3:nano-ZnO composites(2009) P.K. Siwach; Pankaj Srivastava; Jai Singh; H.K. Singh; O.N. SrivastavaWe report the enhanced low field magnetoresistance in broad temperature range (∼70 K) in LCMO:(%x) nano-ZnO (%x = 0, 1, 3, 5, 7 and 10) composites. Nano-ZnO has been synthesized by vapor phase transport technique which then admixed with sol-gel synthesized nano-LCMO that leads to LCMO:nano-ZnO composites. Nano-ZnO admixing does not show any significant change in lattice parameter of the composites. All samples exhibits characteristic insulator-metal (TIM) and para-ferromagnetic (TC) transition temperature, which decreases with increase in nano-ZnO content. Higher nano-ZnO (x ≥ 3%) leads to dual I-M as well as PM-FM transitions. The first transition occurs close to TIM or TC of pure LCMO and another at a significant lower temperature. At x > 7%, the double transitions transforms into a single-broad transition and at then finally disappears for x > 10%. The occurrence of dual transition has been explained on the basis of stress induced phase separation due to nanoglobular ZnO structures that result in formation of intragranular and intergranular regions having different transition temperatures. The interesting and important result for the present LCMO:nano-ZnO composites is the observation of improved MR ∼13-18% at a moderate magnetic field of 3 and 5 kG in a broad temperature range of ∼70 K (80-150 K). The improved broad range low field magnetoresistance (LFMR) has been explained on the basis of grain boundary tailoring due to coating of polar nature nano-ZnO on the LCMO grains. © 2009 Elsevier B.V. All rights reserved.PublicationArticle Structural and Mössbauer spectroscopic investigation of Fe substituted Ti-Ni shape memory alloys(2009) D.K. Rai; T.P. Yadav; V.S. Subrahmanyam; O.N. SrivastavaIn the present investigation the effect of Fe substitution in Ti51Ni49 alloy has been studied. The alloys were synthesized through radio frequency induction melting. The alloy was characterized through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and positron annihilation techniques. It was found that the Fe substitution stabilized the TiNi type cubic (a = 2.998 Å) phase. The microstructure and presence of the oxide phase in Ti51Ni45Fe4 alloy have been investigated by scanning electron microscopy. The positron annihilation measurements indicated a similar bulk electron density in both the as-cast and annealed (1000 °C for 30 h) alloys, typically like that of bulk Ti. Mössbauer spectroscopy studies of as-cast and annealed iron substituted samples showed regions in the samples where nuclear Zeeman splitting of Fe levels occurred and an oxide phase was found to be present in as cast Ti51Ni45Fe4 alloy, while annealed sample indicated the presence of bcc iron phase. © 2009 Elsevier B.V. All rights reserved.PublicationArticle Hydrogen energy in changing environmental scenario: Indian context(2009) M. Sterlin Leo Hudson; P.K. Dubey; D. Pukazhselvan; Sunil Kumar Pandey; Rajesh Kumar Singh; Himanshu Raghubanshi; Rohit. R. Shahi; O.N. SrivastavaThis paper deals with how the Hydrogen Energy may play a crucial role in taking care of the environmental scenario/climate change. The R&D efforts, at the Hydrogen Energy Center, Banaras Hindu University have been described and discussed to elucidate that hydrogen is the best option for taking care of the environmental/climate changes. All three important ingredients for hydrogen economy, i.e., production, storage and application of hydrogen have been dealt with. As regards hydrogen production, solar routes consisting of photoelectrochemical electrolysis of water have been described and discussed. Nanostructured TiO2 films used as photoanodes have been synthesized through hydrolysis of Ti[OCH(CH3)2]4. Modular designs of TiO2 photoelectrode-based PEC cells have been fabricated to get high hydrogen production rate (∼10.35 lh-1 m-2). However, hydrogen storage is a key issue in the success and realization of hydrogen technology and economy. Metal hydrides are the promising candidates due to their safety advantage with high volume efficient storage capacity for on-board applications. As regards storage, we have discussed the storage of hydrogen in intermetallics as well as lightweight complex hydride systems. For intermetallic systems, we have dealt with material tailoring of LaNi5 through Fe substitution. The La(Nil - xFex)5 (x = 0.16) has been found to yield a high storage capacity of ∼2.40 wt%. We have also discussed how CNT admixing helps to improve the hydrogen desorption rate of NaAlH4. CNT (8 mol%) admixed NaAlH4 is found to be optimum for faster desorption (∼3.3 wt% H2 within 2 h). From an applications point of view, we have focused on the use of hydrogen (stored in intermetallic La-Ni-Fe system) as fuel for Internal Combustion (IC) engine-based vehicular transport, particularly two and three-wheelers. It is shown that hydrogen used as a fuel is the most effective alternative fuel for circumventing climate change. © 2009 International Association for Hydrogen Energy.PublicationConference Paper On the synthesis and characterizations of TiO 2 nanotubes(2009) P.K. Dubey; P.R. Mishra; A.S.K. Sinha; O.N. SrivastavaIn the present work, aligned TiO 2 nanotubes have been synthesized by a simple method of electrochemical anodization of high purity, well cleaned, etched and ultrasonicated Ti-sheet (Purity ∼99.99%) in a fluoride mediated electrolytic media consisting of a solution of 0.14 M NaF and a solution of 0.5 M/1.0 M H 3PO 4. Studies on the effects of anodization voltage, time and electrolyte concentration on the formation of TiO 2 nanotubes have been carried out. The TiO 2 nanotube arrays have been synthesized at applied anodization voltages of ∼10 V and ∼20 V. The anodization was carried out for 1 hour and 2 hours at each applied voltage. Structural/microstructural characterizations of TiO 2 nanotubes have been carried out through scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM images of TiO 2 nanotubes showed interesting features relating to morphology, the pore size (diameter of the tubes) and the lengths of the tube. TEM investigations revealed that the as synthesized nanotubes are amorphous in nature and on electron beam annealing, these transformed to crystalline phases (rutile and brookite). The optical characterizations through UV-Visible spectroscopy exhibited that the band gap are ∼3.03 eV and ∼2.87 eV for tubes synthesized at applied anodization voltages of ∼10 V and ∼20 V respectively. A tentative mechanism for the growth of TiO 2 nanotube has been put forward. Copyright © 2009 American Scientific Publishers All rights reserved.