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PublicationArticle Synthesis and hydrogen storage characteristics of the composite alloy La2Mg17-x wt% MmNi4.5Al0.5(1993) K. Dutta; O.N. SrivastavaRecently, composite type materials consisting of known hydrogen storage systems e.g. Mg-LaNi5, ZrFeCr-Mg, etc. 1, H. Nagai et al., J. Less-Common Metals 157, 15 (1990); [2], H. Fujii et al., J. Less-Common Metals 175, 243 (1991) have been shown to have better hydrogen absorbing/desorbing properties. The present paper reports the synthesis and hydrogenation/dehydrogenation behaviour of the composite alloy La2Mg17-x wt% MmNi4.5Al0.5. It has been shown that the optimum hydrogen storage characteristics are possessed by the material with x = 10% i.e. La2Mg17-10% MmNi4.5Al0.5. The hydrogen storage capacity for this material, where effective dehydrogenation occurs at 400°C, is 4.85%. Also, the kinetics for the said composite material are much better than La2Mg17 alone. The x = 10 wt% material also has a faster kinetics-about 3 times faster than that of La2Mg17 alone. Evidence and arguments have been advanced to show that the improved hydrogenation-storage capacity and better kinetics of the composite material arise due to the multiphase nature of the material (besides the majority phase La2Mg17, other minority phases exist, e.g. MgNi2, Mm2Ni7 and Ni are present). © 1993.PublicationArticle Synthesis of nanocrystalline cerium oxide by high energy ball milling(2012) T.P. Yadav; O.N. SrivastavaWe have synthesized pure nanocrystalline CeO 2 powders of nearly spherical shape using high-energy attritor ball mill. Milling parameters such as the milling speed of 400 rpm, ball to powder ratio (40:1), milling time (30 h) and water cooled media were determined to be suitable for synthesizing nanosize (∼10 nm) powders of CeO 2. The powders after milling for various durations (up-to 50 h) were characterized by X-ray Diffraction, Scanning Electron Microscopy, Energy-dispersive X-ray Spectrometry and Transmission Electron Microscopy. An average particle size of 10 nm was obtained at 30 h milling, after which the particle agglomeration started, and a mixture of nanocrystalline and amorphous phase was observed after 50 h milling. © 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.PublicationArticle Nucleation and growth of catalyst-free zinc oxide nanostructures(2005) Jai Singh; Anchal Srivastava; R.S. Tiwari; O.N. SrivastavaThis paper deals with the investigations on the nucleation and growth of Zinc Oxide (ZnO) nanostructures in a catalyst free synthesis. The ZnO nanostructures have been formed by evaporation of Zn (99.99%) in O 2 and Ar atmosphere in single zone furnace under two temperature regions, region A (∼1173-1073 K) and region B (∼873-773 K). Through application of XRD and TEM techniques, it has been shown that first ZnO is formed which changes to ZnO x through creation of oxygen vacancies. The ZnO x acts as self-catalyst and leads to formation of various nanostructures. Those observed in the present investigation are nanotetrapods (1D, diameter ∼ 70-450 nm, length ∼ 2-4.5 μm) nanorods (1D, diameter ∼ 45-95 nm, length ∼ 2.5-4.5 μm), nanoflowers(2D, central core diameter ∼ 90-185 nm, length of petals/nanorod ∼ 1.0-3.5 μm) and nanoparticles (3D, size ∼ 0.85-2.5 μm). These nanostructures have been revealed by SEM explorations. Attempts have been made to explain the formation of the various nanostructures in terms of the creation and distribution of the ZnO x, the temperature as well as oxygenation conditions. Copyright © 2005 American Scientific Publishers All rights reserved.PublicationArticle Study of harmonic generation in Hg(Tl)BaCaCuO thin film(Elsevier, 1998) Neeraj Khare; A.K. Gupta; S.K. Shrivastava; H.K. Singh; O.N. SrivastavaThe generation of higher harmonics in Hg(Tl)BaCaCuO high-Tc thin film is reported, when an ac magnetic field is applied to it. The film has been prepared by the spray pyrolysis technique using two step process and exhibits Tc(R = 0) = 123 K. Application of an ac magnetic field (Hac > Hc1) on the film leads to generation of higher odd harmonics. Further, application of the dc field (Hdc) in addition to the ac field causes generation of even harmonics also. Variations of amplitude of these harmonics as a function of Hac, Hdc and temperature are studied. The variation of second and third harmonic amplitudes with Hdc shows the hysteresis effect. © 1998 Published by Elsevier Science B.V. All rights reserved.PublicationArticle TiH2 as a Dynamic Additive for Improving the De/Rehydrogenation Properties of MgH2: A Combined Experimental and Theoretical Mechanistic Investigation(American Chemical Society, 2018) Ashish Bhatnagar; J. Karl Johnson; M.A. Shaz; O.N. SrivastavaWe report a mechanism for how low concentrations of TiH2 can function as a dynamic additive for improving both the kinetics and the thermodynamics of the Mg/MgH2-TiH2 system. We use a combination of experimental and theoretical methods to study de/rehydrogenation in Mg/MgH2-TiH2 systems. Our experiments show that ball milling MgH2 with 5 wt % TiH2 improves the thermodynamics and kinetics of de/rehydrogenation in Mg/MgH2, and these improvements are robust with respect to cycling. The enthalpy change for the Mg/MgH2-TiH2 system is about 7 kJ/mol H2 lower than that for ball-milled MgH2. Structural (X-ray diffraction, in situ X-ray diffraction) and microstructural (electron microscopy) characterization has been employed to understand the role and state of the catalyst. We have developed a theoretical model that broadly captures the experimental observations and performed first-principles density functional theory and phonon density of states calculations to identify the energetically preferred location of TiH2 in the Mg/MgH2-TiH2 systems. We found that TiH2 located beneath the surface of Mg is energetically more favorable than being located on top of Mg. In contrast, TiH2 on the surface of MgH2 is favored over subsurface configurations. We hypothesize that TiH2 particles migrate from the subsurface to the surface during hydrogenation of Mg and in the opposite direction during dehydrogenation of MgH2. The hypothesis of migration of TiH2 particles during de/rehydrogenation of Mg/MgH2-TiH2 is supported by our microstructural (electron microscopy) and XPS studies. © 2018 American Chemical Society.PublicationConference Paper Hydrogen fueled vehicular transport for Indian and other developing countries(2006) B.K. Gupta; O.N. SrivastavaThe Hydrogen Energy Centre at Banaras Hindu University has been carrying out R&D and initiation of commercialization efforts with assistance from Ministry of Nonconventional Energy Sources (New Delhi, India) and commercial collaboration with International Cars and Motors Ltd. (Punjab, India) on hydrogen fueled vehicular transport prevalent in India, the two and three-wheelers. India has 70% of the vehicles on road of the type of two wheeler (motorcycle / scooters) for personalized transport and three wheelers for hired transport. The IC engine (SI) based two and three wheelers are 100cc, 175cc, 4-stroke vehicles. We have employed timed manifold hydrogen injection in the IC combustion chamber. The timing is provided either through a cam or diaphragm or electronic system. A hydrogen injector system, which can vary hydrogen and air fuel ratio adjusted for knock and backfire free operation has been fabricated. Since the two and three wheelers are small vehicles, high-pressure gaseous storage is not compatible from the point of view of either volumetric compatibility or safety. We have therefore employed in house fabricated AB5 type (Mm:Ni:Fe) hydrides with ∼2.0 wt% storage capacity mounted in a vehicle exhaust coupled heat exchanger tank. The range of the vehicle is 60 to 80 kms, which are compatible for intracity transport in India. What is true of Indian conditions also holds for other developing countries where two and three wheelers are common mode of vehicular transport. Further details on the development of the hydrogen fueled two and three wheelers will be described and discussed.PublicationArticle Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy(1995) N.P. Lalla; R.S. Tiwari; O.N. Srivastava; B. Schnell; G. ThummesThe low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al65Cu20Ru15 quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a {Mathematical expression}-variation, whereas for the range 20 to 200 K a linear T-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ {Mathematical expression} behaviour prevailing over such a wide temperature range has been observed for the first time for Al65Cu20Ru15. A quantitative analysis of the {Mathematical expression}-variation in terms of electron-electron interaction yields a very low density of states at the Fermi level. © 1995 Springer-Verlag.PublicationConference Paper Comparative photoelectrochemical study of PEC solar cell fabricated with n-TiO2 photoelectrodes at different temperatures and under different oxygen flow rates(2006) P.R. Mishra; P.K. Shukla; O.N. SrivastavaPhotoelectrochemical splitting of water induced by solar energy for hydrogen production has been studied in the present investigation. PEC solar cell was fabricated with n-TiO2 photoelectrodes synthesized at different oxidation temperatures e.g. 700°C, 750°C, 800°C and 850°C under oxygen flow rate 200 ml/min, 350 ml/min and 500 ml/min. The optimum oxygen flow rate for all the temperatures was found to be 350 ml/min. This is therefore kept invariant for synthesis of electrodes at different temperatures. The photoelectrochemical characterization of the PEC cell was done in the three-electrode configuration, i.e Ti/n-TiO2/1M-NaOH/Pt. It has been observed that the optimum values of the PEC solar cell parameters are exhibited by the solar cell employing the photoelectrodes prepared at ∼750°C. The XRD and SEM explorations revealed that the TiO2 prepared at ∼750°C is in the nanometric range (∼100-150 nm). The TiO2 films formed at this temperature has been found to exhibit optimum PEC solar cell parameters. The PEC parameters, like photocurrent density, photoconversion efficiency and hydrogen production rate, with this photoelectrode correspond to 0.93 mA/cm2, 0.472% and 4.00 l/hm 2 respectively.PublicationArticle Relaxor ferroelectric phase transition and ac conduction in polycrystalline Gd0.55Ca0.45MnO3 at low temperature(Elsevier Ltd, 2021) Pooja Pant; Harshit Agarwal; Suresh Bharadwaj; O.N. Srivastava; M.A. ShazThe present investigation reports the dielectric phase transition studies in polycrystalline Gd0.55Ca0.45MnO3 along with ac conduction and structural property. Polycrystalline Gd0.55Ca0.45MnO3 has been synthesized by the solid-state synthesis route at 1350 °C which crystallized in the orthorhombic phase with centrosymmetric Pnma symmetry at room temperature. The complex dielectric and modulus properties of polycrystalline Gd0.55Ca0.45MnO3 has been explored in the frequency range of 500 Hz to 1 MHz at low temperatures having the range 6 K–200 K. The real part of dielectric permittivity shows the sharp increase up to 30 K–120 K temperature range and then decreases constantly which confirm the characteristic of relaxor ferroelectric material. The low-temperature dielectric study reveals that Gd0.55Ca0.45MnO3 possesses a high dielectric permittivity and low loss below 120 K. The frequency variation of ac conductivity shows the correlated barrier hopping in the temperature range of 10 K–60 K and 100 K–180 K and gives the signature of small polaron hopping at 60 K–100 K. Thermal variation of activation energy for Gd0.55Ca0.45MnO3 gives the activation energy of 0.0634(2) eV for higher frequency 1 MHz. © 2021 Elsevier B.V.PublicationConference Paper Role of intermetallic materials in hydrogen storage(2008) Rajesh Kumar Singh; Sunil Kumar Pandey; T.P. Yadav; O.N. Srivastava[No abstract available]