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Preparation of high-Tc superconducting thin films of Tl-Ca-Ba-Cu-O by the chemical spray pyrolysis technique
(1992) K.K. Verma; A.K. Sexena; A.K. Singh; R.S. Tiwari; O.N. Srivastava
Thin films of the Tl-Ca-Ba-Cu-O system have been prepared on MgO and YSZ single-crystal substrates by a simple chemical process (the spray pyrolysis technique). Since Tl is toxic, the Tl-Ca-Ba-Cu-O films were prepared by a two step process. Firstly, precursor (Ca-Ba-Cu-O) films were prepared followed by diffusion of Tl derived by vaporizing Tl2O3 in a closed system under oxygen flow. A new feature found in the present investigation is that the Tc (R=0) of the Tl-Ca-Ba-Cu-O films has been found to depend upon the microstructural characteristics of the precursor films which can be varied by annealing these films. The Tc (R=0) of the Tl-Ca-Ba-Cu-O HTSCs films prepared from the annealed precursor films has been found to be higher than in the films prepared from the as-deposited precursor films. As, for example, in the case of a film on a MgO substrate, the Tc (R=0) has been enhanced from approximately 90 K to approximately 98 K. The highest T c obtained by the authors for the films synthesized from the annealed precursor films on YSZ substrates corresponds to approximately 103 K. The XRD reveals the biphasic (2223/2122) nature of the films. The microstructural characteristics of the films as monitored by SEM reveals the partially oriented nature of the Tl-Ca-Ba-Cu-O film.
Synthesis, characterization and hydrogenation behaviour of composite hydrogen storage alloys, LaNi5/La2Ni7, LaNi3
(Elsevier BV, 1999) Sumita Srivastava; O.N. Srivastava
The present study deals with investigations on the composite hydrogen storage materials LaNi5/La2Ni7, LaNi3, for efficient, high energy density Ni-MH batteries. To achieve this goal extensive investigations were carried out to study the effect of presence of secondary phases on the main phase LaNi5 with reference to hydrogenation behaviour and structural-microstructural characterizations using XRD, TEM and SEM techniques. Alloys were synthesized through melt-casting with varying concentrations of secondary phases like La2Ni7, LaNi3, Ni (minor phases), i.e., 55%, 30%, 5% and 0% by a special technique of pellet encapsulation. Structural characterizations using the XRD technique confirmed the formation of secondary phases together with the major phase LaNi5. The diffraction patterns after hydrogenation did not exhibit secondary phase diffraction peaks, instead the resulting phase was found to be single phasic, exhibiting LaNi5 like diffraction patterns with an unusual intensity distribution. TEM investigations showed the presence of two types of modulated phases; one of which corresponds to a≈1.5o, c≈co and the other to a≈1.3ao, c≈co, together with the parent phase (LaNi5), in the as-synthesized and hydrogenated/dehydrogenated versions, corresponding to alloy with secondary phase concentration upto 30%, (ao and Co being lattice constants of the parent LaNi5 structure). The signature of the modulated phases were not present in XRD patterns, but could be seen in the selected area electron diffraction patterns, suggesting that modulation has taken place in local areas of the basal plane, due to ordering of hydrogen atoms. These modulated unit cells can be modeled in terms of the original lattice of LaNi5, by inserting and ordering of hydrogen atoms at viable interstitial sites. Micro structural evaluations through SEM revealed that secondary phases suppressed pulverization. Investigations of the hydrogenation behaviour of these alloys showed that hydrogen storage capacity and desorption kinetics did not decrease for minor concentrations of secondary phases, but a decrease was found for the alloys having higher concentrations of secondary phases. Similar results were obtained for alloy MmNi5, where secondary phases corresponded to Mm2Ni7 and MmNi. © 1999 Elsevier Science S.A. All rights reserved.
Shape and Size-Dependent Magnetic Properties of Fe3O4 Nanoparticles Synthesized Using Piperidine
(Springer New York LLC, 2017) Ashwani Kumar Singh; O.N. Srivastava; Kedar Singh
In this article, we proposed a facile one-step synthesis of Fe3O4 nanoparticles of different shapes and sizes by co-precipitation of FeCl2 with piperidine. A careful investigation of TEM micrographs shows that the shape and size of nanoparticles can be tuned by varying the molarity of piperidine. XRD patterns match the standard phase of the spinal structure of Fe3O4 which confirms the formation of Fe3O4 nanoparticles. Transmission electron microscopy reveals that molar concentration of FeCl2 solution plays a significant role in determining the shape and size of Fe3O4 nanoparticles. Changes in the shape and sizes of Fe3O4 nanoparticles which are influenced by the molar concentration of FeCl2 can easily be explained with the help of surface free energy minimization principle. Further, to study the magnetic behavior of synthesized Fe3O4 nanoparticles, magnetization vs. magnetic field (M-H) and magnetization vs. temperature (M-T) measurements were carried out by using Physical Property Measurement System (PPMS). These results show systematic changes in various magnetic parameters like remanent magnetization (Mr), saturation magnetization (Ms), coercivity (Hc), and blocking temperature (TB) with shapes and sizes of Fe3O4. These variations of magnetic properties of different shaped Fe3O4 nanoparticles can be explained with surface effect and finite size effect. © 2017, The Author(s).
Mechanically induced phase transformation in Al-Cu- Fe-Cr decagonal quasicrystalline alloy by high-energy ball milling
(2011) T.P. Yadav; N.K. Mukhopadhyay; R.S. Tiwari; O.N. Srivastava
In the present investigation, an Al-Cu-Fe-Cr decagonal quasicrystalline alloy synthesized by a slow cooling technique, has been mechanically milled in a high-energy ball mill for various times ranging up to 100 h under liquid hexane medium at the speed of 400 rpm with a ball to powder ratio of 40:1. After milling, the milled product has been characterized in order to understand the evolution of the various metastable phases by using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Powders milled for more than 10 h contained predominantly the B2-type crystalline phase with a lattice parameter of ~ 0.29 nm. No amorphous phase could be identified after a prolonged milling up to 100 h. The disordering in the transformed crystalline phased could be observed. The mechanical stability during milling and the evolution nano phases will be discussed with reference to their relative stability. © CSIR-National Metallurgical Laboratory, Jamshedpur-831007, India, 2011.
Negative resistance in ZrSe2 single crystals
(1993) K.S. Bartwal; O.N. Srivastava
Voltage-controlled negative resistance (VCNR) has been observed in the semiconducting layered compound ZrSe2 at a field of ∼ 50-100 V cm-1, at room temperature. The occurence of VCNR is attributed to the intervalley transfer of electrons in the conduction band. © 1993.
Studies on the synthesis, characterization and hydrogenation behaviour of new Zr1-xMmx(Cr0.8Mo0.2)2 AB2-type hydrogen storage materials
(2003) B.K. Singh; A.K. Singh; M.A. Imam; O.N. Srivastava
This paper deals with the improvement in the hydrogenation behaviour of ZrCr2 type alloys through substitutions at both the Zr and Cr sites. It has been found that the alloy Zr1-xMmx(Cr1-yMoy)2 exhibits better hydrogen storage capacity and kinetics as compared to the parent alloy ZrCr2 or its variant Zr(Cr1-yMoy)2. The optimum material has been found to correspond to Zr0.93Mm0.07(Cr0.8Mo0.2)2 which shows a high hydrogen storage capacity of ∼1.8 wt.% under ambient conditions. This is the highest storage capacity known so far for the ZrCr2 type hydrogen storage system. It should be pointed out that the storage capacity reported previously in the literature for the optimum material Zr(Cr0.8Mo0.2)2 has a value of 1.53 wt.% only. In addition to the higher storage capacity, the material Zr0.93Mm0.07(Cr0.8Mo0.2)2 also exhibits kinetics (5×10-4 m3/s/kg) that are faster than the known kinetics (3×10-4 m3/s/kg) for the ZrCr2 storage system. A correlation between structural/microstructural characteristics and the improved hydrogenation behaviour (higher storage capacity and faster kinetics) has been outlined. © 2003 Elsevier Science B.V. All rights reserved.
Investigations on the structural and hydrogenation characteristics of LaNi5, HoNi5, GdNi5, SmNi5, MmNi5, and CFMmNi4.5Al0.5 thin films
(1985) S.K. Singh; A.K. Singh; K. Ramakrishna; O.N. Srivastava
This communication deals with the investigations made on synthesis, structural characteristics, hydrogen absorption behaviour and electrical resistivity variation with hydrogenation in thin films of rare earth metal pentanickelides-RNi5 where R = La, Sm, Gd, Ho and Mm (Mischmetal) and a related phase cerium-free MmNi4.5Al0.5. The synthesis of the above mentioned intermetallics was accomplished by suitable solid-state interdiffusion and homogenization. The as-synthesized flux were characterized by X-ray diffraction. The thin films prepared by vacuum thermal vapour deposition were characterized by transmission electron microscopy in various modes. These films were amorphous and transformed to crystalline form on annealing. The hydrogen absorption behaviour (both in bulk and thin films) was studied by employing electrolytic method. It was found that amorphous forms absorb more hydrogen than their crystalline counterparts. The variation of electrical resistivity with hydrogen absorption in thin-film form of various intermetallics was also studied. Plausible explanations for the above hydrogenation characteristics have been outlined. © 1985.
Sexual disturbances in temporal lobe epilepsy: A controlled study
(1979) G.D. Shukla; O.N. Srivastava; B.C. Katiyar
Seventy cases each of temporal lobe epilepsy and grand mal epilepsy were studied for their sexual functioning. The 2 groups were similar as regards age, sex, duration of illness, frequency of seizures and menstrual and marital history. A significantly greater number of temporal lobe epileptics were found to be hyposexual. They had a global loss of performance and interest in the sexual sphere and showed no concern over it. One case in the group of temporal lobe epilepsy, as against none in the other group, was hypersexual. There were no cases of sexual deviations in either of the groups. The findings of the study are discussed in the light of the relevant literature.
The formation and characterization of Tl doped Hg-Ba-Ca-Cu-O superconducting films synthesised through spray pyrolysis
(Elsevier, 1996) H.K. Singh; A.K. Saxena; O.N. Srivastava
The Tl doped Hg-Ba-Ca-Cu-O HTSC films have been synthesised by reacting Ba-Ca-Cu-O precursor films deposited by spray pyrolysis with a mixture of Hg-Tl vapour in an evacuated sealed quartz tube. These films (∼ 3 μm in thickness) having 1223 as the majority phase are found to possess Tc(R = 0)'s of 127 K and 122 K on MgO (100) and ZrO2 (100) single-crystal substrates, respectively, and Jc ∼ 105 A/cm2 (both MgO and ZrO2 have 20 K). The observed Tc(R = 0) of 127 K is the highest reported so far for Tl doped Hg-Ba-Ca-Cu-O films. Surface morphological investigations reveal the occurrence of layered spiral-like growth features.
Effect of growth temperature on gallium nitride nanostructures using HVPE technique
(2012) S. Munawar Basha; S.R. Ryu; T.W. Kang; O.N. Srivastava; V. Ramakrishnan; J. Kumar
The growth of hexagonal wurzite one dimensional (1D) gallium nitride (GaN) nanostructures on sapphire substrates using hydride vapor phase epitaxy (HVPE) process was carried out at two different temperatures (973 K and 1023 K). The GaN nanoneedles were formed at 973 K and hexagonal nanorods get formed at 1023 K. The morphologies of these nanostructures were studied using high resolution scanning electron microscope. X-ray diffraction and micro-Raman spectroscopy measurements confirmed that the as grown GaN nanostructures are of hexagonal wurtzite structure without any oxide phase. The emission properties of these nanostructures have been investigated using photoluminescence. © 2012 Elsevier B.V. All rights reserved.

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