2005
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PublicationArticle Synthesis, characterizations and applications of some nanomaterials (TiO2 and SiC nanostructured films, organized CNT structures, ZnO structures and CNT-blood platelet clusters)(Indian Academy of Sciences, 2005) O.N. Srivastava; A. Srivastava; D. Dash; D.P. Singh; R.M. Yadav; P.R. Mishra; J. SinghTiO2 nanostructured films have been synthesized by the hydrolysis of Ti[OCH(CH3)2]4 as the precursor. These films have been utilized for the dissociation of phenol contaminant in water. Free-standing nanostructured film of silicon carbide (SiC) has been synthesized, employing a simple and new route of spray pyrolysis technique utilizing a slurry of Si in hexane. Another study is done on organized carbon nanotube (ONT) structures. These are made in the form of hollow cylinders (50 mm length, 4 mm diameter and 1.5 mm wall thickness). These CNT-based cylinders are made of conventional CNT and bamboo-shaped CNT. The filtrations of heavy hydrocarbons and E. coli bacteria from water have been carried out. In addition to this, ZnO nanostructures have also been studied. Another study concerns CNT-blood platelet clusters. © Indian Academy of Sciences.PublicationArticle Effect of annealing temperature on the structural-microstructural and electrical characteristics of thallium bearing HTSC films prepared by chemical spray pyrolysis technique(Indian Academy of Sciences, 2005) K.K. Verma; R.S. Tiwari; O.N. SrivastavaIn order to get good quality reproducible films of Tl: HTSC system, we have studied the different annealing conditions to finally achieve the optimized annealing condition. In the present investigation, Tl-Ca-Ba-Cu-O superconducting films have been prepared on YSZ (100) and MgO (100) single crystal substrates via precursor route followed by thallination. The post deposition heat treatments of the precursor films were carried out for various annealing temperatures (870°C, 890°C) and durations (1 and 2 min). The optimized thallination procedure occurred at 870°C for 2 min into good quality films with Tc (R = 0) ∼ 103 K for YSZ and Tc (R = 0) ∼ 98 K for MgO substrates, respectively. Further we have correlated the structural/microstructural characteristics of the films. © Indian Academy of Sciences.PublicationArticle Studies on the formation and stability of nano-crystalline Al50Cu28Fe22 alloy synthesized through high-energy ball milling(Elsevier BV, 2005) T.P. Yadav; N.K. Mukhopadhyay; R.S. Tiwari; O.N. SrivastavaThis work reports the formation of nano-crystalline Al50 Cu28Fe22 alloy by high-energy ball milling. The Al50Cu28Fe22 alloy, synthesized through slow cooling of the molten alloy, was subjected to milling in an attritor mill at 400 rpm for 5 h, 10 h, 20 h, 40 h and 80 h with a ball to powder ratio of 40:1 in hexane medium. The X-ray diffraction and transmission electron microscopy observation of milled samples revealed that the milling duration of 5-40 h has led to the formation of nano-phase. The average crystallite size has been found to be of ∼17 nm. When the nano-crystalline alloy was vacuum annealed at a temperature of ∼500 °C for 5-20 h, new structural phases corresponding to the superstructures of crystallite size around 100-200 nm of the parent nano-crystalline phase were identified. The superstructures have been found to correspond to the simple cubic structure with asc = √2ap and the face centred cubic structure with afcc = 2ap (ap: lattice parameter of parent nano-crystalline alloy). It has been proposed that the formation of two different types of superstructures resulting from ball milling followed by annealing is possibly governed by the minimization of free energy of phase constituents. © 2004 Elsevier B.V. All rights reserved.PublicationArticle Effect of Ca doping for Y on structural/microstructural and superconducting properties of YBa2Cu3O7-δ(Elsevier, 2005) Rajiv Giri; V.P.S. Awana; H.K. Singh; R.S. Tiwari; O.N. Srivastava; Anurag Gupta; B.V. Kumaraswamy; H. KishanWe report systematic studies of structural, microstructural and transport properties of Y1-xCaxBa2Cu3O 7-δ bulk samples with 0.20 ≥ x ≥ 0.0. The partial substitution of Ca2+ at Y3+ site in YBa2Cu 3O7-δ leads towards slightly overdoped regime, which along with disorder in CuO2 planes is responsible for decrease in superconducting transition temperature (Tc) of the doped system. The microstructural variants being studied by transmission electron microscopy (TEM) technique in imaging and selected area diffraction modes revealed an increase in the density of twins with increase in Ca concentration despite the fact that there is slight decrease in the orthorhombicity. This is against the general conviction of decrease in twin density with decreasing orthorhombicity. An increase in twin density results in sharpening of twin boundaries with increasing Ca concentration. These sharpened twin boundaries may work as effective pinning centers. A possible correlation between microstructural features and superconducting properties has been put forward and discussed in the present investigation. © 2005 Elsevier B.V. All rights reserved.PublicationArticle Effect of Ba doping on structure and magneto-transport properties of layered manganite La1.4Ca1.6-xBa xMn2O7(2005) H.K. Singh; Ajai. K. Gupta; P.K. Siwach; O.N. SrivastavaThe magneto-transport properties of the double-layer manganite La 1.4Ca1.6-xBaxMn2O 7 (LCBMO) (x=0.0, 0.2, 0.4 and 0.6) have been studied in the temperature range 300-20 K. Polycrystalline bulk samples were prepared by the solid-state reaction route. The X-ray diffraction reveals these samples to be single phased having Sr3Ti2O3-type tetragonal crystal structure. Lattice parameters are found to increase with increasing Ba content. The paramagnetic (PM) to ferromagnetic (FM) transition temperature (TC) is observed to shift to higher values with the increase in the Ba content and TC values are 135, 144, 163 and 185 K, respectively, for Ba contents x=0.0, 0.2, 0.4 and 0.6. The increase in the TC may be due to the reduction of the distortion of the MnO6 octahedron and hence suppression of the Jahn-Teller distortion due to substitution of larger Ba cations. The resistivity of all the samples is found to decrease as the temperature is reduced, and below TC an insulator to metal (IM) transition is observed at a temperature TP which is 125, 131, 133 and 136 K for x=0.0, 0.2, 0.4 and 0.6, respectively. The increased discrepancy between the TC and TP values is due to the increased anisotropy of the FM exchange interaction as a consequence of Ba doping. There is a significant increase (∼30% increase) in the low-field low-temperature magnetoresistance in the Ba-doped samples. At 77 K and Hdc=600Oe, the magnetoresistance values are measured to be 10, 10.7, 11.5 and 13.2%, respectively x=0.0, 0.2, 0.4 and 0.6. © 2004 Elsevier B.V. All rights reserved.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 Effect of la doping on microstructure and critical current density of MgB2(2005) Chandra Shekhar; Rajiv Giri; R.S. Tiwari; D.S. Rana; S.K. Malik; O.N. SrivastavaIn the present study, La-doped MgB2 superconductors with different doping levels (Mg1-xLaxB2; x ≤ 0.00, 0.01, 0.03 and 0.05) have been synthesized by the solid-state reaction route at ambient pressure. Effects of La doping have been investigated in relation to microstructural characteristics and superconducting properties, particularly intragrain critical current density (Jc). The microstructural characteristics of the as-synthesized Mg(La)B2 compounds were studied employing the transmission electron microscopic (TEM) technique. The TEM investigations reveal inclusion of LaB6 nanoparticles within the MgB2 grains, which provide effective flux pinning centres. The evaluation of intragrain Jc through magnetic measurements on the fine powdered version of the as-synthesized samples reveal that Jc values of the samples change significantly with the doping level. The optimum result on Jc is obtained for Mg 0.97La0.03B2 at 5 K; Jc reaches ∼1.4 × 107 A cm-2 in self-field, ∼2.1 × 106 A cm-2 at 1 T, ∼2.5 × 10 5 A cm-2 at 2.5 T and ∼1.8 × 104 A cm-2 at 4.5 T. The highest value of intragrain Jc in the Mg0.97La0.03B2 superconductor has been attributed to the inclusion of LaB6 nanoparticles, which are capable of providing effective flux pinning centres. © 2005 IOP Publishing Ltd.PublicationArticle On the structural, hydrogenation behaviour and computer simulation studies of P-C-T of Zr1-2xMmxTixFe1.4Cr 0.6 (x = 0.03, 0.05, 0.07, 0.09) alloys(2005) Sudipta Roy; B.K. Singh; M.V. Lototsky; O.N. SrivastavaIn this paper an investigation has been made on the effects of vacuum annealing treatment at 1000 °C for 100 h on the hydrogen storage capacity of Mm, Ti and Cr substituted Zr1-2xMmxTixFe 1.4Cr0.6 (x = 0.03, 0.05, 0.07, 0.09) Laves phase alloys. XRD patterns show that annealing has no significant effect on the structure of the alloy. However, the crystal lattice strain and defects decreased and grain size increased for all compositions studied after annealing. These materials exhibit high uptake of hydrogen as compared to those by the as-synthesized alloy. The optimum alloy corresponds to the composition Zr0.9Mm 0.05Ti0.05Fe1.4Cr0.6. It shows a storage capacity of ∼1.94 wt.% and kinetics of ∼5.83 × 10 -4 m3/s/kg. This is one of the highest storage capacity reported for any ZrFe2 type alloy. The unit cell volumes have been found to correlate with storage capacity. The P-C isotherms at various temperatures have been simulated by a suitable mathematical model. For this, the randomized variables ΔH and ΔS have been evaluated experimentally. These have been used as input parameters for simulation. The simulated and experimental curves have been found to match closely. © 2005 Elsevier B.V. All rights reserved.PublicationReview On the evolution of a nanocrystalline phase from the Al-Cu-Fe quasicrystalline alloy during high energy ball milling(2005) T.P. Yadav; N.K. Mukhopadhyay; R.S. Tiwari; O.N. SrivastavaMechanical milling of an Al-Cu-Fe quasicrystalline alloy was performed in a high energy ball mill (Szegvari attritor mill) at a constant speed of 400 rpm for various milling time (from 0.5 to 40 h) under liquid hexane medium with a ball to powder ratio of 40:1. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and x-ray diffraction (XRD) techniques were employed for characterizing the milled and unmilled samples. The evolution of nanocrystalline (NC) phase (bcc, a =0.29 nm) was observed from the quasicrystalline phase after 5h of milling. This nanocrystalline phase, which was quite stable up to 40h of milling, did not transform to amorphous or any other metastable phase. The size of crystalline particles was found to vary from 60nm to minimum 10nm for different milling durations. It is interesting to note that the strain induced in the milled samples tends to increase along with milling time. The possible mechanisms for the formation of nanocrystalline phase will be put forward based on the evolution of the structural and microstructural features.PublicationArticle Investigations on hydrogen storage behavior of CNT doped NaAlH4(2005) D. Pukazhselvan; Bipin Kumar Gupta; Anchal Srivastava; O.N. SrivastavaIn this paper, we have carried out investigations on admixing carbon nanotubes (CNT) to increase the desorption rate of hydrogen in NaAlH 4. So far only transition metal Ti has been attempted as the suitable dopant for making NaAlH4 (purified version) a viable hydrogen storage material by increasing the hydrogen desorption rate. Out of the various materials corresponding to NaAlH4-x mol% CNT (x = 2, 4, 6, 8 and 12), we have found that the material with x = 8 mol% is the optimum material. It shows highest desorption rate leading to 3.3 wt.% of H2 at ∼160 °C within 2 h for the first dissociation reaction. The CNT admixed NaAlH4 has also been found to exhibit good rehydrogenation characteristics (reversibility on hydrogenation up to ∼4.2 wt.%). A feasible mechanism for the improvement of hydrogen desorption characteristics has been put forward. © 2005 Elsevier B.V. All rights reserved.