Browsing by Author "K. Ramakrishna"

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Effect of hydrogenation on the electrical resistivity of LaNi5 films
(Kluwer Academic Publishers, 1987) K. Ramakrishna; O.N. Srivastava
[No abstract available]
Electron microscope studies of structural characteristics of Tl(Bi)-Sr-Ca-Cu-O high temperature superconductor
(1991) G.D. Verma; K. Ramakrishna; O.N. Srivastava
Tl(Bi)-Sr-Ca-Cu-O corresponding to nominal compositions Tl:Bi:Sr:Ca:Cu=(2-x):x:2:2:3 (where x=0.2, 0.4, 0.6 and 1) and Tc ranging between approximately 77 K and approximately 106 K have been synthesized and their structural characteristic explored employing the technique of transmission electron microscopy. An interesting result arising from structural explorations in the present investigation is the presence of a new disordered superstructure a approximately=4.2 AA, c approximately=15.26 AA) embedded in the main monolayer Tl-0, 1212/1223-type structural matrix. Although the new disordered superstructural phase is present for all the compositions studied, its density was found to be highest for the nominal composition Tl 1.6Bi0.4Sr2Ca2Cu3O 10. This also corresponds to the highest Tc approximately 106 K HTS (high-temperature superconductor) material. It has been suggested that the substitution of Bi for Tl produces disorder through the occurrence of a lone pair on Bi3+. Besides the creation of disorder, the presence of Bi3+ may also lead to the optimization of carrier concentration and hence Tc.
Electron microscopic investigations of neutron irradiated YBa2Cu3O7-x high temperature superconducting single crystals
(1991) Ram Janam; K. Ramakrishna; O.N. Srivastava; B.A. Dasannacharya; P.R. Vijayaraghavan
There has been several recent explorations of HTSC materials particularly YBCO elucidating enhancements of critical current density 'Jc' as a result of fast neutron irradiation (∼1 MeV, fluence 1016/cm2) (see e.g. Van Dover et al 1989). Although the enhancements in Jc are known to result from the flux pinning effects arising from the neutron irradiation created defects, the nature of these defects have remained unknown. The present communication elucidates the details of these defects. Employing the technique of transmission electron microscopy in normal and high resolution modes, the nature of the defects have been uncovered. It has been shown that the fast neutron irradiation creates defects corresponding to thermal and displacement spikes. In the thermal spike region the defects correspond to local removal of chain containing CuO layers and consequent creation of linear defect stripe regions with a characteristic of width of ∼ 13.6 Å. After the defect stripe, the displacement spike corresponding to removal and rearrangement of oxygen atoms from CuO chain containing CuO layers are produced. These defects consist of linearly arranged clustures (∼ 100 to 300 Å ). The effectiveness of these defects particularly the defect stripes as flux pinning centres has been outlined. © 1991.
Electron microscopic investigations of the as-synthesised and dissociated Y: 124 HTSC phases
(1992) A.K. Srivastava; K. Ramakrishna; O.N. Srivastava
The Y: 124 was synthesised at ambient oxygen pressure using an oxygen enhancer (NaNO3). The as-synthesised material was thermally dissociated at 915°C for various time spans, e.g., 1, 2 and 5 min. X-ray diffraction (XRD) results showed that the undissociated sample corresponds to the Y: 124 phase, whereas the dissociated samples consisted of mixed phases. For the samples dissociated for 1 min these were Y: 124 and Y: 123 (with some traces of CuO), for dissociation times of 2 and 5 min these were dominantly Y: 123 and CuO. From R-T and L-T (inductance-temperature) measurements, it was found that the undissociated Y: 124 possesses a Tc(R=0) ≈80 K and there is no drop in inductance for dissociated samples (2 and 5 min). Microstructural investigations of Y: 124 revealed that the material dissociated for 1 min is biphasic, consisting of Y: 124 and Y: 123. Besides stacking disorder, local area superstructures along "c" emanating from the native Y: 124 phase were also observed. This is thought to ensue from the random and sequential removal of CuO chains from the Y: 124 structure. The sample subjected to longer dissociation times, i.e., 2 and 5 min consists of Y: 123 and CuO precipitates. For longer dissociation times, e.g. 2 min and above. the Y: 124 phase nearly completely converts into Y: 123 and CuO. © 1992.
Electron microscopic observations of Bi and Tl bearing cuprate high temperature superconductors
(Springer India, 1991) B. Das; K. Ramakrishna; G.D. Verma; R.S. Tiwari; O.N. Srivastava
The second generation of high temperature superconductors typified by Bi2Sr2Ca n-1Cu n O2 n+4 and Tl2Ba2Ca n-1Cu n O2 n+4 exhibits curious structural properties which have direct relevance to the superconducting behaviour particularly transition temperature (T c ). The present paper reports on investigations of structural properties at microlevel in Bi-bearing HTCS. We have found curious structural characteristics which manifests itself in the form of transformation from a p ×a p ×c to (2)1/2 a p ×(2)1/2 a p ×c through the loss of calcium atoms and the formation of five-fold modulated phase along b through the loss of Ca and Sr atoms. We have also found the evidence of high periodicities (n=4) Bi2Sr2Ca3Cu4O12. The microstructural characteristics of HTCS showing the higher T c (R=0) ∼ 120 K exhibits unusual characteristics. © 1991 Indian Academy of Sciences.
Electron microscopic studies of local structures of high temperature YBa2Cu3O7-x superconductor
(1988) K. Ramakrishna; B. Das; A.K. Singh; R.S. Tiwari; O.N. Srivastava
The local structural characteristics of the high temperature superconducting material YBa2Cu3O7-x (YBCO) have been explored employing electron microscope technique. Two special structural characteristics have been obtained. One of these corresponds to a significant local area variation of the c periodicity (∼11.6 to ∼11.8 Å) and the other relates to the domain formation. It has been suggested that the variation of the c periodicity originates due to cation disorder in the chain -Ba-Ba-Y-Ba-Ba-(e.g. -Ba-Ba-Ba-Y-Ba- and -Ba-Ba-Cu-Ba-Y-). The domain formation which represents an important result of the present investigation originates due to ordering of oxygen vacancies on alternative equivalent set of sites in different crystal regions. A domain model together with the inherent defect lamellae has been advanced and the oxygen stoichiometries obtained by different workers e.g. 6.81 and 6.90, have been shown to be a natural artifact of oxygen ordering and domain formation. It has been suggested that the various reported short lived-near room temperature forms of YBCO and related phases may correspond to highly ordered versions of YBa2Cu3O7-x with x=0 or x → 0. © 1988.
Enhanced Tc in Sn-added Bi-2-1-2-2 superconductor
(1992) B.R. Sekhar; N.L. Saini; P. Srivastava; K. Asokan; K.B. Garg; K. Ramakrishna; O.N. Srivastava; G.L. Dwivedi
Addition of Sn to superconducting (BiPb)2Sr2Ca1Cu2Oy, has been studied extensively using EDAX, SEM and electron diffraction. It is found that addition of Sn increases Tc to an extent of 7 K unlike Sn substitution. The present study indicates that Sn occupies the Cu site. © 1992.
Investigations on the formation of new structural phases through stoichiometric deviations on Y sublattice in Y1Ba2Cu3O7±
(1989) I.S. Mulla; A.P.B. Sinha; M.R. Chandrachood; C.C. Tripathi; K. Ramakrishna; A.K. Singh; O.N. Srivastava
Attempts have been made to form new structural phases by introducing Y deficiency in YBa2Cu3O7± structure and to see if the vacancies can be ordered to give new superstructures. Y0.5Ba2Cu3O7± and Y.66Ba2Cu3O7± have been successfully prepared with structure isomorphous to 1:2:3. These compounds are superconducting with Tc between 90-92K. No ordering is, however, observed; instead Ba and Cu ions move and occupy the vacant Y sites. © 1989.
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.
On electron microscopic studies of structural characteristics of the Tl(Bi, Pb)SrCaCuO high-temperature superconductor
(1992) G.D. Verma; K. Ramakrishna; O.N. Srivastava
The Tl(Bi, Pb)SrCaCuO HTSC system corresponding to nominal compositions Tl2-(x+y)BixPbxSr2Ca2Cu3Oz (ifx=0.75, y=0.25; x=0.5, y=0.5; x=0.25, y=0.75 and x=0.25, y=0.25) is synthesized and has been shown to possess a Tc ranging from 77 K to 115 K. The TEM exploration revealed the changes in the structural characteristics with variation of the compositions of Tl(Bi, Pb). For higher Bi concentration a new type of modulated phase with ten times modulation together with the usual incommensurate modulation of the type Bi:2212 is obtained. As the concentration of Bi decreases and of Pb increases the system tends towards the stabilisation of 1223 phase. The density of 1223 phase is found to be maximum for the material synthesized with nominal composition Tl1.0Bi0.5Pb0.5Sr2Ca2Cu3Oz corresponding to which the maximum Tc of ∼ 115 K is obtained. TEM studies for this composition also revealed the presence of an intergrowth type of structure. For higher Pb content only 1212 type of structure was found to be present. It is thought that the substitution of Bi and Pb at the Tl-site in the TlSrCaCuO system leads to the optimization of carrier concentration and hence enhancement in Tc. © 1992.
Studies on structural characteristics of high Tc superconducting BiCaSrCuO phases
(1988) K. Ramakrishna; B. Das; A.K. Singh; R.S. Tiwari; O.N. Srivastava
The recently discovered high Tc superconducting BiCaSrCuO phases show several structural subtilities. In the present investigation evidences and arguments have been advanced to show that the modified perovskite phase with a=b= √2 ap results and the modulated superstructural phase owe their origin to the loss of Ca and Sr atoms respectively. The present investigation also reveals that structural phases with higher periodicities e.g. ∼48 Å, 54 Å, etc. which may correspond to very high Tc e.g. 190 K and higher do really exist in the BCSCO system. © 1988.
Synthesis, characterization and hydrogenation behaviour of Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5-new hydrogen storage composite alloys
(1992) P. Mandal; K. Dutta; K. Ramakrishna; K. Sapru; O.N. Srivastava
New hydrogen storage materials with higher capacity and better suited for applications have been successfully synthesized. The hydriding behaviour of the new composite materials, Mg-ξwt.%FeTi(Mn) and La2Mg17-ξwt.%LaNi5, were studied for various values of ξ (ξ = 10, 20, 30, 40 and 50). The Mg-ξ-%FeTi(Mn) materials were activated under a hydrogen atmosphere (about 33 kgf cm-2) and an optimum storage capacity of about 3.5 wt.% corresponding to room temperature hydrogenation was established for ξ=40. This high storage capacity-almost double the storage capacity of the well-known FeTi(Mn)-has been observed under ambient conditions. The La2Mg17-ξ-%LaNi6 materials were activated at higher temperatures (about 360 °C) in a hydrogen atmosphere. An optimum storage capacity of 4 wt.% in terms of pressure and composition was observed for La2Mg17-20%LaNi5 at 350 °C. In comparison with the native ingredient La2Mg17, much faster (nearly three times) kinetics were found. In order to understand the hydrogenation behaviour and the high storage capacity, structural-microstructural and chemical analyses of these composite materials were carried out. From the structural investigations it has been found that all the synthesized materials are multiphase. The composite material Mg-FeTi(Mn) was found to contain FeTi1-ξ1 magnesium, titanium and Ti-Mg phases. The higher storage capacity (about 3.5 wt.%) in the case of Mg-40%FeTi(Mn) is probably due to FeTi-Mg complexes. The hydrogen molecule is split at the FeTi surface and diffuses into the magnesium matrix via FeTi. In the case of La2Mg17-ξ%LaNi5, the composite material consists of La2Mg17, MgNi2, nickel and LaNi3 phases. Because of the presence of nickel and nickel-containing phases (e.g. Mg-Ni), it is assumed that the dissociation of hydrogen is easier and hence the system La2Mg17-ξ%LaNi5 has better kinetics than its counterpart La2Mg17 alone. © 1992.
The hydrogenation behaviour of RNi5 type materials in thin film and bulk form
(1989) S.K. Singh; K. Ramakrishna; A.K. Singh; O.N. Srivastava
The present communication deals with the electrical resistivity variation in RNi5 (R = rare earth, mischmetal) thin films on hydrogenation, the differential thermal analysis and application of RNi5 in vehicular transport using hydrogen as a fuel. Explanation for the observed behaviour of resistivity variation of RNi5 on hydrogenation has been advanced on the basis of the band structure of LaNi5 and hydride of LaNi5. The differential thermal analysis of the amorphous MmNi4.5Al0.5 has been discussed. © 1989.
The synthesis and hydrogenation behaviour of some new composite storage materials: Mg-xwt% FeTi(Mn) and La2Mg17-xwt% LaNi5
(1994) K. Dutta; P. Mandal; K. Ramakrishna; O.N. Srivastava
The composite alloys La2Mg17-xwt% LaNi5 and Mg-xwt% FeTi(Mn) have been successfully synthesized and the hydriding behaviours of these materials were studied for various values of x. Both these systems have been found to possess characteristics which are superior to the individual storage systems alone. In contrast to La2Mg17, hydrogen storage capacities of 5.24wt% with 3-4 times faster kinetics have been obtained for La2Mg17-xwt% LaNi5. For Mg-40wt% FeTi(Mn), the optimum storage capacity in relation to their reversible characteristic has been found to be about 3.5wt% even at ambient conditions, which is almost double the storage capacity of well-known FeTi(Mn) (∼ 1.9). In order to understand the hydrogenation behaviour and high storage capacity, structural/microstructural characteristics and chemical analyses of these composite materials were carried out employing the XRD, SEM and EDAX techniques. © 1994.