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PublicationArticle 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. SrivastavaThin 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.PublicationArticle Magnetospheric electron temperatures inferred from whistler observations at low latitudes(2008) Krishna K. Singh; A.K. Singh; M. Altaf; M.M. Ahmad; B.L. Koul; Lalmani; R.P. Singh; Jaipal Singh; Balraj KumarWhistler observations during night times made at low latitude Indian ground stations Jammu (geomag.lat.22° 26'N; L= 1.17), Nainital(geomag.lat., 19° 1' N; L= 1.16) and Varanasi (geomag.lat., 14°55'N; L = 1.11) are used to deduced electron temperatures in the vicinity of the magnetospheric equator. The accurate curve fitting and parameter estimation technique are used to compute nose frequency and equatorial electron densities from the dispersion measurements of short whistlers recorded at Jammu, Nainital and Varanasi. These computed parameters are further used to estimate the magnetospheric electron temperatures from the dispersion analysis of short whistlers observed at low latitudes which are in good agreement with the results reported by other workers. © 2008 IACS.PublicationArticle Studies on Photosynthesis, the Associated Electron Transport System and Some Physiological Variables of Chlorella vulgaris Under Heavy Metal Stress(1991) L.C. Rai; A.K. Singh; Nirupama MallickResults are presented to document the effects of different concentrations of zinc (ZnCl2) and mercury (HgCl2) on different processes of Chlorella vulgaris. The effective concentrations of Hg and Zn reducing 50 per cent (LC50) of the population of the test organism were found to be 0.5 and 2.0 µg mL-1, respectively. These concentrations of Hg and Zn reduced growth, CO2 fixation, O2 evolution, ATP content and NR activity by 56, 64, 75, 89 and 91 % and 50, 36, 57, 79 and 90 %, respectively. The test metals were also found to be highly toxic for nutrient (NO3- and NH4+) uptake. PS II was found to be the primary site of action for the two metals. A possible mechanism of Hg and Zn toxicity is discussed. © 1991, Gustav Fischer Verlag, Stuttgart. All rights reserved.PublicationArticle Molecular electrostatic properties using point charges: Ab initio hybridization displacement charges combined with bond-centered charges(2003) A.K. Singh; A. Kumar; P.C. MishraA new approximate scheme to compute hybridization displacement charges (HDC) in molecules at the ab initio level using bond-centered charges arising due to overlapping atomic orbitals has been developed. It has been applied at the SCF/6-31G** and SCF-CIS/6-31G** levels to the ground and excited states, respectively, of several polar and non-polar molecules. It is found that, statistically speaking, HDC computed using the present approach reproduce SCF dipole moments and surface molecular electrostatic potential (MEP) patterns quite satisfactorily. In some cases, HDC reproduce MEP features even somewhat better than the MEP-fitted CHelpG charges, particularly in molecules where MEP values around atoms vary rapidly, e.g. around the boron atom in certain molecules containing it, and homonuclear diatomic molecules. Some interesting information about molecular bonding, not available from other charge distributions, is revealed by the present approach, e.g. in lithium containing molecules. The present approach also appears to describe molecular electrostatic properties and molecular bonding better than that where the Löwdin's symmetry orthogonalized atomic orbitals are used. © 2003 Elsevier Science B.V. All rights reserved.PublicationConference Paper Estimation of D-region electron density using tweeks measurements at Nainital and Allahabad(2010) P. Pant; A.K. Maurya; Rajesh Singh; B. Veenadhari; A.K. SinghLightning generated radio atmospheric that propagates over long distances via multiple reflections through the boundaries of the Earth-ionosphere waveguide (EIWG), shows sharp dispersion near the cut-off frequency ∼1.8kHz of the EIWG. These dispersed atmospherics at lower frequency end are called as 'tweek' radio atmospherics. In order to estimate D-region electron densities at the ionospheric reflection heights we have utilized the dispersive property of tweeks observed at low latitude Indian stations Nainital (Geomag. Lat., 20.29° N) and Allahabad (Geomag. Lat., 16.05° N). Direction finding technique has also been applied to determine the source locations of causative lightning discharge of tweeks. In this perspective, the geographic locations is determined by the intersection of two circles that are drawn by taking the travelled / propagation distance by tweek atmospherics from source location to Allahabad (ALD) and Nainital (NTL) stations. These results are in good agreement with World Wide Lightning Location Network (WWLLN) data. The average D-region electron density along the propagation path varied in the range ∼20-35 el/cc at ionospheric reflection heights of 70-90km. The tweek method has unique advantage of monitoring lower boundary of the D-region over an area of several thousand of km surrounding to the receiving stations. © 2010 American Institute of Physics.PublicationArticle 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. SrivastavaThis 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.PublicationConference Paper The effect of geomagnetic storm on GPS derived total electron content (TEC) at Varanasi, India(Institute of Physics Publishing, 2010) Sanjay Kumar; A.K. SinghIn this paper we studied the effect of geomagnetic storm on Global Positioning System (GPS) derived total electron content (TEC) at low latitude Varanasi (Geomagnetic lat 14°, 55' N, geomagnetic long 154° E) during the period of May 2007 to April 2008. During this period 2 storms were found, which were occurred on 20 November 2007 and 9 March 2008. In this study vertical total electron content (VTEC) of single Pseudorandom Noise (PRN) and average of VTEC of same PRN before 10 days of storm, which is called background TEC, were used to see the effect of these storms on the variation of TEC. From this study this is found that during the storm of March 2008 the TEC increases in main phase of storm while in the case of November 2007 storm, TEC decreases during the main phase of storm but increases in the recovery phase (next day) of storm. © 2010 IOP Publishing Ltd.PublicationArticle 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. SrivastavaThis 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.PublicationArticle Chemical constituents from Zizyphus oenoplia(Indian Chemical Society, 2012) A.K. Singh; V.P. SinghFour compounds, β-sitosterol, β-sitosteryl-β-D-glucoside, luteolin and quercetin have been isolated from the barks of Zizyphus oenoplia and their structures were established on the basis of spectral evidences. This is the first report of these compounds in Z. oenoplia.PublicationArticle P Toluenesulphonyl derivatives of N,N' bis (4 quinolino 4 quinaldino/ 4 quinazolino and 9 acridino) polymethylene diamines as hypoglycemic agents(1975) S.M. Deshpande; K.C. Datta; A.K. Singh[No abstract available]