Now showing 1 - 9 of 9
  • Placeholder Image
    Publication
    Ion-induced carbide formation of TiFe: evidence from XPS and AES studies
    (01-01-1990) ;
    Viswanathan, B.
    ;
    Srinivasan, V.
    X-ray photoelectron and Auger electron spectroscopic studies on TiFe demonstrate that ion bombardment of the alloy not only results in sputter cleaning or even destruction of the surface but also leads to the formation of compact carbide layers. Such carbide species are of considerable importance in elucidating the reaction mechanism of hydrocarbon synthesis. They are also expected to play a significant role in the hydrogen sorption properties of materials. A better understanding of the nature of sputtered carbon is needed for its use as protective (against corrosion) thin films in magnetic recording applications. A comparison of photoelectron and Auger electron spectra of the surface carbides of TiFe would also be of interest since it is unusual for both types of data to be produced on the same samples where chemical shifts are involved. © 1990.
  • Placeholder Image
    Publication
    Surface properties and their consequences on the hydrogen sorption characteristics of certain materials
    (01-01-1990) ;
    Viswanathan, B.
    ;
    Swamy, C. S.
    ;
    Srinivasan, V.
    Surface analyses (X-ray photoelectron spectroscopy, X-ray-induced Auger electron spectroscopy and Auger electron spectroscopy) results of some hydrogen storage intermetallic compounds of the type AB, AB5, A2B and the hydride, A2BH4 (A ≡ Ti, La, Ca or Mg; B ≡ Fe, Ni or Cu) show the presence of numerous oxidation (aerial) products, such as oxides, hydroxides and carbonates. In general, the hydroxide and/or carbonate species are present in small amounts and persist only on the outer passivated surface. However, there are cases where they seem to be predominant, at least in the top few layers. Therefore such species are also expected to play an important role in the surface chemistry of the alloys and hydrides similar to that observed with surface oxides. Accordingly, the activation procedure varies for the different materials. Upon activation the surface may be described as a supported metal system which in turn increases the activity thus explaining the rapid kinetics of the hydriding/dehydriding reactions upon cycling processes. The easy activation of certain alloys could also be explained on the basis of the reducibility and the thickness of the surface oxide layers. In other words, it could be demonstrated based on their oxidation resistance and the segregation behaviour of the metals. The specific reactivity of different systems are related to the segregation, oxidation, and reduction behaviours. Surface enrichment of iron in the TiFe system is demonstrated for the first time. © 1990.
  • Placeholder Image
    Publication
    Oxidation state and superconducting properties of Y1-xPrxSr2Cu2.85Re 0.15O7+δ
    (15-12-1996)
    Murugesan, M.
    ;
    ;
    Sharon, M.
    ;
    Gupta, L. C.
    ;
    Pinto, R.
    ;
    ;
    Gopinath, C. S.
    ;
    Subramanian, S.
    Superconductivity exists in orthorhombic Y1-xPrxSr2Cu2.85Re 0.15O7+δ up to a critical concentration (xcr) of 0.65. A progressive decrease in Tc occurs as x increases from 0 to 0.65. A further increase in x leads to a tetragonal transformation and as a consequence the Tc vanishes; however, the orthorhombicity of these Sr-based compounds is much lower than that observed for the Ba analog, Y1-xPrxBa2Cu3O7-δ and, hence, the Tc. On the one hand, crystal chemistry correlations indicate that the Pr ion is in trivalent state while on the other hand, the stabilizing cation, viz., Re, is in hexavalent state which accounts for the excess oxygen (>7.0) in the system. The high xcr value of the Sr series compared to the Ba series (xcr=0.55) is attributed to the much reduced orbital overlap of the trivalent Pr(4f ) state with the Cu(3dx2-y2)-O(2p) conduction band, via hole localization and/or pair breaking, and is not due to the much discussed hole filling by tetravalent Pr. © 1996 American Institute of Physics.
  • Placeholder Image
    Publication
    The valence state of M-ions in the chemically stabilized YSr2Cu3-xMxO7+δ (M = Mo, W and Re) superconductors
    (01-01-1997)
    Murugesan, M.
    ;
    ;
    Sharon, M.
    ;
    Gupta, L. C.
    ;
    Pinto, R.
    ;
    Gopinath, C. S.
    ;
    Subramanian, S.
    Single phase Ba- free Sr-based YSr2Cu3-xMxO7+δ (M = Mo, W and Re) compounds have been stabilized by chemical doping. Superconductivity is observed for these phases in the range 30-45 K. X-ray diffraction studies suggest a relatively small orthorhombicity compared to Ba-analogue. X-ray photoelectron spectroscopic investigations reveal that the stabilizing cations are in the hexavalent state. The observation of the higher oxidation state of M-ions accounts for the excess oxygen content in these phases which is in accordence with the diffraction results.
  • Placeholder Image
    Publication
    Evidence for the formation of surface carbonates on some hydrogen storage intermetallic compounds: an XPS study
    (01-01-1990) ;
    Viswanathan, B.
    ;
    Srinivasan, V.
    X-Ray photoelectron spectroscopic (XPS) studies of air exposed samples of LaNi5, CaNi5, Mg2Ni, Mg2Cu and FeTi indicate that the interaction of atmospheric carbon dioxide results in the formation of surface carbonates in addition to the usual oxides and hydroxides. The reaction of carbon dioxide, in fact, is favoured by the oxidized surface which in turn produces the carbonate species on the top few layers, hitherto not reported. In particular, the carbonates seem to be predominant in the case of CaNi5. The adsorption of carbon dioxide also gives an idea of the segregated elements of the different systems. Even though such carbonate species on the alloy surfaces seem to be expected rather than surprising, there is no evidence available in the literature. To our knowledge, this forms the first report on such a problem. The results also provide valuable information concerning the understanding of various surface reactions and the nature of activation processes which often differ between materials involved in hydrogen uptake. © 1990.
  • Placeholder Image
    Publication
    Surface properties of LaNi5: A reinvestigation
    (01-01-1991) ;
    Viswanathan, B.
    ;
    Swamy, C. S.
    ;
    Srinivasan, V.
    Although a number of surface analysis reports are available on the LaNi5 system, yet a clear picture of the actual species involved and their consequences on the activation and deactivation processes has not been reached. Moreover, differences of opinion have been seen among the researchers. The present results distinctly indicate that the air exposed samples are completely contaminated with a variety of oxygenated species (such as oxides, hydroxides and carbonates) of the constituent elements, hitherto not reported by others. The protective nature of such species in regard to the durability of the alloy and the formation of supported-metal type species upon activation accounts for its excellent behaviour. The relative higher stability of LaNi5 over CaNi5 under cyclic operating conditions has been explained based on surface energy criteria of the constituent elements rather than the deterioration initiated by hydrogen and/or gaseous impurities alone. In addition, the specific importance of X-ray induced Auger electron spectroscopy (XAES) technique in conjunction with X-ray photoelectron spectroscopy (XPS) has been highlighted in the identification of different species. © 1990.
  • Placeholder Image
    Publication
    A novel mixed ligand Te(IV) complex comprising three halides and a dithiocarbamate; synthesis and crystal structure of triiododiethyldithiocarbamatotellurium(IV), Te{(C2H5)2NCS2}I3
    (01-01-1996) ;
    Aravamudan, G.
    ;
    Udupa, M. R.
    ;
    Seshasayee, M.
    ;
    ;
    Yvon, K.
    The title compound TeIVLI3 [L = diethyldithiocarbamate, (C2H5)2NCS-2], is the first example of a mixed halide-dithiocarbamate complex of TeIV with halide to a dithiocarbamate ratio of 3 : 1. Two different methods have been employed for the preparation of this compound. The first method involves the ligand exchange reaction between TeL2 and Tel4. The second method is the oxidative addition of iodine on TeIILI. The structure of the title compound has been determined and it is found to be polymeric with iodine being the bridging ligand. The polymer is made up of interlinking of edge-sharing pentagonal bipyramidal dimers, which are stacked along a axis of the triclinic unit cell. The three Te-I distances are Te-I(1) 2.982(1), Te-I(2) 2.901(1) and Te-I(3) 2.872(1) Å, and together with two Te-S and two intermolecular Te-I interactions complete the seven coordination with a distorted pentagonal bipyramidal geometry around each tellurium.
  • Placeholder Image
    Publication
    Stabilization of a superconducting TbSr2Cu2.85Re 0.15O7+δ compound and the valence state of terbium
    (01-12-1995)
    Murugesan, M.
    ;
    ;
    Sharon, M.
    ;
    ;
    Gupta, L. C.
    ;
    Pinto, R.
    ;
    Gopinath, C. S.
    ;
    Subramanian, S.
    A single-phase Sr-based Tb-123 phase has been synthesized by chemical stabilization. TbSr2Cu2.85Re0.15O 7+δ exhibits superconductivity at Tc,zero=22 K. X-ray diffraction results suggest a relatively small orthorhombicity and neutron diffraction studies indicate an excess oxygen content (≥7.2) in the sample. Tb is present as a trivalent ion in the superconducting phase as derived from x-ray photoelectron spectroscopy.© 1995 American Institute of Physics.