K. Ramakrishna

Investigations on the structural and hydrogenation characteristics of LaNi5, HoNi5, GdNi5, SmNi5, MmNi5, and CFMmNi4.5Al0.5 thin films

Abstract 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— R Ni 5 where R = La, Sm, Gd, Ho and Mm (Mischmetal) and a related phase cerium-free MmNi 4.5 Al 0.5 . 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.

The synthesis and hydrogenation behaviour of some new composite storage materials: Mg-xwt% FeTi(Mn) and La2Mg17-xwt% LaNi5

Abstract The composite alloys La 2 Mg 17 - x wt% LaNi 5 and Mg- x wt% 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 La 2 Mg 17 , hydrogen storage capacities of 5.24wt% with 3–4 times faster kinetics have been obtained for La 2 Mg 17 - x wt% LaNi 5 . 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.

The hydrogenation behaviour of RNi5 type materials in thin film and bulk form

Abstract The present communication deals with the electrical resistivity variation in RNi 5 (R = rare earth, mischmetal) thin films on hydrogenation, the differential thermal analysis and application of RNi 5 in vehicular transport using hydrogen as a fuel. Explanation for the observed behaviour of resistivity variation of RNi 5 on hydrogenation has been advanced on the basis of the band structure of LaNi 5 and hydride of LaNi 5 . The differential thermal analysis of the amorphous MmNi 4.5 Al 0.5 has been discussed.

Solid State Materials for Hydrogen Storage

This paper seeks to review the hydride/hydrogen technology and to describe the work being done in our laboratory for the development of rare earth pentanickelide (RNi5) type solid state materials for hydrogen storage. To start with a brief review of the basic theme for solid state storage mode of hydrogen has been given. The salient features of this storage mode have been outlined. For the RNi5 type materials, more emphasis has been laid on the discussion of a new result obtained by us. It has been found that it is possible to synthesise amorphous phases of RNi5 (LaNi5, MmNi5) materials and this phase has been found to have a higher hydrogen storage capacity. Another new result in regard to the RNi5 type hydrides is relating to the variation of electrical resistivity (P) as a function of hydrogenation time (t). The curious nature of P-t curve has been explained in terms of the variation of electronic density of states as a result of the hydrogenation process.

Investigations on the formation of new structural phases through stoichiometric deviations on Y sublattice in Y1Ba2Cu3O7

Abstract Attempts have been made to form new structural phases by introducing Y deficiency in YBa 2 Cu 3 O 7± structure and to see if the vacancies can be ordered to give new superstructures. Y 0.5 Ba 2 Cu 3 O 7± and Y .66 Ba 2 Cu 3 O 7± have been successfully prepared with structure isomorphous to 1:2:3. These compounds are superconducting with T c between 90–92K. No ordering is, however, observed; instead Ba and Cu ions move and occupy the vacant Y sites.

Electron microscopic investigations of neutron irradiated YBa2Cu3O7-x high temperature superconducting single crystals

Abstract 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 10 16 /cm 2 ) (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 A. 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 A ). The effectiveness of these defects particularly the defect stripes as flux pinning centres has been outlined.

Electron microscopic studies of local structures of high temperature YBa2Cu3O7−x superconductor

Abstract The local structural characteristics of the high temperature superconducting material YBa 2 Cu 3 O 7− 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 A) 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 YBa 2 Cu 3 O 7− x with x =0 or x → 0.

On electron microscopic studies of structural characteristics of the Tl(Bi, Pb)SrCaCuO high-temperature superconductor

Abstract The Tl(Bi, Pb)SrCaCuO HTSC system corresponding to nominal compositions Tl 2−( x + y ) Bi x Pb x Sr 2 Ca 2 Cu 3 O z (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 T c 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 Tl 1.0 Bi 0.5 Pb 0.5 Sr 2 Ca 2 Cu 3 O z corresponding to which the maximum T c 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 TlSrCaCuO system leads to the optimization of carrier concentration and hence enhancement in T c .

Studies on structural characteristics of high Tc superconducting BiCaSrCuO phases

Abstract The recently discovered high Tc superconducting BiCaSrCuO 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 A, 54 A, etc. which may correspond to very high Tc e.g. 190 K and higher do really exist in the BCSCO system.

Electron microscope studies of structural characteristics of Tl(Bi)-Sr-Ca-Cu-O high temperature superconductor

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 Tl1.6Bi0.4Sr2Ca2Cu3O10. 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.