• O N Srivastava

      Articles written in Bulletin of Materials Science

    • High resolution electron microscopic studies of HoAl3, Er0·5Gd0·5Al3 and Y0·91Er0·09Al3 in thin film form

      Anand Kumar Singh Ajay Kumar Singh M S Gupta O N Srivastava

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      Rare earth trialuminides (RAl3) exhibit an interesting series of structures changing from 2H to 3C in the bulk form. Many of the rare earth trialuminides have been recently found to exhibit curious structural characteristics such as the occurrence of the modulated phases. A detailed investigation of the formation synthesis and characterization of some binary and ternary alloys of the rare earth-aluminium system has been carried out. High resolution microscopic technique has been employed to study the modulated phases for some alloyse.g. HoAl3, Er0·5Gd0·5Al3 and Y0·91Er0·09Al3. With the help of lattice imaging technique, several new modulated phases have been investigated. A possible mechanism for the formation of these phases has been suggested. The details of the results obtained by lattice imaging technique are discussed.

    • Preparation of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O superconducting films by conventional techniques

      K K Verma A K Saxena S Rastogi R S Tiwari O N Srivastava

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      High temperature superconducting films of Y-Ba-Cu-O and Bi-Sr-Ca-Cu-O have been deposited on different substrates using conventional techniques, like flash evaporation and spray pyrolysis. The microstructural investigation of the films by SEM technique reveals the partially oriented nature of the crystallites. In the case of spray-deposited Bi-Sr-Ca-Cu-O HTSC films it has been found that film/substrate mismatch is not the decisive factor for the superconducting transition temperatureTc.

    • Electron microscopic observations of Bi and Tl bearing cuprate high temperature superconductors

      B Das K Ramakrishna G D Verma R S Tiwari O N Srivastava

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      The second generation of high temperature superconductors typified by Bi2Sr2Can−1CunO2n+4 and Tl2Ba2Can−1CunO2n+4 exhibits curious structural properties which have direct relevance to the superconducting behaviour particularly transition temperature (Tc). 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 fromap×ap×c to (2)1/2ap×(2)1/2ap×c through the loss of calcium atoms and the formation of five-fold modulated phase alongb 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 higherTc(R=0) ∼ 120 K exhibits unusual characteristics.

    • Some new structural and electronic characteristics of quasicrystals

      O N Srivastava R S Tiwari N P Lalla R K Mandal M A Shaz

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      The quasicrystals being based on quasiperiodic order other than crystal like periodic translational order and embodying self similarity, present unique condensed matter phases. In addition to their curious structural characteristics the paucity of translational periodicity leads to drastic deviations in their electronic behaviour as compared to crystalline counterparts. This paper describes and discusses some new developments in regard to structural and electronic aspects of quasicrystalline materials. In regard to the structural aspects, two comparatively newer features will be described. One of them relates to the observation of variable strain approximants (VSA) first found in Ti68Fe26NiSi5, qc alloys; the other relates to the structure of decagonal phases. The variable strain approximants correspond to qc phases exhibiting variable strain for the different diffraction spots for the same reciprocal lattice row (possessing linear shifts). The VSA is thought to result from variable phason strain mode locking; this in contrast to RAS which results due to linear phason mode locking. The results obtained in our laboratory on VSA will be described and discussed. Another interesting structural feature emanating in the last few years relate to the development of structural models for the decagonal phases which have nearly answered the question ‘Where are the atoms?’ for this qc variant. High resolution electron microscopy has revealed the existence of two types of atom cluster columns with a diameter of 20Å; a pentagonal cluster column and a decagonal one. The decagonal quasicrystals can be classified into three types of structures according to the space groups and symmetries and arrangements of the cluster columns. These three deca structures have been typified by: deca Al-Co-Cu, deca Al-Mn and deca Al-Fe-Pd types. Some basic features on the structures of decagonal phases as obtained recently will be outlined.

      Since materials owe their practical importance due to their physical behaviours, assessment of qc materials from this point of view is of imperative importance. However, the real physics/science governing properties for qc is not well understood as yet. For example, the fundamental property relating to electronic conductivity and its temperature variation has been attempted to become explicable based on (a) taking qc as disordered materials, (b) assuming qc as hierarchy of clusters and (c) bringing in new concepts governing the wave functions of electrons (critical wave functions) and some other models. However, the same results are not universally reported/reproduced by various workers. Thus the variation of electronic conductivities with temperature of Al65Cu20Ru15, Al70Pd20Re10 and other stable quasicrystals have found varied interpretations e.g. based on QIEs and power law temperature variation. Some results onσ-T and related characteristics for stable qc crystals obtained by us and also by other workers will be analysed in terms of feasible transport mechanisms.

    • Leaching of rapidly quenched Al$_{65}$Cu${20}$Fe$_{15}$ quasicrystalline ribbons

      S S MISHRA T P YADAV N K MUKHOPADHYAY O N SRIVASTAVA

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      In the present work, Al$_{65}$Cu$_{20}$Fe$_{15}$ alloy has been synthesized by melting of pure elements (e.g., Al (99.96%),Cu (99.99%) and Fe (99.98%)), using a radiofrequency induction melting furnace. The as-prepared alloy was subjected torapid solidification by melt spinning technique at $\sim$3500 rpm speed on a copper disk of diameter 14 cm. As a result of themelt spinning, nearly 2 mm wide, 30−40 $\mu$m thick and 4–5 cm long ribbons were formed. The structural and microstructuralcharacterizations were carried out by X-ray diffraction and transmission electron microscopy techniques.We have performedleaching operation using 10 mol NaOH aqueous solution on the surface using a pipette. Leaching was performed for variousdurations ranging from 30 min to 8 h. After leaching, the reflectivity reduces and the surface looks reddish brown. Themicrostructure of the 8 h leached sample shows a breakdown of the quasicrystalline phase but with the evolution of othermetallic phases. Copper (Cu) particles are found to be present on the surface of quasicrystal after 4 h of leaching and relativelymore iron (Fe) evolves during further leaching of 8 h. This low-cost method to prepare a distribution of nano-Cu/Fe metalparticles encourages their uses in catalytic reactions, indicating the possibility of use of quasicrystals as the industrial catalysts.

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