• R K Mandal

      Articles written in Bulletin of Materials Science

    • Dielectric behaviour of glasses and glass ceramics in the system BaO-PbO-TiO2-B2O3-SiO2

      R K Mandal Ch Durga Prasad O M Parkash Devendra Kumar

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      Glasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO2-B2O3-SiO2 were prepared keeping (BaO + PbO)/TiO2 ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO3 crystallizes in lead-rich glasses while BaTiO3 precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO3 does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features.

    • 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.

    • Interfaces in icosahedrally related structures: problems and prospects

      R K Mandal

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      Icosahedral quasicrystals are frequently observed to coexist with their related phases like pentagonal/decagonal quasicrystals and rational approximant structures. Owing to this, they have common interfaces. The crystallography of these interfaces needs consideration for providing an aid for their characterization through experiments. The purpose of this communication is to present examples of heterophase and homophase interfaces in quasicrystalline systems and to discuss their structural details in terms of higher dimensional crystallography. Some of the uncommon aspects of these interfacesvis-a-vis their crystalline counterparts will be highlighted. We shall conclude by identifying the problems and prospects of further research in this area.

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