• D K Chakrabarty

      Articles written in Bulletin of Materials Science

    • Structural and dielectric studies of some niobate-titanate systems

      A M Varaprasad A L Shashi Mohan D K Chakrabarty A B Biswas

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      Ferroelectric ceramics of the general formulax (Li1/2 La1/2 TiO3) (1−x) (LiNbO3) have been found to have tungsten bronze (TTB) and ilmenite type structure in the ranges 0·8≤x≤1·0 and 0≤x≤0·2 respectively. The system is biphasic in the other ranges. All single phase materials exhibit dielectric anomalies in the temperature range 300–600 K. The high dielectric constant of the TTB phase is explained as due to the existence of localised rotational excitons arising from a large number of defect structural units and their coupling with one of the low frequency lattice modes. Information about the lattice modes to which the rotational excitons are coupled has been obtained from Raman spectra.

    • Some barium titanate based dielectrics

      A M Varaprasad D K Chakrabarty A B Biswas

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      Some new potential dielectric materials have been made through the cross-substitution of Ba2+ by a 1:1 molar combinations of tri-(La3+) and mono-valent (Li+, Na+, K+) ions at the Ba2+ site in BaTiO3. Chemical analysis shows that compositions in the potassium series are about 6·1% deficient in their K2O content and exhibit interesting dielectric relaxations. The cross-substitution has lowered theTc down to room temperature. The disorder in the larger ‘A’ cation sublattice of the three systems has been established by infra-red and x-ray data. This leads to microscopic compositional variations, which in turn can account for the diffuseness of the dielectric anomalies observed in the barium titanate-rich compositions (0⩽x⩽0·3).

    • Preparation and characterisation of γ-Fe2O3 as tape recording material

      K Seshan M R Anantharaman Venkatesh Rao A L Shashimohan H V Keer D K Chakrabarty

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      Optimum conditions for the preparation of tape recording quality γ-Fe2O3 by the thermal decomposition of ferrous oxalate dihydrate have been established. Formation of the intermediate Fe3O4 which is most important in forming γ-Fe2O3 takes place only in the presence of water vapour. Various stages of decomposition have been characterised by DTA, TG, DTG, and x-ray powder diffraction. The method for the preparation of acicular γ-Fe2O3 that matches very well with the commercial tape recording material has been developed.

    • Synthesis and characterization of γ-Fe2O3 — a magnetic tape material

      M R Anantharaman K Seshan D K Chakrabarty H V Keer

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      Acicular FeC2O4 · 2H2O was precipitated from glycerol and starch media. Thermal decomposition of this oxalate in dry and moist nitrogen yielded primarily FeO and Fe3O4 respectively. Characterization was attempted through DTA, TG, x-ray diffraction, TEM and magnetization studies. It was found that the oxalate can be completely decomposed to Fe3O4 in moist nitrogen (PH2O ∼ 35 torr) at 775 K and then oxidised by dry air to acicular γ-Fe2O3 at 575 K. The resulting material has saturation magnetization (∼ 70 emu/g), coercive field (∼ 300 Oe) and squareness ratio (∼ 0·60–0·65), which values art comparable with those of the commercial samples.

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