• A M Varaprasad

      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).

    • Magnetic hysteresis and crystallization studies on metallic glass alloy Fe67Co18B14Si1

      A M Varaprasad C Radhakrishnamurthy

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      Magnetic hysteresis studies were carried out on Metglas 2605 Co(Fe67Co18B14Si1) samples, cut parallel and perpendicular to the ribbon axis and annealed at different temperatures. The samples annealed at 450°C for 2 hr show a coercive force of 8·75 Oe and 0·60 Oe in the parallel and perpendicular directions respectively. Electron probe micro analysis studies revealed the onset of primary crystallization at 400°C and a gradual displacement of B and Si from the lattice as the annealing temperature was increased to 450°C. The observed large coercive force is due to an increase in anisotropy resulting from an intergrowth pattern of the type FexCo1 −x, 0·3<x<1·0, including boron containing phases such as (FeCo)B, (FeCo)2B, (FeCo)3B, developed by the displacement of and Si from the original amorphous lattice.

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