• V S TIWARI

      Articles written in Bulletin of Materials Science

    • Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates

      D Pandey V S Tiwari A K Singh Sangeeta Chaudhry

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      The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 − y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 − y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics.

    • Developments in ferroelectric ceramics for capacitor applications

      Dhananjai Pandey A P Singh V S Tiwari

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      A brief overview of the materials and processes for making ceramic capacitors based on BaTiO3 and relaxor ferroelectric compositions is presented with special emphasis on more recent developments.

    • Investigations on vacuum sintered ytterbium-doped YAG ceramic: a laser–host material

      GURVINDERJIT SINGH S S RESHMA RACHNA SELVAMANI V S TIWARI A K KARNAL

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      Laser–host ceramics are a new class of materials and have proved to be a good alternative to the single-crystals. They can be produced in large size with higher concentration of activator ions having more homogeneity which is difficultto achieve with the conventional single-crystal growth technology. Ytterbium (Yb)-doped yttrium aluminium garnet (YAG) is a very attractive laser–host material because of its broad emission band, low quantum defect, no excited state absorption or up-conversion loss and wide range of tunability. In the present work, 10 atm% Yb-doped YAG transparent ceramic was fabricated using nano-powder technology and vacuum sintering. Single-phase nano-powders of Yb:YAG were synthesized by a co-precipitation method and thereafter sintered under vacuum at 1750$^{\circ}$C for 5 h. The as-sintered ceramic samples are ofdark greenish colour which is attributed to the reduction of Yb$^{3+}$ to Yb$^{2+}$ during vacuum sintering. However, after annealing in air at 1350$^{\circ}$C for 8 h they retain their natural colour attributed to the transformations from Yb$^{2+}$ to Yb$^{3+}$. The transmission spectra of the as-sintered polished ceramics show the presence of broad absorption bands near 250, 380, 480 and 630 nm due to the electronic transitions in the Yb$^{2+}$ ion. These bands disappear after annealing in air. The well-established bandsdue to transitions in Yb3+ were observed between 900 and 1000 nm.

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