• J Koshy

      Articles written in Bulletin of Materials Science

    • Superconducting YBa2Cu3O7–𝛿 thick film (𝑇c (0)$ = 92 K) on a newly developed perovskite ceramic substrate

      S U K Nair P R S Warriar J Koshy

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      A complex perovskite oxide, YbBa2NbO6, as a non-reacting substrate for YBa2Cu3O7–𝛿 superconducting film has been developed. The dielectric constant and loss factor values of the material are in the range suitable for its use as substrate for microwave application. A YBa2Cu3O7–𝛿 superconducting thick film dip coated on YbBa2NbO6 substrate gave a 𝑇c (0) of 92 K and current density of ∼ 1.3 × 104 A cm-2.

    • Ba2ErNbO6: A new perovskite ceramic substrate for Bi(2223) superconducting thick films (𝑇c(0) = 110 K)

      S U K Nair P R S Warriar J Koshy

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      Barium erbium niobate (Ba2ErNbO6) has been developed as a new substrate for (Bi,Pb)2Sr2Ca2Cu3O𝑥 [Bi(2223)] superconductor film. Ba2ErNbO6 (BENO) has a cubic perovskite structure with lattice constant, 𝑎 = 8.318 Å. The Bi(2223) superconductor does not show any detectable chemical reaction with BENO even under extreme processing conditions. Dip coated Bi (2223) thick film, Ba2ErNbO6 substrate, gave a 𝑇c (0) of 110 K and current density of ∼ 4 × 103 A cm-2 at 77 K and zero magnetic field.

    • Synthesis, characterization, sintering and dielectric properties of nanostructured perovskite-type oxide, Ba2GdSbO6

      C Vijayakumar H Padma Kumar Sam Solomon J K Thomas P R S Warriar J Koshy

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      Nanoparticles of barium gadolinium antimonate (Ba2GdSbO6), a complex perovskite-type oxide, has been synthesized using an auto ignition combustion process for the first time. The nanoparticles thus obtained have been characterized by powder X-ray diffraction, thermogravimetric analysis, differential thermal analysis, Fourier transform infrared spectroscopy and transmission electron microscopy. The XRD studies have shown that the as-prepared powder is phase pure Ba2GdSbO6 and has a complex cubic perovskite (A2BB′O6) crystalline structure with lattice constant, 𝑎 = 8.449 Å. The TEM image reveals that the particle size of the as-prepared nano powder was in the range 30–60 nm. The nanocrystals of Ba2GdSbO6 synthesized by the combustion technique could be sintered to 96% of the theoretical density by heating at a temperature of 1560°C for a short duration of 3 h. The surface morphology of the sintered pellet has been studied by scanning electron microscope (SEM). The dielectric constant ($\varepsilon_{r}$) was 20 and the loss factor (tan𝛿) was 0.03 at 3 MHz. By the present combustion technique a phase pure nanopowder of Ba2GdSbO6 could be obtained by a single step process without the need of any calcination step.

    • Combustion synthesis and characterization of Ba2NdSbO6 nanocrystals

      V T Kavitha R Jose S Ramakrishna P R S Wariar J Koshy

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      Nanocrystalline Ba2NdSbO6, a complex cubic perovskite metal oxide, powders were synthesized by a self-sustained combustion method employing citric acid. The product was characterized by X-ray diffraction, differential thermal analysis, thermogravimetric analysis, Fourier transform infrared spectroscopy, transmission electron microscopy and scanning electron microscopy. The as-prepared powders were single phase Ba2NdSbO6 and a mixture of polycrystalline spheroidal particles and single crystalline nanorods. The Ba2NdSbO6 sample sintered at 1500°C for 4 h has high density (∼ 95% of theoretical density). Sintered nanocrystalline Ba2NdSbO6 had a dielectric constant of ∼ 21; and dielectric loss = 8 × 10-3 at 5 MHz.

    • Development, characterization, sintering, dielectric and optical properties of NdBa2ZrO5.5 nanocrystals

      V Ratheesh Kumar P R S Wariar R Pazhani J K Thomas R Jose J Koshy

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      Nanocrystalline NdBa2ZrO5.5 has been successfully synthesized through a single step auto-ignition combustion route for the first time. X-ray diffraction and Fourier transform infrared spectroscopy revealed that the combustion product is phase pure and has an ordered cubic perovskite structure. The phase transitions and thermal stability of the nanopowder were investigated by differential thermal and thermogravimetric analyses. Transmission electron microscopy results indicated that the particle sizes are 20–30 nm. Selected area electron diffraction pattern has shown that as-prepared powder is polycrystalline in nature. The optical absorption spectra analysis confirmed that the material falls to the semiconducting range with a bandgap of ∼3.69 eV and therefore, could be used as transparent wide bandgap semiconductor. The relative density of the sintered sample is ∼96% at 1510 °C for 2 h. The surface morphology of the sintered pellet has been studied by scanning electron microscopy and the average grain size observed is ∼0.7 𝜇m. Dielectric constant (𝜀r) of NdBa2ZrO5.5 at 5 MHz is 29.6 and loss factor (tan 𝛿) is 4 × 10-2 at room temperature.

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