• M Thirumal

      Articles written in Bulletin of Materials Science

    • Phase analysis and dielectric properties of ceramics in PbO-MgO-ZnO-Nb2O5 system: A comparative study of materials obtained by ceramic and molten salt synthesis routes

      M Thirumal A K Ganguli

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      Compositions of the type 3PbO-MgO/ZnO-Nb2O5 were synthesized by the ceramic route at 1000°C and sintered at 1200°C. Powder X-ray diffraction studies of the 1000°C heated products show the presence of the cubic pyrochlore and the columbite (Mg/ZnNb2O6) type phase in the ratio of 3 : 1 for all possible combinations of MgO and ZnO. Further heating at 1200°C led to a decrease in the cubic pyrochlore phase and an increase in the columbite phase by around 10%. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthesized using the KCl-NaCl molten salt method at 900°C for 6 h gave a significant amount of the cubic perovskite related phase of the type Pb(Mg/Zn)1/3Nb2/3O3 for all compositions containing MgO. The amount of the perovskite phase was nearly 55% for the Mg rich compositions and decreased with increase in Zn content, the pure Zn composition yielding mainly the cubic pyrochlore phase. On sintering these phases at 1000°C the perovskite phase content decreased. The dielectric constant of the composite materials formed by the ceramic route was in the region of 14 to 20 and varied little with frequency. The composites obtained by the molten salt method, however, showed much larger dielectric constants in the region 40–150 at 500 kHz for various compositions. The dielectric loss tangent of these composites were lower by an order (0.005–0.03 at 500 kHz) compared to the ceramic route.

    • Influence of strontium on the cubic to ordered hexagonal phase transformation in barium magnesium niobate

      M Thirumal A K Ganguli

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      Oxides of the type Ba3-xSrxMgNb2O9 were synthesized by the solid state route. Thex = 0 composition (Ba3MgNb2O9) was found to crystallize in a disordered (cubic) perovskite structure when sintered at 1000C. For higher Sr doping (x ≥ 0.5), there was clearly the presence of an ordered hexagonal phase indicated by the growth of superstructure reflections in the powder X-ray diffraction patterns. In all the compositions there was the presence of a minor amount of Ba5-xSrxNb4O15 phase which increased with Sr substitution up tox = 1 and then it remained nearly constant at about 5%. Samples sintered at 1300C showed the hexagonally ordered phase for the entire range of composition (0 ≤x ≤ 3). The degree of ordering being considerably greater than in the 1000C heated samples as evidenced by several superstructure reflections

    • Synthesis of Ba3ZnNb2O9–Sr3ZnNb2O9 solid solution and their dielectric properties

      M Thirumal A K Ganguli

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      Oxides of the type, Ba3–𝑥Sr𝑥ZnNb2O9 (0 ≤ 𝑥 ≤ 3), were synthesized by the solid state route. Oxides calcined at 1000°C show single cubic phase for all the compositions. The cubic lattice parameter (a) decreases with increase in Sr concentration from 4.0938(2) for 𝑥 = 0 to 4.0067(2) for 𝑥 = 3. Scanning electron micrographs show maximum grain size for the 𝑥 = 1 composition (∼ 2 𝜇m) at 1200°C. Disks sintered at 1200°C show dielectric constant variation between 28 and 40 (at 500 kHz) for different values of 𝑥 with the maximum dielectric constant at 𝑥 = 1.

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