Phase analysis and dielectric properties of ceramics in PbO-MgO-ZnO-Nb_2O_5 system: A comparative study of materials obtained by ceramic and molten salt synthesis routes

摘要

Compositions of the type 3PbO-MgO/ZnO-Nb_2O_5 were synthesized by the ceramic route at 1000 deg C and sintered at 1200 deg C. Powder X-ray diffraction studies of the 1000 deg C heated products show the presence of the cubic pyrochlore and the columbite (Mg/ZnNb_2O_6) type phase in the ratio of 3:1 for all possible combinations of MgO and ZnO. Further heating at 1200 deg C led to a decrease in the cubic pyrochlore phase and an increase in the columbite phase by around 10 percent. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthesized using the KCl-NaCl molten salt method at 900 deg C for 6 h gave a significant amount of the cubic perovskite related phase of the type Pb(Mg/Zn)_(1/3)Nb_(2/3)O_3 for all compositions containing MgO. The amount of the perovskite phase was nearly 55 percent 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 deg 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.