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

Compositions of the type 3PbO-MgO/ZnO-Nb2O5 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/ZnNb2O6) type phase in the ratio 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 %. Compacted pellets sintered further showed the appearance of the perovskite phase. Similar compositions synthesized using 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)1/3Nb2/3O3 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 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 500kHz 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. (author)