Effect of sintering temperature on the microstructure and conductivity of Na0.54Bi0.46Ti0.99Mg0.01O3-δ

The present work examines the effect of sintering temperature on the phase formation and the conductivity of Na0.54Bi0.46Ti0.99Mg0.01O3-delta. The formation of the secondary phase and the poor grain boundary conductivity are a few major concerns in this recently developed material. Polycrystalline bulk specimens are fabricated through conventional pressureless sintering at different temperatures of 930 degrees C, 950 degrees C, 1000.C, and 1050 degrees C for 2 h. A single rhombohedral perovskite phase is detected in the XRD profiles collected on all the sintered samples. However, the SEM micrographs reveal the existence of impurity phase in all the tested samples with the amount increasing considerably with the increase in temperature. The local elemental analysis of the impurity phase suggests the presence of non-stoichiometric Na2xTi2-xMgxO4 compound. However, the chemical composition of this compound varies across different specimens. The samples sintered at 950.C exhibit the maximum bulk and grain boundary conductivities of 13.3 mS.cm(-1) and 3.6 mS.cm(-1), respectively, at 600 degrees C. A lower magnitude of both the conductivities is observed on sintering the samples at higher temperatures. The activation energy for bulk ionic conduction also decreases from 0.67 eV to 0.48 eV with the sintering temperature supporting the conjecture that higher Mg2+ amount is dissolved in the perovskite phase present in the samples sintered at 950 degrees C. The higher total conductivity in the samples sintered at 950 degrees C is attributed to the formation of comparatively less amount of Mg-rich impurity phase. Even though 950 degrees C appears to be an optimum sintering temperature, the grain boundaries of the samples continue to be resistive. At 600 degrees C, the grain boundary conductivity is less than one-third of the bulk conductivity. As a result, grain boundaries contribute a significant portion of the total resistivity offered by the Na0.54Bi0.46Ti0.99Mg0.01O3- delta samples.