Microstructure and dielectric characteristics of tungsten bronze structured SBN70 ceramics: effect of Nb2O5 content

The effect of Nb2O5 content on the microstructure evolution of strontium barium niobate Sr0.7Ba0.3Nb2O6 (SBN70) ceramic was examined from a viewpoint of abnormal grain growth generation. The compositions of stoichiometry, Nb2O5-deficient and Nb2O5-excess SBN70 were prepared and sintered in the temperature range of 1250–1400 °C for 2 h. The sample with Nb2O5-excess composition showed a duplex microstructure with abnormally large grains and fine grains, while the Nb2O5-deficient sample showed a homogeneous and fine-grained microstructure. X-ray diffraction analysis of the Nb2O5-excess and Nb2O5-deficient compositions revealed second phases of Ba3Nb10O28 s.s. (B3N5) and Sr2Nb2O7 s.s. (S2N), respectively. Because the B3N5 phase has a low melting point of 1320 °C, it is believed that the abnormal grain growth generated in the Nb2O5-excess composition originates from the inhomogeneous distribution of the B3N5 liquid phase. These second phases also influence the dielectric characteristics of ceramics such as the Curie temperature, the maximum dielectric constant and diffuse phase transition behavior.