Effects of Nb/Co ratio on the dielectric properties of BaTiO3-based X7R ceramics

The influences of the Nb/Co ratio on dielectrical properties, and microstructural evolution of the BaTiO3-based X7R ceramics were investigated. The Nb/Co ratios based on the same Co3O4 content of 0.34 wt.% were 3:1, 2:1, 1.7:1, 1.5:1 and 1.3:1, respectively. The ceramics in all of Nb/Co ratio were temperature-stable. The highest dielectric constant at room temperature is 4150. The dielectric constants of the ceramics sintered at the same temperature increased with the decreasing Nb/Co ratio, while the temperature coefficients of capacitance (TCC) rotated clockwise around 25 °C with the decrease of Nb/Co ratio. The TCC under 25 °C increased from negative values to positive ones; the TCC above 25oC decreased to nearly −15%. The dielectric properties of the Nb-rich composition (Nb/Co=3:1) were dependent strongly on the sintering times, while those of the Co-rich compositions were independent on that. The lattice constants parameters (a, c, c/a,) of the ceramics were investigated by XRD analysis. SEM studies showed that the Nb-rich ceramic contained a larger amount of secondary phases, Ba6Ti17O40, than other ceramics. So the Nb/Co ratio can affect the dielectric properties and microstructures of BaTiO3 ceramics significantly.