Effect of firing temperatures on properties of BNT-BCTZ-0.007mol%BFCO lead free piezoelectric ceramics synthesized by the solid state combustion method

In this study, the effects of calcination temperature (550-750 °C for 2 h) and sintering temperature (1100-1200 °C for 2 h) on the phase formation, microstructure, electric and magnetic properties of the 0.94Bi0.5Na0.5TiO3-0.06(Ba0.85Ca0.15)(Ti0.90Zr0.10) lead-free piezoelectric ceramic doped with 0.007 mol%Bi2FeCrO6 (BNT-BCTZ-BFCO) were investigated. These ceramics were prepared by the solid state combustion technique using glycine as fuel. The pure perovskite phase was observed in the BNT-BCTZ-BFCO powders attained at the calcined temperature of 650 °C for 2 h. The morphology of the BNT-BCTZ-BFCO powders exhibited an almost-spherical shape and the average particle size increased when the calcination temperature increased from 550 to 750 °C. TEM results of pure BNT-BCTZ-BFCO powder calcined at 650 °C for 2 h showed rounded shapes and the average particle size was ∼ 200 nm. The XRD results of all ceramics exhibited a single perovskite structure with the co-existence of the rhombohedral and tetragonal phases. The average grain size increased with increasing sintered temperature. The density, dielectric constant (ε at Tr and Tm) and piezoelectric constant (d33) increased when the sintered temperature increased up to 1150 °C and then reduced in values. At a sintered temperature of 1150 °C, BNT-BCTZ-BFCO ceramic showed the highest relative density (97.2%), maximum dielectric properties (ε at Tr = 1783 and ε at Tm = 5117), d33 value (178 pC/N), d*33 (549 pm/V) and the remnant polarization (Pr = 34.66 µC/cm2). At higher sintering temperatures, the properties of BNT-BCTZ-BFCO ceramics decreased due to the evaporation of Bi and Na. All ceramics exhibited the paramagnetic behavior and the magnetization increased with increasing sintering temperature.