Microstructure, ferroelectric and piezoelectric properties of MnO2-modified Ba0.70Ca0.30TiO3 lead-free ceramics

Lead-free Ba 0.70 Ca 0.30 TiO 3  +  x mol% MnO 2 (abbreviated as BCTM x , x  = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.0 and 1.2) piezoelectric ceramics were synthesized by the solid-state reaction method. The effects of MnO 2 additives on the phase transition, microstructure, dielectric, ferroelectric, strain behaviors and piezoelectric properties are investigated. BCTM x ceramics ( x  = 0–1.2) exhibit diphasic tetragonal and orthorhombic phases. The tetragonal phase is gradually suppressed by increasing x . As x increases from 0 to 1.2, the Curie temperature ( T C ) decreases monotonically from 128 to 45 °C, while the grain size, dielectric and ferroelectric properties increase and reach the maximum near x  = 0.6. On the other hand, the piezoelectric coefficient ( d 33 ) and the electromechanical coupling coefficient ( k p %) decrease simultaneously, whereas the mechanical quality factor ( Q m ), bipolar and unipolar strain increase and reach the maximum values of 684, 0.28% and 0.24%, respectively, near x  = 0.6. The optimum electrical performance with \(\varepsilon_{r}\)  = 1323, tan \(\delta\)  = 0.08, T C  = 66 °C, P max  = 12.2 μC/cm 2 , P r  = 6.9 μC/cm 2 , E C  = 13.3 kV/cm, d 33  = 116 pC/N, k p % = 0.22%, Q m  = 684 and the normalized strain \(d_{33}^{*}\)  =  S max / E max as high as 480 pm/V were observed for x  = 0.6. The high strain and Q m suggested that the MnO 2 -modified Ba 0.70 Ca 0.30 TiO 3 ceramics are promising candidate for high power applications.