Effect of MnO2 on the dielectric and piezoelectric properties of alkaline niobate based lead free piezoelectric ceramics

Lead free ferroelectric ceramics of the KNN–LiTaO3–LiSbO3 system were prepared using the mixed oxide route. This work reports the effect of doping (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 produced through the conventional solid state sintering method with different amounts of MnO2. With 1 mol% of the dopant, ∼96.5% of the theoretical density of the ceramics was achieved while grain growth inhibition was attained through pinning of the grain boundary movement. A polymorphic phase transition (PPT) was induced in the ceramic from the orthorhombic crystal structure to the tetragonal structure with increasing dopant amount. At lower temperatures, the doped samples had higher epsilon values but there was a decrease in both Tc (from 333 °C to 249 °C) and epsilon value at Tc (from ≈9500 to <6000). At temperatures below 300 °C however, the loss tangent in the doped samples (≈2.5 mol%) was much lower and steady when compared to the undoped one. The ferroelectric properties were slightly lowered with the addition of MnO2. The remnant polarisation (Pr) was lowered from ∼18 μC/cm2 to ∼9 μC/cm2, the coercive field (Ec) from ∼8.5 kV/cm to ∼6.2 kV/cm and the piezoelectric charge coefficient (d33) decreased as well.