Giant electrocaloric effect enhancement due to the polarization flip and influence of Mn⁴⁺ doping on the dielectric, ferroelectric properties in 0.7BiFeO₃-0.3BaTiO₃ ceramics

Abstract

As a kind of ferroelectric and antiferromagnetic coexistent multi-ferroic material, BiFeO₃ (BFO) has a theoretical saturation polarization over 100 μC/cm², and a Curie temperature of 830 ℃, which may offer a huge electrocaloric effect. However, owing to the evaporation of Bi₂O₃ in the sintering process at high temperatures and the variation of chemical valence of iron ions, there are lots of point defects and also a large leakage current existing in BFO, making the ferroelectricity of BFO hard to develop and measure. Although the forming of solid solution with BaTiO₃ (BTO) or other oxide ferroelectrics may mitigate the leakage current, high loss tangent is still existent. This work tries to address this issue by adding manganese ions into the BFO-BTO solid solution. The 0.7(BFO)-0.3(BTO)+<i>x</i>%MnO₂ ceramics are prepared through using the conventional solid-state reaction at high temperature. The microstructure, dielectric characteristic and ferroelectric characteristic are investigated by doping different Mn⁴⁺ ions. Results indicate that the crystallographic structure is of rhombohedral and pseudocubic phase coexistence. It is observed that a certain content of Mn⁴⁺ ions may lead both the loss tangent and the leakage current for BFO-BTO ceramic to decrease, which is due to the compensation of dopant Mn⁴⁺ ions for the oxygen vacancies. In addition, the 0.7BFO-0.3BTO+0.5%MnO₂ ceramic arrives at a maximum polarization of 50.53 μC/cm² at 100 kV/cm. Finally, a direct approach is used to measure the electrocaloric effect. It is found that using the polarization flip method, the ECE temperature change is observed to increase almost 8 times when the electric field changes from 0 to –30 kV/m with respect to that when the electric field decreases from 30 kV/cm to 0. This verifies that the Lu et al’s method is also applicable to polycrystalline first-order phase transition ferroelectrics.