Studies On The Strong Field Electrical Properties Of Barium Titanate-based Lead-free Relaxation Ferroelectric Ceramics

BaTiO₃（BT）based ceramics can be widely used in sensor,energy storage capacitor and solid-state refrigeration due to the excellent dielectric properties,energy storage,energy harvesting and electrocaloric effect.In this paper,BiFeO₃ doped（1-x）（0.5Ba0.8Ca0.2TiO3-0.5BiMg0.5Ti0.5O3）-x（BiFeO₃）（abbreviated as（1-x）BCT-BMT-xBFO,where x = 0.0,0.01,0.02,0.03,0.04,0.05,0.06,0.07）)relaxor ferroelectric ceramics were successfully prepared using a solid-state reaction method.The effects of doping content and aging time on the microstructure,dielectric properties,ferroelectric properties and energy storage of（1-x）BCT-BMT-xBFO ceramics were investigated systematically.Moreover,x（Ba0.9Ca0.1TiO3）-（1-x）（BaSn0.2Ti0.8O3）（abbreviated as xBCT-（1-x）BST,where x= 1.0,0.625,0.60,0.575,0.55,0.525,0.50,0.0）ceramics were synthesized by various sintering methods.The effects of compositions and sintering methods on the microstructure,energy harvesting performance and electrocaloric effect of xBCT-（1-x）BST ceramics were studied systematically.The addition of BiFeO₃ does not bring a new phase into（1-x）BCT-BMT-xBFO ceramics.All samples exhibit almost pure perovskites with the coexistence of orthogonal and tetragonal.The orthogonal phase decreases when x is equal to or less than 0.04,while the tetragonal phase increases gradually.When x is more than 0.04,the orthogonal phase increases gradually,while the tetragonal phase begins to decrease.It is demonstrated that an energy storage density（Wenergy,0.58 J/cm3）with high efficiency（η～80.8%）at a large breakthrough electric field（100 kV/cm）,together with an excellent thermal stability（η>91%）in a broad temperature range（～130 K）when x =0.04.The increasing BiFeO₃ results in a large number of oxygen vacancies in（1-x）BCTBMT-xBFO ceramics.When aged,the oxygen vacancies in x = 0.06 and 0.07 reduced sharply and the breakdown electric field strength was improved greatly,resulting a large enhancement of energy storage property.Two-step sintering is beneficial to refine the grain size of xBCT-（1-x）BST ceramics and to increase the density of the ceramics,and also to improve the breakdown electric field strength.XRD results show that the compositions of xBCT-（1-x）BST ceramics are close to the morphotropic phase boundaries（MPB）,where the rhombohedral,orthogonal and tetragonal coexist.Two-step sintering is also helpful to optimize the energy harvesting performance of xBCT-（1-x）BST ceramics.A maximum value of energy harvesting（WPEH～0.246 J/cm3）with a maximum electrocaloric（EC）temperature change（△T）of～0.66 K are obtained at x= 0.625 at an applied electric field of 40 kV/cm.The electrocaloric strength（AT/AE）of one-step sintered ceramics is higher than that of two-step sintered ceramics due to the existence of defect dipoles and a highest△T/△E（～0.022 K·cm/kV）is obtained at x = 0.575.