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???80.8%?at a large breakthrough electric field?100 kV/cm?,together with an excellent thermal stability??>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.