Investigation On Phase Structure Regulation And Electrocaloric Effect Of Potassium Sodium Niobite Lead-free Ferroelectric Material

Potassium Sodium Niobate(K_0.5Na_0.5NbO₃,KNN)is a ferroelectric material with a rich phase structure,which undergoes a series of phase transitions from cubic phase?C?to tetragonal phase?T?,from tetragonal phase to orthorhombic phase?O?,and from orthorhombic phase to rhombohedral phase?R?with the decrease of temperature.The structure of multi-phase coexistence contributes to increase polarization direction in KNN material,resulting large entropy change under the action of external electric field and expecting to obtain large electrocaloric effect.Firstly,?1–x?K_0.5Na_0.5NbO₃–xBi_0.5Na_0.5ZrO₃ binary solid solution is formed using Bi_0.5Na_0.5ZrO₃?BNZ?and KNN,and compact ceramics are prepared by conventional solid-state sintering.The effects of BNZ content on KNN phase structure,microstructure,dielectric properties,pyroelectric properties and electrocaloric effect are investigated.The results show that the increase of BNZ content drives the KNN phase transition temperature move to the vicinity of room temperature,forming a T phase and R phase coexistence structure,and the typical ferroelectric changes to the relaxer ferroelectric.When the BNZ content is 0.03 mol%,the maximum adiabatic temperature change is 0.22 K under 35 kV cm^–1by direct measurement.Moreover,the pyroelectric coefficient is up to 454.46×10^–4 C·m^–2K^–1 at Curie temperature via direct measurement.Secondly,phase structure and phase transition types are studied in?1–x?(K_0.49Na_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–xCaZrO₃ system by adjusting CaZrO₃?CZ?content.The effects of CZ content on the phase structure,microstructure,dielectric properties and ferroelectricity of the ceramic are investigated,and the electrocaloric effects induced by diffuse ferroelectric–ferroelectric phase transition and relaxor ferroelectric–paraelectric phase transition are investigated.The phase structure of ceramics changes obviously and the grain size decreases monotonously with the increase of CZ content.The ceramics have a structure of multiple ferroelectric phases coexistence at 0.02?x?0.06.The characteristic of diffuse phase transitions is found between different ferroelectric phases,and the diffuse degree of phase transition is gradually enhanced.As the content of CZ increases,the polarization of ceramics increases firstly and then decreases.As the CZ content is 0.05 mol%,the structure with R-O-T phase coexistence is obtained near room temperature,and the maximum adiabatic temperature change is 0.63 K under 80 kV cm^–1,and refrigeration working temperature span is 43 ^oC.The ceramic of this system becomes a relaxer ferroelectric with only a relaxer ferroelectric-paraelectric phase transition at 0.08?x?0.12.The phase transition temperature?T_m?of ferroelectric-paraelectric moves towards room temperature with the increase of CZ content,and the dielectric and ferroelectric properties gradually decrease.The maximum electrocaloric effect in this system is achieved at the CZ content of 0.08 mol%,and the maximum?T value is 0.75 K at 145 ^oC.Finally,ceramic components with CZ content of 0.08 mol%and 0.1 mol%are selected in the?1–x?(K_0.49Na_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–xCaZrO₃ system.Multilayer ceramic capacitors?MLCC?are prepared by tape-casting method.The dielectric properties,ferroelectric properties,and electrocaloric effect of the KNN-based MLCC are investigated.Compared with the same system ceramics prepared by the traditional solid-phase sintering method,the breakdown electric-field of the MLCC is obviously increased,so the electrocaloric effect is significantly enhanced.The maximum?T of KNLNT–0.08MLCC is 1.28 K under 200 kV cm^–1 by direct test method;The maximum?T of KNLNT–0.1 MLCC is 2.45 K under 300 kV cm^–1 by direct test method,the value of?T increased nearly 10 times compared with the ceramic components of the same system.