The Phase Structure And Electrical Properties Of BKH Modified KNN-based Lead-free Piezoelectric Ceramics

As a basic key functional material,piezoelectric ceramics are widely used in important fields such as electronics,medical treatment,biology,and military industry because of their characteristics of mutual conversion of mechanical energy and electrical energy.Because of its excellent piezoelectric properties and high Curie temperature,potassium sodium niobate based?KNN?piezoelectric ceramics have attracted great attention in the research of lead-free piezoelectric ceramics.In recent years,many studies have found that a phase boundary similar to the morphotropic phase boundary?MPB?in lead-containing piezoelectric ceramics can be constructed in KNN-based ceramics to significantly improve the piezoelectric performance of the ceramics,so the phase boundary construction has now become the hotspot of KNN-based piezoelectric ceramic research.Therefore,in order to establish the phase boundary,we designed a new potassium sodium niobate system?1-x?K_0.40Na_0.60Nb_0.96Sb_0.04O₃-xBi_0.5K_0.5HfO₃?abbreviated as KNNS-BKH?by conventional solid-state reaction method.The variation law of crystal structure,micro-morphology and electrical properties of KNN-based ceramics with the increase of Bi_0.5K_0.5HfO₃ doping was discussed.And also,the influence of Bi_0.5K_0.5HfO₃ on the orthogonal-tetragonal phase transition temperature(T_O-T)and trigonal-orthogonal phase transition temperature(T_R-O)was studied through the characterization of Temperature-dependent dielectric properties.Observation by scanning electron microscopy found that with the increase of the addition of BKH,the grain size of the ceramic first increased and then decreased.According to the XRD diffraction pattern and the dielectric-temperature curves,we found that with the addition of BKH,the ceramic has undergone the structural evolution from orthogonal phase to multi-phase coexistence to trigonal phase.Furthermore,one can note that the characteristics of diffusion phase transition shows more obviously with the increase of BKH content.In addition,a boundary of the rhombohedral-tetragonal phase was found at x=0.035,which was caused by the simultaneous change and mixing of the orthogonal-tetragonal phase transition temperature and trigonal-orthogonal phase transition temperature.This ceramic exhibits the greatest piezoelectric performance near the boundary between the rhombohedral-tetragonal phase,where the piezoelectric constant is 230 pC/N and the planar electromechanical coupling coefficient is 0.34.Next,K_0.40Na_0.60Nb_?1-y?Sb_yO₃-0.03Bi_0.5K_0.5HfO₃?abbreviated as KNNS_y-BKH?were prepared with varying Sb doping amount.This series of ceramics was used to study the effect of Sb on the crystal structure,micromorphology and piezoelectric properties of KNNS-BKH ceramics.The XRD diffraction pattern and the dielectric-temperature curves confirmed that with the increase of Sb,the transition temperature of rhombohedral-orthogonal phase and the orthogonal-tetragonal phase phase of the ceramic simultaneously changed,and it was observed a triangular-orthogonal phase boundary at 0.03?y?0.04.At the same time,the grain size tends to decrease with the addition of Sb.The piezoelectric constant d₃₃,electromechanical coupling coefficient k_p and dielectric constant?_r of the ceramics all change regularly with the addition of Sb.When y=0.04,the piezoelectric performance of the ceramic reaches its optimum,in which the piezoelectric constant d₃₃ is 202 pC/N and the planar electromechanical coupling coefficient k_p is 0.32.Finally,we doped SrZrO₃ into KNNS-BKH ceramics to prepare?0.97-x?K_0.40Na_0.60Nb_0.95Sb_0.05O₃-0.03Bi_0.5K_0.5HfO₃-xSrZrO₃[abbreviatedas?0.97-x?KNNS-0.03BKH-xSZ].This ternary ceramics were used to study the microstructure,phase structure,electrical properties and ferroelectric properties of?0.97-x?KNNS-0.03BKH-xSZ ceramics.The XRD diffraction pattern and the dielectric-temperature curves improved that with the increase of SrZrO₃ addition,the orthorhombic-tetragonal phase transition temperature of?0.97-x?KNNS-0.03BKH-xSZ ceramics gradually decreased,while its rhombohedral-orthogonal phase transition temperature did not change significantly.At room temperature,the structure of this ternary ceramic phase transitions from a rhombohedral-orthorhombic mixed phase?0?x?0.005?to form a rhombohedral-tetragonal mixed phase?0.01?x?0.015?and finally forms a rhombohedral phase?0.02?x?0.03?.At the same time,ceramics with a rhombohedral-tetragonal phase structure at room temperature show a strong composition dependence and a significant increase in piezoelectric performance.The ceramic exhibits the best piezoelectric performance at x=0.01,where the piezoelectric constant d₃₃ is 280 pC/N and the planar electromechanical coupling coefficient k_p is0.36.