Study On Doping Modification And Properties Of Potassium-sodium Niobate Based Piezoelectric Ceramics

Based on the concept of environmentally friendly development,lead-free piezoelectric materials have become an important research topic in the field of materials science.In recent years,potassium sodium niobate（KNN）piezoelectric ceramics have been developed rapidly because of their excellent piezoelectric properties and high Curie temperature.However,its comprehensive performance is far from that of lead based piezoelectric ceramics,and there is still a long way to go before large-scale production and market application.Therefore,it is one of the main tasks to improve its piezoelectric performance by various strategies.In this thesis,the second composition introduction,ion substitution and non-stoichiometry defect design were used to improve the piezoelectric properties of sintered ceramics.The relationship between the phase structure,microstructure and macroscopic electrical properties of ceramics was studied in detail.The main contents are described below:First,the R-T phase boundary was successfully constructed at room temperature by regulating the doping content of(Bi_0.5Li_0.45Ag_0.05)(Zr_0.98Hf_0.02)O₃,and the piezoelectric responsewasenhanced.The（1-x）K_0.48Na_0.52Nb_0.96Sb_0.04O₃-x(Bi_0.5Li_0.45Ag_0.05)(Zr_0.98Hf_0.02)O₃-0.2wt%Fe₂O₃（KNNS-x BLAZH-Fe）ceramics were obtained by the solid state sintering method,the relationship between the phase structure,microstructure and electrical properties of the ceramics was studied.The results show that the ceramics are R-T phase coexistence when the doping amount of BLAZH is0.035≤x≤0.05.The optimal comprehensive properties(d₃₃=343 p C/N,d^*₃₃=410 pm/V,T_C=270℃)were obtained in the ceramic with x=0.035.Second,the piezoelectric properties were further enhanced by the substitution of Fe ions on the basis of binary ceramics.The 0.96K_0.48Na_0.52Nb_0.96Sb_0.04O₃-0.04Bi_0.5-xFe_xAg_0.03Na_0.47Zr O₃（KNNS-BF_xANZ）ceramics were prepared by the solid state sintering method,the effects of the substitution of Fe ions on phase transition temperature,grain size and electrical properties were studied in detail.It has been shown that trace Fe ion substitution can adjust the phase transition temperature(T_R-O/T_O-T)and release the O phase without affecting the Curie temperature.At the same time,the comprehensive performance of ceramics can be further improved.The most excellent overall properties(k_p=0.56,d₃₃=464 p C/N and T_C=240°C)were obtained with x=0.06.Third,based on KNNS-BNZ ceramics,the piezoelectric properties of the ceramics are effectively enhanced by non-stoichiometry defects of Zr.In this work,the0.96(K_0.48Na_0.52)(Nb_0.96Sb_0.04)O₃-0.04(Bi_0.5Na_0.5)Zr_1+xO₃（KNNS-BF_xANZ）ceramics were prepared by the solid state sintering method.The effect of zirconium element non-stoichiometry on phase structure and electrical behavior was explored.The results show that the prepared ceramics have pure R phase perovskite structure.The microstructure and macroscopic electrical properties of ceramics were very sensitive to the change of Zr content.The overall performances reach the maximum(d₃₃=351 p C/N,P_r=24.26μC/cm²and E_C=10.77 k V/cm)with x=0.05.Moreover,the defect and domain switching model was proposed to investigate the effect of defects on their performances.Fourth,a new ternary ceramic system was designed on the basis of binary ceramics by introducing Fe In O₃,which effectively enhanced the piezoelectric properties of ceramics.The（0.96-x）(K_0.48Na_0.52Nb_0.96Sb_0.04)O₃-0.04(Bi_0.5Ag_0.5)Zr O₃-x Fe In O₃（KNNS-BAZ-x FI）ceramics were prepared by the solid state sintering method,the effect of Fe In O₃content on ceramic phase structure,microstructure and electrical properties was studied.The results show that the addition of Fe In O₃ reduces the R phase fraction and increases the T phase fraction.At the same time,the density of ceramic was greatly improved.The optimal comprehensive properties(k_p=0.575,d₃₃=552 p C/N)were achieved in the ceramic with x=0.006.