Exploration On The Preparation Process Of KNN-based Lead-free Piezoelectric Ceramics

Piezoelectric ceramics refer to a class of functional materials with piezoelectric effect,which have been widely used in piezoelectric drives,sensors,and transducers.Nowadays,lead zirconate titanate Pb（Zr,Ti）O₃（PZT）-based ceramics are widely used in different fields due to their excellent mechanical and electrical properties,but they contain a large amount of harmful element lead,which seriously pollutes the environment and endangers human health and does not meet the needs of sustainable development.（K,Na）Nb O₃（KNN）-based ceramics have relatively high Curie temperature and good piezoelectric properties,which was expected to replace lead-based piezoelectric ceramics.However,the preparation of KNN-based ceramics is difficult,and it is difficult to obtain high piezoelectric properties.Therefore,the KNN-based ceramics were deeply and systematically studied from three parts in this thesis.In the first part,the（1-x）(Li_0.02Na_0.50K_0.48)(Nb_0.95Sb_0.05)O₃-x Ag Ta O₃（abbreviated as LNKNS-x AT）ceramics were prepared by composition design and two-step sintering process（abbreviated as TSS）.The effects of sintering process and composition on the phase structure,microstructure,and electrical properties of the LNKNS-x AT ceramics were studied in detail.Via introducing TSS for preparing the LNKNS-x AT ceramics,the volatilization of Na and K can be decreased and all the LNKNS-x AT ceramics show high densification.The XRD results show that all the LNKNS-x AT ceramics exhibit a pure perovskite phase structure,and the orthorhombic-tetragonal coexisting phase boundary is successfully constructed.Dielectric spectra above room temperature indicate that the LNKNS-x AT ceramics undergo two-phase transitions from orthorhombic to tetragonal and from tetragonal to cubic.The fitting of Curie-Weiss law and exponential law shows that the LNKNS-x AT ceramics approach a normal ferroelectric with dispersion-type phase transition（DPT）characteristics.After optimizing the sintering process（sintering at 1140℃for 30 minutes and holding at 1050℃for 8 hours）,the 0.97LNKNS-0.03AT ceramics exhibit the best electrical properties,in which（?）_m=5742,T_C=307℃,d₃₃=241 p C/N,K_p=36.4%,and Q_m=72.3.In the second part,the core-shell structure0.95(Li_0.02Na_0.50K_0.48)(Nb_0.95Sb_0.05)O₃-0.05Ag Ta O₃@5 mol%Ba Zr O₃（abbreviated as LNKNSAT@BZ）powder was prepared by sol-gel core-shell process.The crystal structure and microscopic morphology of the powder were studied in detail.The formation of core-shell LNKNSAT@BZ powders was confirmed by TEM analysis,in which the“LNKNSAT”core particle size was 150-200 nm,and the“Ba Zr O₃”shell thickness was about 5-6 nm.The calcination temperature of the powder was determined to be 725-825℃by the TG-DCS thermal analysis.The analysis of XRD patterns of the LNKNSAT@BZ powders calcined at different temperatures shows that the main perovskite crystal structure has been formed in the calcined powders,which meets the needs for sintering ceramics.In the third part,the effects of powder calcination temperature and ceramic sintering temperature on the phase structure,microstructure,and electrical properties of the LNKNSAT@BZ ceramic system prepared by the sol-gel core-shell process were systematically studied.The effects of different shell of LNKNSAT@AB（AB=5 mol%Ba Zr O₃（BZ）,5 mol%Ba Ti O₃（BT）,5 mol%Sr Zr O₃（SZ））on the phase structure,microstructure and electrical properties of KNN-based ceramics were also investigated.The sol-gel core-shell process greatly improves the activity of the LNKNSAT@BZ powder and reduces the calcination temperature of the powders and the sintering temperature of the ceramics.The fitting of Curie-Weiss law and exponential law shows that LNKNSAT@BZ is a normal ferroelectric with dispersion-type phase transition characteristics.With the increase of calcination temperature and sintering temperature,the electrical properties of LNKNSAT@BZ ceramics prepared by the sol-gel core-shell process first increased and then decreased.LNKNSAT@BZ ceramics exhibit the best electrical properties when calcined at 775℃and sintered at 1150℃,in which T_C=184℃,（?）_m=4150,d₃₃=157 p C/N,K_p=30.6%,and Q_m=76.9.All the coated LNKNSAT@AB ceramics exhibit a single perovskite structure.After optimizing the sintering process（calcining at775℃and sintering at 1150℃）,the piezoelectric properties of LNKNSAT@BZ ceramics coated with“Ba Zr O₃”are improved as compared with those of the uncoated LNKNSAT ceramics.Through the research on the phase structure and electrical properties of the LNKNS-x AT ceramics,the better dielectric and piezoelectric properties can be attributed to the reduced evaporation of alkali metal elements during the TSS sintering,which maintains the designing composition and a better piezoelectric response can be obtained.Through the research on the phase structure and electrical properties of the LNKNSAT@BZ ceramics,better dielectric and piezoelectric properties are attributed to the multiphase coexistence at room temperature.The LNKNS-x AT ceramics and LNKNSAT@BZ ceramics exhibit relatively high dielectric constant and excellent piezoelectric properties,which present great application potential in the field of lead-free piezoelectric ceramics.