High-temperature And High-pressure Synthesis Of KNN-based Lead-free Piezoelectric Ceramics And Their Electrical Properties

Piezoelectric ceramics can realize the mutual conversion of mechanical and electrical energy,and are often used in medical ultrasound probes,actuators,and piezoelectric ignition devices because of their good mechanical properties and relatively stable piezoelectric performance.Currently the most widely used is the lead-based piezoelectric ceramics represented by PZT(lead zirconate titanate),although excellent in performance,but lead will have a bad impact on the environment,so the development of lead-free environmentally friendly piezoelectric ceramics is the inevitable development trend.Sodium potassium niobate-based KNa Nb O3(KNN)piezoelectric ceramics are considered as one of the most promising lead-free piezoelectric ceramic systems due to their high piezoelectric coefficient and Curie temperature.However,the volatile nature of alkali metals in the conventional sintering process makes them very limited in real applications.In order to avoid the volatilization of alkali metal during the preparation process and to optimize the piezoelectric properties of KNN ceramics,this paper successfully prepared KNN lead-free piezoelectric ceramics using high-temperature and high-pressure synthesis method,and further investigated the effect of Li element doping on the properties of KNN piezoelectric ceramics.In this paper,the single-phase chalcogenide structure of KNN piezoelectric ceramics was successfully prepared using the high-temperature and high-pressure method.The variable-temperature XRD and dielectric temperature spectra showed that the phase transition temperature of the synthesized KNN ceramics was 210 °C for the orthogonal-quadratic phase and 410 °C for the Curie transition temperature.The measured Ec(coercivity field)was 27 kv/cm,Pr(residual polarization intensity)was18.4 μC/m2,and the piezoelectric constant was 110 p C/N.The KNN ceramics were characterized by Raman spectroscopy.In this paper,the KNN piezoelectric ceramics with different Li doping ratios were also tested in relation to each other.The phase transition temperature of the orthogonal-quadratic phase with different doping concentrations was determined by dielectric temperature spectroscopy.5.5% Li-doped samples decreased the phase transition temperature to 180 °C,while the Curie temperature increased to 427 °C.6.5% Li-doped samples decreased the phase transition temperature to 160 °C,while the Curie temperature increased to 433 °C.13% Li-doped samples decreased the phase transition temperature to 110 °C,while the Curie temperature increased to448 ℃.The ferroelectric properties were tested,and the coercivity field Ec and the remaining polarization Pr of the 5.5% Li-doped sample were Ec = 19.6 kv/cm and Pr= 12.2 μC/m2,respectively,and Pr = 13.7 μC/m2 and Ec = 22.1 kv/cm for the 6.5%Li-doped ceramic and Pr = 8.9 μC/m2 for the 13% Li-doped ceramic.=It can be concluded that the phase transition temperature from the tripartite to the tetragonal phase of the KNN ceramics decreases gradually with the increase of the proportion of Li elements,and the Curie transition temperature Tc increases.The spontaneous polarization and coercivity fields did not change significantly.The phase transition temperatures of the 6.5% Li-doped KNN samples were subsequently investigated by variable temperature Raman spectroscopy,and the results obtained were in good agreement with the dielectric temperature spectrum.