Preparation And Properties Of Lead-free (K_0.5Na_0.5)NbO₃-LiNbO₃ Piezoelectric Ceramics

The development of lead-free piezoelectric ceramics has been intensively studied for environment protection. The sintering characteristic, the electromechanical propreties, the dielectric, piezoelectric and ferroelectric properties of the Lead-free (1-x)(K_0.5Na_0.5)NbO₃ -xLiNbO₃ (abbreviated as KNLN) piezoelectric ceramics were investigated systematically in this thesis in order to obtain an excellent lead-free piezoelectric ceramics. The influences of the composition, additives and processing on the properties of the ceramics were analyzed in order to optimize the processing parameters and the poling conditions of the KNLN based piezoelectric ceramics.Results shows that the KNLN piezoelectric ceramics exhibited pure perovskite structure and the phase structure at room temperature transformed from orthorhombic to tetragonal with the increase of LiNbO₃, besides, the morphortropic phase boundary (MPB) of (1-x)(K_0.5Na_0.5)NbO₃- xLiNbO₃ existed at about 5-7 mol% of LiNbO₃. The sintering temperature and heating rates have little effects on the phase structures but great effects on the microstructure and piezoelectric properties of the KNLN ceramics. The fracture surface of the ceramics transformed from the intergranular fracture mode to a transgranular fracture mode with the increasing of the heating rate. The sinterability of KNLN ceramics was improved and its sintering temperature was lower than that of pure KNN ceramics. Furthermore, the KNLN ceramics were full densed with an excellent piezoelectric properties and the typical ferroelectric characteristic. The (1-x)(K_0.5Na_0.5)NbO₃- xLiNbO₃ (x=0.06) ceramics exhibit an excellent properties with the piezoelectric constant d₃₃, the electromechanical coupling coefficients k_p and k_t of 215pC/N , 40.3% and 49.8%, respectively. The E_c is 13.4kV/cm and the P_r is 20.0μC/cm².The analysis of the poling condition shows that the lattice constant of the KNLN ceramics were increased after poling. The piezoelectric properties of the KNLN ceramics were varied at different poling condition. The optimum poling condition for 0.94(K_0.5Na_0.5)NbO₃-0.06LiNbO₃ ceramics is poling electric field of 3.5kV/mm, poling temperature of 120℃ and poling time of 30min.