Study Of Bi_0.5Na_0.5TiO₃ Based Lead-free Piezoelectric Ceramics

Lead-free piezoelectric ceramics Bi_0.5(Na_1-xK_x)_0.5TiO₃ (BNKT)were prepared by the conventional ceramic sintering technique. The K+ substituted Na+ at the A sites of the perovskite structure. The piezoelectric properties ,dielectric properties, and microstructures of these piezoelectric ceramics were studied. And then, we are engaged to developing the Bi_0.5(Na_1-x-yK_xLi_y)_0.5TiO₃ ( BNKLT) on the base of BNKT by using Li as the third structure, and we research the effective improvement of Li+ in the BNKLT ceramic.We research the relation between the temperature of powder and the piezoelectric properties for the ceramic system , then we found the result that best sintering temperature of the ceramics decrease from 1150～1200℃as the substituting amount of K+ increase. From the result of XRD,the fact that diffract peak moving to the left indicate that K+ can increase the constant of crystal lattice. The measurement results of piezoelectric properties reveal that d33 and the Kp both reach the maximum 175 pC/N and 0.34 at x=0.22,which is the same point that Qm reach the minimum at. The morphotropic phase boundaries (MPB) of the system between rhombohedral and tetragonal locates in the range of x = 0.16 0.24, which exhibit the best piezoelectric properties.First we get the best sintering temperatures for BNKLT ceramics with different amount of Li+. When the x=0.16,y=0.1,the Bi_0.5(Na_1-x-yK_xLi_y)_0.5TiO₃(BNKLT) ceramic get its perfect piezoelectric property with the d33=159 pC/N and the Kp=0.34, which are larger than BNKT without Li+ by 14.4% and 21%. The result on observation of P-E hysteresis loops indicates that BNKLT ceramic have excellent piezoelectric property with its high remanent polarization Pr and low coercive field Ec. From the result of XRD, it was found that Bi_0.5(Na_1-x-yK_xLi_y)_0.5TiO₃ ceramics show a single phase of perovskite structure with a rhombohedral symmetry in the range of y=0.05⁰.20. Other new phases which was harmful to the piezoelectric properties of ceramics were found at y=0.35. From the result of SEM, the best sintering temperature decreases as the substituting amounts of Li+ increase, and Li+ promotes the growth in some directions of crystalline grains. The dielectric loss peaks appear in the temperature of 120²00℃in the dielectric loss-temperature curves, and the dielectric constant peaks appear in the temperature of 300～340℃in the dielectric constant - temperature curves.