Study On Morphotropic Phase Boundary Of The Niobate-based Lead-free Piezoelectric Ceramics

In 1952, Takagi Laboratory at Tokyo Institute of Technology for the first time conducted a thorough investigation into the solid solution PbZr_xTi_1-xO₃ composed of zirconate, PbZrO₃, and titanate PbTiO₃, which laid a foundation for the wide uses of these piezoceramics. Since then, the lead zirconium-titanate solid solution, PZT, has been in the leading position in piezoceramics for half a century. Because the content of PbO in PZT is higher than 60wt%, the Pb evaporation during sintering PZT and the discarded PZT products pollute the environment and do harm to human health. Therefore, it is an urgent task to probe and develop environment-friendly new industrial piezoceramic products.The perovskite-type niobate-based piezoceramics have been considered the best lead-free ceramics substituting for the PZT. In comparison with PZT, the niobate-based piezoceramics also have its shortcomings, the lower piezoelectric constants and electromechanical coupling factors are its major shortcomings. Based on PZT experimental regularity that perovskite-type PZT has good piezoelectric performance at the morphotropic phase boundary (MPB), it was my plans to search the relation of the niobate piezoceramic compositions with the MPB and focus the studies on the piezoelectric, dielectric properties of the niobate piezoceramics at the MPB, so that the new lead-free piezoceramic system with high-piezoelectric properties can be obtained.Based on my experiments and others references, x (Na_0.53K_0.435Li_0.035)NbO₃-y(Na_0.53K_0.435Li_0.035)SbO₃-z(Na_0.53K_0.435Li_0.035)TaO₃ ternary system with a ratio of Na, K, Li: 0.53: 0.435:0.035, was chosen as studied object. The powers of (Na_0.53K_0.435Li_0.035)NbO₃ (abbreviated as NKLN) ,(Na_0.53K_0.435Li_0.035)SbO₃ and (Na_0.53K_0.435Li_0.035)TaO₃ were first prepared by the conventional solid-state reaction method respectively. Then (1-x)(Na_0.53K_0.435Li_0.035)NbO₃-x(Na_0.53K_0.435Li_0.035)SbO₃ (abbreviated as NKLN-xSb),(1-x)(Na_0.53K_0.435Li_0.035)NbO₃-x(Na_0.53K_0.435Li_0.035)TaO₃(abbreviated as NKLN-xTa) and (1-x-y)(Na_0.53K_0.435Li_0.035)NbO₃-x(Na_0.53K_0.435Li_0.035)SbO₃-y(Na_0.53K_0.435Li_0.035)TaO₃ (abbreviated as NKLN-xSb-yTa) were also prepared respectively, their microstructures, electromechanical, piezoelectric and dielectric properties had been systematically investigated. It was found that with increasing Sb content, the piezoelectric constant d₃₃ ofNKLN-xSb increased and then decreased. With Sb increased, the orthorhombic to tetragonal polymorphic phase transition temperature shift towards lower temperatures. It was also found 0.912(Na_0.53K_0.435Li_0.035)NbO₃-0.088(Na_0.53K_0.435Li_0.035)SbO₃ has high-piezoelectric performance, its piezoelectric stress constant d₃₃ =246pC/N ,electromechanical coupling factor k_p=52%, k_t=48% , with its Curie temperature T_c=259℃:and T_O-T =81℃.Relative compact NKLN-xTa lead-free ceramics were prepared by conventional oxide solid-solution technique .The piezoelectric, electromechanical, dielectric properties of the ceramics were evaluated. It was found with the increase of Ta, the dielectric constant increased, the Curie temperature moved to lower temperature. NKLN-0.23Ta had a relatively high piezoelectric constant d₃₃ =271pC/N, highelectromechanical coupling factor k_p=51%, k_t =44%, and a high Curie temperatureof 311℃ These features indicate the ceramics is a promising candidate for substituting PZT in piezoelectric application fields.The NKLN-xSb-yTa (y=0.03, 0.06, 0.09, 0.12, 0.20) ceramic systems were prepared by conventional oxide solid-solution technique. The MPB of NKLN-xSb-yTa had been found and it was shown that the ceramics at MPB have excellent properties, piezoelectric constant d₃₃ ranging in 300-327pC/N. The piezoelectric constant d₃₃ of 327pC/N is higher than the highest one of niobate-based ceramics prepared by conventional processing reported by Japanese (d₃₃=300 pC/N, Salto Y., Takao H., Tani T., et al. Lead-free Piezoceramics. Nature, 2004,432: 84-87).