Preparation And Properties Of Potassium Niobate-based Lead-free Piezoelectric Ceramics

Perovskite structure (K,Na)NbO3-based lead-free piezoelectric ceramics is considered to be the most promising lead-free piezoelectric materials. Preparation and doping of ceramics are discussed on the KNN system in this thesis.Li+ and Ta5+ co-doped KNN-based lead-free piezoelectric ceramics were prepared by three different sintering processes, the effect of Li+ content on the ceramic structure and piezoelectric properties have been studied. The results show that Modified Sintering can decrease oxygen vacancies within ceramic by high temperature annealing at 1040℃. The uniformity and density of KNN-based ceramics are greatly improved, which result in enhancing the piezoelectric properties of KNN-based ceramics. Increasing Li content make the structure transform from orthogonal to tetragonal phase. When Li+ doping content less than 3mol%, the system remains orthorhombic,4～5mol% is orthorhombic-tetragonal two-phase coexistence, and the system into the tetragonal phase at room temperature when the Li+ content more than 6mol%. Piezoelectric constant d33 of KNN-based ceramics after poling by field cooling from 110℃can significantly improve. After optimization of the preparation process, [(K0.48Na0.52)o.96Li0.04](Nb0.85Ta0.15)O3 lead-free piezoelectric ceramics reached a maximum value of piezoelectric constant d33 for 238.9pC/N.The effect of Bi(Mgo.5Tio.5)03 (BMT) doping on phase structure and electrical properties of KNN-based piezoelectric ceramics were investigated. The results show that, doping perovskite BMT in the KNN-based can make significant refinement of grain, increased the Curie temperature, and the phase structure of KNN transform to pseudocubic phase, resulting in a substantial decrease of piezoelectric constant.Morphotropic phase boundary between tetragonal-phase of the KNN-base and rhombohedral-phase of BiScO3 (BS) was built. The results show that, the introduction of BiScO3 can make the grain size decrease, and when BS doping amount within the range of 1～2mol% is the morphotropic phase boundary in this system.Finally, the poling technology of the orthorhombic phase 0.82 (K0.5Na0.5) NbO3-0.18AgNbO3 (KNN-AN) lead-free piezoelectric ceramics was investigated. The results show that, a poling field of 30kV/cm at 120℃for 25 minutes would be the ideal poling condition used in poling KNN-AN ceramics. Also, we qualitatively analyzed the relationship between the electric field and domains shift by using the X-ray diffraction.