Growth And Structure Study Of Sodium-potassium Niobate Based Lead-free Ferroelectric Single Crystals

Ferroelectrics and piezoelectrics are widely applied in our life. Lead-based ferroelectrics such as Lead Zirconium Titanate ?PZT? ceramics possess more than 90% market share of all piezoelectric materials. For the toxic nature of PbO, however, considerable recent work has been focused on lead-free piezoelectric materials from environmental protecting viewpoint.As a member of lead free materials, alkaline niobate ?KxNa1-x?NbO3?KNN?-based solid solutions have been attracting extensive attention as they are believed to provide relatively high piezoelectric response, high electromechanical coupling coefficient and high Curie temperature. KNN-based piezoelectric materials were considered as the most promising candidate for replacing PZT systems. This thesis mainly focuses on the KNN single crystals and crystal growth, defects, electrical properties and structure of KNN crystals are systemically studied.Top seeded solution growth ?TSSG? method were employed to fabricate KNN based single crystals. Pt wire was chosen as seed at first for the lack of KNN crystal, and the quality of crystal was further promoted when KNN crystal seeds were introduced. A series of high-quality and large-sized KNN and ?KxNa1-x?NbO3-x%MnO₂ single crystals have been achieved by carefully controlled TSSG method. The dimension of the as-grown single crystal reached up to ???30×10 mm.The XRD results show that the KNN and KNN-x% MnO₂ single crystals are pure perovskite structures, which are orthorhombic at room temperature without other phase.We also studied the single crystals dielectric, piezoelectric and ferroelelctric properties.Two main dielectric anomaly peaks in the heating ?-T curves were attributed to the ferro-to paraelectric phase transition from the tetragonal to the cubic phase, and from tetragonal to orthorhombic phase. The dielectric constant of the pure KNN single crystal was about 168 and the dielectric loss tan? is 0.2. After the MnO₂ is doped, the dielectric constant increases at room temperature, and the dielectric loss decreases. After doped with a certain amount of MnO₂, the piezoelectric coefficient KNN-0.5% MnO₂ single crystal d33 increases from 161pC/N to 191pC/N. If the crystal was annealed at 600 ?, the piezoelectric constant increased to 261pC/N. After doped with MnO₂, the coercive field of the crystal decreases and the remnant polarization increases.The lattice vibration of KNN single crystals were studied by temperature increasing Raman scattering and the influence of temperature on lattice vibration modes were studied.The phase transition temperature of the orthorhombic phase ?Amm2? to the tetragonal phase?P4bm? and the tetragonal phase to the cubic phase is consistent with the result of the ?-T curves. The phase change is determined by the number of peaks in the temperature rise process and the change of the peak shape. Different Raman modes were successfully separated by employing different scattering configuration.