Characterization of KNN single crystals by slow-cooling technique

Lead free piezoelectric [001]-oriented (K0.56Na0.44)NbO3 single crystals with very high crystallization have been grown by slow-cooling technique using 0.8(K0.5Na0.5)NbO3-0.2K2CO3 powder as starting materials. The dielectric properties, piezoelectric properties and domain structure of KNN single crystals were investigated. Three phase transition temperatures of rhombohedral-to- orthorhombic (R-to-O), orthorhombic-to-tetragonal (O-to-T) and tetragonal-to-cubic (T-to-C) are around -100°C, 214°C and 433°C for KNN single crystal according to the dielectric measurements, respectively. The relationship between dielectric permittivity ε and T(above Tm) is very well represented by a Cuire-Weiss law, where the Cuire constant C = 2.44 × 105K and Curie-Weiss temperature T0 = 647 K (374°C). The average piezoelectric coefficient d33 of [001]-oriented KNN single crystal is 130pC/N at room temperature. The piezoelectric coefficient d33 as a function of temperature was studied. The maximum d33 reached to 220 pC/N around 130°C. Domain structure and the extinction of orthorhombic domain in [001]-oriented KNN single crystal was observed using polarizing light microscope.