Crystal Growth and Piezoelectric Properties of Lead-free Based K0.5Na0.5NbO3 by the Floating Zone Method
Résumé
The floating zone method (FZM) is used to grow large crystal ingots of lead-free (K0.5Na0.5)NbO3 (KNN), (K0.5Na0.5)(Nb0.95Mn0.05)O3 (KNN-Mn), and (K0.5Na0.5)(Nb0.95Ta0.05)O3 (KNN-Ta). FZM allows high quality crystal to be obtained without using a crucible. However, the KNN molten zone is instable at high temperature (∼1200 °C) due to its incongruent melting and alkali segregation. The challenge of this work is to control the stability and the viscosity of the molten zone which allows long growth in order to achieve large volume single crystals. Our work reveals that the use of a slight overpressure of gas (O2 or N2) and doping KNN with Mn and Ta allows the molten zone to be maintained over 50 h. In such conditions, the as-grown crystals are preferentially oriented along the [011] direction and display piezoelectric properties such as kt of 0.48, d33 of 65–70 pC/N, ε33,rS of 123–148, δe of 0.03–0.07, and δm of 0.11–0.21. The ferroelectric measurements show saturated hysteresis loops with a decrease in EC and Pr when attaining single crystal after several hours of growth. High resolution transmission electron microscopy (HRTEM) investigations show complex domain structures that are mainly attributed to the accommodation of the stresses occurring during the crystal growth in the high thermal gradient and in the phase transition from tetragonal to orthorhombic phases during cooling.