Crystal Structure and Microstructural Changes in (K0.5Na0.5)NbO3-SrTiO3 Solid-Solutions
KIM G. 1, MOON K. 1
1 Department of Materials Engineering and Convergence Technology, Gyeongsang National University, Jinju, Korea (Republic of)
The potassium-sodium niobate (K0.5Na0.5NbO3, KNN) system is a promising candidate for the dielectric materials of high-temperature Multilayer Ceramic Capacitors (MLCC) because of the high Curie temperature (Tc, tetragonal↔cubic phase transition temperature, ~ 420°C). Although Curie temperature of KNN is above 400°C, orthorhombic↔tetragonal phase transition (To-t) appears around 200°C, and rhombohedral↔orthorhombic phase transition occurs between -100°C and room temperature [1]. The KNN system has a low dielectric constant and high dielectric loss. It is also challenging to control grain growth and densification behavior.
In the present study, (K0.5Na0.5)NbO3-SrTiO3 (KNN-ST) powder was synthesized as a solid solution to have a relaxor characteristic behavior. Dielectric properties, crystal structure, and microstructure were investigated by adding different contents (0, 0.5, 1.0, and 2.0 mol%) of Dy2O3 and Al2O3 to KNN-ST to enhance the dielectric properties of KNN-ST via control of the grain shape and grain growth behavior. In the case of the sintered sample with Dy2O3, the microstructure was porous as Dy content increased. The Dy-rich phase appeared when 1 mol% Dy2O3 was added. The crystal structure changed from the pseudo-cubic to the orthorhombic system when Dy2O3 was added to KNN-ST. The second phase, DyNbO4 (PDF#22-1094, Monoclinic), appeared at 2 mol% Dy2O3. It could be considered that Dy diffused into the KNN-ST lattice and the crystal structure and microstructure changed. In the case of adding Al2O3, the crystal structure changed from the pseudo-cubic to the orthorhombic system when Al2O3 was added to KNN-ST. The second phase AlNbO4 (PDF#51-0023, Monoclinic) appeared over 1 mol% Al2O3. In addition, the grain shape of KNN-ST changed to a more faceted cube with an increasing Al2O3 concentration. The grain shape change indicates increasing step-free energy and then rising in the critical driving force for appreciable growth. Further, the grain growth behavior will be discussed according to the 2-dimensional nucleation and growth.