Nanocomposites and nanomaterials
Synthesis and sintering of Co-doped K0.5Na0.5NbO3
S. Lemishko1, J. Koruza2, J. Schultheiß2, I. Golovina1
1Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, pr. Nauky 41, 03028 Kyiv, Ukraine            
E-mail: sleeper1986@ukr.net 
2Technische Universität Darmstadt, Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany
Solid solutions of K0.5Na0.5NbO3 (KNN) are a promising alternative to toxic Pb(Zr1-xTix)O3 (PZT). KNN-based ceramics have high dielectric and piezoelectric properties. In addition, high mechanical quality factors (Qm) can be achieved by acceptor doping. While it was previously shown that Co is a useful acceptor dopant for KNN thin films and ceramics [1], the hard-type properties were not well investigated yet. The purpose of the present work is to develop a method for obtaining single-phase doped KNN with a uniform distribution of cobalt.
Synthesis of undoped KNN and three KNN doped compositions with 0.1, 0.5, and 1 mol. % of Co was performed by the mixed-oxide route. After weighing the K2CO3, Na2CO3, Nb2O5, and Co3O4 powders were milled in nylon containers for 4 h using ZrO2 milling balls and isopropanol and then dried. Milled powders were crushed using mortar and pestle, pressed into cylinders and calcined. Although the XRD did not show evidence for secondary phases, the calcined doped samples contained randomly-distributed black inclusions of coagulated cobalt particles. The inclusions indicated inhomogeneity in the processed samples and possibly influenced the homogeneous distribution of the elements. Considerable effort was therefore put into studying the origin of the inclusions and possible approaches for their removal. The SEM study showed that the material near the inclusions was partially sintered even prior to reaching the actual sintering temperature.
Different approaches were used in order to avoid the formation of Co inclusions: temperature and heating rates variation, two step calcination with intermediate milling, deagglomeration by ultrasonic bath, and others. Only after using milling ZrO2-containers and 38 h milling time, KNN powder with homogeneously distributed Co (no black inclusions) has been obtained. The optical dilatometry measurements revealed a decrease of the sintering temperature with increasing Co content.
1.	L. Wang, W. Ren, P. Shi, and X. Wu, Journal of Alloys and Compounds 608, 202–206 (2014).