Growth and Characterization of Single Crystals of Potassium Sodium Niobate by Solid State Crystal Growth

Due to the toxic nature of PbO in Pb(Zr,Ti)O3 (PZT), the most common type of piezoceramic, many studies on lead-free materials are being conducted worldwide (Röedel et al., 2009). One of the most studied groups of lead-free ferroelectric materials is based on a solid solution of potassium sodium niobate, K0.5Na0.5NbO3 (KNN) (Jaffe et al., 1971; Kosec et al., 2008). However, activity in attempts to find a lead-free replacement for PZT really accelerated with the discovery of textured (K,Na,Li)(Nb,Ta,Sb)O3 ceramics, with properties comparable to those of PZT (d33> 300 pC/N, relative permittivity > 1500, planar coupling coefficient kp> 60%)(Saito et al., 2004). One way to enhance the piezoelectric properties of KNN is to grow KNN-based single crystals along certain crystallographic directions, as has been demonstrated for the case of KNbO3 (Wada et al., 2004) and relaxor-based ferroelectric single crystals (Park & Shrout, 1997). The most frequently used methods for growing alkali niobate based single crystals are top seeded solution growth and the flux methods (Chen et al. , 2007; Davis et al., 2007; Kizaki et al., 2007; Lin et al., 2009). However, these methods are not yet fully commercialized due to high costs and poor reproducibility related to compositional inhomogeneity within the crystals. A possible way to improve the homogeneity of crystals with complex composition is to grow them by the low cost Solid State Single Crystal Growth (SSCG) method. The SSCG method is essentially a form of induced abnormal grain growth, a phenomenon which is very well known in the solid state sintering community. A significant breakthrough in the solid state synthesis of lead-free materials has been achieved in recent years (Kosec at al., 2010; Malič et al., 2008a). However, due to the strongly hygroscopic nature of alkaline carbonates usually used in solid state synthesis, different diffusion rates of involved ionic species during annealing, and the high sublimation rates of the alkaline species at high temperatures (Bomlai et al., 2007; Jenko et al., 2005; Kosec & Kolar, 1975; Malič et al., 2008b), it may be rather difficult to obtain compositionally homogeneous alkali niobate-based single crystals by SSCG.