Synthesis of polycrystalline platelike NaNbO3 particles by the topochemical micro-crystal conversion from K4Nb6O17 and fabrication of grain-oriented (K0.5Na0.5)NbO3 ceramics

Abstract

Topochemical microcrystal conversion (TMC) method is a powerful tool to synthesize platelike microcrystal particles with a regular-perovskite crystal structure, which is difficult to be fabricated by conventional flux techniques. By using the TMC method, polycrystalline rectangular-platelike NaNbO3 particles with a orthorhombic perovskite structure were able to be synthesized from platelike precursor particles of layer-structured K4Nb6O17 at 1000°C in molten NaCl-salt. The TMC-synthesized NaNbO3 particles preserved the shape of precursor particles, and had a thickness of about 1 micron and a width of 5-10 microns. However TMC-synthesized platelike NaNbO3 particles had a polycrystalline morphology having a preferred pseudo-cubic {100} orientation. Oriented particulate layer X-ray diffraction (OPL-XRD) analysis revealed that, during the TMC reaction, the crystallographic {010} plane of K4Nb6O17 is converted to the most of {001} plane of polycrystalline NaNbO3 particles in spite of polycrystalline morphology. Using the polycrystalline platelike NaNbO3 particles as a template in the reactive templated grain growth method (RTGG), {001} grain-oriented (K0.5Na0.5)NbO3—1 mol% CuO ceramics having a {001} orientation degree (Logering’s factor) of 45% could be fabricated. The result indicates that not only single crystalline particles, which were generally used, but also the polycrystalline particles can be act as template in the RTGG process. The availability of polycrystalline particles will give a new design of synthesizing templates for texturing of various kinds of perovskite crystal-structured ceramics.