Synthesis of polycrystalline platelike NaNbO_3 particles by the topochemical micro-crystal conversion from K_4Nb_6O_(17) and fabrication of grain-oriented (K_(0.5)Na_(0.5))NbO_3 ceramics

摘要

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 NaNbO_3 particles with a orthorhombic perovskite structure were able to be synthesized from platelike precursor particles of layer-structured K_4Nb_6O_(17) at 1000℃ in molten NaCl-salt. The TMC-synthesized NaNbO_3 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 NaNbO_3 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 K_4Nb_6O_(17) is converted to the most of {001} plane of polycrystalline NaNbO_3 particles in spite of polycrystalline morphology. Using the polycrystalline platelike NaNbO_3 particles as a template in the reactive templated grain growth method (RTGG), {001} grain-oriented (K_(0.5)Na_(0.5))NbO_3-1 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.