Synthesis of polycrystalline platelike KNbO3 particles by the topochemical micro-crystal conversion method and fabrication of grain-oriented (K0.5Na0.5)NbO3 ceramics

Polycrystalline rectangular-platelike KNbO3 particles with an orthorhombic perovskite structure were synthesized by the topochemical micro-crystal conversion (TMC) method from rectangular-platelike precursor particles of layer-structured K4Nb6O17 at 850 °C in molten KCl-salt. TMC-synthesized KNbO3 particles preserved the shape of precursor particles, and had a thickness of about 1 μm, a width of 5–10 μm and a length of 20–40 μm. However, TMC-synthesized rectangular-platelike KNbO3 particles exhibited a polycrystalline morphology having preferred pseudo-cubic {0 0 1} orientation. Oriented particulate layer (OPL) X-ray diffraction analysis revealed that, in the TMC reaction, the crystallographic {0 1 0} plane of K4Nb6O17, which is the largest extensive face, is converted into the {0 0 1} plane of polycrystalline KNbO3 particles, which is the largest developed face, in spite of polycrystalline morphology. Using the polycrystalline rectangular-platelike KNbO3 particles as template in the reactive templated grain growth method, grain-oriented (K0.5Na0.5)NbO3–1 mol% CuO ceramics having a pseudo-cubic {0 0 1} orientation degree (Logeringʹs factor) of 39.7% 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 templated grain growth process. The availability of polycrystalline particles will give a new synthesis design of templates for texturing of various kinds of ceramics.