Synthesis of polycrystalline platelike KNbO 3 particles by the topochemical micro-crystal conversion method and fabrication of grain-oriented (K 0.5Na 0.5)NbO 3 ceramics

Abstract

Polycrystalline rectangular-platelike KNbO 3 particles with an orthorhombic perovskite structure were synthesized by the topochemical micro-crystal conversion (TMC) method from rectangular-platelike precursor particles of layer-structured K 4Nb 6O 17 at 850 °C in molten KCl-salt. TMC-synthesized KNbO 3 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 KNbO 3 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 K 4Nb 6O 17, which is the largest extensive face, is converted into the {0 0 1} plane of polycrystalline KNbO 3 particles, which is the largest developed face, in spite of polycrystalline morphology. Using the polycrystalline rectangular-platelike KNbO 3 particles as template in the reactive templated grain growth method, grain-oriented (K 0.5Na 0.5)NbO 3–1 mol% CuO ceramics having a pseudo-cubic {0 0 1} orientation degree (Logering's 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.