Thermally induced crystallization of mechanically alloyed Na_0.5Bi_0.5TiO₃ and K_0.5Bi_0.5TiO₃ piezoelectric ceramic nanopowders

Thermally induced crystallization of mechanically alloyed K_0.5Bi_0.5TiO₃ and Na_0.5Bi_0.5TiO₃ piezoelectric ceramic nanopowders was investigated. The raw materials with a distinct molar ratio were initially mixed and milled in a high-energy ball mill under air atmosphere at room temperature for 1 h. After the mechanical activation, the resultant powders were annealed between 500 and 700 °C for 1 h to crystallize the mechanosynthesized powders. The results show that the mode of the reaction was progressive and consists of two stages: (i) the decomposition of some reagents to oxides and (ii) the reaction of Bi₂O₃ and TiO₂ with potassium and sodium oxides which led to the formation of fine powders. From the structural point of view, the crystallite size and lattice strain of K_0.5Bi_0.5TiO₃ nanopowder were 30 nm and 0.427%, respectively, while the Na_0.5Bi_0.5TiO₃ powder gave a crystallite size of 37 nm and lattice strain of 0.340%. In accordance with the TEM images, upon heat treatment at 700 °C, coalescence of the grains led to the appearance of relatively large grains, with cubic (55±10 nm) and spheroidal (75±15 nm) morphologies, for the K_0.5Bi_0.5TiO₃ and Na_0.5Bi_0.5TiO₃ powders, respectively.