Formation mechanism of barium zirconate nanoparticles under supercritical hydrothermal synthesis

In this study, supercritical hydrothermal synthesis of BaZrO₃ and its formation mechanism during the synthesis were studied using a continuous flow reactor. The Mono-phase, nano-sized BaZrO₃ was successfully synthesized at a temperature of 400°C and a pressure of 30 MPa using oxy-zirconium nitrate and excess barium hydroxide as the starting materials. The formation mechanism of BaZrO₃ was studied by examining the time dependence of size and composition using XRD, TEM, and ICP. As a result of the time-resolved experiment, the following formation mechanism was revealed. At the first stage of the reaction (∼0.1 s), a perovskite structure forms though it has many defects of Ba site. The particle size increases to 20 nm range by coalescence at the middle stage (∼1 s) and becomes constant at the last stage (1-10 s). Ba site defects are filled by the uptake of Ba with increasing time until the last stage (∼10 s). The elucidated formation mechanism, i.e., the coalescence of nuclei and uptake of Ba, is significant to develop a new methodology for controlling the size and composition of the BaZrO₃ nanoparticles.