In situ TEM observations of growth mechanisms of PbO nanoparticles from a Sm-doped PMN-PT matrix

An excess PbO is usually added to raw materials to compensate for PbO volatilization during high-temperature sintering of a (1 - x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-PT) piezoelectric material. However, the detailed growth mechanism of liquid phase and solid phase PbO due to excess PbO during the sintering process is still unknown. Here, the evolution behavior and growth mechanism of PbO nanoparticles from a Sm-doped 0.70PMN-0.30PT (Sm-PMN-PT) matrix were in situ observed using transmission electron microscopy with the help of electron beam irradiation. It was found that PbO nanodroplets firstly separated from the Sm-PMN-PT matrix, leading to rapid growth of newly formed PbO nanodroplets. Then, these nanodroplets coalesced into solid phase PbO nanoparticles with their size increased. After that, small solid phase nanoparticles further grew into large PbO nanoparticles by either rapidly engulfing adjacent nanodroplets and nanoparticles or slowly merging by matching these same crystal planes of adjacent nanoparticles. Finally, a heterojunction was formed between the formed large PbO nanoparticles and Sm-PMN-PT matrix. Our investigations demonstrate that the excess PbO could provide a liquid environment at the interface of Sm-PMN-PT, and the PbO nanoparticles formed act as the secondary phase at the grain boundaries of the Sm-PMN-PT matrix. This work provides a deep understanding of the role of excess PbO in the synthesis of lead-based piezoelectric materials.