Phase transformation of ultrafine rare earth oxide powders synthesized by radio frequency plasma spraying

Inductively coupled radio frequency plasma spraying was used to prepare ultrafine powders of Sm2O3, Dy2O3, and Lu2O3. These three materials were studied because they are effective dopants in multi-layer ceramic capacitors (MLCC) to improve lifetime. The as-sprayed powders consist of both micron-sized mono-dispersed spherical particles and nano-sized particles in various shapes. In addition to the spheroidization effect, plasma treatment leads to an increase of the monoclinic phase fraction associated with a corresponding decrease of the amount of the cubic phase. The degree of this phase change was found to depend sensitively on the sprayed oxides, decreasing with increasing atomic number of the rare earth element, which is associated with their thermodynamic stability. Furthermore, the monoclinic high-temperature equilibrium phase induced by plasma treatment remains stable at room temperature and even during annealing at a temperature of 900 °C.