Particle Size Effect on the Crystal Structure of Y2O3 Particles Formed in a Flame Aerosol Process

Y₂O₃ is a material with wide applications. Its cubic and monoclinic polymorphs have significantly different mechanical and optical properties. The flame aerosol technique offers unique advantages in the synthesis of Y₂O₃ particles, but it also faces the challenge of phase control for Y₂O₃. One possible strategy for Y₂O₃ phase control is to utilize the particle size effect that causes the formation of the monoclinic phase at atmospheric pressure. In this study, the particle size effect on crystal structure was experimentally investigated for Y₂O₃ particles synthesized in a high-temperature flame aerosol process. A critical particle diameter of approximately 1.5 μm was found by single-particle electron diffraction. At the critical diameter, the probability is 50% for a particle to be either cubic or monoclinic. Particles significantly smaller than the critical diameter were all monoclinic, while those significantly larger were all cubic. The critical diameter was interpreted using an analysis that involves the free energy of a particle, which includes the bulk Gibbs free energy and the surface energy.