The origin of bimodal size distribution of alumina ultrafine particles produced by radio frequency plasma

Alumina ultrafine particles (UFPs) produced by radio-frequency plasma are found to have a bimodal size distribution. Analyses of the UFPs by means of X-ray diffraction and transmission electron microscopy show that the smaller particles around 100 nm in diameter (S particles) are mostly single crystals of γ, δ or θ phase and the larger particles around 800 nm (L particles) are polycrystals of α phase. The single-crystal nature of the S particles is confirmed also by rough coincidence between the particle size obtained from the size distribution curve and the grain size estimated from the width of X-ray diffraction peaks. The L particles have nearly the same grain size as that of raw powder (α phase) fed into the plasma. Their volume fraction in the UFPs determined from the size distribution curve agrees with that of α phase estimated by the external standard method. It is concluded from these experimental results that the S particles are formed by evaporation and condensation of surface shells of the raw powder and the L particles are the remaining unmelted cores.