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 gamma, delta or theta phase and the larger particles around 800 nm (L particles) are polycrystals of alpha phase. The single-crystal nature of the S particles is confirmed also by rou gh coincidence between the particle size obtained from the size distri bution curve and the grain size estimated from the width of X-ray diff raction peaks. The L particles have nearly the same grain size as that of raw powder (alpha phase) fed into the plasma. Their volume fractio n in the UFPs determined from the size distribution curve agrees with that of alpha phase estimated by the external standard method. It is c oncluded from these experimental results that the S particles are form ed by evaporation and condensation of surface shells of the raw powder and the L particles are the remaining unmelted cores.