Initial stage sintering of ultrafine alumina particles

Traditional initial stage sintering models assume a spherical particle shape and isotropic surface energy and diffusion coefficient. While they may be appropriate under special conditions with relatively large particles, these models break down in the sub-micrometer particle size ranges typically encountered in sinterable ceramic powders.; This study extended sintering research into the ultrafine particle (UFP) size range. We produced small faceted alumina particles, 20-50 nm in diameter, by an arc-discharge method, and sintered small clusters of them as they travelled through a flowing gas furnace. Unlike other similar work, the particles stayed in an ultrahigh vacuum (UHV) environment throughout the whole experiment. The UFPs were produced and sintered in a custom designed UHV furnace; a UHV transfer assembly ensured specimen cleanliness during transfer to a UHV high resolution transmission electron microscope, allowing the specimens to be kept clean during examination.; The alumina UFP size increased with gas pressure and showed an approximately log-normal size distribution. The particles formed by nucleation and growth, rather than coalescence, as has been proposed in metal UFP studies. Most particles displayed a cuboctahedron shape with {dollar}{lcub}{dollar}110{dollar}{rcub}{dollar} facets which were decomposed into small {dollar}{lcub}{dollar}111{dollar}{rcub}{dollar} facets. A minimum surface energy calculation explained the existence of these {dollar}{lcub}{dollar}110{dollar}{rcub}{dollar} facets.; Two computer programs were developed to simulate (1) the shape and arrangement of UFPs in three dimensions, and (2) the probabilities of six types of contact for the random contact of two UFPs. The UFPs probably rotated and slid just before or after contact, which resulted in the commonly observed, but highly unlikely, face-to-face contact.; Since no shrinkage was observed in the specimens, surface diffusion was the predominant matter transport mechanism. Sintering temperature and time were found not to be the most important factors controlling the neck size. Instead, the initial contact conditions played a major role.