Structural features of fumed silica and alumina alone, blend powders and fumed binary systems

Fumed silica, initial alumina blend, after mechanochemical activation (MCA), and fumed binary silica/alumina (SA) were studied using infrared spectroscopy, X-ray diffraction (XRD), ultrasoft X-ray emission (USXE) spectroscopy (giving emission bands Si Lα, Al Lα and O Kα related to valence electron transfer onto core levels) and quantum chemistry. The MCA influence on nanoparticle characteristics (sizes, electronic structure) increases with increasing alumina content in the blends due to stronger abrasive effect of alumina nanocrystallites (snagging a surface layer of nanoparticles during MCA) than non-crystalline silica nanoparticles. A difference in Si Lα, Al Lα and O Kα affected by MCA increases with increasing alumina content. It is greater for the top peak of the upper valence band (UVB) than for a lower energy peak in the bottom of the UVB. These spectral changes suggest redistribution of electron density between Si, Al and O atoms depending on alumina content, material type and treatment conditions. The main difference in the properties of the SA blends and binary SA is due to distribution of Si atoms in alumina phase and Al atoms in silica phase in fumed SA (which is amorphous at CAl2O3 ≤ 30 wt.%) in contrast to the SA blends with practically separated silica and alumina nanoparticles.