MECHANICALLY ACTIVATED MOO3 .1. PARTICLE-SIZE, CRYSTALLINITY, AND MORPHOLOGY

Physicochemical changes induced in MoO3 by mechanical activation in a planetary mill were investigated. During the milling process, the BET surface area increases from about 1.3 to 32 m(2)/g. Energy dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS) of the powdered MoO3 samples reveal that no agate is abrased during the mill ing process. An estimation of the mean particle size using the BET dat a or scanning electron microscopic (SEM) images indicates a decrease f rom about 1 mu m to about 50 nm. The primary crystallite size calculat ed from X-ray diffraction (XRD) line broadening also shows a decreasin g size from about 160 nm to about 80 nm. The difference between the pa rticle size of unmilled MoO3, as determined from the BET surface area or by scanning electron microscopy (SEM), and the calculated primary c rystallite size using X-ray line broadening is explained by MoO3 parti cles consisting of smaller primary crystallites. The smaller average p article size of MoO3 milled for 600 min calculated from BET data, on t he other hand, as compared to the XRD primary crystallite size is ascr ibed to the presence of ultrafine amorphous material which is X-ray am orphous and, therefore, does not contribute to the X-ray line broadeni ng. This formation of amorphous material is also indicated by an incre asing amorphous scattering background in the X-ray diffraction pattern s. In SEM pictures, these particles appear to have an amorphous overla yer. The strange behavior of both the X-ray diffraction pattern qualit y and the diffuse scattering background, which do not coincide, during mechanical activation probably indicates a complex process of particl e size reduction. Variation of X-ray reflection profiles, intensity ra tios, and additional X-ray reflections may point toward the formation of shear defects during this process. A Warren-Averbach analysis of th e most intense X-ray reflections of milled MoO3 reveals that internal strain is only marginally enhanced by mechanical activation.