INFLUENCE OF THE NATURE OF MILLING MEDIA ON PHASE-TRANSFORMATIONS INDUCED BY GRINDING IN SOME OXIDES

Polymorphic transformations induced by dry ball milling in an argon atmosphere have been investigated in various oxides (TiO2, SnO2, Y2O3, WO3) by X-ray diffraction, Mossbauer spectroscopy and transmission electron microscopy (TEM). The transformations have been found to depend on the nature of the milling media, particularly when reduction reactions take place between grinding tools and oxide particles. When ground with steel tools, cubic yttria is transformed into a monoclinic modification as reported in the literature, while tin oxide is reduced. When ground with zirconia tools, cubic yttria with a bixbyite type structure is transformed into cubic yttria with a fluorite type structure, while monoclinic tungsten oxide is transformed into a cubic oxide with an ReO3 type structure. Plausible structural explanations are proposed. In all cases, nanometre-sized domains have been observed by TEM even after grinding times as short as some minutes.