Using Discrete Element method (DEM) simulations to reveal the differences in the γ-Al₂O₃ to α-Al₂O₃ mechanically induced phase transformation between a planetary ball mill and an attritor mill

In this work the γ- to α-Al₂O₃ phase transformation induced by mechanical milling was investigated as a function of the bead size for two laboratory scale high-energy mills: a planetary ball mill and an attritor mill. The alumina was texturally characterised, before and after milling, by Scanning Electron Microscopy (SEM) analysis, porosimetry by N₂ physisorption with Brunauer-Emmett-Teller (BET) specific surface area analysis, and structurally characterised by X-Ray Diffraction (XRD), Differential Scanning Calorimetry (TGA-DSC) and Nuclear Magnetic Resonance (²⁷Al-MAS NMR). SEM micrographs highlighted that the planetary ball mill had a better particle size reduction, while the BET surface area results indicated a greater surface area loss compared to the attritor mill. XRD patterns and ²⁷Al MAS NMR spectra showed that depending on the intensity of the milling conditions the phase transformation was mechanically activated and most of the γ-Al₂O₃ was converted into α-Al₂O₃. The attritor mill did not supply the minimum stress energy to induce alumina crystal change; this was in contrast to the planetary ball mill which showed progressively more α-Al₂O₃ formation as bead size increased. For the planetary ball mill samples which did not exhibit phase transformation an exceptional fraction (15%) of pentacoordinate Al ions (AlO₅) was observed, suggesting incipient transformation of the initial γ- Al₂O₃ structure. Consistent with the other characterisation techniques, differential scanning calorimetry (DSC) results showed that the reduction of the transformation temperature was directly proportional to the intensity of the milling conditions. The Discrete Element Method (DEM) simulations were used to model the bead motion inside the two mills and to estimate the specific energy and the stress energy for the considered experimental conditions. The simulation results coupled with the XRD patterns clearly suggested that the formation of α-Al₂O₃ nuclei was controlled by the stress energy. This finding could potentially be utilised as the basis scaling parameter to determine the conditions to reproduce the phase transformation in a different mill or to a different scale.