Vacuum balance studies of milled material and mechanochemical reactions

Abstract

The types of change in surface properties and porosity of milled materials having widely different hardness and crystal structure are reviewed. For more intensive milling, the use of vacuum balance techniques in conjunction with X-ray diffraction, optical- and electron-microscopy enables changes in microstructure and phase composition to be determined during mechanochemical reactions. Metallic or non-metallic harder materials (Mohs scale 8–9), such as transition metal nitrides or silicon nitride and boron carbide, increase their surface on milling largely due to brittle fracture, so that the surface area tends towards an upper limiting value with comparatively little development of porosity. Softer materials (Mohs scale 1–2), such as gypsum, china clay(Kaolinite) and hydrated lime and magnesia undergo plastic deformation and strain hardening on longer milling, so that the surface area passes through a maximum before decreasing to an equilibrium value. This is applicable also to materials of intermediate hardness (Mohs scale 3–5), such as calcite, magnesite and dolomite, provided that the milling is sufficiently intensive; the flow and welding processes during the plastic deformation leave the grains non-porous to nitrogen gas, but adsorption of water vapour causes development of porosity, a phenomenon also observed with lunar fines (mainly silicate minerals) which are of somewhat greater hardness but contain amorphous surface layers (cf. Beilby layer) and nuclear particle damage tracks having tubular pores with narrow constrictions forming micropores. More intensive milling can result in crystal transformation and mechanochemical reactions. Thus gypsum is converted to anhydrite, viz., CaSO 4.2H 2O → CaSO 4.2H 2O → γ—CaSO 4 → β—CaSO 4. Calcite, CaCO 3, is converted to aragonite, while calcitic CaCO 3 and magnesite, MgCO 3, form dolomite CaCO 3.MgCO 3.