Where does the energy go in high energy milling?

Abstract

An attempt is made to analyze as to how the energy is dissipated, stored and distributed in the material during the process of high energy milling. The manifestation of the enhanced potential energy in different forms (point, line and volume defects, surfaces and interfaces, strain and structural disorder) is determined through direct energy measurements, calorimetry, surface area and surface energy measurements. Xray line broadening analysis employing the Hall-Williamson method is used to estimate the non-uniform elastic strain and grain size and the extent of structural disorder is evaluated from integral peak areas of XRD. A close packing of crystallites approximating a tetrakai-decahedron configuration is used to calculate the grain boundary area. The strain energy is calculated using the theory of elasticity. The energy of amorphisation is calculated from the enthalpy of fusion and specific heats of solid and liquid. Mechanical activation of zircon is chosen for the study of the energetics of the process. For the milling of zircon, the energy transferred to the material is found to be 13% of the specific energy input in 6 h of milling in a planetary mill. It is observed that a large part of the energy transferred to the material is lost during the breaking of the bonds and only a small fraction goes towards enhancing the potential energy mainly as elastic strain energy and structural disorder. The energy stored in point and line defects, additional surfaces and grain boundaries are comparatively lesser.