Reactivity of Mechanically Activated Solids

The primary effect of milling a solid is to increase its specific surface area. Most reactions between a solid and another phase are proportional to the extent of specific surface area being in contact. This includes outer surfaces as well as the inner surfaces of pore systems and primary crystallites. Intensive milling decreases the size of the primary crystallites which tend to lower the energy by recrystallization. Therefore, a limiting crystallite size can be reached for a given experimental set-up. Most of the crystallite surface is not accessible for fluid reactants, but changes in the pore system and the outer surface allow for an increased reaction rate. 

Other mechanisms which are related to enhanced reaction rates under mechanical treatment include  

Early explanations about the effect of mechanical energy on the reactivity of solids are the hot-spot-model [23] and the magma-plasma-model [8]. The generation of hot-spof may be used to explain the initiation of a self-sustained reaction such as explosion, deflagration, or decomposition. Temperatures of over 1000 K on surfaces of about 1 pm2 for KM to 10-3 s can be created. These temperatures can also be found near the tip of a propagating crack [24]. Typically nonequilibrium thermodynamics are used to describe these phenomena. The magma-plasma-model allows for local nonequilibrium states on the solid surface during impact however, due to the very short time scale of 1(H s of these states only statistical thermodynamics can describe the behavior. 

Long-lived states including defects and dislocations have been analyzed by equilibrium thermodynamics [14]. Mechanically activated solids have an actual free energy content G which is higher than the equilibrium free energy G  

Knowledge of the concentration of defects and molar disturbance enthalpies would permit calculation of the actual free energy of the solid, and also the chemical potential. These can be measured by using either solution calorimetry or differential scanning calorimetry. An example of the excess energy was given as 20-30 kj mol-i in mechanically activated quartz. Different types of reactions demand different defect types. For example, Boldyrev et al. [25] state a classification and provide examples for solid reactions with different mechanisms and necessary solid alterations. Often, reaction rates in solids depend strongly on the mass transport of matter. Lidi-ard [26] and Schmalzried [27] each provide reviews on transport properties in mechanically treated solids. The increased amount of defects allows a faster transport of ions and atoms in the solid structure. 

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