Thermodynamics of Pore-Boundary Interactions

Whether a pore shrinks or not is determined by the free energy change that accompanies the change in pore size. If the pore shrinks, there will be a decrease in the free energy due to the decrease in the pore surface area, while there is also an increase due to the increase in the grain-boundary area. If the pore shrinks, the former (decrease) must be larger than the latter (increase). The equilibrium shape of the pore is determined by the dihedral angle if/, which is defined by 

In two dimensions, a pore can have a dihedral angle y/ = 120°, which is surrounded by N other grains. The number N is called the pore coordination number (CN). Similar to the case of a grain surrounded by other grains, if IV = 6, the pore has straight sides otherwise, it has convex sides for A 6 and concave sides for IV 6, as shown in Fig. 8.28. The surface of the pore will move toward its center of curvature, so the pore with 6 will shrink, whereas the one with N 6 will expand. The pore is metastable for N = 6, so that the number is called critical pore CN, or Nc-Accordingly, if the pores have convex sides, i.e., N 6, the decrease in the pore 

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