whereas According to Eq. (21), the overall diffusivity is never greater than diffusivity in the solid phase and approaches zero as the porosity approaches the percolation limit, E = */3, above which, in a random composite, the solid phase would no longer be ~o n n e c t e d .~ This behavior is expected for the example considered. However, Eq. (22) yields the physically unacceptable prediction that the overall diffusion coefficient is =Dz ( > > > D , ) for porosities above lh, References ' R. Landauer, "Electrical ResistanceofBinaryMetallicMixtures,"J. Appl. Phys.. M. H. Cohen and J. Jortner, "Effective Medium Theory for the Hall Effect in a J. Jortner and M. H. Cohen, "Metal-Nonmetal Transition in Metal-Ammonia 23 [7] 779-84 (1952). Disordered Materials," Phys. Rev. Len., 30 [15] 6%-98 (1973). Solutions," J . Chem. Phys., 58 Ill] 5170-71 (1973). S. Kirkpatrick, Percolationand Conduction," Rev. Mod. Phys.. 45 [4]574-88 (1 071) \ . , , ' I . N. R;,Kestner and J. Jortner, "Conjecture on Electron Mobility in Liquid Hydro-H. T. Davis, L. R. Valencourt, and C. E. Johnson, "Transport Processes in 'See for example Samuel Glasstone, Textbook of Physical Chemistry, 2d ed.; pp. Per Kofstad, Nonstoichiomeay, Diffusion, and Electrical Conductivity in Binary carbons, J .The system HfO2-EuzO3 was investigated at =5 mol% intervals over a temperature range of %O" to 19WC, by X-ray analysisof quenched specimens. Liquidus temperature measurements made over the entire system showed the lowest liquidus temperfluorite solid solution region was also found to exist at higher
