Diffusion Coefficients

Diffusion Coefficients Подпись: :9.73 ± 1.4 x 10-3 exp Подпись: 56.0 ± 2.4 kcal/mol 1 RT J (5)

Diffusion in zirconia is closely linked to ionic conductivity. Consequently, some diffusion data has already been presented in Sect. 5. This section will include additional results par­ticularly for monoclinic zirconia. Oxygen self-diffusion at a pressure of 300 Torr, as determined by testing zirconia spheres of diameters between 75 and 105 (dm, behaves as shown in Fig. 17 [57], where D is the diffusion coefficient, t is time, and a is the sphere radius. At a pressure of 700 Torr, the behavior changes to that shown in Fig. 18 [58]. In this case D* is the self-diffusion coefficient and the rest of the terms are as defined before, with a = 100-150 (dm. Both of these experiments were per­formed in an oxygen atmosphere of 18O-16O. The self-diffusion coefficients calculated from the diffusion data obey Arrhenius expressions as illustrated in Fig. 19 [57, 58]. The linear fits describing the diffusion coefficient at 300 and 700 Torr, are given by:

Diffusion Coefficients

According to Ikuma et al. [59], surface diffusion and lattice diffusion should be sepa­rated and result in the following diffusion coefficients:

Fig. 19 Arrhenius plot of oxygen self-diffusion in mono­clinic zirconia (adapted from Madeyski and Smeltzer [57] and Keneshea and Douglass [58])

Diffusion CoefficientsПодпись:Подпись: 87.0 kJ/mol RT (8)

These two expressions are not that very different. Hence, the macroscopic diffusion behavior of monoclinic zirconia can be approximated by lattice diffusion, while sur­face diffusion can be ignored.

Diffusion in pure tetragonal and cubic zirconia is experimentally challenging because it requires the higher temperatures at which the two phases are stable. However, simulations at temperatures between 1,273 and 2,673 K have been performed on cubic zirconia, showing noticeable, but not large, oxygen ion diffusion along the grain boundaries and a significant energy barrier to movement from the grain bounda­ries into the bulk, although at higher temperatures diffusion is obviously enhanced. However, even at higher temperatures, diffusion along the grain boundary is not as favorable as that across the grain boundary [60].