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 , 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 . 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:
According to Ikuma et al. , 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  and Keneshea and Douglass )
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 .