The self-diffusion of Ca, Y and Zr in monocrystalline yttria-stabilized zirconia and calcia-stabilized zirconia (containing 10 to 32mol%Y2O3 and 11 or 17mol%CaO, respectively) was measured between 960 and 1700C. The stable isotopes, 44Ca and 96Zr, were used as tracers and the samples were analyzed using secondary ion mass spectrometry. In the case of Y diffusion, the radioactive tracer, 88Y, was used with an abrasive sectioning technique. The results could be described by the relationships:

Zr4+ (Y2O3-stabilized) 1070-1700C:     D (cm2/s) = 4.1 x 10-2 exp[-4.5(eV)/kT]

Zr4+ (CaO-stabilized) 1120-1700C:     D (cm2/s) = 2.5 x 101 exp[-5.3(eV)/kT]

Sc3+ (Y2O3-stabilized) 1270-1700C:     D (cm2/s) = 1.7 x 100 exp[-4.9(eV)/kT]

Y3+ (Y2O3-stabilized) 1400-1676C:     D (cm2/s) = 2.4 x 10-2 exp[-4.2(eV)/kT]

Ca2+ (CaO-stabilized) 960-1700C:     D (cm2/s) = 4.5 x 101 exp[-5.1(eV)/kT]

It was therefore found that the Zr bulk diffusion was slower than Y or Ca bulk diffusion, and that there was a nearly linear correlation of diffusion coefficient with cation radius (figure 40). In yttria-stabilized zirconia, the Zr and Y bulk diffusivities were greatest for a stabilizer content of 10 to 11mol%. In calcia-stabilized zirconia, the Ca and Zr tracer diffusivities were both independent of the Ca content. The activation enthalpy for Y stabilizer bulk diffusion (4.2eV) was (as in calcia-stabilized zirconia) slightly lower than that for Zr bulk diffusion (4.5eV). The Y dislocation-pipe diffusivity was 5 to 6 orders of magnitude faster than the bulk diffusivity (figure 41), and its activation enthalpy (3.5eV) was also smaller than that for bulk diffusion. From the activation enthalpy, and from the concentration dependence of the cation bulk diffusion, it was concluded that cation diffusion occurred either via free vacancies (VZr4+ in yttria-stabilized zirconia) or via bound vacancies ([VZr4+–2VO2*]x in calcia-stabilized zirconia).

Cation Self-Diffusion of 44Ca, 88Y and 96Zr in Single-Crystalline Calcia- and Yttria-Doped Zirconia. M.Kilo, M.A.Taylor, C.Argirusis, G.Borchardt, B.Lesage, S.Weber, S.Scherrer, H.Scherrer, M.Schroeder, M.Martin: Journal of Applied Physics, 2003, 94[12], 7547-52

Figure 41

Volume and Pipe Diffusion of 88Y in ZrO2