Activation Energies in the Diffusional Creep of Polycrystalline Ceramics

A mixed diffusional creep mechanism in a polycrystalline ceramic can cause the apparent activation energy to vary with temperature and grain size. For mechanisms involving parallel transport paths for the same ion (e.g. cation lattice and grain-boundary diffusion), the process with the lowest activation energy will be dominant (i.e. rate-limiting) at low temperatures and the process with the highest activation energy will dominate at high temperatures. However, for mechanisms involving coupled and parallel diffusional steps (e.g. cation lattice and anion grain-boundary diffusion), the process with the lowest activation energy will be dominant at high temperatures whereas the high-activation-energy process will dominate at low temperatures. Examples of these effects are presented for the diffusional creep of polycrystalline MgO and A1₂O₃ doped with Fe. Variations in creep activation energy with grain size and temperature are only significant when the difference in activation energies for the competing processes is significant and the temperature range investigated is large.