grain-boundary diffusion
A polycrystalline ceramic is a mosaic of tiny crystals — grains — pressed together, and where two grains meet the atoms cannot pack in the neat crystal pattern of either. This mismatched, slightly open seam is the grain boundary, and it is a diffusion superhighway. Atoms slip along these disordered interfaces far more easily than through the tight crystal interior, so grain-boundary diffusion is a fast 'short-circuit' path threading through the material.
The reason is structural: the boundary is a thin region (only a nanometre or so wide) of looser, more open atomic packing, so an atom there has more room to move and a lower energy barrier to each jump. That gives grain-boundary diffusion a smaller activation energy than lattice diffusion — its Arrhenius line is shallower. Two consequences follow. First, the boundary D is much larger than the bulk D at any given temperature. Second, because its Q is smaller, its advantage grows as the temperature falls: the steep bulk path shuts down faster on cooling than the gentle boundary path, so boundaries dominate the transport at low and moderate temperatures. The catch is cross-section — boundaries are a vanishingly thin fraction of the volume, so they only dominate the total flux when either the grains are fine (lots of boundary per unit volume) or the temperature is low enough to have crippled the bulk route.
Grain-boundary diffusion is decisive in real ceramics precisely because they are fine-grained. It is often the fastest route for the rate-controlling species, so it can govern how a ceramic densifies during sintering, how a reaction front advances, and how creep proceeds at high temperature. It cuts both ways: the same fast boundaries that help a ceramic sinter can also let impurities segregate and race along the interfaces. A common oversimplification is to quote 'the' diffusion coefficient of a ceramic as if it were a bulk property; in a fine-grained part the boundaries may carry most of the atoms.
Fine-grained alumina densifies far faster than coarse alumina at the same temperature because the abundant grain boundaries give the slow cation a fast short-circuit path; the finer the grains, the more boundary area per unit volume and the more the boundaries dominate transport.
Grain-boundary diffusion: atoms race along the open, mismatched seams between grains — a low-activation-energy shortcut that dominates in fine-grained ceramics and at lower temperatures.
Grain boundaries are fast per unit area but occupy a tiny fraction of the volume. They dominate the total flux only when grains are fine or temperatures are low enough that the bulk path has shut down — in a coarse ceramic at very high temperature, lattice diffusion can still win.