a grain boundary
Where two grains meet, their atomic rows point in different directions, so they cannot line up cleanly. The result is a grain boundary: a thin, disordered seam, usually only a few atoms wide, where the neat crystal pattern of one grain has to negotiate a compromise with the neatly-but-differently-arranged pattern of its neighbour. Picture two sheets of graph paper laid over one another at an angle — along the join, the squares no longer match, and there is a strip of misfit. That strip of misfit is the boundary, and in a polycrystalline ceramic there is a whole three-dimensional foam of them wrapping every grain.
Because the atoms in the boundary are crowded and mismatched, a boundary is a region of higher energy and looser packing than the crystal interior. Three practical things follow. First, atoms and ions move far faster along a boundary than through the bulk lattice — grain-boundary diffusion is a superhighway, which is why boundaries drive sintering and creep. Second, boundaries are where impurities and dopants collect, because a foreign atom that does not fit the perfect lattice is happier in the loose seam; segregated impurities can even melt during firing into a thin glassy film. Third, a boundary scatters electrons, phonons (heat-carrying vibrations) and cracks, so it resists heat and electrical flow and can steer fracture.
The grain boundary is often where a ceramic actually lives or dies in service, even though it is a tiny fraction of the volume. High-temperature creep usually happens by grains sliding on boundaries lubricated by a glassy film. Electrical devices like the zinc-oxide varistor and many capacitors get their whole function from what happens at the boundary, not inside the grain. And a crack in a brittle ceramic frequently runs along boundaries (intergranular fracture) rather than straight through grains (transgranular). Control the boundary chemistry and you control much of the ceramic.
In a boundary-layer capacitor, a titanate ceramic is fired so its grains become semiconducting while a thin insulating layer forms at every grain boundary. The device stores charge across those countless nanometre-thick boundaries as if it were thousands of tiny capacitors in series, giving a huge effective capacitance from a small part.
A grain boundary is a tiny fraction of the volume, yet here it does the whole job.
A grain boundary is a true interface, not a physical crack or gap — there is no void, just misaligned atoms. But a boundary weakened by a soft glassy film or by impurity segregation can behave almost like a pre-crack at high temperature.