liquid-phase sintering

Solid-state sintering is slow because everything has to crawl through the solid by diffusion. Liquid-phase sintering cheats: a small amount of a second material is chosen so that, at the firing temperature, it melts into a thin liquid that wets the ceramic grains — and once there is a liquid threading between the particles, densification becomes far faster and easier. It is like the difference between packing damp sand, which slumps and compacts readily, and packing bone-dry sand, which resists. Most traditional ceramics and a great many advanced ones densify this way, because it lets you reach full density at a lower temperature than the pure solid ever could.

The liquid does its work in three overlapping stages. First, rearrangement: the instant the liquid forms it is pulled into the narrow gaps between particles by capillary force, and that capillary pull yanks the particles into a tighter packing, giving a fast burst of shrinkage almost immediately. Second, solution-reprecipitation: where grains press hard against each other through the liquid, the extra pressure raises the solubility of the solid there, so material dissolves at the loaded contacts, diffuses through the liquid, and reprecipitates on unstressed surfaces — the grains flatten their contacts and pack even more densely, and their centres approach. Third, a final solid-state-like stage where the remaining pores close and grains coarsen. Two conditions must be met for any of this to work: the liquid must wet the solid (a small contact and dihedral angle, so it spreads into the boundaries rather than balling up), and the solid must be at least slightly soluble in the liquid.

Liquid-phase sintering is everywhere: the feldspar in porcelain melts to a glass that pulls the body dense and gives it translucency; silicon nitride and SiC, being stubbornly covalent and nearly impossible to sinter in the pure solid state, are densified with oxide additives (like Y2O3 plus Al2O3) that form a liquid; cemented tungsten carbide uses molten cobalt; ZnO varistors and many capacitor dielectrics rely on a fluxing liquid. The catch is what the liquid leaves behind. On cooling it usually freezes into a glassy grain-boundary phase — a thin amorphous film wetting every grain — and that film is the Achilles' heel of the ceramic at high temperature: it softens and lets the grains slide, driving creep, and it raises dielectric loss. Much modern work goes into crystallizing that boundary phase, or choosing additives that leave a more refractory film, to recover high-temperature performance.