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How does solid-state sintering reduce porosity in advanced ceramics?

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advanced structural ceramics Figure 1: Resistance to cracking in transformation-toughened zirconia. In a ceramic composed of tetragonal zirconia dispersed in a zirconia matrix, the stress field advancing ahead of a propagating crack transforms the small tetragonal particles to larger monoclinic particles. The larger particles exert a crack-closing force in the process zone behind the crack tip, effectively resisting propagation of the crack.

In advanced ceramics, solid-state sintering reduces porosity through the diffusion of matter rather than the formation of a liquid phase. Under the influence of heat and pressure, material from adjacent particles migrates to "neck" regions that form between them, effectively bonding the particles together.1

As the boundaries between grains grow, the material becomes increasingly dense, and porosity progressively decreases until the pores close off and are no longer interconnected. Pores located at these grain boundaries are typically eliminated through grain boundary diffusion.1

Because pores trapped inside growing grains are difficult to remove, chemical additives known as sintering aids are often employed to enhance the process. For example, in the production of alumina lamp envelopes, magnesia is added to slow the migration of grain boundaries. This ensures that pores remain on the boundaries, where they can be successfully eliminated by diffusion, allowing for the achievement of extremely low porosities.1