Grain growth and densification process play the two most crucial roles on the microstructure evolution and the achieved performances during sintering of ceramics. In this thesis, the grain growth of SrTiO₃, BaTiO₃-SrTiO₃ solid solutions and Si₃N₄ ceramics during spark plasma sintering (SPS) were investigated by electron microscopy.

SrTiO₃ ceramics starting from nanopowders were fabricated by SPS. A novel grain growth mechanism was discovered and named as ordered coalescence (OC) of nanocrystals. This mechanism involved nanocrystals as building blocks and is distinguished from atomic layer epitaxial growth (AEG) in classical sintering theory. The results also revealed that the dominant grain growth mechanism can be changed by varying heating rates. Low rate (10°C/min) gives AEG, whereas high rates (≥ 50°C/min) yields three-dimensional coalescence of nanocrystals, i.e. OC.

BaTiO₃-SrTiO₃ sintered bodies were made by SPS of BaTiO₃ and SrTiO₃ nanopowders mixtures. A novel Sr_1-xBa_xTiO₃ “solid solution” with mosaic-like single crystal structure was manufactured by OC of the precursor crystallites. This reveals a new path for preparation of solid solution grains or composites.  

Si₃N₄ ceramics were prepared from α- or β-Si₃N₄ nanopowders at the same SPS conditions. The anisotropic OC of precipitated β-Si₃N₄ crystallites gives elongated β-Si₃N₄ grains at 1650°C using α-Si₃N₄ nanopowder. In contrast, AEG leads to the equi-axed β-Si₃N₄ grains using β-Si₃N₄ nanopowder. The metastable α- to β-Si₃N₄ phase transformation and OC accelerates anisotropic grain growth.

Grain motions contribute to the densification process during pressureless sintered 3Y-ZrO₂ (>87%TD) or SPS of SrTiO₃ (>92%TD) ceramics. This extends the sintering range for active grain re-arrangement over that predicted by classical theory.

In this thesis a new grain growth mechanism (OC) is proved by using SPS and nanopowders. By OC the microstructural evolution can be manipulated.