Fully-dense bulk nanocrystalline ceramics may exhibit interesting physical properties that lack in their counterparts with conventional grain size. However, full densification of nanocrystalline ceramics is still a challenge due to the long term thermal processes. Spark plasma sintering (SPS) was used for superfast densification of ceramic nanopowders. Application of high-density dc current pulses and load lead to rapid densification of ceramic nanopowders compacts. However, significant gap exists between the technological and fabrication achievements to the fundamental understanding of the SPS mechanisms due to the complexity of the thermal, electrical and mechanical processes that may be involved. Several different mechanisms such as vaporization-solidification, plastic deformation, surface-, grain boundary-, and volume-diffusions were assumed for the sintering and densification during the SPS.

Nanopowders of MgO, YAG, Nd-YAG, and Y₂O₃ were densified using SPS at different temperature ranges, pressures and durations. Nanocrystalline MgO and YAG powders were densified to optical transparency at distinctly different homologous temperatures (0.3Tm for nc-MgO and 0.7Tm for nc-YAG). Dense Nd-YAG and Y₂O₃ specimens were translucent with micrometer and nanometer grain size, respectively. Analysis of the density and grain size evolution versus the SPS parameters (i.e. pressure, temperature, duration) showed that densification of these nanopowders proceed either by plastic deformation, grain-rotation coalescence and sliding, aided by softening of the particle surfaces or by accelerated surface diffusion. The active densification mechanism depends on the changes both in the mechanical and electrical properties of the ceramic with temperature.

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