Sintering and densification of nanocrystalline ceramic oxide powders: a review

Observation of the unconventional properties and material behaviour expected in the nanometre grain size range necessitates the fabrication of fully dense bulk nanostructured ceramics. This is achieved by the application of ceramic nanoparticles and suitable densification conditions, both for the green and sintered compacts. Various sintering and densification strategies were adopted, including pressureless sintering, hot pressing, hot isostatic pressing, microwave sintering, sinter forging, and spark plasma sintering. The theoretical aspects and characteristics of these processing techniques, in conjunction with densification mechanisms in the nanocrystalline oxides, were discussed. Spherical nanoparticles with narrow size distribution are crucial to obtain homogeneous density and low pore-to-particle-size ratio in the green compacts, and to preserve the nanograin size at full densification. High applied pressure is beneficial via the densification mechanisms of nanoparticle rearrangement and sliding, plastic deformation, and pore shrinkage. Low temperature mass transport by surface diffusion during the spark plasma sintering of nanoparticles can lead to rapid densification kinetics with negligible grain growth

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y2O3 powders with 18nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600°C. Dense specimens were fabricated at 100MPa and 1400°C for 5min duration. A maximum in density was observed at 1400°C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400°C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150kJmol−1 and associated diffusion coefficients higher by 103 than expected for Y3+ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature