Effect of green density and electric field direction on densification of YAG nano-powders by spark plasma sintering

Nanocrystalline YAG powders with 34-nm average particle size were cold isostatic pressed (CIP) between 10 and 240 MPa to form compacts with 43-52% green densities. Spark plasma sintering (SPS) was performed for 5 min at 1,400 A degrees C and 90 MPa using clusters of three disc specimens packed in series with the electric field direction. The pore size and its volume fraction decreased with the increase in the CIP pressure. The final density varied between 86% and 99% and strongly depended on the specimen location in the series. Density, grain size, and pore size analyses showed that the specimens facing the electric current experienced higher SPS temperatures. Possible temperature gradient due to axial displacement of non-conducting oxide was too low to explain the observed effect. Effect of the green density on the final density was marginal.