Studies on the synthesis of nanocrystalline yttria powder by oxalate deagglomeration and its sintering behaviour

Nanocrystalline yttria powders were synthesized from deagglomerated yttrium oxalate. The precipitation of this oxalate was carried in two different modes viz., addition of aqueous oxalic acid into yttrium nitrate solution (forward strike) and vice versa (reverse strike) followed by ultrasonication in acetone and water. Nanocrystalline yttria was obtained by calcining the oxalate in air at 1073 K. The bulk densities, specific surface area, X-ray crystallite size, size distribution of particles as well as the quantity of carbon residue in these powders were determined. The influence of the deagglomeration medium on the powder properties was analyzed. Scanning electron microscopy (SEM) showed that these powders comprised irregular agglomerates while the high resolution transmission electron microscopy (HRTEM) revealed that the constituent units of these agglomerates were randomly oriented cuboidal nanocrystallites (20–40 nm). These powders were compacted at 120 MPa without any lubricant or binder and their sinterability was studied. Pellets with sintered density as high as 97.5% T.D. (theoretical density) could be obtained at a relatively low sintering temperature of 1873 K. Synthesis of nanocrystalline yttria powders by oxalate deagglomeration route as well as the systematic studies of their properties and sinterabilities are being reported for the first time. It was further demonstrated in this study that higher sintered densities could be obtained with less number of process steps and at a much lower compaction pressure. Samples prepared by reverse strike yielded a powder with characteristics most suitable for fabricating high density yttria bodies. 1673 K would be the optimum temperature for sintering the compacts made out of this powder.