Studies on the synthesis and sintering of nanocrystalline yttria

Abstract

Nanocrystalline yttria powders were synthesized from yttrium nitrate by the citrate gel-combustion technique. The auto-ignition of five different gels with fuel to oxidant (citric acid/nitrate) ratios 0.25, 0.5, 0.75, 1.0 and 1.1 was studied. All these mixtures yielded precursors which upon calcination in air at 1073 K yielded yttria. The bulk densities, specific surface area, X-ray crystallite size, size distribution of particles as well as the residual carbon present in these powders were determined. The influence of the fuel to oxidant ratio on the powder properties was analyzed. Scanning electron microscopy (SEM) showed that these powders were porous while the high resolution transmission electron microscopy (HRTEM) revealed that they consist of randomly oriented cuboidal nanocrystallites with an average crystallite size of 25±7 nm. These powders were compacted at 120 MPa without any lubricant or binder and their sinterability was studied. Pellets with a sintered density as high as 98–99% T.D. (theoretical density) could be obtained at a relatively low sintering temperature of 1673 K. Studies on the dependence of the properties of nanocrystalline yttria powders on the composition of the initial mixture used in the citrate gel-combustion as well as the sintering characteristics of these powders are being reported for the first time. Our investigations revealed that an initial mixture comprising equimolar quantities of the nitrate and citric acid yielded a powder with characteristics most suitable for fabricating yttria crucibles.