Studies on the synthesis of nanocrystalline Y₂O₃ and ThO₂ through volume combustion and their sintering

Volume combustion was observed in the auto-ignition of the citrate gels containing the nitrates of yttrium/thorium for the first time in mixture with a fuel (citric acid) to oxidant (Y³⁺ or Th⁴⁺ nitrate) ratio close to that demanded by the stoichiometry. These nanocrystalline powders were characterized for their bulk density, specific surface area, particle size distribution, carbon residue and X-ray crystallite size and were sintered by both the conventional and the two-step method. The maximum relative sintered density of Y₂O₃ was 98.9% TD. The sintered density of thoria (97.8% TD) is the highest among the values reported so far, for nanocrystalline ThO₂. Characterization of the pellets and powders by using scanning electron microscopy and transmission electron microscopy reaffirmed nanocrystallinity and that the sintered pellets comprised faceted sintered grains. The two-step sintering was found to restrict "runaway" sintering. - Highlights: • Scaled-up synthesis of nanocrystalline Y₂O₃ and ThO₂ using citrate gel-combustion method. • VCR was observed at a fuel to nitrate ratio (R) of 0.125 and 0.17 in mixtures containing Th(NO₃)₄ and Y(NO₃)₃ respectively. • The calcined powders were compacted and sintered by using a novel two-step sintering method. • Sintered densities as high as 97.8% T.D. (ThO₂, T_H = 0.48) and 98.9% T.D. (Y₂O₃, T_H = 0.61) were obtained.