Abstract: With nanometer (75mol%Bi₂O₃+25mol%Y₂O₃) powder prepared by coprecipitation as raw material, the nanocrystalline Bi₂O₃-Y₂O₃ fast ionic conductor was fabricated through
pressureless reactive sintering technique. The study results on formation law of highly conductive phase (nano δ -Bi₂O₃) in the sintering process show that the solid solution reaction
happens at the early stage of sintering process, and the grain growth accords with the rule of para-curve equation (D-D₀)²=K·t. The optimizing domain of sintering technique parameters forming
δ -Bi₂O₃ was optimized by the pattern recognition technique, that is Y>-1.846X+3.433 (X=0.0059T+0.0101t, Y=-0.0059T+0.0101t, where T is sintering temperature, t is sintering time).
At the pressureless reactive sintering conditions of T=600℃, t=2h, the nanocrystalline Bi₂O₃-Y₂O₃ fast ionic conductor can reach to a relative density of higher than 96%,
and its microstructure is compact and homogeneous with few remaining pores or cracks, its average grain size is less than 100nm.

Key words: coprecipitation method, pressureless reactive sintering, nanocrystalline Bi₂O₃-Y₂O₃ fast ionic conductor, technique optimization