Microwave Plasma Sintered Nanocrystalline Bi₂O₃−HfO₂−Y₂O₃ Composite Solid Electrolyte

Processing and characterization of nanocrystalline Bi 2 O 3 −HfO 2 - Y 2 O 3 composite solid electrolyte having high density and conductivity has been investigated and reported in this paper. Nanopowders of mixed bismuth oxide, hafnia, and yttrium oxide in Bi 2 O 3:HfO 2:Y 2 O 3 = 3:2:1 molar ratio have been prepared by a reverse titration chemical coprecipitation from Bi 3+, Hf 4+, and Y 3+ containing aqueous solution. The high density, nanocrystalline Bi 2 O 3 - HfO 2 −Y 2 O 3 solid electrolyte has been synthesized by microwave plasma and pressureless sintering. From the XRD results, the growth behavior indicates that growth of both δ-Bi 2 O 3 and c-HfO 2 crystallites obeys the parabolic rate law, expressed as ( D − D 0 ) 2 = kt, during the sintering process. After the samples were sintered in microwave plasma at 700 °C for 30 min, the relative density was found to be greater than 96%. Moreover, the sintered specimens exhibit considerably finer microstructure and greater densification compared to that of samples sintered by conventional pressureless sintering. Results of AC impedance spectroscopy of the solid electrolyte indicates that the conductivity of nanocrystalline Bi 2 O 3 −HfO 2 −Y 2 O 3 electrolyte was more than 10 - 6 S cm -1 at 350 °C and 10 - 2 S cm -1 at 550 °C, which is significantly higher than that of microcrystalline HfO 2 -based solid electrolyte.