Application Of Flash-sintering Of Proton Conductors For SOFC And Investigation Of The Mechanism

Solid oxide fuel cell （SOFC） is an efficient, clean energy transfer system. However,the resistance of the electrolyte will increase greatly under intermediate or lowtemperature （400-750℃）. Proton conductor as a promising electrolyte of SOFC exhibitstremendous advantages. Additionally, a novel field-assisted sintering technique known asflash-sintering has great potential for sintering ceramics since it was discovered in2010.Compared with1400-1800℃and12-24h in conventional sintering, flash-sintering candensify most electrolytes below1000℃within no more than1hour. In this work, weused flash-sintering to densify BaZr_0.1Ce_0.7Y_0.1Yb_0.1O₃-δ（BZCYYb）, a highly conductiveand stable proton conductor. During this process, we have studied the factors and themechanism. Moreover, we also selected rare earth doped ceria （REDC） as researchmaterials to systematically investigate the flash-sintering behavior, and then proposed amechanism.First, we designed the flash-sintering device and the arrangement of electrodes. Then,BZCYYb powder was prepared via the sol-gel method. The phase structure was examinedby X-ray Diffraction （XRD）. BZCYYb was densified at850℃for the first time. Duringthe flash-sintering of BZCYYb, the applied initial field strength, current density, and theduration time are studied in terms of scanning electron microscope （SEM）, two-probeconductivity, electrochemical impedance spectroscopy （EIS）.To further understand the flash-sintering mechanism, we have densified fourelectrolytes including yttria-stabilized zirconia （YSZ）, Gd-doped ceria （GDC10）,BZCYYb, BaZr_0.8Y_0.2O_3-δ. And we focused on three REDCs including GDC20, GDC10,and SDC20. We first studied their flash-sintering phenomenon, and then compared themwith their conventional sintering behavior, which demonstrated the great advantage offlash-sintering. The phase structure of flash-sintered REDC specimens was characterizedby XRD. Then, we have investigated the effect of total conductivity, electronicconductivity, and ionic conductivity on flash-sintering process. And we found that the electronic conductivity of materials determined the flash-sintering onset.Finally, based on the experimental phenomenon and analyzation, we proposedflash-sintering mechanism. First, we presented the Joule heating effect and black bodyradiation with a chain mechanism. The real temperature of electrolyte ceramics underflash-sintering was estimated by this theory. Additionally, we confirmed the validity of thistheory. Subsequently, based on the finding that electronic conductivity determined theflash-sintering onset, we proposed the spark plasma sintering mechanism.