Densification of plasma sprayed YSZ electrolytes by spark plasma sintering (SPS)

Solid oxide fuel cells (SOFC) are promising candidates for alternative power generation systems due to their high-energy conversion efficiencies, and low emissions of environmentally hazardous by-products. Plasma spray (PS) is an effective, and relatively inexpensive process for fabricating high performance yttria stabilized zirconia (YSZ) electrolyte for SOFC. Yet, because of the numerous inter-granular defects introduced to the electrolyte by the plasma spray process, the electrolyte is not gas tight and consequently, the energy efficiency of the cell is severely curtailed. In order to improve the performance of the SOFC, spark plasma sintering (SPS) is introduced as a post-spray treatment to enhance the density of the PS YSZ electrolyte rapidly, and effectively. In this study, spark plasma sintering (SPS) was performed at 1200, 1400 and 1500 °C. Each sintering cycle had a holding time of 3 min. Single and multiple SPS cycles (3 min at preset temperature per cycle) were used to treat the plasma sprayed yttria stabilized zirconia (PS YSZ) electrolytes. The microstructure of as-received and SPS treated electrolytes as examined by scanning electron microscopy (SEM) demonstrated a microstructure transition above 1200 °C, where the typical plasma sprayed lamella structure transformed to a granular-type structure. The porosity of as-received and SPS post-treated electrolytes, which were determined by a mercury intrusion porosimeter (MIP) revealed a significant reduction in pores at 1500 °C. Average pore size reduced from 0.2 to 0.08 μm. The ionic conductivity of the electrolytes is evaluated by AC impedance spectroscopy to characterize the effect of SPS on enhancing the ionic conductivity of the electrolytes.