Electric current effect on 8Y-zirconia ceramics : pore migration, grain growth and densification

Abstract

The electric current effect was studied in 8 mol% $Y_2O_3$ stabilized $ZrO_2$ ceramics (8YSZ), the leading electrolyte material for Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOECs). Since the electrolyte in SOFC/SOEC is under the electrical load at high temperature for a long operation time, we examined their material stability. The present study focuses on the electrically induced microstructure evolution (pore/bubble migration, densification, and grain growth) and those mechanisms. In Chapter 4, electrically induced movement of neutral second phase (pores/bubbles) was investigated in the cubic stabilized zirconia (8YSZ) ceramics using implanted argon bubbles in dense ceramics, and residual sintering pores in porous (75%, 95%) ceramics. Pores/bubbles migrates against the electric field at low temperature $(as low as 875^\circ C)$, where the cation diffusion is supposed to have frozen. The upstream migration of bubbles/pores was evident by the redistribution of porosity and the distorted grain boundaries. The contorted grains, however, were not observed at lower temperature and higher fields when they apparently passed through grain boundaries without causing any visible distortion. The migration mechanism can be attributed to the relocation of atoms around the pore surface by imbalanced ion diffusivity, which yields ${D_{Zr}}^S = 2.8×10^{-6} exp(-174 [kJ/mol]/RT) m^2/s$ : in essence, it is like an electrical Kirkendall effect. In Chapter 5, electrically enhanced densification, called electro-sintering, without the assistance of Joule heating was investigated in 70% dense 8YSZ ceramics at temperatures well below those for conventional sintering. The favored densification to the cathode side can be attributed to electric effect, not to Joule heating, which would leave the center section the hottest, thereby causing it to experience the most densification. Remarkably, full density can be obtained without grain growth under a wide range...