Incubation time for flash sintering as caused by internal reactions, exemplified for yttria stabilized zirconia

Transient and steady state electrotransport and diffusion of ions being produced and annihilated by internal reactions are discussed and exemplified for Yttria Stabilized Zirconia (YSZ). These phenomena are important for understanding flash sintering. They will also play a role in solid oxide fuel cell (SOFC) and solid oxide electrolysis cells (SOEC), where current densities may exceed the reaction rates with gases at the porous electrodes. The characteristic time for attaining steady state transport contains two parts, one depending on the length of the sample and one depending on the field strength. This characteristic time is derived in this study for a linear increase of temperature for the first time. By assuming that the characteristic time is a measure of the onset of flash sintering, yields - without fitting parameters - incubation times or onset temperatures of flash sintering in good agreement with experimental results for YSZ. Thus concentration changes building up during the incubation period are driving forces for internal reactions generating or consuming holes and electrons. A concomitant increase of conductivity leads to Joule heating which further accelerates reaction rates and ion mobility. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.