Effect of boundary conditions on reduction during early stage flash sintering of YSZ

The onset of flash sintering is generally considered to be predominantly a function of the conductivity of a given ceramic, the furnace temperature, and the electric field. However, the evolution of the point defect profiles in ionic conductors such as yttria-stabilized zirconia (YSZ) can complicate the picture of homogeneous Joule heating and thermal runaway during DC flash sintering. Here, 8 mol% YSZ pellets were partially flash sintered under a DC bias using various current densities and hold times. The electrode geometry was varied to modulate the oxygen ion flux available to the cathode to compare the effects on the resulting oxygen vacancy inhomogeneity. The contribution from fundamental and experimental factors to the cathodic reduction reaction is also discussed, such as interface reaction kinetics and sample geometry. Local reduction of the ceramic was inevitably observed under all current densities and the resulting microstructural inhomogeneity was explained as the result of a transient conductivity asymmetry. This asymmetry was the result of the enhanced electronic conductivity in the cathode region to a value significantly greater than the ionic conductivity of near-stoichiometric YSZ. The link between the local conductivity, voltage and Joule heating is mathematically demonstrated to result in an asymmetric heating profile. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.