Lattice modification of ZrO2-δ and formation of rocksalt structure ZrO and Zr(O,N) after DC electrical loading of 3YSZ

Abstract

It is widely acknowledged that electrochemical reduction occurs during DC flash sintering (FS) of YSZ. In situ experiments have shown the emergence of new phases during FS that disappear again when the current is switched off. In addition, it has recently been reported that YSZ flash sintered in the absence of oxygen has a high electrical conductivity at room temperature. In this investigation electrical loading of YSZ was conducted in Ar and N2 and maintained as the furnace cooled down such that on removing the current, the sample was rapidly quenched, avoiding reoxidation or other diffusion-limited phase transformations. XRD showed that the tetragonality of the ZrO2-δ phase was diminished and the volume of the unit cell increased, the effect of the electrochemical reduction being similar to an increase in Y2O3 content. In addition, rocksalt structure Zr(O,N) with a lattice constant of 0.4580 nm was formed at the surface and retained at room temperature. This phase had a metallic lustre and electrical conductivity approaching those of metals. The results enable several recent results in the literature to be reconciled and the implications for flash sintering are briefly discussed.