Probing High Oxygen Activity in YSZ Electrolyte

Abstract

The redox behavior of terbium and praseodymium doped yttria-stabilized zirconia (YSZ) is studied. The aim is to identify spectroscopic probes and a suitable experimental procedure to monitor the oxygen activity in YSZ electrolytes in solid oxide cells with spatial resolution and at operation conditions (e.g. at high temperatures). Sintered ceramics and crystals with 0.3 to 10 at% content of Pr or Tb ions in YSZ were prepared. Upon equilibration in atmospheres from 10−20 to 100 bar PO2 around 800 °C, the majority of these rare earth ions are in the 3 + oxidation state. At oxygen pressures above 0.001 bar, the small proportion of Tb4+ and Pr4+ formed give rise to intense optical absorption around 300–500 nm and to decreased reflectance. From the reflectance measurements it is shown that the Tb4+ concentration increases as PO2 1/4, as correspond to the trapping of the holes generated upon the oxygen incorporation as Tb4+. This competitive absorption causes a decrease of the Tb3+ luminescence. A quantitative relationship of the Tb3+ luminescence intensity with PO2 at 800 °C has been found, which is compatible with the trapping model. The spatial resolution of the experimental procedure could be very roughly estimated of the order of 100 μm.