As a consequence of its good oxygen ion conductivity stabilized or partially stabilized zirconia is chosen as the solid electrolyte for oxygen sensors operating in a high temperature environment. Experience showed that most of these 5 mol% Y2O3-ZrO2 tubes survived the autoclave environment for an acceptable time. However, some tubes failed after a few days. In order to explain this behaviour the ceramographic structure of several material batches was thoroughly characterized. Subsequently they were exposed to 300 °C water and examined with respect to crack formation. It was found that degeneration by cracking is determined by the size and the amount of intergranular tetragonal precipitates. These range from 1 - 10 μm diameter and 3 vol% in most samples to 30 μm and 7 vol% in others. After exposure it was found that the structure with large precipitates is extremely sensitive to cracking. This is explained by the water catalysed martensitic (tetragonal to monoclinic) transformation of zirconia which involves a volume expansion of 4 %. Transformation of large surface particles, in contact with water causes cracks extending to sub-surface particles. This brings these particles in contact with water and makes the cracking process continue.
|Title of host publication||Water chemistry of nuclear reactor systems|
|Place of Publication||Berlin, Germany|
|State||Published - 15 Sep 2008|
|Event||International conference on water chemistry of nuclear reactor systems - VGB, Berlin|
Duration: 14 Sep 2008 → 18 Sep 2008
|Conference||International conference on water chemistry of nuclear reactor systems|
|Period||2008-09-14 → 2008-09-18|