Pure zirconium oxide (unstabilised) is monoclinic at room temperature but its structure changes to a tetragonal form at around 1000 degrees C. With this phase change there is a large volume change that results in structural cracks leading to a very low thermal shock resistence. Partial stabilisation of zirconia occurs with the addition of a dopant material such as 3-4mol% yttria, 2-3mol% magnesia or 8mol% calcia. This leads to a mixture of structural phases at different temperatures.
A mixture of cubic, monoclinic and some tetragonal phase up to 1000 degrees C. and tetragonal at higher than 1000 degrees. Due to the tetragonal phase at higher temperatures partially stabilised zirconia is also known as tetragonal zirconia polycrystal (TZP). It is generally believed that microcracks and induced stress amongst the different phases are reasons for the toughening in partially stabilised zirconia. Partially stabilised zirconia tends to be used when high temperatures are needed.
The low thermal conductivity ensures a low heat loss and the high melting point (2,700 degrees C.) means stabilised zirconia regreactories can be used in temperatures in excess of 2200 degrees C. Partially stabilised zirconia is also finding increased application in specialist structural/engineering ceramics. Fully stabilised zirconia is generally achieved with dopant levels of 8mol% yttria, 16mol% magnesia or 16mol% calcia. A fully stabilised zirconia has a structure which is a cubic solid solution which has no phase transformation from room temperature up to 2500 degrees C. Fully stabilised zirconia is a very good ion conducting ceramic and is used in applications such as oxygen sensors and solid oxide fuel cells.
