High purity alumina (Al2O3 content > 99%) ceramics are ideal for high temperature applications. Re-crystallised alumina is used in crucibles as it offers the best thermal shock resistance due to the larger grain size (up to 200m m is advantageous).
Such high purity alumina shows considerable resistance to chemical attack due to the lack of glassy phases that tend to determine corrosion resistance.
The electrical and dielectrical properties generally improve with increasing purity. Electrical resistance is greatest and dielectrical loss factor is lowest in materials of highest alumina content.
High purity alumina produces a dense and essentially single-phase ceramic on sintering. Grain growth during sintering can be controlled by small additions of MgO up to 0.05%. The product usually contains a small amount of closed porosity but this can be kept below 0.5% using modern firing techniques.
Zirconia (ZrO2) offers three advantages over alumina: it is more refractory, having a melting point some 500oC above alumina, it gives a high quality surface finish and it is a useful oxygen anion conductor for use in sensors and fuel cells. However it is considerably more expensive than alumina.
Zirconia has the complication of being metastable and needs to be stabilised before it can be put to use. The addition of small quantities of stabilising oxides such as CaO, MgO and best of all Y2O3 allow the high temperature cubic phase to stabilise itself. Partially stabilised zirconia can also be used as a toughening agent in alumina. The zirconia-toughened alumina (ZTA) shows a considerable improvement in strength and more importantly toughness. As a result, these ceramics can be used in areas of extreme mechanical abrasion and thermal shock.
Fully stabilised zirconia (FSZ) produces a high-density ceramic on sintering. The grain size is relatively large at 10-40m m and the ceramics are often translucent in appearance. Recent optimisation of zirconia powders have resulted in a readily sinterable high purity product.