Improved purity levels of specialty alumina play a major role for emerging and developing technical applications. Thus, the impact of the overall purity level of alumina on its properties was analyzed. Furthermore, the impact of specific impurities or dopants (e.g. Na2O, SiO2, MgO) on different properties such as densification and dielectric loss are discussed.
To determine how grain-boundary composition affects liquid phase sintering, different impurity levels of alumina were formed by doping. MgO-free Bayer process aluminas, samples were singly or co-doped with up to 600 ppm Na2O or SiO2. Na2O retards the sample densification only up to hold times of 30 min at 1525°C compared to undoped MgO-free Al2O3 samples. Increasing SiO2 concentration significantly retards densification at all hold times up to 8 h. This effect of SiO2 was found less severe in MgO-doped samples. MgO is observed to reduce the grain boundary thickness during densification, which is shown by the increased solubility of SiO2 in alumina grains. The removal of SiO2 from the grain boundaries is a key process by which MgO enhances the sintering of alumina. Overall, the results indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase.
The dielectric loss is a critical parameter for semiconductor applications and is an example property that is significantly influenced by specific impurities. In general the dielectric loss tangent was found to decrease with increasing purity. Individual impurities (or combinations of impurities) were found to be more important than the overall purity level. For example Na2O and SiO2 were found to have severe effects on the dielectric loss tangent. The awareness that specific impurities and certain combinations, thereof, have a larger impact on the properties of alumina than the overall purity, opens up the possibility for a more economical solution to selectively avoid or remove impurities in alumina that are harmful for the targeted properties, depending on the application.