The role of C-impurities in alumina dielectrics

This work forms part of an extensive study on the effect of impurities on the structure and dielectric loss in alumina ceramics. In this case, the carbon impurity has been chosen since it can be an important contaminant in fusion reactors. Hot-Pressing at 1500 °C in a carbon rich atmosphere has been found to be a powerful method to introduce carbon in two different alumina powders. Due to the presence of C in the samples, they acquire a black colour. To check the ability of C to penetrate into alumina, microstructure (SEM,TEM) and chemical analyses (ICP-AES and EA) have been performed. The existence of pyrolitic carbon at the alumina grain boundaries in the as-sintered state has been observed. The dielectric properties (permittivity and loss tangent) as a function of frequency (from 0.1 Hz to 20 GHz) and DC conductivity of the sintered material were measured by means of several techniques. The most dramatic effect is that the DC conductivity is strongly enhanced when carbon reaches concentrations higher than about 0.5% wt. Oxidation of these samples by a treatment at 1500 °C in air reduces the C content and restores a white exterior colour. In general this treatment strongly reduces the dielectric loss. Changes between reduced and oxidised microstructures were analysed by SEM.