The effect of carbon additives on the dielectric behaviour of alumina ceramics

Alumina samples containing carbon microspheres (from 0.22 to 1.0 wt.%) were hot pressed at 1550 °C in order to study the effect of carbon additives on the dielectric properties. The dielectric study in the frequency range from 0.1 Hz to 18 GHz demonstrates a loss tangent dependency on carbon concentration, which decreases when the carbon microsphere content is increased. Investigation of the microstructure shows a randomised distribution for the carbon microspheres. The microspheres seem to act as nucleation points, capturing the contaminating carbon coming from the graphite moulds during sintering. Grain boundary cleaning causes an improvement of the dielectric properties of this material. The optical properties show defect formation related to carbon content.