Sintered alumina with low dielectric loss

Low dielectric loss materials are required for applications in radio-frequency and microwave communications. Aluminium is the second most abundant element in the Earth’s crust and aluminium oxide (alumina) is one of the commonest ceramics. Single crystals of aluminium oxide, i.e., sapphire, possess one of the lowest dielectric losses of any material. Polycrystalline alumina has a higher loss due to extrinsic factors. The dielectric loss of sintered alumina is studied in an attempt to determine the causes of extrinsic loss. Impurities are shown to play an important role, but the microstructure also is a key factor. High-purity aluminas, sintered to near theoretical density, are found to display very low loss, tan δ=2.7×10−5 at 10 GHz. Doping alumina with titanium dioxide was found to reduce the tan δ=2×10−5.