EFFECT OF POROSITY AND GRAIN-SIZE ON THE MICROWAVE DIELECTRIC-PROPERTIES OF SINTERED ALUMINA

The real part of the permittivity (epsilon') and the tan delta of sint ered alumina (Al2O3) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, a nd sintered grain size. The tan delta is found to depend very strongly on the pore volume, purity, and grain size, epsilon' is far less sens itive to impurities and grain size. The dependence of epsilon' on poro sity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these material s where extrinsic factors such as porosity, random crystal orientation , grain boundaries, microcracks, and impurities dominate. These factor s have been studied in an attempt to describe the tan delta and epsilo n' of sintered polycrystalline alumina, In this work, the tan delta fo r alumina has been studied in near-theoretical density ranges between 9.1 x 10(-5) and 2.4 x 10(-5) depending on grain size.