Effect of Porosity and Grain Size on the Microwave Dielectric Properties of Sintered Alumina

TLDR

Extrinsic factors such as porosity, grain boundaries, microcracks, and impurities dominate losses in sintered alumina, making single‑crystal loss models inadequate. The study aims to characterize how porosity, grain size, and purity affect the dielectric loss (tan δ) and permittivity (ε) of sintered polycrystalline alumina. Dielectric properties were examined as functions of purity, pore volume, and grain size, with tan δ measured across near‑theoretical densities ranging from 9.1 × 10⁻⁵ to 2.4 × 10⁻⁵ at ~9 GHz. Measured ε and tan δ at 9 GHz show that tan δ is strongly influenced by pore volume, purity, and grain size, whereas ε is relatively insensitive to impurities and grain size and follows simple mixture models with porosity.

Abstract

The real part of the permittivity (ε) and the tan δ of sintered alumina (Al 2 O 3 ) at about 9 GHz have been measured. The dielectric properties have been examined as a function of purity, pore volume, and sintered grain size. The tan δ is found to depend very strongly on the pore volume, purity, and grain size. ε is far less sensitive to impurities and grain size. The dependence of ε on porosity can be described by simple mixture models as expected. A model of losses in single crystals cannot be extended easily to these materials where extrinsic factors such as porosity, random crystal orientation, grain boundaries, microcracks, and impurities dominate. These factors have been studied in an attempt to describe the tan δ and ε of sintered polycrystalline alumina. In this work, the tan δ for alumina has been studied in near‐theoretical density ranges between 9.1 times 10 ‐5 and 2.4 times 10 ‐5 depending on grain size.