New potentials for high mechanical strength grades of polycrystalline alumina for EC waves windows

Abstract

A fine grained high purity alumina grade and a composite of alumina and 5% zircona have been characterized as possible candidates to be used in rf windows. Both dielectric and mechanical properties are measured. The room temperature permittivity is around 9.84 and 10.28, respectively without significant frequency dependence. Its temperature dependence shows a T 2 law for both types of materials. The room temperature loss tangent goes from 2 to 15×10 −5 and from 6 to 39×10 −5 respectively in the frequency range between 17 to 145 GHz. The temperature dependence shows a strong decrease at low temperatures for the fine grained alumina and almost no temperature dependence for the alumina/zircona composite. The obtained bending strength data are 370 ± 50 and 584 ± 66 MPa, respectively. These results indicate that the material grades are attractive for use in cryogenically cooled and in backup rf window designs.