Microstructural effects on far-infrared reflection spectra of alumina ceramics

In recent years, use of IR reflection spectroscopy has been demonstrated for microwave characterisation of selected dielectric ceramics. The reflection spectra is analysed by the Kramers-Kronig relation and/or classical dispersion theory. A systematic study was carried out to determine the relative magnitude of the effects of porosity and grain size on the calculated dielectric functions. Al2O3 ceramics, conventional calibration standards in dielectric characterisation, were used. Samples were doped with MgO in order to control the grain size and to provide a uniform grain structure. It was found that the microstructure of the specimens affects the reflectivity data and, in turn, the calculated values of dielectric functions. The porosity seems to degrade the quality factor and dielectric constant. A large grain size apparently increases the values; further studies are needed in this regard. 6 refs.