Dielectric properties of alumina/zirconia composites at millimeter wavelengths

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Abstract
Alumina-zirconia composites with ZrO2 contents up to 20 wt % and negligible porosity were investigated at millimeter (mm) wavelengths to determine the changes appearing in the dielectric properties of pure alumina ceramics when unstabilized or partially stabilized ZrO2 is added to improve the mechanical strength. It is demonstrated that it is essential to distinguish between the contributions of the monoclinic and the tetragonal phase of zirconia (m-ZrO2, t-ZrO2). Permittivity is raised with increasing content of either phase; the effective permittivity can be assessed by the rule of mixtures (Maxwell–Garnett formulation of the generalized Clausius–Mossotti relation) using permittivity values of 10 for Al2O3, 14–21 for m-ZrO2, and 40–45 for t-ZrO2. The permittivity data show only a small variation in the investigated range of 9–145 GHz. For the dielectric loss, there is evidence of a predominant contribution of m-ZrO2; in addition, the marked increase in loss with frequency becomes sharper. The t-ZrO2, which is responsible for strengthening, does not show any significant influence on losses. It is therefore concluded that ZrO2 strengthening of alumina is feasible without affecting mm-wave losses at room temperature as long as the presence of m-ZrO2 is avoided.

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