DIELECTRIC PROPERTY MEASUREMENTS OF ZRO2-STRENGTHENED AL2O3

Ceramic composites of alumina and zirconia with ZrO2 contents up to 20 wt% and negligible porosity were investigated at radio frequencies fr om 10(4) to 10(7) Hz, and in a temperature range from 20 to 550 degree s C. If unstabilized or partially stabilized ZrO2 is dispersed in pure alumina in order to improve the mechanical strength the dielectric pr operties are changed. The admixture of zirconia leads to enhanced diel ectric losses and an increased temperature dependence of the permittiv ity. It is demonstrated that it is essential to distinguish between th e contributions of the monoclinic and the tetragonal phase of zirconia . The presence of the tetragonal ZrO2 phase is correlated with the app earance of two peaks in the temperature dependence of the dielectric l oss and two smoothed steps in the temperature dependence of the permit tivity. This is well described by a Debye model for dipole relaxation. The presence of the monoclinic phase leads to a higher exponential te mperature rise for the dielectric loss as well as for the permittivity , due to an increased ionic conductivity. At higher frequencies the in fluence of ionic conductivity is smaller and the dipole relaxation pro cesses are shifted to higher temperatures. Therefore, for high-frequen cy applications low-loss composites with improved mechanical propertie s can be realized if monoclinic zirconia is kept at low levels. (C) 19 97 American Institute of Physics.