Effect of humidity on microwave dielectric losses of porous alumina

The microwave dielectric loss in highly porous alumina is measured in dry and moist atmospheres. Data are compared with those for sapphire and a fully densified alumina grade. Results indicate that the combination of humidity and porosity gives rise to a very high dielectric loss which is drastically reduced by replacing the moist atmosphere by dry gas. Measurements over a wide frequency range from 1 mHz to 100 MHz indicate that the origin of the microwave loss is due to the high frequency tail of a low frequency process. This low frequency loss peak shifts to lower frequencies with decreasing humidity, explaining the observed dependence of the microwave loss on humidity. (C) 1999 American Institute of Physics. [S0021-8979(99)06403-8].