Use of an open-ended coaxial cavity method to characterize powdery substances exposed to humidity

The effect of moisture adsorption in the characterization of dielectric powders by an indirectly coupled open-ended coaxial cavity method operating at 4.5 GHz was analyzed. SiO2, Al2O3, and ZrO2 powders were exposed to 5% and 100% relative humidity levels and Bruggeman symmetric, Looyenga, and General Mixing Model equations were used to determine changes in permittivity and loss tangent of the inclusions. Low moisture adsorption (0.1-2.5 vol. %) induced only a small change in inclusion permittivity but had a pronounced impact on dielectric losses. Calculated moisture percentages based on responses of the resonator and the General Mixing Model correlated well with traditional mass based determination. The presented characterization method exhibited high sensitivity for the determination of dielectric properties of powders and their moisture content to be further utilized in, e.g., process and quality monitoring. (C) 2013 AIP Publishing LLC.