Permittivity measurement of liquids, powders, and suspensions using a parallel-plate cell

This work describes the measurement of complex permittivity (or dielectric constant) of powders (water insoluble), solutions, and suspensions using a parallel-plate capacitor cell. An impedance analyzer measures the cell's impedance at radio frequencies used in nuclear magnetic resonance and magnetic resonance imaging (10-300 MHz) through an appropriate test fixture. The cell's impedance is fitted to an equivalent circuit using a Matlab script or other software, and the permittivity of the material is extracted after calibration with known materials. The permittivity of the solid material is obtained from that of the powder suspension using known mixing rules. Measurements for most materials tested are in agreement with those obtained using standard coaxial probes. Some discrepancies are observed for loose powders because of the difficulty of controlling the amount of packing. For high-permittivity materials or conductive solutions an enhanced equivalent circuit has been found that characterizes the cell's behavior over the full frequency range. (c) 2016 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 46B: 19-24, 2016