One-sided measurement of material dielectric properties using a liquid dielectric immersion technique

One of the most important factors that limit the accuracy of nondestructive measurements of dielectric and related physical properties of solid electrically insulating materials is the presence of an erratically non-uniform air gap between the sensor head and the material surface due to electrode thickness and surface roughness and deformations. Sensitivity analysis shows that the statistical error of interdigital sensor measurements due to non-ideal contact conditions may be reduced by filling the air gap with a dielectric liquid that has relatively high dielectric permittivity. A feasibility study with several combinations of liquid and solid dielectrics is presented. Two measurement approaches are explored. In the first case, the capacitance of a reference interdigital single-wavelength sensor immersed in a dielectric liquid is compared to the capacitance of a twin sensor in the same dielectric liquid adjacent to the solid dielectric. Borrowing the methodology from more common parallel-plate methods, one mixes two liquids with different properties until matching of the dielectric constant of the solid and liquid dielectrics is achieved. In the second case, multiple penetration depth measurement options are utilized to measure properties of the solid dielectric. The achieved accuracy and the potential improvements of this approach are discussed.