Microdielectrometry

Abstract

The low-frequency dielectric properties of resins provide a useful tool for characterizing both the curing process and the fully cured material. We have used silicon microfabrication technology to develop a miniaturized dielectric sensor that combines a planar interdigitated electrode structure with a pair of matched field-effect transistors to achieve sensitivity at frequencies as low as 1 Hz. The microdielectrometer "chip" can be implanted in a specimen, or a small sample of material (a few milligrams) can be placed on the active area of the device. When combined with an off-chip electronic feed-back system, the device can be used to measure the complex dielectric constant of the sample material either as it cures, or after cure. The device is capable of operating in excess of 200 °C, making it useful for a wide variety of curing and post-cure studies. Device calibration is based on a two-dimensional computer model which has been experimentally confirmed for a variety of control samples. Several typical applications are illustrated by experiments, and the use of the data to follow changes in the dominant low-frequency dielectric relaxation during the cure of a model epoxy resin system is presented.