Study of Dielectric Properties of Decomposing Materials during Heating

Microwave dielectric properties of materials during heating can be studied by methods based on the use of waveguide cavity resonators. However, since the material is located in a closed space during such measurements, determination of dielectric properties of decomposing materials involves errors related to the filling of the resonator and deposition of decomposition products of the material on its inner conducting surfaces. In this paper, we report a study of dielectric properties of decomposing materials during heating with the use of a waveguide cavity resonator by methods that preclude any effect of destruction products on measurement accuracy. To reduce the effect of the vapor released from the sample surface as a result of the destruction process, additional thin quartz plates were placed on the sample surface. In addition, we used cuvettes whose lid prevented the destruction products from entering the resonator. A technique is described for taking into account the effect of the plates and cuvette in calculating the dielectric permittivity and dielectric loss tangent of materials during heating in a cavity resonator. The variation in the dielectric properties of a composite material consisting of quartz fabric impregnated with an aluminum chromium phosphate binder and of fiberglass laminate made of quartz fabric impregnated with a phenol formaldehyde binder has been studied above the destruction onset temperature (up to the temperatures at which the materials lose their structural strength). The proposed technique can be useful in studies of the thermophysical properties of decomposing materials for assessing changes in their dielectric properties under high-temperature service conditions.