Composites Help Improve Uniformity of Microwave Heating

The phenomenon of interaction of electromagnetic fields with the media has been in the core of numerous industrial and scientific endeavors. Research has shown that the dielectric properties of the materials involved directly affect the operating efficiency of corresponding systems apparatuses. While dielectric constant is responsible for the field distribution, and the loss factor dictates what part of the field power is absorbed by the material. Despite recent advances and developments in microwave power engineering, a field in which dielectric materials and their properties play a particularly crucial role, designers of systems for different applications still struggle with the challenge of intrinsic non-uniformity of microwave heating.

In the recent studies, materials with particular dielectric properties were found to be promising systems’ supplementary inserts capable of appropriate redistributing the electromagnetic field and improving (evening up) uniformity of heating. However, the materials possessing desired dielectric constant and the loss factor may not be readily available thereby making the practical utilization of the developed techniques problematic.

Recently, Worcester Polytechnique Institute scientist Dr. Vadim V. Yakovlev in collaboration with Dr. Sébastien Vaucher at EMPA – Swiss Federal Laboratories for Materials Science and Technology introduced a novel approach which could be used for the production of materials characterized by a desired dielectric properties. They applied the inverted power-law mixing rule model to determine volume fractions, in which three or more prime materials were to be taken so as to get, in the resultant homogeneous mixture, the required dielectric constant and the loss factor. Their work is currently published in the research journal, Polymer Engineering and Science.

The authors considered a series of power-law mixing rule models and suggested a choice and calibration of the most suitable one; these steps are based on measurement of complex permittivity of the prime materials and their binary mixtures. Functionality of the proposed approach was demonstrated by production of composites with a polymer matrix loaded with two inorganic fillers. The prime materials used in the experiments were alumina, silicon, and acrylic glass. Small amounts of Si and Al₂O₃ powders were added to the polymer powder (PMMA); as such, the reported demonstration was limited to relatively low values of dielectric constant of the target mixture. The composites were made by mechanically mixing the components and axially hot-pressing and cooling the mixture. For these composites, the Looyenga power-law model was found to be the most adequate and used for determination of the volume fractions.

The authors observed that in the produced samples, the targeted values of dielectric constant were reached with a higher precision than the ones of the loss factor; yet, analysis of the production process and error propagation in the computations suggested that deviations of the resultant complex permittivity fell within the anticipated ranges.

In summary, the study demonstrated a computational procedure and a process of production of materials having the required complex permittivity for a group of ceramic-polymer composites. Their technique was able to determine the volume fractions of those substances which, after thoroughly mixing, were expected to end up in the material with the targeted dielectric constant and the loss factor. While certain diversions of the measured loss factor of the produced composites were explained by the experimental conditions, the resulting dielectric constants were seen to be in excellent agreement with the ones suggested by the model. The proposed approach provides solid support for further development of the optional use of supplementary dielectric inserts for improving heating patterns and, consequently, quality of microwave heating in practical applications.