Composites Help Improve Uniformity of Microwave Heating

Significance 

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.

Materials with Required Dielectric Properties: Computational Development and Production of Polymer-Ceramic Composites - Advances in Engineering
Solid cylindrical disks made by hot-pressing of pure PMMA powder (1) and binary mixture powders: PMMA+Si (2), and PMMA+Al2O3 (3).
Materials with Required Dielectric Properties: Computational Development and Production of Polymer-Ceramic Composites - Advances in Engineering
Samples of PMMA-Si-Al2O3 composites produced as the materials with required complex permittivity; e.g., sample A: required: 6.0 – j0.2; produced: 5.88 – j0.198.

About the author

Sébastien Vaucher was born in Switzerland in 1970. He obtained his chemistry degree in 1993 at the University of Neuchâtel and joined the Tokyo Institute of Technology for one year. After completing his PhD at the University of Neuchâtel in 1998, he explored “soft-chemistry” methods for the synthesis of nanostructured functional molecular magnets at the University of Bristol (UK) as a post-doctoral fellow. Since 2001 he has been with the Swiss Federal Laboratories for Materials Testing and Research (EMPA), where he leads research dedicated to the development of microwave-assisted materials processing technologies.

He has developed in-situ analytical tools to explore the effect of electromagnetic fields on materials such as time resolved X-Ray diffraction, time-resolved tomography, and time-resolved EXAFS under microwave irradiation using the Swiss Light Source Synchrotron facilities. Other developments include volumetric temperature evaluation during microwave heating and in-situ Raman spectroscopy to follow selective heating in composite powders. Docent at the Swiss Polytechnic Institute of Technology (EPFL), he has represented Switzerland at the European level as a domain committee member of European Cooperation in Science and Technology (COST).

Dr. Vaucher was invited to numerous conferences and has co-authored over 30 publications. He engages himself to reconnect science and art and values taking time with other fellows to share green tea in his garden where he grows for many years Japanese maples from seeds and where he enjoys playing music and singing.

About the author

Vadim Yakovlev is an Associate Research Professor in the Department of Mathematical Sciences, Worcester Polytechnic Institute (WPI), Worcester, MA, USA. He is a head of the Industrial Microwave Modeling Group (IMMG) which he established in 1999 as a division of the WPI’s Center for Industrial Mathematics and Statistics (CIMS). He received his Ph.D. degree in Radio Physics from the Institute of Radio Engineering and Electronics (IRE) of the Russian Academy of Sciences (RAS), Moscow, Russia in 1991.

His research interests include multiphysics modeling, microwave power engineering, wireless energy transfer, broadband/multiband antennas, neural-network-based optimization, and microwave imaging. He is an author of more than 150 papers in referred journals and conference proceedings.

Dr. Yakovlev is a Fellow of IMPI, a Senior Member of the IEEE, a Member of AMPERE. In 2013, Dr. Yakovlev served as a Chair of the Technical Program Committee for of the 47th IMPI Microwave Power Symposium. He was the Guest Editor of a special issue on modeling of Journal of Microwave Power & Electromagnetic Energy in 2007. He has been a reviewer for more than 20 journals and a member of program committees of several conferences. He is a founder of a series of international interdisciplinary Seminars/Worshops “Computer Modeling in Microwave Power Engineering” held annually since 2000.

Reference

Sebastien Vaucher, Vadim V. Yakovlev, Hannah Yeung. Materials with Required Dielectric Properties: Computational Development and Production of Polymer-Ceramic Composites. Polymer Engineering and Science 2018, page 319-326.

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