Processing materials with microwave energy

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Materials Science and Engineering A287 (2000) 153 – 158 www.elsevier.com/locate/msea

Processing materials with microwave energy
David E. Clark *, Diane C. Folz, Jon K. West
Department of Materials Science and Engineering, Uni6ersity of Florida, Gaines6ille, FL 32611 -6400, USA

Abstract Microwave energy (microwave frequency, in this case, includes radio frequencies and ranges from 0.3 MHzto 300 GHz) is being developed as a new tool for high-temperature processing of materials. Examples of the advantages associated with microwave processing include: rapid and uniform heating; decreased sintering temperatures; improved physical and mechanical properties; and, unique properties which are not observed in conventional processes. These advantages observed in materials processed usingmicrowave energy are being attributed to ‘microwave effects’ which are particular to this technology. Researchers at the University of Florida are working to identify and to qualitatively and quantitatively define the mechanisms of microwave–material interactions. A new model has been developed based on the molecular orbital model which predicts the behavior of specific pure materials in a microwavefield. Experimental work as well as dielectric property measurements confirm the accuracy of this model in specific cases. Published by Elsevier Science S.A.
Keywords: Microwave processing; Microwave–material interactions; Materials processing

1. Introduction Microwave energy has been developed primarily for communications and some areas of processing such as cooking food, tempering and thawing, andcuring of wood and rubber products. Although there is extensive consumer and industrial use of microwave energy, interaction of microwaves with materials is poorly understood. Furthermore, there are numerous reports in the literature of non-thermal ‘microwave effects’ that accelerate reaction rates, alter reaction pathways and result in unique properties in polymers, ceramics and composites. Theorigins of these microwave effects are unknown. Thus, a fundamental understanding of how microwave energy interacts with materials is the key to unlocking the technology for future and widespread use. The advantages observed with microwave processing warrant serious, focused attention on this technology. Tangible benefits to be produced by microwave–material research include: reduced processingcosts, better production quality, new materials and products, improved human health, reduced hazards to humans and the environment and enhanced quality of life. With
* Corresponding author. 0921-5093/00/$ - see front matter Published by Elsevier Science S.A. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 7 6 8 - 1

proper understanding and control, many technically important materials can be heated rapidly,uniformly, selectively, less expensively and with greater control than is possible with conventional methods. Moreover, the unique internal heating phenomenon associated with microwave energy can lead to products and processes that cannot be achieved using conventional methods. An excellent example of this interaction on a large scale is the Parallam process (developed by McMillan-Bloedel) wherethe penetrating nature of microwave energy is used to rapidly and uniformly cure thick, cross-sectional, polymer/wood composite beams as they are pultruded continuously through a die [1]. Conventional heating methods overcure the surface while undercuring the interior. On-going research at the University of Florida (UF) is focused primarily on high-temperature processing of materials, including:sintering of ceramics; combustion synthesis of composite materials; nucleation and crystallization in glass to form glass–ceramics; microwave absorption/heating in composite susceptor materials; and, remediation and recycling of electronic waste materials. Also, a model is being developed which will help to predict the behavior of materials in a microwave field. This model, based on the...
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