Composites Part A: Applied Science and Manufacturing
Microwave processing: fundamentals and applications
Introduction
In the past 20 years, the microwave oven has become an essential appliance in most kitchens. Faster cooking times and energy savings over conventional cooking methods are the primary benefits. Although the use of microwaves for cooking food is widespread, the application of this technology to the processing of materials is a relatively new development. The use of microwave energy for processing materials has the potential to offer similar advantages in reduced processing times and energy savings.
In conventional thermal processing, energy is transferred to the material through convection, conduction, and radiation of heat from the surfaces of the material. In contrast, microwave energy is delivered directly to materials through molecular interaction with the electromagnetic field. In heat transfer, energy is transferred due to thermal gradients, but microwave heating is the transfer of electromagnetic energy to thermal energy and is energy conversion, rather than heat transfer. This difference in the way energy is delivered can result in many potential advantages to using microwaves for processing of materials. Because microwaves can penetrate materials and deposit energy, heat can be generated throughout the volume of the material. The transfer of energy does not rely on diffusion of heat from the surfaces, and it is possible to achieve rapid and uniform heating of thick materials. In traditional heating, the cycle time is often dominated by slow heating rates that are chosen to minimize steep thermal gradients that result in process-induced stresses. For polymers and ceramics, which are materials with low thermal conductivities, this can result in significantly reduced processing times. Thus, there often is a balance between processing time and product quality in conventional processing. As microwaves can transfer energy throughout the volume of the material, the potential exists to reduce processing time and enhance overall quality.
In addition to volumetric heating, energy transfer at a molecular level can have some additional advantages. Microwaves can be utilized for selective heating of materials. The molecular structure affects the ability of the microwaves to interact with materials and transfer energy. When materials in contact have different dielectric properties, microwaves will selectively couple with the higher loss material. This phenomenon of selective heating can be used for a number of purposes. In conventional joining of ceramics or polymers, considerable time and energy is wasted in heating up the interface by conduction through the substrates. With microwaves, the joint interface can be heated in-situ by incorporating a higher loss material at the interface [1]. In multiple phase materials, some phases may couple more readily with microwaves. Thus, it may be possible to process materials with new or unique microstructures by selectively heating distinct phases. Microwaves may also be able to initiate chemical reactions not possible in conventional processing through selective heating of reactants. Thus, new materials may be created.
In recent literature, many researchers report non-thermal phenomena that have been broadly termed “microwave effects”. Examples of the microwave effect include enhanced reaction rates of thermosetting resins during microwave curing [2] and faster densification rates in ceramics sintering [3]. Although there is considerable debate over the existence of microwave effects, many papers present unexpected results that do not seem to be a consequence of reduced thermal gradients possible within microwave processed materials. Critics of the microwave effect often claim that differences can be attributed to poor temperature measurement and control of experimental conditions that result in systematic error. The existence (or non-existence) of microwave effects continues to be an area of considerable debate and research. Some current literature on the microwave effect is reviewed in later sections.
Although direct heating by microwaves can offer advantages over conventional heat transfer, the different mechanism of energy transfer in microwave heating has also resulted in several new processing challenges. Because energy is transferred by the electromagnetic field, non-uniformity within the electromagnetic field will result in non-uniform heating. As materials are processed, they often undergo physical and structural transformations that affect the dielectric properties. Thus, the ability of microwaves to generate heat varies during the process. Sharp transformations in the ability of microwaves to generate heat can cause difficulties with process modeling and control. Understanding the generation, propagation, and interaction of microwaves with materials is critical. Because the processing equipment determines the electromagnetic field, the design of microwave equipment is particularly important. The properties of the electromagnetic field, chemical composition of the material being processed, structural changes that occur during processing, size and shape of the object being heated, and the physics of the microwave/materials interactions all complicate microwave processing.
Recent interest in microwave processing of materials is highlighted by the number of recent symposia that have been dedicated to microwave processing of materials. To date, the Materials Research Society (MRS) and the American Ceramic Society have held nine symposia that have focused on microwave processing of materials [4], [5], [6], [7], [8], [9], [10], [11], [12]. In addition to expanding the published literature on microwave processing, these symposia have addressed many of the difficulties associated with microwave processing. The recent research in microwave equipment design, microwave/materials interactions, and materials processing continues to expand interest in microwave techniques.
The purpose of this article is to offer an overview of the fundamentals of microwaves, processing equipment, and microwave/materials interactions. Some recent applications of microwave heating to materials processing are also reviewed. It is hoped that this paper will benefit those who are interested in the fundamentals of microwave processing or in the recent advancements in this developing field. The review of literature in this article is not intended to be inclusive, and readers interested in the subject should refer to the bibliography, which gives the key sources of information.
Section snippets
Microwaves
Microwaves belong to the portion of the electromagnetic spectrum with wavelengths from 1 mm to 1 m with corresponding frequencies between 300 MHz and 300 GHz. Within this portion of the electromagnetic spectrum there are frequencies that are used for cellular phones, radar, and television satellite communications. For microwave heating, two frequencies, reserved by the Federal Communications Commission (FCC) for industrial, scientific, and medical (ISM) purposes are commonly used for microwave
Electromagnetic theory
Microwave furnaces consist of three major components: the source, the transmission lines, and the applicator. The microwave source generates the electromagnetic radiation, and the transmission lines deliver the electromagnetic energy from the source to the applicator. In the applicator, the energy is either absorbed or reflected by the material. The theoretical analysis of each of these microwave components is governed by the appropriate boundary conditions and the Maxwell equations:
Microwave/materials interaction
Energy is transferred to materials by interaction of the electromagnetic fields at the molecular level, and the dielectric properties ultimately determine the effect of the electromagnetic field on the material. Thus, the physics of the microwave/materials interaction is of primary importance in microwave processing. The interaction of microwaves with molecular dipoles results in rotation of the dipoles, and energy is dissipated as heat from internal resistance to the rotation. In the following
Ceramics and ceramic matrix composites
An area of microwave processing that has received a lot of attention is ceramic processing. Because ceramics have low thermal conductivities and are processed at high temperatures, many researchers have attempted to take advantage of volumetric heating for sintering, chemical vapor infiltration (CVI), and pyrolysis of polymeric precursors. Other applications include joining [18], [19], [49] fiber processing [27], [28], and plasma pyrolysis [50].
Polymers and polymer matrix composites
Because polymers and their composites also have low thermal conductivity, many of the technical challenges associated with conventional processing of ceramics also exist for polymers. Consequently, there has been much research in the area of microwave processing for polymers and composites. A major barrier in the use of thermosetting composites in many applications is the long cure and post-cure processing times required to achieve the required mechanical properties. As in the case of ceramics,
Conclusions
Microwave processing is a relatively new development in materials processing. The ability of microwaves to couple energy directly to the material is the primary advantage of microwave processing as compared to conventional techniques. The volumetric heating ability of microwaves allows for more rapid, uniform heating, decreased processing time, and often enhanced material properties. The application of microwave heating to the manufacturing of ceramic and polymeric materials has the potential
Acknowledgements
This work was supported by the ARO/URI program at the University of Delaware. Dr. Andrew Crowson is the program director of the ARO/URI program.
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