Microwave processing of materials is a technology that can provide the material processor with a new, powerful, and significantly different tool to process materials that may not be amenable to conventional means of processing or to improve the performance characteristics of existing materials. However, due to the complexity of microwave interactions with materials, simply placing a sample in a microwave oven and expecting it to heat efficiently will seldom lead to success.
Microwaves are electromagnetic waves in the frequency band from 300 MHz (3 × 108 cycles/second) to 300 GHz (3 × 1011 cycles/second). Industrial microwave processing is usually accomplished at the frequencies set aside for industrial use, 915 MHz, 2.45 GHz, 5.8 GHz, and 24.124 GHz.
First controlled and used during the second world war in radar systems, the usefulness of microwaves in the heating of materials was first recognized in 1946. Raytheon introduced the first microwave oven to the marketplace in 1952. During the past two decades, the microwave oven has become a ubiquitous technology, present in more than 60 million homes. Despite this long history and widespread use, there still remains a great deal that is not fully understood about microwaves and their use.
The Department of Defense and the National Aeronautics and Space Administration requested that the National Materials Advisory Board of the National Research Council conduct a study to (1) assess the current status of microwave processing technology; (2) identify applications of microwave technology where resulting properties are unique or enhanced relative to conventional processing or where significant cost, energy, or space savings can be realized; and (3) recommend future activities in microwave processing. The Committee on Microwave Processing of Materials: An Emerging Industrial Technology was established to conduct this study.
A large investment has been made over many years in the development of microwave processing systems for a wide range of product applications. In general, microwave processing systems consist of a microwave source, an applicator to deliver the power to the sample, and systems to control the heating. Microwave generators are generally vacuum tubes, but solid state devices are sometimes used. The magnetron is the most common microwave source in materials processing applications. Microwave energy is applied to samples via microwave applicators. The most common applicators are multimode (e.g., home ovens), where numerous modes are excited simultaneously, and single-mode, where one resonant mode is excited.
Control of temperature in microwave heating processes is generally accomplished through variation of input power or through pulsed sources.
Microwaves possess several characteristics that are not available in conventional processing of materials, including:
controllable electric field distributions;
selective heating of materials through differential absorption; and
These characteristics, either singly or in combination, present opportunities and benefits that are not available from conventional heating or processing methods and provide alternatives for the processing of a wide variety of materials, including rubber, polymers, ceramics, composites, minerals, soils, wastes, chemicals, and powders. The characteristics of microwaves also introduce new problems and challenges, making some materials very difficult to process. First, bulk materials with significant ionic or metallic conductivity cannot be effectively processed due to inadequate penetration of the microwave energy. Second, insulators with low dielectric loss factors are difficult to heat from room temperature due to their minimal absorption of the incident energy. Finally, materials with permittivity or loss factors that change rapidly with temperature during processing can be susceptible to uneven heating and thermal runaway. While the use of insulation or hybrid heating can improve the situation, stable microwave heating of these types of materials is problematic.
The committee found that efforts in microwave process development that succeeded commercially did so because there was a compelling advantage for the use of microwave energy. Failure almost always resulted from simple, general causes e.g., trying to process materials that were not conducive to microwave absorption or trying to use equipment that was not optimized for the particular material and application.
The most likely candidates for future production-scale applications will take full advantage of the unique characteristics of microwaves. For example, chemical vapor infiltration of ceramics and solution chemical reactions are enhanced by reverse thermal gradients that can be established using microwaves. Polymer, ceramic, and composite joining processes and catalytic processes are enabled by selective microwave heating. Powder synthesis of nanoparticles can take full advantage of rapid microwave heating to produce unique formulations and small particle sizes. Thermoplastic composite lamination and composite pultrusion processes are enhanced by rapid and bulk heating and by the ability to tailor the material's dielectric properties to microwave processes. The potential for portability and remote processing also make microwave processing attractive for waste remediation.
Due to the high cost of microwave generators and the relatively poor efficiency of electric power for heating applications, factors other than energy generally account for savings realized from microwave processing. Such factors include process time savings, increased process yield, and environmental compatibility.
KEY FINDINGS AND CONCLUSIONS
The future of microwave processing of materials appears to be strongest in specialty applications, and it will probably be of limited usefulness as a general method of producing process heat. Within the specialized areas, microwave processing has distinct advantages over conventional processing means. Microwave processing will not be applicable to all materials and in fact may be readily applicable only to certain types of materials.
Failure to realize expected benefits from microwave processing is a result of inadequate interaction among researchers, materials engineers, process designers, and microwave engineers. In most cases, the basic equipment (e.g., generators, applicators, power supplies) for microwave processing applications is commercially available. However, the methodology for system integration, including system design, special applicator design, rapid equipment prototyping, and process control, is inadequate. It must be recognized that samples cannot be heated efficiently and uniformly if simply placed in a microwave oven without consideration of specific microwave/materials interactions.
The development of hybrid heating systems that optimally combine microwave sources with conventional sources to balance process variables such as required power, process flow time, tooling requirements, etc., represents a very promising, largely untapped area in process development. Hybrid heating may be provided actively, using a separate conventional heat source, or passively, using higher dielectric loss susceptors, insulation, or coatings that more readily absorb the incident power. Development of hybrid heating systems may be required for full realization of the benefits of microwave technology.
Most of the current research has focused on laboratory-scale, exploratory efforts. In order to realize the potential benefits of microwave and hybrid processes, work is needed to scale-up process and system designs to large-batch or continuous processes. Process scaling includes model simulation, system design and integration, and an understanding of the costs and benefits involved in moving to production scale.
An important element of microwave process development and system design is the capability to model electromagnetic interactions. An understanding of the variation of dielectric properties with temperature and processing state is crucial for simulations and process modeling. Computer modeling can be used to optimize generator or applicator system design, establish achievable processing windows, and conduct realistic process simulations for given dielectric properties, sample size, and desired processing conditions.
Although there is evidence of enhancements of processes due to the effects of microwaves alone (e.g., enhanced ceramic sintering, grain growth, and diffusion rates and faster apparent kinetics in polymers and synthetic chemistry), the evidence is equivocal due to questionable temperature measurement techniques, uncertain process characterization methods, and conflicting evidence.
For particular materials, define the conditions under which microwaves provide uniform, stable processing. These may be developed through appropriate numerical modeling techniques and should be presented as processing charts that contain information on material properties, processing conditions, and specimen size and geometry. This modeling requires characterization of the thermal and physical properties of materials, including thermal conductivity and diffusivity, thermal expansion, and the temperature-dependent dielectric properties. Hybrid heating schemes, in which traditional heating is augmented with microwave heat, should be considered.
Emphasize research work that facilitates the transition of developmental processes to production scale. This may include materials property characterization, process simulation, control schemes, equipment prototyping, and pilot-scale production.
Establish multidisciplinary teams, consisting of materials and process engineers, microwave engineers, equipment designers, and manufacturing specialists, to properly develop microwave processes and procedures.
Provide training in fundamentals of microwave processing technology, including microwave interactions with materials.
Define general specifications for applicator design, and characterize the resulting electromagnetic field to enable users to successfully apply microwaves to materials processing.
Compile existing material-property information on dielectric, magnetic, and thermal properties (including dependence on frequency and temperature) in the range useful in the processing of materials.
Provide more-complete and more-consistent measurements of basic dielectric properties of materials to be processed using microwaves, and develop calibration standards for comparing the various techniques for dielectric properties measurements.
Develop empirically simplified models and ''microwave heating diagrams'' based on measurements and on the extensive data collected from results of numerical simulation to make numerical techniques more accessible to processors.
Establish standards for measurement of temperature to ensure reproducibility. In addition, the techniques and procedures used to measure temperature should be reported in detail, so an evaluation of accuracy can be made. The level of uncertainty in temperature measurements should also be reported. Perform experiments using several techniques for measuring temperature to determine the relative accuracy and reproducibility of the various techniques against a known standard (melting point, phase transition temperature, etc., of well characterized materials).
Develop practical methods to monitor or determine internal temperature and thermal profiles (thermal gradients) within a material during the process cycle.
Conduct detailed and controlled experiments to determine if the "microwave effect" reported for materials is valid. Care should be taken to use a microwave source with predictable and reproducible fields and to have an internal temperature calibration to avoid temperature measurement uncertainties.