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.