Due to the broad range of industries, materials, and processes involved in the application of microwaves, the committee limited the scope of the study to materials processing, especially with regard to industrial materials and advanced materials processes of interest to Department of Defense and the National Aeronautics and Space Administration. Due to the availability of commercial equipment for more mature applications and review articles of previous research, more recent work is emphasized. Developments in these more mature application technologies such as microwave processing of rubber and plasma processing, are reviewed only briefly, emphasizing aspects important to developing commercial applications in other areas. In the case of plasma processing, applications or developments where the use of microwave-frequency plasma had specific benefit were emphasized. The application of microwave processing in wood products, biomedical, pharmaceutical, and food processing industries is not included in the committee's assessment.

PERSPECTIVE

High-frequency heating really started when engineers working on short-wave transmitters contracted artificial fevers. The great virtues of this kind of heat are as follows: The heat is generated directly in the object itself; no transfer of heat is involved. Associated apparatus need not be heated. The surfaces of the material need not be affected. The people who work with the equipment have cooler working conditions. No gases are involved and thus the likelihood of corroded surfaces is eliminated. The material can be heated from the inside-out. Finally, objects of unusual size or shape can be heated.

Scientific American, 1943

It may be useful to provide the reader with some perspective concerning microwave processing in order to facilitate understanding of the more detailed and complete discussion in subsequent chapters. Microwaves were first controlled and used during the second world war as a critical component of radar systems. Although, as described above, the virtues of radio-frequency heating were forecasted earlier, the usefulness of microwaves in the heating of materials was first discovered in 1946, and in 1952 Raytheon introduced the first microwave oven to the marketplace. During the past two decades, the microwave oven has become a ubiquitous technology, with more than 60 million homes having one. Despite this long history and widespread use, there still remains a great deal that is not understood about microwaves.

The principal problems have to do with a lack of understanding, especially by the users, of the basic interactions that occur between materials and microwaves, of the design of equipment to meet the needs of a specific application, and of the inherent limitations (including cost) of microwaves as a processing methodology. Specialists in microwave technology are hindered by an incomplete understanding of some of the basic interactions that occur between materials and microwaves, and they have an incomplete data base to test their theories and



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