Microwave Processing of Materials (1994)

The purpose of this report is 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 Department of Defense and National Aeronautics and Space Administration research and development activities in microwave processing.

Microwave processing of materials is a developing technology that has proven useful in a number of applications. However, microwave processing has not always been as successful as proponents of the technology had hoped. Due to the complexity of microwave interactions with materials, the successful application of microwave processing places a heavier demand on the user to understand the technique than does conventional heating. The blind application of microwave processing will likely lead to disappointment; however, wise application may have greater advantages than had been anticipated. Materials processors are becoming more sophisticated at tailoring the material to the manufacturing process in order to make full use of the capabilities of microwaves.

Much of the body of research in microwave processing of materials is exploratory in nature, often applying to particular materials, sample sizes and geometries, equipment, and processing methods. While providing valuable empirical information, these studies have not advanced the general understanding of microwave processing as much as studies more grounded in the fundamental interactions between microwaves and materials that could more readily lead to scalable, repeatable production processes. In this report, the committee seeks to develop an understanding of microwave processing, starting with fundamental interactions, that includes process development, equipment selection and design, materials evaluation, and applications development.

This report has two primary goals. The first is to provide the information needed for a basic understanding of microwave processing technology and of the strengths and limitations of microwave processing in order to assist materials processors in making wise decisions in using microwaves. Examples of successful applications will be presented to help create an understanding of the conditions necessary for success. The second goal is to identify research and development that will be crucial to the enhancement of microwave processing of materials.

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.

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

models and to provide guidance in designing proper systems for practical use. There is also the barrier between the people who understand electromagnetism, wave theory, and microwaves and the materials specialists who are, in general, limited in their understanding of electromagnetic wave theory and therefore inhibited in utilizing the technology. This report is intended to alleviate these inhibitions by explaining the fundamentals of microwave interaction with materials and by describing and explaining the systems that are used to apply the microwaves to materials, both in a way that will promote an understanding of the basics for the materials processor.

When an electric field interacts with a material, various responses may take place. In a conductor, electrons move freely in the material in response to the electric field, and an electric current results. Unless the material is a superconductor, the flow of electrons will heat the material through resistive heating. However, microwaves will be largely reflected from metallic conductors, and therefore such conductors are not effectively heated by microwaves. In insulators, electrons do not flow freely, but electronic reorientation or distortions of induced or permanent dipoles can give rise to heating. The common experience of using microwaves to heat food is based primarily on the dipole behavior of the water molecule in the food and the dipole's interaction with microwaves. Because microwaves generate rapidly changing electric fields, these dipoles rapidly change their orientations in response to the changing fields. If the field change is occurring near the natural frequency at which reorientation can occur, a maximum in energy consumed is realized, and optimum heating occurs. In the terminology of microwave processing, when this happens it is said the material is well "coupled."

The material properties of greatest importance in microwave processing of a dielectric are the permittivity (often called the dielectric constant), , and the loss tangent, tanδ. A more thorough discussion of materials properties and interactions with microwaves is included in Chapter 2. The complex permittivity is a measure of the ability of a dielectric to absorb and to store electrical potential energy, with the real permittivity, , characterizing the penetration of microwaves into the material and the loss factor, , indicating the material's ability to store the energy. The most important property in microwave processing is tanδ, indicative of the ability of the material to convert absorbed energy into heat. For optimum coupling, a balanced combination of moderate , to permit adequate penetration, and high loss (maximum and tanδ) is required.

The trick in microwave processing is to find a material that is polarizable and whose dipoles can reorient rapidly in response to changing electric field strength. Fortunately, many materials satisfy these requirements and are therefore candidate materials for microwave processing. However, if these materials possess poor thermal conductivity, heat does not rapidly dissipate into the surrounding regions of the material when a region in the solid becomes hot. This difficulty is compounded, because the dielectric loss frequently increases dramatically as the temperature increases. Thus, the hot region becomes even hotter, sometimes resulting in local melting. These "hot spots" are a major difficulty and have led to the use of hybrid systems, combining microwave heating with other heat sources to reduce uneven heating.

There is another consideration that is often overlooked in microwave processing of materials. Microwaves are generated by devices requiring electrical energy. Electrical energy is generated primarily from fossil fuels. The conversion of the energy in the fuel to electrical energy is less than 40 percent efficient. In addition, microwave generators (magnetrons, etc.) are not generally better than 85 percent efficient in converting electric power to microwaves, and the microwaves are not perfectly coupled to the material (90 percent coupling would be very good), so the total energy generated is probably less than 30 percent of the energy content of the fossil fuel used in generating the electricity. This means there are real limitations to the economics of bulk heating. Direct heating with fossil fuels makes much more efficient use of energy, and microwaves can only be economically competitive when electric heating is mandated or the selective heating capability of microwaves, or some other factor, more than compensates for the inefficiency of electric heating. An example, discussed later in this report, is the removal of volatiles from soil, where it is not necessary to heat the soil as is required when heating by conventional means.

The successful use of microwaves requires the processor to have a good understanding of the strengths and limitations of microwaves. Among the strengths are penetrating radiation, controllable electric field distributions, rapid heating, selective heating, and self-limiting reactions. But, simply putting a material into the microwave oven and "zapping" it in the hopes of solving a problem is risky. The materials processor must understand and match the special capabilities of microwave processing to the material and the properties required in order to design an appropriate process. In some cases, incomplete understanding exists, requiring research to improve the knowledge base for using microwaves to process materials.

The information contained in this report is intended to ease the work of those interested in using microwaves to solve a problem or improve a current process. Examples of successful applications are given to illustrate the characteristics of a material and process that are amenable to microwave processing. An equipment section describes the alternatives available and the "setup" required to apply the microwaves to the material. Economic considerations are described, and where possible, costs are provided as an aid in determining the economic consequences of using microwaves. A theory section is provided to help both the materials processor and the equipment designer understand the fundamental limitations and advantages of microwaves in the processing of materials. In addition, the limitations in present understanding are delineated as a caution to users and as a guide for future research activities.