The use of microwave energy to process a wide variety of ceramic, polymeric and composite materials offers many new and exciting opportunities. Although microwave processing is well established in the food, rubber, and textile and wood products industries, it is a relatively new tool for processing inorganic and other organic materials, chemicals, and certain minerals. Much of its potential has yet to be exploited and carried through to industrial and commercial applications.

Microwave processing is fundamentally different from other heating sources such as conventional ovens and furnaces, lasers, plasmas, and electron beams, where heat is applied externally to the surface of the material, and where time is required for the heat to penetrate the cooler, interior regions via conduction or radiation.

Microwave irradiation penetrates and simultaneously heats the bulk of the material. This feature, along with other microwave characteristics, offers new and unique opportunities to process materials with greater flexibility, to improve quality and product properties, to create or synthesize entirely new types of materials that cannot be produced by alternative methods, and to do all this with greater speed and energy savings.

In recent years, the materials community has become increasingly aware of the potential for microwave processing, and there is now an active R&D effort worldwide to explore, develop, and exploit this potential. In 1988 the Materials Research Society held the first international symposium on microwave processing of materials to review and publish some of these new developments. Every year since then, MRS and the American Ceramic Society have alternated sponsoring symposia on this topic.

Computer modeling and simulation of microwave processing systems provide significant advantages that are expected to play an important role in the development, scale-up, commercialization, and effective utilization of microwave heating technology. These advantages include the following:

• Understanding of the physical aspects of microwave interaction with materials. Microwave heating is a complex process, and computer modeling may help identify the role of materials properties, dimensions, input power, heating rates, etc. on the effective use of microwave heating procedures. Hybrid microwave heating often refers to the use of SiC rods, or lining the insulation surrounding the sample with thin SiC coatings, to help stimulate the microwave heating process. This is particularly important when microwaves are used to sinter low-loss materials that do not effectively absorb electromagnetic radiation. This complex hybrid heating process may lead to unpredictable results in routine microwave heating experiments; and many aspects of its procedure may be simulated, evaluated, and possibly optimized using computer modeling. For example, the number, dimensions, and locations of SiC rods in a typical picketfence arrangement and the thickness of SiC sheets that may be used to allow effective stimulation of the heating process, while still not shielding the sample from the microwave energy, may be better understood, and guidelines for experimentations may be developed using numerical simulation.

As applications for polymeric materials continue to expand, cost-effective and environmentally responsible processing methods become vitally important. It is also becoming increasingly apparent that the success of some new materials is tightly coupled to their processing technologies because the development of their properties depends directly on the processing. In thermoplastic-toughened thermoset resins, for example, morphology development during processing is critical to the mechanical performance of the final material.

Microwave processing has enjoyed growing interest primarily because of the potential of uniformly heating relatively large volumes of materials very rapidly; this overcomes limitations due to the poor thermal conductivity of polymeric materials. Microwave radiation has a very large penetration depth in polymeric materials, and absorption follows a simple Beer-Lambert Law. Hence, depending on the material, very uniform heating profiles throughout the thickness of a material can be obtained up to 0.5 m. This bulk absorption of energy allows for substantially higher heating rates than can be obtained using conventional or infrared methods and for the retention of an acceptable heating profile. Other advantages of this technology include (1) more efficient heating and thus efficient use of energy (only the part is heated, not the walls and surrounding air); (2) shorter processing time for batch operations; (3) shorter process lines for continuous operations; (4) environmental superiority; (5) production of properties or morphologies not attainable using other processing techniques; and (6) the possibility of selective heating—the selective absorption of energy by one phase or part of a material—leading to a custom temperature profile within a part.

Highly absorbing ceramics can be heated in a standard multimode microwave oven. However, most of the microwave energy is dissipated in their ceramic surfaces and heat is transported slowly throughout the sample volume by the usual thermal conduction mechanisms. As with conventional methods, this process leads to non-uniform heating. In contrast, ceramics that are fairly transparent to microwave energy are difficult to heat in standard microwave ovens. When they can be heated, reverse temperature gradients often develop. Reverse temperature gradients can be beneficial in some processes, e.g., combustion synthesis, chemical vapor infiltration (CVI) and binder burnout, but in most ceramic processes, temperature gradients are undesirable. These characteristics have presumably been partially responsible for the ceramic industry's failure to examine microwave processing more closely.

The ability of dipoles to respond to the oscillating electric field and their subsequent inability to keep up with rapid reversals in the field result in dielectric losses that are manifested as heat. While some of the mechanisms by which electromagnetic radiation interacts with ceramics are well known, interaction mechanisms in the microwave frequency range are less well understood. Since several mechanisms, including electrical conductivity, can operate simultaneously, the total energy dissipation and heat generation depends on the effective loss factor. The effective loss factor (ε″eff) is described by

where ε′ is the dielectric constant and is a measure of the extent of polarization, and tan δ indicates how responsive the dipoles are to the oscillating field.

Ceramic joining has become an area of widespread and increasing engineering importance as the aerospace, automotive, chemicals, metals, and utilities industries seek improved performance at higher temperature and in more corrosive service environments. The brittle nature of ceramics, the difficulty of fabricating near net-shape components, the expense of ceramic machining (requiring diamondtipped cutting tools), and the lack of reliable in-process nondestructive evaluation (NDE) methods have driven the development of a variety of joining techniques. This article discusses recent advances in ceramic joining using microwave energy as a fast, efficient means to obtain joints indistinguishable from as received material.

Ceramic materials of engineering interest include oxides such as Al2O3 and Al6Si2O13 (mullite), Si3N4, and SiC. Joining applications include attachment of components such as piston liners, combustor cans and turbine rotors in high-efficiency engines; fabrication of tube assemblies for high-temperature, high-pressure heat exchangers and temperature and corrosion resistant radiant burners; and installation of hermetic seals for vacuum system components. The most common joints are mechanical attachments which take advantage of the coefficient of thermal expansion (CTE) difference between ceramics and metals. Ceramic-ceramic and ceramicmetal joining methods using metallic, glassy, and glass-ceramic interlayers and diffusion bonding have also been successfully demonstrated in recent years.

The ability to provide rapid and volumetric heating of the parts to be joined makes microwave joining attractive. The feasibility and simplicity of this approach was first demonstrated at Los Alamos National Laboratory in 1985 by Meek and Blake, who used a home microwave oven to join Al2O3 to itself and to metal with a borosilicate glass interlayer.

Microwave heating offers the unique potential for selectively heating a strongly microwave-absorbing material within a nonabsorbing matrix, providing the mixture is homogeneous and the absorbing phase is diluted enough to retain a penetration depth sufficient for volumetric heating. Mainly for this reason, microwave processing opens up a way to produce entirely new materials and microstructures that cannot be processed by conventional methods. By proper selection of raw materials and microwave heating conditions, processes can be tailored to produce desired materials and structures, such as powders, compacts, or functional gradient materials, as described in this article.

Synthetic powders with controlled morphology, agglomerate structure, and composition are necessary to improve the reliability of ceramic materials. A novel approach to synthesizing such powders is based on microwave heating of metalorganic precursor compounds. In contrast to conventional precursor-based methods for powder syntheses, which use diluted solutions of precursor compounds, the main objective of microwave processing is to develop a method for direct pyrolysis of the precursor compound into a ceramic powder, produced by simultaneous decomposition of the precursor within the whole volume of the reaction mixture. Besides a simple pyrolysis of a single precursor compound, decomposition of precursor mixtures is also possible, eventually followed by further reaction of the components. Furthermore, an inert or reactive powdery matrix-material can be impregnated with the precursor and then converted to an “alloyed” ceramic powder.

A new method for microstructural modeling of ceramic materials combines precursor compounds with heating by microwave radiation to introduce a second phase and to control grain growth.

Microwave processing technology has taken on increasing significance in manufacturing. Numerous equipment configurations that take advantage of heating by coupling microwave radiation to materials have been developed for curing, bonding, and sintering materials. Such industrial microwave systems generally operate at centimeter wavelength frequencies of 0.915 or 2.45 GHz. Although they offer new methods of industrial processing, their application is often limited by the degree of coupling of the microwave energy to the materials to be processed.

Recently, millimeter wavelength generators have become available. They have enabled scientists and engineers to devise new methods for processing materials. Radiation in these superhigh frequency (SHF) domains interacts differently with materials than that at centimeter wavelengths. For many materials, the absorption of microwave energy at millimeter wavelengths is significantly greater than at 0.915 or 2.45 GHz. This in turn offers new opportunities for thermally processing materials in significantly shorter processing times, or for modifying their properties.

The recent collapse of the Soviet Union has opened previously closed laboratories and industrial facilities. Discoveries and equipment that were once closely guarded secrets are now being shown and shared. Institutes, which only a few years ago would have been suspicious of Western contacts now welcome potential investors, licensers, or co-workers. Some of these institutes have made significant advances in microwave instrumentation and its application to industrial processing. The Institute of Applied Physics in Nizhny Novhorod (Gorky), Russia, has developed high-power gyrotrons that operate at millimeter wavelengths.