ZnO varistors are electronic ceramics whose electrical behavior is dominated by grain boundary effects. A careful evaluation of the effect of microwave sintering compared to conventional sintering revealed no significant differences between varistor devices processed by these two methods. A slight apparent enhancement of grain growth was observed for microwave processing, but further evaluation of this effect leads us to believe that it arises from a small systematic shift in temperature derived from the different experimental configurations.

Some kinetics parameters of alumina during microwave sintering were studied and compared with that during conventional sintering. The results demonstrated that the sintering rates for microwave processing were much greater than that for conventional processing, and the grain growth of alumina was rapid with prolonged time at high temperature in a microwave field. It was indicated that the microwave sintering at higher temperatures for a shorter time was favorable for preparing high density and fine-grained alumina ceramics.

The Si3N4 ceramics(91wt%Si3N4+5wt%Y2O3+4wt%Al2O3) of room temperature strength 620--760 MPa and fracture toughness 7 MPa-m½ were prepared by microwave sintering and their microstructure and mechanical properties were studied.The experiment results show that the higher N2 pressure (>4atm) is very necessary for microwave sintering of Si3N4 ceramics; microwave sintering can greatly decrease the sintering temperature to 1500* C, and increase the transformation rate of α to β-Si3N4,and reduce the total sintering time to about 1 hour. The mechanical properties of the Si3N4 ceramics sintered by microwave are better than that by conventional pressureless sintering.

Alumina ceramics of different purities were joined using alumina gel as an interlayer and microwave energy as the heating source. Direct microwave joining of alumina was also accomplished. A comparison between the two joining methods was made. The effect of the gel on joining time and joining temperature was determined. Knoop microhardness was measured across the joint to show the change in the microhardness at the joint region. Joints of hardness as high as that of the bulk were produced when the joining parameters were properly selected. The dielectric properties of the aluminas used in this study were measured. X-ray maps and SEM micrographs were taken for different joints.

Compact samples of alumina and polymethyl methacrylate have been heated in a 2.45 GHz microwave cavity and by conventional heating in an electric furnace. Various heating schedules have been used to effect the removal of the polymeric binder by thermal decomposition. Dielectric properties, porosity and other physical properties have been investigated in order to better understand the binder removal process in a microwave field. Results of the study emphasize the amount of carbon residuals remaining in the bulk.

Sodium alumino-silicate glasses having a constant concentration of alkali ions (15mol%) were prepared. The interdiffusion of potassium ions, K+, for sodium ions, Na+, in the glass composition having AI/Na= 1 was investigated in the presence and absence of a microwave field. Characterization consisted of dielectric loss measurements, electron microprobe analysis (EMP), and microhardness indentation.

Traditionally the heating and curing of cellular concrete takes place in an autoclave. This building material is a thermal insulator so the curing and drying processes are energy and time consuming. Volumetric heating is an attractive method for improving the throughputs.

A feasibility study leads to the conclusion that microwave processing is not likely to be applied in a large scale production process.

Attempts are being made to exploit the inverse temperature profile which can be developed with microwave heating with respect to the processing of certain advanced ceramics. This paper discusses the results obtained to date during the microwave sintering of YBCO high-Tc superconductors and the microwave reaction bonding of silicon nitride.

Morphology, chemistry and microstructure of zirconia and alumina based ceramic powders synthesized by microwave plasma pyrolysis were studied by transmission electron microscopy. Electron diffraction was used to determine the phases present, while conventional bright field and dark field imaging were used to characterize grain size and density variations. Materials analyzed in this study were either pure zirconia, pure alumina, or a ternary zirconia-yttria-alumina oxide. Solid spheres, hollow spheres, and fragments of hollow spheres were observed. All three modifications of zirconia were observed in the pure zirconia powder. In alumina, the predominant phase was cubic, gamma alumina. In the ternary oxide, most structures exhibited cubic zirconia diffraction patterns. Energy dispersive x-ray spectroscopy revealed chemical inhomogeneities in the ternary oxide powder.

Electromagnetic radiation in the microwave range has been utilized for the fabrication of SiC particulates-reinforced aluminum matrix cermets. Powder mixtures with compositions ranging from 70 wt% Al - 30wt% SiC to 100 wt% Al were placed and processed under protective nitrogen atmosphere in a 750 W multimode cavity operating at 2.45 GHz. Temperatures larger than 800°C have been achieved in a time varying with the mixture composition. Microstructures and some physical properties of the composites are presented.

Pyrolytically prepared ceramic powders usually exhibit excellent homogeneity and small grain sizes. The energy efficiency of those methods for synthesis is, particularly in systems heated electrically, very poor. The situation can be improved drastically using a microwave plasma as a source of energy. This has been confirmed by the synthesis of alumina- and zirconia-based ceramic powders for which an efficiency of more than 80% was found. Aqueous solutions of nitrates of zirconium, yttrium, and aluminum were used as starting materials. Electron microscopy revealed that, through proper selection of the experimental conditions, it is possible to obtain nanocrystalline powders with cubic structure. Because of the extreme conditions during synthesis, it is possible to prepare solid solutions in systems without solubility.

A concise overview of the use of microwave heating in organic synthesis is presented. The role of solvents and representative examples from the relevant literature are discussed to illustrate the significant impact this non-conventional heating mode continues to have in this area of materials research.

Graphite fiber-thermoset epoxy resin composite materials are frequently used in aerospace and related industries. Electromagnetic heating may have some advantages over the present autoclave techniques, especially for large objects. However, the loss factor of graphite composites at microwave frequencies (915 MHz and 2.45 GHz) is so high that the depth of penetration is no more than about 0.1 mm. Arcing may be encountered at high heating rates due to strong transverse E-fields. Also, since the curing process is exothermic and the surface is at elevated temperature, thermal runaway may be encountered in the interior spaces and result in over-cured (thermally damaged) material.

We have investigated the heating obtained at radio frequency (nominal 27 MHz) in magnetic induction fields in both experiments and numerical models. Inductive coupling has the advantage that arcing is virtually eliminated, even at high heating rates, for typical cylindrical geometries. Also, induction heating easily accomplishes lap seam welds in thermoplastic composites owing to favorable electric field boundary conditions. Transient temperatures were recorded during heating by optical fiber probes and heating distribution was measured, where possible, using thermographic imaging. Finite difference electrical and thermal model results illustrate the dominance of conduction heat transfer in these materials.

Microwave reflection resonator sensors have been developed to monitor the dynamic, in situ real and imaginary dielectric components (ε′, ε″) of thermoset polymer matrix composites and thermoplastics. These reusable and noninvasive sensors are conformably mounted in the autoclave mold in such a manner that the EM fields are strongly coupled to the resin. Tracking of (ε′, ε″) during the cure provides information about the chemical kinetics such as timing the point of minimum viscosity and monitoring the relative cure index. The sensor is readily adjustable for optimum coupling to the type of material being tested and is rated for temperatures up to 250°C (480*F). The technique is complementary to other analysis techniques such as Differential Scanning Calorimetry (DSC) and Dynamic Mechanical Analysis (DMA). A prototype software and electronics package has been developed to support the sensor.

The microwave heating of composites with complex shapes has been studied. A V-shaped mold was used to fabricate samples which do not lie parallel to the electric field produced in a 7-in. cylindrical microwave applicator operating at 2.45 GHz.

A polyester/glass composite and a polyester/graphite fiber composite were produced for comparison. Heating profiles were made more uniform by applying mode-switching techniques to the processing. Mode-switching is the application of more than one electromagnetic mode to achieve uniform heating.

The samples were tested for heating patterns in different geometric and fiber orientations within a single-frequency, cylindrical microwave applicator operating at 2.45 GHz. The temperature profile for each mode was recorded. Mode-switching provided better temperature profiles than single modes. Modeswitching techniques were more successful with the graphite fiber reinforcement.

Microwave curing of composite materials is a novel technology which can be an alternative to conventional thermal curing and has many advantages over thermal curing, i.e, fast and direct heating, selective heating, and controllability. Currently, the existing technology is more empirical and cannot be easily described by traditional process models. One key enabling step for realizing the advantages of the microwave curing technology is a capability for utilizing all the available existing knowledge in a concise manner. The major elements of a control system which utilizes traditional control techniques and (non-traditional) expert systems techniques in a real time microwave cure monitoring and control is discussed.

Microwave processing has been investigated as an alternative to the conventional processing of polymers and polymer matrix composites. Different studies have demonstrated its viability and advantages, but various aspects related to its industrial applicability are still being investigated.

Two aspects related to the microwave curing of glass fiber/vinyl ester-vinyl toluene composites have been studied. A qualitative evaluation was done to determine the reproducibility of heating patterns of these laminates in different modes of a microwave resonant cavity. The samples were prepared to similar physical and chemical specifications and heated in different single modes of the cavity.

Another evaluation was done to compare the mechanical properties of microwave and thermally cured laminates subjected to flexural load. The samples were processed using similar cure cycles. The microwave samples were cured using a mode-switching technique and the thermal samples were cured in a temperature controlled oven. The void fractions of the samples were evaluated to develop pre-consolidation conditions that yield samples with low void fraction. Results of these two evaluations are discussed.

A process model was developed and coded in FORTRAN to simulate the temperature and the extent of cure profiles across the thickness of the composite during microwave and thermal processing. A simplified five-parameter microwave power absorption model was developed for microwave processing. The computer code was used to predict the behavior of Hercules AS4/3501-6 prepreg during thermal processing using various laminate thickness and boundary conditions. The simulation results show that 4.0 cm is the thickest composite that can be processed with the manufacture's cure cycle. The advantages of microwave processing over thermal processing are fully demonstrated.

The distinct phases in the production process of a foam material are foaming, curing and drying. Cellular materials are insulators to conventional heat. This paper discusses the effectiveness of microwave or high frequency energy for processing a layer of foam material on for example a conveyor belt. The emphasis is on the thermal and drying time constants and on the specific energy consumption. It is demonstrated that a limit is set on time gain when using microwaves.

A case study for applying microwaves in an industrial production line of foam rubber for mattresses according to the above principles is presented.

Dielectric heating technologies (microwave and radio-frequency, RF) bring unique capabilities to the processing of hazardous wastes. Potential opportunities for using these techniques can be described by classifying and matching waste streams to the particular features of dielectric heating. Microwave and RF applications for processing hazardous waste are at various stages of development and include: solvent recovery, regeneration of activated carbon, pyrolysis of rubber tires, soil remediation, and disinfection of hospital wastes. Other solutions to environmental problems can include the replacement of conventional fossil-fired processes (eg, RF-assisted post-baking of biscuits) and microwave heating of catalyst substrate materil for automotive emission control. Examples of dielectric heating systems that are processing hazardous waste are used to illustrate the benefits of these technologies and to characterize future opportunities.