As discussed in Chapter 2, the rapid rise in dielectric loss factor with temperature is the major issue in thermal runaway and temperature nonuniformity. The presence of conducting fillers may inhibit microwave heating by decreasing skin depth. The development of microwave transparent glass and quartz reaction vessels and improved pressure-relief valves has been critical in allowing attainment of higher temperatures and pressures than was possible with low-loss teflon vessels (Baghurst and Mingos, 1992b). Subsequent pulses contribute to the system average power based on the pulse repetition rate and the pulse width (i.e. This method has not received much attention from a microwave processing viewpoint. This preferential heating has been shown to provide an enhancement of the interfacial adhesion between the fibers and the matrix resin (Agrawal and Drzal, 1989) and a subsequent improvement in the fracture properties of microwave-processed composite materials. In the case of silicon nitride, porous powder compacts of silicon have been reacted with nitrogen at elevated temperatures of 1150—1450 ºC (Kiggans et al., 1991). This effect has been observed by several researchers (Mingos, 1993; Baghurst and Mingos, 1992a; Neas, 1992b; Majetich, 1992). Click here to buy this book in print or download it as a free PDF, if available. The specimen may be in direct contact with the plasma, or the effluent of the plasma may be utilized in the processing. The duplexer can be a physical switch or a series of transmission lines that perform the switching function. Thermal runaway was avoided if the concentration of TiC was greater than about 20 percent by weight. A Gunn diode oscillator and waveguide are used as a motion detector for automatic door openers (although these are being replaced by ultrasonic devices). plasma processing. The advantages of this promising process are discussed in more detail in a later section of this report. As in the picket fence arrangement, the silicon carbide is initially heated by the microwaves, transferring heat to the specimen. The characteristics of a starting powder (composition, size, structure, shape, etc.) Hybrid heating schemes may find important usage in this regard. Similarly, silicon oxide films grown on silicon appear comparable to those grown by conventional thermal oxidation at 1000 ºC. The microwave energy is transmitted into a waveguide. Fax: (781) 769-5037, For magazine subscriptions and newsletter customer service: T. S. Laverghetta,"Microwaves and Wireless Simplified, Second Edition," Artech House, Norwood, Mass., 2005. A further advantage is that the microwave plasma can heat the substrate to the temperature required for good deposition conditions (greater than 500 ºC). comminution (grinding) and separation. Hence, they are suitable for various unique and specific applications, starting from short-distance to long-distance communication, from detecting a target at a short distance to identifying start light years away from identifying a small object to mapping the surface of a planet, from the study of a linear particle accelerator to that of plasma, from domestic cooking to industrial heating and many more. In some cases, the conventional methods, in addition to being much slower, did not lead to the crystallization of a pure oxide phase, as shown in Table 5-4 for TiO2. Thus, after 50 minutes the surface temperatures ranged from 536—1190 ºC, whereas after 140 minutes the range was from 1540—1610 ºC. The potential for this process is in the refuting industry for the treatment of the sour gas resulting from hydrodesulfurization of hydrocarbon feedstocks. Materials with lower temperature dependence of dielectric loss factor may be heated stably. The reverse thermal gradients and reaction fronts may enable the synthesis of new and unique structures, composition gradients. The high energy of the microwave rotates the polar molecules of water, fat, and sugars of the foodstuff. A broad range of applications will be discussed. To further understand radar concepts, it is necessary to define some additional terminology: Figure 3 Waveforms; continuous wave (a), pulse (b). firing, but the energy costs were higher for microwave heating because of the relative costs of gas versus electric energy. The chief advantages of microwave plasma processing were high efficiency (80 percent) in transferring thermal energy to the chemical reactions; formation of completely crystalline, spherical particles of ZrO2 (100—500 nm in diameter); and a capability to produce solid-solution particles. One of the aspects that should be explored in more detail is the effect of variable frequency on the chemical and physical processes occurring in the microwave plasma and on interactions with the substrate during deposition, etching, and surface modification. Although some advantageous application of microwave processing in sintering has been demonstrated, the perceived potential of the technology has gone largely unrealized on a production scale. Films deposited at temperatures less than 150 ºC have chemical and physical properties equivalent to films deposited at 900 ºC using conventional chemical vapor deposition processes, and the low-energy ion bombardment does not damage the substrate. Similar relaxation processes are observed in dynamic mechanical properties of polymers, with analogous dispersions in real and imaginary components of viscoelastic response (Ward and Chen, 1992). In many cases, packing powders are required, which would only be acceptable for products of very high value. A similar study of the sintering of ferrites resulted in similar conclusions regarding feasibility (Krage, 1981). Nonconductive additives such as glass fibers and nonconducting metal oxides which are used as pigments (e.g., titanium dioxide), can also influence composite properties through preferential heating mechanisms, depending on their dielectric properties. Microwave ovens become common kitchen appliances in Western countries in the late 1970s, following development of expensive cavity magnetrons. Reductions in sintering temperature and activation energy were greater at 28 GHz than they were at 2.45 GHz. With conventional heating, the vessel wall and liquid surface are generally hotter than the bulk. The energy consumption was half that for gas. With this arrangement, these materials were successfully sintered, albeit at fairly low heating rates (2 ºC/min). FIGURE 5-6 Hydrogen sulfide waste-treatment process utilizing microwave plasma dissociation. They are processed well above their glass transition temperatures or melting points (if the material is semicrystalline) to reduce the melt viscosity and allow flow and to promote adhesion. Applicator design to move the processes to production scale on more complex joints is required (Silberglitt et al., 1993). For this reason, there continues to be a significant effort to develop improved and tailorable powders to meet the increasing demands for a wide range of future, advanced ceramic products (Messing et al., 1987, 1988a, b). Since microwaves create volumetric heating, they have been used to initiate internal ignition in mixtures of exothermic powder compacts (Ahmad et al., 1991). This is because the microwave discharge results in a lower acceleration potential between the plasma and the substrate. The promising future of microwave chemistry to the chemical industry is just beginning to be realized. Of several reported attempts to sinter Al2O3 in multimode cavities, the experiments of Patterson et al. The primary motivation for use of microwave heating has been time savings through rapid heating, rather than any nonthermal effects. FIGURE 5-5 Photomicrograph of pyrite ore (a) nonmicrowaved and (b) microwaved, showing stress cracking. to heat uniformly. While temperature measurement may yet be a problem, it is difficult to imagine a 400 ºC error. The enhanced reaction rate corresponded to a reduction in the activation energy for the reaction from 105 kJ/mole to 55 kJ/mole. The most widespread use of microwaves in chemistry is in analytical laboratories. Duty cycle is the ratio of the pulse width, τ, to the pulse repetition time, T or τ/T. Chemical modification of the surface can be achieved with or without adding reactive components in the plasma. Increased density and improved mechanical properties of microwave-sintered B4C were reported. Applications of Microwaves in the field of Power; A microwave passes (non-ionizing) microwave radiation (at a frequency near 2.45 GHz) through food, causing dielectric heating by absorption of energy in the water, fats and sugar contained in the food. Even within the general class of epoxy resins on which a large amount of work has been performed, the reactivity can vary more than an order of magnitude depending on the resin constituents and formulations. A schematic of the microwave dissociation process is shown in Figure 5-6. The characteristic temperature excursion resulting from the exothermic reaction during epoxy cure was eliminated by using a pulsed system that allowed a higher temperature cure without thermal degradation (Jow et al., 1989; Jow, 1988). The processing of industrial wastes is an area of tremendous promise for the application of microwave energy. Jump up to the previous page or down to the next one. Radiometry, spectrometry for the study of naturally propagated EM waves. Microwave sintered and hot isostatically pressed Si3N4 cutting tools showed significantly improved performance compared with commercially available cutting tools (Patterson, 1992b). Microwave chemistry is a rapidly growing field that has been gaining attention recently (IMPI, 1992; EPRI, 1993). The rate of cross-linking is attributed to the higher dielectric loss due to the presence of the filler. Unfortunately, it is difficult to analyze these data further, since the curing reaction can behave autocatalytically. (1992) found no significant difference in the sintering of ZnO varistor materials in microwaves relative to that in conventional firing, and there was no difference in properties. Application ID: 1424. More-general application of polymeric composites has been hindered by their high cost of orientation (layup) and forming (molding and curing) processes. Finally, Day et al. High-performance polymeric composites, reinforced with carbon, glass, or aramid fibers, have been effectively used by the aerospace and electronics industries in applications requiring light weight, high specific strength and stiffness, corrosion and chemical resistance, and tailorable thermal-expansion coefficients. They recognized many of the potential advantages of microwave sintering, including high thermal efficiency as well as rapid processing, and also discovered many of the problems that have plagued the process, including difficulty in temperature measurement due to temperature gradients and the propensity for thermal runaway.