Among them, the approaches [ 4 — 18 ] based on frequency-to-amplitude mapping are most widely studied by researchers. In these systems, by utilizing an amplitude comparison function ACF which is derived by comparing two frequency to amplitude characteristics, microwave frequency can be calculated. In the approaches such as [ 4 — 8 ], the IFM system is realized by using different modulators or different dispersive media.
The ACF curve can be also obtained by detecting the optical power output from a fiber Bragg grating [ 9 ], which possesses high measurement resolution. However, as always, there is a trade-off between measurement range and resolution. The wider the bandwidth of the instantaneous radio frequency estimation, the less accurate the measurement becomes.
To overcome this problem, several photonic IFM techniques with tunable measurement range and resolution have been proposed. One popular solution is by carefully tuning the wavelength of the laser [ 10 , 11 ], so that the ACF can be adjusted. An approach employing two dispersive media to simultaneously generate multiple ACFs can also extend the measurement range and improve the resolution [ 12 ].
The measurement range is adjustable due to the dispersion variation. When large measurement range is demanded, the wavelength spacing of lasers has to be far apart, so as to get a decent ACF. Researchers have started to explore other tuning mechanisms, which substitute for shifting the laser wavelength. A reconfigurable IFM system based on stimulated Brillouin scattering has been reported in [ 13 ]. The measurement range and the resolution are tunable by varying the reference driving frequency.
With bias voltage control, tunable IFM system can be realized by using one DP-MZM [ 15 ] or a polarization modulator PolM [ 16 ] as well, achieving high resolution for frequency measurement. Moreover, tuning polarization angle is also a good choice for tunable IFM system with high flexibility. In [ 17 ], by simply adjusting a polarization controller after a PolM, the measurement range and resolution can be tuned finely.
But the light waves from two lasers are with the same polarization directions and the power fading functions only have slight difference. This calls for a large wavelength spacing regarding to the lasers. Then we have proposed a simplified IFM prototype based on a single laser source and filter-less architecture [ 18 ]. However, the walk off effect in the dispersion compensating fiber is enhanced due to the impact of the two different polarization states. In this paper, we propose a photonic IFM system with tunable measurement range and resolution in order to address the above problems.
To obtain significantly different frequency to amplitude characteristics at two laser wavelengths and generate a sensitive ACF, we encode these wavelengths in two perpendicular polarizations. A polarization-maintaining fiber Bragg grating PM-FBG , which has distinctive transmission profiles at two polarizations, manipulates these two wavelength components independently, despite their small wavelength spacing.
Firstly, two wavelength components are modulated by a PolM. Then one of their optical carriers is filtered out along the two orthogonally-polarized transmission bands of PM-FBG separately. By adjusting the polarization angle before the PolM, the ACF curve can be shifted so that the measurement range is tunable. Our approach eliminates the requirement of shifting laser wavelengths, commonly used in other tunable IFM systems.
The measurement resolution is also improved by dividing the whole measurement range into several sections. Since the wavelength spacing in a single-polarization dual-wavelength laser is small, there is no requirement for large optical bandwidth for the components.
Therefore, the system is affordable, stable, and reliable with consistent characteristics due to its narrowband nature. Owing to the birefringence effect and high integration, PM-FBG exhibits promising applications in many polarization manipulating systems. The schematic setup of the proposed IFM system with tunable measurement range and resolution is shown in Fig 1A. To obtain two wavelength components from the laser, conventionally two lasers are required. But this IFM system employs only one laser with single-polarization and dual-wavelength [ 19 — 21 ].
The two wavelength components output from the laser are orthogonally polarized with a wavelength spacing. One significant advantage of using this laser lies in its capability of being operated in a dual-wavelength mode of single polarization per wavelength, which has a very good stability. The amplitude variation can be smaller than 1. The wavelength variation can be less than 0.
Even if a change in the temperature of the laser will shift the two wavelengths simultaneously, the wavelength spacing and stability will not be affected [ 19 , 20 ]. Additionally, environmental effects and aging processes act likewise on both wavelengths of this dual-wavelength laser. Thus, the laser is potentially stable while portable, and wavelength tunable while cost effective. The modulator is driven by the unknown microwave signal, so that two wavelength components can be modulated with complementary phase modulation.
The output field can be written as. Due to the birefringence effect, it has two separated and orthogonally-polarized transmission profiles along the fast and slow axes in the fiber, i. After passing through the PM-FBG, the optical carriers of two signals are suppressed separately in the two orthogonal polarizations. The AC terms of the photocurrents are. Taking an example, two wavelengths are set as Since the polarizations of the two modulated wavelength components are separated by the PM-FBG before transmission in the SMF, the walk off effect in the IFM systems [ 18 ] can be eliminated to some extent.
As can be seen, the notch point of ACF is corresponding to the frequency of 8. The ACF decreases monotonically from 0 to 8. Simulations are conducted via an OptiSystem As can be seen, it has two orthogonally-polarized transmission bands and the wavelength difference between the two transmission bands is about 0.
The setup can be found in Fig 1. The laser works at two carrier wavelengths of The laser linewidth is 0. Then the PC1 is used to adjust the polarization angle and the signal is sent into PolM for complementary phase modulation. The PolM is designed via a Matlab program and a programmable module according to its characteristic.
The unknown microwave signal is applied to PolM via its radio frequency port as a driving signal. In order to better illustrate the function of PM-FBG, a polarization beam splitter is connected after the fiber grating, so that the spectra of the output signals in the two orthogonal axes can be observed separately. Therefore, the unwanted frequency components along the two orthogonally-polarized transmission bands of PM-FBG can be removed respectively. The results are shown in Fig 5A. It indicates that the simulated marks and calculated lines results match well.
The ACF decreases monotonically between frequencies from 0 to 8. As can be seen, the notch points of the simulated ACFs are shifted from 8. Based on the relationship between ACF and input frequency, the unknown frequency can be estimated. From the figure we can see, the simulated results dot roughly fits the calculated results line for all the four different measurement ranges, which signifies a high-resolution for the microwave frequency measurement. To further verify the measurement performance by quantitative analysis, we calculate the estimated errors at different polarization angles accordingly.
The results are revealed in Fig 7A — Fig 7D. It can be found that under a small measurement range, the estimated errors are very small. But with the increment of the measurement range, the slope of ACF at low frequencies becomes flat and the estimated errors become larger. Thus, a trade-off problem between measurement range and resolution exists in the IFM system. In order to address this trade-off problem, we try to employ segmentation measurement to lower the estimated errors. Fig 8A illustrates the simulation of estimated frequency, which matches the calculation well.
The estimated errors are shown in Fig 8B. We mark four color regions red, green, purple and orange to represent the four sections. It manifests that in the entire measurement range from 0 to Thus, the measurement resolution can be effectively improved by using segmentation measurement. The measurement errors can be attributed to some factors. One is light fluctuation, which may be caused by using an unstable laser source. Therefore, the stability of laser is of significance for the IFM system. To lower the errors, we first use a single-polarization dual-wavelength laser with good stability.
Then we verify the impact induced by the variation in average input power via changing the laser power from 0 dBm to 12 dBm. The estimated errors are shown in Fig 9A. But when the power becomes as large as 12 dBm, the measurement resolution is impaired badly. As the cure reaction progresses, the temperature may be difficult to control due to the additional heat input caused by the exothermic reaction. Thermoplastics are fully polymerized materials that melt and flow upon application of heat.
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. High-performance, semicrystalline thermoplastic polymers, such as Polyetheretherketone PEEK , can be difficult to heat using microwaves until a critical temperature is reached, where , and therefore the heating rate, increases significantly Chen et al. This critical temperature is related to increased molecular mobility but may not be the same as the glass transition temperature of the polymer.
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The crystallinity of these materials is important; amorphous polymers heat more effectively than semicrystalline polymers DeMeuse, Functionally terminated thermoplastics combine the toughness of thermoplastics with the ease of processing and the creep resistance and solvent resistance of thermosets. These materials undergo a combination of thermoset and thermoplastic processing with the initial heating reducing viscosity and improving flow and ultimately reaction providing a cross-linked network. Microwave processing of functionally terminated thermoplastics offers advantages over conventional processing, particularly in reducing the processing time Hedrick et al.
One of the challenges in the microwave processing of these polymers is that the processing temperature is often very close to its thermal degradation temperature, making temperature control crucial. If the temperature is too high, the polymer undergoes undesirable cross-linking, scission, and oxidation, which can cause significant changes in the mechanical and optical properties of the material. This behavior places a strict requirement on the microwave system to provide very uniform temperature distributions throughout the part being processed and careful control of the temperature of the part.
Although the polymer systems that are candidates for microwave processing are typically not conductive, particles and fibers that are conductive, or have dielectric properties significantly different from the matrix polymer, may be included to aid processing or to modify the mechanical, physical, or optical properties.
The presence of these inclusions can strongly influence the way in which the composite material interacts with the microwave radiation. Conductors also modify the electric field pattern in and around the composite, potentially resulting in very different heating profiles than with the neat resin. Some examples of these conductive additives include carbon black used extensively in rubber formulation ; carbon or metal fibers; and metal flakes, spheres, or needles with sizes ranging from 0.
Although the final composite is not necessarily conductive, the surfaces of the conducting inclusions interact strongly with the microwave radiation. The effect of conductive additives on microwave heating and skin depth of the composite depends on the size, shape, concentration and electrical resistivity of the inclusions and their distribution in the matrix Lagarkov et al.
The presence of conducting fillers may inhibit microwave heating by decreasing skin depth. However, by controlling the nature, orientation, and concentration of the fillers, the microwave response of the material can be tailored over a broad range. For example, carbon fibers have a relatively high resistivity and heat the surrounding matrix very effectively; the thermal profile has a maximum at the surface of the fibers.
This preferential heating has been shown to provide an enhancement of the interfacial adhesion between the fibers and the matrix resin Agrawal and Drzal, and a subsequent improvement in the fracture properties of microwave-processed composite materials. Preferential heating of conducting fillers has also been utilized in the joining of polymers and polymeric composites Varadan, et al. Baziard and Gourdenne a, b report an increased rate of cross-linking in a composite system of an aluminum powder and epoxy resin.
The rate of cross-linking is attributed to the higher dielectric loss due to the presence of the filler. Similar results for carbon-black filled epoxy resin systems have been reported Bouazizi and Gourdenne, Nonconductive additives such as glass fibers and nonconducting metal oxides which are used as pigments e. In addition to the efficient coupling of microwave energy in polar materials and significant depth of penetration, nonthermal ''microwave effects,'' including accelerated apparent kinetics Lewis et al.
The most prevalent reports of microwave effects have been acceleration of reaction rates. There have also been reports in which no effect of the radiation on the kinetics was observed Mijovic et al. One of the difficulties in the comparison and rationalization of these effects is that the experimental conditions and the materials have differed from group to group. A number of the reports of microwave effects are for the curing of epoxy resins and simply measure conversion with time Boey et al. Unfortunately, it is difficult to analyze these data further, since the curing reaction can behave autocatalytically.
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. Furthermore, as the reaction progresses, molecular weight and cross-link density increase, limiting molecular mobility which limits reaction rate and making comparison of reaction kinetics difficult, especially at high conversions.
A meaningful kinetic analysis must account for the development of network structure and the resulting reduction in mobility of reactive groups Wingard and Beatty, ; Woo and Seferis, Two general observations that can be made are that 1 slower-reacting systems tend to show a greater effect under microwave radiation than faster-reacting systems and 2 the magnitude of the observed effect decreases as the temperature of the reaction is increased. The manner in which temperature is measured and controlled is critical in kinetic analysis.
The challenges associated with temperature measurement in a microwave field are discussed in Chapter 3. A number of problems associated with kinetic analysis of a reacting system were avoided in a study of a solution imidization reaction. This reaction followed first-order kinetics, and the reactant and product remained in solution throughout the reaction Lewis et al.
Isothermal conditions were maintained by varying the microwave power or detuning the applicator. An to fold enhancement in the reaction rate was reported over the temperature range studied. A proposed mechanism for the "microwave effect" in polymers suggests a nonequilibrium, nonuniform energy distribution on the molecular level, which results in certain dipoles having a greater energy than the "average" energy of adjacent groups Lewis et al.
The energy couples directly with a reactive polar group in this system and dissipates through adjacent groups by the usual mechanisms. However, if the energy is absorbed faster than it is transferred, at least initially, there will be a nonuniformity present. This mechanism is consistent with some of the recent pulsed-radiation studies in that the rate of energy transfer along the chain may be related to chain relaxations that occur on a similar time scale to the pulse repetition frequency. Because of the range of materials studied, differences in temperature control and measurement methods, and variations in microwave applicators, based on available data it is impossible to determine the effect that microwave processing has on reaction kinetics.
Consistent, controlled experiments, with careful measurement and control of temperature, that account for variations in resin chemistry and changes in reaction mechanisms during cure, are needed to investigate nonthermal microwave effects. 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.
The dielectric properties of glass or high-performance, polymeric fiber-reinforced composites have made them attractive for printed circuit boards and in aviation, marine, and land-based systems radome applications. More-general application of polymeric composites has been hindered by their high cost of orientation layup and forming molding and curing processes. Innovative processing, including automated lamination, rapid consolidation and curing, and out-of-autoclave processing, is being pursued in an attempt to reduce the costs associated with processing.
Microwave processing shows promise for rapid, nonautoclave processing of composite structures. The processing of very thick cross-section parts using conventional processing requires complex cure schedules with very slow thermal ramp rates and isothermal holds to control overheating due to cure reaction exotherms and poor thermal conductivity. Because of microwave penetration and rapid, even heating characteristics, thick composites were initially targeted as ideal applications for microwave processing.
Early studies Lee and Springer, a, b indicated that, while microwave curing of composites in wave-guide applicators was feasible, materials with conducting carbon fibers would be limited to unidirectional composites with less than about 32 plies approximately 7—8 mm thick due to the high reflectivity of the fibers and, hence, poor penetration depth of the radiation into the composite.
Tunable, single-mode resonant cavity applicators with feedback controls to allow the resonant frequency to be changed as material properties vary during processing have been developed to allow more-efficient coupling with composites Asmussen et al. Much of the work accomplished in polymer and composite processing has utilized this type of cavity applicator.
The feasibility of curing thick cross-plied carbon fiber composites was shown when and ply composites were successfully cured using a single-mode resonant cavity Wei et al. 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. Although this process works well for flat parts, tuning of a single-mode cavity containing complex or large parts to provide uniform heating has not yet been accomplished Fellows et al.
A tunable single-mode applicator was used to heat carbon-fiber reinforced PEEK thermoplastic Lind et al. Enough power was absorbed to rapidly heat the PEEK matrix. Based on these results, an applicator was designed and preliminary concepts were developed for an automated tape placement process for fabrication of composite parts Figure Feedback controls to adjust cavity resonance to account for panel curvature are required for scaling.
Although various studies have claimed that the microwave curing processes can have cost advantages over conventional processes Simonian, ; Chabinsky, ; Akyel and Bilgen, , microwave processing of polymers has not found widespread industrial application. However, there are polymer processes that are particularly promising for industrial application. An area where microwave processing has shown promise is composite pultrusion.
In pultrusion, a polymeric composite preform is pulled through a heated die, where the shape is molded and the matrix cured. In conventional processes, the processing chamber consists of a heated die, which is quite long due to the slow heat transfer to the polymer matrix and relatively long cure times. A single-mode resonance cavity has been used to rapidly heat the part using microwave radiation in a significantly shorter process chamber, resulting in less force required to pull the fiber bundle through the die Methven and Ghaffairyan, Since the part configuration that the applicator sees is fixed for each shape, process control should be relatively simple.
In a process analogous to pultrusion, polymeric fibers are drawn through heated dies to increase their axial strength and stiffness through polymer chain orientation. When microwave radiation was utilized for drawing fibers, it was shown that the draw ratio could be increased from approximately The significantly superior mechanical properties of microwave ultradrawn poly oxymethylene fibers over conventionally processed fibers Nakagawa et al, were due to the increased orientation of the polymer chain in the fiber direction.
Because microwaves will couple selectively with materials that contain polar functionalities, it is possible to combine the efficiency and uniformity of heating with the selectivity of materials and accelerate and improve adhesion by coupling microwave energy directly into the adhesion interface. Recently, it has been shown that an intrinsically conducting organic polymer "self-heats" when it is exposed to electromagnetic radiation from a microwave, dielectric, or induction source or when a current is passed through it.
The high dielectric loss tangent of a conducting polymer such as polyaniline loss tangent greater than or equal to 10 -1 at 6. This heat is sufficient to locally melt and weld adjoining thermoplastic parts or cure thermoset polymers, but it does not heat the entire structure, which can result in softening or distortion. This phenomenon can be used to fabricate strong joints of plastics or composites either with each other or with metals.
Extensive work has been done on the microwave welding of high density polyethylene HDPE using conductive gaskets made from a blend of HDPE and conducting polyaniline Wu and Benatar, Under optimum welding conditions, the microwave-welded joint had a tensile strength equal to that of the bulk material. Microwave excitation readily forms plasmas at reduced gas pressures and, under some circumstances, at pressures in excess of 1 atm. Microwave plasmas are being utilized extensively for various applications in microelectronic processing, including deposition and etching for diamond film deposition; for surface modification; and, on an experimental basis, for sintering of ceramics.
An important application of microwave plasmas, analytical spectroscopy, is outside the scope of this study. Plasmas interact with surfaces in one of two ways beyond simply providing thermal energy for heating. Atomic or ionic species in the plasma may react with the substrate to form volatile constituents etching , or species in the plasma may react to form solid materials, which are deposited on the substrate plasma-enhanced chemical vapor deposition.
Plasma surface modification processes may involve either of these interactions. Microwave plasmas are generated in single-or multimode cavities, electron cyclotron resonance cavities, and coaxial torches. Coaxial torches find little use in materials processing. Microwave plasmas, in contrast to parallel plate RF plasmas, do not involve electrodes in contact with the plasma. This avoids contamination arising from sputtering from the electrodes. The specimen may be in direct contact with the plasma, or the effluent of the plasma may be utilized in the processing.
There are significant differences between microwave plasmas and the more common parallel-plate RF plasmas that are used for microelectronics processing. In RF plasmas, one or both of the electrode plates is excited at radio frequency, typically A large DC bias is developed between the plasma and the electrode on which the specimen rests, causing bombardment of the specimen with directed high-energy ions. This phenomenon is utilized in the reactive ion etching RF systems. In a microwave plasma, a much smaller bias is developed between the plasma and the specimen than in RF plasmas.
In addition, the degree of ionization is greater in the microwave plasma. These characteristics have significant consequences in.
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Depending on the process, the differences may be an issue in deciding whether to use a microwave plasma. An excellent review of microwave plasmas has appeared recently Moisan and Pelletier, The current literature on microwave plasma processing is heavily dominated by reports on diamond film formation.
The growth of diamond films requires an abundance of atomic hydrogen, which etches graphitic nuclei in the deposit and leaves the diamond-like nuclei to grow. Plasmas generated by any means are, in general, good sources of this species. There are certain perceived advantages of microwave plasmas over other diamond film-forming methods.
Cited examples include stability and reproducibility of the plasma, energy efficiency, availability of inexpensive magnetrons, and potential for scaling to larger sizes NRC, Microwave plasma processing has had a major impact in microelectronics device processing, where it is a mature art. A state-of-the-art review listed microwave plasma processing as a key technology that was sufficiently developed for imminent implementation in industry NRC, The two major applications are plasma-enhanced chemical vapor deposition and etching, which includes the possibility of high-resolution etching of silicon Moisan and Pelletier, Microwave plasma deposited materials include silicon films, which are amorphous or polycrystalline depending upon the substrate temperature, and silicon oxide and nitride.
In addition, silicon can be oxidized to form silicon oxide films. The primary advantage of microwave plasma-enhanced chemical vapor deposition is reduction in radiation damage compared with conventional RF plasma chemical vapor deposition. This is because the microwave discharge results in a lower acceleration potential between the plasma and the substrate. The electron cyclotron resonance plasma technique is particularly useful in depositing silicon oxide and silicon nitride films on silicon for device processing.
Proper design of equipment, including positioning of feedstock injection, is important to avoid unwanted depositions on the walls of the reactor or other places. The second major application of microwave plasmas is etching in electronic device processing. The principal advantage is that the microwave plasmas are more selective between photoresist and the underlying material. The second advantage is the lower intensity of radiation damage in reactive ion etching compared with conventional plasma etching because of the lower acceleration potential for ions.
Finally, microwave plasma etching is reported to give highly anisotropic etching, although an RF bias is usually required to achieve the desired level of anisotropic etching Moisan and Pelletier, An RF bias to a microwave plasma not only increases the directionality of the etching, it also increases the rate of etching.
Thus the microwave plasma is more selective than the RF plasma, whereas the RF plasma provides better directionality, so a combination of the two is required to obtain the desirable degree of both selectivity and directionality. A third area of use of microwave plasmas is in surface treatment, where it has been applied to polymer fibers, as well as in the microelectronics industry. Chemical modification of the surface can be achieved with or without adding reactive components in the plasma.
It has been demonstrated that treatment of polyamide fibers in a large microwave plasma system improves the bonding between the fiber and the matrix in composites Wertheimer and Schreiber, This results in a dramatically different response to mechanical loads, providing for higher strength but at the same time a lower ballistic strength.
Fiber mechanical properties can be degraded by the microwave plasma treatment. Microwave plasmas are used also to promote adhesion of films in microelectronics processing. Advantage is taken here of the lower degree of radiation damage that is achievable with the microwave plasma than with other plasmas. By using a combination of microwave excitation and RF biasing, it is possible to independently control the relative contribution of the chemical component and the physical component energetic ions, electrons, and photons.
In addition, microwave plasma sources have been used to passivate the surface of GaAs, resulting in superior device properties. The avoidance of direct ion bombardment of the surface was key to the success of this application. The interactions among the physical and chemical components of a microwave plasma system are numerous and not well understood. Further work of a basic nature is required to better elucidate these interactions. Until then the industry and art probably will be dominated by solutions arrived at by trial and error.
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. The minerals and extractive metallurgy industry is a major consumer of energy and contributor to environmental degradation.
For instance, about 4 percent of the carbon dioxide emitted to the atmosphere comes from the worldwide extractive metallurgy industry Forrest and Szekely, Microwave processing may provide substantial benefits in reducing energy consumption and environmental impact by this industry. In mineral processing, the extraction of values in an ore from the waste or gangue is an energy intensive and energy inefficient process.
According to Walkiewicz et al. The energy efficiency of conventional grinding is about 1 percent, and most of the energy is wasted in heat generated in the material and equipment. Microwave processing of ores provides a possible mechanism to induce fractures between the values in the ore and the waste material surrounding it, due to the differential in absorption of microwaves and the differences in thermal expansion among various materials. These differentials induce tensile fractures in the material Figure , and as a consequence, substantially reduce the energy required in grinding to separate the values from the waste material.
A limited amount of work has been done in this area, and it is clear that microwaving of certain sulfide and oxide ores does result in fractures along the interface between the values and the waste material. Grindability tests show improved grindability less energy is required to achieve a given mesh size for the ore for a series of iron ores. However, it is not clear that the reduction in energy required in grinding will balance or exceed the energy expended in the microwave treatment of the ore.
In iron ores the preliminary results indicate a deficit in the energy balance. To justify using microwave processing, it is also necessary to consider wear on grinding mills, cleaner liberation of the values in the ore, and lower chemical emission during the pyrometallurgy and hydrometallurgy processing steps. Additional research needs to be done to determine the efficiency of coupling the microwave energy to the ore, the effect of particle size on susceptibility to cracking, and the effect on the cracking efficiency of using high power sources the present work has been limited to maximum of about 3 kW.
The effect of microwave processing on chemical reactions or processes touches on most of the application areas emphasized elsewhere in this report. Some of these include ceramic sintering and synthesis, polymer curing, plasma processing, and waste remediation. In this section, applications in analytical and synthetic chemistry and extensions of these applications to the chemical industry are considered. The most widespread use of microwaves in chemistry is in analytical laboratories.
Microwave energy has been used in analytical chemistry since the mids, primarily for sample preparation. Applications span a wide range of sample preparation methods including drying, extractions, acid dissolution, decomposition, and hydrolysis. In these applications, microwave heating has been used as a replacement for conventional heating techniques. In general, analytical chemistry involves time-consuming sample preparation steps to get the samples in a suitable form for analysis. Microwave digestion of materials, such as minerals, oxides, glasses, and alloys, is used in laboratories worldwide to prepare samples for chemical analysis.
The decomposition rate of many difficult-to-dissolve materials in closed-reaction vessels is greatly enhanced by using microwave energy; often only a few minutes are required as opposed to the several hours needed for conventional means Kingston and Jassie, a, b. In addition, volatile elements such as selenium and phosphorous can be quantitatively retained in a sealed vessel using microwave decomposition prior to instrumental analysis Patterson et al. Applications in the chemical laboratory generally use relatively simple ovens and controls. Most of the development work in equipment for these applications has involved improving the existing equipment to extend the operating range and to improve safety and reproducibility.
Examples include improved turntables and sample fixtures, pressure vessels fabricated from glass and quartz to allow higher reaction temperatures and pressures than teflon vessels, and optimized pressure relief valves Baghurst and Mingos, b. The result of these advances has been the development of testing standards that are simple, reproducible, and automatable Kingston, A wide variety of organic synthetic reactions have been shown to be enhanced by microwave processing Bose et al.
Using microwave processing, a number of fundamental organic reactions have shown accelerated reaction rates and increased yields over conventional techniques. While these processes have not yet been scaled to production, important advantages have been realized in education, where reactions that took too long to accomplish in a laboratory session using conventional heating can now be completed using microwave heating. The primary motivation for use of microwave heating has been time savings through rapid heating, rather than any nonthermal effects.
Penetrating radiation and reverse thermal gradients , the ability to superheat polar solvents, and the ability to selectively heat reactive or catalytic compounds were responsible for time savings realized in chemical processes.
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Microwave energy penetrates into the interior of the sample without relying on conduction from the surface required in conventional heating methods. This allows the entire sample temperature to be raised rapidly without overheating, and possibly degrading, the surface. Convective heat losses from the surface to the cooler surroundings allow processors to take advantage of reverse thermal gradients. The reaction temperature of solvent diluents can be raised above the ambient boiling points of the diluents in both closed-and open-reaction vessels Baghurst and Mingos, a. This allows for significant increases in reaction rates in a variety of applications Mingos, Reaction rate enhancements were attributable to Arrhenius rate effects due to increased reaction temperature or selective heating of reactants over diluents.
There are no persuasive arguments to support nonthermal reaction enhancements attributable to the use of microwaves Majetich, ; Mingos, In closed vessels, the increased vapor pressure over the liquid suppresses further boiling. Microwave super-heating of volatile solvents can lead to significant acceleration of chemical processes compared with conventional reflux conditions.
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, b.
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In open vessels, most polar solvents have an inherent ability to be heated above their conventional boiling points. This effect has been observed by several researchers Mingos, ; Baghurst and Mingos, a; Neas, b; Majetich, The phenomenon has been explained using a model of nucleation-limited boiling point Baghurst and Mingos, a. During a boiling process, bubbles nucleate preferentially at sites cavities, pits, scratches on the vessel wall, allowing growth of the vapor phase.
With conventional heating, the vessel wall and liquid surface are generally hotter than the bulk. In microwave processes, however, the vessel wall is cooler than the bulk solution due to convective heat losses from the surface, allowing the. As shown in Table , the nucleation-limited boiling point varies with the solvent and with the ability of the solvent to wet the vessel surface, that is, the more effectively the solvent wets the vessel, the more difficult bubble nucleation becomes.
By using microwave heating, the processor is able to target compounds with high dielectric loss over less-lossy compounds. This characteristic has been shown to enhance a number of chemical processes, including catalytic reactions utilizing metallic or dielectric catalysts, gas-phase synthesis of metal halides and nitrides, and metal reduction processes Bond et al.
The promising future of microwave chemistry to the chemical industry is just beginning to be realized. Advantages in the form of time savings, increased reaction yields, and new processes have been demonstrated in the laboratory using simple multimode ovens. Microwave processing for small-scale, custom organic synthesis looks promising due to the relatively modest equipment investment, broad applicability to a variety of reactions, and significantly reduced processing times Bose et al.
The availability of equipment and the long history of use in the laboratory makes these near-term applications low risk. Another area where microwaves can show an advantage is in producing products or intermediates that are needed in small quantities and may be hazardous and expensive to ship, store, and handle Wan and Koch, The processing of industrial wastes is an area of tremendous promise for the application of microwave energy. The types of industrial waste that have been shown to be amenable to microwave processing, at least at laboratory scale, include hazardous waste including toxic and radioactive with high disposal, storage, or treatment cost and nonhazardous waste where the recovery or reuse of a raw material represents a significant cost or energy savings.
Waste processing includes treatment or remediation of process wastes, detoxification or consolidation of stored waste, or cleanup of storage or disposal sites. The application of microwave energy in the processing of industrial waste has seen significant progress in terms of process development and demonstration but limited commercial application. In varying degrees, applications in this area take advantage of unique features of microwave heating: Potential applications of process waste treatment include microwave plasma hydrogen sulfide dissociation, detoxification of trichloroethane TCE through microwave plasma assisted oxidation, and microwave plasma regeneration of activated carbon.
The dissociation of hydrogen sulfide H 2 S in a microwave plasma was first described in Soviet literature Balebanov et al. The potential for this process is in the refuting industry for the treatment of the sour gas resulting from hydrodesulfurization of hydrocarbon feedstocks. Subsequent work at Argonne National Laboratory has validated the applicability Harkness et al.
A schematic of the microwave dissociation process is shown in Figure The attainable conversions, between 40 and 90 percent, were most sensitive to gas flow rate and power. Conversions up to 99 percent are achievable by cycling the residual H 2 S back through the process in multiple passes. Work is continuing to scale the process. The economic viability of the process depends on the sale of recovered hydrogen and is sensitive to required dissociation power.
TCE oxidation and activated carbon regeneration are accomplished through selective heating of lossy components SiC or carbon in a fluidized bed. These processes cause degradation of hazardous organic compounds at significantly lower overall temperatures than conventional heating methods. Additionally, severe corrosion of furnace components caused by the gases released in conventional high-temperature oxidation of chlorinated hydrocarbons is eliminated in the microwave process. Examples of microwave processing of stored waste include ''in-can'' evaporation of water and consolidation of low-level radioactive waste Oda et al.
These processes heat low-level sludge wastes in the final storage containers by applying a microwave field using a slotted waveguide applicator. In-can processes use the rapid, selective heating of. This approach need not be limited to radioactive sludge but can be applied to any evaporative treatment of stored materials. Even though, as mentioned earlier, the efficiency of bulk drying using microwaves is questionable, the cost avoidance realized by reducing handling steps may justify the increased energy costs.
The cleanup of contaminated industrial, disposal, and storage sites is a formidable task due to the large number of waste sites and the complex chemistries and remediation requirements involved. Currently, the majority of site cleanup efforts require removal and incineration of the waste. Since removal and transfer of contaminated materials for incineration may represent an unacceptable risk, innovative processes to cleanup contaminated storage and disposal sites are being investigated.
Microwave processing shows great promise for site cleanup applications, since microwaves can be applied in situ , avoiding costly and risky excavation and transportation, and can target compounds with high dielectric loss for selective heating, for example, moisture in soils Dauerman, Potential applications of microwave processes for cleanup of contaminated sites include removal of volatile organic compounds from soil George et al.
The feasibility of these processes has been demonstrated on a bench scale in a multimode oven. However, the challenge of bringing applicators and sufficient power to waste sites is formidable. If the promise of these applications is to be realized, additional work needs to be done to develop applicators for in situ processing and to show applicability and cost effectiveness on a larger scale in the field.
The cleanup of surface layers 0. Mechanical removal methods create potentially hazardous dust, may drive contamination into the interior, or may create a large waste stream of contaminated water generated in dust amelioration efforts. Microwaves have been shown, in experiments in Japan Yasunaka et al. Work is underway at Oak Ridge National Laboratory to scale-up the microwave concrete-removal process and to develop, build, and test a full-scale prototype.
It is believed that removal rates exceeding those attainable through mechanical techniques are possible with optimized power, frequency, and applicator design. A schematic of the prototype apparatus is shown in Figure While a wide variety of materials have been processed using microwaves, including rubber, polymers, ceramics, composites, minerals, soils, wastes, chemicals, and powders, there are characteristics that make some materials very difficult to process. First, materials with significant ionic or metallic conductivity cannot be effectively processed due to inadequate penetration of the microwave energy.
Second, insulators with low dielectric loss, including oxide ceramics and thermoplastic polymers, are difficult to heat from room temperature due to their low absorption of the incident energy. Since permittivity and loss factors often increase with temperature, hybrid heating may be used to process these types of materials by using alternate or indirect heating to raise the temperature of the parts to where they can be more effectively heated with microwaves.
Finally, materials with permittivity or loss factors that increase rapidly during processing, such as alumina, can exhibit hot spots and thermal runaway. Although insulation or hybrid heating can improve the situation, stable microwave heating of these types of materials is problematic. Enhanced apparent process kinetics due to microwave processing have been claimed for a range of materials, most notably ceramic sintering and polymer curing. However, in most cases, insufficient care was taken in temperature control and measurement and in measurement of critical process variables and material physical properties.
A series of careful experiments with an internal calibration of the temperature is needed to eliminate the doubts that remain about the microwave enhancement effects. Further investigation is needed to develop maps of the regimes of microwave-power absorption characteristics, batch size, heating rate, and other variables where microwave processing can be reproducible and uniform.
This would allow processors to make informed decisions concerning microwave applications and process and equipment selection, while avoiding inefficient heating, uneven heating, and thermal runaway problems that have plagued earlier attempts. In general, the elements required for successful application of microwave processing to industrial materials include selection of materials amenable to microwave processing; an understanding of the process requirements; an understanding of the process economics; characterization of material thermochemical properties; selection of equipment and design of applicators suitable for the application; an understanding of how the parts to be processed will interact with the microwave field; and adequate measurement and control of process variables such as incident power, part temperature, and field strength.
Microwaves can be effectively used in the processing of industrial materials under a wide range of conditions. However, microwave processing is complex and multidisciplinary in nature, and a high degree of technical knowledge is needed to determine how, when, and where the technology can be most profitably utilized. This book assesses the potential of microwave technology for industrial applications, reviews the latest equipment and processing methods, and identifies both the gaps in understanding of microwave processing technology and the promising development opportunities that take advantage of this new technology's unique performance characteristics.
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