Microwave Processing of Materials (1994) / Chapter Skim
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MICROWAVE APPLICATIONS
MICROWAVE APPLICATIONS
Pages 79-116
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From page 79... ...
Food processing and rubber manufacture involve relatively high-volume, continuous processing. Analytical chemistry applications are broad in scope and involve high-volume, repetitive, batch processing, often with long intermediate drying and reaction steps that can be shortened using microwave heating.
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From page 80... ...
Emerging and innovative applications in microwave chemistry, minerals processing, and waste remediation are also reviewed. CERAMICS/CERAMIC MATRIX COMPOSITES The use of microwave energy for processing ceramics and ceramic matrix composites has been the subject of a large amount of exploratory research.
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From page 81... ...
It is generally assumed that since microwave energy is deposited in the bulk, significantly less time is required to heat the part to the sintering temperature than would be required to diffuse the heat from the exterior, particularly for large parts or large batches of small parts. The resulting rapid sintering may lead to smaller grain size at a given density, with consequently better mechanical properties.
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From page 82... ...
. To avoid thermal runaway, applied power had to be carefully controlled as the sintering temperature was approached.
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From page 83... ...
Microwave Applications TABLE 5-2 Selected Microwave Sintering reports. Material Insulation Coupling Reference A1203 None Self (Tian et al., 1988a)
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From page 84... ...
Issues in Microwave Sint;ering Microwave Enhancement Effects There have been numerous reports of enhancement of sintering kinetics when using microwave processing. Probably the most startling is a report of as much as a 400 °C reduction in sintering temperature along with a dramatically reduced activation energy for AtO3 processed in a 28-GHz microwave cavity (lanney and Kimrey, 198S, 1990~.
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From page 85... ...
lnus these materials will absorb microwave energy ~t they can be preheated to a suitable temperature using another source of heat. This has led to the development of passive hybrid heating using higher dielectric loss susceptors, insulation, or coatings that absorb incident microwave power more readily at low temperature.
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From page 86... ...
Although some impressive results have been reported in the hybrid heating of alumina, controlled rapid heating of oxides with both low initial dielectric loss factors and high temperature dependence of dielectric loss factors is difficult to achieve. Success generally has been limited to single specimens of simple geometry in carefully designed sintering chambers.
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From page 87... ...
Microwave Applications (a)
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From page 88... ...
Thermal Runaway As discussed in Chapter 2, the rapid rise in dielectric loss factor with temperature is the major issue in thermal runaway and temperature nonuniformity. Therefore, although microwave heating frequently is touted as providing more uniform heating, nonuniform heating is a reality in many oxides, often at nominal heating rates.
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From page 89... ...
They observed that higher heating rates result in higher density and smaller grains, just as with conventional fast firing. However, higher heating rates were achieved in the hybrid system than was possible with the same specimen size in a conventional furnace.
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From page 90... ...
to 0.5 ,um, with mullite crystallite sizes of 100-200 nm after microwave heating the gel for 5 minutes. At present, the mechanism for the microwave absorption of the aluminosilicate gels is not well understood.
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From page 91... ...
Gas-Phase Reactions Nonoxide powders of A1N, SiC, and Si3N4 have been synthesized by nonthermal microwave plasmas of precursor gases under conditions of laminar flow (Singh et al., 19911. The product particles were ultra-fine (~ 5 nm)
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From page 92... ...
As shown in Figure 5-2. bv using microwave heating of metallor~anic precursors, decomposition is enhanced 7 ~ ~7 =7 ", and occurs at lower macroscopically measured temperatures than conventional thermal processing, so that a wide selection of mono- and polyphasic powders could be synthesized with reasonably high surface areas (10-700 m2/g)
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From page 93... ...
Microwave Applications - _ ~ 80 ~ 60 (a)
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From page 94... ...
in summary, the microwave synthesis of powders is a new era of processing and provides many opportunities for future developments. Table 5-5 presents some of the ceramic powders that have been synthesized using microwave energy.
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From page 95... ...
Microwave Applications TABLE 5-5 Ceramic Powders Synthesized by Microwave Heating 95 Composition Process Composition Process Oxide Nonoxide A12O3 Solution1 CrB Solid-State7 Pyrolysis2 Fe2B Solid-State7 Hydrothermal3 F~O3 Solution ~ZrB2 Solid-State7 Hydrothermal3 TiO2 Solution ~A1N Gas-Phase' Ti2O3 Gas Solid4 Si3N4 Gas-Phase~ ZrO2 Solutions SiC Gas-Phase~ Pyrolysis2 Solid-State9 10 Hydrothermal3 MgAl2O4 Copyrolysis2 TiC Gas-Phase~ Solid-State9 '0 Al6Si2O~3 Sol-gel6 NbC Copyrolysis2 CuAlO2 Copyrolysis2 TaC Gas-Phase~ Solid-State9 10 BaTiO3 Sol-gel ~Composite Hydrothermal3 YBaCu3O7 ,, Solutioni Al2O3/ZrO2/Y2o3 Solutions Solid -S tate7 Mn0.5zro.4Fe2o4 Solution ~SiC/SiO2 Particle + Coating Pyrolysis2 Mn0.6zro.4Fe2o4 Solution' TiC/TiO2 Particle + Coating Pyrolysis2 KVO3 Solid-State7 ZrC/zrO2 Particle + Coating Pyrolysis2 CuFe2O4 Solid-State7 ZrC/SiC Particle + Coating Pyrolysis2 BaWO4 Solid-State7 BN/ZrO2 Particle + Coating Pyrolysis2 La~.85Sr0.~scuo4 Solid-State7 siC/ZrO2 Particle + Coating Pyrolysis2 Al203/ZrO2 Al2O3/CuO Copyrolysis2 Copyrolysis2 2Kladnig and Horn, 1990 3Willert-Porada et al., 1992 4Komarneni et al., 1992 5Bullard and Lynch, 1992 Vollath et al., 1992 7Komarneni et al., 1988 Mingos and Baghurst, 1992 gSingh et al., 1991 (umar et al., 1991 Kozuka and MacKenzie, 1991
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From page 96... ...
Powder Consolidation/Shaping Reaction Sintering Reaction sintering (or boncling) of oxides and silicon nitride using microwaves has been investigated.
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From page 97... ...
While the high cost of microwave energy makes microwave drying inefficient at high moisture contents, at low moisture contents ~ less than 5 percent) the removal of water using conventional processes becomes inefficient, making microwave processes more competitive (Sutton, 19891.
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From page 98... ...
In any event, application of microwave processing will probably be limited to materials that do not show a large temperature dependence of the dielectric loss factor and thus are susceptible to thermal runaway. Significant effort must be directed toward applicator design, specifically addressing the issue of openings in the microwave applicator for introduction and removal of the parts.
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From page 99... ...
As in a home microwave, the materials with the greatest dipole mobilities will exhibit the most efficient coupling. Microwave heating, therefore, will couple most efficiently with the strongest dipole in a system and has the potential to selectively heat polar polymers in mixtures.
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From page 100... ...
High-performance, semicrystalline thermoplastic polymers, such as Polyetheretherketone (PEEK) , can be difficult to heat using microwaves until a critical temperature is reached, where c", and therefore the heating rate, increases significantly (Chen et al., 19891.
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From page 101... ...
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., 1992)
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From page 102... ...
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.
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From page 103... ...
Microwave processing shows promise for rapid, nonautocIave 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 en c!
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From page 104... ...
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, 19921. Since the part configuration that the applicator sees is fixed for each shape, process control should be relatively simple.
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From page 105... ...
Microwave plasmas are generated in single- or multimode cavities, electron cyclotron resonance cavities, and coaxial torches. Coaxial torches find little use in materials processing.
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From page 106... ...
A state-of-the-art review listed microwave plasma processing as a key technology that was sufficiently developed for imminent implementation in industry (NRC, 1986~. 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, 19921.
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From page 107... ...
Fiber mechanical properties can be degraded by the microwave plasma treatment. Microwave plasmas are used also to promote adhesion of films in microelectronics processing.
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From page 108... ...
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.
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From page 109... ...
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 diff1cult-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, 198Sa, b)
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From page 110... ...
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.
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From page 111... ...
. By using microwave heating, the processor is able to target compounds with high ~ ~ 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 etal., 1992)
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From page 112... ...
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: rapid heating, selective coupling with lossy constituents, and reaction steps not possible or practical with other methods.
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From page 113... ...
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 eases released in conventional high-temperature oxidation of chlorinated hydrocarbons is eliminated in the microwave process.
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From page 114... ...
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, 19921. Potential applications of microwave processes for cleanup of contaminated sites include removal of volatile organic compounds from soil (George et al., 1991; Windgasse and Dauerman, 1992)
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From page 115... ...
Although insulation or hybrid heating can improve the situation, stable microwave heating of these types of materials is problematic.
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From page 116... ...
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. Specific processes that show promise for future development include: · ceramic processes including drying, chemical vapor infiltration, reaction bonding of silicon nitride, powder synthesis, and joining; · polymeric composite pultrusion, ultradrawing of polymeric fibers, and adhesive bonding with intrinsically conducting organic polymers; · chemical processes, including custom organic synthesis, hazardous materials processing, solvent extraction, and drying; and · industrial waste processing, including treatment or remecliation of process wastes, detoxification or consolidation of stored waste, and cleanup of storage or disposal sites.
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