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Processing materials with microwave energy
Materials Science and Engineering A287 (2000) 153 – 158 www.elsevier.com/locate/msea
Processing materials with microwave energy David E. Clark *, Diane C. Folz, Jon K. West Department of Materials Science and Engineering, Uni6ersity of Florida, Gaines6ille, FL 32611 -6400, USA
Abstract Microwave energy (microwave frequency, in this case, includes radio frequencies and ranges from 0.3 MHz to 300 GHz) is being developed as a new tool for high-temperature processing of materials. Examples of the advantages associated with microwave processing include: rapid and uniform heating; decreased sintering temperatures; improved physical and mechanical properties; and, unique properties which are not observed in conventional processes. These advantages observed in materials processed using microwave energy are being attributed to ‘microwave effects’ which are particular to this technology. Researchers at the University of Florida are working to identify and to qualitatively and quantitatively define the mechanisms of microwave–material interactions. A new model has been developed based on the molecular orbital model which predicts the behavior of specific pure materials in a microwave field. Experimental work as well as dielectric property measurements confirm the accuracy of this model in specific cases. Published by Elsevier Science S.A. Keywords: Microwave processing; Microwave–material interactions; Materials processing
1. Introduction Microwave energy has been developed primarily for communications and some areas of processing such as cooking food, tempering and thawing, and curing of wood and rubber products. Although there is extensive consumer and industrial use of microwave energy, interaction of microwaves with materials is poorly understood. Furthermore, there are numerous reports in the literature of non-thermal ‘microwave effects’ that accelerate reaction rates, alter reaction pathways and result in unique properties in polymers, ceramics and composites. The origins of these microwave effects are unknown. Thus, a fundamental understanding of how microwave energy interacts with materials is the key to unlocking the technology for future and widespread use. The advantages observed with microwave processing warrant serious, focused attention on this technology. Tangible benefits to be produced by microwave–material research include: reduced processing costs, better production quality, new materials and products, improved human health, reduced hazards to humans and the environment and enhanced quality of life. With * Corresponding author. 0921-5093/00/$ - see front matter Published by Elsevier Science S.A. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 7 6 8 - 1
proper understanding and control, many technically important materials can be heated rapidly, uniformly, selectively, less expensively and with greater control than is possible with conventional methods. Moreover, the unique internal heating phenomenon associated with microwave energy can lead to products and processes that cannot be achieved using conventional methods. An excellent example of this interaction on a large scale is the Parallam process (developed by McMillan-Bloedel) where the penetrating nature of microwave energy is used to rapidly and uniformly cure thick, cross-sectional, polymer/wood composite beams as they are pultruded continuously through a die [1]. Conventional heating methods overcure the surface while undercuring the interior. On-going research at the University of Florida (UF) is focused primarily on high-temperature processing of materials, including: sintering of ceramics; combustion synthesis of composite materials; nucleation and crystallization in glass to form glass–ceramics; microwave absorption/heating in composite susceptor materials; and, remediation and recycling of electronic waste materials. Also, a model is being developed which will help to predict the behavior of materials in a microwave field. This model, based on the molecular orbital model, has been successful in predicting the behavior of water (as liquid, solid and gas), silica and silicon carbide. In
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D.E. Clark et al. / Materials Science and Engineering A287 (2000) 153–158
this paper the results of some of the work performed recently at UF is presented.
2. Fundamentals of microwave heating Generally, there are three qualitative ways in which a material may be categorized with respect to its interaction with the microwave field: transparent (low dielectric loss materials) — microwaves pass through with little, if any, attenuation; opaque (conductors) — microwave are reflected and do not penetrate; and, absorbing (high dielectric loss materials) — absorb microwave energy to a certain degree based on the value of the dielectric loss factor [2]. A fourth type of interaction is that of a mixed absorber. This type of interaction is observed in composite or multi-phase materials where one of the phases is a high-loss material while the other is a low-loss material. Mixed absorbers take advantage of one of the significant characteristics of microwave processing, that of selective heating. The microwaves are absorbed by the component that has high dielectric loss while passing through the low loss material with little drop in energy. In some processes and products heating of a specific component while leaving the surrounding material relatively unaffected would be of great advantage. This selective heating process is not possible in conventional heating environments. Two important parameters for microwave processing are power absorbed (P) and depth of microwave penetration (D). Unlike conventional heating, these parameters are highly dependent on the dielectric properties of the material and, in practice, can provide another degree of process flexibility. Microwave heating is the result of absorption of microwave energy by a material exposed to the electromagnetic field distributed within a reflective cavity. It is based on the power absorbed per unit volume (Eq. (1), [2]): P = s E 2 =2pfo0o¦eff E 2 =2pfo0o%r tand E 2
(1)
where E is the magnitude of the internal electric field, o %eff% is the relative effective dielectric factor, o0 is the permittivity of free space, f is the microwave frequency, s is the total effective conductivity, o %r is the relative dielectric constant, and tan d is the energy loss required to store a given quantity of energy. As can be seen from this equation, the dielectric properties (o %, o %eff% and r tan d) assume a significant role in the extent of power absorbed by a material. The majority of the absorbed microwave power is converted to heat within the material, as shown in Eq. (2): DT 2pfo0o¦eff E 2 = Dt rCp
(2)
where T is the temperature, t is the time, r is the density, and Cp is the heat capacity. Notice that there are no structural parameters (atomic, microstructural or otherwise) in the equation. Structural features are assumed to be accounted for by changes in the dielectric properties (o %, r o %eff% and tan d). The dielectric properties also are important parameters in determining the depth to which the microwaves will penetrate into the material. As can be seen by Eq. (3), the higher the values of tan d and o %, r the smaller the depth of penetration for a specific wavelength: D=
3l0
o%r 8.686p tan d o0
1 2
(3)
where D is the depth of penetration at which the incident power is reduced by one half, l0 is the incident wavelength. The depth of penetration is important since it will determine the uniformity of heating, curing, etc., throughout the material. High frequencies and large values of the dielectric properties will result in surface heating, while low frequencies and small values of dielectric properties will result in more volumetric heating. One of the limitations in using microwave energy to process materials is the lack of dielectric data in the microwave frequency range as a function of temperature. In addition to the need for expanded databases on dielectric properties of materials with respect to temperature and frequency, predictive models need to be developed to allow for more effective use of this technology in manufacturing.
3. Molecular orbital model for microwave–material coupling [3] A microwave absorption model is being developed which is based on the semi-empirical Austin method to calculate molecular structures, heats of formation, transition state energies and infrared spectra for a variety of materials with wide-ranging dielectric properties. The model attempts to combine quantum molecular orbital theory with the theory of coupled oscillators to suggest a possible microwave–material interaction mechanism. Fig. 1 shows a schematic of a coupled oscillator. The primary frequency of the mass is represented by w1 and the coupled frequency for energy transfer between the two masses is given by w2, also known as the coupled frequency. Solving the second order differential equations provides the relationships shown in Eqs. (4) and (5): (w+)2=w 21 + (w2/2)2
(4)
(w − )2=w 21 − (w2/2)2
(5)
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155
Assuming that w1 w2, we can approximate using the binomial theorem to get: w+ = w1 + w2
(6)
and: w − = w1 − w2
(7)
Then, solving for w2: w2 =
Fig. 1. Schematic of a coupled oscillator.
w+ − w− 2
(8)
in Eq. (8), w2 describes the frequency at which direct microwave absorption by the material is allowed. Fig. 2 illustrates the type of absorption spectrum that occurs in the microwave range as a result of a coupled interaction in the infrared range due to ionic vibrations. In applying this equation to water, ice, steam and singlephase silica systems, the frequency predicted by the equation closely matched the optimum frequency for absorption as observed during experimentation. The value of this type of model is that it will allow for a simple and cost effective evaluation of the potential for processing specific materials with electromagnetic energy. However, to date the model has been developed and verified only for a few single crystals, glasses and gases (SiC single crystals, SiO2 glasses and H2O vapor) [3]. Further developments will make this a viable model for multi-component materials systems as well as for systems with various structural (atomic, crystal and microstructure) features.
4. Experimental and discussion
Fig. 2. Calculated (using the molecular orbital mode) microwave absorption spectrum for a 5-member silica ring.
The experimental approach for the UF research team has been to develop projects that will address the basic ‘characteristics’ of microwave processing as well as provide a process or product for specific applications. During the course of the experimental work two different methods of microwave heating were used: direct microwave heating (DMH), and microwave hybrid heating (MHH). With DMH the samples were placed in a microwave cavity and exposed to microwave energy directly. In DMH the microwave energy is absorbed into the bulk of the material and the interior of the sample will heat more rapidly than the exterior due to thermal emissivity. Therefore, in most cases, a microwave transparent refractory is placed around the sample to prevent thermal gradients due to heat loss at the sample surface. Mixed absorbers are the basis for the development of the microwave hybrid heating process [4]. In MHH a sample container (susceptor) is constructed of a highly absorbing phase incorporated into a refractory matrix material of low dielectric loss. The microwaves pass through the matrix with little or no absorption and
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156
Table 1 Relative density vs. processing temperature for microwave (MHH) and conventionally processed 15.0 g Coors AD85 bars Temperature (°C)
Conventional
Microwave
1300 1350 1425 1450 1500
– – 85.3 89.8 95.4
95.4 96 96 – –
impinge upon the absorber. The absorber heats quickly, radiating heat to the sample, and the refractory matrix retains the heat. The MHH technique is a highly effective method for processing materials that are microwave transparent at room temperature, but become absorbers at high temperatures as their dielectric properties change. Radiant heat by the susceptor causes heating until the sample itself becomes an absorber and heats.
4.1. Microwa6e sintering of alumina One microwave characteristic that could have significant impact on high-temperature processing of materials is that densification of some ceramic materials can be achieved at lower temperatures and in shorter times than those required in conventional processing [5–7]. To examine this phenomenon, a study was conducted on sintering of alumina plates used as armor in military applications (alumina supplied by Coors Ceramics, Golden, CO). Since the alumina is a low-loss material (low values of the dielectric loss factor), MHH was used to elevate the samples over 1000°C where the sample itself then begins to absorb the microwave energy. In these experiments alumina bars were prepared as defined by ASTM standards for strength testing using uniaxial pressing techniques. These bars were placed within an MHH susceptor prepared from 20 to 40 wt.% coarse silicon carbide particles distributed in an alumina cement matrix. The temperature of the samples inside the susceptor was measured using a sheathed platinum–rhodium thermocouple which was grounded to the microwave cavity. Heating of up to eight samples
simultaneously was carried out using 3200 W of microwave energy at a fixed frequency of 2.45 GHz. Samples were soaked at temperatures ranging from 1200 to 1500°C for 30 min to 1 h in conventional as well as microwave ovens. Density and strength measurements were performed on the sintered samples using the Archimedes method and 4-point bend tests, respectively. It was shown that, at soak temperatures of up to 300°C lower than those required conventionally, MHH-processed samples produced materials with physical and mechanical properties equal to or superior to those produced conventionally. The relative densities for samples with a soak time of 30 min in a conventional high-temperature furnace and in a MHH environment (20 wt.% SiC susceptor) are shown in Table 1. Strength measurements performed on alumina samples produced conventionally and using MHH are given in Table 2. Note that the samples processed conventionally were soaked at temperatures 100°C higher than those produced using MHH. What is the cause of these significant differences in densification for the microwave versus the conventional samples? How is it that the microwave-processed samples achieved almost the same density at 1200°C that the conventional samples achieved at 1400°C? It appears that one reason for these differences may be the internal heating phenomenon associated with microwave processing. When the inside of the sample is allowed to achieve high density before the surface layers densify, the internal porosity is minimized since fewer pores are ‘trapped’ inside the samples. This phenomenon should be even more evident in the alumina as the sample size is increased, and in fact, that is the case. In a study conducted by De’ et al. in 1990 [8], sample density and cross-sectional uniformity in the density increased when the sample size was increased from 6 to 20 g. These results also attest to the fact that, in MHH, radiant heating is not solely responsible for sample densification. If that were the case the MHH samples should show the same levels of densification as those produced conventionally. Since the samples show significant differences in densification, it must be due to a ‘microwave effect’. During the course of this study it
Table 2 Results of the indented strength tests on Coors AD85 bars Firing technique
Conventional 20 wt.% SiC susceptor 60 wt.% SiC susceptor w/top
Indention load 2 kg
Indention load 10 kg
MOR (MPa)
Uncertainty (MPa)
MOR (MPa)
176 185 185
921.4 914.9 918.0
90.0 107 103
Uncertainty (MPa) 910.1 99.37 99.79
D.E. Clark et al. / Materials Science and Engineering A287 (2000) 153–158
Fig. 3. Penetration of potassium ions into a sodium aluminosilicate glass using conventional and microwave heating (600°C, 30 min).
Fig. 4. Dielectric properties of the lithium disilicate glass system at 2.467 GHz as a function of temperature: (a) relative dielectric constant, and (b) relative loss factor.
was shown that the maximum soak temperatures, heating and cooling rates and amount of microwave energy absorption could be controlled simply by altering the composition of the MHH susceptor and controlling the amount of microwave energy allowed to reach the samples.
4.2. Microwa6e processing of glasses The effects of microwave energy on a number of glass processing operations have been examined, including: surface modification by ion exchange; and, nucleation and crystallization in glasses to form glass–ceramics. In each of these studies, microwave characteristics have been observed: enhanced diffusion, increased process uniformity, and selective heating.
4.2.1. Surface modification by ion exchange [9,10] Glass is intrinsically a strong material with a theoretical strength in the range of 106 psi. However, due to the presence of surface flaws, glass strength is limited to a few thousand psi (8 – 10 ksi) and its failure is always
157
tensile in nature. The strength can be increased significantly by incorporating compressive stresses into the glass surface. The ion exchange process is a surface modification method based on a diffusion-driven chemical reaction. The glass is immersed in a molten salt bath (KNO3) at a temperature well below the softening point of the glass. When ion exchange takes place, smaller ions from the glass surface are replaced by larger ions from the molten salt, resulting in a high compressive stress in the surface, commonly 50–100 ksi. Since all parts of the glass surface is equally exposed to the salt, it is possible to strengthen complex shapes or very thin pieces. The depth of the ion exchange can be controlled by varying time and temperature. Current methods require long periods of time in conventional ovens due to slow ion exchange reactions, making this process costly. During the course of experiments at UF it has been shown that, by using microwave energy as a heat source for ion exchange reactions, the rate of exchange can be enhanced and thicker ion exchange layers can be achieved (Fig. 3).
4.2.2. Nucleation and crystallization of glasses [11,12] Microwave energy has been used at UF to convert an amorphous glass into a polycrystalline glass–ceramic. Once again, the microwave characteristic of selective heating of specific phases of a material can lead to significant advantages for microwave processing over conventional methods. Dielectric property measurements performed on the material before and after crystallization indicated that the dielectric losses were higher in the glassy phase than in the polycrystalline phase. As seen in Fig. 4 the dielectric loss factor for the glass and the glass–ceramic as a function of temperature followed the same curve for heating and cooling. However, for the nucleated glass where crystals had not yet begun to form, the curve of loss factor versus temperature exhibited a hysteresis behavior, following the same trends as the glass until crystallization occurred. At this point, the curve then took on the behavior of the glass– ceramic. This behavior provides a distinct advantage associated with the use of microwave processing of glass–ceramics. The drop in dielectric loss upon crystallization of the glass can serve as a self-limiting step in the forming process; a point in the process where the material becomes transparent to the microwave energy and heating stops. This step could provide an excellent real-time indicator of process completion in manufacturing operations. Also evident in this study is the very rapid crystallization associated with microwave processing at 2.45 GHz, as seen in Fig. 5. The crystal radius as a function of time is shown in this figure. Crystal growth rate can be
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Acknowledgements The University of Florida microwave research team gratefully acknowledges the Electric Power Research Institute, the Defense Advanced Research Projects Agency and the National Science Foundation for partial support of the work presented in this paper.
References Fig. 5. Crystal size vs. processing time for conventional and microwave crystallization at 580°C.
evaluated from the slope of the crystal radius versus time. While the results of crystal growth from conventional heating agreed with the results from Duebener et al. [13], the growth rate observed in microwave heating was greater than in conventional heating.
5. Summary It is evident from these and other studies that microwave energy is associated with specific characteristics which can prove advantageous in many production operations. Some of these characteristics include: rapid and uniform heating; selective heating of microwaveabsorbing phases and components in materials and products; enhanced physical and/or mechanical properties; efficient processing of complex shapes; and, controlled rates and extent of processing. Further research and development of microwave processing techniques will lead to more effective and efficient manufacturing of a wide range of materials and products.
.
[1] M.T. Churchland, Microwaves: Theory and Application in Materials Processing III, American Ceramic Society, Westerville, OH 1995, 1995, p. 63. [2] W.H. Sutton, Am. Ceram. Soc. Bull. 68 (2) (1989) 376–386. [3] J.K. West and D.E. Clark, 23rd Ann. Conf. on Composites, Advanced Ceramics, Materials and Structures, Vol. 101, Cocoa Beach, FL, 1999, pp. 53 – 73. [4] A.S. De’, I. Ahmad, E.D. Whitney, D.E. Clark, Mater. Res. Soc. Symp. Proc. 189 (1991) 283 – 288. [5] H.D. Kimrey, J.O. Kiggans, M.A. Janney, R.L. Beatty, Mater. Res. Soc. Symp. Proc. 189 (1991) 243 – 255. [6] M.A. Janney, H.D. Kimrey, Mater. Res. Soc. Symp. Proc. 189 (1991) 215 – 227. [7] J.M. Moore, Microwave Hybrid Firing of Low-Purity Alumina, PhD Dissertation, University of Florida, Gainesville, FL, December 1999. [8] A.S. De’, I. Ahmad, E.D. Whitney, D.E. Clark, Am. Ceram. Soc. Ceram. Trans. 21 (1991) 319 – 339. [9] Z. Fathi, I. Ahmad, J.H. Simmons, D.E. Clark, A.R. Lodding, Am. Ceram. Soc. Ceram. Trans. 21 (1991) 623 – 630. [10] Z. Fathi, D.E. Clark, A.R. Lodding, Am. Ceram. Soc. Ceram. Trans. 36 (1993) 333 – 340. [11] A. Boonyapiwat, D.E. Clark, R.M. Hutcheon, 1st World Congr. on Microwave Processing, Orlando, Florida, January 1997. [12] A. Boonyapiwat, D.C. Folz, D.E. Clark, 23rd Ann. Conf. on Composites, Advanced Ceramics, Materials and Structures, Vol. 101, Cocoa Beach, FL, Am. Ceram. Soc. Ceram. Trans. pp. 87 – 96. [13] J. Deubener, R. Bruckner, M. Sternitzke, J. Non-Cryst. Solids 163 (1993) 1 – 12.
Processing materials with microwave energy David E. Clark *, Diane C. Folz, Jon K. West Department of Materials Science and Engineering, Uni6ersity of Florida, Gaines6ille, FL 32611 -6400, USA
Abstract Microwave energy (microwave frequency, in this case, includes radio frequencies and ranges from 0.3 MHz to 300 GHz) is being developed as a new tool for high-temperature processing of materials. Examples of the advantages associated with microwave processing include: rapid and uniform heating; decreased sintering temperatures; improved physical and mechanical properties; and, unique properties which are not observed in conventional processes. These advantages observed in materials processed using microwave energy are being attributed to ‘microwave effects’ which are particular to this technology. Researchers at the University of Florida are working to identify and to qualitatively and quantitatively define the mechanisms of microwave–material interactions. A new model has been developed based on the molecular orbital model which predicts the behavior of specific pure materials in a microwave field. Experimental work as well as dielectric property measurements confirm the accuracy of this model in specific cases. Published by Elsevier Science S.A. Keywords: Microwave processing; Microwave–material interactions; Materials processing
1. Introduction Microwave energy has been developed primarily for communications and some areas of processing such as cooking food, tempering and thawing, and curing of wood and rubber products. Although there is extensive consumer and industrial use of microwave energy, interaction of microwaves with materials is poorly understood. Furthermore, there are numerous reports in the literature of non-thermal ‘microwave effects’ that accelerate reaction rates, alter reaction pathways and result in unique properties in polymers, ceramics and composites. The origins of these microwave effects are unknown. Thus, a fundamental understanding of how microwave energy interacts with materials is the key to unlocking the technology for future and widespread use. The advantages observed with microwave processing warrant serious, focused attention on this technology. Tangible benefits to be produced by microwave–material research include: reduced processing costs, better production quality, new materials and products, improved human health, reduced hazards to humans and the environment and enhanced quality of life. With * Corresponding author. 0921-5093/00/$ - see front matter Published by Elsevier Science S.A. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 0 7 6 8 - 1
proper understanding and control, many technically important materials can be heated rapidly, uniformly, selectively, less expensively and with greater control than is possible with conventional methods. Moreover, the unique internal heating phenomenon associated with microwave energy can lead to products and processes that cannot be achieved using conventional methods. An excellent example of this interaction on a large scale is the Parallam process (developed by McMillan-Bloedel) where the penetrating nature of microwave energy is used to rapidly and uniformly cure thick, cross-sectional, polymer/wood composite beams as they are pultruded continuously through a die [1]. Conventional heating methods overcure the surface while undercuring the interior. On-going research at the University of Florida (UF) is focused primarily on high-temperature processing of materials, including: sintering of ceramics; combustion synthesis of composite materials; nucleation and crystallization in glass to form glass–ceramics; microwave absorption/heating in composite susceptor materials; and, remediation and recycling of electronic waste materials. Also, a model is being developed which will help to predict the behavior of materials in a microwave field. This model, based on the molecular orbital model, has been successful in predicting the behavior of water (as liquid, solid and gas), silica and silicon carbide. In
154
D.E. Clark et al. / Materials Science and Engineering A287 (2000) 153–158
this paper the results of some of the work performed recently at UF is presented.
2. Fundamentals of microwave heating Generally, there are three qualitative ways in which a material may be categorized with respect to its interaction with the microwave field: transparent (low dielectric loss materials) — microwaves pass through with little, if any, attenuation; opaque (conductors) — microwave are reflected and do not penetrate; and, absorbing (high dielectric loss materials) — absorb microwave energy to a certain degree based on the value of the dielectric loss factor [2]. A fourth type of interaction is that of a mixed absorber. This type of interaction is observed in composite or multi-phase materials where one of the phases is a high-loss material while the other is a low-loss material. Mixed absorbers take advantage of one of the significant characteristics of microwave processing, that of selective heating. The microwaves are absorbed by the component that has high dielectric loss while passing through the low loss material with little drop in energy. In some processes and products heating of a specific component while leaving the surrounding material relatively unaffected would be of great advantage. This selective heating process is not possible in conventional heating environments. Two important parameters for microwave processing are power absorbed (P) and depth of microwave penetration (D). Unlike conventional heating, these parameters are highly dependent on the dielectric properties of the material and, in practice, can provide another degree of process flexibility. Microwave heating is the result of absorption of microwave energy by a material exposed to the electromagnetic field distributed within a reflective cavity. It is based on the power absorbed per unit volume (Eq. (1), [2]): P = s E 2 =2pfo0o¦eff E 2 =2pfo0o%r tand E 2
(1)
where E is the magnitude of the internal electric field, o %eff% is the relative effective dielectric factor, o0 is the permittivity of free space, f is the microwave frequency, s is the total effective conductivity, o %r is the relative dielectric constant, and tan d is the energy loss required to store a given quantity of energy. As can be seen from this equation, the dielectric properties (o %, o %eff% and r tan d) assume a significant role in the extent of power absorbed by a material. The majority of the absorbed microwave power is converted to heat within the material, as shown in Eq. (2): DT 2pfo0o¦eff E 2 = Dt rCp
(2)
where T is the temperature, t is the time, r is the density, and Cp is the heat capacity. Notice that there are no structural parameters (atomic, microstructural or otherwise) in the equation. Structural features are assumed to be accounted for by changes in the dielectric properties (o %, r o %eff% and tan d). The dielectric properties also are important parameters in determining the depth to which the microwaves will penetrate into the material. As can be seen by Eq. (3), the higher the values of tan d and o %, r the smaller the depth of penetration for a specific wavelength: D=
3l0
o%r 8.686p tan d o0
1 2
(3)
where D is the depth of penetration at which the incident power is reduced by one half, l0 is the incident wavelength. The depth of penetration is important since it will determine the uniformity of heating, curing, etc., throughout the material. High frequencies and large values of the dielectric properties will result in surface heating, while low frequencies and small values of dielectric properties will result in more volumetric heating. One of the limitations in using microwave energy to process materials is the lack of dielectric data in the microwave frequency range as a function of temperature. In addition to the need for expanded databases on dielectric properties of materials with respect to temperature and frequency, predictive models need to be developed to allow for more effective use of this technology in manufacturing.
3. Molecular orbital model for microwave–material coupling [3] A microwave absorption model is being developed which is based on the semi-empirical Austin method to calculate molecular structures, heats of formation, transition state energies and infrared spectra for a variety of materials with wide-ranging dielectric properties. The model attempts to combine quantum molecular orbital theory with the theory of coupled oscillators to suggest a possible microwave–material interaction mechanism. Fig. 1 shows a schematic of a coupled oscillator. The primary frequency of the mass is represented by w1 and the coupled frequency for energy transfer between the two masses is given by w2, also known as the coupled frequency. Solving the second order differential equations provides the relationships shown in Eqs. (4) and (5): (w+)2=w 21 + (w2/2)2
(4)
(w − )2=w 21 − (w2/2)2
(5)
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155
Assuming that w1 w2, we can approximate using the binomial theorem to get: w+ = w1 + w2
(6)
and: w − = w1 − w2
(7)
Then, solving for w2: w2 =
Fig. 1. Schematic of a coupled oscillator.
w+ − w− 2
(8)
in Eq. (8), w2 describes the frequency at which direct microwave absorption by the material is allowed. Fig. 2 illustrates the type of absorption spectrum that occurs in the microwave range as a result of a coupled interaction in the infrared range due to ionic vibrations. In applying this equation to water, ice, steam and singlephase silica systems, the frequency predicted by the equation closely matched the optimum frequency for absorption as observed during experimentation. The value of this type of model is that it will allow for a simple and cost effective evaluation of the potential for processing specific materials with electromagnetic energy. However, to date the model has been developed and verified only for a few single crystals, glasses and gases (SiC single crystals, SiO2 glasses and H2O vapor) [3]. Further developments will make this a viable model for multi-component materials systems as well as for systems with various structural (atomic, crystal and microstructure) features.
4. Experimental and discussion
Fig. 2. Calculated (using the molecular orbital mode) microwave absorption spectrum for a 5-member silica ring.
The experimental approach for the UF research team has been to develop projects that will address the basic ‘characteristics’ of microwave processing as well as provide a process or product for specific applications. During the course of the experimental work two different methods of microwave heating were used: direct microwave heating (DMH), and microwave hybrid heating (MHH). With DMH the samples were placed in a microwave cavity and exposed to microwave energy directly. In DMH the microwave energy is absorbed into the bulk of the material and the interior of the sample will heat more rapidly than the exterior due to thermal emissivity. Therefore, in most cases, a microwave transparent refractory is placed around the sample to prevent thermal gradients due to heat loss at the sample surface. Mixed absorbers are the basis for the development of the microwave hybrid heating process [4]. In MHH a sample container (susceptor) is constructed of a highly absorbing phase incorporated into a refractory matrix material of low dielectric loss. The microwaves pass through the matrix with little or no absorption and
D.E. Clark et al. / Materials Science and Engineering A287 (2000) 153–158
156
Table 1 Relative density vs. processing temperature for microwave (MHH) and conventionally processed 15.0 g Coors AD85 bars Temperature (°C)
Conventional
Microwave
1300 1350 1425 1450 1500
– – 85.3 89.8 95.4
95.4 96 96 – –
impinge upon the absorber. The absorber heats quickly, radiating heat to the sample, and the refractory matrix retains the heat. The MHH technique is a highly effective method for processing materials that are microwave transparent at room temperature, but become absorbers at high temperatures as their dielectric properties change. Radiant heat by the susceptor causes heating until the sample itself becomes an absorber and heats.
4.1. Microwa6e sintering of alumina One microwave characteristic that could have significant impact on high-temperature processing of materials is that densification of some ceramic materials can be achieved at lower temperatures and in shorter times than those required in conventional processing [5–7]. To examine this phenomenon, a study was conducted on sintering of alumina plates used as armor in military applications (alumina supplied by Coors Ceramics, Golden, CO). Since the alumina is a low-loss material (low values of the dielectric loss factor), MHH was used to elevate the samples over 1000°C where the sample itself then begins to absorb the microwave energy. In these experiments alumina bars were prepared as defined by ASTM standards for strength testing using uniaxial pressing techniques. These bars were placed within an MHH susceptor prepared from 20 to 40 wt.% coarse silicon carbide particles distributed in an alumina cement matrix. The temperature of the samples inside the susceptor was measured using a sheathed platinum–rhodium thermocouple which was grounded to the microwave cavity. Heating of up to eight samples
simultaneously was carried out using 3200 W of microwave energy at a fixed frequency of 2.45 GHz. Samples were soaked at temperatures ranging from 1200 to 1500°C for 30 min to 1 h in conventional as well as microwave ovens. Density and strength measurements were performed on the sintered samples using the Archimedes method and 4-point bend tests, respectively. It was shown that, at soak temperatures of up to 300°C lower than those required conventionally, MHH-processed samples produced materials with physical and mechanical properties equal to or superior to those produced conventionally. The relative densities for samples with a soak time of 30 min in a conventional high-temperature furnace and in a MHH environment (20 wt.% SiC susceptor) are shown in Table 1. Strength measurements performed on alumina samples produced conventionally and using MHH are given in Table 2. Note that the samples processed conventionally were soaked at temperatures 100°C higher than those produced using MHH. What is the cause of these significant differences in densification for the microwave versus the conventional samples? How is it that the microwave-processed samples achieved almost the same density at 1200°C that the conventional samples achieved at 1400°C? It appears that one reason for these differences may be the internal heating phenomenon associated with microwave processing. When the inside of the sample is allowed to achieve high density before the surface layers densify, the internal porosity is minimized since fewer pores are ‘trapped’ inside the samples. This phenomenon should be even more evident in the alumina as the sample size is increased, and in fact, that is the case. In a study conducted by De’ et al. in 1990 [8], sample density and cross-sectional uniformity in the density increased when the sample size was increased from 6 to 20 g. These results also attest to the fact that, in MHH, radiant heating is not solely responsible for sample densification. If that were the case the MHH samples should show the same levels of densification as those produced conventionally. Since the samples show significant differences in densification, it must be due to a ‘microwave effect’. During the course of this study it
Table 2 Results of the indented strength tests on Coors AD85 bars Firing technique
Conventional 20 wt.% SiC susceptor 60 wt.% SiC susceptor w/top
Indention load 2 kg
Indention load 10 kg
MOR (MPa)
Uncertainty (MPa)
MOR (MPa)
176 185 185
921.4 914.9 918.0
90.0 107 103
Uncertainty (MPa) 910.1 99.37 99.79
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Fig. 3. Penetration of potassium ions into a sodium aluminosilicate glass using conventional and microwave heating (600°C, 30 min).
Fig. 4. Dielectric properties of the lithium disilicate glass system at 2.467 GHz as a function of temperature: (a) relative dielectric constant, and (b) relative loss factor.
was shown that the maximum soak temperatures, heating and cooling rates and amount of microwave energy absorption could be controlled simply by altering the composition of the MHH susceptor and controlling the amount of microwave energy allowed to reach the samples.
4.2. Microwa6e processing of glasses The effects of microwave energy on a number of glass processing operations have been examined, including: surface modification by ion exchange; and, nucleation and crystallization in glasses to form glass–ceramics. In each of these studies, microwave characteristics have been observed: enhanced diffusion, increased process uniformity, and selective heating.
4.2.1. Surface modification by ion exchange [9,10] Glass is intrinsically a strong material with a theoretical strength in the range of 106 psi. However, due to the presence of surface flaws, glass strength is limited to a few thousand psi (8 – 10 ksi) and its failure is always
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tensile in nature. The strength can be increased significantly by incorporating compressive stresses into the glass surface. The ion exchange process is a surface modification method based on a diffusion-driven chemical reaction. The glass is immersed in a molten salt bath (KNO3) at a temperature well below the softening point of the glass. When ion exchange takes place, smaller ions from the glass surface are replaced by larger ions from the molten salt, resulting in a high compressive stress in the surface, commonly 50–100 ksi. Since all parts of the glass surface is equally exposed to the salt, it is possible to strengthen complex shapes or very thin pieces. The depth of the ion exchange can be controlled by varying time and temperature. Current methods require long periods of time in conventional ovens due to slow ion exchange reactions, making this process costly. During the course of experiments at UF it has been shown that, by using microwave energy as a heat source for ion exchange reactions, the rate of exchange can be enhanced and thicker ion exchange layers can be achieved (Fig. 3).
4.2.2. Nucleation and crystallization of glasses [11,12] Microwave energy has been used at UF to convert an amorphous glass into a polycrystalline glass–ceramic. Once again, the microwave characteristic of selective heating of specific phases of a material can lead to significant advantages for microwave processing over conventional methods. Dielectric property measurements performed on the material before and after crystallization indicated that the dielectric losses were higher in the glassy phase than in the polycrystalline phase. As seen in Fig. 4 the dielectric loss factor for the glass and the glass–ceramic as a function of temperature followed the same curve for heating and cooling. However, for the nucleated glass where crystals had not yet begun to form, the curve of loss factor versus temperature exhibited a hysteresis behavior, following the same trends as the glass until crystallization occurred. At this point, the curve then took on the behavior of the glass– ceramic. This behavior provides a distinct advantage associated with the use of microwave processing of glass–ceramics. The drop in dielectric loss upon crystallization of the glass can serve as a self-limiting step in the forming process; a point in the process where the material becomes transparent to the microwave energy and heating stops. This step could provide an excellent real-time indicator of process completion in manufacturing operations. Also evident in this study is the very rapid crystallization associated with microwave processing at 2.45 GHz, as seen in Fig. 5. The crystal radius as a function of time is shown in this figure. Crystal growth rate can be
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Acknowledgements The University of Florida microwave research team gratefully acknowledges the Electric Power Research Institute, the Defense Advanced Research Projects Agency and the National Science Foundation for partial support of the work presented in this paper.
References Fig. 5. Crystal size vs. processing time for conventional and microwave crystallization at 580°C.
evaluated from the slope of the crystal radius versus time. While the results of crystal growth from conventional heating agreed with the results from Duebener et al. [13], the growth rate observed in microwave heating was greater than in conventional heating.
5. Summary It is evident from these and other studies that microwave energy is associated with specific characteristics which can prove advantageous in many production operations. Some of these characteristics include: rapid and uniform heating; selective heating of microwaveabsorbing phases and components in materials and products; enhanced physical and/or mechanical properties; efficient processing of complex shapes; and, controlled rates and extent of processing. Further research and development of microwave processing techniques will lead to more effective and efficient manufacturing of a wide range of materials and products.
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[1] M.T. Churchland, Microwaves: Theory and Application in Materials Processing III, American Ceramic Society, Westerville, OH 1995, 1995, p. 63. [2] W.H. Sutton, Am. Ceram. Soc. Bull. 68 (2) (1989) 376–386. [3] J.K. West and D.E. Clark, 23rd Ann. Conf. on Composites, Advanced Ceramics, Materials and Structures, Vol. 101, Cocoa Beach, FL, 1999, pp. 53 – 73. [4] A.S. De’, I. Ahmad, E.D. Whitney, D.E. Clark, Mater. Res. Soc. Symp. Proc. 189 (1991) 283 – 288. [5] H.D. Kimrey, J.O. Kiggans, M.A. Janney, R.L. Beatty, Mater. Res. Soc. Symp. Proc. 189 (1991) 243 – 255. [6] M.A. Janney, H.D. Kimrey, Mater. Res. Soc. Symp. Proc. 189 (1991) 215 – 227. [7] J.M. Moore, Microwave Hybrid Firing of Low-Purity Alumina, PhD Dissertation, University of Florida, Gainesville, FL, December 1999. [8] A.S. De’, I. Ahmad, E.D. Whitney, D.E. Clark, Am. Ceram. Soc. Ceram. Trans. 21 (1991) 319 – 339. [9] Z. Fathi, I. Ahmad, J.H. Simmons, D.E. Clark, A.R. Lodding, Am. Ceram. Soc. Ceram. Trans. 21 (1991) 623 – 630. [10] Z. Fathi, D.E. Clark, A.R. Lodding, Am. Ceram. Soc. Ceram. Trans. 36 (1993) 333 – 340. [11] A. Boonyapiwat, D.E. Clark, R.M. Hutcheon, 1st World Congr. on Microwave Processing, Orlando, Florida, January 1997. [12] A. Boonyapiwat, D.C. Folz, D.E. Clark, 23rd Ann. Conf. on Composites, Advanced Ceramics, Materials and Structures, Vol. 101, Cocoa Beach, FL, Am. Ceram. Soc. Ceram. Trans. pp. 87 – 96. [13] J. Deubener, R. Bruckner, M. Sternitzke, J. Non-Cryst. Solids 163 (1993) 1 – 12.