Ind. Eng. Chem. Res. 1998, 37, 2701-2706
2701
Enhanced Microwave Heating of Nonpolar Solvents by Dispersed Magnetic Nanoparticles Arnold Holzwarth, Jianfeng Lou, T. Alan Hatton,*,† and Paul E. Laibinis*,‡ Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
The microwave absorption characteristics of xylene as a model nonpolar solvent are dramatically increased by the incorporation of dispersed cobalt and magnetite nanoparticles. The addition of 1-2 vol % of these colloids to xylene can produce heating rates by microwaves at 2.45 GHz that approach those for water. The particles have diameters of 5-20 nm and contain a coating on their surface to avoid their aggregation and precipitation from solution. The small particle sizes are compatible with a rapid process of heat transfer to the surrounding xylene, thereby minimizing the generation of large temperature gradients around the particles. Cobalt particles are more effective than magnetite particles for enhancing the heating rates of xylene by microwaves, with nanoparticles of cobalt with diameters less than 10 nm exhibiting greater levels of microwave absorption enhancement than nanoparticles of larger diameters. Introduction Microwave irradiation offers numerous potential processing advantages over conventional heating methods for its ability to provide rapid and volumetric heating to an absorbing medium (Clark et al., 1997). For chemical processing, microwaves can increase reaction rates, yields, and selectivities as demonstrated in various laboratory investigations (Abramovitch, 1991; Mingos and Baghurst, 1991). In many chemical processes, multiple heating and cooling steps are required to induce chemical reactions and effect the separation of products from reaction streams. The use of microwave heating for various unit operations within a chemical process could provide improved process control as microwaves are applied remotely and their power can be varied instantaneously. In contrast with the widespread application of microwave heating in the domestic sector (primarily for heating food), its industrial implementation has been less widely embraced and is limited primarily to applications in materials drying and food processing. An important restriction that limits the utility of microwaves in broader applications in chemical processing is that they do not heat nonpolar media efficiently; such nonpolar solvents are used in many chemical processes and liquid-phase reaction systems. In drying and cooking applications, microwaves interact with the dipoles of water to generate heat. For nonpolar solvents, only atomic and electronic polarizations contribute to their dielectric absorption, and heating rates by microwave irradiation are therefore slow (von Hippel, 1995). A practical approach to increase the efficiency of microwave absorption for nonpolar solvents is to disperse polar species throughout the nonpolar medium. A limited number of investigations have examined the ability to modulate the dielectric absorption of chemical systems. Baziard et al. used aluminum powders to increase the microwave heating rates of epoxy resin composites; microparticles with volume fractions greater than 10% were needed to yield † ‡
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accelerated heating (Baziard and Gourdenne, 1988a,b). In another approach, we have used mixtures of polar and nonpolar solvents to tailor the microwave energy absorption of an initially nonpolar phase (Lou et al., 1997). The dielectric absorption of solvent mixtures can be modulated by solution composition, and we have developed dielectric “mixing rules” to predict the dielectric properties of these solvent mixtures based on those of the component liquids and their concentrations. For chemical processes and reactions that require the hydrophobic (and typically chemically inert) properties of hydrocarbon solvents, the use of solvent mixtures may not provide a practical approach for enhancing microwave absorption. Magnetic materials offer superior microwave absorption characteristics over those of metals and polar liquids. As particles, magnetic materials could significantly accelerate the rate of microwave heating due to the effect of ferromagnetic resonance (Griffiths, 1946; Surig and Hempel, 1996). These particles, when coated with surfactants, can be dispersed in nonpolar solvents without changing the polarity and global solvation properties of the liquid phase. In this study, we use dispersions of cobalt and magnetite nanoparticles to modulate the microwave heating rates of nonpolar solvents. In particular, we investigated the effects of particle size and concentration of these particles on the microwave heating rates of xylene. An advantage of using particles of this size is that heat transfer would be rapid to the surrounding medium and reduce the larger temperature gradients associated with bigger particles. We examined the utility of both cobalt and magnetite colloids; in practical applications, the greater magnetic susceptibility for cobalt may compensate for its higher expense. We observed significant increases in the heating rate of xylene by microwaves with less than 1 vol % of these incorporated particles and have related the heating data for these systems to a heattransfer model that yields their corresponding microwave absorption rates; these rates provide a basis for tailoring the microwave absorption properties of nonpolar chemical systems using these dispersed particles.
S0888-5885(97)00819-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/10/1998
2702 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Experimental Section
Energy Balance Analysis
Dispersed magnetite (Fe3O4) particles with average particle diameters of 8.4 and 10.3 nm were obtained from Ferrofluids Inc. (Nashua, NH) in p-xylene and used as received. The stabilizing agents on these particles were proprietary and could not be ascertained. The concentration of the magnetite particles in these colloids was 20.5 wt % (∼3.6 vol %) as supplied, and their saturation magnetization was 200 G. Particle volume fractions of cobalt and magnetite were calculated using densities of 8.89 and 5.7 g cm-3, respectively, and account for only the inorganic component of each colloid. Cobalt colloids were prepared by thermal decomposition of dicobalt octacarbonyl (Strem, Newburyport, MA) in anhydrous p-xylene as follows:
The conservation equation of energy was applied to analyze the heating data for our experiments by considering the liquid sample in the glass container in the microwave oven to be homogeneous and isotropic. Application of a lumped model to this system yielded the following heat equation:
125-170 °C, p-xylene
Co2(CO)898 2Co + 8CO
(1)
where poly(acrylonitrile-styrene) (PAS) copolymer (BASF America, Mount Olive, NJ) or sodium bis(2-ethylhexyl)sulfosuccinate (AOT) surfactant (Sigma, St. Louis, MO) was used as the stabilizer (Hess and Parker, 1966; Paprer et al., 1983a,b). These stabilizers were dried over P2O5 in a desiccator prior to use. The PAS copolymer had an average molecular weight of 130 000 and contained 8% acrylonitrile. The reaction was performed in a 500-mL round-bottomed flask outfitted with a reflux condenser and a mechanical stirrer. Typically, a solution of 1.35 g of PAS or 1.2 g of AOT in 100 mL of anhydrous p-xylene was sparged with N2 for 1 h, and then 12.37 g of Co2(CO)8 was added to the solution. The flask was immersed in a preheated silicone oil bath that was maintained at a temperature between 125 and 170 °C ((2.5 °C). The evolution of CO was observed during the first 1-3 h of reaction. The reaction mixture was heated for 10 h at each selected temperature under a flow of N2 to complete reaction and loss of CO. The reaction mixture was cooled to room temperature, and the cobalt colloid was separated from the precipitated cobalt metal by decantation. The cobalt colloids were stored in sealed bottles under a N2 atmosphere. The concentration of the cobalt colloids was determined by inductively coupled plasma (ICP) spectroscopy (Desert Analytics, Inc., Tucson, AZ), and their average sizes were determined by transmission electron microscopy (TEM). The heating experiments were performed in a Sharp R-4A58 microwave oven operating at 2.45 GHz with a radiation output of 1000 W. Typically, 50 mL of a colloidal dispersion was placed in the microwave oven for each study. The sample was exposed to sequential 0.5- or 1.0-min irradiation at 100% of full power, and its temperature was subsequently measured with a thermocouple. The sample was stirred prior to each temperature measurement to obtain its bulk temperature. The processes of thermal equilibrium and temperature measurement were completed within 20 s. The concentration of magnetic particles in these dispersions was manipulated by dilution and solvent evaporation. In the microwave heating experiments, the samples were contained in the same Erlenmeyer flask and positioned in the center of the oven, and a 0.5-L beaker of water was included in the chamber as a dummy load to absorb excess microwave radiation. Microwave enhancements were determined by comparing data sets obtained using similar time steps of microwave irradiation.
Q4 ) msCps
dT dT + mgCpg + kS(T - T0) dt dt
(2)
where Q4 is the microwave energy absorption rate (J s-1), S is the surface area of the container exposed to microwave irradiation (m2), k is the heat-transfer coefficient (J m-2 °C-1 s-1), T is the temperature of the sample (°C), T0 is the initial temperature of the system (°C), m is mass (kg), Cp is heat capacity (J kg-1 °C-1), t is time (s), and the subscripts s and g denote the liquid sample and the glass container, respectively. The solution of eq 2 is
A T ) T0 + (1 - e-Bt) B
(3)
where A ) Q4 /(msCps + mgCpg) and B ) kS/(msCps + mgCpg). In eq 3, A (°C s-1) and B (s-1) denote the rates of energy absorption and heat loss, respectively, for the microwave-irradiated system. Values of these parameters were obtained for the various heated samples by fitting eq 3 to the experimental data using the Solver function in Microsoft Excel. We performed the fits simultaneously using a common fitted value for B (so that the heat-transfer coefficient was the same for all experiments) and allowing A to take on a different value for each run. The value of B obtained from the fitting procedure was 0.2 s-1 ((10%) under our experimental conditions. The values of A ranged from 4.5 (pure xylene) to 32.5 °C s-1 (a cobalt colloid containing 1.8 vol % of cobalt particles) and reflected the energy absorption rates by the various samples; these values could be compared directly as the sample mass was kept constant and the sample heat capacity was determined mainly by the xylene (>95 vol %). Results and Discussion Preparation of Cobalt Particles. We performed in situ thermal decomposition of Co2(CO)8 to obtain stabilized cobalt particles of various sizes and concentrations in p-xylene (eq 1). In the preparation of the cobalt colloids, the type of stabilizer and the reaction temperature were two key factors that affected the colloidal properties of the resulting magnetic dispersions. In our experiments, the complete decomposition of Co2(CO)8 would yield a concentration of 5.2 wt % (∼0.51 vol %) for the resulting cobalt colloids if all the produced cobalt particles were suspended well in the solution. For the PAS-stabilized colloids, we achieved this concentration within experimental error, suggesting that there was little precipitation of cobalt particles from this colloidal suspension. In contrast, only ∼25-50% of the estimated concentrations were obtained as suspended materials for the AOT-stabilized cobalt colloids. The concentration difference between the PAS- and AOTstabilized colloids suggested that PAS is a better stabilizer than AOT for the preparation of cobalt particles in xylene. The AOT-stabilized cobalt particles
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2703 Table 1. Physical Properties of Cobalt and Magnetite Colloids and Their Microwave Absorption
colloid cobalt
stabilizer AOT
PAS
magnetite not knownd
microwave absorption concn of prepn particle Co or Fe3O4 enhancement temp size (Ar)c (°C) (nm)a (vol %)b 125
7.0
130
7.0
125 145 155 160
17 12 9.9 10
170
10 7.4 9.7
0.13 0.26 0.06 0.13 0.54 0.45 0.54 0.54 1.15 1.81 0.57 0.39 0.77 1.33 0.39 0.77 1.33
0.87 2.3 0.44 1.0 1.0 2.1 2.9 2.5 3.8 6.2 2.7 0.46 1.4 1.9 0.68 1.6 2.1
a Averaged value obtained from TEM. b Values are as determined by ICP spectroscopy for Co colloids and as supplied for magnetite colloids. The relative error is within (10%. c An enhancement of 1 is a doubling of the energy absorption rate of pure xylene. d Proprietary information of Ferrofluids, Inc.
were smaller and had a narrower particle size distribution than did those stabilized by PAS (Table 1 and Figure 1). The difference in yield may reflect a larger steric stabilization provided by the polymeric material. An increase in the preparation temperature tended to decrease the average particle size of the cobalt colloids (Table 1). With the PAS-stabilized colloids, a reaction temperature increase from 125 to 155 °C reduced the average particle diameter of the resulting colloids from 17 to 10 nm. The larger particles prepared at lower temperatures (125-145 °C) tended to aggregate because their corresponding larger magnetic moments increased interparticle attraction. The colloids with average particle diameters of 17 and 12 nm exhibited some settling after 1 and 3 days, respectively; however, these precipitates could be redispersed by sonication. Cobalt colloids with particle diameters of ∼10 nm yielded stable dispersions where no particle sedimentation was observed during a month of storage. The effect of the reaction temperature on particle size, however, was not established for the AOT-stabilized colloids due to our limited number of preparations and the low yield of suspended material for these preparations. The shape of the cobalt particles depended on their size. The TEM images in Figure 1A-D show that the cobalt particles became less spherical and more cubic in shape with increasing particle size. The magnetite colloids (Figure 1E,F) were irregularly shaped and appeared to aggregate on the grids. Both the cobalt and magnetite particles were crystalline as indicated by the lattice planes observed within these particles in highresolution TEM images. Microwave Heating Enhancement by Nanoparticles. The heating rates for polar and nonpolar solvents can differ significantly under microwave irradiation. Figure 2 shows the temperature profiles during microwave irradiation for deionized water (Millipore, 18 MΩ), xylene, and a cobalt colloid in xylene containing 1.2 vol % of cobalt particles. Under the experimental conditions leading to Figure 2, water boiled after ∼2 min of exposure to microwave irradiation while a similar volume of xylene reached only 45 °C
after ∼8 min of irradiation. The cobalt colloidal dispersion in xylene exhibited a much higher heating rate than that of xylene itself, suggesting an enhanced microwave energy absorption by the dispersed cobalt nanoparticles. This enhancement is likely a result of ferromagnetic resonance (Griffiths, 1946; Surig and Hempel, 1996). The magnetic dipoles in their equilibrium states within the magnetic nanoparticles can be excited by microwave irradiation to precess about these equilibrium positions. The coupling between the magnetic dipoles and the microwave field transforms the radiation energy into heat. The energy conversion is at its maximum when the applied microwave frequency is at the resonant frequency of the particle. The resonant frequency depends on the magnetic properties and the shape of the particle; this dependence may provide a way of modulating the microwave absorption by the magnetic dispersions. Figure 3 presents the temperature profiles for two cobalt and magnetite colloids in xylene that contain various concentrations of these particles (up to ∼2 vol %) along with those for water and xylene as references. In Figure 3, the heating rates of these colloids increase with increasing volume fraction of particles, suggesting that the microwave absorption rate is modulated by the concentration of the magnetic particles. Energy absorption rates, A, were obtained for the cobalt and magnetite colloids by fitting eq 3 to the heating data. The microwave absorption enhancement due to the dispersion of the magnetic particles, Ar, was obtained from the values of A for blank xylene (Axyl) and the xylenebased colloids (Acoll) using
Ar )
Acoll - Axyl Axyl
(4)
and are summarized in Table 1. It is important to note that the absolute values of the absorption enhancements will depend on the size and geometry of the heated sample and the power of the incident microwaves; similar trends would be expected for other sample conditions when irradiated at 2.45 GHz. Figure 4 shows that the values of Ar increased linearly with the particle volume fraction for these xylene-based colloids. This linear increase suggests that the heating effect is not due to a cooperative process between the magnetic particles. We observed similar linear behavior between Ar and compositions for the other particle sizes listed in Table 1. These dispersions in xylene are likely to have solvating properties similar to those of the parent solvent but have superior characteristics of microwave absorption for heating applications. The possibility of removing these particles from a process by magnetic filtration may offer an ease of separation and reuse in practical applications. Figure 4 suggests that the microwave heating rate associated with water could be achieved in xylene at volume fractions of ∼0.025 and ∼0.05 of cobalt and magnetite particles, respectively. It should be noted that an increase in the particle concentration would result in a stronger absorption of microwave energy near the surface of the sample normal to the incident radiation and hence in a reduction in the microwave penetration depth. The consequence of the shorter penetration depth is that material in the interior of the irradiated sample will not be subjected to the same level of microwave radiation
2704 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998
Figure 1. TEM images of the cobalt and magnetite particles and their average particle diameters: (A) AOT-stabilized cobalt, 7 nm, prepared at 125 °C; (B) PAS-stabilized cobalt, 9.9 nm, prepared at 155 °C; (C) PAS-stabilized cobalt, 12 nm, prepared at 145 °C; (D) PAS-stabilized cobalt, 17 nm, prepared at 125 °C; (E) magnetite, 7.4 nm; (F) magnetite, 9.7 nm.
Figure 2. Sample temperature as a function of microwave irradiation time for water, xylene, and a cobalt colloid in xylene containing 1.2 vol % of cobalt particles (12 nm).
and hence will be heated to a lesser degree by the incident microwaves. In any application, a compromise must be made with these magnetic dispersions between the total rate of energy absorption and the uniformity of the microwave energy absorption because more absorbing solutions have shorter penetration depths. To assess the relative effectiveness of the various particles for enhancing microwave absorptivity, we determined the microwave absorption enhancement of
the particles on a volume fraction basis. Figure 5 shows these data as a function of particle size for the cobalt and magnetite colloids. The cobalt colloids exhibited greater energy absorption enhancement on a volume fraction basis with decreasing particle size. We could not perform a similar analysis for the magnetite colloids due to our limited data set. The energy absorption enhancement was greater for cobalt than for magnetite, likely reflecting the higher saturation magnetization for cobalt (1400 G vs 485 G for magnetite). At a constant particle volume fraction, the saturation magnetization of a solution is generally independent of the particle size, while the resonant frequency decreases with decreasing particle size (Griffiths, 1946; Surig and Hempel, 1996). A typical resonant frequency for micrometer-sized particles (∼7 GHz) is much higher than the applied microwave frequency of 2.45 GHz in these experiments, and the observed increase in the absorption enhancement for the smaller particle sizes (7-17 nm) (Figure 5) could be due to a decrease in their resonant frequency with decreasing particle size (Viau et al., 1994, 1995). Conclusions The absorption of microwave radiation at 2.45 GHz by nonpolar solvents can be enhanced significantly by the addition of a small volume fraction, typically ∼1 vol %, of magnetic nanoparticles. Both cobalt and magnetite particles are effective, with the former providing greater enhancements at a smaller added volume fraction of the nanoparticles. These dispersed particles have diameters of 5-20 nm, and their small size is compatible with rapid heat transfer to avoid localization
Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2705
Figure 4. Microwave absorption enhancement in xylene (Ar) as a function of volume fraction (φv) of cobalt and magnetite for the colloids. The symbols denote experimental data, and the solid lines are their linear fits. The dashed line provides a reference value for the heating rate of water by microwaves relative to that of xylene using eq 3.
Figure 3. Temperature evolution under microwave irradiation for (a) cobalt (10 nm, PAS as the stabilizer) and (b) magnetite (7.4 nm) colloids in xylene at different particle concentrations. The symbols denote experimental data, and the solid lines represent fits by eq 3. Data for water and xylene are included for comparison.
of microwave heating. For cobalt particles with diameters of 7-17 nm, the smaller particles produced more rapid heating on microwave irradiation for solutions containing similar volume fractions of cobalt particles. These particles can provide a strategy for obtaining heating rates by microwaves with nonpolar solvents such as xylene that are close to those with polar solvents such as water while keeping the nonpolar solvating environment of the particular solvent. From a processing standpoint, the magnetic properties of these particles could provide an additional benefit by offering the possibility of recycling and reuse through magnetic filtration for removal of the nanoparticles from the liquid phase. Acknowledgment A.H. received a fellowship from Deutsche Forschungsgemeinschaft (German Science Foundation). The authors thank Simon Foner (Francis Bitter Magnet Labo-
Figure 5. Microwave absorption enhancement per volume fraction of particles (Ar/φv) as a function of particle size for cobalt and magnetite colloids in xylene.
ratory at MIT) for helpful discussions, Mike Frongillo for TEM images, and Ron Chu (BASF America) for a gift of poly(acrylonitrile-styrene) copolymer. This project was funded by the Electric Power Research Institute and the National Science Foundation under Grant No. CTS-9413894. Literature Cited Abramovitch, R. A. Applications of Microwave Energy in Organic Chemistry. A Review. Org. Prep. Proced. Int. 1991, 23, 685. Baziard, Y.; Gourdenne, A. Crosslinking under Microwaves (2.45 GHz) of Aluminum Powder-Epoxy Resin CompositessI. Electrical Power Dependence. Eur. Polym. J. 1988a, 24, 873. Baziard, Y.; Gourdenne, A. Crosslinking under Microwaves (2.45 GHz) of Aluminum Powder-Epoxy Resin CompositessII. Aluminum Concentration Dependence. Eur. Polym. J. 1988b, 24, 881.
2706 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 Clark, D. E.; Sutton, W. H.; Lewis, D. A. Microwave Processing of Materials. In Microwaves: Theory and Application in Materials Processing IV; Clark, D. E., Sutton, W. H., Lewis, D. A., Eds.; Ceramics Transactions 80; American Ceramic Society: Westerville, OH, 1997; p 61. Griffiths, J. H. E. Anomalous High-Frequency Resistance of Ferromagnetic Metals. Nature 1946, 158, 670. Hess, P. H.; Parker, P. H. Polymers for Stabilization of Cobalt Particles. J. Appl. Polym. Sci. 1966, 10, 1915. Lou, J.; Hatton, T. A.; Laibinis, P. E. Effective Dielectric Properties of Solvent Mixtures at Microwave Frequencies. J. Phys. Chem. A 1997, 101, 5262. Mingos, D. M. P.; Baghurst, D. R. Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry. Chem. Soc. Rev. 1991, 20, 1. Paprer, E.; Horny, P.; Balard, H.; Anthore, R.; Petipas, C.; Martinet, A. The Preparation of a Ferrofluid by Decomposition of Dicobalt Octacarbonyl: I. Experimental Parameters. J. Colloid Interface Sci. 1983a, 94, 207. Paprer, E.; Horny, P.; Balard, H.; Anthore, R.; Petipas, C.; Martinet, A. The Preparation of a Ferrofluid by Decomposition
of Dicobalt Octacarbonyl: II. Nucleation and Growth of Particles. J. Colloid Interface Sci. 1983b, 94, 220. Surig, C.; Hempel, K. A. Interaction Effects in Particulate Recording Media Studied by Ferromagnetic Resonance. J. Appl. Phys. 1996, 80, 3426. Viau, G.; Ravel, F.; Acher, O.; Fievet-Vincent, F.; Fievet, F. Preparation and Microwave Characterization of Spherical and Monodisperse Co20Ni80 Particles. J. Appl. Phys. 1994, 76, 6570. Viau, G.; Ravel, F.; Acher, O.; Fievet-Vincent, F.; Fievet, F. Preparation and Microwave Characterization of Spherical and Monodisperse Co-Ni Particles. J. Magn. Magn. Mater. 1995, 140, 377. von Hippel, A. Dielectrics and Waves; Artech House: Boston, 1995.
Received for review November 21, 1997 Revised manuscript received March 25, 1998 Accepted March 30, 1998 IE970819U
