Efficiency of Microwave Heating of Weakly Loaded Polymeric Nanocomposites

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1 Efficiency of Microwave Heating of Weakly Loaded Polymeric Nanocomosites Chen-Chih Tsai 1, Binyamin Rubin 1, Eugen Tatartschuk 1, Jeffery R.Owens 2, Igor Luzinov 1, Konstantin G. Kornev 1 1 Clemson University, Clemson, South Carolina UNITED STATES 2 Airbase Tech. Div. Airbase Sci. Branch, Air Force Research Laboratory, Panama, Florida UNITED STATES Corresondence to: Konstantin G. Kornev kkornev@clemson.edu ABSTRACT Electrical and thermal conductivity of materials are tyically correlated, while some alications, including thermoelectrics, require these arameters to be controlled indeendently. Such indeendent control of thermal and electromagnetic roerties can be achieved by using nanocomosites. In this study, nanocomosites were roduced by mixing a small amount of carbon nanofibers (CNF), carbon coated cobalt (Co), and nickel nanowires (NiNW) with araffin, which has low thermal and almost zero electrical conductivity. The fraction of nanoinclusions in the araffin matrix was very low (below 1%). We showed that the thermal roerties of nanocomosites are essentially the same as those of ure araffin, while electromagnetic roerties are significantly different. To determine the deendence of the heating rate on filler concentration, araffinbased samles were heated in a microwave oven. We found that the heating rate of nanocomosites made of carbon nanofibers is much greater than that of any other nanocomosites. These findings suggest that at 2.45 GHz frequency, the heating rate is mostly controlled by the electrical losses in the fillers. The theoretical model redicts that the heating rate increases linearly with the article concentration, which is in agreement with the exerimental data. INTRODUCTION Microwave heating can be emloyed for heating many materials in the GHz frequency range. Microwave ovens used in everyday life is a wellknown examle where the ovens oerate at 2.45 GHz. The major advantage of using microwaves for industrial rocessing is a raid heating of the internal arts of the materials. The use of microwaves for chemically modifying textile surfaces rovides significant advantages over conventional systems with regard to reducing the time and energy needs associated with conventional textile finishing. [1] The rimary mechanisms of the microwave absortion include ohmic (Joule) heating, which occurs in conductive materials; hysteresis heating, which occurs in magnetic materials; and diolar heating which occurs in olar liquids and solids where dioles generate heat uon rotation and friction. Adding nanoarticles to the olymer matrix or any other materials, one can raidly heat the material to locally control a chemical reaction. [1-3] The microwave heating rocesses have been extensively utilized and, currently, this method almost relaced the conventional heating/ drying systems used in industry. The advantages are as follows. For heating by convection and radiation, all heat must flow from the surface to the bulk of the material, and this heat transfer occurs through heat conduction only. As in any diffusion rocesses, the heat conduction cannot be intensified and the maximum temerature is always on the body surface. To avoid the surface overheating, normally the temerature of heating source cannot exceed the desired final temerature, thus the overall heating rate is restricted by that of the heating source. Another factor limiting the heating rate is the thermal diffusivity. Although the resonse of the surface temerature to the outside heating source could be very quick, the temerature resonse of the material is limited by the coefficient of thermal diffusivity: simly increasing heat flux from the boundary cannot always result in raid heating of the whole body. With increasing heat flux, the temerature rofile tends to be more nonuniform, but the temerature does not change significantly beyond a characteristic length scale associated with coefficient of thermal diffusivity. Another advantage of microwave heating is its selective heating. In comosite materials, for Journal of Engineered Fibers and Fabrics 42 htt://

2 examle, one can choose a microwave irradiation with a given frequency guaranteeing that the EM waves would enetrate the matrix, but not the articles. That way, the embedded fillers can roduce heat on demand. Therefore, one can control the rate of chemical reaction, for examle, at the scale associated with the size of the inclusion. Or one can roduce heat only at the given sot where the inclusions are being concentrated. The comosite retains all useful mechanical and thermal roerties of the original matrix material, but can be effectively heated by the microwaves. For examle, interactions of carbon nanotubes with microwaves is a subject of active ongoing research, and the mechanism of the microwave absortion by carbon nanotubes is still oorly understood. [4-6] Low thermal conductivity materials with controlled electromagnetic roerties are esecially imortant for many alications, in articular, because these materials constitute a broad class of olymeric coatings. High asect ratio high conductivity carbon nanofibers and magnetic nanosheres allow significantly changing the microwave resonse of the nanocomosite using low fractions of inclusions mixed with the matrix materials. [2, 6] Since the fraction of nanoarticles in the coatings is low, the thermal roerties of nanocomosites are essentially the same as those of ure olymer, while their electromagnetic roerties are significantly different. In this study, we first construct a simle engineering theory exlaining the kinetics of nanocomosite heating. Paraffin has microwave characteristics very similar to many textile materials. Therefore, it has been chosen as a system modeling olymeric fiberbased materials. Several articulate materials were added in araffin to study the mechanism of microwave heating. Then we analyze the thermal roerties of these araffin-based nanocomosites and find secific heat caacity, the most imortant thermodynamic arameter of the nanocomosite needed for microwave heating alications. We then analyze the heating efficiency of different nanocomosites. As exected, the exerimental heating rate of different nanocomosites as a function of nanoarticle concentration was linearly deendent on the concentration. Thus, the use of microwaves for chemically modifying textile can be controlled by changing the concentration of nanoarticles in the olymer. THEORY Balance Equation When a body is exosed to heating, the kinetics of the heat distribution in the absence of convection is described by the following energy balance equation T C ˆ kt P t (1) where ρ is the material density, C is its secific heat, T (t, r) is the temerature at time moment t and oint r, k is the thermal conductivity of the body, and ˆP is the heating ower density. The first term on the left hand side of Eq. (1) describes the energy change and the second one is resonsible for the heat dissiation due to heat transfer inside the body. The boundary conditions on the interface of two bodies state that the temerature must be a continuous function, T (r + 0) = T (r - 0), and the flux must be also continuous: k T T k n n (2) where k + and k - are the thermal conductivities of air, (r + 0) and nanocomosite, (r - 0), resectively. Integrating Eq. (1) over the samle volume V, we obtain T ˆ C T dv k T dv PdV (3) t where we assume that the secific heat is a function of temerature. It is convenient to transform the volume integral into an integral over the nanocomosite surface S: (4) k T dv k T ds n S As seen from Eq. (2), due to the very low thermal conductivity of air, k + /k - <<1, the heat flux is almost zero at the body surface (5) k T dv k T ds 0 n S Therefore, Eq. (3) is simlified as ˆ C T dv PdV (6) Journal of Engineered Fibers and Fabrics 43 htt:// T t

3 Eq. (6) can be further simlified by introducing an average temerature of the body subjected to the EM radiation, see Figure 1 for the exlanation. FIGURE 1. Sketch of the temerature distribution in the samle during the heating (blue). Temerature T can be nonuniform over the samle, but the integral characteristics of the heating rocess can be described by the average temerature T (red). Introducing the average temerature, Eq. (6) takes on very simle form: dt mc T P dt (7) Assuming a uniform distribution of the absorbed ower in the samle, the temerature of the samle can be found by integration of a non-linear equation: mc T dt / P dt (8) In the simle case of a temerature-indeendent secific heat, the heating rate (sloe of the temerature-time curve) is constant. For the host material, we are using araffin, which secific heat deends on temerature (Figure 4). Knowing this deendence we can numerically calculate the integral t C T m T dt (9) P TT0 where C (T) is the measured secific heat caacity. Embedding nanoarticles into the samle, one effectively changes the hysico-chemical constants of the samle. When the concentration of nanoarticles is low and there is no contact between the nanoarticles, we can use the same heat caacity as that of ure araffin. This statement is confirmed in a series of DSC exeriments, see Figure 4. EM Source P In our exerimental set-u, two heating sources can be resent and have to be included into the heating ower P: the (arasitic) infrared radiation from the walls of the microwave oven P IR and the heating owing to the microwave radiation P MW. In the latter, we distinguish two heating mechanisms induced by the microwave radiation: electric and magnetic. The electric heating occurs in the mixtures of nonmagnetic conductive materials and araffin, which mainly interact with the electric field. The magnetic heating is induced by magnetic nanoarticles. A combination of these mechanisms is an attractive way to otimize the heating efficiency of olymeric nanocomosites. We are in osition to derive the EM losses. Assuming that the samle does not change its density during the heating rocess, the first law of thermodynamics for a unit volume is written as du = dq + dw, where U is the internal energy, Q is the heat added to the samle from outside, and W is the EM work done on the system. As shown in Eq. (4) through Eq. (6), the heat exchange with air can be neglected, hence dq=0. The differential EM work is given by [7] dw = H db + E dd, where H is the magnetic field, B is the induction, E is the electric field and D is the dislacement vector. The constitutive equations for D and B vectors can be written in the form D = 0 () E and B = µ 0 µ() H, where 0 = F/m is the ermittivity of free sace, µ 0 = T m/a is the ermeability of free sace, = ' + i" is the comlex-valued dielectric function, and µ = µ' + iµ" is the comlexvalued ermeability, both are functions of cyclic frequency f = /2 of the EM wave. The fields are collinear hence the scalar roducts are reduced to the roducts of the field magnitudes. Since electric and magnetic contributions have similar forms, it is sufficient to find the dissiation ower for electric wave. In EM wave, the magnitude of electric field changes eriodically, E = E 0 cos(t)= Re { E 0 e it }. The magnitude of the dislacement vector is written as D = Re{('+i") E 0 e it }= Re{('+i") E 0 (cos(t)+isin(t))}= E 0 ('cos(t) - "sin(t)). This field has the in-hase comonent roortional to ' and out-of-hase comonent roortional to ". The change of the internal energy er unit volume over one cycle is written as Journal of Engineered Fibers and Fabrics 44 htt://

4 2 / 2 / 2 2 '' cos (10) U EdD E t dt '' E0 multilying this result by f, we obtain the volumetric ower dissiation as P Uf f '' E (11) MW For a dilute disersion with the volume fraction of inclusions, assuming that a single EM wave interacts only with a single nanoarticle, the dielectric function is exressed as [7] (12) m where " is the imaginary art of the dielectric function of the olymeric carrier (araffin) and " m deends on the roerties of inclusions. Conducting nanofibers change the dielectric ermittivity of the nanocomosite and therefore induce the electric heating due to currents. Eq. (11) and Eq. (12) redict that the heating ower should deend linearly on the article concentration. This theoretical rediction needs to be validated for each disersion, because long microwaves are able to interact with many nanoarticles simultaneously, and the scattering effect might change the linear deendence. LHT, Pyrograf), carbon coated cobalt nanosheres (<50 nm, Aldrich), mixtures of nanofibers and cobalt, and nickel nanowires. Nickel nanowires were reared by our grou using temlate electrochemical synthesis (diameter: 200 nm, length: 16±3 µm). [9] Nanocomosites were reared using the following rocedure: Paralast (comound of urified araffin and lastic olymers) ellets were mixed with aforementioned materials in the given weight roortion, then the mixture was laced in a heated (>melting oint of araffin: 56 C) sonicator. After the araffin has been melted and roduced regularly disersed mixture, the heater was turned off and the obtained nanocomosites were slowly cooled while sonicator was still oerating in order to revent the sedimentation of nanoarticles. It should be noticed that ethanol was used to diserse the cobalt nanosheres and nickel nanowires before mixing with araffin, which allowed obtaining a more uniform nanocomosite samle. The roduced nanocomosite samles (Figure 2) had a disk-like shae with a diameter of aroximately 4 cm and a thickness of about 0.5 cm. The morhological surface structure of carbon nanofibers and nickel nanowires were examined with a Hitachi Field Emission scanning electron microscoe (FESEM-Hitachi 4800), and shown in Figure 3. In the resence of magnetic inclusions, one needs to introduce magnetic losses P Uf f '' H (13) MW The imaginary art of the comosite s ermeability, µ", that is resonsible for the losses, has the same form Eq. (12), i.e. it is roortional to the nanoarticle concentration. [7] In the microwave region, magnetic nanoarticles show the ferromagnetic resonance. Therefore, the heating rate is exected to increase in the vicinity of the resonance. [8] FIGURE 2. Samles used in microwave heating exeriments: (a) ure araffin, (b) 0.3 wt% carbon nanofibers in araffin, (c) 0.3 wt% nickel nanowires in araffin, (d) 0.3 wt% carbon coated cobalt nanosheres in araffin, and (e) 0.3 wt% carbon nanofiber/cobalt (1/1) in araffin. EXPERIMENTAL Exerimental measurements of microwave absortion by araffin-based nanocomosite materials were conducting with carbon nanofiber (CNF), carbon coated cobalt (Co), and nickel nanowire (NiNW) comosites. a b Nanocomosites Prearation Four tyes of comosites were used in this study: carbon nanofibers (diameter: 90±5 nm, PR-24-XT- FIGURE 3. Micrograhs of (a) carbon nanofibers obtained from Pyrograf, and (b) Nickel nanowires roduced in our grou. [9] Journal of Engineered Fibers and Fabrics 45 htt://

5 Microwave Heating The nanocomosite samle was heated for a given time in a 0.9 kw microwave oven, oerating at 2.45 GHz. To reduce the heat exchange between the samle and suorting structure, the samle was laced on a hollow thin-walled lastic cylinder. The temerature was measured in the center of the samle using K-tye thermocoule (30 AWG) with NISTtraceable Omega HH506RA data logger thermometer. The samle and the microwave oven were cooled down to room temerature before and after each exeriment. The initial temerature (T i ) and final temerature (T f ) were recorded, and exressed by temerature increment (ΔT = T f - T i ). The temerature increment of each samle was examined twice, and the variation was ±0.3 C. Imedance Measurement Imedance sectroscoy can be used to test if there is ercolation in nanocomosite. The imedance of ure araffin and nanocomosites was determined with the Imedance sectroscoy system that is built in our lab, and it can be used for the analysis of samle reaction on AC fields in 0 to 100 khz frequency range. The system includes an Analog Devices EVAL-AD5933EB imedance converter and a HP- 8757D scalar network analyzer card and uses a twooint measurement technique. Secific Heat Caacity The secific heat of ure araffin and nanocomosites was determined by the Modulated Differential Scanning Calorimetry (MDSC-2920, TA Instruments). All tests were conducted in N 2 urge from 0 to 50 C at a scan rate of 5 C/min. Each samle was tested three runs and the average value was reorted. The results of the measurements are shown in Figure 4. Pure araffin and araffin-based nanocomosites have essentially the same deendence of the secific heat on temerature. The heat caacity of ure araffin and that of nanocomosites is not constant and dislays a nonlinearity in the range of temeratures of interest. FIGURE 4. Measured heat caacity of araffin and araffin-based nanocomosites. RESULTS AND DISCUSSION Imedance Imedance sectra of ure araffin and nanocomosite samles that used in microwave heating exeriments are shown in Figure 5. All samles are non-conductive in the DC field and demonstrate the same caacitance-tye frequencydeendent imedance sectra, decreasing with increasing frequency. No obvious difference was detected between the sectra of ure araffin and nanocomosites. This confirms that no ercolation occurred in these samles. FIGURE 5. The imedance sectra of ure araffin and nanocomosites. It demonstrates that all samles are nonconductive in this range of frequencies. Journal of Engineered Fibers and Fabrics 46 htt://

6 Heating Power rate (P/m) In Figure 6, we demonstrate the temerature of a araffin samle heated at the constant heating ower (P/m) (calculated from Eq. (9)). In the temerature range from 0 to 25 C, where C is close to constant, the temerature changes almost linearly. When the temerature crosses 30 C, the linear deendence is relaced by a nonlinear one. As follows from Eq. (9), the time needed for heating the samle to a certain temerature deends on the inut ower. If the temerature deendence in a given heating exeriment is known, the heating ower, and the EM losses of the samle, can be calculated. FIGURE 6. Heating of ure araffin with a constant ower (P/m=0.1 J/(s g), red, and P/m=0.15 J/(s g), blue): Temerature deendent heat caacity causes nonlinear heating. reventing an irradiative interaction of the samle with the oven walls. By doing so, we simultaneously rohibited the convection heat transfer from the samle. No difference in the temerature curves as comared to the uncovered samle was found. We conclude that the IR radiation and convection can indeed be neglected in the model (P=P MW in Eq. (9)). Next, all nanocomosites with 0.3 wt% nanoarticle concentration were heated in the microwave oven using 50% of ower. The resulting thermal energy gain er unit mass was lotted in Figure 7 and comared with that of ure araffin. In the current configuration, we can conclude that the nickel nanowires (NiNW) in araffin have the smallest heat roduction, which is almost the same with ure araffin. However, it is obvious that the change in the heating mechanism is very significant: since araffin has never been reorted to have magnetic roerties, the loss mechanism must be urely electric. Comaring carbon coated cobalts nanosheres (Co) in araffin and its mixture with carbon nanofibers (CNF); we can also enhance the losses in the electric comonent of electromagnetic waves, interacting with the comosite. The Co/CNF comosite includes 1.5 wt% of nanosheres and nanofibers, resectively. As exected, the dissiation gain is, within the exeriment accuracy, the average value of those for ure nanoshere and ure nanofiber comosites. We numerically calculated the integral of Eq. (9) for 256 values of P/m in the range between 0 and 0.15 J/(sg). Then the theoretical temerature deendencies T num (t) for various heating rates were evaluated numerically by solving a nonlinear equation Eq. (9) for T. From the exeriments, we know the temerature gains of the samles T ex (t i ) for the discrete times t i. Comaring the measured data with the theoretically obtained curves, we can fit the exerimental data using P/m as an adjustable arameter and minimizing the error P/m (J/s/g) Paraffin NiNW Co CNF/Co(1:1) CNF N 2 ex i num i (14) e T t T t i1 FIGURE 7. Heat gain in 0.3 wt% nanocomosites comared to ure araffin. for the given number of measurements, N, and finding the best matching theoretical curve. First, the ure araffin samle was heated in the microwave oven using 50% of its ower. To extract the imact of the IR radiation from the oven walls, a reference measurement has been erformed: the ure araffin samle was covered by a foam dome, In Figure 8a, the temerature change in CNF/araffin and Co/araffin nanocomosites with various concentrations is lotted against time. We started with ure araffin, and increased the concentration gradually to 0.5 wt%. In all cases, the temerature increased almost linearly as it has been redicted by Eq. (11) and (12). We can also see that with increasing the CNF concentration, the heat energy gain is higher. The same tendency can be observed for carbon coated cobalt nanosheres (Figure 8b). Journal of Engineered Fibers and Fabrics 47 htt://

7 a 0.11 CNF Co 0.09 P/m (J/s/g) Concentration (wt%) b FIGURE 8. Temerature change for different (a) carbon nanofiber (CNF) and (b) carbon coated cobalt (Co) concentrations. The heating rate of different nanocomosites as a function of nanoarticle concentration was redicted to be linearly deendent on concentration, Eq. (11) and (12). In Figure 9, we lot the heating ower of nanocomosites with different concentrations. This grah was obtained by fitting the exerimental data of temerature versus time with Eq. (9) and using P/m as an adjustable arameter. This method does not involve any articular theoretical model for the analysis of microwave absortion. Thus, it can be treated as the exerimental method to find the P/m deendence on nanoarticle concentrations. As exected, the exerimental data fall on a linear deendence on concentration confirming that the exerimental heating ower can be fitted well by a straight line. FIGURE 9. Heating rates of various nanocomosites deending on the nanoarticle concentration. CONCLUSIONS The microwave absortion at 2.45 GHz of ure araffin can be significantly imroved by adding a small fraction (< 1 wt%) of nanoarticles. Both carbon nanofibers and cobalt nanosheres showed better enhancement on absortion of microwave radiation comaring with ure araffin. In addition, the heating rate increases significantly with increasing nanoarticle concentration. The deendence is nearly linear, which agrees qualitatively with the theoretical redictions. This confirms that one can control the chemical reactions by changing the concentration of nanoarticles in the olymer. An addition of nanoarticles to the fibrous materials would rovide significant advantages over conventional systems with regard to reducing the time and energy required for materials heat treatment. ACKNOWLEDGEMENT The roject was suorted through the U.S. Air Force contract FA D The authors also acknowledge the assistance of Kim Ivey. REFERENCE [1] Hayn, R.A., et al., Prearation of highly hydrohobic and oleohobic textile surfaces using microwave-romoted silane couling. Journal of Materials Science, (8): [2] Hatton, T.A., et al., Enhanced microwave heating of nonolar solvents by disersed magnetic nanoarticles. Industrial & Engineering Chemistry Research, (7): Journal of Engineered Fibers and Fabrics 48 htt://

8 [3] Li, S.M., et al., Cellulose-silver nanocomosites: Microwave-assisted synthesis, characterization, their thermal stability, and antimicrobial roerty. Carbohydrate Polymers, (2): [4] Vázquez, E. and M. Prato, Carbon Nanotubes and Microwaves: Interactions, Resonses, and Alications. ACS Nano, (12): [5] Ghasemi, A., Remarkable influence of carbon nanotubes on microwave absortion characteristics of strontium ferrite/cnt nanocomosites. Journal of Magnetism and Magnetic Materials, (23): [6] Benitez, R., A. Fuentes, and K. Lozano, Effects of microwave assisted heating of carbon nanofiber reinforced high density olyethylene. Journal of Materials Processing Technology, (1-3): [7] Landau, L.D. and E.M. Lifshitz, Electrodynamics of continuous media. 1960, Oxford: Pergamon [8] Ramrasad, R., et al., Magnetic roerties of metallic ferromagnetic nanoarticle comosites. Journal of Alied Physics, (1): [9] Tokarev, A., et al., Magnetic Nanorods for Otofluidic Alications. AIP Conference Proceedings : AUTHORS' ADDRESSES Chen-Chih Tsai Binyamin Rubin Eugen Tatartschuk Igor Luzinov Konstantin G. Kornev Clemson University 161 Sirrine Hall Clemson, SC UNITED STATES Jeffery R. Owens Airbase Tech. Div. Airbase Sci. Branch Air Force Research Laboratory Tyndall AFB Panama, FL UNITED STATES Journal of Engineered Fibers and Fabrics 49 htt://