Polypyrrole composites for shielding applications

Synthetic Metals 151 (2005) 211–217

Polypyrrole composites for shielding applications ¨ Ozlem Yavuz a,∗ , Manoj K. Ram a , Matt Aldissi a , Pankaj Poddar b , Hariharan Srikanth b b

a Fractal Systems Inc., 200 9th Avenue N., Suite 100, Safety Harbor, FL 34695, USA Materials Physics Laboratory, Department of Physics, University of South Florida, Tampa, FL 33620, USA

Received 11 October 2004; received in revised form 26 January 2005; accepted 6 May 2005

Abstract This article highlights the physical properties of polypyrrole (PPy) coated over MnZn ferrite (MZF), nickel coated over PPy, and PPy coated over Ni-MZF magnetic core particles. The commercial ferrite and Ni-ferrite particles are primarily used, and the PPy-ferrite particles with composite structure are synthesized both via an oxidative polymerization of pyrrole in an aqueous solution, which contains well-dispersed ferrite particles, and electrochemical polymerization technique in acetonitrile (ACN). The materials have been processed in the form of coatings, films, and sheets by blending of conventional polymer such as polyurethane (PU) wherein the composites retain the mechanical properties of the conventional polymers and the electrical conductivity of the conducting polymers. The influence of ferrite content with respect to the electrical and ferromagnetic properties of PPy composites were investigated by electrochemical impedance spectroscopy (EIS) and dc field-cooled, zero-field cooled susceptibilities and M–H loop measurements. Their structural characterization is also discussed based on Fourier transform infrared (FTIR) and the X-ray diffraction (XRD) measurements. The shielding effectiveness (SE) properties will be reported in our future studies. © 2005 Elsevier B.V. All rights reserved. Keywords: Polypyrrole; MnZn ferrite; Composite; Magnetization; Shielding applications

1. Introduction The rapid progress in the field of conducting polymer has inspired much interest for the technological applications in molecular electronics, sensor, battery, light emitting diodes (LED), antistatic coatings, and electromagnetic interference (EMI) shielding where lightweight, flexibility and high conductivity materials are required [1–8]. The conducting polymer has been used as an EMI shielding material where circumvent disadvantages has been seen in the metals [9–12]. Researches in the past have established the ability of polymer composites made with electrically conducting polymers to be suitable as a shield against the electromagnetic interference [13–22]. Most of the studies are performed so far; conducting composites are made by adding carbon black, carbon fiber, nickel-coated graphite fiber, metal powders, or ∗

Corresponding author. Tel.: +1 727 723 1077; fax: +1 727 723 3007. ¨ Yavuz). E-mail address: [email protected] (O.

0379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2005.05.011

metallic particle fillers [23–26]. However, because of certain disadvantages like relatively high cost and time consuming besides the galvanic corrosion phenomenon observed when dissimilar metals are joined, conducting polymer composites were being made which were found suitable for EMI shielding dissipation of electrostatic charge. Polypyrrole (PPy) is an especially promising conductive polymer for commercial applications, due to its high conductivity, good environmental stability, and ease in synthesis. Their use as new materials has opened up entirely new field for polymeric materials. Therefore, several approaches to prepare nanocomposite consisting of magnetic nanoparticles and polypyrrole have been reported [27–31]. Although iron oxide–PPy composites have been successfully prepared by various methods, they still have low room-temperature conductivity, low coercive force, and their structure and properties are difficult to control. Thus, further development of synthetic methods to produce novel electrical–magnetic composites with high conductivity, high coercive force and

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wide frequency range is highly desirable. There is one single material, which satisfies these requirements in a wide range of frequencies; therefore, several materials and/or methods have to employ in order to achieve the desired absorption properties for EMI applications. As a result, a creation of novel synthetic methodology is necessary to develop materials response to wide range frequency with a good absorption or reflection characteristics depending on application. Our approach is to synthesis one single particle, that combines different EMI characteristics, covers wide frequency range and is processed in a magnetic field with novel composition, resulting lighter materials. Recently, magnetic and conducting polyaniline-MnZn ferrite (PANI-MZF) microparticles with composite structure have been prepared by oxidative chemical and electrochemical polymerization technique in our laboratory [32–33]. This work is continuation of our previous work and includes synthesis and characterization of composites by FTIR, impedance, and magnetization. Composite synthesis is performed by three elements of particles: (i) ferrite particles, (ii) a thin coating of nickel, and (iii) a thin layer of a conducting polymer (1–10 wt.%). Coating of Nickel gives high strength, low weight, high aspect ratio and better conductivity, and corrosion resistance. Due to the unprocessibility of conducting polymers with extensive delocaliztion of ␲-electrons, later, the materials have been processed in the form of coatings, films, and sheets as such (the polypyrrole is the binder), and by blending of conventional polymer such as polyurethane (PU) wherein the composites retain the mechanical properties of the conventional polymers and the electrical conductivity of the conducting polymers. The influence of monomer concentration, oxidation potential on the electrical and ferromagnetic properties of the PPy was investigated. The electrical and ferromagnetic properties are discussed based on the structural characterizations including, Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD). Magnetic properties of particles were investigated 10–300 K-temperature range.

3. Instrumentation The AC measurements were performed using electrochemical impedance spectroscopy in the 0.1–104 Hz (EG&G Potentiostat/Galvanostat, 273A), by sandwiching the samples between two stainless steel (SS) cylinders. DC conductivity of the samples was measured using the standard four-point probe technique. The principal transmission bands observed in the FTIR was carried out using Perkin-Elmer Spectrum one instrument. Magnetic characterizations of ferrite and polymer coated ferrite particles were performed by “Physical Property Measurement System” (PPMS) by Quantum Design which is equipped with a 5 T superconducting magnet and could scan from 2 to 400 K temperature range. XRD diagrams were obtained with a Philips using Fe K␣ radiation.

4. Material synthesis 4.1. Synthesis of polypyrrole (PPy) in the presence of MnZn ferrite and Ni-MnZn ferrite PPy was prepared by chemical polymerization technique of pyrrole in presence of MnZn ferrite and/or Ni-MnZn ferrite, p-toluensulfonic acid monohydrate (p-TSA), and FeCl3 at (∼0 ◦ C), and it was kept for 4 h. PPy coated particles were collected on a filter paper, washed using the aqueous acid solution and methanol until the washings were found colorless and later, dried in the vacuum oven at 60 ◦ C. 4.2. Electrochemical synthesis of polypyrrole in the presence of MnZn ferrite and Ni-MnZn ferrite In the electrochemical method, for polypyrrole deposition over the magnetic core cell is shown schematically in Fig. 1. The set-up consists of a rectangular stainless steel container with a flat bottom acting as an anode, and Pt was used a

2. Materials The monomer, pyrrole (Py) (Aldrich), p-tolueneslufonic acid (p-TSA, Aldirch), HCI (Aldrich), acetonitrile (ACN, Aldrich), tetraethylammoniumtetrafluoroborate (TEATFBO4 , Aldrich), lithium tetrafluoroborate (LiTFBO4 , Aldrich), N-methyl-2-pyrrolidinone (NMP), NiSO4 ·6H2 O (Aldrich), NiCl2 ·6H2 O (Aldrich), H3 BO3 (Aldrich), KH2 PO4 (Aldrich) were analytical grade and used without further purification. MnZn ferrite, which has high initial permeability (µi ≈ 10,000) and Ni-MnZn ferrite (Ni-MZF) were a commercial product from Steward Company with an average 5.9 ␮m particle size for MZF and 1.5 wt.% Ni coating of ferrite particles. The Noveon polyurethanes (types 825 and 835) were used for the dispersion of the polymers.

Fig. 1. Electrochemical set-up for metal coating of MZF and polymer particles.

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cathode. The same set up was used for the metal coating. In order to achieve a coating efficiently and directly on the particles in a slurry form, the slurry has to be stirred to form a suspension every 2 min by applying 1.2 V. Deposition time is the critical parameter which determines the coating thickness or polymer to the ferrite ratio. The resulting materials were washed several times using the ethanol, and dried under the vacuum. 4.3. Ni-coating over polypyrrole The metal coating was achieved electrochemically using similar set up of the electro-polymerization technique. The polymer particles used in this process were synthesized by both chemical oxidative polymerization and electrochemical polymerization techniques. The polarity was reversed compared to the polymer coating because of the anodic oxidation nature of conductive polymers. The aqueous metal (nickel) precursor solution containing NiSO4 ·6H2 O = 0.171 mol l−1 , NiCl2 ·6H2 O = 0.063 mol l−1 , H3 BO3 = 0.404 mol l−1 , KH2 PO4 × 0.33 mol l−1 were mixed with the PPy particles, by adjusting and forming slurry at pH 3–3.5 which was direct contact with the cathode plate. Several short (few tens of seconds) stirring and electrodeposition cycles were performed to achieve the desired coating thickness using an applied current (5–10 mA). Longer process times were not desirable, as the particles tended to settle to the bottom. Therefore, repetitive short coating times with stirring after each coating pulse provided more homogeneous materials. The coated particles were then washed with water several times then rinsed with methanol and dried in a vacuum oven. The whole coating process was achieved within few minutes because the low percentage of metal was required for shielding applications. 4.4. Dispersion preparations and processing Three type of dispersion were made: nickel-coated PPy core; the two-layer particles of ferrites, and PPy; and threelayer particles of Ni-MZF with PPy. Polypyrrole acts as an interfacial modifier for the nickel surface. In all cases, the material is lightweight with a density equivalent to that of the conductive polymer. The polyurethanes of different grades as 825 and 835 were mixed in the dispersion solution. Initially, the ferrite particles were used as a 75% where 25% were used as the mixtures of 825 and 835 grades of polyurethanes. The dispersion contained multilayer particles within polymer matrix. Dispersion is prepared by using polymeric particles together with PU in the presence of N-methylpyrrolidone (NMP) as a solvent. Different percentage of polymer-coated ferrite as well as Ni coated polymer particles were added and stirred continuously until the mixture was homogenized. The film/sheet were casted on glass plates. The films/sheets were removed from the glass after 48–72 h of drying in air and tested by using impedence, and FT-IR techniques, respectively.

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5. Material characterization 5.1. FTIR measurements The FTIR spectra at transmission mode of different coated particles were measured using KBr/particles pellet. The FTIR spectra of PPy and the PPy-MZF composite are shown in Fig. 2. The characteristic absorption bands of polypyrrole are observed at 1652, 1560 and 1459 cm−1 . The absorption peaks at 1652, 1560 and 1459 cm−1 were induced in the PPy-MZF composite, by the interaction of MZF and the PPy backbone. The C H in-plane vibrations at 1309, 1044 and 1190 cm−1 and C H out-of-plane vibrations at 786 and 922 cm−1 were observed in Fe3 O4 -PPy, and found to be shifted to a lower wave number. The IR absorption bands in the range between 100 and 1000 cm−1 are usually assigned to vibrations of ions in the crystal lattice. The absorption bands ν1 and ν2 around 600 and 400 cm−1 are attributed to the stretching vibration of tetrahedral and octahedral group complexes of ferrites, respectively. The metal ions in ferrites are situated in two different sub-lattices designated tetrahedral and octahedral according to the geometrical configuration of the oxygen nearest neighbors. Waldron and Hafner, attributed the ν1 (400–450 cm−1 ) band to the intrinsic vibrations of the tetrahedral groups, the ν2 (520–580 cm−1 ) band to the octahedral groups and the ν3 (800 cm−1 ) band to octahedral or tetrahedral environments of ferrites [34–35]. Fig. 3 shows the FTIR spectra of PPy and Ni coated particles. The disappearance of absorption peak at 3400 cm−1 , which belongs to asymmetric stretching of NH indicates interaction of Ni with PPy from NH bond. 5.2. Electrical properties 5.2.1. DC measurements The DC conductivity of ‘chemically polymerized PPy over MZF particles’ and ‘PPy over Ni-MZF particles’ is shown in Table 1. The results in Table 1 reveal a significant change of conductivity value of composite particles by varying the percentage of pyrrole monomer, amount of

Fig. 2. FT-IR spectra of PPy (1) and PPy coated MZF (2) particles.

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Table 3 Conductivity results of Ni coated PPy and PPy coated MZF and Ni-MZF containing polyurethane sheets

Fig. 3. FT-IR spectra of PPy (1) and Ni coated PPy (2) particles. Table 1 Conductivity results of polymers obtained by chemically in the presence of MnZn ferrite (MZF) and 3% Ni-MnZn ferrite (Ni-MZF) Pyrrole (%)

PPy-MZF (−1 cm−1 )

PPy-Ni-MZF (−1 cm−1 )

1a

0.29 0.43 1.02 0.25 1.13 1.47

4.24 1.98 2.08 4.07 1.98 2.18

1b 1c 2a 2b 2c a b c

p-TSA: 0.026 M, FeCI3 : 0.006 M. p-TSA: 0.052 M, FeCI3 : 0.012 M. p-TSA: 0.104 M, FeCI3 : 0.104 M.

oxidant and electrolyte (used for doping PPy). In general, Ni-MZF particles observe significant conductivity. The incremental increase of polypyrrole over Ni-MZF particles shows the better conductivity than MZF particles. However, it can be remarked that the change in conductivity is observed by varying 1 or 2% of pyrrole monomer over the weight of Ni-MZF particles for obtaining PPy-Ni-MZF composite particles. Table 2 shows the results of Ni coated PPy particles. Interestingly, the decrease in conductivity is observed in PPy-Ni-MZF composite particles by varying the amount of dopant (p-TSA), indicating that the concentration of dopant is an important parameter for repositioning Ni anions on the polypyrrole surface. The conductivity of PPy and Ni-MZF with increasing PPy content in polyurethane is summarized in Table 3. The conductivity of the low percentage of PPy containing composites is found to be higher than the bigger percentage of PPy coated MZF in polyurethane. The conductivity increases by increasing PPy in the samples. Pure polypyrrole is a very

Sample

δ (−1 cm−1 )

1% PPy-MZF, 99% PU 2% PPy-MZF, 98% PU 5% PPy/Ni, 95% PU 2% PPy-Ni, 98% PU 1% PPy-Ni, 99% PU

8.15 × 10−6 1.64 × 10−6 2.9 × 10−5 4.01 × 10−5 4.87 × 10−5

lightweight polymer with poor compactness; therefore, in the present samples, 2% pyrrole containing composites are very randomly oriented and interaction with the polymer particles through the grain boundaries is very poor which resulted lower conductivity. As the PPy content in the samples is lowered, the change in compactness become more significant and the weak links between the grains are increasingly improved, and the coupling through the grain boundaries become stronger resulting the improvement in the conductivity [36]. 5.2.2. AC measurements The Fig. 4 shows the log f (frequency (Hz)) versus log Z (impedance ()) plot for the chemically PPy-coated over MZF (1), PPy (2), and PPy-coated over Ni-MZF (3) particles. It has been earlier shown that impedance measurements were performed in two-electrodes set-up by compacting the powder in a cylinder between the two steel rods. The force (5 N) between the steel electrodes has been kept constant in the measurement process. The impedance decreases by increasing the polypyrrole over Ni-MZF similar to conductivity increase by four point probes. No appreciable change in the impedance has been observed at frequency below 103 to 104 Hz similar to observe for the films studied by Beladakere et al. [37]. The conductivity at the region has always found to be content for polypyrrole systems, interestingly, the composite PPy-Ni-MZF does not also shows appreciable change in the impedance at low frequency region. Fig. 5 reveals log f versus log Z plot of MZF (1) and results of electrochemically PPy coated MZF particles (3% (2), 8%

Table 2 Conductivity results of electrochemical Ni coating of PPy particles (synthesis conditions: PPy1: 0.1 M Py, 0.2 M p-TSA, 0.2 M FeCl3 ; PPy2: 0.1 M Py, 0.1 M p-TS, 0.2 M FeCl3 ) Sample

Applied current (mA)

δ (−1 cm−1 )

Ni/PPy1 Ni/PPy1 Ni/PPy2 Ni/PPy2

3 10 3 10

0.21 0.18 1.47 1.28

Fig. 4. Bode plot of chemically PPy coated MZF (1), PPy (2) and chemically PPy coated 3% Ni/MZF (3).

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5.3. X-ray diffraction measurements

Fig. 5. Bode plot of MZF (1) and electrochemically 3% (2), 8% (3), 10% (4) PPy coated MZF by using 0.1 M TEATFBO4 as an electrolyte in ACN at 10 mA.

XRD was performed on a Phillips X-ray diffractometer, using a Cu K␣ radiation source operating at 50 kV and 30 mA. X-ray powder diffraction measurements for polypyrrole particles and polypyrrole (weight 1% pyrrole monomer to Ni-MZF) coated over Ni-MZF was performed as shown in Fig. 7. The X-ray spectrum of PPy (curve 1) does not show sharp peak suggesting not ordered structure in PPy conducting polymer. In fact, polymer displays a diffuse broad peak ranging from 20◦ to 50◦ and broad peaks centered at around 45◦ –55◦ similar to one observed for electrochemically synthesized PPy doped with p-TSA by Ouyang and Li et al. [38], which can be interpreted due to ordering in PPy chains at the inter-planar spacing [39]. The polypyrrole has been reported to be a 95% amorphous material, where the controlling of synthesis, and types of dopant can make slight ordering. Fig. 7 (curve 2) shows the crystalline structure of PPy coated over Ni-MZF particles. In fact, MnZn ferrite shows the spinel crystal structure [40]. The PPy coated Ni-MZF (curve 2) shows the similar characteristics peaks as observed for MnZn ferrite particles, indicating that the crystalline structure can be maintained by coating the PPy conducting polymer. Composite formation of PPy with ferrite shows the spinel structure of MnZn ferrite, and can be maintained by interacting each particle of polypyrrole conducting polymer.

(3), 10% (4)) by using 0.1 M TEATFBO4 as an electrolyte in acetonitrile (ACN) at 10 mA. We have chosen ACN as a solvent for polypyrrole coated over the ferrite. Results show that dispersion of ferrite particles in ACN is very close to dispersion of water, resulting similar conductivity of electrochemical system. The decrease in the Z as a function of PPy coated over particles indicates the increment in dopant amount in the polymer. Fig. 6 shows the log f versus log Z plot of electrochemically PPy coated over Ni-MZF in the presence of different electrolyte (LiTFBO4 and TEATFBO4 ). Results show that the size of dopant affects the polymer properties significantly. The impedance studies for various composites by changing the percentage of polypyrrole do not vary the trend in the impedance studies. This behavior indicates that polypyrrole does not reveal any polarization effect of solvent and water entrapment at the junction of PPy and Ni-MZF particles. The polypyrrole shows the stability at such frequency 103 kHz. Based on impedance measurements, 1 and 2% of pyrrole can be sufficient to obtain desired composites as for obtaining the polypyrrole coating over the MZF particles, which have already, be seen in FTIR and X-ray measurements, respectively.

dc magnetization measurements were performed using a physical property measurement system (PPMS) from Quantum Design. Saturation magnetization versus temperature measurements was performed over the 10–300 Ktemperature range. To study the magnetic properties, assynthesized powdered samples were filled in gelatin capsules that have a small diamagnetic background. Fig. 8 shows the M–H loop measurements on PPy and Ni-coated PPy samples at 300 and 10 K. The PPy sample reveals a weak ferromagnetic behavior at lower H values at 300 K; whereas at

Fig. 6. Bode plot of electrochemically PPy coated Ni-MZF, (electropolymerization is performed in ACN by using 0.1 M LiTFBO4 and 0.1 M TEATFBO4 , 10 mA).

Fig. 7. XRD spectra of PPy and 1% PPy coated 3% Ni-MZF.

5.4. Magnetic properties

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Fig. 8. M–H loop measurements on PPy at 10 and 300 K. In the inset, we have shown the temperature dependence of magnetization.

higher magnetic field the diamagnetic contribution is dominant over the ferromagnetic contribution. At 10 K, the M–H curve shows primarily weak ferromagnetic behavior masking the temperature independent diamagnetic contribution. In principle, magnetically pure PPy should show a diamagnetic signal, we believe that the present behavior is due to small ferromagnetic impurities mixed in our PPy samples due to the processing technique as evident from Fig. 8. In the inset of this figure, we have shown the temperature dependence of the magnetic moments that also shows the evidence of ferromagnetic impurity. A comparative measurement of M–H curves at 300 and 10 K was performed for MZF, MZF coated with PPy, MZF coated with Ni and MZF coated with both Ni and PPy in Fig. 9. The results depict that none of the samples show appreciable coercivity even at 10 K within the resolution of our measurement; these results are in consistent with the soft ferromagnetic nature of MZF. The magnetization values are summarized in Table 4. The coating of PPy reduces the magnetization value pertaining to the dominant diamagnetic nature. At 300 K, MZF samples show saturation magnetization (MS ) of 66 emu/g, which increase up to 140 emu/g at 10 K indicating that MZF samples saturate much faster. The MS of MZF is reduced to 61.7 emu/g after PPy coating, which goes up to 121.3 emu/g at 10 K. It can also be noticed that the magnetization of MZF particles is seen increasing after coating with Ni. Either thin coating of Ni over micron-sized particles of MZF may affect the moments of the surface atoms leading to an increase in the saturation magnetization or the Curie temperature of the Ni-coated MZF Table 4 Summary of the magnetic characterization Sample

PPy MZF PPy-MZF Ni-MZF PPy-Ni-MZF

Magnetization at 15 kOe (emu/g) 300 K

10 K

−0.025 66 61.7 72.7 71.93

0.257 140 121.27 78.20 78.07

Fig. 9. Comparison between magnetization vs. magnetic field results for MZF, PPy-coated MZF, Ni-coated MZF and both PPy and Ni coated MZF particles at 10 and 300 K.

particle is higher than MZF particles. It is well known that the surface chemistry greatly affects the magnetic properties of fine magnetic particles due to relatively larger surface area. After further coating with PPy, the magnetization of these particles is greatly reduced, and this is again due to its diamagnetic contribution. Interestingly, none of these samples show any detectable coercivity. At 300 K, the Nicoated MZF showed the MS of 72.7 emu/g, which increased to 78.2 emu/g at 10 K. The weak temperature dependence of Ni-coated MZF particles partially supports the fact that the Curie temperature of Ni-coated MZF is much higher than uncoated MZF. A high temperature magnetization study is required to know the effect of Ni-coating on the Curie temperature of MZF. The PPy-Ni-coated MZF particles showed an MS of 71.93 emu/g, which increased to 78.07 emu/g at 10 K. The magnetization versus temperature curves at 15 kOe magnetic field are shown in Fig. 10. This field was sufficient to magnetically saturate all the studied samples. The temperature dependence of MS is quite strong for MZF and PPy-coated MZF samples. On the other hand, the temperature dependence is relatively weaker for the Ni-coated and PPy-Ni-coated MZF samples, this again establishes the role of Ni-coating over the surface magnetic moments.

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References

Fig. 10. Comparison between magnetization vs. temperature (M–T) data of MZF, PPy coated MZF, Ni-coated MZF and PPy and Ni-coated MZF.

6. Conclusions We have studied chemical and electrochemical polymerization of pyrrole in the presence of ferrites and investigated resulting physical properties of polypyrrole (PPy) coated over MnZn ferrite (MZF), nickel coated over PPy, and PPy coated over Ni-MZF magnetic core particles which might be responsible for wide range frequency for their absorption or reflection properties. According to polymerization and coating techniques that they are used in this study, the main results are summarized as follows: 1. Chemical and electrochemical polymerization techniques yield homogenous coatings onto ferrite particles with conductivity being dependent on the experimental conditions for each technique. 2. Based on the conductivity and impedance measurements, 1 and 2% of pyyrole is sufficient to obtain desired composites. 3. Composite formation of PPy with ferrite shows the spinel structure of MnZn ferrite, and can be maintained by interacting each particle of polypyrrole conducting polymer. 4. Saturation magnetization versus temperature measurements was performed over the 10–300 K-temperature range. These results showed that saturation magnetization is changed for the samples, which are coated by polymerization and Ni plating technique. This phenomena is explained with the surface chemistry where showed relatively differential surface areas. Acknowledgements This work is funded by the Missile Defense Agency and sponsored by the U.S. Army Space and Missile Defense Command (Contract no. DASG30-01-C-0085). Authors at USF acknowledge support from National Science Foundation through grant no. ECS-0140047.

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