MWCNT

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In Situ Synthesis and Characterization of CuFe10Al2O19/MWCNT Nanocomposites for Supercapacitor and Microwave-Absorbing Applications Pallab Bhattacharya† and Chapal K. Das*,† †

Materials Science Centre, Indian Institute of Technology, Kharagpur-721302, West Bengal, India ABSTRACT: The present work focused on the synthesis and characterization of a magnetic M-type hexaferrite (CuFe10Al2O19) and its coating on the acid-modified multiwall carbon nanotube (MWCNT) through an in situ technique. Thermoplastic polyurethane (TPU)/CuFe10Al2O19/MWCNT nanocomposites, for the fabrication of microwave test plates, were prepared via the solution mixing process. The nanocomposites exhibited a remarkable and improved microwave absorption properties, compared to the pristine MWCNT and CuFe10Al2O19. The enhanced microwave absorption can be explained by the dielectric loss due to MWCNT, and the magnetic loss initiated by CuFe10Al2O19. We have also measured the relative complex permittivity and permeability, which assists to elucidate the improved microwave absorption property of the composites. The nanocomposite also showed a better specific capacitance of 269 F/g, and good electrochemical impedance properties than those of the pristine MWCNT and CuFe10Al2O19. This work can be extended effectively in the future for both supercapacitor and microwaveabsorbing applications.

1. INTRODUCTION In the present days, the term “Electromagnetic Interference (EMI) Problem” is very much well-known to researchers. Because of the EMI problems, the actual performance of the electronic devices becomes very poor. Hence, to solve this serious problem, the usage of microwave absorbing material is increasing day by day. Microwave absorbers also can be used for shielding and stealth technology. In defense, the use of radar-absorbing material (RAM) is been underway since World War II. Therefore, the demand for a suitable microwave absorber is very high in both commercial and military applications. More precisely, the demand is to develop the thin microwave absorber with a high bandwidth.1,2 A microwave absorber absorbs electromagnetic energy and then dissipates it as heat energy, via both dielectric and magnetic losses. Hence, the preparation of composite materials containing both dielectric and magnetic components, and their microwave absorbing properties, have been extensively studied for many years. Carbon nanotubes (CNTs) have generated an immense research interest in wide applications, because of their unique mechanical, electrical, and magnetic properties. Li et al. stated that the energy gaps of CNTs fall into the microwave energy range after the electronic level splitting. They have also reported that high specific surface area and a large number of surface suspension parts of CNTs are the main reason for good interfacial polarization and multiple scattering and, thus, the enhanced absorbing properties of CNTs.3 CNT compounds and their composite materials have already left a good impression on many reports that involve the preparation of CNT-based microwave absorbers.4−7 Here, we have used CNT as a dielectric candidate for the preparation of nanocomposites, because CNT has brilliant thermal stability, resistance to chemical agents, high aspect ratio, excellent electrical conductivity, and good dielectric behavior.3,8−11 Many reports are available on the usage of various magnetic materials, © XXXX American Chemical Society

such as ferrite, metal powder, carbonyl iron powder etc., to prepare this type of composite.12−17 So, people thought that a composite made of CNT and magnetic material could be a potential microwave absorber. Some research has also been reported on this particular topic.18−20 Che et al. reported a maximum reflection loss (RL) of −18 dB at 9 GHz for the CNT−CoFe2O4 nanocomposite prepared by chemical vapor deposition process, whereas, the maximum reflection loss for CNT and CoFe2O4 is −6 dB and −8.3 dB, respectively.21 Yi et al. reported on the microwave absorption properties of multiwall carbon nanotube (MWCNT)/Co/resin composites, which showed the reflection loss of −10 dB.22 Although some reports are accessible, this topic still has a lot of area for improvement, such as the ease of synthetic procedure, performance of the absorber, development of thin and lightweight absorber etc. The M-type hexagonal ferrites are also considered as a potential candidate for this application, because they have good magnetic properties with planar anisotropy, as well as excellent chemical and thermal stability.23 The research that has been reported by Chen et al. revealed the microwave absorption properties of BaAl2Fe10O19 and its composite material with poly(o-toluidine). The composite showed better reflection loss (−29.16 dB) than that shown by only BaAl2Fe10O19 (−25.14 dB).12 The composites based on barium ferrite and polypyrrole24 or poly(3,4-ethylenedioxy thiophene)25 also have been examined as microwave absorbers. Ghasemi et al. reported a maximum reflection loss of −32 dB for a MWCNT/Mg−Co−Zr-substituted barium ferrite/PVC composite.26 To the best of our knowledge, CNT and many hexaferrites24,25,27 have been investigated individually, with Received: February 22, 2013 Revised: May 18, 2013 Accepted: June 20, 2013

A

dx.doi.org/10.1021/ie4005783 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Merck, Ltd., India) was used as an electrolyte for the CV and EIS measurements to be performed. 2.2. Preparation. 2.2.1. Preparation of CuFe10Al2O19. To prepare the crystalline hexaferrite compound of chemical formula CuFe10Al2O19, first, all the desired metal nitrate salts were dissolved in a minimal amount of doubly distilled water in their appropriate stoichiometric amounts. The nitrate solution then was stirred vigorously for 10 min in a magnetic stirrer to make it homogeneous. That solution then was added to another beaker containing concentrated NaOH aqueous solution at room temperature and was stirred for 2 h to complete the process of precipitation at 80 °C. The precipitate then was filtered, washed several times with distilled H2O (until the filtrates becomes neutral, checked by pH paper), and collected for further treatment. The precipitate was divided into three equal portions and calcined slowly in air at three different calcining temperatures: 500, 700, and 900 °C. The time for calcination is 3 h and maintained equally for all portions. 2.2.2. Preparation of CuFe10Al2O19/MWCNT. To make it more dispersive in the solvent, and to activate its surface for reaction, pristine MWCNT was modified by a mixture of HNO3 and H2SO4 (3:1), according to the procedure described by Yuen et al.,32 prior to the treatment of MWCNT by metal salts. A small quantity (0.2 g) of acid-modified MWCNT was dispersed in doubly distilled H2O through sonication for 30 min, in the presence of a cationic surfactant (CTAB). The aqueous solution of metal nitrates then was added to it and further sonicated for 1 h. Then, the in situ precipitation process was initiated by adding the aqueous NaOH solution dropwise to the final solution. The solution was filtered, washed with distilled H2O, and calcined slowly for 3 h at 900 °C under an oxygen atmosphere. It is well-established that coated CNTs can resist degradation at much higher temperatures, compared to pristine CNTs, in an oxidizing atmosphere,33,34 and they can also be treated at such high temperature.28,35 Therefore, we have calcined our as-prepared materials effectively at 900 °C in air. 2.2.3. Preparation of CuFe10Al2O19/MWCNT/TPU. The final nanocomposites were prepared through solution mixing process with three different percentages of CuFe10Al2O19coated MWCNT in the TPU matrix. These TPU-based nanocomposites were necessary for the measurement of the microwave absorption properties of those nanocomposites. The composition details and sample codes for all RAMs are given in Table 1. At first, TPU beads were placed in a beaker containing

regard to their use in preparing many microwave absorbers, but the number of reports considering the combination of both are, to date, very few. Our objective is to determine the easiest procedure to fabricate CNT with a magnetic material (like Mtype hexaferrite) and enhance their reflection loss value effectively over a broad frequency range in the X-band (8.2− 12.4 GHz) region. Li et al. reported the synthesis of CNT/ SrFe12O19 composites via an in situ sol−gel technique using a cotton template.28 Here, we have prepared one new M-type hexaferrite, CuFe10Al2O19, through a simple chemical coprecipitation route and used it as a magnetic component for the preparation of the composite. We have implemented the in situ technique to fabricate MWCNT with the magnetic component, so that the interfacial interaction between MWCNT and CuFe10Al2O19 particle becomes greater. Hence, we have developed a composite material based on one dielectric component (MWCNT) and one magnetic component (CuFe10Al2O19), with thermoplastic polyurethane (TPU) as a polymer matrix. TPU was chosen because of its highly flexible nature.29 The nanocomposites were prepared with different filler percentages and characterized by different analytical techniques. Over the past few years, the demand for clean energy sources such as supercapacitors has been very high, and CNTs have been considered to be promising candidates in this particular field.30 Spinel ferrites have also started to show their importance in the supercapacitor field.31 Thus, we thought that our material may also have good electrochemical properties and could be used as electrode materials for supercapacitor applications. Hence, we are interested to investigate the electrochemical properties of the prepared nanocomposites. We have performed cyclic voltammetry (CV) tests and electrochemical impedance spectroscopy (EIS) in a threeelectrode system for those nanocomposites. Finally, we have explored the effect of MWCNT on both electrochemical and microwave absorption property (in X-band region) of the prepared nanocomposites.

2. EXPERIMENTAL WORK 2.1. Materials. All the required metal nitrates [Cu(NO3)2, Al(NO3)3·9H2O, and Fe(NO3)3·9H2O] to prepare CuFe10Al2O19, were purchased from Merck, Ltd., India. MWCNTs (Guangzhou Jiechuang Trading Co. Ltd., China) having an outer diameter of 20−30 nm and a length of 3−15 μm were used as components for the preparation of nanocomposites. HNO3 and H2SO4, used for the acid modification of MWCNT, were obtained from Loba Chemie PVT, Ltd., India. Tetrahydrofuran (C4H8O, E. Merck Ltd., India) was used as a solvent for the preparation of RAM. Sodium hydroxide (NaOH, Loba Chemie PVT, Ltd., India) was used as a precipitating agent. Cetyltrimethylammonium bromide (CTAB) [(C16H33)N(CH3)3Br, Loba Chemie PVT, Ltd., India] played the role of a surfactant. All the chemicals and materials were used as purchased, without any purification, unless specified. The TPU (Lubrizol Advanced Materials, Thermedics, Inc. Polymer Products, USA) used for developing RAMs belonged with commercial medical-grade aliphatic polyether (TecoflexVR EG 80A, injection grade). Tecoflex EG 80A (∼35% of hard segments) has a shore hardness of 72A, a specific gravity of 1.04, and its constituent formulation contains methylene bis-(cyclohexyl) diisocyanate (HMDI) as the hard segment, and polytetramethylene oxide (PTMO) as the soft segment (molecular weight = 1000 g/mol), and 1,4butane diol (BD) as a chain extender. Potassium chloride (KCl,

Table 1. Composition Used for the Preparation of RadarAbsorbing Materials (RAMs) Component (%) sample code

CuFe10Al2O19/MWCNT

TPU

RAM-1 RAM-2 RAM-3

10 15 20

90 85 80

150 mL of THF and subjected to stirring overnight at room temperature and then this solution was stirred for another 3 h at 60 °C. After the dissolution of TPU in THF, the required filler was added to it, in desired ratios. The stirring was continued by a mechanical stirrer to ensure that the fillers were well-dispersed in the matrix. The mixture then was heated at 70 °C to evaporate the solvent and make the mixture highly concentrated. Then, this highly concentrated mixture was B



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Figure 1. (a) XRD analysis of CuFe10Al2O19 and (b) XRD analysis of (i) MWCNT, (ii) CuFe10Al2O19 prepared at 900 °C, and (iii) CuFe10Al2O19/ MWCNT.

poured onto a Petri dish and heating was continued at 70 °C for complete drying. Final shaping of the prepared nanocomposites was carried out using a compression molding machine at 5 MPa of pressure and a temperature of 170 °C. Finally, the samples were cut into 2.5-mm-thick rectangular slabs with a cross-sectional area of 0.4 in. × 0.9 in. to fit into the X-band waveguide for microwave measurements. To determine the effect of MWCNT on the microwave absorption property of the nanocomposites, we have also prepared CuFe10Al2O19/ TPU nanocomposites.

performed to understand the chemical constituents present in the nanocomposites. 3.7. Magnetization Study. The magnetic properties were measured using a Quantum Design Evercool SQUID-VSM magnetometer with an applied magnetic field strength over the range from −15 kOe to +15 kOe at room temperature. The frequency of VSM used for the measurement was 12.4 Hz. 3.8. Microwave Measurements. Microwave absorption study was done by using a two-port vector network analyzer (ENA E5071C). The microwave absorption property of the prepared RAMs was measured using the Transmission Line method, where the material was placed inside an enclosed rectangular waveguide transmission line. Relative complex permittivity (εr) and permeability (μr) was computed from the measurement of the reflected signal (S11) and transmitted signal (S21). 3.9. Electrochemical Study. CV and EIS of the materials were carried out on a GAMRY instrument (750 mA and 1.6 V), using a three-electrode system where platinum and a saturated calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The electrodes were prepared according to the procedure described by Bhattacharya et al.36 CV measurements were performed in the voltage range of −0.8 V to 0.8 V versus SCE. For the EIS measurements, a frequency range from 1 MHz to 0.1 Hz was applied. 3.10. Thermogravimetric Analysis. Thermogravimetric analysis (TGA) of the composites was performed using a Dupont Model 2100 thermogravimetric analyzer. The TGA measurements were conducted at a heating rate of 10 °C/min under a nitrogen atmosphere from 30 to 800 °C.

3. CHARACTERIZATION 3.1. X-ray Diffraction Analysis. The samples were characterized by X-ray diffraction (XRD), which was conducted on a Rigaku X-ray Diffracrometer (Model ULTIMA III) with Cu Kα radiation (λ = 1.5418 Ǻ ) at a scanning rate of 1°/min. 3.2. X-ray Photoelectron Spectroscopy. X-ray photoelectron spectroscopy (XPS) of the prepared hexaferrite was determined using a PHI Versa Probe 5000 calibrated system with the C 1s peak centered at 284.6 eV. 3.3. Fourier Transform Infrared Spectroscopy. Fourier transform infrared (FTIR) spectroscopy of nanocomposites was carried out using a NEXUS 870 FTIR system (Thermo Nicolet) to detect the presence of different functional groups. The samples were prepared by mixing the composites with potassium bromide (KBr) in a weight ratio of 1:10 and pelletized. 3.4. Field-Emission Scanning Electron Microscopy. A Carl Zeiss-SUPRATM 40 field-emission scanning electron microscopy (FESEM) system with an accelerating voltage of 5 kV was used to understand the morphology of the nanocomposites. For the FESEM analysis to be done, the specimens were coated with a thin layer of gold for electrical conductivity. 3.5. Transmission Electron Microscopy (TEM). The coating of MWCNT by CuFe10Al2O19 was analyzed using a transmission electron microscopy (TEM) system (FEI TM, Type 5022/22, Technai G2 20 S-Twin) operated at a voltage of 200 kV. A slight amount of sample was dispersed in acetone by sonication for 30 min and then it was deposited dropwise on the copper grid to perform the TEM analysis. 3.6. Energy-Dispersive Spectroscopy. Energy-dispersive spectroscopy (EDS), in association with FESEM, was

4. RESULTS AND DISCUSSION 4.1. XRD Analysis. Figure 1a shows the XRD pattern of CuFe10Al2O19 prepared under three different sintering temperatures. Each sample was sintered for 3 h. At sintering temperature of 500 °C, no characteristic peaks for the hexagonal structure was observed. The phase separation started at 700 °C, and at 900 °C, so the characteristic peaks for the hexagonal structure were observed. The material also contains a peak that is due to a secondary phase (marked by an asterisk, *) coming from unreacted hydroxides or hematite.37 The intensity of the peak for secondary phase became weaker at higher sintering temperature, which indicates that the contribution C



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Figure 2. XPS analysis of CuFe10Al2O19 prepared at 900 °C: (a) survey analysis and high-resolution (deconvoluted) spectra for (b) Fe 2p, (c) O 1s, (d) Al 2p, and (e) Cu 2p.

from the secondary phase was decreased at that temperature. At a sintering temperature of 900 °C, the peaks appeared for CuFe10Al2O19 (at 2θ = 23.1°, 30.54°, 32.39°, 34.29°, 37.29°, 40.97°, 42.65°, 54.79°, 57.56°, and 63.17°). The peaks have been assigned to the corresponding 2θ values shown in Figure 1a. The result is in good agreement with the similar types of observations by Chen et al.12 and Hu et al.38 Hence, from the XRD patterns, the M-type hexagonal structure of CuFe10Al2O19

was confirmed. From XRD, we have measured the crystallite size of CuFe10Al2O19 corresponding to peak (114), which is ∼56 nm. The increase in sintering temperature assists to strengthen the relative intensity of the diffraction peaks, which indicates a better structural quality of the materials. Hence, we have proceeded with CuFe10Al2O19, which was calcined at 900 °C for further study with MWCNT. Figure 1b shows the XRD pattern to confirm the formation of CuFe10Al2O19 particles in D



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the presence of MWCNT. MWCNT showed its characteristic (002) peak at 2θ = 25.69°, as shown in Figure 1b(i). Figure 1b(ii) showed the XRD peaks for CuFe10Al2O19, prepared at a calcining temperature of 900 °C. The XRD pattern of the final material containing both MWCNT and CuFe10Al2O19, shown in Figure 1b(iii), possessed all the expected peaks coming from each single component. In the case of Figure 1b(iii), the characteristic (002) peak of MWCNT at 2θ = 25.69° has appeared more sharp than the pristine MWCNT (Figure 1b(i)), which may be due to the exclusion of amorphous impurities from pristine MWCNT during the process of heating.39−44 Hence, from the XRD analysis, the CuFe10Al2O19 particle may adsorb on the surface of MWCNT or the particle is simply present with MWCNT. Now, to further confirm the adsorption of CuFe10Al2O19 particle on the surface of MWCNT, we have done some other characterization, as discussed below. 4.2. X-ray Photoelectron Spectroscopy. Figure 2 shows the X-ray photoelectron spectroscopy (XPS) analysis of the prepared hexaferrite (CuFe10Al2O19). The survey analysis (Figure 2a) showed the peaks of all the elements present in the CuFe10Al2O19 i.e., Cu 2p, Fe 2p, Al 2p, and O 1s. Hence, from the XPS survey analysis, we can confirm the presence of the entire desired element used for the preparation of hexaferrite. The high-resolution spectra (deconvoluted spectra) for each element was also taken and shown in Figures 2b−e. We have calculated the elemental concentration present in CuFe10Al2O19. The ratio of Cu:Al is ∼1:2, which is in good agreement with the stoichiometric ratio of CuFe10Al2O19. The ratio of Cu:Fe is not perfectly 1:10, but it is expected, according to the stoichiometric ratio of CuFe10Al2O19. This is may be due to the presence of a secondary phase (confirmed by XRD analysis) in the prepared hexaferrite. Hence, the result of XPS analysis can be correlated to the result of XRD analysis. Figure 2b showed the deconvoluted spectra for Fe 2p, which contain the characteristic peaks, i.e., 710.6 eV (2p3/2), 724.4 eV (2p1/2), and one satellite peak (denoted by S) at 718.9 eV. Figure 2c showed the high-resolution spectra for O 1s at 530.4 eV. Figure 2d shows the high-resolution spectra for Al 2p, which contain one single peak at 73.4 eV. Figure 2e is the high-resolution spectra for Cu 2p; it contains four peaks at different positions, i.e., 934.2 eV (2p3/2), 953.9 eV (2p1/2), and two satellite peaks at 942.2 eV and 961.8 eV. 4.3. Fourier Transform Infrared Spectroscopy. Figure 3 shows the FTIR analysis to determine the functionalities present in the prepared materials. The FTIR spectrum of acidmodified MWCNT has already been reported by our group previously. 45 That analysis fruitfully showed that the modification process was good enough to incorporate functionalities such as CO, −OH, −COOH, etc. on the surface of MWCNT. The peaks at 439 cm−1, 550 cm−1, and 745 cm−1 are the characteristic peaks of Fe−O bond vibration present in CuFe10Al2O19.46 The bands arise from lattice vibrations of the oxide ions against the cations. The peak at 3443 cm−1 is due to the hydroxyl group47 and indicates that either the reaction was incomplete or the material absorbed moisture from the atmosphere during analysis. The peak at 2368 cm−1 may be assigned to the bending vibration of the adsorbed molecular water.28 In the case of CuFe10Al2O19coated MWCNT, one broad peak is present at 587 cm−1, which may be due to the bonding of MWCNT with metal−oxygen bond of CuFe10Al2O19. CuFe10Al2O19-coated MWCNT has two peaks, at 2843 cm−1 and 2930 cm−1, which are due to the C−H

Figure 3. Fourier transform infrared (FTIR) spectroscopy of CuFe10Al2O19 prepared at 900 °C (blue trace) and CuFe10Al2O19/ MWCNT (red trace).

stretching of H−CO in the carboxyl group present in the MWCNT.48 It also contains one peak at 3443 cm−1, which represents the hydroxyl group. The peak at 1030 cm−1 corresponds to the C−O bond, and 1608 cm−1 is associated with the stretching of the CNT backbone.49 Relative intensity and position of the FTIR peaks of CuFe10Al2O19-coated MWCNT is slightly different from the mother component, which is due to the interaction between the components. Hence, from the FTIR analysis of the prepared materials, it is confirmed that the CuFe10Al2O19 particle was formed on the surface of the MWCNT. 4.4. Morphological Study. Figures 4a and 4b show the FESEM image for a CuFe10Al2O19 particle calcined at 900 °C and CuFe10Al2O19/MWCNT respectively, which further confirmed the result of their XRD and FTIR analysis. Figure 4a clearly shows the hexagonal plates of CuFe10Al2O19 and again confirms their formation. Some agglomeration is also noticed in Figure 4a. This agglomeration may occur due to the magneto dipole interaction among CuFe10Al2O19 particles. Figure 4b shows the FESEM image for the formation of nanocomposites based on CuFe10Al2O19 and MWCNT. This image is quite clear to observe the presence of both components. We have shown the TEM image (Figure 4c) to confirm the coating of MWCNT by CuFe10Al2O19. The characteristics of the TEM image are consistent with the FESEM image (Figure 4c), where both the image showed that some hexaferrites are also formed on the outside of the MWCNT surface. 4.5. Energy-Dispersive Spectroscopy. To further support the FESEM results, we have done the EDS analysis of the prepared substances. EDS analysis helps to know the chemical elements present in the materials of concern. Figures 5a and 5b show the EDS spectra of CuFe10Al2O19 and CuFe10Al2O19/MWCNT, respectively. The EDS spectrum of CuFe10Al2O19 contains all the constituent elements, i.e., oxygen (O), aluminum (Al), iron (Fe), and copper (Cu). However, the EDS spectrum of CuFe10Al2O19/MWCNT contains oxygen, aluminum, iron, and copper, along with carbon (C), which comes from MWCNT. 4.6. Magnetic Property. We have applied an easy and simple technique to confirm the magnetic behavior of the E



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Figure 4. FESEM images of (a) CuFe10Al2O19 prepared at 900 °C and (b) CuFe10Al2O19/MWCNT, and (c) TEM image of CuFe10Al2O19/ MWCNT.

Figure 5. EDS study of (a) CuFe10Al2O19 prepared at 900 °C and (b) CuFe10Al2O19/MWCNT.

CuFe10Al2O19 prepared at 900 °C. The entire process is depicted in Figure 6. The figure shows the picture, both in solid state and solution state (acetone is the solvent), which indicates that the material is attached to the magnet when it comes in close proximity to the material. Figure 6a shows the photograph of the material without the magnet, and Figure 6b shows the photograph after application of the magnet. Therefore, from this study, we can assume that the prepared hexaferrite, CuFe10Al2O19, is a magnetic material. For clear understanding, we have further included the magnetization versus applied magnetic field plot (hysteresis plot) at room temperature; this is shown in Figure 6c. The coercivity of CuFe10Al2O19 is 2395 Oe, which depends upon many factors, such as microstructure, grain shape, composition, and magnetic anisotropya . The presence of hysteresis loop confirms that CuFe10Al2O19 is a magnetic material, which is highly desirable for microwaveabsorbing applications. The remanence magnetization of prepared CuFe10Al2O19 is 0.34 emu/g. The result is consistent with the observation made

by Reena et al.,50 and they have expressed that these types of materials (having low remanence magnetization) can be used for microwave-absorbing applications. For effective microwave absorption, the magnetic materials with low remanence magnetization (soft magnetic materials) are useful.51 Soft magnetic materials are useful for this particular purpose, because the magnetization and demagnetization process will be easy and fast,52 and, hence, the material will absorb much energy from the wave. This can be correlated to many other observations where the remanence magnetization of a magnetic material becomes low after its coating by a nonmagnetic material, and then its microwave absorption increases for the material having low remanence magnetization.53,54 4.7. Microwave Measurements. We have used eqs 1 and 255 to determine the microwave-absorbing properties of the prepared RAMs, in terms of reflection loss (RL): F



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Figure 6. Pictorial presentation showing the magnetic behavior of CuFe10Al2O19 prepared at 900 °C.

Z=

⎛ μr ⎞ ⎤ ⎡ ⎛ 2π ⎞ ⎜ ⎟ tanh⎢ −j⎜ ⎟( με r )fd ⎥ r ⎝ ⎠ ⎦ ⎣ c ⎝ εr ⎠

⎛ Z−1 ⎞ RL (in dB) = − 20 log⎜ ⎟ ⎝ Z+1 ⎠

maximum reflection loss of RL = −11.7 dB at 10.7 GHz. In contrast, CuFe10Al2O19/MWCNT/TPU showed higher reflection loss, compared to the reflection loss shown by CuFe10Al2O19/TPU composite. Hence, MWCNT played an effective role to enhance the reflection loss value of the prepared nanocomposites. This enhancement in reflection loss for RAM-1, RAM-2, and RAM-3 can be due to the high surface area and high electrical conductivity of MWCNT.7,56 This can also be happen due to the both dielectric and magnetic loss initiated by MWCNT and magnetic material CuFe10Al2O19, respectively. RAM-1, RAM-2, and RAM-3 displayed maximum reflection losses of −47.9 dB at 11.96 GHz, −49.54 dB at 11.81 GHz, and −58.51 dB at 11.82 GHz, respectively. The maximum reflection loss is found to move slightly toward the low-frequency region as the filler weight percentage increased, which can be correlated with the result obtained by Qing et al.57 The differences in the RL values of RAM-1, RAM2, and RAM-3 are not so prominent. But still, the reflection loss increases as the loading percentage of fillers increases. In general, we can say that this difference in the RL result may come due to the difference in their ability to absorb the microwave radiation and attenuation. This phenomenon can be explained with the help of Figure 8. As the filler loading percentage of the prepared nanocomposites increases from RAM-1 to RAM-3, the probability of connectivity among the filler particles will also increase with the increase in the loading of fillers. Hence, in RAM-1 (10% filler loading), the connectivity among the filler particles will be less, RAM-2 (15% filler loading) will have moderate connectivity, and in RAM-3 (20% filler loading), the chances of connectivity among the filler particles will be maximum. Now, when the microwave radiation will be incident on RAMs, some part will be absorbed and attenuated (attenuation is shown by the red line in Figure 8) and the remaining part will be transmitted (which means a leakage of radiation) through the absorber. Since the connectivity among the filler particles in RAM-3 is highest, the leakage of radiation (represented by the black arrow in Figure 8) will be minimum or the attenuation path will be maximum, and, hence, the absorption will be maximum (i.e., maximum reflection loss). In the case of RAM-1, the leakage of microwave radiation will be maximum and, hence, the absorption will be minimum, i.e., the reflection loss will also

(1)

(2)

where Z is the normalized input impedance, with respect to the impedance in free space, and reflection loss (RL) is expressed in decibels (dB). εr (εr = ε′ − jε″) and μr (μr = μ′ − jμ″) are the relative complex permittivity and permeability of the absorber, respectively; f and c are the frequency of microwave in free space and the velocity of light, respectively; and d is the sample thickness. The variables ε′ and ε″ are known as the real and imaginary parts of the relative complex permittivity, whereas μ′ and μ″ denote the real and imaginary parts of relative complex permeability respectively. Figure 7 represents the reflection loss vs frequency plot for all the RAMs, which showed the absorbing properties in a wide frequency range in the X-band region. The dip of the curves designates the maximum RL, which means the reflection is minimum or the absorption become maximum at that particular point. CuFe10Al2O19/TPU (with 20% filler loading) showed a

Figure 7. Reflection loss (RL) of the prepared RAMs. G



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Figure 8. Possible mechanism for explaining the difference in microwave absorption by the prepared RAMs.

Figure 9. (a) Real part (ε′), (b) imaginary part (ε″), and (c) loss tangent (tan δε) of the relative complex permittivity. (d) Real part (μ′), (e) imaginary part (μ″), and (f) loss tangent (tan δμ) of the relative complex permeability of the prepared RAMs.

polarization happening in the material is related to the ε′ and ε″ values, which are associated with the energy dissipation.26 The dielectric performance of a RAM may depend on different polarization mechanisms such as ionic, electronic, orientational, and space charge polarization. In case of the microwave frequency range, the effective contribution can only be anticipated by orientational and space charge polarization. The space charge polarization is associated with the heterogeneity presents at the interface between the components of the composites, and the bound charge (dipoles) present in the material is responsible for the orientational polarization.60 Therefore, in this particular case, the dielectric performances of all the RAMs (RAM-1, RAM-2, and RAM-3) are related to both the space charge and orientational polarization. The difference in the conductivity between MWCNT and CuFe10Al2O19 particle is responsible for the generation of space charge and also its polarization. In situ synthesis of this composite helps to generate more interfacial sites between the components, and therefore these two polarization mechanisms occur easily and showed high microwave absorption property. In the case of relative complex permeability, both the real and imaginary parts showed a decreasing trend with increase in frequency. Figure 9d, 9e, and

be minimum. RAM-2 will show a moderate RL value, i.e., between the RL values shown by RAM-1 and RAM-3. With the greater interest in knowing the proper reason for variation in the reflectivity of the RAMs, we have measured the relative complex permittivity and permeability of the RAMs, as shown in Figure 9. Figures 9a, 9b, and 9c respectively show the variation of real, imaginary, and loss tangent (tan δε = ε″/ε′) components of the relative complex permittivity with the increase in frequency. The real part of the complex permittivity showed an increasing trend with the increase in frequency and also with the increase in loading percentage of the fillers in Xband region. This result is in good agreement with the result shown by Al-Hartomy et al., who explained it by considering the percolation theory.58 The increase of ε′, relative to the increase in filler percentage, may be due to the increase in bound charges present in the material, because high bound charges creates high displacement current in the material during microwave treatment and, hence, the real part of the permittivity increases.59 The imaginary part remains almost constant throughout the entire frequency range, with some resonance peaks. The tan δε plot showed almost the same behavior, with some differences in value in different frequencies for all the RAMs. The extent of H



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Figure 10. Cyclic voltammograms of CuFe10Al2O19 prepared at 900 °C and CuFe10Al2O19/MWCNT at three different scan rates.

Table 2. Specific Capacitance, Energy Density, and Power Density Obtained at Different Scan Rates from Cyclic Voltammograms CuFe10Al2O19

CuFe10Al2O19/MWCNT

scan rate (mV/s)

specific capacitance (F/g)

energy density (Wh/kg)

power density (W/kg)

scan rate (mV/s)

specific capacitance (F/g)

energy density (Wh/kg)

power density (W/kg)

10 50 100

4 2.35 1.75

1.42 0.83 0.62

16 47.42 69.75

10 50 100

269 76.6 36.25

95.64 27.24 13

1076 1532.25 1462.5

case of three electrode cells, the capacitance values are the capacitance per electrode. The shape of CV curves is not perfectly rectangular, which indicates the redox behavior of the materials. The CV plots of CuFe10Al2O19 clearly shows the presence of redox peaks, which may arise due to the oxidation and reduction between the metal cations as in the case of spinel ferrites.65 However, in the case of the CuFe10Al2O19/ MWCNT nanocomposites, the redox peaks are not very clear, which may be due to the unavailability of metal cations for redox reaction, because of the coating of CuFe10Al2O19 on the MWCNT surface. Table 2 shows the specific capacitance obtained for CuFe10Al2O19 and CuFe10Al2O19/ MWCNT at various scan rates. CuFe10Al2O19 showed the maximum specific capacitance of 4 F/g, which is due to the insulating character of the material and the maximum specific capacitance of CuFe10Al2O19/ MWCNT was calculated as 269 F/g at 10 mV/s. The presence of MWCNT improves the specific capacitance many-fold. This increase in the Csp value of CuFe10Al2O19/MWCNT can be attributed to the following possible reasons: (1) Pristine MWCNT generally provides double-layer capacitance. On the other hand, CuFe10Al2O19 usually provides pseudo-capacitance. In the nanocomposite, these two individual components contribute to the overall capacitance. Hence, because of the combined contribution of the two components, CuFe10Al2O19/ MWCNT showed better capacitance properties than pristine MWCNT and CuFe10Al2O19 individually. (2) Superior interaction between MWCNT and CuFe10Al2O19 is also responsible for the enhancement of the capacitance properties of the nanocomposite. (3) The coating of CuFe10Al2O19 on the MWCNT surfaces also provided a superior structure for easy electrolyte accessibility and, hence, the capacitance properties of the nanocomposite increased. Because of the easy availability

9f respectively show the variation of the real, imaginary, and loss tangent (tan δμ = μ″/μ′) components of the relative complex permeability with the increase in frequency. The loss mechanisms involved in this case may be hysteresis loss, magnetic domain-wall motion, and spin rotation.58,61 Since the hexaferrites have high magneto-crystalline anisotropy, so the strong coupling between the magnetic dipoles is expected. Now, the fact is that, at the high frequency of the applied field, the induced magnetization and applied field differ from each other and magnetic loss occurs.62,63 Hence, it may happen in this present case also. So, the excellent microwave absorption property of the prepared nanocomposites is due to the cooperation coming from both dielectric and magnetic components. We have already reported the microwave absorption property of MWCNT/TPU composite with the maximum reflection loss of −7.6 dB.4 So, it is obvious that the CuFe10Al2O19/MWCNT/TPU composite showed an improved microwave absorption property than that of both CuFe10Al2O19/TPU and MWCNT/TPU composites. 4.8. Electrochemical Study. 4.8.1. Cyclic Voltammetry. We have performed the CV analysis of CuFe10Al2O19 and CuFe10Al2O19/ MWCNT at various scan rates in the potential range of −0.8 V to 0.8 V versus SCE in 1 M KCl solution as the electrolyte. We have shown the CV plots of those materials in Figure 10. The plot has two regions: the negative current zone refers to the reduction in cathode and the positive current region refers to the oxidation in anode. We have used eq 3 to calculate the specific capacitance of the materials (Csp).64

Csp =

I+ − I − v×m

(3)

where Csp is the specific capacitance obtained by CV measurements; I+ and I− denote the maximum current in the positive and negative voltage scans, respectively; v is the scan rate; and m is the mass of the composite electrode materials. In I



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Figure 11. Nyquist plots of CuFe10Al2O19 prepared at 900 °C (left) and CuFe10Al2O19/MWCNT (right).

Table 3. Fitting Data for Equivalent Electrical Circuit Elements by the Simulation of Impedance Spectra Rs (Ω)

sample CuFe10Al2O19 CuFe10Al2O19/MWCNT

2.622 1.870

Rct (Ω) 2.736 × 10 41

15

1 (CV 2) 2

power density (P) =

⎛E⎞ ⎜ ⎟ ⎝t⎠

CPE (S-sn) × 10−3

n

817 × 10 12.6 × 10−3

0.477 0.045

0.860 0.710

9

45° straight line is known as the Warburg resistance (W), which represents the mass-transfer parameters of the electrochemical doping process. Pure capacitance behavior is represented by a 90° straight line, indicating that the diffusion layer involves the entire electrode mass.36 We have interpreted the Nyquist plots by fitting the experimental impedance spectra to an equivalent electrical circuit. The fitting data is tabulated in Table 3. The suitable equivalent electrical circuit is shown in Figure 12. In

of electrolyte access, the nanocomposite also showed good impedance properties.66 (4) The availability of high surface area, high aspect ratio, excellent electrical conductivity of MWCNT, and moreover, the charge transfer process can occur between hexaferrite particles and MWCNT also helps to improve the electrochemical properties of the composite.67,68 Hence, the specific capacitance of CuFe10Al2O19/ MWCNT nanocomposites is much higher than that of MWCNT (10− 135 F/g) and CuFe10Al2O19 alone.69 We have also calculated the energy density and power density, using eqs 4 and 5, respectively;64 these values are summarized in Table 2. energy density (E) =

W (S-s0.5)

(4)

(5)

Here, C is the specific capacitance (in F/g), V the operating voltage, and t the time required for a complete cycle (in seconds). The energy density and power density are also very important parameters in supercapacitors. CuFe10Al2O19/ MWCNT exhibited excellent energy and power density values, whereas CuFe10Al2O19 is not so impressive. The highest energy density that appeared for CuFe10Al2O19/MWCNT is ∼96 Wh/ kg at a scan rate of 10 mV/s. The highest power density for CuFe10Al2O19/MWCNT is ∼1533 W/kg, at a scan rate of 50 mV/s. Both the energy density and power density are quite high and suitable for supercapacitor applications. 4.8.2. Electrochemical Impedance Spectroscopy. The Nyquist plot at 0 V is shown in Figure 11 to analyze the EIS data obtained for the CuFe10Al2O19 and CuFe10Al2O19/ MWCNT materials. The higher-frequency region refers to the electrolyte properties, and the mid-frequency region is associated with electrode/electrolyte interface processes. The solution resistance (Rs) and charge transfer resistance (Rct) are the intercept of the curve at high frequency and mid frequency with the real axis, respectively. The impedance response of a

Figure 12. Equivalent electrical circuit used in EIS fitting for the prepared materials.

real-world systems, the capacitors always show some sort of deviation from ideal behavior. These imperfections of the capacitors are represented by a constant phase element (CPE), which can be recognized as a depressed semicircle in a Nyquist plot. The CPE can be defined by eq 6:

ZCPE = [Q 0(jω)n ]−1

(6)

0

where Q is a constant related to the surface and electroactive species and is independent of frequency; ω is the angular frequency, and n arises from the slope of a plot of log Z vs log f. The value of n covers a range of 0 to 1. A value of n = 1 indicates ideal capacitor behavior of the CPE element, whereas a value of n = 0 indicates the resistor and a value of n = 0.5 indicates the Warburg behavior. The values of n for J



Article

properties of the nanocomposites, because of the good interfacial interaction between the components of the composites. Hence, the preparation of MWCNT/hexaferrite nanocomposites via the in situ technique can be conducted efficiently in the future for both supercapacitor and microwaveabsorbing applications. The prepared nanocomposites are also useful for high-temperature applications as per the requirements for such uses.

CuFe10Al2O19 and CuFe10Al2O19/MWCNT are 0.860 and 0.710, respectively. Hence, both the materials have n values of >0.5, which indicates ideal capacitor behavior. 4.9. Thermogravimetric Analysis. Good thermal stability of a material is highly desirable for high-temperature applications. Hence, we have monitored the thermal degradation of the prepared materials with the assistance of a thermogravimetric analyzer. Figure 13 shows the residual

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 91-322-028-3978. Fax: +913222-255303. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are thankful to IIT Kharagpur. P.B. is also thankful to Council of Scientific and Industrial Research (CSIR), New Delhi, India for the financial support. The authors also thank DST, Government of India, for providing the XPS facility through the FIST program at IIT Kharagpur, India.

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REFERENCES

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Figure 13. TGA plot of RAM-1, RAM-2, and RAM-3.

weight (as a percentage) versus temperature plot for all of the materials. CuFe10Al2O19 and CuFe10Al2O19/MWCNT showed insignificant thermal degradation up to 800 °C. In the case of RAMs, the TGA plot patterns are different. RAM-1, RAM-2, and RAM-3 are stable up to 330 °C, but after that temperature, they all degraded up to 15% of their initial residual weight at ∼400 °C. They then showed a sharp decrease in their residual weight and degradation ended at 485 °C. All of the RAMs then were stable up to 800 °C, which means that there was no significant thermal degradation from 500 °C to 800 °C. At the end of the measurement, the remaining residue of RAM1, RAM-2, and RAM-3 is ∼10%, ∼15%, and ∼20%, respectively, which is in good agreement with the filler percentage used during the preparation of the nanocomposites. This can also be indirect proof of a good distribution of fillers in the matrix.

5. CONCLUSION In summary, we have established the successful synthesis of an M-type hexaferrite, which showed good magnetic properties and involved in the formation of nanocomposites with MWCNT. The hexagonal structure of the hexaferrite was confirmed by FESEM and XRD analysis. The composite of MWCNT and hexaferrite showed superior capacitance properties. Hence, it can be considered as a potential candidate for supercapacitor applications. CuFe10Al2O19/MWCNT/TPU showed the highest reflection loss of −58.51 dB, which is much higher than that of the individual components of the composites. This improved microwave absorption property is due to the great combination between dielectric and magnetic loss, i.e., good complementarity between relative complex permittivity and permeability. The in situ technique also helps to improve both the electrochemical and microwave absorption K



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