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expanded graphite composites by low-temperature combustion synthesis
Materials Science and Engineering B 185 (2014) 1–6
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Preparation and millimeter wave attenuation properties of NiFe2 O4 /expanded graphite composites by low-temperature combustion synthesis Xiong-biao Wang, Wen-feng Zhu, Xu Wei, Yao-xuan Zhang, Hou-he Chen ∗ School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 19 December 2013 Accepted 5 January 2014 Available online 17 January 2014 Keywords: Nickel ferrite Expanded graphite Low-temperature combustion synthesis Composite
a b s t r a c t In this paper, NiFe2 O4 /expanded graphite (EG) composites are successfully prepared by low-temperature combustion synthesis method. The morphology, structure and millimeter wave (MMW) attenuation properties of the NiFe2 O4 /EG composites are investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and MMV radar measurement device. The effects of fuel and stoichiometric ratio on the composites are also investigated. The results show that NiFe2 O4 /EG composites are uniform and have a good crystallinity. The composites possess better MMW attenuation properties than EG. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Expanded graphite (EG) is a kind of graphite compound with low density, high strength and excellent electrical conductivity [1,2]. It is widely used as a millimeter wave (MMW) absorbing material [3]. Its electromagnetic property can be changed by coating different materials in to it, which leads to potential military use. Liang et al. proved that, magnetic loss can promote microwave absorbing properties of carbon nanotubes [4], which has significance in providing a new technique to improve wave attenuation performances of wave absorbing materials. Nickel ferrite (NiFe2 O4 ) is one of the most versatile soft magnetic material, especially suitable for high-frequency applications with low magnetic coercivity, high electrical resistivity, low eddy current loss and excellent chemical stability [5–8]. Nickel ferrite is widely used in the field of magnetic drug delivery, magnetic information storage device, ferrofluids, sensors, catalysis, microwave absorber [9–14]. It might find potential application in enhancing wave absorbing abilities of MMW. Besides, Kim and his coworkers indicated that composites structure can improve wave absorbing properties obviously [15]. Therefore, based on these findings, we infer that nickel ferrite-included composites probably provide an effective direction for the preparation of high quality millimeter wave attenuating device. However, to our best knowledge, there are few reports about the preparation and
∗ Corresponding author. Tel.: +86 13852291760. E-mail address: [email protected] (H.-h. Chen). 0921-5107/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2014.01.004
MMW absorbing properties of NiFe2 O4 /EG composites. NiFe2 O4 /EG composites can be used as “artificial aerosol” materials, which can be launched into the air above and cover target to protect target from being detected. Up to date, there are plenty of methods available to synthesize NiFe2 O4 including co-precipitation [16], ball milling [17], sol–gel [18], hydrothermal [19], combustion method [20–24]. In particular, low-temperature combustion is a simple, safe and rapid synthesis process wherein the main advantages are energy- and time-saving, process controllable and reactants evenly mixed. It involves an exothermic and self-sustaining chemical reaction between the metal salts and a suitable organic fuel, which has great potential in the preparation of ferrites. A key characteristic of this technique is that the heat essential to drive the process is provided mainly by an exothermal reaction occurring among the reagents, which could be utilized to simultaneously converted graphite intercalated compound (GIC) to EG. Herein, in this paper, for the first time we presented a simple and practical low-temperature combustion synthesis approach for the preparation of NiFe2 O4 /EG composites. The composition, structure and morphology of the as-prepared composites were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectra and scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive Xray (EDX). The 3 and 8 MMW attenuation performances of the NiFe2 O4 /EG composites were also evaluated by MMW radar measurement device. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively,
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Fig. 1. Schematic illustration of the preparation of NiFe2 O4 /EG composites.
indicating that, the NiFe2 O4 /EG composites exhibit better MMW attenuation performances than EG. 2. Materials and methods 2.1. Preparation of graphite intercalated compound (GIC) All the reagents obtained commercially were used as received without further purification. All aqueous solutions were made using deionized water. The experimental procedure for the preparation of NiFe2 O4 /EG composites was as follows: natural flake graphite was dried at 60 ◦ C in drying oven for 24 h before mixed and saturated with sulfuric acid. Then, the appropriate amount of KMnO4 was added to acid saturated graphite at 35 ◦ C with ultrasonic treatment for 1 h. The graphite intercalated compound was obtained by washing the product with water until the pH value reached 6. After that, the product was dried at 60 ◦ C for 24 h for further use. 2.2. Preparation of NiFe2 O4 /EG composites NiFe2 O4 /EG composites were prepared by one-step lowtemperature combustion method. The schematic illustration of the preparation of NiFe2 O4 /EG composites is shown in Fig. 1. Nickel nitrate Ni(NO3 )2 ·6H2 O and iron nitrate Fe(NO3 )3 ·9H2 O were used as the reactants and CO(NH2 )2 (urea) was used as reducing agent. In a typical procedure, firstly, Ni(NO3 )2 ·6H2 O and Fe(NO3 )3 ·9H2 O were dissolved in water under stirring to form uniform solution. The molar ratio of Ni2+ :Fe3+ :CO(NH2 )2 was 1:2:7. GIC and ethylene glycol were then added into the solution. Ethylene glycol is usually used as a surfactant and reducing agent which prevent the agglomeration of nanoparticles and provide more fuel. The precursor mixture was heated by electric furnace to 300 ◦ C. During the heating process, the solution started vaporizing and produce a great deal of foams. When water was evaporated, spark appeared in corner of evaporating dish and spread through to entire mass, yielding a voluminous and white smoke. During this process, GIC was rapidly expanded and NiFe2 O4 /EG composites were obtained. Combustion was attempted with different fuel (carbohydrazide and urea) because the fuel controls the combustion temperature and gas amount thereby has important effects on the characteristics of the final product. 2.3. Characterization The crystalline structure, phase composition and crystallite size of NiFe2 O4 /EG composites were identified from XRD patterns ˚ for 2 value ranges obtained using Cu K␣ radiation ( = 1.541 A)
from 10◦ to 80◦ in X-ray diffractometer (Bruker D8 Advance). The morphology of composites was investigated by scanning electron microscope (JEOLJSM-6380LV) operated at an acceleration voltage of 20 kV. The Fourier transform infrared (FTIR) transmission spectrum of the samples mixed with KBr powder reference were obtained from 1800 cm−1 to 400 cm−1 using Thermo Scientific Nicolet IS-10 spectrophotometer. Raman spectra were collected in backscattering geometry with a Renishaw Invia Reflex system equipped with a charge-coupled device (CCD) detector and a confocal Leica microscope. Excitation of the sample was carried out with a 514.4 nm argon ion laser line, with acquisition time of 20 s and powder at the sample of about 20 mW. Transmission electron microscopy (TEM) images and energy dispersive X-ray (EDX, EDAX Genesis 2000) spectroscopy were preformed on a JEOL JEM-2100 microscope operating at 200 kV. MMW radar measurement device is composed of a transmitter, a receiver and data processing system and illustrative drawing of the experimental set-up for MMW measurement is shown in Fig. 2. MMW samples were prepared by sticking a certain amount of NiFe2 O4 /EG composites to the plastic tape. The rectangle area of plastic tape is 16 cm × 24 cm, 70 mL samples are spread evenly over the plastic tape and the thicknesses of each sample is about 1.8 mm.
3. Results and discussion 3.1. Structure and morphology Fig. 3 shows the X-ray diffraction (XRD) pattern for NiFe2 O4 /EG composites with different molar ratio of NiFe2 O4 /EG. The main peak at 2 = 23.2◦ is corresponding to the standard diffraction pattern of EG, whereas the relatively weak intensity peaks at 30.3◦ , 35.7◦ , 43.3◦ , 57.4◦ and 63.0◦ fit to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of NiFe2 O4 , respectively, indicating the cubic spinel structure of nickel ferrite. A side from the peaks of NiFe2 O4 and EG, no extra peaks are found in Fig. 3, which means only two components exist in the composites. As the molar ratio of NiFe2 O4 /EG increased, the peaks of NiFe2 O4 are enhanced. According to the peak intensity in Fig. 3, the main component in the composites is EG. Besides, the lattice parameters for the NiFe2 O4 calculated from the position of the (3 1 1) peak, according to structures with Materials Date Jade 8.5 software. Also, Scherrer’s formula was used to address the crystallite size of the particles from the broadening FWHM of the (3 1 1) peak in the XRD patterns, using the profile fitting and peak decomposition method of the Materials Date Jade 8.5 software. The crystallite size estimated by Scherrer’s formula is 28 nm. ˚ and it is in good accordance The lattice parameters value is 8.35 A, with the earlier reported of 8.36 A˚ for the nano NiFe2 O4 [25] which prove the efficiency of synthesis technique.
Fig. 2. Schematic diagram of measurement device for millimeter wave attenuation characteristics.
X.-b. Wang et al. / Materials Science and Engineering B 185 (2014) 1–6
3
Fig. 3. X-ray diffraction pattern of NiFe2 O4 /EG composites.
The effect of reactant molar ratio on the morphology of NiFe2 O4 /EG composite is evaluated by SEM as shown in Fig. 4. When the molar ratio of NiFe2 O4 /EG is 1:2 (Fig. 4(a and b)), only small amounts of NiFe2 O4 is successively adhered to the surface of EG with uneven distribution. In addition, as a result of a low content of NiFe2 O4 , the corresponding fuel does not provide enough heat to expand GIC sufficiently. So we adjusted the molar ratio of NiFe2 O4 /EG to 1:1 (Fig. 4(c and d)) and the result indicating that both the loading amount and dispersibility of NiFe2 O4 are improved. When the molar ratio NiFe2 O4 /EG is 2:1 (Fig. 4(e and f)), NiFe2 O4 nanoparticles are found agglomerated due to magnetic dipole interaction among the particles [25]. Fuel types also have an important influence on NiFe2 O4 /EG composites as shown in Fig. 4(g and h). Considerable amount of NiFe2 O4 covers the most part of EG surface when carbohydrazide was used as fuel. But the combination between NiFe2 O4 and EG are not tightly enough which is similar to the mixture of NiFe2 O4 and EG, and NiFe2 O4 powder peel off after shock. This might be due to the large volumes of gas release during the reaction process, facilitating the dispersion of powder but at the same time bring in disturbance detrimental to the connection effect. Finally, urea was chosen as optimal fuel for recombination processes, in which EG surface is firmly coated with NiFe2 O4 and EG expands well. The transmission electron micrographs of NiFe2 O4 /EG composites (obtained under optimal conditions with NiFe2 O4 /EG molar ratio 1:1 and urea as fuel) are shown in Fig. 5. The particles are firmly adhered to EG with even distribution and the size of particles ranges from 30 to 60 nm (Fig. 5). The EDX spectra of NiFe2 O4 /EG composites are shown in Fig. 6 and the composition details are listed in Table 1. The EDX peaks intensity is relative to atom content [26]. Fig. 6 shows C as the main peak with very small Fe and Ni peaks that arise from the coating. The inset figure is the enlargement for Fe and Ni peaks. It is obvious
that the sample contains only C, O, Fe, Ni and no other contamination elements. The atomic ratio of Ni:Fe:O is close to 1:2:4 which matches the exact stoichiometric ratio of nickel ferrite. The infrared (IR) spectrum of NiFe2 O4 /EG composites in the range 400–1800 cm−1 is shown in Fig. 7. The IR spectrum exhibits two intense bands between 454 cm−1 and 562 cm−1 belonging to the stretching vibration modes associated to the metal-oxygen absorption bands Fe O bonds in the crystalline lattice of NiFe2 O4 [27]. They are characteristically pronounced for spinel structures and for ferrites in particular. Intrinsic stretching vibrations of metal at tetrahedral site, Mtetra O are generally observed in the range of 620–550 cm−1 . The stretching vibrations of metal at octahedral site, Mocta O are generally observed in the range of 450–385 cm−1 [19–22]. IR spectrum also shows an absorption band at 1038 cm−1 , corresponding to the C O stretching mode, which belongs to graphite intercalation compound. The Raman spectrum of NiFe2 O4 /EG composites is shown in Fig. 8. From which, we can see the peak at 1580 cm−1 is assigned to the first-order G band, which is related to disordered sp2 -bonded graphitic carbon [28–32]. The 2D band at 2729 cm−1 is consistent with the multi-layer feature of graphite [32]. The results of FTIR and Raman spectra suggest that the composites consist of graphite and NiFe2 O4 .
Table 1 Stoichiometry composition from EDX. Element
Theoretical atomic%
Experimental atomic%
Experimental weight%
C O Fe Ni
73.6 15.1 7.5 3.8
91.4 5.2 2.3 1.1
79.9 6.1 9.3 4.7
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X.-b. Wang et al. / Materials Science and Engineering B 185 (2014) 1–6
Fig. 4. SEM images of NiFe2 O4 /EG composites: (a and b) NiFe2 O4 /EG (1:2) urea as fuel, (c and d) NiFe2 O4 /EG (1:1) urea as fuel, (e and f) NiFe2 O4 /EG (2:1) urea as fuel, (g and h) NiFe2 O4 /EG (1:1) carbohydrazide as fuel.
X.-b. Wang et al. / Materials Science and Engineering B 185 (2014) 1–6
5
Fig. 5. TEM images of NiFe2 O4 /EG composites.
Fig. 8. Raman spectrum of NiFe2 O4 /EG composites. Fig. 6. EDX spectra of NiFe2 O4 /EG composites.
3.2. MMW attenuation properties The 3 and 8 MMW attenuation spectra of EG and NiFe2 O4 /EG composites with different molar ratio (NiFe2 O4 :EG) are presented in Fig. 9 and the corresponding MMW attenuation values are listed in Table 2. Generally, the composites possess a better MMW attenuation performance than EG. With increasing molar ratio of NiFe2 O4 :EG 1:2 to 1:1, MMW attenuation performance is improved. The optimal result is obtained when NiFe2 O4 :EG reaches 1:1. This is probably attributed to the increasing amount of NiFe2 O4 on the surface of EG. When the molar ratio of NiFe2 O4 :EG is further enhanced to 2:1, MMW attenuation performance decreased due to the agglomeration of NiFe2 O4 , which is observed in SEM and TEM images. Table 2 MMW attenuation values of EG and NiFe2 O4 /EG composites with different molar ratio.
Fig. 7. FTIR spectrum of NiFe2 O4 /EG composites.
Sample
3 MMW attenuation value/dB
8 MMW attenuation value/dB
EG NiFe2 O4 /EG (1:2) NiFe2 O4 /EG (1:1) NiFe2 O4 /EG (2:1)
6.3 7.2 8.5 7.8
11.7 13.0 14.6 13.9
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X.-b. Wang et al. / Materials Science and Engineering B 185 (2014) 1–6
Fig. 9. MMW attenuation spectra of EG and NiFe2 O4 /EG composites with different molar ratio: 3 mm (a), 8 mm (b).
Absorption loss is to change electromagnetic energy into thermal energy. It can be divided into resistance loss, dielectric loss and magnetic loss according to different loss mechanisms. Resistance loss is related to resistivity. Magnetic loss mainly comes from magnetic rotation, domain wall motion and natural resonance. Dielectric loss results from the polarizations as electron, ion, molecule and interface [33]. EG absorb MMW by its resistance loss and dielectric loss. By coating nickel ferrite, the composites absorb more MMW energy by increasing the magnetic loss. NiFe2 O4 particles have a high electrical resistivity which avoid the skin effects of conductor under the high frequency, it is beneficial to MMW attenuation [5]. So the NiFe2 O4 /EG composites have a good microwave attenuation performance cause of the good electric and magnetic absorption for electromagnetic wave. 4. Conclusions In summary, the uniform NiFe2 O4 /EG composites are prepared by low-temperature combustion synthesis. The composition, structure, morphology and MMW attenuation properties of NiFe2 O4 /EG composites were studied by XRD, SEM, TEM, EDX, FTIR, Raman spectra techniques and MMV radar measurement device. The results show that the NiFe2 O4 /EG composites are uniform and have a good crystallinity with urea as fuel and proper stoichiometric ratio. The NiFe2 O4 /EG composites possess better MMW attenuation properties than EG. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively. This work provides a better MMW attenuation performance composite by a simple production process which might find extensive application in military, electronic protection and so on. References [1] A. Celzard, S. Schneider, J.F. Mareche, J. Carbon 40 (2002) 2185–2191.
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X. Py, E. Daguerre, D. Menard, J. Carbon 40 (2002) 1255–1265. G. Chen, W. Weng, D. Wu, C. Wu, J. Eur. Polym. 39 (2003) 29–35. R.C. Che, L.M. Peng, X.F. Duan, Q. Chen, Adv. Mater. 16 (2004) 401–405. H.T. Zhao, X.D. Sun, C.H. Mao, J. Phys. B: Condens. Matter 404 (2009) 69–72. C.Y. Tsay, K.S. Liu, T.F. Lin, I.N. Lin, J. Magn. Magn. Mater. 209 (2000) 189. T. Abraham, Am. Ceram. Soc. Bull. 73 (1994) 62. P.S. Anil Kumar, J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date, Mater. Lett. 27 (1996) 293. J. Zhang, J. Shi, M. Gong, J. Solid State Chem. 182 (8) (2009) 2135–2140. I. Safarik, M. Safarikova, Nanostructured Materials, Springer, Vienna, 2002, pp. 1–23. M.P. Pileni, Adv. Funct. Mater. 5 (2001) 323–333. F.Y. Cheng, C.H. Su, Y.S. Yang, C.S. Yeh, C.Y. Tsai, C.L. Wu, M.T. Wu, D.B. Shieh, Biomaterials 26 (2005) 729–738. Q. Song, Z.J. Zhang, J. Am. Chem. Soc. 126 (2004) 6164–6168. J. Sun, S. Zhou, P. Hou, Y. Yang, J. Weng, X. Li, M. Li, J. Biomed. Mater. Res. A 80A (2007) 333–341. S.S. Kim, S.T. Kim, J.M. Ahn, K.H. Kim, J. Magn. Magn. Mater. 271 (2004) 39–45. S. Son, M. Taheri, E. Carpenter, V.G. Harris, M.E. McHenry, J. Appl. Phys. 91 (2002) 7589–7591. J.H. Liu, L. Wang, F.S. Li, J. Mater. Sci. 40 (2005) 2573–2575. P. Lavela, J.L. Tirado, J. Power Sources 172 (2007) 379–387. M.M. Bucko, K. Haberko, J. Eur. Ceram. Soc. 27 (2007) 723–727. M.R. Barati, S.A. Seyyed Ebrahimi, A. Badiei, J. Non-Cryst. Solids 354 (2008) 5184–5185. M.A.F. Ramalho, L. Gama, S.G. Antonio, C.O. Paiva-Santos, E.J. Miola, R.H.G.A. kiminami, A.C.F.M. Costa, J. Mater. Sci. 42 (2007) 3603–3606. A.T. Raghavender, K. Zadro, D. Pajic, Z. Skoko, N. Billiskov, Mater. Lett. 64 (2010) 1144–1146. J. Azadmanjiri, S.A. Seyyed Ebrahimi, H.K. Salehani, Ceram. Int. 33 (2007) 1623–1625. A. Pradeep, P. Priyadharsini, G. Chandrasekaran, Mater. Chem. Phys. 112 (2008) 572–576. T. Prabhakaran, J. Hemalatha, J. Alloys Compd. 509 (2011) 7071–7077. A. Teleki, M.C. Heine, F. Krumeich, M.K. Akhtar, S.E. Pratsinis, Langmuir 24 (2008) 12553–12558. Y. Ahn, E.J. Choi, E.H. Kim, Rev. Adv. Mater. Sci. 5 (2003) 477–480. R. Saito, A. Jorio, A.G. Souza, J. Phys. Rev. Lett. 88 (2002) 027401. L.M. Malard, M.A. Pimenta, G. Dresselhaus, J. Phys. Rep. 473 (2009) 51–87. C. Thomsen, S. Reich, J. Phys. Rev. Lett. 5 (2008) 5214–5217. Y. Guo, H. Wang, C. He, L. Qiu, X. Cao, Langmuir 25 (2009) 4678–4684. A.C. Ferrari, J.C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K.S. Novoselov, S. Roth, A.K. Geim, Phys. Rev. Lett. 97 (2006) 187401. M.S. Zhou, M. Xu, R.Q. Shen, J. Microw. 25 (2009) 84–86.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Short communication
Preparation and millimeter wave attenuation properties of NiFe2 O4 /expanded graphite composites by low-temperature combustion synthesis Xiong-biao Wang, Wen-feng Zhu, Xu Wei, Yao-xuan Zhang, Hou-he Chen ∗ School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
a r t i c l e
i n f o
Article history: Received 1 August 2013 Received in revised form 19 December 2013 Accepted 5 January 2014 Available online 17 January 2014 Keywords: Nickel ferrite Expanded graphite Low-temperature combustion synthesis Composite
a b s t r a c t In this paper, NiFe2 O4 /expanded graphite (EG) composites are successfully prepared by low-temperature combustion synthesis method. The morphology, structure and millimeter wave (MMW) attenuation properties of the NiFe2 O4 /EG composites are investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) and MMV radar measurement device. The effects of fuel and stoichiometric ratio on the composites are also investigated. The results show that NiFe2 O4 /EG composites are uniform and have a good crystallinity. The composites possess better MMW attenuation properties than EG. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Expanded graphite (EG) is a kind of graphite compound with low density, high strength and excellent electrical conductivity [1,2]. It is widely used as a millimeter wave (MMW) absorbing material [3]. Its electromagnetic property can be changed by coating different materials in to it, which leads to potential military use. Liang et al. proved that, magnetic loss can promote microwave absorbing properties of carbon nanotubes [4], which has significance in providing a new technique to improve wave attenuation performances of wave absorbing materials. Nickel ferrite (NiFe2 O4 ) is one of the most versatile soft magnetic material, especially suitable for high-frequency applications with low magnetic coercivity, high electrical resistivity, low eddy current loss and excellent chemical stability [5–8]. Nickel ferrite is widely used in the field of magnetic drug delivery, magnetic information storage device, ferrofluids, sensors, catalysis, microwave absorber [9–14]. It might find potential application in enhancing wave absorbing abilities of MMW. Besides, Kim and his coworkers indicated that composites structure can improve wave absorbing properties obviously [15]. Therefore, based on these findings, we infer that nickel ferrite-included composites probably provide an effective direction for the preparation of high quality millimeter wave attenuating device. However, to our best knowledge, there are few reports about the preparation and
∗ Corresponding author. Tel.: +86 13852291760. E-mail address: [email protected] (H.-h. Chen). 0921-5107/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2014.01.004
MMW absorbing properties of NiFe2 O4 /EG composites. NiFe2 O4 /EG composites can be used as “artificial aerosol” materials, which can be launched into the air above and cover target to protect target from being detected. Up to date, there are plenty of methods available to synthesize NiFe2 O4 including co-precipitation [16], ball milling [17], sol–gel [18], hydrothermal [19], combustion method [20–24]. In particular, low-temperature combustion is a simple, safe and rapid synthesis process wherein the main advantages are energy- and time-saving, process controllable and reactants evenly mixed. It involves an exothermic and self-sustaining chemical reaction between the metal salts and a suitable organic fuel, which has great potential in the preparation of ferrites. A key characteristic of this technique is that the heat essential to drive the process is provided mainly by an exothermal reaction occurring among the reagents, which could be utilized to simultaneously converted graphite intercalated compound (GIC) to EG. Herein, in this paper, for the first time we presented a simple and practical low-temperature combustion synthesis approach for the preparation of NiFe2 O4 /EG composites. The composition, structure and morphology of the as-prepared composites were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectra and scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive Xray (EDX). The 3 and 8 MMW attenuation performances of the NiFe2 O4 /EG composites were also evaluated by MMW radar measurement device. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively,
2
X.-b. Wang et al. / Materials Science and Engineering B 185 (2014) 1–6
Fig. 1. Schematic illustration of the preparation of NiFe2 O4 /EG composites.
indicating that, the NiFe2 O4 /EG composites exhibit better MMW attenuation performances than EG. 2. Materials and methods 2.1. Preparation of graphite intercalated compound (GIC) All the reagents obtained commercially were used as received without further purification. All aqueous solutions were made using deionized water. The experimental procedure for the preparation of NiFe2 O4 /EG composites was as follows: natural flake graphite was dried at 60 ◦ C in drying oven for 24 h before mixed and saturated with sulfuric acid. Then, the appropriate amount of KMnO4 was added to acid saturated graphite at 35 ◦ C with ultrasonic treatment for 1 h. The graphite intercalated compound was obtained by washing the product with water until the pH value reached 6. After that, the product was dried at 60 ◦ C for 24 h for further use. 2.2. Preparation of NiFe2 O4 /EG composites NiFe2 O4 /EG composites were prepared by one-step lowtemperature combustion method. The schematic illustration of the preparation of NiFe2 O4 /EG composites is shown in Fig. 1. Nickel nitrate Ni(NO3 )2 ·6H2 O and iron nitrate Fe(NO3 )3 ·9H2 O were used as the reactants and CO(NH2 )2 (urea) was used as reducing agent. In a typical procedure, firstly, Ni(NO3 )2 ·6H2 O and Fe(NO3 )3 ·9H2 O were dissolved in water under stirring to form uniform solution. The molar ratio of Ni2+ :Fe3+ :CO(NH2 )2 was 1:2:7. GIC and ethylene glycol were then added into the solution. Ethylene glycol is usually used as a surfactant and reducing agent which prevent the agglomeration of nanoparticles and provide more fuel. The precursor mixture was heated by electric furnace to 300 ◦ C. During the heating process, the solution started vaporizing and produce a great deal of foams. When water was evaporated, spark appeared in corner of evaporating dish and spread through to entire mass, yielding a voluminous and white smoke. During this process, GIC was rapidly expanded and NiFe2 O4 /EG composites were obtained. Combustion was attempted with different fuel (carbohydrazide and urea) because the fuel controls the combustion temperature and gas amount thereby has important effects on the characteristics of the final product. 2.3. Characterization The crystalline structure, phase composition and crystallite size of NiFe2 O4 /EG composites were identified from XRD patterns ˚ for 2 value ranges obtained using Cu K␣ radiation ( = 1.541 A)
from 10◦ to 80◦ in X-ray diffractometer (Bruker D8 Advance). The morphology of composites was investigated by scanning electron microscope (JEOLJSM-6380LV) operated at an acceleration voltage of 20 kV. The Fourier transform infrared (FTIR) transmission spectrum of the samples mixed with KBr powder reference were obtained from 1800 cm−1 to 400 cm−1 using Thermo Scientific Nicolet IS-10 spectrophotometer. Raman spectra were collected in backscattering geometry with a Renishaw Invia Reflex system equipped with a charge-coupled device (CCD) detector and a confocal Leica microscope. Excitation of the sample was carried out with a 514.4 nm argon ion laser line, with acquisition time of 20 s and powder at the sample of about 20 mW. Transmission electron microscopy (TEM) images and energy dispersive X-ray (EDX, EDAX Genesis 2000) spectroscopy were preformed on a JEOL JEM-2100 microscope operating at 200 kV. MMW radar measurement device is composed of a transmitter, a receiver and data processing system and illustrative drawing of the experimental set-up for MMW measurement is shown in Fig. 2. MMW samples were prepared by sticking a certain amount of NiFe2 O4 /EG composites to the plastic tape. The rectangle area of plastic tape is 16 cm × 24 cm, 70 mL samples are spread evenly over the plastic tape and the thicknesses of each sample is about 1.8 mm.
3. Results and discussion 3.1. Structure and morphology Fig. 3 shows the X-ray diffraction (XRD) pattern for NiFe2 O4 /EG composites with different molar ratio of NiFe2 O4 /EG. The main peak at 2 = 23.2◦ is corresponding to the standard diffraction pattern of EG, whereas the relatively weak intensity peaks at 30.3◦ , 35.7◦ , 43.3◦ , 57.4◦ and 63.0◦ fit to the (2 2 0), (3 1 1), (4 0 0), (5 1 1) and (4 4 0) planes of NiFe2 O4 , respectively, indicating the cubic spinel structure of nickel ferrite. A side from the peaks of NiFe2 O4 and EG, no extra peaks are found in Fig. 3, which means only two components exist in the composites. As the molar ratio of NiFe2 O4 /EG increased, the peaks of NiFe2 O4 are enhanced. According to the peak intensity in Fig. 3, the main component in the composites is EG. Besides, the lattice parameters for the NiFe2 O4 calculated from the position of the (3 1 1) peak, according to structures with Materials Date Jade 8.5 software. Also, Scherrer’s formula was used to address the crystallite size of the particles from the broadening FWHM of the (3 1 1) peak in the XRD patterns, using the profile fitting and peak decomposition method of the Materials Date Jade 8.5 software. The crystallite size estimated by Scherrer’s formula is 28 nm. ˚ and it is in good accordance The lattice parameters value is 8.35 A, with the earlier reported of 8.36 A˚ for the nano NiFe2 O4 [25] which prove the efficiency of synthesis technique.
Fig. 2. Schematic diagram of measurement device for millimeter wave attenuation characteristics.
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Fig. 3. X-ray diffraction pattern of NiFe2 O4 /EG composites.
The effect of reactant molar ratio on the morphology of NiFe2 O4 /EG composite is evaluated by SEM as shown in Fig. 4. When the molar ratio of NiFe2 O4 /EG is 1:2 (Fig. 4(a and b)), only small amounts of NiFe2 O4 is successively adhered to the surface of EG with uneven distribution. In addition, as a result of a low content of NiFe2 O4 , the corresponding fuel does not provide enough heat to expand GIC sufficiently. So we adjusted the molar ratio of NiFe2 O4 /EG to 1:1 (Fig. 4(c and d)) and the result indicating that both the loading amount and dispersibility of NiFe2 O4 are improved. When the molar ratio NiFe2 O4 /EG is 2:1 (Fig. 4(e and f)), NiFe2 O4 nanoparticles are found agglomerated due to magnetic dipole interaction among the particles [25]. Fuel types also have an important influence on NiFe2 O4 /EG composites as shown in Fig. 4(g and h). Considerable amount of NiFe2 O4 covers the most part of EG surface when carbohydrazide was used as fuel. But the combination between NiFe2 O4 and EG are not tightly enough which is similar to the mixture of NiFe2 O4 and EG, and NiFe2 O4 powder peel off after shock. This might be due to the large volumes of gas release during the reaction process, facilitating the dispersion of powder but at the same time bring in disturbance detrimental to the connection effect. Finally, urea was chosen as optimal fuel for recombination processes, in which EG surface is firmly coated with NiFe2 O4 and EG expands well. The transmission electron micrographs of NiFe2 O4 /EG composites (obtained under optimal conditions with NiFe2 O4 /EG molar ratio 1:1 and urea as fuel) are shown in Fig. 5. The particles are firmly adhered to EG with even distribution and the size of particles ranges from 30 to 60 nm (Fig. 5). The EDX spectra of NiFe2 O4 /EG composites are shown in Fig. 6 and the composition details are listed in Table 1. The EDX peaks intensity is relative to atom content [26]. Fig. 6 shows C as the main peak with very small Fe and Ni peaks that arise from the coating. The inset figure is the enlargement for Fe and Ni peaks. It is obvious
that the sample contains only C, O, Fe, Ni and no other contamination elements. The atomic ratio of Ni:Fe:O is close to 1:2:4 which matches the exact stoichiometric ratio of nickel ferrite. The infrared (IR) spectrum of NiFe2 O4 /EG composites in the range 400–1800 cm−1 is shown in Fig. 7. The IR spectrum exhibits two intense bands between 454 cm−1 and 562 cm−1 belonging to the stretching vibration modes associated to the metal-oxygen absorption bands Fe O bonds in the crystalline lattice of NiFe2 O4 [27]. They are characteristically pronounced for spinel structures and for ferrites in particular. Intrinsic stretching vibrations of metal at tetrahedral site, Mtetra O are generally observed in the range of 620–550 cm−1 . The stretching vibrations of metal at octahedral site, Mocta O are generally observed in the range of 450–385 cm−1 [19–22]. IR spectrum also shows an absorption band at 1038 cm−1 , corresponding to the C O stretching mode, which belongs to graphite intercalation compound. The Raman spectrum of NiFe2 O4 /EG composites is shown in Fig. 8. From which, we can see the peak at 1580 cm−1 is assigned to the first-order G band, which is related to disordered sp2 -bonded graphitic carbon [28–32]. The 2D band at 2729 cm−1 is consistent with the multi-layer feature of graphite [32]. The results of FTIR and Raman spectra suggest that the composites consist of graphite and NiFe2 O4 .
Table 1 Stoichiometry composition from EDX. Element
Theoretical atomic%
Experimental atomic%
Experimental weight%
C O Fe Ni
73.6 15.1 7.5 3.8
91.4 5.2 2.3 1.1
79.9 6.1 9.3 4.7
4
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Fig. 4. SEM images of NiFe2 O4 /EG composites: (a and b) NiFe2 O4 /EG (1:2) urea as fuel, (c and d) NiFe2 O4 /EG (1:1) urea as fuel, (e and f) NiFe2 O4 /EG (2:1) urea as fuel, (g and h) NiFe2 O4 /EG (1:1) carbohydrazide as fuel.
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Fig. 5. TEM images of NiFe2 O4 /EG composites.
Fig. 8. Raman spectrum of NiFe2 O4 /EG composites. Fig. 6. EDX spectra of NiFe2 O4 /EG composites.
3.2. MMW attenuation properties The 3 and 8 MMW attenuation spectra of EG and NiFe2 O4 /EG composites with different molar ratio (NiFe2 O4 :EG) are presented in Fig. 9 and the corresponding MMW attenuation values are listed in Table 2. Generally, the composites possess a better MMW attenuation performance than EG. With increasing molar ratio of NiFe2 O4 :EG 1:2 to 1:1, MMW attenuation performance is improved. The optimal result is obtained when NiFe2 O4 :EG reaches 1:1. This is probably attributed to the increasing amount of NiFe2 O4 on the surface of EG. When the molar ratio of NiFe2 O4 :EG is further enhanced to 2:1, MMW attenuation performance decreased due to the agglomeration of NiFe2 O4 , which is observed in SEM and TEM images. Table 2 MMW attenuation values of EG and NiFe2 O4 /EG composites with different molar ratio.
Fig. 7. FTIR spectrum of NiFe2 O4 /EG composites.
Sample
3 MMW attenuation value/dB
8 MMW attenuation value/dB
EG NiFe2 O4 /EG (1:2) NiFe2 O4 /EG (1:1) NiFe2 O4 /EG (2:1)
6.3 7.2 8.5 7.8
11.7 13.0 14.6 13.9
6
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Fig. 9. MMW attenuation spectra of EG and NiFe2 O4 /EG composites with different molar ratio: 3 mm (a), 8 mm (b).
Absorption loss is to change electromagnetic energy into thermal energy. It can be divided into resistance loss, dielectric loss and magnetic loss according to different loss mechanisms. Resistance loss is related to resistivity. Magnetic loss mainly comes from magnetic rotation, domain wall motion and natural resonance. Dielectric loss results from the polarizations as electron, ion, molecule and interface [33]. EG absorb MMW by its resistance loss and dielectric loss. By coating nickel ferrite, the composites absorb more MMW energy by increasing the magnetic loss. NiFe2 O4 particles have a high electrical resistivity which avoid the skin effects of conductor under the high frequency, it is beneficial to MMW attenuation [5]. So the NiFe2 O4 /EG composites have a good microwave attenuation performance cause of the good electric and magnetic absorption for electromagnetic wave. 4. Conclusions In summary, the uniform NiFe2 O4 /EG composites are prepared by low-temperature combustion synthesis. The composition, structure, morphology and MMW attenuation properties of NiFe2 O4 /EG composites were studied by XRD, SEM, TEM, EDX, FTIR, Raman spectra techniques and MMV radar measurement device. The results show that the NiFe2 O4 /EG composites are uniform and have a good crystallinity with urea as fuel and proper stoichiometric ratio. The NiFe2 O4 /EG composites possess better MMW attenuation properties than EG. The 3 and 8 MMW attenuation performances of NiFe2 O4 /EG composites are 8.5 dB and 14.6 dB, respectively. This work provides a better MMW attenuation performance composite by a simple production process which might find extensive application in military, electronic protection and so on. References [1] A. Celzard, S. Schneider, J.F. Mareche, J. Carbon 40 (2002) 2185–2191.
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