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Enhanced antioxidation and electromagnetic properties of Co-coated flaky carbonyl iron particles prepared by electroless plating
Journal of Alloys and Compounds 637 (2015) 10–15
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced antioxidation and electromagnetic properties of Co-coated flaky carbonyl iron particles prepared by electroless plating Yingying Zhou a,⇑, Wancheng Zhou a, Rong Li a,b, Yang Mu a, Yuchang Qing a a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China No. 603 Faculty, Xi’an Institute of High Technology, Xi’an 710025, China
a r t i c l e
i n f o
Article history: Received 22 October 2014 Received in revised form 2 February 2015 Accepted 1 March 2015 Available online 6 March 2015 Keywords: Electroless plating Carbonyl iron Co coating Antioxidation Electromagnetic property
a b s t r a c t Co was successfully coated on the surface of flaky carbonyl iron particles using an electroless plating method. The morphologies, composition, as well as magnetic, antioxidation and electromagnetic properties of the samples were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), vibrating sample magnetometer (VSM), thermogravimetric (TG) and microwave network analyzer. TG curve shows that the obvious weight gain of carbonyl iron was deferred from 300 to 400 °C after Co-coated. In contrast to raw carbonyl iron, the Co-coated carbonyl iron shows better stability on electromagnetic properties after 300 °C heat treatment for 10 h, demonstrating that the Co coating can act as the protection of carbonyl iron. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic waves have been widely used in both military and civil applications: radar, wireless communication tools, local area networks, personal digital assistant, etc. [1,2]. However, there are many problems caused by the increasing usage of electromagnetic waves. In order to provide solution to electromagnetic interference (EMI) and microwave absorption, the absorbers of electromagnetic waves are becoming very important, which have attracted much attention of many scientists [3–5]. Carbonyl iron, as well as ferrites, has been extensively studied for a long time as magnetic components of polymeric composites for the application of electromagnetic wave absorbers [6]. However, ferrites cannot be used in higher frequency range due to their Snoke’s limit, so the electromagnetic wave absorbers filled with ferrites can only play a good role in a narrow band. In contrast, carbonyl iron has larger values of saturation magnetization and its Snoke’s limit is located at a higher frequency [7]. So carbonyl iron is more suitable to be applied in a broad frequency range. For carbonyl iron particles, the oxidation-prone property usually limits their usages at a higher temperature, especially for the particles with flaky shape [8]. A good way to conquer the limitations is to encapsulate them with other materials, which could effectively protect them from contacting with oxygen. Therefore, some researchers have focused on the using of different kinds of ⇑ Corresponding author. Tel./fax: +86 29 88494574. E-mail address: [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.jallcom.2015.03.014 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
surface coatings, such as Al/iron [9], iron/SiO2 [10] and carbonyl iron/PANI (polyaniline) [11], to protect the carbonyl iron particles from oxidation. However, few works were devoted to the variation of electromagnetic properties/microwave absorption after heat treatment. Abshinova [11] researched the temperature variation of the permittivity spectra of carbonyl iron/PANI (polyaniline). And also, in our previous works, the Ni coated carbonyl iron composite powder has been prepared successfully by electroless nickel plating [12]. Comparing to Ni, Co has higher Curie temperature and better magnetic property [13]. In this paper, Co-coated carbonyl iron was synthesized using an electroless plating method, using flaky carbonyl iron particles as raw materials. This preparation method has the advantages of simplicity, low-cost and high-purity in practical applications. The antioxidation and electromagnetic properties of the Co-coated carbonyl iron are discussed in detail. 2. Experimental and characterization The flaky-shaped carbonyl iron powder, purchased from Xinghua chemical Co., Ltd., Shanxi province, China, was used as the raw material. Carbonyl iron particles were heat-treated at 600 °C for 30 min and then cooled to room temperature in a vacuum furnace to remove the residual organic layer remaining on the surface of particles. Then the carbonyl iron particles were degreased in dilute solution of hydrochloric acid at the pH-value of 3 to remove the oxidation layer. Then the activated carbonyl iron particles were introduced into an electroless plating bath after being washed with distilled water for 3 times. The composition of the plating solution and the reaction conditions were shown in Table 1. The chemical reaction process can be expressed as the following reaction:
Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15 Table 1 Bath composition and operating conditions of electroless cobalt coating. Chemical
Concentration (g/L)
CoSO47H2O NaH2PO27H2O Na3C6H5O NH4Cl Bath temperature pH at 90 °C (adjusting using 10 wt% NaOH)
20 25 50 40 90 °C 9.0
Co2þ þ H2 PO2 þ 3OH ! Co þ HPO2 3 þ 2H2 O
ð1Þ
All the chemical reactants were analytical grade and used as received. The proportion of pre-treated carbonyl iron particles added to the plating bath was 10 g/L. The particles were put into the electroless plating solution when the temperature reached 90 °C under mechanical stirring with a rate of 300rmp in order to mix sufficiently and homogenously. In general, the thickness of Co coating can be controlled by the plating time [14]. The plating time was maintained for 60 min in the present study. After electroless plating, the solution was filtered, washed and dried. Thus, the final coated particles were obtained. The morphology of the raw and Co-coated carbonyl iron particles were examined under a scanning electron microscope (SEM; JEOL JSM-5800 LV SKANNING) attached with a Links Systems energy dispersive spectrometer (EDS). Magnetic measurement was conducted at room temperature using a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). The raw and Co-coated carbonyl iron particles were homogeneously dispersed into the paraffin with a mass fraction of 70% to measure the electromagnetic property. The effective complex permittivity (er = e0 je00 ) and permeability (lr = l0 jl00 ) of the samples were measured using a network analyzer (Agilent technologies E8362B: 10 MHz–20 GHz). The dimension of the samples for electromagnetic measurement was 10.16 22.86 2 mm, which were based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz in the fundamental wave-guide method. In addition, the antioxidation property of the samples up to 800 °C was characterized using a thermal analyzer (Netzsch STA 449 TG–DTA/DSC) in air with the heating rate 10 °C/min.
3. Results and discussion 3.1. Morphologies of the raw and Co-coated carbonyl iron particles Fig. 1 shows the SEM and EDS of carbonyl iron particles before and after coated with Co. As seen from Fig. 1(a), the raw carbonyl iron particles are thin flakes with diameters ranging in 1–5 lm and thickness below 1 lm. In addition, there are small globular granules. Fig. 1(b) shows the SEM image of the Co-coated carbonyl iron. It is observed that the morphology of the Co-coated carbonyl iron is similar to the raw carbonyl iron. It is worth noting that there exist few micro-holes on the surface of Co-coated carbonyl iron particles, which may contributed to the acid pickling process. From the inserts of Fig. 1(b), the carbonyl iron particles are uniformly coated with Co coatings, the size of which are all below 0.2 lm, indicating the core–shell structure of the Co coated carbonyl iron particles. The EDS analysis indicates the presence of Co and P on the surface of carbonyl iron particles after the electroless deposition, as shown in Fig. 1(c) and (d). 3.2. VSM and TG analysis of the raw and Co-coated carbonyl iron particles The field dependence of magnetization for the raw and Cocoated carbonyl iron particles obtained from the typical electroless plating method were measured at room temperature by VSM, as shown in Fig. 2. The saturation magnetization (MS) and coercivity (HC) are 208.54 emu/g and 4.8 Oe for the raw carbonyl iron particles, and 183.78 emu/g and 9 Oe for the Co-coated carbonyl iron particles respectively. According to the theory of ferromagnetism materials: MS = NglBJ. Where N is the number of atoms, g is the g-factor, lB is called the Bohr magneton and J is the total angular moment. Thus, the saturation magnetization value of Co-coated
11
carbonyl iron particles is lower than that of the raw carbonyl iron particles, which can be attributed to the higher Bohr magneton of iron atom [15]. The enhancement of the coercivity HC after Co coated is originated from the disordered crystal structure due to the increasing internal strain by electroless plating process [16]. Fig. 3 shows the TG curves of the carbonyl iron particles with and without Co coating in air from room temperature to 800 °C. TG curve is generally characterized by the region above 400 °C with significant increase of the sample mass due to iron oxidation, and finally a high-temperature plateau above 700 °C after the formation of iron oxides. There is, however, a significant difference in the obvious weight gain temperature between uncoated and Cocoated carbonyl iron particles. The obvious weight gain of the raw carbonyl iron occurs at about 300 °C. Thus, after a single coating with Co, the obvious weight gain temperature is postponed, indicating the Co coating provides a protective overlayer on iron particles that slows the iron oxidation. 3.3. The complex permittivity and permeability spectra Fig. 4 depicts the complex permittivity and complex permeability spectra of the composites filled with the raw and Co-coated carbonyl iron particles (the content of the raw and Co-coated carbonyl iron particles are both 70 wt%). It could be seen from Fig. 4(a) that the values of the real part of permittivity (e0 ) of the composites filled with raw carbonyl iron particles kept almost unvaried (e0 9.5) in the frequency range of 8.2–12.4 GHz, and the real part increased from 9.5 to 11 after Co coating deposition on the surface of carbonyl iron particles. Therefore, it can be deduced from Fig. 4(a) that Co coating improves the storage capability of electric energy, which is consistent with the result of Pan [17]. It also reveals in Fig. 4(a) that drastic increase of the imaginary part of permittivity (e00 ) is observed from 9.8 to 12.4 GHz after electroless Co deposition. As we know, two factors mainly determine the real part of complex permittivity of material in the microwave frequency range: (i) interfacial polarization of the interface [18] and (ii) space charge polarization [19]. For the heterogeneous system such as conductor-loaded composites, the properties of interfaces could have a dominant role in determining dielectric performance, and interfacial polarization induced by the charge along the boundaries of iron and cobalt is an important polarization process and associated relaxation also gives rise to loss mechanism [20]. Besides, the space charge polarization should also be considered. The high e0 value of Co-coated carbonyl iron particles composite is attributed to the space charge polarization, which occurs between adjacent conductive iron and cobalt particles, is more easily appeared for Co-coated carbonyl iron particles [21]. On the other hand, the e00 accounts for the loss energy dissipative mechanisms in the materials, and according to the free electron theory: e00 = r/2pe0f [22]. Where r is the electrical conductivity, f is frequency and e0 is the dielectric constant in vacuum. According to the research of Fert et al. [23], the electrical resistivity of cobalt is lower than the value of iron. Therefore, based on the knowledge that the electrical conductivity is the reciprocal of electrical resistivity, it can be easily deduced that the electrical conductivity of cobalt is higher than the value of iron. In conclusion, the higher electrical conductivity of cobalt results in the higher e00 value of Co-coated carbonyl iron particles composite especially at high frequency range, showed in Fig. 4(a). The real and imaginary parts of permeability of the raw and Co-coated carbonyl iron particles composites are shown in Fig. 4(b). One feature of the data is that the values of real part (l0 ) and imaginary part (l00 ) of complex permeability for both samples all decrease with increasing frequency as shown in Fig. 4(b), which is mainly due to both eddy current loss and ferromagnetic resonance of the carbonyl iron particles [24,25]. It can
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Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15
Fig. 1. SEM and EDS of carbonyl iron particles before and after coated with Co.
Fig. 2. Magnetic hysteresis loop of the raw and Co-coated carbonyl iron particles measured at room temperature. Inset: the expanded low field magnetization curves.
also be seen clearly that both the l0 and l00 values between 8.2 and 12.4 GHz show no obvious difference for these two samples, which is ascribed to the fact that the thickness of Co interlayer among these carbonyl iron particles is sufficiently small not to make a disorder at filler–matrix interface and, thus, to cut-off the inter-particle magnetic interaction [11]. Generally, for ferrite magnetic materials, the microwave magnetic loss of magnetic materials originates mainly from hysteresis loss, domain wall resonance, natural ferromagnetic resonance, and the eddy current effect [26]. The hysteresis loss is usually caused by irreversible magnetization and can be negligible in a weak applied field. It is possible to distinguish the permeability spectra into the spin rotational component and the domain wall motion contribution, using numerical fitting [27]. Resonance resulted from
Fig. 3. TG curves of raw and Co-coated carbonyl iron samples measured in air.
domain wall movement normally occurs at low-frequency region (<2 GHz); however, resonance caused by spin rotational component occurs as high-frequency region. Thus, it could be concluded that the magnetic loss of the two samples in this study may be due to natural ferromagnetic resonance and eddy current effect. It is reasonable that both the dielectric loss and the magnetic loss can be influenced by the ‘‘core–shell’’ microstructure of microwave absorbent. In general, the dielectric loss is attributed to the lags of polarization between the core/shell interfaces as the frequency varies. In addition, the ‘‘core–shell’’ microstructure of microwave absorbent has something to do with eddy current loss that is one of the contributors to magnetic loss [25]. In order to investigate the heat resistance of Co-coated carbonyl iron particles, the raw and Co-coated carbonyl iron particles were heat treated at 300 °C for 10 h, followed by embedded in 30 wt%
Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15
Fig. 4. Complex permittivity and permeability spectra for the composites containing 70 wt.% raw or Co-coated carbonyl iron particles.
paraffin matrix. The complex permittivity and permeability of the composites are presented in Figs. 5 and 6. As seen from Figs. 5 and 6, after heat treatment at 300 °C, the e0 and e00 of the carbonyl iron composite increased considerably compared with the values of Cocoated carbonyl iron composite. The composites filled with raw carbonyl iron particles, however, exhibit a degeneracy of the dispersion law of permeability, which is contributed by a decrease in both real and imaginary parts in the measured frequency region (Fig. 5(b)). Thus, the Co coating changes the character of magnetic spectra of carbonyl iron composites from heat treatment-dependent (Fig. 5(b)) to heat treatment – independent (Fig. 6(b)) at 300 °C. This indicates the suppression of oxidation processes of carbonyl iron due to the presence of Co coating. 3.4. Microwave-absorbing properties According to the transmission line theory, the reflection loss (RL) of single-layer coating can be calculated by the following equation:
Z in Z 0 RL ¼ 20 lg jCj ¼ 20 lg Z in þ Z 0
ð2Þ
where U is the reflection coefficient, Z0 is the impedance of the free space, and Zin is the input impedance of the absorber. There, Zin can be expresses as Eq. (3):
rffiffiffiffiffi Z in ¼ Z 0
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tanh j c er
ð3Þ
13
Fig. 5. Complex permittivity and permeability spectra of the composites containing 70 wt.% raw carbonyl iron particles after 300 °C heat treatment.
where lr and er are the relative permeability and permittivity of the absorber, f is the frequency of incident wave, d is the thickness of the absorber, and c is the velocity of light in free space. In the present investigation, the mass fraction of the filler content and the thickness of the composite are fixed for 70 wt.% and 1.5 mm, respectively, to investigate the influence of Co coating and heat treatment on the RL property of these samples. The calculated RL values, according to the formula (2), (3), of the samples are shown in Fig. 7. It can be seen that the RL values for both composites filled with raw and Co-coated carbonyl iron particles are less than 10 dB can be obtained in the frequency range of 8.2–12.4 GHz, which indicates that the Co coating can help to maintain a good microwave absorbing property for these carbonyl iron particles. It is worth mentioning that the minimum RL value of Co-coated carbonyl iron particles composites turns to lower frequency, which is mainly resulted from the higher permittivity of it, as shown in Fig. 4(a), which is similar to the other’s result [8]. The difference in microwave absorption properties of the Cocoated particles is resulted from the different composition of the coated and uncoated carbonyl iron particles. The Bohr magnetons (lB) of Fe atom and Co atom are 2.22 and 1.72, respectively [28]. So, the magnetism of iron is better than the cobalt [25]. For magnetic microwave absorbers, magnetic loss is the main microwave loss tunnel. The powder with better magnetism results in larger microwave loss [29]. In a word, the decrease of the absolute RL value for the composite with Co coated particles is mainly attributed to the reduction of magnetic loss of the carbonyl iron particles after
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Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15
were heat treated at the same condition, the RL curve shows a small decreasing tendency compared with that of the Co-coated particles without treatment, and its values are still maintained below 10 dB in the whole frequency range. As a result, the microwave absorbing property of the composites is kept at a high level. It can be deduced from the forementioned results that the antioxidation property of the raw carbonyl iron particle can be effectively improved after coated with Co layer. In a word, the prepared Co-coated sample exhibits good absorption performance in the X-band frequency range and has the advantages in serving as a potential microwave absorbing material below 300 °C. 4. Conclusions Carbonyl iron was successfully coated with Co layer using electroless plating method. The electromagnetic properties of raw and Co-coated carbonyl iron particles before and after heat treatment were investigated. The permittivity of the Co-coated particle composite was enhanced by coating Co on the surface of carbonyl iron particle because of the enhanced interfacial polarization and orientation polarization between carbonyl iron and Co. As the Co coating layer was extremely thin compared with the carbonyl iron particle size, the permeability of the Co-coated particle composite kept almost invariable. After heat treatment at 300 °C for 10 h, the Co coating changes the character of permeability of carbonyl iron composites from temperature-dependent to temperature-independent. In a word, the electroless plated with Co greatly improves the antioxidation property of carbonyl iron and has slight influence upon the microwave absorption properties below 300 °C. So the Co-coated carbonyl iron has much better antioxidation property and can also serve as a super thin microwave absorber. Acknowledgements Fig. 6. Complex permittivity and permeability spectra of the composites containing 70 wt.% Co-coated carbonyl iron particles after 300 °C heat treatment.
This work was financially supported by the Chinese National Natural Science Foundation (No. 51072165) and the fund of the States Key Laboratory of the Solidification Processing in NWPU (No. KP201307). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Fig. 7. Microwave absorption of the raw and Co-coated carbonyl iron particles before and after 300 °C heat treatment filled composites with 1.5-mm thickness.
coated with Co. While for the composite with carbonyl iron particles after heat treatment at 300 °C for 10 h, the minimum RL values degrades critically from 25 dB to 15 dB, indicating the deduction of the microwave absorbing property of the carbonyl iron particles. And the RL values in the frequency range up to 11 GHz are above 10 dB. As seen from Fig. 7, When the Co-coated particles
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
K.Y. Park, J.H. Han, S.B. Lee, J.W. Yi, Composites Part A 42 (2011) 573. L. Zhang, C.S. Shi, K.Y. Rhee, N.Q. Zhao, Composites Part A 43 (2012) 2241. Y.C. Qing, W.C. Zhou, F. Luo, D.M. Zhu, J. Magn. Magn. Mater. 321 (2009) 25. Y. Liu, F. Luo, W.C. Zhou, D.M. Zhu, J. Alloys Comp. 576 (2013) 43. S.Y. Tong, M.J. Tung, W.S. Ko, et al., J. Alloys Comp. 550 (2013) 39. M.S. Kim, E.H. Min, J.G. Koh, J. Magn. Magn. Mater. 321 (2009) 581. S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, M. Homma, H. Ota, Y. Houjou, R. Sato, J. Alloys Comp. 330–332 (2002) 301. C.L. Yin, J.M. Fan, L.Y. Bai, F. Ding, F.L. Yuan, J. Magn. Magn. Mater. 340 (2013) 65. F. Jay, V. Gauthier-Brunet, F. Pailloux, J. Mimault, S. Bucher, S. Dubois, Surf. Coat. Technol. 202 (2008) 4302. X.M. Ni, Z. Zheng, X. Hu, X.K. Xiao, J. Colloid Interface Sci. 341 (2010) 18. M.A. Abshinova, N.E. Kazantseva, P. Sáha, I. Sapurina, J. Kovárˇová, J. Stejskal, Polym. Degrad. Stab. 93 (2008) 1826. R. Li, W.C. Zhou, Y.C. Qing, Chin. Phys. Lett. 31 (2014) 097701. S. Wurmehl, G.H. Fecher, H.C. Kandpal, V. Ksenofontov, C. Felser, Phys. Rev. B 72 (2005) 184434. S.L. Cheng, T.L. Hsu, T. Lee, S.W. Lee, J.C. Hu, L.T. Chen, Appl. Surf. Sci. 264 (2013) 732. S.S. Kim, S.T. Kim, J.M. Ahn, K.H. Kim, J. Magn. Magn. Mater. 271 (2004) 39. R. Han, L. Qiao, T. Wang, F.S. Li, J. Alloys Comp. 509 (2011) 2734. X.F. Pan, H.G. Shen, J.X. Qiu, M.Y. Gu, Mater. Chem. Phys. 101 (2007) 505. Y.C. Qing, W.C. Zhou, S. Jia, F. Luo, D.M. Zhu, Physica B 406 (2011) 777. R. Han, X.H. Han, L. Qiao, T. Wang, F.S. Li, Mater. Chem. Phys. 128 (2011) 317. L. Zhou, W.C. Zhou, J.B. Su, F. Luo, D.M. Zhu, Ceram. Int. 38 (2012) 1077. X.G. Liu, D.Y. Geng, H. Meng, P.J. Shang, Z.D. Zhang, J. Phys. D Appl. Phys. 41 (2008) 7. X.G. Liu, D.Y. Geng, H. Meng, P.J. Shang, Z.D. Zhang, Appl. Phys. Lett. 92 (2008) 173117. A. Fert, I.A. Campbell, J. Phys. F: Metal. Phys. 6 (1976) 849.
Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15 [24] D. Wan, X. Ma, Physics of Magnetism, Press of University of Electric Science and Technology of China, Chengdu, 1994, pp. 406–459. [25] Y.J. Li, R. Wang, F.M. Qi, C.M. Wang, Appl. Surf. Sci. 254 (2008) 4708. [26] L.D. Liu, Y.P. Duan, S.H. Liu, L.Y. Chen, J.B. Guo, J. Magn. Magn. Mater. 322 (2010) 1736.
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[27] T. Tsutaoka, J. Appl. Phys. 93 (2003) 2789. [28] K.H.J. Bushow, F.R. De Boer, Physics of Magnetism and Magnetic Materials, seventh ed., Kluwer Academic/Plenum Publishers, New York, 2003. [29] X.F. Pan, G.H. Mu, H.G. Shen, M.Y. Gu, Appl. Surf. Sci. 253 (2007) 4119.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Enhanced antioxidation and electromagnetic properties of Co-coated flaky carbonyl iron particles prepared by electroless plating Yingying Zhou a,⇑, Wancheng Zhou a, Rong Li a,b, Yang Mu a, Yuchang Qing a a b
State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, China No. 603 Faculty, Xi’an Institute of High Technology, Xi’an 710025, China
a r t i c l e
i n f o
Article history: Received 22 October 2014 Received in revised form 2 February 2015 Accepted 1 March 2015 Available online 6 March 2015 Keywords: Electroless plating Carbonyl iron Co coating Antioxidation Electromagnetic property
a b s t r a c t Co was successfully coated on the surface of flaky carbonyl iron particles using an electroless plating method. The morphologies, composition, as well as magnetic, antioxidation and electromagnetic properties of the samples were characterized by scanning electron microscope (SEM), energy dispersive spectrometer (EDS), vibrating sample magnetometer (VSM), thermogravimetric (TG) and microwave network analyzer. TG curve shows that the obvious weight gain of carbonyl iron was deferred from 300 to 400 °C after Co-coated. In contrast to raw carbonyl iron, the Co-coated carbonyl iron shows better stability on electromagnetic properties after 300 °C heat treatment for 10 h, demonstrating that the Co coating can act as the protection of carbonyl iron. Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Electromagnetic waves have been widely used in both military and civil applications: radar, wireless communication tools, local area networks, personal digital assistant, etc. [1,2]. However, there are many problems caused by the increasing usage of electromagnetic waves. In order to provide solution to electromagnetic interference (EMI) and microwave absorption, the absorbers of electromagnetic waves are becoming very important, which have attracted much attention of many scientists [3–5]. Carbonyl iron, as well as ferrites, has been extensively studied for a long time as magnetic components of polymeric composites for the application of electromagnetic wave absorbers [6]. However, ferrites cannot be used in higher frequency range due to their Snoke’s limit, so the electromagnetic wave absorbers filled with ferrites can only play a good role in a narrow band. In contrast, carbonyl iron has larger values of saturation magnetization and its Snoke’s limit is located at a higher frequency [7]. So carbonyl iron is more suitable to be applied in a broad frequency range. For carbonyl iron particles, the oxidation-prone property usually limits their usages at a higher temperature, especially for the particles with flaky shape [8]. A good way to conquer the limitations is to encapsulate them with other materials, which could effectively protect them from contacting with oxygen. Therefore, some researchers have focused on the using of different kinds of ⇑ Corresponding author. Tel./fax: +86 29 88494574. E-mail address: [email protected] (Y. Zhou). http://dx.doi.org/10.1016/j.jallcom.2015.03.014 0925-8388/Ó 2015 Elsevier B.V. All rights reserved.
surface coatings, such as Al/iron [9], iron/SiO2 [10] and carbonyl iron/PANI (polyaniline) [11], to protect the carbonyl iron particles from oxidation. However, few works were devoted to the variation of electromagnetic properties/microwave absorption after heat treatment. Abshinova [11] researched the temperature variation of the permittivity spectra of carbonyl iron/PANI (polyaniline). And also, in our previous works, the Ni coated carbonyl iron composite powder has been prepared successfully by electroless nickel plating [12]. Comparing to Ni, Co has higher Curie temperature and better magnetic property [13]. In this paper, Co-coated carbonyl iron was synthesized using an electroless plating method, using flaky carbonyl iron particles as raw materials. This preparation method has the advantages of simplicity, low-cost and high-purity in practical applications. The antioxidation and electromagnetic properties of the Co-coated carbonyl iron are discussed in detail. 2. Experimental and characterization The flaky-shaped carbonyl iron powder, purchased from Xinghua chemical Co., Ltd., Shanxi province, China, was used as the raw material. Carbonyl iron particles were heat-treated at 600 °C for 30 min and then cooled to room temperature in a vacuum furnace to remove the residual organic layer remaining on the surface of particles. Then the carbonyl iron particles were degreased in dilute solution of hydrochloric acid at the pH-value of 3 to remove the oxidation layer. Then the activated carbonyl iron particles were introduced into an electroless plating bath after being washed with distilled water for 3 times. The composition of the plating solution and the reaction conditions were shown in Table 1. The chemical reaction process can be expressed as the following reaction:
Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15 Table 1 Bath composition and operating conditions of electroless cobalt coating. Chemical
Concentration (g/L)
CoSO47H2O NaH2PO27H2O Na3C6H5O NH4Cl Bath temperature pH at 90 °C (adjusting using 10 wt% NaOH)
20 25 50 40 90 °C 9.0
Co2þ þ H2 PO2 þ 3OH ! Co þ HPO2 3 þ 2H2 O
ð1Þ
All the chemical reactants were analytical grade and used as received. The proportion of pre-treated carbonyl iron particles added to the plating bath was 10 g/L. The particles were put into the electroless plating solution when the temperature reached 90 °C under mechanical stirring with a rate of 300rmp in order to mix sufficiently and homogenously. In general, the thickness of Co coating can be controlled by the plating time [14]. The plating time was maintained for 60 min in the present study. After electroless plating, the solution was filtered, washed and dried. Thus, the final coated particles were obtained. The morphology of the raw and Co-coated carbonyl iron particles were examined under a scanning electron microscope (SEM; JEOL JSM-5800 LV SKANNING) attached with a Links Systems energy dispersive spectrometer (EDS). Magnetic measurement was conducted at room temperature using a vibrating sample magnetometer (VSM, Riken Denshi, BHV-525). The raw and Co-coated carbonyl iron particles were homogeneously dispersed into the paraffin with a mass fraction of 70% to measure the electromagnetic property. The effective complex permittivity (er = e0 je00 ) and permeability (lr = l0 jl00 ) of the samples were measured using a network analyzer (Agilent technologies E8362B: 10 MHz–20 GHz). The dimension of the samples for electromagnetic measurement was 10.16 22.86 2 mm, which were based on the measurements of the reflection and transmission module between 8.2 and 12.4 GHz in the fundamental wave-guide method. In addition, the antioxidation property of the samples up to 800 °C was characterized using a thermal analyzer (Netzsch STA 449 TG–DTA/DSC) in air with the heating rate 10 °C/min.
3. Results and discussion 3.1. Morphologies of the raw and Co-coated carbonyl iron particles Fig. 1 shows the SEM and EDS of carbonyl iron particles before and after coated with Co. As seen from Fig. 1(a), the raw carbonyl iron particles are thin flakes with diameters ranging in 1–5 lm and thickness below 1 lm. In addition, there are small globular granules. Fig. 1(b) shows the SEM image of the Co-coated carbonyl iron. It is observed that the morphology of the Co-coated carbonyl iron is similar to the raw carbonyl iron. It is worth noting that there exist few micro-holes on the surface of Co-coated carbonyl iron particles, which may contributed to the acid pickling process. From the inserts of Fig. 1(b), the carbonyl iron particles are uniformly coated with Co coatings, the size of which are all below 0.2 lm, indicating the core–shell structure of the Co coated carbonyl iron particles. The EDS analysis indicates the presence of Co and P on the surface of carbonyl iron particles after the electroless deposition, as shown in Fig. 1(c) and (d). 3.2. VSM and TG analysis of the raw and Co-coated carbonyl iron particles The field dependence of magnetization for the raw and Cocoated carbonyl iron particles obtained from the typical electroless plating method were measured at room temperature by VSM, as shown in Fig. 2. The saturation magnetization (MS) and coercivity (HC) are 208.54 emu/g and 4.8 Oe for the raw carbonyl iron particles, and 183.78 emu/g and 9 Oe for the Co-coated carbonyl iron particles respectively. According to the theory of ferromagnetism materials: MS = NglBJ. Where N is the number of atoms, g is the g-factor, lB is called the Bohr magneton and J is the total angular moment. Thus, the saturation magnetization value of Co-coated
11
carbonyl iron particles is lower than that of the raw carbonyl iron particles, which can be attributed to the higher Bohr magneton of iron atom [15]. The enhancement of the coercivity HC after Co coated is originated from the disordered crystal structure due to the increasing internal strain by electroless plating process [16]. Fig. 3 shows the TG curves of the carbonyl iron particles with and without Co coating in air from room temperature to 800 °C. TG curve is generally characterized by the region above 400 °C with significant increase of the sample mass due to iron oxidation, and finally a high-temperature plateau above 700 °C after the formation of iron oxides. There is, however, a significant difference in the obvious weight gain temperature between uncoated and Cocoated carbonyl iron particles. The obvious weight gain of the raw carbonyl iron occurs at about 300 °C. Thus, after a single coating with Co, the obvious weight gain temperature is postponed, indicating the Co coating provides a protective overlayer on iron particles that slows the iron oxidation. 3.3. The complex permittivity and permeability spectra Fig. 4 depicts the complex permittivity and complex permeability spectra of the composites filled with the raw and Co-coated carbonyl iron particles (the content of the raw and Co-coated carbonyl iron particles are both 70 wt%). It could be seen from Fig. 4(a) that the values of the real part of permittivity (e0 ) of the composites filled with raw carbonyl iron particles kept almost unvaried (e0 9.5) in the frequency range of 8.2–12.4 GHz, and the real part increased from 9.5 to 11 after Co coating deposition on the surface of carbonyl iron particles. Therefore, it can be deduced from Fig. 4(a) that Co coating improves the storage capability of electric energy, which is consistent with the result of Pan [17]. It also reveals in Fig. 4(a) that drastic increase of the imaginary part of permittivity (e00 ) is observed from 9.8 to 12.4 GHz after electroless Co deposition. As we know, two factors mainly determine the real part of complex permittivity of material in the microwave frequency range: (i) interfacial polarization of the interface [18] and (ii) space charge polarization [19]. For the heterogeneous system such as conductor-loaded composites, the properties of interfaces could have a dominant role in determining dielectric performance, and interfacial polarization induced by the charge along the boundaries of iron and cobalt is an important polarization process and associated relaxation also gives rise to loss mechanism [20]. Besides, the space charge polarization should also be considered. The high e0 value of Co-coated carbonyl iron particles composite is attributed to the space charge polarization, which occurs between adjacent conductive iron and cobalt particles, is more easily appeared for Co-coated carbonyl iron particles [21]. On the other hand, the e00 accounts for the loss energy dissipative mechanisms in the materials, and according to the free electron theory: e00 = r/2pe0f [22]. Where r is the electrical conductivity, f is frequency and e0 is the dielectric constant in vacuum. According to the research of Fert et al. [23], the electrical resistivity of cobalt is lower than the value of iron. Therefore, based on the knowledge that the electrical conductivity is the reciprocal of electrical resistivity, it can be easily deduced that the electrical conductivity of cobalt is higher than the value of iron. In conclusion, the higher electrical conductivity of cobalt results in the higher e00 value of Co-coated carbonyl iron particles composite especially at high frequency range, showed in Fig. 4(a). The real and imaginary parts of permeability of the raw and Co-coated carbonyl iron particles composites are shown in Fig. 4(b). One feature of the data is that the values of real part (l0 ) and imaginary part (l00 ) of complex permeability for both samples all decrease with increasing frequency as shown in Fig. 4(b), which is mainly due to both eddy current loss and ferromagnetic resonance of the carbonyl iron particles [24,25]. It can
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Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15
Fig. 1. SEM and EDS of carbonyl iron particles before and after coated with Co.
Fig. 2. Magnetic hysteresis loop of the raw and Co-coated carbonyl iron particles measured at room temperature. Inset: the expanded low field magnetization curves.
also be seen clearly that both the l0 and l00 values between 8.2 and 12.4 GHz show no obvious difference for these two samples, which is ascribed to the fact that the thickness of Co interlayer among these carbonyl iron particles is sufficiently small not to make a disorder at filler–matrix interface and, thus, to cut-off the inter-particle magnetic interaction [11]. Generally, for ferrite magnetic materials, the microwave magnetic loss of magnetic materials originates mainly from hysteresis loss, domain wall resonance, natural ferromagnetic resonance, and the eddy current effect [26]. The hysteresis loss is usually caused by irreversible magnetization and can be negligible in a weak applied field. It is possible to distinguish the permeability spectra into the spin rotational component and the domain wall motion contribution, using numerical fitting [27]. Resonance resulted from
Fig. 3. TG curves of raw and Co-coated carbonyl iron samples measured in air.
domain wall movement normally occurs at low-frequency region (<2 GHz); however, resonance caused by spin rotational component occurs as high-frequency region. Thus, it could be concluded that the magnetic loss of the two samples in this study may be due to natural ferromagnetic resonance and eddy current effect. It is reasonable that both the dielectric loss and the magnetic loss can be influenced by the ‘‘core–shell’’ microstructure of microwave absorbent. In general, the dielectric loss is attributed to the lags of polarization between the core/shell interfaces as the frequency varies. In addition, the ‘‘core–shell’’ microstructure of microwave absorbent has something to do with eddy current loss that is one of the contributors to magnetic loss [25]. In order to investigate the heat resistance of Co-coated carbonyl iron particles, the raw and Co-coated carbonyl iron particles were heat treated at 300 °C for 10 h, followed by embedded in 30 wt%
Y. Zhou et al. / Journal of Alloys and Compounds 637 (2015) 10–15
Fig. 4. Complex permittivity and permeability spectra for the composites containing 70 wt.% raw or Co-coated carbonyl iron particles.
paraffin matrix. The complex permittivity and permeability of the composites are presented in Figs. 5 and 6. As seen from Figs. 5 and 6, after heat treatment at 300 °C, the e0 and e00 of the carbonyl iron composite increased considerably compared with the values of Cocoated carbonyl iron composite. The composites filled with raw carbonyl iron particles, however, exhibit a degeneracy of the dispersion law of permeability, which is contributed by a decrease in both real and imaginary parts in the measured frequency region (Fig. 5(b)). Thus, the Co coating changes the character of magnetic spectra of carbonyl iron composites from heat treatment-dependent (Fig. 5(b)) to heat treatment – independent (Fig. 6(b)) at 300 °C. This indicates the suppression of oxidation processes of carbonyl iron due to the presence of Co coating. 3.4. Microwave-absorbing properties According to the transmission line theory, the reflection loss (RL) of single-layer coating can be calculated by the following equation:
Z in Z 0 RL ¼ 20 lg jCj ¼ 20 lg Z in þ Z 0
ð2Þ
where U is the reflection coefficient, Z0 is the impedance of the free space, and Zin is the input impedance of the absorber. There, Zin can be expresses as Eq. (3):
rffiffiffiffiffi Z in ¼ Z 0
lr 2pfd pffiffiffiffiffiffiffiffiffi lr er tanh j c er
ð3Þ
13
Fig. 5. Complex permittivity and permeability spectra of the composites containing 70 wt.% raw carbonyl iron particles after 300 °C heat treatment.
where lr and er are the relative permeability and permittivity of the absorber, f is the frequency of incident wave, d is the thickness of the absorber, and c is the velocity of light in free space. In the present investigation, the mass fraction of the filler content and the thickness of the composite are fixed for 70 wt.% and 1.5 mm, respectively, to investigate the influence of Co coating and heat treatment on the RL property of these samples. The calculated RL values, according to the formula (2), (3), of the samples are shown in Fig. 7. It can be seen that the RL values for both composites filled with raw and Co-coated carbonyl iron particles are less than 10 dB can be obtained in the frequency range of 8.2–12.4 GHz, which indicates that the Co coating can help to maintain a good microwave absorbing property for these carbonyl iron particles. It is worth mentioning that the minimum RL value of Co-coated carbonyl iron particles composites turns to lower frequency, which is mainly resulted from the higher permittivity of it, as shown in Fig. 4(a), which is similar to the other’s result [8]. The difference in microwave absorption properties of the Cocoated particles is resulted from the different composition of the coated and uncoated carbonyl iron particles. The Bohr magnetons (lB) of Fe atom and Co atom are 2.22 and 1.72, respectively [28]. So, the magnetism of iron is better than the cobalt [25]. For magnetic microwave absorbers, magnetic loss is the main microwave loss tunnel. The powder with better magnetism results in larger microwave loss [29]. In a word, the decrease of the absolute RL value for the composite with Co coated particles is mainly attributed to the reduction of magnetic loss of the carbonyl iron particles after
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were heat treated at the same condition, the RL curve shows a small decreasing tendency compared with that of the Co-coated particles without treatment, and its values are still maintained below 10 dB in the whole frequency range. As a result, the microwave absorbing property of the composites is kept at a high level. It can be deduced from the forementioned results that the antioxidation property of the raw carbonyl iron particle can be effectively improved after coated with Co layer. In a word, the prepared Co-coated sample exhibits good absorption performance in the X-band frequency range and has the advantages in serving as a potential microwave absorbing material below 300 °C. 4. Conclusions Carbonyl iron was successfully coated with Co layer using electroless plating method. The electromagnetic properties of raw and Co-coated carbonyl iron particles before and after heat treatment were investigated. The permittivity of the Co-coated particle composite was enhanced by coating Co on the surface of carbonyl iron particle because of the enhanced interfacial polarization and orientation polarization between carbonyl iron and Co. As the Co coating layer was extremely thin compared with the carbonyl iron particle size, the permeability of the Co-coated particle composite kept almost invariable. After heat treatment at 300 °C for 10 h, the Co coating changes the character of permeability of carbonyl iron composites from temperature-dependent to temperature-independent. In a word, the electroless plated with Co greatly improves the antioxidation property of carbonyl iron and has slight influence upon the microwave absorption properties below 300 °C. So the Co-coated carbonyl iron has much better antioxidation property and can also serve as a super thin microwave absorber. Acknowledgements Fig. 6. Complex permittivity and permeability spectra of the composites containing 70 wt.% Co-coated carbonyl iron particles after 300 °C heat treatment.
This work was financially supported by the Chinese National Natural Science Foundation (No. 51072165) and the fund of the States Key Laboratory of the Solidification Processing in NWPU (No. KP201307). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Fig. 7. Microwave absorption of the raw and Co-coated carbonyl iron particles before and after 300 °C heat treatment filled composites with 1.5-mm thickness.
coated with Co. While for the composite with carbonyl iron particles after heat treatment at 300 °C for 10 h, the minimum RL values degrades critically from 25 dB to 15 dB, indicating the deduction of the microwave absorbing property of the carbonyl iron particles. And the RL values in the frequency range up to 11 GHz are above 10 dB. As seen from Fig. 7, When the Co-coated particles
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