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Epoxy resin addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl [email protected] composites
Journal of Magnetism and Magnetic Materials 485 (2019) 244–250
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Research articles
Epoxy resin addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl iron@epoxy composites ⁎
T
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Xinlu Guoa,b, Zhengjun Yaoa,b, , Haiyan Linc, Jintang Zhoua,d, , Yuxin Zuoa,b, Xiangyu Xua, Bo Weia,b, Wenjing Chena,b, Kun Qiana,b a
College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Key Laboratory of Material Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology, Nanjing 211100, China c Research Institute of Aerospace Special Materials & Technology, Beijing 100074, China d Jiangsu Key Laboratory of Advanced Metallic Materials, Nanjing 211100, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonyl iron powder EP contents In-situ polymerization Microwave absorption
In this study, novel CIP@EP composites with a core-shell structure were prepared by using the in-situ polymerization method. The phase structure, morphology and thermal stability were investigated by FTIR, XRD, SEM, TEM and TG. The results indicated that EP were densely covered on the surface of CIP particles, and with the increase of EP contents, the EP shell became thicker, which effectively strengthened the thermal stability of the materials. The magnetic and electromagnetic properties of the composites were systematically investigated. It was found that the content of EP played an important role in improving the magnetic and dielectric properties. High EP content can effectively improve the impedance matching between the dielectric and magnetic loss of the absorbers and enhance the EM wave absorption ability of the composites. This study proves that the CIP@30%EP exhibits an excellent EM wave absorption ability, which has higher value of RL and wider absorption width than other CIP@EP composites. The CIP@30%EP has a strong reflection loss peak of −16.1 dB and wide effective absorption bandwidths of 5.4 GHz at the thickness of 2.2 mm. Furthermore, the method utilized to synthesis the CIP@EP composites can be a suitable and efficient way to prepare other spherical microwave absorption materials.
1. Introduction With the development of communication technology, extensive use of electronics and electric apparatus has become an indispensable and important part in the information age. While these devices provide convenience and benefits, the problems of electromagnetic radiation and interference have unfavorable effects on the regular production and application such as electromagnetic (EM)-wave pollution [1–4]. To dissipate or minimize EM waves effectively, the microwave absorption materials has been paid more attention in recent years [5,6]. Usually, microwave absorption materials should possess small thickness, strong absorption capacity over a broadband, and high thermal stabilization [7–11]. Among them, carbonyl iron powder (CIP), a typical magnetic absorber, has been widely used because of the advantages of high permeability, low dispersion effect in real and imaginary parts of permeability, and strong absorption efficiency under low matching thickness [12–14].
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However, due to the high surface activity of CIP, in practical application, it is easy to absorb moisture and agglomerate and is still challenging to be uniformly dispersed in the matrix [15]. Especially in the process of manufacturing microwave absorbing composites, CIP was readily to react in the mostly acid-base environment, resulting in the corrosion and oxidation, which corroded and oxidized, depressing the microwave absorbing properties. At the same time, the higher dielectric constant makes the impedance matching at the interface with air performance worse. To solve these problems, extensive studies have been done to improve the oxidation resistance, acid and alkali resistance and impedance matching of CIP by various methods. At present, researchers generally increase the service temperature of carbonyl iron powder by coating it with organic or inorganic substances. For instance, the coating of Ag, Co and Ni layers on CIPs by electroless plating or surface deposition will significantly increase the service temperature of the material [16–24]. But during the long period of services, it will be
Corresponding authors at: College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China. E-mail addresses: [email protected] (Z. Yao), [email protected] (J. Zhou).
https://doi.org/10.1016/j.jmmm.2019.04.059 Received 17 January 2019; Received in revised form 9 March 2019; Accepted 15 April 2019 Available online 16 April 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 485 (2019) 244–250
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oxidized gradually. Besides, Gong et al. [25] and Liu et al. [26] synthesized ferrite coatings (CoFe2O4, MgFe2O4) on CIPs by heterogeneous precipitation. However, since most of the coatings are granular or scaly, the compactness and continuity need to be improved in the long-term high temperature application. To increase the application temperature, Sedlačík [27], Abshinova [15] fabricated polyaniline layer on the surface of carbonyl iron powder by organic coating. The γ-Fe2O3 layer was obtained finally on the surface of carbonyl iron powder with the reaction of 2Fe3+ + 6OH− → γ-Fe2O3 + 3H2O. Owing to the strict operation and high operating temperature, the proposed method is not suitable for smaller carbonyl iron powder. In order to obtain proper impedance matching of CIPs, surface modification is also an effective way. Bahri-Laleh et al. [7] prepared the polyaniline/carbonyl iron composites via in-situ polymerization, and the composite showed a good EM absorbing property with a weight ratio of 1:6 (PANI:CIPs). Wu et al. [10] combined ball milling and mild hydrothermal treatment to synthesize SnO2-coated carbonyl iron (SCCI) flaky composites, and performed a strong absorption between 7.2 and 18 GHz with the thickness of 7.0–13.0 mm. Moreover, the outstanding microwave absorption property was exhibited even at a low thickness. Li et al. [28] integrated flaked carbonyl iron powder and SiO2 making the composite a core-shell structure, which resulted in a large absorption band. Epoxy resin as a thermosetting resin with low cost and stable performance has been applied in this research. The CIP@EP composites are fabricated via in-situ polymerization method. The rational design of EP shell with core-shell structure is effective in improving the thermal stability and microwave absorption properties. Using epoxy resin as the shell CIPs can be easy to produce by in-situ polymerization which is a simple synthesis method with one step. Due to the excellent adhesion ability, electrical insulation and high stability of the core-shell structure, the composites of CIP particles coated with EP shell will improve the thermal stability, impedance matching property and enhance the ability of EM absorption compared with the uncoated CIPs. The aim of the investigations here is to investigate the influence of the EP addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl iron@epoxy composites.
Fig. 1. FTIR spectra of pure EP, CIP, and CIP@EP composites.
Cu Kα radiation of wavelength λ = 1.5406 Å. The morphology of powders was observed via SEM (Ultra55 FE-SEM, Zeiss) and TEM (FEI Tecnai G2 F20). All samples were characterized by FTIR spectrophotometer using KBr pellets (400–4000 cm−1). The thermogravimetric measurements were performed with a thermogravimetric analyzer (Netzsch STA 449C, Germany). Magnetic measurements were performed using a vibrating sample magnetometer (Lakeshore, Model 7400 series), and the electromagnetic parameters of products were measured by a vector network analyzer (Agilent PNA N5224A) in the frequency range from 0.5 to 18 GHz. 3. Result and discussion 3.1. FTIR spectra Fig. 1 shows the FTIR spectra of CIP (a), pure EP (b), and CIP@EP composites (c)-(f). Fig. 1(a) shows two large bands at 3425 cm−1 and 1084 cm−1 which are assigned to O–H stretching vibration from hy, droxyl groups. The stretching vibration of C–H is corresponding to three broad bands, which are located at 2963, 2924 and 2855 cm−1 As for . pure EP, the characteristic peaks display at 1511 and 1452 cm−1 in Fig. 1(b), which are attributed to the stretching vibration of C]C and C–N bending, respectively. The characteristic C–O and C–O–C bonds are responsible for the broad bands which are located at 1245 and 1036 cm−1, respectively. And at 827 cm−1, there is the bend of out-ofplane deformation of C–H. Fig. 1(c)–(f) shows the CIP@EP composites with different EP additions. For the composite with 20–40% EP additions, the amplitudes of the CIP’s peak are lower than that of the pure CIP at 1084 cm−1. And the peak disappeared for the sample with 50% EP addition. However, the major characteristic bands of EP can be found in all the CIP@EP composites, and with the increase of EP, the peaks of transmittance are stronger. The FTIR results indicate that the chemical reaction can successfully synthesize the composite of EP coating on the CIPs.
2. Experimental section 2.1. Materials In our experiment, carbonyl iron powders (CIPs), silane coupling agent (KH-550), epoxy resin, ethanol, triethylenetetramine and deionized water were used. CIP was commercially bought from Shaanxi Xinghua Group Co., Ltd., with an average particle size of 5 μm. Silane coupling agent (KH-550) and ethanol were purchased from Aladdin Chemical Reagent, China. Epoxy resin and triethylenetetramine were provided by NO. 1 Advanced Materials Americas Inc., China. Deionized water was used for all the experiments. 2.2. Preparation of CIP@EP composites To prepare the CIP@EP composites, the surface of the carbonyl iron powder particles was firstly modified with silane coupling agent to make the carbonyl iron powder more evenly dispersed in the epoxy resin. Secondly, epoxy resin was weighed and dissolved in ethanol for ultrasonic dispersion for 20–30 min. Then the corresponding amount of modified carbonyl iron powder and triethylenetetramine were added to the epoxy resin solution. Heating and stirring the mixture for 6–7 h at 60 °C to obtain the suspension liquid and centrifugal filtering and drying to acquire the CIP@EP powders. CIP@EP composites with mass fraction of 0, 20, 30, 40, and 50 wt% of EP were prepared, respectively.
3.2. X-ray diffraction Fig. 2 shows the XRD patterns of pure EP (a), CIP (b), and CIP@EP (c)–(e) composites. Fig. 2(a) shows a single peak of the polymer chains of EP where 2θ equals 18.45°. Fig. 2(b) exhibits the diffraction peak of carbonyl iron with body-centered cubic structure where 2θ = 44.81°, 65.16°, and 82.43°, respectively, and are matching well with the JCPDS file data (JCPDS 06-0696). As is shown in Fig. 2(c)–(e), CIP@EP composites have the same diffraction peak. At the same time, with the increasing of EP content, the diffraction angle of the peaks did not change
2.3. Characterization The phase composition of the samples was identified by XRD using 245
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Fig. 2. XRD patterns of pure EP, CIP, and CIP@EP composites.
significantly. This confirmed that epoxy resin does not change the phase composition and the crystal structure of CIP particles during the preparation of CIP@EP composites. However, no peak of crystalline EP was found in CIP@EP composites, indicating that the epoxide resin shell coating on the CIP is amorphous state [29]. 3.3. Morphological characterization Fig. 3 shows the FE-SEM and TEM images of uncoated CIP particles and CIP@EP composites. Fig. 3(a) and (b) are the SEM and TEM images of uncoated CIPs with the particle diameter ranging from 0.51 to 4.56 μm. Microstructure of the particles shown in Fig. 3(c), (e), (g) and (i) are the CIP@EP composites with 20%–50% EP addition. From the SEM images, we can observe that the core-shell structure of CIP@EP composites gradually formed with the increase of EP addition. Also, we can clearly observe the sphere configuration of CIP particles in Fig. 3(c) and (e) which are different from the Fig. 3(g) and (i). When the contents of EP ≥ 30%, the morphology of CIP@EP composites is no longer presenting sphere configuration obviously. Since the more EP added in the polymerization process will have the surface of the CIP particles coated with more EP. Fig. 3(d), (f), (h) and (j) show the morphology and microstructure of four CIP@EP composites from TEM. Due to the difference in atomic number, the EP shell appears bright in the TEM image, while the CIP particles appear dark [30]. As seen in the TEM images, as the increase of EP addition, the thickness of EP shell ranges from 40 nm to 490 nm. Besides, the introduction of EP covers the CIP cores tightly and fully that did not change the microstructure of the CIP particles. Thus, the core-shell structure of CIP@EP composites via insitu polymerization method is fabricated successfully and the content of EP will change the thickness of EP shell directly which can decrease the reflectivity of EM waves on the CIP surface and give the CIP@EP composites great microwave absorption properties [31].
Fig. 3. FE-SEM images of uncoated CIP (a) and CIP@EP composites (c, e, g, i); TEM images of uncoated CIP (b) and CIP@EP composites (d, f, h, j).
residual carbon content was only 0.2%. However, the EP-coated CIPs exhibit greater heat resistance ability than pure CIP particles. We can observe from the figure, when adding the EP into CIPs, the weight of the samples first decreased and then increased, Meanwhile, CIP@EP composites have lower gain of weight than CIPs and the 50% EP addition shows the lowest. Meanwhile, with the increase of EP addition, the temperatures when the weight of CIP@EP composites start to rise are getting higher. It suggests that the EP shell on the CIP particles can be a barrier to prevent the oxygen. These results demonstrates that the thermal stability is improved obviously owing to the introduction of EP.
3.4. Thermogravimetric analysis Fig. 4 shows the thermogravimetric curves of CIP, pure EP, as well as the CIP@EP composites. The samples were heated from 100 °C to 800 °C at a heating rate of 15 °C/min in air. As we can see, the weight of pure CIP increases with the increment of the temperature, which is due to the oxidation of the CIPs. As for pure EP, it is unstable beyond the temperature of 280 °C which caused the mass loss of EP since the thermal decomposition of EP happened after 280 °C. Pure EP broke the main chain at about 280 °C and decomposed under a slow rate. Then, the weightlessness rate increases rapidly above 400 °C. At 600 °C, the 246
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decreased, and the spatial spacing of the particles increased, which slowed down the response time of the particles to the external magnetic field, leading to the increasing of Hc. 3.6. Microwave absorption properties Fig. 6 shows the complex permittivity ε′ (a), imaginary parts of the complex permittivity ε″ (b), real parts of the complex permeability μ′ (c) and imaginary parts of the complex permeability μ″ (d) images of the uncoated CIP and CIP@EP composites in the frequency range of 2–18 GHz. Complex permittivity (ε = ε′ − jε″) and permeability (μ = μ′ − jμ″) are parameters to characterize the microwave absorption properties of materials [35]. It can be seen in Fig. 6(a) that the ε′ of CIP and CIP@EP composites have nearly parallel variations in the whole frequency range and the value of ε′ decrease with the increase of EP content. Because of the surface resistivity of the particles increases due to the coating of the EP shell coated on the surface of the CIP particles and the energy barrier between the interfaces are reduced, leading to the weakness of the space-charge polarization of the composites [36]. As shown in Fig. 6(b), the complex permittivity ε″, the value of the CIP and CIP@EP composites are all exhibited an increasing tendency in the whole frequency range. There is no significant change in the frequency range of 2–10 GHz. For CIP and CIP@20%EP to CIP@50%EP, the peak located at the frequency of 11.8 GHz, 15.1 GHz, 17.4 GHz, 15.0 GHz and 15.7 GHz, respectively. The variations in ε' and ε“ suggested a mild dielectric relaxation in the band. Meanwhile, Fig. 6(c) and (d) show the frequency dependence of μ′ and μ″ for the CIP@EP composites coating with 0–50% EP. As the increase of EP content, the value of μ′ and μ″ all decrease. It may arise from the decrease of mass ratio of magnetic particles and specific saturation magnetization [37]. Fig. 7 shows the calculated RL curves of uncoated CIP and four CIP@ EP composites at the thickness of 2.2 mm and the 3D and 2D images of the RL for CIP@30%EP composite. The equations of the reflection loss of the specimens represent as follows [38,39]:
Fig. 4. TG curves of pure EP, CIP, and CIP@EP composites.
3.5. Magnetic analysis Fig. 5 shows the magnetic loops of pure EP, CIP and four CIP@EP composites at room temperature with an applied field of −10,000Oe ≤ H ≤ 10,000Oe measured by a vibrating sample magnetometer (VSM). Both CIP particles and CIP@EP composites are ferromagnetic [32]. The saturation magnetization (Ms) of the pure EP, CIP and four CIP@EP composites were 0.68 emu/g, 145.9 emu/g, 144.8 emu/g, 126.4 emu/g, 121.2 emu/g, 102.6 emu/g, respectively. The coercivity (Hc) of pure EP, CIP and four CIP@EP composites were 0Oe, 49.3Oe, 46.1Oe, 50.5Oe and 50.4Oe, respectively, which indicates that the Ms for the CIP decreased with the increasing of EP content [33] and Hc for the CIP first decreased then increased with the increasing of EP content. The introduction of non-magnetic EP weakened the interaction between the CIP particles and reduced the surface anisotropy of carbonyl iron [34]. Furthermore, the Ms of the CIP@EP composites depends on the mass fraction of CIPs [30]. Therefore, the increasing of EP content in CIP@EP composites weakened the CIP particles’ interaction and reduced the mass fraction of CIPs, resulting in the decreasing of Ms for the CIPs. Furthermore, The Hc decreasing of CIP was beneficial to improve the microwave absorption properties in a lower frequency range [31]. However, when the content of EP ≥ 30%, the EP coating became thicker. The volume ratio of the magnetic core
RL (dB ) = 20log|
Zin =
Zin − 1 | Zin + 1
μr 2πfd tanh ⎛j εr μr ⎞ εr ⎝ c ⎠
(1)
(2)
where Zin is the normalized input impedance of the absorber, c is the velocity of electromagnetic wave in free space, f is the frequency of the microwaves, and d is thickness of the absorbing layer. As shown in Fig. 7(a), we can observe that under the same conditions of the volume fraction and the thickness of the absorbing coating, the RL curve gradually drifted to the high frequency band. This is due to the increase of the thickness of EP shell layer resulting in a decrease of the dielectric constant [40]. Meanwhile, CIP@30%EP composite presented a higher value of RL and wider absorption width than other CIP@EP composites. To further study the influence of thickness on the EM wave absorption abilities. Fig. 7(b) and (c) exhibits the 3D and 2D images of the reflection loss for CIP@30%EP composite. As observed in Fig. 7(b), the composite exhibited the maximum RL value of −16.1 dB at 14.3 GHz with the thickness of 2.2 mm. Besides, the effective absorption bandwidths were up to 5.4 GHz when the specific absorption value was set as −10 dB. That is because a proper thickness of EP shell can adjust the dielectric constant of the magnetic particles, improving the dispersion characteristics of the material. Impedance matching is realized in the true sense to enhance the absorbing property of microwave absorbers. Fig. 8 shows the Impedance matching ratio (a), attenuation constant α (b) and eddy current data C0 (c) of uncoated CIP and four CIP@EP composites. In theory, the microwave absorption property of materials is closely attributed to impedance matching behaviors, dielectric loss and magnetic loss [41]. The impedance matching ratio and attenuation constant α are usually used to represent the abilities of impedance
Fig. 5. Hysteresis loops of pure EP, CIP, and CIP@EP composites. 247
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Fig. 6. The real parts of the complex permittivity ε′ (a), imaginary parts of the complex permittivity ε″ (b), real parts of the complex permeability μ′ (c) and imaginary parts of the complex permeability μ″ (d) images of uncoated CIP and CIP@EP composites.
matching behaviors and dielectric loss, while the dimensional resonance, natural resonance and eddy current resonance are three important factors to analyze the magnetic loss consider [42]. High impedance matching ratio can be seen as a sign of the great microwave absorption ability. The values of the impedance matching ratio of all composites presented in Fig. 8(a) can be defined as [43]:
Z=
Z1 =
Z1 Z0
(3)
μr × Z0 εr
(4)
where Z1 is the impedance matching of the materials, Z0 is the free space of impedance matching, μr is the complex permittivity, and εr is the complex permittivity.
Fig. 7. Reflection loss of uncoated CIP and CIP@EP composites with a thickness of 2.2 mm (a), (b) three-dimensional representation, and (c) two-dimensional representation of the values of reflection loss for CIP@30%EP composite. 248
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Fig. 8. Impedance matching ratio (a), attenuation constant α (b) and eddy current data C0 (c) of uncoated CIP and CIP@EP composites.
Scheme 1. Schematics of synthesis and microwave attenuation mechanism of CIP@EP composites.
demonstrated that the magnetic loss is not caused by the eddy current effect in this case. Taken into account the above factors, CIP@30%EP may have the highest electromagnetic wave absorption properties. In summary, the introduction of EP will enhance the microwave absorption properties of CIP@EP composites. As seen in Scheme 1, more electromagnetic waves entered absorbers rather than reflected on absorbing material surfaces and more electromagnetic waves incident channels formed to make the electromagnetic wave more easily propagate into the composites [49–52].
As is shown in Fig. 8(a), all composites with EP addition exhibited higher impedance matching ratio than CIP particles. In addition, with the increase of EP content, the value of impedance matching ratio also increased. The CIP@20% EP composites demonstrated an obviously higher ratio than that of CIP@30%EP composites at 2–7.8 GHz. It implies that the core-shell structure of CIP@EP composites makes the electromagnetic wave more easily propagate into the composites due to the forming of more EM wave incident channels [44]. Fig. 8(b) shows the value of the attenuation constant α of uncoated CIP and all CIP@EP composites, the calculation of α can be expressed by the equation of [45]:
α=
2 πf × c
4. Conclusion
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2
(5)
A series of CIP@EP composites added with four different EP contents were synthesized via a one-step in-situ polymerization method. Epoxy shell were coated on the surface and formed a core-shell structure, which did not change the microstructure of CIP particles. Meanwhile, with the increase of EP contents, the EP shell became thicker. The thermal stability was improved obviously owing to the EP coating. Moreover, the EM wave absorption ability was significantly improved, the CIP@30%EP composites exhibited higher value of RL and wider absorption width than other CIP@EP composites, which are −16.1 dB and 5.4 GHz, respectively, at the thickness of 2.2 mm. It can be concluded that the core-shell structure of CIP@EP composites with a proper EP content can effectively enhance the EM wave absorption properties of the materials. Resulting in the improvement of the impedance matching and dispersion characteristics of the composites.
where c is the velocity of light in a vacuum. As observed, all the samples exhibited a strong electromagnetic attenuation performance in the whole frequency range. Moreover, the eddy current coefficient C0 has been studied to analyze the magnetic loss, which can be described by the equation as [46]:
C0 = μ'' (μ')−2f −1 = 2πμ0 d 2δ
(6)
The eddy current effect is the factor that prevents electromagnetic wave from entering into the material, which will weak the electromagnetic attenuation. It can be evaluated with C0 and when the C0 varies with frequency increase, the eddy current effect is not the main reason caused the magnetic loss [47,48]. From Fig. 8(c), we can see that the C0 changed in the whole frequency range of all samples, which 249
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Acknowledgements [26]
This work was supported by the National Natural Science Foundation of China (51702158 and 51672129), the Fundamental Research Funds for the Central Universities (NS2017036 and NP2018111), Open Fund of Key Laboratory of Materials Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology No. 56XCA18159-3, and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0322).
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Research articles
Epoxy resin addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl iron@epoxy composites ⁎
T
⁎
Xinlu Guoa,b, Zhengjun Yaoa,b, , Haiyan Linc, Jintang Zhoua,d, , Yuxin Zuoa,b, Xiangyu Xua, Bo Weia,b, Wenjing Chena,b, Kun Qiana,b a
College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Key Laboratory of Material Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology, Nanjing 211100, China c Research Institute of Aerospace Special Materials & Technology, Beijing 100074, China d Jiangsu Key Laboratory of Advanced Metallic Materials, Nanjing 211100, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbonyl iron powder EP contents In-situ polymerization Microwave absorption
In this study, novel CIP@EP composites with a core-shell structure were prepared by using the in-situ polymerization method. The phase structure, morphology and thermal stability were investigated by FTIR, XRD, SEM, TEM and TG. The results indicated that EP were densely covered on the surface of CIP particles, and with the increase of EP contents, the EP shell became thicker, which effectively strengthened the thermal stability of the materials. The magnetic and electromagnetic properties of the composites were systematically investigated. It was found that the content of EP played an important role in improving the magnetic and dielectric properties. High EP content can effectively improve the impedance matching between the dielectric and magnetic loss of the absorbers and enhance the EM wave absorption ability of the composites. This study proves that the CIP@30%EP exhibits an excellent EM wave absorption ability, which has higher value of RL and wider absorption width than other CIP@EP composites. The CIP@30%EP has a strong reflection loss peak of −16.1 dB and wide effective absorption bandwidths of 5.4 GHz at the thickness of 2.2 mm. Furthermore, the method utilized to synthesis the CIP@EP composites can be a suitable and efficient way to prepare other spherical microwave absorption materials.
1. Introduction With the development of communication technology, extensive use of electronics and electric apparatus has become an indispensable and important part in the information age. While these devices provide convenience and benefits, the problems of electromagnetic radiation and interference have unfavorable effects on the regular production and application such as electromagnetic (EM)-wave pollution [1–4]. To dissipate or minimize EM waves effectively, the microwave absorption materials has been paid more attention in recent years [5,6]. Usually, microwave absorption materials should possess small thickness, strong absorption capacity over a broadband, and high thermal stabilization [7–11]. Among them, carbonyl iron powder (CIP), a typical magnetic absorber, has been widely used because of the advantages of high permeability, low dispersion effect in real and imaginary parts of permeability, and strong absorption efficiency under low matching thickness [12–14].
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However, due to the high surface activity of CIP, in practical application, it is easy to absorb moisture and agglomerate and is still challenging to be uniformly dispersed in the matrix [15]. Especially in the process of manufacturing microwave absorbing composites, CIP was readily to react in the mostly acid-base environment, resulting in the corrosion and oxidation, which corroded and oxidized, depressing the microwave absorbing properties. At the same time, the higher dielectric constant makes the impedance matching at the interface with air performance worse. To solve these problems, extensive studies have been done to improve the oxidation resistance, acid and alkali resistance and impedance matching of CIP by various methods. At present, researchers generally increase the service temperature of carbonyl iron powder by coating it with organic or inorganic substances. For instance, the coating of Ag, Co and Ni layers on CIPs by electroless plating or surface deposition will significantly increase the service temperature of the material [16–24]. But during the long period of services, it will be
Corresponding authors at: College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China. E-mail addresses: [email protected] (Z. Yao), [email protected] (J. Zhou).
https://doi.org/10.1016/j.jmmm.2019.04.059 Received 17 January 2019; Received in revised form 9 March 2019; Accepted 15 April 2019 Available online 16 April 2019 0304-8853/ © 2019 Published by Elsevier B.V.
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oxidized gradually. Besides, Gong et al. [25] and Liu et al. [26] synthesized ferrite coatings (CoFe2O4, MgFe2O4) on CIPs by heterogeneous precipitation. However, since most of the coatings are granular or scaly, the compactness and continuity need to be improved in the long-term high temperature application. To increase the application temperature, Sedlačík [27], Abshinova [15] fabricated polyaniline layer on the surface of carbonyl iron powder by organic coating. The γ-Fe2O3 layer was obtained finally on the surface of carbonyl iron powder with the reaction of 2Fe3+ + 6OH− → γ-Fe2O3 + 3H2O. Owing to the strict operation and high operating temperature, the proposed method is not suitable for smaller carbonyl iron powder. In order to obtain proper impedance matching of CIPs, surface modification is also an effective way. Bahri-Laleh et al. [7] prepared the polyaniline/carbonyl iron composites via in-situ polymerization, and the composite showed a good EM absorbing property with a weight ratio of 1:6 (PANI:CIPs). Wu et al. [10] combined ball milling and mild hydrothermal treatment to synthesize SnO2-coated carbonyl iron (SCCI) flaky composites, and performed a strong absorption between 7.2 and 18 GHz with the thickness of 7.0–13.0 mm. Moreover, the outstanding microwave absorption property was exhibited even at a low thickness. Li et al. [28] integrated flaked carbonyl iron powder and SiO2 making the composite a core-shell structure, which resulted in a large absorption band. Epoxy resin as a thermosetting resin with low cost and stable performance has been applied in this research. The CIP@EP composites are fabricated via in-situ polymerization method. The rational design of EP shell with core-shell structure is effective in improving the thermal stability and microwave absorption properties. Using epoxy resin as the shell CIPs can be easy to produce by in-situ polymerization which is a simple synthesis method with one step. Due to the excellent adhesion ability, electrical insulation and high stability of the core-shell structure, the composites of CIP particles coated with EP shell will improve the thermal stability, impedance matching property and enhance the ability of EM absorption compared with the uncoated CIPs. The aim of the investigations here is to investigate the influence of the EP addition on the microstructure, thermal stability and microwave absorption properties of core-shell carbonyl iron@epoxy composites.
Fig. 1. FTIR spectra of pure EP, CIP, and CIP@EP composites.
Cu Kα radiation of wavelength λ = 1.5406 Å. The morphology of powders was observed via SEM (Ultra55 FE-SEM, Zeiss) and TEM (FEI Tecnai G2 F20). All samples were characterized by FTIR spectrophotometer using KBr pellets (400–4000 cm−1). The thermogravimetric measurements were performed with a thermogravimetric analyzer (Netzsch STA 449C, Germany). Magnetic measurements were performed using a vibrating sample magnetometer (Lakeshore, Model 7400 series), and the electromagnetic parameters of products were measured by a vector network analyzer (Agilent PNA N5224A) in the frequency range from 0.5 to 18 GHz. 3. Result and discussion 3.1. FTIR spectra Fig. 1 shows the FTIR spectra of CIP (a), pure EP (b), and CIP@EP composites (c)-(f). Fig. 1(a) shows two large bands at 3425 cm−1 and 1084 cm−1 which are assigned to O–H stretching vibration from hy, droxyl groups. The stretching vibration of C–H is corresponding to three broad bands, which are located at 2963, 2924 and 2855 cm−1 As for . pure EP, the characteristic peaks display at 1511 and 1452 cm−1 in Fig. 1(b), which are attributed to the stretching vibration of C]C and C–N bending, respectively. The characteristic C–O and C–O–C bonds are responsible for the broad bands which are located at 1245 and 1036 cm−1, respectively. And at 827 cm−1, there is the bend of out-ofplane deformation of C–H. Fig. 1(c)–(f) shows the CIP@EP composites with different EP additions. For the composite with 20–40% EP additions, the amplitudes of the CIP’s peak are lower than that of the pure CIP at 1084 cm−1. And the peak disappeared for the sample with 50% EP addition. However, the major characteristic bands of EP can be found in all the CIP@EP composites, and with the increase of EP, the peaks of transmittance are stronger. The FTIR results indicate that the chemical reaction can successfully synthesize the composite of EP coating on the CIPs.
2. Experimental section 2.1. Materials In our experiment, carbonyl iron powders (CIPs), silane coupling agent (KH-550), epoxy resin, ethanol, triethylenetetramine and deionized water were used. CIP was commercially bought from Shaanxi Xinghua Group Co., Ltd., with an average particle size of 5 μm. Silane coupling agent (KH-550) and ethanol were purchased from Aladdin Chemical Reagent, China. Epoxy resin and triethylenetetramine were provided by NO. 1 Advanced Materials Americas Inc., China. Deionized water was used for all the experiments. 2.2. Preparation of CIP@EP composites To prepare the CIP@EP composites, the surface of the carbonyl iron powder particles was firstly modified with silane coupling agent to make the carbonyl iron powder more evenly dispersed in the epoxy resin. Secondly, epoxy resin was weighed and dissolved in ethanol for ultrasonic dispersion for 20–30 min. Then the corresponding amount of modified carbonyl iron powder and triethylenetetramine were added to the epoxy resin solution. Heating and stirring the mixture for 6–7 h at 60 °C to obtain the suspension liquid and centrifugal filtering and drying to acquire the CIP@EP powders. CIP@EP composites with mass fraction of 0, 20, 30, 40, and 50 wt% of EP were prepared, respectively.
3.2. X-ray diffraction Fig. 2 shows the XRD patterns of pure EP (a), CIP (b), and CIP@EP (c)–(e) composites. Fig. 2(a) shows a single peak of the polymer chains of EP where 2θ equals 18.45°. Fig. 2(b) exhibits the diffraction peak of carbonyl iron with body-centered cubic structure where 2θ = 44.81°, 65.16°, and 82.43°, respectively, and are matching well with the JCPDS file data (JCPDS 06-0696). As is shown in Fig. 2(c)–(e), CIP@EP composites have the same diffraction peak. At the same time, with the increasing of EP content, the diffraction angle of the peaks did not change
2.3. Characterization The phase composition of the samples was identified by XRD using 245
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Fig. 2. XRD patterns of pure EP, CIP, and CIP@EP composites.
significantly. This confirmed that epoxy resin does not change the phase composition and the crystal structure of CIP particles during the preparation of CIP@EP composites. However, no peak of crystalline EP was found in CIP@EP composites, indicating that the epoxide resin shell coating on the CIP is amorphous state [29]. 3.3. Morphological characterization Fig. 3 shows the FE-SEM and TEM images of uncoated CIP particles and CIP@EP composites. Fig. 3(a) and (b) are the SEM and TEM images of uncoated CIPs with the particle diameter ranging from 0.51 to 4.56 μm. Microstructure of the particles shown in Fig. 3(c), (e), (g) and (i) are the CIP@EP composites with 20%–50% EP addition. From the SEM images, we can observe that the core-shell structure of CIP@EP composites gradually formed with the increase of EP addition. Also, we can clearly observe the sphere configuration of CIP particles in Fig. 3(c) and (e) which are different from the Fig. 3(g) and (i). When the contents of EP ≥ 30%, the morphology of CIP@EP composites is no longer presenting sphere configuration obviously. Since the more EP added in the polymerization process will have the surface of the CIP particles coated with more EP. Fig. 3(d), (f), (h) and (j) show the morphology and microstructure of four CIP@EP composites from TEM. Due to the difference in atomic number, the EP shell appears bright in the TEM image, while the CIP particles appear dark [30]. As seen in the TEM images, as the increase of EP addition, the thickness of EP shell ranges from 40 nm to 490 nm. Besides, the introduction of EP covers the CIP cores tightly and fully that did not change the microstructure of the CIP particles. Thus, the core-shell structure of CIP@EP composites via insitu polymerization method is fabricated successfully and the content of EP will change the thickness of EP shell directly which can decrease the reflectivity of EM waves on the CIP surface and give the CIP@EP composites great microwave absorption properties [31].
Fig. 3. FE-SEM images of uncoated CIP (a) and CIP@EP composites (c, e, g, i); TEM images of uncoated CIP (b) and CIP@EP composites (d, f, h, j).
residual carbon content was only 0.2%. However, the EP-coated CIPs exhibit greater heat resistance ability than pure CIP particles. We can observe from the figure, when adding the EP into CIPs, the weight of the samples first decreased and then increased, Meanwhile, CIP@EP composites have lower gain of weight than CIPs and the 50% EP addition shows the lowest. Meanwhile, with the increase of EP addition, the temperatures when the weight of CIP@EP composites start to rise are getting higher. It suggests that the EP shell on the CIP particles can be a barrier to prevent the oxygen. These results demonstrates that the thermal stability is improved obviously owing to the introduction of EP.
3.4. Thermogravimetric analysis Fig. 4 shows the thermogravimetric curves of CIP, pure EP, as well as the CIP@EP composites. The samples were heated from 100 °C to 800 °C at a heating rate of 15 °C/min in air. As we can see, the weight of pure CIP increases with the increment of the temperature, which is due to the oxidation of the CIPs. As for pure EP, it is unstable beyond the temperature of 280 °C which caused the mass loss of EP since the thermal decomposition of EP happened after 280 °C. Pure EP broke the main chain at about 280 °C and decomposed under a slow rate. Then, the weightlessness rate increases rapidly above 400 °C. At 600 °C, the 246
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decreased, and the spatial spacing of the particles increased, which slowed down the response time of the particles to the external magnetic field, leading to the increasing of Hc. 3.6. Microwave absorption properties Fig. 6 shows the complex permittivity ε′ (a), imaginary parts of the complex permittivity ε″ (b), real parts of the complex permeability μ′ (c) and imaginary parts of the complex permeability μ″ (d) images of the uncoated CIP and CIP@EP composites in the frequency range of 2–18 GHz. Complex permittivity (ε = ε′ − jε″) and permeability (μ = μ′ − jμ″) are parameters to characterize the microwave absorption properties of materials [35]. It can be seen in Fig. 6(a) that the ε′ of CIP and CIP@EP composites have nearly parallel variations in the whole frequency range and the value of ε′ decrease with the increase of EP content. Because of the surface resistivity of the particles increases due to the coating of the EP shell coated on the surface of the CIP particles and the energy barrier between the interfaces are reduced, leading to the weakness of the space-charge polarization of the composites [36]. As shown in Fig. 6(b), the complex permittivity ε″, the value of the CIP and CIP@EP composites are all exhibited an increasing tendency in the whole frequency range. There is no significant change in the frequency range of 2–10 GHz. For CIP and CIP@20%EP to CIP@50%EP, the peak located at the frequency of 11.8 GHz, 15.1 GHz, 17.4 GHz, 15.0 GHz and 15.7 GHz, respectively. The variations in ε' and ε“ suggested a mild dielectric relaxation in the band. Meanwhile, Fig. 6(c) and (d) show the frequency dependence of μ′ and μ″ for the CIP@EP composites coating with 0–50% EP. As the increase of EP content, the value of μ′ and μ″ all decrease. It may arise from the decrease of mass ratio of magnetic particles and specific saturation magnetization [37]. Fig. 7 shows the calculated RL curves of uncoated CIP and four CIP@ EP composites at the thickness of 2.2 mm and the 3D and 2D images of the RL for CIP@30%EP composite. The equations of the reflection loss of the specimens represent as follows [38,39]:
Fig. 4. TG curves of pure EP, CIP, and CIP@EP composites.
3.5. Magnetic analysis Fig. 5 shows the magnetic loops of pure EP, CIP and four CIP@EP composites at room temperature with an applied field of −10,000Oe ≤ H ≤ 10,000Oe measured by a vibrating sample magnetometer (VSM). Both CIP particles and CIP@EP composites are ferromagnetic [32]. The saturation magnetization (Ms) of the pure EP, CIP and four CIP@EP composites were 0.68 emu/g, 145.9 emu/g, 144.8 emu/g, 126.4 emu/g, 121.2 emu/g, 102.6 emu/g, respectively. The coercivity (Hc) of pure EP, CIP and four CIP@EP composites were 0Oe, 49.3Oe, 46.1Oe, 50.5Oe and 50.4Oe, respectively, which indicates that the Ms for the CIP decreased with the increasing of EP content [33] and Hc for the CIP first decreased then increased with the increasing of EP content. The introduction of non-magnetic EP weakened the interaction between the CIP particles and reduced the surface anisotropy of carbonyl iron [34]. Furthermore, the Ms of the CIP@EP composites depends on the mass fraction of CIPs [30]. Therefore, the increasing of EP content in CIP@EP composites weakened the CIP particles’ interaction and reduced the mass fraction of CIPs, resulting in the decreasing of Ms for the CIPs. Furthermore, The Hc decreasing of CIP was beneficial to improve the microwave absorption properties in a lower frequency range [31]. However, when the content of EP ≥ 30%, the EP coating became thicker. The volume ratio of the magnetic core
RL (dB ) = 20log|
Zin =
Zin − 1 | Zin + 1
μr 2πfd tanh ⎛j εr μr ⎞ εr ⎝ c ⎠
(1)
(2)
where Zin is the normalized input impedance of the absorber, c is the velocity of electromagnetic wave in free space, f is the frequency of the microwaves, and d is thickness of the absorbing layer. As shown in Fig. 7(a), we can observe that under the same conditions of the volume fraction and the thickness of the absorbing coating, the RL curve gradually drifted to the high frequency band. This is due to the increase of the thickness of EP shell layer resulting in a decrease of the dielectric constant [40]. Meanwhile, CIP@30%EP composite presented a higher value of RL and wider absorption width than other CIP@EP composites. To further study the influence of thickness on the EM wave absorption abilities. Fig. 7(b) and (c) exhibits the 3D and 2D images of the reflection loss for CIP@30%EP composite. As observed in Fig. 7(b), the composite exhibited the maximum RL value of −16.1 dB at 14.3 GHz with the thickness of 2.2 mm. Besides, the effective absorption bandwidths were up to 5.4 GHz when the specific absorption value was set as −10 dB. That is because a proper thickness of EP shell can adjust the dielectric constant of the magnetic particles, improving the dispersion characteristics of the material. Impedance matching is realized in the true sense to enhance the absorbing property of microwave absorbers. Fig. 8 shows the Impedance matching ratio (a), attenuation constant α (b) and eddy current data C0 (c) of uncoated CIP and four CIP@EP composites. In theory, the microwave absorption property of materials is closely attributed to impedance matching behaviors, dielectric loss and magnetic loss [41]. The impedance matching ratio and attenuation constant α are usually used to represent the abilities of impedance
Fig. 5. Hysteresis loops of pure EP, CIP, and CIP@EP composites. 247
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Fig. 6. The real parts of the complex permittivity ε′ (a), imaginary parts of the complex permittivity ε″ (b), real parts of the complex permeability μ′ (c) and imaginary parts of the complex permeability μ″ (d) images of uncoated CIP and CIP@EP composites.
matching behaviors and dielectric loss, while the dimensional resonance, natural resonance and eddy current resonance are three important factors to analyze the magnetic loss consider [42]. High impedance matching ratio can be seen as a sign of the great microwave absorption ability. The values of the impedance matching ratio of all composites presented in Fig. 8(a) can be defined as [43]:
Z=
Z1 =
Z1 Z0
(3)
μr × Z0 εr
(4)
where Z1 is the impedance matching of the materials, Z0 is the free space of impedance matching, μr is the complex permittivity, and εr is the complex permittivity.
Fig. 7. Reflection loss of uncoated CIP and CIP@EP composites with a thickness of 2.2 mm (a), (b) three-dimensional representation, and (c) two-dimensional representation of the values of reflection loss for CIP@30%EP composite. 248
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Fig. 8. Impedance matching ratio (a), attenuation constant α (b) and eddy current data C0 (c) of uncoated CIP and CIP@EP composites.
Scheme 1. Schematics of synthesis and microwave attenuation mechanism of CIP@EP composites.
demonstrated that the magnetic loss is not caused by the eddy current effect in this case. Taken into account the above factors, CIP@30%EP may have the highest electromagnetic wave absorption properties. In summary, the introduction of EP will enhance the microwave absorption properties of CIP@EP composites. As seen in Scheme 1, more electromagnetic waves entered absorbers rather than reflected on absorbing material surfaces and more electromagnetic waves incident channels formed to make the electromagnetic wave more easily propagate into the composites [49–52].
As is shown in Fig. 8(a), all composites with EP addition exhibited higher impedance matching ratio than CIP particles. In addition, with the increase of EP content, the value of impedance matching ratio also increased. The CIP@20% EP composites demonstrated an obviously higher ratio than that of CIP@30%EP composites at 2–7.8 GHz. It implies that the core-shell structure of CIP@EP composites makes the electromagnetic wave more easily propagate into the composites due to the forming of more EM wave incident channels [44]. Fig. 8(b) shows the value of the attenuation constant α of uncoated CIP and all CIP@EP composites, the calculation of α can be expressed by the equation of [45]:
α=
2 πf × c
4. Conclusion
(μ″ε″ − μ′ε′) +
(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2
(5)
A series of CIP@EP composites added with four different EP contents were synthesized via a one-step in-situ polymerization method. Epoxy shell were coated on the surface and formed a core-shell structure, which did not change the microstructure of CIP particles. Meanwhile, with the increase of EP contents, the EP shell became thicker. The thermal stability was improved obviously owing to the EP coating. Moreover, the EM wave absorption ability was significantly improved, the CIP@30%EP composites exhibited higher value of RL and wider absorption width than other CIP@EP composites, which are −16.1 dB and 5.4 GHz, respectively, at the thickness of 2.2 mm. It can be concluded that the core-shell structure of CIP@EP composites with a proper EP content can effectively enhance the EM wave absorption properties of the materials. Resulting in the improvement of the impedance matching and dispersion characteristics of the composites.
where c is the velocity of light in a vacuum. As observed, all the samples exhibited a strong electromagnetic attenuation performance in the whole frequency range. Moreover, the eddy current coefficient C0 has been studied to analyze the magnetic loss, which can be described by the equation as [46]:
C0 = μ'' (μ')−2f −1 = 2πμ0 d 2δ
(6)
The eddy current effect is the factor that prevents electromagnetic wave from entering into the material, which will weak the electromagnetic attenuation. It can be evaluated with C0 and when the C0 varies with frequency increase, the eddy current effect is not the main reason caused the magnetic loss [47,48]. From Fig. 8(c), we can see that the C0 changed in the whole frequency range of all samples, which 249
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Acknowledgements [26]
This work was supported by the National Natural Science Foundation of China (51702158 and 51672129), the Fundamental Research Funds for the Central Universities (NS2017036 and NP2018111), Open Fund of Key Laboratory of Materials Preparation and Protection for Harsh Environment (Nanjing University of Aeronautics and Astronautics), Ministry of Industry and Information Technology No. 56XCA18159-3, and Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18_0322).
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