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multi-walled carbon nanotubes composite with enhanced electromagnetic-wave absorption performance
Journal of Alloys and Compounds 731 (2018) 745e752
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hydrothermally synthesized Zn ferrite/multi-walled carbon nanotubes composite with enhanced electromagnetic-wave absorption performance Zhenfeng Liu, Honglong Xing*, Ye Liu, Huan Wang, Hanxiao Jia, Xiaoli Ji School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui Province 232001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 June 2017 Received in revised form 27 September 2017 Accepted 28 September 2017 Available online 29 September 2017
Zn ferrite/multi-walled carbon nanotubes (Zn ferrite/MWCNTs) composite were prepared by one-pot hydrothermal method. Their crystal structure, morphology, composition, magnetic properties, and electromagnetic-wave (EM-wave) absorption performance were measured by X-ray powder diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, vibrating sample magnetometry, and vector network analysis. Results indicated that Zn ferrite particles were distributed on MWCNTs surface with agglomeration, and MWCNTs crucially affected the magnetic properties of Zn ferrite/MWCNTs composite. Compared with Zn ferrite, Zn ferrite/MWCNTs composite showed low addition amount, low coating thickness, and enhanced EM-wave absorption performance. With 60 wt% addition amount and 1.5 mm coating thickness, the minimum reflection loss of Zn ferrite/MWCNTs composite was 42.6 dB at 12.1 GHz. The enhanced EM-wave absorption performance was mainly ascribed to the increased interfacial polarization and dielectric loss that resulting from the introduction of MWCNTs. The result illustrated that the introduction of MWCNTs into magnetic materials can enable the efficient design of excellent EM-wave absorbers with low addition amount and coating thickness. © 2017 Published by Elsevier B.V.
Keywords: Zn ferrite Multi-walled carbon nanotubes Electromagnetic-wave absorption Interfacial polarization
1. Introduction Identifying an efficient way to eliminate or weaken the electromagnetic (EM)-wave pollution that arising from the use of electronic products and communication equipment is a research hotspot [1e3]. EM-wave absorbers, can be used for solve EM-wave pollution conveniently, has been attracted considerable attention because of its easy fabrication, low cost, convenient application, and efficient absorption capabilities [4e7]. EM-wave absorption materials need to transfer EM-wave energy into thermal energy and to effectively prevent the incident EM-wave from being reflected by the absorber surface [8]. Thus, low coating thickness, light weight, strong absorption, and large absorption bandwidth are important features for efficient EM-wave absorption materials. Ferrites, such as Fe3O4 [9], CoFe2O4 [10], and BaFe12O19 [11], are traditional EM-wave absorption materials because of their magnetic properties, low cost, environmental stability, and excellent
* Corresponding author. E-mail address: [email protected] (H. Xing). https://doi.org/10.1016/j.jallcom.2017.09.317 0925-8388/© 2017 Published by Elsevier B.V.
EM-wave absorption performance. However, the large coating thickness and high addition amount (mixed with the matrix for application) of ferrites restrict their application [12]. Owing to the magnetic and dielectric loss of EM-wave absorption materials, magnetic and conductive materials have been combined to enhance the EM-wave absorption capability. To create an excellent EM-wave absorber, conductive materials, such as PANI [13], graphene [14], carbon nanotubes (CNTs) [15,16], and carbon powder [17,18], were introduced into ferrite. Results show that the conductive material decreases the addition amount and coating thickness and enhanced ferrite EM-wave absorption performance. Due to their low density, high conductivity, and unique nanostructure, CNTs are extensively used in EM-wave absorption, but the high conductivity consistently results in poor absorption. Thus, CNTs are combined with magnetic nanoparticles for EM-wave absorption. The EM-wave absorption capability of ultrathin Fe3O4/ CNT sandwich buckypaper was studied by Lu et al. [19]. Their results indicated that the reflection loss (RL) of ultrathin Fe3O4/CNT reached 12.62 dB at 17.72 GHz with absorption thickness of 0.1 mm. Lan et al. prepared Fe3O4/MWCNTs hybrids, the strongest reflection loss values reached 60.7 dB at 11 GHz by optimizing the
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Fig. 1. Illustration for the synthesis of Zn ferrite/MWCNTs composite.
absorber thickness [20]. Zhang et al. reported Boron and nitrogen doped carbon nanotubes/Fe3O4 composite, the composite architectures exhibit the minimum reflection loss at 51.2 dB [21]. Although these methods that combine Fe3O4 with CNTs enhanced their EM-wave absorption performance, but the performance can't be enhanced too much on the basis of previous combination methods. In our previous work, Zn ferrite was synthesized and showed enhanced EM-wave absorption performance than Fe3O4 [22]. The bandwidth corresponding to RL below 10 dB was about 5.0 GHz, which nearly covered the entire S band (2e4 GHz) and C band (4e8 GHz), but the coating thickness was 4.0 mm and the addition amount was 75 wt%. Thus Zn ferrite was used to combine with MWCNTs to decrease the coating thickness, addition amount and
ascertain the potential EM-wave absorption performance. To prepare Zn ferrite/MWCNTs composite, Zhang et al. proposed a complicated template method by using CNTs as template synthesized zinc ferrite/CNTs nanochains [23]. Novelty, an easy hydrothermal method without surfactant was used to synthesize Zn ferrite/MWCNTs composite here, as showing in Fig. 1. The content of MWCNTs was changed to determine its effect on Zn ferrite/ MWCNTs composite EM-wave absorption properties. Also, their crystal structure, morphology, composition, magnetic properties, and EM-wave absorption performance were characterized. They showed enhanced EM-wave absorption performance. In Zn ferrite/ MWCNTs composite with 60 wt% addition amount, the RLmin reached 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. The results suggested that MWCNTs were a key factor in decreasing
Fig. 2. FE-SEM images of Zn ferrite (a), S1 (b), S2 (c), and S3 (d); HRTEM images of Zn ferrite (upper right corner in a) and S3 (e and f).
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the addition amount and coating thickness of Zn ferrite. 2. Experimental section 2.1. Synthesis of Zn ferrite/MWCNTs composite All chemical reagents purchased from Sinopharm Chemical Reagent Co., Ltd. were of analytical grade and used without further purification. Zn ferrite/MWCNTs composite were synthesized by an easy hydrothermal method. MWCNTs were dispersed in 40 mL of distilled water and ultrasonicated for 2 h. Subsequently, FeCl3$6H2O (2.0 mmol), ZnSO4$7H2O (0.4 mmol), and FeSO4$7H2O (2.0 mmol) were dissolved in the above mixture. After adjusting the pH to
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approximately 10.0 using a diluted ammonium hydroxide, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 140 C for 15 h. After the reaction, the obtained product was washed with distilled water and absolute ethanol repeatedly for six times. Finally, the product was dried at 80 C. With different amounts of MWCNTs (0, 30, 40, and 50 mg) added, the products were denoted as Zn ferrite, S1, S2, and S3, respectively. 2.2. Characterization The morphology and nanostructures of Zn ferrite/MWCNTs composite were identified using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution
Fig. 3. XRD patterns of Zn ferrite, S1, S2, and S3 (a), XPS spectra: wide span (b), C 1s spectrum (c), O 1s spectrum (d), Fe 2p spectrum (e), and Zn 2p spectrum (f) of S3.
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transmission electron microscopy (HRTEM, FEI Tecnal G2 F20). The crystal structure was characterized by X-ray powder diffraction (XRD, PANalytical Empyrean) using monochromatic Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) was conducted using a monochromatic Al Ka X-ray source. The magnetic properties were examined with a vibrating sample magnetometer (HH-20) at room temperature. Complex permit00 00 tivity (εr ¼ ε0 jε ) and complex permeability (mr ¼ m0 jm ) were obtained on a vector network analyzer (AV3629D) in the frequency range of 2e18 GHz. For complex permittivity and complex permeability measurement, samples were mixed with paraffin and pressed into a toroid mold with an outer diameter of 7.0 mm and inner diameter of 3.04 mm. Based on complex permittivity and complex permeability, the RL values were calculated using transmission line theory [24]:
Z Z0 RL ¼ 20lg in Zin þ Z0 Zin ¼ Z0
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
(1)
changed to Fe3þ to maintain the electric neutrality of the crystal unit [28]. Thus, the ratio of Fe3þ to Fe2þ in Zn ferrite was higher than that in Fe3O4. The fingerprint shakeup satellite peak at 719.5 eV demonstrated the increasing Fe3þ. In Fig. 3f, the binding energy peaks at 1021.1 and 1044.1 eV of Zn 2p3/2 and Zn 2p1/2 suggested the existence of Zn in this composite [29]. Due to the magnetic loss of the magnetic materials, magnetic property played an important role in EM-wave attenuation capability. Fig. 4 shows the magnetic properties of Zn ferrite, S1, S2, and S3. The remnant magnetization (Mr), coercivity (Hc), and saturation magnetization (Ms) are shown in Table 1. Mr, Hc, and Ms of Zn ferrite/MWCNTs composite were smaller than Zn ferrite. Hc and Mr of Zn ferrite/MWCNTs composite were increased with increasing addition amount of MWCNTs, which suggested the introduced MWCNTs highly affect the anisotropic energy of Zn ferrite nanoparticles [30]. By contrast, Ms of Zn ferrite/MWCNTs composite was decreased with increasing addition amount of MWCNTs, which was attributed to the high content of MWCNTs. The materials' permeability can be expressed as
(2)
3. Results and discussion The morphology and nanostructure of Zn ferrite, S1, S2, and S3 are shown in Fig. 2. In Fig. 2a, Zn ferrite was agglomerated with a size about 25 nm. The Zn ferrite HRTEM image (Fig. 2a upper right corner) indicated that the interplanar spacing was 2.53 Å, which matched well with (311) plane of Zn ferrite [22]. After the introduction of MWCNTs (Fig. 2bed), the agglomeration was weakened with the addition amount of MWCNTs. Due to the larger size, Zn ferrite tended to agglomerate on MWCNTs surface but uniformly covered on it. The nanostructure of S3 was characterized by HRTEM (Fig. 2e and f). Compared with Zn ferrite, the size and shape of Zn ferrite in S3 were almost unchanged, which suggested that the introduction of MWCNTs does not affect its morphology. In Fig. 2f, the interplanar spacings of 2.53 and 3.30 Å correspond to Zn ferrite (311) plane and MWCNTs, respectively. XRD patterns of Zn ferrite, S1, S2, and S3 are shown in Fig. 3a. The diffraction peaks of Zn ferrite, S1, S2, and S3 are matched well with JCPDS Card no. 19-0629, except the peak at 2q ¼ 26.0 . Those peaks located at 2q ¼ 30.2 , 35.4 , 43.2 , 53.5 , 57.0 , and 62.3 are attributed to (220), (311), (400), (422), (511), and (440), respectively. With the increased addition amount of MWCNTs, the intensity of the peak at 2q ¼ 26.0 was increased, which was assigned to MWCNTs [25]. ZnO diffraction peaks were not detected in all samples, because Zn2þ was doped into the Fe3O4 lattice [26]. To further determine the chemical composition of Zn ferrite/MWCNTs composite, XPS was employed. The wide-scan spectrum of S3 is shown in Fig. 3b. The binding energy peaks at 284.5, 530.2, 711.0, and 1020.9 eV are attributed to C 1s, O 1s, Fe 2p, and Zn 2p, respectively, which indicated the presence of C, O, Fe, and Zn elements. In Fig. 3c, the peaks at 284.4, 285.4, and 288.6 eV in the C1s spectrum are assigned to CeC, CeO, and O]CeO in MWCNTs, respectively. The peaks at 529.8e530.4 eV in the O 1s spectrum (Fig. 3d) are associated with the lattice oxygen for Fe3O4 and ZneO [26,27], and binding energy peaks at 531.5 eV are assigned to C]O. As shown in Fig. 3e, the binding energy peaks at 711.0 and 724.9 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively. The binding energy peaks at 710.9 and 712.9 eV are attributed to Fe2þ and Fe3þ for Fe 2p3/2, respectively. Moreover, the peaks at 724.2 and 726.1 eV correspond to Fe2þ and Fe3þ for Fe 2p1/2, respectively [27]. As Fe3þ in the A-site was substituted by Zn2þ, Fe2þin the B-site will be
mi ¼
Ms2 akHc Ms þ blx
(3)
where Ms, Hc, l, k, and x are the saturation magnetization, coercivity, magnetostriction constant, proportion coefficient, and elastic strain parameter of the crystal, respectively; a and b are two constants determined by the material composition [31]. Therefore, the magnetic loss of Zn ferrite/MWCNTs composite was weakened by the increased Hc and the decreased Ms. In general, the introduced MWCNTs have a detrimental effect on Zn ferrite magnetic loss capability. 00 Complex permittivity (εr ¼ ε0 jε ) and complex permeability 00 0 (mr ¼ m jm ) were measured in 2e18 GHz frequency range at room temperature to evaluate the EM-wave absorption perfor00 00 mance of samples. ε0 , ε , m0 , and m of Zn ferrite, S1, S2, and S3 are shown in Fig. 5aed. The real part (ε0 or m0 ) represents the storage 00 00 capability, and the imaginary part (ε or m ) symbolizes the dissipation capability of EM-wave energy [32]. In Fig. 5a, ε0 values of Zn ferrite/MWCNTs composite were increased with the increase of MWCNTs addition amount. However, ε0 was decreased gradually in S1, S2, and S3 with several fluctuations in the frequency range of 2e18 GHz, which can be attributed to the frequency dispersion behavior of carbon materials [33]. By contrast, ε0 in Zn ferrite was
Fig. 4. Magnetic hysteresis loops of Zn ferrite, S1, S2, and S3.
Z. Liu et al. / Journal of Alloys and Compounds 731 (2018) 745e752 Table 1 Remnant magnetization (Mr), coercivity (Hc), and saturation magnetization (Ms) of Zn ferrite, S1, S2, and S3. Sample
Hc/Oe
Ms/emu g1
Mr/emu g1
Zn ferrite S1 S2 S3
64.03 26.43 39.51 50.05
84.13 75.85 73.34 70.82
11.05 6.01 6.52 7.75
maintained at an almost constant level, which is possibly due to the 00 low conductivity of Zn ferrite. In Fig. 5b, ε was increased with the increasing addition amount of MWCNTs in all samples. According
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to free electron theory, ε ¼ 1=ð2prf ε0 Þ, where f, r, and ε0 are the frequency of EM wave, resistivity of samples, and permittivity of a 00 vacuum, respectively [34]. The increasing ε of samples was attributed to the decreasing resistivity that resulted from MWCNTs. 00 ε in S1, S2, and S3 was decreased with the increasing EM-wave frequency in the 2e8 GHz range, with several fluctuations in the 00 8e18 GHz frequency range. However, ε in Zn ferrite was still kept at an almost constant level because of the very weak electronic 00 polarization and dipole polarization. The value of ε in S1 is larger than S2 in the 13e15.2 GHz and 16.7e18 GHz frequency range, suggesting the strong polarization capability of S1 in high fre00 quency [35]. The dielectric loss tangent (tande ¼ ε =ε0 ) was calculated as shown in Fig. 5e. The enhanced dielectric loss capability
Fig. 5. Real (a) and imaginary (b) parts of the complex permittivity as well as real (c) and imaginary (d) parts of the complex permeability, dielectric loss tangent (e), and magnetic loss tangent (f) of Zn ferrite, S1, S2, and S3.
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Fig. 6. C0ef curves (a) and attenuation constant a (b) of Zn ferrite, S1, S2, and S3.
Fig. 7. RL of Zn ferrite and S2 with different addition amounts.
revealed that the introduced MWCNTs improved the dielectric loss capability of Zn ferrite, especially in the 10e18 GHz frequency range. 00 In Fig. 5c and d, m0 and m of S1, S2, and S3 were almost the same in the entire measurement frequency. However, in Zn ferrite, m0 was 00 smaller in the 2e12 GHz frequency range, and m was larger in the 2e8 GHz frequency range than in S1, S2, and S3. The results indicated that the introduced MWCNTs weakened the magnetic loss capability of Zn ferrite in the 2e8 GHz frequency range, which was
consistent with the changes of magnetic properties above. In all samples, m0 showed an abrupt decrease at 2e6.8 GHz and then 00 increased at 7e11 GHz. Meanwhile, m exhibited a decrease in the 00 2e6.8 GHz range as well. These changes of m0 and m in the 2e6.8 GHz range were mainly attributed to natural resonance loss. 00 As m0 and m were retained at nearly constant values in the 00 11e18 GHz and 6.8e18 GHz range, respectively, C0 ¼ m ðm0 Þ2 ðf Þ1 was calculated and shown in Fig. 6a. C0 was kept at an almost constant level at 8e18 GHz, suggesting that the magnetic loss was caused by eddy current loss [36]. The curve of magnetic loss 00 tangent (tandm ¼ m =m0 ) is shown in Fig. 5f, which indicated that MWCNTs negatively influence the magnetic loss capability of Zn ferrite in the 2e8 GHz frequency range. Attenuation constant a is an important factor in EM-wave attenuation capability. It can be calculated using the following formula [37]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 00 00 00 00 00 00 ðm ε m0 ε0 Þ þ ðm ε m0 ε0 Þ2 þ ðm ε m0 ε0 Þ2 a¼ c (4)
Fig. 8. EM-wave absorption process of Zn ferrite/MWCNTs composite.
As shown in Fig. 6b, the increased attenuation constant a revealed that the increased addition amount of MWCNTs enhanced the EM-wave attenuation capability of Zn ferrite. The change trend of attenuation constant a at 2e8 GHz was almost the same as tandm in Fig. 5f. Moreover, the change trend of attenuation constant a at 9e18 GHz was also consistent with tande in Fig. 5e. These results suggested that the EM-wave attenuation
Z. Liu et al. / Journal of Alloys and Compounds 731 (2018) 745e752
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was 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. These results revealed that the introduced MWCNTs not only decreased the addition amount and coating thickness of Zn ferrite but also enhanced its EM-wave absorption performance. In Zn ferrite/MWCNTs composite, MWCNTs can form conductivity net, and enhance the dielectric loss capability. In addition, the interfacial between Zn ferrite and MWCNTs enhanced the interfacial polarization capability. Thus Zn ferrite/MWCNTs composite got an enhanced EM-wave absorption properties at lower addition amount and coating thickness. To understand the EM-wave absorption mechanism clearly, the absorption process is shown in Fig. 8. Without MWCNTs, only magnetic loss and very weak dielectric loss work in Zn ferrite to attenuate the EM-wave that enters the absorber inside. However, after MWCNTs were introduced, the increased conductivity allows the interfacial polarization and dielectric loss to attenuate the EM wave, which results in the enhanced EM-wave absorption capability of Zn ferrite/ MWCNTs composite. According to quarterewave thickness criteria:
tm ¼
Fig. 9. RL (a) (60 wt% and mixed with paraffin) and l/4 thicknesses on frequency (b) of S2.
capability in the 2e18 GHz frequency range was attributed to magnetic and dielectric loss. Based on Equations (1) and (2), the RL of Zn ferrite, S1, S2, and S3 were calculated using the measured complex permittivity and complex permeability. As shown in Fig. 7, the RL of Zn ferrite and S2 with different addition amounts (75 wt%, 50 wt%, 60 wt% and mixed with paraffin) were obtained. The RLmin of Zn ferrite (Fig. 7a) was 10.9 dB at 8.2 GHz with the coating thickness of 3 mm. In Fig. 7b, the RLmin of S2 (with 50 wt% addition amount) was 16.0 dB at 7.7 GHz with the coating thickness of 3 mm. Moreover, with 60 wt% addition amount (Fig. 7c), the RLmin of S2
nl nc pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5……Þ 4 4fm jmr jjεr j
(5)
where tm is the thickness of the samples; l is the wavelength of the electromagnetic wave; fm is the peak frequency of RL values; and c is the velocity of the electromagnetic wave [38]. When the relationship between coating thickness and frequency of incident EMwave was matched well with the quarterewave thickness criteria, the EM-wave reflected from absorber inside would cancel a portion of incident EM-wave. As showing in Fig. 9, simulated thickness (tm) on frequency at n ¼ 1 of S2 was matched well with the coating thickness at the peak frequency of RL values, which suggested that the frequency of RLmin was mainly rely on phase cancellation theory. The RL of S1, S2 and S3, with 60 wt% and mixed with paraffin, are shown in Fig. 10. The RLmin of S1 (Fig. 10a) was 48.4 dB at 9.3 GHz with the coating thickness of 2 mm. In addition, the RLmin of S3 (Fig. 10c) was 14.1 dB at 16.2 GHz with the coating thickness of 1 mm. Compared with the RLmin of S1, S2, and S3, the RLmin of Zn ferrite/MWCNTs composite were on decline with the increasing addition amount of MWCNTs, which mainly attributed to the weakened impedance matching that caused by increased conductivity. In addition, the coating thickness of RLmin was decreased, and the location of RLmin peaks was shifted to higher frequency with the addition amount of MWCNTs. These results suggest that the EMwave absorption capability of Zn ferrite/MWCNTs composite can be tuned by the addition amount of MWCNTs and the coating thickness. Therefore, to obtain a better EM-wave absorption
Fig. 10. RL (60 wt% and mixed with paraffin) of S1 (a), S2 (b), and S3 (c).
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performance of Zn ferrite/MWCNTs composite, the addition amount of MWCNTs should be appropriate.
[15]
4. Conclusions [16]
Zn ferrite/MWCNTs composite were prepared using an easy one-step method. The as-prepared samples suggested that the introduced MWCNTs have a distinct effect on the magnetic properties of Zn ferrite/MWCNTs composite. After the introduction of MWCNTs, the dielectric loss capability in the 9e18 GHz frequency range and the interfacial polarization of Zn ferrite were enhanced. The RLmin of S2 (with 60 wt% addition amount) was 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. Compared with Zn ferrite, Zn ferrite/MWCNTs composite showed lower addition amount, lower coating thickness, and enhanced EM-wave absorption performance. Therefore, the as-prepared Zn ferrite/MWCNTs composite with low addition amount and coating thickness open opportunities to developing new EM-wave absorbing materials.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Hydrothermally synthesized Zn ferrite/multi-walled carbon nanotubes composite with enhanced electromagnetic-wave absorption performance Zhenfeng Liu, Honglong Xing*, Ye Liu, Huan Wang, Hanxiao Jia, Xiaoli Ji School of Chemical Engineering, Anhui University of Science and Technology, Huainan, Anhui Province 232001, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 June 2017 Received in revised form 27 September 2017 Accepted 28 September 2017 Available online 29 September 2017
Zn ferrite/multi-walled carbon nanotubes (Zn ferrite/MWCNTs) composite were prepared by one-pot hydrothermal method. Their crystal structure, morphology, composition, magnetic properties, and electromagnetic-wave (EM-wave) absorption performance were measured by X-ray powder diffraction, field-emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, vibrating sample magnetometry, and vector network analysis. Results indicated that Zn ferrite particles were distributed on MWCNTs surface with agglomeration, and MWCNTs crucially affected the magnetic properties of Zn ferrite/MWCNTs composite. Compared with Zn ferrite, Zn ferrite/MWCNTs composite showed low addition amount, low coating thickness, and enhanced EM-wave absorption performance. With 60 wt% addition amount and 1.5 mm coating thickness, the minimum reflection loss of Zn ferrite/MWCNTs composite was 42.6 dB at 12.1 GHz. The enhanced EM-wave absorption performance was mainly ascribed to the increased interfacial polarization and dielectric loss that resulting from the introduction of MWCNTs. The result illustrated that the introduction of MWCNTs into magnetic materials can enable the efficient design of excellent EM-wave absorbers with low addition amount and coating thickness. © 2017 Published by Elsevier B.V.
Keywords: Zn ferrite Multi-walled carbon nanotubes Electromagnetic-wave absorption Interfacial polarization
1. Introduction Identifying an efficient way to eliminate or weaken the electromagnetic (EM)-wave pollution that arising from the use of electronic products and communication equipment is a research hotspot [1e3]. EM-wave absorbers, can be used for solve EM-wave pollution conveniently, has been attracted considerable attention because of its easy fabrication, low cost, convenient application, and efficient absorption capabilities [4e7]. EM-wave absorption materials need to transfer EM-wave energy into thermal energy and to effectively prevent the incident EM-wave from being reflected by the absorber surface [8]. Thus, low coating thickness, light weight, strong absorption, and large absorption bandwidth are important features for efficient EM-wave absorption materials. Ferrites, such as Fe3O4 [9], CoFe2O4 [10], and BaFe12O19 [11], are traditional EM-wave absorption materials because of their magnetic properties, low cost, environmental stability, and excellent
* Corresponding author. E-mail address: [email protected] (H. Xing). https://doi.org/10.1016/j.jallcom.2017.09.317 0925-8388/© 2017 Published by Elsevier B.V.
EM-wave absorption performance. However, the large coating thickness and high addition amount (mixed with the matrix for application) of ferrites restrict their application [12]. Owing to the magnetic and dielectric loss of EM-wave absorption materials, magnetic and conductive materials have been combined to enhance the EM-wave absorption capability. To create an excellent EM-wave absorber, conductive materials, such as PANI [13], graphene [14], carbon nanotubes (CNTs) [15,16], and carbon powder [17,18], were introduced into ferrite. Results show that the conductive material decreases the addition amount and coating thickness and enhanced ferrite EM-wave absorption performance. Due to their low density, high conductivity, and unique nanostructure, CNTs are extensively used in EM-wave absorption, but the high conductivity consistently results in poor absorption. Thus, CNTs are combined with magnetic nanoparticles for EM-wave absorption. The EM-wave absorption capability of ultrathin Fe3O4/ CNT sandwich buckypaper was studied by Lu et al. [19]. Their results indicated that the reflection loss (RL) of ultrathin Fe3O4/CNT reached 12.62 dB at 17.72 GHz with absorption thickness of 0.1 mm. Lan et al. prepared Fe3O4/MWCNTs hybrids, the strongest reflection loss values reached 60.7 dB at 11 GHz by optimizing the
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Fig. 1. Illustration for the synthesis of Zn ferrite/MWCNTs composite.
absorber thickness [20]. Zhang et al. reported Boron and nitrogen doped carbon nanotubes/Fe3O4 composite, the composite architectures exhibit the minimum reflection loss at 51.2 dB [21]. Although these methods that combine Fe3O4 with CNTs enhanced their EM-wave absorption performance, but the performance can't be enhanced too much on the basis of previous combination methods. In our previous work, Zn ferrite was synthesized and showed enhanced EM-wave absorption performance than Fe3O4 [22]. The bandwidth corresponding to RL below 10 dB was about 5.0 GHz, which nearly covered the entire S band (2e4 GHz) and C band (4e8 GHz), but the coating thickness was 4.0 mm and the addition amount was 75 wt%. Thus Zn ferrite was used to combine with MWCNTs to decrease the coating thickness, addition amount and
ascertain the potential EM-wave absorption performance. To prepare Zn ferrite/MWCNTs composite, Zhang et al. proposed a complicated template method by using CNTs as template synthesized zinc ferrite/CNTs nanochains [23]. Novelty, an easy hydrothermal method without surfactant was used to synthesize Zn ferrite/MWCNTs composite here, as showing in Fig. 1. The content of MWCNTs was changed to determine its effect on Zn ferrite/ MWCNTs composite EM-wave absorption properties. Also, their crystal structure, morphology, composition, magnetic properties, and EM-wave absorption performance were characterized. They showed enhanced EM-wave absorption performance. In Zn ferrite/ MWCNTs composite with 60 wt% addition amount, the RLmin reached 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. The results suggested that MWCNTs were a key factor in decreasing
Fig. 2. FE-SEM images of Zn ferrite (a), S1 (b), S2 (c), and S3 (d); HRTEM images of Zn ferrite (upper right corner in a) and S3 (e and f).
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the addition amount and coating thickness of Zn ferrite. 2. Experimental section 2.1. Synthesis of Zn ferrite/MWCNTs composite All chemical reagents purchased from Sinopharm Chemical Reagent Co., Ltd. were of analytical grade and used without further purification. Zn ferrite/MWCNTs composite were synthesized by an easy hydrothermal method. MWCNTs were dispersed in 40 mL of distilled water and ultrasonicated for 2 h. Subsequently, FeCl3$6H2O (2.0 mmol), ZnSO4$7H2O (0.4 mmol), and FeSO4$7H2O (2.0 mmol) were dissolved in the above mixture. After adjusting the pH to
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approximately 10.0 using a diluted ammonium hydroxide, the solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and maintained at 140 C for 15 h. After the reaction, the obtained product was washed with distilled water and absolute ethanol repeatedly for six times. Finally, the product was dried at 80 C. With different amounts of MWCNTs (0, 30, 40, and 50 mg) added, the products were denoted as Zn ferrite, S1, S2, and S3, respectively. 2.2. Characterization The morphology and nanostructures of Zn ferrite/MWCNTs composite were identified using field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and high-resolution
Fig. 3. XRD patterns of Zn ferrite, S1, S2, and S3 (a), XPS spectra: wide span (b), C 1s spectrum (c), O 1s spectrum (d), Fe 2p spectrum (e), and Zn 2p spectrum (f) of S3.
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transmission electron microscopy (HRTEM, FEI Tecnal G2 F20). The crystal structure was characterized by X-ray powder diffraction (XRD, PANalytical Empyrean) using monochromatic Cu-Ka radiation. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) was conducted using a monochromatic Al Ka X-ray source. The magnetic properties were examined with a vibrating sample magnetometer (HH-20) at room temperature. Complex permit00 00 tivity (εr ¼ ε0 jε ) and complex permeability (mr ¼ m0 jm ) were obtained on a vector network analyzer (AV3629D) in the frequency range of 2e18 GHz. For complex permittivity and complex permeability measurement, samples were mixed with paraffin and pressed into a toroid mold with an outer diameter of 7.0 mm and inner diameter of 3.04 mm. Based on complex permittivity and complex permeability, the RL values were calculated using transmission line theory [24]:
Z Z0 RL ¼ 20lg in Zin þ Z0 Zin ¼ Z0
rffiffiffiffiffi mr 2pfd pffiffiffiffiffiffiffiffiffi tanh j mr εr c εr
(1)
changed to Fe3þ to maintain the electric neutrality of the crystal unit [28]. Thus, the ratio of Fe3þ to Fe2þ in Zn ferrite was higher than that in Fe3O4. The fingerprint shakeup satellite peak at 719.5 eV demonstrated the increasing Fe3þ. In Fig. 3f, the binding energy peaks at 1021.1 and 1044.1 eV of Zn 2p3/2 and Zn 2p1/2 suggested the existence of Zn in this composite [29]. Due to the magnetic loss of the magnetic materials, magnetic property played an important role in EM-wave attenuation capability. Fig. 4 shows the magnetic properties of Zn ferrite, S1, S2, and S3. The remnant magnetization (Mr), coercivity (Hc), and saturation magnetization (Ms) are shown in Table 1. Mr, Hc, and Ms of Zn ferrite/MWCNTs composite were smaller than Zn ferrite. Hc and Mr of Zn ferrite/MWCNTs composite were increased with increasing addition amount of MWCNTs, which suggested the introduced MWCNTs highly affect the anisotropic energy of Zn ferrite nanoparticles [30]. By contrast, Ms of Zn ferrite/MWCNTs composite was decreased with increasing addition amount of MWCNTs, which was attributed to the high content of MWCNTs. The materials' permeability can be expressed as
(2)
3. Results and discussion The morphology and nanostructure of Zn ferrite, S1, S2, and S3 are shown in Fig. 2. In Fig. 2a, Zn ferrite was agglomerated with a size about 25 nm. The Zn ferrite HRTEM image (Fig. 2a upper right corner) indicated that the interplanar spacing was 2.53 Å, which matched well with (311) plane of Zn ferrite [22]. After the introduction of MWCNTs (Fig. 2bed), the agglomeration was weakened with the addition amount of MWCNTs. Due to the larger size, Zn ferrite tended to agglomerate on MWCNTs surface but uniformly covered on it. The nanostructure of S3 was characterized by HRTEM (Fig. 2e and f). Compared with Zn ferrite, the size and shape of Zn ferrite in S3 were almost unchanged, which suggested that the introduction of MWCNTs does not affect its morphology. In Fig. 2f, the interplanar spacings of 2.53 and 3.30 Å correspond to Zn ferrite (311) plane and MWCNTs, respectively. XRD patterns of Zn ferrite, S1, S2, and S3 are shown in Fig. 3a. The diffraction peaks of Zn ferrite, S1, S2, and S3 are matched well with JCPDS Card no. 19-0629, except the peak at 2q ¼ 26.0 . Those peaks located at 2q ¼ 30.2 , 35.4 , 43.2 , 53.5 , 57.0 , and 62.3 are attributed to (220), (311), (400), (422), (511), and (440), respectively. With the increased addition amount of MWCNTs, the intensity of the peak at 2q ¼ 26.0 was increased, which was assigned to MWCNTs [25]. ZnO diffraction peaks were not detected in all samples, because Zn2þ was doped into the Fe3O4 lattice [26]. To further determine the chemical composition of Zn ferrite/MWCNTs composite, XPS was employed. The wide-scan spectrum of S3 is shown in Fig. 3b. The binding energy peaks at 284.5, 530.2, 711.0, and 1020.9 eV are attributed to C 1s, O 1s, Fe 2p, and Zn 2p, respectively, which indicated the presence of C, O, Fe, and Zn elements. In Fig. 3c, the peaks at 284.4, 285.4, and 288.6 eV in the C1s spectrum are assigned to CeC, CeO, and O]CeO in MWCNTs, respectively. The peaks at 529.8e530.4 eV in the O 1s spectrum (Fig. 3d) are associated with the lattice oxygen for Fe3O4 and ZneO [26,27], and binding energy peaks at 531.5 eV are assigned to C]O. As shown in Fig. 3e, the binding energy peaks at 711.0 and 724.9 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively. The binding energy peaks at 710.9 and 712.9 eV are attributed to Fe2þ and Fe3þ for Fe 2p3/2, respectively. Moreover, the peaks at 724.2 and 726.1 eV correspond to Fe2þ and Fe3þ for Fe 2p1/2, respectively [27]. As Fe3þ in the A-site was substituted by Zn2þ, Fe2þin the B-site will be
mi ¼
Ms2 akHc Ms þ blx
(3)
where Ms, Hc, l, k, and x are the saturation magnetization, coercivity, magnetostriction constant, proportion coefficient, and elastic strain parameter of the crystal, respectively; a and b are two constants determined by the material composition [31]. Therefore, the magnetic loss of Zn ferrite/MWCNTs composite was weakened by the increased Hc and the decreased Ms. In general, the introduced MWCNTs have a detrimental effect on Zn ferrite magnetic loss capability. 00 Complex permittivity (εr ¼ ε0 jε ) and complex permeability 00 0 (mr ¼ m jm ) were measured in 2e18 GHz frequency range at room temperature to evaluate the EM-wave absorption perfor00 00 mance of samples. ε0 , ε , m0 , and m of Zn ferrite, S1, S2, and S3 are shown in Fig. 5aed. The real part (ε0 or m0 ) represents the storage 00 00 capability, and the imaginary part (ε or m ) symbolizes the dissipation capability of EM-wave energy [32]. In Fig. 5a, ε0 values of Zn ferrite/MWCNTs composite were increased with the increase of MWCNTs addition amount. However, ε0 was decreased gradually in S1, S2, and S3 with several fluctuations in the frequency range of 2e18 GHz, which can be attributed to the frequency dispersion behavior of carbon materials [33]. By contrast, ε0 in Zn ferrite was
Fig. 4. Magnetic hysteresis loops of Zn ferrite, S1, S2, and S3.
Z. Liu et al. / Journal of Alloys and Compounds 731 (2018) 745e752 Table 1 Remnant magnetization (Mr), coercivity (Hc), and saturation magnetization (Ms) of Zn ferrite, S1, S2, and S3. Sample
Hc/Oe
Ms/emu g1
Mr/emu g1
Zn ferrite S1 S2 S3
64.03 26.43 39.51 50.05
84.13 75.85 73.34 70.82
11.05 6.01 6.52 7.75
maintained at an almost constant level, which is possibly due to the 00 low conductivity of Zn ferrite. In Fig. 5b, ε was increased with the increasing addition amount of MWCNTs in all samples. According
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to free electron theory, ε ¼ 1=ð2prf ε0 Þ, where f, r, and ε0 are the frequency of EM wave, resistivity of samples, and permittivity of a 00 vacuum, respectively [34]. The increasing ε of samples was attributed to the decreasing resistivity that resulted from MWCNTs. 00 ε in S1, S2, and S3 was decreased with the increasing EM-wave frequency in the 2e8 GHz range, with several fluctuations in the 00 8e18 GHz frequency range. However, ε in Zn ferrite was still kept at an almost constant level because of the very weak electronic 00 polarization and dipole polarization. The value of ε in S1 is larger than S2 in the 13e15.2 GHz and 16.7e18 GHz frequency range, suggesting the strong polarization capability of S1 in high fre00 quency [35]. The dielectric loss tangent (tande ¼ ε =ε0 ) was calculated as shown in Fig. 5e. The enhanced dielectric loss capability
Fig. 5. Real (a) and imaginary (b) parts of the complex permittivity as well as real (c) and imaginary (d) parts of the complex permeability, dielectric loss tangent (e), and magnetic loss tangent (f) of Zn ferrite, S1, S2, and S3.
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Fig. 6. C0ef curves (a) and attenuation constant a (b) of Zn ferrite, S1, S2, and S3.
Fig. 7. RL of Zn ferrite and S2 with different addition amounts.
revealed that the introduced MWCNTs improved the dielectric loss capability of Zn ferrite, especially in the 10e18 GHz frequency range. 00 In Fig. 5c and d, m0 and m of S1, S2, and S3 were almost the same in the entire measurement frequency. However, in Zn ferrite, m0 was 00 smaller in the 2e12 GHz frequency range, and m was larger in the 2e8 GHz frequency range than in S1, S2, and S3. The results indicated that the introduced MWCNTs weakened the magnetic loss capability of Zn ferrite in the 2e8 GHz frequency range, which was
consistent with the changes of magnetic properties above. In all samples, m0 showed an abrupt decrease at 2e6.8 GHz and then 00 increased at 7e11 GHz. Meanwhile, m exhibited a decrease in the 00 2e6.8 GHz range as well. These changes of m0 and m in the 2e6.8 GHz range were mainly attributed to natural resonance loss. 00 As m0 and m were retained at nearly constant values in the 00 11e18 GHz and 6.8e18 GHz range, respectively, C0 ¼ m ðm0 Þ2 ðf Þ1 was calculated and shown in Fig. 6a. C0 was kept at an almost constant level at 8e18 GHz, suggesting that the magnetic loss was caused by eddy current loss [36]. The curve of magnetic loss 00 tangent (tandm ¼ m =m0 ) is shown in Fig. 5f, which indicated that MWCNTs negatively influence the magnetic loss capability of Zn ferrite in the 2e8 GHz frequency range. Attenuation constant a is an important factor in EM-wave attenuation capability. It can be calculated using the following formula [37]:
rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf 00 00 00 00 00 00 ðm ε m0 ε0 Þ þ ðm ε m0 ε0 Þ2 þ ðm ε m0 ε0 Þ2 a¼ c (4)
Fig. 8. EM-wave absorption process of Zn ferrite/MWCNTs composite.
As shown in Fig. 6b, the increased attenuation constant a revealed that the increased addition amount of MWCNTs enhanced the EM-wave attenuation capability of Zn ferrite. The change trend of attenuation constant a at 2e8 GHz was almost the same as tandm in Fig. 5f. Moreover, the change trend of attenuation constant a at 9e18 GHz was also consistent with tande in Fig. 5e. These results suggested that the EM-wave attenuation
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was 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. These results revealed that the introduced MWCNTs not only decreased the addition amount and coating thickness of Zn ferrite but also enhanced its EM-wave absorption performance. In Zn ferrite/MWCNTs composite, MWCNTs can form conductivity net, and enhance the dielectric loss capability. In addition, the interfacial between Zn ferrite and MWCNTs enhanced the interfacial polarization capability. Thus Zn ferrite/MWCNTs composite got an enhanced EM-wave absorption properties at lower addition amount and coating thickness. To understand the EM-wave absorption mechanism clearly, the absorption process is shown in Fig. 8. Without MWCNTs, only magnetic loss and very weak dielectric loss work in Zn ferrite to attenuate the EM-wave that enters the absorber inside. However, after MWCNTs were introduced, the increased conductivity allows the interfacial polarization and dielectric loss to attenuate the EM wave, which results in the enhanced EM-wave absorption capability of Zn ferrite/ MWCNTs composite. According to quarterewave thickness criteria:
tm ¼
Fig. 9. RL (a) (60 wt% and mixed with paraffin) and l/4 thicknesses on frequency (b) of S2.
capability in the 2e18 GHz frequency range was attributed to magnetic and dielectric loss. Based on Equations (1) and (2), the RL of Zn ferrite, S1, S2, and S3 were calculated using the measured complex permittivity and complex permeability. As shown in Fig. 7, the RL of Zn ferrite and S2 with different addition amounts (75 wt%, 50 wt%, 60 wt% and mixed with paraffin) were obtained. The RLmin of Zn ferrite (Fig. 7a) was 10.9 dB at 8.2 GHz with the coating thickness of 3 mm. In Fig. 7b, the RLmin of S2 (with 50 wt% addition amount) was 16.0 dB at 7.7 GHz with the coating thickness of 3 mm. Moreover, with 60 wt% addition amount (Fig. 7c), the RLmin of S2
nl nc pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5……Þ 4 4fm jmr jjεr j
(5)
where tm is the thickness of the samples; l is the wavelength of the electromagnetic wave; fm is the peak frequency of RL values; and c is the velocity of the electromagnetic wave [38]. When the relationship between coating thickness and frequency of incident EMwave was matched well with the quarterewave thickness criteria, the EM-wave reflected from absorber inside would cancel a portion of incident EM-wave. As showing in Fig. 9, simulated thickness (tm) on frequency at n ¼ 1 of S2 was matched well with the coating thickness at the peak frequency of RL values, which suggested that the frequency of RLmin was mainly rely on phase cancellation theory. The RL of S1, S2 and S3, with 60 wt% and mixed with paraffin, are shown in Fig. 10. The RLmin of S1 (Fig. 10a) was 48.4 dB at 9.3 GHz with the coating thickness of 2 mm. In addition, the RLmin of S3 (Fig. 10c) was 14.1 dB at 16.2 GHz with the coating thickness of 1 mm. Compared with the RLmin of S1, S2, and S3, the RLmin of Zn ferrite/MWCNTs composite were on decline with the increasing addition amount of MWCNTs, which mainly attributed to the weakened impedance matching that caused by increased conductivity. In addition, the coating thickness of RLmin was decreased, and the location of RLmin peaks was shifted to higher frequency with the addition amount of MWCNTs. These results suggest that the EMwave absorption capability of Zn ferrite/MWCNTs composite can be tuned by the addition amount of MWCNTs and the coating thickness. Therefore, to obtain a better EM-wave absorption
Fig. 10. RL (60 wt% and mixed with paraffin) of S1 (a), S2 (b), and S3 (c).
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performance of Zn ferrite/MWCNTs composite, the addition amount of MWCNTs should be appropriate.
[15]
4. Conclusions [16]
Zn ferrite/MWCNTs composite were prepared using an easy one-step method. The as-prepared samples suggested that the introduced MWCNTs have a distinct effect on the magnetic properties of Zn ferrite/MWCNTs composite. After the introduction of MWCNTs, the dielectric loss capability in the 9e18 GHz frequency range and the interfacial polarization of Zn ferrite were enhanced. The RLmin of S2 (with 60 wt% addition amount) was 42.6 dB at 12.1 GHz with the coating thickness of 1.5 mm. Compared with Zn ferrite, Zn ferrite/MWCNTs composite showed lower addition amount, lower coating thickness, and enhanced EM-wave absorption performance. Therefore, the as-prepared Zn ferrite/MWCNTs composite with low addition amount and coating thickness open opportunities to developing new EM-wave absorbing materials.
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