Preparation and electromagnetic shielding effectiveness of cobalt ferrite nanoparticles/carbon nanotubes composites

Abstract

Here, the cobalt ferrite/carbon nanotubes nanocomposites were directly synthesized by a facile hydrothermal method at 200°C using benzyl alcohol as the solvent. The crystal and morphological structures as well as electromagnetic shielding performance at Ku band (12–18 GHz) were thoroughly investigated. Cobalt ferrite nanoparticles with a diameter around 6–10 nm were anchored on the surface of carbon nanotubes with some aggregation. It is found that the as-prepared nanocomposites exhibit excellent electromagnetic shielding performance with values 22–25 dB in the Ku frequency range with a thickness of 2 mm. The small cobalt ferrite nanoparticles offer a large number of polarization and magnetization active sites to improve the electromagnetic performance by the incorporation of carbon nanotubes.

Introduction

With the fast development of wireless telecommunication systems and electronic equipment and devices, the problem of electromagnetic interference (EMI) has become increasingly serious, which can not only cause interruption in electronic system but also lead to potential harm to human health. Therefore, a growing and widespread research interest has been focused on designing and developing new types of electromagnetic absorbing and shielding materials. Among the various candidates, ferrite material is gaining popularity because of its good chemical stability, large saturation magnetic flux density, and good matching for magnetic and dielectric parameters. ¹ However, ferrite material, such as cobalt ferrite (CoFe₂O₄), itself usually possesses high density, low electrical conductivity, and due to the Snoek’s limit, the permeability of ferrite decreases rapidly when the frequency is higher than 1 GHz. ² These drawbacks lead ferrite material to be seldom applied in high-frequency region. Hence, CoFe₂O₄ is difficult to meet the requirements of lightweight, thin thickness, and wide bandwidth for electromagnetic functions, ³ which motivates further explorations.

In order to overcome the abovementioned problems, the ferrite materials were coupled with conductive fillers to improve its conductivity and decrease its bulk density. Carbon materials, including carbon nanotubes (CNTs), ^{4 –6} carbon fiber, ^7,8 and graphene, ^{9 –11} are at the forefront of conductive materials and blend systems for electromagnetic functionality studies. ¹² Particularly, CNTs and their hybrids have been considered as the most promising conductive nanofillers in the fields of electronic and electromagnetic applications, ¹³ due to their low density and excellent mechanical, thermal, electronic, and chemical properties. ¹⁴ To date, intensive investigations have been carried out to its potential applications in solving electromagnetic pollution. For instance, Cao’s group fabricated cadmium sulfide decorating CNTs composites, which gave a microwave absorption peak of −47 dB with a thickness of 2.6 mm. The enhanced absorption was derived from the abundant interface polarization due to the large surfaces of CNTs. ⁶ In addition, the expanded graphite/barium ferrite composite with CNTs showed an exceptional electromagnetic absorption peak of −45.8 dB at 14.1 GHz with the effective absorption band of 4.2 GHz. ¹⁵ Our previous work reported the synthesis of core–shell CNTs@MnOOH nanocomposites, which exhibited an electromagnetic shielding effectiveness (SE) of 13–15 dB in the frequency range of 8–18 GHz. ¹⁶ Although the CNTs/CoFe₂O₄ nanocomposites have been prepared as microwave absorbers, ¹⁷ the synthesis of CoFe₂O₄ was tedious because of the requirement of thermal treatment at a high temperature. Hence, a facile one-step route is highly desired to prepare CoFe₂O₄/CNTs nanocomposites and further study their electromagnetic shielding properties.

On the other hand, by taking organic component as solvent, the final products could be adjustable in terms of their particle size, shape, and assembly properties. Benzyl alcohol has been widely used as a solvent for different metal oxides preparation, ^18,19 which plays multiple roles as reaction medium, oxygen supplying, and capping agent as well. In this article, the CoFe₂O₄/CNTs nanocomposites with CoFe₂O₄ nanoparticles decorating on CNTs were successfully prepared using benzyl alcohol route together with organic metal salt. Furthermore, their EMI SE in the frequency range of Ku band (12–18 GHz) was discussed in detail. A total SE value over 22 dB were obtained in the whole Ku band with a thickness of 2 mm.

Experiment

Materials

The CNTs and the chemical reagents used in this experiment were purchased from commercial sources. The CNTs were first pretreated with 1.0-M nitric acid and then rinsed with absolute ethanol and deionized water. The CoFe₂O₄/CNTs nanocomposites were prepared by a facile hydrothermal method using benzyl alcohol as solvent. Typically, 0.1-g CNTs was fully dispersed in 30-ml benzyl alcohol under vigorous stirring for 2 h. Then, 0.5-mmol Cobalt (III) acetylacetonate and 1-mmol Iron (III) acetylacetonate were added. After stirring for another 1 h, the precursor was transferred into a Teflon autoclave with a total volume of 50 ml. The autoclave was sealed and then heated at 200°C for 24 h in oven. After naturally cooling down to room temperature, the solid products were repeatedly centrifuged and washed with deionized water and ethanol. Finally, the products were collected after drying at 60°C in air.

Characterization

The crystal structure and phase identification of the as-synthesized CoFe₂O₄/CNTs nanocomposites were performed by X-ray diffraction (XRD, Rigaku TTRIII, Japan) using copper K_α radiation (with an incident X-ray wavelength of 1.54056 Å). All the measurements were carried out in the 2θ range from 10° to 80° in steps of 0.02°. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) measurements were recorded on a JEM-2100 (JEOL, Japan) at operating acceleration voltages of 200 kV. The carbon-coated copper grids were used as the sample holders, and the products were dispersed in ethanol and then casted on the copper grids.

The electromagnetic properties of the CoFe₂O₄/CNTs nanocomposites were investigated by complex permittivity and permeability analysis. The nanocomposites were homogeneously mixed with paraffin wax with a mass ratio of 1:1. After that, the mixture was pressed into toroidal-shaped samples (ϕ _in = 3.04 mm, ϕ _out = 7.00 mm) in a thickness of 2 mm. An Agilent N5230A (Keysight Technologies, Inc. USA) vector network analyzer was used to examine the complex permittivity and permeability in the frequency range of 12–18 GHz using a coaxial cable method.

Results and discussions

XRD patterns

Figure 1 shows the XRD patterns of the as-synthesized sample, from which it is obvious that the product can be mainly indexed to CoFe₂O₄ phases (JCPDS 22-1086). Specifically, the diffraction peaks at 18.3°, 30.8°, 35.4°, 43.1°, 53.5°, 57.0°, and 62.6° can be indexed to the (111), (220), (311), (400), (422), (511), and (440) crystal planes of CoFe₂O₄ phases with an Fd3m(227) space group. The diffraction peak at 2θ = 26.4° is estimated to be the (111) plane of CNTs. ¹⁷ The XRD result indicates both a high crystallinity and purity of the CoFe₂O₄/CNTs composite obtained by the one-step synthesis at 200°C using benzyl alcohol as solvent. The average crystallite size of the as-synthesized CoFe₂O₄ was also estimated based on the Debye–Scherrer formula from the most intense peak ²⁰

Figure 1.

XRD patterns of the as-synthesized product. XRD: X-ray diffraction.

$D=\frac{k\lambda }{\beta \text{cos}\theta }$

1

where D is the average crystallite size, k is a constant (0.90), β is the full width at half-maximum, and θ is the diffraction angle, respectively. Taking the diffraction peak at 35.4° as the basis for calculation, the crystallite size of the CoFe₂O₄ was estimated to be about 12.25 nm.

Morphological structures

The morphological structures of the as-prepared CoFe₂O₄/CNTs composites were determined by TEM and HRTEM, as shown in Figure 2. The CNTs were well dispersed and most of the CoFe₂O₄ particles are united together to form aggregates due to their small sizes of 6–10 nm. The smaller aggregates are well-dispersed and anchored on the surface of the CNTs, while there are still aggregates with larger sizes interconnected between the CNTs, as shown in Figure 2(a) and (b). Figure 2(c) illustrates the magnified view of the CoFe₂O₄/CNTs composites. It is clearly seen that the tiny CoFe₂O₄ particles are attached on the surface of a single CNT. The diameters of the CoFe₂O₄ particles and the CNT are about 7 and 30 nm, respectively. Figure 2(d) presents the HRTEM images of the CoFe₂O₄/CNTs composites, which confirms the well crystallinity of the CoFe₂O₄ particles. The interplanar spaces of 0.25 and 0.38 nm, as displayed in the insets of Figure 2(d), can be assigned to the (311) plane of CoFe₂O₄ and the (111) plane of carbon, respectively. The HRTEM results are in good consistence with the XRD analysis as shown in Figure 1.

Figure 2.

TEM ((a) to (c)) and HRTEM (d) images of the CoFe₂O₄/CNTs composites. The insets in (d) are the HRTEM images of CNTs and CoFe₂O₄ nanoparticles. TEM: transmission electron microscopy; HRTEM: high-resolution transmission electron microscopy; CoFe₂O₄: cobalt ferrite; CNT: carbon nanotube.

Electromagnetic results

The EMI shielding performances of the CoFe₂O₄/CNTs composites were characterized using the S parameters and its SE is calculated through the following equations

$SE=20\text{lg}\left(E_{\text{in}}/E_{\text{tr}}\right)=S{E}_{\text{A}}+S{E}_{\text{R}}+S{E}_{\text{M}}\left(\text{dB}\right)$

2

where SE_A, SE_R, and SE_M represent shielding effectiveness from absorption, reflection, and multi-reflection, respectively. E _in and E _tr are the electric intensities of the incident and transmission waves, respectively. In practical applications, SE_M can be usually ignored when the SE_A values are larger than 10 dB. Thus, the SE_A and SE_R are given using the following relations ^21,4

$S{E}_{\text{R}}=-10\text{lg}\left(1-R\right),S{E}_{\text{A}}=-10\text{lg}(T/\left(1-R\right))$

3

Here, T and R are the transmission coefficient and reflection coefficient, respectively, which can be obtained from the following equations based on S parameters ^22,23

$T={|\frac{E_{tr}}{E_{\mathrm{in}}}|}^2={|S_{21}|}^2={|S_{12}|}^2, R={|\frac{E_{\mathrm{re}}}{E_{\mathrm{in}}}|}^2={|S_{11}|}^2={|S_{22}|}^2$

4

The obtained SE values of the CoFe₂O₄/CNTs composites with a thickness of 2 mm are shown in Figure 3. A value of 24.7 dB is observed at 12 GHz, indicating a shielding efficiency of 94.18% of the incident electromagnetic wave. With the frequency increasing from 12 GHz to 18 GHz, the SE decreases to 21.7 at 15 GHz and then increases again to 24.6 dB at 18 GHz. The SE values are mainly derived from SE_A (Figure 3), which takes up over 80% of the total SE in the whole Ku band. That is to say, the shielding performance of the CoFe₂O₄/CNTs composites are dominated by absorption.

Figure 3.

The shielding effectiveness of the CoFe₂O₄/CNTs composites in the frequency range of 12–18 GHz. CoFe₂O₄: cobalt ferrite; CNT: carbon nanotube.

To explore the underlying shielding mechanisms of the CoFe₂O₄/CNTs composites, the electromagnetic parameters, that is, complex dielectric permittivity ( ${\epsilon }_r={\epsilon \prime }_{\mathrm{r}}-\mathrm{j}{\epsilon ″}_r$ ) and magnetic permeability ( ${\mu }_r={\mu \prime }_{\mathrm{r}}-\mathrm{j}{\mu ″}_r$ ) are displayed in Figure 4, along with the attenuation coefficients (α) and the magnetic contributions (C ₀). From Figure 4(a), a decreasing ${\epsilon \prime }_{\mathrm{r}}$ and ${\epsilon ″}_{\mathrm{r}}$ with the raising frequency is found from 12 GHz to 18 GHz. When the frequency is 12 GHz, the real and imaginary parts of ε _r are about 47.3 and 40.5, respectively, which is 10 times larger than those of pure CoFe₂O₄ nanocrystals. ²⁴ The dielectric loss tangent, illustrating as $\text{tan}{\delta }_{\epsilon }={\epsilon ″}_{\mathrm{r}}/{\epsilon \prime }_{\mathrm{r}}$ , is also displayed in Figure 4(a). It is clearly shown that the dielectric loss is in the range of 0.85–1.0, representing a high loss property. Both the dielectric permittivity and loss properties are much higher than CoFe₂O₄/reduced graphene oxide (10 wt%) nanohybrids. ¹¹ This high dielectric performance may be attributed to the combination of CNTs fillers, which has greatly improved the conductivity and dielectric loss properties of pure CoFe₂O₄.

Figure 4.

The dielectric (a), magnetic (b), attenuation coefficient (c) performances and the magnetic domination (d) of the CoFe₂O₄/CNTs composites. CoFe₂O₄: cobalt ferrite; CNT: carbon nanotube.

The effective magnetic permeability parameters are shown in Figure 4(b), demonstrating an increasing tendency of ${\mu \prime }_{\text{r}}$ and ${\mu ″}_{\text{r}}$ with the frequency. The ${\mu \prime }_{\text{r}}$ values vary from 1.10 to 1.15 when the frequency is rising from 12 GHz to 18 GHz, while the ${\mu ″}_{\text{r}}$ changes from 0.06 to 0.15. Based on the permeability values, the magnetic loss tangents are obtained, as shown in Figure 4(b), which indicates a much larger magnetic property of the CoFe₂O₄/CNTs composites. A tan δ_μ of 0.06 is observed when the frequency is 12 GHz, and then, a pronounced increase is observed with the rising of the frequency. While the frequency is increased to 18 GHz, the tanδ_μ value has been turned to about 0.13. The magnetic results show that the CoFe₂O₄/CNTs composites still have excellent magnetic performances at high frequency range, which is much different from that of pure CoFe₂O₄.

Based on the dielectric and magnetic loss values, the electromagnetic attenuation coefficient (α) is calculated through equation (5) ²² , and the results are plotted in Figure 4(c)

$\alpha =\frac{\pi f}{c}\sqrt{{\mu \prime }_r{\epsilon \prime }_r}\sqrt{\text{tan}{\delta }_{\epsilon }\text{tan}{\delta }_{\mu }-1+\sqrt{1+{\text{tan}}^2{\delta }_{\epsilon }+{\text{tan}}^2{\delta }_{\mu }+{\text{tan}}^2{\delta }_{\epsilon }{\text{tan}}^2{\delta }_{\mu }}}$

5

where f and c present the frequency and electromagnetic wave velocity in free space, respectively. Clearly, α has a close relationship with dielectric permittivity, magnetic permeability, and their loss tangents. A high attenuation coefficient of 550–920 Np/m is observed for CoFe₂O₄/CNTs composites, which is derived from the combination of CNTs, and thus it contributes greatly to the electromagnetic attenuation performance of the composite.

As a typical magnetic material, it has been reported that its magnetic loss mainly originates from hysteresis, domain wall resonance, natural ferromagnetic resonance, and eddy current effect. ²⁵ However, hysteresis loss is almost negligible in the weak field, and domain wall resonance loss usually occurs at much lower frequency (MHz). ²⁶ With respect to the eddy current loss, it can be evaluated by the following equation

${\mu ″}_{\mathrm{r}}=\frac{2\pi {\mu }_0}{3}{\left({\mu \prime }_{\mathrm{r}}\cdot d\right)}^2\sigma f$

6

where $\sigma =2\pi f{\epsilon ″}_{\mathrm{r}}{\epsilon }_0$ is the electrical conductivity of the material and µ ₀ = 4π × 10⁻⁷ H/m is the permeability in free space. In practice, the parameter $C_0={\mu ″}_{\mathrm{r}}{\left({\mu \prime }_{\mathrm{r}}\right)}^2{f}^{-1}$ is often used to examine the eddy current loss domination of a magnetic material. If the magnetic loss only results from the eddy current effect, the values of C ₀ will be constant and independent on the frequency range. ²⁵ Figure 4(d) demonstrates that the values of C ₀ vary remarkably with the frequency, confirming the complicated loss mechanisms apart from eddy current loss.

Taken together, the shielding performances of the CoFe₂O₄/CNTs composites are understood in the following points. First, large specific surfaces and plentiful interface areas between the CoFe₂O₄ particles and the CNTs lead to abundant active polarization and magnetization sites. ²⁷ Besides, the incorporation of CNTs with much larger aspect ratio greatly ameliorates the agglomeration of the CoFe₂O₄ particles and thus improves the electrical conductivity of the CoFe₂O₄/CNTs composite, which greatly enhances the electromagnetic absorption of the composite. Moreover, the tiny particle size also has significant effect on the right-shift of the magnetic resonance frequency of the CoFe₂O₄ material to gigahertz band, ²⁴ leading to excellent absorption performance even in the frequency range of Ku band.

Conclusions

CoFe₂O₄/CNTs composites were synthesized successfully with a nonaqueous method using benzyl alcohol as the solvent. The CoFe₂O₄ particles are in the size of 6–10 nm, which aggregates together to some extent to anchor on the surface of CNTs. Electromagnetic measurements show that the composites present excellent SE values of 22–25 dB in the frequency range of 12–18 GHz with a thickness of 2 mm. The shielding performance is absorption dominated, which is mainly attributed to the ameliorated electrical conductivity from the incorporation of CNTs. Moreover, the tiny CoFe₂O₄ particles endow the composites plentiful polarization and magnetization active sites to improve the electromagnetic performance in high-frequency range.