Facile synthesis of a CNT@Fe@SiO₂ ternary composite with enhanced microwave absorption performance

Abstract

Magnetic/dielectric core–shell structures have been regarded as ideal high-performance electromagnetic absorption materials due to their novel multiple-loss mechanism. However, the poor impedance matching property of a dielectric shell may lead to the high reflection of electromagnetic waves from the interface of the shell. Thus, we ingeniously use the magnetic material as the shell while the dielectric material is used as the core. Such a change not only decreases the electromagnetic wave reflection, but also causes a strong interface polarization. Subsequently, the wave-transparent material SiO₂ was further coated on the surface of the magnetic shell which not only protected it from oxidation but also increased the impedance matching performance. Based on the above design, in this study, we fabricated a CNT@Fe@SiO₂ ternary core–shell structure composite using a simple two-step approach consisting of pyrolysis and decomposition processes. As compared with pure CNTs and CNT@Fe materials, the obtained CNT@Fe@SiO₂ composite shows obviously enhanced electromagnetic absorption properties. In particular at a thin thickness of 1.5 mm, the optimal reflection loss value is as high as −14.2 dB which is better than most of the reported CNT based absorbers. The improved electromagnetic absorption properties can be attributed to the perfect impedance matching behavior and the multiple interface polarization effect.

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1. Introduction

In recent years, core–shell structures have aroused extensive attention due to their potential applications in sensor, catalysis, and lithium battery fields.^1,2 Currently, great efforts have demonstrated that composites with a core–shell structure are suitable as electromagnetic radiation absorbers.^3,4 Generally, an electromagnetic absorber is a kind of functional material which can let incident electromagnetic waves into the absorber and then attenuate them, producing thermal energy.^5,6 Depending on how the attenuation occurs, we may classify an electromagnetic absorber into two parts, one is a magnetic loss material, like Fe, Co, Ni, ferrite, while the other is a dielectric loss material including graphene, SiC, ZnO, CuS and so on.^7,8 The dielectric/magnetic core–shell composites have two big advantages in the electromagnetic absorption field. On the one hand, the sample presents high chemical stability owing to the dielectric shell. On the other hand, the attenuation occurs in multiple ways as compared with single magnetic materials or dielectric loss materials.^9,10 As a result; these kinds of absorbers exhibit obviously enhanced electromagnetic absorption properties. For example, both Fe₃O₄ and SnO₂ show poor electromagnetic absorption properties for which the optimal reflection loss values were less than −10 dB. Whereas the minimum reflection loss value of Fe₃O₄@SnO₂ was increased to −36.5 dB.¹¹ Similar results can also be observed in other absorbers including Fe₃O₄ based core–shell structures, such as Fe₃O₄@TiO₂,^12,13 Fe₃O₄@ZnO,¹⁴ Fe₃O₄@C¹⁵ and Ni based composites e.g.: Ni@Al₂O₃,¹⁶ Ni@ZnS,¹⁷ Ni@ZnO¹⁸ or FeCo@C,¹⁹ and Co@ZnO.²⁰ Unfortunately, the optimal RL_min values of these magnetic@dielectric composites are hardly acceptable at thin thicknesses (<2 mm) due to the poor impedance matching properties of high frequency. That is the main drawback in magnetic@dielectric materials which can be explained by the impedance matching theory:^21–23

Z = Z₁/Z₀ (1)

Z₁ = (μ_r/ε_r)^1/2Z₀ (2)

μ_r = μ′ − jμ′′ (3)

ε_r = ε′ − jε′′ (4)

|ε| = (ε′² + ε′′²)^1/2 (5)

|μ| = (μ′² + μ′′²)^1/2 (6)

where Z₁ stands for the impedance matching value of the absorbent, Z₀ is the free space of impedance matching. ε_r and μ_r are the complex permittivity and permeability values, respectively. From eqn (1–6), we can conclude that a poor impedance matching ratio results from a big ε_r value and a small μ_r value. Meanwhile, eqn (5) also indicates that the large ε_r (or μ_r) value is related to the big ε′ and ε′′ (or μ′ and μ′′) values. For most non-magnetic dielectric materials, the ε′ and ε′′ values are large while the μ_r value is equal to 1 (for non-magnetic materials, the μ′ value is equal to 1 and the μ′′ value is close to 0 which makes the μ_r value approach 1). Such unmatched ε_r and μ_r values may lead to poor impedance matching properties. For instance, the ε′ value of carbon is about 40–100 and the ε′′ value is of the range 10–50.²⁴ This is the main reason why dielectric loss materials can rarely obtain excellent electromagnetic absorption properties. Thus, a dielectric material is not suitable for the shell. Different from the dielectric materials, the magnetic materials exhibit an obvious advantage for the impedance matching properties. In fact, a higher saturation magnetization value (M_s) for a magnetic material is always attributed to a higher μ′ value based on the following eqn (7):^25,26

(μ₀ − 1)f₀ = (1/3π)M_s (7)

More importantly, a high M_s value is better for the magnetic loss ability which results in the increase of μ′.

In this study, we introduce a new type of dielectric/magnetic core–shell absorber in which CNTs serve as the core and Fe nano-particles, originated from a Fe(CO)₅ decomposition process, are used as the shell (see Fig. 1). It is not difficult to understand that Fe shows a higher M_s value (220 emu g⁻¹) than most other magnetic materials, like Co (180 emu g⁻¹), Ni (55 emu g⁻¹), Fe₃O₄ (100 emu g⁻¹) and ferrite (less than 100 emu g⁻¹).²⁷ As a classic carbon material, CNTs possess a better dielectric ability than other carbon materials (i.e.: graphene, carbon spheres, etc.) or other dielectric materials.²⁸ The relatively lower density of CNTs is also considered to be a big advantage for light-weight absorbers. What’s more, the as-prepared CNT@Fe composite is further coated by SiO₂ to avoid the possible Fe oxidation process. It should be pointed out that pure SiO₂ has no obvious electromagnetic absorption; however, its excellent transmissivity and high impedance matching make it suitable for the novel magnetic/dielectric absorbing composite. This kind of ternary core–shell structure is apt to reach excellent electromagnetic absorption properties at a thin coating thickness, which greatly satisfies the demand for light-weight absorbers.

Fig. 1 Synthetic scheme for the preparation of CNT@Fe@SiO₂.

2. Experimental section

2.1 Materials

Tetraethoxysilane (TEOS, Si(OC₂H₅)₄, 95 wt%), ammonium (NH₄OH, 28%) and methanol were purchased from the Sinopharm Chemical Reagent Co. Iron pentacarbonyl (Fe(CO)₅) was purchased from the Beijing XinDingTengFei Co. Kerosene was purchased from Chengdu ChengTai Co. The CNTs used in this study were purchased from Shenzhen Nano Technical Co. with dimensions of 10–20 μm in length, 10–15 nm in inter diameter, and ∼40–50 nm in external diameter. All of the chemical reagents used in this study were analytically pure and were used without further purification.

2.2 Preparation of CNT@Fe core–shell structure

The Fe coating layer was prepared by a heat-decomposed process. In detail, 400 mg of CNTs was dissolved into a four-neck flask, which contained 500 mL of kerosene, with ultrasonic treatment for 1 h to ensure that the CNTs were dispersed well. Afterward, the flask was heated at 200 °C for 6 h. During the pyrolysis process, the four-neck flask was connected with a reflux unit, mechanical agitator and flow tube, and temperature controller. Fe(CO)₅ was not added into the four-neck flask at that early stage. Owing to the N₂ steam and relatively high temperature (60 °C), Fe(CO)₅ flowed into the four-neck flask. The speed of the Fe(CO)₅ flow can be controlled by the flow rate of N₂ (10 mL min⁻¹). After the temperature had cooled to room temperature, the CNT@Fe material was obtained by magnetic separation and was washed 5 times with ethanol. The iron coating layer was generated according to the following chemical equation (eqn (8)):²⁹

Fe(CO)₅ = Fe + 5CO (8)

2.3 Preparation of CNT@Fe@SiO₂ ternary composite

The silica oxide coating of CNT@Fe was produced via a modified Stober method.^30,31 Typically, 200 mg of the CNT@Fe sample was dispersed in a mixture of 80 mL of distilled water, 20 mL of absolute ethanol and 2 mL of ammonium. Next, every 30 min, 0.5 mL of TEOS (total of 4 mL) was dropped into the mixed solution. After 8 h of mechanical stirring, the final sample was collected by centrifugation and was washed 5 times with ethanol and dried at 60 °C for 24 h in a vacuum environment.

2.4 Characterization

The crystal structure was obtained by powder X-ray diffraction (XRD) patterns (Bruker D8 ADVANCE X-ray diffractometer) using Cu Kα radiation (λ = 0.154178 nm with a 40 kV scanning voltage, 40 mA scanning current and a scanning range from 10° to 70°). A Hitachi S4800 type scanning electron microscope (operated at an acceleration voltage of 3.0 kV) was used to observe the size and morphological features. The sample was further characterized by transmission electron microscopy (TEM, JEM JEOL 2100). The magnetic properties were measured by a vibrating sample magnetometer (VSM, Lakeshore, Model 7400 series) at room temperature. The chemical bonds were analysed using a KBr pellet technique by Fourier Transform Infrared spectroscopy (FT-IR, Perkin-Elmer IR spectrometer). The weight ratio was calculated using inductively coupled plasma (ICP) (Optima 5300 DV). The S parameter containing S11, S12, S21 and S22 can be tested by an Agilent PNA N5224A vector network analyzer using the coaxial-line method in which the samples are prepared by homogeneously mixing paraffin wax and the sample (mass ratio = 1 : 1) and then pressing into toroidal-shaped samples (Φ_out: 7.0 mm, Φ_in: 3.04 mm). Then, a software which has been installed in Agilent PNA can calculate the ε′, ε′′, μ′, μ′′ values. Finally, the RL values with different thickness (d) can be calculated by the following formulas (eqn (9) and (10)).^32,33

Z_in = Z₀(μ_r/ε_r)^1/2 tanh[j(2πfd(μ_rε_r)^1/2/c)] (9)

RL (dB) = 20 log|(Z_in − Z₀)/(Z_in + Z₀)| (10)

where Z_in is the input impedance of the absorber, f is the frequency of the electromagnetic wave, d is the coating thickness of the absorber while c is the velocity of the electromagnetic wave in free space. ε_r (ε_r = ε′ − jε′′) and μ_r (μ_r = μ′ − jμ′′) are the complex permittivity and permeability values of the absorber.

3. Results and discussion

3.1 The crystal structure and morphologies of the CNT@Fe composite

The crystal structure of the CNTs and the CNT@Fe composite was measured by XRD, as displayed in Fig. 2. The pure CNTs have a broad diffraction peak at 26.4° which is assigned to the (002) plane of CNTs. Whereas, this CNT characteristic peak is quite weak in the CNT@Fe composite. It is noted from the CNT@Fe sample that the diffraction peak at 44.6° is indexed to the (110) crystal plane of cubic Fe (JCPDS card no.: 06-0696). Nevertheless, the nanometer-sized Fe is not stable in air for a long time. Thus, the presence of the weak diffraction peaks at 35.7 and 62.7° may be assigned to the (311) and (400) crystal planes of Fe₂O₃ (JCPDS card no.: 02-1047) after being exposed to air for 5 days. Just because of this, the next SiO₂ coating process is quite important to avoid the Fe oxidation.

Fig. 2 The XRD patterns of the CNTs and CNT@Fe composites.

The coating thickness and morphological features of the CNTs and the CNT@Fe sample were also determined via SEM and TEM analysis. As can be seen from Fig. 3a and b, the CNTs are hollow nanotubes with a ∼40 nm outer diameter and a ∼10 nm inner diameter. After numerous ∼10 nm iron nanoparticles were deposited on the CNTs (see Fig. 3e), the smooth tube walls became rough (Fig. 3c and d). Meanwhile, the corresponding thickness increased to 150 nm, indicating a Fe layer close to 55 nm.

Fig. 3 The SEM/TEM images: (a and b) CNTs; (c and d) CNT@Fe composite, (e); pure Fe nanoparticles.

In order to explore the relationship between pyrolysis time and the coating thickness, a series of time-dependent experiments were also carried out, as observed in Fig. 4. It is apparent that the coating thickness is gradually increased with a prolonged decomposition time (Fig. 4a–c). From Fig. 4d, the diameter of the CNT@Fe sample is 100, 180 and 230 nm with 4, 8, and 14 h of pyrolysis, respectively.

Fig. 4 The SEM images of the CNT@Fe sample prepared from different pyrolysis times (a) 4 h, (b) 8 h, (c) 14 h and (d) is the curve of thickness vs. time.

3.2 The crystal structure and morphologies of CNT@Fe@SiO₂

The XRD pattern of the CNT@Fe@SiO₂ ternary composite is displayed in Fig. 5a. Compared with Fig. 2, the new appearance of the broad diffraction peak around 22–28° can be ascribed to SiO₂ coverage which is consistent with the previous result provided by Hekmatara et al.³⁴ It should be pointed out that the increase in the diffraction peak intensity of Fe₂O₃ is the result of a long time exposure to air. Meanwhile, a tiny amount of Fe will be oxidised during the SiO₂ preparation process. However, under the SiO₂ protection, these diffraction peaks exhibit almost no change after the sample was exposed in air for 5 days.

Fig. 5 The XRD patterns (a) and FT-IR diagram (b) of the CNTs, CNT@Fe and CNT@Fe@SiO₂ samples.

The FT-IR spectra were further utilized to demonstrate the presence of SiO₂ (Fig. 5b). It was found that two adsorption peaks at 465 and 1091 cm⁻¹ were present for the CNT@Fe@SiO₂ sample, and were associated with the Si–O–Si stretching vibration peaks of SiO₂.³⁵ We also found a peak at 570 cm⁻¹ in the CNT@Fe and CNT@Fe@SiO₂ samples, which was attributed to the Fe–O bond.

The microstructure features of the CNT@Fe@SiO₂ ternary composite prepared under 6 h of pyrolysis are shown in Fig. 6. As we know, SiO₂ prepared by the Stober method easily forms a nano-sphere structure. From Fig. 6a, we find that there are numerous nano-spheres of 170–200 nm in diameter. Furthermore, these nano-spheres gather and assemble into necklace-like structures which the tube-like CNTs regard as the axis. Thus, the novel sphere-like three layered structure composed of CNTs and Fe nanoparticles as well as SiO₂ was fabricated firstly for the magnetic/dielectric core–shell electromagnetic absorbing material. The enlarged diameter also indicates that the coating thickness of SiO₂ is about 20 nm (see Fig. 6b). The weight ratios of Fe, the CNTs, Fe₂O₃ and SiO₂ were obtained via the following steps. At the first stage, the percentage weight ratio of the CNTs was found using HCl (0.1 M). 100 mg of CNT@Fe@SiO₂ was added into a 100 mL HCl solution and then the precipitate was collected by centrifugation. Then, the precipitate was further dissolved by HF. Finally, the black insoluble substance was representative of the mass of CNTs. The results revealed that the weight ratio of CNTs is equal to 23.1 wt%. Next, the Fe element mass weight was calculated by the magnetization value of CNT@Fe@SiO₂ (60 emu g⁻¹). It is worth mentioning that the traditional technologies including TG and ICP are hardly suitable to calculate the Fe element mass ratio due to the interference of Fe₂O₃ and the CNTs. Whereas, the magnetization value of this composite only comes from Fe. Theoretically, the saturation magnetization value of pure Fe is equal to 218 emu g⁻¹. In the composites, the magnetization value decreases as the Fe element mass ratio decreases. As a result, we can calculate that the Fe element weight ratio is close to 27.5 wt% (60/218 = 0.275). Subsequently, ICP was applied to characterize the Si mass ratio. Similarly, 10 mg of the composite was dissolved in a 10 mL HF solution. Then, the solution was transferred to a 100 mL volumetric flask. The ICP data show that the Si concentration is 22.1 mg L⁻¹. Thus, the SiO₂ weight ratio is 45.2 wt%. Finally, the mass weight of Fe₂O₃ is 4.2 wt%. Actually, the influence of Fe₂O₃ on the final electromagnetic absorption can be ignored with the following reasoning: the electromagnetic parameters of Fe₂O₃ (ε′ = 3–4, ε′′ = 0) are almost equal to those of SiO₂, which is good for the impedance matching behavior and weak attenuation. As a result, Fe₂O₃ has no contribution to the attenuation of the electromagnetic waves.

Fig. 6 (a) Low and (b) high resolution SEM images of the CNT@Fe@SiO₂ composite.

3.3 Magnetic and electromagnetic absorption properties of the samples

The magnetic properties of the CNT@Fe and CNT@Fe@SiO₂ composites were tested by a vibrating sample magnetometer (Fig. 7). Obviously, the magnetization values mainly originate from the presence of Fe in these two samples. The CNT@Fe sample shows a high magnetization value of 103 emu g⁻¹, which is higher than CNT@Fe@SiO₂ (60 emu g⁻¹). However, for many magnetic/dielectric materials, their magnetization values are no more than 50 emu g⁻¹ due to the lower saturation magnetization.

Fig. 7 The M–H loop of the CNT@Fe and CNT@Fe@SiO₂ composites.

The calculated reflection loss data is displayed in Fig. 8. An excellent electromagnetic absorption material should have a low reflection loss value, a broad effective frequency range and be lightweight. In general, reflection loss values of less than −10 dB (corresponding to less than 10% of reflection, RL_min) are as broad as possible. In order to satisfy the lightweight requirement, the coating thickness (t) should not be too thick (for practical application, t < 2 mm). It is easy to find that the RL_min values of pure CNTs at all thicknesses (1.5–3.5 mm) do not even exceed −6 dB, which indicates that the pure CNT sample is not a good candidate for practical use (see Fig. 8a). Clearly, after coating with Fe, the RL_min value exhibits an obvious improvement (Fig. 8b). A minimum reflection loss value of −12.8 dB is obtained with a thin coating thickness of 1.5 mm. At the same time, the peak shifts to a lower frequency region with an increase in the coating thickness. This can be explained by the 1/4 wavelength equation (eqn (11)):^36,37

t_m = nc/4f_m(ε_rμ_r)^1/2 (11)

where t_m and f_m represent the matching thickness and frequency of the RL_min peaks and c is the velocity of light. As far as the CNT@Fe@SiO₂ sample is concerned, the RL_min value increases to −14.2 dB at the identical 1.5 mm thickness which is much higher than most of the other reported dielectric/magnetic composites, including Ni@CuO (−11 dB),³⁸ Co/CoO (−14 dB),³⁹ Fe@SnO₂ (−10.1 dB),⁴⁰ Fe₃O₄@CuSiO₃ (<−10 dB)^41,42 and Fe₃O₄@ZrO₂ (<−10 dB)⁴³. In addition, the reflection loss for the ternary composite also shows a remarkable advantage at the other coating thicknesses. For example, the RL_min value of CNT@Fe@SiO₂ reaches −22.3 dB while the CNT@Fe value is only −11.1 dB with an identical 3 mm thickness. The effective frequency is another important factor to evaluate the electromagnetic absorption, as described in Fig. 8d–f. The yellow and green areas stand for the effective frequency regions. From Fig. 8d, one can find that the pure CNTs have no yellow area. The as-prepared CNT@Fe composite has three small areas which are located at 3.8–4.2, 4.6–4.9 and 10.5–12.5 GHz with the corresponding thicknesses of 3.45–3.5, 2.95–3.05 and 1.5–1.7 mm, respectively (Fig. 8e). The CNT@Fe@SiO₂ ternary composite exhibits the largest effective frequency region. In detail, the corresponding frequency regions are located at 5.9–15.7 and 16.9–18 GHz with the corresponding thicknesses of 1.8–3.5 and 1.5–1.7 mm.

Fig. 8 The RL data and effective frequency regions, (a) and (d): CNTs; (b) and (e): CNT@Fe; (c) and (f) CNT@Fe@SiO₂.

In order to analyze the difference in the electromagnetic absorption properties, the corresponding electromagnetic parameters are illustrated in Fig. 9. In the electromagnetic absorption field, the real and imaginary parts of permittivity represent the storage and loss abilities of the electromagnetic wave, respectively. As can be seen from Fig. 9a, both the Fe and SiO₂ coating layers do contribute to the ε′ value. For the pure CNTs, ε′ is around 90–100. But, the large ε′ value obviously decreases upon introducing the Fe and SiO₂ layers. Such a decrease is beneficial for electromagnetic absorption. The ε′ value of CNT@Fe is only around 20–30 while the CNT@Fe@SiO₂ sample has the smallest value of 10–12. As far as the imaginary part of the permittivity value is concerned, the three samples share the same tendency as the real part, for which the values are arranged in the following order: CNTs > CNT@Fe > CNT@Fe@SiO₂. It is worth noting that a high ε′′ value will result in a strong attenuation wave ability. Although the CNT@Fe@SiO₂ sample shows the lowest ε′′ value of 3–5, it is still bigger than most pure magnetic materials such as the hollow-like Fe₃O₄ (0–2),⁴⁴ hexagonal-like FeCo (1–2.8),⁴⁵ and coin-like Fe (0–2).⁴⁶

Fig. 9 The real (a) and imaginary part (b) of permittivity of the three samples.

Nevertheless, it is difficult to judge the electromagnetic absorption properties just relying on the electromagnetic parameters. In fact, the enhanced electromagnetic absorption properties may directly attribute to the impedance matching properties and attenuation constant α, which are calculated by the following eqn (12):⁴⁷

The attenuation constant α symbolizes the integral attenuation ability. In other words, a strong magnetic loss and dielectric loss may lead to an increased attenuation constant α, but a high α value does not indicate a strong dielectric or magnetic loss ability. Different from the attenuation constant α, the impedance matching value directly reveals the interface reflection properties, as shown in Fig. 10a. The pure CNTs show a poor impedance matching value. This is the main reason why the high ε′′ value of the CNTs is still hardly likely to obtain ideal electromagnetic absorption properties. After the Fe coating, the impedance matching ratio has significantly improved. Fig. 10a also indicates that the impedance matching ratio of the CNT@Fe sample is further increased after being modified by the SiO₂ coating layer. From Fig. 10b, the CNT@Fe sample exhibits the highest attenuation constant α while the pure CNT sample has the lowest.

Fig. 10 The impedance matching (a) and attenuation loss α (b).

As a result, we can deduce that the CNT@Fe@SiO₂ composite has the optimal electromagnetic absorption, which may originate from the factors depicted in Fig. 11. At the first stage, the high impedance matching property is able to make most of the incident electromagnetic waves absorb into the coating layer. Then, the multi-interface polarization effects, in particular the Fe–C polarization, play a vital role in the electromagnetic wave attenuation. It is well-known that the hexagonal crystal structure of carbon with sp² hybridised bonds exhibits many free electrons.⁴⁸ After the Fe is coated over the carbon, the Fe atoms are easily induced by these free electrons which may cause momentary separation of the Fe atomic nucleus and the extranuclear electrons (Fe atom = Fe atomic nucleus + extranuclear electrons). The generated electrons from Fe may be further spread among the Fe atoms and then induce an outer layer of Fe polarization. Such a polarization and electron transmission procedure is dynamic and needs the electromagnetic wave to provide energy. With the electromagnetic waves incident to the surface carbon, the high electrical conductivity of carbon enables electromagnetic wave transfer in the form of a microcurrent. A small part of the electromagnetic waves may be attenuated during the transmission process. The SiO₂ wave-transparent layer should not be ignored. On the one hand, the presence of the SiO₂ coating layer will let more electromagnetic waves absorb into the Fe layer and the CNTs. On the other hand, without the protection of SiO₂, the Fe layer will be oxidized deeply after being exposed to air. If Fe exists in the form of Fe³⁺ or Fe²⁺, the interface polarization between Fe–C may obviously decrease, which is bad for electromagnetic wave attenuation. The SiO₂ coating layer also contributes to the suppression of the eddy current loss of the Fe layer.

Fig. 11 The possible electromagnetic wave attenuation mechanism.

4. Conclusion

In summary, a CNT@Fe@SiO₂ composite has been synthesized by a two-step method in which the Fe was generated by a Fe(CO)₅ decomposition approach. Then, the Stober method was applied to coat SiO₂ onto the CNT@Fe sample. Such a ternary composite presents an excellent electromagnetic absorption performance at a thin thickness. An optimal reflection loss value of −14.2 dB was obtained at only 1.5 mm thickness, which easily meets the demands of being light-weight. The possible absorption mechanism has also been discussed in this study.

Acknowledgements

Financial support from the Aeronautics Science Foundation of China (no. 2014ZF52072), the National Natural Science Foundation of China (no. 11575085), and the Priority Academic Program Development of Jiangsu Higher Education Institutions is gratefully acknowledged. This work is also supported by the Funding for Outstanding Doctoral Dissertation in NUAA(BCXJ15-09).

Notes and references

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