Doping Strategy To Boost the Electromagnetic Wave Attenuation

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Doping Strategy to Boost the Electromagnetic wave Attenuation ability of Hollow Carbon Spheres at Elevated Temperatures Hualiang Lv, Yuhang Guo, Zhihong Yang, Tengchao Guo, Hongjing Wu, Gu Liu, Liuying Wang, and Renbing Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03857 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Sustainable Chemistry & Engineering

Doping Strategy to Boost the Electromagnetic Wave Attenuation Ability of Hollow Carbon Spheres at Elevated Temperatures Hualiang Lv,† Yuhang Guo,‡ Zhihong Yang,*,† Tengchao Guo,† Hongjing Wu,§ Gu Liu,ǁ Liuying Wang,ǁ and Renbing Wu*,#,¶ †

College of Material Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China ‡

School of Materials Science and Engineering, Jiangsu University of Science and Technology,

Zhenjiang, Jiangsu 212003, P. R. China §

Department of Applied Physics, Northwestern Polytechnical University, Xi’an 710072, P. R.

China ǁ

Xi'an Research Institute of High Technology, Xi'an 710025, P. R. China

#

Department of Materials Science, Fudan University, Shanghai 200433, P. R. China

¶

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University,

Xi’an, 710072, PR China *E-mail address: [email protected] (Z. Yang); Email: [email protected] (R. Wu).

ACS Paragon Plus Environment

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ABSTRACT

Currently, the Electromagnetic (EM) wave absorbers usually suffer severe performance degradation when they work for a while due to the generated heat issue. Developing a highperformance EM absorber with flexibility and adjustability that can effectively absorb the EM energy and convert into thermal energy at elevated temperature is highly desired but still remains a significant challenge. Herein, we demonstrate S-doped hollow carbon nanospheres used as fillers to fabricate flexible and controllable EM absorber towards this challenge. Owing to the insertion of S-based polar groups in the graphitization area of carbon spheres, this EM absorber exhibits outstanding electromagnetic wave absorption capability with elimination of X-band EM wave performance at temperature range of 298−423K. Almost 90 % of X-band EM wave can be dissipated at 373 K, while the effective absorption rate of 75% can still be achieved at 423 K. KEYWORDS: flexible EM wave absorber, elevated temperature, sulfur-doped hollow carbon sphere, polarization loss, lightweight INTRODUCTION Developing advanced electromagnetic (EM) wave absorber with flexible and lightweight features that can be used at elevated temperatures is highly demanded in last decade since the increasingly electromagnetic interference (EMI) issue.1−3 Graphene-based EM materials have the lightweight characteristic. However, the high dielectric feature at temperature region of 273−423 K restricts their applications in the EM wave absorption due to the high reflection of incident EM wave at the graphene’s surface. As a result, the incident electromagnetic wave cannot be efficiently depleted and converted into thermal energy by the dielectric loss in the graphene according to the previous investigations.4


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Alternately, as another important member in the carbon family, amorphous carbon has already been widely applied in the catalysis, lithium-ion batteries and many other fields because of their moderate electrochemical activity.5,6 The electrochemical activity of amorphous carbon is primarily originated from the numerous of graphitization areas within the whole carbon matrix, which also play a key role on their dielectric properties.7 It is widely accepted that most of carbon atoms in amorphous carbon adopt the sp3 hybrid form and the remained part may employ the sp2 hybrid to form the honeycomb structure. In that way, the small portion of ordered honeycomb region will be well distributed in the whole amorphous matrix and can be denoted as the graphitization area.8 These graphitization areas may determine the electrochemical activity and conductive performance of the amorphous carbon due to the movement of the free electrons.9 Generally, these free electrons may move toward to a certain direction and generate the annular micro-currents after exciting under the external energy field such as the electromagnetic field.10 Simultaneously, the applied external energy can be partially eliminated during the movement process of these free electrons and such movement can be strongly enhanced by the elevated temperature. Based on this consideration, amorphous carbon may be a promising candidate for the fabrication of EM wave attenuation materials to solve the EMI issue at the operation of thermal environment. However, there are rarely work have been reported on the performance of EM wave absorber at the elevated temperature. Until now, the absorption performance of electromagnetic wave and conversion from EM to the thermal energy in amorphous carbon was investigated at room temperature. The related absorption rate could be further quantified by the reflection loss. For example, Zhou et al. found that the carbon spheres have the optimal reflection loss value of −50.8 dB which means almost 99.9% of the total incident EM energy can be absorbed and transferred into heats at this


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frequency point.11 Rong reported that the reflection loss value was up to −39.4 dB in the C@C spherical-shaped composites.12 Despite the great progress that has been made, the reported excellent attenuation performances are all in the room temperature range (298 K) and almost no work involved in the elevated temperature range. As we know, after continue working for a period, the temperature of the EM absorbers will be increased by the thermal energy generated during the energy conversion process. In this case, the remained reflection loss value of this EM absorber at the elevated temperature will directly affects the efficiency of its EM attenuation and energy dissipation rate. Thus, the reflection loss value at elevated temperature should also be carefully considered. Theoretically, the intensity of micro-currents in the graphitization area would become even stronger at high temperature. However, the incident electromagnetic energy cannot be efficient attenuated due to the ultralow resistivity in the graphitization area. As a whole, the integral energy dissipation performance caused by the amorphous carbon will be degraded at high temperature. Therefore, in order to take full advantages of the electromagnetic energy attenuation by the graphitization areas of amorphous carbon and further enhance their energy consumption ability, an effective way is to insert a spot of polar bonds into these graphitization areas and these embedded functional groups can be served as resistance during the EM attenuation process.13 Doping is considered to be the simplest way to introduce such groups into these active areas. A series of doping elements including N, B and etc. are able to enter the honeycomb structure and replace the C atoms, thus the density of free electrons would be sharply decreased as well.14−16 Sulfur as another kind of doping element is different from others which would primarily to form polar groups on (C-S bond) honeycomb structure and is more suitable to be selected for doping in the amorphous carbon.17 Currently, the related work on S-doping on graphene has been


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reported while is rarely on the hollow carbon sphere, which may possess the proper dielectric properties and in that way have the good EM attenuation and energy dissipation performance. In this study, we have developed a highly efficient EM wave absorber with flexibility, low density and adjustability based on the S-doped hollow carbon spheres. The dielectric properties of spheres have been studied and their corresponding electromagnetic energy attenuation and consumption performance at elevated temperature have also been investigated. EXPERIMENTAL SECTION Materials. Tetraethylorthosilicate (TEOS), absolute ethanol, ammonia (NH4OH (~30 wt%), phenolic resin (PR), thioacetamide (TAA), formaldehyde and silicon resin were received from commercial producers and used without further purification. Preparation of Silica Template. The SiO2 template was prepared at the early stage according to the modified Stöber method. In a typical synthesis process of ~300 nm silica, 5 mL of ammonia was added into the mixture solution which contains 6 mL TEOS, 30 mL of distilled water and 100 mL absolute ethanol. After the mixture was magnetic stirred at room temperature for 2h, the white products were washed and dried in air. Synthesis of S-Doped Hollow Carbon Spheres. Typically, 50 mg of the SiO2 spheres were well dispersed in the solution with 36 mL of ethanol, 18 mL of distilled water and 0.84 mL of ammonia. Then, 180 mg phenolic resin and 0.16 mL formaldehyde were added into the mixture and kept stirring for 2 h. The products were collected and washed with distilled water for serval times and then dried in air for 24 h. The RS (resorcinol) shell was converted into amorphous carbon by the heat treatment at 500 °C. Subsequently, the SiO2 template could be removed by immersing in the 1 M NaOH solution. In order to form the S-doped hollow carbon spheres, the as-prepared hollow carbon spheres were mixed with 30 mg TAA and then sintered at 800 °C for


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12 h and the TAA would be served as S source during the S-doping process. Meanwhile, the Sdoping solid carbon spheres were also synthesized through the same process without adding the SiO2 template. Characterizations. X-ray diffraction (XRD, Bruker D8 ADVANCE X-ray diffractometer) was applied to detect crystal phase of the S-doped samples, using Cu Kα radiation (λ=0.154178 nm with 40 kV scanning voltage, 40 mA scanning current and scanning range from 20° to 70°. The morphology, element mapping, selected diffraction of S-doped hollow carbon sphere were characterized by a transmission electron microscopy (TEM, JEM JEOL 2100). The graphitization level of the sample was examined by the Raman spectrum (Jobin Yvon HR 800 confocal Rama system). The X-ray photoelectron energy spectrum was obtained with a PHI 5000 VersaProbe systems with an Al Kα X-ray source operating at 150 W. Complex permeability and permittivity of the composite samples were measured by using an Agilent VNA (Vector Network Analyzer) N5232A with a reflection-through-line calibration, over 8-12 GHz (covering whole Xband), using a set of 7 mm coaxial air-line with length of 49.96 mm. Frequency dependence of reflection loss (RL) of the composites was estimated from their complex permittivity (ɛr=ε'–jε") and permeability (µr = µ'r – jµ"r) according to the following equations (1) and (2) (Test standard:ASTM 893-1997, American Society Testing and Materials).18−21 Z in / Z 0 =

µr  2π ft  tanh  j µr ε r  εr  c 

RL(dB ) = 20 log10

Z in − Z 0 Zin + Z 0

(1) (2)

where Zin is the input impedance of the EM samples backed by a ground plane, ƒ is the frequency of EM samples, t is the absorption laryer thickness and c is the velocity of electromagnetic wave in free space. εr (εr=ε'–jε'') and µr (µr=µ'–jµ'') are the complex permittivity and permeability of the EM samples. The samples were made by mixing 20 wt% products with


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silicon resin and then pressing this into a toroidal ring with an outer diameter 7.0 mm and inner diameter 3.04 mm. RESULTS AND DISCUSSION Figure 1a shows the formation process of the S-doped hollow carbon sphere (S/HCS). Firstly, SiO2 nanospheres with the average sizes of ~ 300 nm were produced according to the modified stober approach,22,23 as seen in Figure S1 (Supporting Information). The SiO2@RS (resorcinol) sphere was subsequent obtained by a simple liquid polymerization reaction. Furthermore, to treat this product at 500 °C in the Ar atmosphere, the RS shell was converted into amorphous carbon and the SiO2@C sphere was formed. Subsequently, the SiO2 template could be easily removed after immersing into the NaOH solution. Finally, S/HCS was gained by a sulfidation reaction at 800 °C using TAA as the S source. The distinct uniform hollow feature of this S/HCS with welldistribution are descripted in Figure 1b and c. Also, the polycrystalline ring presents in the selected area electron diffraction (SAED) image demonstrates the amorphous state of this S/HCS product (inset in Figure 1b). This result is matched well with the XRD pattern (Figure S2, Supporting Information), in which the hollow carbon sphere (without sulfidation) displays the same diffraction peak with the S/HCS product. A broad diffraction peak at 21.6° can be found, indicating the poor crystallinity of the sample. The thickness of S/HCS can also be exactly tuned by adjusting the adding amounts of SiO2 templates. Figure 1 d-g show the S/HCS samples obtained with different adding amounts of templates. The S/HCS products with the thicknesses of ~ 4, 8, 23, 30 nm are gained when the adding amounts of SiO2 templates are of 100, 75, 25, 10 mg, respectively (detailed data is shown in Figure S3, Supporting Information). The provided elemental mapping analysis in Figure 1h-k reveal the uniformly distribution of C, S, O elements within the whole hollow carbon sphere matrix.


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Figure 1. (a) schematic illustration the formation of S-doped carbon; (b) low-magnified and (c) high-magnified TEM image of the S-doped hollow carbon spheres (S/HCS) with the inset in (b) showing the SAED image; (d-g) the close TEM images of the S/HCS products with various thickness; (h-k) elemental mapping analysis of the S/HCS samples. For the applications in the EM wave absorption purpose, the as-synthesized S/HCS powder was mixed with silicon resin (seen in Figure 2a) and then the mixture was coated on a piece of glass substrate with spin coater. It should be noted that the used S/HCS product has an average thickness of ~ 8 nm. Depending on the concentration, spinning speed (from 2000 to 8000 rpm) and layer by layer, the thickness of this EM wave absorption sheet can be varied from 10 µm to


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even more than a few millimeters. In order to boost the flexible behavior of this EM sheet, it was treated at a temperature of 120 ºC for 10 minutes and then could be further peeled off from the glass substrate as shown in Figure 2b. Subsequently, for applying on different kinds of subjects which need EM wave absorption and interference, this flexible and lightweight EM sheet could be easily cut into different shapes such as the disk-like shape was achieved in this work, as displayed in Figure 2c. Furthermore, other types of EM wave absorbers including complicated shapes with exactly sizes could also be prepared by using the steel molds and the procedure for preparing the toroid shape EM wave absorber was demonstrated in Figure 2d−j.

Figure 2. The digital photos of (a-b) the preparation of flexible and adjustable EM wave absorber, (c) the absorber with different shapes, (d-e): The mould of prepared toroidal-shaped structure (The EM absorption properties were tested by the coaxial-line method, in which the samples were prepared by homogeneously mixing the adhesive and composite in a certain


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weight ratio and then pressed into toroidal shape); (f-j) the digital images of the toroidal-like sample with fixed sizes (Figure 2g and 2h showing the actual outer diameter and thickness of the toroidal sample prepared by this mould, respectively). XPS spectrum as an efficient way is analysis the probably chemical bonds of C, S and O elements.24−27 In Figure S4 (Supporting Information), C, O and S elements can be found for the S/HCS product and the S doted amount is closed to 9.4 wt%. As shown in Figure 3a, the tested carbon peak can be divided three peaks with the binding values of 284.6, 285.6 and 286.6 eV, corresponding to the graphitization carbon (sp2 hybrid carbon), C-S and C-O polar bonds, respectively. In the high-resolution XPS spectrum of S 2p, the polar groups of C-S, C-S-C, CSOx are observed with the binding values of 162.6, 163.6, 169.9 eV for the S element (Figure 3b). O element can be divided into C-O-C (530.8 eV) and S-O-S bonds (532.6 eV) (Figure 3c).28 Figure 3d shows the Raman spectrum, in which two broad peaks centered at 1350 and 1592 cm‒1 are observed, corresponding to typical D- and G-bands of carbon, respectively.29,30 The relative intensity ratio of the D and the G band (ID/IG) represents the graphitization level.31‒33 The ID/IG ratio value of S/HCS sample is 0.95, larger than that of hollow carbon sphere (0.87). The improvement of ID/IG value indicates the decrease of the graphitization level for the S/HCS product, confirming the partial polar groups have been inserted into the graphitization area.34,35


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Figure 3. The XPS spectra of the S/SCS products: (a) C 1s, (b) S 2p and (c) O 1s; and (d) the Raman spectra of the HCS and S/HCS. As we know, when the incident electromagnetic (EM) wave transmit into the EM wave absorber, the EM wave can be strongly absorbed through the dielectric loss or magnetic loss of the absorber and corresponding absorbed EM wave is converted into heats and dissipated into the air. The related electromagnetic wave absorption performance can be evaluated by the reflection loss values which can be calculated from the complex permittivity and permeability parameters. And furthermore, due to the electromagnetic energy consumption and conversion to heat, the internal temperature of the absorber itself would be increased and the EM absorption performance at elevated temperature may be depressed for the conventional EM absorbers. So in this study the reflection loss (RL) of the EM composites, made of mixing 20 wt% of S/HCS powder with silicon resin, at elevated temperatures are measured and presented in Figure 4. The


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strongest RL of S/HCS composites at room temperature is up to −18.8 dB when the thickness of the EM absorber is fixed at 2.5 mm, as shown in Figure 4a. While at thinner thickness of 2.0 mm, the RL at 8−12 GHz is far from the qualified value (< −10 dB), indicating the poor absorption of electromagnetic energy performance. When the temperature is increased to 323 K, the reflection loss obtained at the thickness of 2.0 mm could reach −12.3 dB (Figure 4b). This S/HCS based EM absorber at 378 K has the smallest RL value of −37.3 dB with the same thickness of 2.0 mm, as shown in Figure 4c. Further increasing the temperature to 423 K, the RL value at thickness of 2.0 mm increases to −18 dB, as illustrated in Figure 4d.

Figure 4. The reflection loss (RL) values of the S/HCS based EM absorber measured at elevated temperatures: (a) 298 K, (b) 323 K, (c) 373 K and (d) 423 K.


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In order to further investigate and understand the possible mechanism for the enhanced electromagnetic wave absorption performance at elevated temperature of the S doped hollow carbon sphere samples, the control experiment has also been conducted by the development of Sdoped solid carbon sphere (S/SCS). The related XRD patterns, low-resolution TEM image as well as the element mapping images are provided in Figure S5−S7 (supporting Information) which can fully confirm the successfully doping of S elements on the solid carbon spheres. The lowest reflection loss values of S/SCS based EM composites with same weight loading at different thicknesses are shown in Figure S8 (Supporting Information). It can be seen that the lowest RL vales at the room temperature are all lower than −10 dB when the thicknesses of absorbers are larger than 2.5 mm. And the corresponding RL values changes when increasing the testing temperature from room temperature to 423 K. For the S/SCS based EM absorber, the optimal thickness is 3.0 mm which the lowest RL values at all the temperatures regions are lower than −18 dB. To further estimate the EM attenuation performance of this absorber, the effective coverage rates, which is defined as the ratio of the frequency region (RL value

Doping Strategy To Boost the Electromagnetic Wave Attenuation

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