Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10570-10579
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Tunable Dielectric Performance Derived from the Metal−Organic Framework/Reduced Graphene Oxide Hybrid with Broadband Absorption Xiaohui Liang,† Bin Quan,† Guangbin Ji,*,† Wei Liu,† Huanqin Zhao,† Sisi Dai,† Jing Lv,† and Youwei Du‡
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College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing 210016, P.R. China ‡ Laboratory of Solid State Microstructures, Nanjing University, No. 22 Hankou Road, Nanjing 210093, P.R. China
ABSTRACT: In terms of microwave absorption, dielectric performance acts vital but has negative characteristics in attenuation and impedance matching. In this study, ZnO/nanoporous carbon (NPC)/reduced graphene oxide (RGO) materials have been fabricated through a simple and valid hydrothermal method derived from Zn metal−organic frameworks (MOFs). By changing the molar ratio of the precursors, the permittivity of the ZnO/NPC/RGO can be calculated, and the greatest balance between energy conservation and impedance matching eventually emerged with the addition of 4 mL of GO. It could be found that, at 14 GHz, a thin sample consisting of 40 wt % ZnO/NPC/RGO in the wax matrix exhibited minimum reflection loss of −50.5 dB with a thickness of 2.4 mm, and with a thickness of 2.6 mm, the effective microwave absorption bandwidth coverage is from 9.6 to 17 GHz. It is worth mentioning that we have also interpreted the relationships between the highest reflection loss values and matching thicknesses. This work not only proposes that ZnO/NPC/RGO samples are able to function as a perfect absorbent with broad frequency bandwidth and strong absorption but also provides better candidates in designing other lightweight microwave absorbents. KEYWORDS: Dielectric performance, ZIF-8, Effective electromagnetic wave absorption bandwidth, Matching thickness, Lightweight
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INTRODUCTION Electromagnetic wave absorbing materials have drawn increasing attention around the world with swift evolution of wireless communications and high frequency equipment. An ideal electromagnetic wave absorber should be lightweight with high absorption efficiency in a broad frequency band at a low filler loading ratio.1 As the evolution of nanotechnology, nanosized magnetic or dielectric materials, for instance, ZnO (cagelikestructure,2 nanorod3), CuS,4 α-MnO2,5 dendrite-like Fe3O4, γFe2O3, and Fe6 have been fabricated to improve the electromagnetic wave absorption performance. Because of the dielectric constant and high conductivity, the traditional absorbers, for instance, metals, have strong absorption performances, but drawbacks such as bad anticorrosion, valuableness, and high density confine their practical application.7,8 © 2017 American Chemical Society
In recent years, significant efforts have already been devoted to graphene and reduced graphene oxide (RGO) due to their impressive electromagnetic wave-absorbing ability.9−11 However, sole graphene suffers from poor dispersion in the matrix,12,13 interfacial impedance mismatching,14,15 and limited loss mechanism.16,17 Therefore, incorporation of other lossy materials has been widely studied as the imperative solution to improve its microwave absorption performance.18 ZnO hollow spheres enwound by reduced graphene oxide sheets exhibited −45.05 dB with maximum reflection loss and 3.3 GHz of effective EM absorption bandwidth.19 RGO/α-Fe2O3 composite with absorption bandwidth of 6.4 GHz and minimum Received: July 28, 2017 Revised: October 3, 2017 Published: October 10, 2017 10570
DOI: 10.1021/acssuschemeng.7b02565 ACS Sustainable Chem. Eng. 2017, 5, 10570−10579
Research Article
ACS Sustainable Chemistry & Engineering reflection loss (RLmin) of −33.5 dB reported by Tian et al.20 The defects of a thick thickness (>2.0 mm) and two-by-four effective frequency bandwidth ( ZS2 > ZS3, and α is αS3 > αS2 > αS1 in the overall frequency. S1 owns the best impedance matching capability and worst attenuation ability because of its relatively low complex permittivity; contrarily, S3 owns the poorest impedance matching activity and superior attenuation property. Moreover, both Zr and α of sample S2 are in the middle sequences among the three ZnO/NPC/RGO samples. An outstanding absorbent should consider both energy conservation and impedance matching. Therefore, we conclude that S2 has superior electromagnetic wave absorption properties with attenuation ability and moderate impedance matching characteristics. For the microwave absorption properties of ZnO/NPC/ RGO composites to be studied in more detail, ZnO, ZnO/ NPC, and RGO RL values were simulated based on the transmit-line theory.25 Zin of EM wave absorption is presented as eq 14. 10576
DOI: 10.1021/acssuschemeng.7b02565 ACS Sustainable Chem. Eng. 2017, 5, 10570−10579
Research Article
ACS Sustainable Chemistry & Engineering
Figure 11. RL curves of S2 at different thicknesses (a); RL curve of S2 at thicknesses of 2.4 (b) and 2.6 mm (c) and f E of S2 with different coating thicknesses.
wax composite (Figure 6c). Clearly, it is the multirelaxation that improves microwave absorption performance of S2-wax composites. Normally, the EM waves are dissipated.40 If the thickness of absorber (tm) at peak frequency (f m) meets the formula tm = nc /(4fm (|μr ||εr|)1/2 ) (n = 1, 3, 5...)
(16)
where c represents the velocity of light in the free space, and |μr| and |εr| are the moduli of μr and εr. To demonstrate the maximum RL value appearing at 2.4 mm, as shown in Figure 12, we conduct the simulations of absorber tm at the minimum RL values versus peak frequency (f m) for S2 under λ/4 conditions.41 The black curve is on behalf of the simulation thickness (tfitm) at 2−18 GHz via violet dots, and quarter wavelength principles are the experimental matching thickness (texpm) at f m. We found that the value of texpm at 2.4 mm fits well with the simulation tfitm, whereas the texpm deviates from tfitm to variable extents. Hence, superior microwave absorption activity present at 2.4 mm is illustrated via the geometrical effect. The optimal microwave absorption property is derived from both attenuation loss ability and moderate impedance matching character. Moreover, aside from magnetic and dielectric loss, interference hardening loss is another significant dissipation factor, and the quarter-wave principle is an effective method for offering important direction in the thickness design of the EM wave absorbent. Electromagnetic wave absorption properties of ZnO/NPC/ RGO hybrid absorber together with other RGO-based materials reported recently are summarized in Table 1. In comparison with the recently reported RGO-based materials, ZnO/NPC/ RGO-wax composites exhibit superior performance at a rather broadband effective frequency, demonstrating the promising perspective of ZnO/NPC/RGO hybrid absorber in the
Figure 12. Comparison of tm for S2 under λ/4 conditions at f m.
no characteristic of Debye relaxation was observed in the Cole−Cole plots of S1 and S3 (Figure 6b and d, respectively). This is in sharp contrast to that observed in the case of the S210577
DOI: 10.1021/acssuschemeng.7b02565 ACS Sustainable Chem. Eng. 2017, 5, 10570−10579
Research Article
ACS Sustainable Chemistry & Engineering Table 1. Microwave Absorption Performance of the Similar Materials filler
RL (dB)
thickness (mm)
frequency range (GHz)
effective bandwidth (
