One-pot synthesis of biomass-derived carbonaceous spheres for excellent microwave absorption at the Ku band

Abstract

Micro carbonaceous spheres have been successfully synthesized from watermelon via a one-pot hydrothermal reaction. After a quick annealing process to remove the surface oxygen groups, the as-prepared spheres presented an excellent microwave absorption performance at the Ku band (12–18 GHz), with a loading ratio of 25 wt% in a paraffin-based composite at 2.0 mm thickness.

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Microwave absorption (MA) has attracted increasing attention due to its important role in blocking undesirable microwave irradiation from the electronic devices and communication apparatus, which has recently aroused great concern with the rapid development in civil, commercial, military and airspace technology. Compared with conventional metal oxides, carbonaceous materials have more advantages such as excellent corrosion resistance, light weight and low cost.¹ During the past few years, carbonaceous nanofillers such as carbon fibres (CFs), carbon nanotubes (CNTs), carbon nanocoils (CNCs) and graphene have been considered as ideal substitutes for metal oxides in electromagnetic pollution management.^1–27 For example, Kong et al. grew CNTs on a graphene surface, and their poly(dimethyl siloxane) (PDMS) composite showed a maximum reflection loss (RL) of −55 dB under 2.75 mm of thickness.¹⁶ Wang et al. reported that CNCs or graphene coated with multi-magnetic materials by atomic layer deposition (ALD) reveal excellent potential in MA;^20,21 Song et al. mixed only 5 wt% of porous carbon in a paraffin-based composite, and 4.5 GHz of effective MA bandwidth (below −10 dB) could be reached.²⁴ However, CFs, CNTs and chemical vapour deposition (CVD) graphene have a predominant conductivity, and thus their composites are mainly used for electromagnetic interference (EMI) shielding rather than absorption.^1–4 Chemically or hydrothermally reduced graphene usually reveals semi-conductivity properties, due to the remaining oxygen groups. After some macro- or nano-particles were added or crystallized onto the graphene surfaces, promising MA performances were achieved.^5–19,25 But these are often costly, and difficult to produce on a large scale and often need complicated purification/functionalization steps.^28,29

An inexpensive carbonaceous MA material would not only provide a cost advantage over the inherently more costly inorganic materials but would also be truly green. Watermelons are globally cultivated and a favourite fruit; in 2009 the output of watermelon was 26.7 million tons all over the world, as reported by the Food and Agriculture Organization (FAO) of the United Nations.³⁰ Although researchers have prepared carbonaceous materials for multi-applications, such as pollution management,^31,32 EMI shielding,³³ supercapacitor,^34–36 and lithium ion battery³⁷ from biomaterials, these carbonaceous materials more or less contain inorganic materials, or are prepared by high temperature for hours and a strong acid or base. In this work, we synthesized carbonaceous spheres (CS) from watermelon using a hydrothermal method without any additives. Compared with seeds or rinds of watermelon, watermelon juice without pulp fibre only contains carbohydrate, especially small sugars, such as glucose, sucrose and fructose. According to the research of Sun et al. and Li et al., these small sugars can polymerize and intermolecularly dehydrate to form micro or nano CS.^38,39 With a loading ratio of 25.0 wt% in a paraffin-based composite, 90% of the microwave energy at the Ku band (12–18 GHz) was absorbed at only a 2.0 mm thickness without enhancement of any inorganic materials.

The pulp of watermelon mainly contains glucose, sucrose and fructose, which have been analysed through high-performance liquid chromatography (HPLC).^40,41 After squeezing and filtration, these soluble small sugars were dissolved into juice. According to the previous similar research,^38,39,42 the growth of CS seems to conform to the LaMer model;⁴³ these small sugars were polymerized firstly and then dehydrated to form CS. As shown in typical SEM images (Fig. 1a), CSs were found with a diameter mainly around 4–5 μm. Further TEM imaging results (Fig. 1b–d) revealed that these spheres were separated without conglutination. EDX analysis (Fig. S1†) and XPS surveys (Fig. 2a and b) explained that these spheres were mainly made up of the elements C and O, and the atom ratio was 4.43 : 1. The C 1s spectra of CS has a peak at 286.6 eV, reflecting that their surface contains hydroxyl (–OH) groups. Considering the C/O ratio and the surface chemical structure of the as-prepared CS, we can't help comparing them with graphene oxide (GO).^44,45 GO can be reduced by a quick annealing process,⁴⁵ and thus these CSs were put into a tubular furnace at 1000 °C for only 30 s under an argon flow. After annealing, most of the hydroxyl groups had been removed, and the C/O ratio reached 9.76 : 1 (Fig. 2c and d). The Raman and XRD spectra (Fig. S3†) also illustrate that the crystallinity improved during the annealing process.

Fig. 1 SEM (a) and FE-HRTEM (b–d) images of CS.

Fig. 2 (a and b) XPS survey and C 1s spectra of CS; (c and d) XPS survey and C 1s spectra of CS after annealing at 1000 °C.

The measured complex permittivity (ε_r) was utilized to determine the MA of the CS/paraffin composites. Considering the weak magnetic properties of CS, a complex permeability (μ_r) was taken as 1.²⁴ Fig. 3a and b show the real (ε′) and imaginary (ε′′) parts of ε_r measured in 2–18 GHz for the samples loaded with 5.0, 15.0 and 25.0 wt% of CS in paraffin. It was found that the value of ε′ obviously increased with the increasing filler loading ratio, and this can be attributed to the increment of dipolar polarization and electrical conductivity.¹⁶ The ε′′ is related to the dissipation of microwave energy,²⁹ and thus, when a material has relatively high value of ε′′, it may have potential in MA. The value of ε′′ also increased with an increasing filler loading ratio, and the main vibration peaks were found in the Ku band. We compared the dielectric loss tangent (tan δ = ε′′/ε′), which is related to microwave attenuation in dielectric materials,²⁹ and Fig. 3c shows the tan δ of each sample. It is easy to find that the composite with 25.0 wt% of filler loading has the maximum value in the tested frequency. The relatively high values of ε′′ and tan δ imply that this sample has the best MA performance.

Fig. 3 The real part (a) and imaginary part (b) of the relative complex permittivity of CS and paraffin-based composites and their dielectric loss tangent (c).

The optimal thicknesses of each sample were taken into account for the higher MAs and the results are exhibited in Fig. 4. It is not surprising that the composites with 5.0 and 15.0 wt% of filler loading have a very poor MA performance, since the values of ε_r and tan δ explain this phenomenon (Fig. 4a and b). It can be clearly seen that composites with 25.0 wt% of filler loading exhibit and effective MA performance. Fig. 4c and 5a suggest that the composite of a 2.0 mm thickness has the highest reflection loss (RL), up to −37.2 dB at 13.72 GHz with an effective bandwidth of ∼5.72 GHz (12.28–18.0 GHz). The contour plot (Fig. 5b) shows that when the thickness is between 2.0 and 2.66 mm, the MA could be higher than −10 dB, which means that more than 90% of the microwave energy has been absorbed at the Ku band. The noticeable peaks of RL mainly refer to the contribution of quarter-wave-length attenuation.^46,47 Observation of the peak shift with thickness change could be explained by the fact that the formation of quarter-wavelength attenuation requires the absorbing thickness to meet the phase match conditions.⁴⁸ Table 1 lists some outstanding composites with typical carbonaceous fillers and their MA performance. Although the filler loading ratio is a little higher than that in previous work, the facile synthesis, low thickness and large effective bandwidth make this composite an excellent MA material.

Fig. 4 The calculated RL for paraffin composites with (a) 5.0 wt%, (b) 15.0 wt% and (c) 25.0 wt% of CS (after annealing).

Fig. 5 3D plot (a) and contour plot (b) of RL versus the Ku band (8–12 GHz) and thickness (1.5–4.0 mm). The composite was loaded with 25.0 wt% of CS.

Table 1 Typical carbonaceous composites for MA (HOPC: highly ordered porous carbon; PVDF: polyvinylidene; PU: polyurethane)

Filler	Matrix	Filler loading	RLmax (dB)	Thickness (mm)	Frequency range (GHz) (RL below −10 dB)	Effective bandwidth (GHz) (RL below −10 dB)	Ref.
Biomass-derived CS	Paraffin	25.0 wt%	−37.2	2.0	12.28–18.0	5.72	This work
RGO	PVDF	3.0 wt%	−25.6	4.0	8.48–12.8	4.32	5 (2014)
RGO/MnFe2O4	PVDF	5.0 wt%	−29.0	3.0	8.0–12.88	4.88	12 (2014)
RGO/ZnO	Paraffin	10.0 wt%	−24.8	2.5	11.6–18.0	6.40	25 (2014)
RGO/PEDOT	Paraffin	10.0 wt%	−35.5	2.0	11.5–16.5	5.0	15 (2014)
RGO/MWCNTs	PDMS	5.0 wt%	−55.0	2.75	8.2–11.7	3.5	16 (2014)
HOPC	Paraffin	5.0 wt%	−17.4	2.0	11.7–16.2	4.5	24 (2014)
SWCNTs	PU	5.0 wt%	−18.5	2.0	9.0–12.0	3.0	26 (2007)
MWCNTs	Silica	5.0 wt%	−30.72	4.0	Unknown	4.2	27 (2013)

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The relationship between ε′ and ε′′ can be expressed as:

Thus, the plot of ε′ versus ε′′ would be a single semicircle, generally denoted as the Cole–Cole semicircle. One semicircle corresponds to one Debye relaxation process. The ε′–ε′′ curve is shown in Fig. 6; three semicircles implies that other kinds of relaxation occur in the CS/paraffin mixture, such as Maxwell–Wagner relaxation and electron polarization.^15,16

Fig. 6 ε′–ε′′ curve of composites loaded with 25.0 wt% of CS.

Furthermore, in order to evaluate the real MA performance, an epoxy-based composite of 2.0 mm thickness and 18 × 18 cm² which contained 25.0 wt% of CS was prepared, placed on iron substrate and measured under an NRL Arch instrument (Fig. 7a). In Fig. 7b, the epoxy/CS composite shows a lower RL (RL_max = −23.4 dB) and a narrower effective MA broadband (5.1 GHz, 12.9 GHz to 18.0 GHz). This may be attributed to the difference between the matrix materials and the defect during the fabrication process.

Fig. 7 (a) Picture of the NRL Arch instrument; (b) the epoxy matrix dispersing 25.0 wt% of CS; the thickness and area are 2.0 mm and 18 × 18 cm², respectively.

Conclusions

To summarize, we created a high microwave absorption (MA) based on micro carbonaceous spheres/paraffin composites. These carbonaceous spheres are entirely from a single precursor of biomass, watermelon. The composites with 25.0 wt% of sphere loading have presented excellent MA performances. Especially, when the thickness is 2.0 mm, 90% of microwave energy can be absorbed by this composite in the Ku band. This green and low energy utilization method to synthesize ordered carbonaceous spheres can be not only used in MA, but they also have potential in energy transform and storage.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (51403236, 51021001) and the Opening Project of State Key Laboratory of Disaster Prevention & Mitigation of Explosion & Impact (DPMEIKF201310). We also thank Mr S. Yang and Miss H. L. Fan for help in SEM and EDX test and Mrs P. Xu for help in XPS test.

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Footnotes

† Electronic supplementary information (ESI) available: See DOI: 10.1039/c5ra06307h

‡ These authors contributed equally.

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