A high-performance eletromagnetic wave absorption material based on biochar derived from pomelo peel

Abstract

As a kind of renewable resources with abundant reserves, research on the utilization of biochar has been paid more and more attention. However, the use of pomelo peel biomass carbon in the field of eletromagnetic wave absorption (EWA) has never been reported. In this study, the pomelo peel of natural recovery were carbonized at temperature of 700°C for 1 h in a high vacuum of 300 pa. The as-prepared biomass carbon were uniformly mixed with the paraffin by heat-assisted impregnation to prepare small ring-shaped devices, and the EWA characteristics of the small devices were investigated. The absorbing measurements were performed with the filling mass fraction of 30% and 50% of the biochar, respectively. The biochar with a mass fraction of 50% showed the excellent absorbing properties with the maximum reflection loss up to –44.3 dB at 9.84 GHz in the thickness of 2.0 mm, which is expected to be used as light, broadband and efficient EWA material.

Keywords

Biochar pomelo peel carbon vacuum calcination eletromagnetic wave absorption

1 Introduction

With the rapid arising of information technology, humans are facing severe threat from electromagnetic wave (EW) radiation pollution, which has detrimental effects on community health and accurate operation of electronic devices [1 –3]. Therefore, the high-performance electromagnetic wave absorbing (EWA) materials possess great applications in modern electronic information industry, healthcare, and national defense security [4]. Recently, considerable attention has been paid to the development of low density and thin thickness EWA materials [5, 6].

Carbon is very versatile in its behavior and is a key substance in a great number of substances, including most of the compounds essential to life. Carbon-based materials with various textures and morphologies have attracted much attention as multifunctional materials due to their unique properties such as availability, thermal stability, and chemical stability [7, 8]. Therefore, many efforts have been devoted to enhance the ability to prepare various carbon structures, for example, graphene [9, 10], carbon nanotubes (CNT) [11], nanofibers (CNF) [12], etc. However, most of these preparation methods rely on specialized reagents or complicated process control [13]. In addition, these materials usually have too high dielectric constants, resulting in poor impedance matching characteristics, which limits their application as electromagnetic wave absorbing (EWA) materials [14, 15].

Biochar refers to such a series of carbon materials that a solid material obtained from thermochemical conversion of biomass in an oxygen limited environment. The main component of biochar is carbon, but unlike other common carbon materials, generally according to different biomasses, biochar contains a small amount of other elements such as calcium, magnesium, phosphorus, boron, nitrogen, potassium, sodium, etc. Up to now, amorphous carbons obtained from biomass sources such as coconut shell [16], rice husk [17], pomelo peel [18, 19] and wood [20] have been reported. Pomelo peel (Citrus Grandis) is a kind of common renewable biomass resources that can be seen everywhere. In recent years, pomelo peel is directly engineered to serve as functional materials, such as catalytic or photocatalytic materials [21, 22], supercapacitors [23, 24], lithium-ion batteries [25, 26], solar steam generators [27], etc. However, to the best of my knowledge, pomelo peels used as EWA materials has never been reported. Recently, some works related to carbon composites have been reported as EWA materials [28 –30], indicating that carbon materials have advantages and prospects in the field of functional materials.

Herein, a simple, efficient and economical route was explored to obtain biomass-pyrolized carbon materials through vacuum thermal annealing method derived from recycled pomelo peel. The enhanced EWA performance as a porous thin-layer absorber of this biochar material was studied in the frequency range of 2–18 GHz, indicating superior behaviours as a type of low-cost EWA material for potential applications.

2 Experimental

2.1 Materials and methods

Pomelo peels (denoted as PP) raw materials were collected and cut into small pieces (about 1.0 cm²) then washed using deionized water, the as-obtained materials were dried in a hot air oven at 60°C for 24 h. Briefly, PP were employed to be carbonized at 700°C for 1 h in high vacuum of 300 pa with a heating ramp of 5°C min^–1, and then obtained the active carbon (denoted as PPC). The black product was cooled naturally and then thoroughly ground in an agate mortar for further use. The PPC powders/paraffin composites were prepared by uniformly mixed with the paraffin substrate by heat-assisted impregnation and then pressing the mixture into a cylindrical-shaped compact with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm and thickness of 2 mm.

2.2 Characterization

XRD was identified by X-ray diffraction measurement on MiniFlex600 with Cu Kα radiation. Raman spectrum was obtained on a Laser Raman spectrometer (LabRAM HR Evolution, the laser wavelength: 532 nm; the power: 5%; the scan time: 3 times per 20 seconds). The chemical states were investigated by X-ray photoelectron spectroscopy (XPS, ESCALAB-MKII). TG measurements were performed on a simultaneous thermal analyzer (Setaram, Labsys evo). The morphologies of the samples were characterized by a scanning electronic microscope (SEM) of Phenom proX with CeB₆ filament and working at 10 kV. The complex permittivity (ɛ_r =ɛ^′ –jɛ^′′) and the complex permeability (μ_r =μ^′–jμ^′′) of as-prepared samples/paraffin were measured by Agilent E5071C vector network analyzer in the range of frequency 1–18 GHz, and corresponding reflection loss (RL) was calculated according to the transmission line theory.

3 Results and discussion

X-ray diffraction (XRD) experiments were carried out to reveal the structures of the original pomelo peel (PP) and the calcined (PPC) sample from pomelo peel (shown in Fig. 1). PP and PPC represent the samples before and after the vacuum calcination process, respectively, and both exhibit highly broadened (002) Bragg reflections near 22.5 °, which are typical peaks of nongraphitic carbon material with disordered crystalline structures [31]. However, a peak (100) at 43.5 ° could be clearly observed in PPC, indicating that PPC consisted of small domains assembled by stacked graphene sheets [32]. The half-graphite carbon will be beneficial for achieving excellent dielectric loss due to its good electronic conductivity [33].

Fig. 1

The XRD patterns of PP and PPC.

The Raman spectra curves of PP and PPC are shown in Fig. 2. Two strong carbon characteristic peaks were observed at 1340 and 1597 cm^–1. The peak at 1340 cm^–1 (D band) was assigned to disordered carbon, and the peak at 1597 cm^–1 (G band) was assigned to the sp^2- bond carbon atoms in a two-dimensional hexagonal graphitic layer. Here, the D/G intensity ratio intensity approximating to 1 indicates that PPC was composed primarily of disordered carbon and also had a partially graphitized structure [34]. Moreover, the Raman spectroscopy of PPC appears as the second-order 2D peak (2887.24 cm^–1) caused by the second-order zone boundary phonons [35], further indicating that the PPC has a more ordered graphitic structure than that of the PP.

Fig. 2

Raman spectra of PP and PPC.

To further investigate the elementary composition of PPC, XPS was employed. The XPS survey spectra in Fig. 3 suggests that the PPC contains primarily carbon (92.97%) and oxygen (7.03%) which indicates that vacuum calcination results in a high degree of carbonization of biochar, wherein the presence of oxygen may be ascribed to the incomplete carbonization of the carbohydrates in the pomelo peel. The high-resolution C 1s spectrum can be resolved into four individual peaks centered at 289.15, 286.72, 285.20 and 284.54 eV, referring to O=C–O, C–O, sp³ C and sp² C, respectively [36 –38]. These results further demonstrate the higher graphitization degree in PPC, which is consistent with the above analyses.

Fig. 3

(a) XPS survey spectrum and (b) C 1s spectrum of PPC.

The process of heat annealing was monitored by the TG under insert gas (Ar) and air are shown in Fig. 4. According to the two TG curves, the original pomelo peel (PP) starts to lose weight from 90°C, the proposal is that the process of free water evaporation occurs. The weight loss of PP mainly happens in the range 200 700°C, which may be due to the removal of the bound water in the material, the carbonization of PP and the decomposition of various active ingredients in the grapefruit skin (cellulose, hemicellulose, oxycarbide, lignin and other substances) [39]. The weight loss process curve in Ar atmosphere is basically stabilized at 700°C and the total weight loss in this process is ∼68%. However, in air, the PP sample reaches the 68% weight loss in the first stage at 400°C. With increasing temperature, the curve continues to drop rapidly. At about 480°C, the weight loss reaches the balanced point of around 88%. It is assumed that the extra weight loss is due to the fact that the carbon in the material combines with the oxygen in the air and releases carbon dioxide gas.

Fig. 4

The TG plots show the residual masses of PP in thermal weight loss processes under Ar and air atmosphere, respectively.

To further examine this half-graphite biochar, the morphology and structure of the PPC product were also investigated. Figure 5a shows the representative SEM images of the original ground PP. As can be seen from Fig. 5a, the raw material displays featureless wrinkle morphology, indicating the fact of no obvious microporous character in the PP sample. As a comparison, the PPC sample also exhibits irregular and amorphous structure in the low magnification (Fig. 5b). The PPC has shown almost no difference from the PP sample before vacuum calcination, indicating that the micro morphology of the carbon material is relatively stable, and the layered structure of biochar provides a suitable free space for electromagnetic wave reflection and scattering, increasing the propagation path of electromagnetic waves which may be beneficial to electromagnetic wave absorption (EWA) [40]. The SEM image under the high magnification shows that the lamellar structure may be composed of fine fibrous carbon material (Fig. 5c). In addition, the Brunauer–Emmett–Teller (BET) surface area of the PPC product is 301.57 m² g^–1 and a pore volume of 0.1428 cm³ g^–1 with many pores generally smaller than 5 nm were obtained from nitrogen adsorption–desorption isotherm analysis (shown in S1).

Fig. 5

SEM images of (a) original PP, (b) and (c) PPC.

In accordance with transmit line theory, the reflection loss (RL) is optimized by using the following equations [41, 42]: $RL = 20 \log | \frac{Z_{in} - Z_{0}}{Z_{in} + Z_{0}} |$ (1) $Z_{in} = Z_{0} \sqrt{\frac{μ_{r}}{ɛ_{r}}} \tanh [j (\frac{2 π}{c}) fd \sqrt{μ_{r} ɛ_{r}}]$ (2)

where Z_in is the input impedance of the absorber, c is the light velocity of electromagnetic waves in free space, f is the electromagnetic wave frequency and d is the absorber layer thickness.

With the PPC filling mass fraction of 30% (Fig. 6a), the RL_min of PPC with the thickness of 2 mm on EWA is –30.49 dB at the frequency of 10.02 GHz. In particular, the maximum reflection loss value of electromagnetic waves exceeds –20 dB at each thickness, achieving ultra-strong absorption of electromagnetic waves with a frequency of 2 to 18 GHz. The PPC-filled sample was also tested for a mass fraction of 50%. It can be seen from Fig. 6b that an optimal RL value of –44.283 dB is reached at 9.84 GHz with the sample thickness of 2.0 mm, thus an excellent EWA line with respect to the requirements of the thin PPC sample, which showed excellent absorbing properties comparing with other carbon materials and biomass listed in Table 1. The EWA shows dual peaks with the thickness of 1.5 mm, the bandwidth of absorption exceeding –10 dB is about 3.9 GHz (in the range of 12.4–16.3 GHz). Meanwhile, the RL value of the 30% and 50% samples both shift to lower frequency with increasing thickness, suggesting that the range of absorption frequency can be modulated by adjusting the composite thickness.

Fig. 6

RL curves of (a) 30 wt% and (b) 50 wt% PPC in paraffin composites with different thicknesses (mm).

Table 1

Microwave absorption properties of some carbon materials and biomass

Compounds	Maximum \|RL\|	EAB (GHz)	Thickness (mm)	References
Graphene/Carbon sphere	28.1(dB)	14	2.0	46
Graphene/PEO	38.8(dB)	16	1.8	47
C@C	34.8(dB)	5	2.0	48
Carbon fiber	12.0(dB)	3	3.0	49
Graphene	2.4(dB)	14.5	1.5	30
Graphene microflower	42.9(dB)	6	1.0	50
Tobacco stem biomass	36(dB)	2.4	—	51
Pomelo peel biochar	44.2(dB)	10	2.0	This work

The complex permittivity (ɛ_r =ɛ^′-jɛ^′′) and permeability (μ_r =μ^′-jμ^′′) of the PPC are plotted versus frequency in Fig. 7. In the complex permittivity, the real part (ɛ^′) and imaginary part (ɛ^′′) correspond to the storage and loss capability of electric energy, respectively [43, 44]. It can be seen that the ɛ^′ value decreases from 23.755 to 12.625 with increasing frequency and exhibits a broad peak at around 15 GHz. And the ɛ^′′ value declines from 11.39 to 4.05 with fluctuations. The real part (μ^′) and imaginary part (μ^′′) of the complex permeability are very close to 1 and 0 with fluctuations, therefore, the reflection loss (RL) of the PPC sample is mainly a result of dielectric loss. The matching between the complex permittivity and permeability (shown in Fig. 7c) is mainly reflected in the comparison of the dielectric dissipation factors (tanδ_ɛ) and magnetic dissipation factors (tanδ_μ) [45]. In the range of 1–18 GHz, the tanδ_ɛ value of PPC fluctuates around 0.4, and the tanδ_μ value is basically stable at 0. Therefore, it also shows that the dielectric loss is the main factor determining the EM wave absorption performance, and the influence of magnetic loss can be ignored.

Fig. 7

Real (ɛ^′, μ^′) and imaginary (ɛ^′′, μ^′′) parts of the relative complex permittivity (a) and permeability (b), as well as the tangent loss of both dielectric tangent loss (tanδ_ɛ) and magnetic tangent loss (tanδ_μ) (c) of 50 wt% PPC composite.

4 Conclusion

In summary, functional biomass carbon material was successfully fabricated using a biological waste, pomelo peel, as biochar precursor by a simple high-temperature vacuum calcination method. The BET specific area of 301.57 m² g^–1 and pore volume of 0.1428 cm³ g^–1 shows indeed the biochar is a porous material. As an electromagnetic wave absorbent, the pomelo peel derived carbon material possesses the maximum RL value of –44.283 dB at a thickness of 2.0 mm under the experimental conditions. The excellent results confirm the applicability of the biochar materials derived from pomelo peel. Thus, the well-treated biochar is a promising material with eminent absorption, high stability, low-cost and easy access for practical applications.

Footnotes

Acknowledgments

Author thanks Prof. Q. Wang for kind discussion.

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