C Composites from Waste Wood with

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Synthesis of porous 3-D Fe/C composites from waste wood with tunable and excellent electromagnetic wave absorption performance Zhichao Lou, Yanjun Li, He Han, Huanhuan Ma, Lian Wang, Jiabin Cai, Lintian Yang, Chenglong Yuan, and Jing Zou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04045 • Publication Date (Web): 17 Sep 2018 Downloaded from http://pubs.acs.org on September 23, 2018

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ACS Sustainable Chemistry & Engineering

Synthesis of porous 3-D Fe/C composites from waste wood with tunable and excellent electromagnetic wave absorption performance Zhichao Lou1,2, Yanjun Li1*, He Han1, Huanhuan Ma1, Lian Wang1, Jiabin Cai1, Lintian Yang1, Chenglong Yuan1, Jing Zou1 1

College of Materials Science and Engineering, Nanjing Forestry University, #159

Longpan Road, Xuanwu District, Nanjing 210037, China 2

State Key Laboratory of Bioelectronics, Jiangsu Key Laboratory for Biomaterials

and Devices, School of Biological Science and Medical Engineering, Southeast University, #2 Sipailou Road, Gu lou District, Nanjing 210096, China *To whom correspondence should be addressed. Yanjun Li, Email: [email protected]

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Abstract Here, we synthesized Fe/C composites with different crystal structures (Fe3O4@C, Fe3O4/Fe@C, or Fe3C@C) by carbonizing iron (III) 2,4-pentanedionate (Fe(acac)3) pre-enriched forestry waste wood at different pyrolysis temperatures from 400°C to 1000°C. The obtained samples are porous 3D bio-chars inlaid with varied Fe phases. The corresponding EMW absorbing properties are proved to be dependent on the pyrolysis temperature, as well as the resulting crystal structures and graphitization degrees. Among them, Fe3C@C obtained at 1000°C possesses excellent EMW absorbing properties with a minimum RL value of -57.64 dB at 6.92 GHz and a wide response bandwidth of 5.00 GHz covering 6.36-11.36 GHz. This good performance is due to continuous substances of Fe3C covering the inner surface of the conductive bio-chars, permitting optimal impedance matching along with strongest dielectric loss and optimal magnetic loss. Moreover, more defects are generated at 1000°C which act as the dipole center and product dipole relaxation polarization. The dipole relaxation is proved to be positive to improve the EMW absorbing performance together with the interface polarization between C-Fe3C.

Keywords: Pyrolysis, Magnetic bio-char, Electromagnetic wave absorption, Electromagnetic interference shielding, Composite

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Introduction With the rapid advancement of communication technology, electromagnetic interference (EMI) irradiation pollution has become a novel environmental pollution besides of water and air pollutions since it not only influences information security in civil and military fields1 but also can break DNA, affect biological immune systems and therefore threaten human health2. To address EMI radiation problems, the application of electromagnetic wave (EMW) absorbers which can convert EMW energy into thermal energy, is proved to be an effective method

3,4

. An ideal EMW

absorber should possess advantages of lightweight, strong absorption capacity, and a broad frequency bandwidth5,6. Magnetic materials, such as Fe3O4, Fe3C and Fe2O3, are one group of the most attractive absorbers, which are of advantages of high magnetic permeability, environmental benignity, non-toxicity and low-cost7–9. However, because of the poor electromagnetic impedance matching characteristics, these monophasic magnetic materials have the disadvantages of narrow absorbing bandwidth, large thickness requirement and high density, which limit their practical applications in EMW absorbing fields. One approach to ensure a good EMW absorption performance is to load other conductive materials to fabricate multiphase composite magnetic materials. By doing this, the lower dielectric value of the traditional magnetic materials and relatively higher permittivity value of the conductive materials will lead to a moderate permittivity value, ensuring better electromagnetic impedance matching. Due to the advantages of relatively low density, abundant resources, good electric properties, facile manipulations, and relatively excellent chemical and thermal stabilities, carbon materials with various structures and phases are the most promising candidates among the dielectric materials10–15. There are many related studies on fabrication of magnetic carbon composites, but most corresponding synthesis methods involve templates, complex surface functionalization, or coupling agents, and require pre-prepared magnetic particles16–18. Furthermore, the inter-particle dipolar forces easily lead to aggregation of magnetic 3 ACS Paragon Plus Environment


particles during the fabrication processes. Besides, due to the lack of effective method of synthesizing magnetic materials in large quantities, and due to the high preparing cost of the carbon phase such as graphene19, carbon nanotubes20 and carbon nanofibers21, the practical utilization of such magnetic carbon composites is still few. Thus, the challenge is to develop a low-cost, green and scalable method to product magnetic carbon composites with outstanding magnetism, conductivity, and high EMW absorbing performance. As we know, biomass is a natural matrix for composites and is a sustainable natural carbon source. Take China for example, about 900 million dry tones of straw, 50 million dry tones of nut shell, and 300 million dry tones of forestry waste wood is available every year, indicating a great potentials for bio-energy and bio-chemicals production. However, there are merely researches on the synthesis of magnetic bio-chars as EMW absorbers based on the utilization of waste biomass. Here, we successfully synthesized Fe/C composites with different crystal structures by in-situ pyrolysis of iron (III) 2,4-pentanedionate (Fe(acac)3) in the inner surface of the lumen walls in forestry waste wood at different pyrolysis temperatures from 400℃ to 1000°C. In advance, Fe(acac)3 is transported into wood through a vacuum impregnation followed by a pressure impregnation. The EMW absorption properties of the obtained magnetic bio-chars are greatly dependent on the pyrolysis temperature, as well as the resulting varied crystal structures and graphitization degrees. Among them, Fe3C@C composites obtained at the pyrolysis temperature of 1000°C possess the most excellent absorption properties, which is due to continuous substances of Fe3C covering the inner surface of the conductive bio-chars, permitting optimal impedance matching, the strongest dielectric loss, and optimal magnetic loss. Moreover, the more defects generated at 1000°C act as the dipole center and product the dipole relaxation polarization. The latter is positive to improve the microwave absorption performance together with the interface polarization between C-Fe3C.

Materials and Methods Chemicals. Iron (III) 2,4-pentanedionate (98%) and N,N-Dimethylformamide (DMF, 4 ACS Paragon Plus Environment

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99%) was purchased from Aldrich. The forestry waste wood from furniture production were obtained from JM Arts & Crafts Co. in Fuyang, Anhui, China, which was dominated with fast-growing poplar wood. Preparation of porous 3-D Fe/C composites. First, the obtained waste wood was size-classified, selected, and then heated in distilled water several times until the water turned clear. Then, the dried specimens were extracted with a solvent mixture of alcohol/toluene (1:2, v/v) overnight to remove the wood extractive compounds and to improve the connectivity among pores. The end-matched specimens were dried again in an air-dry oven at 105℃ until the moisture content was ~10%. The obtained specimens were impregnated in DMF solution containing Fe(acac)3 with an exact concentration of 0.12 g/mL. The impregnation was performed under vacuum (0.07 MPa) for 2.5 hours, followed by pressure impregnation (0.80 MPa) for another 2.5 hours. Then, the impregnated specimens were sensoned under natural state and air seasoning state for 1.0 hour, followed by being air-dried at 55°C for another 4.0 hours. In succession, the treated wood was placed into a tube furnace under N2 flow. After air being purged and N2 being filled in the furnace for 30 min, the samples were heated to 220°C at the heating rate of 5°C/min, and refluxed for 1.0 hour. Then the heating temperature was increased to 300°C with a heating rate of 5°C/min, and kept for 1.0 hour. Fe3O4 nanoparticles are supposed to be formed via high-temperature decomposition of Fe(acac)3 under 300°C22. The samples were further heated to an exact temperature (400, 700 and 1000°C) at a rate of 5°C/min and kept for 4.0 hours. After carbonization process, the samples were cooled down to ambient temperature under N2 protection, and the named as CFe-400, CFe-700 and CFe-1000. Characterization of Synthesized Materials. The characterization details of the synthesized materials are applied in the Supporting Information, such as SEM observations, AFM, XRD, FTIR, XPS analyses, Raman spectroscopy and magnetic measurements. Electromagnetic parameters measurement. The relative complex permittivities 5 ACS Paragon Plus Environment


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(ε’, ε’’) and relative complex permeabilities (µ’, µ’’) of the specimens were measured by the coaxial-line method with an Agilent PNA N5224A vector network analyzer in the frequency range of 2-18 GHz. Toroidal-shaped samples (Φout: 7.0 mm, Φin: 3.04 mm) for the testing were prepared by homogeneously mixing 85 wt% of paraffin wax and 15 wt% samples (filled ratio = 1:4) and then pressed. Finally, the electromagnetic wave absorbing performance of the specimens can be evaluated by the reflection loss (RL), which can be defined with the following equations on the basis of transmission line theory: = ⁄ / tan ℎ[2⁄ ⁄ ]

= 20

|

− / + |

(1) (2)

where Z0 is the characteristic impedance of free space, Zin is the input impedance of the absorber, εr is the relative complex permittivity ( = $ − $$ ), µr is the relative

complex permeability ( = $ − $$ ), f is the frequency of the microwaves, d is the thickness of the specimen, and c is the velocity of light.

Results and Discussion Figure 1 shows the SEM observations of the cross-sections of natural wood (Figure 1A) and pyrolyzed natural wood (Figure 1B), and the cross- (Figure 1C) and radial(Figure 1D) sections of CFe-1000, respectively. And the corresponding SEM images of the same sections of these three samples in large areas are also shown in Figure S1, S2 and S3, respectively. Figure S4 and S5 show the radial-sections of CFe-400 and CFe-700, respectively. From the images we may see that particles are formed on the inner surfaces of the carbon matrices obtained at all the three pyrolysis temperatures, 400ºC, 700ºC and 1000ºC. Comparing among these cross-section SEM images, it is obvious that the natural wood possesses tough lumen walls which turn smoother after the pyrolysis processes (Figure 1B and 1C). And the black areas observed in both pyrolyzed samples (non-impregnated and impregnated) demonstrate presence of voids, indicating the typical porous structural characteristics of the carbonized wood. In addition, being dramatically different from the empty pores observed in Figure 1B and Figure S2D, a puffy layer consisting of small particles is observed to attach onto 6 ACS Paragon Plus Environment

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the inner cell lumen surfaces of CFe-1000 in Figure 1C(2). And from the radial-section SEM image in Figure 1D, we may see that this visible and continuous layer is formed by clusters of particles while there is no similar clusters observed in the radial-section SEM images of either non-pyrolyzed or pyrolyzed natural wood in Figure S1C and S2C. Besides, the thickness (d) of the defined layer on the lumen wall (deciphered by orange lines in Figure 1C(2)) appears to be 1-2 µm while the particles are only 50-100 nm in size as shown in Figure 1D(2), indicating that multilayered films are formed by these particles on the inner surface of the porous structures of CFe-1000. The detailed morphologies of the inner surfaces of the natural wood, pyrolyzed natural wood and CFe-1000 are also investigated by AFM as shown in Figure S6. From the AFM results, we may see that the obtained clusters of CFe-1000 in SEM image are constituted by individual nanosize particles. According to the corresponding 3D AFM image, the height value of the particle film in CFe-1000 is around 2µm. All the AFM results are in accordance with the SEM results. The porous nature of CFe-1000 is confirmed by Brunauer-Emmett-Teller (BET). The N2 adsorption-desorption isotherms result is shown in Figure S7. A hysteresis loop is observed in the range of 0.43-1.0 P/P0, implying that the obtained bio-char attains porosity. The specific surface area of the micropores is determined by t-plot method as 226.31 m2/g, while the mesopores is measured by the Barret-Joyner-Halenda (BJH) method as 115.513 m2/g. This high surface area generated reflects the micro- and mesoporous architecture of the magnetic bio-char. The EDS elemental mapping results in Figure 1, S1, S2, and S3 provide further insights on the elemental compositions and distributions of the obtained samples. As expected, both C and O elements are uniformly dispersed in all the three samples while the signal intensities of O elements obviously weaken after the pyrolysis processes. Compared with natural wood (the insert in Figure 1A(2)) and pyrolyzed natural wood (the insert in Figure 1B), Fe signals are only observed in CFe-1000 as shown in Figure 1C(3), 1D(3) and Figure S3. And it is worth noting that Fe elements are mainly distributed in the puffy layer which attaches to the inner surface of the 7 ACS Paragon Plus Environment


porous structures of CFe-1000, and their localizations are observed to be coincide with the clusters as pointed with green arrows in Figure 1D(2), implying that these clusters consist of Fe element.

Figure 1. (A) and (B) The SEM images of the cross-sections of the natural wood and the carbonized wood, and the corresponding Fe and C EDS signals images inserted. (C) and (D) The SEM images of the cross-section and radial-section of CFe-1000 and the corresponding Fe EDS signal image. XRD is introduced to investigate the crystalline structures of the samples obtained at different pyrolysis temperatures, as shown in Figure 2A. From the XRD curves we may see that, the components of the pyrolyzed products varied with increasing temperatures. For 400°C, the peaks at 2θ=30.0o, 35.3o, 43.0o, 56.9o and 62.5o correspond to the (220), (311), (400), (511) and (440) planes of cubic Fe3O4 with the () (JCPDS 16-629).23 When the temperature increases to space group of %&' 700°C, an additional peak at 2θ=44.6o is observed for CFe-700, corresponding to the (110) plane of the body-centered cubic structure of α-Fe (JCPDS 6-696).24 Thus, CFe-700 is a composite structure consisting of bio-char, Fe3O4, and Fe. For CFe-1000, the observed peaks at 2*=37.6o, 39.8o, 40.6o, 42.8o, 43.7o, 44.5o, 45.0o, 45.8o, 48.6o, 8 ACS Paragon Plus Environment

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49.1o and 51.8o correspond to the atomic plane-reflections of (210), (002), (201), (211), (102), (220), (031), (131), (221) and (122) respectively, indicating the conversion from Fe/Fe3O4 to Fe3C.24 Additionally, the peak at 26.1o gradually grows sharper, corresponding to a graphite-like structure which is attributed to the carbonization of wood at a high temperature.

Figure 2. The XRD (A), XPS (B), FT-IR (E) and Raman (F) spectra of CFe-400, CFe-700 and CFe-1000, respectively. (C) and (D) is the corresponding high-resolution XPS spectra in Fe 2p and C 1s, respectively. The XPS spectra of Fe 2p are resolved by deconvolution and Gaussian curve fitting, and show two fitting peaks as labeled by pink and green in (C). XPS can detect and distinguish the core electron lines of both ferrous and ferric ions.25 Based on this, we introduce XPS to further confirm the chemical compositions in the resultant magnetic pyrolyzed wood. Figure 2B is the survey XPS spectra of the samples obtained at 400°C, 700°C, and 1000°C, respectively. It is obvious that all of the as-prepared samples contain C, O, and Fe elements. Figure 2C and 2D are the high-resolution XPS spectra of Fe 2P and C 1s, respectively. As shown in Figure 2C, the spectra of Fe 2p of all the three samples are resolved by deconvolution and 9 ACS Paragon Plus Environment


Gaussian curve fitting. The results show that all the three samples possess double satellite signals with different binding energies. For CFe-400 and CFe-700, the binding energies at ~711.0 eV and ~725.0 eV are the characteristic doublet from Fe 2p3/2 and Fe 2p1/2 core-level electrons, indicating the formation of Fe3O4 in the samples. In addition, the absence of the shoulder peak between Fe 2p3/2 and Fe 2p1/2, which is the major characteristic for λ-Fe2O3, excludes the presence of λ-Fe2O3 during the pyrolysis processes. Although metallic iron is proved to be formed at 700°C by XRD (Figure 2A), there is no binding energy observed at ~707.0 eV in Figure 2C. This is because the metallic iron is probably formed at the interfaces between the attaching Fe3O4 on the inner surfaces and the bio-char matrix, resulting in partly measurement of the Fe interlayer since thickness of the Fe3O4 layer and the bio-char are observed to be 1~2 µm (Figure 1B and 1C) which is larger than the limit of the analysis depth of XPS (less than 20 nm). When heated to 1000°C, the peaks of Fe 2p shift from high binding energies to low ones at ~722.7 eV and ~709.1 eV which are the characteristic doublet for Fe3C.26 This is in accordance with the results of XRD. The peak at around 285.0 eV in Figure 2D is attributed to C 1s. At lower pyrolysis temperature, the spectra for CFe-400 is broad because of the presence of C-C, C-H, C-O and C=O groups which will be discussed in details according to the FTIR results. With the increase of the temperature, the C 1s lineshapes become sharper, indicating the higher-graphitization of the as-prepared bio-char at higher temperature (1000°C). This observed phase transformation from magnetite (Fe3O4) to Fe/Fe3O4, and to Fe3C in the obtained absorbers is caused by the reactions taking place at the interfaces between the carbon matrix and the attaching Fe-containing layers on the inner surface of the porous structures (Figure S8).27 This thermal treatment activates chemical reactions are also well known in Metallurgy.28,29 Firstly, it is reported that Fe3O4 nanoparticles can be formed by thermal decomposition of Fe(acac)3 at around 300°C.30 This explains the formation of magnetite phase in CFe-400. When the pyrolysis temperature increases to 700°C, the following reaction occurs at the 10 ACS Paragon Plus Environment

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interfaces between the carbon matrix and attaching Fe3O4 layer where the temperature is supposed to be below 600°C: +,- ./ → 32 − +, + 2.

(3)

This explains the results of XRD (Figure 2A) and XPS (Figure 2C) that at 700°C, α-Fe is only observed between the carbon matrix and attaching Fe3O4 layer. And when the pyrolysis temperature further increases to 1000°C, two interactions possibly happen at the interfaces as below: +,- ./ + 3 → +,- 3 + 2.

(4)

2 − +, + 3 → +,- 3

(5)

As a result, cementite (Fe3C) is dominant in CFe-1000. This proposed mechanism may also be proved by the TGA results of the impregnated samples as shown in Figure S9. From Figure S9 we may see that, the TGA equilibrium mass percentage decreases from 27.82% at 400°C to 22.80% at 700°C, and to 12.04% at 1000°C, which is mainly due to the further carbonization of the carbon matrices and the further reactions of iron containing compounds. Our previous report proves that the oxygen-containing function groups can produce electronic dipolar polarization which is beneficial to EMW absorbing behavior of absorbers.31 Here, FT-IR spectra of the as-prepared Fe-containing bio-chars is introduced to investigate their function-group features. As shown in Figure 2E, it is obvious that the alkyl aromatic structures with oxygen-containing functional groups which correspond to the absorption bands at around 1693 cm-1 (C=O groups) and 1593~1213 cm-1 (C=O, C-O), are destroyed with the carbonization processes of the wood waste biomass. In details, the strong and broad bands with sub-bands between 1700 cm-1 and 1000 cm-1 as a result of skeletal stretching and bending modes of aromatic structures containing residual O heteroatoms, can be observed for CFe-400. The intensities of these corresponding bands diminish with the pyrolysis temperature increase from 400°C to 700°C, and can be hardly detected at 1000°C. The characteristic C-H out-of-plane bending modes between 900 and 700 11 ACS Paragon Plus Environment


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cm-1 have the same variation tendency as a function of pyrolysis temperature. Additionally, the presence of Fe3O4 can be confirmed by the absorption bands between 700 and 400 cm-1 for CFe-400, which disappear for CFe-1000. These data are consistent with XRD and XPS results, and indicates that the probable electronic dipolar polarization can only be considered for CFe-400 but not for CFe-700 and CFe-1000. The graphitization and aromatic nucleus features of the bio-char matrices are supposed to determine the conductive properties of the EMW absorbers and further to affect their absorption capacities. Raman spectroscopy is an effective means in this field since it is particularly sensitive to sp2 carbon structures and corresponding features measured on the scale of nanometers. The typical Raman spectra of CFe-400, CFe-700 and CFe-1000 in 1800-1000 cm-1 region are shown in Figure 2F. All the three spectra exhibit two bands near 1340 cm-1 and 1590 cm-1, referring to the vibration of sp3 atoms of disordered graphite (D-band) and the in-plane vibration of sp2 atoms in a 2D hexagonal lattice (G-band), respectively.32 For CFe-400, additional peaks are observed at around 1275 cm-1 and 1520 cm-1, assigning to the lumped Raman

signal

for

the

sp2-sp3

carbonaceous

structures

and

aryl-alkyl

ether/para-aromatics structures (S band) and the typical structures of the relatively small aromatic ring systems consisting 3-5 fused benzene rings (V band), respectively. It is obvious in Figure 2F that, the intensities of S and V bands decrease with the pyrolysis temperature increase from 400°C to 1000°C, implying a continuous removal of amorphous carbon structures in the as-prepared absorbers. On the other hand, the ID/IG value has been extensively used as an important parameter to study the crystalline or graphite-like carbon structures. Here, a higher peak area ratio (1.26 for CFe-1000) means a higher graphitization degree to the amorphous state. Besides, the increase in the ratio value from 0.48 to 1.26 reveals a rise of defect or edge-bonded graphite-like structures, which is supposed to be attributed to the self-carbonization of waste wood at high temperatures and the catalysis of Fe/Fe3O433.

This increase

indicates an increase in the concentrations of aromatic clusters in the obtained carbon 12 ACS Paragon Plus Environment

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matrices. As we know, the appearance of D band requires the presence of ordered carbon structures (≧6 benzene rings) in close proximity, being beneficial to the conductive properties of the as-prepared absorber. All these results imply the conversion from the amorphous structures of CFe-400 to the more ordered graphite-like structures of CFe-1000 with increasing conductive properties which can be obviously confirmed as the bulb brightness shown in Figure S10. Besides, the observed defects in the as-prepared absorbers will act as dipoles and are supposed to be with great benefits for the absorbing behavior through the dipolar polarization effect. Figure S11A shows the VSM curves of CFe-400, CFe-700 and CFe-1000. It is obvious in Figure S11A that both the saturation magnetization (Ms) and the coercive field (Hc) values increase dramatically with the increasing pyrolysis temperature from 400°C to 1000°C. As shown in Figure S11A, the Ms values for CFe-400, CFe-800 and CFe-1000 are 2.27 emu/g, 19.35 emu/g and 54.45 emu/g, respectively. And the Hc values for CFe-400, CFe-800 and CFe-1000 are 61.09 Oe, 130.60 Oe and 612.82 Oe, respectively. The corresponding magnetic performances are shown in Figure S11B. This increasing tendency is attributed to the phase change and the reduction in sample mass caused by carbonization process which is proved by the FT-IR results. Larger Ms and Hc values are proved to predicate a larger magnetocrystalline anisotropy (Keff) according to Stoner-Wohlfarth theory, and the latter is supposed to be beneficial to the EMW absorption behavior.31,34 The three-dimensional (3D) curves for the EMW loss RL values of the absorbers with coating thicknesses of 1.00 mm to 5.00 mm are shown in Figure 5, respectively. From the RL curves, CFe-400 does not present effective absorption performance (RLA

B + $$ = =

>? C>A

B

(7)

where D and E are the static permittivity and relative permittivity in higher frequency region respectively. Thus, the plot of ε’ versus ε’’ is supposed to be a single semicircle termed as Cole-Cole semicircle, with each semicircle corresponding to a Debye relaxation process.36 Figure 5 shows the plots of ε’-ε’’ for the Fe/C composites carbonized at different temperatures of 400, 700 and 1000°C, respectively. It is obvious in Figure 5D that the plots of CFe-1000 display more distorted Cole-Cole semicircles. For CFe-1000, interfacial polarization could be generated at interfaces between the attaching Fe3C particles on the inner surface of the porous structures and the carbon matrix. Besides, the Raman results demonstrate that more defects generated in CFe-1000 which can act as dipole center and bring out dipole relaxation polarization. Therefore, the multiple dipole and interfacial polarizations correspond to the Cole-Cole semicircles for CFe-1000. Compared with CFe-400 and CFe-1000, CFe-700 own more interfaces (Fe-C and Fe-Fe3O4 interfaces), but there is no similar peaks observed in the corresponding ε’’ curves in Figure 4B. This is because that due to the presence of more conductive Fe layer covering the carbon matrix, a part of the EMW will convert into the micro-current upon the surface of the as-prepared absorber, leading to the decrease of skin depth and consequently resulting in fewer interface polarization processes. In addition, it can be seen in Figure 5A that the plot of ε’ versus ε’’ for the three absorbers shifts to a higher value with the increase of the 16 ACS Paragon Plus Environment

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pyrolysis temperature, implying an enhanced contribution of Debye relaxation to dielectric loss.

Figure 5. (A) Survey of the plots of ε’ vs ε’’ for the three absorbers. Typical Cole-Cole semicircles (ε’ vs ε’’) for (B) CFe-400, (C) CFe-700 and (D) CFe-1000. Figure 4C and 4D reveal that both the µ’ and µ’’ values of CFe-1000 are higher than the ones of CFe-400 and CFe-700, 1.06 at 12.24 GHz and 0.12 at 16.72 GHz respectively. According to the following equations (8) and (9)37: $ = 1 + F/G cos J

$$ = 1 + F/G sin J

(8) (9)

this better magnetic loss ability of CFe-1000 is attributed to its higher saturation magnetization value as shown in Figure S11. In addition, several resonant peaks can be observed in the µ’’ curves of CFe-1000 as shown in Figure 4D. According to our previous report, the formation of these peaks are attributed to exchange resonance, eddy current resonance and natural resonances.38 Among them, the eddy current resonance has a negative impact on the EMW absorption behavior of an absorber since it prevents the EMW from coming into the absorber. We introduce C0 value to evaluate the effect of eddy current resonance on the magnetic loss based on the 17 ACS Paragon Plus Environment


following equation: 3 = $$ $ C C

(10)

The criterion of skin effect suggests that the magnetic loss is only contributed to the eddy current loss if the C0 values remain a constant with the frequency. However, the calculated C0 value of CFe-1000 varies with frequency as shown in Figure S12, indicating that the natural resonance and exchange resonance may be the main contributors to the magnetic loss of CFe-1000. The dielectric-loss tangent ( tan L> = $$ / $ ) and the magnetic loss tangent

(tan LM = $$ /$) are usually used to estimate the loss ability of the microwave. The

dielectric loss properties of the three as-prepared absorbers are evaluated by the loss tangent as shown in Figure 6A. It is obvious that CFe-1000 possesses the largest tan L> value of 0.89 at 9.68 GHz, indicating its largest dielectric loss and highest capacity of converting the EMW to the energy in other forms. As shown in Figure 6B, CFe-1000 also possesses the highest tan LM value of 0.12 at 16.72 GHz. Besides, the

tan LM value is lower than the tan L> value, implying that the dielectric loss which mainly comes from the dipolar and multiple interfacial polarizations plays a dominant role in the attenuation of electromagnetic energy.

Figure 6. Frequency dependences of tan L> (A) and tan LM (B) of CFe-400, CFe-700, and CFe-1000, respectively. On the basis of the transmission line theory, the input impedance of the EMW incident at the interface is Zin as below: 18 ACS Paragon Plus Environment

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M

= N O tan ℎ = >O

PQ R

√ ∙ B

(11)

where Z0 is the impedance of air, d is the thickness of the single-layer absorber, and c is the velocity of light. As we know, 1 of Zin/Z0 value implies no EMW reflection happening on the air-absorbent interfaces, which is supposed as the achievement of well-matched impedance for the EMW absorber. According to Equation (11), the Zin/Z0 values of CFe-400, CFe-700 and CFe-1000 are calculated and displayed in Figure 7A, at a representative thickness of 3.95 mm. Among the three absorbers, CFe-1000 shows the optimal impedance matching, which implies that the incident EMW can enter CFe-1000 with minimum reflection. This explains why the minimum RL of CFe-1000 is obtained at the thickness of 3.95 mm.

Figure 7. (A) Plots of / vs. frequency for all three as-prepared absorbers, CFe-400, CFe-700, and CFe-1000, respectively. (B) Plots of a vs. frequency for the three as-prepared absorbers, CFe-400, CFe-700, and CFe-1000. Respectively. In addition, the attenuation constant α is supposed to evaluate the attenuation abilities of the samples, which can be calculated according to the following formula: α=

√PQ R

∗ N $$ $$ − $ $ +

C Composites from Waste Wood with

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