Synthesis of Magnetic Wood with Excellent and Tunable

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Synthesis of magnetic wood with excellent and tunable electromagnetic wave absorbing properties by a facile vacuum/pressure impregnation method Zhichao Lou, He Han, Ming Zhou, Jingquan Han, Jiabin Cai, Caoxing Huang, Jing Zou, Xiaoyan Zhou, Hongjie Zhou, and Zhaobin Sun ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03332 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Sustainable Chemistry & Engineering

1

Synthesis of magnetic wood with excellent and tunable

2

electromagnetic wave absorbing properties by a facile

3

vacuum/pressure impregnation method

4

Zhichao Lou1,2*, He Han1, Ming Zhou1, Jingquan Han1, Jiabin Cai1, Caoxing Huang3,

5

Jing Zou1, Xiaoyan Zhou1, Hongjie Zhou4, Zhaobin Sun5 1

6 7 8 9 10 11 12 13 14 15 16

College of Materials Science and Engineering, Nanjing Forestry University, Longpan Road #159, 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, 1 Sipailou #2, Nanjing 210096, China 3Co-Innovation Center for Efficient Processing and Utilization of Forest Products, Nanjing Forestry University, Longpan Road 159#, Nanjing 210037, China 4 JM Arts & Crafts Co., Ltd., Xingye Road 68#, Fuyang, Anhui 236000, China 5 College of Forestry, Hebei Agricultural University, Yuling Temple Street 289#, Baoding, Hebei 071000, China

17

*To whom correspondence should be addressed.

18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Zhichao Lou, Email: [email protected]

40

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1

Abstract

2

Herein, magnetic wood was successfully prepared by in-situ synthesizing Fe3O4 in wood,

3

through co-precipitation chemical interactions. A facile impregnation method, vacuum

4

impregnation followed by pressure impregnation, was introduced to transport the

5

adequate amount of ferric salt precursor and to further shorten the required production

6

cycle. It was demonstrated that the obtained products exhibited out-standing microwave

7

absorbing properties. The best electromagnetic interference (EMI) absorbing properties

8

could reach -64.26 dB at 14.36 GHz with the matching thickness of only 2.25 mm and

9

broad absorbing bandwidth (|| > 10 dB) of 5.20 GHz covering 12.80-18.00 GHz. The

10

subsequent thoroughly investigations proved that this good shielding property was due to

11

the distinctive self-assembling morphology of Fe3O4 formed in the inner surface of the

12

lumen walls in wood, which permitted optimal impedance matching, strongest dielectric

13

loss, optimal magnetic loss, and interconnected conductive network for electron hopping

14

and migrating. This synthetic process for magnetic wood is quite facile, and the resulted

15

EMI absorbing properties is tunable by the concentrations of the iron precursor solutions

16

and the thickness values. This kind of synthetic magnetic wood can be potentially used as

17

lightweight, flexible and strong absorbing performance shielding materials for

18

construction, furniture, decoration and packing.

19 20

Keywords: Electromagnetic wave absorbing, Magnetic wood, Vacuum/pressure

21

impregnation, Interface polarization

22 23 24 25 26 27 28 29 30 31


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1

Introduction

2

With the rapid development of wireless communication and the rapid growth of the

3

wireless electronic equipment applications, such as LAN, mobile phones and family

4

robots, electromagnetic (EM) radiation and interference have caused serious EM

5

pollution with great threats to the health of human beings and information safety.1–5 Thus,

6

the materials for EM interference (EMI) shielding and absorbing are of great interest over

7

the past years. Among them, Fe3O4, as a typical magnetodielectric material with both

8

magnetic loss and dielectric loss, is one of the most attractive microwave absorbing

9

materials.6,7 However, practical applications of most Fe3O4-involved materials are

10

restricted by their narrow absorbing bandwidth, large thickness and high density.8,9

11

Manufacturing Fe3O4 or Fe3O4 composites with different geometries is proved to be a

12

good method to improve the microwave absorbing performance of these Fe3O4 based

13

samples10–13, but the corresponding preparing processes are all with organic solvents,

14

templates, surfactants, or other harsh reaction conditions such as high temperature. Thus,

15

designing an environment-friendly method to manufacture Fe3O4-involved EMI

16

absorbing materials with light-weight, thin-thickness and strong attenuation, is urgent

17

demand.

18

An efficient method is to combine the magnetic materials with light weight dielectric

19

materials, taking advantage of the synergistic effect between the two components.6,8,14

20

Wood as a renewable material has been widely used for construction, furniture, and

21

decoration. Oka group firstly proposed magnetic wood which is basically the

22

combination of wood with a magnetic fluid or powder.15,16 This magnetic wood has been

23

proved to possess strong magnetic characteristics and a wave-absorbing function.17–19 In

24

addition, this magnetic wood also offers a wood texture, low specific gravity, and is very

25

easy to process, which is expected as an indoor wave absorber to extend the wood supply,

26

improve the value of wooden products and preserve natural resources from over-

27

exploitation.20 Among the reported three types of magnetic woods (impregnated, powder

28

and coating types), the impregnated magnetic wood is proved to be with the best wood

29

texture.21 However, these studies of manufacturing magnetic wood were performed by

30

impregnating the prepared magnetic fluid into the wood, which cause the difficulties in



1

ensuring a sufficient amount of the magnetic materials as well as their even distribution

2

in the wood.

3

Based on the better characteristics of ferric irons than synthesized Fe3O4 magnetic

4

nanoparticles, such as smaller size and better permeability in wood, some groups

5

impregnated the pretreated wood in the mixed solution of Fe3+ and Fe2+ under

6

atmospheric pressure, followed by the impregnation of ammonia solution.22 By doing this,

7

Fe3O4 nanoparticles were synthesized via in-situ co-precipitation chemical reaction in the

8

wood, manufacturing the magnetic wood. Furthermore, Youming Dong et al. introduced

9

the processing method of vacuum impregnation followed by atmospheric impregnation

10

instead of direct atmospheric impregnation, successfully obtaining magnetic wood and

11

greatly shortening the time required for the iron precursor solution impregnation.23

12

Although these attempts mentioned above successfully obtained magnetic wood, the

13

products are still hardly of out-standing EMI absorbing properties due to the restrictions

14

of the amount of magnetic nanoparticles in wood. And the microscopic principle

15

especially the inherent relationship between the distribution characteristics of the

16

magnetic particles and the resulted EMI absorbing features is not thoroughly studied,

17

which is very important for the choice of the industrial procedure and the determination

18

of related parameters, to extend the application of the functional magnetic wood.

19

Herein, magnetic wood was obtained by in-situ synthesizing Fe3O4 in the inner surface

20

of the lumen walls in wood, through co-precipitation chemical interactions. A novel

21

impregnation method, vacuum impregnation followed by pressure impregnation, was

22

introduced to transport the adequate amount of ferric salt precursor and to further shorten

23

the required production cycle. SEM and AFM were used to observe the morphologies of

24

the slice surfaces and the distribution characteristics of the magnetic particles. FT-IR,

25

XRD, EDS, and XPS were used to investigate the composition of the synthesized

26

samples. VSM and vector network analyzer were conducted to demonstrate the magnetic

27

and electromagnetic parameters of the synthesized samples. As a comparison, the

28

specimens obtained by atmospheric impregnation were also tested here. The obtained

29

magnetic wood in our work is proved to be of excellent and tunable shielding property,

30

which is due to the distinctive self-assembling morphology of Fe3O4 formed in the inner

31

surface of the lumen walls in wood, permitting optimal impedance matching, strongest


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1

dielectric loss, optimal magnetic loss, and interconnected conductive network for electron

2

hopping and migrating.

3

Materials and Methods

4

Materials. FeCl3•6H2O, FeCl2•4H2O and NH3•H2O (25%) were purchased from Aldrich

5

(Germany). All aqueous solutions were prepared with de-ionized water. The straight-

6

grained sapwood portions of fast-growing poplar wood (hardwood) with ~151% initial

7

moisture content were obtained from Fuyang, Anhui province in China.

8

Preparation of magnetic wood. First of all, the moisture content of the obtained

9

wood was artificially controlled at ~10% in an air-dry oven. Then the wood was cut into

10

80 mm X 80 mm in longitudinal section and 9 mm in thickness. The end-matched

11

specimens were selected and heated in distilled water several times until the water turned

12

clear. Then the dried specimens were extracted with a solvent mixture of alcohol/toluene

13

(1:2, v/v) over night to remove the wood extractive compounds such as gums, tropolones,

14

fats and fatty acids, and to improve the connectivity among pores and cellular affinity for

15

the precursor. The specimens were dried in an air-dry oven at 105°C until the moisture

16

content was ~10% again, and divided into 4 groups: A, B, C and D.

17

Magnetic wood was fabricated by chemical coprecipitation but via two different

18

immersion methods, atmospheric impregnation and pressure impregnation. Mixture

19

solution of FeCl3•6H2O and FeCl2•4H2O (molar ratio of Fe3+:Fe2+=2:1) were dissolved

20

in distilled water to form iron precursor solutions with a concentration of 0.6 mol/L (P1)

21

and 1.2 mol/L (P2) ferric chloride, respectively. Group A was treated as control (Sample

22

A). Group B was impregnated in a beaker containing P1 iron solution for 48 h, and then

23

was immersed to the 25% ammonia solution for 24 h. After filtrated and washed several

24

times with distilled water until reaching neutral pH, the specimens were then oven-dried

25

at 65°C for 24 h and labeled as Sample B.

26

Groups C and D were impregnated in P1 and P2 iron solution respectively, under

27

vacuum (0.07 MPa) for 2.5 h, followed by pressure impregnation (0.8 MPa) for 2.5 h.

28

And then the specimens were immersed to the 25% ammonia solution under vacuum

29

(0.07 MPa) for 2.5 h, followed by pressure impregnation (0.8 MPa) for 2.5 h. After

30

filtrated and washed several times with distilled water until reaching neutral pH, the



1

specimens were then oven-dried at 65°C for 24 h and labeled as Sample C and D,

2

respectively.

3

Characterization and Property Measurements of Synthesized Materials.

4

Details are shown in the Supporting Information.

5 6

Results and Discussion

7

Characterization of Magnetic Wood. Figure 1 shows the macro-structure of Sample A

8

(A(1)), Sample B (B(1)), Sample C (C(1)) and Sample D (D(1)). It was obvious that the

9

magnetic wood was dark brown compared with the untreated wood. In order to accurately

10

determine the difference of the samples in color, we performed a pilot study with a

11

colorimeter. The results are shown in Table S1. From Table S1, we may see that the L*

12

value of the untreated wood was much larger than that of the magnetic wood, indicating

13

that the samples became darker after the magnetization treatment. Accordingly, the

14

dramatic increase of a* and b* values indicated that the color of the samples shift to red

15

and yellow after the treatment. The color changes can be attributed to the partial

16

decomposition of the wood components and the presence of Fe3O4 nanoparticles that

17

might be generated during the treatment. Besides, the L* values decreasing from 48.33 to

18

40.99 may implied the increase of the amount of the Fe3O4 nanoparticles in the treated

19

samples with the increasing immersion time. Besides, the densities of the obtained

20

magnetic wood were measured and shown in Table S2. The major reason for the increase

21

of the density was the increased amount of Fe3O4. Compared with the other conventional

22

electromagnetic wave absorbing materials, we may conclude that our obtained materials

23

are lightweight.

24

Figure 1 also shows the SEM observations of the timber cross sections (2) and radial

25

sections (3) of the untreated wood (Figure 1A) and the magnetic wood (Figure 1B-D),

26

respectively. From the SEM images of the timber cross sections, no differences were

27

observed in the morphologies between the untreated wood and treated wood, indicating

28

there is no collapse of the wood natural structure. This phenomenon is in accordance with

29

the findings by the previous report24. On the contrary, the morphologies of the untreated

30

wood and treated wood were dramatically different in radial sections. In Figure 1A(3),

31

the lumen walls of untreated wood were obviously smooth, while clusters of particles


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1

were observed to attach to the inner surface of the lumen walls of the magnetization

2

treated wood in Figure 1B(3) to D(3). Besides, the samples showed vastly different

3

degrees of particle attachments, with the pressure immersion and higher iron

4

concentration inducing a higher degree of surface immobilized particles. Confirmed by

5

EDS mapping images in Figure 1A(4) to D(4), the distribution densities of Fe signals

6

were in accordance with those of the attached particles, implying that these particles

7

might be Fe3O4 particles adhering to the cell lumen walls. The detailed percentage of

8

each element (C, N, O, Cl and Fe) in the magnetic wood was shown in Table S3 and

9

Figure S2. From the EDS results, the contents of Fe and O both increased while the

10

contents of C decreased from Sample B to Sample D. This is attributed to two reasons:

11

the dissolution of lignin and hemicellulose during the ammonia immersion process and

12

the formation of Fe3O4 nanoparticles during the crystallization process. The contents of

13

the biomass materials in the treated/untreated wood were analyzed and collected in Table

14

S4, which are consistent with the SEM-EDS results.

15

The composition details of the adhering clusters were carefully characterized by

16

atomic force microscopy (AFM) (Figure 1E and 1F). From the AFM results, we may see

17

that the clusters were constituted by individual nano-size particles. According to the

18

height values obtained by AFM, the average dimension of the particles was 13.21±0.16

19

nm (Figure S3). The height values of the nanoparticle films were observed to increase

20

from 27.10 nm to 95.71 nm, indicating that multi-layered film were formed by these

21

particles on the inner surface of the lumen walls. From these characterizations, the in-situ

22

synthesized nanoparticles could be seen to form discrete aggregates when the iron

23

precursor was limit (Figure 1B(3)). By introducing the pressure immersion method, more

24

ferric and ferrous ions penetrated into the cell lumens, and the aggregates increased and

25

formed films (Figure 1C(3) and Figure 1E). And with increasing the iron concentration in

26

the impregnating solution, the aggregates kept increasing and formed multi-layered films

27

(Figure 1D(3) and Figure 1F).



1 2

Figure 1. Macro- and micro-structure of sample A (A), sample B (B), sample C (C) and

3

sample D (D). A(2)-D(2) are SEM images of the transections of the specimens. A(3)-D(3)

4

are SEM images of the longitudinal sections of the specimens, and A(4)-D(4) are the

5

corresponding EDS images of the Fe element distributions. (E) and (F) are AFM images

6

of sample C and D, respectively.

7 8

Figure 2A shows the XRD patterns of the untreated wood and the resultant magnetic

9

wood (Sample D). Both of the samples display two primary diffraction peaks at 16.0° and

10

22.5°, which can be assigned to the (100) and (002) planes of cellulose, respectively25.

11

The magnetic wood shows the additional diffraction peaks at 2θ=30.0, 35.3, 43.0, 53.4,

12

56.9 and 62.5, corresponding to the (220), (311), (400), (422), (511), and (440) planes of

13

Fe3O4 in a cubic phase, respectively.26 According to the Scherrer equation with the

14

corresponding parameters from the most intense peak (311), the average size of Fe3O4

15

nanoparticles is 13.4 nm, which is consistent to the AFM results in Figure S2 indicating

16

that the particles are nano-size. Besides, there were no obvious diffraction peaks from

17

FeCl2 or FeCl3 for the magnetic wood, possibly because the contents of these species

18

were very small and were below the detection limit.

19

FT-IR measurements could hardly distinguish the magnetic wood from the untreated

20

wood as the characteristic absorption bands at the wavenumbers lower than 800 cm-1,

21

which result from the v1 and v2 band of the Fe-O bond of magnetite and demonstrate the


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1

presence of Fe3O4 nanoparticles27,28, were overlapped by the broaden peak of the C-H

2

characteristic band from the wood at 610 cm-1 (Figure 2B). However, in the case of

3

magnetic wood, the intensities of the absorption bands from 896.76 cm-1 to 1509.99 cm-1,

4

which were attributed to the methylene and methyl groups in the saturated hydrocarbons

5

of the natural wood, obviously decreased while the O-H absorption band at 3448.05 cm-1

6

inversely increased. This was attributed to the fact that during the magnetic treatment, the

7

acid condition of the ferric and ferrous chloride solution and the subsequent base

8

condition of the aqueous ammonia solution lead to the partial hydrolysis of the C-O

9

groups, inducing large amounts of hydroxyl groups.

10

The magnetic properties of the prepared magnetic wood were measured by VSM

11

(Figure 2C). As could be seen, the typical characteristics of magnetic behavior are

12

observed. It is obvious that the saturation magnetization values (MS) were 2.26, 10.27 and

13

30.46 emu/g for Sample B, C and D, respectively. Their magnetic performances are

14

obviously in Figure S4. The curves also show that all the samples exhibit a clear

15

hysteretic behavior. The coercivity (HC) changes little from Sample B to C, while it

16

increases a lot for Sample D. According to Stoner-Wohlfarth theory, Ms is related to the

17

magnetocrystalline anisotropy (Keff) of the specimen:

18

= /2 (1)

19

where µ0 is a constant of permeability.29 Thus, the increase of MS and HC values among

20

the specimens should be interpreted as an increase in Keff with the increasing of Fe3O4

21

attachment. Surface anisotropy (Ks), which is another relevant parameter to coercivity30

22

and contributes to the effective Keff :

23

= + (6/) (2)

24

where Kb is the bulk anisotropy and is probably not changed in this work. d is the

25

diameter of the Fe3O4 nanoparticles. In this equation, we may see that Ks is positively

26

correlated with Keff, indicating that Ks also increase with the increasing of the Fe3O4

27

attachment. However, Ks is supposed to be maximum for the free surfaces and is reduced

28

by solid coverage.31 This unusual increasing of Ks should be attributed to the fact that

29

more Fe3O4 nanoparticles aggregated in the wood cell inducing a rougher inner surface

30

and further affecting the contribution of the surface anisotropy (Ks) (Figure 1E and 1F).

31

And therefore, the coercivity changes a lot for Sample D. In fact, this special anisotropic



1

internal surface also determines the electromagnetic wave absorbing properties of the

2

prepared magnetic wood in the following part.

3 4

Figure 2. XRD patterns (A) and FT-IR spectra (B) of the untreated wood and magnetic

5

wood (Sample D). (C) VSM curves of Sample B (black), Sample C (blue) and Sample D

6

(red). (D) Survey XPS spectrums of Sample A (black), Sample B (green), Sample C (blue)

7

and Sample D (red).

8 9

The possible peak associated to the (210) plane of λ-Fe2O3 at around 24° is covered by

10

the broaden peak of cellulose, and Fe3O4 and λ-Fe2O3 have many diffraction peaks in

11

common, which makes difficult to exclude the presence of λ-Fe2O3 in the resultant

12

magnetic wood only relying on the XRD data. The presence of λ-Fe2O3 is excluded

13

according to the obtained results of XPS measurements because core electron lines of

14

ferrous and ferric ions can both be detected and are distinguishable from each other in

15

XPS.2,32 It is obvious that the surface of natural wood contained mainly C and O, while

16

the surfaces of magnetic wood contained C, O, Fe and trace amount of Cl (Figure 2D).

17

To determine the oxidation states of the elements, high-resolution XPS spectra of Fe 2p,

18

O 1s, Cl 2p and C 1s are highlighted in Figure 3. As shown in Figure 3A, the binding

19

energies at ~711 eV and ~725 eV are the characteristic doublet from Fe 2p3/2 and Fe


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1

2p1/2 core-level electrons, respectively. The absence of the shoulder peak between Fe

2

2p3/2 and Fe 2p1/2, which is a major characteristic for λ-Fe2O3,33 excluding the existence

3

of λ-Fe2O3. Figure 3B displays the high-resolution XPS spectra of O 1s. Two

4

characteristic peaks of O 1s were observed for magnetic wood. The peak at around

5

~532.5 eV was attributed to the C 1s for carbohydrate, which was also observed for the

6

natural wood. The peak at around ~530.5 eV was attributed to Fe3O4, which was absent

7

in the natural wood. As shown in Figure 3C, only one characteristic peak of Cl 2p was

8

observed at ~198.3 eV. This peak confirmed the existence of FeCl2 or FeCl3. Combining

9

with the SEM-EDS results in Table S3 and Figure S2, the incompletely-reacted

10

ferric/ferrous chloride was in trace amount. To be mentioned, there was no other peak

11

observed in Figure 3C, indicating the absence of NH4Cl which was the supposed product

12

of in-situ coprecipitation reaction. Based on the characterization results, the reaction

13

equation was confirmed as below:

14 15

∆

2FeC ∙ 6 ! + "#$ ∙ 4 ! + 8' ∙ !

→ "# !* ↓ +8' ↑ +8$ ↑ +20 ! ↑

(3)

16 17

Figure 3. High-resolution XPS spectra of Sample A (black), Sample B (green), Sample C

18

(blue) and Sample D (red): (A) Fe 2p, (B) O 1s, (C) Cl 2p and (D) C 1s.



Page 12 of 23

1 2

EMI Absorbing Performances of Magnetic Wood. The electromagnetic wave

3

absorbing performance of the specimens can be evaluated by the reflection loss (RL),

4

which can be defined with the following equations on the basis of transmission line

5

theory:34–37

6 7

-./ = - ( 0 /10 )2/ tan ℎ[8(29:/;)( 0 10 )2/ ] = 20 log2 |(-./ − - )/(-./ + - )|

(4)

(5)

8

where Z0 is the characteristic impedance of free space, Zin is the input impedance of the

9

absorber, εr is the relative complex permittivity ( 10 = 1 A − 81 AA ), µr is the relative

10

complex permeability ( 0 = A − 8 AA ), f is the frequency of the microwaves, d is the

11

thickness of the specimen, and c is the velocity of light. Figure 4 shows the calculated

12

reflection-loss properties of the specimens with thickness from 1.0 mm to 5.0 mm in the

13

frequency range of 2-18 GHz. As shown in Figure 4A-C, the immersion method has

14

dramatic effects on the electromagnetic wave absorbing capacities of the specimens.

15

When the thickness exceeds 1.25 mm, Sample B exhibit weaker absorbing capacities

16

with || > 20 dB only for partial thickness values (1.75 mm and 1.80 mm), while

17

Sample C and D exhibit strongest adsorption capacities with || > 20 dB for most of

18

the thickness values. As we know, the || > 10 dB means 90% attenuation of

19

electromagnetic wave, which has been an effective RL values.38 And an ideal EMW

20

absorber is required to have not only a strong absorbing, but also a wide absorbing

21

bandwidth where corresponding || is larger than 10 dB. It is obvious that the ||

22

values of the three samples are all larger than 10 dB with the thickness from 1.50 mm to

23

5.00 mm. Besides, with the introducing of pressure impregnation and the increasing of

24

the concentration of iron precursor solution, the maximum RL value increases, indicating

25

the improvement of the absorbing properties. Sample B which was obtained through

26

atmospheric impregnation possesses the maximum RL of -22.89 dB at 15.28 GHz, while

27

the maximum RL for Sample A was only -2.01 dB at 16.88 GHz (Figure S5). As the

28

immersion method changed to pressure impregnation, the maximum RL reaches -58.65

29

dB at 5.92 GHz (Sample C). When increasing the iron concentration of the precursor

30

solution, the maximum RL for the obtained specimens (Sample D) is up to -64.26 dB at

31

14.36 GHz, with the matching thickness as thin as 2.25 mm. This electromagnetic


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1

absorbing property of Sample D is excellent compared with the obtained magnetic wood

2

by Oka group (Maxlaminated-type=45.18 dB at 2.62 GHz).18,19 Moreover, the effective

3

absorbing bandwidths (|| > 10 dB) of all the specimens (Sample B to D) are wide at

4

small thickness. For example, a broad absorbing bandwidth of 3.48 GHz covering 14.20-

5

17.68 GHz can be achieved for Sample B with the thickness of 1.75 mm. Broad

6

absorbing bandwidths of 3.80 GHz covering 14.16-17.96 GHz and 5.20 GHz covering

7

12.80-18.00 GHz are achieved for Sample C and D with the same thickness of 1.50 mm,

8

respectively. And their corresponding || values are 13.48 dB, 35.79 dB and 26.86 dB,

9

respectively.

10

In addition, for each thickness, the RL peaks show a shift from high frequencies to

11

lower frequencies for the obtained magnetic wood (Figure S6). This is supposed to be

12

attributed to the fact that, the higher coercivity value of the specimen (as shown in Figure

13

2C) induces a higher frequency resonance, and then results in the shift of the RL peak at

14

the same thickness.3,39 Moreover the matching thickness (tm) vs. the peak frequency were

15

directly extracted from the RL curves in Figure 4A-C and shown in Figure 4D-F. It is

16

obvious that either Sample B, C or D, their matching frequency (fm) shifts to the low-

17

frequency region with increasing thickness. This can be explained by the following 1/4

18

wavelength cancelation law:

19

BC = D;/4:C (10 0 )2/ (n=1,3,5,…)40,41

(6)

20

when tm and fm satisfy this equation, the reflected electromagnetic microwaves both from

21

the air-absorber interface and the absorber-conductive background interface are out of

22

phase by 180°, inducing an extinction of them on the air-absorber interface, and then

23

resulting in a maximum RL value.



1 2

Figure 4. Frequency dependences of reflection losses of magnetic wood: (A) Sample B,

3

(B) Sample C, and (C) Sample D. (D-F) Dependence of matching thickness (tm) on

4

matching frequency (fm) of the magnetic wood.

5 6

Based on the transmission line theory mentioned above, the electromagnetic wave

7

absorbing properties of the specimens are dependent on the relative complex permittivity

8

and the relative complex permeability, where ε’ and µ’ represent the storage capability of

9

the electric and magnetic energy, and ε” and µ” stand for the loss capability of the

10

electric and magnetic energy.42,43 Only when εr value is close to µr value, the intrinsic

11

impedance becomes close to the free space impedance and the reflection of

12

electromagnetic waves at the surface of the absorber is reduced, resulting a perfect

13

electromagnetic wave absorbing property. Figure 5A shows the frequency dependences

14

of complex permittivity of the prepared magnetic wood. As observed in Figure 5A,

15

Sample D exhibits a highest ε’ value (ranging from 13.23 to 10.62) and a largest ε” value

16

(ranging from 11.6 to 4.09), inducing not only the good impedance matching behavior

17

but also the strong ability for the electromagnetic attenuation. Besides, the ε’ of the

18

magnetic wood gradually decreases with the increasing frequency, which is attributed to

19

the fact that the lagging of polarization increases with respect to the electric-field change

20

at high frequency.44,45 Compared with the variation trend of ε’, the ε” values intensively

21

fluctuate following the frequency change. Several resonant peaks are observed in the ε”

22

curves, indicating that multiple polarization relaxation processes occur in the composites

23

under alternating electromagnetic field. Here, the XPS results shows that the inner


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1

surface of the lumen walls of the magnetic wood is functioned with oxygen-containing

2

chemical bonds (Figure 3B), these function groups can produce electronic dipolar

3

polarization. Meanwhile, the SEM and AFM results (Figure 1) show previously that

4

Fe3O4 nanoparticles aggregate on the inner surface of the lumen walls and their amounts

5

increase from Sample B to Sample D. The interfacial polarization comes mainly from the

6

interface between Fe3O4 nanoparticles and the carbohydrates from the cell walls. With

7

the aggregation of Fe3O4 nanoparticles, the corresponding interface increases and then

8

further induces the increasing of the interfacial polarization. Therefore, the increasing of

9

the real permittivity should be attributed to the addition of dipolar polarization and

10

interfacial polarization.

11

In addition, the dielectric loss tangent (tan EF = 1 AA /1 A ) is one of the two important

12

factors evaluating the microwave absorbing properties for the absorber, the other is the

13

magnetic loss tangent (tan EG = AA / A ). The dielectric-loss properties of the magnetic

14

wood are evaluated by the loss tangent as shown in Figure 6A. It is obvious that the

15

tan EF value increases from Sample B to Sample D. This variation trend indicates that the

16

Sample D owns the largest dielectric loss, and a highest capacity of converting the

17

electromagnetic waves to the energy in other forms, which is very important to the

18

electromagnetic wave absorbing properties. According to the SEM and AFM results in

19

Figure 1, the higher loss tangent in terms of permittivity is attributed to the larger

20

quantity of Fe3O4 nanoparticles as the conductive component in sample, which fills the

21

wood holes and store up on the cell wall. And the conduits and bordered pits form an

22

interconnected conductive network spreading over the magnetic wood for electron

23

hopping and migrating.

24



1

Figure 5. Frequency dependences of (A) real parts (ε’) and imaginary parts (ε’’) of

2

complex permittivities, and (B) real parts (µ’) and imaginary parts (µ’’) of complex

3

permeabilities of the magnetic wood: Sample B (black), Sample C (blue), and (Sample D

4

(red).

5 6

Figure 5B shows the magnetic loss abilities of the three kinds of magnetic wood. At

7

2-18 GHz, the µ’ and µ” values gradually increases from Sample B to Sample D.

8

According to the following equations:46

A = 1 + (/) cos J

9

AA = 1 + (/) sin J

10

(7) (8)

11

where M is the magnetization, H is the intensity of the external magnetic field, and σ is

12

the phase lag angle, the increased trend of the permeability is attributed to the increasing

13

of saturation magnetization of the magnetic wood. As we know, the bigger µ’ and µ”

14

values should induce a better impedance-matching behavior. Besides, according to the

15

following equations:34

16 17

29:0 = LM

(9)

M = 4NO N/3

(10)

18

where fr is the natural resonance frequency, r is gyromagnetic ratio, and Ha is the

19

anisotropy energy, a high HC value induces the shift of fr from low frequency to high

20

frequency. Hence, the resonant peaks can be obviously observed in the µ” curve of

21

Sample D. In general, the magnetic loss of an absorber in the GHz range mainly derives

22

from natural resonance, exchange resonance and eddy currents.47 The effect of the eddy

23

currents on the magnetic loss is evaluated by analyzing the variation trend of the C0

24

values ($ = AA ( A )Q : Q2 ) following the frequency from 2 GHz to 18 GHz. It is known

25

that if the C0 value keeps constant with the change of frequency, the eddy currents will be

26

the only reason for the magnetic loss of the absorber. However, as shown in Figure 7A,

27

the C0 values vary with frequency, indicating that the eddy currents effect could be

28

excluded, and the natural resonance and exchange resonance may be the main

29

contributors to the magnetic loss. The magnetic loss tangent is also investigated in Figure

30

6B. As shown in Figure 6B, Sample D possesses the highest tan EG value of 0.38 at 11.60

31

GHz. This value is slightly lower than the tan EF values, suggesting that the dielectric


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1

loss which mainly comes from the interfacial polarization plays a dominant role in the

2

attenuation of electromagnetic energy.

3 4

Figure 6. Frequency dependences of tan EF and tan EG of magnetic wood: Sample B

5

(black), Sample C (blue), and Sample D (red).

6 7

To be concluded, Sample D, which was obtained through pressure impregnation

8

method and with higher concentration iron precursor, possesses the optimal impedance

9

matching. And the electromagnetic energy can be converted to heat energy in the

10

magnetic wood, through the strongest dielectric loss caused by the interfacial polarization

11

between the Fe3O4 nanoparticles and the cell walls, the electron hopping and migrating,

12

and the optimal magnetic loss caused by natural resonance and exchange resonance of

13

Fe3O4 nanoparticles (Figure S7). Hence, the attenuation of the electromagnetic wave of

14

these three specimens is evaluated by the following equation:39,48

15

R=

√ T V( AA 1 AA U

− A 1 A ) + W( AA 1 AA − A 1 A ) + ( A 1 AA + AA 1 A )

(11)

16

As shown in Figure 7B, all three kinds of magnetic wood possess strong electromagnetic

17

wave attenuation abilities at high frequency. And Sample D has the biggest a value in the

18

whole frequency range, indicating its largest attenuation ability together with the optimal

19

impedance matching induces the strongest electromagnetic wave absorbing performance.



1 2

Figure 7. The C0 curves (A) and the attenuation constants (B) of magnetic wood: Sample

3

B (black), Sample C (blue), and Sample D (red).

4 5

Conclusions

6

In this study, Fe3O4 NPs were prepared by in-situ co-precipitation on the inner surface of

7

the lumen walls of wood, manufacturing magnetic wood. A facile impregnation method,

8

vacuum impregnation followed by pressure impregnation, was introduced to transport the

9

ferric salt precursor into wood, followed by the transportation of ammonia. The EMI

10

absorbing properties of the obtained magnetic wood reached -64.26 dB at 14.36 GHz

11

with the matching thickness of only 2.25 mm and broad absorbing bandwidthof 5.20

12

GHz covering 12.80-18.00 GHz. The subsequent thoroughly investigations proved that

13

this out-standing EMI absorbing property was due to the optimal impedance matching,

14

the strongest dielectric loss, the optimal magnetic loss and the interconnected conductive

15

network for electron hopping and migrating fabricated by the assembling Fe3O4 NPs. The

16

resulted EMI absorbing properties of the magnetic wood obtained by the method here, are

17

tunable by the iron precursor concentrations and the thickness values. We hope that this

18

vacuum/pressure impregnation method for manufacturing magnetic wood could be used

19

for the re-usage of wood shavings, fibers and other wood wastes.

20

Associated Content

21

The Supporting Information is available free of charge in the online version:

22

Details of the characterization, schematic diagram of the EMI absorbing test,

23

colorimeter results, atomic constitution of the primary elements, fm values for each

24

thickness, and scheme of primary EMI absorbing processes in the magnetic wood.


Page 18 of 23

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1

Acknowledgment

2

Financial support from the National Natural Science Foundation of China (No. 61601227,

3

31770609), Nature Science Foundation of Jiangsu Province (BK20160939), Natural

4

Science Foundation of the Jiangsu Higher Education Institutions of China (16KJB180010,

5

17KJB220007), Research and Demonstration of Green Integrated Technology of Fast-

6

growing poplar (1704a07020076), the Qing Lan Project and Priority Academic Program

7

Development of Jiangsu Higher Education Institutions (PAPD).

8

Reference

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For Table of Contents Use Only


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1 2

Synopsis: Magnetic wood with excellent and tunable electromagnetic wave absorption

3

properties were synthesized by a facile vacuum/pressure impregnation method. (18 words)


Synthesis of Magnetic Wood with Excellent and Tunable

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