1 Synthesis, Electromagnetic and Microwave Absorption Properties of

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Synthesis, Electromagnetic and Microwave Absorption Properties of Monodispersive Fe3O4/#-Fe2O3 Composites Jindou Ji, Yue Huang, Jinhua Yin, Xiuchen Zhao, Xingwang Cheng, Shuli He, Xiang Li, Jun He, and Jiping Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00703 • Publication Date (Web): 06 Jul 2018 Downloaded from http://pubs.acs.org on July 7, 2018

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ACS Applied Nano Materials

Synthesis, Electromagnetic and Microwave Absorption Properties of Monodispersive Fe3O4/α-Fe2O3 Composites Jindou Jia,b, Yue Huanga, Jinhua Yinb, Xiuchen Zhaoa, Xingwang Chenga, Shuli Hec, Xiang Lia*, Jun Hed* and Jiping Liua a

School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China b

School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China c

d

Department of Physics, Capital Normal University, Beijing 100048, China

Division of Functional Materials, Central Iron and Steel Research Institute, Beijing 100081, China

ABSTRACT: Fe3O4/α-Fe2O3 composites synthesized by one step way. The composition and their performances modified by adjusting the concentration of Fe3+ (FeCl3·6H2O) in the precursor solution, effectively. Compared with single-phase Fe3O4 and Fe2O3, the effective microwave absorbing ability of Fe3O4/α-Fe2O3 composites are much widened in range of 1-18 GHz. , which could be stemmed from the enhanced dipolar polarization and interfacial polarization due to lattice dislocations at the interface of Fe3O4/α-Fe2O3. The minimum reflection loss(RL) of Fe3O4/α-Fe2O3 composites reaches about -43.1 dB at a thickness of 2.0 mm, with effective absorption band reaches 3.4 GHz (9.8-13.2 GHz). In the thickness of 1.5-3.2 mm, the width of the RL reaches 9.9 GHz (3.5-13.4 GHz). The results demonstrate that Fe3O4/α-Fe2O3 composites could be candidate to be used as absorbers with much widened the microwave absorption band. *

Corresponding authors. E-mail address: [email protected] (X. Li). E-mail address: [email protected] (J. He).

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KEYWORDS: Microwave absorption; Electromagnetic properties; Polyol method; Dipolar polarization; Interfacial polarization.


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1. INTRODUCTION With the advanced in microwave technology, electromagnetic (EM) protection and detection have caused great attention [1-6], and stimulate the development of novel EM wave absorbers with low thickness, light weight, high performance, and broad effective absorption band [2,7-9]. Iron oxide ferrites can be used as EM wave absorbers favor to restrain the skin effects in high frequency electromagnetic bands because of their high resistivity [10]. As a result, EM radiation can effectively enters into the absorbers and well absorbed. Moreover, the high dielectric properties, good permeability and higher saturated magnetization make iron oxide ferrites is a kind of an effective microwave absorber.[6,11-16] therefore, a great deal of studies have been done to design novel absorbers with advanced performances.[13,15-18] Hematite (α-Fe2O3) is an n-type semiconductor [19,20] with rhombic structure and weak ferromagnetism at room temperature [21,22]. Besides, Fe2O3 has a low real part of the permittivity (3-5), which is conducive to impedance matching.[23] Magnetite (Fe3O4) has an inverse spinel cubic structure, is an important magnetic absorber. In the structure of Fe3O4 (AB2O4), half of Fe3+ occupies the tetrahedral position (A position), and Fe2+ and the other half of Fe3+ locate at the octahedral position (B position). In the position of B, electronics can move quickly between Fe2+ and Fe3+ [24, 25], such a characristic contributes to a great microwave absorption performance of Fe3O4. Furthmore, as the frequency is high enough, Fe2+ will be easily polarized, and induces a high EM wave loss [13]. Besides, the natural resonance loss [2, 26, 27] and eddy current loss [17,26-28] of the Fe3O4 also contribute greatly to EM wave absorption.



However, the microwave absorption performance of single-phase iron oxide is relatively weak, such as the impedance mismatching in the high-frequency of Fe3O4 [29] and the poor magnetic loss of α-Fe2O3 [23]. These problems can be effectively solved by constructing composites, such as α-Fe2O3@CoFe2O4 [23], rGO/α-Fe2O3 [20], B(OH)3/α-Fe2O3-CMSs [30], Fe3O4@C [31], Fe3O4@SiO2 [26], Fe3O4@SnO2 [32] and RGO/Fe3O4/ZnO [33] et al. In this paper, one-step reduction hydrothermal method [34] was used to synthesize α-Fe2O3 and Fe3O4 composites and the excellent EM absorption performance has been reached with the smallest RL of -43.1 dB near 2.0 mm, and the effective bandwidth of absorbing is 3.4 GHz (9.8-13.2 GHz). 2. EXPERIMENTAL SECTION Synthesis of Fe3O4/α-Fe2O3 nanoparticles. A certain amount of FeCl3·6H2O and sodium citrate dihydrate (Na3C6H5O7·2H2O, 0.34 mmol) were dissolved in 2.69 mol/l ethylene glycol (MEG, C2H6O2), and at the room temperature, the mixture was stirred by a magnetic stirrer to form a steady and well-proportioned solution. Subsequently, diethylene glycol (DEG, C4H10O3) and sodium acetate trihydrate (C2H9NaO5, 29 mmol) were added to the solution, and deionized water was added to form 80 ml uniformly stable solution. Then, the solution was shifted into a Teflon-lined stainless-steel autoclave (100 mL capacity). The autoclave was sealed and kept at 200 °C for 16 hours and then descended to room temperature naturally. The products were washed several times with ethanol and deionized water,. Finally, the composite materials dried in oven under 70 °C for 3 hours. In the solvent thermal reaction process (Scheme 1 (a) and (b)), all chemicals are of commercial analytical grade with no further purification, and hydrothermal reactions are as following: [34] Fe 3+ + 3OH - = Fe(OH) 3


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Fe(OH)3 → FeOOH + H 2 O

2FeOOH → Fe2O3 + H 2 O

FeOOH + H 2 O + e- → Fe(OH)2 + OH −  2FeOOH + Fe(OH)2 → Fe3O 4 + 2H 2 O

In the solution, Fe3+ may undergo hydrolysis reaction to form a stable reddish brown colloidal Fe(OH)3, which decomposes into iron hydrate (FeOOH) in a high temperature environment. When the Fe3+ content is low, it will be reduced to Fe(OH)2 by MEG and DEG, and eventually Fe3O4 is formed. However, if the content of Fe3+ is high enough, FeOOH will decompose to Fe2O3. As a result, one-step synthesis of Fe3O4 and Fe2O3 composites achieved by Managed concentration of Fe3+ in the precursor solution. The details of sample preparation shows in Scheme 1. (a) Schematic Representation for Synthetic Route to Fe3O4/α-Fe2O3 composites; (b) mechanism of reaction. Characterizations. The composition and strcuture of the samples were examined by powder X-ray spectrometer (XRD, Smartlab(3), Rigaku, Japan) with Cu Kα radiation source (λ= 0.15406 nm, at operation voltage of 35 KV and current of 40 mA) over scanning range 10°—80°. The feature of surface of samples were characterized by highresolution transmission electron microscope (TEM, JEM-2100F, JEOL, Belgium) operating with 200 kV at room temperature. The magnetic performance of the composites was studied by vibration sample magnetometer (VSM, Versolab, Quantum Design, USA) with an external field of -20~20 KOe at room temperature. The complex permittivity/permeability of the samples were studied by vector network analyzer (VNA, N5230C, Agilent technologies, USA) in range of 1~18 GHz, and it, the test samples were prepared by uniformly mixing with paraffin wax with a mass ratio of 4:1. The the test samples formed to shape of a cylindrical ring with an inner and out diameters of 3.0 snd 7.0 mm , respectively. 3. RESULTS AND DISCUSSION



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The XRD patterns of Fe3O4 and Fe3O4/α-Fe2O3 particles synthesized under different concentration of Fe3+ displayed in Fig. 1. Fig. 1(a) is the details XRD peaks of sample that can be assigned to the cubic lattice crystal structure of Fe3O4 (JCPDS card No. 190629), respectively. Nevertheless, when the concentration of Fe3+ increases, more

and

more characteristic peaks are observed, which corresponds to the diffractions of the planes of α-Fe2O3 (JCPDS card No. 33-0664). therefore, the phase of sample A is Fe3O4, and increase in the concentration of Fe3+ in the solutions results in the content of Fe2O3 in the sample gradually increased and its diffraction peak gradually increased. But when concentration of Fe3+ is more than 0.1625 mol/l (sample E), it seems that the sample does not contain Fe3O4, which is due to Fe(OH)3 is almost completely decomposed into Fe2O3 during the high temperature hydrothermal reaction. No other substances obviously observed except for Fe3O4 and Fe2O3 in all samples. To better confirm the compositional changes of Fe3O4 and Fe2O3 in the samples, the XRD patterns near 35° were graphed in Fig. 1(b). Fig. 1(b) reveals that as the Fe3+ increases, the intensity of the peak at 35.42° gradually decreases and eventually disappears; in contrast, the diffraction intensity at 35.60° of Fe2O3 (110) plane continues to increase, and the increase of the Fe2O3 also be detected by the analysis of XPS as indicated in Fig S1-S3. Fig.S1is the global XPS of samples, Fig.S2 and Fig.S3are the XPS of O and Fe element with the fine fitting. the fitted result in Fig.S2 indicates that the O comes from Fe-O bond and hydroxyl and H2O , the Fe chemical states are of Fe2+ and Fe3+ which constructed in Fe2O3 and Fe3O4. According to the fine fitting results, it is clear to see that When Fe2O3 phase occurs in Fe3O4 the binding energy of Fe3+ shifts to low energy, which may be caused by the interface of Fe3O4 and α-Fe2O3, consistent with our XRD results.(the details can be seen in supporting informations)


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All the diffraction peaks of samples B,C and D shifts slightly in global (shifted to a high angle by 0.03°), therefore, the Fe3O4 peak deviation could be due to errors in the measurement process.The morphology of Fe3O4, Fe3O4/α-Fe2O3 and α-Fe2O3 were studied by high-resolution TEM and displayed in Fig. 2. Fig. 2(a), (c) and (e), the nanoparticles are monodispersive rather than agglomerated. For Fe3O4 sample, the TEM image (Fig. 2(a)) shows mulitishapes, including square, long rods, and triangles, and the particle size on average of the sample is about 86.5 nm with a wide range distribution. The low concentration of Fe3+ in the precursor leads to a relatively low density of the nuclei of Fe3O4 growth. Meanwhile a large amount of hydroxyl groups of MEG and DEG are accumulated around the nuclei, promoting a further growth of the nuclei, in addition, some small nuclei were absorbed by the large nuclei to form large grains. Considering the difference in the adsorption energy of Fe3+ chelated DEG and MEG on the surfaces of the nuclei [35], the exchange energy of Fe3+ between the DEG, MEG and the nuclei are different, which induces the anisotropic growth behavior of the Fe3O4 grains. When the concentration of Fe3+ is high enough, the nucleation density will increase. Accordingly, the hydroxyl at the surface of the nuclei will decrease and behave as isotropic slow growth model [14]. The size of nanoparticles of the corresponding samples reduced and the uniform structure enhanced, which is indicated by the Gaussian distribution statistic results illustrated in form of histograms in the inset of Fig. 2(a), (c) and (e). It is clear to see that the grain size of the sample decreases and the distribution of grain size narrows, which might be due to the monodispersion of DEG [35]. The average grain sizes of the sample A, B and E are 86.5, 59.9 and 59.5 nm, respectively, which is consistent with that obtained by XRD results (D=Kλ/βcosθ) [36]. In samples B, C and D, only part of Fe3+ have be reduced to Fe2+ due to a high



concentration of Fe3+ in the precursor solution, in turn, Fe3O4/α-Fe2O3 composite synthesized by one step, and some of the composite grains have Fe3O4/α-Fe2O3 heterogeneous structures (the inset of Fig. 2(c) and (d)). The microwave absorption properties of Fe3O4/α-Fe2O3.samples shows much widened absorption band due to the presence of the interfaces effects between Fe3O4 and α-Fe2O3. The high-resolution TEM (HRTEM) images shown in Fig. 2(b), (d) and (f) correspond to the TEM Fig. 2(a), (c) and (e), respectively. Fig. 2(b) shows that the span of lattice fringes is of 0.253 nm which can be specified as the (311) facet of the cubic spinel crystal Fe3O4. As shown in Fig. 2(d), t, Fe3O4/α-Fe2O3 can be observed clearly. The lattice fringes with span of 0.368 and 0.162 nm can be assigned to the (012) planes of α-Fe2O3 and the (511) planes of Fe3O4 respectively. The two structures seem to be bonded together (the inset of Fig. 2(c) and Fig. 2(d)). When Fe3O4 and Fe2O3 form composites, Fe3+ in Fe2O3 may diffuse into Fe3O4, which increases the charge transfer at the interface. In this process, Fe3O4 and Fe2O3 heterostructures could formed (the inset 2 of Fig. 2(d)) [37]. As shown in the inset 2 of Fig. 2(d), lattice dislocations observed clearly at the region of interfaces between Fe3O4 and Fe2O3 grains. While the contrast of the lattice fringes in the interface region is much higher than these beyond the interface regions. The fast Fourier transform (FFT) of the lattice fringes shown in the inset 1 and 3 of Fig. 2(d) confirm that the corresponding regions are Fe2O3 and Fe3O4 grains, respectively. Considering the dislocations and vacancies exist near the interface between Fe3O4 and Fe2O3 grains, which could cause an accumulation of charge, the bright lattice finger observed under HRTEM could be clue to suggest the accumulation of charge at the interface of Fe3O4 and α-Fe2O3 domains, which is similar to Li et al [38].


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Fig. 3 displays the applied external field dependent magnetization hysteresis (M-H) loops of the samples measured between -20 and 20 kOe at room temperature.

Fig. 3

demonstrated, when the content of Fe3+ is 0.0625 mol/l, the sample A has a highest saturated magnetization, and the saturated magnetization of this sample nearly meets that on bulk Fe3O4 (98 emu/g) [39,40]. However, when the content of Fe3+ increases, the saturation magnetization value obviously decreases, which is mainly due to the increase of the content of Fe2O3 in the samples. But when the sample is Fe2O3 (sample E), it can be seen the saturation magnetization value is nearly zero, indicating the sample E is αFe2O3, which is consistent with the XRD shown in Fig. 1. It is well known that at room temperature, the bulk α-Fe2O3 is weak ferromagnetic with saturation magnetization less than 1 emu/g [41]. nano α-Fe2O3 is paramagnetic due to he dangling bond and defects on the surface ,which induces in noncompensated spins. Therefore, the presence of Fe2O3 reduces the saturation magnetization of the samples.



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Based upon the magnified graphs of M-H near zero applied magnetic field shown in the inset of Fig. 3, the coercivity of the samples gradually decreases as the Fe3+ concentration increases, and the saturation magnetization (Ms) value and coercivity (Hc) of the samples are shown in Table 1. It is well known the grain size, geometry and magnetocrystalline anisotropy have a significant effect on the coercivity of the magnetic material [42-44]; meanewhile, the plentiful defects near the grain boundary of

Table 1. Saturation Magnetization (Ms) and Coercivity (Hc) of Nano-Materials Prepared under Different Concentrations of Fe3+ in the Precursor Solution.

Samples A

Saturation magnetization (Ms) (emu/g) 88.0

Coercivity (Hc) (Oe) 120

B

79.9

115

C

49.6

95

D

30.5

90

E

-

-

nanopaticals worked as anti-magnetization nucleation and determine the coercivity, namely, the defects and grain boundaries have pining effects on the magnetic domain movement. And the impurity or non-magnetic inclusions leads to the generation of grain boundaries through the magnetic domain wall pinning to improve the coercivity [45]. However, in our samples, the grains of sample B-D changed little, therefore, the decrease in coercivity of the samples could be induced by: (1) the change of the geometry, (2) the interface coupling between Fe3O4 and Fe2O3 in the composites, (3) decrease of magnetocrystalline anisotropy of Fe3O4 due to formation of Fe2O3.


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Fig. 4 displays complex permittivity(permeability) of the samples synthesized at the different concentration of Fe3+ at the frequency of 1~18 GHz. It is known, The perforance of EM radiation absorption on dielectric/ferromagnetic composite absorbers mainly comes from dielectric magnetic losses. In other words, the complex permittivity (εr=ε'-iε") and complex permeability (µr=µ'-iµ") of the absorbers are crucial to determine its absorption performance. As known, the real part ε' and µ' represent the storage capacity of the electromagnetic field energy collected by absorbers, and the larger real part of the material, the greater the energy density of the energy storage capacity; the imaginary part ε" and µ" manifest the electromagnetic loss capability of the absorber. Increase in ε" (µ") indicates the increase in the loss ability, therefore, improvies the converting of electromagnetic energy into heat energy, thus ,promote the performance of the EM absorption of absorber [46]. Fig. 4(a) & (b) show that real and imaginary part of complex permittivity of sample A (Fe3O4) are 13 and 0.3, respectively. The large real part of permittivity indicates that Fe3O4 has a high ability of the energy storage. When the samples contain Fe3O4 and Fe2O3, the real parts of the complex permittivity of samples gradually decrease, which suggests that the capacity of energy storage the respective samples reduced. The frequency dependent the real parts of the permittivity of samples B~D decrease with increasing frequency at low frequencies band, which could be attributed to the polarization induced by electric dipolar and interfacial effects of the samples [2, 26] As a response, the imaginary parts exhibit the same properties, which is shown in Fig. 4(b). However, when the sample is α-Fe2O3 (sample E), the permittivity is the lowest (ε'=6.7, ε"=0.2), which indicates that it is the presence of Fe2O3 results in the decrease of the permittivity of the composites. As shown in Fig. 4(b), there are multiple peaks in the



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imaginary part of ε" of sample B, indicating that there are various losses, which is beneficial to the improvement of its microwave absorbing properties in a much enhanced waveband. According to the theory of free electronic [42],

ε′′ = 1 / 2πε0 ρf

(1)

where ε0，f andρ are the vacuum dielectric constant, frequency of EM wave, and the resistivity of aborber, respectively. According to equation (1), ε" is proportional to the conductivity of the absorber at the specified frequency. As known, Fe2+ at B site of Fe3O4 is easily oxidized to Fe3+, which induce the increase in vacancies of the sample, in other hand, the vacancies limit the movement of electrons and reduces the conductivity, correspondingly, ε" decreases. However, the resistivity can be affected by composition, size, morphology and polarization [47]. As revealed in Fig. 4(b), the ε" of the sample B and C does not decrease. Considering the lattice dislocation and defects caused by the interface Fe3O4 and Fe2O3 could serve as a polarization center and introduce additional polarization [48], which means that more energy needed to alternate the direction of the dipoles. Moreover, both the increase in the plentiful of unsaturated dangling bonds on the surface of the sample caused by the reduced of crystalline size [49] and the enhancing in internal stress at the interface may contribute to polarization. Compared with the dielectric loss induced by the resistance, the losses induced by polarization is more prominent in our samples. Such a result is also supported by the HRTEM (Fig. 2(d)). Nevertheless, sample C and D exhibits weak EM wave absorption ability with high conductivities, which is due to their weak impedance matching [50]. In general, the factors that cause dielectric losses are mainly from space charge polarization, electron polarization, ion polarization, orientation polarization and interfacial polarization [42, 51]. Among them, electron polarization and ion polarization


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work above the terahertz band [42], which can be ignored in our work. Therefore, the dielectric loss should arise from the polarization induced by interfacial effects, orientation polarization and space charge polarization. As shown in Fig. 4(c), in low the frequency band, the dielectric loss tangent (tanδe=ε"/ε') of the sample B, C and D is relatively high, but that of Fe3O4 (sample A) and α-Fe2O3 (sample E) are very low, indicating the dielectric loss caused by the interfacial polarization (Fe3O4/α-Fe2O3, Fe3O4/paraffin and α-Fe2O3/paraffin) and space charge polarization (Fe3O4/α-Fe2O3). Besides, the dielectric loss peak at 11 GHz indicates that there exists a dielectric polarization of the samples. For the ferrite, the presence of Fe2+ tends to cause an increase in the complex permitivity [27], which is due to the exchange resonance of the electron jump between Fe2+ and Fe3+. And dipole relaxation from the interface of αFe2O3 and Fe3O4 may also contribute to dielectric loss, which can be owe to the charge accumulation caused by dislocations (Fig. 2(d)) and XPS result in Fig.S3. In Fig.S3, the change of the binding energy of Fe2+and Fe3+ suggests that the charges accumulated near the interfaces betweenα-Fe2O3 and Fe3O4 (see the supporting information). In range of 10-18 GHz, both real and imaginary parts of complex permitivity increases with frequency, which is similar with the report by Liu et al [17], indicating that there is dielectric loss at high frequencies. This is most likely because of the orientation polarization of Fe2+ at high frequency [27]. The ferrous ions (Fe2+) are polarized easily, when the sample contains more Fe3O4, the polarization strength increases, accordingly, the complex permitivity and dielectric loss increase. If the frequency is higher, the polarization of Fe2+ cannot keep up with the change of frequency, and it will exhibit relaxation at high frequency.



Fig. 4(d), (e) and (f) show the dependence of the complex permeability (µ', µ"), magnetic loss tangent (tanδm=µ"/µ') of the samples and frequency, respectively. In this paper, the real part of the permeability of the sample A can reach 3.0 and the imaginary part reaches 1.6, indicating that the permeability of Fe3O4 is relatively higher. The high imaginary part of the permeability shows that sample A has a great magnetic loss capability. But when the content of Fe3+ increases in the sample, the permeability gradually decreases, indicating that the magnetic properties of the sample is weakened because of the increase the content of α-Fe2O3 in the sample. For α-Fe2O3 (sample E), the permeability is minimum (µ'=1.5, µ"=0.4), indicating a weak performance in magnetic loss, which is also confirmed by VSM shown in Fig. 3. In 1~10 GHz range , the real part of the permeability of sample A decreases sharply from 3.0 to 0.6, the imaginary part decreases from 1.6 to 0.3, which is attributed to Snoek’s limitation in the GHz frequency range, and this phenomenon was also observed by Zhao et al [52]. However, the frequency dependent permeability of samples B~E changes more and more slowly, which confirms that the introduction of α-Fe2O3 reduces the magnetic properties (Fig. (3)). In frequency range higher than 10 GHz, the real parts for all samples almost do not vary, and the imaginary parts decrease slowly, indicating that at high frequencies, magnetic does not work. Similar with the permittivity, there are also many factors that may cause magnetic loss, including hysteresis, the movement of domain wall, eddy current and natural resonance [53, 54]. Wherein the loss of hysteresis is usually negligible for it is irreversible in the external weak magnetic field, domain wall resonance usually occurs at low frequencies (1-100 MHz). Therefore, magnetic loss shall be caused by resonance and eddy current loss. Based on the ferromagnetic resonance theory [49, 55], the magnetic anisotropy energy of the ferromagnetic material


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has a great influence on the natural resonance frequency, and the correlation between natural resonant frequency fr and anisotropic energy Ha is:

2πf r = γHa

(2)

H a = 4 K / 3µ 0 M s

(3)

K = µ0 Ms Hc / 2

(4)

Where γ, K, µ0 , Hc and Ms are the gyromagnetic ratio, the crystal anisotropy constant, the vacuum permeability (4π×10-7N•A-2), the coercivity, and saturation magnetization, respectively. As shown in Fig. 4(e), the natural resonance frequency of sample A is of 1.8 GHz, but the resonance frequencies of the sample B~E move to low frequencies, which is likely stemmed from the reduction of the anisotropy energy (eq (2)) of the corresponding samples. According to equation (3) and (4), the anisotropy energy is involved in the anisotropy constant that is determined by saturation magnetization and coercivity. In combination with Table 1, as the Fe2O3 increases, the coercivity of the sample decreases, as a result, the anisotropy energy decreases. According to equation (2), the natural resonance shifts to low frequency. As shown in Fig. 4(f), the sample A and sample B have high magnetic loss tangent, and there is a large magnetic loss peak at 6 GHz, indicating their high magnetic loss capability. However, the magnetic losses of sample C~E are weak. To deeply reveal the EM absorbing performances, frequeny dependent reflection loss (RL) curves of the sample were calculated by using the complex permittivity and permeability at the given frequency and thickness based on the transmission line theory [51]: RL = 20 lg

Z in − Z 0 Z in + Z 0

(5)



{

Z in = Z 0 (µ r / ε r )1 / 2 tanh j( 2πfd / c)(µ r ε r )1 / 2

}

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(6)

where Zin is the input impedance, Z0=(µ0/ε0)1/2 is the f vacuum impedance, c is the velocity of electromagnetic waves in vacuum, d is thickness of absorber, f is microwave frequency, and εr(µr) is respectively relative complex permeability ( permittivity) of samples. Fig. 5 shows the behavior of the microwave reflection loss against frequency of the samples. It can be seen that the values of RL peaks shift to low frequencies with the increase of the thickness of the samples, indicating that the loss ability of thick absorber takes effect mainly in the low frequency region [56], and it can be understood by the equation of fm=c/2πµ"d [26], where fm represents the matching frequency. As shown in Fig.5, the matching frequency for the RL is strongly frequency and thickness dependent. When the thickness in 2.2 mm, The maximal reflection loss ( RL) of

Sample A

reaches -33.9 dB. However, the width of effective absorbing band is about 2.3 GHz (9.84-12.14 GHz). When the samples contain Fe2O3, sample B seems to have a better absorbing property. The best matching thickness is 2.0 mm, and the RL is -43.1 dB. What’s more, the width of effective absorbing band is of 3.4 GHz (9.8-13.2 GHz), indicating that the improvement of microwave absorption performance. Fig. 5(b) shows that there is a fixed absorption peak at 11 GHz for different thicknesses. It can be attributed to intrinsic properties of the composites, in which the lattice dislocation of the interface (Fig. 2(d)) plays a major role. Moreover, the composites satisfy the impedance matching conditions to the maximum extent by adjusting εr and µr and increase the effective width of the sample. As a result, in the thickness of 1.5-3.2 mm, the width of the reflective loss of sample B can reach 9.9 GHz, almost covers the entire C-band and X-band. The addition of a small amount of Fe2O3 leads to a decrease in the complex


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permittivity (Fig. 4(a) and (b)) of the absorber, which facilitates its impedance matching (eq 6). Through the analysis and integration of the latest literature (Table 2), we have found that our materials have greatly improved the wave absorption width in terms of a relatively low thickness. But when the content of Fe2O3 in the sample is too more, the microwave absorption performance is reduced due to its weak magnetic loss performance, such as the matching thickness increases (higher than 2.6 mm), the value of the reflective loss peak decreases and the effective bandwidth becomes narrow. All of these is induced from the weak matching in permittivity and permeability (eq 6), which is also known as the weak impedance matching of the material, correspondingly, their microwave absorb performance is reduced.

Table 2. Electromagnetic Absorption parameters of Some Representative Fe3O4 or αFe2O3 Composites. Freque ncy (GHz)

Thickn ess (mm)

Frequency coverage (GHz) (RL

1 Synthesis, Electromagnetic and Microwave Absorption Properties of

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