Constructing flower-like porous Bi0.9La0.1FeO3 microspheres for excellent electromagnetic wave absorption performances

Journal of Alloys and Compounds 745 (2018) 761e772

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Constructing flower-like porous Bi0.9La0.1FeO3 microspheres for excellent electromagnetic wave absorption performances Ying Lin, Qian Wang, Shuya Gao, Haibo Yang*, Lei Wang School of Materials Science and Engineering, Shaanxi University of Science and Technology, 710021, Xi'an, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 28 November 2017 Received in revised form 31 January 2018 Accepted 18 February 2018 Available online 21 February 2018

Uniform flower-like porous Bi0.9La0.1FeO3 microspheres etched for 30, 60 and 90 min (named as pBLFO30, pBLFO-60 and pBLFO-90, respectively) have been successfully constructed by a facile chemical etching process based on Bi0.9La0.1FeO3 particles (BLFO) as precursors. In particular, Sample pBLFO-60 has the largest specific surface area of 75.09 m2 g
1, which leads to the most outstanding electromagnetic wave attenuation performance. The optimal reflection loss (RL) value reaches
57.9 dB when the matching thickness is 2.9 mm at 6.9 GHz and the effective absorption bandwidth (RL <
10 dB) is 2.7 GHz (6.1e8.9 GHz). The three-dimensional porous architecture and enhanced magnetic loss make great contribution to the electromagnetic wave absorption performance, among which the porous microstructure ensures multiple scattering and enhances the magnetic loss contributing to better impedance matching. Meanwhile, the electromagnetic wave absorption performance of porous BLFO microspheres are superior to those of most porous electromagnetic wave absorbers, implying their potential application as novel electromagnetic wave absorbing materials. © 2018 Elsevier B.V. All rights reserved.

Keywords: Flower-like porous Bi0.9La0.1FeO3 microspheres Etching process Magnetic loss Electromagnetic wave absorption

1. Introduction Recently, porous or hollow microstructures with large surface area and adjustable pore size have drawn tremendous attention in various aspects such as catalysis [1,2], supercapacitors [3,4], battery anodes [5] and electromagnetic wave absorption materials [6e8] owing to the unique structure with numerous pores. Notably, electromagnetic (EM) wave absorption materials have been urgently required to eliminate the expanded EM wave that could interrupt the normal operation of electronic equipment as well as pose a threat to human health [9e11]. Thus, it is imperative to explore ideal EM wave absorption materials, which possess strong absorption performance, broad absorption bandwidth, and more importantly, light weight and thin coating thickness [12e15]. Varieties of absorbing materials have been exploited, which present the potential application in the EM wave absorption field. As traditional EM wave absorption materials, Fe3O4, Fe2O3 and other magnetic metal materials exhibit excellent EM wave absorption performance [16,17]. Especially for ferrites, magnetic materials are demonstrated to have both strong EM wave absorption

* Corresponding author. E-mail address: [email protected] (H. Yang). https://doi.org/10.1016/j.jallcom.2018.02.237 0925-8388/© 2018 Elsevier B.V. All rights reserved.

performance and broad absorption frequency bandwidth by virtue of their large magnetic permeability [18,19]. Nonetheless, most of magnetic materials have large densities, which hinders their practical application. It is well known that the microstructures of absorbers play a key role in the EM wave absorption performance [20,21]. The porous or hollow structured materials with light weight are supposed to be a hopeful candidate for EM wave absorption performance [22e28]. Zhou et al. reported a new strategy of in-situ polymerization to prepare ordered mesoporous carbon and the maximum RL value was
27 dB with the matching thickness of 2 mm [24]. Sun et al. fabricated the porous Fe3O4@ZnO sphere decorated graphene with uniform pore size and the minimal RL value could reach almost
40 dB with a broad absorption bandwidth up to
11.4 GHz [25]. Zhao et al. synthesized 3D honeycomb-like SnO2 foams by using 322 nm polystyrene spheres as sacrificial template and the optimal RL was
37.6 dB with an absorber thickness of 2 mm [26]. Yang et al. designed silica coated mesoporous Fe (Fe@SiO2) microcubes and the RL value of
54 dB could be obtained at a matching thickness of 4.5 mm, in which the mesoporous iron microcubes reduced the thickness of silica resin composites [27]. Xu et al. prepared mesoporous carbon hollow microspheres with designable mesoporous shell (pore size 4.7 nm) and interior void by a €ber templating approach and a polysis-etching facile in-situ sto

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process and the composite exhibited outstanding EM wave absorption performance [28]. Therefore, it can be concluded that the mesoporous or hollow microstructures are conducive to the EM wave absorption performance based on the reported work. In this work, porous BLFO microspheres with light weight can be synthesized by utilizing BLFO particles obtained from a molten-salt approach as precursors and subsequently a facile chemical etching process. The ferric (III) of partial BLFO crystals are reduced to the ferric (II) with the action of hydrazine and the ferric (II) subsequently coordinated with methyl mercaptoacetate to construct flower-like porous BLFO microspheres. The evolution of microstructures in BLFO can be easily controlled by adjusting the etching time. Remarkably, the porosity and specific surface area of porous BLFO microspheres are enhanced significantly. A succession of gradient experiments related to etching times were also performed. Particularly, Sample pBLFO-60 exhibits the most superior EM wave absorption property, in which the optimal RL value reaches
57.9 dB at 6.9 GHz with an absorber thickness of 2.9 mm and the effective absorption bandwidth is 2.7 GHz, ranging from 6.1 GHz to 8.9 GHz. The given merits of porous BLFO microspheres, such as strong absorption, broad responding bandwidth, easy preparation and fairly low density, can bring them a promising prospect in the EM wave absorption field. 2. Experimental Reagents including Bi2O3, La2O3, Fe2O3 (starting materials) and NaCl-KCl (molten salts system) were purchased from Sinopharm Group. DimethyFormamide (DMF), hydrazine (a reducing agent) and methyl mercaptoacetate (a complexing agent) were also obtained from Sinopharm Group. All the chemicals were of analytical grade. BLFO particles were synthesized via a molten salt method. Bi2O3, La2O3, Fe2O3 and NaCl-KCl were mixed, with a molar ratio of [Bi]/ [La] ¼ 9:1 and a molar ratio of BLFO/NaCl-KCl ¼ 1:10. The mixture was ground in ethanol for 4 h. The mixture was transferred to a crucible after drying, and then loaded into a furnace. The phase of BLFO can be obtained after heating the mixture at 750  C for 2 h and washing the mixture several times with deionized water to remove the NaCl-KCl salt. Porous BLFO microspheres were prepared via a one-step etching method. A certain amount of BLFO particles were dispersed ultrasonically into a solution of DMF, heated in a water-bath at 80  C, and hydrazine and methyl mercaptoacetate were then added. N2was used to prevent the reaction between methyl mercaptoacetate and O2. The reaction was terminated by cold ethanol after etching for different times (30, 60, 90 min). The black powders were ultimately obtained after washing by deionized water and ethanol five times and drying in vacuum for 12 h. The phase structure of all the samples was identified by an X-ray diffractometer with Cu-Ka radiation (Rigaku D/MAX-2400, Japan). Raman spectra were collected using a microscopic Laser-Raman spectrometer (Renishaw-invia, England) with a 514 nm radiation. The morphological characterizations of all the samples were obtained using scanning electron microscopy (Hitachi S-4800, Japan) and transmission electron microscope (FEI Tecnai G2 F20, America). The surface areas of porous BLFO microspheres were determined using a Brunauer-Emmett-Teller (BET) surface analyzer (ASAP2460, Micromeritics Instrument Corp, USA) and the pore size distribution was estimated according to Horvaih-Kawazoe (HK) theory. X-ray photoelectron spectroscopy (XPS, recorded on an AXIS SUPRA X-ray photoelectron spectrometer, UK) spectra were measured by using an ultrahigh vacuum VG Scientific Corp MK-II electron spectrometer. The magnetic hysteresis loops of porous BLFO microspheres were measured by a vibrating sample magnetometer (VSM, Lake

Fig. 1. XRD patterns of BLFO particles prepared by molten salt method and porous BLFO microspheres etched for different times.

Shore 7410, USA). The EM wave absorption performances of the obtained absorbers were also investigated. The paraffin was selected as the matrix and the mixture of paraffin and 60 vol% porous BLFO microspheres were heated, mixed and then molded into rings, with an outer diameter of 7 mm, an inner diameter of 3 mm and a height of about 3 mm for EM measurement. The complex permittivity (ε0 , ε00 ) and permeability (m0 , m00 ) were tested with the system composed of a vector network analyzer (VNA) (HP8720ES) and a coaxial fixture. 3. Results and discussion The XRD patterns of all the samples are shown in Fig. 1. It is found that the as-prepared BLFO synthesized by the molten salt route are highly crystallized as a single phase and no other phases such as Bi2Fe4O9 and Bi25FeO40 could be detected, which is in agreement with JCPDS card No.14-0181, pertaining to the composition of Bi0.93La0.07FeO3. Meanwhile, the calculated lattice parameters of BLFO are a ¼ b ¼ 5.58 Å and c ¼ 13.81 Å, which coincides with the literature result [29]. The XRD patterns of porous BLFO

Fig. 2. Raman spectra of BLFO particles and porous BLFO microspheres with different etching times.

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Fig. 3. SEM images of (a) BLFO particles and porous BLFO microspheres with different etching times (b) pBLFO-30 (c) pBLFO-60 and (d) pBLFO-90.

microspheres with different etching times show that the phase constitution keeps unchanged, which indicates that the structure of perovskite is still well preserved. However, the diffraction peak intensities significantly decrease with enlarging the etching time, which is attributed to the formation of porous BLFO microspheres that contain large amounts of nanoslices and numerous micropores.

The Raman scattering spectra in Fig. 2 provide the information about ionic substitution of BLFO particles and porous BLFO microspheres. It can be observed that BLFO exhibits A1-1, A1-2, A1-3 and E
4 modes, peaking at 140, 176, 226 and 273 cm
1 respectively, in which the Bi-O covalent bonds are predominant in the A1-1, A1-2 and A1-3 modes [30]. The peak positions of porous BLFO microspheres remain unchanged while the intensities present a trend of

Fig. 4. TEM images of (a) BLFO (b) pBLFO-60 and HRTEM images of (c) BLFO (d) pBLFO-60.

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Fig. 5. N2 adsorption-desorption isotherms and pore size distribution of BLFO particles and porous BLFO microspheres with different etching times.

Table 1 Porosity data of all the samples based on N2 sorption. Sample

SBET (m2 g
1)

Total pore volume (cm3 g
1)

Dpore (BJH)/nm

BLFO pBLFO-30 pBLFO-60 pBLFO-90

1.44 56.01 75.09 61.65

0.0039 0.2564 0.2785 0.2470

16.04 14.10 9.16 10.21

decline with increasing the etching time, corresponding to the XRD result, which indicates the stabilization of crystal structure and the reduction of crystallinity in porous BLFO microspheres. The morphology and microstructure of BLFO particles and porous BLFO microspheres are detected by the scanning electron microscope (SEM), as shown in Fig. 3. It can be clearly found that BLFO particles are produced homogeneously into a blocky structure and the average grain size is about 0.7e2.0 mm. Great changes have taken place on the surface of BLFO particles after etching, in which the generation of 2D nanoslices outline the shape of a flower. Amounts of nanoslices with diameter of 45e120 nm appear on the surface of BLFO particles and the surface of BLFO particles become rough when etching for 30 min. With a gradual increase of etching time to 60 min, the generated nanoslices are more and more noticeable and BLFO particles are completely etched into tiny fragments, which gather together and then construct flower-like

porous BLFO microspheres. This can also be evidenced by the generation of numerous mesopores with a reaction time of 60 min. When the etching time is up to 90 min, a growing number of slices accumulate in Sample pBLFO-60 microspheres, blocking some mesopores and resulting in the decrease of porosity. To further investigate the internal structure of porous BLFO microspheres and the reaction process of etching, the transmission electron microscope (TEM) is also performed. As evidenced by Fig. 4a, BLFO particles display a typical blocky shape, which is consistent with SEM image in Fig. 3a. Fig. 4b shows the TEM image of Sample pBLFO-60, where the nanoslices on the surface give an evidence of the formation of porous BLFO microspheres. Shown in Fig. 4c and d are the high-resolution TEM (HRTEM) images of Sample BLFO and Sample pBLFO-60. It is obviously found that the HRTEM image of Sample BLFO presents clear lattice spacing of 0.28 nm, indicating the (012) crystal plane of R3c phase. The interplanar spacings of 0.27 nm and 0.28 nm are indexed as (110) and (012) crystal plane of Sample pBLFO-60 respectively. Nitrogen (N2) adsorption-desorption isotherms and pore size distribution of all the samples are displayed in Fig. 5. It can be noted that the N2 adsorption-desorption isotherm of BLFO is type II, according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which illustrates the characteristics of nonporous adsorbent. Additionally, the isotherms of porous BLFO microspheres exhibit Type IV behaviors, indicating the presence of micropores [31]. The physical parameters of N2 adsorptiondesorption isotherms, including the Brunauer-Emmett-Teller surface area (SBET), total pore volume and average pore size, are presented in Table 1. It can be concluded that the relationship between the SBET of porous BLFO and etching time are not strictly positive linear. With enlarging the etching time, the SBET increases accordingly and reaches the maximum value of 75.09 m2 g
1 after etching for 60 min, significantly larger than that of BLFO (1.44 m2 g
1), which indicates that the generation of micropores elevates the surface area effectively. However, the specific surface area of Sample pBLFO-90 decreases to 61.65 m2 g
1, which is due to the fact that some micropores are blocked by the nanoslices, corresponding to the SEM result, as shown in Fig. 3d. The X-ray photoelectron spectroscopy (XPS) analysis is investigated to give a better insight into the chemical composition and surface state of as-synthesized Sample BLFO and Sample pBLFO-60, as shown in Fig. 6. It can be seen that both BLFO particles and porous BLFO microspheres mainly consist of Bi, La, Fe and O elements in Fig. 6a. The spectra obtained over the Bi 4f, Fe 2p, O 1s and La 3d are shown respectively in Fig. 6bee. The Bi ion of BLFO exist in two states: one is 4f photo-electron emissions located at 158.4 and 164.1 eV resulted from a major proportion of Bi2O3-like species and another is 4f photoelectrons located at 159.1 and 163.9 eV caused by the minority of Bi2O3þx-like species [32]. By comparison, the Bi 4f peaks of pBLFO-60 are deconvoluted into two main peaks at 158.5 and 163.7 eV, referred to the Bi 4f7/2 and Bi 4f5/2 respectively. In Fig. 6c, the La 3d peaks located at 835.9 and 852.1 eV of Sample BLFO and Sample pBLFO-60 are in agreement with the binding energies of La 3d5/2 and La 3d7/2, which are assigned to the lanthanum (III) [33]. Besides, it can be found from Fig. 6d that the peaks at 710.4 and 724.5 eV of BLFO are coincident with Fe 2p3/2 and Fe 2p1/2, which are ascribed to the ferric (III) [34]. In contrast, the high resolution of Fe 2p spectrum in Sample pBLFO-60 presents two Fe 2p peaks at lower binding energies (the Fe 2p3/2 peak at 708.9 eV and another peak of Fe 2p1/2 at 722.8 eV), corresponding to the ferric (II) of Fe 2p [35]. This indicates that the ferric (III) is reduced to the ferric (II) by hydrazine. It is noted that the O 1s peaks of BLFO are deconvoluted into three Lorentzian-Gaussian lines at 529.2, 531.5 and 533.3 eV, which can be assigned to oxygen vacancy, ordered lattice oxygen ions and surface absorbed H2O,

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Fig. 6. Fitted high-resolution XPS spectra of Sample BLFO and Sample pBLFO-60 (a. Survey spectrum b. Bi 4f c La 3d d Fe 2p and e O 1s).

respectively [36e38]. Besides, the O 1s peaks of Sample pBLFO-60 show a slight shift towards the higher binding energy as well as a strengthened peak at 531.5 eV, indicating a growing number of lattice oxygen ions on porous BLFO microspheres, which further confirms the reduction of the ferric (III). The results concluded by XPS spectra clarify elaborately the reaction mechanism, of which the ferric (III) of partial BLFO particles are reduced to the ferric (II) by hydrazine cooperated with methyl mercaptoacetate, leading to the formation of porous BLFO microspheres. A sketchy formation scheme of porous BLFO microspheres based

on XPS results is presented to investigate distinct changes of BLFO particles before and after etching as shown in Scheme 1. A kind of reaction mechanism is put forward, of which the process involves two steps: (i) the reduction of the ferric (III) to ferric (II) by hydrazine and the generation of numerous nanoslices on the surface of BLFO particles; (ii) the sulfhydryl group coordinated with ferric (II) to facilitate the dissolution in DMF and the aggregation of numerous nanoslices to construct porous BLFO microspheres [39]. Therefore, a portion of BLFO crystals was removed, giving rise to the characteristic of porous structure.

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Scheme 1. A possible schematic illustration of the formation of porous BLFO microspheres.

Fig. 7. Magnetic hysteresis (MeH) loops of BLFO particles and porous BLFO microspheres with different etching times.

The room temperature magnetization hysteresis loops up to a magnetic field of 8000 Oe were measured for BLFO particles and porous BLFO microspheres, as shown in Fig. 7. The ferromagnetic properties of porous BLFO microspheres increase to different levels, among which the Ms of Sample pBLFO-60 is 0.09 emu/g. Meanwhile, the M-H curve of porous BLFO microspheres shows a noteworthy hysteresis loop compared with that of BLFO. The increased remnant magnetization and saturation magnetization could be due to the size effect resulted from the nanoslices of porous BLFO microspheres. It is well known that uncompensated spins can be obtained when the characteristic size is inferior to 62 nm and the reason is that the long cycle spin spiral modulation is inhibited on the surface of BLFO [40]. The microwave absorption performance of materials is dependent upon the complex permittivity (εr ¼ εr0-jεr00 ), complex permeability (mr ¼ mr0-jmr00 ) and the electromagnetic impedance matching. Fig. 8 shows the permittivity (ε0 , ε00 ) and permeability (m0 , m00 ) of BLFO particles and porous BLFO microspheres. Overall, the observed real permittivity (ε0 ) presents a slightly fluctuation in the frequency range of 13e18 GHz and the value of ε0 keeps a stationary value in Fig. 8a. Also, the ε0 of porous BLFO microspheres are

generally lower than that of BLFO. It is well known that the charge asymmetric distribution appear in Bi ion site owing to the long pair of Bi ion, leading to the intrinsic dipole polarization in both BLFO particles and pBLFO microspheres. In addition, the existence of oxygen vacancy defect (concluded by XPS analysis) in BLFO particles break the charge balance of crystal structure, in which BLFO particles possess defect dipole polarization. In contrast, the concentration of oxygen vacancy decrease sharply in pBLFO microspheres from the high resolution of O 1s spectrum in Fig. 6e, which implies the decrease of ε0 after etching. The imaginary part ε00 of all the samples show a trend of decline (Fig. 8c). Similarly, the ε00 of porous BLFO microspheres are generally lower than that of BLFO, corresponding to the real part of permittivity. Fig. 8b and d show the complex permeability of all the samples. It can be seen that the real and imaginary permeability (m0 , m00 ) of porous BLFO microspheres are higher than that of BLFO. This can be attributed to the enhanced magnetization of porous BLFO microspheres after etching, which is confirmed by the increase of Ms. The dielectric loss (tandε ¼ ε00 /ε0 ) and magnetic loss (tandm ¼ m00 / 0 m ) are identified to illustrate the EM wave absorption performance, as shown in Fig. 9. The value of tandε is in the range of
0.5-0.3, and the value of tandm is in the range of 0.05e2.6. Larger values of magnetic loss are obtained compared with dielectric loss, demonstrating that the magnetic loss is dominant in the EM wave absorption mechanism of porous BLFO microspheres. Especially for Sample pBLFO-60, it has the highest tandm value in the frequency range from 6 to 18 GHz, signifying that strong EM wave absorption performance would be obtained. The EM wave absorption performances of the ferromagnetic materials are often limited to regression resulted from the eddy current effect in GHz range. The eddy current loss concerned with thickness and electric conductivity are presented by the equation as the following [41]:

.

m ðm0 Þ
2 f
1 ¼ 2pm0 dd2 3 00

(1)

Where f is the applied frequency, d is the electric conductivity and m0 is the permeability in vacuum. Fig. 10 shows the values of m00 (m0 )
2f
1 as a function of frequency for all the samples. It is known that the value of m00 (m0 )
2f
1 is a constant if the eddy current loss is the only inducement of the magnetic loss when the frequency is varied [42]. The values of m00 (m0 )
2f
1 for both BLFO particles and porous BLFO microspheres have a fluctuation in the frequency from

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Fig. 8. Frequency dependence of the complex permittivity and complex permeability of BLFO particles and porous BLFO microspheres with different etching times (a. real permittivity b. real permeability c. imaginary permittivity and d. imaginary permeability).

Fig. 9. Loss factors represented by (a) dielectric loss and (b) magnetic loss of BLFO particles and porous BLFO microspheres with different etching times.

2 to 3.7 GHz, indicating that the magnetic loss is resulted from the nature resonance [43]. For BLFO, the value of m00 (m0 )
2f
1 keeps nearly stationary with increasing frequency, which implies that only the eddy current loss is resulted from magnetic loss in the frequency ranged from 3.7 to 18 GHz. For Sample pBLFO-30, pBLFO60 and pBLFO-90, the variation of m00 (m0 )
2f
1 present noticeable peaks ranging from 5.2 to 6 GHz, from 6.3 to 8 GHz and from 5.6 to 7.8 GHz respectively, as shown in Fig. 10 b, c and d. According to Aharonai's theory, the obvious peak can be ascribed to the

exchange resonance which is related to the shape and size of nanoparticles [44]. Therefore, the eddy current loss, exchange resonance and nature resonance work together in the magnetic loss of porous BLFO microspheres. The EM wave absorption performances of porous BLFO microspheres are also investigated and they can be appraised by the reflection loss (RL) values. According to the transmission line theory, the RL values can be measured using the equations followed by Refs. [45,46]:

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Fig. 10. Values of m00 (m0 )
2f
1 as a function of frequency for BLFO particles and porous BLFO microspheres with different etching times.

Zin

  qffiffiffiffiffiffiffiffiffiffiffi ¼ mr=ε tanh j 2pfd pffiffiffiffiffiffiffiffiffi m ε r r r c

RL ¼ 20 log

jZin
1j jZin þ 1j

(2)

(3)

where Zin is the input impedance of the absorber, c is the light velocity, f is the EM wave frequency, d is the thickness of the absorber, εr is the complex permittivity, and mr is the complex permeability. It is recognized that the RL value below
10 dB indicates exceeding 90% of EM wave absorbed by the material that can be regarded as effective EM wave absorber [47,48]. The threedimensional diagrams in Fig. 11 show the evolution of RL values versus frequency and absorber thickness. It is found that the peak of RL values shifts towards a lower frequency when the absorber thickness is increased from 2 to 4 mm. Meanwhile, the maximum RL value and the effective bandwidth of all the samples and typical porous materials are listed in Table 2. Sample BLFO displays a poor EM wave absorption performance, of which the maximum RL value only arrives at
19.5 dB in the range of 9.2e11.3 GHz with a thickness of 2.5 mm. Interestingly, the EM wave absorption performance is significantly enhanced with porous BLFO microspheres obtained from the etching process. The optimal EM wave absorption performance is achieved with Sample pBLFO-60, for which the RL value exceeding
10 dB is in 3.8e7.8 GHz at 2.2e4.0 mm, and

the optimal RL value can reach
57.9 dB at 6.9 GHz with an absorber thickness of 2.9 mm. Besides, Sample pBLFO-30 displays a maximum RL value of
35.8 dB at 5.2 GHz with an effective bandwidth of 2.6 GHz (4e6.6 GHz). For Sample pBLFO-90, the RL value inferior to
10 dB is obtained in the frequency range of 3.2e7 GHz, and the maximum RL value is
45.6 dB at 6.6 GHz with the absorber thickness of 3.0 mm. All the results prove that the EM wave absorption performances of porous BLFO microspheres are tremendously improved compared with the unetched BLFO particles. Fig. 12 shows the dependence of RL peak on absorber thickness for all the samples. The frequency of the dips in RL can be explained by the quarter-wavelength (l/4) condition [49]:

tm ¼

nl nc pffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðn ¼ 1; 3; 5:::Þ ¼ 4 4f m jεr jjmr j

(4)

where tm is the matching thickness, fm is the peak frequency of a dip, εr and mr are the complex permittivity and complex permeability and c is the light velocity. It can be seen that the effective bandwidth of porous BLFO microspheres completely covers C-band (4e8 GHz) and X-band (8e12 GHz) with the matching thickness ranging from 2 to 4 mm, which implies that the EM wave absorption performance of porous BLFO microspheres in different frequency can be adjusted by the matching thickness. Generally, the RL peak shows a shift towards lower frequency with increasing the absorber thickness when the value of n is 1 [50].

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Fig. 11. Three-dimensional representation of RL values for (a) BLFO (b) pBLFO-30 (c) pBLFO-60 and (d) pBLFO-90.

Table 2 Electromagnetic wave absorption performance of typical porous materials. Sample

Maximum RL value Maximum peak position Thickness (dB) (GHz) (mm)

Frequency range (GHz) (RL <
10 dB)

Efficient bandwidth (GHz) (RL <
10 dB)

Refs

BLFO


19.5

10.6

2.5

9.2e11.3

2.1

pBLFO-30


35.8

5.2

3.5

4e6.6

2.6

pBLFO-60


57.9

6.9

2.9

6.0e8.9

2.7

pBLFO-90


45.6

6.6

3.0

5.94e7.94

2.2

SiO2@mesoporous Fe Co/porous carbon Surface-modified mesoporous carbon Graphene-pFe3O4@ZnO Fe/nanoporous carbon


54
40
29

3.2 4.2 10.7

4.5 5 2

2.8e3.9 3.5e5.0 9.2e12.4

1.1 1.5 3.2

This work This work This work This work [23] [19] [20]


40
29.5

11.1 17.2

5 2.5

3.0e6.8, 10.4e18 13.7e18.0

11.4 4.3

[21] [8]

To understand deeply the EM wave absorption mechanism of porous BLFO microspheres, the attenuation constant and impedance matching are also necessary to be investigated. The attenuation constant a that embodys the dissipation ability of materials can be determined as [51]:

a¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi pffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2pf  ðm00 ε00
m0ε0Þ þ ðm00 ε00
m0ε0Þ2 þ ðm0ε00 þ m00 ε0Þ2 c (5)

where a is the attenuation constant, c is the light velocity and f is the electromagnetic wave frequency. As shown in Fig. 13a, Sample

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Fig. 12. RL values on frequency at various thickness for (a) BLFO, (c) pBLFO-30, (e) pBLFO-60 (g) pBLFO-90 and dependence of l/4 thickness for (b) BLFO, (d) pBLFO-30 (f) pBLFO-60 and (h) pBLFO-90.

pBLFO-30 has the highest a value in the whole frequency range and the a value of Sample pBLFO-60 is in the middle among all the samples. In general, a high a value may be attributed to high imaginary permittivity and imaginary permeability, which corresponds to strong dielectric loss and strong magnetic loss. However,

intensive dielectric loss and magnetic loss are not the whole factors which lead to a high a value, and the surface architecture of porous BLFO microspheres may also contribute to a high a value. The suitable surface microstructure and porosity volume can facilitate multiple scattering and thus the absorption of EM wave, which

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materials and the latter facilitates the EM wave attenuation by magnetic loss and dielectric loss. So with those concluding remarks, the outstanding EM wave absorption performance of porous BLFO microspheres can be ascribed to two aspects. One is the formation of large amounts of nanoslices in the process of etching, leading to the increase of magnetization and permeability, which is conducive to an optimized impedance matching. The other is the three-dimensional architecture of porous BLFO microspheres with large surface area that gives rise to multiple scattering and absorption of EM wave. 4. Conclusion Flower-like porous Bi0.9La0.1FeO3 microspheres have been prepared via a facile one-step etching method, which includes two steps: the reduction of the ferric (Ⅲ) to the ferric (Ⅱ) in BLFO and the conjugation between the ferric (Ⅱ) and the methyl mercaptoacetate, leading to enhanced specific surface area. The superiority of porous BLFO microspheres can be manifested from a comparison to unetched BLFO particles, in which the maximum RL value only reaches to
19.5 dB at 10.6 GHz with the matching thickness of 2.5 mm. The EM wave absorption performances of porous BLFO microspheres have been significantly improved. Especially for Sample pBLFO-60, the maximum RL value can be achieved to be
57.9 dB with a thinner thickness of 2.9 mm and the effective absorption bandwidth is 2.7 GHz, ranging from 6.1 to 8.9 GHz. More importantly, the maximum RL value still achieves to
45.6 dB with the matching thickness of 3 mm when the etching time is up to 90 min. The porous architecture and increased magnetic loss of porous BLFO microspheres can be responsible for the remarkable improvement of EM wave absorption performance. Therefore, porous BLFO microspheres show excellent EM wave absorption performances such as strong and wide band absorption and can be promising candidates as the new type of EM wave absorbers. Fig. 13. Attenuation constant and impedance versus frequency of BLFO particles and porous BLFO microspheres with different etching times.

Acknowledgements

plays a key role in the enhancement of EM wave absorption properties. The impedance matching means the efficient complementarity between dielectric loss and magnetic loss, and Zim can be calculated by the following formula [52]:

This work is supported by the National Natural Science Foundation of China (Grant No. 51772177), the Chinese Postdoctoral Science Foundation (Grant No. 2016M590916), the Science and Technology Foundation of Weiyang District of Xi'an City (Grant No. 201605), the Industrialization Foundation of Education Department of Shaanxi Provincial Government (Grant No. 16JF002).

rffiffiffiffiffiffirffiffiffiffiffi Zim ¼

m0 mr ε0

εr

(6)

The impedance matching condition should satisfy Z ¼ 1, which means that the characteristic impedance should be as close as possible to the free space when the EM wave almost enters into the EM wave absorber. Fig. 13b shows the Zim value of all the samples. It can be seen that the Zim value of BLFO is 0.21e0.36, far less than 1 in the whole frequency range, implying a larger reflection on the surface of Sample BLFO. By comparison, the Zim values of porous BLFO microspheres increase obviously and they are more close to 1, indicating that more EM wave can easily penetrate the interior of porous BLFO microspheres, which leads to a better EM wave absorption performance. Particularly, the Zim value of Sample pBLFO60 are larger than those of Sample pBLFO-30 and Sample pBLFO-90, implying a better impedance matching of Sample pBLFO-60. This is in agreement with the optimal EM wave absorption performance. Therefore, it can be concluded that an excellent EM wave absorption material should satisfy the impedance matching condition and good EM wave attenuation ability simultaneously, among which the former ensures the penetration of EM wave into the interior of

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