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One-pot synthesis and microwave absorbing properties of ultrathin SrFe12O19 nanosheets
Accepted Manuscript One-pot synthesis and microwave absorbing properties of ultrathin SrFe12O19 nanosheets Shenhui Dong, Chucheng Lin, Xianfeng Meng PII:
S0925-8388(18)34821-7
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.265
Reference:
JALCOM 48908
To appear in:
Journal of Alloys and Compounds
Received Date: 23 October 2018 Revised Date:
4 December 2018
Accepted Date: 21 December 2018
Please cite this article as: S. Dong, C. Lin, X. Meng, One-pot synthesis and microwave absorbing properties of ultrathin SrFe12O19 nanosheets, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2018.12.265. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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One-pot Synthesis and Microwave Absorbing Properties of Ultrathin SrFe12O19 Nanosheets Shenhui Donga,#, Chucheng Linb,#, Xianfeng Menga*
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a. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
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b. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050,
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China These authors contributed equally to this work.
Abstract
Ultrathin SrFe12O19 nanosheets were successfully synthesized by modified
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hydrothermal method. The structure, morphology and properties were characterized and investigated by using X-ray diffractometer (XRD), Fourier transform infrared
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spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Vibrating sample magnetometer (VSM) and
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Vector network analyzer (VNA). The results revealed that the temperature and diethylene glycol were responsible for ultrathin structure and good dispersity of SrFe12O19 nanosheets. The SrM nanosheets showed strong magnetocrystaline anisotropy and large coercive force, which enhanced microwave absorbing properties of SrFe12O19. The reflection loss (RL) of a composite absorber with thickness of 3.5 mm reached a maximum value of -28.6 dB and the effective absorption bandwidth (RL < -10 dB) was
ACCEPTED MANUSCRIPT 0.92 GHz at the high frequency region, exhibiting excellent microwave absorbing properties as promising microwave absorber. Keywords: Magnetic materials, Strontium hexaferrite, Ultrathin nanosheet, Microwave
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absorber 1. Introduction
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Electromagnetic interference and pollution are posing serious threat to human health and electronic devices[1]. To resolve this problem, M-type SrFe12O19 (SrM) has
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gained immense interest due to its large anisotropy field, high coercive force, low cost and excellent chemical stability [2-4]. The excellent properties make SrM ferrite useful for electromagnetic shielding and microwave absorption materials [5]. Therefore, many strategies such as advanced synthesis techniques, cation substitution etc, have been
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employed to improve microwave absorbing properties of SrM ferrite [6-9]. However, the radiation absorption performance of SrM ferrite still remains relatively modest. It is
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well known, the magnetic properties and microwave absorbing properties of SrM ferrite depend strongly on its microstructure, grain size and particle shape [10]. Unfortunately,
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most reported SrM particles are suffering from the uncontrolled particle sizes and agglomeration. Thus so far, the preparation of SrM nanoparticles with nice dispersity and pure phase is still maintain the challenge. Herein, we reported ultrathin SrFe12O19 nanosheets with high aspect ratio and good dispersity, which were synthesized by modified hydrothermal method. The effect of temperature on structure, morphology and
ACCEPTED MANUSCRIPT magnetic properties were discussed. Subsequently, the microwave absorbing properties were investigated in detail. 2. Experimental procedures
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The SrFe12O19 nanosheets were prepared using analytical Fe(NO3)3·9H2O and Sr(NO3)2 as raw materials. The synthesis route for SrFe12O19 nanosheets was shown in
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Fig.1. In a typical process, 20.2 g Fe(NO3)3·9H2O and 1.51g Sr(NO3)2 were dissolved in 40 mL mixture with diethylene glycol (DEG, C4H10O3) and water (the mass ratio of
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DEG and water was 1:2) under stirring. Here, DEG was applied to control morphology and dispersity of products. Subsequently, NaOH solution was added dropwise to above solution until pH is equal to 10. Afterwards, the mixture solution were poured into a 50 ml reactor and heat-treated at various temperature for 24 h. Finally, the samples were
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cooled, washed and dried. The obtained samples at 140 °C, 160 °C, 180 °C and 200 °C were marked as SrM14, SrM16, SrM18 and SrM20, respectively.
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The structure, morphology, electromagnetic parameters of samples were respectively characterized using X-ray diffractometer (XRD, RigakuD/Mmax 2500PC),
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Fourier transform infrared spectroscopy (FTIR, Nexu670), Field emission scanning
Fig.1 Schematic illustration of the synthesis route for SrFe12O19 nanosheets.
ACCEPTED MANUSCRIPT electron microscopy (FESEM, HitachiS4800), Transmission electron microscopy (TEM, JEOL-2100), Vibrating sample magnetometer (VSM, HH-15) and Vector network
wt% SrM nanosheets and 60 wt% paraffin. 3. Results and discussion
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analyzer (VNA, N5224A). The specimens for VNA measurement were prepared with 40
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Fig.2 (a) shows XRD diffraction patterns of SrM samples. It can be seen that the characteristic peaks corresponding to SrM (JCPDS no. 80-1198) appear in all samples,
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indicating the formation of hexagonal SrM crystallites. However, except for SrM18
Fig.2 (a) the XRD patterns, (b) the refined XRD patterns of SrM14 sample, and (c) FTIR spectra of samples with different hydrothermal temperature.
ACCEPTED MANUSCRIPT sample, a small fraction of diffraction peaks corresponding to SrCO3, α-Fe2O3 and Sr4Fe6O13 phases are also observed. The presence of SrCO3 secondary phase can be attributed to the reaction between Sr(NO3)2 and CO2 in air, the formation of α-Fe2O3
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may be originated from a higher disorder in the structure of SrFe2O3, and Sr4Fe6O13 phase may be attributed to the reaction between SrCO3 and SrFe12O19 [11, 12]. These
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results authenticate that the formation of SrM phase is significantly sensitive to hydrothermal temperature. For quantitative calculation of phase quantities, the
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refinement of SrM14 sample is carried out by the Rietveld method, as shown in Fig.2 (b). The refined results show that phase quantities corresponding to SrFe12O19, SrCO3 and α-Fe2O3 in SrM14 sample are 78.6 wt%, 13.3 wt% and 8.1 wt%, respectively. To further explore microstructure of samples, the FTIR spectra are recorded as shown in
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Fig.2 (c). The specific adsorption peaks at 590 cm−1, 551 cm−1 and 438 cm−1 correspond to symmetric stretching and out of plane bending vibration of tetrahedral and octahedral
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sites in SrM, respectively [13]. Similar to the XRD pattern, two narrow absorption peaks at 856 cm−1 and 668 cm−1 belonging to SrCO3 and of α-Fe2O3 compounds are also
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observed in the FTIR spectrum of SrM14 sample, which further confirms that the hydrothermal temperature acts as a key role in synthesis of SrM nanosheets. The typical SEM, TEM images and EDS spectrum of samples are shown in Fig.3.
As can be seen, although hexagonal SrM nanosheets have been formed at below 160 °C, some sphere-like and needle-like particles are also observed, which are assigned to α-Fe2O3 and SrCO3 phases. In addition, the samples display a certain degree of
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Fig.3 The SEM, TEM images and EDS spectrum of samples prepared at different hydrothermal temperatures (a) SrM14, (b) SrM16, (c, e, f) SrM18, (d) SrM20. aggregation, which is associated with the magnetic dipole interactions between nanoparticles. As expected, with increasing the hydrothermal temperature to 180 °C, the
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SrM samples (Fig. 3c) show regular hexagonal sheet structure. Especially, the samples exhibit better dispersibility than SrM particles as reported in [14-16], suggesting that the
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DEG plays an important role for controlling mophology and dispersibility of SrM nanosheets. The mean particle sizes are about 25 nm in thickness and 2 µm in diameter,
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which make SrM nanosheets possess large aspect ratio of about 80. What is more, the Fe/Sr molar ratio of 11.13 from EDS spectrum is very closed to stoichiometry of SrM, suggesting the pure phase of SrM nanosheets. However, further increasing the hydrothermal temperature to 200 °C, the particle size, especially along c-axis direction, displays a rapid increase. It indicates that the higher hydrothermal temperature is conducive to orientation growth of SrM nanosheets.
ACCEPTED MANUSCRIPT The magnetic hysteresis loops and corresponding magnetic parameters of samples at room temperature are shown in Fig.4. It can be seen that the Ms value increases monotonously from 37.61 A·m2·kg-1 to 73.6 A·m2·kg-1 with increasing hydrothermal
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temperature. This behavior is mainly due to decrease of nonmagnetic phase and improvement of crystallinity, which result in higher magnetic order within and surface
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of the particles. In addition, the Ms value of 73.6 A·m2·kg-1 is close to that of SrM single crystal prepared by ceramic route, indicating the modified hydrothermal method
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is appreciate to enhance magnetic properties of SrM [17]. Different from the change of Ms value, Hc value first increases and reaches maximum value of 510.8 kA·m-1 at 180 °C and then decreases. The variation of Hc is strongly related to magnetocrystaline anisotropy and particle size. Firstly, with increase of hydrothermal temperature,
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reduction of secondary phase and improvement of crystallinity enhance the spin-orbit coupling of Fe3+ ions in SrM structure, which lead strong magnetocrystaline anisotropy
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[18]. Secondly, the small particle size and large aspect ratio further improve uniaxial
Fig.4 (a) the magnetic hysteresis loops of SrM nanosheets, and (b) variation of Ms and Hc with hydrothermal temperature for SrM nanosheets.
ACCEPTED MANUSCRIPT anisotropy along c-axis of SrM, which gives rise to the increase of Hc value [19]. To evaluate microwave absorbing properties of SrM samples, electromagnetic parameters are measured. The real parts (ε', µ') and imaginary parts (ε", µ") stand for
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storage and inner dissipation capability of microwave absorber for electron and magnetic energies, respectively [20]. The variation of real (ε', µ') and imaginary (ε", µ")
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parts with frequency are illustrated in Fig.5. For complex permittivity, the ε' and ε" values of all samples do not obvious change with the increase of temperature. This is
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because that the dielectric properties of polycrystalline ferrite depend mainly on interfacial polarization and intrinsic dipole polarization, which occur as consequence of heterogeneous structure and electrons hopping between ions, respectively [21]. However, there are hardly presence of heterogeneous structure and occurance of
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electrons hopping during the synthesis of SrM nanosheets. For complex permeability, the µ' remains nearly constant from 7.0 GHz to 18.0 GHz, while µ" decreases
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monotonously. It can be explained by ferromagnetic resonance theory, the µ" is
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concluded and expressed as follows [22]:
µ＂ =
γMs 4παf
(1)
where γ, Ms and α is are the gyromagnetic ratio, the saturation magnetization and the extinction coefficient, respectively. Obviously, µ" is inversely related to f. It is noteworthy that, for SrM18 sample, a clear resonance peak at around 13.7 GHz is observed, which is mainly attributed to the large anisotropy field of SrM nanosheets. What is more, the tanε and tanµ values represent the electromagnetic wave dissipation
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Fig.5 Frequency dependence of (a, c) the real parts, (b, d) the imaginary parts of complex permittivity and permeability, (e) the dielectric loss tangent and (f) the magnetic loss tangent for SrM nanosheets.
and absorption capability of SrM nanosheets, respectively. The tanµ curves in Fig.5 (f) show that value of tanµ for SrM18 sample is larger than that of other samples, meaning the strongest electromagnetic wave absorption capability of SrM18 sample. Furthermore, the tanµ values of all samples are larger than their tanε values, which
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electromagnetic wave absorber.
To further explore the microwave absorbing properties of SrM nanosheets, based on the transmission line theory, RL value is calculated by following equations [1]:
{j ( 2 π fd
/ c )( µ r ε r ) 1 / 2
}
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Z in = Z o ( µ r / ε r ) 1 / 2 tanh
(2)
RL = 20 log ( Z in − Z o ) /( Z in + Z o )
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(3)
where, f, d, µr, εr, c, Z0 and Zin are respectively frequency, thickness, complex permeability, complex permittivity, light velocity, impedance of air and input impedance of sample. Generally, to achieve the largest reflection loss, the absorber must satisfy two conditions:
impedance
matching
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fundamental
characteristic
and
attenuation
characteristic [25]. Hence, to find the optimal matching thickness, the variation of RL
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with frequency at different thickness for SrM18 composite sample is characterized, as
Fig.6 Frequency dependence of reflection loss (RL) for SrM nanosheets (a) with different thickness and (b) with different temperature.
ACCEPTED MANUSCRIPT presented in Fig.6 (a). It can be clearly seen that the SrM composite sample with 3.5 mm thickness exhibits the largest RL value of -28.6 dB at matching frequency of 13.4 GHz, and the effective absorption bandwidth (RL < -10 dB) is 0.92 GHz. Furthermore,
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the relationship of hydrothermal temperature and RL is investigated, as shown in Fig.6 (b). It can be observed that SrM18 sample exhibits the largest reflection loss and the
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widest bandwidth. This can be explained by two reasons: firstly, the SrM18 sample possess the strongest magnetocrystaline anisotropy and the largest Hc value, which are
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proportional to natural resonance frequency. Secondly, the SrM18 sample is not only nanoscale in thickness, but also has large aspect ratio, which can produce remarkable surface effect. The surface effect leads to disordered state of surface spins, which helps
4. Conclusions
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enhance magnetic loss [26].
In summary, ultrathin SrM nanosheets were prepared by modified hydrothermal
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method. The mophology and dispersity could be controlled by adjusting diethylene glycol and hydrothermal temperature. When the hydrothermal temperature was 180 °C,
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the SrM nanosheets possessed uniform hexagonal structure and large aspect ratio. Benefiting from ultrathin structure and monodispersity, the SrM nanosheets showed outstanding microwave absorbing properties. The RL of SrM nanosheets reached maximum value of -28.6 dB at the thickness of 3.5 mm, and the absorption bandwidth for RL < -10 dB was 0.92 GHz at the high frequency region. The ultrathin SrM nanosheets, as microwave absorbing absorber, exhibited outstanding properties, which
ACCEPTED MANUSCRIPT indicated a potential application in electromagnetic interference. Acknowledgements This work is supported by National Natural Science Foundation of China (No.
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51202091) and Six Talent Peaks Project in Jiangsu Province (2016-XCL-015). Reference
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[3] X.F. Meng, Q.X. Han, Y.J. Sun, Y.F. Liu, Synthesis and microwave absorption properties of Ni0.5Zn0.5Fe2O4/BaFe12O19@polyaniline composite, Ceram. Inter. 45
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ACCEPTED MANUSCRIPT [6] W. Chen, X.X. Zhu, Q.Y. Liu, M. Fu, Preparation of urchin-like strontium ferrites as microwave absorbing materials, Mater. Lett. 209 (2017) 425-428. [7] X.F. Meng, J .L. Mi, Q. Li, B. Christian, M.D. Dong, One dimensional
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Magn. Mater. 426 (2017) 604-614.
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[10] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191-1334.
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[11] M. Jean, V. Nachbaur, J.Bran, J.M.L.Breton, Synthesis and characterization of SrFe12O19, powder obtained by hydrothermal process, J. Alloy. Compd. 496, 1
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[12] M.A.P. Buzinaro, N.S. Ferreira, F. Cunha, M.A. Macêdo, Hopkinson effect, structural and magnetic properties of M-type Sm3+-doped SrFe12O19 nanoparticles produced by a proteic sol-gel process, Ceram. Inter. 42 (2016) 5865-5872. [13] S. Kong, P. Zhang, X. Wen, P. Pi, J. Cheng, Z. Yang, J. Hai, Influence of surface modification of SrFe12O19 particles with oleic acid on magnetic microsphere
ACCEPTED MANUSCRIPT preparation, Particuology 6 (2008) 185-190. [14] B. Grindi, Z. Beji, G. Viau, A. BenAli, Microwave-assisted synthesis and magnetic properties of M-SrFe12O19 nanoparticles, J. Magn. Magn. Mater. 449 (2018)
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[16] T.T. Viet Nga, N.P. Duong, T.T. Loan, T.D. Hien, Key step in the synthesis of ultrafine strontium ferrite powders (SrFe12O19) by sol-gel method, J. Alloy. Compd. 610 (2014) 630-634.
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Structure, Handb. Ferromagn. Mater. 3 (1982) 305-391. [18] A. Ghasemi, A. Morisako, Static and high frequency magnetic properties of
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laTorre, R. Valenzuela, Synthesis of M-type SrFe12O19 by mechanosynthesis
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[22] R. Shams Alam, M. Moradi, H. Nikmanesh, J. Ventura, M. RostamiGh, Magnetic and microwave absorption properties of BaMgx/2Mnx/2CoxTi2xFe12-4xO19 hexaferrite
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Hong, X.Q. Wang, H.L. Ge, Structural, magnetic and electromagnetic properties of SrFe12O19 ferrite with particles aligned in a magnetic field, J. Alloy. Compd. 690 (2017) 936-941.
[24] S. Hazra, B.K. Ghosh, M.K. Patra, R.K. Jani, S.R. Vadera, N.N. Ghosh, One-pot
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synthesis of (NiFe2O4)x-(SrFe12O19)1-x nanocomposites and their microwave absorption properties, J. Nanosci. Nanotechnol. 15(2015) 6559-6567.
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[25] B.S. Zhang, Y. Feng, J. Xiong, Y. Yang, H.X. Lu, Microwave-absorbing properties of de-aggregated flake-shaped carbonyl-iron particle composites at 2-18 GHz, IEEE
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Highlights The purity SrFe12O19 nanosheets are synthesized by modified hydrothermal method.
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The SrFe12O19 nanosheets showed ultrathin structure and better dispersity.
The SrFe12O19 nanosheets exhibited outstanding magnetic properties.
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The SrFe12O19 nanosheets showed appropriate reflection loss and
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aborption bandwidth.
S0925-8388(18)34821-7
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.265
Reference:
JALCOM 48908
To appear in:
Journal of Alloys and Compounds
Received Date: 23 October 2018 Revised Date:
4 December 2018
Accepted Date: 21 December 2018
Please cite this article as: S. Dong, C. Lin, X. Meng, One-pot synthesis and microwave absorbing properties of ultrathin SrFe12O19 nanosheets, Journal of Alloys and Compounds (2019), doi: https:// doi.org/10.1016/j.jallcom.2018.12.265. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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One-pot Synthesis and Microwave Absorbing Properties of Ultrathin SrFe12O19 Nanosheets Shenhui Donga,#, Chucheng Linb,#, Xianfeng Menga*
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a. School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China
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b. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050,
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China These authors contributed equally to this work.
Abstract
Ultrathin SrFe12O19 nanosheets were successfully synthesized by modified
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hydrothermal method. The structure, morphology and properties were characterized and investigated by using X-ray diffractometer (XRD), Fourier transform infrared
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spectroscopy (FTIR), Field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), Vibrating sample magnetometer (VSM) and
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Vector network analyzer (VNA). The results revealed that the temperature and diethylene glycol were responsible for ultrathin structure and good dispersity of SrFe12O19 nanosheets. The SrM nanosheets showed strong magnetocrystaline anisotropy and large coercive force, which enhanced microwave absorbing properties of SrFe12O19. The reflection loss (RL) of a composite absorber with thickness of 3.5 mm reached a maximum value of -28.6 dB and the effective absorption bandwidth (RL < -10 dB) was
ACCEPTED MANUSCRIPT 0.92 GHz at the high frequency region, exhibiting excellent microwave absorbing properties as promising microwave absorber. Keywords: Magnetic materials, Strontium hexaferrite, Ultrathin nanosheet, Microwave
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absorber 1. Introduction
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Electromagnetic interference and pollution are posing serious threat to human health and electronic devices[1]. To resolve this problem, M-type SrFe12O19 (SrM) has
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gained immense interest due to its large anisotropy field, high coercive force, low cost and excellent chemical stability [2-4]. The excellent properties make SrM ferrite useful for electromagnetic shielding and microwave absorption materials [5]. Therefore, many strategies such as advanced synthesis techniques, cation substitution etc, have been
TE D
employed to improve microwave absorbing properties of SrM ferrite [6-9]. However, the radiation absorption performance of SrM ferrite still remains relatively modest. It is
EP
well known, the magnetic properties and microwave absorbing properties of SrM ferrite depend strongly on its microstructure, grain size and particle shape [10]. Unfortunately,
AC C
most reported SrM particles are suffering from the uncontrolled particle sizes and agglomeration. Thus so far, the preparation of SrM nanoparticles with nice dispersity and pure phase is still maintain the challenge. Herein, we reported ultrathin SrFe12O19 nanosheets with high aspect ratio and good dispersity, which were synthesized by modified hydrothermal method. The effect of temperature on structure, morphology and
ACCEPTED MANUSCRIPT magnetic properties were discussed. Subsequently, the microwave absorbing properties were investigated in detail. 2. Experimental procedures
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The SrFe12O19 nanosheets were prepared using analytical Fe(NO3)3·9H2O and Sr(NO3)2 as raw materials. The synthesis route for SrFe12O19 nanosheets was shown in
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Fig.1. In a typical process, 20.2 g Fe(NO3)3·9H2O and 1.51g Sr(NO3)2 were dissolved in 40 mL mixture with diethylene glycol (DEG, C4H10O3) and water (the mass ratio of
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DEG and water was 1:2) under stirring. Here, DEG was applied to control morphology and dispersity of products. Subsequently, NaOH solution was added dropwise to above solution until pH is equal to 10. Afterwards, the mixture solution were poured into a 50 ml reactor and heat-treated at various temperature for 24 h. Finally, the samples were
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cooled, washed and dried. The obtained samples at 140 °C, 160 °C, 180 °C and 200 °C were marked as SrM14, SrM16, SrM18 and SrM20, respectively.
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The structure, morphology, electromagnetic parameters of samples were respectively characterized using X-ray diffractometer (XRD, RigakuD/Mmax 2500PC),
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Fourier transform infrared spectroscopy (FTIR, Nexu670), Field emission scanning
Fig.1 Schematic illustration of the synthesis route for SrFe12O19 nanosheets.
ACCEPTED MANUSCRIPT electron microscopy (FESEM, HitachiS4800), Transmission electron microscopy (TEM, JEOL-2100), Vibrating sample magnetometer (VSM, HH-15) and Vector network
wt% SrM nanosheets and 60 wt% paraffin. 3. Results and discussion
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analyzer (VNA, N5224A). The specimens for VNA measurement were prepared with 40
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Fig.2 (a) shows XRD diffraction patterns of SrM samples. It can be seen that the characteristic peaks corresponding to SrM (JCPDS no. 80-1198) appear in all samples,
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indicating the formation of hexagonal SrM crystallites. However, except for SrM18
Fig.2 (a) the XRD patterns, (b) the refined XRD patterns of SrM14 sample, and (c) FTIR spectra of samples with different hydrothermal temperature.
ACCEPTED MANUSCRIPT sample, a small fraction of diffraction peaks corresponding to SrCO3, α-Fe2O3 and Sr4Fe6O13 phases are also observed. The presence of SrCO3 secondary phase can be attributed to the reaction between Sr(NO3)2 and CO2 in air, the formation of α-Fe2O3
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may be originated from a higher disorder in the structure of SrFe2O3, and Sr4Fe6O13 phase may be attributed to the reaction between SrCO3 and SrFe12O19 [11, 12]. These
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results authenticate that the formation of SrM phase is significantly sensitive to hydrothermal temperature. For quantitative calculation of phase quantities, the
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refinement of SrM14 sample is carried out by the Rietveld method, as shown in Fig.2 (b). The refined results show that phase quantities corresponding to SrFe12O19, SrCO3 and α-Fe2O3 in SrM14 sample are 78.6 wt%, 13.3 wt% and 8.1 wt%, respectively. To further explore microstructure of samples, the FTIR spectra are recorded as shown in
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Fig.2 (c). The specific adsorption peaks at 590 cm−1, 551 cm−1 and 438 cm−1 correspond to symmetric stretching and out of plane bending vibration of tetrahedral and octahedral
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sites in SrM, respectively [13]. Similar to the XRD pattern, two narrow absorption peaks at 856 cm−1 and 668 cm−1 belonging to SrCO3 and of α-Fe2O3 compounds are also
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observed in the FTIR spectrum of SrM14 sample, which further confirms that the hydrothermal temperature acts as a key role in synthesis of SrM nanosheets. The typical SEM, TEM images and EDS spectrum of samples are shown in Fig.3.
As can be seen, although hexagonal SrM nanosheets have been formed at below 160 °C, some sphere-like and needle-like particles are also observed, which are assigned to α-Fe2O3 and SrCO3 phases. In addition, the samples display a certain degree of
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Fig.3 The SEM, TEM images and EDS spectrum of samples prepared at different hydrothermal temperatures (a) SrM14, (b) SrM16, (c, e, f) SrM18, (d) SrM20. aggregation, which is associated with the magnetic dipole interactions between nanoparticles. As expected, with increasing the hydrothermal temperature to 180 °C, the
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SrM samples (Fig. 3c) show regular hexagonal sheet structure. Especially, the samples exhibit better dispersibility than SrM particles as reported in [14-16], suggesting that the
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DEG plays an important role for controlling mophology and dispersibility of SrM nanosheets. The mean particle sizes are about 25 nm in thickness and 2 µm in diameter,
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which make SrM nanosheets possess large aspect ratio of about 80. What is more, the Fe/Sr molar ratio of 11.13 from EDS spectrum is very closed to stoichiometry of SrM, suggesting the pure phase of SrM nanosheets. However, further increasing the hydrothermal temperature to 200 °C, the particle size, especially along c-axis direction, displays a rapid increase. It indicates that the higher hydrothermal temperature is conducive to orientation growth of SrM nanosheets.
ACCEPTED MANUSCRIPT The magnetic hysteresis loops and corresponding magnetic parameters of samples at room temperature are shown in Fig.4. It can be seen that the Ms value increases monotonously from 37.61 A·m2·kg-1 to 73.6 A·m2·kg-1 with increasing hydrothermal
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temperature. This behavior is mainly due to decrease of nonmagnetic phase and improvement of crystallinity, which result in higher magnetic order within and surface
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of the particles. In addition, the Ms value of 73.6 A·m2·kg-1 is close to that of SrM single crystal prepared by ceramic route, indicating the modified hydrothermal method
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is appreciate to enhance magnetic properties of SrM [17]. Different from the change of Ms value, Hc value first increases and reaches maximum value of 510.8 kA·m-1 at 180 °C and then decreases. The variation of Hc is strongly related to magnetocrystaline anisotropy and particle size. Firstly, with increase of hydrothermal temperature,
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reduction of secondary phase and improvement of crystallinity enhance the spin-orbit coupling of Fe3+ ions in SrM structure, which lead strong magnetocrystaline anisotropy
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[18]. Secondly, the small particle size and large aspect ratio further improve uniaxial
Fig.4 (a) the magnetic hysteresis loops of SrM nanosheets, and (b) variation of Ms and Hc with hydrothermal temperature for SrM nanosheets.
ACCEPTED MANUSCRIPT anisotropy along c-axis of SrM, which gives rise to the increase of Hc value [19]. To evaluate microwave absorbing properties of SrM samples, electromagnetic parameters are measured. The real parts (ε', µ') and imaginary parts (ε", µ") stand for
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storage and inner dissipation capability of microwave absorber for electron and magnetic energies, respectively [20]. The variation of real (ε', µ') and imaginary (ε", µ")
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parts with frequency are illustrated in Fig.5. For complex permittivity, the ε' and ε" values of all samples do not obvious change with the increase of temperature. This is
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because that the dielectric properties of polycrystalline ferrite depend mainly on interfacial polarization and intrinsic dipole polarization, which occur as consequence of heterogeneous structure and electrons hopping between ions, respectively [21]. However, there are hardly presence of heterogeneous structure and occurance of
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electrons hopping during the synthesis of SrM nanosheets. For complex permeability, the µ' remains nearly constant from 7.0 GHz to 18.0 GHz, while µ" decreases
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monotonously. It can be explained by ferromagnetic resonance theory, the µ" is
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concluded and expressed as follows [22]:
µ＂ =
γMs 4παf
(1)
where γ, Ms and α is are the gyromagnetic ratio, the saturation magnetization and the extinction coefficient, respectively. Obviously, µ" is inversely related to f. It is noteworthy that, for SrM18 sample, a clear resonance peak at around 13.7 GHz is observed, which is mainly attributed to the large anisotropy field of SrM nanosheets. What is more, the tanε and tanµ values represent the electromagnetic wave dissipation
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Fig.5 Frequency dependence of (a, c) the real parts, (b, d) the imaginary parts of complex permittivity and permeability, (e) the dielectric loss tangent and (f) the magnetic loss tangent for SrM nanosheets.
and absorption capability of SrM nanosheets, respectively. The tanµ curves in Fig.5 (f) show that value of tanµ for SrM18 sample is larger than that of other samples, meaning the strongest electromagnetic wave absorption capability of SrM18 sample. Furthermore, the tanµ values of all samples are larger than their tanε values, which
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electromagnetic wave absorber.
To further explore the microwave absorbing properties of SrM nanosheets, based on the transmission line theory, RL value is calculated by following equations [1]:
{j ( 2 π fd
/ c )( µ r ε r ) 1 / 2
}
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Z in = Z o ( µ r / ε r ) 1 / 2 tanh
(2)
RL = 20 log ( Z in − Z o ) /( Z in + Z o )
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(3)
where, f, d, µr, εr, c, Z0 and Zin are respectively frequency, thickness, complex permeability, complex permittivity, light velocity, impedance of air and input impedance of sample. Generally, to achieve the largest reflection loss, the absorber must satisfy two conditions:
impedance
matching
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fundamental
characteristic
and
attenuation
characteristic [25]. Hence, to find the optimal matching thickness, the variation of RL
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with frequency at different thickness for SrM18 composite sample is characterized, as
Fig.6 Frequency dependence of reflection loss (RL) for SrM nanosheets (a) with different thickness and (b) with different temperature.
ACCEPTED MANUSCRIPT presented in Fig.6 (a). It can be clearly seen that the SrM composite sample with 3.5 mm thickness exhibits the largest RL value of -28.6 dB at matching frequency of 13.4 GHz, and the effective absorption bandwidth (RL < -10 dB) is 0.92 GHz. Furthermore,
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the relationship of hydrothermal temperature and RL is investigated, as shown in Fig.6 (b). It can be observed that SrM18 sample exhibits the largest reflection loss and the
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widest bandwidth. This can be explained by two reasons: firstly, the SrM18 sample possess the strongest magnetocrystaline anisotropy and the largest Hc value, which are
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proportional to natural resonance frequency. Secondly, the SrM18 sample is not only nanoscale in thickness, but also has large aspect ratio, which can produce remarkable surface effect. The surface effect leads to disordered state of surface spins, which helps
4. Conclusions
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enhance magnetic loss [26].
In summary, ultrathin SrM nanosheets were prepared by modified hydrothermal
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method. The mophology and dispersity could be controlled by adjusting diethylene glycol and hydrothermal temperature. When the hydrothermal temperature was 180 °C,
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the SrM nanosheets possessed uniform hexagonal structure and large aspect ratio. Benefiting from ultrathin structure and monodispersity, the SrM nanosheets showed outstanding microwave absorbing properties. The RL of SrM nanosheets reached maximum value of -28.6 dB at the thickness of 3.5 mm, and the absorption bandwidth for RL < -10 dB was 0.92 GHz at the high frequency region. The ultrathin SrM nanosheets, as microwave absorbing absorber, exhibited outstanding properties, which
ACCEPTED MANUSCRIPT indicated a potential application in electromagnetic interference. Acknowledgements This work is supported by National Natural Science Foundation of China (No.
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51202091) and Six Talent Peaks Project in Jiangsu Province (2016-XCL-015). Reference
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Highlights The purity SrFe12O19 nanosheets are synthesized by modified hydrothermal method.
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The SrFe12O19 nanosheets showed ultrathin structure and better dispersity.
The SrFe12O19 nanosheets exhibited outstanding magnetic properties.
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The SrFe12O19 nanosheets showed appropriate reflection loss and
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aborption bandwidth.