- Email: [email protected]
ferrite composites
Journal Pre-proof Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl / ferrite composites Chenglong Lei, Youwei Du PII:
S0925-8388(20)30037-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153674
Reference:
JALCOM 153674
To appear in:
Journal of Alloys and Compounds
Received Date: 22 October 2019 Revised Date:
28 December 2019
Accepted Date: 3 January 2020
Please cite this article as: C. Lei, Y. Du, Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153674. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Credit Author Statement Chenglong Lei: Conceptualization, Methodology, Writing- Original draft preparation, Funding acquisition. Youwei Du: Conceptualization, Project administration, Supervision, Writing - Review & Editing.
Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites Chenglong Lei1,3,* and Youwei Du2 1
College of Engineering and Applied Science, Nanjing University, Nanjing 210093, China.
2
Department of Physics, Nanjing University, Nanjing 210093, China
3
Haian Institute of High-Tech Research, Nanjing University, Nanjing 210093, China
Abstract Flakey FeSiAl/ferrite composites were prepared via ball milling-assisted co-precipitation to evaluate the microstructure, morphology, and microwave absorption. Measurement of the electromagnetic properties indicated that the permittivity and permeability of flakey FeSiAl composites can be well tuned by adjusting the amount of Ni-Zn ferrite loads. We found an extraordinary tunable dielectric-type loss that enhanced microwave absorption due to the interface between FeSiAl and ferrites. This enhanced microwave absorption of flakey FeSiAl composites is derived from the synergy of metal-ferrite interface and dielectric matching. Controlled by ferrite loads of 8wt.% content, enhanced microwave absorption is achieved with the reflection loss –29.2 dB and bandwidth 4 GHz at matching thickness 2.5 mm. Furthermore, the extraordinary tunable dielectric-type loss is thickness-independent. This promising result revealed a great opportunity for flakey FeSiAl/ferrite composites in microwave absorption applications. Keywords: Flakey FeSiAl, Ferrite coating, ball milling, Co-precipitation, Microwave absorption 1. Introduction Flakey FeSiAl powders have attracted considerable interest for applications in shielding from electromagnetic interference and in microwave absorption due to their low cost, large anisotropy, high permittivity, and good temperature stability. Furthermore, the flakey particles can have a higher resonance frequency above *
Correspondence to: [email protected]
Snoek’s limit in the gigahertz frequency range due to the strong anisotropy in shape [1]. However, a highly complex permeability accompanied by a higher complex permittivity of FeSiAl results in poor impedance matching and relatively narrow microwave absorption bandwidth. Thus, the composite of a low electrical conductivity material is key for an FeSiAl alloy to improve the matching impedance. When compared to a single type of absorber, the composites always exhibit a wider spectrum range of microwave absorption along with stronger reflection loss[2-6]. To date, the composite of nonmagnetic or magnetic dielectric materials on the surface of FeSiAl alloy has been an effective method used to regulate dielectric loss and improve the microwave absorption properties. The microwave loss mechanism of the above material can be classified into two categories, namely dielectric loss and magnetic loss material. The nonmagnetic dielectric loss candidates are rather versatile: BaTiO3 [7], Al2O3 [8, 9], ZnO [10, 11], MgO [12], graphite [13, 14], MWCNT [15], and SiO2 [16, 17]. These has been widely used to enhance the electrical resistivity of the FeSiAl alloy. Unfortunately, the nonmagnetic composite leads to a decrease in permeability. The magnetic composites such as Fe3O4 [18, 19], NiCuZn ferrite [20], ZnFe2O4 [21], and Mn-Zn ferrites[22] possess both strong permeability and relative high resistivity. Therefore, combining magnetic ferrite materials with FeSiAl sheets not only may retain the property of a high impedance matching but also may increase the ability for integral dielectric loss. Previous research has mostly focused on the influence of magnetic dielectric materials on the permeability of flakey FeSiAl materials through a simple stirring mix or ball milling approach. Generally, high-performance magnetic absorbers are designed through interface strategies of composites between magnetic and dielectric materials. The interface polarization effect is the most effective method to improve dielectric loss, especially at high frequencies[23]. To date, the excellent microwave absorbing properties of FeSiAl/ Ni-Zn ferrite sheets are not apparent and remain to be further investigated through controllable interface growth. In this article, we report an approach to generate metal−ferrite interfaces through growing ferrites onto flakey FeSiAl. Because of the formation of coatings with a
continuous and homogenous structure and the simple preparation process of chemical co-precipitation, the electromagnetic properties of FeSiAl/Ni-Zn ferrite composites can be well tuned by the controllable metal−ferrite interface. An extraordinary tunable dielectric-type loss was found to enhance the microwave absorption of composites between FeSiA and Ni-Zn ferrites. 2. Experiment procedure Flakey morphology of FeSiAl alloy powder was prepared via mechanical ball milling in triolein solvent. The irregular FeSiAl raw powders (Si 9.6 wt.%-Al 5.4 wt.%-balance Fe, purity > 99.9%, particle size < 300 mesh) were purchased from Changsha Tianjiu Co. Ltd. The ball-to-powder weight ratio of 10:1 was applied, and milling was conducted for 15 h at 400 rpm. Then the products were dried in an oven for 2 h at 60°C and collected. Flakey FeSiAl/Ni0.5Zn0.5Fe2O4 composites with different amounts of ferrite (the quality scores are 2%, 4%, 6%, 8%, 10%) were achieved by co-precipitation. The synthesis was carried out using commercially available reagents: ferric nitrate (Fe(NO3)3·9H2O,
AR),
nickel
nitrate
(Ni(NO3)2·6H2O,
AR),
zinc
nitrate
(Zn(NO3)2·6H2O, AR) and sodium hydrate. The prepared flakey FeSiAl powders were put into deionized water and stirred at 150 r/min, and then the analytical grade of the reagents in different amounts were added to the powders. After the precipitate agent of NaOH began dripping at 5 ml/min into the mixtures, the bath was continuously stirred for 30 min to expose each particle in the solution so that there was uniformly deposited precipitation on the surface of the flakey FeSiAl powders. The composite particles were filter cleaned in deionized water until PH=7 was reached, and then they were dried in an oven for 2 h at 60°C. Finally, the brown red products were annealed at 600°C for 2 h under an argon atmosphere. The crystallographic structure, microstructure, and morphology were studied by X-ray powder diffraction ((Bruker D8, Bruker, Germany) with Cu-Ka radiation (λ = 0.1541 nm) and scanning electron microscopy (SEM, Hitachi S3400, Japan), respectively. Either FeSiAl flakes or ferrite coated FeSiAl flakes were mixed with
paraffin in a weight ratio 3:2 of the powder to paraffin for dynamic electromagnetic measurements, resulting in a toroid with an inner diameter of 3.0 mm and an outer diameter of 8.0 mm. Finally, the complex relative permittivity and permeability of the samples were measured by using Agilent N5244A network vector analyzer within a frequency range of 1-18 GHz. According to the transmission line theory, the theoretical reflection loss curves (RL) at a given thickness can be calculated. 3. Results and discussion Figure 1 shows the XRD pattern of the obtained composite FeSiAl alloy sheets with different amounts of Ni−Zn ferrite. It shows the Ni-Zn ferrite crystal planes at 30.1, 35.5, 43, 53.3, 57.0, 62.7, 75 which are the characteristic peaks corresponding to the PDF card JCPDS#52-0278. The evident diffraction peaks of a-FeSiAl alloy with bcc structure are at 45, 65.5, and 83, and the DO3 super-lattice structures are at 27, 31.5, and 53, corresponding to the PDF card JCPDS #45-1206. It is clear that the spinel structure of ferrites appear in the crystal structure of the FeSiAl alloy, even though the loads of (Ni0.5Zn0.5)Fe2O4 are at or around 2 wt.%. This is different from the results previously reported in the literature that the ferrite loads exceeding 6 wt.% cannot be detected just by mechanically stirring FeSiAl and Ni-Zn ferrites [24]. The possible reason may be that co-precipitation generates the continuous and homogenous coatings of ferrite on the surface of FeSiAl sheets. Subsequently the SEM morphology characterization confirms this.
.
Fig. 1 XRD pattern of FeSiAl flakes composites with various weight ratios of ferrite. Figure 2 shows the SEM images of the obtained composite FeSiAl alloy powders with different amounts of Ni-Zn coated with ferrite. As is shown in Fig. 2 (a), (c), (e), (g) and (i), the results show that the composite FeSiAl powders are flakey. The nano-ferrites are in-situ growths on the surface of FeSiAl alloy flakes. In Fig. 2(b), we can observe the massive nano-sheet of ferrites deposited on the surface of flakey FeSiAl when the weight ratio is 2 wt.%. The thickness of the milled FeSiAl alloy sheets are less than 1.5 µm and the aspect ratios such as length/thickness are more than 15. It is expected that the large aspect ratio of the FeSiAl sheets favors increasing the magnetic loss in high-frequency region by overcoming the Snoek’s limit. Moreover, the thickness of the sheet is below the typical skin depth of traditional FeSiAl alloy ~2 µm [25]. From Fig. 2(d), (f), (h) and (g), we can also observe that the density of the surface area changes as the loads increase from 4 wt.% to 10 wt.%. The energy dispersive spectrometer (EDS) of the composite material with 8 wt.% loads obtained from the inset of Fig. 2(k) shows that the elements on the surface contain Fe, Si, Al, Ni, Zn and O. Because the raw materials were iron, silicon and aluminum, it is easily to recognize that the coating layer is the Ni0.5Zn0.5Fe2O4 ferrite.
Fig. 2 SEM images of flakey FeSiAl/ferrite composites particles with various amounts of coating: (a and b) 2 wt.%, (c and d) 4 wt.%, (e and f) 6 wt.%, (g and h) 8 wt.%, (i and j) 10 wt.%, and (k) EDS of composites with 8 wt.% Ni-Zn ferrite. Figure 3 shows the complex permittivity and permeability of the milled FeSiAl and the flakey FeSiAl/ferrite composites with different amounts of loading. The frequency dependence of the real and imaginary permittivity of composite samples is shown in Fig. 3(a) and (b). The real permittivity values of composites are found to decrease when compared with flakey FeSiAl. For all the FeSiAl/ferrite composite samples the real permittivity is found to be nearly constant for 2 wt.%, 4 wt.%, 6 wt.%, 8 wt.%, 10 wt.%, with values of 7.2, 7.8, 7.5, 8.6, and 7.7, respectively, for a frequency range of 2-10 GHz. However, the values display an obvious resonant peak in 10 GHz-12 GHz regime, while the loads exceeding 4 wt.% correspond to the matching frequency zone. Similarly, the imaginary permittivity value for all composites have an approximate value 0.2 (at 2 GHz) and also exhibit those resonant peaks in the corresponding frequency range. Further the real and imaginary permittivity of flakey FeSiAl composites with a loading of 8 wt.% NiZn ferrites show higher resonance among the composites. The real and imaginary complex permeability of composite samples are shown in Fig. 3(c) and (d). The real and
imaginary permeability decrease monotonically as the frequency increases from 2 to 18 GHz for all composite samples. However, the complex permeability dependence of the amount of load is not obvious. The real and imaginary permeability for all composites are 1.4-1.6 (at 2GHz) and 0.48-0.56 (at 2GHz), respectively. There is also a resonant peak at 10-12 GHz for the FeSiAl composites coated with 8 wt.% Ni-Zn ferrites.
Fig. 3 Real part (a) - (c) and imaginary part (b) - (d) of complex permittivity and complex permeability of flakey FeSiAl/ferrite composites with different amounts of coating. Based on these complex relative permittivity and permeability of the composites, the microwave reflection loss curves (RL) of the composite absorbers were calculated. The reflection loss curves with changed thickness (d) and the impedance were drawn by Equations (1) and (2):
(1)
,
(2)
where f is the frequency, d is the thickness of the absorber layer, c is the speed of an
electromagnetic wave in free space.
and
are the
complex permeability and permittivity of the absorber, respectively. According to the theory of impedance matching, the impedance z of ideal materials should be closest to 1. Generally, for most of the absorption materials, the value of permittivity (
) is
larger than permeability ( ), so the impedance matching cannot be ideally achieved. It is known that magnetic materials offer improvement of the complex permeability parameters. A higher permeability value will result in a larger impedance matching value, and thus, an electromagnetic wave can easily be absorbed. Figure 4 shows the calculated RL of the milled FeSiAl alloys and FeSiAl/ferrite composites in the range of 2-18 GHz. From Fig. 4(a), we see that the ferrite loads have important effects on the microwave absorbing performance of the flakey FeSiAl alloys. The microwave absorption is enhanced by coating the ferrites and shifting to high frequency. The quarter-wavelength cancellation model can be applied to explain the relationship between the RL peaks (fm) and the matching thickness (dm), which is determined by Equation (3): .
(3)
From Fig. 3, we can observe the relatively high complex permeability and permittivity for loads of 8 wt.% ferrite compared with other composites; hence, the reflection loss peaks of 8 wt.% ferrite loads shown in Fig. 4 (a) shift to low frequency based on Eq. (3). In addition, the results indicated that the position and intensity of the peaks are sensitive to the weight ratio of ferrite loads. The maximum RL value of −29.2 dB is observed at 10.7 GHz with matching thickness 2.5 mm while the amount of ferrite loading is 8 wt.%. The microwave absorption properties of the FeSiAl/ferrite composites (8 wt.%) together with other flakey Fe-based and FeSiAl alloys composites reported in recent literature are summarized in Table 1. In Fig. 4(b) - (d), the effective RL curves for 6 wt.%, 8 wt.%, and 10 wt.%, respectively, show a small attenuation peak along with the main attenuation peak at thickness 2 mm or 3 mm. Subsequently, it disappears, and the intensity of reflection loss reaches a maximum
when the matching thickness is 2.5 mm. Meanwhile, the attenuation of the small peak is most obvious when loading ferrites have a weight ratio of 8 wt.%. Similar phenomena of the small resonant peak have also been observed in the microwave absorption materials such as Fe@ZnFe2O4 and Sm1.5Y0.5Fe17-x Six [3, 21, 26]. The thickness-independent peak almost fixed at specific frequency. It is noted the peak of the imaginary parts of the permittivity is also about 10.7 GHz in Fig.3b, suggesting that the fixed attenuation peak originates from the dielectric-type loss mechanism [26]. The shift of the peak towards the low-frequency range with increasing thickness can be explained by the quarter wave theory. The quarter wave theory suggests an interference type loss, which is the intrinsic relation between the absorbing performance and the magnetic properties of the composites.
Fig. 4 Reflection loss curves of flakey FeSiAl/ferrite composites (a) in thickness 2.5 mm with different weight ratio, (b) with weight ratio of 6%, (c) 8% and (d) 10% in different thickness (2-3 mm). Table 1. Microwave Absorption Properties of Flakey Alloy Composites Reported in a Recent Paper.
Filler
Thickn ess (mm)
Resonance frequency (GHz)
RLmin (dB)
Bandw idth (GHz)
Synthesis approach
Ref.
FeSiAl FeSiAl/SiO2 FeSiAl/ZnO FeSiAl/Al2O3 FeSiAl/BaTiO3 FeSiAl
2 2.2 1.9 4 2.5
12.5 13.7 10.3 9.4 3.1 1.25
-20.5 -39.4 -40.5 -34.2 -18.1 -10
5.6 3.5 3.5 1.9 2.8 -
[27] [17] [10] [8] [7] [28]
Aligned FeSiAl FeSiAl/graphite Fe-based amorphous FeSiAl/MnZnFe2O4 FeSiAl/NiZnFe2O4 FeSiAl/NiCuZn ferrite
2.5 3 2 6
1.5 6.7 11.6 1.4
-8.8 -21 -27.3 -20
1.8 2 4.5 1.2
Ball milling ultrasonic ultrasonic Hot-pressed stirring Meltquenching ball milling Ball milling Ball milling Precipitation Stirring Sol-gel
FeSiAl/NiZn ferrtie
2.5
10.7
-29.2
3.75
Precipitation
This work
[29] [13] [30] [22] [24] [20]
Figure 5 shows reflection loss curves of flakey FeSiAl coated with 8 wt.% ferrites and the variation of the bandwidth of microwave absorption (RL > 10 dB) for composites at different thicknesses (2.0-5.0 mm). The effective RL of more than 10 dB with an amount of 8 wt.% ferrite could be achieved over a frequency range of 3.5-17.5 GHz while varying thickness from 2 to 5 mm. As shown in Fig. 5(a), the reflection loss curves of flakey FeSiAl/ferrite composites with loads of 8 wt.% at different thickness all exhibit small attenuation peaks at a frequency around 10.5 GHz. Although the accompanying attenuation peaks have been observed in other materials as well; the puzzle is that the position of the attenuation peak barely varies with the thickness shown in the shadow region of Fig. 5(a). The possible explanation is that the resonant peaks of the complex permittivity and permeability originates from the dielectric-type loss. Therefore, the RL peak is thickness-independent and almost fixed at 10-12 GHz region. In Fig. 5(b), the absorption bandwidth of composites increase and then decrease with the increase of thickness. However, an enhanced absorption bandwidth is observed for ferrite loads exceeding 8 wt.% with maximum 4 GHz bandwidth at matching thickness 2.5-3.0 mm. The bandwidth of absorption (more
than 10 dB) can be tuned from 1.8 to 4 GHz, while varying the thickness of composites from 2.0 to 5.0 mm, respectively.
Fig. 5 (a) Reflection loss curves of coated 8 wt.% ferrites with a small attenuation peaks shown in shaded area and (b) bandwidth of flakey FeSiAl/ferrite composites coated with different weight ratios varying with thickness. 4. Conclusions Flakey FeSiAl/Ni-Zn ferrite composites were prepared via ball milling-assisted co-precipitation. The structure and morphology show that the spinel ferrites grow on the surface of FeSiAl sheets. As the loads increase, so does the surface density of flakey FeSiAl. An extraordinary dielectric-type microwave absorption was found due to generating metal−ferrite interfaces through growing ferrites on FeSiAl sheets. The dielectric-type loss to enhance microwave absorption can be well tuned by the controllable Ni-Zn ferrite loads. Enhanced microwave absorption is achieved with the minimum reflection loss –29.2 dB at a frequency of 10.7 GHz. The absorption bandwidth is observed at a maximum of 4 GHz at matching thickness 2.5-3.0 mm for ferrite loads exceeding 8 wt.%. Effective microwave absorption as the reflection loss (RL) < -10 dB bandwidth over a frequency range of 3.5-17.5 GHz at a thickness of 2-5 mm reveals an excellent opportunity for flakey FeSiAl/ferrite composites in applications for microwave absorption. Acknowledgments: The authors acknowledge the financial support from the National Natural Science Foundation (No. 51801097), the Industrial Innovation of Applied
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1. Density and homogenous, flakey FeSiAl/Ni-Zn ferrite composites were fabricated via ball milling-assisted co-precipitation 2. Controlled by ferrite loads, enhance microwave absorption is achieved with the reflection loss –29.2 dB and bandwidth 4 GHz at matching thickness 2.5 mm 3. An extraordinary tunable dielectric-type loss was found to enhance the microwave absorption which is thickness-independent
Declaration of Interest Statement On behalf of my coauthors, I would like to submit the manuscript entitled “Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites” for publication in Journal of Alloys and Compounds. I hereby certify that this paper consists of original, unpublished work which is not under consideration for publication elsewhere. The authors claim no conflicts of interest.
S0925-8388(20)30037-2
DOI:
https://doi.org/10.1016/j.jallcom.2020.153674
Reference:
JALCOM 153674
To appear in:
Journal of Alloys and Compounds
Received Date: 22 October 2019 Revised Date:
28 December 2019
Accepted Date: 3 January 2020
Please cite this article as: C. Lei, Y. Du, Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153674. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.
Credit Author Statement Chenglong Lei: Conceptualization, Methodology, Writing- Original draft preparation, Funding acquisition. Youwei Du: Conceptualization, Project administration, Supervision, Writing - Review & Editing.
Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites Chenglong Lei1,3,* and Youwei Du2 1
College of Engineering and Applied Science, Nanjing University, Nanjing 210093, China.
2
Department of Physics, Nanjing University, Nanjing 210093, China
3
Haian Institute of High-Tech Research, Nanjing University, Nanjing 210093, China
Abstract Flakey FeSiAl/ferrite composites were prepared via ball milling-assisted co-precipitation to evaluate the microstructure, morphology, and microwave absorption. Measurement of the electromagnetic properties indicated that the permittivity and permeability of flakey FeSiAl composites can be well tuned by adjusting the amount of Ni-Zn ferrite loads. We found an extraordinary tunable dielectric-type loss that enhanced microwave absorption due to the interface between FeSiAl and ferrites. This enhanced microwave absorption of flakey FeSiAl composites is derived from the synergy of metal-ferrite interface and dielectric matching. Controlled by ferrite loads of 8wt.% content, enhanced microwave absorption is achieved with the reflection loss –29.2 dB and bandwidth 4 GHz at matching thickness 2.5 mm. Furthermore, the extraordinary tunable dielectric-type loss is thickness-independent. This promising result revealed a great opportunity for flakey FeSiAl/ferrite composites in microwave absorption applications. Keywords: Flakey FeSiAl, Ferrite coating, ball milling, Co-precipitation, Microwave absorption 1. Introduction Flakey FeSiAl powders have attracted considerable interest for applications in shielding from electromagnetic interference and in microwave absorption due to their low cost, large anisotropy, high permittivity, and good temperature stability. Furthermore, the flakey particles can have a higher resonance frequency above *
Correspondence to: [email protected]
Snoek’s limit in the gigahertz frequency range due to the strong anisotropy in shape [1]. However, a highly complex permeability accompanied by a higher complex permittivity of FeSiAl results in poor impedance matching and relatively narrow microwave absorption bandwidth. Thus, the composite of a low electrical conductivity material is key for an FeSiAl alloy to improve the matching impedance. When compared to a single type of absorber, the composites always exhibit a wider spectrum range of microwave absorption along with stronger reflection loss[2-6]. To date, the composite of nonmagnetic or magnetic dielectric materials on the surface of FeSiAl alloy has been an effective method used to regulate dielectric loss and improve the microwave absorption properties. The microwave loss mechanism of the above material can be classified into two categories, namely dielectric loss and magnetic loss material. The nonmagnetic dielectric loss candidates are rather versatile: BaTiO3 [7], Al2O3 [8, 9], ZnO [10, 11], MgO [12], graphite [13, 14], MWCNT [15], and SiO2 [16, 17]. These has been widely used to enhance the electrical resistivity of the FeSiAl alloy. Unfortunately, the nonmagnetic composite leads to a decrease in permeability. The magnetic composites such as Fe3O4 [18, 19], NiCuZn ferrite [20], ZnFe2O4 [21], and Mn-Zn ferrites[22] possess both strong permeability and relative high resistivity. Therefore, combining magnetic ferrite materials with FeSiAl sheets not only may retain the property of a high impedance matching but also may increase the ability for integral dielectric loss. Previous research has mostly focused on the influence of magnetic dielectric materials on the permeability of flakey FeSiAl materials through a simple stirring mix or ball milling approach. Generally, high-performance magnetic absorbers are designed through interface strategies of composites between magnetic and dielectric materials. The interface polarization effect is the most effective method to improve dielectric loss, especially at high frequencies[23]. To date, the excellent microwave absorbing properties of FeSiAl/ Ni-Zn ferrite sheets are not apparent and remain to be further investigated through controllable interface growth. In this article, we report an approach to generate metal−ferrite interfaces through growing ferrites onto flakey FeSiAl. Because of the formation of coatings with a
continuous and homogenous structure and the simple preparation process of chemical co-precipitation, the electromagnetic properties of FeSiAl/Ni-Zn ferrite composites can be well tuned by the controllable metal−ferrite interface. An extraordinary tunable dielectric-type loss was found to enhance the microwave absorption of composites between FeSiA and Ni-Zn ferrites. 2. Experiment procedure Flakey morphology of FeSiAl alloy powder was prepared via mechanical ball milling in triolein solvent. The irregular FeSiAl raw powders (Si 9.6 wt.%-Al 5.4 wt.%-balance Fe, purity > 99.9%, particle size < 300 mesh) were purchased from Changsha Tianjiu Co. Ltd. The ball-to-powder weight ratio of 10:1 was applied, and milling was conducted for 15 h at 400 rpm. Then the products were dried in an oven for 2 h at 60°C and collected. Flakey FeSiAl/Ni0.5Zn0.5Fe2O4 composites with different amounts of ferrite (the quality scores are 2%, 4%, 6%, 8%, 10%) were achieved by co-precipitation. The synthesis was carried out using commercially available reagents: ferric nitrate (Fe(NO3)3·9H2O,
AR),
nickel
nitrate
(Ni(NO3)2·6H2O,
AR),
zinc
nitrate
(Zn(NO3)2·6H2O, AR) and sodium hydrate. The prepared flakey FeSiAl powders were put into deionized water and stirred at 150 r/min, and then the analytical grade of the reagents in different amounts were added to the powders. After the precipitate agent of NaOH began dripping at 5 ml/min into the mixtures, the bath was continuously stirred for 30 min to expose each particle in the solution so that there was uniformly deposited precipitation on the surface of the flakey FeSiAl powders. The composite particles were filter cleaned in deionized water until PH=7 was reached, and then they were dried in an oven for 2 h at 60°C. Finally, the brown red products were annealed at 600°C for 2 h under an argon atmosphere. The crystallographic structure, microstructure, and morphology were studied by X-ray powder diffraction ((Bruker D8, Bruker, Germany) with Cu-Ka radiation (λ = 0.1541 nm) and scanning electron microscopy (SEM, Hitachi S3400, Japan), respectively. Either FeSiAl flakes or ferrite coated FeSiAl flakes were mixed with
paraffin in a weight ratio 3:2 of the powder to paraffin for dynamic electromagnetic measurements, resulting in a toroid with an inner diameter of 3.0 mm and an outer diameter of 8.0 mm. Finally, the complex relative permittivity and permeability of the samples were measured by using Agilent N5244A network vector analyzer within a frequency range of 1-18 GHz. According to the transmission line theory, the theoretical reflection loss curves (RL) at a given thickness can be calculated. 3. Results and discussion Figure 1 shows the XRD pattern of the obtained composite FeSiAl alloy sheets with different amounts of Ni−Zn ferrite. It shows the Ni-Zn ferrite crystal planes at 30.1, 35.5, 43, 53.3, 57.0, 62.7, 75 which are the characteristic peaks corresponding to the PDF card JCPDS#52-0278. The evident diffraction peaks of a-FeSiAl alloy with bcc structure are at 45, 65.5, and 83, and the DO3 super-lattice structures are at 27, 31.5, and 53, corresponding to the PDF card JCPDS #45-1206. It is clear that the spinel structure of ferrites appear in the crystal structure of the FeSiAl alloy, even though the loads of (Ni0.5Zn0.5)Fe2O4 are at or around 2 wt.%. This is different from the results previously reported in the literature that the ferrite loads exceeding 6 wt.% cannot be detected just by mechanically stirring FeSiAl and Ni-Zn ferrites [24]. The possible reason may be that co-precipitation generates the continuous and homogenous coatings of ferrite on the surface of FeSiAl sheets. Subsequently the SEM morphology characterization confirms this.
.
Fig. 1 XRD pattern of FeSiAl flakes composites with various weight ratios of ferrite. Figure 2 shows the SEM images of the obtained composite FeSiAl alloy powders with different amounts of Ni-Zn coated with ferrite. As is shown in Fig. 2 (a), (c), (e), (g) and (i), the results show that the composite FeSiAl powders are flakey. The nano-ferrites are in-situ growths on the surface of FeSiAl alloy flakes. In Fig. 2(b), we can observe the massive nano-sheet of ferrites deposited on the surface of flakey FeSiAl when the weight ratio is 2 wt.%. The thickness of the milled FeSiAl alloy sheets are less than 1.5 µm and the aspect ratios such as length/thickness are more than 15. It is expected that the large aspect ratio of the FeSiAl sheets favors increasing the magnetic loss in high-frequency region by overcoming the Snoek’s limit. Moreover, the thickness of the sheet is below the typical skin depth of traditional FeSiAl alloy ~2 µm [25]. From Fig. 2(d), (f), (h) and (g), we can also observe that the density of the surface area changes as the loads increase from 4 wt.% to 10 wt.%. The energy dispersive spectrometer (EDS) of the composite material with 8 wt.% loads obtained from the inset of Fig. 2(k) shows that the elements on the surface contain Fe, Si, Al, Ni, Zn and O. Because the raw materials were iron, silicon and aluminum, it is easily to recognize that the coating layer is the Ni0.5Zn0.5Fe2O4 ferrite.
Fig. 2 SEM images of flakey FeSiAl/ferrite composites particles with various amounts of coating: (a and b) 2 wt.%, (c and d) 4 wt.%, (e and f) 6 wt.%, (g and h) 8 wt.%, (i and j) 10 wt.%, and (k) EDS of composites with 8 wt.% Ni-Zn ferrite. Figure 3 shows the complex permittivity and permeability of the milled FeSiAl and the flakey FeSiAl/ferrite composites with different amounts of loading. The frequency dependence of the real and imaginary permittivity of composite samples is shown in Fig. 3(a) and (b). The real permittivity values of composites are found to decrease when compared with flakey FeSiAl. For all the FeSiAl/ferrite composite samples the real permittivity is found to be nearly constant for 2 wt.%, 4 wt.%, 6 wt.%, 8 wt.%, 10 wt.%, with values of 7.2, 7.8, 7.5, 8.6, and 7.7, respectively, for a frequency range of 2-10 GHz. However, the values display an obvious resonant peak in 10 GHz-12 GHz regime, while the loads exceeding 4 wt.% correspond to the matching frequency zone. Similarly, the imaginary permittivity value for all composites have an approximate value 0.2 (at 2 GHz) and also exhibit those resonant peaks in the corresponding frequency range. Further the real and imaginary permittivity of flakey FeSiAl composites with a loading of 8 wt.% NiZn ferrites show higher resonance among the composites. The real and imaginary complex permeability of composite samples are shown in Fig. 3(c) and (d). The real and
imaginary permeability decrease monotonically as the frequency increases from 2 to 18 GHz for all composite samples. However, the complex permeability dependence of the amount of load is not obvious. The real and imaginary permeability for all composites are 1.4-1.6 (at 2GHz) and 0.48-0.56 (at 2GHz), respectively. There is also a resonant peak at 10-12 GHz for the FeSiAl composites coated with 8 wt.% Ni-Zn ferrites.
Fig. 3 Real part (a) - (c) and imaginary part (b) - (d) of complex permittivity and complex permeability of flakey FeSiAl/ferrite composites with different amounts of coating. Based on these complex relative permittivity and permeability of the composites, the microwave reflection loss curves (RL) of the composite absorbers were calculated. The reflection loss curves with changed thickness (d) and the impedance were drawn by Equations (1) and (2):
(1)
,
(2)
where f is the frequency, d is the thickness of the absorber layer, c is the speed of an
electromagnetic wave in free space.
and
are the
complex permeability and permittivity of the absorber, respectively. According to the theory of impedance matching, the impedance z of ideal materials should be closest to 1. Generally, for most of the absorption materials, the value of permittivity (
) is
larger than permeability ( ), so the impedance matching cannot be ideally achieved. It is known that magnetic materials offer improvement of the complex permeability parameters. A higher permeability value will result in a larger impedance matching value, and thus, an electromagnetic wave can easily be absorbed. Figure 4 shows the calculated RL of the milled FeSiAl alloys and FeSiAl/ferrite composites in the range of 2-18 GHz. From Fig. 4(a), we see that the ferrite loads have important effects on the microwave absorbing performance of the flakey FeSiAl alloys. The microwave absorption is enhanced by coating the ferrites and shifting to high frequency. The quarter-wavelength cancellation model can be applied to explain the relationship between the RL peaks (fm) and the matching thickness (dm), which is determined by Equation (3): .
(3)
From Fig. 3, we can observe the relatively high complex permeability and permittivity for loads of 8 wt.% ferrite compared with other composites; hence, the reflection loss peaks of 8 wt.% ferrite loads shown in Fig. 4 (a) shift to low frequency based on Eq. (3). In addition, the results indicated that the position and intensity of the peaks are sensitive to the weight ratio of ferrite loads. The maximum RL value of −29.2 dB is observed at 10.7 GHz with matching thickness 2.5 mm while the amount of ferrite loading is 8 wt.%. The microwave absorption properties of the FeSiAl/ferrite composites (8 wt.%) together with other flakey Fe-based and FeSiAl alloys composites reported in recent literature are summarized in Table 1. In Fig. 4(b) - (d), the effective RL curves for 6 wt.%, 8 wt.%, and 10 wt.%, respectively, show a small attenuation peak along with the main attenuation peak at thickness 2 mm or 3 mm. Subsequently, it disappears, and the intensity of reflection loss reaches a maximum
when the matching thickness is 2.5 mm. Meanwhile, the attenuation of the small peak is most obvious when loading ferrites have a weight ratio of 8 wt.%. Similar phenomena of the small resonant peak have also been observed in the microwave absorption materials such as Fe@ZnFe2O4 and Sm1.5Y0.5Fe17-x Six [3, 21, 26]. The thickness-independent peak almost fixed at specific frequency. It is noted the peak of the imaginary parts of the permittivity is also about 10.7 GHz in Fig.3b, suggesting that the fixed attenuation peak originates from the dielectric-type loss mechanism [26]. The shift of the peak towards the low-frequency range with increasing thickness can be explained by the quarter wave theory. The quarter wave theory suggests an interference type loss, which is the intrinsic relation between the absorbing performance and the magnetic properties of the composites.
Fig. 4 Reflection loss curves of flakey FeSiAl/ferrite composites (a) in thickness 2.5 mm with different weight ratio, (b) with weight ratio of 6%, (c) 8% and (d) 10% in different thickness (2-3 mm). Table 1. Microwave Absorption Properties of Flakey Alloy Composites Reported in a Recent Paper.
Filler
Thickn ess (mm)
Resonance frequency (GHz)
RLmin (dB)
Bandw idth (GHz)
Synthesis approach
Ref.
FeSiAl FeSiAl/SiO2 FeSiAl/ZnO FeSiAl/Al2O3 FeSiAl/BaTiO3 FeSiAl
2 2.2 1.9 4 2.5
12.5 13.7 10.3 9.4 3.1 1.25
-20.5 -39.4 -40.5 -34.2 -18.1 -10
5.6 3.5 3.5 1.9 2.8 -
[27] [17] [10] [8] [7] [28]
Aligned FeSiAl FeSiAl/graphite Fe-based amorphous FeSiAl/MnZnFe2O4 FeSiAl/NiZnFe2O4 FeSiAl/NiCuZn ferrite
2.5 3 2 6
1.5 6.7 11.6 1.4
-8.8 -21 -27.3 -20
1.8 2 4.5 1.2
Ball milling ultrasonic ultrasonic Hot-pressed stirring Meltquenching ball milling Ball milling Ball milling Precipitation Stirring Sol-gel
FeSiAl/NiZn ferrtie
2.5
10.7
-29.2
3.75
Precipitation
This work
[29] [13] [30] [22] [24] [20]
Figure 5 shows reflection loss curves of flakey FeSiAl coated with 8 wt.% ferrites and the variation of the bandwidth of microwave absorption (RL > 10 dB) for composites at different thicknesses (2.0-5.0 mm). The effective RL of more than 10 dB with an amount of 8 wt.% ferrite could be achieved over a frequency range of 3.5-17.5 GHz while varying thickness from 2 to 5 mm. As shown in Fig. 5(a), the reflection loss curves of flakey FeSiAl/ferrite composites with loads of 8 wt.% at different thickness all exhibit small attenuation peaks at a frequency around 10.5 GHz. Although the accompanying attenuation peaks have been observed in other materials as well; the puzzle is that the position of the attenuation peak barely varies with the thickness shown in the shadow region of Fig. 5(a). The possible explanation is that the resonant peaks of the complex permittivity and permeability originates from the dielectric-type loss. Therefore, the RL peak is thickness-independent and almost fixed at 10-12 GHz region. In Fig. 5(b), the absorption bandwidth of composites increase and then decrease with the increase of thickness. However, an enhanced absorption bandwidth is observed for ferrite loads exceeding 8 wt.% with maximum 4 GHz bandwidth at matching thickness 2.5-3.0 mm. The bandwidth of absorption (more
than 10 dB) can be tuned from 1.8 to 4 GHz, while varying the thickness of composites from 2.0 to 5.0 mm, respectively.
Fig. 5 (a) Reflection loss curves of coated 8 wt.% ferrites with a small attenuation peaks shown in shaded area and (b) bandwidth of flakey FeSiAl/ferrite composites coated with different weight ratios varying with thickness. 4. Conclusions Flakey FeSiAl/Ni-Zn ferrite composites were prepared via ball milling-assisted co-precipitation. The structure and morphology show that the spinel ferrites grow on the surface of FeSiAl sheets. As the loads increase, so does the surface density of flakey FeSiAl. An extraordinary dielectric-type microwave absorption was found due to generating metal−ferrite interfaces through growing ferrites on FeSiAl sheets. The dielectric-type loss to enhance microwave absorption can be well tuned by the controllable Ni-Zn ferrite loads. Enhanced microwave absorption is achieved with the minimum reflection loss –29.2 dB at a frequency of 10.7 GHz. The absorption bandwidth is observed at a maximum of 4 GHz at matching thickness 2.5-3.0 mm for ferrite loads exceeding 8 wt.%. Effective microwave absorption as the reflection loss (RL) < -10 dB bandwidth over a frequency range of 3.5-17.5 GHz at a thickness of 2-5 mm reveals an excellent opportunity for flakey FeSiAl/ferrite composites in applications for microwave absorption. Acknowledgments: The authors acknowledge the financial support from the National Natural Science Foundation (No. 51801097), the Industrial Innovation of Applied
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1. Density and homogenous, flakey FeSiAl/Ni-Zn ferrite composites were fabricated via ball milling-assisted co-precipitation 2. Controlled by ferrite loads, enhance microwave absorption is achieved with the reflection loss –29.2 dB and bandwidth 4 GHz at matching thickness 2.5 mm 3. An extraordinary tunable dielectric-type loss was found to enhance the microwave absorption which is thickness-independent
Declaration of Interest Statement On behalf of my coauthors, I would like to submit the manuscript entitled “Tunable dielectric loss to enhance microwave absorption properties of flakey FeSiAl /ferrite composites” for publication in Journal of Alloys and Compounds. I hereby certify that this paper consists of original, unpublished work which is not under consideration for publication elsewhere. The authors claim no conflicts of interest.