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Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites
Author’s Accepted Manuscript Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites Wangchang Li, Junjun Lv, Xiang Zhou, Jingwu Zheng, Yao Ying, Liang Qiao, Jing Yu, Shenglei Che www.elsevier.com/locate/jmmm
PII: DOI: Reference:
S0304-8853(16)32406-4 http://dx.doi.org/10.1016/j.jmmm.2016.11.119 MAGMA62195
To appear in: Journal of Magnetism and Magnetic Materials Received date: 30 September 2016 Revised date: 25 November 2016 Accepted date: 25 November 2016 Cite this article as: Wangchang Li, Junjun Lv, Xiang Zhou, Jingwu Zheng, Yao Ying, Liang Qiao, Jing Yu and Shenglei Che, Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.119 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 galley proof before it is published in its final citable 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.
Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites Wangchang Lia, Junjun Lvb, , Xiang Zhoua, Jingwu Zhenga, Yao Yinga, Liang Qiaoa, Jing Yua, Shenglei Chea, a
Research Center of Magnetic and Electronic Materials, College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310000, China b
Institute of Chemical Material, CAEP, Mianyang 621900, China
In order to achieve a broad bandwidth absorber at high frequency, the composites of M-type ferrite BaCo1.0Ti1.0Fe10O19 (BaM) with flaked carbonyl iron powders (CIP) and flaked Fe50Ni50 were prepared to optimize the surface impedance in broadband frequency, respectively. The diameter of the flaked carbonyl iron powders (CIP) and Fe50Ni50 is in the range of 5~10 μm and 10~20 μm and the thickness of the CIP and Fe50Ni50 is close to 200 nm and 400 nm, respectively. The complex permeability and permittivity show that the addition of BaM obviously reduces the values of real part of permittivity and imaginary part of the permeability which can enhance the matched-wave-impedance. The absorption bands less than -10 dB of CIP-BaM and FeNi-BaM absorber approach to 5.5 GHz (5.7~11.2 GHz) and 7 GHz (11~18 GHz) at 1.5 mm. However, the bands of CIP and FeNi are only 1.9 GHz (4.7~6.6 GHz) and 2.1 GHz (4.0~6.1 GHz). Hence, the electromagnetic match property is greatly improved by BaM ferrites, and this composite shows a broaden absorption band. Key words: Microwave absorbing materials, M-type ferrite, Flaked FeNi mixture, Mechanical ball milling
Corresponding author. E-mail:[email protected](Shenglei Che) Phone:+86 (0)571-88320450 and E-mail [email protected] (Junjun Lv) Phone: +86 (0)13096175001
1. Introduction Microwave absorbing material is the essential material in the civil electromagnetic shielding applications and is also widely used in the military technology. In the last decade, various novel absorbers with matching complex permittivity and permeability have been developed, including magnetic metallic fillers[1], ferrites[2, 3], nanosized particles[4], carbon nanomaterials[5], conductive fibers[6], frequency selective surface[7], and even metamaterials[8]. However, these materials are restricted to the narrow band absorption and low ferromagnetic resonance frequency. To our knowledge, carbonyl iron powder (CIP) is widely used and is considered to be the most acceptable and excellent material for its large scale production, high magnetic resonance frequency and remanence loss[9, 10]. But it is
2 still limited to the Snoek’s law f r ( r 1) M s which illustrates the relationship 3 between the permeability and resonance frequency[11]. Many works have been reported that flaked CIP could obtain the large value of the permeability and improved ferromagnetic resonance for the planar anisotropy[12]. Nevertheless it is still deficient for the low frequency and impedance matching. Barium ferrite possesses the high response frequency and low conductivity which is depended on the element composition[13]. Hence, this ferrite is usually mixed with CIP to solve the defect of CIP[14, 15]. Moreover, alloying as the other way to enhance the absorbing properties is also widely carried out. It is reported that the permeability of the FeNi alloy with the particles size of 2.34 μm can reach 6.32 in 1.81 GHz[16]. The resonance frequency of the CoNi alloy could reach 6 GHz with particle size of 0.22
μm. In order to avoid oxidation and high permittivity of the metallic particles, coating is considered to be another effective way to modify the materials[17]. The permeability of the CIP@SiO2 core-shell particles decreases slightly while the permittivity decreases dramatically leading to the proper electromagnetic impedance match and outstanding absorbing property. In this work, we aim to synthesize flaked CIP and FeNi alloys and their composite with BaCo1.0Ti1.0Fe10O19 ferrites to develop the high frequency microwave absorber. The morphology and composition of related electromagnetic properties are studied. It is found that addition of barium ferrite and flake shape can obviously enhance the high frequency absorbing property due to the optimized electromagnetic parameter.
2. Experimental 2.1. Preparation of flaked CIP and FeNi alloys The raw materials were sphere-shaped CIP and thorny sphere-shaped carbonyl nickel powders purchased from Commercial company in China. The powders were manufactured through the decomposition of Fe(CO)5 and Ni(CO)4 in the gaseous state, respectively. The flake-shaped Fe was prepared by mechanical ball milling in ethanol medium for 24 h and the ball-to-powder weight ratio was 10:1. The flake-shaped Fe50Ni50 alloy was prepared by mechanical ball milling and subsequent calcination processes. Mixture of carbonyl iron and carbonyl nickel with the required weight rate 1:1 was prepared through mechanical ball milling process in ethanol medium for 24 h. Then, the washed and dried mixture was calcinated at 900℃ for 2 h in the N2 atmosphere.
2.2. Preparation of Ba ferrites The sample of BaM was synthesized using conventional ceramic techniques. Powders of BaCO3 (99 wt%, industry purity), TiO2(99 wt%), Co2O3 (99 wt%), Fe2O3 (99 wt%) were used as raw materials. The raw materials were milled in deionized water for 8 h at the angular velocity of 120 rpm and the ball-to-power weight ratio of 15:1. The mixed powder was dried, sifted and presintered at 1310 ℃ for 3 h in a muffle furnace. The presintered product was comminuted into particles by using a vibrating mill firstly, then the powder was sintered at 1300 ℃ for 2h in the muffle furnace, finally milled in deionized water for 24 h. The powder was dried at 110 ℃ for 12 h in an oven. After drying, the sample was sifted using 200 mesh sieve. The presintered and sintered processes were heated at a rate of 3 ℃/min. 2.3. Preparation of the composition The above prepared BaM and flake-shaped CIP and flake-shaped Fe50Ni50 was mixed by mechanical ball milling method in ethanol medium for 6 h. The weight ratio of the powder was 3:7 and the ball-to-powder weight ratio was 15:1. Then the washed and dried mixture was annealed at 500℃ for 2 h in the N2 atmosphere. 2.4. Characterization The X-ray diffraction (XRD) analysis of prepared composite samples was carried out by using a Panalytical X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Scanning electron microscopy (SEM, SU-1510) was used to observe the morphologies and microstructures of the particles. The prepared flake-shaped CIP, flake-shaped FeNi, CIP-BaM and FeNi-BaM were homogenously dispersed in
paraffin at a mass ratio of 8:2, respectively, and formed to a cylindrical sample (7 mm in outer diameter and 3 mm in inner diameter) with a coaxial mould. The effective complex permittivity and permeability of these composites in the frequency range from 2 GHz to 18 GHz were measured with the vector network analyzer Agilent E8363B.
3. Results and discussion
Fig. 1. SEM images of raw material powders: (a) carbonyl iron, and (b) carbonyl nickel.
Fig. 2. SEM images of the (a) flaked CIP, (b) flaked Fe50Ni50 and (c) BaCo1.0Ti1.0Fe10O19 (BaM).
Fig. 3. SEM images of the (a) flaked CIP-BaM and (b)flaked FeNi-BaM.
Fig. 1 shows the morphologies of the two kinds of the raw material powders. The raw materials of carbonyl iron and carbonyl nickel are sphere and thorny sphere as shown in Fig. 1(a) and (b). The particle sizes are in the range of 0.5～6 μm and 1～7 μm, respectively. Fig. 2 shows the prepared product flake-shaped CIP, flake-shaped Fe50Ni50 and the barium ferrite BaM. From Fig. 2(a) and (b), it could be found that the CIP and Fe50Ni50 are flaked particles and the diameter is mostly in the range of 5~10 μm and 10～20 μm, respectively. The thickness of the CIP and Fe50Ni50 is close to 200 nm and 400 nm, respectively. Fig. 2(c) shows that the BaCo1.0Ti1.0Fe10O19 (BaM) barium ferrite is a hexagonal flake and the particle size of the barium ferrite powder ranges from 0.5 to 4 μm. Because of the small size of the BaM, there are many agglomerations. Fig. 3 shows the morphologies of the composite sample flaked CIP-BaM and flaked FeNi-BaM. It can be clearly seen that the flaked powder and barium ferrite are physically mixed.
Fig. 4 XRD patterns of the particles: (a) Flaked CIP, (b) Flaked Fe50Ni50 and (c) BaCo1.0Ti1.0Fe10O19 (BaM)
Fig. 5. XRD patterns of the composite particles: (a) CIP-BaM, (b) FeNi-BaM
Fig. 4 shows the XRD patterns of the particles which include flaked CIP, flaked Fe50Ni50 and BaM. All the diffraction peaks are very consistent with those particles without any other impurity. From the Fig .4(a), it can be observed that the peaks located at 44.1°, 65.2°, 82.3° are associated with the (110), (200) and (211) lattice
planes of bcc Fe with the lattice constant a=2.857 Å, and the peaks located at 43.9°, 51.1°, 75.4° are associated with the (111), (200) and (220) planes of fcc FeNi alloy with the lattice constant a=3.560 Å as shown from blue curve. The lattice constant of FeNi alloy is 3.560 Å which is a bit higher than that of fcc Fe (2.857 Å). This can be ascribed to the dissolution of Fe atoms with large radius[18]. When iron and nickel formed solid solution phase, crystal lattice constant of the single cell expanded gradually and the microstructure changes from fcc to bcc[19]. The main diffraction peaks (006), (110), (107), (114), (203) and (2011) of BaM have been clearly revealed in Fig. 4(c). Fig. 5 shows the XRD patterns of the composite particles showing the composition of CIP-BaM and FeNi-BaM. It is very consistent with the SEM images of the Fig. 3 and also indicating that the particles are only physically mixed after milling and annealing.
Fig. 6. Effective complex permittivity and permeability of the flaked CIP, CIP-BaM, flaked FeNi
and FeNi-BaM paraffin composite, respectively. (a) real part ε′,(b) imaginary part ε″,(c) real part μ′, and (d) imaginary part μ″.
It is well known that the complex permittivity ε[20] and complex permeability μ[21] determine the efficiency of EM wave absorbing material. In a detailed description, the real parts ε′ and μ′ represent the storage of the EM energy, and the imaginary parts ε″ and μ″ represent the loss of the energy, which play an important role in the EM wave absorbing properties[22, 23]. In order to evaluate electromagnetic resonance and microwave absorbing properties, the complex permittivity and permeability of the prepared sample are measured in the frequency range of 2~18 GHz as shown in Fig. 5. It is clear that the complex permittivity and permeability are all influenced with the composition and frequency. The real part of the permittivity of the composite seems to be an approximate constant in the range of 2~18 GHz with a slight fluctuation as revealed in Fig. 5(a). To achieve low reflection, the impedance matching between the material and free space, the ratio of μ′/ε′ should be close to unity.
' 1 '
(1)
According to equation (1), if a considerable dielectric loss is present, a high value of permeability is needed for matched-wave-impedance[24]. However, improving the microwave permeability is difficult, thus lower values of ε′′ and higher values of ε′ are required. The real part of the permittivity (ε′) of flaked FeNi alloys disperses at 27 which is higher than that of flaked CIP at 22. The ε′ of the composite of FeNi-BaM and
flaked CIP-BaM are 17 and 11, respectively, which is lower than that of the sample without BaM. Dielectric polarizations can be induced by asymmetric charge distribution either in a molecule structure or in a meso-scopic dielectric interface. They mainly consist of the electron polarization, the ion polarization, and the electric dipolar polarization. However, in microwave range, the dielectric polarization is mainly come from electric dipolar polarization. Here, in our work, the conductive particles are the electric dipolar and the morphology, conductivity, and interface property determine the electric dipolar properties. The strength of dipolar polarization of the FeNi-BaM and flaked CIP-BaM are much lower than the metallic sample. The imaginary part of the permittivity indicates the dielectric loss in high frequency. The flaked FeNi alloys have highest value ~5.0 of the imaginary part of permittivity in high frequency which increases linearly from ~1.5 in low frequency indicating the dielectric loss increases as the frequency increases. The imaginary part of permittivity of flaked CIP and FeNi-BaM decrease first and then increase dispersing in the range of 0~3.5. While that of the CIP-BaM is near zero in the whole frequency. It can be seen that the value of the ε″ has severe fluctuation after adding BaM which belongs to the dielectric loss of material[25]. As a typical magnetic magnetic loss absorber, complex permeability is a critical parameter to evaluate the properties. Generally, the permeability spectra is explained by hysteresis loss, domain-wall resonance, eddy current effect and natural resonance[26]. The hysteresis loss can be excluded because the applied microwave field is weak. The domain-wall resonance usually occurs below the gigahertz range.
The skin depth of iron in the microwave frequency range is about 1 μm, which is much greater than the typical thickness of samples, so eddy current is also ignored[27]. As stated above, the permeability spectra of samples are mainly determined by natural resonance. It is shown from Fig. 5(c) that all the real part of the permeability decreases with the frequency increases. The real part of the permeability of flaked FeNi alloy and flaked CIP are ~3.3 at 2 GHz which is higher than the CIP-BaM (~2.3) and FeNi-BaM (~2.6). But the permeability of flaked FeNi alloy drops sharply to ~1.0 at 8 GHz while that of the flaked CIP is ~1.0 at 12 GHz. It is reported that BaM has a high resonance frequency[28]. However, in our work the real part of the permeability of the BaM composite is just 2.3 and 2.6, respectively. The permeability also decreases quickly to 1.0 at ~10 GHz. The imaginary part of the permeability depicts the magnetic loss and the peak of the curve indicates the ferromagnetic resonance. From Fig. 5(d), the ferromagnetic resonance frequencies of the all sample are all located between 4 and 10 GHz. The peak of the flaked FeNi reaches to 2.0 at 5 GHz indicating the superior loss characteristics. However, the peak of the flaked CIP is board. Compared to the above mentioned flaked absorber, the BaM composite shows low loss and broadband frequency. The natural resonance of the BaM may lead to the frequency spectra of the μ′ and μ″[14]. Microwave absorber with a suitable complex permittivity and permeability was prepared by adding magnetic particles into the polymer matrix with proper thickness to meet the low reflection over a broad or special frequency. Here, the reflection loss were calculated from complex permittivity and permeability at the given thickness
based on transmission line theory[29, 30]. The input impedance of the composite layer is:
Zin Z 0
0 r 2 fd tanh j r r , Z 0 r 0 c
(2)
So the reflection loss is:
RL 20log
Zin Z 0 Zin Z 0
(3)
where Z0 is the impedance of free space, Zin is the input impedance, ε0 is the dielectric constant of free space, μ0 is the permeability of free space, f is the frequency of the incident wave, d is the thickness of absorber, and c is the velocity of light in free space (3×108 m·s-1). Fig. 6 shows the typical relationship between RL and frequency with thickness from 1 mm to 5 mm. It shows that the absorption peaks of all samples are mainly located at low frequency range. The reflection less than -10 dB of CIP is in the range of 3.2~5.4 GHz at thickness of 2 mm and 4.7~6.6 GHz at thickness of 1.5 mm. In addition, the reflection less than -10 dB of FeNi is in the range of 3.0~4.4 GHz at thickness of 2 mm and 4.0~6.1 GHz at thickness of 1.5 mm. Flaked FeNi has great absorption at low frequency. This could be ascribed to two reasons. One is the surface impedance match which leads to the cancellation of surface reflected wave and ground metal plane reflected wave. The other is the low magnetic resonance frequency and dielectric loss revealed in the sharp curve of imaginary part of the permeability and permittivity. The absorption band of the flaked CIP is broader than that of the FeNi particles which could be correspond to the broad peak of the
imaginary part of the permeability. Comparing Fig. 6 (c) and (d) with Fig. (a) and (b), the BaM added samples would have broad absorption peak and high frequency especially in the layer thickness of 1.5~2.5 mm. The reflection less than -10 dB of CIP-BaM is in the range of 4.3~7.6 GHz at thickness of 2 mm and 5.7~11.2 GHz at thickness of 1.5 mm. Moreover, the reflection less than -10 dB of FeNi-BaM is dispersed in the range of 6~12 GHz at thickness of 2 mm and 11~18 GHz at thickness of 1.5 mm. The -10 dB microwave absorption bandwidth of the BaM added samples is broadened compared with the CIP and FeNi. The hysteresis loss and dielectric loss of the BaM added sample are lower than the flaked CIP and FeNi. Hence, we believe that this high frequency and broad band absorption is ascribed to the surface impedance match for the cancellation of reflected wave. The permittivity of the BaM added samples is near 10 and 12 in the whole frequency which leads to the electromagnetic match for incident wave and reflected wave.
FIG. 6. Frequency dependence of the RL of composite containing (a) flake CIP, (b)flake FeNi, (c) CIP-BaM, and (d) FeNi-BaM.
4. Conclusion In our work, flake-shaped Fe and flake-shaped Fe50Ni50 were prepared by mechanical ball milling and subsequent calcination method. The BaM sample synthesized by using conventional ceramic techniques was mixed with the flaked CIP and FeNi, respectively. The values of the electromagnetic parameter of the BaM added samples are almost reduced in the whole frequency which leads to the electromagnetic match for incident wave and reflected wave. Compared to the above mentioned flaked absorber, the BaM composite shows low loss and broadband frequency. The reflection less than -10 dB of CIP and CIP-BaM is in the range of 3.2~5.4 GHz at thickness of 2mm, 4.7~6.6 GHz at thickness of 1.5 mm and 4.3~7.6 GHz at thickness of 2 mm, 5.7~11.2 GHz at thickness of 1.5 mm, respectively. The reflection less than -10 dB of FeNi and FeNi-BaM is in the range of 3.0~4.4 GHz at thickness of 2 mm, 4.0~6.1 GHz at thickness of 1.5 mm and 6~12 GHz at thickness of 2 mm, 11~18 GHz at thickness of 1.5 mm, respectively. It can be found that the
maximum absorption peak moves to high frequency and has larger bandwidth after added BaM at the same thickness. Because of the broadband nature of absorber, this can be used for the high frequency fields of microwave materials and eliminating the EMI in the electron device.
Acknowledgments This work was supported by National Nature Science Foundation of China through Grant No. 11204270 and Zhejiang Provincial Natural Science Foundation of China through Grant No. LQ15E020003.
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Research Highlights 1. In order to achieve a broad bandwidth absorber at high frequency, the composites of M-type ferrite BaCo1.0Ti1.0Fe10O19 (BaM) with flaked carbonyl iron powders (CIP) and flaked Fe50Ni50 were prepared to optimize the surface impedance in broadband frequency.
2. The complex permeability and permittivity show that the addition of BaM obviously reduces the values of real part of permittivity and imaginary part of the permeability which can enhance the matched-wave-impedance. 3. The structure, morphology and electromagnetic properties are studied in detail by using XRD, SEM and so on. The optimized properties are ascribed to the strength of dipolar polarization and high frequency magnetic resonance.
PII: DOI: Reference:
S0304-8853(16)32406-4 http://dx.doi.org/10.1016/j.jmmm.2016.11.119 MAGMA62195
To appear in: Journal of Magnetism and Magnetic Materials Received date: 30 September 2016 Revised date: 25 November 2016 Accepted date: 25 November 2016 Cite this article as: Wangchang Li, Junjun Lv, Xiang Zhou, Jingwu Zheng, Yao Ying, Liang Qiao, Jing Yu and Shenglei Che, Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.11.119 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 galley proof before it is published in its final citable 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.
Enhanced and broadband microwave absorption of flake-shaped Fe and FeNi composite with Ba ferrites Wangchang Lia, Junjun Lvb, , Xiang Zhoua, Jingwu Zhenga, Yao Yinga, Liang Qiaoa, Jing Yua, Shenglei Chea, a
Research Center of Magnetic and Electronic Materials, College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310000, China b
Institute of Chemical Material, CAEP, Mianyang 621900, China
In order to achieve a broad bandwidth absorber at high frequency, the composites of M-type ferrite BaCo1.0Ti1.0Fe10O19 (BaM) with flaked carbonyl iron powders (CIP) and flaked Fe50Ni50 were prepared to optimize the surface impedance in broadband frequency, respectively. The diameter of the flaked carbonyl iron powders (CIP) and Fe50Ni50 is in the range of 5~10 μm and 10~20 μm and the thickness of the CIP and Fe50Ni50 is close to 200 nm and 400 nm, respectively. The complex permeability and permittivity show that the addition of BaM obviously reduces the values of real part of permittivity and imaginary part of the permeability which can enhance the matched-wave-impedance. The absorption bands less than -10 dB of CIP-BaM and FeNi-BaM absorber approach to 5.5 GHz (5.7~11.2 GHz) and 7 GHz (11~18 GHz) at 1.5 mm. However, the bands of CIP and FeNi are only 1.9 GHz (4.7~6.6 GHz) and 2.1 GHz (4.0~6.1 GHz). Hence, the electromagnetic match property is greatly improved by BaM ferrites, and this composite shows a broaden absorption band. Key words: Microwave absorbing materials, M-type ferrite, Flaked FeNi mixture, Mechanical ball milling
Corresponding author. E-mail:[email protected](Shenglei Che) Phone:+86 (0)571-88320450 and E-mail [email protected] (Junjun Lv) Phone: +86 (0)13096175001
1. Introduction Microwave absorbing material is the essential material in the civil electromagnetic shielding applications and is also widely used in the military technology. In the last decade, various novel absorbers with matching complex permittivity and permeability have been developed, including magnetic metallic fillers[1], ferrites[2, 3], nanosized particles[4], carbon nanomaterials[5], conductive fibers[6], frequency selective surface[7], and even metamaterials[8]. However, these materials are restricted to the narrow band absorption and low ferromagnetic resonance frequency. To our knowledge, carbonyl iron powder (CIP) is widely used and is considered to be the most acceptable and excellent material for its large scale production, high magnetic resonance frequency and remanence loss[9, 10]. But it is
2 still limited to the Snoek’s law f r ( r 1) M s which illustrates the relationship 3 between the permeability and resonance frequency[11]. Many works have been reported that flaked CIP could obtain the large value of the permeability and improved ferromagnetic resonance for the planar anisotropy[12]. Nevertheless it is still deficient for the low frequency and impedance matching. Barium ferrite possesses the high response frequency and low conductivity which is depended on the element composition[13]. Hence, this ferrite is usually mixed with CIP to solve the defect of CIP[14, 15]. Moreover, alloying as the other way to enhance the absorbing properties is also widely carried out. It is reported that the permeability of the FeNi alloy with the particles size of 2.34 μm can reach 6.32 in 1.81 GHz[16]. The resonance frequency of the CoNi alloy could reach 6 GHz with particle size of 0.22
μm. In order to avoid oxidation and high permittivity of the metallic particles, coating is considered to be another effective way to modify the materials[17]. The permeability of the CIP@SiO2 core-shell particles decreases slightly while the permittivity decreases dramatically leading to the proper electromagnetic impedance match and outstanding absorbing property. In this work, we aim to synthesize flaked CIP and FeNi alloys and their composite with BaCo1.0Ti1.0Fe10O19 ferrites to develop the high frequency microwave absorber. The morphology and composition of related electromagnetic properties are studied. It is found that addition of barium ferrite and flake shape can obviously enhance the high frequency absorbing property due to the optimized electromagnetic parameter.
2. Experimental 2.1. Preparation of flaked CIP and FeNi alloys The raw materials were sphere-shaped CIP and thorny sphere-shaped carbonyl nickel powders purchased from Commercial company in China. The powders were manufactured through the decomposition of Fe(CO)5 and Ni(CO)4 in the gaseous state, respectively. The flake-shaped Fe was prepared by mechanical ball milling in ethanol medium for 24 h and the ball-to-powder weight ratio was 10:1. The flake-shaped Fe50Ni50 alloy was prepared by mechanical ball milling and subsequent calcination processes. Mixture of carbonyl iron and carbonyl nickel with the required weight rate 1:1 was prepared through mechanical ball milling process in ethanol medium for 24 h. Then, the washed and dried mixture was calcinated at 900℃ for 2 h in the N2 atmosphere.
2.2. Preparation of Ba ferrites The sample of BaM was synthesized using conventional ceramic techniques. Powders of BaCO3 (99 wt%, industry purity), TiO2(99 wt%), Co2O3 (99 wt%), Fe2O3 (99 wt%) were used as raw materials. The raw materials were milled in deionized water for 8 h at the angular velocity of 120 rpm and the ball-to-power weight ratio of 15:1. The mixed powder was dried, sifted and presintered at 1310 ℃ for 3 h in a muffle furnace. The presintered product was comminuted into particles by using a vibrating mill firstly, then the powder was sintered at 1300 ℃ for 2h in the muffle furnace, finally milled in deionized water for 24 h. The powder was dried at 110 ℃ for 12 h in an oven. After drying, the sample was sifted using 200 mesh sieve. The presintered and sintered processes were heated at a rate of 3 ℃/min. 2.3. Preparation of the composition The above prepared BaM and flake-shaped CIP and flake-shaped Fe50Ni50 was mixed by mechanical ball milling method in ethanol medium for 6 h. The weight ratio of the powder was 3:7 and the ball-to-powder weight ratio was 15:1. Then the washed and dried mixture was annealed at 500℃ for 2 h in the N2 atmosphere. 2.4. Characterization The X-ray diffraction (XRD) analysis of prepared composite samples was carried out by using a Panalytical X-ray diffractometer with Cu Kα radiation (λ = 0.154178 nm). Scanning electron microscopy (SEM, SU-1510) was used to observe the morphologies and microstructures of the particles. The prepared flake-shaped CIP, flake-shaped FeNi, CIP-BaM and FeNi-BaM were homogenously dispersed in
paraffin at a mass ratio of 8:2, respectively, and formed to a cylindrical sample (7 mm in outer diameter and 3 mm in inner diameter) with a coaxial mould. The effective complex permittivity and permeability of these composites in the frequency range from 2 GHz to 18 GHz were measured with the vector network analyzer Agilent E8363B.
3. Results and discussion
Fig. 1. SEM images of raw material powders: (a) carbonyl iron, and (b) carbonyl nickel.
Fig. 2. SEM images of the (a) flaked CIP, (b) flaked Fe50Ni50 and (c) BaCo1.0Ti1.0Fe10O19 (BaM).
Fig. 3. SEM images of the (a) flaked CIP-BaM and (b)flaked FeNi-BaM.
Fig. 1 shows the morphologies of the two kinds of the raw material powders. The raw materials of carbonyl iron and carbonyl nickel are sphere and thorny sphere as shown in Fig. 1(a) and (b). The particle sizes are in the range of 0.5～6 μm and 1～7 μm, respectively. Fig. 2 shows the prepared product flake-shaped CIP, flake-shaped Fe50Ni50 and the barium ferrite BaM. From Fig. 2(a) and (b), it could be found that the CIP and Fe50Ni50 are flaked particles and the diameter is mostly in the range of 5~10 μm and 10～20 μm, respectively. The thickness of the CIP and Fe50Ni50 is close to 200 nm and 400 nm, respectively. Fig. 2(c) shows that the BaCo1.0Ti1.0Fe10O19 (BaM) barium ferrite is a hexagonal flake and the particle size of the barium ferrite powder ranges from 0.5 to 4 μm. Because of the small size of the BaM, there are many agglomerations. Fig. 3 shows the morphologies of the composite sample flaked CIP-BaM and flaked FeNi-BaM. It can be clearly seen that the flaked powder and barium ferrite are physically mixed.
Fig. 4 XRD patterns of the particles: (a) Flaked CIP, (b) Flaked Fe50Ni50 and (c) BaCo1.0Ti1.0Fe10O19 (BaM)
Fig. 5. XRD patterns of the composite particles: (a) CIP-BaM, (b) FeNi-BaM
Fig. 4 shows the XRD patterns of the particles which include flaked CIP, flaked Fe50Ni50 and BaM. All the diffraction peaks are very consistent with those particles without any other impurity. From the Fig .4(a), it can be observed that the peaks located at 44.1°, 65.2°, 82.3° are associated with the (110), (200) and (211) lattice
planes of bcc Fe with the lattice constant a=2.857 Å, and the peaks located at 43.9°, 51.1°, 75.4° are associated with the (111), (200) and (220) planes of fcc FeNi alloy with the lattice constant a=3.560 Å as shown from blue curve. The lattice constant of FeNi alloy is 3.560 Å which is a bit higher than that of fcc Fe (2.857 Å). This can be ascribed to the dissolution of Fe atoms with large radius[18]. When iron and nickel formed solid solution phase, crystal lattice constant of the single cell expanded gradually and the microstructure changes from fcc to bcc[19]. The main diffraction peaks (006), (110), (107), (114), (203) and (2011) of BaM have been clearly revealed in Fig. 4(c). Fig. 5 shows the XRD patterns of the composite particles showing the composition of CIP-BaM and FeNi-BaM. It is very consistent with the SEM images of the Fig. 3 and also indicating that the particles are only physically mixed after milling and annealing.
Fig. 6. Effective complex permittivity and permeability of the flaked CIP, CIP-BaM, flaked FeNi
and FeNi-BaM paraffin composite, respectively. (a) real part ε′,(b) imaginary part ε″,(c) real part μ′, and (d) imaginary part μ″.
It is well known that the complex permittivity ε[20] and complex permeability μ[21] determine the efficiency of EM wave absorbing material. In a detailed description, the real parts ε′ and μ′ represent the storage of the EM energy, and the imaginary parts ε″ and μ″ represent the loss of the energy, which play an important role in the EM wave absorbing properties[22, 23]. In order to evaluate electromagnetic resonance and microwave absorbing properties, the complex permittivity and permeability of the prepared sample are measured in the frequency range of 2~18 GHz as shown in Fig. 5. It is clear that the complex permittivity and permeability are all influenced with the composition and frequency. The real part of the permittivity of the composite seems to be an approximate constant in the range of 2~18 GHz with a slight fluctuation as revealed in Fig. 5(a). To achieve low reflection, the impedance matching between the material and free space, the ratio of μ′/ε′ should be close to unity.
' 1 '
(1)
According to equation (1), if a considerable dielectric loss is present, a high value of permeability is needed for matched-wave-impedance[24]. However, improving the microwave permeability is difficult, thus lower values of ε′′ and higher values of ε′ are required. The real part of the permittivity (ε′) of flaked FeNi alloys disperses at 27 which is higher than that of flaked CIP at 22. The ε′ of the composite of FeNi-BaM and
flaked CIP-BaM are 17 and 11, respectively, which is lower than that of the sample without BaM. Dielectric polarizations can be induced by asymmetric charge distribution either in a molecule structure or in a meso-scopic dielectric interface. They mainly consist of the electron polarization, the ion polarization, and the electric dipolar polarization. However, in microwave range, the dielectric polarization is mainly come from electric dipolar polarization. Here, in our work, the conductive particles are the electric dipolar and the morphology, conductivity, and interface property determine the electric dipolar properties. The strength of dipolar polarization of the FeNi-BaM and flaked CIP-BaM are much lower than the metallic sample. The imaginary part of the permittivity indicates the dielectric loss in high frequency. The flaked FeNi alloys have highest value ~5.0 of the imaginary part of permittivity in high frequency which increases linearly from ~1.5 in low frequency indicating the dielectric loss increases as the frequency increases. The imaginary part of permittivity of flaked CIP and FeNi-BaM decrease first and then increase dispersing in the range of 0~3.5. While that of the CIP-BaM is near zero in the whole frequency. It can be seen that the value of the ε″ has severe fluctuation after adding BaM which belongs to the dielectric loss of material[25]. As a typical magnetic magnetic loss absorber, complex permeability is a critical parameter to evaluate the properties. Generally, the permeability spectra is explained by hysteresis loss, domain-wall resonance, eddy current effect and natural resonance[26]. The hysteresis loss can be excluded because the applied microwave field is weak. The domain-wall resonance usually occurs below the gigahertz range.
The skin depth of iron in the microwave frequency range is about 1 μm, which is much greater than the typical thickness of samples, so eddy current is also ignored[27]. As stated above, the permeability spectra of samples are mainly determined by natural resonance. It is shown from Fig. 5(c) that all the real part of the permeability decreases with the frequency increases. The real part of the permeability of flaked FeNi alloy and flaked CIP are ~3.3 at 2 GHz which is higher than the CIP-BaM (~2.3) and FeNi-BaM (~2.6). But the permeability of flaked FeNi alloy drops sharply to ~1.0 at 8 GHz while that of the flaked CIP is ~1.0 at 12 GHz. It is reported that BaM has a high resonance frequency[28]. However, in our work the real part of the permeability of the BaM composite is just 2.3 and 2.6, respectively. The permeability also decreases quickly to 1.0 at ~10 GHz. The imaginary part of the permeability depicts the magnetic loss and the peak of the curve indicates the ferromagnetic resonance. From Fig. 5(d), the ferromagnetic resonance frequencies of the all sample are all located between 4 and 10 GHz. The peak of the flaked FeNi reaches to 2.0 at 5 GHz indicating the superior loss characteristics. However, the peak of the flaked CIP is board. Compared to the above mentioned flaked absorber, the BaM composite shows low loss and broadband frequency. The natural resonance of the BaM may lead to the frequency spectra of the μ′ and μ″[14]. Microwave absorber with a suitable complex permittivity and permeability was prepared by adding magnetic particles into the polymer matrix with proper thickness to meet the low reflection over a broad or special frequency. Here, the reflection loss were calculated from complex permittivity and permeability at the given thickness
based on transmission line theory[29, 30]. The input impedance of the composite layer is:
Zin Z 0
0 r 2 fd tanh j r r , Z 0 r 0 c
(2)
So the reflection loss is:
RL 20log
Zin Z 0 Zin Z 0
(3)
where Z0 is the impedance of free space, Zin is the input impedance, ε0 is the dielectric constant of free space, μ0 is the permeability of free space, f is the frequency of the incident wave, d is the thickness of absorber, and c is the velocity of light in free space (3×108 m·s-1). Fig. 6 shows the typical relationship between RL and frequency with thickness from 1 mm to 5 mm. It shows that the absorption peaks of all samples are mainly located at low frequency range. The reflection less than -10 dB of CIP is in the range of 3.2~5.4 GHz at thickness of 2 mm and 4.7~6.6 GHz at thickness of 1.5 mm. In addition, the reflection less than -10 dB of FeNi is in the range of 3.0~4.4 GHz at thickness of 2 mm and 4.0~6.1 GHz at thickness of 1.5 mm. Flaked FeNi has great absorption at low frequency. This could be ascribed to two reasons. One is the surface impedance match which leads to the cancellation of surface reflected wave and ground metal plane reflected wave. The other is the low magnetic resonance frequency and dielectric loss revealed in the sharp curve of imaginary part of the permeability and permittivity. The absorption band of the flaked CIP is broader than that of the FeNi particles which could be correspond to the broad peak of the
imaginary part of the permeability. Comparing Fig. 6 (c) and (d) with Fig. (a) and (b), the BaM added samples would have broad absorption peak and high frequency especially in the layer thickness of 1.5~2.5 mm. The reflection less than -10 dB of CIP-BaM is in the range of 4.3~7.6 GHz at thickness of 2 mm and 5.7~11.2 GHz at thickness of 1.5 mm. Moreover, the reflection less than -10 dB of FeNi-BaM is dispersed in the range of 6~12 GHz at thickness of 2 mm and 11~18 GHz at thickness of 1.5 mm. The -10 dB microwave absorption bandwidth of the BaM added samples is broadened compared with the CIP and FeNi. The hysteresis loss and dielectric loss of the BaM added sample are lower than the flaked CIP and FeNi. Hence, we believe that this high frequency and broad band absorption is ascribed to the surface impedance match for the cancellation of reflected wave. The permittivity of the BaM added samples is near 10 and 12 in the whole frequency which leads to the electromagnetic match for incident wave and reflected wave.
FIG. 6. Frequency dependence of the RL of composite containing (a) flake CIP, (b)flake FeNi, (c) CIP-BaM, and (d) FeNi-BaM.
4. Conclusion In our work, flake-shaped Fe and flake-shaped Fe50Ni50 were prepared by mechanical ball milling and subsequent calcination method. The BaM sample synthesized by using conventional ceramic techniques was mixed with the flaked CIP and FeNi, respectively. The values of the electromagnetic parameter of the BaM added samples are almost reduced in the whole frequency which leads to the electromagnetic match for incident wave and reflected wave. Compared to the above mentioned flaked absorber, the BaM composite shows low loss and broadband frequency. The reflection less than -10 dB of CIP and CIP-BaM is in the range of 3.2~5.4 GHz at thickness of 2mm, 4.7~6.6 GHz at thickness of 1.5 mm and 4.3~7.6 GHz at thickness of 2 mm, 5.7~11.2 GHz at thickness of 1.5 mm, respectively. The reflection less than -10 dB of FeNi and FeNi-BaM is in the range of 3.0~4.4 GHz at thickness of 2 mm, 4.0~6.1 GHz at thickness of 1.5 mm and 6~12 GHz at thickness of 2 mm, 11~18 GHz at thickness of 1.5 mm, respectively. It can be found that the
maximum absorption peak moves to high frequency and has larger bandwidth after added BaM at the same thickness. Because of the broadband nature of absorber, this can be used for the high frequency fields of microwave materials and eliminating the EMI in the electron device.
Acknowledgments This work was supported by National Nature Science Foundation of China through Grant No. 11204270 and Zhejiang Provincial Natural Science Foundation of China through Grant No. LQ15E020003.
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Research Highlights 1. In order to achieve a broad bandwidth absorber at high frequency, the composites of M-type ferrite BaCo1.0Ti1.0Fe10O19 (BaM) with flaked carbonyl iron powders (CIP) and flaked Fe50Ni50 were prepared to optimize the surface impedance in broadband frequency.
2. The complex permeability and permittivity show that the addition of BaM obviously reduces the values of real part of permittivity and imaginary part of the permeability which can enhance the matched-wave-impedance. 3. The structure, morphology and electromagnetic properties are studied in detail by using XRD, SEM and so on. The optimized properties are ascribed to the strength of dipolar polarization and high frequency magnetic resonance.