Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding

Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding

Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178 Contents lists available at SciVerse ScienceDirect Journal of Magnetism and Magneti...

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Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178

Contents lists available at SciVerse ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Enhanced microwave absorption of Fe flakes with magnesium ferrite cladding N. Tian, J.W. Wang, F. Li, Z. Mei, Z.X. Lu, L.L. Ge, C.Y. You n School of Materials Science and Engineering, Xi0 an University of Technology, Xi0 an 710048, PR China

a r t i c l e i n f o

abstract

Article history: Received 16 April 2012 Received in revised form 29 June 2012 Available online 1 August 2012

Surface cladding of Magnesium ferrite (MgFe2O4) was found to be helpful for effectively decreasing the permittivity of Fe flakes. So the electromagnetic impedance matching was improved, resulting in a good microwave absorption. Careful characterizations showed that the giant decreases of permittivity were ascribed to the high resistivity of surface due to the MgFe2O4 cladding. A high frequency microwave absorption property with thin absorber thickness was obtained for the Fe flakes with 10 wt% MgFe2O4. & 2012 Elsevier B.V. All rights reserved.

Keywords: Fe flake Dielectric Microwave absorption

1. Introduction Rapidly expanding communication devices, such as mobile telephone, local area network systems and radar system, require high performance microwave absorption materials with a large permeability and less reflection loss. According to Snoek’s limit [1], the permeability values are limited by the saturation magnetization of materials. Because of the high saturation magnetization, Fe-based metallic magnetic materials have been expected to be helpful for getting a high permeability as an inclusion in the composite microwave absorber. Deduced from the MaxwellGarnett mixing law [2], the real permeability m00 is also decided by the volume fraction (P) of metallic particles and its demagnetizing factor (Nd) 0 0o

m

P þ1  m00 ðmaxÞ ð1PÞN d

ð1Þ

As we know, the shape anisotropy can be introduced to adjust the demagnetizing factor to achieve a high permeability. Flake shaped ferromagnetic microwave absorbers have been successfully prepared to increase the complex permeability [3–6]. In addition, metallic magnetic materials with flake-like shapes could exceed the Snoek’s limit due to its planar-anisotropy [7]. However, the permittivity of metallic magnetic flakes would be very large because of the large surface polarization [8]. Limited by the electromagnetic impedance matching condition [9], the unilateral increase of the permeability or the permittivity would lead to a poor reflection loss (RL). Surface modification must be done to n

Corresponding author. Tel.: þ86 29 82312090; fax: þ86 29 82312994. E-mail addresses: [email protected], [email protected] (C.Y. You).

0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.07.043

decrease the permittivity of metallic magnetic flakes. It was found that the surface modification by H2O2 can suppress the eddy current effect and reduce the permittivity of the Fe50Ni50 particles to realize a high improvement of reflection loss properties [10]. Surface oxidization and coating with C are also efficient ways to improve electromagnetic impedance matching [11,12]. In this work, magnesium ferrite (MgFe2O4) was used as surface modifier of Fe flakes, considering that MgFe2O4 is a soft magnetic semiconductor and possesses high resistivity [13]. The experimental results showed that the MgFe2O4 cladding significantly reduced the permittivity of Fe flakes to get good microwave absorptions as a result of the improved impedance matching.

2. Experimental procedure Flake-like Fe particles were prepared by mechanical milling with a commercially available Fe powders (99% purity). The mechanical milling was performed using a planetary GN-& ball mill with a weight ratio (20:1) of ball/powder for 2 h. The shape of the Fe particles was controlled using the mixed process control agents, which consisted of Ethanol and oleic acid. Ethanol is 50 wt% of Fe powders and oleic acid is 15 wt% of Fe powders. The oleic acid is easy to form a thin film on the particle surface to effectively avoid the cold welding. On the other hand, the thick oleic acid film could also weaken the flattening of powders, which is not good for getting a high aspect ratio. The current agent ratio (ethanol to oleic) and milling parameters are the optimum process to get flaky shape. The powder size was evaluated by using a laser particle size analyzer (BT-2003). The MgFe2O4 cladding was carried out through mixing the flakes with the trifluoroacetic magnesium sol–gel followed by vacuum annealing

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at 350 1C for 30 min. The cladding with 1 wt% and 10 wt% of Fe flakes was fabricated to investigate the effect of MgFe2O4 cladding on the microwave absorption. Flake morphology and surface cladding were analyzed by scanning electron microscopy (SEM) and energy dispersion spectra (EDS). Phase constituents were analyzed by X-ray diffraction (XRD) with Cu Ka radiation. The toroidal composites (the weight ration of powders/paraffin is about 4:1) were analyzed within the frequency range of 1–18 GHz on a vector network analyzer (Agilent 8722 ES) to extract the complex permittivity and complex permeability.

3. Results The original powder exhibit conglobated particle shape from several tens micrometer to hundred micrometer, as shown in the inset of Fig. 1(a). The inset of Fig. 1(b) presents the low magnification SEM image of the powders with 10 wt% cladding. With using process control agents, the shape of Fe particles was changed to the flake. By means of the laser particle size analyzer, the mean size of the flakes is around 30.4 mm, in agreement with the SEM observation. Fig. 1(a) and (b) gives the vertical and plane images of a single Fe flake. Fe flakes with 10 wt% MgFe2O4 cladding exhibit a thickness of around 0.3 mm and width of about 30 mm, achieving a high aspect ratio close to 100. Both sides of flakes were attached with cladding as seen in Fig. 1(a) and (c) gives the EDS results detected from the flake surface marked in Fig. 1(b) with a square. It is clear that the spectra obviously show the peaks of Mg, Fe and O elements. The signal of C might come from the environment. Fig. 2 gives the XRD patterns of the original, as-milled, 1 wt% and 10 wt% powders, respectively. The original powders contain minor oxides of FeO due to the less purity of the commercial products. After wet-milling, the diffraction peaks of main phase a-Fe were broadened owing to the grain refinement. For the samples with cladding, the new phases precipitated after vacuum annealing. Taking account of the existence of Mg on the surface of Fe flakes, it can be deduced that the magnesium ferrite MgFe2O4 was formed. The experimental

diffraction patterns are well matched with the MgFe2O4 JPCPDS file (Nos. 36–0398). The diffraction intensity of ferrites became stronger with increasing cladding, indicating the increased volume of MgFe2O4. Fig. 3 gives the frequency dependence of the complex permittivity of three samples. Regarding the flakes without cladding, the complex permittivity possesses very high real part (e0 ) and imaginary part (e00 ). The complex permittivity exhibits a significant decrease with frequency. The real part of permittivity decreases from 179 at 1 GHz to 58 at 18 GHz and the imaginary part decreases from 150 at 1 GHz to 68 at 18 GHz, respectively. After coating 1 wt% MgFe2O4, the permittivity was seriously decreased, but still showing high values of e0 of 101 and e00 of 97 at 1 GHz, 63 and 34 at 18 Hz, respectively. While the cladding of MgFe2O4 reaches 10 wt%, the permittivity presents very low values of e0 of 27 and e00 of 5 at 1 GHz, 20 and 6 at 18 Hz. The real part decreases gently with frequency for the sample with

Fig. 2. XRD patterns of the original, as-milled, 1 wt% and 10 wt% cladding samples.

Fig. 1. Vertical view (a) and plane view (b) SEM images of Fe flakes with 10 wt% MgFe2O4 cladding. The insets are the image of the original powders and low magnification image of Fe-flakes, respectively. (c) EDS spectra detected from the square marked region of image (b).

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Fig. 3. Complex permittivity as a function of frequency.

Fig. 5. Frequency dependence of the microwave reflection loss (RL).

and absorber thickness with the equations below [9] RL ¼ 20log ðZ in Z 0 Þ=ðZ in þZ 0 Þ , Z in ¼ Z 0 ðmr =er Þ1=2 tanh½jð2pf d=cÞðmr er Þ1=2 

Fig. 4. Complex permeability as a function of frequency.

10 wt% MgFe2O4. Especially for the imaginary part of permittivity, the change tendency with frequency is totally different, which shows two resonance peaks at 9.8 GHz and 17.7 Hz. Carefully clarifying the curve of imaginary part of complex permittivity of sample with 1 wt%, it seems that there are also two weak resonance peaks marked with arrows, which occur at a little lower frequencies. In contrast to the complex permittivity, the permeability did not present a significant deduction with MgFe2O4 cladding as shown in Fig. 4. As to the real part (m0 ) of complex permeability, it slightly decreased at low frequency with cladding, but showing a bigger value at high frequency with cladding. The imaginary part (m00 ) of permeability exhibits different change tendency with cladding. After 1 wt% MgFe2O4 cladding, the imaginary part presents two weak resonances at around 2.4 and 10.7 GHz. With increasing the cladding to 10 wt%, the resonance becomes very obvious and resonance frequencies increase to 3.4 and 11.0 GHz. Above effects of cladding on electromagnetic properties were directly reflected to the microwave absorption. The reflection loss (RL) of normal incident electromagnetic wave was simulated from the complex permeability and permittivity at a given frequency

ð2Þ

Where f is the frequency of the electromagnetic wave, t is the thickness of the absorber, c is the velocity of light, Z0 is the impedance of air and Zin is the input impedance of the absorber. Fig. 5 gives the frequency dependence of the RL of above three specimens. The specimen without cladding presents a very poor RL with a minimum value of 2.6 dB. By coating 1 wt% MgFe2O4, the RL properties were improved a little to get a minimum value of  3.5 dB. However, the significant improvement was gained by coating 10 wt% MgFe2O4. A minimum RL of 16 dB was obtained with an absorber thickness of 5 mm. There are few variations with the absorber thickness. Even for a thin absorber of 1.5 mm, the minimum RL is lower than  10 dB, indicating a higher than 90% microwave absorption.

4. Discussions As well known, the permittivity is highly related to the surface resistivity and polarization [11]. High polarization could bring a high permittivity. On the other hand, the high surface resistivity would weaken the surface polarization resulting in a low permittivity [11]. In terms of the free electron theory [14], the resistivity can be evaluated from the imaginary component of complex permittivity

e00 ¼ 1=ð2pe0 rf Þ

ð3Þ

where e0 is the vacuum permittivity, r is the resistivity, and f is frequency. In terms of the deduction of the imaginary part of complex permittivity, MgFe2O4 cladding significantly increased the surface resistivity of Fe flakes, which could be understood from the non-conductive feature of MgFe2O4 in comparison to the metallic Fe flakes. Moreover, the high aspect ratio of naked Fe flakes would also bring a high surface polarization due to the broken symmetry or surface defects. With MgFe2O4 cladding, the

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structural defects and surface symmetry could be released to a degree, which was useful for decreasing the permittivity. One of interesting feature is the appearance of resonance for the specimen with 10 wt% MgFe2O4 cladding. The dielectric properties are similar to the properties of capacitances to some extent [15,16]. The Fe flakes without cladding are a conductor and thus shortcircuit the capacitance, showing no dielectric resonance. With 1 wt% MgFe2O4 cladding, Fe flake could not be fully coated to form a good capacitance, resulting in a weak resonance. With increasing MgFe2O4 cladding to 10 wt%, the Fe flakes were better coated, exhibiting a clear resonance. The complex permeability is mainly decided by magnetic matrix component within the specimen, causing a less influence with MgFe2O4 cladding. In general, the magnetic hysteresis, domain-wall resonance, eddy-current loss, natural resonance and exchange resonance can all contribute to the magnetic loss. As shown in Fig. 4, the imaginary part of complex permeability shows resonance feature with MgFe2O4 cladding and resonance feature become very clear with increasing the cladding. The magnetic hysteresis and domain-wall resonance can be excluded since their influences are very weak and occur at low frequency [17]. On the other hand, the resonance only happened for the specimens with cladding, the contribution of the eddy-current loss can be avoided. Finally, the resonance features would be only ascribed to the natural resonance and exchange resonance. Commonly, the exchange resonance occurred at higher frequency than natural resonance. Previous researches have shown a natural resonance around 1.3–3 GHz for the Fe-based magnetic particles or flakes [10,11]. Moreover, the natural resonance (f r ) [18] will vary with effective anisotropic field (Ha) with a relationship 2pf r ¼ gHa

ð4Þ

where g is gyromagnetic ratio. So in this work, it is reasonable to think that the first resonance corresponds to the natural resonance and the high frequency resonance corresponds to the exchange resonance. The dispersion of resonance could attribute to the variation of the effective anisotropic field due to possible size dispersion of flakes. Good microwave absorption comes from the proper impedance matching between the complex permittivity and permeability of materials. As shown in Figs. 3 and 4, the MgFe2O4 cladding significantly decreased the permittivity and mildly affected the permeability, which brought a better impedance matching. Current work proposed that MgFe2O4 is a good cladding to optimize the microwave absorption of Fe flakes.

5. Conclusions Fe flakes were successfully fabricated with a high aspect ratio close to 100. With 10 wt% MgFe2O4 cladding, the complex

permittivity of Fe flakes decreased from 179 to 27 for real part, from 150 to 5.3 for imaginary part at a frequency of 1 GHz. However, the real part of the complex permeability exhibited a high value at high frequency with a slight degradation at low frequency after 10 wt% MgFe2O4 cladding. Due to the improved electromagnetic impedance matching, the microwave absorption was significantly decreased from 2.6 dB (2.2 GHz) to 13 dB (10.3 GHz) with an absorber thickness of 1.5 mm by 10 wt% MgFe2O4 cladding. This work proposed a good way to optimize the electromagnetic microwave properties of Fe flakes.

Acknowledgment This work was in part supported by the National Natural Science Foundation of China (No. 51001085, No. 51171148), the Educational department of Shaanxi Provincial Government (No. 2010JK766), the Doctoral Course Foundation of Ministry of Education of China (No. 20106118120015) and Shaanxi Provincial Project of Special Foundation of Key Disciplines.

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