Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178
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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
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 , 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 , the real permeability m00 is also decided by the volume fraction (P) of metallic particles and its demagnetizing factor (Nd) 0 0o
P þ1 m00 ðmaxÞ ð1PÞN d
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 . However, the permittivity of metallic magnetic ﬂakes would be very large because of the large surface polarization . Limited by the electromagnetic impedance matching condition , the unilateral increase of the permeability or the permittivity would lead to a poor reﬂection loss (RL). Surface modiﬁcation must be done to n
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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 . 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 . 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
N. Tian et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178
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).
N. Tian et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178
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  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
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 . 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 . In terms of the free electron theory , the resistivity can be evaluated from the imaginary component of complex permittivity
e00 ¼ 1=ð2pe0 rf Þ
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
N. Tian et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 4175–4178
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 . 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 )  will vary with effective anisotropic ﬁeld (Ha) with a relationship 2pf r ¼ gHa
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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