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(Ni0.5Zn0.5)Fe2O4 magnetic sheet composite with tunable electromagnetic properties for enhancing magnetic field coupling efficiency
Journal of Alloys and Compounds 729 (2017) 277e284
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
FeSiAl/(Ni0.5Zn0.5)Fe2O4 magnetic sheet composite with tunable electromagnetic properties for enhancing magnetic field coupling efficiency X. Jin*, Q. Wang, W.Q. Khan, Y.Q. Li, Zh.H. Tang School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2017 Received in revised form 7 September 2017 Accepted 8 September 2017 Available online 8 September 2017
Flaky FeSiAl alloy (9.5 wt% Si, 5.5 wt% Al and Fe balance) was prepared by using mechanical milling method. Fine ferrite powders were produced by oxalate co-precipitation process. Flexible composite sheets were fabricated via tape-casting method by mixing the flake-shape sendust and various weight fraction of NiZn ferrite. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were used to characterize the samples. Electromagnetic properties measurement results indicated that permittivity, permeability, power loss of flaky FeSiAl/NiZn ferrite composite can be well tuned by adjusting the amount of loading NiZn ferrite particles. It is found that the magnetic field coupling efficiency is enhanced by attaching the composite with the mix of ferrites. Consequently, the composite filled with 14 wt% of ferrite particles showed the best coupling ratio at 1 MHz-1GHz. This research work shows promising results and making FeSiAl/NiZn ferrite composite a potential magnetically conductive material for energy transmission applications, such as NFC. © 2017 Elsevier B.V. All rights reserved.
Keywords: Flaky FeSiAl alloy Magnetic field coupling Magnetic loss Dielectric loss Soft magnetic composite Tape-casting
1. Introduction FeSiAl alloys are well known for excellent soft magnetic properties with relatively high permeability, high saturation magnetization and low cost [1]. It has been widely applied in electromagnetic interference (EMI) filtration, EM noise suppression [2e4] and microwave absorption [5,6]. However, the high electrical conductivity of FeSiAl alloy caused a high eddy current loss under alternating electromagnetic field which restricts its application in magnetic energy transmission system such as wireless power transfer [7,8] and near field communication (NFC) [9e11]. With attached soft magnetic sheets, it can improve magnetic field coupling efficiency of those systems by segregating magnetic field interference to and from other devices, shielding effect of environment such as metal backplane and guiding the magnetic flux. It is desired to obtain a material with minimal losses under alternating electromagnetic field for magnetic energy transmission system. Low electrical conductivity is a key for the FeSiAl alloy to reduce eddy current loss in high frequencies. A soft magnetic composite (SMC) can be used well to combine the advantages of
* Corresponding author. E-mail address: [email protected] (X. Jin). https://doi.org/10.1016/j.jallcom.2017.09.088 0925-8388/© 2017 Elsevier B.V. All rights reserved.
various elements and may overcome the limitations of a single material, for instance to reduce the electrical conductivity [12]. The addition of nonmagnetic AlN, Al2O3, Ti and MgO has been used to enhance the electrical resistivity of FeSiAl alloy [13e15]. On the contrary this will harm the permeability, saturation magnetization of the FeSiAl alloy and may significantly increase magnetic loss. Ferrites are magnetic materials with low electrical conductivity, negligible losses and high frequency characteristic, however it has relatively low saturation magnetization [16]. MnZn, NiZn spinel and hexaferrites are the most common radio frequency soft ferrite material in the advanced industry [17,18]. Various ferrites have their advantages and can be applied to actual application according to different working frequency. Specifically, high permeability MnZn ferrite is applied in low frequency (1 MHz or less) conditions, NiZn ferrite is used when the frequency exceeds 1 MHz and hexaferrites is employed as the frequency is more than 1 GHz. The addition of fine ferrite particles can overcome those limitations effectively. The composite containing suitable ferrite powders and FeSiAl flakes possess both strong permeability and relative high resistivity. The comprehensive performance of magnetic composite can be achieved by adjusting the amount of ferrites. Prepared spherical FeSiAl mixed with manganese zinc (MnZn) ferrite, after low temperature curing exhibited the less loss and higher
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resistivity [19]. Fe-Si coated with nickel-zinc (NiZn) ferrite composite was produced by spark plasma sintering in order to obtain low total loss of composite [20]. Many researchers tried to get a material with high permeability under alternating electromagnetic field. The permeability is mainly dependent on the shape of particles [21]. Flaky shaped FeSiAl alloy particles with larger aspect ratio can improve both permeability and resonance frequency [22e24]. The permeability of Sendust flake particle composites will increase 5e6 times higher than Sendust bulk particle composites [25]. Moreover, flattened sendust having thickness less than the skin depth can suppress eddycurrent effectively [26]. In order to keep large aspect ratio of inplane aligned Sendust flakes within the matrix, tape-casting method is the better option to fabricate the SMC as compared to compact molding method [27e29]. Furthermore, this method is simple, cost efficient and appropriate for large-scale production to facilitate the practical application of flake shape FeSiAl composites [30,31]. This study attempts to obtain a flexible SMC by combining the FeSiAl flakes with varying fractions (wt%) of NiZn ferrite particles, which possess higher frequency characteristic than MnZn ferrite. Electromagnetic parameters of the composites were tuned by adjusting the additional weight fractions of NiZn ferrite. Excellent magnetic field coupling performance was achieved in the composite having moderate amount of NiZn ferrite particles (14 wt.%). The possible loss mechanisms related with the electromagnetic parameters of the composite and the combined effect of NiZn ferrite and alloy were discussed in detail. 2. Experimental details Flaky morphology of FeSiAl alloy was obtained by the mechanical ball milling of raw powders (9.5 wt% Si, 5.5 wt% Al and Fe balance, BGRIMM Materials & Technology Co, Ltd.) in anhydrous ethanol solvent under protective atmosphere of nitrogen. The ballto-powder weight ratio of 10:1 was applied and milling was conducted for 10 h at 460 rpm. The then products were collected, washed and dried in an oven for 6 h at 70 C. The dry flakes were annealed using vacuum furnace for 2 h at 400 C. Ferrite powders were prepared by oxalate co-precipitation process. All chemicals were of reagent grade and used as received without further purification. Nickel sulfate hexahydrate (NiSO4 6H2O), znic sulfate heptahydrate (ZnSO4 7H2O), ferric sulfate heptahydrate (FeSO47H2O), ammonium oxalate ((NH4)2 C2O4 H2O), sulfuric acid (H2SO4), were acquired from Sigma Aldrich. The aqueous solutions of NiSO4 6H2O, ZnSO4 7H2O, FeSO47H2O and (NH4)2 C2O4 H2O were prepared according to the ratio required for NiZn ferrite. 0.1 wt% sulfuric acid solution was confected as a solvent to inhibit hydrolysis of ferrous ion. The precursor was prepared by using NiSO4, ZnSO4 and FeSO4 as raw material with (NH4)2 C2O4 H2O as the precipitator through precipitation. It was thermally decomposed at 700 C for 30 min by high temperature calcination to obtain ferrite fine powders. SMC with appropriate amounts of FeSiAl alloy flakes and NiZn ferrite particles was prepared by using tape-casting method. A mixture of polyvinyl butyral, dibutyl phthalate (Butvar B-98, Monsanto Co. St. Louis, MO, USA), and ethanol was mixed well in a mechanical stirring setup for 6hrs. Then the varying amounts of NiZn ferrite powder and flakes of FeSiAl were added along with glycerol trioleate as a dispersant in mechanical stirring equipment for 7hrs. The slurry was vacuum treated to get rid of froth for 120 min with the stirring at a speed of 60 rpm and casted onto a polymer substrate 100 mm thick having molecular formula (C10H8O4) n. by using a tape casting machine. Composite with thickness around 0.3e0.4 mm in tape sheet form was obtained after
the solution was dried in a vacuum oven at 50 C. Six samples of composites with various weight fractions of NiZn ferrite, 0%, 6%, 10%, 14%, 18%, and 22% have been produced. Phase structure of ferrites, sendust flakes and composite sheets were characterized by X-ray diffraction (XRD, SHIMADZU 7000) using Cu-Ka source and morphologies of the composites and FeSiAl flakes were observed by a FEI Quanta 200 scanning electron microscope (SEM, HITACHI S-4800). Particle size of the ferrite powder was measured by laser particle size analyzer (Mastersizer Micro). The static magnetic properties of the composite were measured at room temperature by a vibrating sample magnetometer (Quantum Design, VersaLab). The complex permeability and permittivity of the composite were measured on a Network spectrum impedance analyzer (Agilent, Network spectrum impedance analyzer 4396B) in the frequency range from 1 MHz to 1 GHz with a dielectric material test fixture (Agilent, 16453A) and magnetic material test fixture (Agilent, 16454A) respectively. The toroid samples with outer and inner diameter of 8 mm and 3 mm respectively were cut from the composite sheet with a particularly designed cutter. Samples (20 40 0.4 mm) were placed in the middle of microstrip line (MSL) with a characteristic impedance of 50U for power loss (PL) evaluation. Both ends of the MSL are connected to a vector network analyzer (CETC, AV36850A), which can measure scatter parameter in the frequency range from 1 MHz to 1 GHz. Magnetic field coupling efficiency was analyzed by an in-house developed method, two near filed probes (RF 400-1/2), one serving as a transmission coil connecting to signal generator (Agilent, E8257D) and the second as a receiver coil connecting to EMC analyzer (Agilent, E7405A). When an alternating current passes through the transmission coil, magnetic lines of field were generated and passed through the receiving coil creating an induced electromotive force around it resulting in magnetic field couple. By attaching the composites to the transmission coil as a backplane, the values were measured from EMC receiver which can evaluate the efficiency of magnetic field couple. 3. Results and discussion 3.1. Principal of precursor preparation Fine ferrite powders having average particle size of 2e5 mm were prepared by oxalate co-precipitation process. Laboratory grade salts of NiSO4, ZnSO4 and FeSO4 containing cations (Ni2þ, Zn2þ and Fe2þ) and no anions were used as reactants. After the complete reaction between salts, basic carbonate precursor was obtained having very fine crystalline grains, size was confirmed by laser particle size analyzer. Precursor was then calcined at 700 C for 30 min. The precipitation and calcination reactions are as follows:
0:5Ni2þ þ 0:5Zn2þ þ 2Fe2þ þ 3C2 O2 4 þ 6H2 O/ðNi0:5 Zn0:5 ÞFe2 ðC2 O4 Þ3 $6H2 OY
700 C
ðNi0:5 Zn0:5 ÞFe2 ðC2 O4 Þ3 $6H2 O ! ðNi0:5 Zn0:5 ÞFe2 O4 þ 4CO2 [ 30 min
þ 6H2 O
3.2. Phase structure and morphology (XRD & SEM) Fig. 1 shows the XRD pattern of the obtained ferrites, sendust flakes and composite sheets with different NiZn ferrite contents. As observed in Fig. 1(a), the peak position and relative intensities of
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(Ni0.5Zn0.5)Fe2O4 diffraction spectra were analyzed with X'pert software and matched well with the standard patterns. It shows the NiZn ferrite crystal planes at 2q ¼ 30.1, 35.5 , 57.0 are at most preferred orientations of (113), (222), and (333), JCPDS#00-0520278. Evident diffraction peaks of a-FeSiAl alloy with bcc structure are at 44.9 and 65.5 , and DO3 superlattice structure are at 27.1, 31.4 and 53.3 , JCPDS # 00-045-1206. It displays that FeSiAl alloy flakes with the DO3 superlattice structure exhibit good soft magnetic properties due to the appropriate proportion of ordered and disordered lattices [32]. Fig. 1(b) depicts the XRD patterns of FeSiAl/(Ni0.5Zn0.5)Fe2O4 composites with different NiZn ferrites content. It is clear that the mixing of ferrites does not change the crystal structure of the FeSiAl alloy. After the NiZn ferrite content was increased to 10 wt%, characteristic peaks from NiZn ferrite can be clearly detected. All the peaks are sharp confirming the crystalline nature of composite. When the quantity of (Ni0.5Zn0.5)Fe2O4 is at or around 10 wt%, the minor peaks of ferrite appeared showing the minimum detectable wt% is ±10. The intensity of the ferrite spectra is highest at 22 wt% and the slight broadening of peaks base was observed. Fig. 2 shows the typical morphologies of FeSiAl alloy flakes with large aspect ratio (length/thickness). The microstructures of DO3 superlattice are ordered structure while FeSiAl flakes are bcc disordered structure [33e35]. A homogeneous distribution of flake sizes was achieved. A typical flake indexed in the figure had a thickness of around 0.7 mm and a length of approximately 100 mm. The corresponding aspect ratio is more than 140:1, which is favorable to enhance the saturation magnetization on the parallel directions of the particle plane by decreasing the demagnetizing field and suppressing the negative effect of eddy current in alternating electromagnetic field since the thickness of the particle is less than skin depth of sendust [36]. Fig. 3 shows the SEM images of two composites having 0 wt% and 14 wt% of ferrites. In the composite, most of the flakes can be well aligned in the plane, and still keeps a high aspect ratio. Thus, thin flakes in the direction parallel to the electromagnetic field increase the magnetized density and its anisotropic microstructure will increase the permeability value of the composite. The ferrite powders are closely bonded to the surface of FeSiAl alloy flakes. The fine ferrite particles with small dimensions can readily segregate interconnected flakes which closes the conductive paths in matrix, and thus benefit to prohibit the ohmic loss. With the increasing
279
amount of ferrites attached to the surface, the separation of flakes becomes more effective. Clearly, parts of the ferrites are well dispersed as shown in Fig. 4(c), only few portions are heavily segregated with flake plates, Fig. 4(d). 3.3. Static magnetic properties The magnetization curves of composites with the various amounts of ferrite at room temperature are shown in Fig. 4. The hysteresis loops were measured in-plane of the film with respect to the applied magnetic field. All the samples exhibit representative soft magnetic characteristics with quite small values of remnant magnetization and coercivity [37]. The saturation magnetizations (Ms) are 117 emu/g, 110 emu/g, 94 emu/g, 83 emu/g, 74 emu/g and 64 emu/g for the composites with the corresponding NiZn ferrites fractions of 0%, 6%, 10%, 14%, 18%, and 22%, respectively. The range of Ms decreased with increasing the amount of ferrites, because the saturation magnetization of ferrite is significantly lower than that of FeSiAl alloy. 3.4. The complex permittivity and permeability Fig. 5 shows the complex permittivity spectra measured for composite films with different wt% of NiZn ferrite within the frequency range of 1 MHz-1GHz. According to Debye relaxation equation, the relationship between permittivity and frequency of the composites follow the equations [38]
ε0 ðuÞ ¼ ε∞ þ 00
ε ðuÞ ¼
εs ε∞ 1 þ u2 t2
ðεs ε∞ Þut 1 þ u2 t 2
(1)
(2)
Here, t is relaxation time, u is angular frequency, εs and ε∞ represent the static permittivity and permittivity at infinite frequency, respectively. Equations (1) and (2) show that permittivity is proportional to relaxation time and frequency. The real and imaginary part (ε0 and ε") of all samples have same trend of decreasing with frequency in the range 1 MHz to 1 GHz. Changing the values of ferrite leads to the significantly difference of both ε0 and ε" which denotes the storage and dissipation abilities of electric energy [39], respectively. The value of relaxation strength belongs to dielectric
Fig. 1. The XRD pattern of ferrites and sendust flakes (a), composite sheets with varying wt% of (Ni0.5Zn0.5) Fe2O4 (b).
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Fig. 2. SEM images of FeSiAl alloy flakes (a), cross section of flakes (b).
Fig. 3. SEM images of composites with varying ferrites 0 wt% (a) and (b), 14 wt% (c) and (d).
transition (εs-ε∞)u decrease with the weakened polarization of composite [38]. This confirms our results in Fig. 5 that amount of ferrites have weakened the polarization of composite, so that ε0 and ε" of the composite decrease with the increasing fraction of ferrites. Additionally, according to Maxwell-Wagner-Sillars (MWS), the relationship between ε" of the composite and frequency are similar to Debye equations, except there is one more consideration of electrical conductivity in equation (2), as shows in the following equation [40].
00
ε ¼
ðεs ε∞ Þut s þ u 1 þ u2 t 2
(3)
Where, s is the electrical conductivity of composite. Equation (3) shows that there is conductive and polarization relaxation loss, and ε" is proportional to electric conductivity, which obviously decreased with increasing weight fraction of ferrites in the composite. Mainly because of the fact that sendust flakes are isolated from each other by the addition of very fine ferrite particles in the matrix, which blocks the conductive paths for electron hopping and migrating. For flaky shape particles possessing higher aspect ratio,
X. Jin et al. / Journal of Alloys and Compounds 729 (2017) 277e284
me 1 mi 1 ¼p 1 þ nðme 1Þ 1 þ nðmi 1Þ
281
(4)
where p, n and mi are the volume fraction, averaged shape factor and permeability of inclusions, respectively. The permeability of the composite is me that can be solved from Eq. (4)
me ¼ 1 þ
p nð1 pÞ þ 1=ðmi 1Þ
(5)
The value of me decreases with the reduction of mi within the certain volume fractions. This confirms our results in Fig. 6 that amount of ferrites have reduced the mi of composite, so that the m0 and m" of composite decrease with the increasing fraction of ferrites. From the above analysis, we can conclude that the addition of ferrite particles can also cause less loss.
Fig. 4. Hysteresis loops of the composites.
polarization mainly originates from the interfacial polarization at the interfaces of FeSiAl/PVB and FeSiAl/FeSiAl causing polarization relaxation loss [41]. The addition of ferrite particles hasn't introduced any significant polarizations in the composites. Which is depicted by the curves of all samples almost having same profile [39,42], Fig. 5(b). From the above analysis, it is confirmed that the addition of ferrite particles can reduce loss to a great extent. Complex permeability spectra of the composite films filled with different wt% of NiZn ferrite within the frequency range of 1 MHz1GHz is showed in Fig. 6. The real part of permeability (m0 ) value of all the kind of FeSiAl/NiZn ferrite composites decreased as the frequency increased. The imaginary part of permeability (m") of all kind of FeSiAl/NiZn ferrite composites increased slightly in the frequency 1e200 MHz, the maximum value obtained at 200 MHz, then m" decreased in the following frequency. All of the above phenomenon was consistent with the other research works [43e46]. The m0 and m" differ greatly depending on the filled weight fraction of ferrites. It reducing significantly with increasing filled ferrites in the composite. For the flake FeSiAl/NiZn ferrite composites, the permeability could be explained according to the Maxwell Garnett equation [47]
3.5. Power loss of composites Fig. 7 shows Power Loss (PL) of the composites associated with varying weight fractions of ferrite. The value of PL, as given by Eq. (6), increased with frequency and reduced with weight loading fraction of ferrites. Frequency dependence of power loss (PL) of the composites were estimated from their Scattering-parameters (S11 and S21) according to the following equation [48]: S11 S21 Ploss ¼ 1 10 10 10 10 Pin
(6)
The total losses of the MSL with the film attached is the sum of the dielectric loss and magnetic loss of film, dielectric loss of the MSL substrate and radiation loss [49]. Dielectric loss of the MSL substrate and radiation loss can be disregarded after calibrating the equipment and the losses from the composite samples can be defined and calculated according to the following equations. I) Dielectric loss of the sheet
Pdie ¼
1 T
ZtþT t
00
uε ε0
Z
E2 dV dt
DV
Fig. 5. Complex permittivity spectra of the composite films with different ferrites loading, real part (a) and imaginary part (b).
(7)
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Fig. 6. Complex permeability spectra of the composite films with different ferrites loading, real part (a) and imaginary part (b).
Fig. 7. Power loss for FeSiAl/NiZn ferrite composites.
II) Magnetic loss of the sheet
Pmag ¼
1 T
ZtþT t
00
um m0
Z H2 dV dt
(8)
DV
Where T is the time period of one cycle, V is the volume of the film of composite and ε0 and m0 are the permittivity and permeability of free space, respectively. Equations (7) and (8) show that the material loss increases with the angular frequency, imaginary part of permittivity (ε") and permeability (m"). These mathematical expressions are in consistent well with the phenomenon explains in Fig. 7. According to Figs. 5 (b) and 6 (b), it is expected to obtain lower imaginary part of permittivity (ε") and permeability (m") for reducing losses from the magnetic composites. 3.6. Magnetic field coupling efficiency Collectively the aim was to improve magnetic field coupling efficiency which had positive correlation with the inductance of the device (such as a coil). By attaching the composites to the transmission coil as a backplane, attenuation of the inductance is mainly
the current loss (induced and displacement current) and magnetic loss of the composite. The current loss is related to ε" and s of the composite while magnetic loss is related to m" of the composite. We intend to obtain relatively less ε" and m" values of the FeSiAl/NiZn composites indicate the lower dielectric, ohm and magnetic loss. On the other hand, the improved inductance is the advanced ability of attached composite gathering the magnetic field on account of its high m0 and Ms values of the FeSiAl/NiZn composites while the geometrical dimensions of the coil is governed by the design. Above analysis reveals that, increasing the %age of additive ferrites, the relationship between less values of ε" and m" and high values of m0 and Ms for FeSiAl/NiZn ferrite composites is reversed. It suggests that optimal fraction of ferrite maybe decided by balancing the values of ε", m", m0 and Ms in order to get the highest magnetic field coupling efficiency. Magnetic field coupling efficiency is shown in Fig. 8 by attaching with and without the composites to the transmission coil as a backplane. As observed in Fig. 8(a), with and without attaching the FeSiAl/NiZn ferrite composite leads to the great change of received power values which represent the function of magnetic sheet. After attaching the composite without loading the ferrite particles, the value of received power is less as compared to without attaching. This result in magnetic and dielectric loss from composites is significantly higher than the ability of composite gathering the magnetic field. By increasing the amount of ferrites from 0% to 6%, the enhancement of the received power values shows the loaded NiZn ferrite particles can reduce the loss of FeSiAl/NiZn ferrite composites. That confirms the dielectric and magnetic loss from composites is lower than the ability of composite gathering the magnetic field while the m0 and Ms are decreasing sharply. After loading ferrite particles with 6%, 10% and 14%, the slight increase in received power values shows that the influence from reducing magnetic and dielectric loss of FeSiAl/NiZn ferrite composites is higher than the weakened ability of composite gathering the magnetic field. After loading ferrites with 18% and 22%, the gradual decrease of the received power values means the much reduced m0 and Ms that leads to weaken the ability of FeSiAl/NiZn ferrite composites to gather magnetic field, although the material loss is still decreasing sharply. Consequently, the value of received powder is optimized at of 14% of NiZn ferrite fraction. NFC antennas were then used as transmission and received devices with the central work frequency of 13.56 Mhz. The
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Fig. 8. Measured near field probe (a) and NFC (b) antenna with and without FeSiAl/NiZn ferrite composites.
phenomenon observed in Fig. 8(b) is in consistent well with Fig. 8(a) that 14% of NiZn ferrites represent optimal value. From the above analysis, we can conclude that low material loss and the strong Ms and m0 contributes to the excellent magnetic field coupling efficiency of FeSiAl/NiZn ferrite (14 wt %) composite. 4. Conclusions Flexible composite produced during the present work containing NiZn ferrite powder with FeSiAl flakes possess both relatively strong permeability and high resistivity. Increasing the amount of ferrites from 0% to 6%, 10%, 14%, 18% and 22%, imaginary part of permittivity and permeability decreases which leads to less power loss of the composite, and Ms and real part of permeability decreases which results in the reduction of magnetic field coupling efficiency. The composites without loading the ferrite particles shows the lowest magnetic field coupling efficiency because of significantly high magnetic loss and dielectric loss of the composites. By adjusting the weight percentage of NiZn ferrite, the composite having 14 wt% NiZn ferrite demonstrates optimized high magnetic field coupling efficiency due to relatively low material loss, strong Ms and m0 of composite. Therefore, the Sendust flakes mixed with NiZn ferrite particles could be promising candidate in both wireless power transfer and nearfield communication fields. Notes The authors declare no competing financial interest. Acknowledgement This work was supported by National Key R&D Plan of China [grant number 2016YFB1200602-37]. References [1] W. Wang, T. Ma, M. Yan, Microstructure and magnetic properties of nanocrystalline Co-doped Sendust alloys prepared by melt spinning, J. Alloy. Comp. 459 (1) (2008) 447e451. [2] S. Yoshida, M. Sato, E. Sugawara, et al., Permeability and electromagnetic interference characteristics of Fe-Si-Al alloy flakes polymer composite, J. Appl. Phys. 85 (8) (1999) 4636e4638. [3] L. Liu, Z.H. Yang, C.R. Deng, Z.W. Li, M.A. Abshinova, L.B. Kong, High frequency properties of composite membrane with in-plane aligned Sendust flake prepared by infiltration method, J. Magn. Magn. Mater. 324 (10) (2012)
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shaped FeSiAl/graphite composites, J. Alloy. Comp. 548 (2013) 18e22. [25] Z. Zhang, J. Wei, et al., Effect of shape of Sendust particles on their electromagnetic properties within 0.1-18 GHz range, Phys. B Condens. Matter 406 (20) (2011) 3896e3900. [26] S.S. Kim, S.T. Kim, Y.C. Yoon, K.S. Lee, Magnetic, dielectric, and microwave absorbing properties of iron particles dispersed in rubber matrix in gigahertz frequencies 97 (10) (2005), 10F905eF910F905-3. [27] S. Yoshida, S. Ando, Y. Shimada, K. Suzuki, K. Nomura, K. Fukamichi, Crystal structure and microwave permeability of very thin Fe-Si-Al flakes produced by microforging, J. Appl. Phys. 93 (10) (2003) 6659. [28] H. Ono, T. Ito, S. Yoshida, Y. Takase, O. Hashimoto, Y. Shimad, Noble magnetic films for effective electromagnetic noise absorption in the gigahertz frequency range, IEEE Trans. Magn. 40 (4) (2004) 2853. [29] T.D. Zhou, P.H. Zhou, D.F. Liang, L.J. Deng, Structure and electromagnetic characteristics of flaky FeSiAl powders made by melt-quenching, J. Alloy. Comp. (2009) 484. [30] H. Ono, S. Yoshida, S. Ando, Y. Shimada, Improvement of the electromagneticnoise suppressing features for Fe-Si-Al composite sheets by dc magnetic field biasing, J. Appl. Phys. 93 (10) (2003) 6662. [31] K. Oh, H. Lee, W. Lee, M. Yoo, W. Kim, H. Yoon, Effect of viscosity on the magnetic permeability of Sendust-filled polymer composites, J. Magn. Magn Mater. 321 (2009) 1295. [32] T.D. Zhou, P.H. Zhou, D.F. Liang, Structure and electromagnetic characteristics of flaky FeSiAl powders made by melt-quenching, J. Alloy. Comp. 484 (2009) 545e549. [33] M. Abshinova, Factors affecting magnetic properties of Fe-Si-Al and Ni-Fe-Mo alloys, Procedia Eng. 76 (2014) 35e44. [34] S. Yoshida, S. Ando, Y. Shimada, Crystal structure and microwave permeability of very thin Fe-Si-Al flakes produced by microforging, J. Appl. Phys. 93 (10) (2003) 6659e6661. [35] M.P.C. Kalita, A. Perumal, A. Srinivasan, Structural analysis of mechanically alloyed nanocrystalline Fe75Si15Al10 powders, Mater. Lett. 61 (3) (2007) 824e826. [36] R. Han, L. Qiao, T. Wang, et al., Microwave complex permeability of planar anisotropy carbonyl-iron particles, J. Alloy. Comp. 509 (2011) 2734e2737.
[37] A. Boglietti, A. Cavagnino, M. Lazzari, Predicting iron losses in soft magnetic materials with arbitrary voltage supply: an engineering approach, Magn. IEEE Trans. on 39 (2) (2003) 981e989. [38] Peter Hedvig, Dielectric Spectroscopy of Polymers, Akademiai Kiado, Budapest, 1977. [39] L. Huang, X. Liu, C. Dan, Flaky FeSiAl alloy-carbon nanotube composite with tunable electromagnetic properties for microwave absorption, Sci. Rep. 6 (2016) 35377. [40] Lee Sang Eui, O. Choi, H.T. Hahn, Microwave properties of graphite nanoplatelet/epoxy composites, J. Appl. Phys. 104 (2008) 397-R. [41] X. Lu, G. Liang, Q. Sun, C. Yang, High-frequency magnetic properties of FeNi3SiO2 nanocomposite synthesized by a facile chemical method, J. Alloy. Comp. 509 (16) (2011) 5079e5083. [42] O. Bomatí-Miguel, M.P. Morales, P. Tartaj, Fe-based nanoparticulate metallic alloys as contrast agents for magnetic resonance imaging, Biomaterials 26 (28) (2005) 5695e5703. €ssbauer [43] M. Han, D. Liang, K.N. Rozanov, Microwave permeability and mo spectra of flaky Fe-Si-Al particles, IEEE Trans. Magn. 49 (3) (2013) 982e985. [44] S. Yoshida, M. Sato, E. Sugawara, Permeability and electromagneticinterference characteristics of Fe-Si-Al alloy flakes-polymer composite, J. Appl. Phys. 85 (8) (1999) 4636e4638. [45] L. Liu, Z.H. Yang, C.R. Deng, High frequency properties of composite membrane with in-plane aligned Sendust flake prepared by infiltration method, J. Magn. Magn Mater. 324 (10) (2012) 1786e1790. [46] J.L. Wallace, Broadband magnetic microwave absorbers: fundamental limitations, IEEE Trans. Magn. 29 (6) (1993) 4209e4214. [47] K.N. Rozanov, A.V. Osipov, D.A. Petrov, The effect of shape distribution of inclusions on the frequency dependence of permeability in composites, J. Magn. Magn Mater. 321 (7) (2009) 738e741. [48] S. Yoshida, S. Ando, H. Ono, Permeability characteristics and conduction noise suppression of magnetic sheets made of micro-forged Fe-Si-Al flakes embedded in a polymer matrix, J. Magnetics Soc. Jpn. 26 (6) (2002) 843e849. [49] H. Ono, T. Ito, S. Yoshida, Noble magnetic films for effective electromagnetic noise absorption in the gigahertz frequency range, IEEE Trans. Magn. 40 (4) (2004) 2853e2857.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
FeSiAl/(Ni0.5Zn0.5)Fe2O4 magnetic sheet composite with tunable electromagnetic properties for enhancing magnetic field coupling efficiency X. Jin*, Q. Wang, W.Q. Khan, Y.Q. Li, Zh.H. Tang School of Materials Science and Engineering, Beijing University of Technology, Beijing 100124, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 June 2017 Received in revised form 7 September 2017 Accepted 8 September 2017 Available online 8 September 2017
Flaky FeSiAl alloy (9.5 wt% Si, 5.5 wt% Al and Fe balance) was prepared by using mechanical milling method. Fine ferrite powders were produced by oxalate co-precipitation process. Flexible composite sheets were fabricated via tape-casting method by mixing the flake-shape sendust and various weight fraction of NiZn ferrite. X-ray diffraction, scanning electron microscopy and vibrating sample magnetometer were used to characterize the samples. Electromagnetic properties measurement results indicated that permittivity, permeability, power loss of flaky FeSiAl/NiZn ferrite composite can be well tuned by adjusting the amount of loading NiZn ferrite particles. It is found that the magnetic field coupling efficiency is enhanced by attaching the composite with the mix of ferrites. Consequently, the composite filled with 14 wt% of ferrite particles showed the best coupling ratio at 1 MHz-1GHz. This research work shows promising results and making FeSiAl/NiZn ferrite composite a potential magnetically conductive material for energy transmission applications, such as NFC. © 2017 Elsevier B.V. All rights reserved.
Keywords: Flaky FeSiAl alloy Magnetic field coupling Magnetic loss Dielectric loss Soft magnetic composite Tape-casting
1. Introduction FeSiAl alloys are well known for excellent soft magnetic properties with relatively high permeability, high saturation magnetization and low cost [1]. It has been widely applied in electromagnetic interference (EMI) filtration, EM noise suppression [2e4] and microwave absorption [5,6]. However, the high electrical conductivity of FeSiAl alloy caused a high eddy current loss under alternating electromagnetic field which restricts its application in magnetic energy transmission system such as wireless power transfer [7,8] and near field communication (NFC) [9e11]. With attached soft magnetic sheets, it can improve magnetic field coupling efficiency of those systems by segregating magnetic field interference to and from other devices, shielding effect of environment such as metal backplane and guiding the magnetic flux. It is desired to obtain a material with minimal losses under alternating electromagnetic field for magnetic energy transmission system. Low electrical conductivity is a key for the FeSiAl alloy to reduce eddy current loss in high frequencies. A soft magnetic composite (SMC) can be used well to combine the advantages of
* Corresponding author. E-mail address: [email protected] (X. Jin). https://doi.org/10.1016/j.jallcom.2017.09.088 0925-8388/© 2017 Elsevier B.V. All rights reserved.
various elements and may overcome the limitations of a single material, for instance to reduce the electrical conductivity [12]. The addition of nonmagnetic AlN, Al2O3, Ti and MgO has been used to enhance the electrical resistivity of FeSiAl alloy [13e15]. On the contrary this will harm the permeability, saturation magnetization of the FeSiAl alloy and may significantly increase magnetic loss. Ferrites are magnetic materials with low electrical conductivity, negligible losses and high frequency characteristic, however it has relatively low saturation magnetization [16]. MnZn, NiZn spinel and hexaferrites are the most common radio frequency soft ferrite material in the advanced industry [17,18]. Various ferrites have their advantages and can be applied to actual application according to different working frequency. Specifically, high permeability MnZn ferrite is applied in low frequency (1 MHz or less) conditions, NiZn ferrite is used when the frequency exceeds 1 MHz and hexaferrites is employed as the frequency is more than 1 GHz. The addition of fine ferrite particles can overcome those limitations effectively. The composite containing suitable ferrite powders and FeSiAl flakes possess both strong permeability and relative high resistivity. The comprehensive performance of magnetic composite can be achieved by adjusting the amount of ferrites. Prepared spherical FeSiAl mixed with manganese zinc (MnZn) ferrite, after low temperature curing exhibited the less loss and higher
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resistivity [19]. Fe-Si coated with nickel-zinc (NiZn) ferrite composite was produced by spark plasma sintering in order to obtain low total loss of composite [20]. Many researchers tried to get a material with high permeability under alternating electromagnetic field. The permeability is mainly dependent on the shape of particles [21]. Flaky shaped FeSiAl alloy particles with larger aspect ratio can improve both permeability and resonance frequency [22e24]. The permeability of Sendust flake particle composites will increase 5e6 times higher than Sendust bulk particle composites [25]. Moreover, flattened sendust having thickness less than the skin depth can suppress eddycurrent effectively [26]. In order to keep large aspect ratio of inplane aligned Sendust flakes within the matrix, tape-casting method is the better option to fabricate the SMC as compared to compact molding method [27e29]. Furthermore, this method is simple, cost efficient and appropriate for large-scale production to facilitate the practical application of flake shape FeSiAl composites [30,31]. This study attempts to obtain a flexible SMC by combining the FeSiAl flakes with varying fractions (wt%) of NiZn ferrite particles, which possess higher frequency characteristic than MnZn ferrite. Electromagnetic parameters of the composites were tuned by adjusting the additional weight fractions of NiZn ferrite. Excellent magnetic field coupling performance was achieved in the composite having moderate amount of NiZn ferrite particles (14 wt.%). The possible loss mechanisms related with the electromagnetic parameters of the composite and the combined effect of NiZn ferrite and alloy were discussed in detail. 2. Experimental details Flaky morphology of FeSiAl alloy was obtained by the mechanical ball milling of raw powders (9.5 wt% Si, 5.5 wt% Al and Fe balance, BGRIMM Materials & Technology Co, Ltd.) in anhydrous ethanol solvent under protective atmosphere of nitrogen. The ballto-powder weight ratio of 10:1 was applied and milling was conducted for 10 h at 460 rpm. The then products were collected, washed and dried in an oven for 6 h at 70 C. The dry flakes were annealed using vacuum furnace for 2 h at 400 C. Ferrite powders were prepared by oxalate co-precipitation process. All chemicals were of reagent grade and used as received without further purification. Nickel sulfate hexahydrate (NiSO4 6H2O), znic sulfate heptahydrate (ZnSO4 7H2O), ferric sulfate heptahydrate (FeSO47H2O), ammonium oxalate ((NH4)2 C2O4 H2O), sulfuric acid (H2SO4), were acquired from Sigma Aldrich. The aqueous solutions of NiSO4 6H2O, ZnSO4 7H2O, FeSO47H2O and (NH4)2 C2O4 H2O were prepared according to the ratio required for NiZn ferrite. 0.1 wt% sulfuric acid solution was confected as a solvent to inhibit hydrolysis of ferrous ion. The precursor was prepared by using NiSO4, ZnSO4 and FeSO4 as raw material with (NH4)2 C2O4 H2O as the precipitator through precipitation. It was thermally decomposed at 700 C for 30 min by high temperature calcination to obtain ferrite fine powders. SMC with appropriate amounts of FeSiAl alloy flakes and NiZn ferrite particles was prepared by using tape-casting method. A mixture of polyvinyl butyral, dibutyl phthalate (Butvar B-98, Monsanto Co. St. Louis, MO, USA), and ethanol was mixed well in a mechanical stirring setup for 6hrs. Then the varying amounts of NiZn ferrite powder and flakes of FeSiAl were added along with glycerol trioleate as a dispersant in mechanical stirring equipment for 7hrs. The slurry was vacuum treated to get rid of froth for 120 min with the stirring at a speed of 60 rpm and casted onto a polymer substrate 100 mm thick having molecular formula (C10H8O4) n. by using a tape casting machine. Composite with thickness around 0.3e0.4 mm in tape sheet form was obtained after
the solution was dried in a vacuum oven at 50 C. Six samples of composites with various weight fractions of NiZn ferrite, 0%, 6%, 10%, 14%, 18%, and 22% have been produced. Phase structure of ferrites, sendust flakes and composite sheets were characterized by X-ray diffraction (XRD, SHIMADZU 7000) using Cu-Ka source and morphologies of the composites and FeSiAl flakes were observed by a FEI Quanta 200 scanning electron microscope (SEM, HITACHI S-4800). Particle size of the ferrite powder was measured by laser particle size analyzer (Mastersizer Micro). The static magnetic properties of the composite were measured at room temperature by a vibrating sample magnetometer (Quantum Design, VersaLab). The complex permeability and permittivity of the composite were measured on a Network spectrum impedance analyzer (Agilent, Network spectrum impedance analyzer 4396B) in the frequency range from 1 MHz to 1 GHz with a dielectric material test fixture (Agilent, 16453A) and magnetic material test fixture (Agilent, 16454A) respectively. The toroid samples with outer and inner diameter of 8 mm and 3 mm respectively were cut from the composite sheet with a particularly designed cutter. Samples (20 40 0.4 mm) were placed in the middle of microstrip line (MSL) with a characteristic impedance of 50U for power loss (PL) evaluation. Both ends of the MSL are connected to a vector network analyzer (CETC, AV36850A), which can measure scatter parameter in the frequency range from 1 MHz to 1 GHz. Magnetic field coupling efficiency was analyzed by an in-house developed method, two near filed probes (RF 400-1/2), one serving as a transmission coil connecting to signal generator (Agilent, E8257D) and the second as a receiver coil connecting to EMC analyzer (Agilent, E7405A). When an alternating current passes through the transmission coil, magnetic lines of field were generated and passed through the receiving coil creating an induced electromotive force around it resulting in magnetic field couple. By attaching the composites to the transmission coil as a backplane, the values were measured from EMC receiver which can evaluate the efficiency of magnetic field couple. 3. Results and discussion 3.1. Principal of precursor preparation Fine ferrite powders having average particle size of 2e5 mm were prepared by oxalate co-precipitation process. Laboratory grade salts of NiSO4, ZnSO4 and FeSO4 containing cations (Ni2þ, Zn2þ and Fe2þ) and no anions were used as reactants. After the complete reaction between salts, basic carbonate precursor was obtained having very fine crystalline grains, size was confirmed by laser particle size analyzer. Precursor was then calcined at 700 C for 30 min. The precipitation and calcination reactions are as follows:
0:5Ni2þ þ 0:5Zn2þ þ 2Fe2þ þ 3C2 O2 4 þ 6H2 O/ðNi0:5 Zn0:5 ÞFe2 ðC2 O4 Þ3 $6H2 OY
700 C
ðNi0:5 Zn0:5 ÞFe2 ðC2 O4 Þ3 $6H2 O ! ðNi0:5 Zn0:5 ÞFe2 O4 þ 4CO2 [ 30 min
þ 6H2 O
3.2. Phase structure and morphology (XRD & SEM) Fig. 1 shows the XRD pattern of the obtained ferrites, sendust flakes and composite sheets with different NiZn ferrite contents. As observed in Fig. 1(a), the peak position and relative intensities of
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(Ni0.5Zn0.5)Fe2O4 diffraction spectra were analyzed with X'pert software and matched well with the standard patterns. It shows the NiZn ferrite crystal planes at 2q ¼ 30.1, 35.5 , 57.0 are at most preferred orientations of (113), (222), and (333), JCPDS#00-0520278. Evident diffraction peaks of a-FeSiAl alloy with bcc structure are at 44.9 and 65.5 , and DO3 superlattice structure are at 27.1, 31.4 and 53.3 , JCPDS # 00-045-1206. It displays that FeSiAl alloy flakes with the DO3 superlattice structure exhibit good soft magnetic properties due to the appropriate proportion of ordered and disordered lattices [32]. Fig. 1(b) depicts the XRD patterns of FeSiAl/(Ni0.5Zn0.5)Fe2O4 composites with different NiZn ferrites content. It is clear that the mixing of ferrites does not change the crystal structure of the FeSiAl alloy. After the NiZn ferrite content was increased to 10 wt%, characteristic peaks from NiZn ferrite can be clearly detected. All the peaks are sharp confirming the crystalline nature of composite. When the quantity of (Ni0.5Zn0.5)Fe2O4 is at or around 10 wt%, the minor peaks of ferrite appeared showing the minimum detectable wt% is ±10. The intensity of the ferrite spectra is highest at 22 wt% and the slight broadening of peaks base was observed. Fig. 2 shows the typical morphologies of FeSiAl alloy flakes with large aspect ratio (length/thickness). The microstructures of DO3 superlattice are ordered structure while FeSiAl flakes are bcc disordered structure [33e35]. A homogeneous distribution of flake sizes was achieved. A typical flake indexed in the figure had a thickness of around 0.7 mm and a length of approximately 100 mm. The corresponding aspect ratio is more than 140:1, which is favorable to enhance the saturation magnetization on the parallel directions of the particle plane by decreasing the demagnetizing field and suppressing the negative effect of eddy current in alternating electromagnetic field since the thickness of the particle is less than skin depth of sendust [36]. Fig. 3 shows the SEM images of two composites having 0 wt% and 14 wt% of ferrites. In the composite, most of the flakes can be well aligned in the plane, and still keeps a high aspect ratio. Thus, thin flakes in the direction parallel to the electromagnetic field increase the magnetized density and its anisotropic microstructure will increase the permeability value of the composite. The ferrite powders are closely bonded to the surface of FeSiAl alloy flakes. The fine ferrite particles with small dimensions can readily segregate interconnected flakes which closes the conductive paths in matrix, and thus benefit to prohibit the ohmic loss. With the increasing
279
amount of ferrites attached to the surface, the separation of flakes becomes more effective. Clearly, parts of the ferrites are well dispersed as shown in Fig. 4(c), only few portions are heavily segregated with flake plates, Fig. 4(d). 3.3. Static magnetic properties The magnetization curves of composites with the various amounts of ferrite at room temperature are shown in Fig. 4. The hysteresis loops were measured in-plane of the film with respect to the applied magnetic field. All the samples exhibit representative soft magnetic characteristics with quite small values of remnant magnetization and coercivity [37]. The saturation magnetizations (Ms) are 117 emu/g, 110 emu/g, 94 emu/g, 83 emu/g, 74 emu/g and 64 emu/g for the composites with the corresponding NiZn ferrites fractions of 0%, 6%, 10%, 14%, 18%, and 22%, respectively. The range of Ms decreased with increasing the amount of ferrites, because the saturation magnetization of ferrite is significantly lower than that of FeSiAl alloy. 3.4. The complex permittivity and permeability Fig. 5 shows the complex permittivity spectra measured for composite films with different wt% of NiZn ferrite within the frequency range of 1 MHz-1GHz. According to Debye relaxation equation, the relationship between permittivity and frequency of the composites follow the equations [38]
ε0 ðuÞ ¼ ε∞ þ 00
ε ðuÞ ¼
εs ε∞ 1 þ u2 t2
ðεs ε∞ Þut 1 þ u2 t 2
(1)
(2)
Here, t is relaxation time, u is angular frequency, εs and ε∞ represent the static permittivity and permittivity at infinite frequency, respectively. Equations (1) and (2) show that permittivity is proportional to relaxation time and frequency. The real and imaginary part (ε0 and ε") of all samples have same trend of decreasing with frequency in the range 1 MHz to 1 GHz. Changing the values of ferrite leads to the significantly difference of both ε0 and ε" which denotes the storage and dissipation abilities of electric energy [39], respectively. The value of relaxation strength belongs to dielectric
Fig. 1. The XRD pattern of ferrites and sendust flakes (a), composite sheets with varying wt% of (Ni0.5Zn0.5) Fe2O4 (b).
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Fig. 2. SEM images of FeSiAl alloy flakes (a), cross section of flakes (b).
Fig. 3. SEM images of composites with varying ferrites 0 wt% (a) and (b), 14 wt% (c) and (d).
transition (εs-ε∞)u decrease with the weakened polarization of composite [38]. This confirms our results in Fig. 5 that amount of ferrites have weakened the polarization of composite, so that ε0 and ε" of the composite decrease with the increasing fraction of ferrites. Additionally, according to Maxwell-Wagner-Sillars (MWS), the relationship between ε" of the composite and frequency are similar to Debye equations, except there is one more consideration of electrical conductivity in equation (2), as shows in the following equation [40].
00
ε ¼
ðεs ε∞ Þut s þ u 1 þ u2 t 2
(3)
Where, s is the electrical conductivity of composite. Equation (3) shows that there is conductive and polarization relaxation loss, and ε" is proportional to electric conductivity, which obviously decreased with increasing weight fraction of ferrites in the composite. Mainly because of the fact that sendust flakes are isolated from each other by the addition of very fine ferrite particles in the matrix, which blocks the conductive paths for electron hopping and migrating. For flaky shape particles possessing higher aspect ratio,
X. Jin et al. / Journal of Alloys and Compounds 729 (2017) 277e284
me 1 mi 1 ¼p 1 þ nðme 1Þ 1 þ nðmi 1Þ
281
(4)
where p, n and mi are the volume fraction, averaged shape factor and permeability of inclusions, respectively. The permeability of the composite is me that can be solved from Eq. (4)
me ¼ 1 þ
p nð1 pÞ þ 1=ðmi 1Þ
(5)
The value of me decreases with the reduction of mi within the certain volume fractions. This confirms our results in Fig. 6 that amount of ferrites have reduced the mi of composite, so that the m0 and m" of composite decrease with the increasing fraction of ferrites. From the above analysis, we can conclude that the addition of ferrite particles can also cause less loss.
Fig. 4. Hysteresis loops of the composites.
polarization mainly originates from the interfacial polarization at the interfaces of FeSiAl/PVB and FeSiAl/FeSiAl causing polarization relaxation loss [41]. The addition of ferrite particles hasn't introduced any significant polarizations in the composites. Which is depicted by the curves of all samples almost having same profile [39,42], Fig. 5(b). From the above analysis, it is confirmed that the addition of ferrite particles can reduce loss to a great extent. Complex permeability spectra of the composite films filled with different wt% of NiZn ferrite within the frequency range of 1 MHz1GHz is showed in Fig. 6. The real part of permeability (m0 ) value of all the kind of FeSiAl/NiZn ferrite composites decreased as the frequency increased. The imaginary part of permeability (m") of all kind of FeSiAl/NiZn ferrite composites increased slightly in the frequency 1e200 MHz, the maximum value obtained at 200 MHz, then m" decreased in the following frequency. All of the above phenomenon was consistent with the other research works [43e46]. The m0 and m" differ greatly depending on the filled weight fraction of ferrites. It reducing significantly with increasing filled ferrites in the composite. For the flake FeSiAl/NiZn ferrite composites, the permeability could be explained according to the Maxwell Garnett equation [47]
3.5. Power loss of composites Fig. 7 shows Power Loss (PL) of the composites associated with varying weight fractions of ferrite. The value of PL, as given by Eq. (6), increased with frequency and reduced with weight loading fraction of ferrites. Frequency dependence of power loss (PL) of the composites were estimated from their Scattering-parameters (S11 and S21) according to the following equation [48]: S11 S21 Ploss ¼ 1 10 10 10 10 Pin
(6)
The total losses of the MSL with the film attached is the sum of the dielectric loss and magnetic loss of film, dielectric loss of the MSL substrate and radiation loss [49]. Dielectric loss of the MSL substrate and radiation loss can be disregarded after calibrating the equipment and the losses from the composite samples can be defined and calculated according to the following equations. I) Dielectric loss of the sheet
Pdie ¼
1 T
ZtþT t
00
uε ε0
Z
E2 dV dt
DV
Fig. 5. Complex permittivity spectra of the composite films with different ferrites loading, real part (a) and imaginary part (b).
(7)
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Fig. 6. Complex permeability spectra of the composite films with different ferrites loading, real part (a) and imaginary part (b).
Fig. 7. Power loss for FeSiAl/NiZn ferrite composites.
II) Magnetic loss of the sheet
Pmag ¼
1 T
ZtþT t
00
um m0
Z H2 dV dt
(8)
DV
Where T is the time period of one cycle, V is the volume of the film of composite and ε0 and m0 are the permittivity and permeability of free space, respectively. Equations (7) and (8) show that the material loss increases with the angular frequency, imaginary part of permittivity (ε") and permeability (m"). These mathematical expressions are in consistent well with the phenomenon explains in Fig. 7. According to Figs. 5 (b) and 6 (b), it is expected to obtain lower imaginary part of permittivity (ε") and permeability (m") for reducing losses from the magnetic composites. 3.6. Magnetic field coupling efficiency Collectively the aim was to improve magnetic field coupling efficiency which had positive correlation with the inductance of the device (such as a coil). By attaching the composites to the transmission coil as a backplane, attenuation of the inductance is mainly
the current loss (induced and displacement current) and magnetic loss of the composite. The current loss is related to ε" and s of the composite while magnetic loss is related to m" of the composite. We intend to obtain relatively less ε" and m" values of the FeSiAl/NiZn composites indicate the lower dielectric, ohm and magnetic loss. On the other hand, the improved inductance is the advanced ability of attached composite gathering the magnetic field on account of its high m0 and Ms values of the FeSiAl/NiZn composites while the geometrical dimensions of the coil is governed by the design. Above analysis reveals that, increasing the %age of additive ferrites, the relationship between less values of ε" and m" and high values of m0 and Ms for FeSiAl/NiZn ferrite composites is reversed. It suggests that optimal fraction of ferrite maybe decided by balancing the values of ε", m", m0 and Ms in order to get the highest magnetic field coupling efficiency. Magnetic field coupling efficiency is shown in Fig. 8 by attaching with and without the composites to the transmission coil as a backplane. As observed in Fig. 8(a), with and without attaching the FeSiAl/NiZn ferrite composite leads to the great change of received power values which represent the function of magnetic sheet. After attaching the composite without loading the ferrite particles, the value of received power is less as compared to without attaching. This result in magnetic and dielectric loss from composites is significantly higher than the ability of composite gathering the magnetic field. By increasing the amount of ferrites from 0% to 6%, the enhancement of the received power values shows the loaded NiZn ferrite particles can reduce the loss of FeSiAl/NiZn ferrite composites. That confirms the dielectric and magnetic loss from composites is lower than the ability of composite gathering the magnetic field while the m0 and Ms are decreasing sharply. After loading ferrite particles with 6%, 10% and 14%, the slight increase in received power values shows that the influence from reducing magnetic and dielectric loss of FeSiAl/NiZn ferrite composites is higher than the weakened ability of composite gathering the magnetic field. After loading ferrites with 18% and 22%, the gradual decrease of the received power values means the much reduced m0 and Ms that leads to weaken the ability of FeSiAl/NiZn ferrite composites to gather magnetic field, although the material loss is still decreasing sharply. Consequently, the value of received powder is optimized at of 14% of NiZn ferrite fraction. NFC antennas were then used as transmission and received devices with the central work frequency of 13.56 Mhz. The
X. Jin et al. / Journal of Alloys and Compounds 729 (2017) 277e284
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Fig. 8. Measured near field probe (a) and NFC (b) antenna with and without FeSiAl/NiZn ferrite composites.
phenomenon observed in Fig. 8(b) is in consistent well with Fig. 8(a) that 14% of NiZn ferrites represent optimal value. From the above analysis, we can conclude that low material loss and the strong Ms and m0 contributes to the excellent magnetic field coupling efficiency of FeSiAl/NiZn ferrite (14 wt %) composite. 4. Conclusions Flexible composite produced during the present work containing NiZn ferrite powder with FeSiAl flakes possess both relatively strong permeability and high resistivity. Increasing the amount of ferrites from 0% to 6%, 10%, 14%, 18% and 22%, imaginary part of permittivity and permeability decreases which leads to less power loss of the composite, and Ms and real part of permeability decreases which results in the reduction of magnetic field coupling efficiency. The composites without loading the ferrite particles shows the lowest magnetic field coupling efficiency because of significantly high magnetic loss and dielectric loss of the composites. By adjusting the weight percentage of NiZn ferrite, the composite having 14 wt% NiZn ferrite demonstrates optimized high magnetic field coupling efficiency due to relatively low material loss, strong Ms and m0 of composite. Therefore, the Sendust flakes mixed with NiZn ferrite particles could be promising candidate in both wireless power transfer and nearfield communication fields. Notes The authors declare no competing financial interest. Acknowledgement This work was supported by National Key R&D Plan of China [grant number 2016YFB1200602-37]. References [1] W. Wang, T. Ma, M. Yan, Microstructure and magnetic properties of nanocrystalline Co-doped Sendust alloys prepared by melt spinning, J. Alloy. Comp. 459 (1) (2008) 447e451. [2] S. Yoshida, M. Sato, E. Sugawara, et al., Permeability and electromagnetic interference characteristics of Fe-Si-Al alloy flakes polymer composite, J. Appl. Phys. 85 (8) (1999) 4636e4638. [3] L. Liu, Z.H. Yang, C.R. Deng, Z.W. Li, M.A. Abshinova, L.B. Kong, High frequency properties of composite membrane with in-plane aligned Sendust flake prepared by infiltration method, J. Magn. Magn. Mater. 324 (10) (2012)
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