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Particle Size and Concentration Effect on Permeability and EM-Wave Absorption Properties of Hybrid Ferrite Polymer Composites R. Dosoudil, M. Uˇsáková, J. Franek, J. Sláma, and A. Grusková Faculty of Electrical Engineering and Information, Technology Department of Electromagnetic Theory, Slovak University of Technology, Bratislava, 812 19 Slovak Republic Hybrid MnZn/NiZn/PVC polymer composite materials have been prepared using a dry low-temperature hot-pressing process and the influence of particle size, concentration, and the fraction ratio of the dual MnZn/NiZn ferrite filler on their complex permeability and electromagnetic wave (EM-wave) absorbing properties were investigated within the frequency range of 1–3000 MHz. The observed frequency dispersion of permeability was of a relaxation type and caused by a resonance of vibrating domain walls and a natural ferromagnetic resonance of precessing magnetic moments in domains. The complex permeability and resonance frequency res at low frequency of hybrid composites have been affected mainly by changes in concentration and fraction ratio of ferrite filler: the decreased and res shifted towards the higher frequency region with the decrease of ferrite concentration and with the configuration change from the MnZn/PVC composite to NiZn/PVC one. Measured values of complex permeability were used to determine the return loss RL using a model of a single-layered EM-wave absorber backed by a perfect conductor. In the designed single layer absorbers, the for dB, and the minimum of return loss RL as well as the matching frequency m , matching thickness m , bandwidth min seem to be strongly dependent on particle size, concentration, and the fraction ratio of the dual MnZn/NiZn ferrite filler.
=
j
1
(RL)
RL
20
Index Terms—Absorption property, electromagnetic interference, hybrid composite materials, permeability, return loss.
TABLE I THREE SETS OF FABRICATED COMPOSITE SAMPLES USED IN EXPERIMENTS
I. INTRODUCTION
HE PROGRESS in electronic technologies and especially in wireless communications has lead to an increase of electromagnetic interference (EMI) problems. One of the possible solutions is to use microwave absorbing materials. Among the candidates, ferrite polymer composites consisting of ferrite particles embedded in a non-magnetic polymeric matrix play an important role. A relatively new composite is the hybrid composite material, which is obtained by means of two or more different kinds of fillers in a single matrix [1]. Hybrids have a better all-round combination of properties than composites containing only a single filler type. In our previous studies, we have investigated ferrite polymer composite materials based on polyvinylchloride (PVC) polymer matrix and various types of single ferrite fillers (NiZn [2], [3], MnZn [4]–[6], and LiZn [7]) as well as dual ferrite fillers (MnZn/NiZn [8] and MnZn/LiZn [9]), and have found out that the complex permeability had a characteristic frequency dispersion and was attributed to two kinds of magnetic resonance mechanisms: the resonance of oscillating domain walls and the natural ferromagnetic resonance of precessing magnetic moments in domains. In most cases, the dispersion of permeability of sintered ferrites was of a resonance type and changed to relaxation type of dispersion in the case of ferrite polymer composites. We have also studied the electromagnetic wave (EM-wave) absorbing properties of ferrite polymer composites [3], [9] only as a function of ferrite content and found these materials as suitable candidates for single-layer absorber design.
T
Manuscript received June 15, 2009; revised September 02, 2009; accepted September 22, 2009. Current version published January 20, 2010. Corresponding author: R. Dosoudil (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2033347
In the present work, a more detailed study is carried out to examine the influences of particle size, concentration and fraction ratio of the dual MnZn/NiZn ferrite filler in hybrid composites with PVC matrix on the frequency dispersion of their complex permeability and microwave absorption performances within the frequency interval of 1–3000 MHz. II. EXPERIMENT The commercially available Mn Zn Fe O ferrite and Ni Zn Fe O ferrite synthesized by a ceramic route at 1200 C for 4 h in air have been used as magnetic fillers. The hybrid ferrite polymer composites were prepared from the powder fillers and PVC polymer matrix by a dry low-temperature hot-pressing process (at a temperature of 135 C and at a pressure of about 5 MPa). Three sets (A, B, and C) of fabricated composite samples with different particle sizes (Set A), concentrations (Set B), and fraction ratios (Set C) of the dual MnZn/NiZn ferrite filler are listed in Table I. All the prepared composite samples had a toroidal form with an inner diameter of 3.05 mm, an outer diameter of 7 mm, and
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TABLE II RESONANCE FREQUENCY FOR COMPOSITES OF SET A AND SET B
TABLE III RESONANCE FREQUENCY FOR COMPOSITES OF SET C
Fig. 1. Frequency dependences of real and imaginary parts of complex permeability of prepared composites for different (a) particle sizes, (b) concentrations, and (c) fraction ratios of the double MnZn/NiZn ferrite filler.
a height of 3 mm. The complex permeability of hybrid composite samples was measured by a short-circuit coaxial transmission line method within the frequency interval of 1–3000 MHz using two different vector network analyzers (HP 4191 A: 1–1000 MHz and Agilent 8714ET: 0.1–3 GHz). III. RESULTS AND DISCUSSIONS The frequency dependences of complex permeability for the MnZn/NiZn/PVC composites are pre-
sented in Fig. 1: with variable particle size in Fig. 1(a), with variable concentration in Fig. 1(b), and with variable fraction ratio of the dual MnZn/NiZn ferrite filler in Fig. 1(c). The real of complex permeability decreased with increasing part frequency. In composites containing larger ferrite particles m), Fig. 1(a), the values rapidly decreased from about ( 90 MHz, and the remaining composites with smaller particles m) had frequency ranges, in which they maintained ( values, which began to decrease near 130 MHz. The their resonance frequency , at which has its maximum value, increases with decreasing filler particle size from 247 to 361 MHz, as shown in Table II. In case of composites with different values increased with filler concentrations, Fig. 1(b), the increasing ferrite particle concentration in the low frequency range below 400 MHz due to the increase of filler particle amount. All composites had nearly similar value in the high frequency range above 400 MHz. However, B1 composite with larger filler particle concentration (65 vol%) had the lowest value in the high frequency range due to the rapid decrease of . The value of increased with decreasing filler particle content from 247 to 710 MHz. Fig. 1(c) depicts the complex permeability of hybrid MnZn/NiZn/PVC composites with constant particle size 80–250 m and constant total filler concentration 65 vol%, in which only the fraction ratio of the dual MnZn:NiZn ferrite filler varied from 1:0 to 0:1 (see also Table I). In this case the permeability changed continuously with the change of ferrite filler fraction ratio between two decreased from about types of ferrite fillers. The value of 27 for bicomponent MnZn/PVC composite to about 16 for increased from 238 MHz NiZn/PVC one. The value of for MnZn/PVC composite to 462 MHz for NiZn/PVC one, as shown in Table III. From the results presented in Fig. 1, it follows that the observed frequency dispersion of permeability is of a relaxation type in contrast to the sintered MnZn and/or NiZn ferrite [8] and is principally caused by the domain wall resonance (vibrating Bloch’s walls due to the force acting on walls in the presence of high frequency external ac magnetic field), the natural ferromagnetic resonance (the forced precession of magnetization vectors in domains due to the presence of effective magnetic anisotropy), and the relaxation of magnetization [8]–[10]. The
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decrease of filler particle size and/or concentration in composdue ites resulted in the decrease of value and the raise of to the demagnetization effect of ferrite particles dispersed in a polymer matrix. In addition, small particles may have shorter domain wall length than large particles which can result in reducing the width of domain wall vibration and increasing vivalue in low frequency bration frequency. The decrease of region and the increase of in composite containing small particles are caused by the increase of demagnetization field effect and the diminution of the magnetization effect induced by domain wall motion. On the other hand, the demagnetization , formatted by magnetic poles in the surfaces of filler field particles in the composite, leads to an increase of resonance fre, quency according to the equation: H/m the permeability of free space, the with gyromagnetic ratio, and the magnetocrystalline anisotropy is field. In case of fraction ratio variations only, the value of also affected by the change of filler from MnZn to NiZn ferrite: shifts higher due to the contribution of the decreases and magnetocrystalline anisotropy field to the . and imaginary parts of comMeasured values of real as depicted in Fig. 1 were used plex permeability to the calculation of return loss in the prepared composite samples utilizing a model of a single-layered EM-wave absorber proposed by Naito and Suetake [11]. In this model, the input at the air/absorber interface is given by wave impedance the relation (1) with the wave impedance of free space, the comm/s the speed of light, and the plex permittivity, frequency. If we assume that the thickness of the composite absorber is small enough compared with the wavelength of the incident transversal electromagnetic (TEM) wave, then the can be determined using the following siminput impedance plified relation (2) The return loss RL in decibels (dB) is then calculated as (3) The impedance-matching condition representing the perfectly . This condition is absorbing properties is given as and a matching satisfied at a particular matching thickness , where minimum return loss occurs. frequency The frequency dependences of return loss RL for the fabricated composites can be found in Fig. 2: with variable particle size in Fig. 2(a), with variable concentration in Fig. 2(b), and with variable fraction ratio of the dual MnZn/NiZn ferrite filler in Fig. 2(c). Some absorption parameters such as the matching frequency , matching thickness , bandwidth for dB, and the minimum of return loss are listed in Table IV. As the particle size of the double MnZn/NiZn ferrite filler decreases from 80–250 to 0–40 m, the matching frequency increases from 566 to 1569 MHz, matching thickness decreases from 9.8 to 7.9 mm, the bandwidth raises from 183 to 799 MHz, and the minimum of return loss decreases from to dB. The similar behaviour was achieved when
Fig. 2. Frequency dependences of return loss RL of prepared composites for different (a) particle sizes, (b) concentrations, and (c) fraction ratios of the double MnZn/NiZn ferrite filler.
the volume concentration of the ferrite filler decreased from 65 to 45 vol% except for the minimum of return loss, which into dB. Accordingly, the return loss creased from of hybrid composites was found to depend sensitively on both the filler particle size and volume concentration: the minimum of RL shifted to high frequency when the particle size as well as filler concentration decreased. In addition, with the configuration change from MnZn/PVC composite structure to NiZn/PVC increased from 419 to 1259 one, the matching frequency
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caused mainly by the change of magnetocrystalline anisotropy: increased and decreased with the configuration change from MnZn/PVC composite to NiZn/PVC due to the increase of (see Table III) caused by the contribution of magnetocrys. The talline anisotropy to the total effective anisotropy field presented results showed that the hybrid ferrite polymer composites could fruitfully be used in applications such as electromagnetic interference noise reduction in portable and wireless radio communication systems.
TABLE IV EM-WAVE ABSORPTION PROPERTIES OF SINGLE-LAYER ABSORBERS
ACKNOWLEDGMENT This work was supported by the VEGA agency of Slovak Republic under Grant 1/0575/09. REFERENCES
MHz, the matching thickness decreased from 12.5 to 5.9 mm, the bandwidth for dB raised from 129 to changed be775 MHz, and the minimum of return loss and dB. tween (and also ) variation in hybrid comThe reason for posites according to particle size, concentration, and fraction ratio of the ferrite filler may be found in the basic principles for designing EM-wave absorbers. The following relationship , and can be written [12] between (4)
value is affected (mainly) by This relation states that . From Figs. 1 and 2, it follows that increasing filler particle size and concentration causes the increase of , which results value. Also, decreases with inin the decrease of creasing filler particle size and concentration. Although the decrease of has an influence on reducing absorber thickness , has larger effect on absorber thickness in this the variation of is much work. That is because the degree of decreasing in larger than that of increasing . On the other hand, because the decrease of is due to arising of demagnetizing fields of ferrite and is due particles in hybrid composites, the variation of according to ferrite particle size and conto the variation of centration (see Table II). The absorption parameters variation due to the modification of fraction ratio of the dual MnZn/NiZn ferrite filler in composites (see Fig. 2(c) and Table IV) has been
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