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Magnetic behaviour of composites containing polyaniline-coated manganese–zinc ferrite
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
Magnetic behaviour of composites containing polyaniline-coated manganese–zinc ferrite N.E. Kazantsevaa,b, J. VilWa! kova! b, V. Kr$esa! lekb, P. Sa! hab, I. Sapurinac, J. Stejskald,* a
Institute of Radio-Engineering and Electronics, Russian Academy of Sciences, Friasino, Moscow region 141190, Russia b Polymer Centre, Faculty of Technology, Tomas Bata University in Zlin, Zlin 762 72, Czech Republic c Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia d Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Received 23 January 2003; received in revised form 14 May 2003
Abstract Polycrystalline manganese–zinc ferrite has been coated with polyaniline (PANI) and embedded into a polyurethane matrix. The complex permeability of the composites was studied in the frequency range 1 MHz–3 GHz. The conductivity of PANI coating was adjusted by controlled protonation with picric acid. Large shifts in the resonance frequency were observed as a function of varying PANI conductivity. The changes in the magnetic properties of the PANI-coated composite material are due to the change of the boundary conditions of the microwave field at the interface between the ferrite particle and polymer matrix. This effect is observed especially when the magnetic anisotropy of ferrite is low. r 2003 Elsevier B.V. All rights reserved. Keywords: Ferrite; Permeability; Coated particles; Polyaniline; Conducting polymer
1. Introduction New magnetic materials have received considerable attention, especially in studies of composite materials with different types of ferromagnetic fillers. The main challenge is to design the structure, which would have the greatest effect on the magnetic and dielectric properties and thus could be used in their control.
*Corresponding author. Tel.: +420-296-809-351; fax: +420296-809-410. E-mail address: [email protected] (J. Stejskal).
It is well known that the frequency dispersion of the complex permeability of a ferrite material strongly depends on the magnetic substance [1,2]. Up to the present time, resonance phenomena in ferrites have been studied extensively and described by two different mechanisms of magnetization–spin resonance and domain-wall motion [3,4]. For a polycrystalline ferrite, the permeability spectra can be described by the superposition of both contributions. Furthermore, the complex permeability in the high-frequency region is greatly affected by the spin-rotation component [5,6]. It has been found that, in ferrites, the relaxation of magnetic oscillations is strongly affected by
0304-8853/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-8853(03)00557-2
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
structural disorder, both crystallographic and geometrical, such as pores, cracks, defects, and surface roughness. Composite materials, in which magnetic particles are embedded in insulating binders, introduce an additional disorder in the form of filler–matrix interfaces. This makes the magnetic relaxation behaviour still more complex. This additional disorder brings about changes in the internal magnetic fields by reducing exchange and dipole interactions through demagnetization fields. As a result of dipole interactions, various effects have been observed, e.g., the anomalous broadening of the resonance band, the shift of resonance to higher frequencies, a considerable decrease in complex permeability, the spreading of ferromagnetic–paramagnetic transition temperature range, etc. [7–10]. At present, there is no adequate procedure to predict the dependence of the magnetic properties on the structure of composites. Very few data have been published on the magnetic and dielectric properties of composites with multicomponent magnetic powders, such as mixtures of ferrites with conducting fillers and coated ferromagnetic particles. Such composite materials have special interest because of new and often unusual electromagnetic properties. Applications of ferrites and ferroelectric mixtures, which have their own relaxation process, make it possible to achieve more or less the same value of complex permittivity, e ; and complex permeability, m ; in different regions of the frequency range [11,12]. For example, the electrochemical deposition of iron on spinel-type Ni0.45Zn0.55Fe2O4 ferrite changes the frequency dispersion of e and m of composite materials [13]. But it is difficult to control the uniform thickening of the iron overlayer during the deposition process. When the thickness of the iron layer is too large, both the real and the imaginary parts of the complex permeability of the composite material, m0 and m00 ; dramatically decrease. One of the promising alternatives for the coating of ferrite particles is the deposition of conducting polymers on their surface. Conducting polymers, such as polyaniline (PANI) and polypyrrole, are easy to prepare and they are environmentally stable. The materials
31
containing such polymers and magnetic component are represented by the preparation of nanocomposites of ferric oxide and PANI [14,15] or polypyrrole [16,17]. The surface modification techniques providing the coating of various substrates with conducting polymer nanofilms are simple and well developed [18,19]. They are, in principle, well suited also for the treatment of ferrite powders. There are many technologically important ferrite materials with different sorts of magnetocrystalline anisotropy. We have selected manganese–zinc (MnZn) ferrites because of their high initial permeability, low value of effective magnetic-field anisotropy, and low core losses. Ferrites of this type have widely been used in electronic devices. This article reports the effect of ferrite content and particle size on the level of permeability. The studies included the surface modification of ferrite particles with conducting PANI coatings resulting in the shift of frequency resonance from the MHz to the GHz-frequency region.
2. Experimental procedure 2.1. Ferrites The bulk MnZn spinel ferrite in polycrystalline form was prepared by heating mixed powders of oxides (Fe2O3, MnO and ZnO) at 1300 C for 6 h and annealing at 200 C h1 cooling rate at gradually reduced air pressure. The composition of ferrite used in the present experiments was 53.2 mol% Fe2O3, 26.2 mol% MnO and 20.6 mol% ZnO. Ferrite powders of 30 and 60 mm average particle sizes were prepared by the mechanical grinding of sintered ferrite. Particle morphology was observed by scanning electron microscopy using JEOL JEM-200CX. Toroidal samples of outer diameter of 8 mm, inner diameter 3.1 mm, and 3 mm thick were cut from bulk ferrite plate with a diamond saw on a cutter at 1000 rpm with water cooling. The magnetic properties of MnZn ferrite compacts, relative intrinsic permeability m0 B3000 and the magnetic-field anisotropy HA ¼ 104 A m1, were
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N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
determined by standard techniques with a vibrating-sample magnetometer (VSM, PARR, model 4500). An applied magnetic field of 5 kOe was employed to evaluate the saturation magnetization MS ¼ 3:5 kG s and coercitivity HC ¼150 A m–1. Curie temperature TC ¼473 K was derived from the temperature dependence of permeability. The ferrite conductivity sf ¼ 2 102 S cm1 was determined by four-point method using a silver paste for contacts. Archimedes method was used to find the density, rf ¼ 5:2 g cm3. The frequency dependences of complex permeability were determined for ferrite and composite samples in the range 1 MHz–3 GHz with an RF Impedance/Material Analyser (Agilent E49991A).
Table 1 Conductivity of polyaniline, s; protonated with picric acid to various level, xa x
s (S cm1)
x
s (S cm1)
0 0.1 0.2 0.3 0.4 0.5
6.0 1011 1.2 107 1.8 105 4.5 104 4.5 103 2.1 102
0.6 0.7 0.8 1.0 1.5 2.0
7.1 102 9.8 102 1.3 101 1.4 101 2.0 101 2.1 101
a Number of moles of acid per mole of constitutional units of PANI base. PANI base (362 mg, 2 mmol) was suspended in 100 ml of water containing dissolved picric acid for 24 h.
2.3. Conductivity of polyaniline coatings 2.2. Coating of ferrite powder with polyaniline PANI was prepared by the oxidation of aniline hydrochloride with ammonium peroxydisulphate in an aqueous medium [20]. Any material in contact with the reaction mixture used for the preparation of PANI becomes coated with a film of this polymer [18]. The typical film thickness is 100–200 nm [19]. This fact has been used for the coating of ferrite powder. MnZn ferrite (20 g, 60-mm size) was immersed in 50 ml of aqueous solution containing 2.59 g (=20 mmol) of aniline hydrochloride, at room temperature. The polymerization of aniline was started by introducing 5.70 g (=25 mmol) of ammonium peroxydisulphate also in 50 ml of aqueous solution. Next day, ferrite particles coated with conducting PANI hydrochloride were collected on a filter, rinsed with dilute (0.2 M) hydrochloric acid, followed by acetone, and dried. The coated particles contained B10 wt% of PANI hydrochloride and had a conductivity of 0.34 S cm1 after being compressed into a pellet. PANI hydrochloride prepared in the absence of ferrite had a conductivity of 4.4 S cm1 [20]. A portion of the PANI-coated ferrite powder was suspended in an excess of 1 M ammonium hydroxide. The ammonia caused the deprotonation of the conducting polymer coating to the nonconducting PANI base [21]. The coated ferrite had a conductivity of 4.3 107 S cm1.
Ferrites coated with PANI base were immersed in aqueous solutions of picric acid (2,4,6-trinitrophenol; Fluka, Switzerland). By adjusting the concentration of the acid, the degree of protonation of the PANI coating and, consequently, also its conductivity was controlled over several orders of magnitude. The similar immersion of PANI base in the solutions of picric acid was used to prepare the material for the determination of conductivity measured on dry samples compressed into pellets by four-point or two-probe methods (Table 1).
2.4. Polyurethane–ferrite composites Composite materials have been prepared by mixing ferrite particles with a polyurethane prepolymer (AXSON UR 3420, Axson, France) and pressing between two metallic plates separated with a 3 mm spacer. Samples were kept for 4 h at 80 C in vacuum. The toroidal samples were cut from composite plates by means of screw press. The volume fraction of ferrite in composite was calculated from the density by assuming the additivity of densities of the constituents: polycrystalline MnZn ferrite (rf ¼ 5:20 g cm3) or PANI-coated ferrite (rfc ¼ 3:43 g cm3) and polyurethane matrix (rm ¼ 1:02 g cm3). Composites containing 40 and 50 vol% of ferrites were prepared.
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
3. Results A polycrystalline ferrite particle consists of separate grains in the form of polyhedrons with clear-cut boundaries (Fig. 1a). The total area of the interface between grains reaches a significant value, thus weakening the grain contacts and affecting the magnetic properties of MnZn ferrite. After coating with PANI, the continuous overlayer of a conducting polymer has been produced on the ferrite particle surface (Fig. 1b). PANI is also deposited at the crystallite boundaries and covers the surface defects, such as pores and cracks. The effects of ferrite loading, ferrite particle size, and of the surface modification of ferrite particles
Fig. 1. The surface of MnZn ferrite particle before (top) and after (bottom) coating with PANI.
33
with conducting polymer on permeability spectra have been investigated. The permeability spectrum of MnZn ferrite composites (Fig. 2) has resonance-type frequency dispersion exhibiting a maximum in the magnetic losses (Fig. 2, bottom). This indicates that the MnZn ferrite samples have a natural resonance of spin rotation and the vibration resonance of magnetic domain walls. As expected, the permeability of the composites is lower compared with that of a ferrite alone and the resonance frequencies have moved to higher frequencies (Fig. 2a). For the ferrite, the real part of the permeability m0 B1300 in the low-frequency region but it begins to decrease above a frequency of 1 MHz. The imaginary part has a maximum of m00 B800 at B1.4 MHz frequency. At the same time, the m0 values of composite materials with polycrystalline MnZn ferrite begin to decrease at about 100 MHz and the m00 peak is shifted to 590 MHz and 1 GHz. The effect of the loading is more clearly visible in semi-logarithmic plots (Fig. 2b). Besides the ferrite content (Fig. 2), the resonance peak frequency also depends on the ferrite particle size (Fig. 3). The larger particles produce composite materials of higher permeability and they have therefore been used in the subsequent coating with a PANI overlayer. When PANI-base powder is suspended in the aqueous solution of picric acid, partial protonation of PANI, accompanied by an increase in the conductivity is observed [22] (Table 1). This procedure has also been used for the conductivity tuning of PANI coatings on ferrite particles. An example of the variation of permeability spectra with the conductivity of the PANI coating is shown in Fig. 4. The resonance frequency fr has increased about six times as the conductivity of PANI coating grew (Fig. 5) but the maximum in magnetic loss m00 was much less affected by the conductivity of the coating (Fig. 6). The effects we observe are connected with demagnetization at the surface of the coated particles. If we prepare a composite in which PANI is not deposited on ferrite but freely dispersed in the polyurethane matrix along with ferrite, the permeability spectra are different,
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
34
12 50 vol.%
10
103 Ferrite
8
µ'
µ'
102 101
50 vol.%
6
40 vol.%
4
40 vol.%
100 2 10-1
106 106
108
107
107
109
108
109
f, Hz
f, Hz
4
Ferrite
102
100
50 vol.%
µ"
µ"
3 50 vol.%
2
40 vol.%
40 vol.%
10-2
1
106 (a)
107
108
0
109
f, Hz
106 (b)
107
108 f, Hz
109
Fig. 2. (a) Frequency dependence of real (top) and imaginary (bottom) part of complex permeability of MnZn ferrite and its composites with polyurethane. Ferrite particle size, 60 mm. The content of ferrite is given at the individual curves. (b) The detail of frequency dependence of real (top) and imaginary (bottom) parts of complex permeability of MnZn ferrite composites with polyurethane.
because the structure of composite has changed. The position of resonance frequency, however, is completely unaffected by the conductivity of PANI powder (Fig. 7, curves 2,3). The same results are obtained with conducting PANI hydrochloride and non-conducting PANI base powders. The resonance frequency of the 40 vol% uncoated MnZn ferrite in a polyurethane composite (Fig. 2b) is exactly the same as in the systems including PANI powders (Fig. 7). This confirms that these are the surface phenomena in PANI coatings on ferrite that control the resonance frequency.
4. Discussion The resonance-frequency shift and the decrease in m are direct consequences of microscopic demagnetizing fields in the composite in connection with modifications of the dipolar-type interactions on the one hand and of the particle arrangement on the other. This is quite consistent with the results of other studies, which have also clearly highlighted the basic role of the demagnetizing effects on the spin-rotation resonance in composite materials with various types of ferromagnetic fillers [10,23].
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
35
15
5 60 µm
1
4
10
µ'
µ'
2
3
30 µm
3
5
2
1
0
106
107
108
106
109
107
108
109
f, Hz
f, Hz
2.0
1
6
2
1.5 µ"
60 µm
4
1.0
µ"
3
30 µm 2
0.5
0.0 106 0 106
107
108 f, Hz
109
Fig. 3. The frequency dependence of real (top) and imaginary (bottom) parts of permeability for ferrite particles of 30 and 60 mm average size. 50 vol% of MnZn ferrite in polyurethane matrix.
For spin rotation, the general model of Snoek [3] predicts the resonance frequency to be fr ¼ ðg=2pÞ HA ; where g is the gyromagnetic ratio and HA is the magnetic-field anisotropy. It is known that, in composite materials, as the volume fraction of the magnetic component decreases, the local demagnetization fields add to the total anisotropy field and, consequently, the spin-rotation resonance frequency shifts to a high-frequency range. This can be represented by the relation [5,10,13] fr ¼ ðg=2pÞ (HA þ HD ), where HD is the demagnetizing field. The latter quantity is strongly affected by structural disorder in the
107
108
109
f, Hz Fig. 4. The frequency dependence of real (top) and imaginary (bottom) parts of complex permeability of polyurethane composite containing 50 vol% of MnZn ferrite of 60 mm size coated with PANI of various conductivity, s: 1 – PANI coating s ¼ 1:2 107 S cm1 (fr ¼ 1:5 GHz), 2 s ¼ 4:5 104 S cm1 (fr ¼ 1:7 GHz), 3 s ¼ 4:4 S cm1 (fr > 2 GHz).
ferrite, i.e., by both the crystallographic and the geometrical structure, such as cracks and surface roughness of the ferrite. The PANI coating can produce a healing effect with respect to defects and to smooth the ferrite surface. A speculation is offered with respect to the interaction of magnetic material and conducting polymer. Oligomeric aniline intermediates are adsorbed on the ferrite surface immersed in the reaction mixture used for the preparation of PANI coating [19]. PANI chains subsequently grow from these primary nucleation centres in an oxidized
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
36 3
5
4 1 2
µ'
fr, GHz
2 3
3
2
1 Uncoated ferrite
1 106
0 -12 10
-9 10
-6 10
-3 10
107
0
108
109
f, Hz
10
σ , S cm -1
Fig. 5. The dependence of resonance frequency fr on the conductivity of PANI coating, s; on ferrite. 50 vol% of coated MnZn ferrite in polyurethane matrix.
2 1 2
6
µ"
3
1
Uncoated ferrite
µ " max
4
0 106
2
107
108
109
f, Hz
0 -11
10
-8
10
-5
-
10 2
10
σ , S cm
1
10
-1
Fig. 6. The dependence of maximum magnetic loss, m00max ; on the conductivity of PANI coating, s; on ferrite. 50 vol% of PANI-coated MnZn ferrite in polyurethane matrix.
pernigraniline form [21]. This paramagnetic structure is represented by a polymer chain, in which each second aniline constitutional unit carries a positive polaron, i.e., it includes electrons with unpaired spins. Thus, on one hand, it is not ruled out that the internal magnetic field of a ferrite may influence the polymerization of aniline at the surface, thus modifying the structure and
Fig. 7. The frequency dependence of the real (top) and imaginary (bottom) parts of complex permeability. 1–50 vol% of MnZn ferrite coated with PANI hydrochloride, 2–40 vol% of uncoated ferrite and 10 vol% of conducting PANI hydrochloride, 3–40 vol% of uncoated ferrite and 10 vol% of non-conducting PANI base.
properties of the PANI film. On the other hand, due to a possible charge transfer between the ferrite surface and the PANI film, analogous to the corrosion protection of metals by depositing PANI [24], the PANI films may change the electron density at the ferrite surfaces and thus affect magnetic relaxation processes in the system. The presence of conducting layer on the surface of ferrites, which changes the boundary conditions for the microwave field at the interface between the ferrite particle and polymer matrix, is another factor that can influence the resonance frequency.
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
The same effect is known for thin ferrite plates in which the even metallization of one of the sides increased the natural frequency [25]. The marked effect of resonance-frequency shifts may be expected when surface layer on a ferrite particle has much higher conductivity than the bulk of ferrite. Thus, the interactive processes passing between the ferrite surface and the PANI film may increase the conductivity at ferrite surface. We expect that the resonance-frequency shifts will be determined by the coating thickness and morphology, as well as by the conductivity of the particle coating. In order to estimate and predict the values of resonance-frequency shifts, a problem of natural oscillations of ferrite sphere coated with conducting overlayer of finite thickness has to be solved.
5. Conclusions Polycrystalline MnZn ferrite and the same material after coating with PANI have been used for the preparation of composites with a polyurethane matrix. Composites based on ferrite particles of average size 60 mm had higher permeability than those with 30 mm particles. The former particles were coated with an overlayer of conducting polymer, PANI. The resonance frequency is greatly affected by the conductivity of the PANI coating deposited on the surface of the ferrite particles. A higher resonance frequency, enhanced approximately 6-fold, is observed in composites containing ferrite coated with conducting PANI hydrochloride compared with those based on uncoated specimens. The changes in magnetic properties of composites containing PANI-coated ferrite are due to the demagnetizing field generated by the magnetic dipoles and by the changes of boundary condition of the microwave field at the interface between the ferrite particle and polymer matrix. This is confirmed by the fact that simple mixtures of ferrites with PANI powders in a polyurethane matrix do not exhibit any resonance frequency shifts dependent on the conductivity of the PANI.
37
Acknowledgements The financial support of Ministry of Education, Youth, and Sports of the Czech Republic (MSM 265 2000 15 and ME 539) and Grant Agency of the Academy of Sciences of the Czech Republic (A4050313) is gratefully acknowledged. Authors are also indebted to Drs. V. Cheparin and Shakirzyanov from Moscow Power Engineering Institute for providing the samples of ferrites. References [1] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. [2] A.G. Gurevich, G.A. Melkov, Magnetisation, Oscillations and Waves, CRC Press, London, 1996. [3] J.L. Snoek, Physica 14 (1948) 207. [4] G.T. Rado, Rev. Mod. Phys. 25 (1953) 81. [5] H.J. Kwon, J.Y. Shin, J.H. Oh, J. Appl. Phys. 75 (1994) 6109. [6] T. Nakamura, T. Tsutaoka, H. Hatakeyama, J. Magn. Magn. Mater. 138 (1995) 319. [7] C. Kittel, Phys. Rev. 155 (1948) 155. [8] T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 (1995) 3983. [9] N.E. Kazantseva, A.T. Ponomarenko, V.G. Shevchenko, C. Klasson, Electromagnetics 20 (2000) 453. [10] A. Chevalier, M. Le Floc’h, J. Appl. Phys. 90 (2001) 3462. [11] N.E. Kazantseva, N.G. Ryvkina, I.A. Chmutin, J. Commun. Technol. Electron. 48 (2003) 173. [12] A.N. Yusoff, M.H. Addullah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [13] V.I. Ponomarenko, V.N. Berzhanskiy, Radiotekh. Elektron. 35 (1990) 2208. [14] M. Wan, W. Zhou, J. Li, Synth. Metals 78 (1996) 27. [15] J.G. Deng, X.B. Ding, W.C. Zhang, Y.X. Peng, J.H. Wang, X.P. Long, P. Li, A.S.C. Chan, Polymer 48 (2002) 2179. [16] R. Gangopadhyay, A. De, Eur. Polym. J. 35 (1999) 1985. [17] R. Gangopadhyay, A. De, J. Appl. Phys. 87 (2000) 2363. [18] A. Malinauskas, Polymer 42 (2001) 3957. [19] I. Sapurina, A.Yu. Osadchev, B.Z. Volchek, M. Trchov!a, A. Riede, J. Stejskal, Synth. Metals 129 (2002) 29. [20] J. Stejskal, R.G. Gilbert, Pure Appl. Chem. 74 (2002) 857. [21] J. Stejskal, P. Kratochv!ıl, A.D. Jenkins, Polymer 37 (1996) 367. [22] J. Stejskal, I. Sapurina, M. Trchov!a, J. Proke$s, I. K$rivka, E. Tobolkov!a, Macromolecules 31 (1998) 2218. [23] T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 (1995) 3985. [24] M. Fahlman, S. Jasty, A.J. Epstein, Synth. Metals 85 (1997) 1323. [25] W.L. Bongiani, J. Appl. Phys. 43 (1972) 2541.
Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
Magnetic behaviour of composites containing polyaniline-coated manganese–zinc ferrite N.E. Kazantsevaa,b, J. VilWa! kova! b, V. Kr$esa! lekb, P. Sa! hab, I. Sapurinac, J. Stejskald,* a
Institute of Radio-Engineering and Electronics, Russian Academy of Sciences, Friasino, Moscow region 141190, Russia b Polymer Centre, Faculty of Technology, Tomas Bata University in Zlin, Zlin 762 72, Czech Republic c Institute of Macromolecular Compounds, Russian Academy of Sciences, St. Petersburg 199004, Russia d Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic Received 23 January 2003; received in revised form 14 May 2003
Abstract Polycrystalline manganese–zinc ferrite has been coated with polyaniline (PANI) and embedded into a polyurethane matrix. The complex permeability of the composites was studied in the frequency range 1 MHz–3 GHz. The conductivity of PANI coating was adjusted by controlled protonation with picric acid. Large shifts in the resonance frequency were observed as a function of varying PANI conductivity. The changes in the magnetic properties of the PANI-coated composite material are due to the change of the boundary conditions of the microwave field at the interface between the ferrite particle and polymer matrix. This effect is observed especially when the magnetic anisotropy of ferrite is low. r 2003 Elsevier B.V. All rights reserved. Keywords: Ferrite; Permeability; Coated particles; Polyaniline; Conducting polymer
1. Introduction New magnetic materials have received considerable attention, especially in studies of composite materials with different types of ferromagnetic fillers. The main challenge is to design the structure, which would have the greatest effect on the magnetic and dielectric properties and thus could be used in their control.
*Corresponding author. Tel.: +420-296-809-351; fax: +420296-809-410. E-mail address: [email protected] (J. Stejskal).
It is well known that the frequency dispersion of the complex permeability of a ferrite material strongly depends on the magnetic substance [1,2]. Up to the present time, resonance phenomena in ferrites have been studied extensively and described by two different mechanisms of magnetization–spin resonance and domain-wall motion [3,4]. For a polycrystalline ferrite, the permeability spectra can be described by the superposition of both contributions. Furthermore, the complex permeability in the high-frequency region is greatly affected by the spin-rotation component [5,6]. It has been found that, in ferrites, the relaxation of magnetic oscillations is strongly affected by
0304-8853/03/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0304-8853(03)00557-2
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
structural disorder, both crystallographic and geometrical, such as pores, cracks, defects, and surface roughness. Composite materials, in which magnetic particles are embedded in insulating binders, introduce an additional disorder in the form of filler–matrix interfaces. This makes the magnetic relaxation behaviour still more complex. This additional disorder brings about changes in the internal magnetic fields by reducing exchange and dipole interactions through demagnetization fields. As a result of dipole interactions, various effects have been observed, e.g., the anomalous broadening of the resonance band, the shift of resonance to higher frequencies, a considerable decrease in complex permeability, the spreading of ferromagnetic–paramagnetic transition temperature range, etc. [7–10]. At present, there is no adequate procedure to predict the dependence of the magnetic properties on the structure of composites. Very few data have been published on the magnetic and dielectric properties of composites with multicomponent magnetic powders, such as mixtures of ferrites with conducting fillers and coated ferromagnetic particles. Such composite materials have special interest because of new and often unusual electromagnetic properties. Applications of ferrites and ferroelectric mixtures, which have their own relaxation process, make it possible to achieve more or less the same value of complex permittivity, e ; and complex permeability, m ; in different regions of the frequency range [11,12]. For example, the electrochemical deposition of iron on spinel-type Ni0.45Zn0.55Fe2O4 ferrite changes the frequency dispersion of e and m of composite materials [13]. But it is difficult to control the uniform thickening of the iron overlayer during the deposition process. When the thickness of the iron layer is too large, both the real and the imaginary parts of the complex permeability of the composite material, m0 and m00 ; dramatically decrease. One of the promising alternatives for the coating of ferrite particles is the deposition of conducting polymers on their surface. Conducting polymers, such as polyaniline (PANI) and polypyrrole, are easy to prepare and they are environmentally stable. The materials
31
containing such polymers and magnetic component are represented by the preparation of nanocomposites of ferric oxide and PANI [14,15] or polypyrrole [16,17]. The surface modification techniques providing the coating of various substrates with conducting polymer nanofilms are simple and well developed [18,19]. They are, in principle, well suited also for the treatment of ferrite powders. There are many technologically important ferrite materials with different sorts of magnetocrystalline anisotropy. We have selected manganese–zinc (MnZn) ferrites because of their high initial permeability, low value of effective magnetic-field anisotropy, and low core losses. Ferrites of this type have widely been used in electronic devices. This article reports the effect of ferrite content and particle size on the level of permeability. The studies included the surface modification of ferrite particles with conducting PANI coatings resulting in the shift of frequency resonance from the MHz to the GHz-frequency region.
2. Experimental procedure 2.1. Ferrites The bulk MnZn spinel ferrite in polycrystalline form was prepared by heating mixed powders of oxides (Fe2O3, MnO and ZnO) at 1300 C for 6 h and annealing at 200 C h1 cooling rate at gradually reduced air pressure. The composition of ferrite used in the present experiments was 53.2 mol% Fe2O3, 26.2 mol% MnO and 20.6 mol% ZnO. Ferrite powders of 30 and 60 mm average particle sizes were prepared by the mechanical grinding of sintered ferrite. Particle morphology was observed by scanning electron microscopy using JEOL JEM-200CX. Toroidal samples of outer diameter of 8 mm, inner diameter 3.1 mm, and 3 mm thick were cut from bulk ferrite plate with a diamond saw on a cutter at 1000 rpm with water cooling. The magnetic properties of MnZn ferrite compacts, relative intrinsic permeability m0 B3000 and the magnetic-field anisotropy HA ¼ 104 A m1, were
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N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
determined by standard techniques with a vibrating-sample magnetometer (VSM, PARR, model 4500). An applied magnetic field of 5 kOe was employed to evaluate the saturation magnetization MS ¼ 3:5 kG s and coercitivity HC ¼150 A m–1. Curie temperature TC ¼473 K was derived from the temperature dependence of permeability. The ferrite conductivity sf ¼ 2 102 S cm1 was determined by four-point method using a silver paste for contacts. Archimedes method was used to find the density, rf ¼ 5:2 g cm3. The frequency dependences of complex permeability were determined for ferrite and composite samples in the range 1 MHz–3 GHz with an RF Impedance/Material Analyser (Agilent E49991A).
Table 1 Conductivity of polyaniline, s; protonated with picric acid to various level, xa x
s (S cm1)
x
s (S cm1)
0 0.1 0.2 0.3 0.4 0.5
6.0 1011 1.2 107 1.8 105 4.5 104 4.5 103 2.1 102
0.6 0.7 0.8 1.0 1.5 2.0
7.1 102 9.8 102 1.3 101 1.4 101 2.0 101 2.1 101
a Number of moles of acid per mole of constitutional units of PANI base. PANI base (362 mg, 2 mmol) was suspended in 100 ml of water containing dissolved picric acid for 24 h.
2.3. Conductivity of polyaniline coatings 2.2. Coating of ferrite powder with polyaniline PANI was prepared by the oxidation of aniline hydrochloride with ammonium peroxydisulphate in an aqueous medium [20]. Any material in contact with the reaction mixture used for the preparation of PANI becomes coated with a film of this polymer [18]. The typical film thickness is 100–200 nm [19]. This fact has been used for the coating of ferrite powder. MnZn ferrite (20 g, 60-mm size) was immersed in 50 ml of aqueous solution containing 2.59 g (=20 mmol) of aniline hydrochloride, at room temperature. The polymerization of aniline was started by introducing 5.70 g (=25 mmol) of ammonium peroxydisulphate also in 50 ml of aqueous solution. Next day, ferrite particles coated with conducting PANI hydrochloride were collected on a filter, rinsed with dilute (0.2 M) hydrochloric acid, followed by acetone, and dried. The coated particles contained B10 wt% of PANI hydrochloride and had a conductivity of 0.34 S cm1 after being compressed into a pellet. PANI hydrochloride prepared in the absence of ferrite had a conductivity of 4.4 S cm1 [20]. A portion of the PANI-coated ferrite powder was suspended in an excess of 1 M ammonium hydroxide. The ammonia caused the deprotonation of the conducting polymer coating to the nonconducting PANI base [21]. The coated ferrite had a conductivity of 4.3 107 S cm1.
Ferrites coated with PANI base were immersed in aqueous solutions of picric acid (2,4,6-trinitrophenol; Fluka, Switzerland). By adjusting the concentration of the acid, the degree of protonation of the PANI coating and, consequently, also its conductivity was controlled over several orders of magnitude. The similar immersion of PANI base in the solutions of picric acid was used to prepare the material for the determination of conductivity measured on dry samples compressed into pellets by four-point or two-probe methods (Table 1).
2.4. Polyurethane–ferrite composites Composite materials have been prepared by mixing ferrite particles with a polyurethane prepolymer (AXSON UR 3420, Axson, France) and pressing between two metallic plates separated with a 3 mm spacer. Samples were kept for 4 h at 80 C in vacuum. The toroidal samples were cut from composite plates by means of screw press. The volume fraction of ferrite in composite was calculated from the density by assuming the additivity of densities of the constituents: polycrystalline MnZn ferrite (rf ¼ 5:20 g cm3) or PANI-coated ferrite (rfc ¼ 3:43 g cm3) and polyurethane matrix (rm ¼ 1:02 g cm3). Composites containing 40 and 50 vol% of ferrites were prepared.
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
3. Results A polycrystalline ferrite particle consists of separate grains in the form of polyhedrons with clear-cut boundaries (Fig. 1a). The total area of the interface between grains reaches a significant value, thus weakening the grain contacts and affecting the magnetic properties of MnZn ferrite. After coating with PANI, the continuous overlayer of a conducting polymer has been produced on the ferrite particle surface (Fig. 1b). PANI is also deposited at the crystallite boundaries and covers the surface defects, such as pores and cracks. The effects of ferrite loading, ferrite particle size, and of the surface modification of ferrite particles
Fig. 1. The surface of MnZn ferrite particle before (top) and after (bottom) coating with PANI.
33
with conducting polymer on permeability spectra have been investigated. The permeability spectrum of MnZn ferrite composites (Fig. 2) has resonance-type frequency dispersion exhibiting a maximum in the magnetic losses (Fig. 2, bottom). This indicates that the MnZn ferrite samples have a natural resonance of spin rotation and the vibration resonance of magnetic domain walls. As expected, the permeability of the composites is lower compared with that of a ferrite alone and the resonance frequencies have moved to higher frequencies (Fig. 2a). For the ferrite, the real part of the permeability m0 B1300 in the low-frequency region but it begins to decrease above a frequency of 1 MHz. The imaginary part has a maximum of m00 B800 at B1.4 MHz frequency. At the same time, the m0 values of composite materials with polycrystalline MnZn ferrite begin to decrease at about 100 MHz and the m00 peak is shifted to 590 MHz and 1 GHz. The effect of the loading is more clearly visible in semi-logarithmic plots (Fig. 2b). Besides the ferrite content (Fig. 2), the resonance peak frequency also depends on the ferrite particle size (Fig. 3). The larger particles produce composite materials of higher permeability and they have therefore been used in the subsequent coating with a PANI overlayer. When PANI-base powder is suspended in the aqueous solution of picric acid, partial protonation of PANI, accompanied by an increase in the conductivity is observed [22] (Table 1). This procedure has also been used for the conductivity tuning of PANI coatings on ferrite particles. An example of the variation of permeability spectra with the conductivity of the PANI coating is shown in Fig. 4. The resonance frequency fr has increased about six times as the conductivity of PANI coating grew (Fig. 5) but the maximum in magnetic loss m00 was much less affected by the conductivity of the coating (Fig. 6). The effects we observe are connected with demagnetization at the surface of the coated particles. If we prepare a composite in which PANI is not deposited on ferrite but freely dispersed in the polyurethane matrix along with ferrite, the permeability spectra are different,
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
34
12 50 vol.%
10
103 Ferrite
8
µ'
µ'
102 101
50 vol.%
6
40 vol.%
4
40 vol.%
100 2 10-1
106 106
108
107
107
109
108
109
f, Hz
f, Hz
4
Ferrite
102
100
50 vol.%
µ"
µ"
3 50 vol.%
2
40 vol.%
40 vol.%
10-2
1
106 (a)
107
108
0
109
f, Hz
106 (b)
107
108 f, Hz
109
Fig. 2. (a) Frequency dependence of real (top) and imaginary (bottom) part of complex permeability of MnZn ferrite and its composites with polyurethane. Ferrite particle size, 60 mm. The content of ferrite is given at the individual curves. (b) The detail of frequency dependence of real (top) and imaginary (bottom) parts of complex permeability of MnZn ferrite composites with polyurethane.
because the structure of composite has changed. The position of resonance frequency, however, is completely unaffected by the conductivity of PANI powder (Fig. 7, curves 2,3). The same results are obtained with conducting PANI hydrochloride and non-conducting PANI base powders. The resonance frequency of the 40 vol% uncoated MnZn ferrite in a polyurethane composite (Fig. 2b) is exactly the same as in the systems including PANI powders (Fig. 7). This confirms that these are the surface phenomena in PANI coatings on ferrite that control the resonance frequency.
4. Discussion The resonance-frequency shift and the decrease in m are direct consequences of microscopic demagnetizing fields in the composite in connection with modifications of the dipolar-type interactions on the one hand and of the particle arrangement on the other. This is quite consistent with the results of other studies, which have also clearly highlighted the basic role of the demagnetizing effects on the spin-rotation resonance in composite materials with various types of ferromagnetic fillers [10,23].
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
35
15
5 60 µm
1
4
10
µ'
µ'
2
3
30 µm
3
5
2
1
0
106
107
108
106
109
107
108
109
f, Hz
f, Hz
2.0
1
6
2
1.5 µ"
60 µm
4
1.0
µ"
3
30 µm 2
0.5
0.0 106 0 106
107
108 f, Hz
109
Fig. 3. The frequency dependence of real (top) and imaginary (bottom) parts of permeability for ferrite particles of 30 and 60 mm average size. 50 vol% of MnZn ferrite in polyurethane matrix.
For spin rotation, the general model of Snoek [3] predicts the resonance frequency to be fr ¼ ðg=2pÞ HA ; where g is the gyromagnetic ratio and HA is the magnetic-field anisotropy. It is known that, in composite materials, as the volume fraction of the magnetic component decreases, the local demagnetization fields add to the total anisotropy field and, consequently, the spin-rotation resonance frequency shifts to a high-frequency range. This can be represented by the relation [5,10,13] fr ¼ ðg=2pÞ (HA þ HD ), where HD is the demagnetizing field. The latter quantity is strongly affected by structural disorder in the
107
108
109
f, Hz Fig. 4. The frequency dependence of real (top) and imaginary (bottom) parts of complex permeability of polyurethane composite containing 50 vol% of MnZn ferrite of 60 mm size coated with PANI of various conductivity, s: 1 – PANI coating s ¼ 1:2 107 S cm1 (fr ¼ 1:5 GHz), 2 s ¼ 4:5 104 S cm1 (fr ¼ 1:7 GHz), 3 s ¼ 4:4 S cm1 (fr > 2 GHz).
ferrite, i.e., by both the crystallographic and the geometrical structure, such as cracks and surface roughness of the ferrite. The PANI coating can produce a healing effect with respect to defects and to smooth the ferrite surface. A speculation is offered with respect to the interaction of magnetic material and conducting polymer. Oligomeric aniline intermediates are adsorbed on the ferrite surface immersed in the reaction mixture used for the preparation of PANI coating [19]. PANI chains subsequently grow from these primary nucleation centres in an oxidized
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
36 3
5
4 1 2
µ'
fr, GHz
2 3
3
2
1 Uncoated ferrite
1 106
0 -12 10
-9 10
-6 10
-3 10
107
0
108
109
f, Hz
10
σ , S cm -1
Fig. 5. The dependence of resonance frequency fr on the conductivity of PANI coating, s; on ferrite. 50 vol% of coated MnZn ferrite in polyurethane matrix.
2 1 2
6
µ"
3
1
Uncoated ferrite
µ " max
4
0 106
2
107
108
109
f, Hz
0 -11
10
-8
10
-5
-
10 2
10
σ , S cm
1
10
-1
Fig. 6. The dependence of maximum magnetic loss, m00max ; on the conductivity of PANI coating, s; on ferrite. 50 vol% of PANI-coated MnZn ferrite in polyurethane matrix.
pernigraniline form [21]. This paramagnetic structure is represented by a polymer chain, in which each second aniline constitutional unit carries a positive polaron, i.e., it includes electrons with unpaired spins. Thus, on one hand, it is not ruled out that the internal magnetic field of a ferrite may influence the polymerization of aniline at the surface, thus modifying the structure and
Fig. 7. The frequency dependence of the real (top) and imaginary (bottom) parts of complex permeability. 1–50 vol% of MnZn ferrite coated with PANI hydrochloride, 2–40 vol% of uncoated ferrite and 10 vol% of conducting PANI hydrochloride, 3–40 vol% of uncoated ferrite and 10 vol% of non-conducting PANI base.
properties of the PANI film. On the other hand, due to a possible charge transfer between the ferrite surface and the PANI film, analogous to the corrosion protection of metals by depositing PANI [24], the PANI films may change the electron density at the ferrite surfaces and thus affect magnetic relaxation processes in the system. The presence of conducting layer on the surface of ferrites, which changes the boundary conditions for the microwave field at the interface between the ferrite particle and polymer matrix, is another factor that can influence the resonance frequency.
ARTICLE IN PRESS N. Kazantseva et al. / Journal of Magnetism and Magnetic Materials 269 (2004) 30–37
The same effect is known for thin ferrite plates in which the even metallization of one of the sides increased the natural frequency [25]. The marked effect of resonance-frequency shifts may be expected when surface layer on a ferrite particle has much higher conductivity than the bulk of ferrite. Thus, the interactive processes passing between the ferrite surface and the PANI film may increase the conductivity at ferrite surface. We expect that the resonance-frequency shifts will be determined by the coating thickness and morphology, as well as by the conductivity of the particle coating. In order to estimate and predict the values of resonance-frequency shifts, a problem of natural oscillations of ferrite sphere coated with conducting overlayer of finite thickness has to be solved.
5. Conclusions Polycrystalline MnZn ferrite and the same material after coating with PANI have been used for the preparation of composites with a polyurethane matrix. Composites based on ferrite particles of average size 60 mm had higher permeability than those with 30 mm particles. The former particles were coated with an overlayer of conducting polymer, PANI. The resonance frequency is greatly affected by the conductivity of the PANI coating deposited on the surface of the ferrite particles. A higher resonance frequency, enhanced approximately 6-fold, is observed in composites containing ferrite coated with conducting PANI hydrochloride compared with those based on uncoated specimens. The changes in magnetic properties of composites containing PANI-coated ferrite are due to the demagnetizing field generated by the magnetic dipoles and by the changes of boundary condition of the microwave field at the interface between the ferrite particle and polymer matrix. This is confirmed by the fact that simple mixtures of ferrites with PANI powders in a polyurethane matrix do not exhibit any resonance frequency shifts dependent on the conductivity of the PANI.
37
Acknowledgements The financial support of Ministry of Education, Youth, and Sports of the Czech Republic (MSM 265 2000 15 and ME 539) and Grant Agency of the Academy of Sciences of the Czech Republic (A4050313) is gratefully acknowledged. Authors are also indebted to Drs. V. Cheparin and Shakirzyanov from Moscow Power Engineering Institute for providing the samples of ferrites. References [1] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. [2] A.G. Gurevich, G.A. Melkov, Magnetisation, Oscillations and Waves, CRC Press, London, 1996. [3] J.L. Snoek, Physica 14 (1948) 207. [4] G.T. Rado, Rev. Mod. Phys. 25 (1953) 81. [5] H.J. Kwon, J.Y. Shin, J.H. Oh, J. Appl. Phys. 75 (1994) 6109. [6] T. Nakamura, T. Tsutaoka, H. Hatakeyama, J. Magn. Magn. Mater. 138 (1995) 319. [7] C. Kittel, Phys. Rev. 155 (1948) 155. [8] T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 (1995) 3983. [9] N.E. Kazantseva, A.T. Ponomarenko, V.G. Shevchenko, C. Klasson, Electromagnetics 20 (2000) 453. [10] A. Chevalier, M. Le Floc’h, J. Appl. Phys. 90 (2001) 3462. [11] N.E. Kazantseva, N.G. Ryvkina, I.A. Chmutin, J. Commun. Technol. Electron. 48 (2003) 173. [12] A.N. Yusoff, M.H. Addullah, S.H. Ahmad, S.F. Jusoh, A.A. Mansor, S.A.A. Hamid, J. Appl. Phys. 92 (2002) 876. [13] V.I. Ponomarenko, V.N. Berzhanskiy, Radiotekh. Elektron. 35 (1990) 2208. [14] M. Wan, W. Zhou, J. Li, Synth. Metals 78 (1996) 27. [15] J.G. Deng, X.B. Ding, W.C. Zhang, Y.X. Peng, J.H. Wang, X.P. Long, P. Li, A.S.C. Chan, Polymer 48 (2002) 2179. [16] R. Gangopadhyay, A. De, Eur. Polym. J. 35 (1999) 1985. [17] R. Gangopadhyay, A. De, J. Appl. Phys. 87 (2000) 2363. [18] A. Malinauskas, Polymer 42 (2001) 3957. [19] I. Sapurina, A.Yu. Osadchev, B.Z. Volchek, M. Trchov!a, A. Riede, J. Stejskal, Synth. Metals 129 (2002) 29. [20] J. Stejskal, R.G. Gilbert, Pure Appl. Chem. 74 (2002) 857. [21] J. Stejskal, P. Kratochv!ıl, A.D. Jenkins, Polymer 37 (1996) 367. [22] J. Stejskal, I. Sapurina, M. Trchov!a, J. Proke$s, I. K$rivka, E. Tobolkov!a, Macromolecules 31 (1998) 2218. [23] T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 (1995) 3985. [24] M. Fahlman, S. Jasty, A.J. Epstein, Synth. Metals 85 (1997) 1323. [25] W.L. Bongiani, J. Appl. Phys. 43 (1972) 2541.