BaFe12O19 composites

Synthetic Metals 159 (2009) 695–699

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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Preparation and characterization of electromagnetic functionalized polyaniline/BaFe12 O19 composites Jing Jiang a,∗ , Lun-Hong Ai a , Da-Bin Qin a , Hui Liu b , Liang-Chao Li b,∗ a b

Laboratory of Applied Chemistry and Pollution Control Technology, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, China Zhejiang Key Laboratory for Reactive Chemistry on Solid Surface, Department of Chemistry, Zhejiang Normal University, Jinhua 321004, China

a r t i c l e

i n f o

Article history: Received 7 August 2008 Received in revised form 27 November 2008 Accepted 26 December 2008 Available online 11 February 2009

a b s t r a c t Electromagnetic functionalized polyaniline/BaFe12 O19 composites were synthesized by in situ polymerization of aniline in the presence of BaFe12 O19 particles. The structure and morphologies of products were characterized by X-ray diffraction, infrared spectra, scanning electron microscopy and transmission electron microscopy. In the electromagnetic measurements, it was found that the ac conductivity of BaFe12 O19 particles enhanced while the saturation magnetization and coercivity decreased after polyaniline coating. © 2008 Elsevier B.V. All rights reserved.

Keywords: Composite Polymer Magnetic property Electrical property

1. Introduction Inherently conducting polymers are attractive materials, as they cover a wide range of functions from insulators to metals and retain the mechanical properties of conventional polymers [1,2]. The considerable electrochemical and physicochemical properties result in conducting polymers having various practical applications [3–6]. Among the conducting polymer, polyaniline (PANI) has received a great deal of attention in recent years due to its easy synthesis, excellent environmental stability, and high electrical conductivity. It is well known that conducting polymers can effectively shield electromagnetic waves generated from an electric source, whereas electromagnetic waves from a magnetic source can be effectively shielded only by magnetic materials [7]. Thus, incorporation of magnetic constituents and conducting polymeric materials opens new possibilities for the achievement of good shielding effectiveness for various electromagnetic sources. Barium hexaferrite BaFe12 O19 has been currently magnetic material with great scientific and technological interest, and have been widely used for permanent magnets, magnetic recording media and microwave absorbers, due to its high stability, excellent high-frequency response, large magnetocrystalline anisotropy and large magnetization as well [8]. In recent years, barium hexaferrites have displayed a promising application in microwave absorption

∗ Corresponding authors. Tel.: +86 817 2568081; fax: +86 817 2224217. E-mail address: [email protected] (J. Jiang). 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.12.021

due to their dielectric and magnetic losses in microwave frequency band [9–11]. Up to now, many reports have focused on choosing the cubic spinel ferrite as magnetic component in the polyanilinebased composites [12–14]. To our best of knowledge, little work has been reported on the preparation and electromagnetic properties of polyaniline-based hexaferrite composites [15]. In this article, the electromagnetic functionalized PANI/ BaFe12 O19 composites, where BaFe12 O19 particles were magnetic core obtained by a citrate sol–gel combustion process and PANI was the conducting shell, were synthesized by in situ polymerization of aniline in the presence of BaFe12 O19 particles. The samples were characterized by various experimental techniques, and the electromagnetic properties of composites were investigated. 2. Experimental 2.1. Materials Aniline was distilled twice under reduced pressure and stored below 0 ◦ C. Citric acid, ammonia, Fe(NO3 )3 ·9H2 O, Ba(NO3 )2 , (NH4 )2 S2 O8 (APS) were all of analytical purity and used without further purification. 2.2. Preparation of hexaferriteBaFe12 O19 (BF) particles Hexaferrite BaFe12 O19 was prepared by a citrate sol–gel combustion process. Stoichiometric amounts of Fe(NO3 )3 ·9H2 O and Ba(NO3 )2 were dissolved in a minimum amount of deionized water

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by stirring on a hotplate at ca. 50 ◦ C with the ratio of iron to barium being set at 11.5. Citric acid was then added to the mixture solution to chelate Ba2+ and Fe3+ . The molar ratios of citric acid to metal ions used were 1:1. An ammonia solution was added to adjust the pH value to 7. The clear solution was slowly evaporated at 80 ◦ C with constant stirring and then the viscous gels were formed. By increasing the temperature up to 200 ◦ C, the gel precursors were combusted to form the brown-colored loose powders. Finally, the as-burnt powders were calcined at 900 ◦ C for 2 h. The hexaferrite BaFe12 O19 with particle size in the range of 90–140 nm were obtained. 2.3. Preparation of polyaniline/BaFe12 O19 composites Polyaniline/BaFe12 O19 composites were prepared by in situ polymerization of aniline in the presence of BaFe12 O19 particles. The whole experiment was operated in an ultrasonic apparatus (Model KQ-250DB, Kunshan Ultrasonic Instrument Co. Ltd.), using a power of 100 W and operated at 50 kHz. In a typical procedure, 1 ml aniline monomer was injected into 35 ml of 0.1 M HCl solution containing a certain amount of BaFe12 O19 particles under ultrasonic stirring for 30 min 2.49 g APS in 20 ml of 0.1 M HCl solution was then slowly added dropwise to the above mixture. The polymerization was carried out by ultrasonic stirring for 8 h at room temperature. The composites were obtained by filtering and washing the reaction mixture with deionized water and ethanol, and dried under vacuum at 60 ◦ C for 24 h. The PANI/BaFe12 O19 composites containing different content of BaFe12 O19 were synthesized by using 10 wt% (PB-1) and 20 wt% (PB-2)of BaFe12 O19 hexaferrite with respect to aniline monomer. 2.4. Characterization The XRD patterns of the samples were collected on a Philips X’pert Pro MPD diffractometer with Cu K␣ radiation ( = 0.15418 nm). The working voltage of the instrument was 40 kV, and the current was 40 mA. Infrared spectra were recorded on a Nicolet Avatar 360 spectrometer in the range of 400–4000 cm−1 using KBr pellets. The field emission scanning electron microscopy (FESEM) was conducted on a Hitachi S4800 field emission scanning electron microscope. The high-resolution transmission electron microscopy (HRTEM) observations were performed on a JEOL JEM2010 transmission electron microscope at an accelerating voltage of 200 kV. Magnetic measurements were carried out at room temperature using a vibrating sample magnetometer (VSM, Lakeshore 7404) with a maximum magnetic field of 15 kOe. The ac conductivity of samples at room temperature was performed on an Agilent E4991A RF Impedance/Material Analyzer in the frequency range from 1 MHz to 1 GHz.

Fig. 1. XRD patterns of (a) BaFe12 O19 particles, (b) PANI/BaFe12 O19 composite (PB-1) and (c) PANI.

polymerized by ammonium persulfate as an oxidizing agent at room temperature. 3.2. Structural characterization Fig. 1 shows the XRD patterns of the BaFe12 O19 particles and PANI/BaFe12 O19 composite (PB-1). As shown in Fig. 1(a), the XRD pattern of the BaFe12 O19 particles presents the magnetoplumbite structure with no extra reflections, and is perfectly indexed to (1 1 0), (1 0 7), (1 1 4), (2 0 3), (2 0 5), (2 1 7), (2 0 1 1) and (2 2 0) crystal plane of hexagonal BaFe12 O19 (JCPDS Card No. 84-0757). The typical XRD pattern of PANI (Fig. 1c) shows two broad diffraction peaks centered at 2 = 20.4◦ and 25.4◦ , which can be ascribed to the periodicity parallel and perpendicular to the polymer chains, respectively [17]. Fig. 1(b) shows the XRD pattern of PANI/BaFe12 O19 composite which contains the characteristic peaks of PANI and BaFe12 O19 including the peaks at 2 = 30.4◦ , 32.1◦ , 34.1◦ , 37.1◦ , 40.3◦ , 55.1◦ , 56.6◦ and 63.1◦ . In addition, the intensities of broad diffraction peaks corresponding to PANI in the composite become weakened with introducing BaFe12 O19 particles, which indicates that BaFe12 O19 particles have an effect on the crystallinity of PANI. Fig. 2 shows the IR spectra of PANI and PANI/BaFe12 O19 composite (PB-1). The characteristic peaks of PANI occur at 1562, 1479,

3. Results and discussions 3.1. Polymerization mechanism It is known that the surface charge of metal oxide is positive below the pH of the point of zero charge (PZC), while it is negative above that. Since the surface of barium ferrite has PZC of pH ≈4.2 [16], it is positively charged in the acidic conditions. Therefore, adsorption of an amount of the anions may occur and compensate the positive charges on barium ferrite surface. Meanwhile, the specific adsorption of these anions on the barium ferrite surface may also take place. In this approach, aniline monomers are converted to cationic anilinium ions in acidic conditions. Thus, the electrostatic interactions appear between anions adsorbed on the barium ferrite surface and cationic anilinium ions. The aniline monomers electrostatically complexed to the barium ferrite surface are then

Fig. 2. IR spectra of (a) PANI and (b) PANI/BaFe12 O19 composite (PB-1).

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Fig. 3. FESEM images of (a) BaFe12 O19 particles, (b) PANI and PANI/BaFe12 O19 composites: (c) PB-2 and (d) PB-1. TEM images of (e) BaFe12 O19 particles and PANI/BaFe12 O19 composite: (f) PB-2.

1300, 1109 and 803 cm−1 . The peaks at 1562 and 1479 cm−1 are attributed to the characteristic C C stretching of the quinoid and benzenoid rings, the peaks at 1300 cm−1 is assigned to C–N stretching of the benzenoid ring, the broad peak at 1109 cm−1 which is described by MacDiarmid et al. as the “electronic-like band” is associated with vibration mode of N = Q = N (Q refers to the quinonic-type rings) [18], and the peak at 803 cm−1 is attributed to the out-of-plane deformation of C–H in the p-disubstituted benzene ring. The IR spectra of PANI/BaFe12 O19 composite (Fig. 2b) are almost identical to that of PANI. In addition, there are no characteristic peaks of BaFe12 O19 particles in the IR spectra of composite, indicating the well wrapping of BaFe12 O19 particles with PANI in the composite [19]. 3.3. Morphology The morphology of the obtained PANI/BaFe12 O19 composites has been studied by FESEM and TEM, as shown in Fig. 3. The FESEM (Fig. 3a) and TEM (Fig. 3e) images of BaFe12 O19 parti-

cles indicate that the BaFe12 O19 particles appear the plate-like shape with the random grain orientation, and the particle sizes of obtained BaFe12 O19 particles are estimated to be in the range of 90–140 nm. Agglomeration appears unavoidable due to higher annealing temperature and interaction between magnetic particles. The FESEM image of pristine PANI displayed in Fig. 3b indicates the regular sticks with an average diameter of about 40 nm and the length of about 150 nm or longer. Fig. 3(c and d) shows the FESEM images of PANI/BaFe12 O19 composites containing different content of BaFe12 O19 particles (PB-1 and PB-2), which present different morphologies compared with that of pristine PANI and bare BaFe12 O19 particles. It can be observed that the PANI/BaFe12 O19 composites exist as globular agglomerates (Fig. 3d) at low content of BaFe12 O19 particles (PB-1). This may be attributed to the large proportion of free bulk/solution polymerized PANI (existing in agglomerated form) as compared to aniline polymerized over BaFe12 O19 surface. However, with the increase in BaFe12 O19 content, there is a change in morphology from aggregated globules (Fig. 3d) towards uniformly coated plates (Fig. 3c). TEM image of

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PANI/BaFe12 O19 composite (PB-2), as shown in Fig. 3f, indicates that the BaFe12 O19 particles are embedded in the PANI matrix. The black region is magnetic hexaferrite BaFe12 O19 particle, and the greycolored shell is PANI in the composite, due to the different electron penetrability. 3.4. Magnetic properties Fig. 4 shows the hysteresis loops of the BaFe12 O19 particles and PANI/BaFe12 O19 composites at room temperature. PANI/BaFe12 O19 composites under applied magnetic field exhibit a clear hysteretic behavior. As seen from Fig. 4, the saturation magnetization (MS ) and coercivity (HC ) of PANI/BaFe12 O19 composites are lower than that of BaFe12 O19 particles and decrease with decreasing the BaFe12 O19 content. The observed decrease in MS of PANI/BaFe12 O19 composites with decreasing the BaFe12 O19 content shows the BaFe12 O19 particles are responsible for the magnetic behavior of the composites. According to the equation MS = ϕmS , MS is related to the volume fraction of the particles (ϕ) and the saturation moment of a single particle (mS ) [20]. It can be considered that the MS of PANI/BaFe12 O19 composite depends mainly on the volume fraction of the magnetic hexaferrite particles, due to the non-magnetic PANI coating layer contribution to the total magnetization, resulting in a decrease in the saturation magnetization. Magnetic properties observed for magnetic particles are a combination of many anisotropy mechanisms. An effective anisotropy constant (K) could be obtained by adding the bulk anisotropy and surface contributions, and the following expression has been used to account for K [21]: K = Kb +

6 d

KS

(1)

where Kb is the bulk magnetocrystalline anisotropy, KS is the surface anisotropy and d is the particle diameter. KS is usually maximum for free surfaces and is reduced by solid coverage. The decrease in KS resulting from the particle coverage by the PANI reduces the effective magnetocrystalline anisotropy (K) and therefore decreases HC . 3.5. Electrical properties Electrical conductivity measurements are known to be very sensitive for the study of electronic properties of materials. In amorphous systems, ac conductivity measurements provide useful information concerning various relaxation phenomenon related to the electrical polarization process. Fig. 5 shows the variation of

Fig. 5. Variation of  ac of BaFe12 O19 particles, PANI/BaFe12 O19 composites and PANI as a function of frequency.

ac conductivity ( ac ) of PANI and PANI/BaFe12 O19 composites as a function of frequency. Within the measurable frequency range,  ac tends to remain constant for all samples up to about 107 Hz, and thereafter increases with frequency. This is characteristic of disordered material where conductivity is due to hopping of charge carriers between localized states [22]. It is well known fact that the conductivity of composite depends, apart from frequency and temperature, on degree of protonation, percent crystallinity, crystalline domain size and order in crystalline and amorphous regions have a relationship with the delocalization length. A observed decrease in  ac of the PANI/BaFe12 O19 composites with increasing the content of BaFe12 O19 could be due to an increase in the disorderliness of composites, leading to a reduction in the delocalization length and particle blockage the conduction path of PANI [23,24]. Also, increasing the BaFe12 O19 contents leads to a larger number of polarons where the interpolaron coupling gets progressively stronger even though disorder present, resulting in severe pinning of polarons, thus restricting their contribution at higher frequencies, hence in the reduction of conductivity [25]. 4. Conclusion In summary, the electrical–magnetic multifunctional PANI/ BaFe12 O19 composite was successfully synthesized by a facile in situ polymerization of aniline in the presence of BaFe12 O19 particles. The structure of PANI/BaFe12 O19 composite was characterized by XRD, FT-IR, FESEM and TEM. The electromagnetic measurements revealed that the MS and HC of BaFe12 O19 particles decreased, while  ac increased after polyaniline coating. Acknowledgement This work was supported by the Scientific Research Start-up Foundation of China West Normal University (07B008, 07B005). References

Fig. 4. Hysteresis loops of BaFe12 O19 particles and PANI/BaFe12 O19 composites.

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