J Polym Res (2011) 18:2017–2021 DOI 10.1007/s10965-011-9610-x
ORIGINAL PAPER
Magnetic and dielectric properties of barium ferrite fibers/poly(vinylidene fluoride) composite films Xiaojing Jing & Xiangqian Shen & Haojie Song & Fuzhan Song
Received: 10 November 2010 / Accepted: 30 March 2011 / Published online: 20 April 2011 # Springer Science+Business Media B.V. 2011
Abstract Barium ferrite fibers/poly(vinylidene fluoride) (BaFe12O19/PVDF) composite films were prepared from poly(vinylidene fluoride) (PVDF) resin with different weight percentages (1, 5, 10, 20, 30 wt%) of M-type barium ferrite fibers using N,N-dimethylformamide as a solvent. The structure and morphology of the BaFe12O19/ PVDF composite films were characterized by scanning electron microscopy and X-ray diffraction. These results show that BaFe12O19 fibers with diameters around 1 μm and an aspect ratio (length/diameter) of about 50 are well dispersed in the PVDF resin and the dispersed fibers result in a structural change of the PVDF from α to β phase. Measurements of the magnetization of the composite films by using a vibrating sample magnetometer show that these BaFe12O19/PVDF composite films possess a hard magnetic characteristic. The specific saturation magnetization and dielectric loss increase with BaFe12O19 content, whereas the coercivity and dielectric constant of the composite films are less affected. These BaFe12O19/PVDF composite films can combine magnetic, dielectric, and mechanical properties of the BaFe12O19 and PVDF phases. Keywords Barium ferrite fibers . Poly(vinylidene fluoride) (PVDF) . Composite films . Magnetic property . Dielectric property
Introduction Poly(vinylidene fluoride) (PVDF) and its copolymers have been extensively studied over the last three decades owing X. Jing : X. Shen (*) : H. Song : F. Song School of Materials Science and Engineering, Jiangsu University, Zhenjiang 212013, China e-mail:
[email protected]
to their excellent pyroelectric and piezoelectric properties [1–3]. PVDF is a semicrystalline polymer with uncommon polymorphism among polymeric materials, as it shows at least four crystalline phases called α, β, γ, and δ [4, 5]. The β-PVDF phase exhibits very good piezoelectric, pyroelectric, and dielectric properties. PVDF has shown great potential as a polymeric matrix for functional applications. Magalhães et al. [6] prepared electroactive β-PVDF membranes by the isothermal crystallization route. Dillon et al. [7] showed that β-PVDF could be obtained in all the nanocomposites (PVDF-nanoclay) regardless of the nanoclay morphologies and contents. Both crystallization and melting temperatures of PVDF were increased with the addition of nanoclay, possibly because of the formation of the β-phase PVDF. Although a great number of works were reported on the preparation and characterization of different PVDF phases, the properties available can not meet the needs of the modern communications industry and electronic technology development. The barium ferrite (BaFe12O19) has a stable hexagonal magnetoplumbite (M-type) structure, with high coercive force and energy, uniaxial magnetocrystalline anisotropy, high magnetic loss tangent, etc. Its absorption of electromagnetic waves largely relies on the hysteresis loss, domain wall resonance, and residual loss magnetic mechanism [8], with a strong absorption and a wide frequency bandwidth, and the barium ferrite materials can be applied in millimeter-wave devices, such as millimeter-wave circulators and isolators [9, 10]. However discrete devices such as traditional microwave circulators and isolators are difficult to integrate into advanced microwave systems. The key to solve this problem technologically is to develop functional thin or thick films instead of blocks. Chen et al. [11] prepared barium ferrite permanent thick films by the screen-printing technique. The barium ferrite thick film fabricated by hot press sintering at 1,200 °C has promising
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properties of a high saturation magnetization, and the remanence ratio can reach up to 0.95. But, because of their ceramic characteristics, large area barium ferrite films are difficult to obtain and this limits their applications. The preparation of composites of inorganic and polymeric matter is an important route to obtain multifunctional films. Kobayashi et al. [12] reported high dielectric thin films comprising hybrids of nanometer-sized barium titanate (BaTiO3) and PVDF. These BaTiO3/PVDF composite films had a high dielectric constant and a dissipation factor of as low as 0.05 at 104 Hz. But, up to now, there has been no report on the BaFe12O19/PVDF composite films. Our present work reports the preparation of BaFe12O19/PVDF composite films with excellent dielectric and magnetic properties by the solution casting method. The structure and morphology of the composite films were studied by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furthermore, their magnetic and dielectric properties were analyzed. These BaFe12O19/PVDF composite films can be used in multifunctional sensors.
Experimental Preparation of barium ferrite fibers Barium ferrite fibers were prepared by the organic gel precursor transformation method and the details were described in our previous work [13]. The starting reagents were analytical grade Ba (NO3)2, Fe (NO3)3·9H2O, and citric acid. According to the stoichiometry, citric acid and metal nitrates were dissolved in deionized water with continuous magnetic stirring. The final solution was magnetically stirred for 20–24 h at room temperatures and was transferred to a rotary evaporator and evaporated in a vacuum at 60–70 °C to remove surplus water until a viscous liquid was obtained. The gel fibers were drawn from the gel by the drawing machine and dried in a vacuum oven at 80 °C for about 24 h. The dried gel fibers were then put in an alumina crucible and subsequently were calcined at 900 °C for 2 h under an ambient atmosphere to form the ferrite fibers. Figure 1 shows the SEM morphology of the barium ferrite fiber calcined at 900 °C, with diameters around 1 μm and a ratio of length to diameter of about 50.
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Fig. 1 SEM morphology of barium ferrite fibers calcined at 900 °C
DMF with continuous magnetic stirring at 50 °C. The initial concentration of solution was 20 wt% of PVDF. The barium ferrite fibers were first ultrasonically dispersed in DMF. Then the well-dispersed barium ferrite fibers and PVDF solutions were mixed and stirring for 5 h to produce a homogeneous BaFe12O19/PVDF suspension. The content of BaFe12O19 in the composite films ranged from 0 to 30 wt%. This BaFe12O19/PVDF suspension was then cast on a mold and followed by a heat treatment at 70 °C in a vacuum oven for 3 h to remove the solvent. The BaFe12O19/PVDF composite films obtained had various contents of BaFe12O19 and a thickness of 0.5–0.7 mm. Characterization The XRD patterns of the fibers and composite films were collected on a BrukerD8 Advance powder diffractometer with CuKα radiation. SEM (performed on a Jeol JSM5600LV) was used to characterize the morphology of the fibers and composite films. The magnetic properties of the composite films were investigated at room temperatures using a vibrating sample magnetometer (VSM). The dielectric properties of composite films were characterized using a TH2828 LCR digital meter (Huizhou) at a frequency of 1 kHz at room temperature.
Results and discussion XRD analysis
Preparation of BaFe12O19/PVDF composite films The PVDF was bought from East Fluorine Chemical Technology Co. Ltd. (Shanghai) and used without a further purification. Chemical-grade N,N-dimethylformamide (DMF) was used as solvent. The BaFe12O19/PVDF composite films were prepared by the following procedure. The PVDF powder was dissolved in an appropriate amount of
Figure 2 shows the XRD patterns of pure BaFe12O19 (a), BaFe 12 O 19 /PVDF composite films with 30 wt% of BaFe12O19 (b), and pure PVDF (c). In Fig. 2a, peaks in the XRD spectrum of barium ferrite fibers are observed at 2θ=30.3, 32.2, 34.1, 37.1, 40.3, 55.0, 56.5, and 63.0, which belong the M-type BaFe12O19 (JCPDF No.84-0757), indicating the formation of the single M-type ferrite phase
Magnetic/dielectric properties of BaFe12O19/PVDF composite films
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Morphology of BaFe12O19/PVDF composite films
Fig. 2 XRD patterns of BaFe12O19 (a), BaFe12O19/PVDF composite films with 30 wt% of BaFe12O19 (b), and pure PVDF (c)
at 900 °C for 2 h. As seen in Fig. 2c, there are three broad peaks at 18.4, 19.9, and 26.6, which clearly correspond to α-phase PVDF [7]. The broad peaks mean that the PVDF crystallite size is small. In Fig. 2b, it can be observed that the corresponding peak strength of the barium ferrite fibers is decreased relative to that in Fig. 2a, and a broad peak occurs only at 20.4, which can be attributed to β-PVDF [14]. It can be deduced that the formed composite film consists of a binary phase of M-type barium ferrite and β-PVDF. It is interesting to note that the dispersed barium ferrite fibers in the PVDF matrix result in a phase transformation from α- to β-PVDF. A similar phenomenon was observed by Dillon et al. [7] for the PVDF/nanoclay nanocomposite. Fig. 3 SEM images of bare PVDF (a) and BaFe12O19/ PVDF composite films with different barium ferrite contents (wt%): 5 (b); 10 (c); 30 (d)
Figure 3 shows that the SEM morphology of BaFe12O19/ PVDF composite films with different barium ferrite contents of 0 wt%, 5 wt%, 10 wt%, and 30 wt%. It can be seen that the bare PVDF film (Fig. 3a) is composed of spherelike particles of about 12 μm in diameter, with many voids among the particles. At lower barium ferrite contents (5 wt% and 10 wt%; Fig. 3b, c), the number of the film surface voids is reduced and the barium ferrite fibers are well dispersed in the PVDF matrix, with a good compatibility between the barium fibers and PVDF matrix. Whereas, at the high content of BaFe 12 O 19 fibers (30 wt%; Fig. 3d), the fibers in the composite film tend to aggregate and the film has a relatively rough surface. Magnetic properties The hysteresis loops of BaFe12O19/PVDF composite films with different ferrite contents are shown in Fig. 4. It can be seen that BaFe12O19/PVDF composite films have a hard magnetic characteristic. The specific saturation magnetization (Ms) of BaFe12O19/PVDF composite films continuously increases with increasing the BaFe12O19 content from 1 to 30 wt%, with an Ms value of 13.1 emu g−1 for the BaFe12O19/PVDF composite films with 30 wt% barium ferrite fibers (Ms =62.3 emu g−1 for pure BaFe12O19), which is very consistent with results reported by Makled et al. [15]. According to the equation Ms = φms, Ms is related to the
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Fig. 4 Hysteresis loops of BaFe12O19/PVDF composite films with different barium ferrite contents (wt%): a 1; b 5; c 10; d 20; and e 30
volume fraction of the magnetic particles (φ) and the saturation moment (ms) of a single particle [16]. The saturation magnetization of BaFe12O19/PVDF composite films depends mainly on the fraction of the magnetic BaFe12O19 fibers. Besides, as the PVDF is nonmagnetic, it plays a part in isolating the magnetic fibers, leading to the transformation of the collinear ferromagnetic order of the ferrite into a noncollinear arrangement with a disruption of ferromagnetic order. Because PVDF is the matrix, the demagnetization effect is notable. Therefore, the specific saturation magnetization for the composite films is lower than that of pure barium ferrite fibers, and increases with barium ferrite fibers contents. However, the coercivity (Hc) of the composite films exhibits a different behavior from the saturation magnetization, and it is less influenced by the barium ferrite fibers content (ranging from 0 to 30 wt%), with a value of about 5,045–5,144 Oe (Hc =5,001 Oe for pure BaFe12O19). Many factors affect the composite film coercivity such as interface structure, constitution, particle size and shape, magnetic anisotropy, and magnetostriction [17]. The coercivity of the composite films is slightly higher than that of the pure BaFe12O19. This can be mainly attributed to the interface structure because of the PVDF segregation among the barium ferrite fibers. This nonmagnetic phase of PVDF acts as a barrier for domain interactions and domain misalignment, its effect in some degree is similar to nonmagnetic oxide additives in ferrites [18, 19], which results in a higher coercivity.
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Fig. 5 Effect of barium ferrite content on dielectric constant and dielectric loss of BaFe12O19/PVDF composite films, measured at 1 kHz
content (ca. 1.0 wt%), the dielectric constant of the composite films is dramatically increased to about 12.8, compared to that for the pure PVDF (7.7) [20]. It is known that β-phase PVDF has very good dielectric properties. So this increase can be attributed to the addition of barium ferrite fibers and the PVDF phase transformation as evidenced by the above XRD analysis. Then, the further increase of BaFe12O19 content up to 30 wt% has little influence on the dielectric constant of the composite films, whereas the dielectric loss increases remarkably with increasing the BaFe12O19 content, possibly arising from the interface effect and the porosity in BaFe12O19/PVDF composite films [21]. Figure 6 shows the dependence of the dielectric constant and dielectric loss of BaFe12O19/PVDF composite films with 10 wt% barium ferrite fibers as a function of frequency (log f) in the range of 50 Hz to 1 MHz. Figure 6 shows that the dielectric constant of the composite films basically
Dielectric properties The effect of BaFe12O19 content on the dielectric constant and dielectric loss of BaFe12O19/PVDF composite films is shown in Fig. 5. At a very low content of BaFe12O19
Fig. 6 Frequency dependence of dielectric constant and dielectric loss of BaFe12O19/PVDF composite films (10 wt% of BaFe12O19)
Magnetic/dielectric properties of BaFe12O19/PVDF composite films
decreases with frequency owing to interfacial polarizations [22]. Whereas, at the frequency range of 50 to 10 kHz, the dielectric loss initially decreases continuously, reaches its minimum value near 10 kHz, and then exhibits an obvious increase at frequencies up to 1 MHz.
Conclusions BaFe12O19/PVDF composite films with various BaFe12O19 contents were successfully prepared by the solution casting method with M-type barium ferrite fibers and PVDF resin. The XRD and SEM characterization indicates that the barium ferrite microfibers are well dispersed in the PVDF matrix and the PVDF structure is transformed from α to β phase arising from the filled BaFe12O19 fibers. The composite films have a hard magnetic characteristic, with a high coercivity of about 5,045 Oe (Hc =5,001 Oe for pure BaFe12O19 ). The specific saturation magnetization and dielectric loss of BaFe12O19/PVDF composite films continuously increase with increasing the BaFe12O19 content, whereas the coercivity and dielectric constant of the composite films are less affected by the BaFe12O19 content. With the frequency increase, the dielectric constant is reduced, and the dielectric loss initially decreases, reaches its minimum value near 10 kHz, and then exhibits an obvious increase up to 1 MHz. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 50674048), China Postdoctoral Science Foundation (Grant No. 20090451169), Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20103227110006), and the Jiangsu Province's Postgraduate Cultivation and Innovation Project (Grant No.CX10B-257Z).
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