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Preparation and microwave absorbing property of Ni–Zn ferrite-coated hollow glass microspheres with polythiophene
Journal of Magnetism and Magnetic Materials 417 (2016) 349–354
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation and microwave absorbing property of Ni–Zn ferrite-coated hollow glass microspheres with polythiophene Lindong Li, Xingliang Chen, Shuhua Qi n Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 25 May 2016 Accepted 30 May 2016 Available online 31 May 2016
The composite of hollow glass microspheres (HMG) coated by Ni0.7Zn0.3Fe2O4 particles was fabricated via sol–gel method, and then the ternary composite (HMG/Ni0.7Zn0.3Fe2O4/PT) was synthesized by in situ polymerization. The electrical property, magnetic performance and reflection loss of the composites were measured, and the results suggest that the conductivity and the saturation magnetization (Ms) of HMG/Ni0.7Zn0.3Fe2O4/PT reach 6.87 10 5 S/cm and 11.627 emu/g, respectively. The ternary composite has good microwave absorbing properties (Rmin ¼ 13.79 dB at 10.51 GHz) and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz) in X band (8.2–12.4 GHz). The morphology and chemical structure of the samples were measured through scanning electron microscopy (SEM), X-Ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). This paper also analyzes the relationship between the reflection loss of the absorber and its thickness. & 2016 Published by Elsevier B.V.
Keywords: Microwave absorption Reflection loss Hollow glass microsphere Polythiophene
1. Introduction With the rapid development of modern science and technology, especially in electronic industrial technology, people's living spaces are filled with different frequencies of electromagnetic radiation, the ecological environment have been destroyed, and electromagnetic pollution has been created [1,2]. Only by using absorbent materials can the electromagnetic waves be converted into heat energy or other forms of energy, in order to effectively eliminate the electromagnetic pollution [3]. So the research and application of absorbent materials is paid great attention [4]. Because the ferrite has the advantages of high efficiency absorption and thin coating, the research and application of the ferrite has a long history [5]. Because of its relatively high density, ferrite makes parts heavier, which affects the overall performance. Its absorption at high frequencies is also not ideal. Furthermore, ferrite's electromagnetic parameters make it difficult to bring relative magnetic permeability as close as possible to the principle of the relative dielectric constant [6]. It is difficult for a single material to meet the requirements of broad band-width and low density. Often ferrite powder is dispersed into other materials to make composite materials [7,8]. By adjusting ferrite grain size, composition, and so on, the electromagnetic parameters are changed, in order to improve the absorbing properties of ferrite [9]. For instance, Liu et al. successfully synthesized hollow glass microspheres/Fe3O4 n
Corresponding author. E-mail address: [email protected] (S. Qi).
http://dx.doi.org/10.1016/j.jmmm.2016.05.101 0304-8853/& 2016 Published by Elsevier B.V.
(HGMs/Fe3O4) composites with low density by solvothermal method. When the thicknesses of these HGMs/Fe3O4 composites are more than 1.5 mm, they all exhibit strong absorption peaks (lower than 10 dB). A possible mechanism of the improved microwave absorption properties was discussed [10]. The main chemical composition of hollow glass microspheres is SiO2 and Al2O3.With stable chemical performance, high temperature resistance and other properties, it is a good base material for preparing absorbent materials [11,12]. But it is lack of dielectric and magnetic loss properties. Hollow glass microspheres coated with the ferrite can solve the above mentioned problems in preparing absorbent material [13]. Fu et al. reported that spinel CoFe2O4 coating on the surface of HGMs with low density was synthesized by co-precipitation method and a remarkable absorption (about 8.5 dB) appeared at 18 GHz [12]. With the advantages of low density, adjustable electromagnetic parameters, good compatibility, easily processed molding and industrialized production, etc., conductive polymer materials have been widely applied in the research of absorbing materials [14]. For the lack of magnetic loss and its narrow absorbing frequency, the absorbing intensity and bandwidth of polymer material cannot meet the needs of the practical application [15]. Wang et al. proposed a route of controllable deposition of Fe3O4 and polyaniline on HGMs and study its conductive and magnetic properties [16]. The idea of this research was that through materials' multiple absorbing mechanisms, the absorption band can significantly increase, the maximum reflection loss also increases significantly, and the effective absorption bandwidth of the materials can be broadened.
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HMG was obtained. After heat treatment at 450 °C for 2 h and 850 °C for 2 h in the muffle furnace, the desired sample HMG/Ni0.7Zn0.3Fe2O4 composites was prepared.
2. Experiment 2.1. Materials In this study, thiophene (C4H4S, 99%) was purchased from Aladdin Chemistry Co. Ltd. Thiophene was freshly distilled prior to use. The hollow glass microspheres (HMG) were purchased from the Yongqing Shunchang glass beads Co. Ltd, Hebei, China. An international researcher can order HMG from PQ (PHiladelphea Quart Company) or 3 M (Minnesota Mining and Manufacturing Company) and refer to the previous reports about the preparation method of HMG [17]. Nickel nitrate (Ni(NO3)2 6H2O), citric acid (C6H8O7), iron nitrate (Fe(NO3)3 9H2O), Zinc nitrate (Zn(NO3)2 6H2O), methylene chloride (CH2Cl2) and sodium hydroxide (NaOH) of analytical reagent grade were obtained from the Tianjin Fuyu Fine Chemical Co. Ltd. All reagents were used without further purification and used as received unless otherwise noted. Distilled water was used in all the experiments.
2.2.3. Preparation of HMG/Ni0.7Zn0.3Fe2O4/PT composites 0.25 g HMG/Ni0.7Zn0.3Fe2O4 was added into 50 mL chloroform and carry out ultrasonic treatment for 1 h. 3.8554 g FeCl3 was added into the solution and undergo ultrasonic dispersion for 30 min. 0.5 g Th monomer was added drop by drop into the above mixture, and reacted for 10 h under ice–water bath. The mass ratio among HMG, Ni0.7Zn0.3Fe2O4 and PT is 1:2:6. After that, the solution was filtered by a water circulating multi-purpose vacuum pump. Using the purification method of polythiophene (PT), the filtered solid matter was purified [19]. The solid powders were washed with deionized water until the pH value of the filtrate reached 7. Finally, the HMG/Ni0.7Zn0.3Fe2O4/PT powders were dried under vacuum at 60 °C for 24 h (Fig. 1). 2.3. Characterization
2.2. The preparation of the absorbent material The FT-IR spectra was obtained by a WQF-510 FTIR spectrometer (Ruili, China) with the KBr method. The structure was measured by an X-ray diffractometer (XRD, PANalytical, Holland) with Cu Kα radiation in the 2θ range from 5° to 85°. The surface morphologies of HMG, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT were checked by a scanning electron microscope (SEM; JSM-6390, HITACHI, Japan). The conductivities of samples were measured by a SZ-82 digital four probes resistance tester (Suzhou Electronic Equipment Factory, China). The products were prepared as circular samples with a diameter of 15 mm and a thickness of 2 mm by casting them in stainless forms and cold-pressing to measure electrical properties. The magnetic properties and the electromagnetic parameters were analyzed by a Lake Shore 7307 vibrating sample magnetometer (VSM) and a HP8753 Dvector network analyzer, respectively.
2.2.1. Surface treatment of hollow glass microspheres In the preparation process, there are residual organic groups on the surface of hollow glass microspheres. These groups will affect the ferrite in the deposition and the uniformity of coating on the surface of the HMG. Therefore, it needs to be surface-treated before use. Processing method is as follows: 10 g HMG was soaked in 100 mL CH2Cl2 solution for 10 min to remove residual organics on the surface of HMG. After washing with water, HMG was put into 100 mL NaOH solution (0.5 mol/L) and ultrasonic cleaning for 1 h to improve the surface activity of HMG. Then, HMG was washed with water and dried for the next step. 2.2.2. Preparation of HMG/Ni0.7Zn0.3Fe2O4 composites The Ni–Zn ferrite magnetic material was prepared by the sol–gel auto-combustion method, as reported elsewhere [18]. 1.7222 g Ni(NO3)2 6H2O, 0.755 g Zn(NO3)2 6H2O and 6.8362 g Fe(NO3)3 9H2O were dissolved in 100 mL deionized water. Adding 4.8769 g citric acid in the above solution, the pH was adjusted with ammonia to a value of about 7, then evaporated and stirred at 70 °C, until the sol reached a certain viscosity, at which point the pre-treated HMG (4 g) was added into the sol, continuously stirred for 2 h and dried. A dry gel containing
3. Results and discussion 3.1. Surface morphology SEM analysis was used to investigate the surface morphologies of HMG, pure PT, HMG/ Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT
(a) S
FeCl3,0
2n S
CHCl3
S
n
(b)
Fig. 1. Schematic illustration of the procedure for (a) polymerization of thiophene and to and (b) synthesize HMG/Ni0.7Zn0.3Fe2O4/PT composites.
L. Li et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 349–354
3.2. X-ray diffraction Phase investigation of the products was performed by XRD, and Fig. 3 presents the XRD patterns of PT, Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4 /PT. From Fig. 3(a), it can be seen that the characteristic diffraction peak of PT is at 2θ ¼ 17° 23° which presents a broad, amorphous diffraction peak. Some diffraction peaks at 18.25°, 30.28°, 35.50°, 43.23°, 53.89°, 57.10° and 62.87° can be seen in Fig. 3(b) and (c), which relate to (111), (220), (311), (400), (422), (511) and (531) planes of the spinal crystal structure of Ni–Zn ferrite [20]. In addition, the peaks at 43.23° and 53.89° in Fig. 3(c) are stronger than them in Fig. 3(b), which may be the influence of glass beads. Simultaneously, the related diffraction peaks of HMG are not obviously shown in Fig. 2 (c), perhaps because more Ni–Zn ferrite coats the surface of HMG. In Fig. 3(d), the intensity of characteristic diffraction peaks of PT in
(311)
PT
(400)
(220)
(511)
(d)
(531)
(311) (220)
(400)
(531) (511)
(111)
Intensity (a.u.)
composites. The SEM micrographs of these materials are shown in Fig. 2. It can be observed that the surface of hollow glass microspheres are very smooth, and there are some holes left on the surface after pretreatment, as shown in Fig. 2(a, f). Fig. 2(b) shows the irregularly stacked PT. Fig. 2(c, d) indicates that Ni0.7Zn0.3Fe2O4 nanoparticles coat on the surfaces of HMG. Fig. 2(d) is SEM image of Ni0.7Zn0.3Fe2O4 coating on the broken hollow glass microsphere, which further illustrates the morphology after combination of HMG and Ni0.7Zn0.3Fe2O4. From Fig. 2(e), we can clearly see that PT, which has a granular-like morphology, forms disorderly stacks on the HMG/Ni0.7Zn0.3Fe2O4 particles in the ternary composite's structure. It can be seen from the SEM micrograph that a bridge was formed between the HMG/Ni0.7Zn0.3Fe2O4 and PT. Such special types of structures may be beneficial in improving the electrical properties of the ternary composite.
351
(c)
(422)
(311) (531) (220) (400)
(111)
(511)
(b)
(422)
PT
10
20
(a)
30
40 50 2 Theta (degree)
60
70
80
Fig. 3. XRD patterns of (a) PT, (b) Ni0.7Zn0.3Fe2O4, (c) HMG/Ni0.7Zn0.3Fe2O4 and (d) HMG/Ni0.7Zn0.3Fe2O4/PT composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
the composites is weakened and some diffraction peaks of Ni–Zn ferrite have also disappeared. The weak peak is attributed to the diluted crystal concentration of Ni–Zn ferrite. 3.3. FT-IR analysis Fig. 4 shows the FT-IR spectra of the PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites. In Fig. 4(a), there is a less intense peak in the range of 2800–3100 cm 1 due to the aromatic C–H stretching vibrations. The peak at 1706, 1645, 1003, 790, 701 and 472 cm–1 is the fingerprint peaks of PT. Owing to the partial oxidization of the PT ring, there was a carbonyl stretching
Fig. 2. SEM images of (a) HMG, (b) PT (c, d), HMG/Ni0.7Zn0.3Fe2O4, (e) HMG/Ni0.7Zn0.3Fe2O4/PT composites and (f) untreated HMG.
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electromagnetic characteristics. Electromagnetic waves can go into composites, which can enhance their absorbing properties.
(c)
Transmittance (%)
3.5. Magnetic property
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 4. FT-IR spectra of (a) PT, HMG/Ni0.7Zn0.3Fe2O4/PT composites.
(b)
HMG/Ni0.7Zn0.3Fe2O4
and
(c)
vibration peak at 1706 cm 1. 1645 and 1003 cm 1 belongs to C–C asymmetric stretching vibration of the thiophene ring and C–H in the bending band, respectively. 472 cm 1 is assigned to C–S–C ring deformation [19,21]. From Fig. 4(b), it can be seen that the FT-IR spectrum of the Ni–Zn ferrite and HMG exhibits the characteristic absorption at about 500 to 1200 cm 1. The peaks at 1080 and 795 cm 1 are the characteristic absorption peaks of the glass beads. 1080 cm 1 is attributed to the stretching vibration of Si–O. 795 cm 1 is due to the Al–O stretching vibration mode [16]. 577 cm 1 is the Ni–Zn ferrite characteristic absorption peak, which is attributed to the Zn–O stretching mode [17]. Compared with Fig. 3(b), 1402 cm 1, which belongs to symmetric stretching vibration of thiophene ring, exhibits red shift in Fig. 3(c). In Fig. 4 (c), we can also see that the peak at 1004 cm 1 splits into two smaller peaks and the strength of 801 and 578 cm 1 increases and has slight red shift. Moreover, there is a new weak absorption peak at about 1303 cm 1, which may belong to new atomic vibrations. These indicate that there are some interactions between Ni–Zn ferrite and PT and the composites are composed of HMG, Ni–Zn ferrite and PT. 3.4. Electrical conductivity The electrical conductivity values of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites are listed in Table 1. The conductivity of HMG/Ni0.7Zn0.3Fe2O4 (5.32 10 7 S/cm) indicates that it is not a conductive material, but the introduction of PT into HMG/Ni0.7Zn0.3Fe2O4 has enhanced the electrical properties and the conductivity of composites reaches 6.87 10 5 S/cm [22]. After combining PT with HMG/Ni0.7Zn0.3Fe2O4 together, electrons are able to move freely through the network in composites. So the conductivity of composites increases from 5.32 10 7 S/cm to 6.87 10 5 S/cm, which is higher than that of HMG/Ni0.7Zn0.3Fe2O4. After combining PT and HMG/Ni0.7Zn0.3Fe2O4, it increases the conductivity of composites and achieves a good match between Table 1 The electrical conductivity of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites. Samples
Conductivity (S/cm)
PT 7.35 10 4 HMG/Ni0.7Zn0.3Fe2O4 5.32 10 7 HMG/ Ni0.7Zn0.3Fe2O4/PT 6.87 10 5
Fig. 5 shows that the hysteresis loops of the samples were measured by VSM at room temperature. The hysteresis loop of PT shows very weak saturation magnetization (Ms¼1.7873 emu/g) and suggests that PT shows paramagnetism. In addition, Fig. 5 shows that Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites have weak remanence and coercivity, indicating that they are soft magnetic materials, and their Ms values are 33.142, 18.994 and 11.627 emu/g, respectively. Due to low field-related error, there are several points where jumping occurred in low field, which is difficult to avoid in the measurement process. According to the equation Ms¼ ϕms, Ms is related to the volume fraction of the particles (ϕ), and the saturation moment of a single particle (ms). The Ms of composites is mainly due to the volume fraction of the magnetic Ni–Zn ferrite. The Ms of the composites decreases compared to that of Ni–Zn ferrite. The phenomenon can be attributed to the presence of PT and HMG in composites. 3.6. Microwave absorbing properties Electromagnetic wave absorbing properties of the material mainly is characterized by reflection loss (R). According to the transmit-line theory, when the electromagnetic wave vertically incidents upon the samples, the reflection loss can be calculated by the following equations:
R (dB) = 20 log
Zin =
Zin − 1 Zin + 1
⎛ 2πfd ⎞ μr tanh ⎜ j μ r εr ⎟ ⎝ c ⎠ εr
(1)
(2)
where Zin is the normalized input impedance relating to the impedance in free space; ϵr =ϵ′−j″ and μr =μ′−j″ are the relative complex permeability and permittivity of the material; d is the thickness of the absorber; c and f are the velocity of light and the frequency of microwave, respectively. It indicates that the absorption capacity of the materials is closely related to electromagnetic parameters ε′, ε′′, μ′, μ′′, d and f. The reflection loss of the same material at the same frequency depends mainly on the thickness of the material [23]. Fig. 6 shows the reflection losses of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites at the best match thickness. PT is not a good microwave absorber. The reason may be that the reflection loss of PT mainly depends on polarized properties and dielectric loss. The HMG/Ni0.7Zn0.3Fe2O4/PT composite exhibits the best microwave absorption performance in comparison with PT and HMG/Ni0.7Zn0.3Fe2O4. The excellent absorbing property of composites is attributed to the following factors. On the one hand, PT increases the dielectric loss of composites. It enhances the complex permittivity, improves impedance matching of composites and reduces the reflection of electromagnetic waves on the incident interface. On the other hand, electromagnetic wave can be reflected many times in HMG after passing through the coating and absorbed many times by Ni–Zn ferrite and PT [13]. It strengthens the absorption of electromagnetic waves that have entered the media and avoided the electromagnetic wave returning to the interface. Moreover, electromagnetic wave attenuation could be caused by hysteresis loss, cavity effect and so on, which further enhance the microwave absorption ability of composites. In Fig. 7, it is obvious that the best match thickness of
L. Li et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 349–354
353
Fig. 5. Magnetization hysteresis loss of (a) Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4/PT composites and (b) PT.
0
(a)
-2
-2 (b)
Reflection Loss (dB)
Reflection Loss (dB)
-4 -6 -8
(c)
-10 -12
-4 -6 -8 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm
-10 -12
-14
-14 8
9
10
11
12
8
Frequecy (GHz) Fig. 6. Reflection loss of (a) PT, (b) HMG/Ni0.7Zn0.3Fe2O4 HMG/Ni0.7Zn0.3Fe2O4/PT composites at the best match thickness.
(
)1/2
10
11
12
Frequency (GHz) and
(c)
composites is 3.0 mm in X band and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz). The minimum reflection loss reaches 13.79 dB at 10.51 GHz. When the thickness of the absorber increases, the minimum value of reflection loss appears in different frequency. The dips of minimal reflection for composites are also shifted towards a lower frequency with an increase in thickness. This phenomenon is caused by the resonance of electromagnetic waves interfering with the absorber. The propagating wavelength in a material ( λm ) is expressed by the following equation:
λm = λ o/ μ r εr
9
(3)
where λο is the free space wavelength and μr and εr are the moduli of μr and εr , respectively [24]. When the thickness of the absorber is equal to an odd multiple of a quarter wavelengths of electromagnetic waves, the resonance of electromagnetic waves interfering with the absorber will occur. When electromagnetic waves reflected on the upper surface and electromagnetic waves on the lower surface are superimposed, interference occurs and weakens the energy of the electromagnetic wave. With the thickness increasing, the absorption peaks appear successively. When the frequency of the electromagnetic wave increases, the wavelength of the electromagnetic wave becomes shorter. Therefore, when the frequency is higher, the absorption peak shifts to the lower frequency [25]. According to the above theory, no matter how poor the absorbing property of the material is, because of resonant interference, it will have an absorption peak. Therefore, according to the range of the frequency, it is necessary to control
Fig. 7. Reflection loss of HMG/Ni0.7Zn0.3Fe2O4/PT composites at the different thicknesses.
the thickness of the absorbing material in order to design absorbing material.
4. Conclusions In this article, we synthesized the ternary composite (HMG/Ni0.7Zn0.3Fe2O4/PT) and its properties were measured and analyzed. The results indicate that HMG can be coated by Ni0.7Zn0.3Fe2O4 and PT successfully. Compared with HMG/Ni0.7Zn0.3Fe2O4, the conductivity of the composite increases and reaches 6.87 10 5 S/cm. The Ms value of it is 11.627 emu/g. At the best match thickness (3.0 mm), the minimum reflection loss of the composite reaches 13.79 dB at 10.51 GHz and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz). It appears that the ternary composite can be a candidate for use as a kind of absorber.
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Preparation and microwave absorbing property of Ni–Zn ferrite-coated hollow glass microspheres with polythiophene Lindong Li, Xingliang Chen, Shuhua Qi n Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an 710072, China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 January 2016 Received in revised form 25 May 2016 Accepted 30 May 2016 Available online 31 May 2016
The composite of hollow glass microspheres (HMG) coated by Ni0.7Zn0.3Fe2O4 particles was fabricated via sol–gel method, and then the ternary composite (HMG/Ni0.7Zn0.3Fe2O4/PT) was synthesized by in situ polymerization. The electrical property, magnetic performance and reflection loss of the composites were measured, and the results suggest that the conductivity and the saturation magnetization (Ms) of HMG/Ni0.7Zn0.3Fe2O4/PT reach 6.87 10 5 S/cm and 11.627 emu/g, respectively. The ternary composite has good microwave absorbing properties (Rmin ¼ 13.79 dB at 10.51 GHz) and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz) in X band (8.2–12.4 GHz). The morphology and chemical structure of the samples were measured through scanning electron microscopy (SEM), X-Ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). This paper also analyzes the relationship between the reflection loss of the absorber and its thickness. & 2016 Published by Elsevier B.V.
Keywords: Microwave absorption Reflection loss Hollow glass microsphere Polythiophene
1. Introduction With the rapid development of modern science and technology, especially in electronic industrial technology, people's living spaces are filled with different frequencies of electromagnetic radiation, the ecological environment have been destroyed, and electromagnetic pollution has been created [1,2]. Only by using absorbent materials can the electromagnetic waves be converted into heat energy or other forms of energy, in order to effectively eliminate the electromagnetic pollution [3]. So the research and application of absorbent materials is paid great attention [4]. Because the ferrite has the advantages of high efficiency absorption and thin coating, the research and application of the ferrite has a long history [5]. Because of its relatively high density, ferrite makes parts heavier, which affects the overall performance. Its absorption at high frequencies is also not ideal. Furthermore, ferrite's electromagnetic parameters make it difficult to bring relative magnetic permeability as close as possible to the principle of the relative dielectric constant [6]. It is difficult for a single material to meet the requirements of broad band-width and low density. Often ferrite powder is dispersed into other materials to make composite materials [7,8]. By adjusting ferrite grain size, composition, and so on, the electromagnetic parameters are changed, in order to improve the absorbing properties of ferrite [9]. For instance, Liu et al. successfully synthesized hollow glass microspheres/Fe3O4 n
Corresponding author. E-mail address: [email protected] (S. Qi).
http://dx.doi.org/10.1016/j.jmmm.2016.05.101 0304-8853/& 2016 Published by Elsevier B.V.
(HGMs/Fe3O4) composites with low density by solvothermal method. When the thicknesses of these HGMs/Fe3O4 composites are more than 1.5 mm, they all exhibit strong absorption peaks (lower than 10 dB). A possible mechanism of the improved microwave absorption properties was discussed [10]. The main chemical composition of hollow glass microspheres is SiO2 and Al2O3.With stable chemical performance, high temperature resistance and other properties, it is a good base material for preparing absorbent materials [11,12]. But it is lack of dielectric and magnetic loss properties. Hollow glass microspheres coated with the ferrite can solve the above mentioned problems in preparing absorbent material [13]. Fu et al. reported that spinel CoFe2O4 coating on the surface of HGMs with low density was synthesized by co-precipitation method and a remarkable absorption (about 8.5 dB) appeared at 18 GHz [12]. With the advantages of low density, adjustable electromagnetic parameters, good compatibility, easily processed molding and industrialized production, etc., conductive polymer materials have been widely applied in the research of absorbing materials [14]. For the lack of magnetic loss and its narrow absorbing frequency, the absorbing intensity and bandwidth of polymer material cannot meet the needs of the practical application [15]. Wang et al. proposed a route of controllable deposition of Fe3O4 and polyaniline on HGMs and study its conductive and magnetic properties [16]. The idea of this research was that through materials' multiple absorbing mechanisms, the absorption band can significantly increase, the maximum reflection loss also increases significantly, and the effective absorption bandwidth of the materials can be broadened.
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HMG was obtained. After heat treatment at 450 °C for 2 h and 850 °C for 2 h in the muffle furnace, the desired sample HMG/Ni0.7Zn0.3Fe2O4 composites was prepared.
2. Experiment 2.1. Materials In this study, thiophene (C4H4S, 99%) was purchased from Aladdin Chemistry Co. Ltd. Thiophene was freshly distilled prior to use. The hollow glass microspheres (HMG) were purchased from the Yongqing Shunchang glass beads Co. Ltd, Hebei, China. An international researcher can order HMG from PQ (PHiladelphea Quart Company) or 3 M (Minnesota Mining and Manufacturing Company) and refer to the previous reports about the preparation method of HMG [17]. Nickel nitrate (Ni(NO3)2 6H2O), citric acid (C6H8O7), iron nitrate (Fe(NO3)3 9H2O), Zinc nitrate (Zn(NO3)2 6H2O), methylene chloride (CH2Cl2) and sodium hydroxide (NaOH) of analytical reagent grade were obtained from the Tianjin Fuyu Fine Chemical Co. Ltd. All reagents were used without further purification and used as received unless otherwise noted. Distilled water was used in all the experiments.
2.2.3. Preparation of HMG/Ni0.7Zn0.3Fe2O4/PT composites 0.25 g HMG/Ni0.7Zn0.3Fe2O4 was added into 50 mL chloroform and carry out ultrasonic treatment for 1 h. 3.8554 g FeCl3 was added into the solution and undergo ultrasonic dispersion for 30 min. 0.5 g Th monomer was added drop by drop into the above mixture, and reacted for 10 h under ice–water bath. The mass ratio among HMG, Ni0.7Zn0.3Fe2O4 and PT is 1:2:6. After that, the solution was filtered by a water circulating multi-purpose vacuum pump. Using the purification method of polythiophene (PT), the filtered solid matter was purified [19]. The solid powders were washed with deionized water until the pH value of the filtrate reached 7. Finally, the HMG/Ni0.7Zn0.3Fe2O4/PT powders were dried under vacuum at 60 °C for 24 h (Fig. 1). 2.3. Characterization
2.2. The preparation of the absorbent material The FT-IR spectra was obtained by a WQF-510 FTIR spectrometer (Ruili, China) with the KBr method. The structure was measured by an X-ray diffractometer (XRD, PANalytical, Holland) with Cu Kα radiation in the 2θ range from 5° to 85°. The surface morphologies of HMG, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT were checked by a scanning electron microscope (SEM; JSM-6390, HITACHI, Japan). The conductivities of samples were measured by a SZ-82 digital four probes resistance tester (Suzhou Electronic Equipment Factory, China). The products were prepared as circular samples with a diameter of 15 mm and a thickness of 2 mm by casting them in stainless forms and cold-pressing to measure electrical properties. The magnetic properties and the electromagnetic parameters were analyzed by a Lake Shore 7307 vibrating sample magnetometer (VSM) and a HP8753 Dvector network analyzer, respectively.
2.2.1. Surface treatment of hollow glass microspheres In the preparation process, there are residual organic groups on the surface of hollow glass microspheres. These groups will affect the ferrite in the deposition and the uniformity of coating on the surface of the HMG. Therefore, it needs to be surface-treated before use. Processing method is as follows: 10 g HMG was soaked in 100 mL CH2Cl2 solution for 10 min to remove residual organics on the surface of HMG. After washing with water, HMG was put into 100 mL NaOH solution (0.5 mol/L) and ultrasonic cleaning for 1 h to improve the surface activity of HMG. Then, HMG was washed with water and dried for the next step. 2.2.2. Preparation of HMG/Ni0.7Zn0.3Fe2O4 composites The Ni–Zn ferrite magnetic material was prepared by the sol–gel auto-combustion method, as reported elsewhere [18]. 1.7222 g Ni(NO3)2 6H2O, 0.755 g Zn(NO3)2 6H2O and 6.8362 g Fe(NO3)3 9H2O were dissolved in 100 mL deionized water. Adding 4.8769 g citric acid in the above solution, the pH was adjusted with ammonia to a value of about 7, then evaporated and stirred at 70 °C, until the sol reached a certain viscosity, at which point the pre-treated HMG (4 g) was added into the sol, continuously stirred for 2 h and dried. A dry gel containing
3. Results and discussion 3.1. Surface morphology SEM analysis was used to investigate the surface morphologies of HMG, pure PT, HMG/ Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT
(a) S
FeCl3,0
2n S
CHCl3
S
n
(b)
Fig. 1. Schematic illustration of the procedure for (a) polymerization of thiophene and to and (b) synthesize HMG/Ni0.7Zn0.3Fe2O4/PT composites.
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3.2. X-ray diffraction Phase investigation of the products was performed by XRD, and Fig. 3 presents the XRD patterns of PT, Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4 /PT. From Fig. 3(a), it can be seen that the characteristic diffraction peak of PT is at 2θ ¼ 17° 23° which presents a broad, amorphous diffraction peak. Some diffraction peaks at 18.25°, 30.28°, 35.50°, 43.23°, 53.89°, 57.10° and 62.87° can be seen in Fig. 3(b) and (c), which relate to (111), (220), (311), (400), (422), (511) and (531) planes of the spinal crystal structure of Ni–Zn ferrite [20]. In addition, the peaks at 43.23° and 53.89° in Fig. 3(c) are stronger than them in Fig. 3(b), which may be the influence of glass beads. Simultaneously, the related diffraction peaks of HMG are not obviously shown in Fig. 2 (c), perhaps because more Ni–Zn ferrite coats the surface of HMG. In Fig. 3(d), the intensity of characteristic diffraction peaks of PT in
(311)
PT
(400)
(220)
(511)
(d)
(531)
(311) (220)
(400)
(531) (511)
(111)
Intensity (a.u.)
composites. The SEM micrographs of these materials are shown in Fig. 2. It can be observed that the surface of hollow glass microspheres are very smooth, and there are some holes left on the surface after pretreatment, as shown in Fig. 2(a, f). Fig. 2(b) shows the irregularly stacked PT. Fig. 2(c, d) indicates that Ni0.7Zn0.3Fe2O4 nanoparticles coat on the surfaces of HMG. Fig. 2(d) is SEM image of Ni0.7Zn0.3Fe2O4 coating on the broken hollow glass microsphere, which further illustrates the morphology after combination of HMG and Ni0.7Zn0.3Fe2O4. From Fig. 2(e), we can clearly see that PT, which has a granular-like morphology, forms disorderly stacks on the HMG/Ni0.7Zn0.3Fe2O4 particles in the ternary composite's structure. It can be seen from the SEM micrograph that a bridge was formed between the HMG/Ni0.7Zn0.3Fe2O4 and PT. Such special types of structures may be beneficial in improving the electrical properties of the ternary composite.
351
(c)
(422)
(311) (531) (220) (400)
(111)
(511)
(b)
(422)
PT
10
20
(a)
30
40 50 2 Theta (degree)
60
70
80
Fig. 3. XRD patterns of (a) PT, (b) Ni0.7Zn0.3Fe2O4, (c) HMG/Ni0.7Zn0.3Fe2O4 and (d) HMG/Ni0.7Zn0.3Fe2O4/PT composites. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
the composites is weakened and some diffraction peaks of Ni–Zn ferrite have also disappeared. The weak peak is attributed to the diluted crystal concentration of Ni–Zn ferrite. 3.3. FT-IR analysis Fig. 4 shows the FT-IR spectra of the PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites. In Fig. 4(a), there is a less intense peak in the range of 2800–3100 cm 1 due to the aromatic C–H stretching vibrations. The peak at 1706, 1645, 1003, 790, 701 and 472 cm–1 is the fingerprint peaks of PT. Owing to the partial oxidization of the PT ring, there was a carbonyl stretching
Fig. 2. SEM images of (a) HMG, (b) PT (c, d), HMG/Ni0.7Zn0.3Fe2O4, (e) HMG/Ni0.7Zn0.3Fe2O4/PT composites and (f) untreated HMG.
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electromagnetic characteristics. Electromagnetic waves can go into composites, which can enhance their absorbing properties.
(c)
Transmittance (%)
3.5. Magnetic property
(b)
(a)
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm ) Fig. 4. FT-IR spectra of (a) PT, HMG/Ni0.7Zn0.3Fe2O4/PT composites.
(b)
HMG/Ni0.7Zn0.3Fe2O4
and
(c)
vibration peak at 1706 cm 1. 1645 and 1003 cm 1 belongs to C–C asymmetric stretching vibration of the thiophene ring and C–H in the bending band, respectively. 472 cm 1 is assigned to C–S–C ring deformation [19,21]. From Fig. 4(b), it can be seen that the FT-IR spectrum of the Ni–Zn ferrite and HMG exhibits the characteristic absorption at about 500 to 1200 cm 1. The peaks at 1080 and 795 cm 1 are the characteristic absorption peaks of the glass beads. 1080 cm 1 is attributed to the stretching vibration of Si–O. 795 cm 1 is due to the Al–O stretching vibration mode [16]. 577 cm 1 is the Ni–Zn ferrite characteristic absorption peak, which is attributed to the Zn–O stretching mode [17]. Compared with Fig. 3(b), 1402 cm 1, which belongs to symmetric stretching vibration of thiophene ring, exhibits red shift in Fig. 3(c). In Fig. 4 (c), we can also see that the peak at 1004 cm 1 splits into two smaller peaks and the strength of 801 and 578 cm 1 increases and has slight red shift. Moreover, there is a new weak absorption peak at about 1303 cm 1, which may belong to new atomic vibrations. These indicate that there are some interactions between Ni–Zn ferrite and PT and the composites are composed of HMG, Ni–Zn ferrite and PT. 3.4. Electrical conductivity The electrical conductivity values of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites are listed in Table 1. The conductivity of HMG/Ni0.7Zn0.3Fe2O4 (5.32 10 7 S/cm) indicates that it is not a conductive material, but the introduction of PT into HMG/Ni0.7Zn0.3Fe2O4 has enhanced the electrical properties and the conductivity of composites reaches 6.87 10 5 S/cm [22]. After combining PT with HMG/Ni0.7Zn0.3Fe2O4 together, electrons are able to move freely through the network in composites. So the conductivity of composites increases from 5.32 10 7 S/cm to 6.87 10 5 S/cm, which is higher than that of HMG/Ni0.7Zn0.3Fe2O4. After combining PT and HMG/Ni0.7Zn0.3Fe2O4, it increases the conductivity of composites and achieves a good match between Table 1 The electrical conductivity of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites. Samples
Conductivity (S/cm)
PT 7.35 10 4 HMG/Ni0.7Zn0.3Fe2O4 5.32 10 7 HMG/ Ni0.7Zn0.3Fe2O4/PT 6.87 10 5
Fig. 5 shows that the hysteresis loops of the samples were measured by VSM at room temperature. The hysteresis loop of PT shows very weak saturation magnetization (Ms¼1.7873 emu/g) and suggests that PT shows paramagnetism. In addition, Fig. 5 shows that Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites have weak remanence and coercivity, indicating that they are soft magnetic materials, and their Ms values are 33.142, 18.994 and 11.627 emu/g, respectively. Due to low field-related error, there are several points where jumping occurred in low field, which is difficult to avoid in the measurement process. According to the equation Ms¼ ϕms, Ms is related to the volume fraction of the particles (ϕ), and the saturation moment of a single particle (ms). The Ms of composites is mainly due to the volume fraction of the magnetic Ni–Zn ferrite. The Ms of the composites decreases compared to that of Ni–Zn ferrite. The phenomenon can be attributed to the presence of PT and HMG in composites. 3.6. Microwave absorbing properties Electromagnetic wave absorbing properties of the material mainly is characterized by reflection loss (R). According to the transmit-line theory, when the electromagnetic wave vertically incidents upon the samples, the reflection loss can be calculated by the following equations:
R (dB) = 20 log
Zin =
Zin − 1 Zin + 1
⎛ 2πfd ⎞ μr tanh ⎜ j μ r εr ⎟ ⎝ c ⎠ εr
(1)
(2)
where Zin is the normalized input impedance relating to the impedance in free space; ϵr =ϵ′−j″ and μr =μ′−j″ are the relative complex permeability and permittivity of the material; d is the thickness of the absorber; c and f are the velocity of light and the frequency of microwave, respectively. It indicates that the absorption capacity of the materials is closely related to electromagnetic parameters ε′, ε′′, μ′, μ′′, d and f. The reflection loss of the same material at the same frequency depends mainly on the thickness of the material [23]. Fig. 6 shows the reflection losses of PT, HMG/Ni0.7Zn0.3Fe2O4 and HMG/Ni0.7Zn0.3Fe2O4/PT composites at the best match thickness. PT is not a good microwave absorber. The reason may be that the reflection loss of PT mainly depends on polarized properties and dielectric loss. The HMG/Ni0.7Zn0.3Fe2O4/PT composite exhibits the best microwave absorption performance in comparison with PT and HMG/Ni0.7Zn0.3Fe2O4. The excellent absorbing property of composites is attributed to the following factors. On the one hand, PT increases the dielectric loss of composites. It enhances the complex permittivity, improves impedance matching of composites and reduces the reflection of electromagnetic waves on the incident interface. On the other hand, electromagnetic wave can be reflected many times in HMG after passing through the coating and absorbed many times by Ni–Zn ferrite and PT [13]. It strengthens the absorption of electromagnetic waves that have entered the media and avoided the electromagnetic wave returning to the interface. Moreover, electromagnetic wave attenuation could be caused by hysteresis loss, cavity effect and so on, which further enhance the microwave absorption ability of composites. In Fig. 7, it is obvious that the best match thickness of
L. Li et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 349–354
353
Fig. 5. Magnetization hysteresis loss of (a) Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4, HMG/Ni0.7Zn0.3Fe2O4/PT composites and (b) PT.
0
(a)
-2
-2 (b)
Reflection Loss (dB)
Reflection Loss (dB)
-4 -6 -8
(c)
-10 -12
-4 -6 -8 1.5mm 2.0mm 2.5mm 3.0mm 3.5mm
-10 -12
-14
-14 8
9
10
11
12
8
Frequecy (GHz) Fig. 6. Reflection loss of (a) PT, (b) HMG/Ni0.7Zn0.3Fe2O4 HMG/Ni0.7Zn0.3Fe2O4/PT composites at the best match thickness.
(
)1/2
10
11
12
Frequency (GHz) and
(c)
composites is 3.0 mm in X band and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz). The minimum reflection loss reaches 13.79 dB at 10.51 GHz. When the thickness of the absorber increases, the minimum value of reflection loss appears in different frequency. The dips of minimal reflection for composites are also shifted towards a lower frequency with an increase in thickness. This phenomenon is caused by the resonance of electromagnetic waves interfering with the absorber. The propagating wavelength in a material ( λm ) is expressed by the following equation:
λm = λ o/ μ r εr
9
(3)
where λο is the free space wavelength and μr and εr are the moduli of μr and εr , respectively [24]. When the thickness of the absorber is equal to an odd multiple of a quarter wavelengths of electromagnetic waves, the resonance of electromagnetic waves interfering with the absorber will occur. When electromagnetic waves reflected on the upper surface and electromagnetic waves on the lower surface are superimposed, interference occurs and weakens the energy of the electromagnetic wave. With the thickness increasing, the absorption peaks appear successively. When the frequency of the electromagnetic wave increases, the wavelength of the electromagnetic wave becomes shorter. Therefore, when the frequency is higher, the absorption peak shifts to the lower frequency [25]. According to the above theory, no matter how poor the absorbing property of the material is, because of resonant interference, it will have an absorption peak. Therefore, according to the range of the frequency, it is necessary to control
Fig. 7. Reflection loss of HMG/Ni0.7Zn0.3Fe2O4/PT composites at the different thicknesses.
the thickness of the absorbing material in order to design absorbing material.
4. Conclusions In this article, we synthesized the ternary composite (HMG/Ni0.7Zn0.3Fe2O4/PT) and its properties were measured and analyzed. The results indicate that HMG can be coated by Ni0.7Zn0.3Fe2O4 and PT successfully. Compared with HMG/Ni0.7Zn0.3Fe2O4, the conductivity of the composite increases and reaches 6.87 10 5 S/cm. The Ms value of it is 11.627 emu/g. At the best match thickness (3.0 mm), the minimum reflection loss of the composite reaches 13.79 dB at 10.51 GHz and the bandwidth less than 10 dB can reach 2.6 GHz (from 9.4 to 12.0 GHz). It appears that the ternary composite can be a candidate for use as a kind of absorber.
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