J Mater Sci (2013) 48:6048–6055 DOI 10.1007/s10853-013-7401-y

Solvothermal synthesis of hollow glass microspheres/Fe3O4 composites as a lightweight microwave absorber Qiangchun Liu • Zhenfa Zi • Min Zhang • Peng Zhang • Angbo Pang • Jianming Dai Yuping Sun

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Received: 13 March 2013 / Accepted: 23 April 2013 / Published online: 3 May 2013  Springer Science+Business Media New York 2013

Abstract Hollow glass microspheres/Fe3O4 (HGMs/ Fe3O4) composites with low density were successfully synthesized by solvothermal method at 160 C. The morphology, composition, and microstructure of the samples were characterized by scanning electron microscopy and X-ray diffraction, respectively. The results show that the HGMs/Fe3O4 composites exhibit compact and continuous Fe3O4 particles coating on the surface of HGMs. The complex permeability and permittivity of HGMs/Fe3O4 composites obtained at 160 C for different reaction times were measured in the frequency range of 1–18 GHz by vector network analysis. The microwave absorption properties were well-elucidated by the traditional coaxial line method. The as-prepared HGMs/Fe3O4 composites show excellent microwave absorption properties. 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.

Introduction In recent years, with the rapid spread of wireless communication devices using electromagnetic waves in the range around gigahertz, e.g., mobile phone (0.8–2.0 GHz), local Q. Liu  Z. Zi  P. Zhang  A. Pang  J. Dai School of Physics and Electronics Information, Huaibei Normal University, Huaibei 235000, People’s Republic of China Q. Liu  Z. Zi  M. Zhang  J. Dai (&)  Y. Sun Key Laboratory of Materials Physics, Institute of Solid State Physics, Chinese Academy of Sciences, Hefei 230031, People’s Republic of China e-mail: [email protected]

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area network system (LAN, 2.45, 5.2 GHz), electronic toll collection in intelligent transport system (ETC, 5.8 GHz), satellite broadcast systems (11.7–12.0 GHz), and so on [1, 2], several problems such as electromagnetic interference and information leakage have emerged [3, 4]. Therefore, the electromagnetic wave absorption materials with properties of wide frequency range, strong absorption, low density, and thin thickness have been attracting much attention [5–7]. Among the candidates for electromagnetic wave absorption materials, the soft magnetic ferrites can attenuate electromagnetic waves efficiently due to their larger values of saturation magnetization and higher Snoek’s limit [8–10]. In addition, due to their higher efficiency and lower price than those of other materials, soft magnetic ferrites have been used as the most popular conventional magnetic fillers. However, ferrite absorbents have higher density, which restricts their wide applications in microwave absorbers. Carbon nanotubes and conductive polymers, displaying low density and good microwave absorption performance, often involve complicated preparation processes and high price, which is also unfavorable for putting such absorption materials into practical application [11]. Metal-based absorbers are susceptible to corrosion in harsh environments (e.g., high-temperature, highhumidity, and acidic environments) that will severely deteriorate the absorption performance [12]. In order to reduce the density and overcome the disadvantages of other absorbents, synthesis of ferrites on light substrate may be an effective way [13]. Hollow glass microspheres (HGMs) are a kind of inorganic materials and have attracted considerable academic and technological attention because of their low density, thermal and chemical stability, high strength, wear resistance, and fine fluidity [14]. If these HGMs are coated with a layer of magnetic ferrites, the microwave absorption

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Then, the autoclave was sealed and kept at 160 C for 2–12 h without stirring. After cooling down to room temperature, the resulted black powders were washed with distilled water and ethanol three times, respectively, and collected with the aid of a magnet. The washed precipitates were dried in a vacuum oven at 60 C for 8 h.

performance of the hollow core–shell structure composite can be improved because of the magnetic loss of magnetic ferrites. Different methods have been used to fabricate magnetic composite microspheres by coating magnetic particles on the surface of the HGMs. Wei et al. [15] reported the synthesis of Fe3O4 magnetic films on the HGMs using ferrite plating. An et al. [16] synthesized the novel core/shell HGMs/a-Fe2O3 precursor by a solvothermal process and then reduced the precursor at 360 C under a continuous H2/Ar gas flow to obtain the HGMs/ Fe3O4 composite. Fu et al. [17] 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. Wang et al. [18] proposed a route of controllable deposition of Fe3O4 and polyaniline on HGMs. Yang et al. [19] presented a facile way to synthesize unique hollow polyaniline/nano-Fe3O4 composite microspheres by decorating the surface of hollow polyaniline/sulfonated polystyrene microspheres with various amounts of Fe3O4 magnetic nanoparticles using SPS as a so-called ‘‘hard template,’’ followed by the removal of the template with the solvent tetrahydrofuran. However, there are some disadvantages in preparing these composites, such as high temperature or complicated preparation process [17, 19]. Therefore, investigation on the synthesis and the microwave absorption properties of magnetic core–shell composites is of great interest for harvesting the combined advantages of the hollow core–shell structural effects and the intrinsic property of the magnetic oxide. Herein, we report a facile solvothermal method to synthesize the HGMs/Fe3O4 composites with low density and excellent microwave absorption performances at 160 C.

The phase identification was performed by X-ray diffraction (XRD) on a Bruker Advance D8 X-ray diffractometer ˚ ) in the range from with CuKa radiation (k = 1.5418A 2h = 10 to 80 at a scanning rate of 1.5 min-1. The morphologies and surface characteristics were observed by employing JEOL-6610LV scanning electron microscopy (SEM). The absorption properties of electromagnetic wave were investigated by a vector network analyzer (VNA, AV3629D) in the range of 1–18 GHz. The as-prepared HGMs/Fe3O4 composites were mixed uniformly with 30 wt% molten paraffin wax and compressed into toroidalshaped specimens with outer diameter of 7.00 mm, inner diameter of 3.04 mm, and thickness of 2 mm. The complex dielectric permittivity and magnetic permeability were obtained by measuring the S11 and S21 parameters using transmission/reflection mode. A full two-port calibration (SHORT-OPEN-LOAD-THRU) was carried out before measurement on the test setup to reduce or remove errors due to the directivity, source match, load match, isolation and frequency response in both the forward and reverse measurements. The non-dispersive complex permittivity of epoxy resin is measured for 3.5 - j0, while the complex permeability is 1.0 - j0 in 1–18 GHz. The density of composites was measured by the method of pycnometer.

Experimental

Results and discussion

Synthesis of HGMs/Fe3O4 composites

Figure 1 depicts the XRD patterns of the HGMs and HGMs/Fe3O4. As shown in Fig. 1a, the diffraction peaks can be indexed as quartz (JCPDS card No. 86-1630) and mullite (JCPDS card No. 74-2419). However, the intensity of diffraction is relatively low, which may be ascribed to the complicated components and relatively low crystallinity in the HGMs. From Fig. 1b, it can be seen clearly that all the peaks, except for the weak diffraction peaks of HGMs marked by asterials, can be indexed as face centered ˚ , which cubic Fe3O4 with the lattice constant a = 8.394 A is in good agreement with JCPDS card No. 85-1436. No evidence of impurities such as a-Fe2O3 or FeO is found in the XRD patterns. Therefore, it could be concluded that a kind of HGMs/Fe3O4 composites was formed. The morphologies of the HGMs and HGMs/Fe3O4 are shown in Fig. 2. As shown in Fig. 2a, the HGMs used in

The commercially available HGMs (composed of 52.20 % SiO2, 42.90 % Al2O3, and 4.90 % other components, particle density 0.83 g/cm3, diameter 50–60 lm, and wall thickness 1.5–2.5 lm) were provided by Datang Huaibei Power Plant. The HGMs were treated by 0.5 M NaOH aqueous solution for 30 min and then washed with distilled water to be neutral before use. Fe(CO)5 was purchased from Jiangsu Tianyi Co., Ltd. All the other reagents used in the experiments were of analytical grade (purchased from Shanghai Chemical Reagent Industrial Company) and used without further purification. In a typical synthesis, 10 g pretreated HGMs, 0.5 g NaOH, 10 mL Fe(CO)5, 180 mL ethanolamine, and 10 mL hydrazine hydrate (85 %) were added into a 250 mL stainless steel autoclave, respectively.

Characterization

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Fig. 1 XRD patterns of HGMs (a) (The peaks denoted by Q and M are attributed to quartz and mullite, respectively), and HGMs/Fe3O4 composites (b) (The positions of the peaks of the HGMs are marked with asterisks (*))

the present study possess nearly uniform spherical-shape with diameter about 60 lm and own quite smooth surfaces. A broken glass microsphere shown in Fig. 2b proves that the raw material is hollow. In addition, the high-magnified SEM micrograph of the HGMs in Fig. 2b also exhibits that some small cavities or pores exist on the surface of the HGMs. From the cross-sectional SEM image of HGMs (inset in Fig. 2b), we can clearly see that the wall thickness

Fig. 2 SEM image of HGMs (a); a high-magnification SEM image of HGMs in hollow (b) (inset the cross-section SEM image of HGMs); SEM image of HGMs/Fe3O4 composite preparation at 160 C for 8 h

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is about 1.5 lm and some small cavities or pores also exist in the wall of the HGMs. Figure 1c illustrates a SEM micrograph of HGMs/Fe3O4 composites prepared at 160 C for 8 h. This image reveals the presence of a complete, homogeneous and rough Fe3O4 shell on the surfaces of the HGMs. Increasing the reaction time to 12 h, regular spherical-like Fe3O4 particles (shown in Fig. 2d) attach on the inner and outer sides of the HGMs. Moreover, one can also observe that increasing the reaction time, the average particle diameters have increased. The densities of composites measured by the method of pycnometer increase from 0.94 to 1.03, 1.44 and 2.17 g cm-3, respectively, when the reaction times increase from 2 to 4 h, 8 and 12 h, which also indicates the growth of Fe3O4 crystals with the increasing reaction times. The overall possible procedure for preparing HGMs/ Fe3O4 composites is shown in Fig. 3. At the initial reaction stage, there is a strong absorption interaction between Fe(CO)5 and HGMs (Fig. 3a). When NaOH and N2H4H2O are added, HGMs also absorbs the OH anions and N2H4H2O because of their porous structures (Fig. 3b). When the temperature of the reaction system increases to 160 C, some Fe3? ions are reduced to Fe2? ions by N2H4H2O, and then Fe3? ions react with Fe2? ions to form original Fe3O4 nuclei on the surfaces of HGMs in alkali condition (Fig. 3c). This insures the directed assembly of the Fe3O4 crystallites onto the surfaces of the HGMs, rather than self-assembled into isolated particles. With the

(c); SEM image of HGMs/Fe3O4 composite preparation at 160 C for 12 h (d). The scale bar is 5 lm

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Fig. 3 A schematic drawing illustrated the formation process of lightweight composite of HGMs/Fe3O4

Fig. 4 SEM image of HGMs/Fe3O4 composite preparation at 160 C for 2 h (a); 4 h (b). The scale bar is 5 lm

progress of the reaction, the original particles start to agglomerate into larger clusters on the surfaces of HGMs as observed in Fig. 3d to reduce the surface free energy. The SEM images of HGMs/Fe3O4 composites obtained at 160 C for different reaction times are shown in Figs. 2 and 4. When the reaction time is 2 h, small amount of Fe3O4 particles attach on the surfaces of the HGMs (Fig. 4a). With an increase in time to 4 h, the size of Fe3O4 particle becomes large and the number of Fe3O4 particles attached on the surfaces of the HGMs increases (Fig. 4b). The sample obtained at 8 h (Fig. 2c) indicates that Fe3O4 particles have covered all the outer surfaces of HGMs. When the reaction proceeds for 12 h, regular spherical-like Fe3O4 particles (shown in Fig. 2d) attach on the inner and outer sides of the HGMs. Figure 5 shows the complex permittivity and permeability variation of HGMs/Fe3O4 composites obtained at 160 C for different reaction times in the frequency range of 1–18 GHz. As shown in Fig. 5a and b, the real part (e0 ) of the complex permittivity increases from 11.1 to 19.3 and 20.4 and the imaginary part (e00 ) of the complex permittivity increases from 0.51 to 1.21 and 1.94 at 1 GHz, respectively, when the reaction times increase from 2 to 4 and 12 h. However, when the reaction time is 8 h, the values of e0 and e00 exhibit an abnormal variation. The values of e0 and e00 are lower than those of HGMs/Fe3O4 composites with reaction time of 4 h. The higher values of e0 and e00 indicate the higher storage and loss capability of the electric energy owing to the enhanced electrical conductivity of Fe3O4 arising from its inverse spinel-type

crystal structure. Formally, the formula of Fe3O4 can be written as Fe3?[Fe2?, Fe3?]O4. The first Fe3? (A type) is tetrahedrally coordinated and the bracketed Fe2? and Fe3? ions occupy octahedrally coordinated sites(B type). At room temperature, hopping of the minority-spin ‘‘extra’’ electron between the B site Fe2? and Fe3? ions enhances the electronic conductivity [20, 21]. According to the free electron theory [22], e00 ¼ 1=qxe0 , where x, e0 and q are the angular frequency, the dielectric constant of free space and the resistivity, respectively, the value of e00 is closely related to the electric resistivity of absorption materials. The presence of Fe3O4 crystals with hopping of the minority-spin ‘‘extra’’ electron between the B site Fe2? and Fe3? ions enhances the electronic conductivity of HGMs/ Fe3O4 composites. Consequently, the higher electronic conductivity results in the relatively higher e00 of HGMs/ Fe3O4 composites when the reaction times increase. In addition, some peaks in the measured frequency range can be observed in the e00 plots, implying a resonance behavior. The similar multi-resonance behaviors have been reported in the others literatures, which are ascribed to the existence of polarizable Fe2? ions [8, 20]. Figure 5c and d shows the changes with frequency in the real part (l0 ) and the imaginary part (l00 ) of the complex permeability for HGMs/Fe3O4 composites obtained at 160 C for different reaction times in the frequency range of 1–18 GHz. The values of l0 of four samples are in the 0.65–2.75 range, and decrease abruptly with increasing frequency from 1.0 to 7.5 GHz and retain an approximate constant over 7.5–18 GHz. Meanwhile, the values of l0 of

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Fig. 5 The frequency dependence real part e0 (a), imaginary part e00 (b) of the complex permittivity and real part l0 (c), imaginary part l00 (d) of the complex permeability

HGMs/Fe3O4 composites synthesized at 160 C for 2 h are lower than those of HGMs/Fe3O4 composites synthesized at 160 C for 4 h, 8 h, or 12 h in 1.0–6.0 GHz frequency range, and higher in 6.0–18.0 GHz frequency range. The values of l00 of four samples decrease gradually with increasing frequency from 1.0 to 18.0 GHz. Along with the prolonging the reaction times from 2 to 8 h, the values of l00 of HGMs/Fe3O4 composites increase, which is ascribed to the increased eddy current loss due to the increasing thickness of magnetic Fe3O4. When the reaction time increases to 12 h, the value of l00 decreases, which might be related to the high density of Fe3O4 and duplex structure Fe3O4. In addition, Fig. 5d shows the resonance peak at about 5.0 GHz for all the four samples, associated with natural resonance of Fe3O4. According to the Kittel equation [23]: fc = cHa, where fc is the resonance frequency, c is the gyromagnetic ratio (c = 28 GHz T-1), and Ha is the effective anisotropy field (Ha ¼ 4jK1 j=3l0 Ms ), the frequency of natural resonance of sphere-shaped magnet can be calculated. The saturation magnetization l0Ms is 0.547 T and the anisotropy coefficient K1 for the fcc-type bulk magnetite is about -(9 9 103) J m-3, so the theoretical calculation of natural resonance frequency should be

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cHa = 1.2 GHz. The shift of natural resonance may be ascribed to the following two factors. First, the saturation magnetization of the as-synthesized HGMs/Fe3O4 composites decreases due to the no-magnetic HGMs. Second, the small pore in the HGMs may act as a magnetic inactive layer and cause the demagnetizing field, which generates an additional anisotropy field and leads to the shift to higher frequency of the natural resonance frequency [24]. In general, the electromagnetic parameters (e0 , e00 , l0 and l00 ) of materials are strongly dependent on the composition, dimension, morphology, conductivity, and polarization model. In this study, the electromagnetic parameters of HGMs/Fe3O4 composites synthesized at 160 C for 8 h exhibit an abnormal variation, which maybe attribute to the morphology and microstructure variation. However, the mechanism about the abnormal variation is still unclear and needs further study. To further reveal the influence of constituent and morphology of the composites on the electromagnetic wave absorption properties, the reflection loss (RL) can be calculated from the electromagnetic parameters at various sample thicknesses by means of the following expressions [25]:

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RL ¼ 20 logjðZin  Z0 Þ=ðZin þ Z0 Þj

ð1Þ

Z0 ¼ ðl0 =e0 Þ1=2    rffiffiffiffiffi lr 2pft pffiffiffiffiffiffiffiffi Zin ¼ Z0 tanh j lr er c er

ð2Þ ð3Þ

where Z0 and Zin are the impedance of free space and the input impedance at free space and absorption materials, t is the thickness of the absorber, c is the velocity of the light, f is the frequency, lr and er are the relative complex permeability and permittivity, respectively. The RL value of -10 dB indicates that 90 % of the introduced electromagnetic waves are absorbed, which is the target value for electromagnetic absorbers from an industrial point of view. The RL values of the HGMs/Fe3O4 composites obtained at 160 C for different reaction times as a function of frequency for various layer thicknesses are shown in Fig. 6. It is clearly found that the minimum RL of all samples moves toward lower frequency band with the thickness increasing from 1.0 to 3.5 mm. Taking an overview of the RL of the four samples, these HGMs/Fe3O4 composites all have the superior microwave absorption properties. When the thicknesses of these HGMs/Fe3O4 composites are larger than 1.5 mm, they all have absorption peaks lower than

-10 dB. For the HGMs/Fe3O4 composite obtained at 160 C for 2 h, RL values (B-20 dB, corresponding to 99 % attenuation) are observed in the range of 6.0– 11.8 GHz with thicknesses of 2.0–3.5 mm, whereas the minimum RL value of -36.2 dB appears at 7.3 GHz, corresponding to a 3.0 mm matching thickness (Fig. 6a). Increasing the reaction time to 4 h (i.e., increasing the content and size of magnetic Fe3O4 particle), the minimum RL value of -26.2 dB appears at 12.0 GHz with a 1.5 mm matching thickness (Fig. 6b), which is superior than that of other reported absorbers in previous literatures, such as HGMs/CoFe2O4 [17], HGMs/Ni [26], HGMs/Co [27], HGMs/TiO2/BaFe12O19 [28]. Further increasing the reaction time to 8 or 12 h, the intensity of absorption peaks does not distinctly improve (Fig. 6c, d). Nevertheless, two strong absorption peaks appear in Fig. 6c, d with a 3.5 mm matching thickness. Actually, apart from the dielectric and magnetic loss, the electromagnetic wave may also be absorbed via ‘‘geometrical effect’’ [29, 30]. It means if the thickness of absorber satisfies the following two equations: pffiffiffiffiffiffiffiffiffiffiffiffiffi d ¼ lkm =4 (l = 1, 3, 5……) and km ¼ k0 = jlr jjer j. In the above equations, km is the wavelength at certain frequency, lr and er are the complex permeability and permittivity of composites measured by transmission line method, and k0

Fig. 6 Reflection loss with different thicknesses of HGMs/Fe3O4 composite prepared at 160 C for 2 h (a); 4 h (b); 8 h (c); and 12 h (d)

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is the wavelength in the free space. The minimum RL or the dip occurs when the thickness is about an odd number multiple of one quarter of the wavelength thickness of the material, where the sum of the emerging waves and the reflected electromagnetic waves in the material are out of phase by 180, resulting in the total cancellation at the airmaterial interface. To illustrate the double reflection loss of HGMs/Fe3O4 composites in Fig. 6c, we suppose that the microwave absorption ability is coming from the geometrical effect. At matching frequency fm = 3.5 GHz and fm = 16.0 GHz, the complex permittivity and permeability are er = 16.55 - j0.53, er = 17.64 - j1.07 and lr = 1.85 - j0.91, lr = 0.97 - j0.004, respectively. Substituting these values into above two equations, the calculated values of d are 3.64 mm (l = 1) and 3.39 mm (l = 3), respectively, almost agree with the thickness of the sample (3.5 mm) within experimental errors. Consequently, the above calculation validates that the double-frequency microwave absorption ability of HGMs/Fe3O4 composites arises from not only dielectric loss and magnetic loss but also from geometrical effect. The HGMs/Fe3O4 composites exhibit excellent microwave absorption property than Fe3O4 reported in the literature [31, 32]. The possible reasons can be interpreted as follows. First, spinel structure Fe3O4 can generate dielectric loss and magnetic loss as mentioned above. Second, electromagnetic wave can hardly be reflected on the surface HGMs/Fe3O4 composites because of a better impedance match, which originates from the low permittivity of quartz and mullite in HGMs. Quartz is high transparency to microwave and uses as a window material of radar transmitters. Third, the hollow structure of HGMs/Fe3O4 composites confines the incident electromagnetic wave within the hollow sphere and causes multiple scattering and reflection inside the hollow sphere as shown in Fig. 7, resulting in enhanced electromagnetic loss.

Fig. 7 A schematic representation for the possible dissipation route of electromagnetic wave in the HGMs/Fe3O4 composite

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Conclusions In summary, low-density composites of hollow glass spheres coated with Fe3O4 magnetic particles were successfully synthesized by a simple solvothermal method. The microwave absorption properties of HGMs/Fe3O4 composites obtained at 160 C for different reaction times were fully investigated. The as-prepared HGMs/Fe3O4 composites show excellent microwave absorption properties. When the thicknesses of these HGMs/Fe3O4 composites are more than 1.5 mm, they all exhibit strong absorption peaks (lower than -10 dB). These results may be important for preparation and potential applications in industry, commerce and military affairs as lightweight and highly effective microwave absorbers. Acknowledgements This study was financially supported by the National Nature Science Foundation of China (51002156, 11274314, and 11104098).

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