Preparation and microwave absorbing properties of hollow glass microspheres/Fe₃O₄/Ag composites with core–shell structure

Abstract

The low-density, conductive and magnetic hollow glass microspheres (HGM)/Fe₃O₄/Ag composites have been successfully synthesized via co-precipitation and chemical plating method. The morphology, composition, microstructure, magnetic and microwave absorbing properties of the composites were investigated based on the analyses of the results using scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, vibrating sample magnetometer and vector network analyzer. The results showed that the HGM/Fe₃O₄ composites were successfully prepared, and the coating layers on the surface of HGM are compact and continuous. Moreover, the final composites were completely covered with Ag nanoparticles. With the addition of Ag nanoparticles, the saturation magnetization of the HGM/Fe₃O₄ composites reduces from 32.08 to 14.77 emu/g, whereas its conductivity increases to 0.48 S/cm. The reflection loss (R) of HGM/Fe₃O₄/Ag composites is lower than −10 dB at 8.2–8.7, 9.6–10.8 and 11.4–11.9 GHz, and the minimum loss value is −19.1 dB at 9.9 GHz.

1 Introduction

Recently, the materials with hollow spherical structure on micro and nano scale have attracted much attention due to their unique structures and outstanding properties [1, 2]. Among these materials, hollow glass microspheres (HGM, the product of fly ash) are now playing a greater role because of their low-density, strong filling capacity, excellent mobility, thermal resistance and chemical inertness [3–5]. In general, the main chemical compositions of HGM are Al₂O₃ and SiO₂, which cannot meet the increasing demand for functional materials, therefore, more and more works have been focused on the preparation of functionalized HGM [6].

Ferrite (Fe₃O₄) in nanometers is a traditional microwave absorbent owing to its superior magnetic properties and the ease-preparation; therefore, it has been widely applied in the field of magnetic separation, electromagnetic shielding, orientation control, and biological applications, etc. [7–9]. However, the density of Fe₃O₄ is fairly high and it is also a non-conductive material, which restricts its potentiality in applications requiring lightweight mass and electrical conductivity [10, 11]. Coating Fe₃O₄ nanoparticles on HGM might be a feasible way to reduce its density; meanwhile, the chemical plating method can usually improve the conductivity of the material [12–14]. In this way, a light-weight and electromagnetic composite can be obtained, such outstanding properties would significantly contribute to fulfill the requirements for next generation magnetic materials. As a consequence, the material might be a promising candidate with low-density and excellent conductive particle in the field of microwave absorbing materials. To our knowledge, this kind of material is seldom reported.

In this work, we reported a versatile approach to prepare high-performance HGM/Fe₃O₄/Ag composites via two-step method. Firstly, HGM/Fe₃O₄ composites were synthesized via co-precipitation method to obtain the low-density and magnetic microspheres. Secondly, the HGM/Fe₃O₄/Ag composites were prepared by chemical plating process. Finally, the structures of HGM, HGM/Fe₃O₄, and HGM/Fe₃O₄/Ag were characterized using scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD), and conductivity, magnetic properties and microwave absorption properties were also measured.

2 Materials and methods

2.1 Materials

Hollow glass microspheres (10–80 μm) were purchased from Qinhuangdao Qinhuang Glass Microsphere Co. Ltd. Ferric chloride (FeCl₃·6H₂O), silver nitrate (AgNO₃), palladium chloride (PdCl₂), stannous chloride dihydrate (SnCl₂·2H₂O) and sodium dodecyl benzene sulfonate (SDBS) were supplied by the Chemical Company of Tianjin. Ferrous sulfate (FeSO₄·7H₂O), and ammonia (NH₃·H₂O), 36 %, formaldehyde were purchased from the Chemical Company of Xi’an.

2.2 Synthesis of HGM/Fe₃O₄ composites

HGM was first pretreated with NaOH solution (0.5 mol/L) for 30 min to improve their surface activity, and then added to a flask. Some amounts of FeSO₄·7H₂O and FeCl₃·6H₂O (mole ratio 1:2) were dissolved into 200 mL deionized water and then also added in the flask. When the solution heated to 30 °C, a mixture of NH₃·H₂O and SDBS was added dropwise into the mixture until the pH value reached 10. The reaction was carried out at 90 °C for 3 h. When the reaction was complete, the crude products were washed, then dried under vacuum at 80 °C for 24 h, and finally calcined at 600 °C for 2 h under argon gas protection.

2.3 Synthesis of HGM/Fe₃O₄/Ag composites

HGM/Fe₃O₄ composites were first sensitized with SnCl₂·2H₂O for 30 min. Then the sensitized HGM/Fe₃O₄ composites were activated by PdCl₂ to improve the adhesion between HGM/Fe₃O₄ composites and silver coating. After that, the treated HGM/Fe₃O₄ composites were immersed in Ag(NH₃)₂OH (0.5 mol/L, 100 mL) solution while HCHO (2.15 mL) was added dropwise, the solution was then stirred for 0.5 h under ultrasonic treatment. After washed and dried, the HGM/Fe₃O₄/Ag composites were obtained [15, 16]. The process is shown as Fig. 1.

Fig. 1

Schematic illustration of the process for the preparation of HGM/Fe₃O₄/Ag composites

2.4 Characterization

The morphology of composites was characterized using SEM (JSM-6390, HITACHI, Japan), the composition of composites was analyzed via EDS analyzer (JED-2200 Series) and the crystal structure of composites was measured by XRD (PANalytical, Holland).The magnetic property was measured by Lake Shore7307 vibrating sample magnetometer (VSM). The electromagnetic parameters of the composites were also analyzed using a HP8753D vector network analyzer, and the samples had a dimension of 22.86 mm × 10.16 mm × 2 mm.

3 Results and discussion

Figure 2 presents the SEM images of the samples at different magnifications. As shown in Fig. 2a, b, the diameter of the HGM is in the range of 10–80 μm, and the microspheres display very smooth surfaces. The panoramic images of Fig. 2c, d demonstrate that a coating is grown on the surface of HGM after the co-precipitation reaction, which encapsulated completely and uniformly. Figure 2e, f show the appearance of HGM/Fe₃O₄/Ag composites, it is obvious that HGM/Fe₃O₄ composites are fully covered with Ag particles.

Fig. 2

SEM images of HGM (a, b), HGM/Fe₃O₄ (c, d) and HGM/Fe₃O₄/Ag (e, f)

The EDS patterns of HGM/Fe₃O₄ composites and HGM/Fe₃O₄/Ag composites are presented in Fig. 3. As shown in Fig. 3a, it appears that Fe exists in the coating, which suggests that Fe₃O₄ was co-precipitated on the surface of the HGM. The elements of Al and Si are attributed to the HGM itself. Figure 3b shows the presence of Ag in the HGM/Fe₃O₄/Ag composites, with a high mass content in the composites. In addition, the conductivity increases as Ag content increases.

Fig. 3

EDS of HGM/Fe₃O₄ (a) and HGM/Fe₃O₄/Ag (b)

Figure 4a is the X-ray patterns of pristine HGM, a broad diffraction peak is appeared at 2θ = 20°–35° indicating the amorphous structure of HGM. In Fig. 4b, peaks of Fe₃O₄ appear at 2θ = 30.10°, 35.40°, 44.08°, 53.64°, 57.96° and 62.80° ascribing to (220), (311), (400), (422), (511) and (440) (JCPDS No. 03-0863) [17], indicated the formation of Fe₃O₄. Figure 4c is the X-ray patterns of HGM/Fe₃O₄/Ag composites. The typical peaks of silver (111), (200), (220) and (311) at 2θ = 38.14°, 44.28°,64.46° and 77.68° (JCPDS, File No. 04-0783) [18] appear in the curve, and the peaks of Fe₃O₄ can be observed as well. These results indicate that the HGM/Fe₃O₄/Ag composites were successfully obtained, which can also verify the results of SEM and EDS analysis.

Fig. 4

XRD patterns of the HGM (a), HGM/Fe₃O₄ (b) and HGM/Fe₃O₄/Ag (c)

The strength of HGM/Fe₃O₄/Ag composites peaks is weaker than those of HGM and HGM/Fe₃O₄ composites. The reason could be attributed to that the surface of composites is completely covered with silver particles, and the strength of Ag peaks is very strong, other peaks will appear to be relatively weak in the same coordinate.

Conductivities of HGM/Fe₃O₄, Ag and HGM/Fe₃O₄/Ag are shown in Table 1. The conductivity of HGM/Fe₃O₄ is only 6.89 × 10⁻⁷ S/cm, indicated that HGM/Fe₃O₄ is a non-conductive material, whereas the conductivity of Ag can reach to 8.75 × 10⁵ S/cm. After coating Ag nanoparticles on the surface of HGM/Fe₃O₄, the conductivity of composites increases to 0.48 S/cm, which is higher than HGM/Fe₃O₄ but lower than Ag. With the addition of Ag particles, the conductivity of HGM/Fe₃O₄ can enhance prominently.

Table 1 Conductivity values of samples (S/cm)

To characterize the magnetic properties, the M–H hysteresis loops of the HGM/Fe₃O₄ and HGM/Fe₃O₄/Ag composites were measured with VSM. Figure 5 shows that the curves are typical of soft-magnetic materials. The saturation magnetizations (Ms) of HGM/Fe₃O₄ composites are 32.08 emu/g, while the Ms of HGM/Fe₃O₄/Ag composites are 14.77 emu/g. From the VSM results, we can see that both HGM/Fe₃O₄ and HGM/Fe₃O₄/Ag composites possessed some magnetic property to a certain extent. However, Ms of the HGM/Fe₃O₄/Ag composites decrease, mainly attributing to the contribution of the volumes of the non-magnetic silver to the total sample volume [19].

Fig. 5

Magnetization curves of HGM/Fe₃O₄ (a) and HGM/Fe₃O₄/Ag (b)

The mechanism of microwave energy loss in a material is the result of its magnetic and electronic properties, which are related to the complex permittivity (ε* = ε′ − jε″), complex permeability (μ* = μ′ − jμ″), dielectric loss (tan δ_e = ε″/ε′) and magnetic loss (tan δ_m = μ″/μ′). Figure 6 shows the electromagnetic parameters of Fe₃O₄, HGM/Fe₃O₄ and HGM/Fe₃O₄/Ag composites. In the X band, the ε′ of HGM/Fe₃O₄/Ag is much higher than those of Fe₃O₄ and HGM/Fe₃O₄, confirmed that the HGM/Fe₃O₄/Ag composites display a higher conductivity, whereas the ε″, μ′, μ″, of HGM/Fe₃O₄/Ag and HGM/Fe₃O₄ composites are similar. The tan δ_e and tan δ_m of HGM/Fe₃O₄/Ag and HGM/Fe₃O₄ composites are higher than those of Fe₃O₄, indicated that the composites may have better microwave absorption properties.

Fig. 6

Magnetic parameters of Fe₃O₄ (a), HGM/Fe₃O₄ (b) and HGM/Fe₃O₄/Ag (c)

The microwave absorbing properties of materials can be calculated based on the measured magnetic parameters; the equation is as follows [20]:

$${\text{R}} = 20\log \left| {\frac{{Z_{in} - Z_{0} }}{{Z_{in} + Z_{0} }}} \right|$$

(1)

where R (dB) is the reflection loss, Z ₀ = (μ ₀ ε ₀)1/2 = 377 Ω and Z _in can be described as:

$$Z_{\text{in}} = Z_{0} \left( {\frac{{\mu_{r} }}{{\varepsilon_{r} }}} \right)^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} \tanh \,\left[ {j\left( {\frac{2\pi ft}{c}} \right)(\mu_{r} \varepsilon_{r} )^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} } \right]$$

(2)

where t is the thickness of the absorber in millimeter, f is the microwave frequency in hertz; c is the velocity of light e, and μ _r and ε _r is relative complex permeability and permittivity, respectively.

The reflection losses of Fe₃O₄, HGM/Fe₃O₄ and HGM/Fe₃O₄/Ag are shown in Fig. 7. It is obvious that the reflection loss of Fe₃O₄ is about −6.5 dB, this result suggests that a single Fe₃O₄ is not a good microwave absorbing materials; Fig. 7b shows that after coating Fe₃O₄ on the surface of HGM, the microwave absorbing properties significantly improve, the reflection loss of HGM/Fe₃O₄ is below −10 dB (90 % absorption) at 8.6–10.9 and 11.5–11.9 GHz, and the minimum loss value is −13.7 dB at 9.2 GHz. This reason is mainly about that after coating Fe₃O₄ on the HGM surface, a cavity was formed in the composite, and electromagnetic waves attenuate gradually after multiple reflection in the cavity, thus the absorbing properties of composites significantly improve; Fig. 7c shows that HGM/Fe₃O₄/Ag composites have excellent microwave absorbing properties in a certain frequency, the reflection loss is below −10 dB at 8.2–8.7, 9.6–10 .8 and 11.4–11.9 GHz, and the minimum loss value occurs at 9.9 GHz, reaching to −19.1 dB, which is nearly 99 % absorption. This result show that conductive particles can improve the microwave absorbing properties of composites in a certain frequency, the reason is mainly about that the good electrical conductivity makes it easier for electromagnetic wave to enter the material, so the microwave absorbing property is higher. From these results, we can conclude that HGM/Fe₃O₄/Ag composites are excellent candidates for application as a microwave absorber.

Fig. 7

Reflection losses of Fe₃O₄ (a), HGM/Fe₃O₄ (b) and HGM/Fe₃O₄/Ag (c)

4 Conclusions

In this contribution, the low-density, magnetic and conductive HGM/Fe₃O₄/Ag composites have been successfully synthesized via co-precipitation and chemical plating methods. The obtained composites exhibit smooth, compact and continuous Fe₃O₄ coating on the surface of the HGM, and HGM/Fe₃O₄ composites are fully covered with silver particles. The HGM/Fe₃O₄/Ag composites are soft-magnetic materials, and with the addition of the non-magnetic particles of silver, the saturation Ms of the composite reduce to 14.77 emu/g, whereas its conductivity increases to 0.48 S/cm. The HGM/Fe₃O₄/Ag composites have excellent microwave absorbing properties in a certain frequency, the reflection loss is below −10 dB at 8.2–8.7, 9.6–10.8 and 11.4–11.9 GHz, and the minimum loss value is −19.1 dB at 9.9 GHz.