Characterization of a microwave absorbent prepared by coprecipitation reaction of iron oxide on the surface of graphite nanosheet
Introduction
In recent years, microwave absorbing materials have been widely used in military and civil electromagnetic interference areas for their many good properties [1], [2], [3], [4], [5]. According to the absorbing mechanisms, microwave absorbing materials can be divided into two types. The mechanism of the magnetic materials is magnetic loss while it is dielectric loss to the conductive materials. The absorbing properties of the microwave absorbing materials can be expressed by the following parameters: complex permittivity (ɛr = ɛ′ − jɛ′′), complex permeability (μr = μ′ − jμ′′), dielectric loss (tanδe = ɛ′′/ɛ′) and magnetic loss (tanδm = μ′′/μ′) [6], [7], [8], [9]. The higher the imaginary parts of the complex permittivity (ɛ′′) and the complex permeability (μ′′) as well as the higher tanδe and tanδm, the higher the absorbing properties of the materials are.
As popular magnetic materials, ferrites can be used as microwave absorbing materials for their unique magnetic properties and can be easily prepared [10], [11], [12], [13]. Recently, researchers have focused their attention on the synthesis of Fe3O4 nanoparticles [14], [15]. Much effort has been done to the preparation of magnetic Fe3O4 with dendrite patterns [16], nanodiscs [17] and nanotube arrays [18]. But the heavy weight and easily-to-oxidize have limited their application. At the same time, single Fe3O4 cannot meet the characteristics such as thin thickness, light weight, broad frequency and strong absorbing peak that the microwave absorbing materials should have. So Fe3O4 are usually used after combination with some magnetic or conductive materials such as metal powders [19], carbon fibers [20], [21], carbon nanotube [22], [23] and resins [24], [25].
Nowadays, much attention has been paid to graphite because of its unique mechanical, chemical and electrical properties. Graphite is well known to be a layered material with high conductivity and can be intercalated by chemical reagents such as nitric acid or sulphuric acid and form into another kind of compound called oxidized graphite (OG). By rapid thermal treatment, OG can expand several hundreds times the volume of its original and get expanded graphite (EG). As to the conductive filler, EG has been widely used [26], [27], [28].
Ma et al. [29] has prepared the graphite nanosheets decorated with Fe3O4 nanoparticles. The means such as TEM, FTIR, XRD, and XPS are used to characterize them and their application in the immobilization of glucoamylase has been intensively investigated. But the microwave absorbing properties of the composites have not been discussed. In our work, we try to prepare the composites Fe3O4/NanoG to study the microwave absorbing properties of the electromagnetic materials and investigate the matching attribute of the electronic materials and the magnetic materials.
In this paper, graphite nanosheet (NanoG) was prepared by ultrasonic dispersion of EG in an aqueous ethanol solution. Then a kind of composite Fe3O4/NanoG was prepared by coprecipitation reaction of Fe3O4 on the NanoG's surface. The structure of NanoG, Fe3O4 and Fe3O4/NanoG was characterized by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), respectively. Properties such as thermal stability, conductivity and microwave absorbing of them were measured.
Section snippets
Materials
OG was supplied by Shandong Qingdao Graphite Company (Qingdao, China). Ferric chloride (FeCl3·6H2O) and sodium dodecyl benzene sulfonate (SDBS) were bought from Chemical Company of Tianjin. Sodium hydroxide (NaOH), ammonia (NH3·H2O), aminopropyltriethoxysilane (KH550), ferrous sulfate (FeSO4·7H2O) and ethanol at analytical grade were bought from Chemical Company of Xi’an.
Synthesis of Fe3O4
An amount of FeSO4·7H2O and FeCl3·6H2O (mole ratio 2:1) were dissolved in distilled water (100 mL) under a nitrogen atmosphere
SEM images of NanoG, Fe3O4 and Fe3O4/NanoG
NanoG is prepared by ultrasonic dispersion of EG in an aqueous solution of 70% ethanol and 30% distilled water. Fig. 2(a) shows the surface morphology of NanoG. It can be seen that EG has been efficiently exfoliated to ultrathin transparent graphite nanosheets with a width of about 1–20 μm and a thickness of about 30–90 nm, indicating a large aspect ratio (300–500) of them. The higher the aspect ratio of the material, the lower the filler content as well as the higher electrical conductivity they
Conclusions
In this paper, Fe3O4/NanoG was fabricated by precipitation reaction of Fe3O4 on the NanoG surface. The structures of NanoG, Fe3O4 and Fe3O4/NanoG were characterized by SEM, EDS, FTIR and XRD. Results show that NanoG has a great aspect ratio (300–500) and can be tightly wrapped up by Fe3O4. There are chemical bands between Fe3O4 and NanoG. TG analysis shows that both Fe3O4 and Fe3O4/NanoG have good thermal stability. According to the four-point-probe test, the conductivities of Fe3O4/NanoG
Acknowledgements
The work is supported by graduate starting seed fund of Northwestern Polytechnical University (z2012155) and the Basic Research Foundation of Northwestern Polytechnical University under Grant no. JC20110230.
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