Microwave absorption properties of Ni/(C, silicides) nanocapsules
© Jiang et al.; licensee Springer. 2012
Received: 18 November 2011
Accepted: 3 March 2012
Published: 1 May 2012
The microwave absorption properties of Ni/(C, silicides) nanocapsules prepared by an arc discharge method have been studied. The composition and the microstructure of the Ni/(C, silicides) nanocapsules were determined by means of X-ray diffraction, X-ray photoelectric spectroscopy, and transmission electron microscope observations. Silicides, in the forms of SiOx and SiC, mainly exist in the shells of the nanocapsules and result in a large amount of defects at the ‘core/shell’ interfaces as well as in the shells. The complex permittivity and microwave absorption properties of the Ni/(C, silicides) nanocapsules are improved by the doped silicides. Compared with those of Ni/C nanocapsules, the positions of maximum absorption peaks of the Ni/(C, silicides) nanocapsules exhibit large red shifts. An electric dipole model is proposed to explain this red shift phenomenon.
Magnetic nanocapsules have attracted increasing attention in the area of electromagnetic wave absorption in the last decades because of their peculiar structural characteristics [1–4]. The magnetic nanocapsules, such as Fe/ZnO  and FeCo/C  nanocapsules, consist of different core and shell materials, and they show outstanding electromagnetic wave absorption performance in the gigahertz range. Zhang et al. [3, 4] have prepared Ni/C nanocapsules using methane as the carbon source and observed excellent electromagnetic properties, and they also studied the effect of inter-particle distance on the electromagnetic wave attenuation mechanism. Recently, Wang et al. (unpublished work) have reported that changing the thickness of the graphite shell of Ni/C nanocapsules can modulate the effective permittivity.
Both carbon and silicon dioxide are widely used as shell materials, in which the former has a layered structure and the latter is a good insulator . SiC with a high dielectric constant is an interesting wave absorption material . As we know, SiO2 can react with carbon at high temperatures to produce semiconducting SiC . We expect to improve the electromagnetic wave absorption properties of nanocapsules by changing the shell constituents of nanocapsules. In this paper, the electromagnetic wave absorption properties of Ni/(C, silicides) nanocapsules and Ni/C nanocapsules are studied in the frequency range of 2 to 18 GHz. Although the tendencies of the frequency dependence of the complex permeability of the Ni/(C, silicides) and Ni/C nanocapsules in the frequency range of 2 to 18 GHz are similar, compared with the electromagnetic absorption spectra of the Ni/C nanocapsules, the absorption peak of the Ni/(C, silicides) nanocapsules shows a large red shift of 0.7 to 1.9 GHz at different thicknesses of absorption layer. An electric dipole model is proposed to explain this red shift phenomenon.
Ni/(C, silicides) nanocapsules were fabricated by the arc discharge method as described in . A mixture of nickel and silicon dioxide powders with a mole ratio of Ni/SiO2 = 96:4 was compacted into a target and used as an anode. A graphite needle served as a cathode. A mixture gas of high-purity Ar (2.0 × 104 Pa) and H2 (4.0 × 103 Pa) was introduced into the evacuated chamber (5.0 × 10−3 Pa) serving as the source of plasma. Twenty milliliters of 99.7% C2H5OH was introduced into the chamber as the carbon source. During the experimental process, the current was maintained at 80 A, and the voltage, at 18 V. The products were collected from the chamber after passivation for 12 h in Ar atmosphere. In order to study the effects of inclusions of silicides on the absorption properties of Ni/(C, silicides) nanocapsules, Ni/C nanocapsules were also prepared using a pure Ni target as an anode under the same conditions.
Phase analysis was performed by means of X-ray diffraction (XRD). X-ray photoelectron spectroscopy (XPS) was used to determine the elements’ chemical states and the valance band spectrum. The morphology and microstructure of the samples were characterized using a transmission electron microscope (TEM; JEOL-2100, JEOL Ltd., Akishima, Tokyo, Japan) with an emission voltage of 200 kV. The sample preparation for complex permittivity and permeability measurements is described in detail elsewhere [1, 9]. The mass fraction of the nanocapsules in paraffin was set at 50 wt.%. The coaxial method was used to determine the EM parameters of the toroidal samples (see details for the preparation of toroidal samples in ) in the frequency range of 2 to 18 GHz using an Agilent 8722ES vector network analyzer (VNA; Santa Clara, CA, USA) with a transverse electromagnetic mode. The complex permittivity and complex permeability were derived from the S-parameters tested by the calibrated VNA, using a simulation program for the Reflection/Transmission Mu and Epsilon (Nicholson-Ross-Weir model) . According to the transmission line model , the reflection losses (RLs) of the Ni/(C, silicides) and Ni/C nanocapsules were calculated from the complex permittivity and complex permeability measured on the nanocapsules dispersed in paraffin.
Results and discussion
The distance between adjacent nanoparticles may play an important role in the electromagnetic response of Ni nanocapsule-paraffin composites . According to , the inter-particle distance can be estimated by , where D is the average diameter of the nanoparticles and φ is the space occupancy ratio of the nanocapsules in paraffin. If the average particle size D is taken as 27 nm and φ = 50 wt.% for Ni/(C, silicides) nanocapsules, the inter-particle distance is about 25 nm for the Ni/(C, silicides) system. This inter-particle distance is much larger than the critical distance of about 11 nm . As a result, the interaction between the Ni cores in Ni/(C, silicides) nanocapsules can be ignored, and only individual cores are involved during the microwave absorption process. This assumption is critical for the following discussion because band theory requires the negligence of inter-particle interaction. Belavin et al.  have calculated the electronic structure of defect and defect-free CNTs using the tight-binding method, and their results show that lattice defects can create localized states near the Fermi level. For Ni/(C, silicides) nanocapsules, the inclusion of silicides introduces large amounts of defects into the shells, such as amorphous silicides and carbon, vacancies, and twists in the layered carbon network. These defects in the shells of Ni/(C, silicides) nanocapsules cause the localization of electron density and generate additional energy levels near the Fermi level, which may reduce the electron transition energy. When the electromagnetic wave penetrates into absorbents, the wave is absorbed through electron transitions from quasi-continuous states induced by the defects near the Fermi level . Because of the reduction of the electron transition energy, the transition can occur at a lower frequency, which leads to the red shift of the absorption peak at different thicknesses of absorption layer.
We have prepared Ni/(C, silicides) nanocapsules and Ni/C nanocapsules. XPS measurements reveal that the silicides mainly occur in the shells of nanocapsules. The two kinds of nanocapsule-paraffin composites demonstrate attractive microwave absorption properties. When inclusions of silicides are introduced into Ni/(C, silicides) nanocapsules, the absorption peaks of the Ni/(C, silicides) nanocapsule-paraffin composite have a red shift of 0.7 to 1.9 GHz compared with those of the Ni/C nanocapsule-paraffin composite. We propose a simple electric dipole model which semi-quantitatively explains the red shift phenomenon. The RL peak value for the Ni/(C, silicides)-paraffin composite with a layer thickness of 2 mm has increased to about −37 dB at 13.8 GHz compared with that of −13 dB at 15.6 GHz for the Ni/C nanocapsules-paraffin composite.
Dr DL is a professor in the Institute of Metal Research, the Chinese Academy of Sciences. His main research interests include: synthesis of nanocrystals with novel properties; magnetic nanocapsules and nanostructures; magnetic, electrical transport and electromagnetic-wave absorption properties of functional nanomaterials.
The work is supported by the National Natural Science Foundation of China under grant no.51171185 and by the National Basic Research Program (no. 2012CB933103) of China, Ministry of Science and Technology China. Teng Yang acknowledges the IMR SYNL-T.S. Ke Research Grant for the support.
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