In this article, SnO2nanowires (NWs) have been prepared and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the SnO2NWs/paraffin composites have been measured in a frequency range of 0.1–18 GHz, and the measured results are compared with that calculated from effective medium theory. The value of maximum reflection loss for the composites with 20 vol.% SnO2NWs is approximately −32.5 dB at 14 GHz with a thickness of 5.0 mm.
KeywordsNanowires Permittivity Microwave absorption Effective medium theory
In recent years, electromagnetic (EM) wave absorbing materials have aroused great interest because of more and more civil, commercial, and military applications in electromagnetic interference (EMI) shielding and radar cross section (RCS) reduction in the gigahertz (GHz) band range [1, 2]. Traditionally, EM wave absorbing materials, which are composed of magnetic metals or alloys particles, are restricted in application because of high specific gravity and formulation difficulty. It is hence desirable to have microwave absorbing materials that are lightweight, structurally sound, and flexible and show good microwave-absorbing ability in a wide frequency range. In terms of these criteria, one-dimensional nanostructures, which have a tremendous surface area and more dangling bond atoms on surface, appear to be good candidates . Recently, carbon nanotubes (CNTs) [4–6], magnetic-particle-doped CNTs , magnetic nanowires (NWs) , nanostructured ZnO [9, 10], and Mn3O4 were intensively studied and found to be promising microwave absorbing materials. Many groups found ZnO nanomaterials with different morphologies show excellent microwave absorption behavior, and partly attributed to its semiconductor character [9, 10, 12]. Microwave absorption property of ZnO has been investigated thoroughly in previous reports. In this work, microwave absorption behavior of another important semiconductor SnO2 was investigated in detail.
SnO2 has been paid attention in a variety of applications in chemical, optical, electronic, and mechanical fields, due to its unique high conductivity, chemical stability, photoluminescence, and gas sensitivity [13–16]. However, the research on its dielectric property and microwave absorption has not been reported. Here, both the complex permittivity (εr = ε′ − j ε″) and permeability (μr = μ′ − j μ″) of the SnO2 NWs/paraffin composites with different loading proportion were studied, and the measured results are compared with calculation results from effective medium theory (EMT). The effective permittivity of composite has linear increase with increment of SnO2 NWs proportion. Their microwave reflection loss curves were simulated according to transmission line theory. The excellent absorbing properties of the NW–paraffin were revealed, and the relationship between absorption property and the proportion between SnO2 NWs and paraffin were also investigated.
SnO2NWs were prepared by a normal chemical vapor deposition (CVD) method. Briefly, a small amount of Sn powder (purity: ≥99%, about 3 g) was placed in an alumina crucible. A porous aluminum oxide (AAO) template coated with Au film of about 10 nm was used as substrate, which was positioned about 5 cm downstream from the precursor. Then, the crucible was put into a quartz tube that was located at the center of an electronic resistance furnace. One end of the quartz tube was connected with a mass-flow controller, which introduces a constant mixed carrier gas flow of Ar and O2at a flow rate of 100 and 10 sccm, respectively; the other end of the quartz tube was evacuated by a pump. The furnace was heated to 1,000 °C and kept for 1 h. After the furnace was cooled naturally down to room temperature, white wool-like products in high yield were found on the substrate.
The powder samples were characterized by high resolution transmission electron microscopy (TEM) and selected-area electron diffraction (SAED) on a JEM-2010 transmission electron microscope operated at 100 kV. Field emission scanning electron microscopy (FESEM) observation was performed on a Hitachi S-4800 field emission scanning electron microscope. The products were mixed with paraffin wax with different volume fraction and pressed into toroidal-shaped samples (φout = 7 mm, φin = 3. 04 mm) for microwave absorption tests. The real part and imaginary part of the complex permittivity and permeability of the samples were measured using the transmission/reflection coaxial method by an Agilent E8363B vector network analyzer working at 0.1–18 GHz.
Results and Discussion
Figure 3c shows the microwave reflection loss of composite with 50 vol.% loading at different composite thicknesses. With matching thicknesst m = 7 mm, the maximum reflection lossR maxis ca. −16 dB at 7 GHz. Att = 2 mm, the bandwidth corresponding to reflection loss below −10 dB (i.e., over 90% microwave absorption) is higher than 1.5 GHz.
The dominant dipolar polarization and the associated relaxation phenomena of SnO2 constitute the loss mechanisms. Composite materials, in which semiconductor NWs are coated with a dielectric nanolayer, introduce additional interfaces and more polarization charges at the surface [18, 19]. The interfacial polarization is an important polarization process and the associated relaxation will also give rise to a loss mechanism. It is reasonable to expect that the dielectric loss may be due to significant contributions of the interfacial polarization. It is well known that SnO2 NWs have excellent gas sensitivity and can form space charge layer of several nanometers on the surface. Molecular dipoles formed at the NWs surface interact with the microwave field, leading to some absorption losses through heating .
From Fig. 4c–f, it can be seen that composite of 10, 20, 30, 40 vol.% loading proportion have their approximate reflection loss R max at 11.5, 10, 8.5, 8 GHz at thickness t = 7 mm. With the increase of proportion in the nanocomposites, the matching frequency tends to shift to the lower frequency region, and similar results have been gained on CNTs [1, 2] and ZnO NWs . Fan et al. pointed out that with an increase of CNT content in composite, the electric field of short-distance resonance multipoles leads to dominance of reflection property rather than absorption property. They reported that ε increase with increasing CNT concentration, resulting in a shift of reflectivity peak toward lower frequency . The revelation is important because it suggests that the range of absorption frequency can be easily tuned by changing the SnO2 NWs content of composites. Thus, wideband absorption could be achieved by coupling SnO2 NWs/paraffin layers of different SnO2 NWs contents. So, it is of great significance to calculate real and imaginary part of complex permittivity at different loading proportion of SnO2 NWs.
Φ is either of the real part and imaginary part of the complex permittivity ε and complex permeability μ. Φe, Φi, Φm correspond to the parameter of the effective medium, the insulator, and the semiconductor particles, respectively. p is the volume fractions of SnO2 NWs in the components. The insulator is paraffin in our experiment, real part and imaginary part of the complex permittivity are 2 and 0.01, respectively, as shown in Fig. 4a, b
The real and the imaginary parts of the permittivity increase with the volume concentration. Our measured results show approximately a homogeneous increase across different proportion. BG formula predicts a distinct increase happening at around 30 vol.%, which results from the semiconductor–insulator transition at the percolation threshold , and a linear increase after percolation, which is the same as measured results but with a different slope. BG formula is often used in the case of spherical inclusions whose diameter d is much smaller than the incident wavelength λ. In our experiment, SnO2 NWs are around 100 nm in width and up to micron scale in length; the aspect ratio is so large that error may be brought and result in the difference in slope. As BG formula has difficulty in dealing with composite with percolation, we find that EMT can be only used in qualitative analyses, and leads to large error in quantitative analyses.
In conclusion, SnO2NWs have been prepared by a CVD method and their microwave absorption properties have been investigated in detail. Complex permittivity and permeability of the SnO2nanostructures and paraffin composites have been measured in a frequency range of 0.1–18 GHz, the value of both real part ε′ and imaginary part ε″ of the permittivity increase significantly with increasing SnO2NWs loading, and the variation curve of every content has the similar shape. The value of maximum reflection loss for the composites with 20 vol.% SnO2NWs is −32.5 dB at 14 GHz with a thickness of 5.0 mm. The measured results are compared with results calculated with EMT. We find that BG equation can be only used in qualitative analyses, and leads to large error in quantitative analyses.