Conductivity-dependent dielectric properties and microwave absorption of Al-doped SiC whiskers
Graphical abstract
Introduction
In recent years, electromagnetic wave absorbing materials with three-dimensional (3D) network structure have attracted increasing interesting because they can provide an additional attenuation excepted for their intrinsic absorption. Generally, these 3D network structures are mainly formed through the connection of one-dimensional (1D) carbon nanotubes [1], carbon fibers [2], metal nanowires [3] and their composites [4], [5], with large aspect ratio and good electrical conductivity. And the charges will be excited to form dissipative current along each 1D structure when exposed to electromagnetic wave. So, compared with the equiaxial granules, materials with 1D structure are able to provide longer transport paths for dissipative current, resulting in a larger conduction loss. Moreover, 1D structure is much easier to connect with each other to form a 3D network structure, which can further extend the path for dissipative current. Thus, the absorbing materials with 3D network structures exhibit more efficient electromagnetic wave absorption than that of equiaxial granules. Due to these attractive features, Lu et al. selected carbon nanotubes to construct a conduction network structure in an insulated matrix, which shows excellent microwave absorption ability [6]. Cao et al. demonstrated a good microwave attenuation ability of the carbon fiber network structure [7]. Wen et al. also believed the reduced graphene oxides with the similar conduction network structure dominated its high-efficiency absorption performance [8].
In addition to the above mentioned materials, SiC whisker (SiCw) is also a good candidate for constructing 3D network structure to be used as electromagnetic wave absorber. Especially, SiCw can be processed at a higher temperature and harsh environment compared to the 1D carbon and ferromagnetic materials [9], [10], [11], [12]. However, SiC is a semiconductor with a relatively low electrical conductivity, and its conduction loss is not so high even with constructed 3D network structure by 1D SiC whiskers or wires. Research work indicates that aluminum doping can effectively increase its carriers concentration as well as the electrical conductivity [13]. Many attempts have been performed on this topic. Li et al. prepared Al-doped SiC nano-powders by combustion synthesis, and revealed that the dielectric loss increased with the increase in dopant concentration [14]. Similar results were also reported by Su et al. and Jin et al. [15], [16], [17]. As all known, compared to the Al-doped SiC equiaxial granules, Al-doped SiCw can construct an interconnect 3D conduction network more easily in the matrix when the SiC loading level is the same. Meanwhile the SiC whiskers prepared under microwave irradiation possess high density stacking faults (SF), resulting in a larger dipolar loss [18]. Both of the merits make Al-doped SiCw very promising for microwave absorption. However, to the best of our knowledge, such a case has seldom been reported. Here, inspired by the above analysis, the Al-doped SiCw with different electrical conductivity has been prepared by microwave heating method, and then their dielectric permittivity and microwave absorbing properties in frequency range of 2–18 GHz has been investigated.
Section snippets
Preparation of Al-doped SiC whiskers
Firstly, silica sol (wSiO2 = 30%) and activated carbon (AR, <50 μm, Beijing-Chem Co.) were mixed and stirred for 3 h and then were dehydrated at 250 °C for 24 h. The obtained mixture with the aluminum powders (AR, <50 μm, Beijing-Chem Co.) were well homogenized by ball-milling in a polyethylene milling jar with zirconia balls for 8 h to get the starting powder mixtures. The molar content of aluminum in the raw materials was set at x = 0, 0.01, 0.02 and 0.03 (AlxSi1−xC), respectively. Then the
Morphology analysis
Fig. 1(a–d) shows the micro-morphologies of Al-doped SiCw after concentrated. It is very clear that the SiCw is the main constituent. EDS analysis shows the aluminum dopant concentrations in SiCw are 0.21 at.%, 0.39 at.% and 0.66 at.% corresponding to the x = 0.01, 0.02 and 0.03 samples respectively, revealing the different dopant concentrations were achieved. The diameter and length of as-synthesized SiCw are approximately 50–200 nm and >10 μm respectively. Thus, its aspect ratio reached more
Conclusions
SiC whiskers doped with different concentrations of aluminum were prepared and exhibited conductivity-dependent dielectric properties. The aluminum doping increased the carrier concentration of SiC whiskers and enhanced the electrical conductivity. At the same time, a 3D network of conductive path for the dissipative current can be constructed by connecting up the Al-doped SiC whiskers, thus further increase the electrical conductivity and thereby remarkably enhanced the conduction loss. As a
Acknowledgments
This work is supported by the National Natural Science Foundation of China under Grant No. 51572018, and China Postdoctoral Science Foundation under Grant No. 2015M571135.
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