Microwave absorption properties of MWCNT-SiC composites synthesized via a low temperature induced reaction

In recent years, electromagnetic (EM) interference pollution is becoming a serious problem with marvelous development and application of electronic devices and communication instruments in commercial, military, industrial and scientific fields, such as computers, wireless network system, mobile phones and wireless antenna systems. Also, EM radiation can cause health hazards especially for expectant mothers and children. To effectively solve the EM interference pollution problem, worldwide attention has been paid to explore efficient microwave absorbing materials. On the other hand, the rising development of stealth technology in military applications also prompts intensive investigations on EM absorbing materials in gigahertz (GHz) band range.

Among the candidates for EM wave absorbers, carbon and magnetic particle doped carbon, such as CNTs,^1,2 Fe encapsulated CNTs,³ FeCo-filled CNTs,⁴ porous carbon/Co composite,⁵ and ordered mesoporous C-Fe-SiO₂ composites,⁶ have been reported to exhibit outstanding EM absorbing properties. As an alternative candidate for microwave absorbing materials, SiC possesses high chemical stability, superior mechanical strength and hardness, good fracture toughness and abrasive properties at high temperatures. Based upon literature, SiC nanowires exhibited good microwave absorption ability with a minimum reflection loss of -31.7 dB.⁷ Ni-Co-P coated SiC composite powder possessed a minimum microwave loss of -32 dB at the frequency of 6.30 GHz.⁸ SiC/polyester resin mixture with a diamond structure had high absorption ability.⁹ SiC-based woven fabrics had promising potentials for EM wave absorption applications in the 17–40 GHz frequency range with a ∼ 90% absorption.¹⁰ As a combination of the two materials of carbon and SiC, it is reasonable to speculate that the C-SiC composites should also possess outstanding EM absorbing properties. However, to the best of our knowledge, few researches are so far related to the EM absorbing properties of C-SiC composites.

In this work, we prepared MWCNT-SiC composites by the reaction of Si powders and MWCNTs induced by that of Na and sulfur, and investigated the microwave absorbing performances of the composites. Besides the energy-saving and time-saving preparation, the lightweight MWCNT-SiC composites exhibit excellent microwave absorbing properties. It is worthwhile to explore what factors give rise to the improvement in microwave absorbing performance of the product. Associated with the characterization on structure and morphology, the possible microwave absorbing mechanism was discussed.

As has been reported in our previous work,¹¹ the reaction between Na and sulfur could induce that between Si and graphite to form SiC at low temperatures. Similarly, in this work MWCNT-SiC composites were synthesized by the reaction of Si powders and MWCNTs (40-60nm in diameter, several micrometers in length) induced also by that of Na and sulfur. The reactants of 1.0 g Si, 0.2 g MWCNT, 5.0 g Na and 1.6 g sulfur were put into autoclaves of 30 mL in capacity with a molar ratio of 3:1 for sulfur:MWCNT. The reactions were performed at 200, 500 and 600 °C for 5 h, respectively. The products were washed in turn with anhydrous ethanol, concentrated sodium hydroxide solution, hydrochloric acid and distilled water, and then dried at 50 °C for 6 h. Dark grey powders were ultimately obtained. The sample prepared at a temperature of T was designated as S-T, for example, S-600 represents the sample prepared at 600 °C for 5 h.

X-ray diffraction (XRD) patterns were obtained on a Dmax-rc diffractometer with Ni-filtered Cu Kα radiation (V = 40 kV, I = 50 mA) at a scanning rate of 4 °/min. The morphology of the product was examined using a Hitachi SU-70 field-emission scanning electron microscope (FESEM) at an accelerating voltage of 15 kV and a JEOL JEM-2100 high-resolution transmission electron microscope (HRTEM) operated at 200 kV.

Toroidal shaped samples (φ_out = 7 mm, φ_in = 3 mm) were prepared by mixing 0.1 g MWCNT-SiC composites into 0.3 g paraffin, i.e. the mass percentage of the MWCNT-SiC composites in the samples is 25 wt %. The relative complex permittivity (ɛ_r) and permeability (μ_r) of the samples were measured by a transmission/reflection coaxial method in the range of 2-18 GHz using an E8363B PNA Series Network Analyzer.

The microwave absorbing properties of the products prepared at 200, 500 and 600 °C were determined by measuring their electromagnetic parameters, i.e. the relative complex permittivity (ɛ_r=ɛ^′-jɛ^′′) and permeability (μ_r=μ^′-jμ^′′), and by calculating the dielectric loss tangent (tgδ_E=ɛ^′′/ɛ^′) and magnetic loss tangent (tgδ_M=μ^′′/μ^′). The real part (μ^′) and imaginary part (μ^′′) of complex permeability are about 1.0 and 0.0, respectively, just the same as those for pure carbon or SiC materials,^5,12 suggesting that the products are nonmagnetic materials and almost no magnetic loss may occur, so the relative complex permeability is not shown here.

Figure 1 depicts the relationship between the relative complex permittivity of the product/paraffin composites and wave frequency. For S-600, ɛ^′ declines from 8.0 to 6.9 in the range of 2-8.8 GHz followed by a peak at about 10.8 GHz with a peak value of 7.4, thereafter ɛ^′ almost keeps unchanged. A relatively complex variation occurs for ɛ^′′ and tgδ_E. The ɛ^′′ decreases from 1.4 to 1.1 in the range of 2-6.2 GHz followed by two overlapped strong peaks in the range of 6.2-13 GHz and a weak peak at about 16 GHz. The variation of tgδ_E is almost the same as that of ɛ^′′. By carefully comparing the relative complex permittivity of S-500 with that of S-600, in spite of some similarity in curve appearance, distinct differences can be found both in peak position and in peak intensity. The peak values of ɛ^′, ɛ^′′ and tgδ_E for S-500 are lower than those for S-600 concomitant with a peak shift to higher frequency. However, for S-200, the ɛ^′ value decreases, while the ɛ^′′ and tgδ_E values increase gradually from 2 to 18 GHz with some weak fluctuations occurring between 8 and 14 GHz. These EM parameters imply that the products may exhibit excellent EM absorbing properties.

According to the EM wave transmission line theory,^13,14 the reflection loss of the product/wax composites with various coating thicknesses were calculated, as shown in Figure 2. It is obvious that the microwave absorbing peaks become strong and shift to lower frequency with increasing the thickness. For S-600 and S-500, there are three absorbing peaks in the frequency range of 2-18 GHz, as shown in Figures 2(a) and 2(b). The minimum reflection loss is -38.7 dB around 12.9 GHz with a coating thickness of 7 mm for S-600 and -24.0 dB around 15.9 GHz with a coating thickness of 6 mm for S-500, and the absorption bandwidths lower than -10 dB (90% absorption) are about 2 GHz and 2.5 GHz, respectively. The weak peaks around 11.3 GHz in Figure 2(a) is consistent with those for S-600 in Figure 1, and the peaks around 12.2 GHz in Figure 2(b) with those S-500 in Figure 1, indicative of the interdependence between these peaks. Note that these peaks do not change their positions with varying coating thickness, thus should be resulted from the interfacial polarizations. For S-200, only two peaks occur in the range of 2-18 GHz, as shown in Figure 2(c), and the minimum reflection loss is -15.2 dB at about 10.6 GHz with a coating thickness of 7 mm. These results indicate that the microwave absorbing properties of S-600 is superior to those of S-500 and S-200. Therefore, it is necessary to clarify the reason responsible for the remarkable differences in microwave absorbing abilities, and a series of characterizations on the products were carried out by XRD, FESEM and HRTEM.

As a matter of convenience for comparison, the XRD patterns of S-600, S-500 and S-200 are depicted in Figure 3. The diffraction peaks at 2θ = 35.5, 41.4, 59.9 and 71.7º correspond to the (111), (200), (220) and (311) planes of β-SiC with a calculated lattice constant a= 4.364 Å, which are in good agreement with those in JCPDS 29-1129. The peak marked with “SF” arises from the stacking faults in β-SiC crystals, and the one at 2θ=26.6º from the (002) plane of carbon. Besides β-SiC and carbon, no other diffractions can be detected, indicating that the products are composites comprised of SiC and carbon. Though the C-SiC composites can be synthesized at a temperature as low as 200 °C, the crystallinity and yield of β-SiC as well as the stacking faults in the crystals increase with the rise of reaction temperature,¹¹ while the relative amount of carbon remained in the product decreases. From the XRD patterns, it seems that the improvement in microwave absorbing properties for S-600 is associated with the high crystallinity of β-SiC involving high density of stacking faults, and the relative amount of β-SiC in the product.

Figure 4 shows the FESEM and TEM images of S-600, S-500 and S-200. From the FESEM image of S-600 shown in Figure 4(a), a large number of spheres about 300-800 nm in diameter are mixed with some one-dimensional (1D) structures. The average diameter of the 1D structure is 50 nm or so, almost the same as that of the pristine MWCNTs (see Figure S1(a) in supplementary material).²⁶ The spheres exhibit accidented surfaces, which are particularly true in the magnified image shown at the bottom left of Figure 4(a). In fact, the spheres are comprised of a lot of nanoparticles. Figures 4(b)–4(d) are the TEM images to identify the microstructures of the 1D structure and the spheres. From Figure 4(b), it can be determined that the 1D structure is actually the unreacted MWCNTs with some spheres attached to them. The spheres are somewhat transparent under TEM, as displayed in the magnified image at the bottom left of Figure 4(b), indicative of their loose structure. From the selected area electron diffraction pattern (SAED) shown at the top right of Figure 4(b), besides the inner weak ring arising from the (002) plane of MWCNTs, the other strong diffraction rings match respectively with the (111), (220) and (311) planes of β-SiC, demonstrating that the spheres are β-SiC polycrystals. The lattice fringe image of a single sphere is shown in Figure 4(c), from which SiC crystallites with an average diameter of 10nm or so can be clearly distinguished, and stacking faults can be examined in the crystallites as marked by “SF”. Furthermore, some pores present in the spheres, verifying that the loose SiC spheres possess a porous structure. Figure 4(d) shows the interface between SiC crystallites and a single MWCNT, the perfect interface bonding gives a confirmation that some SiC spheres formed on the MWCNTs. So the MWCNT-SiC composites combine several features favorable to their microwave absorbing properties, such as porous SiC spheres, high density of stacking faults in SiC crystallites, interfaces between MWCNTs and SiC spheres, and grain boundaries between SiC crystallites.

Figure 4(e) is the FESEM image of S-500, both the quantity and size of the spheres decrease markedly in comparison with those in S-600. The spheres are about 200 nm in average diameter with relatively smooth surfaces. And a lot of MWCNTs retained in the product. For S-200, only a spot of small SiC spheres and nanoparticles distribute in bulk quantities of MWCNTs. The characterization by FESEM and TEM further confirms that the high yield of β-SiC with high density of stacking faults was achieved in S-600, in accordance with the XRD result.

Taking into comprehensive consideration the microwave absorbing properties and the structure and morphology of the product, SiC spheres play an important role in improving the properties, so we will simply analyze the formation process of the SiC spheres. No reaction happened between Si and MWCNTs in the absence of of Na and sulfur or in the presence of either of them even at 600°C, so the main feature of our synthetic route is to employ the high heat energy released from the reaction of Na and sulfur to induce the reaction between Si and MWCNTs. It is very likely that the formation of SiC spheres is related to some intermediate products, such as silicon sulfide and carbon sulfide. It is known that the reaction between Na and sulfur happens at low temperatures accompanying with the release of a large amount of heat energy which could induce the reactions between Si, CNTs and sulfur to generate intermediate products. According to literature, sulfur vapor reacts with Si at 600 °C, and with carbon at 750 °C.¹⁵ The subsidiary reactions could be verified by examining the morphology changes of MWCNTs, as displayed in Figures S1(b) and (c) in supplementary material.²⁶ The intermediate products could then be reduced by Na to yield Si and carbon with high reactivity, and the reduced Si could react with CNTs to form SiC on them, while the reduced carbon could react with Si to generate SiC around Si powders. Of course, the reduced Si may also react directly with the reduced carbon to form SiC. The continuous deposition of SiC resulted in the formation of SiC spheres. When the products were washed with anhydrous ethanol, concentrated sodium hydroxide solution, hydrochloric acid and distilled water to remove the unreacted Na, Si and the byproduct of Na₂S, porous SiC spheres and the unreacted CNTs were remained, namely the MWCNT-SiC composites were obtained. The preparation of MWCNT-SiC composites can be simply depicted in Figure 5. The higher the reaction temperature, say 600 °C, the more favorable the subsidiary reactions, and the bigger the SiC spheres, as shown in Figure 4(a). At a temperature of 200 °C, the weak subsidiary reactions induced by that of Na and sulfur only gave rise to a spot of small SiC spheres and nanoparticles distributing in bulk quantities of MWCNTs, as shown in Figure 4(f).

As stated above, the microwave absorbing properties of the nonmagnetic MWCNT-SiC composites could be ascribed to the dielectric loss rather than magnetic absorption. Combined with the characterization on microstructure and morphology, the microwave absorbing properties strongly depend upon the SiC spheres. The low yield of SiC spheres with small sizes results in poor microwave absorbing abilities, for instance, the microwave absorption of S-200 is dominantly resulted from MWCNTs rather than from SiC. However, the high yield of porous SiC spheres comprised of nanocrystals in S-600 is responsible for the excellent microwave absorbing abilities. And the main contributions may include the porous SiC spheres, high density of stacking faults in SiC crystallites, interfaces between MWCNTs and SiC spheres, grain boundaries between SiC nanocrystals, as well as the interfacial polarizations aroused therefrom. As referred in literature, porous structure may exhibit good microwave absorbing properties.^5,6,16–18 When microwave transmits into the porous spheres, multiple reflections and scattering will occur, leading to gradual dissipation of EM energy. The bulk quantities of defects, such as stacking faults, grain boundaries and interfaces, which can give rise to space charge polarization and relaxation, are also favorable to improving microwave absorption.^19–22 Meanwhile, the nanoparticles constituting the SiC spheres could be repeatedly polarized under an alternating EM field, leading to strong interfacial polarization relaxation loss and Ohmic loss, and consequently to the enhancement in dielectric properties.^21,23 Furthermore, CNTs have good electric conductivity,^24,25 their bonding to the SiC spheres could also increase the microwave attenuation.

In summary, the MWCNT-SiC composites synthesized by the reaction of MWCNTs and Si powders induced by that of Na and sulfur at 600°C exhibit outstanding microwave absorbing performances, and a minimum reflection loss of -38.7 dB can be reached around 12.9 GHz. The excellent properties are closely related to the porous SiC spheres comprised of nanocrystals, high density of stacking faults in SiC crystallites, interfaces between MWCNTs and SiC spheres, grain boundaries between SiC nanocrystals, as well as the interfacial polarizations aroused therefrom. The light-weight MWCNT-SiC composites are promising for cost-efficient, abrasion and corrosion resistance microwave absorbing materials.

This work was supported by the National Natural Science Foundation of China (No. 50972076, 50872072 and 50772061), Shandong Provincial Natural Science Foundation, China (Y2008F26 and Y2008F40), Science and Technology Development Project of Shandong Province (2009GG10003001), Graduate Independent Innovative Fund of Shandong University (31370071613052).