Carbon nanotubes materials and their application to guarantee safety from exposure to electromagnetic fields

Epikote 828 was degassed at 80 °C for 2 h to eliminate air bubbles. CNTs were dispersed in ethanol in an ultrasonic bath for 1 h at room temperature then dried at 80 °C, pressure 100 mbar for 2 h to evaporate the solvent. After drying MWCNTs and adding them to epoxy resin, the mixture was ultrasonicated for 1 h. Epicure hardener was added into the mixture in the known weight ratios. The composite was molded into a flat plate and cured at 80 °C for 2 h.

At first, the MWCNTs were dispersed in ethanol in an ultrasonic bath for 1 h at room temperature then dried at 80 °C, pressure 100 mbar for 2 h to evaporate the solvent. After drying, MWCNTs were ground in a planetary ball mill with PMMA resin, plasticizer and xylen.

Using a network analyzer (HP E8363B) to measure the reflection losses and transmission losses, vector network was connected to antennas. The test frequency was from 8 to 12 GHz. Dimensions of sample are 100 mm ×100 mm.

The frequency was in the range of 100 MHz–14 GHz, in an electromagnetic chamber (EMC). The sample was put in a closed wooden box (EUT) with the size 50 cm × 50 cm × 50 cm. The outside of the box was painted by a layer of PMMA/CNTs about 100 μm thick. The optical sensor was placed in the EUT and connected with an EMI test receiver. The EUT was put on a turntable, 0.8 m high from the floor and the distance from the antenna to the EUT is 1 m. The transmitting antenna was rotated vertically and horizontally (figures 1 and 2).

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Effective EMI shielding materials must have electrical conductivity up to 10 S cm⁻¹ and more [10]. As we know, the ability of the material to achieve electrical percolation threshold and to become an electrical conductor is basically related to the nanotube–nanotube distances and polymer–nanotube interactions. In fact, direct connection and overlapping of the CNTs is not necessary, it means that CNTs do not need to touch each other physically. However, CNTs should be close enough to allow the electron tunneling effect. This mechanism requires the CNTs–CNTs distance to be less than their diameter [16]. For this reason, CNTs loading and dispersion in polymer matrix determine the electrical conductivity of the materials.

Electromagnetic interference shielding effectiveness (EMI SE) was obtained following ASTM D 4953–89 using a network analyzer and set over a frequency range of 8–12 GHz. EMI SE value expressed in dB was calculated from the ratio of incident power to transmitted power of the electromagnetic wave as follows:

$$\begin{equation} {\rm{SE}} = 10\log \left| {\frac{{P_{{\rm{inc}}} }}{{P_{{\rm{trans}}} }}} \right| = 20\log \left| {\frac{{E_{{\rm{inc}}}^2 }}{{E_{{\rm{trans}}}^2 }}} \right|\;, \end{equation} \tag{ 1 }$$

where P_inc(E_inc) and P_trans(E_trans) are the power (electric field) of incident and transmitted electromagnetic waves, respectively, at a measuring point. The reflectance (R_e) and the transmittance (T_r) of the material were measured and the absorbance (A_b) was calculated by using the following equations:

$$\begin{equation} R_{\rm{e}} = \left| {\frac{{E_{\rm{r}} }}{{E_{\rm{i}} }}} \right|^2 \; = \left| {S_{11} } \right|^2 \quad {\rm or}\;\quad R_{\rm e} = \left| {\frac{{E_{\rm{r}} }}{{E_{\rm{i}} }}} \right|^2 \; = \left| {S_{22} } \right|^2 , \end{equation} \tag{ 2 }$$

$$\begin{equation} T_{\rm{r}} = \left| {\frac{{E_{\rm{t}} }}{{E_{\rm{i}} }}} \right|^2 \; = \left| {S_{21} } \right|^2 \quad {\rm or}\;\quad T_{\rm r} = \left| {\frac{{E_{\rm{t}} }}{{E_{\rm{i}} }}} \right|^2 \; = \left| {S_{12} } \right|^2 , \end{equation} \tag{ 3 }$$

where R_e and T_r were obtained by the measurement of S-parameters, S₁₁ (or S₂₂) and S₁₂ (or S₂₁) for the reflection and the transmission, respectively. In ideal conditions, without scattering, we have

$$\begin{equation} R_{\rm e}+T_{\rm r}+A_{\rm b}=1. \end{equation} \tag{ 4 }$$

Electrical conductivity measurements were performed by using a standard four-point probe method. The MWCNTs/epoxy composites material had conductivities of several orders of magnitude higher than conductivities of neat epoxy. As seen in figure 3, filling MWCNTs increases the electrical conductivity of pristine epoxy (10⁻¹⁴ S m⁻¹) by ten orders of magnitude. To achieve the threshold of electromagnetic shielding, the electrical conductivity of the composites should reach about 10 S m⁻¹ when the concentration of MWCNTs filler was not less than 25 wt%.

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Figure 4 shows the EMI SE of composites based on epoxy resin with 1–25 wt% MWCNTs loadings, thickness of 5 mm. We find that SE increases with increasing MWCNTs loading at a fixed frequency. At low loadings, the MWCNTs/epoxy composites exhibit an almost frequency-independent EMI SE performance. At higher loadings, the EMI SE values tend to fluctuate considerably. With 25 wt% CNTs, the SE effectiveness of epoxy/CNTs composite attained an average value 22 dB.

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Figure 5 shows that PMMA coating is completely incapable of shielding electromagnetic waves while PMMA/CNTs coating with 25 wt% CNTs and thickness of 100 μm has the shielding effectiveness up to 30 dB in the frequency range 8–12 GHz; that is, more than 99.99% of electromagnetic energy was shielded over the tested frequency range.

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Figure 6 shows the EMI shielding characteristics of PMMA coating in the frequency range of 8–12 GHz. It can be seen that the reflectance and absorbance can be considered as negligible. The electromagnetic wave almost 100% passes through this coating.

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With a rising frequency, the reflectance of PMMA/CNTs coating decreases and the absorbance increases (figure 7). This is due to reflectance by mismatching impedance between materials and electromagnetic waves. Impedance of highly conductive material is low and the impedance between PMMA/CNTs coating and PMMA coating is large, so that reflectance is great (average 0.3). However, characteristic absorption is dominated (near to 0.7). Since EMI SE is the total outcome by reflectance and absorbance, it is nearly impossible for electromagnetic waves to pass through this coating.

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The test was in the range of frequency from 100 MHz to 14 GHz and the sample was put in a closed wooden box sized 500 mm × 500 mm × 500 mm; the outside of the box was painted by PMMA/CNTs coating with the thickness of 100 μm.

The EMI SE of PMMA/CNTs coating determined by using a network analyzer is higher than the results in compliance with experimental models in EMC at X–band (8–12 GHz). It can be explained as follows: the first test shows the absolutely precise results in laboratory conditions, the sample dimensions are equal to those of transmitting and receiving speakers, the layer of PMMA/CNT coating is uniform. Meanwhile, the second test is more likely to be a practical model, EUT is bigger, and the PMMA/CNTs layers are less homogeneous. In addition, EUT having slots (for electrical cables) permits the electromagnetic wave to go through partly. Nevertheless, it can be proved that by using PMMA coating with CNTs loading of 25 wt% for the purpose of electromagnetic field exposure protection, in practical conditions one can achieve an EMI SE of 20 dB in both X–band and RF (table 1).