id="f5">Figure 5. Variation of the coated-layer thickness for different samples as a function of the number of coatings.
Figure 6. The variation of the shielding effectiveness according to the samples at ~1.5 mm composite layer thickness.
Figure 7. The variation of the shielding effectiveness according to the samples at ~2.0 mm composite layer thickness.
Figure 8. The variation of the shielding effectiveness according to the samples at ~2.8 mm composite layer thickness.
Meanwhile, reflection, absorption, and multiple reflections are thought as the major three mechanisms to shield EMI . In the conducting materials like metals, the shielding is usually enhanced by the electrical conductivity of the materials  . So, reflection is understood to work as the main shielding mechanism to protect EMI  . For the materials having the high electric constants or the magnetic permeability, absorption was known as the major shielding mechanism . For the multiple reflections, the small-sized fillers having the high surface area in the composite usually gave rise to the better shielding performance . These three mechanisms can complementary and/or supplementary operate to shield the EMI.
Considering only the reflection and the absorption effects as the main shielding mechanisms for EMI of this work, the SE of EMI for the electrically conductive polymer composites can be estimated by the empirical equation of Simon :
where SE is in dB, r is the volume resistivity (Wcm) at room temperature, t is the thickness of the sample (cm), f is the measurement frequency, respectively. In this equation, the combined first and second terms, namely 50 + 10log10(rf)−1, shows the SE only by reflection mechanism. The third term, namely 1.7t(f/r)1/2 indicates the SE only by absorption mechanism.
We first compared the SE of d-CC@PU composites for samples A - C and the SE of g-CMC@PU composites for samples D - F under the similar composite layer thickness as shown in Figures 6-8. The samples C, F seems to have the highest SE among the respective sample group. It indicates that the reflection effect may work as the SE mechanism for the EMI in this case, because only the d-CCs and g-CMCs can have the electrical conductive characteristics in d-CC@PU composite and in g-CMC@PU composites. As indicated by Simon’s equation, the SE from reflection decreases with increasing the measurement frequency, while the SE from absorption increases with increasing the measurement frequency. For the measuring frequency dependence of the SE for the samples, the samples show the increase of SE with increasing the measurement frequency in the range of 0.25 - 4.0 GHz. These results inform that the absorption may work as the main SE mechanism for the EMI of these composites.
For the composite layer thickness dependence, the third term of Simon’s equation, 1.7t(f/r)1/2), indicates that the SE by absorption mechanism increases with increasing the thickness (t). Figure 9 and Figure 10 show the variation of the SE as a function of the measurement frequency for samples C and F, respectively. These figures clearly indicate that the SE of the sample increases with increasing the coated layer thickness. Therefore, we confirm that the absorption would play as the main shielding mechanism to protect EMI for in the d-CC@PU composites as well as in the g-CMC@PU composites.
On the whole frequency range in this work, the g-CMC@PU composite is more effective to work as a shielding
Figure 9. The variation of the shielding effectiveness of sample C according to the different thickness of the coated layers.
Figure 9. The variation of the shielding effectiveness of sample F according to the different thickness of the coated layers.
material of EMI. The absorption is the main mechanism to shield the EMI for d-CC@PU composite as well as g-CMC@PU composite.
Cite this paper
Gi-Hwan Kang,Sung-Hoon Kim,Saehyun Kim, (2015) Enhancement of the Electromagnetic Wave Shielding Effectiveness by Geometry-Controlled Carbon Coils. Journal of Materials Science and Chemical Engineering,03,37-44. doi: 10.4236/msce.2015.31006