Therefore, the high loss tangent for the CBC ABT 888 composites signifies that they have good attenuating properties. Figure 3 Real (a) and imaginary (b) parts of permittivity for the composites with 20 wt.% CBC loadings. Figure 4 shows the dielectric permittivities of the CBC paraffin wax composites with 5 to 30 wt.% CBC pyrolyzed at 1,200°C. It is evident
that both the real and imaginary permittivities increased rapidly with CBC concentration. The complex permittivity spectra reveal the behavior of electrical conduction and dielectric relaxation of the composites. The rapid increase in the permittivities with concentration is attributed to the onset of percolation, similar Salubrinal in vitro to that of the CNTs [17, 18]. Figure 5 is a plot of DC conductivity of the CBC/paraffin wax composites versus the amount of the CBC loading pyrolyzed at 1200°C. One can see a sharp increase of conductivity when CBC loading was increased from 1 to 7.5 wt.%. The conductivity of the GSK1904529A CBC was of 2 × 10-9 S/cm for 1 wt.% and 0.02 S/cm for 7.5 wt.% and reached a relatively high value of 0.5 S/cm for 15 wt.%. This implies that such a composite has a percolation threshold of about 7.5 wt.%. Figure 4 Frequency dependencies of (a) real and (b) imaginary permittivities. Figure 5 DC conductivity of CBC/paraffin wax composites versus CBC loading pyrolyzed at 1,200°C. For microwave
absorption, the elelctromagnetic parameters should be appropriate, and the optimal filler U0126 clinical trial concentration is always around the percolation threshold. Theoretical RL values in the sample with 7.5 wt.% CBC loading were calculated according to the transmission line theory . (1) (2) where Z in is the normalized impedance at the absorber surface. Figure 6a shows the frequency dependences of the RL at various sample thickness (t = 1.8, 1.9, 2.0, and 2.1 mm). An optimal RL of -40.9 dB was observed at 10.9 GHz with the -20 dB bandwidth over the frequency range of 10.4 to 11.4 GHz for t = 2.0 mm. The minimum RL obviously shifts to lower frequency range with increased thickness, which can be understood according to the geometrical effect
matching condition in which the thickness of the layer is a quarter wavelength thickness of the material. It is interesting that microwave absorption properties do not change dramatically for the thicknesses of 1.8 to 2.1mm. Figure 6 Frequency dependences of the RL at various sample thickness (a) and the EMI shielding efficiency (b). For EMI shielding, the total shielding effectiveness SE T is always expressed by SE T = 10 lg(P in/P out) = SE A + SE R + SE I , where P in and P out are the power incident on and transmitted through a shielding material, respectively. The SE A and SE R are the absorption and reflection shielding efficiencies, respectively, and can be described as SE A = 8.686 αt and SE R = 20 lg |1 + n|2/4|n| . For the composite with 30 wt.