Third, an important mechanism for dielectric loss material to absorb electromagnetic wave arises from the relaxation process. According to the Debye relaxation expression, the complex permittivity can be written as<br>(ε′−ε∞)2+(ε′′)2=(εs−ε∞)2,<br>(3)<br>where εs and ε∞ are the static dielectric constant and the dielectric constant at infinite frequency, respectively. Thus, the plot of ε′′ versus ε′ would be a single semicircle, which can be defined as the Cole–Cole semicircle.21,32 Figure 3 shows the curve characteristic of ε′′ versus ε′, which presents a clear segment of three overlapped Cole–Cole semicircles for r-GO but only one semicircle for graphite. This suggests that there is sole relaxation process for graphite and ternary relaxation processes for r-GO, with each semicircle corresponding to one Debye relaxation process.
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