## 3 Results and Discussion
In addition, the dielectric loss tangent (tan \(\delta_{\rm{r}}=\epsilon^{\prime\prime}/\epsilon^{\prime}\)) and the magnetic loss tangent (tan \(\delta_{\rm{\mu}}=\mu^{\prime\prime}/\mu^{\prime}\)) can be used to describe the dielectric loss capacity and the magnetic loss capacity [17]. Compared with the CNC, the  
Figure 4: The room-temperature hysteresis loops of all composites. Figure 5: The Raman spectra of CNCG-2. (**a**) The XPS spectrum of CNCG-2; (**b**) The Co 2p spectrum of CNCG-2; (**c**) The C 1s spectrum of CNCG-2; (**d**) The N 1s spectrum of CNCG-2. values of \(\varepsilon_{\rm r}\) of CNCG significantly enhance in the frequency range of 2-18 GHz (Figure 6a,b). Due to the introduction of appropriate amount of the 3D-graphene, the values of \(\varepsilon_{\rm r}\) of CNCG-2 are obviously better than that of the other composites. As the frequency changes from 2 to 18 GHz, the values of \(\mu_{\rm r}\) of all composites have the same variation trend, and obvious resonance phenomena can be observed (Figure 6c,d). In addition, it can be obviously observed that the values of \(\mu_{\rm r}\) are less affected by the change of the raw material content. It is worth noting that the values of \(\mu^{\prime\prime}\) appear negative in the high-frequency region, which indicates that, when the incident wave enters the composite, the electrons in the composite will generate eddy currents under the action of the Lorentz force, thus generating an additional induced magnetic field and radiating electromagnetic energy outward at the high frequency [18]. In addition, it can be observed that the values of \(\tan~{}\delta_{\rm\varepsilon}\) are significantly greater than that of \(\tan~{}\delta_{\rm\mu}\) at the same frequency (Figure 6e,f), which indicates that dielectric loss plays a leading role in the loss process of incident wave. According to the transmission line theory, the values of RL in the frequency range of 2-18 GHz were calculated, and then the microwave absorption performances of all composites were evaluated. Generally speaking, when the values of RL are lower than \(-10\) dB, it means that the incident wave can be effectively absorbed by the composite, and the corresponding frequency range is called the effective absorption bandwidth (EAB). The CNC shows weak microwave absorption performances, and the maximum RL (RL\({}_{\text{max}}\)) is only \(-5.87\) dB (Figure 7a). For CNCG-1, the RL\({}_{\text{max}}\) is \(-11.70\) dB at 12.60 GHz when the thickness is 2.50 mm (Figure 7b). The microwave absorption performances of CNCG-2 are better than that of CNCG-1. The RL\({}_{\text{max}}\) can reach \(-58.72\) dB at the thickness of 1.64 mm. In addition, the RL\({}_{\text{max}}\) can reach \(-46.79\) dB, and the maximum EAB (EAB\({}_{\text{max}}\)) can achieve 5.74 GHz (12.26-18.00 GHz) at the 1.79 mm thickness (Figure 7c). The microwave absorption performances of CNCG-3 are obviously weaker than that of CNCG-2. The RL\({}_{\text{max}}\) can reach \(-15.96\) dB and the EAB\({}_{\text{max}}\) can achieve 5.64 GHz (12.36-18.00 GHz) at the 1.64 mm thickness (Figure 7d). Through the comprehensive analysis of above results, adding appropriate amount of 3D-graphene into the raw materials can effectively improve the microwave absorption performances of the composite. The mechanism of dielectric loss can be explained by the Debye theory. The relationship between \(\epsilon^{\prime}\) and \(\epsilon^{\prime\prime}\) satisfies the following Formula [19]:  
\[\left(\epsilon^{\prime}-\left(\varepsilon_{\text{s}}+\varepsilon_{\infty} \right)/2\right)^{2}+\left(\epsilon^{\prime\prime}\right)^{2}=\left(\left( \varepsilon_{\text{s}}-\varepsilon_{\infty}\right)/2\right)^{2} \tag{3}\]  
where \(\varepsilon_{\text{s}}\) is the static permittivity in the high frequency limit and \(\varepsilon_{\infty}\) is the relative permittivity. The \(\epsilon^{\prime}-\epsilon^{\prime\prime}\) curves of all composites have some semicircles, indicating that there are multiple relaxation processes under the action of incident wave (Figure 8).