## 3 Results and Discussion  
### Effect of Graphene Content on the Property of Graphene/PLA Composite Absorbers  
The reflection loss of graphene/PLA composite absorbers with different graphene content is presented in Figure 1. It can be seen that, in the frequency range 10.0-15.8 GHz, the reflection loss of 7 wt% graphene/PLA composite absorber is lower than \(-\)4 dB and the minimum reflection loss value is \(-\)8.3 dB at the frequency of 13.2 GHz; in the frequency range 10.2-15.2 GHz, 9 wt% graphene/PLA composite absorber has a reflection loss of less than \(-\)4 dB and a minimum reflection loss of \(-\)7.3 dB at the frequency of 12.2 GHz. The reflection loss of 5 wt% graphene/PLA composite absorber is higher than \(-\)4 dB in the whole frequency band tested, reaching the minimum reflection loss value of \(-\)3.1 dB at the frequency of 14.2 GHz. As shown in Figure 2, the real and imaginary parts of the complex permittivity of graphene/PLA composite absorber decrease with an increase of frequency. In Figure 2a, as the mass fraction of graphene increases, the real part of the complex permittivity of the composite absorber increases. When the graphene content is 9 wt%, the real part of the complex permittivity is the highest, and at 5 wt% it is the lowest, because as the mass fraction of graphene increases, the number of electric dipoles stored in the composite absorber increases, and the polarization of the electric dipole increases, resulting in an increase in the real part of the complex permittivity [9; 10]. As shown in Figure 2b, when the graphene content is 7 wt%, the composite absorber has the highest complex permittivity imaginary  
Figure 1: Reflection loss of graphene/PLA composite absorbers with different graphene content. part, followed by 9 wt% and the lowest which is 5 wt%. According to the theory of free electrons [11], \(\varepsilon^{\prime\prime}=\sigma/\omega\varepsilon_{0}\), where \(\sigma\) is the electrical conductivity of the material, \(\omega\) is the pi, and \(\varepsilon_{0}\) is the complex permittivity of free space. It can be concluded that the imaginary part of the complex permittivity is positively correlated with the conductivity. The electrical conductivity of the composite is \(2.06\times 10^{-4}\) S/cm when the content of graphene is 5 wt%. When the amount of graphene added is 7 wt%, the electrical conductivity is significantly increased to \(2.71\times 10^{-3}\) S/cm, indicating that the density of the three-dimensional conductive network is increased when the graphene content is increased from 5 wt% to 7 wt%. When the graphene content is 9 wt%, the electrical conductivity of the composite is 2.59 \(\times 10^{-3}\) S/cm. Due to the enhanced adsorption between graphene, the dispersion state of graphene in the matrix is affected and the conductive network structure is destroyed, and thus the conductivity is dropped. The SEM sectional view of PLA absorber and graphene/PLA composite absorbers are shown in Figure 3. It can be seen from Figure 3a that the pure PLA absorber has a relatively flat section. It can be seen from Figure 3b that the graphene is coated on the surface of the PLA matrix. Due to the stacking phenomenon between the graphene sheets, the wrinkles are more obvious when the amount of graphene is 7 wt% (as shown in Figure 3c). When the amount of graphene added is 9 wt%, as shown in Figure 3d, the graphene powders are agglomerated together and the absorbing property of the composite is weakened due to the presence of electron scattering. It can be seen that in the 8.2 to 18.0 GHz band, when the thickness of the composite part is constant, the absorbing property of the 7 wt% graphene/PLA composite is relatively better. Figure 2: Complex permittivity of graphene/PLA composite absorbers with different graphene content. (**a**) The real part of the complex permittivity; (**b**) the imaginary part of the complex permittivity.