## 3 Results and Discussion
### Effect of Open-Cell Structure on the Microwave Absorption Performance
#### 3.4.2 Effect of Cell Size on Microwave Absorption Performance  
Furthermore, the effect of cell size of OCGA on microwave absorption performance was also studied, which could be adjusted by the rotational speeds when the GO Pickering emulsions were prepared. The OCGAs with different cell sizes were obtained by applying different rotational speeds of 10k rpm, 15k rpm, 20k rpm and 25k rpm, which were named OCGA-10K, OCGA-15K, OCGA-20K and OCGA-25K, respectively. The morphology of OCGAs was observed, and their SEM images were shown in the Figure 6. The graphene cells are regular, opening spheres and the size of the cells decreases from 30 in OCGA-10K to 4.5 in OCGA-25K, which corresponds to the microphotograph of their Pickering emulsions (Figure S2). So, the cell size can be controlled easily by the rotation speed. The electromagnetic parameters of OCGAs with different cell sizes were measured, and their RL were also calculated. As shown in Figure 6, the peak location of RLmin rises from 8.42 GHz for OCGA-10K to 10.48 GHz for OCGA-15K, 11.38 GHz for OCGA-20K and 17.87 GHz for OCGA-25K in order. In addition, the matching thickness of the RLmin is reduced from 5.0 mm for OCGA-10K to 3.0 mm for OCGA-25K (Figure 6). This is due to the progressive enhancement of the dielectric loss of OCGA to microwaves as the diameter decreases (Figure S3). Furthermore, the RLmin of OCGAs decreases at first and then increases, which reach a minimum value of \(-\)51.22 dB at OCGA-20K (Figure 6). Thus, the microwave absorption performance of OCGAs achieves the optimum state of impedance matching characteristics (Figure S4) and attenuate ability when the diameter of cells is about 6. Moreover, we find an interesting phenomenon that when the thickness of OCGAs is fixed, the change of cells size has very little effect on the peak location of RL, which only changes peak strength (Figure 6). The regular methods of adjusting microwave absorption performance, such as changing the composition or deformation of materials, usually affect both the peak location and strength of RL, which can hardly be controlled individually. In our opinion, this property of OCGA is due to the constant volume fraction of graphene in the different OCGAs, causing an almost indistinguishable modulus of complex permittivity (Figure 6). Owing to fixed matching thickness, constant modulus of the complex permittivity and permeability, the change of cell size has little effect on the peak location according to Equation (5). This characteristic greatly expands the application of OCGA. For example, if an absorber for 11 GHz to 12 GHz is demanded, the RLmin of our material can be adjusted freely from about \(-23\) dB to \(-45\) dB, and meanwhile there is little interference to other frequency bands. The compressive strength of graphene aerogels is also obviously improved when the cell structure is introduced. The compression performance of GA and OCGAs with different cell sizes was tested. The results show that compression strength of GA without cell structure is 3.6 kPa. When the cell structure exists, the compressive strength of OCGAs increases obviously, and its maximum value reaches 9.5 kPa of OCGA-20K with a 16% increase (Figure 7a). Based on this, the OCGA-20K can withstand a 1.6 \(\times\) 10\({}^{4}\) times heavier weight than its own mass. To visualize the increase of compressive strength, a  
Figure 6: Characterization of OCGA with different cell diameters. SEM images of (**a**) OCGA-10K, (**b**) OCGA-15K, (**c**) OCGA-20K and (**d**) OCGA-25K, showing an average diameter of about 30 \(\upmu\)m, 9 \(\upmu\)m, 6 \(\upmu\)m and 4.5 \(\upmu\)m, respectively. (**e**) RLmin of the OCGAs, reaching \(-35.89\) dB, \(-44.65\) dB, \(-51.22\) dB and \(-41.61\) dB, respectively. (**f**) Matching thickness of OCGA at RLmin. (**g**) RL of OCGA at the matching thickness of 4 mm. (**h**) Modulus of the complex permittivity of OCGA. counterweight (200 g) was placed on different graphene aerogel.