Globular Flower-Like Reduced Graphene Oxide Design for Enhancing Thermally Conductive Properties of Silicone-Based Spherical Alumina Composites

The enhancement of thermally conductive performances for lightweight thermal interface materials is a long-term effort. The superb micro-structures of the thermal conductivity enhancer have an important impact on increasing thermal conductivity and decreasing thermal resistance. Here, globular flower-like reduced graphene oxide (GFRGO) is designed by the self-assembly of reduced graphene oxide (RGO) sheets, under the assistance of a binder via the spray-assisted method for silicone-based spherical alumina (S-Al2O3) composites. When the total filler content is fixed at 84 wt%, silicone-based S-Al2O3 composites with 1 wt% of GFRGO exhibit a much more significant increase in thermal conductivity, reduction in thermal resistance and reinforcement in thermal management capability than that of without graphene. Meanwhile, GFRGO is obviously superior to that of their RGO counterparts. Compared with RGO sheets, GFRGO spheres which are well-distributed between the S-Al2O3 fillers and well-dispersed in the matrix can build three-dimensional and isotropic thermally conductive networks more effectively with S-Al2O3 in the matrix, and this minimizes the thermal boundary resistance among components, owning to its structural characteristics. As with RGO, the introduction of GFRGO is helpful when decreasing the density of silicone-based S-Al2O3 composites. These attractive results suggest that the strategy opens new opportunities for fabricating practical, high-performance and light-weight filler-type thermal interface materials.


Introduction
With the development of microelectronics, there is a tide towards miniaturization and multifunctionalization in the area of modern electric devices. It means that more heat is generated by electronic components in the devices. In order to ensure reliable operation, efficient heat removal from devices is being taken increasingly seriously. Thermal interface materials, which serve as heat transfer bridges, have played a part in the thermal management of packaging and heat-generating electronic The GFRGO/S-Al 2 O 3 /PDMS composites were prepared by an in situ blending method. Firstly, PDMS mixed with a small amount of DOWSIL 11-100 additive was stirred at 30 • C for 10 min. The dosage of DOWSIL 11-100 additive was 0.9%, based on the weight of GFRGO and S-Al 2 O 3 . Secondly, GFRGO and S-Al 2 O 3 , with different weight ratios, were compounded with the above mixture for 4 min at the speed of 2200 rpm, in the automatic rev-rot gravity mixer (VM300SA20, SINOMIX, Mianyang, China). The mixing process was repeated 3-6 times to get a well-dispersed slurry. Subsequently, the as-received slurry was milled at 80 • C using a three-roller machine. In order to obtain a homogeneous compound, the as-prepared GFRGO/S-Al 2 O 3 /PDMS composites was milled 3-6 times. The overall preparation process for GFRGO/S-Al 2 O 3 /PDMS composites is provided in Figure 1. For comparison, S-Al 2 O 3 /PDMS and RGO/S-Al 2 O 3 /PDMS composites were prepared using the same method. Nanomaterials 2019, 9, x FOR PEER REVIEW 3 of 13

Preparation of GFRGO
The GFRGO was obtained by the spray-drying granulation technique, chemical pre-reduction and thermal annealing procedure. Firstly, 2 g GO were added into 1000 mL deionized water by ultrasonication for 180 min. Secondly, 0.4 g 5 wt% PVA aqueous solution was added drop wise into the dispersion, with magnetic stirring for 40 min at 65 °C . After that, the mixture was nebulized into small droplets under 150 °C by the spray dryer. The atomized droplets evaporated in a few seconds and converted into dried globular flower-like GO (GFGO) granules, with the help of the binder PVA. The dried GFGO powders were gathered in the collector. The sample was treated with 0.1 g/L L-AA solution at room temperature until its surface turned brownish black. Next, they were transferred into a tube furnace in argon at 1400 °C to remove organic additives and obtain sintered globular flower-like GFRGO. Finally, the GFRGO was collected for further application. For comparison, RGO was prepared via the same chemical treatment and thermal annealing procedure by using GO as a raw material.

Preparation of GFRGO/S-Al2O3/PDMS Composites
The GFRGO/S-Al2O3/PDMS composites were prepared by an in situ blending method. Firstly, PDMS mixed with a small amount of DOWSIL 11-100 additive was stirred at 30 ℃ for 10 min. The dosage of DOWSIL 11-100 additive was 0.9%, based on the weight of GFRGO and S-Al2O3. Secondly, GFRGO and S-Al2O3, with different weight ratios, were compounded with the above mixture for 4 min at the speed of 2200 rpm, in the automatic rev-rot gravity mixer (VM300SA20, SINOMIX, Mianyang, Chian). The mixing process was repeated 3-6 times to get a well-dispersed slurry. Subsequently, the as-received slurry was milled at 80 ℃ using a three-roller machine. In order to obtain a homogeneous compound, the as-prepared GFRGO/S-Al2O3/PDMS composites was milled 3-6 times. The overall preparation process for GFRGO/S-Al2O3/PDMS composites is provided in Figure  1. For comparison, S-Al2O3/PDMS and RGO/S-Al2O3/PDMS composites were prepared using the same method.

Morphology and Structure of S-Al 2 O 3 and GFRGO
The morphologies of S-Al 2 O 3 , RGO and GFRGO at different magnifications are displayed in Figure 2. It is obvious that the particles of pristine S-Al 2 O 3 possess an ultra-high spherical rate in Figure 2a. The diameter of most of the spheres is about 10 µm. The size S-Al 2 O 3 can contribute to the major thermally conductive pathways in PDMS matrix. There are a tiny proportion of the smaller size S-Al 2 O 3 . It can be served as the point of junction between large sizes of S-Al 2 O 3 to create more contact. Figure 2b is a picture of the selected blue and rectangular region of S-Al 2 O 3 under high magnification. The surface of S-Al 2 O 3 is a bit rough. A few nanoparticles adhere to it. The surface of S-Al 2 O 3 makes it possible for conjunctions between S-Al 2 O 3 and other fillers to occur, which facilitate heat transportation. The SEM image of as-synthesized RGO after ultrasonic treatment is shown in Figure 2c. Some RGO sheets with a few layers are randomly scattered on the conductive film. Figure 2d is an enlarged image of the green and rectangular zone in Figure 2c. The edges of the bare RGO partially curl and the RGO exhibits some folds on its surface in Figure 2d. The graphene structure is favorable for assembling into a special structural filler. Figure 2e shows the picture of the sintered GFRGO. The as-prepared GFRGO has a globular flower-like shape, with the size of about 14 µm. The shape and size of GFRGO are good for improving the thermally conductive properties of silicone-based S-Al 2 O 3 composites. The high magnified image in Figure 2f reveals that GFRGO consists of the highly folded RGO, which interlock with each other. The wrinkled RGO bonds together to form a three-dimensional and crumpled cluster configuration, with ridges and vertices aided by PVA, as a binder via a spray-drying procedure. The spheroidal flower-shaped GFRGO is strong enough to limit disintegration from the mechanical blending during the mixing with polymer matrix. This structure is beneficial for forming an efficient heat conduction channel with S-Al 2 O 3 in PDMS.
The structure of S-Al 2 O 3 , RGO and GFRGO were probed by the XRD patterns and Raman spectra ( Figure 3). The intense peaks in Figure 3a conform to that of α-alumina, indicating the as-used S-Al 2 O 3 with well-crystallized structure is α-alumina [30][31][32]. The high crystalline quality of S-Al 2 O 3 fillers is crucial for the thermal conductive properties of silicone-based S-Al 2 O 3 composites. Figure 3b shows the XRD spectrums of RGO and GFRGO. The RGO spectrum has a broad peak at 25.9 • , signifying the typical multilayered graphene after chemical and thermal reduction. Compared to the RGO, the GFRGO curve shows a moderate sharp peak at 25.9 • , a small peak at 43.6 • and a weak at 53.9 • , corresponding to (002), (100) and (004), and proving only the loose and disordered stacking of self-folded graphene [26]. The significant change of the spectrums means that RGO spliced into GFRGO. The Raman spectra analysis also give information about the structure of GFRGO (Figure 3c). It can be observed that RGO and GFRGO mainly have three characteristic peaks, D peak (1352 cm −1 ), G peak (1595 cm −1 ) and 2D peak (2706 cm −1 ) [27]. The I 2D /I G ratio of GFRGO has a significant decrease compared with RGO, which indicates that RGO sheets build the globular flower-like microstructure of GFRGO. The Raman results confirmed that the GFRGO has been successfully synthesized through adhesive effect by PVA. The architecture of the GFRGO provides the prior condition for the preparation of isotropic silicone-based composites with S-Al 2 O 3 . The structure of S-Al2O3, RGO and GFRGO were probed by the XRD patterns and Raman spectra ( Figure 3). The intense peaks in Figure 3a conform to that of α-alumina, indicating the as-used S-Al2O3 with well-crystallized structure is α-alumina [30][31][32]. The high crystalline quality of S-Al2O3 fillers is crucial for the thermal conductive properties of silicone-based S-Al2O3 composites. Figure 3b shows the XRD spectrums of RGO and GFRGO. The RGO spectrum has a broad peak at 25.9°, signifying the typical multilayered graphene after chemical and thermal reduction. Compared to the RGO, the GFRGO curve shows a moderate sharp peak at 25.9°, a small peak at 43.6° and a weak at 53.9°, corresponding to (002), (100) and (004), and proving only the loose and disordered stacking of self-folded graphene [26]. The significant change of the spectrums means that RGO spliced into GFRGO. The Raman spectra analysis also give information about the structure of GFRGO (Figure 3c).

Density of GFRGO/S-Al2O3/PDMS Composites
Density is a consideration for the silicone-based S-Al2O3 composites in the practical application for thermal management. Low density suggests the lightweight feasibility of the product [33]. Figure  4a,b show the variation of the density with weight contents of S-Al2O3, GFRGO/S-Al2O3 and RGO/S-Al2O3. For the S-Al2O3/PDMS composites in Figure 4a, the density increases linearly with increasing S-Al2O3 content at the S-Al2O3 mass fraction below 84 wt%, and afterwards increases significantly, which is due to the fact that the density of S-Al2O3 is higher than that of PDMS. When the S-Al2O3 content was beyond 88 wt%, the viscosity of S-Al2O3/PDMS composites became very large, so that the S-Al2O3/PDMS composites lost their mobility at ambient temperature. In comparison to S-Al2O3, graphene can be used to prepare the goal-oriented composites with low density and good heat conduction performances, due to its merits of lightweight and high thermal conductivity. Figure 4b shows the density of GFRGO/S-Al2O3/PDMS and RGO/S-Al2O3/PDMS composites with graphene, of which the total filler content is fixed at 84 wt%. It is seen that the density of the two kinds of siliconebased S-Al2O3 composites decreases slightly with the increment of graphene compared to S-Al2O3/PDMS composites. There is no palpable difference in density between the S-Al2O3/PDMS composites filled with GFRGO and RGO. The density of S-Al2O3/PDMS composites is 2.55 g/cm 3 at the S-Al2O3 content 84 wt%. With 1.0 % graphene, the density of GFRGO/S-Al2O3/PDMS composites is about 2.49 g/cm 3 , reduced by about 2.3%. These results indicate that GFRGO can be used as a thermal conductivity enhancer, which has an advantage in the density for silicone-based S-Al2O3 composites.

Density of GFRGO/S-Al 2 O 3 /PDMS Composites
Density is a consideration for the silicone-based S-Al 2 O 3 composites in the practical application for thermal management. Low density suggests the lightweight feasibility of the product [33]. Figure 4a Figure 4a, the density increases linearly with increasing S-Al 2 O 3 content at the S-Al 2 O 3 mass fraction below 84 wt%, and afterwards increases significantly, which is due to the fact that the density of S-Al 2 O 3 is higher than that of PDMS. When the S-Al 2 O 3 content was beyond 88 wt%, the viscosity of S-Al 2 O 3 /PDMS composites became very large, so that the S-Al 2 O 3 /PDMS composites lost their mobility at ambient temperature. In comparison to S-Al 2 O 3 , graphene can be used to prepare the goal-oriented composites with low density and good heat conduction performances, due to its merits of lightweight and high thermal conductivity. Figure 4b shows the density of GFRGO/S-Al 2 O 3 /PDMS and RGO/S-Al 2 O 3 /PDMS composites with graphene, of which the total filler content is fixed at 84 wt%. It is seen that the density of the two kinds of silicone-based S-Al 2 O 3 composites decreases slightly with the increment of graphene compared to S-Al 2 O 3 /PDMS composites. There is no palpable difference in density between the S-Al 2 O 3 /PDMS composites filled with GFRGO and RGO. The density of S-Al 2 O 3 /PDMS composites is 2.55 g/cm 3 at the S-Al 2 O 3 content 84 wt%. With 1.0% graphene, the density of GFRGO/S-Al 2 O 3 /PDMS composites is about 2.49 g/cm 3 , reduced by about 2.3%. These results indicate that GFRGO can be used as a thermal conductivity enhancer, which has an advantage in the density for silicone-based S-Al 2 O 3 composites.

Density of GFRGO/S-Al2O3/PDMS Composites
Density is a consideration for the silicone-based S-Al2O3 composites in the practical application for thermal management. Low density suggests the lightweight feasibility of the product [33]. Figure  4a,b show the variation of the density with weight contents of S-Al2O3, GFRGO/S-Al2O3 and RGO/S-Al2O3. For the S-Al2O3/PDMS composites in Figure 4a, the density increases linearly with increasing S-Al2O3 content at the S-Al2O3 mass fraction below 84 wt%, and afterwards increases significantly, which is due to the fact that the density of S-Al2O3 is higher than that of PDMS. When the S-Al2O3 content was beyond 88 wt%, the viscosity of S-Al2O3/PDMS composites became very large, so that the S-Al2O3/PDMS composites lost their mobility at ambient temperature. In comparison to S-Al2O3, graphene can be used to prepare the goal-oriented composites with low density and good heat conduction performances, due to its merits of lightweight and high thermal conductivity. Figure 4b shows the density of GFRGO/S-Al2O3/PDMS and RGO/S-Al2O3/PDMS composites with graphene, of which the total filler content is fixed at 84 wt%. It is seen that the density of the two kinds of siliconebased S-Al2O3 composites decreases slightly with the increment of graphene compared to S-Al2O3/PDMS composites. There is no palpable difference in density between the S-Al2O3/PDMS composites filled with GFRGO and RGO. The density of S-Al2O3/PDMS composites is 2.55 g/cm 3 at the S-Al2O3 content 84 wt%. With 1.0 % graphene, the density of GFRGO/S-Al2O3/PDMS composites is about 2.49 g/cm 3 , reduced by about 2.3%. These results indicate that GFRGO can be used as a thermal conductivity enhancer, which has an advantage in the density for silicone-based S-Al2O3 composites.   Figure 5a. At the S-Al 2 O 3 content of 84 wt%, its thermal conductivity is 1.39 W·m −1 ·K −1 and the workability of S-Al 2 O 3 /PDMS composites is strong. When the S-Al 2 O 3 content has been further increased to 88%, its thermal conductivity is 1.68 W·m −1 ·K −1 , increased by 21%. At the higher S-Al 2 O 3 loading, the thermal conductivity of the silicone-based S-Al 2 O 3 composites indeed increases, while its workability become weaker because of the increasing viscosity. An ideal silicone-based S-Al 2 O 3 composites possesses strong workability and high thermal conductivity. Combination of two kinds of fillers in composites has been demonstrated to be an effective method to enhance thermal conductivity. In order to maintain strong workability, graphene with higher thermal conductivity was introduced to elevate the thermal conductivity of silicone-based S-Al 2 O 3 composites. The values of thermal conductivity of GFRGO/S-Al 2 O 3 /PDMS composites are greater than that of RGO/S-Al 2 O 3 /PDMS composites in Figure 5b. For GFRGO/S-Al 2 O 3 /PDMS composites, the contact between globular flower-like GFRGO and spherical S-Al 2 O 3 is stronger than that between platelet-like RGO and S-Al 2 O 3 in the RGO/S-Al 2 O 3 /PDMS composites, which reduce the phonon scattering at the interface [28]. For both the GFRGO/S-Al 2 O 3 /PDMS and RGO/S-Al 2 O 3 /PDMS composites, at the graphene content from 0.2% to 0.6%, the proportion of graphene is too low, so that the thermal conductivity of them increases slowly. However, the thermal conductivity of GFRGO/S-Al 2 O 3 /PDMS composites increases faster than that of RGO/S-Al 2 O 3 /PDMS composites and the gap of enhancement between GFRGO/S-Al 2 O 3 /PDMS and RGO/S-Al 2 O 3 /PDMS composites in the thermal conductivity have widened at the graphene content from 0.8% to 1.0%. Table 1 displays the some polymer-based S-Al 2 O 3 composites and their thermal conductivity enhancement. By adding a small portion of other fillers with higher thermal conductivity into polymer-based S-Al 2 O 3 composites for replacing the same content of S-Al 2 O 3 , the thermal conductivity of them is increased compared with that of the corresponding polymer-based S-Al 2 O 3 composites, due to the synergistic effect. In our work, the enhanced ability of GFRGO reached 48%, about 2.1 times that of the RGO in filled S-Al 2 O 3 /PDMS composites (23%), at the filling ratio 1.0%. The GFRGO is a more effective enhancer than RGO for enhancement in the thermal conductivity of silicone-based S-Al 2 O 3 composites, which is attributed to the formation of the isotropic, continuous and stable heat-conductive pathways by the 3D near-spherical GFRGO and the spherical S-Al 2 O 3 , as well as the synergistic effect of the binary-filler hybrid [2,34,35]. Thus, the structure and dimensions of GFRGO are in favor of the improvement in the thermal conductivity of silicone-based S-Al 2 O 3 composites. Compared with this work, several studies reported higher values of composites with graphene or graphene and boron nitride was achieved in the heat conduction properties at the high filler loading [36][37][38]. These results were quite enlightening for developing silicone-based S-Al 2 O 3 composites, with better heat conduction properties in the later studies. The thermal conductivity of silicone-based S-Al2O3 composites is one of important thermally conductive performances. The thermal conductivity of S-Al2O3, GFRGO/S-Al2O3 and RGO/S-Al2O3 filled PDMS composites are shown in Figure 5a,b. A continuous increase in thermal conductivity of silicone-based S-Al2O3 composites is observed, with increasing S-Al2O3 loading in Figure 5a. At the S-Al2O3 content of 84 wt%, its thermal conductivity is 1.39 W· m -1 · K -1 and the workability of S-Al2O3/PDMS composites is strong. When the S-Al2O3 content has been further increased to 88 %, its thermal conductivity is 1.68 W· m -1 · K -1 , increased by 21%. At the higher S-Al2O3 loading, the thermal conductivity of the silicone-based S-Al2O3 composites indeed increases, while its workability become weaker because of the increasing viscosity. An ideal silicone-based S-Al2O3 composites possesses strong workability and high thermal conductivity. Combination of two kinds of fillers in composites has been demonstrated to be an effective method to enhance thermal conductivity. In order to maintain strong workability, graphene with higher thermal conductivity was introduced to elevate the thermal conductivity of silicone-based S-Al2O3 composites. The values of thermal conductivity of GFRGO/S-Al2O3/PDMS composites are greater than that of RGO/S-Al2O3/PDMS composites in Figure  5b. For GFRGO/S-Al2O3/PDMS composites, the contact between globular flower-like GFRGO and spherical S-Al2O3 is stronger than that between platelet-like RGO and S-Al2O3 in the RGO/S-Al2O3/PDMS composites, which reduce the phonon scattering at the interface [28]. For both the GFRGO/S-Al2O3/PDMS and RGO/S-Al2O3/PDMS composites, at the graphene content from 0.2% to 0.6 %, the proportion of graphene is too low, so that the thermal conductivity of them increases slowly. However, the thermal conductivity of GFRGO/S-Al2O3/PDMS composites increases faster than that of RGO/S-Al2O3/PDMS composites and the gap of enhancement between GFRGO/S-Al2O3/PDMS and RGO/S-Al2O3/PDMS composites in the thermal conductivity have widened at the graphene content from 0.8% to 1.0 %. Table 1 displays the some polymer-based S-Al2O3 composites and their thermal conductivity enhancement. By adding a small portion of other fillers with higher thermal conductivity into polymer-based S-Al2O3 composites for replacing the same content of S-Al2O3, the thermal conductivity of them is increased compared with that of the corresponding polymer-based S-Al2O3 composites, due to the synergistic effect. In our work, the enhanced ability of GFRGO reached 48%, about 2.1 times that of the RGO in filled S-Al2O3/PDMS composites (23%), at the filling ratio 1.0 %. The GFRGO is a more effective enhancer than RGO for enhancement in the thermal conductivity of silicone-based S-Al2O3 composites, which is attributed to the formation of the isotropic, continuous and stable heat-conductive pathways by the 3D near-spherical GFRGO and the spherical S-Al2O3, as well as the synergistic effect of the binary-filler hybrid [2,34,35]. Thus, the structure and dimensions of GFRGO are in favor of the improvement in the thermal conductivity of silicone-based S-Al2O3 composites. Compared with this work, several studies reported higher values of composites with graphene or graphene and boron nitride was achieved in the heat conduction properties at the high filler loading [36][37][38]. These results were quite enlightening for developing silicone-based S-Al2O3 composites, with better heat conduction properties in the later studies.   The thermal resistance of silicone-based S-Al 2 O 3 composites is also a crucial thermally conductive property index. Figure 6a Figure 6a. At a filler content of 84%, the thermal resistance of S-Al 2 O 3 /PDMS composites is 0.262 • C/W. While when 88% S-Al 2 O 3 is added, it drops to 0.225 • C/W, only reduced by about 14%, which is accompanied by the decline of constructability and the augment of density. The lower the thermal resistance, the more effective the heat conduction is. The loading of graphene in silicone-based S-Al 2 O 3 composites is expected to induce a diminution in its thermal resistance. GFRGO/S-Al 2 O 3 /PDMS and RGO/S-Al 2 O 3 /PDMS composites do have the same declining trend in the thermal resistance, with the addition of GFRGO and RGO. Indeed, this highlights the role of graphene having different shapes, which can replace a fraction of S-Al 2 O 3 and serve as thermally conductive enhancer. It is illustrated that new and effective heat-conductive paths were established to reduce interfacial thermal resistance between the binary-filler hybrid and PDMS [45]. When the total filler content is fixed at 84%, the thermal resistance of GFRGO/S-Al 2 O 3 /PDMS composites with GFRGO of 1% is presented with lower values, with a reduction of 42%, when compared to that without graphene. Meanwhile, it is observed that GFRGO significantly outperforms that of RGO counterparts, too. The decline of thermal resistance of the GFRGO/S-Al 2 O 3 /PDMS composites is greater than that observed for RGO/S-Al 2 O 3 /PDMS composites in the same content of graphene. The thermal resistance value of the GFRGO/S-Al 2 O 3 /PDMS composites declines to a value of 0.152 • C/W with GFRGO of 1%, lower than 0.212 • C/W of the RGO/S-Al 2 O 3 /PDMS composites. Compare to flaked RGO, near-spherical GFRGO and spherical S-Al 2 O 3 can form stronger interface interaction with PDMS and built up more favorable isotropic thermal conductivity pathways [46]. The size of GFRGO is greater than RGO, so has more energetic effects on the S-Al 2 O 3 /PDMS composites [47]. Therefore, regarding thermal resistance, the GFRGO is more suitable for S-Al 2 O 3 /PDMS composites, which benefit from the shape and size of the enhancer, compared with RGO.

Thermally Conductive Properties of GFRGO/S-Al2O3/PDMS Composites
An infrared thermal imaging technique is used for investigating the heat diffusivity of silicone-based S-Al 2 O 3 composites directly. GFRGO/S-Al 2 O 3 /PDMS composites with GFRGO of 1% and S-Al 2 O 3 of 83%, RGO/S-Al 2 O 3 /PDMS composites with RGO of 1% and S-Al 2 O 3 of 83%, and S-Al 2 O 3 /PDMS composites with S-Al 2 O 3 of 84% were subjected to cycles of heating and cooling. Figure 7 displays their surface temperature variations with time, during a thermography test by an infrared thermal imager. To investigate the heat absorption property, they were put on a heating round-platform (90 • C), heated for 5 min and the changes in temperature were observed. Detailed temperature distribution images acquired subsequently at various times during the heating process are shown in Figure 7a All specimens stabilize at an invariable temperature over time, because of their stable state heat conduction [48]. Therefore, we can conclude that the heat absorption property of GFRGO/S-Al 2 O 3 /PDMS composites is best in the midst of them. To study their heat dissipation property, the specimens were withdrawn from heat source and placed on the disk to cool down. The quick and obvious color shift of specimens was observed in the cooling course. The colors of GFRGO/S-Al 2 O 3 /PDMS composites at the same time were lighter than RGO/S-Al 2 O 3 /PDMS and S-Al 2 O 3 /PDMS composites, reflecting the improved heat release. Three temperature-cooling time lines of the corresponding specimens are displayed in Figure 7b. All specimens cooled down at varying heat dissipation speeds and exhibited relatively big decreasing amplitude in the surface temperature in the beginning. After that, they showed a gradual reduction in temperature change. An infrared thermal imaging technique is used for investigating the heat diffusivity of siliconebased S-Al2O3 composites directly. GFRGO/S-Al2O3/PDMS composites with GFRGO of 1 % and S-Al2O3 of 83 %, RGO/S-Al2O3/PDMS composites with RGO of 1 % and S-Al2O3 of 83 %, and S-Al2O3/PDMS composites with S-Al2O3 of 84 % were subjected to cycles of heating and cooling. Figure  7 displays their surface temperature variations with time, during a thermography test by an infrared thermal imager. To investigate the heat absorption property, they were put on a heating roundplatform (90 °C), heated for 5 min and the changes in temperature were observed. Detailed temperature distribution images acquired subsequently at various times during the heating process are shown in Figure 7a. Obviously, GFRGO/S-Al2O3/PDMS composites can absorb the quantity of heat most efficiently from the hot-stage with the express and noticeable color variance, followed by capability, because of their superiority in thermal conductivity and thermal resistance [49][50][51]. It follows that GFRGO could help in improving the capacity of heat transfer of silicone-based S-Al2O3 composites effectively.

Conclusions
In this work, GFRGO, with a globular flower-like structure, has been prepared successfully by a spray-assisted self-assembly, investigated as a thermal conductivity enhancer for silicone-based S-Al2O3 composites. Like RGO, the introduction of GFRGO is conducive to the density of silicone-based S-Al2O3 composites. In contrast to RGO, GFRGO, with its superb micro-structure, enhances the thermally conductive properties of silicone-based S-Al2O3 composites more effectively, which can be ascribed to the formation of the 3D isotropic thermally conductive network with S-Al2O3. When the content of S-Al2O3 and graphene is fixed at 84 wt%, the thermal conductivity enhancement of GFRGO/S-Al2O3/PDMS composites with 1 wt% of GFRGO reached 48%, about 2.1 times that of RGO/S-Al2O3/PDMS composites (23%). Meanwhile, the thermal resistance of the GFRGO/S-Al2O3/PDMS composites declined to 0.152 °C/W from 0.262 °C/W of the S-Al2O3/PDMS composites, lower than the 0.213 °C/W of the RGO/S-Al2O3/PDMS composites. Importantly, GFRGO/S-

Conclusions
In this work, GFRGO, with a globular flower-like structure, has been prepared successfully by a spray-assisted self-assembly, investigated as a thermal conductivity enhancer for silicone-based S-Al 2 O 3 composites. Like RGO, the introduction of GFRGO is conducive to the density of silicone-based S-Al 2 O 3 composites. In contrast to RGO, GFRGO, with its superb micro-structure, enhances the thermally conductive properties of silicone-based S-Al 2 O 3 composites more effectively, which can be ascribed to the formation of the 3D isotropic thermally conductive network with S-Al 2 O 3 . When the content of S-Al 2 O 3 and graphene is fixed at 84 wt%, the thermal conductivity enhancement of GFRGO/S-Al 2 O 3 /PDMS composites with 1 wt% of GFRGO reached 48%, about 2.1 times that of RGO/S-Al 2 O 3 /PDMS composites (23%). Meanwhile, the thermal resistance of the GFRGO/S-Al 2 O 3 /PDMS composites declined to 0.152 • C/W from 0.262 • C/W of the S-Al 2 O 3 /PDMS composites, lower than the 0.213 • C/W of the RGO/S-Al 2 O 3 /PDMS composites. Importantly, GFRGO/S-Al 2 O 3 /PDMS composites display better thermal management capability than the other two composites during the heat transfer process. Thus, GFRGO is a candidate for thermal conductivity enhancing of the light-weight and high-performance thermal interface materials.