## 3 Results and Discussion
### Thermal-Mechanical Properties  
The effect of the dispersed fillers on the thermo-mechanical properties of crosslinked silicone composites was evaluated by DMTA. As reported in Table 2, the addition of the filler did not significantly influence the Tg of the crosslinked polymer matrix. Indeed, all the measured Tg lay within \(-125\)\({}^{\circ}\)C and \(-115\)\({}^{\circ}\)C. Figure 5 shows, as an example, the tan\(\delta\) curves for the Sylgard containing spheroidal copper (all results are reported in Supplementary Materials Figures S6-S11). The increased Tg can be explained by the hindrance effect of the filler to the motion of PDSM chains. This result was in accordance with some previous researches [5; 6; 18]. Tensile tests were carried out on carbon-based composites. The effect of the addition of the filler is shown in Figure 6. The filler influenced the rigidity of the matrix increasing the Young Modulus. The Young Modulus for pristine Sylgard was measured to be 1.16 MPa. The dispersion of CB induced an enhancement of the value to 1.84 MPa and 2.72 MPa respectively for the silicone composite containing 2.2 vol\(\%\) and 4.4 vol\(\%\). When CNTs were dispersed a Young modulus of 1.90 MPa was achieved in the presence of 0.06 vol\(\%\) of CNT, a value of 2.00 MPa for the composite containing 0.3 vol\(\%\) and a value of 2.55 MPa for the sample containing 0.6 vol\(\%\) of the filler. The strain at break point was reduced for the composites with respect to the pristine crosslinked Sylgard. Considering these results, it can be assumed that the mechanical effect of the filler was the increasing of rigidity of the  
Figure 4: SEM imagines for composite, the red arrows highlight the filler and in the picture I the voids; (**a**) pristine Sylgard, \(100\times\); (**b**) Cu15D, \(400\times\); (**c**) Cu15S, \(4000\times\); (**d**) Ni15, \(400\times\); (**e**) NiL15, \(100\times\); (**f**) BN6.6, \(10{,}000\times\); (**g**) CNT0.6, \(10{,}000\times\); (**h**) CB4.4, \(400\times\); (**i**) CB4.4 \(100\times\). The thermal conductivity of pristine crosslinked Sylgard was measured, obtaining a value of 0.19 W/mK. The addition of metal particles increased k values and the data are collected in Figure 7. The dispersion of metal fillers induced an increase of k as a function of filler volume content. Both spheroidal copper and dendritic copper powders led to a large increase of thermal conductivity (Figure 8), it was possible to achieve an enhancement of the k value of Sylgard by 105% when spheroidal copper was added (CuS15), and about 300%, when dendritic copper was dispersed in the silicon matrix (CuD15). While in the case of spheroidal copper it was not possible to observe a linear increase of k as a function of filler volume content, the decrease of the  
of filler content, this is evident in the case of dendritic copper, showing the important role played by the shape of the particles, as it will be further discussed below. These results were consistent with previous studies reported in literature [24; 25], where it was observed that spheroidal Cu particles had a lower impact compared to elongated forms of Cu particles on increasing thermal conductivity of polymer composites. This study showed higher % increment respect to the pristine matrix if compared with the previous data cited in literature and report a new composite considering the PDMS as polymer matrix. Figure 8: Percentage of k increment related to pristine Sylgard of the different metal-based composites. Figure 7: k values of the different composite measured by Hot Disk. Similarly, the Ni particles dispersion within the polymeric matrix showed an enhancement of the k value, achieving higher values by using larger particles (see data collected in Figures 7 and 8) at a content of 15 vol% with a successful enhancement of k of around 150%. These results are in agreement with the data reported in literature [4; 10; 33], where larger particles are more likely to produce a heat-conductive path in the materials with a higher enhance the thermal conductivity.