## 3 Results and Discussion  
### Characterization of Fillers  
MWCNTs have a diameter of 15-40 nm and a length of 3-5 \(\upmu\)m (Figure 1a-c). From the images, it can be seen that the MWCNT sample is represented by long nanotubes with a large number of walls and a hollow channel inside. The diameter of the channel was below 10 nm. Additionally, there was some amount of carbon nanofibers with the cup-stacked structure in the sample. According to TEM, the sample also contained catalytic nanoparticles. The particle size distribution of the MWCNT sample is shown in Figure 1d. The data show that the sample consisted of aggregates of carbon nanotubes, with the predominant particle size ranging from 5 to 280 \(\upmu\)m. Since it was not possible to determine the particle size of EG using sedimentation analysis, the grain size was determined using sieves. There were the following fractions of the sample of EG (wt.%): above 500 \(\upmu\)m--47.4%, 250-500 \(\upmu\)m--45.3%, 200-250 \(\upmu\)m--4.3%, 100-200 \(\upmu\)m--2.5%, below 100 \(\upmu\)m--0.5%. According to data of TG obtained in the oxidative atmosphere, the ash content in the material was 8.37% (Figure 1e). At the same time, the ash content of the EG sample was only 5.14% which corresponds to a relatively lower amount of impurities and a high fraction of carbon in the material. SEM images of the EG sample are shown in Figure 2b-d. Figure 2a shows the worm-like particles obtained as a result of the rapid heating of intercalated graphite. The structure of graphite was porous. The pores were formed during the release of the gas phase under heating of intercalated graphite and therefore the sample is represented by the strongly-entangled graphite platelets. Figure 3 shows the Raman spectra of MWCNTs and EG. There are two main peaks in the Raman spectra. The D peak corresponds to defect structure in carbon materials. The G peak corresponds to the ordered graphite structure in the material [40,41]. The positions of the D and G peaks for the MWCNTs were 1345 cm\({}^{-1}\) (full width at half maximum (FWHM) 50 cm\({}^{-1}\)) and 1572 cm\({}^{-1}\) (FWHM 42 cm\({}^{-1}\)), respectively. Expanded graphite showed the shift of positions towards higher angles, D (1356 cm\({}^{-1}\), FWHM 84 cm\({}^{-1}\)) and G (1584 cm\({}^{-1}\), FWHM 20 cm\({}^{-1}\)), confirming the well-ordered structure of the material, which is closer to graphite. The ratio of intensities I(D)/I(G) makes it possible to estimate the disorder degree of the MWCNTs and EG, and it was 0.84 and 0.1, respectively. The Raman spectrum of MWCNTs shows their relatively high defectiveness closer to the level of defects of carbon nanofibers [42], whereas the expanded graphite has a low intensity of D peak that confirms the high graphitization degree of the material. Investigation of textural characteristics showed that the MWCNT sample is fully mesoporous with the surface area 128 m\({}^{2}\)/g. The average pore diameter and total pore volume were 10.8 nm and 0.346 cm\({}^{3}\)/g. The EG sample had a higher porosity compared to the MWCNTs. Its surface area was 593 m\({}^{2}\)/g with the domination of mesopores (448 m\({}^{2}\)/g) and a lower fraction of micropores (145 m\({}^{2}\)/g). The average pore diameter was lower compared to the MWCNTs (4.3 nm) and the total pore volume was also lower (0.198 cm\({}^{3}\)/g). The role of surface oxygen-containing functional groups is important for the formation of an interface between epoxy resin and fillers; therefore, it is necessary to determine the concentrations of these groups using X-ray photoelectron spectroscopy (XPS). An XPS spectrum of the MWCNT sample was mentioned in [43] (original sample name was iMWCNT). The XPS spectrum of the EG sample is in the Supplementary Materials (Figure S1). Both samples contain C-O, C=O, and O-C=O functional groups. The O/C ratio was 0.1 (EG) and 0.04 (MWCNTs), showing the higher concentration of oxygen on the surface of the expanded graphite sample. However, it can be noted that the concentration of functional groups was relatively low.