## 4 Conclusions  
Within the scope of this work was a study of the effect of CNT and CB dispersions in DiglycidylI Ether of bisphenol-A (DGEBA) epoxy resin on the electrical and mechanical properties. A parametric study was performed to investigate the effects of filler content/type, and dispersion duration by applying IS and mechanical measurements.  
IS results for CNT binary protocols indicated the formation of the conductive network for concentrations over 0.3 wt. % MWCNTs, designated by a step drop of impedance magnitude. CNTs0.5_III exhibited optimal electrical properties between CNT binary protocols, while the CNTs0.5_IV showed an increase of the impedance after the fourth hour of dispersion by almost half an order of magnitude, probably caused by CNTs structural degradation. The second binary group consisted of CB dispersion did not present any changes due to the increase of the CB concentration and the dispersion duration. Ternary systems revealed ameliorated electrical properties in comparison to binary systems, manifested by a decrease of the magnitude of the impedance. This behavior was caused  
Figure 8: Scanning electron micrographs recorded from the fracture surface of a CNTs0.5_CB2-IV SENB specimen at (**a**) \(\times\)100, (**b**) and (**d**) \(\times\)500 magnifications, and (**c**) \(\times\)1000.  
by the synergistic effects of the CNT/CB network. Both ternary systems exhibited an improvement in electrical properties during the evolution of the dispersion process.  
The modification of the epoxy resin DGEBA increased its fracture toughness. Systems with CNTs exhibited improved mean values of \(\mathrm{K_{ic}}\) and \(\mathrm{G_{ic}}\) at 3 h dispersion in most cases. Binary systems with CB exhibited insignificantly enhanced mean values of \(\mathrm{K_{ic}}\), with no correlation between that dispersion duration. However, in most of the cases, \(\mathrm{G_{ic}}\) values decreased or remained stable. Ternary systems exhibited increased \(\mathrm{K_{ic}}\) as filler percentage and dispersion time increased. Significant improved of \(\mathrm{G_{ic}}\) was found for CNTs0.5_CB2-III and CNTs0.5_CB2-IV.  
Overall, nano- and micro- fillers such as CNTs and CB improved the mechanical properties of epoxics. \(\mathrm{K_{ic}}\) values, which represent the fracture toughness of the epoxy, increased in all systems, with the most valuable protocol being the CNTs0.5_CB2-IV, due to the synergistic effect that CNTs and CB produce, as mentioned before. In terms of \(\mathrm{G_{ic}}\), which represent the maximum energy required to fracture, the most beneficial system was CNTs in all percentages. This could be attributed to the excellent properties of CNTs, such as their high aspect ratio (length-to-diameter ratio).  
IS and mechanical measurements revealed a direct correlation between the electrical and mechanical properties of CNTs/CB reinforced epoxy resin, while fractography confirmed the aforementioned results. Various fracture mechanisms were observed in both binary systems (CNTs and CB) such as crack pinning, particle pull-out and deflection. In addition, in ternary systems synergistic effects of the nano- and micro-fillers led to a combination of the above mechanisms.  
G.F.: conceptualization, methodology, analysis, writing--original draft preparation, and editing. K.T.: conceptualization, methodology, analysis, writing--review and editing. L.T.: analysis, writing--review and editing. D.B.: conceptualization, writing--review and editing. A.S.P.: conceptualization, supervision, project administration, review and editing.  
This research has been co-financed by the Operational Program \"Human Resources Development, Education and Lifelong Learning\" and is co-financed by the European Union (European Social Fund) and Greek national funds.  
DGEBA, Epikote 828 epoxy resin and Epicure 541 hardener were kindly provided by Dichem Polymers, Greece.  
The authors declare no conflict of interest.