# Carbon Nanotube-Based Composite Filaments for 3D Printing of Structural and Conductive Elements
## 4 Conclusions  
In this study, filaments based on ABS/CNT matrices were prepared for the FDM additive process. It was shown that with the FDM technique, we can fabricate complex shapes with the use of these composites. The results demonstrate that CNT content has a significant and nonlinear impact on mechanical and electrical properties. During the extrusion and printing of the developed filaments, it is important to apply optimal process parameters to obtain high-quality filaments and 3D printed elements. The key parameters are extrusion/print speed and head temperature. The speed must not be too high, as the appropriate amount of material is not applied. However, when it is very low, the print time is much longer, which is significant for industrial applications. On the other hand, if the temperature value is too low, the filament does not melt properly. The tensile strength test showed that a small amount of CNT in the composite could decrease the maximum tensile strength of printed elements compared to those printed with pure ABS. Above the estimated mechanical percolation threshold, the addition of 9.09 wt.% of CNTs results in a 12.6% increase in tensile strength. This counterintuitive phenomenon of strength value decrease must be taken into consideration during the elaboration of new composite materials. On the other hand, the high content of CNTs favored for greater mechanical and electrical properties can be problematic for the processing of composite materials. Electrical conductivity measurements showed that elements printed with composites containing 4.76 and 9.09 wt.% CNT are electrically conductive. For a fixed voltage value equal to 12 V, samples average resistivity reduces to 2.5 \(\Omega\)m for 4.76 wt.% CNT and to 0.15 \(\Omega\)m for 9.09 wt.% CNT. Current-voltage characteristics were determined for the developed materials. A significant influence of the tunneling mechanism on the electrical properties of ABS/CNT composites was observed. Non-linearity of the characteristics proves that the electrical conductivity is based in such composites on quantum tunneling. For materials with higher CNT content, the occurrence of physical contacts between individual nanotubes forming conductive pathways inside the composite becomes more and more critical, resulting in a decrease in resistance at higher values of measurement voltages than for composites with lower CNT content. For material containing 4.76 wt.% CNTs, the decrease in resistance value during a voltage change from 2 to 18V was 83% and for material containing 9.09 wt.% CNTs it was 88%. The research also showed the  
Figure 9: Electrical resistance values of conductive composite samples under different values of voltage applied and different schemes of I–V measurement (increasing and decreasing voltage load applied). occurrence of resistivity hysteresis, depending on the applied voltage. It was observed that the resistivity of the composite is lower when the voltage value decreases compared to the resistivity for an increasing voltage. This relation is directly related to the hysteresis of the I-V relation occurring during the tunneling mechanism. Additional work needs to be done to explore the influence of the other carbon and metal fillers on mechanical and electrical properties of composites, printing parameters (speed, temperature, infill, layer height etc. ), and the porosity of the printed components. Yet, at this moment the obtained and presented results demonstrate that ABS/CNT composite filaments are promising materials for FDM additive manufacturing and may open the possibilities for many new applications in advanced industrial fields such as structural electronics, intelligent structures, and embedded sensors. Conceptualization, B.P. and A.S.; methodology, B.P., P.M., A.S., and M.S. ; validation, P.M. and A.S.; formal analysis, B.P., A.S., and M.S. ; investigation, P.M. and A.S.; resources, A.S. and M.S. ; writing--original draft preparation, B.P. ; writing--review and editing, M.S. ; visualization, B.P. ; supervision, A.S. and M.S.