# Carbon Nanotube-Based Composite Filaments for 3D Printing of Structural and Conductive Elements
## 3 Results
### Electrical Properties  
Direct currentresistivity measurements of 3D printed samples were carried out using a four-probe method. Rohde & Schwarz HM8112 multimeters were used during measurements. It should be noted that samples with electrical resistance above 200 M\(\Omega\) were considered as non-conductive. To calculate samples' resistivity, Equation (1) was used, where \(\rho\) is the volume resistivity, \(S\) is the cross-sections area, \(R\) is the resistance of the measured sample, and \(l\) is a length of the measured sample. \[\rho=\frac{SR}{l} \tag{1}\]Our first attempts of resistance measurements was conducted at a constant voltage of 12 V. Tests showed that components printed from composites containing 0.99 and 1.96 wt.% of CNT exhibited high resistance (above 200 M\(\Omega\)) and therefore were treated as non-conductive. When the CNT content increases to 4.76 wt.%, the electrical resistivity reduces to 2.5 \(\Omega\)m. With the addition of 9.09 wt.% CNT, average resistivity reduces to 0.15 \(\Omega\)m, which is a value that allows the material to be used in many practical applications. This phenomenon indicates that an increase of CNT content in ABS/CNT can promote the formation of multiple conductive paths on the surface and inside 3D printed elements due to the excellent electrical conductivity of CNT. This is at least twice the improvement in results than for the previously reported ABS/CNT filaments [43; 44]. With the same rule of thumb as previously, we have estimated the electrical percolation threshold at the CNT content of 4.76 wt.%. It is worth mentioning that the electrical percolation threshold is much lower than the mechanical percolation. Composite with a CNT content of 4.76 wt.% shows electrical conductivity even though the mechanical percolation threshold is at much higher filler loading. The presence of a tunneling mechanism causes this phenomenon. Electrical connections between two conductive particles arise from two different mechanisms: mechanical contact between particles or electron tunneling effect. According to quantum tunneling theory, electrons can pass through insulator material, causing electrical current flow, under specific conditions. In other words, it is possible that a pair of conductive inclusions (CNT) dispersed in an insulating polymer matrix (ABS) is electrically connected, and electrons can pass from one inclusion to another, even if there is no physical connection between them [45; 46]. It follows that even a smaller amount of the conductive functional phase of CNT can improve the conductive properties of a composite. However, only a larger amount of CNT can improve the mechanical properties, because to increase the tensile strength of a composite, it is necessary to form long carbon fibers from single nanotubes, and that is why it is necessary to connect them physically (Figure 7). Figure 6: Comparison of ultimate tensile strength values for composite samples with different CNTs loading (from 0.99 to 9.09 wt.%), and with ABS reference samples (0 wt.% of CNTs). In order to confirm the occurrence of the tunneling effect in the developed composites, current-voltage characteristics of the printed elements were determined. In the case where the leading type of conductivity in the material is based on the tunnelling effect, the current-voltage characteristics are not linear. The non-linearity is due to the fact that CNTs are semiconductor materials, but also because the change in the electrical properties of the percolation network depends on external factors such as temperature or electric field manifesting as the decrease of junction resistance [47]. Figure 8 shows I-V characteristics for specimens printed with ABS/CNT filaments containing 4.76 and 9.09 wt.% of CNT. The characteristics show a decrease in resistance as the voltage increases. This phenomenon is mainly caused by the increasing value of electrical current increasing the temperature of the composites, causing a decrease in the resistance of samples for higher voltages.