# Zinc Oxide-Based Rotational-Linear Triboelectric Nanogenerator
## 3 Results and Discussion  
### Operating Principle of Tribogenerators in Contact-Separation Mode  
As we have described above, the RL-TENG is based on the transformation of rotational motion to linear motion. The output voltage of RL-TENG is a combination of the output voltages of each triboelectric pair T1, T2 and T3 that operate in contact-separation mode. In this operation mode, the movable part (active TENG in Figure 4a) is moving in a vertical direction relative to the stationary part (base TENG in Figure 4a). When the two triboelectric surfaces (Kapton and ZnO-on-Kapton) are in contact, charges with different polarities appear on the two materials, due to contact electrification. The type of charge that appears on each material depends on the electronegativity of the two materials. In our case, the surface of ZnO is charged positively, and the surface of Kapton is charged negatively, due to the differences in the electronegativities of the two materials. As the two triboelectric surfaces start moving away from each other, a charge build up will appear at the electrodes, located at the backside of the triboelectric materials, and a current will flow in the external circuit. A detailed theoretical analysis of the triboelectric nanogenerators in contact-separation mode has been presented by [PERSON] et al. [20] al and [PERSON] et al. [21].  
Figure 4a illustrates schematically the TENG operation in contact-separation mode. Assuming that there are already charges developed on the active and the base surfaces of the tribogenerator, we can distinguish five different phases during an operation cycle.  
* Phase 1: The active area is stationary at its balance position. No current flows through the external circuit.
* Phase 2: The active area begins moving until it reaches the position of maximum displacement. Current flows to the external circuit due to electrostatic induction. The current flow takes place for the duration of the movement of the active area.
* Phase 3: The active area remains still at the maximum displacement for a short amount of time. No current flows through the external circuit.
* Phase 4: The active area starts moving backwards towards its balance position. Current flows to the external circuit due to electrostatic induction, but in the opposite direction relative to phase P2.
* Phase 5: The active area reaches its maximum position and stops moving. No current flows through the external circuit.  
During all the phases described above, the reference surface (TENG-Base) remains in the same position.  
To analyze the electrical performance of the tribogenerator during these phases, we have modeled and simulated the device in COMSOL 5.0. For the simulation, the following parameters were used: thickness of Kapton: 50 \(\upmu\)m; relative permittivity of Kapton: 3; and surface charge density: 50 \(\upmu\)C/m\({}^{2}\). ZnO layer was omitted in the simulation since  
Figure 4: (**a**) Different phases of a triboelectric generator in contact-separation mode; (**b**) simulation results of the modeled tribogenerator, showing the output voltage during each phase.  
it is several orders of magnitude smaller compared to Kapton. Figure 4b shows the output voltage of the tribogenerator, connected to an external load resistance R\({}_{\text{ext}}\) (=1 M\(\Omega\)), during the various phases. As expected, voltage develops between the electrodes, due to electrostatic induction, when the two electrodes are in relative motion.