Triboelectric Power Generation from Paper Vibration Induced by Sonic Waves

Materials

Paperboard was applied as both a sound propagation material and a substrate to support an electret and electrodes. PTFE (Teflon, Du Pont, USA) and copper tape with electroconductive acrylate adhesive (CCH, Chomerics, USA) were applied as an electret and electrodes, respectively. Table 1 shows the physical properties of the materials used for fabricating the paperboard TriboEG.

Fabrication of the Paperboard TriboEG (Out-of-Plane Motion)

Figure 1 shows the design configuration of the paperboard TriboEG with an out-of-plane motion (Tao et al. 2015). Both the fixed and moving parts of the two paperboards were mounted on a stand such that the periphery would not be shifted and the narrow gap between the two paperboards would be maintained. To design the paperboard TriboEG, the condenser microphone structure (Sessler et al. 1962) was introduced. The vertical arrangement of this structure, which is popular for microphone applications, was equipped with a membrane functioning as a diaphragm to transfer sound-induced vibration into an electric signal via dielectric polarization. One of the advantages of this structure is its high flexibility and its applicability to paper and PTFE sheets. Paper is neutral in terms of electric charge. A PTFE sheet functions as an electret because it is a dielectric material with a permanent negative electric field created by corona discharge or simply rubbing it with another triboelectric material. Electrets have often been used effectively in conjunction with micromachining technology for electrostatic self-powered devices.

Figure 1:

Design configuration of the paperboard TriboEG (out-of-plane motion).

Fabrication of the Paperboard TriboEG (In-Plane Motion)

The propagation of a longitudinal wave was considered, and a paperboard TriboEG with an in-plane motion mechanism was designed using comb-shaped electrodes as shown in Figure 2. Two stiff paperboards were employed to fix the whole structure. The moving part at the top of this figure moves horizontally on a pair of guide rails with bearings on both sides for smooth linear motion (not shown in the figure), keeping a constant distance from the fixed part at the bottom. In the experiment of power generation, the moving part was quickly moved back and forth twice by hand.

Figure 2:

Design configuration of the paperboard TriboEG (in-plane motion) and photograph of apparatus.

Data Acquisition of Voltage and Power Generated by the Paperboard TriboEG

To vibrate the moving part, a vibrator (U56001 Vibration generator, 3B Scientific Physics, Germany) with a signal source from a multifunction generator (WF1973, NF Corporation, Japan) was used for the TriboEG (out-of-plane motion). To record the generated voltage, an oscilloscope (DSO1052B, Agilent Technologies, USA) was used for both the out-of-plane and in-plane motion modes. The voltage generated by the forced vibration from the vibrator was recorded by the oscilloscope under the conditions shown in Table 2 and analyzed using the fast Fourier transform (FFT) to determine the natural frequency of the paperboard and separate the generated electricity from the electric noise sourced by the power outlet in the room. The FFT calculation was conducted by using the analysis function of a spreadsheet application (Excel 2010, Microsoft Corp., USA). As shown in the next chapter, each Fourier coefficient (amplitude) of the background wave was subtracted from those of the measured voltage waves at every frequency in the Fourier space.

For output power measurements, electrostatic charges were provided to electret sheets, initially. A high voltage Corona discharge treatment (GC50S, Green Techno, Inc, Japan) with a bias needle voltage of –50 kV was applied back and forth 30 mm away from the surface of a PTFE electret with an area of 23,000 mm² and a thickness of 0.5 mm before attaching it on the paperboard TriboEG. Lee et al. (2011) reported that the charge density of electrets did not improve significantly without a high bias needle voltage. The default voltage –50 kV of the corona discharge in this work was high enough. Thus, the discharge time was set to 2 min and the surface potential started to be measured 1 min after discharge with a surface potential meter (digital static meter KSD-2000, Kasuga Denki, Inc., Japan).

Subsequently, output power was measured with a power meter (PW3335, Hioki E.E. Corp., Japan) with load resistances varied from 10 kΩ to 10 MΩ at 39 Hz in forced vibration for suitable vibration stability and achievement of a maximum power, 20 Vp-p in vibrator driving voltage, and 40 µm in displacement from the fixed paperboard to moving paperboard of the TriboEG.

Voltage and Power Generated by the Paperboard TriboEG (Out-of-Plane Motion)

Figure 3 shows the recorded voltage generated by the paperboard TEG due to out-of-plane motion at each frequency of forced vibration with a 2-mm gap between the two paperboards. The background noise consisted of a 51-Hz sinusoidal wave. (The specified frequency of a commercial power source in eastern Japan is 50 Hz). Therefore, all generated voltages include this background noise.

Figure 3:

Voltages generated by paperboard TriboEG at a gap of 2 mm and voltage of background electric noise.

To distinguish the voltage generated by the vibrational motion, FFT was applied. The results of the FFT analysis of the voltages after normalization (background noise removal) are shown in Figure 4. Among the various forced frequencies, the highest amplitude of voltage, 2.3 V, was obtained at 46 Hz. This forced vibration is considered to provide resonance to the paperboard to meet the condition set in this configuration.

Figure 4:

Spectra of voltage amplitude generated by paperboards at a gap of 2 mm with background noise subtracted.

The gap between the fixed and moving parts should be as small as possible for effective energy generation by static electricity, because a maximum voltage is obtained at the maximum changing rate of the electric field. At a narrower gap of 1 mm, the paperboard TEG generated higher voltages as shown in Figure 5.

Figure 5:

Voltages generated by paperboard triboelectric generator at a gap of 1 mm and voltage of background electric noise.

Figure 6 shows FFT spectra calculated from those generated voltages. The TriboEG system tended to be practically ineffective at forced vibration frequencies of 10 and 20 Hz; the highest peak voltage for both was only 0.1 V. From the 30 Hz forced vibration, the 1-mm gap configuration generated 1.1 V, which was more than double that of the 2-mm gap (0.4 V). The TriboEG system achieved the highest peak voltage of 3.6 V at 47 Hz by a 46 Hz forced vibration. The peak voltage decreased for forced vibrations greater than 46 Hz to 2.8 and 1.9 V at 50 and 55 Hz, respectively. Large harmonic vibrations were observed at 24 and 26 Hz at forced vibrations of 46 and 50 Hz, respectively. The frequencies of those harmonic vibrations were located at approximately half the fundamental frequencies. It is unknown whether those harmonic vibrations may have contributed to power generation; however, in those few times, the higher frequencies were small and unlikely to contribute. Consequently, the maximum resonance seems to have appeared at a vibration frequency of approximately 46 Hz.

Figure 6:

Voltages generated by paperboards at a gap of 1 mm with amplitude of background noise subtracted.

Figure 7 shows the measured output power with a maximum value of 11.8 µW at a load of 2 MΩ. Although the µW order power seems fairly large, it is high enough to light an LED. The maximum value is attained when the load resistance is equal to the internal resistance of the TriboEG circuit as is usually large.

Figure 7:

Output voltage and power profiles with load resistance.

Figure 8 shows surface potentials of PTFE sheets rubbed against copy paper for 1 min, corona-discharge-treated, and untreated for comparison. The surface potential of the untreated sheet was –0.09 kV. It was found that the rubbed PTFE sheet was provided with a high negative surface potential and kept it higher for about 10 min. Therefore, continuous rubbing action is required to keep the potential. Although the corona-discharge-treated PTFE sheet shows a rapid decrease in the negative potential for initial 30 min, it still kept a much higher negative charge than the rubbed PTFE sheet. Thus, corona-discharge-treated PTFE sheets can be applicable for active power generation. In electrochemical experiments, however, electrostatic charges on rubbed PTFE sheets were directly identified as electrons rather than ions (Liu et al. 2008). The realization of this prototype dielectric material is suitable for interactive power generation.

Figure 8:

Surface potential changes of variously-treated PTFE sheets with time.

Voltage Generated by the Paperboard TriboEG (In-Plane Motion)

The paperboard TriboEG system for in-plane motion generated voltage with a sliding action by hand as shown in Figures 9 and 10 with a gap of 3 and 1 mm between the paperboards, respectively. In the generated voltage wave to the left of Figure 7, each of the four remarkable pulse waves were found to consist of 12 oscillations, corresponding to the number of teeth of the comb-shaped electrodes during the two round trips. The generated voltage wave to the left of Figure 8 shows a similar tendency except for a larger amplitude caused by the higher changing rate of the electric field induced by the narrower gap. The generated voltage momentarily reached 1.3 and 8.2 V at 3 and 1 mm gaps, respectively. The FFT amplitudes to the right of Figures 7 and 8 show the highest peaks at approximately 60–70 Hz. Those frequencies depended on the moving speed of the examiner’s hand and the interval of the electrode teeth. Practical longitudinal waves of sound are weak, and their displacement is much smaller than that in this experiment. However, this result proved that in-plane displacement can generate electricity. This result further indicated that microscopic electrodes and electret printed two sheets of paper with smoothly finished surfaces at a high resolution could be an electric power source for microscopic electronics.

Figure 9:

Voltage generated by paperboard TriboEG (in-plane mode) and voltage amplitude spectrum calculated by FFT at a gap of 3 mm.

Figure 10:

Voltage generated by paperboard TriboEG (in-plane mode) and voltage amplitude spectrum calculated by FFT at a gap of 1 mm.