Embedded Triboelectric Active Sensors for Real-Time Pneumatic

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Embedded triboelectric active sensors for real-time pneumatic monitoring Xianpeng Fu, Tianzhao Bu, Fengben Xi, Tinghai Cheng, Chi Zhang, and Zhong Lin Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08687 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 27, 2017

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Embedded triboelectric active sensors for real-time pneumatic monitoring

Xian Peng Fu,†,‡ Tian Zhao Bu,†,§, ǂ Feng Ben Xi,†,§, ǂ Ting Hai Cheng,*,‡ Chi Zhang,*,†,§ and Zhong Lin Wang*,†,§,#

†Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing, 100083, China ‡School of Mechanical and Electrical Engineering, Changchun University of Technology, Changchun 130012, Jilin, China §National Center for Nanoscience and Technology (NCNST), Beijing, 100190, China ǂUniversity of Chinese Academy of Sciences, Beijing 100049, China #School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, United States

ABSTRACT Pneumatic monitoring sensors have great demands for power supply in cylinder systems. Here we present an embedded sliding triboelectric nanogenerator (TENG) in air cylinder as active sensors for position and velocity monitoring. The embedded TENG is composed of a circular PTFE polymer and the triangular copper electrode. The working mechanism as triboelectric active sensors and electric output performance are systematically investigated. By integrating into the pneumatic system, the embedded triboelectric active sensors have been used for real-time air pressure/flow monitoring and energy storage. Air pressures are measured from 0.04 to 0.12 MPa at a step of 0.02 MPa with a sensitivity of 49.235 V/MPa, as well as air flow from 50 to 250 L/min at a step of 50 L/min with a sensitivity of 0.002 µA·min/L. This work has first demonstrated triboelectric active sensors for pneumatic monitoring and may promote the development of TENG in intelligent pneumatic system. 1

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KEYWORDS: triboelectric nanogenerator; active sensor; pneumatic monitoring; air pressure; air flow

INTRODUCTION In recent years, the rapid development of pneumatic driving technology has drawn considerable attention in the field of mechanical automation. The pneumatic driving technology is widely used due to the advantages of low cost, compact size, high power-to-weight ratio, reliability, low maintenance and so on1–4. Currently, the pneumatic driving system is very intelligent with kinds of monitoring sensors, in which the motion sensor has played a significant role1,5. The traditional motion sensors in pneumatic system utilize position switches or magnetic sensors to detect the piston motion6,7, with the limitations in real-time or working environment. In addition, batteries or wired power supplies are necessary for the traditional motion sensors8, which has more urgently requested for a real-time self-powered motion sensor. Recently, the triboelectric nanogenerator (TENG)9–16, which could be parallel or possibly equivalently important as the traditional electromagnetic induction generator17,18, has been invented as a new energy technology for harvesting mechanical energy19–27. Alternatively, the TENG can also be used as a self-powered sensor for actively detecting the static and dynamic processes by using the voltage and current output signals of the TENG28–33, respectively, which is promising to be applied in the pneumatic driving system. In this paper, we introduce an embedded triboelectric active sensor for real-time pneumatic monitoring which integrates a TENG in an air cylinder. The embedded TENG is composed of two friction layers with opposite triboelectric polarities (PTFE and copper) in single-electrode sliding mode. Owing to the linearly proportional relationship between the displacement/velocity and output voltage/current signals, the 2

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embedded triboelectric active sensor can be used to monitor piston position and moving speed. By integrating into the pneumatic system, the embedded triboelectric active sensors have been used for real-time air pressure/flow monitoring and energy storage. It is a potential and more sensitive method for the pressure and flow detection. This work has first demonstrated triboelectric active sensors for pneumatic monitoring and may promote the development of TENG in intelligent pneumatic system.

RESULTS AND DISCUSSION Figure 1 shows the basic structure of the embedded triboelectric active sensor based on a single-electrode sliding mode TENG. A PTFE polymer film is located on the piston as the sliding layer, while two triangular copper thin foils are located on the cylinder wall, which acts as both the triboelectric surface and the electrode (Figure 1a). Thus, the electric signals can be generated when the piston (PTFE film) passed through the cylinder wall (copper film). The detailed fabrication process of the embedded triboelectric active sensor is introduced in the Experimental Section. In order to enhance the output characteristics of the embedded TENG, the inductively coupled plasma (ICP) is used to create PTFE nanostructures on the PTFE surface. A SEM image of the PTFE film with nanostructures is shown in Figure 1c, in which the nanostructures have been uniformly distributed on the surface of the PTFE film. The working mechanism of the embedded triboelectric active sensor is shown in Figure 2. By coupling of the contact electrification and the relative-sliding-induced charge transfer, the working process is shown in Figure 2a. At the original state as shown in Figure 2a (I), the piston is in the left extreme position where the overlapping area of the fixed triangular copper electrode and the PTFE polymer film is the largest. Since PTFE is easier to gain electrons than copper according to the triboelectric series17,34, electrons will be injected from copper surface to PTFE surface. The equal density of negative and positive charges will be generated on the PTFE and copper surfaces, respectively. At this state, the TENG is in the electrostatic equilibrium state and no electron flowing through the external circuit. At the next state as shown in 3

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Figure 2a (II), when the piston slides along the fixed copper surface, the overlapping area is reduced. The produced potential difference will drive electrons to flow from the ground to the copper electrode for achieving new electrostatic balance. This electricity generation process will last until the piston slide to the right extreme position, in which the copper and PTFE polymer film separate with little overlap area, as shown in Figure 2a (III). At this state, nearly all the positive triboelectric charges on the copper film have been neutralized by the electrons from the ground. In the following, the piston slides along the opposite direction, the PTFE film will contact with copper film again (state IV). The overlapping area starts to increase, resulting in an increase of the tribo-charge-induced potential difference. The electrons on the copper film will flow back to the ground through external circuit in order to re-establish the electrostatic equilibrium, which creates the current in reverse direction. Until the piston arrives at the left extreme position (state I), all the electrons will flow back to the ground and return to the electrostatic equilibrium again, as elaborated in state I. Electrons flow back and forth between the copper electrode and the ground in each reciprocating motion cycle, generating an alternating current signal. Furthermore, the different potential differences are generated between copper and PTFE films when piston at different positions. A finite element simulation was carried out to compare the generated potential difference between the copper and PTFE films when piston passed through the cylinder wall from the left side to the right side. The diameter and width of the circular PTFE film are 80 mm and 30 mm, respectively. Both the length and height of the two triangular copper films are 100 mm. As shown in Figure 2b, the piston at the original state and the calculated open-circuit voltage is 127 V. As the piston slides to the position where PTFE and copper films have minimum overlapping area in Figure 2c, the calculated open-circuit voltage decreased to -49.9 V, which might be ascribed to the influence of the negative charges on the PTFE surface. The simulation results indicate that the output open-circuit voltage decreases as the overlapping area decreases and demonstrate the feasibility that TENG served as embedded triboelectric active sensors. 4

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The electrical output characteristics of the embedded TENG, as shown in Figure 3, are measured by connecting the piston rod to a line motor with a linear velocity and distance of 0.6 m/s and 6 cm, respectively. The short-circuit current and open-circuit voltage were measured respectively. Figure 3a and Figure 3b show that the peak current and voltage can reach 2.02 µA and 21.50 V, respectively. We systematically study the output current and voltage on a series of resistances, from 10 kΩ to 1000 MΩ. As indicated in Figure 3c, the output current declines with the increase of the resistance, while the voltage shows a reversed tendency. Figure 3d shows the calculated instantaneous power with variable resistances. The instantaneous power initially rises at a low resistance region and then drops rapidly at a higher resistance region, showing a maximum value of 1.70 µW at 30 MΩ. The embedded TENG can be served as triboelectric active position and velocity sensors. Figure 4a-c show characteristics of the embedded TENG served as a triboelectric active position sensor. When the PTFE film pass through the copper film with different distances from 1 to 6 cm at a step of 1 cm with a velocity value of 0.6 m/s. The output open-circuit voltage increases with the increase of sliding distance. As shown in Figure 4b, the output open-circuit voltage has a good linear relationship with the sliding distance, indicating that it has clear superiority for position sensing. The displacement resolution is one of the key performance parameter, which is also systematically studied, as shown in Figure 4c. When the PTFE film sliding along the copper film by steps of 0.1 mm, the change of output open-circuit voltage signal is 0.03 V, which indicates the displacement resolution of the position sensor is obviously superior to 0.1 mm. Furthermore, we have systematically investigated the response of the embedded TENG at a series of reciprocating velocity of the piston, serving as a triboelectric active velocity sensor, as shown in Figure 4d-f. Figure 4d shows the output signals of short-circuit current, when the PTFE film sliding in copper film with different velocity from 0.1 to 0.6 m/s at a step of 0.1 m/s. The output short-circuit current increases with the increasing reciprocating velocity. As shown in Figure 4e, the output short-circuit current has a good linear relationship with the sliding velocity, indicating 5

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that it has clear superiority for speed sensing. The active velocity sensor has excellent measurement accuracy, in which the maximal standard deviation is 0.0196 m/s with 25 times repeated measurement at each different velocity, as shown in Figure 4f. To investigate the potential applications of the embedded triboelectric active sensors, it has been used to detect air parameters in pneumatic system. We systematically investigated the response of the embedded TENG to different air flows, as shown in Figure 5a-c. The chamber can be divided into rod and rodless chamber according to the piston rod position. The piston would slide along the cylinder wall when compressed air was continually injected into the rodless chamber through the inlet while the rod chamber was communicated with external environment. The electrical signal will be generated when the PTFE and copper films sliding friction under the action of air-flow, as exhibited in Figure 5a. If the flow of the compressed air is Q, the sectional area of piston is A1, the velocity of PTFE film can be described as: V = Q / A1. So the velocity of the PTFE film is positively correlated with the flow of the air-flow according to the formula, which can be represented for a flow sensor. Figure 5b shows the output signals of current, as the air flow increase from 50 to 250 L/min at a step of 50 L/min. The output peak short-circuit current increases with the increase of air flow, as shown in Figure 5c, which has a good linear relationship with the air flow. The experimental results are good agreement with the theoretical analysis results, which indicates that the embedded TENG can be used to detect the air flow. The sensor has a high sensitivity of 0.002 µA·min/L, which is important for the flow monitoring and servo control of pneumatic systems. On the other hand, the embedded TENG can also be served as a pressure sensor, as shown in Figure 5d-f. The rod chamber was sealed when the outlet was cut off, as shown in Figure 5d. If the compressed air was continually injected into the rodless chamber, the PTFE film would slide along the copper film due to the effect of pressure difference between two chambers, and then stopped at a certain location where the two chamber pressure reached a new equilibrium state. At the pressure balance position, the force equilibrium state of the piston can be described as: p1 A1 = p2 (A1 - A2), where p1 is the chamber pressure or air pressure in the rodless, p2 is the chamber pressure or air 6

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pressure in the rod chamber, and A2 is the sectional area of rod. So the pressure of the compressed air can be expressed as: p1 = p2 (A1 - A2) / A1, which indicated each pressure value corresponds to a different pressure balance position or different output open-circuit voltage signals. As Figure 5e-f given depicts, when the pressure of the compress air varies from 0.04 to 0.12 MPa at a step of 0.02 MPa, the output open-circuit voltage of TENG linearly increases. When the pressure is lower than 0.04 MPa, the piston cannot be driven because of the friction force between piston and cylinder. While the pressure of the rodless and rod chambers cannot reach the equilibrium state in the limited piston stroke when the pressure is over 0.12 MPa. Therefore, the embedded TENG can be used as a pressure sensor within a certain pressure range. The sensitivity of the pressure sensor is 49.235 V/MPa. The embedded triboelectric active sensor offers a method for pneumatic system detection with a few unique advantages. Firstly, the TENG can serve as the embedded active position or velocity sensors for monitoring the piston moving state (position and velocity) in real-time. Compared with the traditional pneumatic system motion sensor, it has higher efficiency and integration, which is beneficial to the servo control of pneumatic system. Secondly, for measuring principle, compared with the current pressure and flow sensors, it does not change the flow of compressed air on working. It is a potential and more sensitive method for the pressure and flow detection. Thirdly, the embedded active sensor can work by using the electric output generated by itself without other external power source which may be more suitable to work in harsh environments. The proposed TENG-based device not only can work as an embedded triboelectric active sensor, but also can work as a power source by converting mechanical energy into electricity. The TENG generated AC electricity can be charged to the capacitor or powering for the low-power components after converted to DC with the rectification circuit35. As shown in Figure 5g, the charging of capacitors with different capacitance values have been studied experimentally. It is observed that the charging time increases gradually with the capacitance increase when the capacitor charged to the 2 V. The charging time are ~19 s, ~61 s, ~112 s and ~275 s for the 1 µF, 7

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3.3 µF, 4.7 µF and 10 µF capacitor, respectively. The generated electricity can light up a commercial LCD screen, as depicted in the optical photo in Figure 5h. Therefore, the proposed TENG-based device is capable of being an energy harvesting unit and powering the electronic devices in the pneumatic system.

CONCLUSION In summary, we have demonstrated a novel embedded triboelectric active sensor based on single-electrode sliding TENG for real-time pneumatic monitoring, which are composed of two friction layers (copper and PTFE films) with opposite triboelectric polarities. The proposed embedded triboelectric active sensor can not only monitor the piston position for air pressure measurement, but also monitor the piston moving speed for air flow measurement in the pneumatic system without any other additional power source. It has been demonstrated to measure the air pressure from 0.04 to 0.12 MPa at a step of 0.02 MPa with a sensitivity of 49.235 V/MPa. As well as the air flow from 50 to 250 L/min at a step of 50 L/min with a sensitivity of 0.002 µA·min/L. Additional, the embedded TENG can convert mechanical energy into electricity for powering electronic devices in the pneumatic system. This study has great potentials in self-powered pneumatic monitoring system and will expand a new application area of TENG as active sensors in precision mechanics.

EXPERIMENTAL SECTION Fabrication of the nanostructure on the surface of the PTFE film. The PTFE film (100 µm thick) was cleaned with isopropyl, alcohol and deionized water in sequence. It was blown dry with compressed air and deposited a layer of gold particles as the mask by sputtering. Subsequently, the PTFE film was processed by the inductively coupled plasma (ICP) for 40 s and the nanostructure was created on the surface. Specifically, Ar and O2 gases were imported in the chamber with the flow of 5.0 and 55.0 sccm, respectively. One power source of 440 W was used to generate a large 8

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density of plasma and the other power source of 100 W was used to accelerate the plasma ions. The SEM image of the nanostructure was taken by Hitach S-5500. Fabrication of the Embedded triboelectric active sensor. Two pieces of copper adhesive tape (100 µm thick) were cleaned and cut into triangle. Both the length and height of the two triangular copper film are 100 mm. A commercial cylinder (Topwinner: SC80 × 100) is used as a substrate. Then the two pieces of copper adhesive tape were pasted on the inner wall of the cylinder. An insulating layer is arranged between the inner wall of the adhesive tape and the cylinder. The cylinder wall is grounded to eliminate charge shielding effect. The PTFE film (100 µm thick) is pasted on the surface of piston as the moving object. The diameter and width of the circular PTFE film are 80 mm and 30 mm, respectively. Finally, there were assembled into a cylinder.

Figure 1. The basic structure of the embedded sliding TENG in air cylinder. (a) Schematic diagram showing the structural design of the embedded sliding TENG. (b) Optical photo of the device, including piston and inwall. (c) SEM image of the PTFE surface with etched nanostructure.

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Figure 2. Working mechanism of the TENG as an embedded triboelectric active sensor. (a) The sketches that illustrate a detailed step by step electricity generation processes. State I: Original state with the PTFE film in the left extreme position; State II: PTFE film sliding to the right; State III: PTFE film slide to the right extreme position; State IV: PTFE film sliding to the left. (b, c) Finite element simulation results of the potential difference between the PTFE and copper films at different positions: (b) The original state; (c) The position where PTFE and copper films have minimum overlapping area.

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Figure 3. Electrical output characteristics of the embedded sliding TENG. (a) The output signal of short-circuit current, when the PTFE film passed through the copper film with a velocity value of 0.6 m/s. (b) The output signal of open-circuit voltage, when the PTFE film passed through the copper film with the distance value of 6 cm. (c) The output voltage and current across an external load with variable resistances. (d) The calculated instantaneous power output of the embedded TENG with variable resistances.

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Figure 4. The TENG served as embedded triboelectric active position and velocity sensors. (a-c) The TENG served as an embedded triboelectric active position sensor: (a) The output signals of open-circuit voltage, when the PTFE film passed through the copper film from 1 cm to 6 cm step by 1 cm with a velocity value of 0.6 m/s. (b) Relationship between the output open-circuit voltage and the sliding distance. The linear correlation coefficient is 0.997. (c) The displacement resolution of the embedded triboelectric active position sensor. (d-f) The TENG served as an embedded triboelectric active velocity sensor: (d) The output signals of short-circuit current, when the PTFE film passed through the copper film with a velocity from 0.1 m/s to 0.6 m/s step by 0.1 m/s. (e) Relationship between the output short-circuit current and the sliding velocity. The linear correlation coefficient is 0.999. (f) Standard deviation of velocity as an embedded triboelectric active velocity sensor with different sliding velocities. 12

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Figure 5. Applications of the embedded triboelectric active sensor in pneumatic system. (a-c) The TENG served as the embedded triboelectric active flow sensor: (a) Schematic diagram showing the embedded triboelectric active flow sensor. (b) The output signals of short-circuit current, when the air flow from 50 L/min to 250 L/min step by 50 L/min. (c) The relationship 13

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between the output short-circuit current and the air flow. The linear correlation coefficient is 0.972. (d-f) The TENG served as the embedded triboelectric active pressure sensor: (d) Schematic diagram showing the embedded triboelectric active flow sensor. (e) The output signals of open-circuit voltage, when the pressure from 0.04 MPa to 0.12 MPa step by 0.02 MPa. (f) The relationship between the output open-circuit voltage and the pressure. The linear correlation coefficient is 0.994. (g-h) The TENG served as a power source: (g) The charging curve of the capacitance. (h) Optical photo of the TENG served as source powered for the LCD screen.

AUTHOR INFORMATION *E-mail: [email protected] (C.Z.) *E-mail: [email protected] (T.H.C.) *E-mail: [email protected] (Z.L.W.)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the support of National Natural Science Foundation of China (No. 51475099), Beijing Natural Science Foundation (No. 4163077), Beijing Nova Program (No. Z171100001117054), the Youth Innovation Promotion Association, CAS (No. 2014033), the "thousands talents" program for the pioneer researcher and his innovation team, China, and National Key Research and Development Program of China (2016YFA0202704).

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Mechanical Energy into Electricity and Light Emissions. Adv. Mater. 2016, 28 (14), 2744–2751. (21)

Kanik, M.; Say, M. G.; Daglar, B.; Yavuz, A. F.; Dolas, M. H.; El-Ashry, M. M.; Bayindir, M. A Motion- and Sound-Activated, 3d-Printed, Chalcogenide-Based Triboelectric Nanogenerator. Adv. Mater. 2015, 27 (14), 2367–2376.

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Kim, T.; Chung, J.; Kim, D. Y.; Moon, J. H.; Lee, S.; Cho, M.; Lee, S. H.; Lee, S. Design and Optimization of Rotating Triboelectric Nanogenerator by Water Electrification and Inertia. Nano Energy. 2016, 27, 340–351.

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Chen, H. T.; Miao, L. M.; Su, Z. M.; Song, Y.; Han, M. Di; Chen, X. X.; Cheng, X. L.; Chen, D. M.; Zhang, H. X. Fingertip-Inspired Electronic Skin Based on Triboelectric Sliding Sensing and Porous Piezoresistive Pressure Detection. Nano Energy. 2017, 40, 65–72.

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