Dynamic wear sensor array based on single-electrode triboelectric nanogenerators

Journal Pre-proof Dynamic wear sensor array based on single-electrode triboelectric nanogenerators Yilong Ren, Guoxu Liu, Hang Yang, Tong Tong, Shaohang Xu, Lin Zhang, Jianbin Luo, Chi Zhang, Guoxin Xie PII:

S2211-2855(19)31010-9

DOI:

https://doi.org/10.1016/j.nanoen.2019.104303

Reference:

NANOEN 104303

To appear in:

Nano Energy

Received Date: 9 October 2019 Revised Date:

7 November 2019

Accepted Date: 14 November 2019

Please cite this article as: Y. Ren, G. Liu, H. Yang, T. Tong, S. Xu, L. Zhang, J. Luo, C. Zhang, G. Xie, Dynamic wear sensor array based on single-electrode triboelectric nanogenerators Nano Energy, https:// doi.org/10.1016/j.nanoen.2019.104303. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier Ltd. All rights reserved.

Yilong Ren. Yilong Ren received his B.S. degree in Materials Science and Engineering from University of Science and Technology Beijing in 2018, and he is currently perusing a Ph.D. from Tsinghua University. His research interests are mainly about solid lubricating materials and wear monitoring technology.

Guoxu Liu. Guoxu Liu received his master degree in material science engineering from Tianjin University of Technology in 2016. Now he is a Ph.D. student at Beijing Institute of Nanoenergy and Nanosystems, Chinese Academic Science. His current research mainly focuses on energy harvesting and fabrication of nanodevices.

Hang Yang. Hang Yang received his B.S. degree from Huazhong University of Science and Technology, Wuhan, China, in 2016. Now, he is a postgraduate student in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His research interests are triboelectric nanogenerator and nanodevices.

Tong Tong. Tong Tong received his bachelor's degree in HeFei University of Technology in 2018. She is currently pursuing a master's degree in Beijing Institute of Nanoenergy and Nanosystems under the supervision of researcher Chi Zhang. Her research interests include power management circuit and self-drive system.

Shaohang Xu. Shaohang Xu received his B.S. degree from University of Jinan, Jinan, China, in 2017. Now, he is a postgraduate student in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences. His research interests are triboelectric nanogenerator and tribotronics

Lin Zhang. She received her Ph.D. degree at Beijing University of Chemical Technology, in 2017, majoring in Chemical Engineering and Technology. She is currently a post-doctor at the State Key Laboratory of Tribology of Tsinghua University. Dr. Zhang has published 6 papers in international journals indexed by SCI. Her research interests include the self-lubricating polymer composites, self-healing materials and core-shell materials, etc.

Jianbin Luo. He received his BEng degree from Northeastern University in 1982, and got his MEng degree from Xi’an University of Architecture and Technology in 1988. In 1994, he received his PhD degree from Tsinghua University and then joined the faculty of Tsinghua University. Prof. Jianbin Luo is an academician of the Chinese Academy of Sciences and a Yangtze River Scholar Distinguished Professor of Tsinghua University, Beijing, China. He was awarded the STLE International Award (2013), the Chinese National Technology Progress Prize (2008), the Chinese National Natural Science Prize (2001), and the Chinese National Invention Prize (1996).

Chi Zhang. He received his Ph.D. degree from Tsinghua University in 2009. After graduation, he worked in Tsinghua University as a postdoc research fellow and NSK Ltd., Japan as a visiting scholar. He now is the principal investigator of Tribotronics Group in Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences (CAS), Fellow of the NANOSMAT Society, Member of Youth Innovation Promotion Association, CAS. Prof. Chi Zhang's research interests are triboelectric nanogenerator, tribotronics, self-powered MEMS/NEMS, and applications in sensor networks, human-computer interaction and new energy technology. He has been awarded the National Science Fund for Excellent Young Scholars.

Guoxin Xie. He received his Ph.D. degree at Tsinghua University,China, in 2010, majoring in Mechanical Engineering. Since 2014, he has worked at Tsinghua University as an associate Professor. His research interests include intelligent self-lubrication, electric contact lubrication, etc. He has published more than 50 referred papers in international journals. He won several important academic awards, such as the Excellent Doctoral Dissertation Award of China, and Ragnar Holm Plaque from KTH, Sweden.

1

Dynamic wear sensor array based on single-electrode

2

triboelectric nanogenerators

3

Yilong Ren a,1, Guoxu Liu b,c,1, Hang Yang b,c,1, Tong Tongb,c, Shaohang Xu b,c, Lin Zhang a,

4

Jianbin Luo a, Chi Zhang b,c, *, Guoxin Xie a,*

5 6

a

7

Beijing 100084, China

8

b

9

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing

State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University,

CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and

10

100083, China

11

c

12

100049, China

13

* Corresponding author at: Tsinghua University.

14

E-mail addresses: [email protected] (C. Zhang), [email protected], (G. X. Xie).

15

1

School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing

These authors contributed equally to this work.

16

1

1 2

ABSTRACT:

3

Smart sensor is the foundation and core of intelligent manufacturing, which is developing for

4

miniaturization, integration and self-powering. Here, we report an active sensor array based

5

on single-electrode triboelectric nanogenerators (TENGs) for dynamic wear monitoring and

6

positioning. The sensor unit is fabricated by embedding the electrode into the core-shell

7

composite with polytetrafluoroethylene (PTFE) as the core and polymethylmethacrylate

8

(PMMA) as the shell. The working mechanism and performances of the sensor unit with

9

different parameters including the thickness of PTFE@PMMA layer, reciprocating frequency,

10

sliding displacement, electrode width and the diameter of copper bar are systematically

11

investigated and discussed. By integrating into the sensor array, the dynamic wear monitoring

12

and positioning have been realized, which can be used to detect the wear states of multiple

13

regions in the sliding bearing system. This work has extended the application of the TENGs to

14

determine the wear states of polymer interface and may promote the great development of

15

intelligent bearing.

16 17

Keywords:

18

Core-shell composite, Triboelectric nanogenerator, Sensor array, Wear monitoring sensor,

19

Intelligent bearing

20

2

1

1. Introduction

2

Sensor can acquire the physical condition information of the operating machines, which is

3

essential for intelligent monitoring systems. Polymer-based sliding bearings are the widely

4

used components in mechanical equipment owing to its superiorities in high load capacity,

5

low friction and high wear resistance. Wear failure of the sliding bearing can lead to

6

abnormal operation or even damage to the entire mechanical equipment. Hence, monitoring

7

the wear state of the sliding bearing is significant for determining the working state or failure

8

mode, which is crucial for the machine monitoring in intelligent manufacturing. Traditional

9

wear sensors mainly rely on the signals such as current, vibration, ultrasound, temperature

10

or luminescence spectra to determine the wear condition [1-4]. However, most of these

11

sensors confront a critical deficiency that their work requires an external power supply. Due

12

to the shortages, including large size, limited life and possible environment pollution [5, 6],

13

the conventional chemical battery is insufficient to address this problem. It is highly desirable

14

to develop the self-powered sensors to address the above issues.

15

Triboelectric nanogenerator (TENG) is a promising technology that converts ambient

16

mechanical energy into electricity based on the coupling of triboelectric effect and

17

electrostatic induction [7-15]. Additionally, the output signals of TENG are correlating with

18

the mechanical and environmental stimuli. Thus, TENG has been used as self-powered

19

sensors for various applications, including pressure sensors [16, 17], vibration sensors

20

[18-20], motion/trajectory sensors [21-24] and chemical sensors [25,26]. The TENG with the

21

advantages of diverse structures, various working modes and good integration [27-32] has

22

offered a prospective solution to the design of a novel self-powered and embedded wear

23

sensor array.

24

In this work, we report an active sensor array based on single-electrode TENGs for

25

dynamic wear monitoring and positioning. The sensor unit is fabricated by embedding the

26

electrode into the core-shell composite with polytetrafluoroethylene (PTFE) as the core and

27

polymethylmethacrylate (PMMA) as the shell. The working mechanism and performances of

28

the sensor unit with different parameters including the thickness of PTFE@PMMA layer,

29

reciprocating frequency, sliding displacement, electrode width and the diameter of copper 3

1

bar are systematically investigated and discussed. By integrating into the sensor array, the

2

dynamic wear monitoring and positioning have been realized, which can be used to detect the

3

wear states of multiple regions in the sliding bearing system. This work has extended the

4

application of the TENGs to determine the wear states of polymer interface and may promote

5

the great development of intelligent bearing.

6

2. Results and discussion

7

2.1. The synthesis procedure of PTFE@PMMA

8

In our previous research, we demonstrated a PTFE@PMMA core-shell nanocomposite,

9

which had excellent mechanical properties, such as exceptionally high strength and modulus,

10

low creep, as well as ultralow friction and wear (Fig. S1, S2). Such a type of composite is

11

very promising for solid lubrication in mechanical components, and hence it is of high

12

practical implication to monitor the wear of this type of polymer based on the TENGs and

13

study its triboelectric properties. In this instance, the PTFE@PMMA was chosen as the

14

dielectric layer of TENG for wear monitoring and positioning, which combines mechanical

15

properties, lubrication performance, and electrification performance. The PTFE@PMMA

16

was synthesized by the seed emulsion polymerization [33]. The methylmethacrylate (MMA)

17

monomer was polymerized around the PTFE particles with the participation of the initiator

18

and eventually every PTFE particle was completely coated by PMMA, as shown in Fig. 1a.

19

Detailed synthesis progress is demonstrated in Experimental Section. The SEM images in

20

Fig. S3a, b, show that the PTFE particles are rod-shaped, while the PTFE@PMMA particles

21

are spherical, proving the successful preparation of the core-shell structure. For further

22

verification, the thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy

23

(XPS) analysis were employed. The result of TGA (Fig. S3c) shows two decomposition stages

24

corresponding to the decomposition of PMMA and PTFE [34], which reveals that the mass

25

ratio of PTFE and PMMA is 1:3.4. Because only PTFE contains fluorine, the core-level

26

spectra of fluorine 1s track electron (F 1s spectra) at different depths (0-109.0 nm) were

27

measured and the result is displayed in Fig. S3d, e. At the surface, the intensity of F 1s peak is

28

invisible, suggesting the PTFE is completely wrapped by PMMA. As the sputtering depth

29

increasing, the intensity of F 1s peak increases firstly and then decreases, reaching the

30

maximum value at 65.4 nm. It can be inferred that the average thickness of the PMMA shell 4

1

was about 65 nm. The shift of the binding energy of F 1s might be attributed to the C-F

2

species [35, 36], which generated from the PTFE dissociation caused by the bombardment of

3

the high energy argon ions (Ar+) [37].

4

2.2. The fabrication process of the single-electrode wear sensor unit

5

The single-electrode wear sensor (SWS) unit was fabricated by embedding the nickel

6

electrode into core-shell composite PTFE@PMMA. Fig. 1b illustrates the fabrication process

7

for the SWS. The nickel electrode was tailored to strip, and the thin fluorinated ethylene

8

propylene (FEP) film with the same size was pasted onto the nickel fabric strip with a copper

9

lead was connected. Then, the bottom PTFE@PMMA powder, the pretreated electrode, the

10

upper PTFE@PMMA powder were paved in the mold sequentially (Fig. 1b i-iii), followed by

11

the hot press forming to prepare the wear sensor (Fig. 1b iv,v). The working process (Fig. 1b

12

vi) of the SWS is that the copper bar reciprocates on it, and the output signals can be

13

recorded by a Keithley 6514 electrometer.

14

2.3. The working mechanism of SWS

15

In the SWS, the copper and the PTFE@PMMA act as two triboelectric materials and the

16

nickel strip acts as electrode. When the copper bar slides on the PTFE@PMMA surface, the

17

opposite triboelectric charges generate on the surface of copper bar and PTFE@PMMA,

18

which are attributed to the electron or ion transfer between two contacting materials [38, 39].

19

However, the triboelectrification is a rather complex process and its results are sensitive to

20

the surface properties of materials. The surface damage of PTFE@PMMA caused by sliding

21

can also affect the triboelectrification process [40]. To fully understand the working

22

mechanism of SWS, the surface potentials of the synthetic PMMA and PTFE@PMMA were

23

measured after rubbing against the copper bar. The surface potential distribution maps (Fig.

24

S4a, b) demonstrate the PMMA and PTFE@PMMA were with positive charges. The

25

triboelectric properties of PTFE@PMMA were consistent with synthetic PMMA, because the

26

mass fraction of PMMA in PTFE@PMMA was 77.2% and PMMA wrapped outside PTFE.

27

Thus, when the copper with negative charges reciprocated on the PTFE@PMMA with

28

positive charges, the induced potential difference as well as the charge transfer between the

29

electrode and the ground generated, and they changed periodically. The detailed working

30

principle of the SWS in one reciprocating cycle is demonstrated in Fig. S5 and Supporting 5

1

Information. Fig. 2a schematically shows the working principle of monitoring the wear state

2

of PTFE@PMMA layer through SWS. As the wear of PTFE@PMMA layer progressed, the

3

thickness of PTFE@PMMA layer became smaller. Thus, the movement of the surface charges

4

caused a larger induced potential difference and the transfer of more electron, as shown in

5

Fig. 2a ii, iii. When the PTFE@PMMA layer and FEP film were worn out (Fig. 2a iv), the

6

induced potential difference dropped sharply owing to the contact of two metals: copper and

7

nickel. However, when the copper contacted the FEP film (Fig. 2a iii), the mechanism was

8

different from that in wearing state 1, because FEP had the opposite triboelectric properties

9

with the PTFE@PMMA. Fig. S4c presents the potential distribution in wearing state 2, where

10

the FEP film was negative and the PTFE@PMMA was positive. The detailed mechanism of

11

the wearing state 2 is shown in Fig. S6. In summary, the potential difference and the current

12

between the electrode and the ground could be very effective in reflecting the wear condition

13

of the PTFE@PMMA layer.

14

The SWSs with PTFE@PMMA layers of different thickness were prepared to represent the

15

different wear depths, and the output characteristics (open-circuit voltage VOC and

16

short-circuit current ISC) were measured. The main sliding and structural parameters of the

17

SWS including sliding displacement (X), copper bar diameter (D), electrode width (L), and

18

thickness of PTFE@PMMA layer (H) are also demonstrated in Fig. 2d. It can be clearly seen

19

from Fig. 2b, d that VOC exponentially increases from 2.2 V to 7.0 V with the decreasing

20

thickness of the PTFE@PMMA layer from 4.5 mm to 0.04 mm, and it drops to 0.2 V when no

21

PTFE@PMMA layer exists. Similarly, as shown in Fig. 2c, e, the similar law that the current

22

increases from 6.0 nA to 26 nA as the thickness of the PTFE@PMMA layer decreases from

23

4.5 mm to 0.04 mm, and drops to 1.7 nA when there is no PTFE@PMMA layer. Based on the

24

electrostatic field and superposition principle, theoretical analyses were established to

25

understand the relationship between the VOC or ISC and the thickness of the PTFE@PMMA

26

layer, which could be described as the following equation (Detailed derivation is shown in

27

Supporting Information):
 = − ln ℎ +  (1)

28

where A and B are the coefficients related to the surface charge density and the dielectric 6

1

constant of the triboelectric materials. The nickel electrode and the ground could be

2

considered as a capacitance, and hence the relationship among QSC, C, VOC, ISC could be

3

given by:

4 5 6


 (2) 

 =

 (3) ∆

where QSC is defined as the transferred charges and ISC represents the current at the short circuit condition. Substituting Equations 1 and 2 into Equation 3, ISC could be obtained by:  =

7

 =


   =− ln h + (4)  ∙ ∆  ∙ ∆  ∙ ∆

As the other parameters remained unchanged, including reciprocating frequency, electrode

8

width, copper bar diameter and sliding displacement, the capacitance C and the charge

9

transfer time t could be adopted as the fixed constants. Thus, the relationship between

10

current and thickness could be expressed by the same function, as shown below.  = −E ln ℎ +  (5)

11

where E and G are the coefficients determined by coefficients A and B, equivalent and

12

electron transfer time. Equation 1 was applied to fit the experimental results of VOC, as shown

13

in Fig. 2d, and the goodness of fit R2 is 0.93. Fig. 2e displays the fit of Equation 5 to the

14

results of ISC, and the goodness of fit R2 is 0.94. Higher goodness of fit proved the credibility

15

of the theoretical model, and the model could be applied to estimate the thickness of

16

PTFE@PMMA layer, depending on VOC and ISC. Additionally, simulation through COMSOL

17

was employed to investigate the law of the electric signals with the thickness. The simulation

18

results of the VOC show the similar exponential changes (Fig. S8). The sensors were tested

19

under the same condition after several months to verify the data reproducibility. As shown in

20

Fig. S9, the VOC and ISC show the similar values and exponential changes, indicating the good

21

stability and reproducibility.

22

2.4. The performances of SWS

23

Taking into consideration that the output characteristics could be affected by the 7

1

parameters apart from thickness, a systematic research was carried out to reveal the

2

relationship between VOC or ISC and the sliding and structural parameters such as

3

reciprocating frequency, sliding displacement (X), copper bar diameter (D) and electrode

4

width (L). In a full cycle of motion, the copper bar slid from left to right, causing the change

5

in the position of the electric field, which was necessary for generating the electric signals.

6

Thus the frequency and displacement of copper bar are the important parameters. Fig. 3a

7

shows the VOC and ISC for a SWS with an electrode width of 3 mm, a PTFE@PMMA thickness

8

of 0.7 mm and a copper bar diameter of 5 mm at different frequencies. VOC is steady at

9

around 3.3 V with the frequency increasing from 1 to 5 Hz, being consistent with the previous

10

study [41]. Because VOC is the result of the electrostatic field established by triboelectric

11

charges, which is independent of speed. However, it is worth noting that ISC is proportional to

12

the frequency (). The time of charge transfer can be calculated as follow: 1 ∆ = (6) 

13

Combining Equation 3, ISC can also be given by:  =  ∙  (7)

14

The behavior of ISC can be easily explained with Equation 7. Since QSC is constant, ISC is

15

linearly proportional to the frequency. Therefore, VOC is more suitable as the signal

16

indicating the wear condition owing to its speed-independent characteristic. The relationship

17

between VOC and the sliding displacement (X) was studied for the SWS with an electrode

18

length of 5 mm, a PTFE@PMMA layer thickness of 0.7 mm and a copper diameter of 5 mm at

19

3 Hz. As shown in Fig. 3a, VOC increases gradually and then slightly fluctuates around the

20

maximum value, being consistent with the finite element method (FEM) calculation results of

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VOC in the sliding-separation mode single-electrode TENGs [42]. The SWS continuously ran

22

for about 10000 cycles to determine its stability. As can be seen from the inset of Fig. 3b,

23

there is no obvious decay of VOC after the durability test.

24

The output voltages of the sensors with different electrode widths under the stimuli of

25

different size copper bars were systematically tested. As shown in Fig. 3c, VOC increases with

26

the increasing copper bar diameter, while the variation of VOC with the electrode width is

27

complicated. When the electrode width is narrow, VOC is proportional to the electrode width,

28

which is confirmed by the linear fit presented in Fig. 3c. VOC starts to stabilize or drop when 8

1

the electrode width is wide. The complicated behavior of VOC is consistent with the analysis

2

about the effect of area size based on the capacitance model of single-electrode TENGs in

3

previous researches [42]. Apart from the sliding and structure parameters, the effect of the

4

polymer material on the output performance was considered. The PTFE and PMMA

5

composite (PTFE/PMMA) prepared by mechanically mixing was used to fabricate the SWS.

6

Fig. 3d compares VOC of the SWSs made by PTFE/PMMA and PTFE@PMMA. The

7

performances of the sensors with PTFE/PMMA are chaotic due to the uneven distribution of

8

PTFE and PMMA, which has a severe influence on the surface charge density after

9

triboelectrification. Conversely, the uniformity of PTFE@PMMA contributes to the regular

10

VOC of the SWSs composed of PTFE@PMMA. Moreover, the VOC of the four kinds of SWSs,

11

prepared with PTFE@PMMA, PTFE/PMMA, pure PMMA, and pure PTFE films with the

12

same thickness of 0.7 mm, were tested (Fig. S10). The VOC of SWS with PTFE@PMMA is

13

similar to that of SWS with pure PMMA, and is slightly lower than that of SWS with pure

14

PTFE.

15

2.5. Planar wear monitoring and positioning by WSA

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In order to monitor the wear conditions of multiple areas simultaneously, a crossed

17

electrode array was embedded into the PTFE@PMMA to produce a wear sensor array

18

(WSA). The advantage of a cross-electrode array is that multi-pixel resolution can be

19

achieved with fewer output channels. A photograph of a fabricated planar WSA with 3×3

20

electrode array is demonstrated in Fig. 4a, which has a diameter of 60 mm. The 3×3

21

electrode array consists of six outputs (X1, X2, X3, Y1, Y2, Y3), each of which contains two

22

electrodes with a length of 44 mm and a width of 2.5 mm, achieving a nine pixels (A-I)

23

resolution. The detailed structure is presented in Fig. 4b, where each node is a woven

24

structure, preventing the electrostatic shielding effect between the upper and lower electrodes.

25

In order to eliminate the signal crosstalk caused by the direct contact between electrodes, the

26

FEP film was used as the insulator layer in the overlapping part of the upper and lower

27

electrodes. As the analysis above, when the copper bar slid on the surface of WSA, the

28

induced potential difference would generate between the electrode and the ground, and the

29

signals can be detected from the corresponding outputs. To characterize the WSA’s response

30

to the sliding motion, the copper bar slid on the WSA driven by hands, as shown in Fig. 4c, 9

1 2

and VOC of each output was acquired by the multichannel measurement system (MMS). As exhibited in Fig. 4d, when the copper bar slides on the node E, VOC of the outputs X2

3

and Y2 are recorded, while the other outputs maintain the state of no signals. In this regard,

4

the position of the sliding copper bar and the wear state of the E node can be explicitly

5

indicated by the voltage signals of six outputs. For a clearer display of the output signals, the

6

voltage of each pixel is demonstrated by the three-dimensional histogram (Fig. 4e). In the

7

histogram, the color represents the voltage value, where the voltage of each node is equal to

8

the sum of the voltages of two corresponding outputs. Since the outputs X2 and Y2 generate

9

the voltage signal, the pixels (B, D, E, F, H) related to these outputs have signals. The sliding

10

position is corresponding to the pixel with the largest value (red color), which can be

11

determined conveniently from Fig. 4e. In a similar way, the output voltage signals and the

12

further processing diagram can effectively reflect the sliding position when the copper bar

13

slid on the node B (Fig. 4f,g) and node I (Fig. 4h,i). To promote the application of the WSA, a

14

Python-based program was designed to process the measured signals of the outputs. The

15

program can also visualize the signals for the pixels by mapping the pixels matrix with color

16

representing signal intensity. A demonstrating video (Supplementary Video. S1) of the

17

position monitoring relying on the integration of the program into MMS is provided. It is

18

shown that the outputs signals can reflect the information about the wear state and position of

19

the PTFE@PMMA.

20

2.6. The monitoring and positioning of the sliding bearing wear state

21

The WSA can be potentially used in such sliding bearing systems as wing and trailing edge

22

of airplane, marine propeller bearing, solar power bearing, as indicated in Fig. 5a. A

23

simplified sliding bearing was established (Fig. S11) and the schematic diagram of the

24

working process and the detailed structure are displayed in Fig. 5a. The sliding bearing was

25

produced by the PTFE@PMMA and seven electrodes consisting of three ring electrodes (R1,

26

R2, R3) and four axis electrodes (A1, A2, A3, A4) was attached on the outer surface, forming

27

a 4×3 sensor array to monitor 12 areas of a sliding bearing. As mentioned above, when the

28

copper bar slides across the electrode, there will be electric signals on the electrode. In order

29

to satisfy this demand, the copper shaft was machined into the struture shown in Fig. 5a.

30

When the shaft rotates, the four rows of protrusions distributed along the axis will slide 10

1

through the electrode A1-A4. Additionally, all protrusions are distributed along a spiral

2

curve, and hence there will be a displacement across the ring electrodes during one rotation

3

cycle. The PTFE@PMMA bearings with two different thicknesses (2.5 mm, 1.0 mm) were

4

prepared to represent the different wear states. The VOC of each electrode for the two

5

bearings were measured when the shaft speed was 300 r/min, as presented in Fig. 5b. The VOC

6

of the seven electrodes for the bearing (the thickness is 1.0 mm) are all larger than that for

7

the bearing with a thickness of 2.5 mm, indicating the successful application in determining

8

the wear state through the VOC signal. Furthermore, a clearer way to display the VOC signal in

9

real time is demonstrated. Using color to represent the magnitude of VOC can effectively

10

achieve this function. Thus, the correlation of thickness, VOC, and color can be described as

11

the VOC magnitude increases when the thickness decreases, and the color changes from blue

12

to red (Fig. 5c). Based on this principle, a 3D display interface was applied to demonstrate

13

the VOC signals of all the pixels. The interface consists of a 3D model of the bearing divided

14

into 12 parts, and it displays the signals of 12 nodes, indicating the thickness of the materials

15

for 12 areas. Similar to the display method in Fig. 4, the VOC of the nodes is equal to the sum

16

of the VOC of the two crossed electrodes. Fig. 5d demonstrates the interface of the two

17

bearings mentioned above when the shaft speed is 300 r/min. When the thickness is 2.5 mm,

18

the colors of the 12 parts are green and the colors of all 12 parts are orange or red when the

19

thickness is 1.0 mm. A photograph of the wear monitoring system including the test equipment

20

and the 3D display interface of the sliding bearing are shown in Fig. S11. The demonstrating

21

videos (Supplementary Video. S2, S3) of the tests of the two bearing are provided in

22

Supporting Information. This system can be applied as the wear monitoring system to

23

provide early bearing failure warning if problems occur during operation.

24 25 26

3. Conclusion In summary, this work demonstrates an active wear sensor array based on the

27

single-electrode TENGs, which is fabricated by embedding the electrode into the core-shell

28

composite PTFE@PMMA. From both theoretical analysis and experimental demonstration,

29

the output electric signals (VOC and ISC) of the sensor unit are proven to be very effective in 11

1

reflecting the wear condition of the PTFE@PMMA layer. A systematical study on the

2

influence of other parameters indicates that VOC increases with the increase of sliding

3

displacement, copper bar diameter and electrode width within a certain range. In addition,

4

the wear sensor array is further explored to achieve dynamic wear monitoring and

5

positioning, which has been successfully applied to detect the wear states of a sliding bearing

6

system. This work opens up the new practical application of TENGs as the wear sensor array

7

and promotes the high quality development of intelligent bearings.

8 9

4. Experimental section

10

Preparation of the PTFE@PMMA core-shell composite and the mechanical mixed

11

PTFE/PMMA powder: The PTFE@PMMA core-shell composite is synthesized by seed

12

emulsion polymerization, which is carried out in a four-neck jacketed reactor (1 L) equipped

13

with a condenser, a mechanical stirrer and inlets of nitrogen and monomer. The 20 mL PTFE

14

latex is added to the reactor which contains deionized water (500 mL) and stirs at 400 r/min

15

under room temperature. Subsequently the reactor is heated to 80 and the 70 mL MMA

16

monomer is added to obtain a mixture of PTFE and MMA. After 20 min stirring, 10 mL

17

potassium persulfate aqueous solution (2 mg/mL) is added. Then, polymerization reaction (12

18

h) is carried out at 400 rpm, and 80 under nitrogen atmosphere protection. The product is

19

isolated and then dried at 85 for 5 h. Finally, the uniform PTFE@PMMA powder is obtained

20

after screening through a 4000 mesh sieve.

21

The mechanically mixed powder marked as PTFE/PMMA is prepared by ball milling the

22

mixture of PTFE powder and PMMA powder for four hours, and the PTFE@PMMA mixture

23

has the same mass ratio as the core-shell structured PTFE@PMMA.

12

1

Fabrication of the wear sensor based on single-electrode TENGs: The nickel electrode is

2

tailored into the strips with the specified size (L mm*15 mm), and then, the FEP films and

3

copper leads are attached on it. Afterwards, the PTFE@PMMA or the PTFE/PMMA powder

4

is filled into the mold with square cavity and the electrode is placed after compressing the

5

powder. The powder is further added on the electrode. Then, the hot pressing process at 160 ℃

6

and 10 MPa is carried out for 60 min, following the cold pressing process at 40 MPa for 10

7

min. The thickness of the polymer layer is controlled by the quantity of the powder.

8

Characterization methods and electrical measurements: Field emission scanning electron

9

microscopy (SEM, SU8220 5.0 kV) is conducted to reveal the morphologies of the PTFE and

10

PTFE@PMMA nanoparticles. Thermogravimetric analysis (TGA Q5000 V3.17 Build 265) is

11

employed to determine the mass ratio of the PTFE and PMMA in the PTFE@PMMA. X-ray

12

photoelectron spectroscopy (XPS, PHI Quantera SXM) is performed to confirm that the PTFE

13

nanoparticles were completely coated by PMMA. VOC and ISC are measured by a Keithley

14

6514 electrometer. The surface potential distribution is measured by the electrostatic

15

voltmeter (Model 244A ISOPROBE® Electrostatic Voltmeter). The movement including the

16

changes of displacement and frequency of the copper bar during testing is driven by a linear

17

motor. The multichannel signals are collected by a PXI system and a Python-based program.

18 19

Conflict of interests

20

The authors declare no conflict of interest.

21 22 23

Acknowledgements

24

This work was supported by the National Natural Science Foundation of China (Grant No.

25

51822505), Tsinghua University Initiative Scientific Research Program (Grant No.

26

2019Z08QCX11). National Natural Science Foundation of China (Nos. 51922023), and Beijing

27

Natural Science Foundation (No. 4192070).

28 29

Appendix A. Supporting information

30

Supplementary data associated with this article can be found in the online version at doi: 13

1 2

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Figures:

Fig. 1. Fabrication process, device structure, working process of the single-electrode wear sensor (SWS). (a) Schematic diagram illustrating the synthesis procedure of the core-shell composite with polytetrafluoroethylene (PTFE) as the core and polymethylmethacrylate (PMMA) as the shell. (b) Schematic diagram illustrating the fabrication (i-iii) and working (iv-v) process of the SWS.

Fig. 2. Working principle and output characteristics of the SWS with PTFE@PMMA layers of different thicknesses. (a) Working principle of the SWS in different states. (b-c) The open-circuit voltage (VOC) and the short-circuit current (ISC) of 10 SWSs with PTFE@PMMA layers of different thicknesses ranging from 4.5 mm to 0 mm. (d-e) The output peak electrical characteristics of the SWS with different thickness of PTFE@PMMA layer from 4.5 mm to 0 mm: (d) VOC; (e) ISC. Note: the other test parameters are shown in (d) and (e).

Fig. 3. SWS performances with different sliding and structural parameters. (a) Dependence of VOC and ISC on sliding frequency of the copper bar, with certain structure parameters (an electrode width of 3 mm, a PTFE@PMMA layer thickness of 0.7 mm, a copper bar diameter of 5 mm and a sliding displacement of 8 mm). (b) Dependence of VOC on sliding displacement of a copper bar at 3 Hz. The inset shows the output voltage curves before and after ∼10,000 cycles. (c) The summarized relationship between VOC and the electrode width for the copper bars with various diameters (3.0 mm, 5.0 mm, 7.0 mm) with a PTFE@PMMA layer thickness of 1.0 mm and a sliding displacement of 8 mm at 3 Hz. (d) The VOC of the SWS made by core-shell PTFE@PMMA and mechanical mixing PTFE/PMMA with the same parameters, respectively.

Fig. 4. Device structure of wear sensor array (WSA) and detection of sliding position. (a) A photograph of WSA with crossed electrodes. (b) Schematic diagram of the crossed electrode with six output ports (X1, X2, X3, Y1, Y2, Y3) and nine nodes(A-I). (c) Schematic diagram of testing process. (d, f, h) The VOC curves of six output ports when the copper bar slid on E,

B, I nodes. (e, g, i) The VOC of nine nodes when the copper bar slid on E, B, I node. Note: the copper bar is driven by hand

Fig. 5. Demonstration of the WSA for monitoring the wear state of a sliding bearing. (a) Structure diagram of the sliding bearing, the shaft and the potential application of the wear sensor. (b) The VOC from the 7 output ports (A1-A4, R1-R3) at 300 rpm. (i) The thickness of bearing is 2.5 mm; (ii) The thickness of bearing is 1.0 mm. (c) The demonstration of the relationship between the display color and the thickness of the PTFE@PMMA. (d) The real-time 3D display interface of the measured VOC of 12 nodes at 300 rpm: (i) The thickness of bearing is 2.5 mm; (ii) The thickness of bearing is 1.0 mm.

Research highlights




By embedding the electrode into the core-shell composite with polytetrafluoroethylene (PTFE) as the core and polymethylmethacrylate (PMMA) as the shell, the sensor unite is fabricated.




A planar sensor array based on single-electrode triboelectric nanogenerators (TENGs) for dynamic wear monitoring and positioning is realized.




By integrating into the sensor array, the dynamic wear monitoring and positioning have been realized, which can be used to detect the wear states of multiple regions in the sliding bearing system.

1

Dynamic wear sensor array based on single-electrode

2

triboelectric nanogenerators

3

Yilong Ren a,1 Guoxu Liu b,c,1, Hang Yang b,c,1, Tong Tongb,c, Shaohang Xu b,c, Lin Zhang a, Jianbin

4

Luo a, Chi Zhang b,c, *, Guoxin Xie a,*

5 6

a

7

Beijing 100084, China

8

b

9

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing

State Key Laboratory of Tribology, Department of Mechanical Engineering, Tsinghua University,

CAS Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-nano Energy and

10

100083, China

11

c

12

100049, China

13

* Corresponding author at: Tsinghua University.

14

E-mail addresses: [email protected] (C. Zhang), [email protected], (G. X. Xie).

15

1

School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing

These authors contributed equally to this work.

16

1

1 2

ABSTRACT:

3

Smart sensor is the foundation and core of intelligent manufacturing, which is developing for

4

miniaturization, integration and self-powering. Here, we report an active sensor array based

5

on single-electrode triboelectric nanogenerators (TENGs) for dynamic wear monitoring and

6

positioning. The sensor unit is fabricated by embedding the electrode into the core-shell

7

composite with polytetrafluoroethylene (PTFE) as the core and polymethylmethacrylate

8

(PMMA) as the shell. The working mechanism and performances of the sensor unit with

9

different parameters including the thickness of PTFE@PMMA layer, reciprocating frequency,

10

sliding displacement, electrode width and the diameter of copper bar are systematically

11

investigated and discussed. By integrating into the sensor array, the dynamic wear monitoring

12

and positioning have been realized, which can be used to detect the wear states of multiple

13

regions in the sliding bearing system. This work has extended the application of the TENGs to

14

determine the wear states of polymer interface and may promote the great development of

15

intelligent bearing.

16 17

Keywords:

18

Core-shell composite, Triboelectric nanogenerator, Sensor array, Wear monitoring sensor,

19

Intelligent bearing

20

2

1

1. Introduction

2

Sensor can acquire the physical condition information of the operating machines, which is

3

essential for intelligent monitoring systems. Polymer-based sliding bearings are the widely

4

used components in mechanical equipment owing to its superiorities in high load capacity,

5

low friction and high wear resistance. Wear failure of the sliding bearing can lead to

6

abnormal operation or even damage to the entire mechanical equipment. Hence, monitoring

7

the wear state of the sliding bearing is significant for determining the working state or failure

8

mode, which is crucial for the machine monitoring in intelligent manufacturing. Traditional

9

wear sensors mainly rely on the signals such as current, vibration, ultrasound, temperature

10

or luminescence spectra to determine the wear condition [1-4]. However, most of these

11

sensors confront a critical deficiency that their work requires an external power supply. Due

12

to the shortages, including large size, limited life and possible environment pollution [5, 6],

13

the conventional chemical battery is insufficient to address this problem. It is highly desirable

14

to develop the self-powered sensors to address the above issues.

15

Triboelectric nanogenerator (TENG) is a promising technology that converts ambient

16

mechanical energy into electricity based on the coupling of triboelectric effect and

17

electrostatic induction [7-15]. Additionally, the output signals of TENG are correlating with

18

the mechanical and environmental stimuli. Thus, TENG has been used as self-powered

19

sensors for various applications, including pressure sensors [16, 17], vibration

20

sensors[18-20], motion/trajectory sensors [21-24] and chemical sensors [25,26]. The TENG

21

with the advantages of diverse structures, various working modes and good integration

22

[27-32] has offered a prospective solution to the design of a novel self-powered and

23

embedded wear sensor array.

24

In this work, we report an active sensor array based on single-electrode TENGs for

25

dynamic wear monitoring and positioning. The sensor unit is fabricated by embedding the

26

electrode into the core-shell composite with polytetrafluoroethylene (PTFE) as the core and

27

polymethylmethacrylate (PMMA) as the shell. The working mechanism and performances of

28

the sensor unit with different parameters including the thickness of PTFE@PMMA layer,

29

reciprocating frequency, sliding displacement, electrode width and the diameter of copper 3

1

bar are systematically investigated and discussed. By integrating into the sensor array, the

2

dynamic wear monitoring and positioning have been realized, which can be used to detect the

3

wear states of multiple regions in the sliding bearing system. This work has extended the

4

application of the TENGs to determine the wear states of polymer interface and may promote

5

the great development of intelligent bearing.

6

2. Results and discussion

7

2.1. The synthesis procedure of PTFE@PMMA

8

In our previous research, we demonstrated a PTFE@PMMA core-shell nanocomposite,

9

which had excellent mechanical properties, such as exceptionally high strength and modulus,

10

low creep, as well as ultralow friction and wear (Fig. S1, S2). Such a type of composite is

11

very promising for solid lubrication in mechanical components, and hence it is of high

12

practical implication to monitor the wear of this type of polymer based on the TENGs and

13

study its triboelectric properties. In this instance, the PTFE@PMMA was chosen as the

14

dielectric layer of TENG for wear monitoring and positioning, which combines mechanical

15

properties, lubrication performance, and electrification performance. The PTFE@PMMA

16

was synthesized by the seed emulsion polymerization [33]. The methylmethacrylate (MMA)

17

monomer was polymerized around the PTFE particles with the participation of the initiator

18

and eventually every PTFE particle was completely coated by PMMA, as shown in Fig. 1a.

19

Detailed synthesis progress is demonstrated in Experimental Section. The SEM images in

20

Fig. S3a, b, show that the PTFE particles are rod-shaped, while the PTFE@PMMA particles

21

are spherical, proving the successful preparation of the core-shell structure. For further

22

verification, the thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy

23

(XPS) analysis were employed. The result of TGA (Fig. S3c) shows two decomposition stages

24

corresponding to the decomposition of PMMA and PTFE [34], which reveals that the mass

25

ratio of PTFE and PMMA is 1:3.4. Because only PTFE contains fluorine, the core-level

26

spectra of fluorine 1s track electron (F 1s spectra) at different depths (0-109.0 nm) were

27

measured and the result is displayed in Fig. S3d, e. At the surface, the intensity of F 1s peak is

28

invisible, suggesting the PTFE is completely wrapped by PMMA. As the sputtering depth

29

increasing, the intensity of F 1s peak increases firstly and then decreases, reaching the

30

maximum value at 65.4 nm. It can be inferred that the average thickness of the PMMA shell 4

1

was about 65 nm. The shift of the binding energy of F 1s might be attributed to the C-F

2

species [35, 36], which generated from the PTFE dissociation caused by the bombardment of

3

the high energy argon ions (Ar+) [37].

4

2.2. The fabrication process of the single-electrode wear sensor unit

5

The single-electrode wear sensor (SWS) unit was fabricated by embedding the nickel

6

electrode into core-shell composite PTFE@PMMA. Fig. 1b illustrates the fabrication process

7

for the SWS. The nickel electrode was tailored to strip, and the thin fluorinated ethylene

8

propylene (FEP) film with the same size was pasted onto the nickel fabric strip with a copper

9

lead was connected. Then, the bottom PTFE@PMMA powder, the pretreated electrode, the

10

upper PTFE@PMMA powder were paved in the mold sequentially (Fig. 1b i-iii), followed by

11

the hot press forming to prepare the wear sensor (Fig. 1b iv,v). The working process (Fig. 1b

12

vi) of the SWS is that the copper bar reciprocates on it, and the output signals can be

13

recorded by a Keithley 6514 electrometer.

14

2.3. The working mechanism of SWS

15

In the SWS, the copper and the PTFE@PMMA act as two triboelectric materials and the

16

nickel strip acts as electrode. When the copper bar slides on the PTFE@PMMA surface, the

17

opposite triboelectric charges generate on the surface of copper bar and PTFE@PMMA,

18

which are attributed to the electron or ion transfer between two contacting materials [38, 39].

19

However, the triboelectrification is a rather complex process and its results are sensitive to

20

the surface properties of materials. The surface damage of PTFE@PMMA caused by sliding

21

can also affect the triboelectrification process [40]. To fully understand the working

22

mechanism of SWS, the surface potentials of the synthetic PMMA and PTFE@PMMA were

23

measured after rubbing against the copper bar. The surface potential distribution maps (Fig.

24

S4a, b) demonstrate the PMMA and PTFE@PMMA were with positive charges. The

25

triboelectric properties of PTFE@PMMA were consistent with synthetic PMMA, because the

26

mass fraction of PMMA in PTFE@PMMA was 77.2% and PMMA wrapped outside PTFE.

27

Thus, when the copper with negative charges reciprocated on the PTFE@PMMA with

28

positive charges, the induced potential difference as well as the charge transfer between the

29

electrode and the ground generated, and they changed periodically. The detailed working

30

principle of the SWS in one reciprocating cycle is demonstrated in Fig. S5 and Supporting 5

1

Information. Fig. 2a schematically shows the working principle of monitoring the wear state

2

of PTFE@PMMA layer through SWS. As the wear of PTFE@PMMA layer progressed, the

3

thickness of PTFE@PMMA layer became smaller. Thus, the movement of the surface charges

4

caused a larger induced potential difference and the transfer of more electron, as shown in

5

Fig. 2a ii, iii. When the PTFE@PMMA layer and FEP film were worn out (Fig. 2a iv), the

6

induced potential difference dropped sharply owing to the contact of two metals: copper and

7

nickel. However, when the copper contacted the FEP film (Fig. 2a iii), the mechanism was

8

different from that in wearing state 1, because FEP had the opposite triboelectric properties

9

with the PTFE@PMMA. Fig. S4c presents the potential distribution in wearing state 2, where

10

the FEP film was negative and the PTFE@PMMA was positive. The detailed mechanism of

11

the wearing state 2 is shown in Fig. S6. In summary, the potential difference and the current

12

between the electrode and the ground could be very effective in reflecting the wear condition

13

of the PTFE@PMMA layer.

14

The SWSs with PTFE@PMMA layers of different thickness were prepared to represent the

15

different wear depths, and the output characteristics (open-circuit voltage VOC and

16

short-circuit current ISC) were measured. The main sliding and structural parameters of the

17

SWS including sliding displacement (X), copper bar diameter (D), electrode width (L), and

18

thickness of PTFE@PMMA layer (H) are also demonstrated in Fig. 2d. It can be clearly seen

19

from Fig. 2b, d that VOC exponentially increases from 2.2 V to 7.0 V with the decreasing

20

thickness of the PTFE@PMMA layer from 4.5 mm to 0.04 mm, and it drops to 0.2 V when no

21

PTFE@PMMA layer exists. Similarly, as shown in Fig. 2c, e, the similar law that the current

22

increases from 6.0 nA to 26 nA as the thickness of the PTFE@PMMA layer decreases from

23

4.5 mm to 0.04 mm, and drops to 1.7 nA when there is no PTFE@PMMA layer. Based on the

24

electrostatic field and superposition principle, theoretical analyses were established to

25

understand the relationship between the VOC or ISC and the thickness of the PTFE@PMMA

26

layer, which could be described as the following equation (Detailed derivation is shown in

27

Supporting Information):
 = − ln ℎ +  (1)

28

where A and B are the coefficients related to the surface charge density and the dielectric 6

1

constant of the triboelectric materials. The nickel electrode and the ground could be

2

considered as a capacitance, and hence the relationship among QSC, C, VOC, ISC could be

3

given by:

4 5 6


 (2) 

 =

 (3) ∆

where QSC is defined as the transferred charges and ISC represents the current at the short circuit condition. Substituting Equations 1 and 2 into Equation 3, ISC could be obtained by:  =

7

 =


   =− ln h + (4)  ∙ ∆  ∙ ∆  ∙ ∆

As the other parameters remained unchanged, including reciprocating frequency, electrode

8

width, copper bar diameter and sliding displacement, the capacitance C and the charge

9

transfer time t could be adopted as the fixed constants. Thus, the relationship between

10

current and thickness could be expressed by the same function, as shown below.  = −E ln ℎ +  (5)

11

where E and G are the coefficients determined by coefficients A and B, equivalent and

12

electron transfer time. Equation 1 was applied to fit the experimental results of VOC, as shown

13

in Fig. 2d, and the goodness of fit R2 is 0.93. Fig. 2e displays the fit of Equation 5 to the

14

results of ISC, and the goodness of fit R2 is 0.94. Higher goodness of fit proved the credibility

15

of the theoretical model, and the model could be applied to estimate the thickness of

16

PTFE@PMMA layer, depending on VOC and ISC. Additionally, simulation through COMSOL

17

was employed to investigate the law of the electric signals with the thickness. The simulation

18

results of the VOC show the similar exponential changes (Fig. S8). The sensors were tested

19

under the same condition after several months to verify the data reproducibility. As shown in

20

Fig. S9, the VOC and ISC show the similar values and exponential changes, indicating the good

21

stability and reproducibility.

22

2.4. The performances of SWS

23

Taking into consideration that the output characteristics could be affected by the 7

1

parameters apart from thickness, a systematic research was carried out to reveal the

2

relationship between VOC or ISC and the sliding and structural parameters such as

3

reciprocating frequency, sliding displacement (X), copper bar diameter (D) and electrode

4

width (L). In a full cycle of motion, the copper bar slid from left to right, causing the change

5

in the position of the electric field, which was necessary for generating the electric signals.

6

Thus the frequency and displacement of copper bar are the important parameters. Fig. 3a

7

shows the VOC and ISC for a SWS with an electrode width of 3 mm, a PTFE@PMMA thickness

8

of 0.7 mm and a copper bar diameter of 5 mm at different frequencies. VOC is steady at

9

around 3.3 V with the frequency increasing from 1 to 5 Hz, being consistent with the previous

10

study [41]. Because VOC is the result of the electrostatic field established by triboelectric

11

charges, which is independent of speed. However, it is worth noting that ISC is proportional to

12

the frequency (). The time of charge transfer can be calculated as follow: 1 ∆ = (6) 

13

Combining Equation 3, ISC can also be given by:  =  ∙  (7)

14

The behavior of ISC can be easily explained with Equation 7. Since QSC is constant, ISC is

15

linearly proportional to the frequency. Therefore, VOC is more suitable as the signal

16

indicating the wear condition owing to its speed-independent characteristic. The relationship

17

between VOC and the sliding displacement (X) was studied for the SWS with an electrode

18

length of 5 mm, a PTFE@PMMA layer thickness of 0.7 mm and a copper diameter of 5 mm at

19

3 Hz. As shown in Fig. 3a, VOC increases gradually and then slightly fluctuates around the

20

maximum value, being consistent with the finite element method (FEM) calculation results of

21

VOC in the sliding-separation mode single-electrode TENGs [42]. The SWS continuously ran

22

for about 10000 cycles to determine its stability. As can be seen from the inset of Fig. 3b,

23

there is no obvious decay of VOC after the durability test.

24

The output voltages of the sensors with different electrode widths under the stimuli of

25

different size copper bars were systematically tested. As shown in Fig. 3c, VOC increases with

26

the increasing copper bar diameter, while the variation of VOC with the electrode width is

27

complicated. When the electrode width is narrow, VOC is proportional to the electrode width,

28

which is confirmed by the linear fit presented in Fig. 3c. VOC starts to stabilize or drop when 8

1

the electrode width is wide. The complicated behavior of VOC is consistent with the analysis

2

about the effect of area size based on the capacitance model of single-electrode TENGs in

3

previous researches [42]. Apart from the sliding and structure parameters, the effect of the

4

polymer material on the output performance was considered. The PTFE and PMMA

5

composite (PTFE/PMMA) prepared by mechanically mixing was used to fabricate the SWS.

6

Fig. 3d compares VOC of the SWSs made by PTFE/PMMA and PTFE@PMMA. The

7

performances of the sensors with PTFE/PMMA are chaotic due to the uneven distribution of

8

PTFE and PMMA, which has a severe influence on the surface charge density after

9

triboelectrification. Conversely, the uniformity of PTFE@PMMA contributes to the regular

10

VOC of the SWSs composed of PTFE@PMMA. Moreover, the VOC of the four kinds of SWSs,

11

prepared with PTFE@PMMA, PTFE/PMMA, pure PMMA, and pure PTFE films with the

12

same thickness of 0.7 mm, were tested (Fig. S10). The VOC of SWS with PTFE@PMMA is

13

similar to that of SWS with pure PMMA, and is slightly lower than that of SWS with pure

14

PTFE.

15

2.5. Planar wear monitoring and positioning by WSA

16

In order to monitor the wear conditions of multiple areas simultaneously, a crossed

17

electrode array was embedded into the PTFE@PMMA to produce a wear sensor array

18

(WSA). The advantage of a cross-electrode array is that multi-pixel resolution can be

19

achieved with fewer output channels. A photograph of a fabricated planar WSA with 3×3

20

electrode array is demonstrated in Fig. 4a, which has a diameter of 60 mm. The 3×3

21

electrode array consists of six outputs (X1, X2, X3, Y1, Y2, Y3), each of which contains two

22

electrodes with a length of 44 mm and a width of 2.5 mm, achieving a nine pixels (A-I)

23

resolution. The detailed structure is presented in Fig. 4b, where each node is a woven

24

structure, preventing the electrostatic shielding effect between the upper and lower electrodes.

25

In order to eliminate the signal crosstalk caused by the direct contact between electrodes, the

26

FEP film was used as the insulator layer in the overlapping part of the upper and lower

27

electrodes. As the analysis above, when the copper bar slid on the surface of WSA, the

28

induced potential difference would generate between the electrode and the ground, and the

29

signals can be detected from the corresponding outputs. To characterize the WSA’s response

30

to the sliding motion, the copper bar slid on the WSA driven by hands, as shown in Fig. 4c, 9

1 2

and VOC of each output was acquired by the multichannel measurement system (MMS). As exhibited in Fig. 4d, when the copper bar slides on the node E, VOC of the outputs X2

3

and Y2 are recorded, while the other outputs maintain the state of no signals. In this regard,

4

the position of the sliding copper bar and the wear state of the E node can be explicitly

5

indicated by the voltage signals of six outputs. For a clearer display of the output signals, the

6

voltage of each pixel is demonstrated by the three-dimensional histogram (Fig. 4e). In the

7

histogram, the color represents the voltage value, where the voltage of each node is equal to

8

the sum of the voltages of two corresponding outputs. Since the outputs X2 and Y2 generate

9

the voltage signal, the pixels (B, D, E, F, H) related to these outputs have signals. The sliding

10

position is corresponding to the pixel with the largest value (red color), which can be

11

determined conveniently from Fig. 4e. In a similar way, the output voltage signals and the

12

further processing diagram can effectively reflect the sliding position when the copper bar

13

slid on the node B (Fig. 4f,g) and node I (Fig. 4h,i). To promote the application of the WSA, a

14

Python-based program was designed to process the measured signals of the outputs. The

15

program can also visualize the signals for the pixels by mapping the pixels matrix with color

16

representing signal intensity. A demonstrating video (Supplementary Video. S1) of the

17

position monitoring relying on the integration of the program into MMS is provided. It is

18

shown that the outputs signals can reflect the information about the wear state and position of

19

the PTFE@PMMA.

20

2.6. The monitoring and positioning of the sliding bearing wear state

21

The WSA can be potentially used in such sliding bearing systems as wing and trailing edge

22

of airplane, marine propeller bearing, solar power bearing, as indicated in Fig. 5a. A

23

simplified sliding bearing was established (Fig. S11) and the schematic diagram of the

24

working process and the detailed structure are displayed in Fig. 5a. The sliding bearing was

25

produced by the PTFE@PMMA and seven electrodes consisting of three ring electrodes (R1,

26

R2, R3) and four axis electrodes (A1, A2, A3, A4) was attached on the outer surface, forming

27

a 4×3 sensor array to monitor 12 areas of a sliding bearing. As mentioned above, when the

28

copper bar slides across the electrode, there will be electric signals on the electrode. In order

29

to satisfy this demand, the copper shaft was machined into the struture shown in Fig. 5a.

30

When the shaft rotates, the four rows of protrusions distributed along the axis will slide 10

1

through the electrode A1-A4. Additionally, all protrusions are distributed along a spiral

2

curve, and hence there will be a displacement across the ring electrodes during one rotation

3

cycle. The PTFE@PMMA bearings with two different thicknesses (2.5 mm, 1.0 mm) were

4

prepared to represent the different wear states. The VOC of each electrode for the two

5

bearings were measured when the shaft speed was 300 r/min, as presented in Fig. 5b. The VOC

6

of the seven electrodes for the bearing (the thickness is 1.0 mm) are all larger than that for

7

the bearing with a thickness of 2.5 mm, indicating the successful application in determining

8

the wear state through the VOC signal. Furthermore, a clearer way to display the VOC signal in

9

real time is demonstrated. Using color to represent the magnitude of VOC can effectively

10

achieve this function. Thus, the correlation of thickness, VOC, and color can be described as

11

the VOC magnitude increases when the thickness decreases, and the color changes from blue

12

to red (Fig. 5c). Based on this principle, a 3D display interface was applied to demonstrate

13

the VOC signals of all the pixels. The interface consists of a 3D model of the bearing divided

14

into 12 parts, and it displays the signals of 12 nodes, indicating the thickness of the materials

15

for 12 areas. Similar to the display method in Fig. 4, the VOC of the nodes is equal to the sum

16

of the VOC of the two crossed electrodes. Fig. 5d demonstrates the interface of the two

17

bearings mentioned above when the shaft speed is 300 r/min. When the thickness is 2.5 mm,

18

the colors of the 12 parts are green and the colors of all 12 parts are orange or red when the

19

thickness is 1.0 mm. A photograph of the wear monitoring system including the test equipment

20

and the 3D display interface of the sliding bearing are shown in Fig. S11. The demonstrating

21

videos (Supplementary Video. S2, S3) of the tests of the two bearing are provided in

22

Supporting Information. This system can be applied as the wear monitoring system to

23

provide early bearing failure warning if problems occur during operation.

24 25 26

3. Conclusion In summary, this work demonstrates an active wear sensor array based on the

27

single-electrode TENGs, which is fabricated by embedding the electrode into the core-shell

28

composite PTFE@PMMA. From both theoretical analysis and experimental demonstration,

29

the output electric signals (VOC and ISC) of the sensor unit are proven to be very effective in 11

1

reflecting the wear condition of the PTFE@PMMA layer. A systematical study on the

2

influence of other parameters indicates that VOC increases with the increase of sliding

3

displacement, copper bar diameter and electrode width within a certain range. In addition,

4

the wear sensor array is further explored to achieve dynamic wear monitoring and

5

positioning, which has been successfully applied to detect the wear states of a sliding bearing

6

system. This work opens up the new practical application of TENGs as the wear sensor array

7

and promotes the high quality development of intelligent bearings.

8 9

4. Experimental section

10

Preparation of the PTFE@PMMA core-shell composite and the mechanical mixed

11

PTFE/PMMA powder: The PTFE@PMMA core-shell composite is synthesized by seed

12

emulsion polymerization, which is carried out in a four-neck jacketed reactor (1 L) equipped

13

with a condenser, a mechanical stirrer and inlets of nitrogen and monomer. The 20 mL PTFE

14

latex is added to the reactor which contains deionized water (500 mL) and stirs at 400 r/min

15

under room temperature. Subsequently the reactor is heated to 80 and the 70 mL MMA

16

monomer is added to obtain a mixture of PTFE and MMA. After 20 min stirring, 10 mL

17

potassium persulfate aqueous solution (2 mg/mL) is added. Then, polymerization reaction (12

18

h) is carried out at 400 rpm, and 80 under nitrogen atmosphere protection. The product is

19

isolated and then dried at 85 for 5 h. Finally, the uniform PTFE@PMMA powder is obtained

20

after screening through a 4000 mesh sieve.

21

The mechanically mixed powder marked as PTFE/PMMA is prepared by ball milling the

22

mixture of PTFE powder and PMMA powder for four hours, and the PTFE@PMMA mixture

23

has the same mass ratio as the core-shell structured PTFE@PMMA.

12

1

Fabrication of the wear sensor based on single-electrode TENGs: The nickel electrode is

2

tailored into the strips with the specified size (L mm*15 mm), and then, the FEP films and

3

copper leads are attached on it. Afterwards, the PTFE@PMMA or the PTFE/PMMA powder

4

is filled into the mold with square cavity and the electrode is placed after compressing the

5

powder. The powder is further added on the electrode. Then, the hot pressing process at 160 ℃

6

and 10 MPa is carried out for 60 min, following the cold pressing process at 40 MPa for 10

7

min. The thickness of the polymer layer is controlled by the quantity of the powder.

8

Characterization methods and electrical measurements: Field emission scanning electron

9

microscopy (SEM, SU8220 5.0 kV) is conducted to reveal the morphologies of the PTFE and

10

PTFE@PMMA nanoparticles. Thermogravimetric analysis (TGA Q5000 V3.17 Build 265) is

11

employed to determine the mass ratio of the PTFE and PMMA in the PTFE@PMMA. X-ray

12

photoelectron spectroscopy (XPS, PHI Quantera SXM) is performed to confirm that the PTFE

13

nanoparticles were completely coated by PMMA. VOC and ISC are measured by a Keithley

14

6514 electrometer. The surface potential distribution is measured by the electrostatic

15

voltmeter (Model 244A ISOPROBE® Electrostatic Voltmeter). The movement including the

16

changes of displacement and frequency of the copper bar during testing is driven by a linear

17

motor. The multichannel signals are collected by a PXI system and a Python-based program.

18 19

Conflict of interests

20

The authors declare no conflict of interest.

21 22 23

Acknowledgements

24

This work was supported by the National Natural Science Foundation of China (Grant No.

25

51822505), Tsinghua University Initiative Scientific Research Program (Grant No.

26

2019Z08QCX11). National Natural Science Foundation of China (Nos. 51922023), and Beijing

27

Natural Science Foundation (No. 4192070).

28 29

Appendix A. Supporting information

30

Supplementary data associated with this article can be found in the online version at doi: 13

1 2

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Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: