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Cheng Yang’s research group decouples piezoelectric-triboelectric signals

Contact electrification (CE) as a universal electron transfer phenomenon that can occur between any material in different physical states (solid, liquid, and gas). Recently, studies have confirmed that CE can take place in piezoelectric devices during a variety of motions, including interfacial shear friction, nanofiller slippage, pre-existing static charge and contact friction. Simultaneously, the existence of triboelectric signals generated by the contacted objects and piezoelectric devices can be strong and shouldn’t be ignored, especially under repeated pressing. However, previous studies have not addressed this kind of triboelectric signal and regarded the output from piezoelectric devices as a single piezoelectric signal, leading to its performance being overstated. The lack of a reliable method to differentiate piezoelectric signals and triboelectric signals prevents us from understanding the origin of hybrid signals, resulting in the inability to analyze the influence of triboelectric signals and evaluate the true contribution of the piezoelectric effect. Because of this, an incorrect evaluation on piezoelectric performance is obtained. Therefore, developing a method to identify and extract the piezoelectric component from a “piezoelectric” signal is particularly important for quantitatively evaluating the performance of piezoelectric materials.

Recently, Research Associate Professor Cheng YANG from Tsinghua Shenzhen International Graduate School (Tsinghua SIGS) and Professor Zhong Lin WANG from the Beijing Institute of Nanoenergy and Nanosystems published a paper, proposing a method to separate the piezoelectric component from piezoelectric-triboelectric hybrid signals, achieving the accurate evaluation of piezoelectric materials.

In this study, the research group analyzed the CE process that happened between loading objects and the encapsulating materials, and developed an effective method to identify the piezoelectric part from a hybrid triboelectric-piezoelectric output in a sandwich-structured poly(vinylidene fluoride) (PVDF)-based piezoelectric device system. Based on experimental observation, the triboelectric signals can take up a large portion of the output, which should not be ignored. The researchers investigated the force-time curve of the piezoelectric process as a strong supplement rather than solely analyzing the electric signals, which is the source of the electric signals. By comparing these two curves simultaneously, it can be found that before and after the device-object contact, the recorded electric signals belong to the triboelectric contribution, and the electric signals acquired when they are contacted can be ascribed to the piezoelectric effect. Then the piezoelectric charge transfer was quantitatively extracted from the hybrid output and the effective piezoelectric coefficient (d33) was calculated, such results were consistent with those measured by other methods, which are elucidated as follows. In this way, it is convenient to separate the combined signals clearly, and the accuracy is only limited by both force-time and charge-time resolutions.

Fig. 1 Schematic of the existence of both SE-TENG and PENG during practical application of piezoelectric devices, but the time at which the signal is generated has distinct differences.

Fig. 2 Verifying the existence of SE-TENG in a compression test.

Fig. 3 Using loading force signal to differentiate the sole triboelectric signals and the sole piezoelectric signals.

Fig. 4 The triboelectric-piezoelectric hybrid output generated by the PVDF-based device with the negative polarization direction.

Fig. 5 Evaluating the piezoelectric performance of PVDF film by extracting the piezoelectric charge transfer from the hybrid output.

This paper has recently been published in the journal Nature Communicationsand is titled “A method for quantitatively separating the piezoelectric component from the as-received ‘Piezoelectric’ signal.” The corresponding authors are Prof. Cheng YANG and Prof. Zhong Lin WANG. The first author is Chaojie CHEN. The second author is Shilong ZHAO. The authors of the paper also include Fangcheng WANG, Prof. Caofeng PAN from Beijing Institute of Nanoenergy and Nanosystems, and Prof. Yunlong ZI from the Chinese University of Hong Kong. This work was supported by the National Natural Science Foundation of China, the Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program, Shenzhen Technical Project, and the China Postdoctoral Science Foundation.

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Written by Chaojie Chen

Edited by Alena Shish & Yuan Yang