Hybrid capacitive/piezoelectric visualized meteorological sensor based on in-situ polarized PVDF-TrFE films on TFT arrays

https://doi.org/10.1016/j.sna.2020.112286Get rights and content

Highlights

  • PVDF-TrFE piezoelectric film is combined with a captive CMOS TFT array.

  • The hybrid sensor can realize the real-time detection of wind speed, rain drop frequency.

  • The hybrid sensor can realize the visualization of rain drops.

Abstract

A novel type of hybrid capacitive/piezoelectric sensor was designed and fabricated for real-time monitoring of the size, rate, and frequency of rainfall. The design is based on in situ polarized poly (vinylidene fluoride–trifluoroethylene) (PVDF-TrFE) films on thin-film transistor (TFT) arrays. The polarization of the PVDF-TrFE film with an area of 200 × 200 mm2 can be achieved within 5 min using an in-situ polarization process, and the piezoelectric coefficient d33 can reach an average value of 27 pC/N with a uniform distribution across the area. The PVDF-TrFE piezoelectric sensor exhibits high sensitivity, with a raindrop frequency error margin of 1%. The capacitive sensor is composed of the PVDF-TrFE films and TFT arrays, which can be used to determine the size and shape of raindrops on the sensor surface. In addition, the sensor can also be used for high-precision detection of water level, with a resolution of 50 μm.

Introduction

High-accuracy, real-time monitoring of meteorological conditions such as wind [1] and rainfall has become increasingly important because it can provide useful information regarding current weather conditions, warn people of potential meteorological disasters, and facilitate production efficiency in agriculture applications. Real-time monitoring of meteorological information is based mostly on different types of meteorological sensors. Among the various types of meteorological sensors, those for rainfall detection have been successfully applied in the fields of automobile traffic [[2], [3], [4], [5], [6]] and agricultural and landscape irrigation [[7], [8], [9], [10], [11]].

The traditional rainfall detection sensors used in automobile traffic are based mostly on optical systems, such as infrared systems [12] or cameras [3,4] with image processing analysis technology, as provided in Table 1. Infrared sensors consist of infrared emitters and receivers. The infrared light reflected by the front windshield decreases with increasing rainfall, because the infrared light is scattered by raindrops. Sensors based on cameras with image processing analysis technology capture the optical image from the front windshield, and obtain raindrop information by algorithm processing and comparison and analysis with images in the database. However, because of the limited sample quantities in the database and the complexity of background images, such rainfall detection sensors exhibit poor performance in complex environments, especially in bustling cities and other brightly lit areas. Furthermore, such sensors require an external data analysis process to detect rainfall, and do not make full use of the characteristics of rainfall, such as the kinetic energy of the drops falling on the surface. Therefore, there is an urgent need to develop the next generation of sensors that can detect information such as raindrop frequency and speed with high performance under various environments.

Piezoelectric materials [[13], [14], [15]] can convert mechanical energy to electrical signals to detect micro-mechanical forces. Because raindrops can generate micro-mechanical forces when they fall to the ground, piezoelectric materials enable the development of novel rainfall detection sensors. Generally, piezoelectric materials fall into two categories: piezoelectric inorganics and piezoelectric polymers. Piezoelectric inorganics, such as piezoelectric ceramics, have higher piezoelectric coefficients but are stiff, whereas piezoelectric polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers [[16], [17], [18], [19]], are flexible and stretchable. Recently, with the rapid development of micro-electromechanical systems (MEMS), the properties of miniaturization, flexibility, and self-power have been prime concerns for low-power-drive sensors [[24], [25], [26]]. Thus, piezoelectric polymers seem to be the best choice for the fabrication of rainfall detection sensors. Ong and co-workers [20] used a lead zirconate titanate (PZT) piezoelectric cantilever beam to harvest energy by guiding water droplets to a syringe to convert random raindrops into single water droplets to control the position of the drops falling on the piezoelectric beam. The performance of this energy harvesting device has been improved by at least 208%. Ilyas et al. [21] analyzed in detail the change in the voltage output curve caused by raindrop impacts on piezoelectric film. They found that there is an impact phase during the droplet impact process, and as the energy stored in the collector is dissipated, a decay phase occurs. Vatansever et al. [22] studied the effects of material dimensions, drop mass, release height of the drops, and wind speed on the voltage output. They showed that piezoelectric polymer materials can produce higher voltage than ceramic-based piezoelectric materials, which proves that it is feasible to use piezoelectric polymer materials to produce energy from raindrops and wind, which are renewable energy sources. However, these studies examined only the electric response to the micro-mechanical force generated by rainfall and ways to improve the efficiency of raindrop energy harvesting devices; they rarely refer to properties of the rainfall, such as the detection and graphical display of raindrop shapes. Thus, there still remains many opportunities for further improvement of rainfall detection sensors.

A thin-film transistor (TFT) is a thin-film field effect transistor; TFT arrays, which are composed of a large number of TFT pixels, are commonly used in the field of liquid crystal displays to drive the illumination of liquid crystal pixel dots [[27], [28], [29], [30], [31]]. With a TFT pixel array, the capacitance of different pixel points of the PVDF film can be detected separately; hence, the values of the PVDF local capacitance caused by the water droplets on the PVDF surface can be collected by a pixelated acquisition circuit. The electrical signals are visualized by a computer to finally obtain the raindrop image on the surface of the PVDF film. Thus, the combination of piezoelectric polymer and TFT pixel arrays can detect both rainfall capacity from the electric response of the piezoelectric polymer and the size and the distribution of raindrops from a TFT-based rainwater image recognition system.

In this study, a hybrid capacitive/piezoelectric sensor for rainfall and wind detection with PVDF-TrFE films on TFT pixel arrays was fabricated and evaluated. The hybrid sensor can measure not only the speed and frequency of raindrops by its piezoelectric sensor, but also visualizing the distribution and size of raindrops by its capacitive sensor. In addition, the hybrid sensor realizes a high accurate meteorological detection by the piezoelectric sensing and capacitive visualization such as raindrop and wind. The proposed system shows great potential as a next-generation rainfall and wind detection and recognition sensor.

Section snippets

Materials

P(VDF-TrFE) copolymer powder was obtained from Arkema Co., Ltd., and butanone was supplied by Chengdu Kelong Chemical Co., Ltd.

Preparation of PVDF-TrFE solution

In a typical process, the calculated PVDF-TrFE copolymer was dispersed into methylethyl ketone (MEK, 99.8%) with a mass ratio of 17:100. Then, the solution was magnetically stirred at room temperature for 4 h, after which the magnetic stirring rate was reduced to remove bubbles from the solution. Finally, a clarified PVDF-TrFE butanone solution was obtained.

Preparation of PVDF-TrFE film

The

Results and discussions

The 200 × 200 mm2 PVDF-TrFE films were prepared with thicknesses of 10 μm (10.1 ± 0.7 μm tested by step profiler). SEM images of the PVDF-TrFE surface morphology with 10-μm thickness obtained under vacuum at ambient temperature and under atmospheric pressure at 100 °C are shown in Fig. 3(a) and (b). It is obvious that the surface of the PVDF-TrFE film obtained by vacuum drying has better flatness and fewer defects compared with that of PVDF-TrFE obtained by atmospheric drying. Furthermore, the

CRediT authorship contribution statement

Qian Zhang: Conceptualization, Writing - original draft. Shuai Liu: Methodology, Investigation. Hongqiang Luo: Methodology, Data curation. Zhen Guo: Methodology, Data curation. Xiaoran Hu: Supervision, Writing - review & editing. Yong Xiang: Funding acquisition.

Declaration of Competing Interest

The authors reported no declarations of interest.

Acknowledgements

This work was supported by National Key Research and Development Program of China (2017TFB0702800).

Qian Zhang was born in China, in 1989. He received his B.S. in Solid-State Electronics and his M.S. in Electrical Engineering from University of Electronic Science and Technology of China, Chengdu, China, in 2012 and 2015, respectively. After graduation, he worked as R&D Engineer at Goodix, Shenzhen, China and HiSilicon, Shenzhen, China. Currently, he is a Sensor Research Engineer at University of Electronic Science and Technology of China, Chengdu, China. His research interests include

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  • Cited by (0)

    Qian Zhang was born in China, in 1989. He received his B.S. in Solid-State Electronics and his M.S. in Electrical Engineering from University of Electronic Science and Technology of China, Chengdu, China, in 2012 and 2015, respectively. After graduation, he worked as R&D Engineer at Goodix, Shenzhen, China and HiSilicon, Shenzhen, China. Currently, he is a Sensor Research Engineer at University of Electronic Science and Technology of China, Chengdu, China. His research interests include switching mode power supplies for piezoelectric films and ultrasonic sensors.

    Shuai Liu was born in China, in1993. He received his M.S in Electronic Science and Technology from the from University of Electronic Science and Technology of China. His research interests include electronic circuits and PCB layout design.

    Hongqiang Luo was born in China, in 1987. He received his B.S. in Material Science and Engineering from Fuzhou University in 2010. Currently he is a graduate student of University of Electronic Science and Technology of China. His research interests include OLED display and touch sensor.

    Zhen Guo was born in China, in 1994. He received his M.S. in Material Science and Engineering from University of Electronic Science and Technology of China. His research interests includes electronic materials.

    Xiaoran Hu received his B.S. and Ph.D. in Materials Science and Engineering from Beijing University of Chemical Technology (BUCT) in 2012 and 2017, respectively. Since 2019, he has been a Associate Professor at School of Materials and Energy, University of Electronic Science and Technology of China (UESTC), Chengdu, China. His research interests include smart functional materials and device, and multifunctional bio-based polymers.

    Yong Xiang received his B.S. in Mechanical Engineering from University of Science and Technology of China, Hefei, China, in 2000, and his M.S. and Ph.D. in Materials Science from Harvard University, Cambridge, MA, USA, in 2001 and 2005, respectively. He was a Senior Engineer and Project Manager at Intel Corporation, Santa Clara, CA, from 2006 to 2009. Since 2009, he has been a Professor of Energy, Materials Science, and Microelectronics at University of Electronic Science and Technology of China (UESTC), Chengdu, China. His research interests include materials genome, thin films, sensors, solid-state batteries, and power electronics. He holds an adjunct position at the State Key Lab of Electronic Thin Films and Integrated Devices and serves as the Associate Director of its Zhuhai Branch. At UESTC, he served as the Assistant Dean of the School of Microelectronics and Solid-State Electronics, UESTC from 2009 to 2011, the Associate Dean of the School of Energy Science and Engineering from 20,011 to 2015, and the Dean of the School of Materials and Energy from 20,017 to 2018. He is currently the Dean of Advanced Energy Institute and the Director of Sichuan Flexible Display Materials Genome Engineering Center at UESTC. He is also members of several national scientific consulting expert panels in China.

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