Self-powered sound detection and recognition sensors based on flexible polyvinylidene fluoride-trifluoroethylene films enhanced by in-situ polarization

Highlights

• We enhanced its piezoelectric performance by in-situ polarization technique that polarized the sample in 5 min.

• SDRS can accurately identify and detect sound waves with a frequency of 20hz-20khz, and the relative error does not exceed 1%.

• SDRS’s characteristics of flexibility, miniaturization and high accuracy can make it applied to various fields.

Abstract

A flexible, accurate and highly sensitive sound detection and recognition sensor was fabricated in this study; this was achieved by employing a polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) film and in-situ polarization. The PVDF-TrFE film was polarized using our polarization system within 5 min without heating; further, this system has the advantages of high production efficiency, excellent piezoelectric performances, and good uniformity compared to traditional poling approaches. The sound detection and recognition sensor responded to various types of sound waves in a wide range of frequencies with an error of less than 1% by producing corresponding voltage signal; these can be used to accurately identify complex sound signals with different frequencies and different intensities. Furthermore, the sensor was flexible and self-powered. All the above features render it hold great potential in various applications, such as microphones, biometrics, environmental protection and artificial intelligence.

Introduction

Sound detection and recognition technology is an important approach to send and/or receive information in everyday life. Most sound detection and recognition technologies function by employing an acoustic–electrical conversion method, which transforms sound waves into measurable electrical signals; this method is widely used in security protection, target location, biomedical applications, environmental protection and other scientific research fields [[1], [2], [3], [4], [5]]. With the development of big data technology and the Internet in more vertical industries, the combination of acoustic sensors with computer technology and communication technology has begun to be applied to intelligent devices. Tsujimoto et al. [6] used acoustic sensors to intelligently monitor the fluidization of particles in a fluidized bed granulator. With the gradual arrival of the intelligent era, sensors will become more irreplaceable. At present, various types of sound detection and recognition sensors (SDRS) have been developed based on different acoustic–electrical conversion approaches, which incorporate electrostatic, capacitive, electromagnetic, electric and piezoelectric techniques [[7], [8], [9], [10], [11]]. However, sound detection and recognition sensors based on electrostatic, capacitive, electromagnetic or electric methodologies have numerous disadvantages, such as large sizes, their need forexternal power supplies, and low acoustic–electric conversion efficiencies. Furthermore, the rapid development of microelectromechanical systems (MEMS) has rendered miniaturization, flexibility, and self-powering capabilities asthe primary factors in the production of SDRS [[12], [13], [14]]. However, piezoelectric sensorsexhibit significant potential over other types of SDRS. This is because of their high acoustic–electric conversion efficiency, small size, flexibility, and high signal-to-noise ratio; further, these sensors display a self-powering capability without any external power supply, long service life, and a receiving sensitivity that is high over a wide temperature range [[15], [16], [17], [18], [19]]. Therefore, piezoelectric materials have become a significant subject for research as a novel acoustic and electrical sensor. Generally, in most piezoelectric SDRS, inorganic piezoelectric materials are used due to their high piezoelectric coefficient. However, the wide use of inorganic piezoelectric materials has been impeded by their material inflexibility, high polarization electric fields, and environmentally polluting properties (such as lead in piezoelectric transducers [20,21]. Recently, piezoelectric polymers with good flexibility, processing ability and biocompatibility have shown great potential in the fabrication of far-SDRS; these include polyvinylidene fluoride (PVDF) and its copolymers such as polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE). For example, Li et al. [22] used a PVDF-based piezoelectric cantilever as a micro-generator with a quarter-wave straight tube acoustic resonator in its proximity. The generator exhibited excellent performance, including high acoustic–electric conversion, high efficiency and high voltage output. Dias et al. [23] used PVDF strips that were bond together to form an arch at each end; further, the elastic knit fabric was tightly woven without pores. This material can be embedded in a variety of cloth products and used as a speaker film material for active control of automotive noise; the reverse piezoelectric effect provides passengers with a quiet interior environment. Cheong et al. [24] succeeded in fabricating high-performance PVDF acoustic actuators using chemical-vapor-deposited (CVD) graphene electrodes in combination with various ZnO structures; thesedemonstrated high sensitivity in the low frequency range and reduced the total harmonic distortion. The material exhibits potentialfor use in telephone microphones. Benech and Novakov [25] produced a PVDF sensor that uses electroacoustic conversion to detect the presence of air bubbles in ducts and the acoustic impedance between the PVDF. Further, the tube was realized in a simple manner, and the high excitation voltage compensated for the low sensitivity of the PVDF; therefore, this sensor has great potential in ultrasonic flaw detection fields.

To the best of our knowledge, little research has been conducted into PVDF-based sound detection and recognition sensors. Moreover, the fabrication of PVDF-based piezoelectric sensors is still limited by the poling process of PVDF. The commonly used corona polarization requires heating, and a relatively long period of time that leads to the formation of only small pieces of PVDF. Recently, new polarization methods have been developed to deal with these issues. An in-situ polarization system (IPS) was used by our research group, and a 200 mm × 200 mm PVDF film with good piezoelectric performance and uniform distribution was fabricated (d33 = 28 ± 2 pC/N) within only 5 min at room temperature. The details are provided in the methods section. In this study, we prepared a novel PVDF-TrFE-based SDRS. The sensor can accurately identify the frequency and intensity of a variety of sounds with a wide frequency response range. The sensor also has a high sensitivity with a relative error of less than 1%. in the frequency response of the sound signal.