Preparation And Flexoelectric Properties Of Polyvinylidene Fluoride Films With Rough Surfaces

Polyvinylidene fluoride(PVDF)is widely used in medical devices,internet of everything and wearable electronics because of its good flexibility and piezoelectricity.However,the preparation of PVDF piezoelectric films requires tedious processing such as film formation,stretching and polarization,which has the disadvantages of low efficiency and high energy consumption.In addition,when used in sensing devices,they usually have pyroelectric effects and tend to lose their piezoelectric properties at higher temperatures,making them difficult to use in applications where temperatures are constantly changing and at high temperatures.The flexoelectric effect is an electromechanical coupling effect caused by the strain gradient of the material,which can effectively improve the apparent piezoelectric properties of the material.However,current research mainly focuses on achieving a strong flexoelectric response at low frequency and large strain conditions.In this paper,we focus on non-polar PVDF films and use the template replication method to construct gradient micro-and nanostructures with certain roughness on the surface of the films to study their flexoelectric properties.The PVDF thin-film pressure sensor based on flexoelectric effect was experimentally verified to maintain good thermal stability and sensing performance in the range of 0-170℃,and was utilized for finger compression test with knuckle and elbow joint bending test in low frequency and large strain scenarios,throat vibration signal test in low frequency and small strain scenarios,and acoustic signal test in higher frequency and small strain scenarios.A preliminary application in multiple scenarios was realized.The main research contents of this paper are as follows:(1)The relationship between surface roughness and strain gradient in longitudinal compression mode of PVDF films was analyzed by COMSOL numerical simulation software,and it was found that the strain gradient showed an increasing and then decreasing trend with the decrease of surface roughness.The simulation results show that PVDF films with certain surface roughness form strain gradients internally under longitudinal stress,producing a strong flexoelectric response.(2)The piezoelectric voltage coefficient of the PVDF film with a surface roughness of 1.46 μm was found to be 2-3 times higher than that of the electrically polarized PVDF piezoelectric film at a frequency of 5 Hz.The output voltage is constant over the temperature range from room temperature to 170℃ and does not drift with temperature changes,thus having strong thermal stability.In order to study the practical value of the device at lower frequencies,finger press test,knuckle and elbow joint bending test and throat vibration signal test were conducted to reflect its application potential in human kinetic energy recognition,human motion detection and voice recognition,it was found that the flexoelectric signal output from the device reflects the magnitude and frequency information of the stress very well.(3)To investigate the practical value of PVDF thin film pressure sensor devices based on flexoelectric effect at higher frequencies,acoustic wave tests were conducted to reflect their potential application in acoustic sensing.It was found that the device can detect the sound intensity and frequency information of acoustic waves well,and maintain the same good sensing performance in the application scenario of higher frequency.These findings provide new ideas for the development of flexible pressure sensors with high thermal stability.