| With the rapid advancement of flexible electronics in the fields of intelligence and portability,the development and utilization of sustainable and high-performance power supplies have become hot research topics.Traditional chemical battery has a short lifespan,thus frequent charging or battery replacement is required to maintain the normal operation of these electronics.Besides,a large amount of battery disposal can also cause serious environmental pollution issues due to the toxic substances contained in batteries,such as lithium,cobalt,and toxic electrolytes.As one of the novel energy harvesting devices,nanogenerators are capable of converting mechanical energy from the ambient environment to electricity,and have become one of the ideal methods to realize self-powered and portable electronics with the advantages of flexibility,portability and miniaturization.However,the output power density and current typically are relatively low,thereby restricting their practical applications as a sustainable power source in electronic products.Thus,the development of flexible and sustainable nanogenerators possessing high output power is of great significance for energy saving and emission reduction.This thesis uses polyvinylidene fluoride(PVDF)copolymers as the research objects.A series of flexible nanogenerators with high output power were fabricated by doping nanofillers,selecting triboelectric/piezoelectric polymers with large polarities differences,and adjusting intensities/frequencies of the forces/ultrasound,which can be applied in self-powered sensing system,self-driving human-machine interaction system,and biomedicine.The main contents are as follows:(1)Poly(vinylidene fluoride-co-hexafluoropropylene)(PVDF?HFP)membrane and nylon 6,6(PA)membrane were fabricated under electrospinning and used as tribonegative materials and tribopositive materials,respectively.The arch-structured triboelectric nanogenerators(TENGs)were developed through the assembling of the two triboelectric layers.The mechanism of the tribopolarity differences of the materials on the electrical output performance of TENG was investigated.Besides,the detailed mechanism of the morphology and structure on the electrical output performance of TENGs were investigated by adjusting the solution concentrations,the layer numbers of the nanofibrous membranes,and the intensities/frequencies of the forces.The results showed that the fabricated PVDF–HFP/PA TENG yielded the highest triboelectric outputs when two-layers PA membranes with 10 wt%concentration was used as the tribopositive material,and one-layer PVDF–HFP membrane with14 wt%concentration was used as tribonegative material.The open-circuit voltage and short-circuit current generated from the TENG were 141.6 V and 20.4?A,respectively.Additionally,the unique arch-structured TENG can be used not only to power portable and miniaturized electronics as a sustainable power supply,but can also readily detect the triboelectric signals generated by finger pressing and effectively switch small electronics on and off,thereby realizing the real-time communication between a TENG-based calculator,a signal processing circuit and a computer.(2)To improve the triboelectricity of the tribopositive/tribonegative materials,the polylactic acid(PLA)membrane was fabricated under electrospinning and used as the tribopositive material,and the poly(vinylidene fluoride-co-trifluoroethylene)(P(VDF?TrFE))membrane with piezoelectric effect and triboelectric effect was used as the tribonegative material.The mechanism of the electrical output performance under triboelectric–piezoelectric effects of the P(VDF?TrFE)membranes was investigated with the assembling of the P(VDF?TrFE)/PLA TENG.Besides,polyethylene glycol monomethyl ether(mPEG)with oxygen-containing groups was added to improve the tribopositivity of PLA.The results showed that PLA/mPEG membrane had a stronger electron-donating ability compared with PLA membrane.When the external load resistance was 10~6?,the TENG achieved a maximum power density of 116.21 W/m~2.After 10~4 press–release cycles,the output voltage and current signals did not get obvious attenuation and exhibited excellent stability and durability.When the TENG was applied under different forces,TENG had the ability to monitor the real-time output signals which could be applied as pressure sensing and human motion recognition.In addition,seven separate arch-structured TENGs were acted as seven notes.When the TENG-based note was pressed,the TENG can detect the output signal generated from the finger pressing and transmit it to the signal processing system to realize the music performance using the buzzer.(3)To investigate the electrical outputs of the piezoelectric membranes under high-frequency ultrasonic stimulation,the flexible P(VDF–TrFE)/1.5BT membranes with piezoelectric effect were fabricated using an electrospinning process with the addition of barium titanate(BT)was added to the P(VDF-TrFE)matrix,and the piezoelectric nanogenerators(PENGs)were assembled by sandwiching the membranes with two copper electrodes.The mechanism of the nanofillers concentrations,ultrasonic intensities and frequencies on the electrical output performance were investigated.When the PENGs were implanted under different depths of the pork tissues,the energy conversion efficiency of the ultrasound in pork tissues and the electrical outputs of the PENGs were investigated.P(VDF–TrFE)/1.5BT PENG can generate an output voltage of 8.22 V even at a depth of 4.5 cm,and can be used as a sustainable power source to achieve rapid charging of LEDs and capacitors.In addition,the mechanism of ultrasound-activable piezoelectric membranes on stimulating cell proliferation and migration behavior were conducted when the fibroblasts were seeded on the membranes.Under ultrasound vibration,the electric field generated from the piezoelectric membranes can promote the proliferation and migration of fibroblasts cells,thereby enhancing the wound healing rates.When the ultrasound power intensity was 1.0 W/cm~2,the cell migration rate of the fibroblasts seeded on the P(VDF–TrFE)/1.5BT membrane reached 92.6%after 24 h.Besides,the wound healing rate in the ultrasound-stimulated P(VDF-TrFE)/1.5BT group was higher than that in the control group.(4)To investigate the electrical outputs of the piezoelectric membranes under high-frequency ultrasound and low-frequency forces,the functionalized barium titanate(BT-C-PFTU)was added to the P(VDF-TrFE)matrix,and then the P(VDF–TrFE)/BT-C-PFTU piezoelectric membrane was prepared under electrospinning and assembled with two electrodes to prepare PENGs.The low-frequency periodic forces generated from the shaker and the high-frequency ultrasound generated from the ultrasound probe were used as two mechanical sources,and the mechanism under two different mechanical sources on the output performance of the PENGs were investigated.The results demonstrated that PENGs under ultrasonic stimulation produced higher output signals compared with that under periodic forces,and the output current was increased by 10~3 orders of magnitude.Furthermore,the PENGs under ultrasonic stimulation had higher energy conversion efficiency,and could quickly charge a 33?F capacitor to 2.5 V in 21.5 s,thereby realizing a rapid charging for electronics.Besides,the P(VDF–TrFE)/BT-C-PFTU membranes had a good biocompatibility and could be applied to the wound healing behavior of mice.The wound healing rate was evaluated by analyzing the wound sizes under different days in the ultrasound/non-ultrasound treated groups.The wounds in the ultrasound treated P(VDF–TrFE)/BT-C-PFTU group healed within 10 days,indicating that the piezoelectric membranes under ultrasound stimulation could effectively promote the wound healing process. |
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