Phase Transition And Piezoelectric Properties Of Carbon Quantum Dot/poly(vinylidene Fluoride) Hybrid Composites Crystallized At High Pressure

Developing clean energy is one of the most crucial problems we are facing today.Since consumption of fossil fuels must be cut down for their well-known environmental hazards and restrictions from environmental protection laws,there have been increasing concerns over the utilization of renewable energy.While piezoelectric material,possessing the ability to capture mechanical energy with different amplitudes and frequencies from the surrounding environment and convert it into electrical energy,is a potential candidate with wide applications.Like the commonly used inorganic piezoelectric materials such as SiO₂,ZnO,PZT,BaTiO₃,etc.,organic piezoelectric polymers including PVDF,PP,PFA,PTFE,and epoxy resin also show high coupling coefficient,dielectric constant,and mechanical flexibility.Nanocomposites with high toughness,excellent piezoelectric performance and long service life could be prepared by introducing some inorganic micro/nano particles into the polymer matrixes.PVDF is a polymorphous semi-crystalline polymer of high toughness,excellent impact strength and good wear resistance,with its piezoelectric and ferroelectric properties largely depending upon the?crystal formation.Carbon quantum dots,belonging to the carbon nanomaterial family,have a near spherical shape and particle size of less than 10 nm.They exhibit such desirable properties as high carrier mobility,excellent conductivity,good biocompatibility,light stability,high fluorescence intensity,strong quantum size effect and surface effects,making it promsing agents in the fields of mechanical measurement,electronic equipment,LED optoelectronic device,medical and energy collection.High-content?-phase PVDF can be obtained by nanoparticle induction,high electric field polarization,high-pressure crystallization,etc.Herein,PVDF was processed using a high-pressure crystallization method,and the introduction of CQDs also in situ induced the self-assembly of polycrystalline substructures of PVDF crystals under high pressure,resulting in a novel non-polarized piezoelectric PVDF/CQDs system with stable multicolor photoluminescent behaviour and high electro-mechanical conversion efficiency.In this thesis,PVDF/CQDs composites were prepared by a solution casting method.High-pressure treatment was performed for PVDF/GQDs with the variation of pressure,temperature,crystallization time and GQDs concentration.DSC,WAXD,ATR-FTIR,SEM and LSCM were used to characterize the crystallization behaviours of the hybrid composites under different experimental conditions.On the basis of such studies,we have suscessfully prepared PVDF/CQDs based piezoelectric nanogenerators of different component ratioes,and studied their piezoelectric and fluorescent properties.Main conclusions are as follows:?1?Crystallization behaviors of PVDF/CQDs composites at high pressureAccording to the results of TEM characterization,the CQDs of PVDF/CQDs?99/1,wt/wt?composites prepared by the solution film forming method were evenly dispersed in polymer matrix.Under 400 MPa high pressure conditions,when the crystallization temperature was at 230°C and 245°C,the material mainly produced spherulite structure.Nevertheless,a sample with highly crystalline?phase was obtained when the temperature was further increased to 260°C under the condition of holding time of 10 min.Also,a large number of?crystals of three-dimensional nano-array structure were generated in the sample,due mainly to the in-situ self-assembly induced by the catalytic action of the CQDs under high pressure.When the crystallization time was within 30-60 min,the?crystals in the composite decreased with the increase of the holding time,indicating that the long crystallization time was not conducive to the formation of high-content?crystals.The composite material mainly generated spherulite structure at 300 MPa,whereas a large number of oriented?crystals,and a small number of one-dimensional nanowire structures were formed at 500 MPa,suggesting the increased pressure favored the growth of?and?crystals of the polymer.?2?Piezoelectric and fluorescence properties of pressure-crystallized PVDF/CQDs compositesReferring to the academic research and application of piezoelectric polymers,we have prepared high pressure crystallized PVDF/CQDs composites based piezoelectric nanogenerators containing different amounts of CQDs.Experiments showed that CQDs were uniformly dispersed in the PVDF/CQDs?99/1,wt/wt?composite sample,while agglomeration within nanoscale occured in the samples with 2%and 3%CQDs concentrations,respectively.As the introduction of CQDs in-situ induced the self-assembly of polycrystalline sub-structures of PVDF crystals under high pressure,with the increase of the concentration of CQDs,three-dimensional nanosheet arrays,unidimensional nanorods and unidimensional nanowires with piezoelectric effect were formed.Without any polarization treatment,the maximum open-circuit voltage output density of the developed PVDF/GQDs nanogenerator reached 19.2 V/cm²,and the short-circuit current output density reached 560 nA/cm²,far exceeding that of pure PVDF.The stability test showed that the short-circuit current output density of the material gradually increased with the increase of operation time,and after 1000working circles,piezoelectric output of the sample remained stable.At the same time,due to the addition of CQDs,the compound exhibited excellent fluorescent behaviour with high stability,and its multi-color photoluminescence was well preserved even after the endurance for a rigorous treatment at high pressure and high temperature.