Prenaration And Dielectric Properties Of TiO₂/PVDF Nanocomposite Films

Nanocomposite dielectric materials have found wide applications in the fields such as pulse systems,sensors,energy storages and transistors.The properties of the nanocomposite dielectric materials are closely related to their cmponents,structures and morphologies.Therefore,the nanocomposite fabrication based on these factors is cracial to improve the properties of the dielectric nanocomposites.In this work,ferroelectric polymers(polyvinylidene fluoride)and mono-oxides(titanium dioxide)were selected as two main compennets.The effects of crystal structure,microstructure,chemical compositions and interface states on the dielectric properties of composites,as well as the optimum chemical composition and electrical properties of titanium dioxide and polyvinylidene fluoride composites were investigated.The main results are as follows:(1)Zero-dimensional TiO2 particles and one-dimensional TiO2 nanorods were synthesized by the aqueous hydrolysis of tetrabutyl titanate(TBOT).The pH value,temperature and sodium dodecyl sulfate(SDS)during solution reaction play a crucial role in the formation of rutile TiO2(R-TiO2).When the temperature is lower than 200°C and the pH is between 0.3-0.8,SDS addition can promote the phase formation of R-TiO2,but pure R-TiO2 cannot yet be obtained.When the temperature is higher than 200°C,the temperature becomes the dominated factor which has serious effect on the hydrothermal reaction,while the surfactants and pH value have less influences on the resulting products.SDS can promote the R-TiO2 transition from zero-dimensional to one-dimensional shape,that is,appparently orientated growth for R-TiO2 was induced by SDS introduction.The composite of R-TiO2/PVDF was faricated.The dielectric constant of the composite with orientated R-TiO2 fillers can be high as 33,much larger than that of the composite with zero-dimensional R-TiO2 fillars,consistent with the theoretical results.(2)The R-TiO2 microspheres were synthesized by hydrolysis of titanium tetrachloride(TiCl4).The microspheres are composed of R-TiO2 nanowires with a diameter of about 50-60 nm and a length distribution of 200-500 nm.The resulting products are pure R-TiO2 when the reaction consitions:TiCl4 concentration was set to 1-2 mol/L,the reaction time was 6-24 h.SDS has no effects on the phase and morphology of R-TiO2 microspheres because of the differences in nucleation-growth process.SDS could lead to the formation of an organic layer on the microspheres’ surface,which also significantly changed the interface states of the R-TiO2/PVDF composites,resuting in the decrease of the dielectric constant with a maximum value of 25 as compared to those without SDS.(3)Polycrystalline titanium dioxide(TiO2)nanowires were prepared by electrospinning method.Control over the electrospinning voltage,speed and distance,TiO2 nanowires were spun with using the concentrations of 7%-10%PVP and 30%-50%TBOT.The pure R-TiO2 was obtained after sintering on the as-spun products at 800°C-900°C.When PVP concentration was 7%,the dielectric constant of polycrystalline R-TiO2 composites reaches 30,slightly higher than that of the zero-dimensional R-TiO2 based composite.(4)The R-TiO2 arrays were prepared by using the conductive glass(FTO)as substrate to tune the morphology,size and performance of R-TiO2 nanorod arrays.The XRD results comfirm the pure phase of the R-TiO2 arrays.The R-TiO2 arrays with different sizes were synthesized by adjusting precursor concentration from 0.05 mol/L to 0.25 mol/L,and reaction temperature was increased from 120°C to 180°C,and reaction time was increased from 2h to 12h.At low temperatures,high-concentration precursors are more easily to grow R-TiO2 arrays with larger aspect ratio.When the temperature is increased,low concentration and short reaction time more easily favor the growth of R-TiO2 arrays with larger aspect ratio.All the as-sythesized R-TiO2 arrays grow preferentially along the[001]direction.The dielectric constant of the R-TiO2 array/PVDF composite can only reach 30 which may be due to the R-TiO2 arrays with relatively poor adhesion.