| In the future,pulse capacitors need dielectric materials with high working voltage,dielectric constant,energy density and short-term charging/discharging times.It is difficult for a single kind of material,such as ceramics or polymers,to achieve this goal.Although polymer materials have good flexibility,processability and low loss,their dielectric constant is usually low.At present,organic/inorganic composites,especially nano-modified polymer matrix composites,haved much attention due to their powerful properties.While retaining the advantages of polymer materials,this kind of material can greatly improve the dielectric constant of materials,and has considerable application prospects in the field of electronic components.In this paper,silver nanoparticles with rod,linear,spherical and cubic morphologies were prepared by solution method,using silver nitrate as raw material and polyvinylpyrrolidone as protective agent.The effects of temperature,reaction time,kinds of inhibitors and molar ratio of silver source to protective agent on the morphology and size of Ag particles were systematically analyzed.The optimum preparation process of rod,linear,spherical and cubic Ag particles was obtained.The experimental results show that the diameter of Ag spherical particles is about 50 nm when the molar ratio of silver nitrate to polyvinylpyrrolidone is 1:2 at 150℃and the reaction time is1 h.At 150℃,the molar ratio of silver nitrate to polyvinylpyrrolidone is 1:2,and the reaction time is 20 min.When sodium chloride is used as inhibitor,the size of cubic Ag particles is smaller than that of cubic Ag particles.The diameter of Ag rod-like particles was 200 nm and the ratio of length to diameter was about 50 when the reaction time was 1 h and the molar ratio of silver nitrate to polyvinylpyrrolidone was 1:2 at 160℃.The molar ratio of silver nitrate to polyvinylpyrrolidone was 1:2 at 160℃.Copper chloride was used as inhibitor and the reaction time was 15 min.The linear Ag particles have a diameter of 100nm and a length-diameter ratio of about 200.TEM analysis shows that when the surface of Ag particles is coated with SiO2,the thickness of the coating is between 10 and 20 nm.On this basis,using tetraethyl orthosilicate as precursor,the Ag particles with different morphology were prepared by sol-gel method,and the core-shell Ag@SiO2 nanoparticles were prepared.A series of Ag@SiO2/PVDF composite films with different filler contents were prepared by adding them into flexible polyvinylidene fluoride（PVDF）polymers.The structure and properties of the composite films were characterized,and the effects of different morphologies and contents of Ag@SiO2 particles on the structure and properties of the composite films were discussed.XRD results show that the introduction of Ag@SiO2 particles promotes the phase transition of PVDF films from multiphase coexistence to excellent ferroelectric properties.The results of broadband dielectric spectroscopy analysis show that the dielectric constant of the composite films is positively correlated with the content of Ag@SiO2 core-shell samples.The dielectric constant of the composite films increased from 7.61 to 10.1（spherical Ag@SiO2/PVDF）,11.7（cubic Ag@SiO2/PVDF）,10.4（rod Ag@SiO2/PVDF）and 14（linear Ag@SiO2/PVDF）respectively when the filler content reached 20 wt%at 100 Hz.It can be seen that under similar morphologies,the dielectric properties of the composite films can be improved obviously by larger particles.The reason may be that the larger the particle size,the more conducive to the formation of an effective dielectric network in the composite film,the more obvious polarization of PVDF molecules around the particles,thus improving the overall dielectric constant of the composite film.The experimental results further show that the dielectric loss of the composite films is also significantly correlated with the content and morphology of Ag@SiO2 particles.The dielectric loss of the composite films increased from 0.008 to 0.015（spherical Ag@SiO2/PVDF）,0.051（cubic Ag@SiO2/PVDF）,0.019（rodAg@SiO2/PVDF）and0.025（linear Ag@SiO2/PVDF）respectively when the filler content reached 20 wt%at 100 Hz.The test results of breakdown field strength show that the breakdown field strength of the composite film is improved to a certain extent compared with that of pure PVDF film.At 20 wt%Ag@SiO2 content,the breakdown field strengths increased by 53.7%,25.8%,32.6%and 70.4%respectively.On the basis of the above results,the maximum energy storage densities of Ag@SiO2composite films with different morphologies were calculated to be 0.044 J/cm3,0.034 J/cm3,0.033 J/cm3 and 0.075 J/cm3. |
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