Preparation And Dielectric Properties At Elevated Temperature Of Polyimide-based Composites

With the advantages of rapid charging and discharging and power density,polymer based composites have been intensively investigated as dielectrics for storing energy electrostatically,especially in modern electronics and electric power systems.However,common polymer dielectrics can only serve at low temperature,so it is hard to meet the new rising command for electricity working at harsh environment such as in the applications of electric vehicles.Besides,in some gentle conditions like embedded capacitors,with more functions in electric products the power dissipation increases and temperature raises,so it brings a new challenge for dielectrics’thermal stability.Among all polymers,polyimide(PI)is one of polymers with best thermal stability.However the dielectric constant of PI is only about 3 and the breakdown strength is lower than 300kV/mm,thus its energy density is very low.In order to obtain the PI based composites with high energy density,the preparation method of pure PI is optimized and the pure PI with high breakdown strength(451 kV/mm)is synthesized.The prepared PI exhibits high energy density(5.2 J/cm~3).Besides,the dielectric behavior of pure PI at high temperature up to 200~oC is investigated.Both breakdown strength and energy density decrease at high temperature due to the rising conduction loss.Considering the low dielectric constant of pure PI,barium titanate nanoparticles(BTNPs)are introduced into the PI matrix,resulting in the enhanced dielectric constant(6.8)and low loss(0.012).However the breakdown strength of the BTNPs/PI composites is damaged because of the inhomogeneous electric field caused by the large difference of permittivity of BTNPs and PI.Although the energy density of BTNPs/PI is enhanced at low electric field,it decreases dramatically at high electric field.The dielectric behavior of the composites at high temperature is investigated carefully.To overcome the drawback of the BTNPs,barium titanate nanofibres(BTNFs)are fabricated by electrospinning method and BTNFs/PI composites are prepared by simple direct mixing.The BTNFs/PI composites show high dielectric constant(8.3)and breakdown strength(553 kV/cm),an enhancement of~168%and~23%over that of pure PI.The energy storage performance is also enhanced by the introduction of BTNFs,and the energy density and discharge efficiency of the composites respectively are 5.8 J/cm~3 and 72.8%.Furthermore,BTNFs/PI composites’dielectric behavior is investigated and they show high energy density(2.7 J/cm~3)and discharge efficiency(80.3%),which are promising polymer based dielectrics used under both high electric field and high temperature.