Study On High Temperature Energy Storage Properties Of Polyimide-based Composite Dielectrics

Energy storage dielectric is an important part of capacitors in hybrid vehicles,oil and gas exploration and aerospace propulsion systems,while the actual working environment of capacitors exceeds 100 ?.Currently commercial biaxially oriented polypropylene(BOPP)(maximum operating temperature ?100 ?)is difficult to meet the requirements.usually need to introduce active cooling system and redesign and remanufacture of the control system,which greatly increases the physical space and cost,so develop energy storage composite dielectrics for high temperature applications is imperative.In this paper,polyimide(PI)with good electrical insulation properties and excellent structural temperature stability is selected as the matrix material.It has high breakdown strength and working temperature(350 ?).The high temperature energy storage properties of high thermal conductivity hexagonal boron nitride(h-BN)/PI single layer composite films were prepared and studied.It was found that the composite film maintained excellent field strength stability at given temperature range,but its efficiency decreased sharply at high temperature when volume fraction is low.The decreased efficiency can be alleviated with the increasing of high thermal conductivity h-BN,but the maximum field strength decreased as the volume fraction continued to increase.At the same time,the high temperature properties of high dielectric constant zirconium titanate strontium titanate(BZT-BCT)nanofiber/PI composite film were studied.It was found that the dielectric strength decreased significantly with the increase of volume fraction and temperature,resulting in a severely reduced energy storage density.On the basis of this,the sandwiched-structure was designed: the composite film of h-BN/PI with a volume fraction of 5% is selected on both sides,and the high thermal conductivity and the excellent insulation performance of h-BN are used to improve the high temperature dielectric strength and reduce the heat loss of the composite film at high temperature;different volume fractions of high dielectric BZT-BCT nanofibers are used as the filling phase in the middle to increase the electrical displacement of the composite films.A three-layer structured composite films with excellent temperature stability was prepared.The study found that when the intermediate layer volume fraction BZT-BCT is less than 7%,the dielectric strength of the three-layer composite medium can be relatively stable in the temperature range,and its electrical displacement decreases less at high temperature and maintains a higher energy storage density.The storage density of the composite films at 150 ? can be kept above 76% of that at room temperature,and has a high discharge efficiency.