Preparation And Dielectric Properties Of Donor-Doped SiO₂/Polyimide Composites

In the insulation system of electrical equipment,due to the long-term strong electric field of dielectric materials,material microstructure damage is easy to occur,serious,and can even lead to material breakdown and other failures,so it is very important to improve the dielectric properties of polymer materials.Polymer composites with high breakdown strengths usually have a low dielectric constant,and vice versa,due to the complexity of their internal microstructure and environmental factors.Therefore,the synthesis of polymers with high breakdown strength and a high dielectric constant has become a hot research topic in the field of insulation in recent years.In this paper,Nb⁵⁺and Si⁴⁺were successfully substituted in Nb_xSi_1-1.25xO₂（x=0,0.5,1,1.5,and 2 mol%）（NSO）particles via high temperature sintering and other processes,and a new particle modified by donor doping was prepared as a filler to improve the polymer’s performance.NSO/PI composite films were prepared by in-situ polymerization with polyimide（PI）.The microstructure and interface characteristics of NSO particles and composite films were studied by SEM,XPS,XRD,FTIR,and SAXS.The results show that NSO particles do not detect the diffraction peaks of other phases except the characteristic diffraction peaks of SiO₂,which proves that no second phase is generated after sintering at high temperature.XPS results show that the oxygen vacancy content is about 18.2%in the undoped state and about 3.8%in the doped amount of 0.5 mol%.The obvious reduction of oxygen vacancies indicates that Nb⁵⁺has successfully replaced Si⁴⁺and NSO particles have been successfully prepared.The results of XRD,FTIR,and SAXS showed that the imine reaction of pure PI and NSO/PI composite films was complete,and the chemical properties of the films did not change after the addition of modified fillers.The dielectric properties of the composite films at room temperature and high temperature,short-term and long-term breakdown,and mechanical properties were studied.The test results show that because the donor doping affects the carrier transport characteristics of the composite film,the E_bof the composite material reaches a maximum value of 527.3 k V/mm,which is 64.2%higher than that of pure PI and 34.8%higher than that of SiO₂/PI composite film.When NSO particles are added to PI,the corona resistance life of the composite material is increased with the increase in doping concentration.The corona resistance life of the composite material is as high as 31.5h at 1 mol%,which is 2.4 times the corona resistance life of pure PI film.When the content of NSO is 1.25 vol%and the content of Nb₂O₅is 1 mol%,the dielectric constant of the composite film at 1 Hz at room temperature reaches 3.9,the dielectric loss is as low as 0.0028,and the composite film still maintains excellent dielectric stability at high temperature.Combined with the structural characterization and performance testing results,it is found that the insulation performance improvement of NSO/PI composite films is attributable to the increase of the interface barrier,the increase of conduction electrons introduced by donor doping,the scattering of the Coulombic potential field inside the lattice,and the interface defects between the particles and the film.