Preparation And Dielectric Properties Of Nano-SiO₂/Polyamide-imide Composites

Polyamide-imide（PAI）is widely used in the electromagnetic enameled lacquer of frequency conversion motor,which has excellent dielectric properties and heat resistance.With the popularization and development of frequency conversion motor,traditional PAI enameled lacquer has been more and more difficult to meet the new high-tech requirements.Nano modification technology and multi-layer composite structure design can greatly improve the performance of materials,and provide an effective way to enhance the performance of traditional insulating materials,which has aroused widespread concern.In this paper,nano-SiO₂ dispersions were prepared by hydrothermal method,and a series of single-layer hybrid films and triple-layer composite films with different nano-SiO₂ contents were prepared by mechanical blending with PAI resin.The composite films and their process performance were characterized and tested by using scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Fourier transform infrared spectroscopy（FT-IR）,thermogravimetric analysis（TGA）and rotational viscometer.The dielectric properties of the composites were investigated by using breakdown strength test device,DC conductivity test device,dielectric spectrum analyzer and corona resistant test device,and the test results were analyzed and discussed.The results of TEM and SEM show that the nano-SiO₂ particles are uniformly dispersed in the PAI matrix.The size of nano-particles are less than 20nm,the triple-layer structure interface of nano SiO₂/PAI composite film is clear and the thickness of each layer is suitable.FT-IR tests show that the Si-O-Si structure is successfully introduced into the PAI matrix and the raw materials reacted completely.TG test result shows that the thermal stability of nano-SiO₂/PAI composites is higher than pure PAI films.Viscosity test shows that the addition of SiO₂ dispersing solution increases the viscosity of the resin mixture.The breakdown strength test shows that the breakdown strength of single-layer hybrid films decreases with the increase of doping content,and the breakdown strength of pure PAI film is maximum（250.42 k V/mm）.The breakdown of the triple-layer composite films increases first and then decreases with the doping content,which is larger than that of the single-layer hybrid films.When the doping content is 8 wt.%,the breakdown strength reaches the maximum（277.65 k V/mm）which larger than pure PAI film.Electric current test results show that the conduction current and the electric degradation threshold of composites were increased with the increase of nano-SiO₂ content.Moreover,electric degradation threshold of triple-layer composite films is larger than single-layer hybrid film with the same doping content which is affected by the carrier trap density,interface and film laying process.Dielectric spectroscopy tests show that the dielectric constant and dielectric loss of nano-SiO₂/PAI composites change regularly with the influence of doping content and frequency.The dielectric constant of nano-SiO₂/PAI composites increases with the increase of doping content,but decreases with the increase of frequency.The dielectric loss increases with the increase of the doping amount or frequency.Corona resistance test results show that the corona resistance of single-layer hybrid film and triple-layers composite films both increases with the increasing of doping content.The corona resistance of 20 wt.%single-layer hybrid film and triple-layer composite film is 17.65 h and 16.50 h respectively,which is about 8 times than pure PAI film.When the doping amount is less than 16 wt.%,the corona resistance of the triple-layer composite films is higher than that of the single layer films.