Fabrication And Electrical Properties Of Polyimide Nanocomposite Films With Good Corona Resistance

Polymer nanocomposites have attracted wide interest as a method of enhancing polymer properties and extending their applications. In polymer nanocomposites, chemically dissimilar components are combined at the nanometer scale, and stronger interactions between the polymer and nanoparticles produce markedly improved materials with better electrical, mechanical, thermal, and rheological performances than the conventional filled polymer composites. In recent years, polymer nanocomposites have been extensively studied in optical, thermal and mechanical properties, but there has been relatively little research into dielectric properties.Due to the rapid development of electrical engineering and electronic technology, more and more electrical design possibilities in traction motors, transformers, and electrical rotating machines are being propounded. Despite traditional insulation materials have special properties, they do not always provide reliable and long-lasting protection against high-voltage "corona" attack. Novel polymer materials with excellent insulating characteristic at high temperature are paid much more attention. Polyimide (PI), which can be prepared by a variety of synthesis techniques, possesses high thermal stability, high chemical resistance and excellent mechanical properties so that it has previously been used as an insulating material.The present paper takes the structure and performance as a master line. A use nanotechnology and the appropriate method of preparation were proposed to synthesize inorganic-organic nanocomposites duo to high corona resiatance of inorganic filler and high heat-resistance of polymer insulation. The failure mechanism and the performance enhancement were studied under the special electric field. The influence of space charge formation and distribution on the electric field and the migration and accumulation in the dielectrics were also studied. The novel corona-resistant materials and insulating structures were developed by controlling the dielectric properties and microstructures of materials. The main conclusions are as follows.Firstly, two approaches (solution mixing technique, in-situ polymerization technique) were adopted to fabricate the PI/TiO2 nanocomposites. SEM images reveal a homogeneous dispersion of TiO2 nanoparticles in the PI matrix and all of the particles are separate in size of nanoscale using in-situ dispersive polymerization. The in-situ polymerization process helps to disperse the nano-TiO2 fillers into PI precursor solution homogeneously, and the PI/TiO2 nanocomposite films show good dielectric properties and corona-resistant properties.Influences of frequency, temperature and the nano-TiO2 particles loading concentration on dielectric permittivities of the PI/TiO2 nanocomposite films were studied. The values of the dielectric permittivity and loss tangent of the PI/TiO2 nanocomposite films changed as functions of the concentration of nano-TiO2 particles and frequency in an alternating current (AC) electric field. The dielectric permittivities and dielectric loss tangent increased with nano-TiO2 concentration.loading in the PI/TiO2 nanocomposite films. Temperature dependence of the dielectric properties shows that temperature has an effect on the mobility of polymer chains and thermal expansion of the PI polymer. Two broad peaks both in dielectric permittivity and loss were presented when the concentration of the nano-TiO2 particles was high (-25 wt.%). A shift of the dielectric relaxation peak towards a lower temperature and a reduced dielectric loss tangent with increasing frequencies were indicative of interfacial polarization interaction. Interfacial polarization or Maxwell-Wagner-Sillars (MWS) relaxation was evident in the dielectric spectrum of the PI/TiO2 nanocomposite films, which was due to the interfaces between PI matrix and the nano-TiO2 particles. The relaxation peaks have been found to follow the Cole-Davidson approach for the distribution of the relaxation times. Considering the variations of the dielectric permittivities were small at low frequencies, a complex dielectric modulus (M*) which is defined as the inverse complex dielectric permittivity (ε*), was introduced in the Cole-Cole plot. it has been used for the investigation of electrical relaxation effects in polymers. The breakdown strengths of the PI/TiO2 nanocomposite films with different concentrations of TiO2 particles as a function of corona aging time are also studied in this paper. After these films are aged under the external electrical fields in the air, they were deteriorated in different times. All the breakdown strengths of nanocomposite films decreased with increasing the corona aging time. The breakdown strength of pure PI film decreased from 210 MV·m-1 to 130 MV·m-1 after 20 h corona aging. While the breakdown strength of the PI/TiO2 nanocomposite film with 25 wt.% nano-TiO2 loading decreased from 110 MV·m-1 to 100 MV·m-1. In comparison to pure PI films, we noticed though the breakdown strength of the nanohybird films decreased (still satisfy the need in practice use), the decreased value in breakdown strength was very weak after 20 h corona aging.Effects of the concentration of nano-TiO2 fillers and the corona aging time on dielectric properties of the films were explored. Results showed that the corona resistance of the hybrid films could be improved with increasing the concentration of nano-TiO2 fillers. The corona resistance was dependent on the dielectric permittivity of the hybrid films. The mechanism of the corona resistance was also discussed by analyzing the change of electric field and the electron action of nano-TiO2 fillers based on the Fermi-Dirac Statistic.The space charge dynamics in these nanocomposite films have been investigated by using the Pulsed Electroacoustic Method (PEA) technique. Bipolar charge injection has taken place in pure PI film. The amount of space charge injected increases with the duration of applied voltage. The space charge characteristics in nanocomposites containing the nano-TiO2 particles are very different from the pure PI film. Heterocharges dominate the distribution indicating ionization occurrence. The addition of the nano-TiO2 particles into PI matrix may also hinder charge injection process. As space charge dynamics are closely related to the electrical performance, it is anticipated that the electrical performance of polyimide-based nanocomposites will be different from the pure PI film. This will be carried out in our future research.The surface potential decays in these nanocomposite films (both one-layer and two-layer) have been investigated by using the corona charging setup. The surface potential of the samples (both one-layer and two-layer) with initial high potential decays faster than that with lower surface potential. There is a significant influence of nanoparticles on the surface potential decay characteristics of PI nanocomposite films. The surface potential decay curves depend also on the amount of nano-TiO2 particles. The incorporation of the nano-TiO2 particles can build up a conductive network in the nanocomposite film, which increases the chance of charge transporting in the bulk. The corona resistance of the nanocomposite films could be improved with increasing the concentration of TiO2 fillers.