The Dielectric Characteristic Research Of Nano-Composite Polyimide Film

With the development of power electronic technology and new semiconductor devices, more and more AC frequency conversion adjustable-speed motors have been widely used especially after 1980's. However, the life of the motor was only about one or two years due to partial discharge, what is more, the insulations in some motors were breakdown during the test running. The progress in nano-science and technology provided a new train of thought and a way for developing new materials and modifying the present materials. The appearance of nanodielectrics opened a new application field of the dielectrics. The electrical, thermal and mechanical properties of nanodielectrics were better than those of conventional ones. Cooperating with ABB and Siemens, Dupont developed the corona-resistant polyimide film, the corona-resistant time of which could be up to 105 hours under 20kV/mm field, approaching the level of mica paper. The further research on the mechanism of corona resistance from the dielectric theory has not reported.In order to research the effect of inorganic hybrid on the dielectric characteristic of polyimide film, the electrical strength, direct current volume resistivity, conduction current, dielectric spectrum, and corona-resistant time of Dupont original and corona-resistant polyimide films (100HN and 100CR) were measured. In addition, the dielectric parameters of polyimide/nano Al₂O₃ composite films with the content of 5wt%, 10wt%, 12.5wt%, 15wt% and 20wt% in weight ratio prepared by sol-gel reaction were measured.By measuring the dielectric parameters of 100HN film and 100CR film, the results are as follows: (1) The electrical strengths of 100HN film and 100CR film are 428kV/mm and 368kV/mm. (2) The direct current volume resistivities of 100HN film and 100CR film are 1.9×10¹⁷Ω·cm and 2.05×10¹⁶Ω·cm. (3) With the increasing of temperature, the conduction current increases, and the threshold of 100HN film increases, while that of 100CR film decreases. The conduction current of 100CR film is much larger than that of the 100HN film. From calculation, the trap capture levels of 100HN film and 100CR film are 0.9eV and 0.7eV. (4) In the frequency range 102Hz¹05Hz and temperature range 30℃¹50℃, the permittivity decreases with the increasing of frequency and temperature, and the dissipation factor increases with the increasing of frequency, but decreases at first and then increases with the increasing of temperature. The permittivity and dissipation factor of 100CR film are larger than that of 100HN film. (5) The corona-resistant times of 100HN film and 100CR film are 12.5h and 86.3h.The results obtained from the measurement of dielectric parameters of polyimide/nano Al₂O₃ composite films with different content show that: (1) when the content of nano Al₂O₃ is less than 5wt%,the electrical strength gets a little increase when the content of nano Al₂O₃ increases,and is up to maximum when the content is 5wt%,however,when the content is more than 5wt%,it decreases rapidly. (2) With the content of nano Al₂O₃ increasing, the direct current volume resistivity of polyimide/ nano Al₂O₃ composite film decreases. (3) With the content of nano Al₂O₃ increasing, the threshold of polyimide/ nano Al₂O₃ composite film decreases, conduction current density increases. (4) In the frequency range 102Hz¹05Hz and temperature range 30?C¹50?C, for the films with different nano Al₂O₃ content, their dielectric spectrum characteristics are consistent with Dupont films. With the content of nano Al₂O₃ increasing, the permittivity increases, so the dissipation factor does. When the content is up to 12.5%,both of them increase with a high speed. (5) The corona-resistant time of the film prolongs with the increase of nano Al₂O₃ content, and up to maximum when the content is 15wt%, then it shortens.