Wenbin Bu

Study on physical and chemical structure changes of polyimide caused by corona ageing

Modified isothermal discharge current method (MIDC) was introduced to investigate the trap level distribution in Kapton 100HN and 100CR (nanodielectrics) PI films before and after corona ageing. The physical and chemical changes were studied by AFM and FT-IR respectively. Experiments showed that the quantity of traps was increased evidently after corona ageing and lots of deeper traps were produced in 100HN film, whereas for 100CR, changes were not very significant. Further IR spectra analysis showed that −COOH group was introduced in both 100HN and 100CR films while the general chemical structures were kept unchanged. AFM graph also found that surface morphology of 100HN PI films changed obviously by corona ageing while that of 100CR has no significant changes. Thus conclusions can be drawn that, corona ageing is a complicated process leading to surface degradation of PI film and addition of nanometer inorganic particles in polyimide can improve corona-resistance ability remarkably. In addition, since both physical and chemical degradation can introduce lots of defects and change the trap properties, so trap energy level and quantity changes can be used as an indicator maker for determining the ageing extent of polymer dielectrics.

A new method for direct determination of trap level distribution from TSC measurement

Thermally stimulated current (TSC) is developed and a new method for direct determination of trap level distribution in polymer film is proposed. In this method, a new function is defined to weight the contribution of a trap level to the current at any temperature. The demarcation energy is used to study the trap empty process. Analysis shows that only electrons with trap levels very close to the demarcation energy significantly contribute to the external circuit. Based on this method, the trap level distribution of polyimide is investigated. Experiments found that a broad peak appeared at about 440K on TSC spectrum. The trap level distribution exhibits a density peak at about 1.35eV. This is in accordance with the theoretical analysis by the proposed method which shows that only electrons with trap levels close to 1.3eV contribute to the current at 440K around.

Effect of nanometer inorganic particles on DC breakdown characteristics of Al 2 O 3 /PI films

In order to investigate the roles of inorganic nanoparticles on improving breakdown behavior of Al<inf>2</inf>O<inf>3</inf>/PI film, the DC breakdown characteristics were studied in this paper. The morphology and element distribution of zones near the breakdown holes were observed by SEM and electron spectroscope for both the original PI film and Al<inf>2</inf>O<inf>3</inf>/PI nanocomposite film. The experimental results showed that the electrical breakdown strength was enhanced significantly for the sample with 5% Al<inf>2</inf>O<inf>3</inf> content. The damaged region of breakdown hole for PI is larger than that of Al<inf>2</inf>O<inf>3</inf>/PI nanocomposite and PI film were much more seriously destroyed. There are larger distinctness for the distribution of C and O elements in zones near breakdown holes for both PI and Al<inf>2</inf>O<inf>3</inf>/PI nanocomposite films.

Structure characteristics and dielectric properties of PI/MMT nanocomposite films

Recent developments in polymeric dielectric nanocomposites have shown that these novel materials can improve design of high voltage components and systems. MMT(Montmorillonite)has an excellent corona performance and PI/MMT nanocomposite films can be prepared by in-situ polymerization process in this paper. Dielectric properties and trap level characteristic were measured by broadband dielectric spectrometer, thermally stimulated current (TSC) and the system of corona resistance. The microstructures of the films are observed by SEM. The experimental results show that the trap densities of nanocomposite films are more than that PI. With the MMT component increasing, the permittivity of nanocomposite films become smaller and dielectric loss of nanocomposite films increase obviously in the range of low frequency (10-3~100Hz), corona aging life of nanocomposite films are longer than that PI. The layered MMT nanoparticles doping in PI matrix provide a lot of traps which have an important influence on the electrons transplanting, furthermore, affecting dielectric properties of films. The research show that the corona aging life and dielectric properties can be enhanced by increasing proper trap density and depth provided by nanoparticles.

Numerical analysis of space charge electric field and its effect on TSDC measurement in thin polymer films

The internal electric field and potential in thin polymer film caused by injected charges were numerically analyzed and some paradoxical TSDC experimental results and the effect of internal electric field on the trap parameters obtained from TSDC measurement were discussed. Its found that, under short-circuit of the external electrodes, one zero electrical field plane and one potential peak appeared within the film and their positions depended on the spatial distribution of the space charges. The internal electric filed can vary from −3×107V/m to 5×107V/m in the case of a space charge density 100C/m3 at the interface between cathode and polymer film. The large internal electric field has an important impact on the space charge transportation due to Poole-Frenkel effect, which can change the trap depth. In addition, the reversed current peak in TSDC spectrum, the reduced space charge quantity that can be measured by TSDC may all be resulted from the internal electric field and potential distribution.

Effect of electrode materials on breakdown of Al 2 O 3 /PI films

Nanohybrid polymers have many excellent properties, such as high dielectric constant, high dielectric strength and low loss and so on, which apply in the electricity and electron fields widely. In this paper, the nanohybrid Al2O3/PI films were prepared by Sol-Gel process, then, Au and Al were sputtered on the films surface respectively as electrode. The dielectric properties (dielectric constant, loss tangent and dielectric breakdown strength) of the samples were measured and the surface morphologies of the breakdown holes were observed by SEM. The experimental results show that the electrode materials effect on the dielectric properties and breakdown characteristics of nanohybrid PI films. The dielectric properties of the films deposited by Au are better than that of Al, and the size of breakdown hole is smaller.

The breakdown characteristic of Al 2 O 3 /PI by light transmittance

Inorganic nanohybrid polyimide (PI) film has great potential for applications in the areas of electrics and electronics because of its excellent electrical performance, such as the resistance to aging and corona and so on. The breakdown strength of insulating materials is generally measured by DC, AC or impulse voltage. However, the influence of laser irradiation to Al2O3/ PI film has not been reported so far. In this paper, the Al2O3/ PI film with different Al2O3 component is irradiated by semiconductor laser, and the output power is 100mW. The influences of some factors (such as the distance of sample and laser, irradiation time) on luminousness have been studied. The experimental results show that the light transmittance of the films and the size of the breakdown hole are affected by the Al2O3 component in films. In this paper, the relationship between breakdown performance and light transmittance is researched and the method of the breakdown performance of Al2O3/ PI film represented by light transmittance is also investigated.