Polyimide Used in Space Applications

Polyimide (PI) is an interesting material for space applications as it offers excellent thermal properties. However, due to its dielectric properties, charge storage and release can be at the origin of electrostatic discharges that are hazardous for the surrounding electrical equipment. Depending on the spacecraft orbit, it is necessary to study the impact of specific surrounding environment. In any cases, the effect of vacuum and temperature variations can be combined with electrons and protons’ irradiation, atomic oxygen erosion, and photons impact from UV exposure. On the market, there exist many types of PI, and since several years, composite are also developed. The main properties that are usually observed are the conductivity that is analyzed from surface potential decay, the photoemission and the ability to initiate and propagate surface flashover. Since several years, the space charge storage analysis by the pulse electro-acoustic method has been developed as an interesting complementary tool. It is important to remember that experimental characterization needs to be representative to the space environment especially because it has been observed that PI can recover its original properties in air in a couple of hours depending on the ageing degree

Round Robin Tests of Electron Irradiated Polymers via Pulsed Electroacoustic Measurements

Charge accumulation and migration can be studied using the pulsed electroacoustic (PEA) method to directly measure internal charge distributions in dielectric materials. This study aims to compare measurements using PEA systems constructed in labs at Utah State University and Université Paul Sabatier to establish confidence in comparing PEA results between different PEA systems. While there is good agreement in data measured for pristine samples with DC bias applied and no charge embedded, there are discrepancies in the data when measuring irradiated samples with embedded charge. The overall characteristics of charge distributions measured with both systems is clearly the same, but there are small differences observed in the distance scaling and larger differences in the magnitude of the charge measured

Behavior of Space Charge in Polyimide and the Influence on Power Semiconductor Device Reliability

Polyimide is widely used in film form as a passivation material for power semiconductor devices such as Si, SiC, and GaN. The magnitude of the electric field at the edge termination area of these semiconductor devices is becoming higher due to the increase of operational voltage and/or demand for shrinking the edge termination area to increase device active area. Hence, it is concerned that the accumulation of space charge in the encapsulation and passivation material may affect the insulation performance of these devices, for example, the degradation of withstand voltage due to distortion of the internal electric field caused by space charge accumulation. To design space charge resistance of semiconductor devices, it is important to understand the space charge behavior in polyimide films with a thickness of several to several tens of micrometers. This chapter addresses practical implementation, specifications, and issues on space charge in polyimide insulation on power semiconductor devices focusing on the space charge measurements in thin polyimide films using the latest developed LIMM method and DC conductivity measurements

The effects of high electric fields on an Epoxy resin

The aim of this work was to determine some of the effects caused by the application of a high electric field on a filler-free epoxy resin material. Two types of sample geometry were moulded. Films (55 to 300μm) were prepared to work under uniform field configurations. A frame of parallel wires (5 to 25μm radius) was introduced into the bulk to work under divergent fields. Three non-destructive complementary methods of investigation were used. In all cases the first measurements were performed before the application of thermal-electric stress to define the properties of the material itself. Measurements were repeated after a controlled period of stress. Dielectric spectroscopy measurements were used to determine the main relaxation processes. To evaluate the impact of charge injection and localise the build up of space charge, the pulsed-electro acoustic method was chosen. Luminescence experiments were carried out to investigate the luminescence excitation parameters and pathway of radiative relaxation that are found to be directly linked with the charge injection and extraction phenomena. In addition spectral analysis was performed to complete the investigation. Light emission is associated with relaxations dependent on the chromophores that are present and that are likely to be affected by an external stress. After thermal-electric stress one relaxation processes was detected by dielectric spectroscopy that was dependent on the stress. This was associated with a change in the local arrangement of the chemical network, which is altered by the field. A mechanism of trapping and de-trapping of charge was determined by the analysis of the PEA response, light emission analysis coupled with external current. The studies were completed by a computer simulation of the effect of injected charge on the electric field. The three techniques were found to be useful complementary tools to determine the effects of stress on this insulation material

Combined effects of charging, electron irradiation and visible light on polyimide films

International audiencePolyimide is widely used for spatial applications as it offers good thermal and electrical properties. In this paper an analysis of the photoconduction effect on charge transport in Kapton® HN films is proposed. Space charge distribution recorded by the Pulse Electro Acoustic Method and surface potential measurements may be explained by a simple model based on light absorption. Evidence is provided that, for strongly absorbed wavelengths, photoconductivity is leading to progressive charge injection, rather than charge vanishing under screening. A stable photoelectret may therefore be produced by light exposure on a charged sample. It may also be responsible for a strong return voltage effect on an apparently neutral film

Space charge buildup and dissipation in charged polyimide films exposed to visible light

International audiencePolyimide is photoconductive under visible light, whose blue component is strongly absorbed. A consequence of this absorption is to induce a heterogeneous conductivity within the film. We present space charge and surface potential measurements which provide evidence for a progressive motion of the charge distribution towards the back electrode, in good accordance with a simple model, assuming stability of the deposited charge, fast retrapping of the photoinduced carriers, as well as a conductivity proportional to light absorption. These results may suggest new ideas for characterization of the trapping structure and distribution of the space charge using different light wavelengths

New design of the pulsed electro-acoustic upper electrode for space charge measurements during electronic irradiation

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