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Etude des matériaux isolants d'encapsulation pour la montée en température des modules de puissance haute tension

Abstract : This thesis presents a selection and characterization of electrically insulating materials able to assure the encapsulation for high voltage components and modules operating at high junction temperatures. This work makes a survey of the solid insulating materials that are commercially available. The initial review outlines that a compromise needs to be achieved between the material physical limits at high temperature and its hardness. So a first objective of the present study is to identify and characterize a flexible material for volume encapsulation of high voltage devices with a minimum operating temperature of 250 °C. A second objective is to identify and characterize another material for surface encapsulation suitable for lower voltage applications (<1,2 kV) with a higher operating temperature between 300 and 350 °C. Two silicone-based elastomers with SiO2 dispersed particules, and a semi-crystalline polymer (fluorinated parylene, PA-F), are both retained for this study. The analysis of the high temperature dielectric properties, are performed for the first time on these materials. A correlation between the electrical properties and the physico-chemical and structural evolution of these materials is realized. For the study on silicone-based elastomers with SiO2 dispersed particules, the thermal characterizations allow to determine their temperature working range from - 60 °C to 250 °C in air. The dielectric properties have a similar behavior for both elastomers and the various phase transitions at low temperatures are identified (-150 °C to 25 °C). At high temperatures (25 °C to 300 °C) and at low frequencies, a relaxation around 120 °C appears, that is related to the absorption of humidity, thus sensitive to the preconditioning of the material. It was possible to evacuate the absorbed water through a suitable annealing at temperatures above 120°C. The DC conductivity at 300 °C is in the range of 10 -13 Ω-1.cm-1. These low values show the interest of these silicone-based elastomers for electrical insulation at high temperatures. As regards to the PA-F, the thermal characterizations confirm that this material is stable for temperatures up to 350 °C in air. The electrical measurements allow to determine the values of DC conductivity up to 350 °C. The DC conductivity value is lower than 10-12 Ω-1 cm-1, meaning that PA-F is an insulating material even at such high temperatures. Moreover the PA-F exhibits dielectric breakdown strength between 2 and 4 MV.cm-1 up to 350°C. Besides, when increasing the film thickness up to 50 μm, the PA-F shows an improvement of the dielectric properties at low fields. This effect is attributed to an increase of the volume fraction of the crystalline phase driven by the film thickness. The decreases in the DC conductivity as well as an increase in the dielectric breakdown strength are correlated to the crystallization kinetics during a high temperature annealing. This phenomenon appears beneficial for the electrical insulation properties. So the PA-F exhibits exceptional initial dielectric properties up to 350 °C, offering a possible solution for surface encapsulation. Further works should confirm this with a long term properties stability study at high temperature
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Submitted on : Friday, November 25, 2011 - 5:10:22 PM
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Mireille Bechara Keyrouz. Etude des matériaux isolants d'encapsulation pour la montée en température des modules de puissance haute tension. Energie électrique. Université Paul Sabatier - Toulouse III, 2011. Français. ⟨tel-00645018⟩

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