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| Analysis of High-Temperature Charge Dynamics of Organic Semiconductor/ Silicone Elastomer Nanocomposites under DC and Square Wave Voltages |
| Wang Qilong1, Chen Xiangrong1, He Xinjun2, Du Haosen2, Tanaka Yasuhiro2 |
1. College of Electrical Engineering Zhejiang University Hangzhou 310027 China;
2. Measurement and Electric Machine Control Laboratory Tokyo City University Tokyo 158-8557 Japan |
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Abstract High-temperature and high-voltage power modules achieve higher power density by increasing operating temperature, voltage tolerance, frequency, and miniaturization, expanding their application in new energy transportation and power systems. Silicone elastomer (SE) is the preferred insulation for packaging above 150℃. Electric field concentration near the triple points in power modules can inject space charges under high temperatures and high electric fields, threatening module safety. To suppress space charge accumulation in the SE under these conditions, the NTCDA/SE nanocomposites are prepared by grafting polyhedral oligomeric silsesquioxane (POSS) and doping organic semiconductor (NTCDA). Given the complex operating conditions of power modules, typically running under DC or high-frequency square wave voltages, the high-temperature space charge characteristics of the pure SE and its composite under these voltages are studied.
Firstly, SEM cross-sectional morphology characterization and thermogravimetric analysis are performed on the pure SE and its composite. Secondly, the space charge distributions of the pure SE and its composite are tested under DC voltage at various temperatures (room temperature (RT) and 150℃) and electric fields (10 kV/mm and 20 kV/mm). Based on the average charge density decay curve under DC short-circuit conditions, the apparent mobility and trap distribution of the pure SE and its composite at 150℃ and 20 kV/mm are analyzed. Thirdly, the effects of different voltage frequencies (50 Hz, 250 Hz and 500 Hz), temperatures (RT and 150℃), and electric fields (10 kV/mm and 20 kV/mm) on the space charge distributions and average charge densities of the pure SE and its composite are studied under square wave voltage. Finally, the charge transport mechanisms under high temperature and high frequency are proposed based on quantum chemical calculations.
The experimental results show that grafting small amounts of POSS alters the crosslinking process of SE, forming a nanostructure in the NTCDA/SE nanocomposite. This reduces the thermal decomposition rate and increases the decomposition temperature by 26.6℃, inhibiting molecular relaxation and degradation at high temperatures, thus enhancing the thermal stability of the SE nanocomposite's crosslinked structure. At 10 kV/mm and RT, both the pure SE and SE-P-N show no obvious space charge accumulation under a square wave voltage, though pure SE exhibits some negative charge accumulation near the cathode under a DC voltage. At 20 kV/mm and RT or 10 kV/mm and 150℃, both materials show some negative charge accumulation near the cathode, with the pure SE accumulating more than the SE-P-N. At 20 kV/mm and 150℃, the pure SE accumulates significant negative charges within the bulk and near the electrodes, causing severe field distortion near the anode, while SE-P-N shows only minor negative charge accumulation near the cathode, maintaining a more uniform field distribution slightly higher than the applied field. Under both DC and square wave voltages, the SE-P-N exhibits half the average charge density and an order of magnitude less negative charge injection depth than the pure SE at 150℃ and 20 kV/mm. Grafting POSS and doping NTCDA together reduce apparent mobility and increase trap energy levels and density in the SE.
The injection depth and accumulated amount of negative charges, as well as the amplitude of distorted electric field, increase with temperature or applied electric field magnitude, and decrease with increasing square wave voltage frequency. Compared to the square wave voltages, both the pure SE and SE-P-N accumulate more negative charges near the cathode under the DC voltages. Grafting nano-POSS and doping NTCDA inhibit space charge injection, migration, and accumulation in the SE under high temperatures and high electric fields for both the DC and square wave voltages.
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Received: 21 May 2024
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2024, Vol. 39 
