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Thermal Stability of Al2O3-Filled Silicone-Based Elastomers for Power Electronic Encapsulation

Fig. 1. Change of weight in polymer matrix of unfilled and Al2O3-filled silicones during isothermal (250°C) aging.
Silicones are widely used for encapsulating power electronic modules due to their high dielectric strength and thermal stability. However, silicones are found to be insufficiently stable at 250°C, which is the maximum junction temperature of commercial SiC devices. In this study, the thermal stability of pure silicone and its Al2O3 fiber-filled composites were investigated by thermogravimetric analysis (TGA) and isothermal (250°C) soak tests. Both tests revealed that Al2O3 fiber improved the thermal stability of silicone (Fig. 1). TGA results indicate that the temperature of degradation onset increased from 330°C to 379°C with a fiber loading of 30 wt%. In the isothermal soak test, pure and 30 wt%-filled silicones lost 10% of polymer weight in 700 and 1,800 hours, respectively. Improved thermal stability was explained by a restrained chain mobility caused by hydrogen bonds formed between the Al2O3 surface and the polymer matrix. Removal of hydrogen bonds impaired thermal stability, increasing the initial weight-loss rate from 0.025 to 0.036 wt%/hour.

Fig. 2. Initial rate of weight loss (in 250°C isothermal aging) of unfilled and Al2O3-filled silicones.
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