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Polypropylene (PP) is the most widely used dielectric material for capacitors, improving its insulation performance is of great significance to improve the energy storage density and reliability of capacitors. In this paper, the action law and mechanism of crystal form on breakdown characteristics of films were studied systematically by adding β-nucleating agent to polypropylene resin, so as to obtain the method of improving the insulation performance which can be popularized in industry. Firstly, X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests were conducted on the polypropylene resin for its aggregate structure. The results show that the crystallization activation energy of polypropylene resin decreases and the crystallinity increases after adding the β-nucleating agent. Secondly, on the basis of the practical application requirements of capacitor dielectric materials, polypropylene resin was bi-axial oriented to obtain bi-axial oriented polypropylene (BOPP) films. It is found that there are no obvious difference in crystal morphology and surface morphology of the resins with different crystal type after stretching,while the electric strength of the BOPP film containing β-nucleating agent is 7.1% lower than that of the BOPP film without β-nucleating agent. Finally, the BOPP films were subjected to vacuum heat treatment at different temperatures for 1 000 h. The electric strength of the two BOPP films increases with the increase of heat treatment temperature, and the electric strength of the film containing β-nucleating agent increases more significantly, which exceeds that of the resin with α-crystal as the main crystal after heat treatment at 120℃. It is speculated that the reason for this result is that the β-crystal is not conducive to the optimal arrangement of molecular chains during the tensile process, resulting in the migration of electrons more easily and thus reduce the electric strength. However, the vacuum heat treatment near the crystallization temperature can promote the movement of molecular chain segments, optimize the aggregation structure, and ultimately make the electric strength incrase.
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| 科 Family | 属数 Number of genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) | 属 Genus | 种数 Number of species | 占总种数比例 Percentage of total species (%) |
|---|---|---|---|---|---|---|
| 鹅膏菌科Amanitaceae | 2 | 11 | 5.26 | 鹅膏菌属 Amanita | 10 | 4.78 |
| 小菇科 Mycenaceae | 2 | 12 | 5.74 | 丝盖伞属 Inocybe | 5 | 2.39 |
| 多孔菌科 Polyporaceae | 8 | 14 | 6.70 | 蜡蘑属 Laccaria | 5 | 2.39 |
| 红菇科 Russulaceae | 3 | 23 | 11.00 | 小皮伞属 Marasmius | 6 | 2.87 |
| 小菇属 Mycena | 11 | 5.26 | ||||
| 光柄菇属 Pluteus | 5 | 2.39 | ||||
| 红菇属 Russula | 17 | 8.13 | ||||
| 栓菌属 Trametes | 5 | 2.39 |
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