Polymers intrinsic breakdown

In order to predict absolute dielectric strengths we need to have more detailed information than is yet available about electronic states and mobilities in polymers. For the present we can only conclude that there is satisfactory agreement between the form of the theoretical results, based on a rather general electronic model, and the best experimental results. To the extent that the model is a very reasonable one, we can say that we can understand intrinsic breakdown behaviour. Measurement of pre-breakdown currents, especially with pointed electrodes which impose regions of very high field strength at their tips when embedded in the material, suggests that electronic carrier production either by injection from the electrodes (Schottky emission) or from impurities (Poole-Frenkel effect) may play a part in the breakdown process. More work is required, however, before this can be fully understood. 

The DS is high for many insulating polymers and may be as high as 103 mV/ m. The upper limit of the DS of a material is dependent on the ionization energy present in the material. Electric or intrinsic decomposition (breakdown) occurs when electrons are removed from their associated nuclei this causes secondary ionization and accelerated breakdown. The DS is reduced by mechanical loading of the specimen and by increasing the temperature. 

Chemical degradation refers to the breakdown of polymer molecules, either through short-term attack by contaminants, such as oxygen and iron, or through longer-term attack to the molecular backbone by processes such as hydrolysis. The latter is caused by the intrinsic instability of molecules even in the absence of oxygen or other attacking species. In other words, polymer chemical stability is mainly controlled by oxidation-reduction reactions and hydrolysis. 

In the thermal breakdown in polymers, many factors influence the breakdown voltage geometry and size (especially thickness) thermal conductivity and specific heat of the polymer and associated materials ambient temperature rate of voltage increase or its steady value magnitude of tan S and its change with temperature magnitude of e and its change with temperature intrinsic... 

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