Charge distribution and stability in electret materials

摘要

The objective of the work presented in this Ph.D. thesis is to give a broader understanding of which key parameters influence the charge stability of polymer electrets, and how the electrical charges are distributed. This has been achieved using polypropylene as an electret polymer model system.Theoretical considerations have been made concerning the effect, of the size of the crystalline areas known as spherulites and the degree of crystallinity, on the charge retention. The considerations showed that small spherulites and a high degree of crystallinity is favouring a high charge retention. This was also showed experimentally where the size of the spherulites was controlled through different cooling methods, and the degree of crystallinity was controlled by mixing atactic-polypropylene (a-PP) and isotactic-polypropylene (i-PP). The reason why the crystallinity have been controlled by mixing a-PP and i-PP, is because the charge retention is extremely sensitive to the sample preparation. This was seen in regard to the thermal history of the samples and the influence of micron and nano size particles in the polymer electret.Through adding micron and nano size calcium carbonate and aluminium oxide particles in the polymer matrix is was seen that the charge retention could be enhance compared to samples with no particles. However, these results also showed that the thermal history for the samples played an equivalent importance role regarding the charge retention.The morphology of the spherulites at the surface was visualised after a selective etch through scanning electron microscopy. The selective etch was to enhance the contrast between the amorphous and crystalline regions. Spherulites in three different size interval was seen, 50 μm to 100 μm, 3 μm to 7 μm, and 0.7 μm to 1.5 μm respectively.By means of kinetic rate theory the discharge behaviour could be explained for polypropylene when thermally stimulated. This resulted in the determination of several activation energies, which could be used for describing the discharging seen at isothermal conditions. This theory is a powerful tool of predicting the lifetime of an electret at various thermal conditions.Through the experimental obtained release currents, for different polypropylene samples, the critical temperature was determined from the largest current peak. If an electret is to avoid significant discharging it should be kept well below its critical temperate.