We have used the probe technique together with the thermally stimulated discharge (TSD) method in order to determine the potential distribution V(x) in poly(methyl methacrylate) (PMMA) thermoelectrets polarized at temperatures below the glass transition temperature . We have obtained current - voltage (j - V) characteristics for PMMA at temperatures on the basis of the V(x) distribution and some theoretical considerations. This characteristic is of sub-ohmic shape, namely , where n<1 and g= constant. On the basis of the above relation, a two-layer condensor (PMMA with one-sided metallized and an air gap) was considered as a model for the formation of PMMA thermoelectrets. It follows from this model that the interfacial charge density changes nonlinearly with the applied polarizing voltage . The theoretical results have been compared with experimental data of PMMA thermoelectrets. The problems of the mean real charge depth and the charge decay in these electrets have also been considered. In particular we can state that the isothermal absorption current, current - voltage characteristic and, moreover, charge decay after poling of PMMA and of the composite are interrelated.
In this paper we suggest preparation of piezoelectric laminates with using PVDF polymer film (a) , polyester urethane elastomer (PUE) film (b), PMMA film (c) and (PMMA+BaTi03) composite film (a). The systems of three-layered structure of (aba), (aca) and (ada) have been made, then conventional electrets based on these laminates have been prapared. The modification of internal layers in the systems leads to the change of their dielectric properties and also the mechanical parameters which determine the piezoelectricity. The origins of piezoelectricity in these systems have been considered on the base of TSD thermograms.
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