Space charge characteristics of layered and functionally graded insulator

Functionally graded materials (FGM) have recently been proposed for effective stress control in insulators. FGM are materials in which one or several material properties vary continuously as a function of space. Their main advantage lies in the absence of discrete changes (as found in layered composites) in the properties of the material. Layered insulator composites suffer from the drawback of charge accumulation at the interface of any two layers. Due to this, local field enhancement may occur in the materials leading to their degradation and eventual breakdown. In this work, we fabricate layered dielectric specimens and continuously graded FGM dielectric specimens for comparison. Bisphenol-A epoxy resin is used as the base material and alumina (AhO3, having average diameter of 9 μm) as filler. By changing the volume fractions of fillers, layers of different permittivity are manufactured. To obtain layered samples, one layer is deposited over another. A layered sample having very low differences in the properties of adjacent layers is hot pressed to obtain approximately `graded' specimens. Space charge accumulation is measured by the Pulsed Electro Acoustic - PEA method (TechImp, Italy). In the graded specimens, reduction in the accumulated space charge is observed.