Functionally graded material composites for effective stress control in insulators

Functionally graded materials (FGM) are materials in which one or several material properties vary continuously as a function of space. In this work, we are interested in a specific spatial distribution of dielectric permittivity with the objective of efficient electric stress control. This is of significance to solid insulators like spacers for GIS (Gas Insulated Switchgears), bushings, cable terminations, winding insulation, etc. In order to create an FGM, micro-sized fillers are incorporated into a base polymer matrix to obtain a spatial gradation of the filler percentage, resulting in a consequent gradation in the permittivity. Here, one dimensional (1D) FGM specimens are prepared by a sedimentation process. Bisphenol-A (LY556) epoxy resin is used as the base polymer and micro sized alumina (Al₂O₃) is used as a filler material. Relative permittivity is measured along the direction of the gradient of the volume fraction of microfillers, using an LCR meter. A linear variation of permittivity is observed. The experimental variation is matched against values obtained from a Finite Element Method (FEM) based numerical model developed to compute permittivity of the composite based on filler volume fraction. Further, the values are compared to those also predicted by several existing theoretical models. The consequent field distribution is also obtained.