74 / 2018-08-23 16:21:53

Effect of Silica Modification on Charge Trapping Behavior of PP blend/Silica Nanocomposites

Dielectric Materials,Silica Surface Modification,Insulation Systems,Thermally Stimulated Depoarization Current,Nanocomposite Dielectrics,Charge Trapping,Trap Depth

Various dielectric nanocomposite materials are studied in the frame of the European Commission funded project - GRIDABLE. This project has the aim to develop DC cable extruded insulation and medium and low voltage DC capacitor films exhibiting enhanced performance with respect to presently used materials. The nanocomposites are based on polypropylene (PP) blend filled with surface modified nano-silica particles. The surface modification is carried out via the state-of-the-art solution method using 3-aminopropyltriethoxysilane (APTES) as the modifying agent. APTES - as an aminosilane – is prone to alter the electronic behavior of the silica/PP blend interphase which, as a consequence, can change the dielectric behavior of the nanocomposites. It is assumed that by the introduction of the amine functionality to the silica surface, due to its polarity, charge trapping properties of the dielectric material undergoes meaningful changes, which might cause beneficial effects on the ultimate insulation characteristics of the nanocomposite materials.
Thermogravimetric Analysis (TGA) results show a higher weight loss for the modified silicas compared to the untreated one, which is an indication of the successful modification of the silica surface. Different reaction conditions were utilized to achieve various levels of silane modification. From TGA results, a range of weight loss, from 4% to 8%, was achieved by altering the reaction conditions.
The nanocomposites were prepared by addition of the different silica samples into the polymer blend matrix in a twin-screw micro extruder, and subsequently, injected into a square mold using a mini-injection molding machine.
Thermally Stimulated Depolarization Current (TSDC) measurements were carried out in order to study the charge trapping behavior of the nanocomposite samples. TSDC results indicate that the addition of the nano-silica, even untreated, reduces the density of the deep traps, significantly. On the one hand, by the introduction of the amine functionality to the surface of the silica, the suppression of the deep trap density is more pronounced. In addition, a deeper trap appears for the samples with functionalized silica. It seems that the depth and the density of this newly introduced trap increases by increasing the level of the modification i.e. the amount of the grafted silane on the silica surface. However, this trend is not entirely dominant as there are some repeatable exceptions observed. For these samples, increasing the amount of the grafted silane results in an increase and a decrease in trap depth and density, respectively. In other words, it is speculated that the depth and density of the traps are not only a function of the level of silica surface treatment, but also other parameters, such as filler dispersion and distribution, silane layer morphology, micro-structure and crystallization characteristics of the compound. In order to get a clearer image of the functionalization of the silica and its effect on the charge trapping behavior of dielectric nanocomposites, studying the aforementioned factors is necessary.

Acknowledgements: This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 720858. The authors also would like to thank the ECIU Researcher Mobility Fund for supporting this research work.