In nanocomposites, different surface states of nanoparticles can potentially provide different interactions with the base polymer and in turn change the bulk properties. Aluminium nitride (AlN) nanoparticles were surface functionalised with three different silane coupling agents (SCAs) with varying organofunctional and hydrolysable groups. The effects of the filler surface chemistry on the resulting AlN/polypropylene (PP) nanocomposites were examined and compared with an unfilled reference system. It is observed that different organofunctional groups can provide different nucleating effects and the dispersion states of nanoparticles while the hydrolysable group is not the dominant factor. The dielectric spectroscopy results show the hydrolysable group of SCA will also result in a difference of the interphase since the trimethoxy silane treated systems show much higher imaginary permittivity than the triethoxy silane treated systems when the frequency is below 1 Hz. The grafted organofunctional layer on the particle surface can provide a significant improvement of the thermal conductivity of the composite materials, e.g. 15 % improvement in thermal conductivity was observed when adding 10 wt% methacrylate silanes treated nano-AlN into PP, while the untreated counterpart only has 5 % improvement.

The influence of water immersion and silane treatment on the AC breakdown and the complex dielectric response of polypropylene/nano-aluminium nitride (PP/nano-AlN) composites has been investigated. The as-received filler was examined to have a nanoscale particle distribution with a hexagonal shape and slight hydrolysation. Grafting the aluminium nitride with an octyl silane reduces the weight increase in samples filled with 10 wt% of aluminium nitride during water immersion by 3, from 0.29 to 0.09%. The results suggest that the AC breakdown strength and complex permittivity of ''wet'' composite samples are related to the silane treatment of the nanofiller. The AC breakdown strength of octyl silane-treated samples after 9 days of water immersion shows comparable results to the dry samples, while a reduction can be seen on non-treated samples. Although silane-treated samples still show an increased dielectric loss at low frequency after water immersion, a significant reduction in low-frequency dispersion of real and imaginary permittivity can be seen when compared to the non-treated composites. This indicates that significant gains can be obtained for PP/nano-AlN composites by suitable silane treatments.

Polypropylene (PP) is considered a competitive candidate to replace widely used cross-linked polyethene (XLPE) in cable material application, as it can potentially withstand higher operating temperature and has excellent electrical properties. Besides, it is a more environment-friendly material, since PP does not require crosslinking and, therefore, can be easily recycled at the end of its life. This paper investigated the influence on the tensile strength, space charge and breakdown behaviours of the PP by adding polyolefin elastomer (POE) and Magnesium Oxide (MgO) nanoparticles. 10 or 20 wt. % of POE and 5 or 10 wt. % of nano-MgO were introduced into PP. Results show that the PP with 10 wt. % of PE02 and 5 wt. % of surface treated-MgO has the highest breakdown strength among all systems. Also, the introduction of POE and nano-MgO can significantly improve the mechanical flexibility. In combination with the observed suppression of space charge accumulation, which shows that PP/POE/MgO nanocomposites are a viable option for high voltage (HV) cable insulation material in the future.

Polymer nanocomposites can potentially provide many advantages and the interaction region between polymer and nanoparticles-the so-called interphase is usually considered to be responsible for the change of properties. In this work, nanoaluminium nitride powders with various surface states obtained by three different silane coupling agents (SCA) and an untreated powder, were blended with isotactic polypropylene. AC breakdown strength and dielectric spectroscopy were used to study the effect of different particle surfaces and polymerparticle interphases. The nano-aluminium nitride powder grafted with different organofunctional groups can provide a number of potential interactions with the matrix polymer. The results show that it can not only affect the dispersion state of nanoparticles but will provide different impacts on the dielectric properties of the bulk material. The hydrolysable groups of SCA, notably, can also influence the dielectric properties through altering the surface chemistry of nanoparticles.

In this work, four loading amounts of trimethoxy (octyl) silane (C8-M) were used in preparing C8-M treated AlN/polypropylene nanocomposites. Although the thermal gravimetric analysis shows a significant difference in the amount of C8-M grafted on the AlN surface, and a saturation point can be reached as the amount of C8-M added increases, the AC breakdown strengths of composites present virtually identical results. Similar behaviours can be observed in the real and imaginary permittivity from 0.01 Hz to 100 kHz, although the upturns of the imaginary permittivity at low frequency are observed increasing with the amount of C8-M used. Seemingly contradictory to a previously reported work on epoxy with nanosilica, AlN/polypropylene is largely unaffected by the amount of the silane coupling agent applied during the surface treatment.

This paper deals with the role played by the interface and bulk volume of the nanofiller about affecting the electrical properties of a nanocomposite material. For this purpose, a simple and completely amorphous matrix, polystyrene (PS), is used as base material, and core-shell quantum dots are exploited for simulating the structure of nanocomposites: CdSe core and CdSe-ZnS core-shell semiconductor quantum dots (QDs) are added into a PS matrix. The latter is to highlight the effect of the ZnS interface and as contrast to the core material. Dispersion and distribution of QDs are first microscopically observed and optimized, by including isopropyl alcohol in the manufacturing phase as an additional solvent. Among electrical properties the focus is on space charge accumulation, tested by means of the pulsed electroacoustic technique at 10 kV/mm and 50 kV/mm on CdSe and CdSe-ZnS doped PS composites. Results are then compared with a reference PS without QDs. Trap depth and density are also obtained by space charge measurement results. When CdSe QDs are added to PS, the trap density increases with respect to the baseline values measured on the unfilled polymer. In contrast, the ZnS shell around the CdSe core creates an additional trap level with lower trap depth, which increases charge mobility, thus turning homocharge into heterocharge accumulation. Therefore, the surface shell-structure of QD nanocrystals appears to significantly influence the space charge behavior of the nanocomposite, independently of the polymer. Index Termsquantum dot (QD), polystyrene, polymer, space charge, nanocomposites INTRODUCTIONNANOCOMPOSITES have been showing unexpected changes to bulk properties, i.e. permittivity, charge transport behaviour, and the enhancement of key dielectric parameters like the breakdown strength and long-term ageing behaviour (treeing resistance). A number of models attempted to explain the effects observed in nanocomposites, e.g. the electrical double layer model [1], the multi-core model [2], an organic and inorganic composites hybrid network model [3], the polymer chain alignment model [4], the interphase volume model [5], the multi-region structure around spherical nanoparticles [6], the

Polypropylene (PP) which is recyclable and easy to process, has the potential to be a promising alternative for conventional cross-linked polyethylene (XLPE) for using in high voltage power cable insulation. This paper aims to determine the effects of different types of nitride-based nanoparticles on the dielectric and mechanical properties of PP. Thus, three nitridebased fillers, boron nitride, aluminium nitride and silicon nitride are selected. To evaluate the influence of different filler concentrations, PP nanocomposite systems containing 5 or 10 wt.% of nitride-based nanoparticles were prepared by a modified solution blending method. Meanwhile, 5 g of neat PP was made with the same procedure to be the reference system. For determining the most suitable fillers in terms of electromechanical properties, three experiments namely, AC breakdown, dielectric spectroscopy and tensile test were conducted. The results of these experiments indicate PP with 5 wt.% of boron nitride has the best performance on dielectric and mechanical properties among nanocomposites.