Polypropylene-based nanodielectrics for HVDC cables

Abstract

Due to the limited recyclability and operating temperature of the cross-linked polyethylene (XLPE) insulation for high voltage direct current (HVDC) cables, renewable next-generation insulation materials have drawn great attention during recent years, especially polypropylene-based nanodielectrics. The filler-matrix interphase within nanodielectrics is believed to be the determining factor in bringing unique features, which show great potential in tailoring the dielectric and thermal properties of the insulations. However, it is still unsatisfactory in understanding how interphase works. In this study, polypropylene (PP) and aluminium nitride (AlN) nanocomposites have been investigated regarding their potential application for HVDC cable insulation. A conceptional model (two-side model) is proposed to explain the behaviours of PP nanocomposites contains the silane coupling agents (SCA) treated fillers. The novelty of this model is to divide the surface chemistry of fillers into two sides (the near- and far-particles sides) at the molecular level, which differs from the conventional theory that the interphase properties are dominated by the interaction between the organofunctional tails (far-particle side) of SCA and polymer matrix. The bonding structure between SCA and particle surface (near-particle side) also executes a strong influence on the filer-matrix interaction by modifying the polarity of the particle surface, e.g., the monolayer of SCA on nanoparticles surface can maintain the polarity of the AlN then provide an inter spherulitesboundary-free structure, thus giving a higher dielectric break down strength compared to systems contains AlN with a multilayer of SCA. Four SCA were used to analyse the effect of the AlN surface chemistry that differ in the near- and far-particle structures on the morphology, thermal, and dielectric properties of nanocomposites. The introduction of AlN shows a nucleating and hindrance effect, increasing the nuclei density but maintaining the crystallinity of the nanocomposites. AlN with higher surface polarity presents a higher nucleating effect and can induce an inter-spheruliteboundary-free structure where the AC breakdown strength is improved. Such so called surface polarity is positively related to both the near- and far-particles sides. A water absorption analysis was used to investigate the interphase properties of the nanocomposites, as the absorbed water molecules will mainly be located at the interfacial region. The AlN filled PP was found to reduce the hydrophobicity of PP, result in an increased permittivity and dielectric loss. This issue can be mitigated by using tri-alkoxy silanes to form a silane network that repels water molecules; meanwhile, the mono-alkoxy silane cannot increase the hydrophobicity of the nanocomposites. Despite the AlN dispersion can be improved by SCA treatment, large agglomerations (> 20 µm) can still be observed in silane treated samples caused by the hydrolysis of AlN. The removal rather than dispersing of those large agglomerations by a centrifugal separation method gives us an excellent opportunity to investigate the relationship between bulk properties and the degree of AlN agglomeration. It is found that the AC breakdown strength is significantly improved after the separation process compared to the PP with non-separated AlN. The improved nanoparticle dispersion reduced the defects and cavities within nanocomposites; benefit from that, the samples still exhibit an excellent hydrophobicity even the interfacial area is increased. The addition of SCA increases the thermal conductivity of the PP/AlN nanocomposites, where 10 % of the increase can be achieved. Although the total interfacial thermal resistance increases with the increased AlN dispersion after the centrifugal separation processes, there is a higher chance for the fillers to percolate, thus showing a comparable bulk conductivity. Overall, this study investigated the influence of the material interphase on the morphology, thermal, and electrical properties of nanocomposites using a proposed two-side model (near- and farparticle side), which is expected to serve as a valuable tool in the future design and application of nanodielectrics.

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Identifiers

ORCID for Xinyu Wang: orcid.org/0000-0001-9434-2906

ORCID for Thomas Andritsch: orcid.org/0000-0002-3462-022X

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