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  • 1.
    Aljure, M.
    et al.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Becerra, M.
    KTH, School of Electrical Engineering (EES), Electromagnetic Engineering.
    Pallon, L. K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Electrical conduction currents of a mineral oil-based nanofluid in needle-plane configuration2016In: 2016 IEEE CONFERENCE ON ELECTRICAL INSULATION AND DIELECTRIC PHENOMENA (IEEE CEIDP), IEEE conference proceedings, 2016, p. 687-690Conference paper (Refereed)
    Abstract [en]

    We present experiments and simulations on the electrical conduction currents of purified transformer oil with and without surface-modified MgO nanoparticles. Results show that on the injection regime of the voltage-current characteristics, nanoparticles increase the charge production in the fluid. It is also found that the conduction currents in the space-charge-limited regime increased at a lower rate as a function of the voltage in the presence of nanoparticles. The numerical simulations suggest electron attachment is increased due to the nanoparticles, leading to larger accumulation of negative ionic space charge close to the needle in the space-charge-limited regime. It is concluded that electron attachment may be significantly increased with nanoparticles, becoming an important process of electrical conduction in nanofluids.

  • 2.
    Borgani, Riccardo
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Haviland, David
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Local Charge Injection and Extraction on Surface-Modified Al2O3Nanoparticles in LDPE2016In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 16, no 9, p. 5934-5937, article id 10.1021/acs.nanolett.6b02920Article in journal (Refereed)
    Abstract [en]

    We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

  • 3. Hoang, A. T.
    et al.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Serdyuk, Y. V.
    Gubanski, S. M.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Charge transport in LDPE nanocomposites part I-experimental approach2016In: Polymers, ISSN 2073-4360, E-ISSN 2073-4360, Vol. 8, no 3, p. 1-19Article in journal (Refereed)
    Abstract [en]

    This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material's direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

  • 4.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, A. T.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nilsson, Fritjof
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gubanski, S. M.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Influence of Nanoparticle Surface Coating on Electrical Conductivity of LDPE/Al2O3 Nanocomposites for HVDC Cable Insulations2017In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 24, no 3, p. 1396-1404Article in journal (Refereed)
    Abstract [en]

    LDPE/metal oxide nanocomposites are promising materials for future high-voltage DC cable insulation. This paper presents data on the influence of the structure of the nanoparticle coating on the electrical conductivity of LDPE/Al2O3 nanocomposites. Al2O3 nanoparticles, 50 nm in size, were coated with a series of silanes with terminal alkyl groups of different lengths (methyl, n-octyl and n-octadecyl groups). The density of the coatings in vacuum was between 200 and 515 kg m(-3,) indicating substantial porosity in the coating. The dispersion of the nanoparticles in the LDPE matrix was assessed based on statistics for the nearest-neighbor particle distance. The electrical conductivity of the nanocomposites was determined at both 40 and 60 degrees C. The results show that an appropriate surface coating on the nanoparticles allowed uniform particle dispersion up to a filler loading of 10 wt.%, with a maximum reduction in the electrical conductivity by a factor of 35. The composites based on the most porous octyl-coated nanoparticles showed the greatest reduction in electrical conductivity and the lowest temperature coefficient of electrical conductivity of the composites studied.

  • 5.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, A.T
    Chalmers University of Technology.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gubanski, Stanislaw
    Chalmers University of Technology.
    Ohlsson, Richard
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Influence of nanoparticle surface coating on electrical conductivity of polyethylene/aluminium oxide nanocomposites for HVDC cable insulations2016In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Article in journal (Refereed)
  • 6.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Zhang, P.
    Diaz, A.
    Holler, M.
    Schneider, K.
    Olsson, Richard
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Yu, Shun
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Cavitation in strained polyethylene/aluminium oxide nanocomposites2017In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 87, p. 255-265Article in journal (Refereed)
    Abstract [en]

    The incorporation of metal oxide (e.g. Al2O3) nanoparticles has a pronounced positive effect on low-density polyethylene (LDPE) as an insulating material for high-voltage direct-current (HVDC) cables, the electrical conductivity being decreased by one to two orders of magnitude and charge species being trapped by the nanoparticles. The risk of debonding between the nanoparticles and the polymer matrix leading to electrical treeing via electrical discharges in the formed cavities was the motivation for this study. Scanning electron microscope (SEM), small-angle X-ray scattering (SAXS) and X-ray ptychographic tomography were used to study a series of LDPE nanocomposites which contained Al2O3 nanoparticles treated with silanes having terminal alkyl groups of different lengths (methyl, octyl and octadecyl). When specimens were subjected to a tensile strain (a typical specimen stretched beyond the onset of necking consisted of three zones according to SEM of specimens that were studied after removal of the external force: an essentially cavitation-free zone with low local plastic strain, a transitional zone in which local plastic strain showed a marked increase and the revealed concentration of permanent cavities increased with increasing plastic strain and a highly strained zone with extensive cavitation), the cavitation occurred mainly at the polymer-nanoparticle interface according to SEM and X-ray ptychographic tomography and according to SEM progressed with increasing plastic strain through an initial phase with no detectable formation of permanent cavities to a period of very fast cavitation and finally almost an order of magnitude slower cavitation. The polymer/nanoparticle interface was fractal before deformation, as revealed by the profile of the Porod region in SAXS, presumably due to the existence of bound polymers at the nanoparticle surface. A pronounced decrease in the interface fractal dimension was observed when the strain exceeded a critical value; a phenomenon attributed to the stress-induced de-bonding of nanoparticles. The strain-dependence of the interface fractal dimension value at low strain levels between composites containing differently treated nanoparticles seems to be an indicator of the strength of the nanoparticle-polymer interface.

  • 7.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Zhang, Peng
    Deutsches Elektronen-Synchrotron (DESY).
    Diaz, Ana
    Paul Scherrer Institut.
    Holler, Mirko
    Paul Scherrer Institut.
    Schneider, Konrad
    Leibniz Institut für Polymerforschung Dresden.
    Olsson, Richard
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Yu, Shun
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Cavitation in strained polyethylene/aluminium oxide nanocomposites2016In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945Article in journal (Refereed)
  • 8.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Andersson, Richard L.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Gedde, Ulf
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Morphology and properties of silica-based coatings with different functionalities for Fe3O4, ZnO and Al2O3 nanoparticles2015In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 5, no 59, p. 48094-48103Article in journal (Refereed)
    Abstract [en]

    A facile single-step method for obtaining 2–3 nm thick silsesquioxane coatings on metal oxide nanoparticles using different carbon-functional silane precursors is presented. Iron oxide nanoparticles 8.5 nm in diameter were used as a model to evaluate the possibilities of forming different uniform carbon-functional coatings, ranging from hydrophobic to hydrophilic in character. Electron microscopy showed that all the coated nanoparticles could be described as core-shell nanoparticles with single Fe3O4 cores and carbon-functional silsesquioxane shells, without any core-free silicone oxide phase. Steric factors strongly influenced the deposited silicon oxide precursors with octyl-, methyl- or aminopropyl functionalities, resulting in coating densities ranging from 260 to 560 kg/m3. The methyl-functional coatings required several layers of silsesquioxane, 3–4, to build up the 2 nm structures, whereas only 1-2 layers were required for silsesquioxane with octyl groups. Pure silica coatings from tetraethoxysilanes were however considerably thicker due to the absence of steric hindrance during deposition, allowing the formation of 5–7 nm coatings of ca. 10 layers. The coating method developed for the iron oxide nanoparticles was generic and successfully transferred and up–scaled 30 and 325 times (by volume) to be applicable to 25 nm ZnO and 45 nm Al2O3 nanoparticles.

  • 9.
    Liu, Dongming
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Sanchez, Carmen Cobo
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Fogelström, Linda
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Malmström, Eva
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene2016In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 125, p. 21-32Article in journal (Refereed)
    Abstract [en]

    Polyethylene composites based on metal oxide nanoparticles are emerging materials for use in the insulation of extruded HVDC cables. The short-term electrical performance of these materials is adequate, but their stability for extended service needs to be assessed. This study is focussed on the capacity of the nanoparticles to adsorb polar species (water, dicumyl peroxide and byproducts from peroxide-vulcanisation, acetophenone and cumyl alcohol) that have an impact on the electrical conductivity of nanocomposites, the oxidative stability by adsorption of phenolic antioxidants on the nanoparticles and the potential transfer of catalytic impurities from the nanoparticles to the polymer. The adsorption of water, dicumyl peroxide, acetophenone, cumyl alcohol and Irganox 1076 (phenolic antioxidant) on pristine and coated (hydrophobic silanes and poly(lauryl methacrylate)) Al2O3, MgO and ZnO particles ranging from 25 nm to 2 gm was assessed. Composites based on low-density polyethylene and the particles mentioned (<= 12 wt.%) were prepared, the degree of adsorption of Irganox 1076 onto the particles was assessed by OIT measurements, and the release of volatile species at elevated temperature was assessed by TG. The concentration of moisture adsorbed on the particles at 25 degrees C increased linearly with both increasing hydroxyl group concentration on the particle surfaces and increasing relative humidity. Dicumyl peroxide showed no adsorption on any of the nanoparticles. Acetophenone and cumyl alcohol showed a linear increase in adsorption with increasing concentration of hydroxyl groups, but the quantities were much smaller than those of water. Irganox 1076 adsorbed only onto the uncoated nanoparticles. Uncoated ZnO nanoparticles that contained ionic species promoted radical formation and a lowering of the OIT. This study showed that carefully coated pure metal oxide nano particles are not likely to adsorb phenolic antioxidants or dicumyl peroxide, but that they have the capacity to adsorb moisture and polar byproducts from peroxide vulcanisation, and that they will not introduce destabilizing ionic species into the polymer matrix. Low contents of dry, equiaxed ZnO and MgO particles strongly retarded the release of volatile species at temperatures above 300 degrees C.

  • 10.
    Nilsson, Fritjof
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Karlsson, Mattias
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Giacinti, Marco
    Olsson, Richard T.
    Venturi, Davide
    Gedde, Ulf W
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Influence of water uptake on the electrical DC-conductivity of insulating LDPE/MgO nanocomposites2017In: Composites Science And Technology, ISSN 0266-3538, E-ISSN 1879-1050, Vol. 152, p. 11-19Article in journal (Refereed)
    Abstract [en]

    Low-density polyethylene (LDPE), typically in cross-linked form, is currently the main insulation material for extruded high voltage cables. The DC-conductivity of LDPE can be reduced 100 times by adding 1-3 wt% well-dispersed metal-oxide nanoparticles (MgO, ZnO, Al2O3), but the underlying physics remain unclear. One of several feasible explanations is that the nanoparticles attract electrical charges, polar molecules (H2O and crosslinking by-products) and ions (H+, OH-, salts and ionic species originating from the crosslinking by-products), and thus clean the polymer. Effective media FEM simulations, assuming that the polymer conductivity is proportional to the moisture content, were used in order to examine this hypothesis. Water sorption measurements for LDPE and MgO/LDPE nanocomposites were conducted as experimental input. The simulations could conceptually predict the experimentally measured composite conductivities. The hypothesis was further strengthened by DC-conductivity measurements on LDPE and MgO/LDPE nanocomposites at 0 and 50% relative humidity (RH), showing a 100-fold conductivity increase for the nanocomposite at the elevated humidity. The DC-conductivity of the most insulating composite (3 wt% MgO) was below 10(-16) S/m after 64 h at 60 degrees C and 0% RH, using an electric field of ca 30 kV/mm. The long-term insulation efficiency of an insulating polymer nanocomposite is thus optimal if the material is carefully dried and surrounded by an impenetrable moisture barrier before use.

  • 11.
    Pallon, L. K. H.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, A. T.
    Pourrahimi, A. M.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nilsson, Fritjof
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gubanski, S.
    Gedde, U. W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    The impact of MgO nanoparticle interface in ultra-insulating polyethylene nanocomposites for high voltage DC cables2016In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 4, no 22, p. 8590-8601Article in journal (Refereed)
    Abstract [en]

    Low density polyethylene (LDPE) nanocomposites with a reduced conductivity of two orders of magnitude are reported as a novel insulation material for high voltage distribution of renewable energy. The key to the high insulation capacity was to provide 70 nm hexagonal MgO nanoparticles with relatively tong, preferably 18 units long, hydrocarbon functional silsesquioxane coatings. This rendered the surface of the particles completely hydrophobic and also served as a protective layer against adsorption of polar low molecular weight atmospheric substances (H2O and CO2). The elimination of trace amounts of water, in combination with the provided carbon functionality, dramatically improved the dispersion of MgO nanoparticles. The lowest volume conductivity was ca. 7 x 10(-16) s m(-1) for 3 wt% surface coated nanoparticles. Extensive electron microscopy characterization was further used to relate the measured volume conductivity, acquired under conditions that resemble 800 kV high voltage direct current (HVDC) cables, to the distribution of the nanoparticles in the polymer matrix. The results show that an appropriate surface-modification approach yielded uniformly dispersed MgO nanoparticles up to contents as high as 9 wt%, with maintained 10-100 times reduced volume conductivity. Simulations of the MgO nanoparticles distribution revealed that the required interaction radius of the MgO-phase was 775 nm, setting a lower limit of particle amount to effectively work as electrical insulation promoters. The reduced volume conductivity values and scalable processing chemistry reported allow for the production of the next generation insulation material for HVDC cables.

  • 12.
    Pallon, Love
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Polyethylene/metal oxide nanocomposites for electrical insulation in future HVDC-cables: probing properties from nano to macro2016Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Nanocomposites of polyethylene and metal oxide nanoparticles have shown to be a feasible approachto the next generation of insulation in high voltage direct current cables. In order to reach an operationvoltage of 1 MV new insulation materials with reduced conductivity and increased breakdown strengthas compared to modern low-density polyethylene (LDPE) is needed.In this work polyethylene MgO nanocomposites for electrical insulation has been produced andcharacterized both from an electrical and material perspective. The MgO nanoparticles weresynthesized into polycrystalline nanoparticles with a large specific surface area (167 m2 g–1). Meltprocessing by extrusion resulted in evenly dispersed MgO nanoparticles in LDPE for the silane surfacemodified MgO as compared to the unmodified MgO. All systems showed a reduction in conductivityby up to two orders of magnitude at low loading levels (1–3 wt.%), but where the surface modifiedsystems were able to retain reduced conductivity even at loading levels of 9 wt.%. A maximuminteraction radius to influence the conductivity of the MgO nanoparticles was theoretically determinedto ca. 800 nm. The interaction radius was in turn experimentally observed around Al2O3 nanoparticlesembedded in LDPE using Intermodulation electrostatic force microscopy. By applying a voltage on theAFM-tip charge injection and extraction around the Al2O3 nanoparticles was observed, visualizing theexistence of additional localized energy states on, and around, the nanoparticles. Ptychography wasused to reveal nanometre features in 3D of electrical trees formed under DC-conditions. Thevisualization showed that the electrical tree grows by pre-step voids in front of the propagatingchannels, facilitating further growth, much in analogy to mechanical crack propagation (Griffithconcept). An electromechanical effect was attributed as possible mechanism for the formation of the voids.

  • 13.
    Pallon, Love K. H.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Nilsson, Fritjof
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Yu, Shun
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Diaz, Ana
    Holler, Mirko
    Chen, Xiangrong R.
    Gubanski, Stanislaw
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Three-Dimensional Nanometer Features of Direct Current Electrical Trees in Low-Density Polyethylene2017In: Nano letters (Print), ISSN 1530-6984, E-ISSN 1530-6992, Vol. 17, no 3, p. 1402-1408Article in journal (Refereed)
    Abstract [en]

    Electrical trees are one reason for the breakdown of insulating materials in electrical power systems. An understanding of the growth of electrical trees plays a crucial role in the development of reliable high voltage direct current (HVDC) power grid systems with transmission voltages up to 1 MV. A section that contained an electrical tree in low-density polyethylene (LDPE) has been visualized in three dimensions (3D) with a resolution of 92 nm by X-ray ptychographic tomography. The 3D imaging revealed prechannel-formations with a lower density with the width of a couple of hundred nanometers formed around the main branch of the electrical tree. The prechannel structures were partially connected with the main tree via paths through material with a lower density, proving that the tree had grown in a step-by-step manner via the prestep structures formed in front of the main channels. All the prechannel structures had a size well below the limit of the Paschen law and were thus not formed by partial discharges. Instead, it is suggested that the prechannel structures were formed by electro-mechanical stress and impact ionization, where the former was confirmed by simulations to be a potential explanation with electro-mechanical stress tensors being almost of the same order of magnitude as the short-term modulus of low-density polyethylene.

  • 14.
    Pallon, Love K. H.
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Liu, D.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pourrahimi, Amir Masoud
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, A. T.
    Gubanski, S.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Formation and the structure of freeze-dried MgO nanoparticle foams and their electrical behaviour in polyethylene2015In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 14, p. 7523-7534Article in journal (Refereed)
    Abstract [en]

    Electrically insulating low-density polyethylene (LDPE) nanocomposites based on dispersed MgO nanoparticle foams are reported. The foams were obtained via freeze-drying aqueous suspensions of precipitated ca. 40 nm wide and 10 nm thick Mg(OH)(2) nanoparticles and dewatering (calcining) at 400 degrees C, resulting in a 25 times more voluminous powder compared to conventionally dried nanoparticles. This powder handling prior to extrusion melt-processing greatly facilitated the nanocomposite preparation since no particle grinding was necessary. Large quantities of particles were prepared (>5 g), and the nanoparticle foams showed improved dispersion in the LDPE matrix with 70% smaller aggregate sizes compared to the conventionally dried and ground nanopowders. The nature of the nanoparticle foams was evaluated in terms of their dispersion on Si-wafers using ultrasonication as a dispersing aid, which showed to be detrimental for the nanoparticle separation into solitary particles and induced severe aggregation of the calcined nanoparticles. The grind-free MgO nanoparticles/LDPE-composite was evaluated by electrical measurement. The prepared composite showed an initial ca. 1.5 orders of magnitude lower charging current at 10(2) s, and a 4.2 times lower charging current after 16 hours compared to unfilled LDPE. The results open a way for improved insulation to be implemented in the future high-voltage cable system and present a new promising nanoparticle powder handling technique that can be used on a large scale.

  • 15.
    Pallon, Love
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Yu, Shun
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Diaz, Ana
    Paul Scherrer Institute.
    Holler, Mirko
    Paul Scherrer Institute.
    Nilsson, Fritjof
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Chen, Xiangrong
    Chalmers University of Technology .
    Gubanski, Stanislaw
    Chalmers University of Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Three-dimensional nanometre features of direct current electrical trees in low-density polyethyleneManuscript (preprint) (Other academic)
  • 16.
    Pourrahimi, Amir Masoud
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, Tuan A
    Chalmers.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K H
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gubanski, Stanislaw
    Chalmers.
    Olsson, Richard T
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Highly Efficient Interfaces in Nanocomposites Based on Polyethylene and ZnO Nano/Hierarchical Particles: A Novel Approach toward Ultralow Electrical Conductivity Insulations.2016In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 28, no 39, p. 8651-8657Article in journal (Refereed)
    Abstract [en]

    Polyethylene nanocomposites based on functionalized ZnO nano/hierarchical particles with highly effective interfacial surface area are presented, for the next generation of ultralow transmission-loss high-voltage DC insulating materials.

  • 17.
    Pourrahimi, Amir Masoud
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Andersson, Richard L.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Martinez Abad, A.
    Lagaron, J. -M
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Ström, Valter
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.
    Water-based synthesis and cleaning methods for high purity ZnO nanoparticles - comparing acetate, chloride, sulphate and nitrate zinc salt precursors2014In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 4, no 67, p. 35568-35577Article in journal (Refereed)
    Abstract [en]

    A low temperature (60 degrees C) aqueous synthesis method of high purity ZnO nanoparticles intended as fillers for ultra-low electrical conductivity insulations is described. Particles were prepared under identical conditions from different zinc salts based on nitrate, chloride, sulphate or acetate to compare their abilities to form high yields of sub-50 nm particles with narrow size distribution. The acetate salt gave uniform 25 nm ZnO particles with a conical prism shape. The chloride and sulphate derived particles showed mixed morphologies of nanoprisms and submicron petals, whereas the nitrate salt yielded prisms assembled into well-defined flower shapes with spiky edges. The micron-sized flower shapes were confirmed by Xray diffraction to consist of the smaller prism units. Photoluminescence spectroscopy showed emission in the blue-violet region with little variation depending on precursor salt, suggesting that the spectra were dependent on the primary nanoprism formation and rather independent of the final particle morphology. Microscopy revealed that the salt residuals after the reaction showed different affinity to the particle surfaces depending on the type of salt used, with the acetate creating ca. 20 nm thick hydrated shells; and in falling order of affinity: chloride, sulphate and nitrate. An acetate ion shielding effect during the synthesis was therefore assumed, preventing nanoparticle fusion during growth. Varying the concentrations of the counter-ions confirmed the shielding and only the acetate anions showed an ability to stabilize solitary nanoprisms formation in reaction yields from 2 to 10 g L-1. Ultrasonic particle surface cleaning was significantly more efficient than water replacement, resulting in a stable aqueous dispersion with a high zeta potential of 38.9 mV at pH 8.

  • 18.
    Pourrahimi, Amir Masoud
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Pallon, Love K. H.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Liu, Dongming
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Hoang, Tuan Anh
    Gubanski, Stanislaw
    Hedenqvist, Mikael S.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Olsson, Richard T.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Gedde, Ulf W.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture Resistant MgO Nanoparticles2016In: ACS Applied Materials and Interfaces, ISSN 1944-8244, E-ISSN 1944-8252, Vol. 8, no 23, p. 14824-14835Article in journal (Refereed)
    Abstract [en]

    The use of MgO nanoparticles in polyethylene for cable insulation has attracted considerable interest, although in humid media the surface regions of the nanoparticles undergo a conversion to a hydroxide phase. A facile method to obtain MgO nanoparticles with a large surface area and remarkable inertness to humidity is presented. The method involves (a) low temperature (400 degrees C) thermal decomposition of Mg(OH)(2), (b) a silicone oxide coating to conceal the nanoparticles and prevent interparticle sintering upon exposure to high temperatures, and (c) heat treatment at 1000 degrees C. The formation of the hydroxide phase on these silicone oxide-coated MgO nanoparticles after extended exposure to humid air was assessed by thermogravimetry, infrared spectroscopy, and X-ray diffraction. The nanoparticles showed essentially no sign of any hydroxide phase compared to particles prepared by the conventional single-step thermal decomposition of Mg(OH)(2). The moisture-resistant MgO nanoparticles showed improved dispersion and interfacial adhesion in the LDPE matrix with smaller nanosized particle clusters compared with conventionally prepared MgO. The addition of 1 wt % moisture-resistant MgO nanoparticles was sufficient to decrease the conductivity of polyethylene 30 times. The reduction in conductivity is discussed in terms of defect concentration on the surface of the moisture-resistant MgO nanoparticles at the polymer/nanoparticle interface.

1 - 18 of 18
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