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| 2. |
- Liu, Dongming, 1987-
(författare)
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Polyethylene – metal oxide particle nanocomposites for future HVDC cable insulation : From interface tailoring to designed performance
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Low-density polyethylene (LDPE) nanocomposites containing metal oxide nanoparticles are considered as promising candidates for insulating materials in future high-voltage direct-current (HVDC) cables. The significant improvement in dielectric properties compared with unfilled polymer is attributed to the large and active interface between the nanoparticles and the polymer. The nanoparticles may also initiate cavitation under stress and potential accelerated aging risks due to the adsorption and inactivation of the protecting antioxidants. This study is focused on the possibilities of achieving well-balanced performances of the polyethylene nanocomposites for HVDC insulation via tailoring the particle interface in the nanocomposites.A facile and versatile surface coating method for metal oxide particles was developed based on silane chemistry. The developed method was successfully applied to 8.5 nm Fe3O4, 25 nm ZnO and 50 nm Al2O3 particles, with the aim to develop uniform coatings that universally could be applied on individual particles rather than aggregates of particles. The surface properties of the coatings were further tailored by applying silanes with terminal alkyl groups of different lengths, including methyl (C1-), octyl (C8-) and octadecyl (C18-) units. Transmission electron microscopy, infrared spectroscopy and thermal gravimetric analysis confirmed the presence of uniform coatings on the particle surface and importantly the coatings were found to be highly porous.The capacity of metal oxide particles to adsorb relevant polar species (e.g. moisture, acetophenone, cumyl alcohol and phenolic antioxidant) was further assessed due to its potential impact on electrical conductivity and long-term stability of the nanocomposites. The oxidative stability of the nanocomposites was affected by the adsorption of phenolic antioxidants on particles and transfer of catalytic impurities (ionic species) from metal oxide particles to polymer matrix. It was found that carefully coated metal oxide particles had much less tendency to adsorb antioxidants. They could, however, adsorb moisture, acetophenone and cumyl alcohol. The coated particles did not emit any destabilizing ionic species into the polymer matrix. The inter-particle distance of the nanocomposites based on C8-coated nanoparticles showed only a small deviation from the ideal, theoretical value, indicating a good particle dispersion in the polymer. Scanning electron microscopy of strained nanocomposite samples suggested the cavitation mainly occurred at the polymer/nanoparticles interface. The microstructural changes at polymer/nanoparticle interface were studied by small-angle X-ray scattering coupled with tensile testing. The polymer/nanoparticle interface was fractal before deformation due to the existence of the bound polymers at the nanoparticle surface. Extensive de-bonding of particles and cavitation were observed when the nanocomposites were stretched beyond a critical strain. It was found that the composites based on carefully coated particles showed higher strain at cavitation than the composites based on uncoated particles. The composites based on C8-coated nanoparticles showed the largest decrease in electrical conductivity and the lowest temperature coefficient of the electrical conductivity among the composite samples studied.
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| 3. |
- Nilsson, Fritjof, 1978-
(författare)
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Meso-scale modelling of composites and semi-crystalline polymers
- 2009
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis covers the first few steps of a multi-scale computer simulation strategy for predicting physical properties of complex polymers like composites and semi-crystalline polymers. Meso-scale simulations of crystallization and solvent diffusion in polyethylene as well as simulations examining the geometrical impact on the effective permittivity of composites have been performed. These meso-scale models will in the near future be coupled to molecular dynamics models for increased realism and accuracy. The first paper was focused on solvent diffusion in spherulitic semi-crystalline polyethylene. Geometrical models of polyethylene spherulites were constructed and Monte-Carlo random walker simulations were used to estimate the geometrical impedance factor as function of volume crystallinity, mean free path and other geometry properties. Novel numerical off-lattice algorithms made it possible to increase the maximum volume crystallinity from 40 to 55%, to decrease the computation time a factor 100 and to use shorter and more realistic diffusion jump-lengths. The simulation results were in good agreement with experimental results and new analytical formulas were found that could be neatly fitted to both simulation data and experimental data. It was noticed that the geometrical impedance factor was proportional to the polymers mean free path length rather than its length/width aspect ratio and that the traditional Fricke formula for oblate spheroids was not able to correctly predict the diffusion behaviour in complex geometries like spherulites at medium-high volume crystal fractions. The second paper was focused on the electrostatics of composites. Geometrical models of layered composites were first obtained and the finite element method was then used to calculate the effective composite permittivity as function of particle content, particle shape, degree of mixing and other geometrical issues. Analytical lamellae formulas for 2- and 3-phase composites were formulated with clearly better correlation to corresponding finite element data than all other previously known analytical formulas. The analytical 3-phase formula was successfully compared with experimental data for mica/polyimide and it was noted that the influence of water and air was significant.
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| 4. |
- Alipour, Nazanin, 1978-
(författare)
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Structure and Mechanical/Transport properties of Single and Multilayer Polyethylene-based Materials
- 2014
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current study discusses the structure, mechanical and transport properties of polyethylene-based materials into two parts. The first part deals with the migration and chemical depletion of active substance such as insecticides from moulded polyethylene sheets. Deltamethrin (DM) and synergist piperonyl butoxide (PBO) are often used for insect control purpose. It was found that DM as a powder was incapable of recrystallization and remained in liquid state after cooling to room temperature, and that the evaporation of a DM/PBO solution was greater than that predicted from the evaporation rates of pristine separate material components. Infrared spectroscopy and liquid chromatography showed that the loss of DM and PBO through polyethylene sheets was negligible over 30 days, when aged in air at 80 °C (60 and 80 %RH). However, significant migration of the active species was observed in aged polyethylene sheets which were exposed in liquid water (at 80 and 95 °C). In the second part, the structure and properties of multi–layered polymer films were studied in terms of crystallization kinetics, mechanical and transport properties. Previously, it has been shown that when the layer thickness decreases from micrometre-scale to nanometre-scale, leading to improvement of the film performance such as crack propagation and oxygen barrier properties. In this work, two multi-layered systems were considered based on compatible (i) or incompatible layers (ii). In the first case (i), metallocene polyethylene (mPE) and low-density polyethylene (LDPE) where investigated as 2, 24, and 288 adjacent layers. In the second case (ii) poly(ethylene-co-vinyl alcohol) (EVOH) and polyethylene adhesive was evaluated as 5 and 19 layers. The crystallization kinetic studies showed that the crystallization rate was retarded as the layers became thinner with increasing number of layers in the multi-layered films as compared to the reference films (2 and 5 layers). The observation was suggested to stem from greater association between layers (inter layer mixing) in the case of mPE/LDPE films with 2 layers. Furthermore, the crack growth resistance increased with increasing number of layers. The x-ray scattering and tensile testing showed that the films were orientated more in extrusion direction than in the transverse direction, besides the EVOH films (the incompatible system) showed higher orientation in the extrusion direction than mPE/LDPE films. The uptake of n-hexane was reduced significantly in multi-layered EVOH films due to the effective protective role of EVOH. Furthermore, it was revealed that non-homogenous swelling causing a folding/curling of bilayer films when exposed to the vapour of the solvent.
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| 6. |
- Pallon, Love, 1984-
(författare)
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Polyethylene/metal oxide nanocomposites for electrical insulation in future HVDC-cables : probing properties from nano to macro
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 7. |
- Svensson, Marie, 1980-
(författare)
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Modification and Utilization of Wood Hydrolysates
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polysaccharides have been used for many years in a variety of products such as food, fuel supply and industrial products. During recent years there has been an increasing interest in a group of polysaccharides, hemicelluloses, due to their highly availability, renewability and potentially low price. It is however a complicated process to extract highly purified hemicellulose and a more practical process is required so that a less refined resource can be used. This thesis presents a study of the utilization of a simply filtrated and upgraded renewable biomass from the forest industry, so called wood hydrolysate. The wood hydrolysate is rich in hemicelluloses and contains some lignin. Microspheres were successfully produced in a benign way from four different hydrolysates; two from softwood and two from hardwood. The microspheres were prepared in a water-in-oil emulsion and different parameters were regulated. The concentration of the dispersed phase, the temperature and the volume ratio of the dispersed phase and the continuous phase were changed and screened in a multi-variate statistical analysis program. A series of hydrogels based on renewable wood hydrolysate were modified and their ability to swell was assessed. An unsaturated alcohol was activated with N,N’-carbonyldiimidazole (CDI), coupled to the hydroxyl group on the hemicellulose backbone and then radically crosslinked with acrylic acid as co-monomer to form a network. Two of the four wood hydrolysates were modified, one from softwood and one from hardwood, as well as the pure hemicelluloses. Hydrogels was successfully produced with the wood hydrolysate from hardwood and pure hemicellulose and the gels from the modified wood hydrolysate swelled the most, almost 30 % of its own weight. In conclusion, this thesis shows that it is possible to use the renewable biomass, wood hydrolysate, in a variety of forms for future applications.
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| 8. |
- Ekelund, Maria, 1977-
(författare)
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Long-term Performance of PVC and CSPE Cables used in Nuclear Power Plants : the Effect of Degradation and Plasticizer migration
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Enormous amounts of low voltage cables installed in a Swedish nuclear power plant are reaching their expected lifetimes. Since the cables are crucial to operational safety, it is of great importance that the actual condition of the installed cables is determined. In this study, cables based on poly(vinyl chloride) plasticized with di(2-ethylhexyl)phthalate (DEHP) were examined with respect to the degradation mechanisms responsible for the ageing of the insulation. This was achieved by studying samples that underwent accelerated ageing by different analytical methods, such as indenter modulus measurements, tensile testing, infrared spectroscopy, differential scanning calorimetry and liquid chromatography, to assess the condition of the cables. The results were unambiguous; the main deterioration mechanism differed for the jacket and the core insulation. The immediate increase in stiffness of the jacket insulation suggests that loss of plasticizer was the dominant cause for degradation. The core insulation on the other hand showed much smaller changes in the mechanical properties due to thermal ageing with an activation energy of the change in the indenter modulus matching that of the dehydrochlorination process. The electrical functionality during high-energy line break accident was correlated to the mechanical properties of the cable and this correlation was used to establish a lifetime criterion. The mechanical data showed Arrhenius temperature dependence with activation energies of 80 kJmol-1 and 100 kJmol-1 for the jacketing and 130 kJmol-1 for the core insulation. These activation energies were used to extrapolate the lifetimes to service temperatures (20 °C to 50 °C). Plasticizer migration was determined as the lifetime controlling mechanism at the service temperatures. Experimental data, obtained by extraction of DEHP followed by liquid chromatography, were analysed by numerical methods to gain a better understanding of the migration. The analysis showed that the transport of DEHP to the surrounding environment was diffusion controlled at temperatures between 120 °C and 150 °C, with an activation energy of 89 kJmol-1. At lower temperatures, HTML clipboard ≤100 °C, the loss of plasticizer was controlled by evaporation, with an activation energy of 99 kJmol-1. Under the latter conditions, the rate of plasticizer loss from the PVC cable was very similar to that from the pure plasticizer, suggesting that a film of plasticizer was formed on the PVC surface. The evaporation of DEHP showed a clear dependence on the rate of ventilation of the gas phase surrounding the cable. The ability to monitor the condition of the installed cables is dependent on good techniques for the remaining lifetime assessment. The condition monitoring technique, Line Resonance Analysis, was applied to chlorosulfonated polyethylene cables. A clear correlation between LIRA and indenter modulus data obtained and LIRA and tensile testing results was found. This is of interest since existing lifetime criteria used in the nuclear plants are based on these two testing techniques.
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| 9. |
- Krämer, Roland H., 1977-
(författare)
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Melt flow and surface stability effects on polymer flammability : A study on polystyrene, flexible polyurethane foam and polyolefin composites
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Our ability to accurately determine the flammability properties of polymers is hampered bythe erratic effects of deformation and liquid flow that occur during polymer combustion.This study aims to improve the understanding of such effects at several levels: The often obvious impact of melt flow on standardized flammability test methods forelectrical appliances is described to identify the need and to motivate others to worktowards an improved material characterization. The production of a burning degraded melt results from both, melting anddecomposition of thermoplastic polymers. The degradation of the polymer chains yieldsboth combustible volatiles that feed the flame and smaller molecules with a low viscositythat might spread to a burning pool. The molar mass is the property of the material that linksboth processes. Changes in molar mass at the surface of specimens exposed to fire‐like heatfluxes were analyzed, using polystyrene as a model thermoplastic material. Analyticalexperiments were combined with bench‐scale gasification and gasification/melt flowexperiments, in order to determine the extent of viscosity reduction by melting and bypolymer decomposition. A strong volume contraction to a low viscous liquid is characteristic for the combustionof flexible polyurethane foam, a low density cellular material used for cushions in furnitureand mattresses. Foam decomposition is initiated at low temperature (< 300 °C), yieldingignitable gases and leading to a fast flame spread that results in the formation of a burningliquid pool. The foam morphology changes rapidly during combustion and the impact ofthese changes on the heat release rate was analyzed. The distribution of flame heat transferled to strong variations in the burning rate, which shaped the specimen surface. The shapeof the surface affected in turn the distribution of flame heat flux, leading to an interlockingprocess. A custom‐made vertical test arrangement gave the ability to study the effect of heatfeedback from the burning liquid. Foam composites with carbon nanofibers and organicallymodified clay were prepared with the aim to eliminate liquefaction. Incorporation of anetwork of carbon nanofibers in the foam struts is demonstrated to prevent foamliquefaction and collapse. An approach to prevent melt‐dripping is the use of high filling levels of inorganicparticles in polyolefin cable materials and to promote reactions between the filler and thepolymer. The transition from a polymer melt to an immobile inorganic residue was studiedfor blends of acrylic copolymers of ethylene with chalk and silicone. The mechanical stabilityof the inorganic surface layer that formed during combustion was decisive for the materialsflammability properties. In‐situ thermocouple measurements and ex‐situ analysis of partiallyburned specimen surfaces were used to explain the coupling between the degradationmechanism of the polymer and the heat shield effect of the inorganic residue layer.
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| 10. |
- Linde, Erik, 1985-
(författare)
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Polymeric materials in nuclear power plants : Lifetime prediction, condition monitoring and simulation of ageing
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Nuclear power plants generate a significant part of the world’s electrical power consumption. However, many plants are nearing the end of their designed lifetime, and to extend the lifetime it is important to verify that every component can withstand the added service time. This includes polymeric materials, which become brittle with time. By predicting their lifetime and monitoring their condition, unnecessary downtime of the plant can be avoided, and secure operation can be ensured. The lifetime can be predicted by extrapolating results from accelerated ageing to service conditions, or by simulation of the degradation process.In this study, lifetime predictions through extrapolation were performed on samples of a polyvinyl chloride (PVC) core insulation and an acrylonitrile butadiene rubber (NBR) membrane, which were thermally aged in air. The lifetime of the PVC cable was predicted using Arrhenius extrapolation, and using a method based on Langmuir, Clausius-Clapeyron, and Kirchhoff’s equations.The lifetime of the NBR membrane was predicted using extrapolation in the temperature domain using an Arrhenius approach coupled with an extrapolation in pressure-domain, yielding realistic lifetimes.Two cable insulations, one made from crosslinked polyethylene (XLPE) and the other from ethylene propylene rubber (EPR) were aged under the simultaneous effect of elevated temperature and γ-radiation investigated using several condition monitoring techniques. In particular, two non-destructive techniques, dielectric spectroscopy and nuclear magnetic resonance, showed promising results be developed and used in situ.Finally, a computer model simulating the diffusion and consumption of oxygen in XLPE was developed, based on assumptions that diffusion, consumption and solubility were dependent on the total degree of oxidation. The model showed promise for further development.
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