Rebecca Hollertz

Improvement of toughness and electrical properties of epoxy composites with carbon nanotubes prepared by industrially relevant processes

The addition of carbon nanotubes (CNTs) to polymeric matrices or master batches has the potential to provide composites with novel properties. However, composites with a uniform dispersion of CNTs have proved to be difficult to manufacture, especially at an industrial scale. This paper reports on processing methods that overcome problems related to the control and reproducibility of dispersions. By using a high pressure homogenizer and a three-roll calendaring mill in combination, CNT reinforced epoxies were fabricated by mould casting with a well dispersed nanofiller content from 0.1 to 2 wt%. The influence of the nano-carbon reinforcements on toughness and electrical properties of the CNT/epoxies was studied. A substantial increase of all mechanical properties already appeared at the lowest CNT content of 0.1 wt%, but further raising the nanofiller concentration only led to moderate further changes. The most significant enhancement was obtained for fracture toughness, reaching up to 82%. The low percolation thresholds were confirmed by electrical conductivity measurements on the same composites yielding a threshold value of only about 0.01 wt%. As corroborated by a thorough microscopic analysis of the composites, mechanical and electrical enhancement points to the formation of an interconnected network of agglomerated CNTs.

Epoxy based materials containing micro and nano sized fillers for improved electrical characteristics

In this paper the effect on dc and ac electrical properties of silica nano and micro particles dispersed in epoxy resins is discussed. In particular, space charge, conductivity, dielectric strength and partial discharge resistance is analyzed. The results show that nanostructured materials exhibit smaller space charge accumulation with respect to both base and microfilled materials. Regarding PD resistance, micro + nano filled materials display longer lifetimes with respect to base epoxy resin and materials including nanofillers or microfillers alone.

Effect of composition and Morphology on the dielectric response of cellulose-based electrical insulation

Different wood-fibre based papers were characterized by dielectric spectroscopy, mechanical testing and microscopy. The data obtained were utilized to investigate the relationship between the chemistry, morphology and density of a paper and its permittivity and dielectric loss. The density strongly influences the dielectric response, but the response is not affected by the way the density has been achieved; by pressing the paper during drying or by mechanical treatment of the fibres before sheet preparation. The chemical composition of the pulp influences the polarization, dielectric loss and charge transport. It was found that paper-vacuum and paper-oil combinations can be represented by series-equivalent circuits. The permittivity of paper made from electrical grade kraft pulp, used in e.g. high voltage transformers, without any porosity is estimated to be 5.3 and tan δ to 0.01- 0.02 at 50 Hz and 70 °C. The lignin and hemicellulose content of the kraft pulp do not affect the real part of the permittivity significantly, but the dielectric losses increase with increasing lignin and hemicellulose content in both oil and vacuum at 50 Hz and 70 °C.

Novel cellulose nanomaterials

Papers, foams and gels from nanofibrillated cellulose (NFC) have emerged as promising materials for various applications. In this study NFC from a Kraft Pulp used in traditional electrical insulation was produced with the aid of a high pressure homogenizer. Papers were manufactured and their mechanical properties as well as their dielectric responses in oil were measured. The disintegration results in a durable, flexible papers with high strength and density while the dielectric response correlate to that of Kraft Paper with similar density. This paper also includes a description on how inorganic nanoparticles was used to modify the properties of the fibres through a topochemical modification. In this latter technique a Layer-by-Layer technology was used where the charges of the fibres are treated with consecutive layers of oppositely charged polyelectrolytes and nanoparticles.

Dielectric response of kraft paper from fibres modified by silica nanoparticles

Papers have been prepared from fibres that were modified by physical adsorption of silica nanoparticles. Cationic and anionic nanoparticles were adsorbed either directly onto wood fibres or using the layer-by-layer (LbL) technique where silica nanoparticles and a polyelectrolyte of opposite charge were adsorbed in consecutive layers. It was shown that it is possible to tailor the dielectric and mechanical properties of kraft paper by utilizing the pH-dependence of the charge density of both the nanoparticles and the polyelectrolyte during the build-up of layers onto wood fibres. With only one layer of cationic silica nanoparticles, 6.0 wt% of nanoparticles were adsorbed at pH 6, leading to almost complete coverage of the wood fibre surface, resulting in a paper with low dielectric losses and improved inplane tensile properties.

Dielectric properties and resistance to corona and ozone of epoxy compositions filled with micro- and nano-fillers

This paper describes activities aiming to evaluate and compare the resistance to a prolonged corona and ozone exposure for a range of epoxy based compositions filled with micro- and nano-fillers of silica at different proportions. It has been earlier shown for this system that substitution of 5 wt% of micro-filler by nano-filler slightly improved mechanical properties, yielding an increase in Youngs modulus, tensile strength and elongation at break. At the same time, the toughness of the systems decreased with the addition of nano-fillers indicating a poor particle-matrix interaction, which was in accordance with the prior art claiming that this interaction is weaker in case of SiO2 nano-particles, as for example compared to A12O3 nano-particles. For the exposure to corona and ozone a methodology employing a multiple-needle electrode system was used, as recommended by CIGRE working group WG D1.14. Measurements of surface resistivity, bulk resistivity as well as dielectric response (DR) at broad frequency range (10 4-103 Hz) were performed on new samples of the investigated compositions and after each of two sequences of the corona-ozone treatment, lasting 100 hours each. It was found that the long-term corona-ozone exposure had obvious effect on surface resistivity for all the compositions investigated and the contents of the micro- and nano-fillers played a significant role in the observed changes. On the other hand, bulk resisitivity, dielectric permittivity as well as dissipation factor, all being the properties of material bulk, did not exhibit high sensitivity to the exposure.

Measurements of the charge of streamers propagating along transformer oil-solid interfaces

This paper presents an experimental study of the positive streamer charge in transformer oil and oil-solid interfaces in a point-plane gap (5mm) under impulse voltage 50ns\1800μs (maximum peak voltage 24 kV). The experiment is intended to compare the charge of streamers propagating in transformer oil with and without a solid barrier along their path. The solid barrier constricting the streamer volume consists of two parallel strips of either impregnated pressboard or a polymeric film (PA6) installed at both sides of the point electrode. The maximum cumulative charge of the streamer is found for the case when the impregnated pressboard is used and the minimum cumulative charge for the case of the polymer as a barrier.

On the initiation of negative streamers at mineral oil-solid interfaces

This paper reports the probability distribution of negative streamers initiated in mineral oil with and without a solid interface. In addition, the charge injected by conduction currents prior to the streamer inception is presented. Impregnated paper and polymeric films made of PET, PTFE and PVDF are tested as solid materials. Comparison of the conduction charge for the different oil-solid interfaces is presented. It is found that the permittivity of the material used at the solid interface does not influence significantly the condition for streamer initiation in mineral oil. A nonlinear increasing of the charge readings for the impregnated paper, PET and PVDF cases are observed.

Influence of paper properties on streamers creeping in mineral oil

This work presents an experimental study of second mode positive streamers propagating along mineral oilpaper interfaces. A point-plane arrangement immersed in mineral oil with the paper inclined 60 degrees to the plane electrode is used to create the liquid-solid interface. Kraft paper and a kraft fibril paper, made from cellulosic micro and nano fibrils, with higher density and lower surface roughness are used as the solid materials. High speed shadowgraphy and charge recordings are used to compare the propagation of second mode positive streamers along the mineral oil-kraft paper and mineral oil-kraft fibril paper. Streamers creeping along the mineral oil-kraft paper interface propagate mainly into the liquid, with one or two main filaments. In comparison, the streamers propagating along the kraft fibril paper show a strong reduction of the branching; these streamers consist of a single filament that propagates exactly on the solid surface. Streamers along the kraft fibril paper also have longer propagation time than for the case with kraft paper. Mutual electrostatic shielding between filaments is observed for the streamers creeping on the kraft paper. An electrostatic analysis of the influence of permittivity, density and surface roughness of the solid in the electrical properties of the streamer filaments is also performed.

First mode negative streamers along mineral oil-solid interfaces

This document presents an experimental study on the propagation of first mode negative streamers along mineral oil-solid interfaces. Samples made of an oil impregnated kraft paper and a low-porosity paper made from cellulosic micro and nano fibrils, as well as different polymeric films (low density polyethylene (LDPE), polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF)) are used as the solid. A comparison of the length, charge and velocity of streamers for all different mineral oil-solid interfaces is reported. It is shown that streamers propagate longer and faster along mineral oil-solid interfaces with low surface roughness, low porosity and higher electrical permittivity than mineral oil. Those streamers show a quasi-continuous injection of charge in the early stage of their propagation. This quasi-continuous charge injection consists of a sequence of small charge steps separated by few tens of nanoseconds in between. In comparison, the streamers that propagate along surfaces with similar permittivity to the mineral oil have lower injection of charge and higher stopping voltage conditions than streamers propagating free in the liquid without any solid barrier.