Suvi Virtanen

Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite

This paper presents the results of the dielectric properties and partial discharge endurance measurements conducted on polypropylene (PP)-silica nanocomposite. The material compounds were analyzed with micro-Raman spectroscopy, X-ray tomography and transmission electron microscopy (TEM). ac and dc breakdown strength of the materials was measured. Dielectric response, capacitance and loss factor of the film samples were measured as a function of temperature and frequency. Partial discharge (PD) endurance of the reference PP and PP Silica nanocomposite was studied as a function of ac voltage. Material surfaces were analyzed after PD stress with optical microscopy. All dielectric measurements were done for oriented thin films with a thickness of 11-23 μm. The results were analyzed statistically to determine the effects of the additive on the properties of PP. The paper discusses the potential of PP Silica nanocomposite with a view to high voltage applications, especially power capacitors.

Dielectric breakdown strength of epoxy bimodal-polymer-brush-grafted core functionalized silica nanocomposites

The central goal of dielectric nanocomposite design is to create a large interfacial area between the matrix polymer and nanofillers and to use it to tailor the properties of the composite. The interface can create sites for trapping electrons leading to increased dielectric breakdown strength (DBS). Nanoparticles with a bimodal population of covalently anchored molecules were created using ligand engineering. Electrically active short molecules (oligothiophene or ferrocene) and matrix compatible long poly(glycidyl methacrylate) (PGMA) chains comprise the bimodal brush. The dielectric breakdown strength was evaluated from recessed samples and dielectric spectroscopy was used to study the dielectric constant and loss as a function of frequency. The dielectric breakdown strength and permittivity increased considerably with only 2 wt% filler loading while the dielectric loss remained comparable to the reference epoxy.

Large-area dielectric breakdown performance of polymer films – Part II: Interdependence of filler content, processing and breakdown performance in polypropylene-silica nanocomposites

In this study, large-area dielectric breakdown performances of various bi-axially oriented polypropylene (BOPP)-silica nanocomposite films are studied by utilizing the self-healing multi-breakdown method presented in the Part I of this publication. In particular, the effects of silica filler content, pre-mixing method, co-stabilizer content and film processing on the large-area breakdown performance are analyzed. Nanostructural and film cross-sectional analyses are correlated to the breakdown responses. The optimum silica filler content is found to reside at the low fill fraction level (~1 wt-%) and automatic pre-mixing of the raw materials and the optimization of the orientation temperature are found to be preferable. The co-stabilizer Irgafos 168 is found to have a significant effect on the breakdown distribution homogeneity of the reference BOPP films. The breakdown response of the silica nanocomposites is found to be not only dependent on the active measurement area but also on the voltage ramp rate, indicating that the silica nanocomposites exhibit altered internal charge behavior under DC electric field. The area- and ramp-rate-dependence results exemplify the importance of careful breakdown strength evaluation of dielectric polymer nanocomposites. Above all, the results emphasize the fact that a thorough understanding and the optimization of the film processing parameters are crucial for achieving improved breakdown response in dielectric polymer nanocomposite films.

The Effect of Curing Conditions on the Electrical Properties of an Epoxy Resin

Epoxy resins have been used in many high voltage applications like cable terminations and cast resin transformers, where the performance of the associated devices depends critically on the effectiveness of the epoxy insulation. The curing process is key to epoxy electrical performance and, therefore, it is necessary to understand the effect of variations of curing conditions on the electrical properties of the epoxy insulation that results. The work described here is focused on studying the effect of the curing conditions of an epoxy resin, based upon measurements of the influence of curing protocol on the AC breakdown strength, DC conductivity, Tg and permittivity of a bisphenol-A system. This research is of potential significance for understanding the behavior of the epoxy resin, because it evaluates the extent to which various curing conditions - and hence network structures - can affect properties.

Balanced nanocomposite thermosetting materials for HVDC and AC applications

There is a need to develop materials with controlled electrical resistivity, reduced space charge accumulation, higher thermal conductivity, higher dielectric strength and enhanced voltage endurance to cope with DC stresses in High Voltage Direct Current (HVDC) transmission systems in addition to HVAC requirements. If the balance of properties, performance and process requirements are achieved this may lead to HVDC insulation systems and equipment having a reduced footprint, larger power densities, and greater multi-stress resilience with longer service lifetimes. It reports findings of a project that is engaging this challenge and investigates the development and scaling of new thermoset based nanocomposite electrical insulation materials for HVDC power transmission applications. Some of the results such as increased electrical breakdown strength and reduced electrical conductivity for reactively surface functionalised nanosilica, and increased thermal conductivity for nano boron nitride and their significance in regard to the wider application of these electrical insulation materials are also discussed. With sufficient understanding of these properties, their trade-offs and process requirements it is possible to tailor balanced materials for specific use in HVAC or HVDC components.

Influence of low amounts of nanostructured silica and calcium carbonate fillers on the large-area dielectric breakdown performance of bi-axially oriented polypropylene

Influence of low amounts (1.0-2.0wt-%) of nanostructured silica and calcium carbonate fillers on the large-area dielectric breakdown performance of bi-axially oriented polypropylene (BOPP) is analyzed. A multi-breakdown measurement method based on the self-healing breakdown capability of metallized film is utilized for the breakdown characterization in order to cover relatively large total film areas, thus leading to results of higher relevance from the practical point-of-view. The dispersion and distribution qualities of filler particles at the nanoscale are evaluated with transmission electron microscopy (TEM) imaging. Weibull statistical analysis suggests that the breakdown distribution homogeneity can be improved with both the filler types. The 1.0wt-% silica-BOPP composite also shows a shift of the weakest points towards higher dielectric strength in comparison to the neat BOPP. However, with increasing filler content, new failure modes are introduced into the nanocomposites, hence decreasing the overall breakdown performance in the >5% breakdown probability region in comparison to the un-filled reference BOPP film.

Dielectric breakdown strength and electrical conductivity of low density polyethylene octylnanosilica composite

One challenge in studying nanodielectric composites is to produce reliable, reproducible samples. A common strategy to suppress aggregation and make the particles more compatible with the polymer matrix is to modify the nanoparticle surface chemistry but, often, evaluation of the effectiveness of the chosen surface functionalization process can prove difficult. In this paper the emphasis is on feasible ways to monitor the production of silane coupled nanosilica low density polyethylene (LDPE) composites, using Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA). The AC-breakdown properties of the resulting composites is studied and the field dependency of the DC-conductivity is measured and also calculated using a space charge limited conduction (SCLC) model together with densities of states obtained from ab initio calculations. For composites containing 13 wt% of nanosilica, breakdown strengths some 18 % higher than that of the unfilled LDPE were obtained. However, the results are not stable over time. This appears to be related to how extensively the composite is dried at elevated temperatures under vacuum.