Developing novel nano-structured dielectric materials for energy applications using gel coating nanotechnology

Rawlings, J.T., 2020. Developing novel nano-structured dielectric materials for energy applications using gel coating nanotechnology. PhD, Nottingham Trent University.

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Sustainable systems are required in order to tackle the major issue of energy saving. The motivation in this project is focused on energy conversion rates within electrical generators, with particular focus on the electrical insulation material. While the current state-of-the-art technology uses nanocomposite technology, nanoparticle fillers embedded within a polymer matrix, current generators employ microcomposite technology, typically formed of polymer and micro sized mica flakes. While this technology shows great promise as insulation material, it is not to say there are no drawbacks. One of the biggest challenges for the production of filler loaded composite materials, are producing well dispersed matrices, as the particles tend to agglomerate and fail to provide the predicted performance. Along with this issue there are also problems with compatibility between filler particles and polymer matrices. For example, hydrophilic particles show poor affinity within a hydrophobic polymer network.

The aim of this study is to develop a novel nano-structured dielectric material, through the use of gel coatings, to produce a continuous layer-structured material. Following this approach allows filler dispersion problems to be overcome, and compatibility can be increased through surface modification, without removing the dielectric properties that are desired.

In this investigation the focus is on nanoclay particles from within the smectite family, due to both their self-cross linking behaviour and use within the field of clay-polymer nanocomposites. The gel forming ability of two types of commercial nanoclay, Cloisite Na+ and Lucentite SWN, and an understanding of their di↵ering gel forming ability is discussed. Of particular importance in nanoclay materials, and other 2-D structured materials, the separation of the layers is a particular challenge. Through the use of liquid shear exfoliation techniques, the exfoliation of clay layers was successfully achieved within water, providing a method that produces high yield of single layers, while avoiding the common use of organic solvents in order to achieve the result. Dynamic light scattering, zeta potential and rheological studies of the colloidal nature of these clay show a dependence on the colloid concentration for particle interaction and aggregation to form a gel network. The differing ability to form a gel between the two types of clay, has been determined by the use of neutron compton scattering and is related to the nature of particle-particle and particle-water bonding interactions. The smaller aspect ratio SWN shows a greater tendency to form particle-particle bonds, likely attributed to the smaller path length for collisions and also the larger number density of particles at a given concentration in comparison to Cloisite Na+. Using the lower gelling concentration ability of SWN, hybrid clay gels have also been formed, allowing for lower mass loadings of clay within the gel used for coating, confirmed by thermogravimetric methods. The formation of hybrid gel systems shows the promise that SWN can be used as an initiator with non-gelling nanoparticle species such as hexagonal boron nitride, to form hybrid gel coatings. This allows further additions to the coating properties to be provided such as greater thermal conductivity or non-linear conduction behaviour.

The produced gel materials were used to produce coatings on low density polyethylene (LDPE) films, due to both the use of LDPE within insulation materials and also the challenges associated with dispersal of hydrophilic nano fillers within the hydrophobic LPDE polymer matrix. Through a combination of UV irradiation (! = 184.9 and 253.7 nm) and layer-by-layer surface modification methods, the surface properties were transformed to become more hydrophilic, as shown by measurement of the contact angle of water on the surface, allowing the simple production of a continuous gel coating upon the polymer. The loading of clay within the coating was determined through the use of thermogravimetric analysis, and a relation is seen between the relative mass percentage of the coating and the applied coating thickness. Observation of coated polymers observed by scanning electron microscopy show that there is a dependence upon the applied thickness to preventing the surface forming cracks, which subsequently effect the electrical performance. Preliminary results on the dielectric behaviour show a poorer than predicted performance in the electrical breakdown strength, with only SWN coating showing an increase to the DC breakdown strength. Dielectric spectroscopy shows the samples possess loss peaks centred around 50 Hz, which would lead to issues for domestic power application, however it is expected this peak could be present due to water molecules hydrogen bonded to the hydroxyl surface groups within the clay. Analysis of the surface properties shed insight into the reasoning for this. Mapping of the surface uniformity through contact angle measurements showed regions of surface inhomogeneity for samples with applied coating of 75 μm, which when increased to a 200 μm coating significant improvement to sample uniformity is seen. Thus, it is proposed the surface uniformity plays a significant role in the dielectric properties, and thicker applied films should lead to the improved performance predicted. Further factors specific to the use of nanoclay materials is careful consideration of any excess charge carrying species such as adsorbed water molecules and cations, present within the clay structure. Vacuum drying techniques shows that the water content within the film can be reduced and this is expected to reduce undesirable conduction behaviour. The results presented within this study show a deep understanding of the gelling properties of two commercial nanoclay materials, that can be exploited to produce surface coatings for new dielectric materials. However, these are preliminary results of this technology, and subsequent study of the coating thickness, water content and adsorbed species will allow greater understanding of the electrical behaviour, and help to deliver the desired dielectric performance.

Item Type: Thesis
Creators: Rawlings, J.T.
Date: September 2020
Rights: This work is the intellectual property of the author, and may also be owned by Nottingham Trent University. You may copy up to 5% of this work for private study, or personal, non-commercial research. Any re-use of the information contained within this document should be fully referenced, quoting the author, title, university, degree level and pagination. Queries or requests for any other use, or if a more substantialcopy is required, should be directed in the owner(s) of the Intellectual Property Rights.
Divisions: Schools > School of Science and Technology
Record created by: Linda Sullivan
Date Added: 10 May 2021 15:05
Last Modified: 31 May 2021 15:03

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