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Dielectric properties of hexagonal boron nitride polymer nanocomposites
No full textThere is a growing research interest in polymer nanocomposite materials due to their potential in enhancing dielectric properties. However, a considerable amount of variability exists in the literature regarding the electrical performance of polymer nanocomposites, and therefore the underlying mechanisms underpinning their electrical properties are still far from fully understood. Possible reasons for the existing inconsistencies could be due to different material preparation techniques, different nanoparticle dispersion states, unknown filler content, inconsistent sample storage conditions, and unknown water level content in the samples. Determining the principal factors that dominate the electrical behaviour of polymer nanocomposites could allow engineers to tailor the electrical properties of dielectrics for their specific application. As a result, the work reported in this thesis was mainly set out to explore the factors governing the electrical properties of polymer nanocomposites such that the inconsistencies in the literature can be better understood, and consequently eliminated. This thesis investigated the performance of hexagonal boron nitride (hBN) nanocomposites based on two thermoplastic polymers: polystyrene and polyethylene.Prior to producing any nanocomposites, the hBN particles were characterised using different techniques. The characterisation primarily revealed that the boron nitride particles are in the hexagonal form and the surface of hBN contains a scarce amount of hydroxyl groups. Polystyrene nanocomposites were prepared containing identical amounts of hBN dispersed in different solvents in an attempt to obtain different dispersion states, as a result of different hBN/solvent interactions. The effect of solvent processing was negligible on the dispersion state of the hBN in the polystyrene; no observable difference in the dispersion and electrical properties was reported although the presence of hBN resulted in a slight increase in the breakdown strength relative to the unfilled polystyrene. A range of polyethylene nanocomposites were produced containing different amounts of hBN to understand the effect of the dispersion or aggregation state of the hBN on the breakdown strength. The results revealed that the nanocomposites, regardless of the morphology, exhibited a monotonic increase in breakdown strength with increasing hBN content from 2 wt % to 30 wt %, while maintaining the low dielectric losses of the unfilled polyethylene. While the hBN was found to have a strong nucleating effect on the polyethylene, it was determined that the local change in morphology was not the cause of the enhanced breakdown strength as both the polyethylene nanocomposites obtained by rapid crystallisation, where the development of spherulites was suppressed, and the amorphous polystyrene nanocomposites, also exhibited an improved breakdown strength. Further experiments indicated that the polyethylene nanocomposites did not absorb any moisture from the environment in ambient conditions, and absorbed a very small amount of water even in the 30 wt % polyethylene/hBN nanocomposite when completely immersed in water. Dielectric spectroscopy measurements revealed that the surface hydroxyl groups on the hBN are most likely located only on the edge surfaces of the hBN rather than basal surfaces. The water was most likely loosely bound to the hBN particles, where local water clusters formed. It was remarkable that a percolating water network was not formed in a nanocomposite consisting of an already percolating hBN network, which was largely attributed to the surface chemistry of hBN. Despite the presence of water in the system, the hBN nanocomposites continued to exhibit an enhanced breakdown strength in comparison to the unfilled polyethylene. Therefore, this thesis demonstrated that the electrical behaviour of polymer nanocomposites is most likely dominated by the surface state of the nanoparticles and how the particles interact with the charge carriers; any other effects due to local morphological changes or nanoparticle dispersion are considered to be secondary reasons for changes in the electrical properties
Introducing particle interphase model for describing the electrical behaviour of nanodielectrics
Get PDFThis study proposes a new model for describing the electrical behaviour of nanocomposites. Unlike other models in the literature, this model has concentrated on the role of an interphase layer within the boundaries of nanoparticles. The experimental part investigates this role by filling an epoxy matrix with two types of surface-modified silicon nitride nanofiller: (a) the particles were dried at 200 °C, and (b) the particles were calcinated at 1050 °C. Electrical characterization showed that the epoxy which was filled with the calcinated particles has considerably better dielectric performance. Given that thermal and dielectric spectroscopy results demonstrate that the matrix molecular dynamics and polar content are comparable for all the investigated samples, the variations in the dielectric performance point to the particle interphase as an essential reason. As shown by infrared spectroscopy, the complex surface chemistry of the dried particles suggests a particle interphase with a high concentration of localized electronic states, which may enhance charge transport through hopping/tunnelling conduction. On the other hand, calcinating the particles results in a particle interphase with wider band gap, which may work as an energy barrier for charge movement. Consequently, this study highlights the paramount importance of particle interphase for designing dielectric properties of nanodielectrics.</p
Effect of resin/hardener stoichiometry on electrical behavior of epoxy networks
No full textBy changing the ratio of resin to hardener, a series of epoxy resin samples has been produced with differing network structures and different retained chemical functionalities. The resulting materials were characterized by thermal analysis, dielectric spectroscopy, DC conductivity, and DC and AC breakdown strength measurements, to explore the effect of network structure and chemical composition on molecular dynamics and electrical properties. Differential scanning calorimetry showed that the glass transition temperature is primarily determined by the crosslinking density and indicates that, under the range of conditions employed here, side reactions, such as etherification or homopolarization, are negligible. Conversely, changes in DC conductivity with resin stoichiometry appear to occur as a result of changes in the chemical content of the system, rather than variations in network structure or dynamics. Specifically, we suggest that the DC conductivity is markedly affected by the residual amine group concentration in the system. While DC conductivity and DC breakdown appear broadly to be correlated, AC breakdown results indicated that this parameter does not vary with changing stoichiometry, which suggests that the AC and DC breakdown strengths are controlled by different mechanisms
The effect of water absorption on the dielectric properties of polyethylene hexagonal boron nitride nanocomposites
No full textThe effect of water absorption on the dielectric response of polyethylene/hexagonal boron nitride nanocomposites has been studied by dielectric spectroscopy. The nanocomposites have been prepared with hBN concentrations ranging from 2 wt% to 30 wt%. Fourier transform infrared spectroscopy and thermogravimetric analysis revealed a very small amount of hydroxyl groups on the surface of hBN. Mass loss measurements showed that the nanocomposites did not absorb any water under ambient and dry conditions while there was some water absorption under wet conditions. The dielectric spectroscopy results showed a broad relaxation peak, indicative of different states of water with water shells of different thickness, which moved to higher frequencies with increasing water content. However, the dielectric losses were significantly lower than the losses reported in the literature of nanocomposites under wet conditions. In addition, all the absorbed water was successfully removed under vacuum conditions which demonstrated that the interactions between the water and the nanocomposites were very weak, due to the hydrophobic nature of the hBN surface. This is a highly useful property, when considering these materials for applications in electrical insulation
Influence of filler/matrix interactions on resin/hardener stoichiometry, molecular dynamics, and particle dispersion of silicon nitride/epoxy nanocomposites
No full textThe addition of nanofillers can have a significant influence on the resin stoichiometry of thermosetting polymer systems. Based on differential scanning calorimetry (DSC) results, it is estimated that the inclusion of 2 wt% and 5 wt% of silicon nitride nanofiller displaces the resin/hardener stoichiometry of an epoxy/amine network by 6.5% and 18%, respectively. Dielectric spectroscopy results confirm the above findings, in that the spectra of the nanocomposite samples were found to be equivalent to the spectra of unfilled samples when the above stoichiometric effect was taken into account. Therefore, this study provides clear evidence that the presence of a nanofiller can directly and significantly affect the curing process of an epoxy network. Consequently, this should always be considered when introducing nanofillers into thermosetting matrices. These results indicate the presence of covalent bonding between the nanoparticles and the surrounding polymer and, therefore, provide an opportunity to explore the influence of this bonding on the molecular dynamics of the polymer layer around the particles. However, the obtained DSC and dielectric spectroscopy results suggest that, in the system considered here, either the covalent bonding does not have an appreciable influence on the segmental dynamics of the polymer, as revealed by these techniques, or that the thickness of the affected layer is less than 1 nm and therefore too small to be distinguished from experimental uncertainties
Enhanced dielectric properties of polyethylene/hexagonal boron nitride nanocomposites
Get PDFA range of nanocomposites based on a polyethylene polymer and hexagonal boron nitride (hBN) filler has been explored in this study. The dielectric properties of the nanocomposites, which consisted of 2 wt %, 5 wt %, 10 wt %, 20 wt %, and 30 wt % of hBN, have been compared to the dielectric properties of the unfilled polyethylene blend. Scanning electron microscopy (SEM) revealed that the hBN was uniformly distributed in the polyethylene matrix, although large amounts of agglomerates were present in the nanocomposites containing more than 10 wt % of hBN. The incorporation of hBN into polyethylene resulted in a highly disordered morphology in comparison to the unfilled polyethylene, in which this effect was more pronounced with increasing hBN content. This is consistent with the increasing crystallisation temperature as the hBN content increases, as shown by differential scanning calorimetry (DSC), where the hBN acted as a highly effective nucleating due to the strong interactions between the polyethylene and the hBN. This strong interaction is again reflected in the thermal decomposition temperature which similarly increases with increasing hBN content. The study demonstrates the remarkable electrical properties of the prepared nanocomposites, where the breakdown strength monotonically increased as a function of hBN content, even with a very high 30 wt % of hBN. The improvement in electrical properties, even at high hBN concentrations, is contradictory to the reported results in the literature and are mainly attributed to the hydrophobic surface of the hBN particles
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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