J. Keith Nelson

Internal Charge Behaviour of Nanocomposites.

The incorporation of 23 nm titanium dioxide nanoparticles into an epoxy matrix to form a nanocomposite structure is described. It is shown that the use of nanometric particles results in a substantial change in the behaviour of the composite, which can be traced to the mitigation of internal charge when a comparison is made with conventional TiO2 fillers. A variety of diagnostic techniques (including dielectric spectroscopy, electroluminescence, thermally stimulated current, photoluminescence) have been used to augment pulsed electro-acoustic space charge measurement to provide a basis for understanding the underlying physics of the phenomenon. It would appear that, when the size of the inclusions becomes small enough, they act co-operatively with the host structure and cease to exhibit interfacial properties leading to Maxwell-Wagner polarization. It is postulated that the particles are surrounded by high charge concentrations in the Gouy-Chapman-Stern layer. Since nanoparticles have very high specific areas, these regions allow limited charge percolation through nano-filled dielectrics. The practical consequences of this have also been explored in terms of the electric strength exhibited. It would appear that there was a window in which real advantages accrue from the nano-formulated material. An optimum loading of about 10% (by weight) is indicated.

Graphene Oxide Filled Nanocomposite with Novel Electrical and Dielectric Properties

IO N The globally increasing demand of energy is a technical challenge for the electrical generation, transmission and distribution systems. This requires often contradictory features such as increasing voltage levels in combination with more compact designs in urban environments. This leads to an increased electric stress on the insulation systems. This can be addressed by using insulating materials with tunable non-linear conductivity, as well as high dielectric constant and low loss, for electric fi eld grading applications. [ 1 ] Current fi eld grading materials consist of polymer, semi-conducting ceramic particles such as SiC, ZnO etc., as well as lower amounts carbon black, embedded in a polymer matrix. [ 2 , 3 ] The nonlinear electrical conductivity is created by the percolated structure of these particles within the matrix, at a typically volume loading of 30% to 40%. As a result of the high fi ller level, current fi eld grading materials are limited in terms of poor mechanical properties, high weight and tendency to overheat at elevated electric fi elds. Graphene oxide (GO) is commonly prepared from natural fl ake graphite by the Hummers method and serves as an intermediate product to prepare graphene. [ 4 ] It has disrupted sp 2

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.

The influence of moisture on the electrical properties of crosslinked polyethylene/silica nanocomposites

Crosslinked polyethylene (XLPE)/silica nanocomposites are promising candidates for future cable insulation. While a significant number of studies have demonstrated improved dielectric properties in nanocomposites compared to XLPE, the performance of polyolefin nanocomposites in humid environments has not received much attention. This paper presents and explains the dielectric behavior of XLPE/silica nanocomposites in humid environments such as the decrease in AC breakdown strength, increases in loss and space charge formation, and the significant reduction in water tree aging. XLPE/silica nanocomposites are found to have an increased moisture uptake compared to the XLPE base polymer due to inclusion of silica particles. It is hypothesized that the formation of a concentric shell surrounding the particle with a high concentration of water (water shell), and the change in the inter-particle/cluster distances are two major factors governing the dielectric behavior in wet XLPE/silica nanocomposites. The dispersion and distribution of the nanofillers were quantified using a new tool and a method to reconstruct the 3D structure was used to determine the size of the water shell required for percolation. It was found that a water layer thickness of tens of nanometers could initiate percolation in the XLPE/silica nanocomposites studied. Notwithstanding that, water tree growth was substantially reduced in the XLPE/silica nanocomposites, and some speculative explanations are provided on the basis of the characteristics observed.

Preparation of BaTiO3/low melting glass core–shell nanoparticles for energy storage capacitor applications

A core–shell nano-scale mixing technique was applied to fabricate BaTiO3/glass nanocomposites in order to preserve the nano-grain dielectric properties of BaTiO3 after sintering and enhance the bulk composite energy storage capability. Coating layers of low melting glasses of lead borosilicate glass (65PbO–20B2O3–15SiO2, mol%) and bismuth borosilicate glass (65Bi2O3–20B2O3–15SiO2, mol%) were deposited onto BaTiO3 nanoparticles in chemical solution by a sol-precipitation method under ultrasonic agitation. Transmission electron microscopy (TEM) results confirmed the formation of core–shell nanostructures with controllable shell thicknesses between 2 and 18 nm. X-ray diffraction (XRD) patterns showed that no crystalline peaks were detected from the glass coating layer. Fourier transform infrared (FT-IR) spectra indicated a glass network structure of lead borosilicate glass and bismuth borosilicate glass, respectively. High densifications were achieved for both composites by sintering at low temperatures (≤900 °C). Noticeable grain growth was observed for the lead borosilicate glass-coated BaTiO3 (Pb-BT) composite while almost no grain growth was observed for the bismuth borosilicate glass-coated BaTiO3 (Bi-BT) nanocomposite. This disparity was attributed to the different interactions between the BaTiO3 core and two glasses during the sintering process, as revealed by the XRD study. Dielectric properties and energy storage capability of the Bi-BT nanocomposite were investigated in detail. The Bi-BT nanocomposite showed high polarization, high dielectric breakdown strength (≥1000 kV cm−1), postponed polarization saturation, and low remnant polarization with the discharge energy density of ∼10 J cm−3 at 1000 kV cm−1. Thus, the Bi-BT core–shell nanocomposite appears to be a promising material system for energy storage capacitor applications.

Estimation of particulate charging and migration for pulsed precipitator applications

Abstract A simulation is described which permits the evaluation of dispersed particulate behaviour under the transient ion density and space-charge fields appropriate to the operation of electrostatic precipitators with pulsed energization. Field, field-assisted diffusion and pure diffusion charging mechanisms are used to describe the charging and a novel stochastic scheme is introduced to evaluate particle charge density distributions in both space and time when turbulent diffusion is important. Supporting experimental measurements are introduced to measure the particle charge distributions and to estimate the spatial changes in dust concentrations as a result of the migration processes within a precipitator.

Simulation and measurement of corona for electrostatic pulse powered precipitators

The use of pulse powering for electrostatic precipitators has shown several advantages but the underlying physics has been insufficiently understood to allow any optimization. A numerical model has been assembled which allows the controlling corona process to be simulated in both time and space to permit estimation of the ion densities and field distributions to be expected for a wide range of transient electrical parameters and gas properties. Supporting laboratory measurements have been undertaken under dc and pulse conditions both to verify the simulation and to provide corona spot density data not readily obtainable by other means.

Background, Principles and Promise of Nanodielectrics

Electrical insulation based on nanodielectric technology stems from the use of fillers in polymeric materials which has been common practice since polymer insulation was first introduced. However, when the filler size is reduced to nanometric size, the internal interface becomes dominant and unique properties can be obtained. The basic processes and effects are introduced in order to provide background and context for the remaining chapters.

Investigation of dielectric breakdown in silica-epoxy nanocomposites using designed interfaces

Adding nano-sized fillers to epoxy has proven to be an effective method for improving dielectric breakdown strength (DBS). Evidence suggests that dispersion state, as well as chemistry at the filler-matrix interface can play a crucial role in property enhancement. Herein we investigate the contribution of both filler dispersion and surface chemistry on the AC dielectric breakdown strength of silica-epoxy nanocomposites. Ligand engineering was used to synthesize bimodal ligands onto 15nm silica nanoparticles consisting of long epoxy compatible, poly(glycidyl methacrylate) (PGMA) chains, and short, π-conjugated, electroactive surface ligands. Surface initiated RAFT polymerization was used to synthesize multiple graft densities of PGMA chains, ultimately controlling the dispersion of the filler. Thiophene, anthracene, and terthiophene were employed as π-conjugated surface ligands that act as electron traps to mitigate avalanche breakdown. Investigation of the synthesized multifunctional nanoparticles was effective in defining the maximum particle spacing or free space length (Lf) that still leads to property enhancement, as well as giving insight into the effects of varying the electronic nature of the molecules at the interface on breakdown strength. Optimization of the investigated variables was shown to increase the AC dielectric breakdown strength of epoxy composites as much as 34% with only 2wt% silica loading.

Nanodielectrics - the first decade and beyond

This contribution chronicles the development of nanodielectrics with particular reference, with examples, to the enhancement of useful properties which can be brought about by the inclusion of nanoparticulates and by surface functionalization. Emphasis is given to the importance of proper processing and critical quantification of these 21st century materials. The impact of the establishment of internal pathways on the aqueous behaviour is also reviewed, and tests are outlined which permit a mechanistic understanding. By considering the part played by the inherent interface structure in a polymer nanocomposite, the paper explores the possibilities for enhancing multifunctionality. In particular, the means to alter the interface in order to impart particular properties is examined and tested using specific examples. In this way it is shown that it is quite possible to tailor materials for particular applications using this technology. It is suggested that, looking forward, these methods may provide the basis for a new generation of “smart materials” in which a functionalized nanophase inclusion may be used to bring about adaptability in an electrically stressed polymer.