Love K. H. Pallon

Water-based synthesis and cleaning methods for high purity ZnO nanoparticles – comparing acetate, chloride, sulphate and nitrate zinc salt precursors

A low temperature (60 °C) aqueous synthesis method of high purity ZnO nanoparticles intended as fillers for ultra-low electrical conductivity insulations is described. Particles were prepared under identical conditions from different zinc salts based on nitrate, chloride, sulphate or acetate to compare their abilities to form high yields of sub-50 nm particles with narrow size distribution. The acetate salt gave uniform 25 nm ZnO particles with a conical prism shape. The chloride and sulphate derived particles showed mixed morphologies of nanoprisms and submicron petals, whereas the nitrate salt yielded prisms assembled into well-defined flower shapes with spiky edges. The micron-sized flower shapes were confirmed by X-ray diffraction to consist of the smaller prism units. Photoluminescence spectroscopy showed emission in the blue-violet region with little variation depending on precursor salt, suggesting that the spectra were dependent on the primary nanoprism formation and rather independent of the final particle morphology. Microscopy revealed that the salt residuals after the reaction showed different affinity to the particle surfaces depending on the type of salt used, with the acetate creating ca. 20 nm thick hydrated shells; and in falling order of affinity: chloride, sulphate and nitrate. An acetate ion shielding effect during the synthesis was therefore assumed, preventing nanoparticle fusion during growth. Varying the concentrations of the counter-ions confirmed the shielding and only the acetate anions showed an ability to stabilize solitary nanoprisms formation in reaction yields from 2 to 10 g L−1. Ultrasonic particle surface cleaning was significantly more efficient than water replacement, resulting in a stable aqueous dispersion with a high zeta potential of 38.9 mV at pH 8.

Highly Efficient Interfaces in Nanocomposites Based on Polyethylene and ZnO Nano/Hierarchical Particles: A Novel Approach toward Ultralow Electrical Conductivity Insulations

Polyethylene nanocomposites based on functionalized ZnO nano/hierarchical particles with highly effective interfacial surface area are presented, for the next generation of ultralow transmission-loss high-voltage DC insulating materials.

Polyethylene Nanocomposites for the Next Generation of Ultralow-Transmission-Loss HVDC Cables: Insulation Containing Moisture-Resistant MgO Nanoparticles

The use of MgO nanoparticles in polyethylene for cable insulation has attracted considerable interest, although in humid media the surface regions of the nanoparticles undergo a conversion to a hydroxide phase. A facile method to obtain MgO nanoparticles with a large surface area and remarkable inertness to humidity is presented. The method involves (a) low temperature (400 °C) thermal decomposition of Mg(OH)2, (b) a silicone oxide coating to conceal the nanoparticles and prevent interparticle sintering upon exposure to high temperatures, and (c) heat treatment at 1000 °C. The formation of the hydroxide phase on these silicone oxide-coated MgO nanoparticles after extended exposure to humid air was assessed by thermogravimetry, infrared spectroscopy, and X-ray diffraction. The nanoparticles showed essentially no sign of any hydroxide phase compared to particles prepared by the conventional single-step thermal decomposition of Mg(OH)2. The moisture-resistant MgO nanoparticles showed improved dispersion and interfacial adhesion in the LDPE matrix with smaller nanosized particle clusters compared with conventionally prepared MgO. The addition of 1 wt % moisture-resistant MgO nanoparticles was sufficient to decrease the conductivity of polyethylene 30 times. The reduction in conductivity is discussed in terms of defect concentration on the surface of the moisture-resistant MgO nanoparticles at the polymer/nanoparticle interface.

Morphology and properties of silica-based coatings with different functionalities for Fe3O4, ZnO and Al2O3 nanoparticles

A facile single-step method for obtaining 2–3 nm thick silsesquioxane coatings on metal oxide nanoparticles using different carbon-functional silane precursors is presented. Iron oxide nanoparticles with 8.5 nm in diameter were used as a model to evaluate the possibilities of forming different uniform carbon-functional coatings, ranging from hydrophobic to hydrophilic in character. Electron microscopy showed that all the coated nanoparticles could be described as core–shell nanoparticles with single Fe3O4 cores and carbon-functional silsesquioxane shells, without any core-free silicone oxide phase. Steric factors strongly influenced the deposited silicon oxide precursors with octyl-, methyl- or aminopropyl functionalities, resulting in coating densities ranging from 260 to 560 kg m−3. The methyl-functional coatings required several layers of silsesquioxane, 3–4, to build up the 2 nm structures, whereas only 1–2 layers were required for silsesquioxane with octyl groups. Pure silica coatings from tetraethoxysilanes were however considerably thicker due to the absence of steric hindrance during deposition, allowing the formation of 5–7 nm coatings of ca. 10 layers. The coating method developed for the iron oxide nanoparticles was generic and successfully transferred and up-scaled 30 and 325 times (by volume) to be applicable to 25 nm ZnO and 45 nm Al2O3 nanoparticles.

Charge Transport in LDPE Nanocomposites Part I—Experimental Approach

This work presents results of bulk conductivity and surface potential decay measurements on low-density polyethylene and its nanocomposites filled with uncoated MgO and Al2O3, with the aim to highlight the effect of the nanofillers on charge transport processes. Material samples at various filler contents, up to 9 wt %, were prepared in the form of thin films. The performed measurements show a significant impact of the nanofillers on reduction of material’s direct current (dc) conductivity. The investigations thus focused on the nanocomposites having the lowest dc conductivity. Various mechanisms of charge generation and transport in solids, including space charge limited current, Poole-Frenkel effect and Schottky injection, were utilized for examining the experimental results. The mobilities of charge carriers were deduced from the measured surface potential decay characteristics and were found to be at least two times lower for the nanocomposites. The temperature dependencies of the mobilities were compared for different materials.

Local Charge Injection and Extraction on Surface-Modified Al2O3 Nanoparticles in LDPE.

We use a recently developed scanning probe technique to image with high spatial resolution the injection and extraction of charge around individual surface-modified aluminum oxide nanoparticles embedded in a low-density polyethylene (LDPE) matrix. We find that the experimental results are consistent with a simple band structure model where localized electronic states are available in the band gap (trap states) in the vicinity of the nanoparticles. This work offers experimental support to a previously proposed mechanism for enhanced insulating properties of nanocomposite LDPE and provides a powerful experimental tool to further investigate such properties.

Three-Dimensional Nanometer Features of Direct Current Electrical Trees in Low-Density Polyethylene

Electrical trees are one reason for the breakdown of insulating materials in electrical power systems. An understanding of the growth of electrical trees plays a crucial role in the development of reliable high voltage direct current (HVDC) power grid systems with transmission voltages up to 1 MV. A section that contained an electrical tree in low-density polyethylene (LDPE) has been visualized in three dimensions (3D) with a resolution of 92 nm by X-ray ptychographic tomography. The 3D imaging revealed prechannel-formations with a lower density with the width of a couple of hundred nanometers formed around the main branch of the electrical tree. The prechannel structures were partially connected with the main tree via paths through material with a lower density, proving that the tree had grown in a step-by-step manner via the prestep structures formed in front of the main channels. All the prechannel structures had a size well below the limit of the Paschen law and were thus not formed by partial discharges. Instead, it is suggested that the prechannel structures were formed by electro-mechanical stress and impact ionization, where the former was confirmed by simulations to be a potential explanation with electro-mechanical stress tensors being almost of the same order of magnitude as the short-term modulus of low-density polyethylene.

Influence of nanoparticle surface coating on electrical conductivity of LDPE/Al 2 O 3 nanocomposites for HVDC cable insulations

LDPE/metal oxide nanocomposites are promising materials for future high-voltage DC cable insulation. This paper presents data on the influence of the structure of the nanoparticle coating on the electrical conductivity of LDPE/Al2O3 nanocomposites. Al2O3 nanoparticles, 50 nm in size, were coated with a series of silanes with terminal alkyl groups of different lengths (methyl, w-octyl and n-octadecyl groups). The density of the coatings in vacuum was between 200 and 515 kg m−3, indicating substantial porosity in the coating. The dispersion of the nanoparticles in the LDPE matrix was assessed based on statistics for the nearest-neighbor particle distance. The electrical conductivity of the nanocomposites was determined at both 40 and 60 °C. The results show that an appropriate surface coating on the nanoparticles allowed uniform particle dispersion up to a filler loading of 10 wt.%, with a maximum reduction in the electrical conductivity by a factor of 35. The composites based on the most porous octyl-coated nanoparticles showed the greatest reduction in electrical conductivity and the lowest temperature coefficient of electrical conductivity of the composites studied.