M. Praeger

Soft-x-ray wavelength shift induced by ionization effects in a capillary

Coherent soft x rays are produced by high-harmonic generation in a capillary filled with Ar gas. We demonstrate that the tuning of the harmonic wavelengths with intensity and chirp arises from changes in the Ar ionization level. Control over the tuning can be achieved either by changing the average intensity of the laser pulse or by varying the quadratic spectral phase of the laser pulse. We observe an ionization-dependent blueshift of the fundamental wavelength that is directly imprinted on the harmonic wavelengths. The harmonic tuning is shown to depend on nonlinear spectral shifts of the fundamental laser pulse that are due to the plasma created by ionization, rather than directly on any chirp imposed on the fundamental wavelength.

Effect of water absorption on dielectric properties of nano-silica/polyethylene composites

The effect of moisture content on the dielectric properties of polymer/nano-silica blends was investigated. It was found that the DC breakdown strength, electrical conductivity and complex permittivity were all strongly influenced by absorbed water. However, a control sample without nano-silica was largely unaffected by changes in moisture content. This has important implications for researchers and cable designers.

Electrical properties of polymer nano-composites based on oxide and nitride fillers

Four polyethylene based nano-composites containing either silica or silicon nitride were prepared. After verifying their compositions and morphologies, their dielectric properties were followed as a function of conditioning (absorbed water content). The dielectric loss and DC breakdown strength were found to be strongly dependent on conditioning whilst the properties of a control sample (with no nano-filler) were found to be invariant. Under ambient conditions, silicon nitride provides a composite with reduced dielectric loss and increased breakdown strength compared to an analogous system employing silica. Silicon nitride based systems exhibit improved breakdown strength relative to the host polymer when dried and therefore hold significant potential for use in future HVDC cables.

Fabrication of nanoscale glass fibers by electrospinning

We report the experimental realization of glass nanofibers by electrospinning directly from a melt, demonstrating the viability of electrospinning fibers from non-polymer materials with high melting temperatures and higher surface tensions. The nanofiber material (B2O3) is molten on the tip of a gold wire, and voltage applied to the tip causes a jet to form, resulting in solid glass fibers with diameters of ∼100 nm.

The effects of surface hydroxyl groups in polyethylene-silica nanocomposites

Logically, the surface chemistry of filler particles must be a key factor that governs how they interact with a polymer matrix, determining for example, how strongly the particles are bound into the matrix and how easy or difficult it is to achieve a homogenous dispersion of filler particles. This second point is surely one of the most basic challenges when producing a nanocomposite (poor dispersion is frequently stated as the cause of undesirable results). Many attempts have been made to modify the surface chemistry of filler particles through surface functionalization. Typically, this is achieved by chemically attaching polymer chains to the surface of the filler particles. In this paper we try a more direct approach; the surface chemistry of silica nanoparticles is modified by processing them at high temperature. This procedure removes hydroxyl groups from the surface of the filler particles, leaving siloxane groups which are stable at room temperature. Polyethylene composites were produced using both “as delivered” and high temperature processed nanosilica. After heat treatment the particles become hydrophobic which reduces the propensity for water uptake in the resulting nanocomposite and significantly modifies the dielectric response of the material.

The breakdown strength and localised structure of polystyrene as a function of nanosilica fill-fraction

In this work the amorphous matrix of polystyrene provides a homogenous basis into which nanosilica particles are added. Composites are made with four different types of nanosilica particles which are subsequently compared. The DC breakdown strength of the resulting nanocomposite materials is measured as a function of filler fraction with loadings between 0 and 10 %. One advantage of using a polystyrene matrix for this study is its compatibility with permanganic etching. This technique is used to remove part of the polystyrene matrix and render the configuration of the nanofiller particles within the composite amenable to examination by scanning electron microscope (SEM). The simple sample preparation protocol employed here resulted in significant nanofiller agglomeration and the DC breakdown strength was found to decrease with increasing filler fraction.

On the effect of functionalizer chain length and water content in polyethylene/silica nanocomposites: Part I — Dielectric properties and breakdown strength

A series of nanoparticles was prepared by functionalizing a commercial nanosilica with alkylsilanes of varying alkyl tail length, from propyl to octadecyl. By using a constant molar concentration of silane, the density of alkyl groups attached to each system should be comparable. The effect of chain length on the structure of the resulting nanosilica/polyethylene nanocomposites was examined and comparison with an unfilled reference system revealed that, other than through a weak nucleating effect, the inclusion of the nanosilica does not affect the matrix structure. Since water interacts strongly with applied electric fields, water was used as a dielectric probe in conjunction with dielectric spectroscopy to examine the effect of the nanofiller and its surface chemistry on the system. Sets of samples were prepared through equilibrating under ambient conditions, vacuum drying and water immersion. While the water content of the unfilled polymer was not greatly affected, the water content of the nanocomposites varied over a wide range as a result of water accumulation, in a range of states, at nanoparticle interfaces. The effect of water content on breakdown behavior was also explored and, in the unfilled polymer, the breakdown strength was found to depend little on exposure to water (∼13% reduction). In all the nanocomposites, the increased propensity for these systems to absorb water meant that the breakdown strength was dramatically affected (>66% reduction).

A simple theoretical model for the bulk properties of nanocomposite materials

Nanocomposites may be produced simply by combining two materials in such a manner as to produce domains of nanometric scale in the resulting composite [1]. True nanocomposites are distinct from simple mixtures in that they exhibit material properties that do not vary monotonically in proportion to the ratio of the constituent materials - throughout this paper this behavior will be labeled as a “nano effect”. It is widely supposed that “nano effects” are produced by interactions that occur at the interface of the nanometric domains [2]. In typical polymer-nanofiller systems, it is proposed that these interactions act to modify the material properties in a region of the polymer matrix near to the surface of the nanoparticle fillers. We shall refer to this volume of modified material as the interphase. A simple theoretical model is presented which links the interphase volume (and the nature of the material within that volume) with the externally measured properties of the nanocomposite. An equation for the probability that inserting an additional nanoparticle will increase the interphase volume is defined. This equation is applied in a Monte Carlo type calculation to evaluate the interphase volume as a function of filler loading. The resulting properties of the nanocomposite are calculated simply by combining the material properties of the constituents (nanoparticle, matrix and interphase) in the appropriate volume ratios. The strength of this approach is that its simplicity both minimises the number of free-parameters and ensures wide applicability. In this work the model is fitted to measured values of permittivity in nanodielectrics, however, the same approach may readily be applied to a range of other material properties. Statistical calculations are provided that demonstrate the generality of this result. Analysis of the model parameters is shown and provides insight into the extent and type of modification that occurs within the interphase.

Dielectric Studies of Polystyrene-based, High- permittivity Composite Systems

In this paper, we report on a dielectric study of a series of composite systems based upon a polystyrene matrix (έm ~2.7) containing dispersed particles of lead zirconate titanate (PZT) and barium titanate (BaTiO3) (in both cases, έp ~1000). These components were chosen to give a high dielectric contrast within the system and, because polystyrene is readily etched using permanganic reagents, the size and distribution of particles within the matrix could be readily determined by scanning electron microscopy. We report on the effect of composition on the effective permittivity of the composite; the precise composition of each system was determined by thermogravimetric analysis. The results obtained are compared with theoretical models. We show that, in systems with high dielectric contrast between the matrix and the particles, the variation in relative permittivity with filler loading is best described by the Lichtenecker Rother model.

The Effects of Water on the Dielectric Properties of Aluminum-Based Nanocomposites

A series of polyethylene nanocomposites was prepared utilizing aluminum nitride or alumina nanopowders with comparable morphologies. These were subsequently subjected to different conditioning regimes, namely prolonged storage in vacuum, the ambient laboratory environment or in water. The effect of filler loading and conditioning (i.e., water content) on their morphological and dielectric properties was then examined. Measurements indicated that, in the case of aluminum nitride nanocomposites, none of the conditioning regimes led to significant absorption of water and, as such, neither the dielectric properties nor the DC conductivity varied. Conversely, the alumina nanocomposites were prone to the absorption of an appreciable mass of water, which resulted in them displaying a broad dielectric relaxation, which shifted to higher frequencies, and a higher DC electrical conductivity. We ascribe these different effects to the interfacial surface chemistry present in each system and, in particular, the propensity for hydrogen bonding with water molecules diffusing through the host matrix. Technologically, the use of nanocomposites based upon systems such as aluminum nitride, in place of the commonly used metal oxides (alumina, silica, etc.), eliminates variations in dielectric properties due to absorption of environmental water without resorting to the adoption of techniques such as surface functionalization or calcination in an attempt to render nanoparticle surface chemistry hydrophobic.