Nanostructured materials are attracting increased interest and applications. Exciting perspectives may be offered by electrical insulation. Polymeric nanofilled materials may find new and/or upgraded applications in the electrical and electronic industry, replacing conventional insulation to provide improved performances in electrical apparatus, as regards, e.g., reliability, environmental compatibility and power rating. This paper shows that electrical properties of nanocomposite insulating materials for DC applications, specifically space charge, conductivity and breakdown voltage, can improve significantly with respect to the basis, unfilled materials. Reference is made to two polymeric materials, i.e. poly(ethylene-covinylacetate) (EVA) and polypropylene (PP), that are widely used as electrical insulation, e.g. for cables and capacitors. The nanofiller consists of an organophilic layered silicate, specifically an extra-pure synthetic fluorohectorite modified by means of interlayer exchange of sodium cations for protonated octadecylamine NH/sub 3//sup +/ (ODA), in a weight concentration of maximum 6%. In both materials the space charge accumulation rate as a function of applied electric field is significantly reduced, while the electrical conductivity is raised. The breakdown voltage can increase for the nanofilled materials.

Modification of electrical properties and performance of EVA and PP insulation through nanostructure by organophilic silicates

The goal of this report is to provide a comprehensive guide for researchers in the area of measurements of lossy dielectric and magnetic materials with the intent of assembhng the relevant information needed to perform and interpret dielectric measurements on lossy materials in one place. The report should aid in the selection of the most relevant methods for particular applications. We emphasize the metrology aspects of the measurement of lossy dielectrics and the associated uncertainty analysis. We present measurement methods, algorithms, and procedures for measuring both commonly used dielectric and magnetic materials, and in addition we present the fundamentals for measuring negative-index materials.

Measuring the Permittivity and Permeability of Lossy Materials: Solids, Liquids, Metals, Building Materials, and Negative-Index Materials

Effect of carbon nanofibres dispersion on the microwave absorbing properties of CNF/epoxy composites

The absorption and conduction currents in synthetic insulating polymers, viz., polyethylene, polypropylene, polyethylene-terephthalate and polyvinylidenefluoride, with dc fields may be electronic or ionic both and it may be difficult to determine their origins unambiguously. The charges which are present in the bulk of the polymer may be responsible for both the absorption and the conduction current. A limited displacement of charges together with the rotation of side chains may provide a decaying transient current in the presence of an externally impressed field whereas the conduction current may originate from a hopping of charge carriers in which the charges transfer between localized states. The complex dielectric response of these polymers with ac field may also be explained by intra- and inter cluster charge movement which would be consistent with the unified model, stated above for the case of dc fields.

Conduction mechanisms and high-field effects in synthetic insulating polymers

Understanding of very high field phenomena in polymeric dielectric is reviewed, based on the concepts of field-dependent conductivity and space charge limited field, which leads to the conclusion that the electric field distribution can be predicted with reasonable accuracy under high field conditions. Other phenomena, such as photon emission, cannot be predicted reliably, probably because the concepts underlying the entire theory, that of "traps" or "impurity levels" within the band gap, are not understood, i.e., while the theory is written in terms of such concepts, no convincing physical basis for these phenomena has been provided. This issue is discussed and some suggestions possible directions for future research are suggested.

Very high field phenomena in dielectrics

The electrical conduction of various kinds of low-density polyethylene (LDPE) has been studied above the melting point. LDPEs are characterized by the amount and types of branches, double bonds, and oxygen-containing groups. Two components of conduction currents were found: one obeyed Ohm's law in the low field range and the other was proportional to the square of the field at lower temperatures in the high field range, and was also inversely proportional to sample thickness for a constant field. The conduction mechanism of the latter component is ascribed to space charge limited current (SCLC). Among the features of the LDPE molecular structure, only the oxygen-containing groups were well correlated with the SCLC. In fact, the oxygen-containing groups reduced the SCLC, suggesting that they act as traps even in the molten state. Branches and double bonds are also known to act as traps in the solid state, but they bore no consistent relation to the electrical conduction in the molten state. >

Effects of molecular structure on electrical conduction in low-density polyethylene above its melting point

Degradation of dielectric properties polyethylene under the combined environment of ?-irradiation and thermal aging has been studied. To exclude the effect of additive agents and to minimize the effect of the process limited by diffusion of oxygen on the degradation, a 25 ?m thick low-density polyethylene ene film free from additives has been used. Dielecc re relaxation is used as a tool to study the oxidative degradation. It is found that the primary dispersion of polyethylene shifts to higher temperatures with oxidation. There is some difference nce in temperature of primary dispersion according to the order of application of thermal aging and ?-irradiation. The reason for it is explained in terms of chain scission, cross linking and dipole-dipole interaction.

Degradation of Dielectric Properties of Polyethylene by Combined Y-Irradiation and Thermal Stresses

The hybrids prepared from polydimethylsiloxane (PDMS) and tetraethoxysilane (TEOS) have been well known to be rubbery hybrid materials. In this report, the authors have investigated the electrical insulating and heat resistive properties of the hybrids by changing the molar ratio of TEOS to PDMS. The electrical insulating properties are evaluated by volume resistivity and AC breakdown strength at room temperature. The resistivity is about 1013 Ω · m and AC breakdown strength is about 25 kV/mm irrespective of the molar ratio. Heat-resistive properties are evaluated by the weight loss after keeping for 480 h at 200°C in air. The weight loss becomes smaller as the TEOS content increases, and it is less than 3%. It has been clarified that the hybrids have good electrical insulating and heat-resistive properties.

Electrical Insulating and Heat-Resistive Properties of PDMS-TEOS Hybrid with Different Molar Ratio of TEOS to PDMS

In this article, the changes of capacitance C and dielectric dissipation factor tan /spl delta/ of a non-penetrated water-treed XLPE sheet sample with voltage stress are discussed based upon the channel dynamism model. It has been found that the increase in tan /spl delta/ with high voltage stress is caused by the growth of the conductive water-filled channel layer. The gradual decrease in tan /spl delta/ with time of exposure to voltage stress is explained by a shift of relaxation frequency to higher range caused by the change in the conductivity of the water-filled channel layer.

Changes of capacitance and dielectric dissipation factor of water-treed XLPE with voltage

Thermal conductive composites made with several types of filler have been studied based on the electrical and mechanical properties. It is clarified that the thermal conductivity of the composite made with a boron nitride increases by further introducing carbon black into the matrix resin.

S. Nakamura

Papers

Effects of molecular structure on electrical conduction in low-density polyethylene above its melting point

Degradation of Dielectric Properties of Polyethylene by Combined Y-Irradiation and Thermal Stresses

Electrical Insulating and Heat-Resistive Properties of PDMS-TEOS Hybrid with Different Molar Ratio of TEOS to PDMS

Changes of capacitance and dielectric dissipation factor of water-treed XLPE with voltage

Properties of high-thermal conductive composite with two kinds of fillers