R.J. Fleming

Electrical conductivity and space charge in LDPE containing TiO/sub 2/ nanoparticles

Electrical conductivity (DC) and space charge accumulation were studied in samples of low density polyethylene to which nano-sized and micro-sized TiO/sub 2/ (anatase) particles and a dispersant had been added. Sample thicknesses were in the range 150-200 /spl mu/m. At applied field strengths of 10 and 20 kV/mm, the conductivity at 30 /spl deg/C, measured in vacuum in samples containing 10 % w/w nano-sized TiO/sub 2/, decreased by 1-2 orders of magnitude relative to samples with dispersant but without TiO/sub 2/, and by three orders of magnitude at 70 /spl deg/C. In air at 30 /spl deg/C the corresponding decrease was an order of magnitude at 10 kV/mm, and a factor of four at 20 kV/mm. In samples containing 10 % w/w micro-sized TiO/sub 2/ the conductivity increased in air and in vacuum, but only by factors in the range 2-10 depending on temperature and field. Space charge profiles were obtained using the laser-intensity-modulation-method (LIIMM), irradiating both surfaces of the sample. The micro-sized TiO/sub 2/ particles are associated with increased charge injection from the electrodes and increased charge trapping in the sample bulk, increasing the conductivity overall. The nano-sized particles generate very little charge in the sample bulk, but render the electrodes partially-blocking and so lower the conductivity.

Space charge profile measurement techniques: recent advances and future directions

Some of the advances in space charge measurement techniques over the last 5 years are described. It is concluded that significant further improvements in spatial resolution are unlikely in the next 5 years, except in the case of the LIMM technique implemented using very short laser pulses, where spatial resolution approaching 100 nm might be achievable. It is suggested that existing space charge techniques be combined with conducting atomic force microscopy (CAFM), with the aim of correlating 3-dimensional space charge profiles and 2-dimensional conductivity maps and thus providing a more complete picture of charge transport through dielectrics, particularly in future work on nanodielectrics. Since CAFM samples are typically 5-10 nm thick, it would be necessary to operate the equipment at higher voltage (say 1000 V) so that samples not less than 10 /spl mu/m thick, e.g. polymers, could be investigated. This seems feasible.

Physical properties of polypyrrole films containing sulfonated metallophthalocyanine anions

Abstract Thin films of polypyrrole (PPy) containing tetrasulfonated metallophthalocyanine anions (MTsPc 4− , MCu, Ni and Co) were prepared electrochemically from aqueous solution in a single-compartment cell. The films were deposited on a platinum anode and had thicknesses in the range 20–100 μm. Their room temperature electrical conductivities were of the order 1–10 S cm −1 and were compatible with a variable range hopping model of interchain charge transport between 77 and 300 K. The conductivities fell by approximately 50% when the films were exposed to laboratory air for one week; this reduction appears to be due to irreversible oxygen attack. ESR measurements suggested intercalation of the MTsPc anions between the PPy chains, while X-ray diffraction data indicated a level of structural order comparable with that of PPy- para -toluenesulfonate (PPyPTS).

Physical properties of polypyrrole films containing trisoxalatometallate anions and prepared from aqueous solution

Abstract Polypyrrole (PPy) thin films containing trisoxalatometallate anions (M(ox) 3 3− , M = Cr and Al; ox = C 2 O 4 2− ) were prepared electrochemically from aqueous solution in a single-compartment cell. Thicknesses were in the range 30–70 μm. They were characterized by elemental analysis and diffuse specular reflectance FT-IR spectroscopy. Their structural and morphological features were probed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Room-temperature electrical conductivities (11–50 S cm −1 ) were comparable to those of the well-studied PPy– p -toluenesulfonate system. Some limited studies of the effect of oxygen and nitrogen on the variation of conductivity with time were made. Some of the films displayed excellent conductivity stability over a period of 50 days, comparable to the stability found in other commonly reported polymers.

Physical properties of polypyrrole films containing dicyanoaurate(I) anions, PPy–Au(CN)2

Polypyrrole dicyanoaurate, Ppy–Au(CN)2, has been synthesized in thin-film form by electrochemical oxidation of pyrrole in aqueous or non-aqueous media. The film samples have been characterized by elemental analysis, IR spectroscopy and X-ray photoelectron spectroscopy (XPS). The room-temperature conductivity of one of the films (303 S cm–1) is one of the highest reported to date for any Ppy–anion system. X-Ray diffraction studies show that the preferred orientation of the PPy chains in Ppy–Au(CN)2 is parallel to the anode surface. The films show excellent stability in air at room temperature, their conductivity decreasing by less than 2% over 50 days. The electrochemical properties of PPy–Au(CN)2-coated platinum electrodes have been probed in detail using cyclic voltammetric (CV) methods.

LIMM study of space charge in crosslinked polyethylene

Electric field/space charge profiles obtained using the laser-intensity-modulation-method (LIMM) are presented for XLPE samples machined from power cable insulation cross-linked using dicumyl peroxide. The samples, 110 mum thick with vacuum-evaporated aluminium electrodes, were subjected to DC fields of 48 kV/mm and both polarities at room temperature for at least 24 hr. The LIMM measurements were made at room temperature, and at various times after the end of poling. Electric field and space charge profiles obtained using the scale-transformation and polynomial regularization methods were in fair quantitative agreement over much of the sample thickness, except when the electric field showed a maximum or minimum. Simulation data for assumed electric field profiles strongly suggested that the polynomial regularization method is more accurate. Most of the space charge was concentrated near the electrodes, with densities of order 5-10 Cm-3 in those regions. Charge of the same sign tended to accumulate adjacent to a given physical electrode, for both poling polarities

Activation energies and electron transport in LDPE

Space charge measurements were made on low density polyethylene samples to which dc fields had been applied. Some measurements were performed under isothermal conditions, and others with a temperature gradient established across the sample thickness. Isochronal current density. was also measured as a function of temperature and field. The activation energy associated with the hopping transport of electrons through the sample volume was around 0.09 eV, whereas the activation energies deduced from the temperature dependence of the isochronal current density were in the range 0.60-1.40 eV, depending on the applied field strength and the impurity content of the samples. It was concluded that the isochronal current densities were controlled by electron injection from the cathode, the most likely injection mechanism being tunnelling between the Fermi level of the metallic electrode and acceptor sites within the dielectric close to the interface.

Physical properties of polypyrrole films containing tetracyanonickelate(II) anions, PPy–Ni(CN)4

Polypyrrole–tetracyanonickelate, PPy–Ni(CN)4, has been synthesized in thin-film form by electrochemical oxidation of pyrrole in aqueous or non-aqueous media. The room-temperature electrical conductivity of one of the films (122 S cm–1) is the highest reported to date for any PPy–metal anion system. The temperature dependence of the conductivity is consistent with the Mott variable-range hopping model. Some of the films show good stability in air at room temperature, their conductivity decreasing by less than 7% over 50 days.

A physical and spectroscopic study of polypyrrole films containing tetrasulfonated metallophthalocyanine and transition metal EDTA chelates

Summary form only given. The effect of the incorporated counteranion on the structure and properties of polypyrrole is of fundamental interest. In the present study, polypyrrole films containing tetrasulfonated metallophthalocyanine (MPcTs, M = Co and Fe) and transition metal EDTA (MEDTA, M= Co, Cr, and Fe) counteranions are examined by a variety of techniques including elemental analysis, conductivity, X-ray diffraction, SEM, TEM, EPR and Mossbauer spectroscopy. The conductivity stability of PPyCrEDTA and PPyFeEDTA is superior to that of both PPyN4PcTs and the well known p - toluenesulfonate system (PPyPTS). TEM studies of PPyCoEDTA reveal some crystallinity. By taking advantage of the presence of paramagnetic and Mossbauer active metal ions, the environment of the counteranion within PPyMPcTs and PPyMEDTA is examined by means of EPR and Mossbauer spectroscopy. The former technique reveals a strong magnetic interaction between the unpaired electrons of the polymer and the paramagnetic metal ion centres, and the origin of this interaction is discussed. Structural models for PPyMPcTs and PPyMEDTA are presented.

Charge injection into cross linked polyethylene studied by thermally stimulated depolarization

The authors present some initial results of a thermally stimulated depolarization (TSD) study of charge injection into three different types of cross-linked polyethylene (XLPE), DC-polarized at room temperature and heated to 90 degrees C. The three types of XLPE samples were: (1) type A, containing dicumyl peroxide (DCP) with a PE:DCP weight ratio of 100:2; type B, which is cross-linked using silane, the gel content and thickness being 52.7 wt.% and 50 mu m, respectively; and type C, which is cross-linked by electron beam irradiation at an accelerating voltage of 150 kV and which has a gel content and thickness of 55 wt.% and 50 mu m, respectively. Type A material seems to possess a much greater concentration of electron traps emptying between 20 and 90 degrees C than either type B or type C material.<<ETX>>