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Principles Of Fluorescence Spectroscopy

Ferroelectric properties of copolymers of vinylidene fluoride with trifluoroethylene and tetrafluoroethylene are described with special interest in their polarization reversal and phase transition behavior. The ferroelectric phase consists of all-trans molecules packed in a parallel fashion while molecules adopt irregular TT, TG, T[Gbar] conformations in the paraelectric phase. In the ferroelectric phase, polarization reversal occurs at very high fields (> 100 MV/m) as a result of eventual 180° rotations of individual chain molecules around their axes. The switching time ranges from sec to nsec depending upon the strength of the applied field according to an exponential law with a particularly large activation field (∼ 1 GV/m). The value of the observed remnant polarization is consistent with prediction from a simple dipole sum implying a minor contribution from the Coulomb interaction. The ferroelectric-to-paraelectric transition appears most clearly for copolymers containing 50-80 mol% vinylide...

Ferroelectric properties of vinylidene fluoride copolymers

Magnetoresponsive Therapy Nohyun Lee, Dongwon Yoo, Daishun Ling,†,‡,⊥ Mi Hyeon Cho, Taeghwan Hyeon,*,†,‡ and Jinwoo Cheon* †Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, Korea ‡School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Korea Department of Chemistry, Yonsei University, Seoul 120-749, Korea School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, Korea Institute of Pharmaceutics, College of Pharmaceutical Sciences, Zhejiang University, Hangzhou 310058, PR China

Iron Oxide Based Nanoparticles for Multimodal Imaging and Magnetoresponsive Therapy.

HV composite polymeric insulators are being accepted increasingly for use in outdoor installations by the traditionally cautious electric power utilities worldwide. They currently represent -60 to 70% of newly installed HV insulators in Nortb America. The tremendous growth in the applications of non-ceramic composite insulators is due to their advantages over the traditional ceramic and glass insulators. These include light weight, higher mechanical strength to weight ratio, resistance to vandalism, better performance in the presence of heavy pollution in wet conditions, and comparable or better withstand voltage than porcelain or glass insulators. However, because polymeric insulators are relatively new, the expected lifetime and their long-term reliability are not known and therefore are of concern to users. Additionally they might suffer from erosion and tracking in the presence of severe contamination and sustained moisture. This leads to the development of dry band arcing that under certain circumstances could lead to failure of polymer insulators. In this paper a review is presented of the recent performance experience of HV composite polymeric insulators in outdoor service, testing methods, aging, the ranking of the materials, the role of fillers, the role of low molecular weight components present in the insulators, the mechanisms responsible for the loss and recovery of hydrophobicity, one of the most important properties of polymers, the mechanisms of failure, detection of faults, type and quantity of natural contaminants, effects of exposure to rain, hydrocarbons, stationary air and wind, various methods to optimize the electrical performance and a relatively new method for evaluating the performance status of polymeric insulators in the field.

Outdoor HV composite polymeric insulators

This is a laboratory study on the aging of SIR and hydrophobicity recovery. UV radiation, corona discharges, acid rain, dry-band arcing are employed as sources of the aging. Chemical and morphological analysis is used to detect the surface chemical and structural changes derived from these stresses. The nature of highly mobile low molecular weight (LMW) chains achieving a quick recovery of the hydrophobicity is carefully examined. Their generation and extinction caused by these stresses are investigated. From this study, it is shown that oxidation that induces crosslinking, branching, interchanging and a formation of silanol groups are the most dominant chemical reactions during the aging of SIR. Furthermore, silanol groups that are byprodncts of oxidation restrict the diffusion of mobile LMw chains, which decreases the recovery speed of hydrophobicity and accelerates the aging. A typical aging scenario of SIR from the installation to the end of their life is drawn.

Electrical and environmental aging of silicone rubber used in outdoor insulation

Tracking and erosion of high temperature vulcanizing (HTV)-silicone rubber (SIR) and the suppression mechanism of alumina trihydrate (ATH) filler were investigated in the present study. The tracking and erosion resistance of HTV-SIR filled with 0 to 60%wt ATH was evaluated by employing an IEC 587 inclined plane (IP) tracking and erosion test, during which leakage current pulses on HTV-SIR were counted. Surface temperature distributions and the occurrences of thermal spots >400/spl deg/C also were observed by means of an infrared thermovision study. We employed thermogravimetry (TG)-differential thermal analysis (DTA)-mass spectrometer (MS) to observe the thermal degradation of unfilled and filled HTV-SIR in both air and argon. The test results indicate that 40%wt is a critical ATH level and whether tracking and erosion is allowed in the IP tracking and erosion test. Highly filling ATH (>40%wt) reduces the number of low unit silicone oligomer precursors which promote dry-band arcing as well as the presence of residual carbon which leads to carbonization. Chemical modifications of water vapor liberated from heated ATH to methyl groups, which occurs at the thermal decomposition temperature of silicone rubber, were found to result in the above process. The protection mechanisms of ATH for the tracking and erosion of HTV-SIR are formulated herein.

Tracking and erosion of HTV silicone rubber and suppression mechanism of ATH

Leakage current produced on contaminated ceramic and polymeric insulating surfaces was analyzed using a wavelet transform technique providing time-frequency domain information. Salt-fog tests with and without the deposition of non-soluble contamination were conducted in a laboratory, during which the leakage current was separated into sinusoidal, local arc, or their transition components per halfwave of power frequency. The wavelet transform allowed the detection of the zero-cross point of the base current at 50 Hz without the need for voltage information. The source information regarding the distortion level (I/sub 150 Hz//I/sub 50 Hz/) of the current half-wave separated by the zero-cross point and a time lag to onset in current attributable to arcs are used in order to obtain the above three components. The time-integrated values (cumulative charges) of these components were found to correlate with hydrophobicity and contamination level. It was shown that time variations of the cumulative charges and of their component ratios were useful for estimating the conditions of ceramic and polymeric insulating surfaces.

Leakage current characterization for estimating the conditions of ceramic and polymeric insulating surfaces

This review describes the development and status of tracking research in Japan. First, the fundamentals of tracking and the influences of environmental stresses on tracking will be treated. Then, the principal tracking test methods currently in use will be characterized. Finally, some techniques for improving the tracking resistance of outdoor polymer insulating materials will be discussed.

Research in Japan on the tracking phenomenon of electrical insulating materials

Acid rain causes one of the more severe environmental contaminants, and there is an increasing interest in the degradation of outdoor polymer insulating materials in severe pollution conditions such as acid rain. The performance of high temperature vulcanizing (HTV) silicon rubber insulators in accelerated aging of artificial acid rain is investigated by the salt fog method. Experimental results and theoretical analysis show that acid rain exerts a large erosion effect on the silicone rubber insulator. The inorganic filler alumina trihydrate (ATH) in the silicone rubber easily dissolves in acid rain, and the increasing discharge heat from dry band arcing due to acid rain can further cause the loss of ATH. The hydrophobic groups of hydrocarbon branch (ie. methyl in silicone rubber), are also degraded in acid rain. Therefore, the surface of aged insulators becomes rough, they become less hydrophobic, and the surface discharge current increases. However, in a subsequent dry environment, the low molecular weight (LMW) polymer silicone liquid or oil in the bulk of the insulator diffuses and migrates to the surface to provide a hydrophobic layer so that the degraded surface of silicone rubber insulator is compensated, and the contamination performance is restored to some extent. Such recovery of contamination indicates why silicone rubber insulators can be used for long times under severe environmental stress.

Noboru Yoshimura

Papers

Electrical and environmental aging of silicone rubber used in outdoor insulation

Tracking and erosion of HTV silicone rubber and suppression mechanism of ATH

Leakage current characterization for estimating the conditions of ceramic and polymeric insulating surfaces

Research in Japan on the tracking phenomenon of electrical insulating materials

Contamination performances of silicone rubber insulator subjected to acid rain