Spark decomposition of SF6: chemical and biologicalstudies

Because electric arcs, sparks or corona can decompose SF₆ insulators into byproducts having chemical properties different from SF₆, environmental concerns arise regarding inadvertent human exposures to electrically decomposed SF₆. Biological assays using mammalian cell culture systems have revealed that SF₆ , spark-decomposed under specific experimental conditions, can produce cell death. Chemical analysis of spark-decomposed SF₆ has identified the major decomposition pathways and byproducts. Biological testing of individual byproduct mixtures has indicated that these major decomposition products may not account for the majority of the cell-killing effects seen in the assays. Further experiments have suggested that S₂F₁₀ may be produced and accumulate under the specific decomposition conditions and that this compound may be a major contributor to the observed cell lethality. It is concluded that testing of samples from commercial facilities and assays of decomposed gas after ameliorative treatments would both be appropriate investigations     

Minor groove-directed and intercalative ligand-DNA interactions in the poisoning of human DNA topoisomerase I by protoberberine analogs

Spectroscopic, calorimetric, DNA cleavage, electrophoretic, and computer modeling techniques have been employed to characterize the DNA binding and topoisomerase poisoning properties of three protoberberine analogs, 8-desmethylcoralyne (DMC), 5,6-dihydro-8-desmethylcoralyne (DHDMC), and palmatine, which differ in the chemical structures of their B- and/or D-rings. DNA topoisomerase-mediated cleavage assays revealed that these compounds were unable to poison mammalian type II topoisomerase. By contrast, the three protoberberine analogs poisoned human topoisomerase I according to the following hierarchy: DHDMC > DMC > palmatine. DNA binding by all three protoberberine analogs induced negative flow linear dichroism signals as well as unwinding of the host duplex. These two observations are consistent with an intercalative mode of protoberberine binding to duplex DNA. However, a comparison of the DNA binding properties for DMC and DHDMC, which differ only by the state of saturation at the 5,6 positions of the B-ring, revealed that the protoberberine analogs do not "behave" like classic DNA intercalators. Specifically, saturation of the 5-6 double bond in the B-ring of DMC, thereby converting it to the DHDMC molecule, was associated with enhanced DNA unwinding as well as a reversal of DNA binding preference from a DNA duplex with an inaccessible or occluded minor groove {poly[d(G-C)]2} to DNA duplexes with accessible or unobstructed minor grooves {poly[d(A-T)]2 and poly[d(I-C)]2}. In addition, a comparison of the DNA binding properties for DHDMC and palmatine revealed that transferring the 11-methoxy moiety on the D-ring of DHDMC to the 9 position, thereby converting it to palmatine, was associated with a reduction in binding affinity for both duplexes with unobstructed minor grooves as well as for duplexes with occluded minor grooves. These DNA binding properties are consistent with a "mixed-mode" DNA binding model for protoberberines in which a portion of the ligand molecule intercalates into the double helix, while the nonintercalated portion of the ligand molecule protrudes into the minor groove of the host duplex, where it is thereby available for interactions with atoms lining the floor and/or walls of the minor groove. Furthermore, saturation at the 5,6 positions of the B-ring, which causes the A-ring to be tilted relative to the plane formed by rings C and D, appears to stabilize the interaction between the host duplex and the minor groove-directed portion of the protoberberine ligand. Computer modeling studies on the DHDMC-poly[d(A-T)]2 complex suggest that this interaction may involve van der Waals contacts between the ligand A-ring and backbone sugar atoms lining the minor groove of the host duplex. The hierarchy of topoisomerase I poisoning noted above suggests that this minor groove-directed interaction may play an important role in topoisomerase I poisoning by protoberberine analogs. In the aggregate, our results presented here, coupled with the recent demonstration of topoisomerase I poisoning by minor groove-binding terbenzimidazoles [Sun, Q., Gatto, B., Yu, C., Liu, A. , Liu, L. F., & LaVoie, E. J. (1995) J. Med. Chem. 38, 3638-3644], suggest that minor groove-directed ligand-DNA interactions may be of general importance in the poisoning of topoisomerase I.     

High voltage studies of dielectric materials for HTS power equipment

The discovery of high temperature superconductors (HTS) has triggered renewed interest in the study of dielectric materials at cryogenic temperatures. While considerable work was done in the 1970s and 1980s on dielectrics immersed in liquid helium for low temperature superconducting applications, there remains a need for dielectric research at liquid nitrogen temperature for HTS applications, requiring experimental data oriented toward practical situations. We report on AC breakdown (puncture and/or flashover), and impulse breakdown of solid materials in either vacuum or in liquid nitrogen. Solid materials which we examined, include fiberglass reinforced plastics, epoxies with and without filler, and polymeric tape. Combinations of some of these materials have also been studied at low temperatures. Additionally we have measured permittivity and dissipation factor for materials for which these parameters are not available at 77 K. Finally, we also discuss specific applications for HTS cables including breakdown and aging studies on model cables, with lapped tape electrical insulation, immersed in liquid nitrogen.     

Effect of polymer–nanoparticle interactions on the glass transition dynamics and the conductivity mechanism in polyurethane titanium dioxide nanocomposites

We report on the glass transition dynamics and the conductivity properties of a nanodielectric system composed of pre-synthesized TiO₂ nanoparticles embedded in thermoplastic polyurethane. Increase of TiO₂ loading results in enhanced segmental mobility of the composites and less steep temperature dependence, i.e., lower fragility index. The decrease in the fragility index and glass transition temperature is discussed based on the FTIR results. We observe different behavior of conductivity for temperatures above and below the glass transition temperature. At high temperatures the composites exhibit conductivity values more than 2 orders of magnitude higher than those in the pristine matrix. At the same time, at sub-T_g temperatures composites are characterized by superior electrical insulation properties compared to pristine matrix material. Such drastic temperature dependence of the conductivity/insulating ability of the flexible and light-weight, low-T_g composite material can be utilized in various applications including sensing and temperature switching materials.     

Physical properties of epoxy resin/titanium dioxide nanocomposites

A polymeric nanocomposite system (nanodielectric) was fabricated, and its mechanical properties were determined. The fabricated nanocomposite was composed of low concentrations of monodispersed titanium dioxide (TiO₂) nanoparticles and an epoxy resin specially designed for cryogenic applications. The monodispersed TiO₂ nanoparticles were synthesized in an aqueous solution of titanium chloride and polyethylene glycol and subsequently dispersed in a commercial‐grade epoxy resin (Araldite® 5808). Nanocomposite thin sheets were prepared at several weight fractions of TiO₂. The morphology of the composites, determined by transmission electron microscopy, showed that the nanoparticles aggregated to form particle clusters. The influence of thermal processing and the effect of filler dispersion on the structure–property relationships were identified by differential scanning calorimetry and dynamic mechanical analysis at a broad range of temperatures. The effect of the aggregates on the electrical insulation properties was determined by dielectric breakdown measurements. The optical properties of the nanocomposites and their potential use as filters in the ultraviolet–visible (UV–vis) range were determined by UV–vis spectroscopy. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers     

Investigation of S2F10 production andmitigation in compressed SF6-insulated power systems

The Cooperative Research and Development Agreement (CRADA), established to study the production and mitigation of S₂F₁₀ (disulfur decafluoride), one of a number of toxic by-products formed in electric discharges in the insulating gas SF ₆, is described. The particular concern about S₂F ₁₀ is due to its highly toxic nature, the ceiling limit value being 10 parts per billion (ppb, or 1 part in 10⁸), and the need for development of sensitive detection techniques down to this level. In the presence of an electrical discharge such as an arc, spark, or corona, a portion of the SF₆ decomposes into lower fluorides of sulphur which can react to form a number of chemically active by-products including SOF₂ and SO₂F₂ . During the maintenance or repair of SF₆-insulated equipment, the handling of these gaseous is a matter of concern. Preliminary arc experiment results, reported health-related incidents caused by SF₆ by-products, and ongoing studies are discussed