Constantine T. Dervos

Bio: Constantine T. Dervos is an academic researcher from National Technical University of Athens. The author has contributed to research in topics: Electrical contacts & Dielectric. The author has an hindex of 14, co-authored 62 publications receiving 825 citations. Previous affiliations of Constantine T. Dervos include National and Kapodistrian University of Athens.

Sulfur hexafluoride (SF6): global environmental effects and toxic byproduct formation.

Abstract: This work provides information concerning possible global environmental implications and personnel safety aspects that should be considered during the commercial uses of sulfur hexafluoride (SF6). SF6 is an anthropogenically produced compound, mainly used as a gaseous dielectric in gas insulated switchgear power installations. It is a potent greenhouse gas with a high global warming potential, and its concentration in the earth atmosphere is rapidly increasing. During its working cycle, SF6 decomposes under electrical stress, forming toxic byproducts that are a health threat for working personnel in the event of exposure. Several precautions are recommended to avoid personnel exposure to toxic byproducts: oxyfluoride levels or other byproduct concentrations in the operating gas matrix should be traced to predetermine the overall gas toxicity; contaminants should be systematically considered during maintenance, chamber evacuation and system opening process; small SF6 quantities leaking into air or stagnated pollutant concentrations in the operating field should be analyzed and compared to the threshold limit values and permissible exposure levels. New system design rules (i.e., hermetically sealed gas compartments, gas recycling or disposal in the field area) and different handling policies--both during maintenance and final disposal--now should be considered globally to provide for environmental and personnel safety.

Dielectric properties of nanopowder dispersions in paraffin oil

Abstract: This work contributes to the study of nanofluids, by investigating polarization phenomena induced by nanoparticle inclusions in paraffin oil, an insulating organic liquid. Fine metal oxide powders and nanopowders of Al2O3, TiO2, CuO, Cu2O and Fe2O3 were dispersed at concentrations up to 5% w/v in the liquid matrix, using ultrasonic treatment. The relative dielectric constant and loss tangent (tanδ) of both pure oil and the prepared nanofluids were recorded in the 20 Hz-1 MHz frequency range. Results depend both on grain size and on the specific compound. In the case of alumina nanoparticles, dielectric behavior is dominated by grain surface polarization phenomena induced by adsorbed water. This effect can be partially cancelled out by the addition of titania nanoparticles. Titanium and (to a lesser extent) ferric oxide increased the dielectric constant at middle and high frequencies. Cupric and cuprous oxides exhibit a distinct relaxation mechanism at the high end of the examined frequency range.

Dielectric characterization of power transformer oils as a diagnostic life prediction method

Abstract: The monitoring of the complex permittivity of transformer oils as a function of frequency and temperature provides information concerning the state of the insulation. In this work, it has been investigated as a quality control method, providing service life estimations of power transformations. The formation of databases recording polarization effects in a wide frequency and temperature range may be used effectively as a working tool for service engineers. Reliable insulation monitoring reduces accidents in aged HV transformers, some of which have been in operation since the 1960s. These transformers always impose an environmental threat because of possible accidental leaks or fires, especially when containing traces of PCBs.

The Environmental Protection Agency

Abstract: The Environmental Protection Agency (EPA) provides access to information on a variety of topics related to the environment and strives to inform citizens of health risks. The EPA also has an extensive library network that consists of 26 libraries throughout the United States, which provide access to a plethora of information to EPA employees, scientists, and researchers. The EPA implemented a reorganization project to digitize their materials so they would be more accessible to a wider range of users, but this plan was drastically accelerated when the EPA was threatened with a budget cut. It chose to close and reduce the hours and services of some of their libraries. As a result, the agency was accused of denying users the “right to know” by making information unavailable, not providing an adequate strategic plan, and discarding vital materials. This case study explores the background of the digitization project, the practices implemented, and the critiques of the project.

Densification mechanisms in spark plasma sintering of nanocrystalline ceramics

Abstract: Effect of the particle size on the possible electric discharge during the SPS was examined. Nanoparticle compacts enable accumulation of high electric charge, and discharge under conventional voltages used for the SPS. The critical particle size for the electric discharge is both morphological and material dependent. The early stages of densification of the nanocrystalline powder compact proceed either by the plastic deformation or grain-rotation coalescence and sliding, aided by softening of the particle surfaces. The active densification mechanism depends on the changes both in the mechanical and electrical properties with temperature. Densification of 11 nm nc-MgO particles with low yield stress proceeds by plastic deformation already at 700 °C. However, densification of 34 nm nc-YAG particles with high yield stress proceeds by nano-grain rotation aided by particle surface softening. Densification at the final stages of SPS is associated with diffusional processes, where curvature driven grain growth predominates.