Constantine T. Dervos

Vacuum heat treatment of electroless Ni-B coatings

Abstract Electroless nickel–boron plating with subsequent rapid heat treatment in vacuum has been tried on steel substrate in an effort to obtain hard engineering coatings. By selected conditions of heat treatment in a high vacuum environment (a type of rapid quenching), the plating can acquire chromium equivalent hardness without the effluents of the hard chromium plating process. The resulting surfaces were examined and characterized under an optical and a scanning electron microscope. XRD analysis was also performed to investigate re-crystallization effects. The fabricated materials were also tested under corrosion conditions and linear polarization measurements were performed. The results show that after a 5-min heat treatment in vacuum, the plating acquires a crystalline structure with very good adhesion to the substrate material, obtained so far by several weeks/months thermal treatment.

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.

Thermal stability of SF6 associated with metallic conductors incorporated in gas insulated switchgear power substations

SF6 is generally treated as thermally stable and inert for applications below 500 °C. This work investigates the thermal stability of pure SF6 gas under 1.2 atm pressure between 200 and 450 °C in the presence of construction metals (Cu, Al), without any applied electric field. The obtained experimental results indicate that SF6 may react with metallic surfaces forming solid and gaseous by-products, either in the gas matrix or diffused in the metallic surfaces. The phenomenon is enhanced in the presence of adsorbed moisture. For copper surfaces, sulfide layers are formed. By-products are not formed for pure Al surfaces. However, when Al is covered by a few micrometres thick Al2O3 film, hot SF6 molecules have a structure change effect, i.e. reduce porosity in the oxide and in the substrate, provide smooth transition layers Al/Al2O3 and increase the Al2O3 layer width. In the presence of moisture this phenomenon is significantly intensified and a diffused overlayer of AlF3 also forms. The by-products in the gas matrix are mainly sulfur oxides for hot spot temperatures below 300 °C, while at higher temperatures oxyfluorides SO2Fx and HF are mainly formed. These by-products are either toxic or corrosive. Thus, the thermal stability issue of SF6 may have to be reconsidered.

Dielectric properties of nanopowder emulsions in paraffin oil

This work investigates 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 tested at concentrations up to 5% w/v in the liquid matrix. Emulsification was attained by ultrasound treatment. The relative dielectric constant and loss tangent (tand) of both pure oil and emulsions 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. Copper (I) and (II) oxides exhibit a distinct relaxation mechanism at the high end of the frequency range.

Corrosion effects on the electrical performance of silver metal contacts

In this work, commercial silver metal contacts welded on top of silver plated brass or brass substrates have been exposed to air rich in NaCl. Scanning electron microscopy and energy dispersive analysis of the exposed contact surfaces were performed to identify the corrosion by‐products on top of the silver contacts, suspending wafers, and welding materials. Surface corrosion products were mainly found to consist of small spherules of Cu‐Zn or Ag‐Cu compounds which cover the surface of the contact proper with low adhesion properties. They mainly originate from the underplating wafer or welding materials. Electrical characterization of the contacting materials was based on dc temperature overheat tests, current switching cycle tests, and energy storage during ac current excitation. The experimental results display that the operating environment is indeed a very significant parameter determining the overall performance of the electrical contacts. New design rules as well as material selection properties may have to be systematically considered to allow for electrochemical induced degradation in saline operating environments.

The effect of accelerated UV-ageing on the dielectric properties of PVC, PTFE and HDPE

In this work the dielectric response of three commonly used polymeric insulating materials, namely polyvinyl chloride (PVC), polytetrafluoroethylene (PTFE) and high-density polyethylene (HDPE), was measured after applying combined action of UV-radiation and condensation (humidity and heat) in an Accelerated Weathering Tester apparatus. Test samples were planar with a 2 mm thickness. For both aged and non-aged samples, the dielectric properties (relative dielectric constant κ and dissipation factor tanδ) were recorded in a frequency range 20 Hz–1 GHz. The induced changes on complex permittivity after applying UV/condensation cycles were described. The accelerated ageing led to surface color differentiations for PVC and HDPE. Finally, Fourier Transform Infrared (FTIR) spectroscopy measurements on the sample surfaces investigated the chemical stability of the tested materials.

SF6 Handling and Maintenance Processes Offered by Quadrupole Mass Spectrometry

High Voltage (HV) insulation relying on the use of sulfur hexafluoride SF6 has passed the required qualification tests to become the most commonly used insulating gas in electrical systems to date1. Gas Insulated Switchgear (GIS) power substations have all of their components interconnected and insulated via compressed SF6, i.e. circuit breakers, dissconnectors, power switches, bushbars, current transformers, power transformers, cable insulation etc2, 3. Due to their compactness and steel shielding, they offer significant savings in land use, are aesthetically acceptable, have relatively low radio and audible noise emission levels, and enable substation installation in cities very close to the loads.

The Effect of Al2O3 and Li2O on the Anatase to Rutile Phase Transformation

The utilization of Al2O3 and Li2O as dopants that promote the anatase-to-rutile (A-R) phase transition in TiO2 nanoparticles during calcinations is studied. X-Ray Diffraction and SEM techniques were employed for the evaluation of phase transformation and particle size coarsening in pure TiO2, TiO2-Al2O3 and TiO2-Li2O mixtures. For the Li-Ti-O pseudobinary systems some complex oxides may be formed during phase transformation that occurs at significantly lower temperatures compared to pure TiO2 or TiO2-Al2O3 mixtures. Al2O3 doping in TiO2 only increases the anatase-to-rutile transition rate once the phase transformation has been initiated.

Byproducts in the Insulating Gaseous Matrix of a Gis

Sulfur hexaflouride (SF6) is a man-made gas, offering excellent electrical, thermal, and acoustical insulation properties, and as such, it has been extensively employed for a variety of applications, i.e. power distribution (mainly), semiconductor foundries, metallurgy, tracer gas, sound sealing (double glazing, torpedo noise silencer), thermal sealing, diving equipment, aeronautical, wind tunnels, telecommunications, relays, antennas, medical (ultrasounds, ophthalmology) etc1 . However, SF6 has been identified to be one of the strongest greenhouse gases, having a global warming potential 25000 greater, compared to that of CO2, and an exceptionally long lifetime (estimated between 1800 and 3200 years). SF6 is primarily employed worldwide (approximately 80% of its total production) as a gaseous insulator for high-voltage systems (GIS) including switchgear, gas insulated transmission lines, and power transformers. The current trend is to reduce SF6 quantities within electrical equipment by mixing it with natural gases. As a result, the interest for possible SF6/gas mixtures for power applications has been reignited. Most of the research work is now focused on SF6/N2 gas mixtures2, as it appears to be suitable for industrial non uniform electric field applications, combining environmental compatibility with comparable dielectric performance of pressurized SF6 and overall cost effectiveness. Other possible mixtures of SF6 that have been recently proposed are the SF6/CO2/N2 instead of the SF6/CO2 mixtures for transformer applications, and also, SF6 with dielectric mixtures of CHF3 and CF4 diluted at 50% to 75% with N2 for possible applications at lower temperature environments, thus combining environmental compatibility with reduced gas decomposition rates3.

CORROSION OF SILVER ELECTRICAL CONTACTS: THE SUBSTRATE INFLUENCE

Silver and silver based alloy (90% Ag10% CdO) contacts welded on different substrate materials have been exposed in hostile S 0 2 environments in simulated laboratory conditions. The corrosion byproducts on the surface have been monitored by SEM and EDS spectroscopy. Their electrical characterization was based upon dc overheat tests, current switching cycle tests and interfacial energy storage during ac current excitation. The experimental results demonstrate that the heat dissipation rates strongly depend on the substrate material selection as well as, the exposure environments. These parameters strongly affect the nominal current values. The dynamic contact resistance during make-break electrical contact operations depends upon the conductivity of the new surface layers formed as a result of chemical equilibrium established by the reaction between the environment and the primary surface material, the substrate, and the welding layers. Corrosion byproducts are formed over the contact joint and are transported to the silver surface of the electrical contact by creep. The resulting byproducts, may be either loosely attached or water-soluble. Corrosion products originate from the welding zone, the substrate material, and the underplating layer by diffusing through the pores of the plating; they aggregate on the surface and affect the heat dissipation rate and consequently the electrical performance of the component.