C.T. Dervos

Dielectric properties of nanopowder dispersions in paraffin oil

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.

Physical interpretations concerning nonlinear conductivity phenomena across no-load switching contacts

An experimental survey concerning physical interpretations of the dominant phenomena which may develop during the operation of no-load switching contacts in laboratory environments is presented. Measurements are performed on industrial switches with the following nominal values: isolation switches 400 V/100 A, isolation switches 20 kV/200 A, and fuse isolators 20 kV/ 100 A. It is shown that beyond a threshold field intensity (corresponding to smaller currents than the nominal values) a nonlinear current-voltage relationship may develop, leading to bistability and negative differential conductance phenomena. This has been attributed to the dominating tunnelling current component between oxidized (or nonideally contacted) metallic surfaces at high electric fields, resulting in an increased effective area of contact.<<ETX>>

Bifurcation evolution across metal-metal contacts sustaining high charge injection rates

In real world even the simplest of the electrical components may exhibit unpredictable characteristics, and it seems that the theory of chaos touches all disciplines. In this work, the contact potential instabilities induced by the high electronic injection rates between metal-to-metal contacts are experimentally investigated in an I-V phase space using state-of-the-art data logging systems. The mechanically contacted metals are energized by a sinusoidal power source, and their response is systematically studied within a 50-Hz cycle. The obtained results convincingly demonstrate their chaotic nature. Two entirely different instability types have been observed for such systems. Their classification may lead to a better understanding of negative differential resistance (NDR) formation, which is frequently observed across highly injecting interfaces. An equivalent circuit for the examined dynamic system is provided, and an effective test for the industrial no-load switching contacts is proposed. >

Effect of water on permittivity of nanodielectrics exposed to the atmosphere

Relative dielectric constant and dissipation factor (tan¿) values as a function of frequency have been measured for a variety of water types. The effect of water absorption on the permittivity properties of crystalline powders or nanopowders when exposed to various humidity levels is also examined. Powdered materials were characterized by XRD and were consisting of calcite (CaCO3), anatase, rutile (TiO2), ¿- and ¿-alumina (Al2O3). The induced polarization changes in the materials by the applied dehydration process, i.e. either high vacuum of 10-4 Pa (10-6 mbar) drying, or calcination in air at temperatures of up to 600°C, were investigated. The advantage offered by vacuum dehydration is that nano-dielectrics maintain their particle size and crystal integrity.

Temperature dependent dielectric spectroscopy in frequency domain of high-voltage transformer oils compared to physicochemical results

The temperature dependent dielectric spectroscopy in frequency domain is compared to physicochemical and electrical tests that are periodically applied on highly insulating transformer oils. Complex permittivity and tandelta data are obtained as a function of frequency and operation temperatures, to demonstrate the polarization phenomena and the induced loss intensification by the thermally stimulated currents. The acquired dielectric data of oil samples collected from 201 operating high voltage (HV) power transformers, were correlated to their corresponding physicochemical ones in order to demonstrate the future potential employment of the proposed measuring technique in the field of high voltage engineering, towards reliability monitoring of oil filled electrical equipment (OFEE)

The effect of contact capacitance on current-voltage characteristics of stationary metal contacts

This paper investigates experimentally the significance of the effective contact capacitance, i.e. the interfacial capacitance during the current flow, for a wide range of stationary metal contacts operating under high charge injection rates. The effective capacitance of metallic interfaces depends on the ratio between the apparent contact area (which is optically determined) and the effective contact area (which injects the electronic charges). Silver contacts having series resistance values significantly less than the contact resistance were subjected to a.c. high current densities (up to 500 A/mm/sup 2/). The obtained i(t) and v(t) profiles were further analyzed to obtain I-V curves. Due to the phase shift between i(t) & v(t) profiles the I-V curve within a single period of the stimulating current will produce a closed loop. The area of the loop determines the interfacial electrical energy. According to the obtained results the electrical energy storage at a given metal contact, increases at: (a) higher ampacity values, (b) lower operating temperatures and (c) higher clamping forces between the joints (elastic deformation regime) each of the above parameters acting independently. The experimental results were obtained for AgSnO2 and OFHC contacts operated in a wide temperature range, varying between -130/spl deg/C and +40/spl deg/C. The observed response of the electrical contacts is mainly characterized by the implications of the asperity contact model and dominating charge transport processes across the metallic interfaces. When standard simple equivalent circuits are used to determine contact impedance, the effective capacitance of current carrying metal contacts acquires exceptionally high values.

Nickel matrix composite electrocoatings as electrical contacts

The work presented in this paper explores the possibility of using composite nickel matrix electrodeposits prepared under direct current (DC) and pulse current (PC) conditions as electrical contact materials. The codeposition of liquid containing microcapsules (/spl mu/caps) along with the metal matrix from electrolytic baths leads to the development of new type of materials with prespecified properties: the liquid-containing metal electrocoatings. The incorporation, in the nickel matrix, of polymeric wall /spl mu/caps containing oil ensures a self-lubrication and cooling action during fretting or transient operation. The electrical properties of composite nickel electrodeposits have been investigated. Different preparation conditions have been employed. Their contact resistance and interfacial temperature have been monitored for a great number of operation cycles during the simultaneous application of mechanical and electrical fatigue. SEM and XRD analysis were applied toward investigating the structural characteristics of the inter-electrode surfaces.

Implications of positive ion layer formation across highly injecting metal contacts: current instabilities and electrostatic repulsion

This paper is a theoretical and experimental investigation of the implications for the contact resistance of highly injecting metal contacts, whenever monolayers of positive ions are formed over the cathode. The major ion neutralization processes for a metal+oxide+gas interface are theoretically discussed since they principally determine whether fixed positive ion monolayers can or cannot form. The formation of positive ion monolayers on cathode surfaces will reduce the effective barriers required for electronic field emission. Practical implications may appear as additional current components, current instabilities and blow-open forces between the joints. Current instabilities have been experimentally investigated for a variety of components operating in the air or in high vacuum environment. The obtained results clearly demonstrate non-linear conductivity phenomena and hysteresis effects between the interfacial field and the injected charge in the examined structures. The evolution of blow-open forces between highly injecting metals has been investigated by analyzing transient current and voltage waveforms during spontaneous contact opening at reduced clamping forces. The importance of interfacial cavities and screening charges between the electrodes has been investigated as they may determine the over-current values during the spontaneous contact opening process. Fundamental aspects in controlling the above effects on heavy duty industrial components are pointed out.

Reliability of the asperity contact model in determining charge injection across interfaces

Current profiles across mechanically contacted materials usually differ from the ones obtained by corresponding evaporated contacts. The asperity contact model has been brought up to cover such discrepancies. However, there is a lack of experimental evidence concerning its applicability on electronic injection across the interfaces. The work presented in this paper uses I-V curves of a well-documented device, the metal-semiconductor contact, as a tool to examine the validity of the asperity contact model and the implications of the interfacial layer, the axial contact force, the interfacial field and the relative permittivity of the surrounding space, on the injection process. Namely, the influence of interfacial layers has been studied in ultra-high-vacuum (UHV) environment (10/sup -10/ mbar) using cleaved silicon samples, contacted by hemispherical metal electrodes (Au, Cu, In, Al) covered in situ by fresh overlayers. The applied axial forces were controlled by electromagnets which displaced stainless steel electrodes to contact chemically prepared and cleaved [110] Si samples in a UHV environment. The importance of the interfacial fields has been examined by using Si and GaAs samples having specific surface profiles, i.e., mesas with 10 /spl mu/m diameter and 1 /spl mu/m height, fabricated by plasma etching or wet chemistry processes. Finally, the effect of the relative permittivity of the surrounding space has been investigated by applying sinusoidal 50-Hz high current densities on metal-metal contacts in the laboratory and high vacuum (10/sup -6/ mbar) environments. Utilizing the framework of the theory of the asperity contact model, the obtained results are in good agreement with the expected implications of the examined factors. >

The effect of water on permittivity properties of nanodielectrics exposed to the atmosphere

Relative dielectric constant and dissipation factor (tandelta) values as a function of frequency have been measured for a variety of water types. The effect of water absorption on the permittivity properties of crystalline powders or nanopowders when exposed to various humidity levels is also examined. Powdered materials were characterized by XRD and were calcite (CaCO3), anatase, rutile (TiO2), gamma and delta alumina (Al2O3). The change induced in the polarization of the powders by the applied dehydration procedures, i.e. high vacuum 10-6 mbar and calcination at temperatures up to 500degC, was investigated.