Olivier Gallot-Lavallée

Space charge distribution measurement methods and particle loaded insulating materials

In this paper the authors discuss the effects of particles (fillers) mixed in a composite polymer on the space charge measurement techniques. The origin of particle-induced spurious signals is determined and silica filled epoxy resin is analysed by using the laser-induced-pressure-pulse (LIPP), the pulsed-electroacoustic (PEA) and the laser-induced-thermal-pulse (LITP) methods. A spurious signal identified as the consequence of a piezoelectric effect of some silica particles is visible for all the methods. Moreover, space charges are clearly detected at the epoxy/silica interface after a 10-kV/mm poling at room temperature for 2 hours.

Dark current measurements in humid SF6: influence of electrode roughness, relative humidity and pressure

The measurement of ‘dark current’ in pressurized SF6 at high electric field is performed using electrodes with a coaxial geometry. To identify the main mechanisms involved in measured currents, the influences of electrode roughness, gas pressure and relative humidity have been investigated. The experimental results reveal that charge injection from the electrode constitute the predominant process responsible for the dark current. The latter is nearly identical in positive and negative polarities, and shows an exponential increase versus the relative humidity and the electric field. The analysis of results shows that under high electric field, the emission of charged water clusters from the water films adsorbed on electrodes probably constitutes the main mechanism of charge emission.

Dark current measurements in pressurized SF 6 : Influence of relative humidity and temperature

Current measurements in pressurized gases at high electric field are performed using electrodes with coaxial geometry. To identify the main mechanisms involved in “dark current” measured, the influences of electrode roughness, gas pressure, relative humidity and temperature have been investigated. The experimental results reveal that charge injection from the electrode constitute the main mechanism responsible for dark current. The latter is considerably modified when relative humidity RH is changed via the influence of pressure and temperature. The measured current shows an exponential increase versus electric field and also versus RH. It is thus assumed that water adsorbed on the electrodes induces an enhancement of charge injection from the electrode surface.

Dielectric spectroscopy on a PHBV bio-polymer

This article is focused on the characterization of dielectric properties such as permittivity, dissipation factor and conductivity. These materials are tested under Nitrogen (N2) in a controlled temperature. The dielectric study of this PHBV, identified three main mechanisms of relaxations classed in order of increasing temperatures here after. Relaxation related to the orientation of the carbonyl groups deemed polar inducing a cooperative motion of methyl and ester groups in the amorphous regions, at T<;Tg (beta-type). Relaxation due to the mobility of entire chains and associated segments directly related to crossing the Tg (alpha-type). Relaxation of space charge accumulated at the interfaces of the amorphous and crystal regions, at Tg<;T<;Tm (MWS-type). Finally a mechanism of ionic conduction seems to settle above 60[°C] at 10[mHz].

Study of surface resistivity of an epoxy/gas interface versus electric field and relative humidity

Surface resistivity of solid insulators is usually determined by measuring the current induced when an electric field tangential to the surface is applied. This classical method can become questionable when the electric field is raised. In fact, a current may also flow above the interface through the humid gas, in addition to currents flowing at the interface and below the interface within the solid bulk. This paper presents an experimental study of the variation of the surface resistivity versus electric field and relative humidity RH of the surrounding gas. The results show that in dry gas (RH <; 1%) and for an electric field up 8 kV/mm, the surface gets no specific properties compared to the volume. However, in humid gas (1% <; RH <; 10%), a decrease of the net surface resistivity (by subtracting the current above and below the interface) is detected.

Dielectric study of Polycaprolactone: A biodegradable polymer

Dielectric spectroscopy measurements of Polycaprolactone over a frequency range of 12 mHz - 2.6 MHz were conducted for temperatures ranging from -160 °C to 40 °C. Relative permittivity, dissipation factor, ac conductivity, and modulus curves are plotted and discussed. Four relaxations: γ (- 150 °C to -140 °C; 12 mHz to 0.5 Hz), β (-110 °C to -100 °C; 12 mHz to 0.5 Hz), α (-60°C to -30 °C; 12 mHz to 2.6 MHz), and Maxwell-Wagner-Sillars (0°C to 30 °C; 12 mHz to 0.5 Hz) polarization peaks are observed in the increasing order of temperature in the measured frequency region. Melting temperature (T_m=64 °C), and degree of crystallinity (X_c=43 %) was obtained by differential scanning calorimetry analysis. Glass transition temperature (T_g=-49 °C) estimated by dynamic mechanical thermal analysis is in coherence with α peak.

Dark current measurements in pressurized air, N 2 , and SF 6

Dark current measurements in pressurized gases at high electric field are performed in a coaxial electrode geometry. Three main parameters are investigated: the influence of electrode surface roughness, the nature and pressure of gas, the presence of water. Currents are measured in both negative and positive polarities. A large influence of electrode surface roughness and of the gas water content is observed. Most of results suggest that charge injection from the electrode constitute the basic mechanism of high field conduction through pressurized gases, and this mechanism is considerably enhanced when water is present.

Screening on research-activity on biodegradable polymers for dielectric properties

The review-article is based on the census of publications on the dielectric properties of biodegradable polymers up-to-date. The introduction section highlights why it is important today to enhance our knowledge on electrical properties of biodegradable polymers as a possible alternative to classical non eco-friendly polymers. Following, biodegradable polymers polled for this study is briefly presented; some semantics are precised, and bibliographic research methodology is defined. The review-article describes the chronology of publications, the main contributors, the major publishers, the most studied biodegradable polymers, and the most analyzed dielectric properties. It is observed that Poly(lactic acid) is extensively studied; Year 2007 has been peak for the number of research-reports communicated; the major-contributor and publisher is Japan and IEEE respectively. Around 81 % of publications come from academic initiatives. Dielectric properties such as permittivity and conductivity are found to be reported for all the biodegradable polymers. Electrical breakdown is reported for all except Polyhydroxybutyrate. Loss tangent is least discussed limited to only four biodegradable polymers.

Overview on thermal and electrical properties of biodegradable polymers

The present review-study outlines thermal and electrical properties of ten biodegradable polymers and its comparison with conventional polymers. The phenomenon of biodegradation and method of obtaining different bio-degradable polymers is discussed briefly including their chemical identity. Some of the biodegradable polymers in the review-study are biobased or from renewable resources, while others originate from fossil fuel resources. Thermal properties such as glass transition and melting temperature of both biodegradable and classical polymers are compiled and demonstrated in figures. Electrical properties such as permittivity, loss tangent; apparent conductivity, and electrical breakdown: AC and DC are reported. Therefore, the review-study allows the comparison of electrical properties of biodegradable and conventional polymers.