Thierry Lebey

Conductivity spectroscopy in aromatic polyimide from 200to400°C

Dielectric relaxation spectroscopy experiments have been performed in a polyimide (PI) in order to measure its electrical conductivity from 200to400°C. The high temperature static (dc) conductivity in PI appears as superimposed by electrode polarization due to the building up of thin space-charge capacitor layers at the bulk-electrode interfaces of the metal-insulator-metal structure making the conduction phenomenon difficult to identify in dielectric spectra. However, the dynamic (ac) conductivity plot versus frequency allows for an approximation of the dc conductivity in PI which was reported here up to 400°C with values reaching 10−6Ω−1m−1 at 400°C.

Development of a new off-line test procedure for low voltage rotating machines fed by adjustable speed drives (ASD)

Using adjustable speed drives employing fast switching components to supply low voltage asynchronous motors may be detrimental to their reliability in some cases. The premature failures of the insulating materials are often attributed to the existence of partial discharges (PD) in the windings. Nevertheless, a clear evidence of any PD existence in such systems is far from being established. Also, their location may not be in the turn-to-ground or phase-to-phase insulation but in the turn-to-turn insulation due to the voltage waveforms, and as such, the classical AC test is no longer applicable. The aim of this paper is to present an off-line test procedure capable of overcoming this problem. The results of the test carried out on the representative population of different motors are presented.

Impact of the surface roughness on the electrical capacitance

A new hybrid approach consists to use the advantages of both systems namely the high geometric aspects of the electrodes of the ultracapacitor and the high dielectric strength of polymer materials used in dielectric capacitors. The surface roughness of the electrodes of the ultracapacitor is manufactured with nano-porous materials; activated carbon and carbon nanotubes (CNTs). Many compositions of both carbonaceous materials are tested with different insulating materials (liquid and solid) to constitute the hybrid capacitor. It appears that the capacitance increases with the carbonaceous composition: An increasing from 15 to 40% is observed as compared to a plane capacitor, it can be twice with a 100 wt% of CNTs content. But, the impregnation of the insulating material in the surface roughness remains the key point of the realization of the hybrid capacitor. The roughness accessibility is a major property to optimize in order to improve the impregnation of the insulating material to increase the electrical capacitance.

Behavior modeling of a CaCu3Ti4O12 ceramic for capacitor applications

In pursuit of high permittivity materials for microelectronics application, there has been a considerable interest recently in the dielectric properties of the cubic perovskite related CaCu3Ti4O12 (CCT), since this material has a so called giant permittivity value (~100000). In this paper a model of the electrical behavior of the capacitor realized with this material is presented. This model is particularly well suited for time domain simulations. Two types of electrical measurements are realized: conduction current measurements (giving static I(V) characteristics) and impedance spectroscopy measurements for different levels of static polarization. The proposed non linear electrical model includes three components: a constant resistor in serial with a capacitor having Cole-Davidson relaxations in parallel with a non linear resistor (describing the non linear dependency of the conduction current when the voltage increases). A good accuracy is obtained for the description of the dynamical behavior in a wide range of frequency and bias voltage, with only five parameters. This frequency model is converted into a time domain model (state space representation) using the Diffusive Representation tool. This approach allows predicting losses and reveals itself efficient to get simple input-output behavioral models which are useful for general nonlinear circuit simulations in the field of electrical engineering.

Partial discharge in electric motor fed by a PWM inverter: off-line and on-line detection

This paper deals with Partial Discharge (PD) detection in Pulse Width Modulation (PWM) inverter fed electric motors. The aim of this work is to detect PD activity in random wound motors, which is the main cause for premature breakdown of insulation system. Detection is performed thanks to a non-intrusive electromagnetic sensor. A high voltage power supply PWM has been built in order to test, offline, electric stators in conditions as close as possible to real electrical conditions. Then, on-line PD measurements have also been successfully carried out, in an engine test bench used to test and control electric vehicle (EV) powertrain.

Degradation and Dielectrics measurements of rotating machines insulation at high temperature (200-400°C)

Due to a continuous power density increase, the thermal stresses endured by the electrical insulation of rotating machines are also increasing. However, up to now, most of the characterizations (breakdown voltage, permittivity, loss tangent) are usually carried out in a temperature range which rarely exceeds the thermal class. Consequently, there is a lack of knowledge on the behaviour of these materials when they are submitted to temperatures which may reach 50°C above their thermal class. In this paper, experimental results regarding the electrical behaviour of enamels are presented and discussed. Hence, permittivity and loss tangent are measured from room temperature to 400°C in the frequency range of 50Hz-100 kHz. The samples consist in 50¿m films of various polymers (polyester-imide, polyamide-imide and polyimide) deposited on different conductive plates (copper or stainless steel). To find the impact of the interfaces, some samples are composed of two insulating layers (same or different varnishes). The differences observed between the different polymers are correlated to their chemical structures obtained thanks to chemicals measurements like ATG and FTIR.

Dielectric and thermal properties of Polyamide-imide (PAI) films

Polyamide-imide (PAI) materials appear as potential candidates for the encapsulation of wide band gap semiconductor power devices operating at high temperature (>200 °C). However, electrical properties of PAIs are under-known compared to those of polyimides (Pis). We propose to evaluate the dielectric properties of two PAI materials with two different glass transition temperatures Tg. The aim is to observe the benefit of increasing Tg, from 280 up to 335 °C for a novel kind of PAIs. So, the broadband dielectric relaxation spectroscopy has been used for measuring the dielectric parameters versus temperature. This paper highlights the role of the glass transition phenomenon in the dielectric properties evolution of PAIs at high temperature. It appears that the glass transition temperature has a strong influence in both the increase of the permittivity, the loss factor and the dc conductivity of several orders of magnitude, and in the characteristic temperature shift when these increases occur.

Weibull Statistical Dielectric Breakdown in Polyimide up to 400°C

In spite of the growing maturity of silicon carbide (SiC) technology, several difficulties still remain and limit its use for high temperature applications up to 200degC. Due to its excellent physical properties, SiC material offers the ability to design electronic devices working at junction (or ambient) temperature and at power level much higher than those of the present silicon based semiconductors. Thus, the environment of the SiC die will endure severe electrical and thermal stresses. Particularly, the passivation layer must present good thermal stability of its dielectric strength, which must remain high enough to ensure a proper electrical insulation on top of the SiC surface. In this study, polyimide material has been chosen as a candidate for SiC power device passivation. This paper presents the changes of the dielectric strength of a BPDA/PPD polyimide for temperatures ranging up to 400degC.

Investigations on DC conductivity and space charge in silicone gel

In power electronics, the development of new IGBT power modules allow them to switch kV and kA during /spl mu/s. However, such components need to be cooled and ceramic materials are used as an interface between the electronic devices (the chips) and the cooling. Another key point of the reliability of the packaging of these devices is their encapsulation made of silicone gel to prevent any discharge on the substrate surface and also in the vicinity of the bondings. In order to identify the physical mechanisms involved in these materials, different experiments are performed. The results of conductivity and space charge measurements (using the LIPP method) under HVDC electrical stress at different temperatures are presented and analysed.

Time‐resolved measurement of space charge in polymeric material under prebreakdown ac field

The buildup of space charge following carrier injection under a prebreakdown field is often considered as a possible degradation process of a polymeric material. The energy released during trapping (phonons and photons) might be at the origin of bond breaking, even if the details of this process are not completely elucidated. Space‐charge formation is of particular importance during ac stressing, since the nature of the trapped charge depends on stress polarity. We present in the following a very sensitive and new experimental setup that allows for the first time the analog, time‐resolved measurement of trapped charge during ac polarization at high field in a dielectric material.