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International audience; This work deals with the study of thermoset-thermoplastic blend forming an epoxy-amine/poly-etherimide phase separated to assess the dielectric and thermal performances. These materials would be good candidates to replace the passivation layer in semiconductors, particularly ones used as switches in power electronic applications. Polyimide and Parylene are usually quoted and studied. Mixtures based on polymers would be a novel candidate that can manage to be an insulator for the system. The modified and non-modified systems where characterized by transmission electron microscopy, scanning electron microscopy, differential scanning calorimetry, thermogravimetric analysis, dynamic mechanical analysis, dielectric analysis and analytical simulation was carried out on dielectric measurements. These complementary techniques were used first to investigate the presence of the phase separation phenomenon and second to quantify the separated nodules size. The effect of this phase separation was examined and showed enhancement in the dielectric values compared to the pure epoxy system. It was finally simulated to show a close assumption of what is found experimentally.

This paper presents an experimental study of Partial Discharges and light emission with ceramic substrates embedded in silicone oil, under ac and square high voltage. Two types of ceramic materials used in power electronics modules are considered: Aluminum Nitrite (AlN) and Alumina (Al2O3). With AlN, a large light emission due to the electroluminescence of the ceramics occurs during the negative dV/dt. This phenomenon is not observed with Al2O3. With Al2O3 and silicon oil, the light emitted is only due to PDs, and recording under square voltage wave shows that PDs are more numerous and/or more intense compared to ac. Square voltage is more relevant to actual working conditions within power electronics modules than ac.

In Insulated gate Bipolar Transistor (IGBT) modules, ceramic substrates are used as insulation between semiconductor chips and the ground potential. The substrate, semiconductor chips and connections are covered with silicone gel to prevent partial discharges (PDs) from occurring. In this work, partial discharges are investigated on laboratory samples made of metalized substrates embedded in silicone gel and various liquids. Substrates are made of aluminum nitride (AlN), alumina (Al2O3), or glass/epoxy composite. The objective is to determine the actual origin of PDs, by changing the nature of encapsulating material and substrate. The various liquids used are first characterized using a point-plane geometry. Experiments show that a rather large discrepancy on the PD behavior can be due to the ceramic substrate itself. "Good" and "bad" substrates can be identified on the basis of PD measurements. The influence of the sample preparation (degassing procedure) is also evidenced. The main conclusion obtained in this paper shows that partial discharges do not occur within the encapsulating material, contrary to most previous hypotheses, but within pores existing in sintered ceramic materials. Changing the substrate nature has a large influence on PD behavior, whereas a modification of the encapsulating material has almost no effect.

Injection and transport of space charges have been observed in a semi-crystalline PET under 6.25 kV/mm dc at 60°C. Measurements were made 30 s after volt-off along a 74 h poling using the Laser-Induced Pressure Pulse (LIPP) method. They revealed only negative charges that spread throughout the inter-electrode gap and accumulated near the anode. The space charge distribution stabilized after about 30 hours. Once stabilized, the distribution of charges led to an increase of the field at the anode of 30% and a decrease of the field at the cathode of 20%. The measured density of charges held at the anode was in good agreement with the computed densities of charges induced by the space charges. The measured transient current is similar to the current derived from the charges induced at the anode. This suggests that in these conditions the transient current is a kind of displacement current entirely due to the transport of the space charges. A criterion taking into account the fields at the anode and at the cathode is proposed to explain the end of the transient regime.

This paper presents an experimental study of streamer inception in a silicone gel. Many parameters are able to influence the inception of discharges in a gel: voltage wave shape, electrode geometry, temperature, etc. In this work, streamer inception is studied in various conditions of electrode shapes (from sharp points up to quasi-uniform field), voltage (ac, dc, impulse voltage) and temperature. Streamer inception is detected by the measurement of breakdown voltages in selected conditions. The results obtained show that streamer inception field varies when the electrode shape is changed, in a similar way as in liquids: the inception field decreases when the electrode surface increases. On the other hand, temperature and voltage wave shape have almost no influence on the inception field. Results are discussed in comparison with data already obtained in liquids in similar conditions.

Silicone gels are widely used to encapsulate power electronics modules. The objective of this paper is to study the partial discharge (PD) mechanism in a silicone gel, using electrical measurements and fast visualization. Experiments are carried out in a pointplane geometry, using either impulse voltage or ac. Under impulse voltage, the visualization shows that the first discharges recorded in a new sample are due to the initiation and propagation of "streamers", with features close to those previously reported in viscous liquids. Under ac, a stable PD regime rapidly establishes: a streamer creates a cavity with a long lifetime, and PDs occur in this cavity during the subsequent ac waves. Since slow streamers develop during long times (some ms), classical PD measurements do not provide an adequate evaluation of the actual discharge magnitude: very small PDs (1 pC) are recorded, whereas large cavities with a total charge in excess of 10 pC are actually present in the gel. When PDs are repeated permanent degradations of the gel rapidly occur, showing the limited self-healing capabilities of silicone gels.

Thanks to the dielectric relaxation spectroscopy method we present results of measurements of dielectric losses versus frequency in alumina substrates before and after thermal treatment. This one consists in applying thermal step of negative and positive values. This type of stress corresponds to those encountered in railway applications. The role of thermal cycling is evaluated, and is rather important. Losses are essentially due to polarization, while the effect of conduction phenomena is less important. For the low frequency range, interfacial phenomena are attributed to the polarization. Results are discussed using the Maxwell-Wagners-Sillars law and Havriliak-Negami's model. On the other hand, the electrical field dependence of losses shows not non-linear phenomenon in the range of voltage [0-3.5kV].

Charging and discharging currents have been measured in an alumina substrate before and after thermal treatment consisting in applying thermal step of negative and positive values (?20°C-50°C). Both transient and steady-state current have been analysed. The average applied voltage ranged from 0 to 5kV. Transient currents suggested a phenomenon generating space charges in the bulk of the material at high fields. Below the high field regime, ohmic and sub-ohmic regime are been identified. Anyway, it was obvious that the thermal treatment increased hugely the conduction current with a factor of almost two decades. No usually Schottky-type injection or tunnel-type was found from our measurements.

In order to increase the knowledge of the processes involved during the degradation of insulating silicone gels under ac stress, recordings of the activity of partial discharges (PD) are performed. Silicone gels are commonly used for the insulation and packaging of high voltage semiconductors such as IGBT modules. We worked on a commercially available dielectric silicone gel. Electrical and optical apparatus allows the measurement of PD's and their location, either on actual power module substrates, or in a point-plane gap. The temperature dependence of PD inception voltage is moderate, while the number of PD's and their charge steeply increases versus temperature. We show that the formation of cavities constitutes the main factor responsible for the degradation under ac conditions. The actual self-healing capabilities of silicone gels are also studied and discussed.

Silicone gels are used for the packaging and insulation of power electronic components such as high voltage IGBT modules, mainly due to their good mechanical and thermal properties. Nevertheless, the knowledge of their dielectric properties is still uncompleted. The aim of this paper is to present the results of measurements of dielectric losses versus frequency, as well as breakdown field under a large temperature range (up to 160 degC). Losses are essentially due to conduction phenomena, while the effect of polarization is less important. The influence of temperature is rather large, and a marked effect of time (ageing) is also recorded, especially when the temperature is raised. Results are discussed with the perspective of use in high temperature power semiconductors.