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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.

In power electronics, electrical insulation encapsulation at high temperatures and/or high voltages is a challenge for the systems such as IGBT. Different materials are used and tested in industry and literature. Silicone elastomers for example associated with a primary passivation layer of polyimide are considered to be good candidates but are limited to 180 – 200 °C. Epoxy thermosets are known to be good insulator to protect electrical devices from moisture, dust and short circuits electrical breakdown. Thermoset/thermoplastic blends can be another good candidate for electrical insulation properties. The thermoplastic, its percentages as well as the curing conditions control the final morphology (phase separation with disperse thermoplastic phase, bicontinuous phases or inverted structure …) and thus the electrical properties of the sample. In our study, polyetherimide (PEI) and Polystyrene (PS) thermoplastics were introduced with different percentages into the epoxy system. Electronic microscopy images showed similar phase separation phenomenon with different size of nodules of the thermoplastics. The effect of the morphology on mechanical, electrical and thermal properties has been investigated. The addition of 6 wt% of PS and 10%wt of PEI gives nodules of thermoplastic with a micrometer size inside the epoxy amine network where it did not affect either the storage modulus or the thermal resistance under air or nitrogen atmosphere. Whereas, the dielectric spectroscopy points out that, the addition of polystyrene into the epoxy system increased the relative permittivity of the blend. Contrariwise, the addition of polyetherimide into the epoxy system has decreases the relative permittivity by 15 % in all the temperature and frequency ranges compared with the pure epoxy amine network.

Among materials used for electrical insulation in power electronics systems like IGBT, silicone elastomers used are considered as good candidates but are limited to 180–200°C. In this work, new thermoset/thermoplastic composites are investigated. The addition of some thermoplastics to epoxy amine systems can lead to system exhibiting phase separation. The thermal degradation under some conditions reaches 300°C, which is interesting for power electronics applications. The mixture of thermoplastic inside epoxy prepolymer governs the final microstructure. For example, the addition of 10 % wt of PEI gives inclusions of thermoplastic with a micrometer size. Even if the T alpha of epoxy matrix was changed upon the addition of PEI, the dielectric spectroscopy points out that the addition of some thermoplastic decreases the permittivity compared with the pure epoxy amine network.

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.

Aluminium nitrite (AlN) or alumina (Al 2 O 3 ) substrates are widely used in power electronics modules, due to their ability to provide both electrical insulation, and heat conduction properties. A silicon gel usually covers the substrate, semiconductor chips, and bondings to prevent partial discharges (PD) from occurring within the module. However, at high voltage PDs can be observed in high electric field regions, mainly at the sharp edges of copper tracks on the substrate. In this study, we try to determine the origin of these PDs. Phase resolved PD recordings (PRPD) and optical visualization at very high sensitivity are carried out on substrates embedded in gel or insulating liquids. PD features are very different from those obtained in needle-plane geometry in gel or liquid, and do not depend on the nature of the liquid or gel. The main conclusion is that PDs recorded with substrates originate from the ceramic material itself, due to the presence of numerous μm-sized pores in sintered materials.

Aluminium nitrite (AlN) or alumina (Al 2 O 3 ) substrates are widely used in power electronics modules, due to their ability to provide both electrical insulation, and heat conduction properties. A gel layer usually covers the substrate, semiconductor chips, and bondings to prevent partial discharges (PD) from occurring within the module. However, at high voltage PDs can be observed in high electric field regions, mainly at the sharp edges of copper tracks on the substrate. In this study, we try to determine the origin of these PDs. Phase resolved PD recordings (PRPD) and optical visualisation at very high sensitivity are carried out on substrates embedded in gel or insulating liquids. PD features are very different from those obtained in needle-plane geometry in gel or liquid. The main conclusion is that PDs of low amplitudes probably originate from the ceramic itself, due to the presence of numerous µm-sized pores in sintered materials.

In this paper, we have investigated the behavior of dielectric properties (partial discharges) of liquids of different natures (silicon oil, aromatic liquid). Partial discharges measurements have been done over the voltage range (0-25 kVpp). Two types of devices have been analyzed. The first one consists of a ceramic covered by liquid, the second one of a needle-plane system embedded in liquid. The origin of partial discharges for the both devices is discussed. A mixture of corona effect and discharge in micro voids appear for the ceramic based system whereas only the corona effect was identified for the second one. Results are then discussed in correlation with potential applications in power electronic components where rubber and gel are used for IGBT module.

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.

Using the LIPP method, negative homocharges have been observed in PET sheets after a few hours poling under 6kV/mm DC at 60 °C. This observation was made with various electrode metals: gold (Au), aluminium (Al) and silver (Ag). With gold electrodes, the injection is much slower than with the other two metals. Furthermore, the gold metallization is the only one leading to the injection of positive space charges in the same conditions. Similarly, at 55 °C under 7kV/mm, positive homocharges have been observed in epoxy sheets only with gold-deposited electrodes and not with aluminium electrodes.

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.

International audience; This paper presents an investigation on the current and breakdown voltage measurements in thermoset-thermoplastic blends based on an epoxy-amine/poly-etherimide phase separated material. Thermosetthermoplastic separated blends could be a novel material for insulation applications and can be compared to epoxy/inorganic composites systems. Pure epoxy network as well as the blends with 10 wt% of PEI were studied in terms of conduction and transient currents. An ohmic behavior below the threshold field (E th) and space-chargelimited conduction (SCLC) above E th are pointed out. Contact emission phenomenon was investigated by means of Schottky model and seems to be valid for both materials. The addition of 5 and 10 wt% of PEI into the epoxy system showed an increase in the values of breakdown voltage with the increase of the PEI amount.