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Développement de nouvelles pâtes à base de nanoparticules métalliques pour du frittage basse température

Abstract : Metallic nanoparticles have the particularity to sinter at lower temperatures compared to microparticles. Silver (Ag) nanoparticles based sintering pastes are commercially available for assembling power electronics chips to their substrates. The assembly is performed between 200 and 300°C, generally under pressure (Hot Pressing) and the resulting metallic joint has excellent thermal and electrical conductivity properties. The theoretical melting temperature of the resulting densified joint corresponds to the melting temperature of bulk silver (962°C), making the silver nanoparticles an alternative for "high temperature" power electronics compared to traditional solder. Nevertheless, the cost of Ag, which is a precious metal, remains a barrier to the use of these sintering pastes. The cost can be reduced by replacing the silver nanoparticles with copper (Cu) nanoparticles. Copper has conductive properties very close to silver. The major hurdle to the integration of copper nanoparticles in sintering pastes is the proneness of copper to oxidation. The oxidation of Cu nanoparticles prevents sintering and greatly reduces the mechanical properties and conductivity of the final metallic joint. Moreover, copper is less reactive during sintering and requires higher temperatures to densify. We chose to protect copper nanoparticles by encapsulation. In a first step copper nanoparticles were synthetized at laboratory scale and semi-industrial scale. In a second step the copper nanoparticles were encapsulated either with a polymer or very thin layer of Ag. The oxidation properties of the core-shell nanoparticles were studied. In a third step the Cu@Ag nanoparticles were formulated in a paste in order to obtain metallic joints. The sintering and density properties of the metallic joints were evaluated and positively compared to the joints obtained with a commercial Ag based paste. The Cu@Ag core-shell system prevents oxidation but also improves the sintering process.
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Thomas Michaud. Développement de nouvelles pâtes à base de nanoparticules métalliques pour du frittage basse température. Matériaux. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAI050⟩. ⟨tel-02887095⟩

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