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Germanite derivative materials : synthesis, crystallographic structure from multi-scale characterizations and thermoelectric properties

Abstract : The work presented in this Ph.D. thesis deals with the synthesis, the structural and electronic properties characterization of the Cu22Fe8Ge4S32 copper sulfide, a material derived of the germanite mineral with promising thermoelectric properties. The first two chapters are dedicated to the optimization of the thermoelectric properties. The last chapter is an in-depth structural study of Cu22Fe8Ge4S32. First, the specific synthesis conditions to yield a ‘‘pure’’ germanite sample by sealed tube are investigated by the means of in situ reactions. Then, two different powder synthesis approaches are compared, namely mechanical alloying and conventional sealed tube synthesis, combined with two different densification methods: spark plasma sintering and hot pressing. This study drags attention to the process impact on the transport properties of complex Cu-based sulfides. Second, the series of compounds Cu22-xZnxFe8Ge4S32 (0 ≤ x ≤ 2) and Cu22Fe8Ge4-xSnxS32 (0 ≤ x ≤ 4) were investigated in the hope to enhance the TE properties through enhanced phonon scattering due to differences in atomic mass. In fact, in addition to lowering the κ_Latt, the Cu by Zn substitution in Cu22-xZnxFe8Ge4S32 leads to a decrease in the concentration of hole carriers. In addition, a reduction of κ_Lattis observed with the Sn-incorporation due to point defect scattering enhancement of the heat carrying phonons as a result of mass, size, and bonding strength disparities. Finally, a new structural model for synthetic germanite was proposed with respect to the space group and lattice parameter of the mineral material, P4 ̅3n and a ≈ 10.595 Å. The crystal structure is proposed based on the complementarity from powder and single crystal XRD, 57Fe Mössbauer spectroscopy and resonant scattering. The originality of this work lies in the experimental approach that was developed to overcome the inherent complexity of germanite cationic distribution.
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  • HAL Id : tel-02519876, version 2

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Laura Paradis Fortin. Germanite derivative materials : synthesis, crystallographic structure from multi-scale characterizations and thermoelectric properties. Cristallography. Normandie Université, 2019. English. ⟨NNT : 2019NORMC249⟩. ⟨tel-02519876v2⟩

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