12According to my research plan, the process influence in strategic materials has been investigated but using different and complementary approaches.For (TE) materials development, two distinct materials have been investigated: kesterite (Cu2ZnSnS4) and digenite (Cu1.8S). Both materials subscribed to the strategy to control the process to enhance the overall thermoelectric performance of the metal-sulfide ceramic zT (= PFκ−1 = S2σTκ−1 with T, absolute temperature; S, Seebeck coefficient; σ, electrical conductivity; κ, thermal conductivity) by tunning defect/disorder.For the transparent materials development, the strategic zinc sulfide-ZnS material has been investigated. This approach has for purpose of developing an understanding of the process to promote high stoichiometric, well-crystallized, and dense metal-sulfide indispensable for transparent properties.high-performance Kesterite (Cu2ZnSnS4-CZTS):For the CZTS, I focused on the synthesis process, and especially the sintering process investigation, and developed an approach using machine learning assistance to improve the TE properties. I built an initial database by investigating the kinetics of heating/cooling and the temperature in a reasonable range (50-166K/min for kinetic, and 773-973K for the temperature) agreeing with the literature report.1 Then I performed several cycles of Bayesian optimization. As a first meaningful achievement, I have evidence once more that the process (especially the reactive sintering by SPS) influence massively the final properties of the metal-sulfide Research Digest thermoelectric ceramic and I have successfully developed a process specification to obtain a zT performance enhancement by more than 30% with a zTmax = 0.4@675K just by control it in this strategic material.Digenite-Cu1.8S:By comparison with the CZTS, the digenite is a binary compound which makes it more suitable for the investigation of the influence of the synthesis approach by various methods compared to the quaternary CZTS more sensible to the formation of parasite binary phases (Cu-S, Zn-S, Sn-S) while employing wet chemistry approach or solid-state synthesis which will create high complexity in the understanding of the influence of the synthesis in the sulfur off-stoichiometry.I investigated the mechanical alloying, conventional solid-state reaction, and wet chemistry synthesis in parallel to compare their respective influences on the binary digenite structure, microstructure, composition, and resulting properties.Zinc Sulfide- ZnS:Finally, for the transparent materials development, the strategic ZnS material has been investigated. I engaged in an ultra-short process with a few hours of mechanical alloying synthesis (1-3h) followed by a short SPS process (less than 1h), previously developed on the commercial powder, and successfully reached the formation of the ZnS structure with a transmittance lying in a range of 5 to 15% in the visible range. Then, as a proof of concept, the Cu-doped and Al-doped samples have been synthesized to screen the possibility of turning this ceramic conductive. The preliminary study reveals that is possible to obtain the ZnS structure in both cases and I showed that the doping contributes to reducing or canceling the formation of the hexagonal ZnS phase. It is worth to inform that the hexagonal ZnS phase negatively affects the transparent properties of ZnS which is currently one of the main challenges faced in this material.2 Therefore, the discovery that the dopants play a role in the single-phase formation of the ZnS is a major insight.References1. Jiang, Q. et al. Colossal thermoelectric enhancement in Cu2+xZn1-xSnS4 solid solution by local disordering of crystal lattice and multi-scale defect engineering. J Mater Chem A Mater 8, 10909-10916 (2020).2. Chen, Y. et al. Fabrication of transparent ZnS ceramic by optimizing the heating rate in spark plasma sintering process. Opt Mater (Amst) 50, 36-39 (2015).1. Outline of ResearchThe study of sulfide materials as functional ceramics is highly strategic. As ceramics, sulfide-based compounds have particularly attracted huge interest in the development of transparent materials and are excellent candidates for energy-saving applications such as battery storage or thermoelectricity (TE). However, the influence of the sulfur off-stoichiometry induced by processing is not well investigated because it is mostly considered as a ‘negative’ impact and harms the image of sulfide ceramics. The scientific research objective of my project is to gain a better understanding of the synthesis/sintering process effect in functional sulfide-based ceramics to produce order or defect materials as desired for thermoelectric or transparent ceramics applications.2. Research ActivitiesTunable Structural Order/Disorder Functional Ceramics: A Novel Prospect of Sulfides-based MaterialsCédric BOURGÈS
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