ICYS Annual Report 20231. Outline of Research2. Research ActivitiesCédric BOURGÈSThe 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.According to my research plan, the process influence in strategic materials has been investigated but using different and complementary approaches to establish empirical connection as well as developing tool for sulfide development in strategics fields.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 to develop 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 (ALMLBO) to improve the TE properties. It led to a first meaningful achievement as I have evidence once more that the process (especially the reactive sintering by SPS) influence massively the final properties of the metal-sulfide 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. I worked massively on the understanding of the ALMLBO convergence and identify that the approach thermoelectric applied converges to the process’s parameter suitable for promoting the nanostructuration which is a strategy applied on TE materials in order to improve the performance and rationalized it by theoretical calculations. It acts as a proof of concept that combining experiment/AI/theory approaches is a proficient way to identify adapt strategy and relationship favorable for optimizing individual metal-sulfide TE materials. Besides, I extend the approach to the Cu2+xZn1+ySn1+zS4+d composition optimization using the same ALMLBO strategy which is undergoing a similar improvement of zT.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.I investigated the influence of 3 methods: 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. Aside from the normalization of the synthesis effect on the transport properties of the digenites, extensive investigations have been conducted on the physico-chemical properties especially on the structural and thermal stability of this material to rationalize the influence of each synthesis approach on the final ceramic. This point has never been considered in the literature where most of the report measured the digenite TE properties up to 773K and even up to 800K without considering the real thermal and chemical stability of the mineral.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. However, in a priority concern, the work has been focused toward using the ALMLBO approach on 3 commercials powders in order to bring a new proof of concept that the optimization of the sintering process can be extended on the transparent properties of a sulfide ceramics.Research Digest 11Tunable Structural Order/Disorder Functional Ceramics: A Novel Prospect of Sulfides-based Materials
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