PROJET ATTRACTIVITE LABEX-EMC3-FEDER-2016
“COmplex Sulphides-based Thermoelectrics”
COST
Contexte, positionnement et objectifs du projet
1.1.Contexte et état de l'artde la proposition de projet
The environmental impact of climate change due to the combustion of fossil fuels which is becoming increasingly alarming, as reported by the UN’s Intergovernmental Panel on Climate Change (IPCC), is a major concern. Recent IPCC data indicate that global warming is a potentially serious threat which demands an urgent global response, across many sectors of industry and society. Major energy saving and CO2-reducing solutions need to be implemented within the next decade, in line with the present Kyoto Protocol and, more importantly, with the coming Conference of the Parties to The United Nations Framework Convention on Climate Change (COP21/CMP11) which will be held in Paris in July.
One way to significantly improve the sustainability of our energy base and reduce our CO2 emissions is through the scavenging of waste heat with thermoelectric (TE) generators. Recent advances in (i) the scientific understanding of nanostructuring effects on thermoelectric properties and (ii) modern materials manufacturing and characterization technologies have now combined to create the prospect of advanced thermoelectric materials with potential conversion efficiencies above the present range of 5-8%. The advent of these new TE materials offers new opportunities to recover waste heat more efficiently and economically using highly reliable systems that have no moving parts and produce neither noise nor vibration. The need to research and develop improved thermoelectric material with its beneficial effect on reducing the carbon footprint of a wide range of industrial activities is self-evident and supports the new Energy Challenge for Secure, Clean and Efficient Energy (Challenge 2)announced in the ANR 2015 call and H2020 EU program.
The COSTteam is fully committed to contributing to this important R&D endeavour and the team will deliver new, affordable, robust and transformational TE materials for real-world and potential industrial applications. For the latter, it is critical to focus attention on thermoelectric materials that are relatively cheap, widely available, non-toxic, practicable in size, vibration-tolerant and scalable for industry. To meet these very tight requirements, it has been decided at the outset of this proposal to concentrate on complex sulphide-based TE materials, more especially on ternary or quaternary synthetic or mineral sulphide compounds. Besides applicative purpose, let’s stress on a fundamental point of view that among all families of solid state materials, sulphides is one of the richest in terms of crystal structure complexities and diversity of physical properties making them very relevant candidate to establish rules and correlations between crystal and electronic structures and TE properties.
The logical rationale for choosing these materials is supported by:
1.Performance-wise, p-type complex tetrahedriteCu12Sb4S13 has been recently reported as a very promising thermoelectric candidate for medium temperature application.The material exhibits very low thermal conductivity, allowing to reach thermoelectric figure-of-meritZT values up to 0.8-1.0 at 700 K, which means that the additional features of chemically substituting, doping and processing will be very promising for increasing both ZT values and crystal structure stability at high temperature. Recent unpublished results have also demonstrated very low thermal conductivity in n-type semi-metallic Cu4Sn7S16 samples, making this system an ideal platform for further enhancement and n-type coupling module elementwith the former material. Alternative ternary and quaternary sulphides compounds, as for example bornite mineral, thiostannates,thiogermanatessulfosaltsare as many candidates for high ZT materials.
2. Complex structures - allow optimization of bulk phases through chemical modification of the materials and control of processing parameters. Therefore complex structures with intrinsically low thermal conductivity and optimized electrical transport properties (low band gap) can be created (without the need to grind to nano-sized powders) that enable the ZT to be maximized.
3.Low Cost - sulphides offer massive opportunity to create very low cost and non toxic thermoelectric solution given the relative cost of Bi2Te3 (€90/kg) with respect to sulphide based Cu12Sb4S13 synthetic tetrahedrite (€8/kg), or Cu4Sn7S16 (€13/kg).
The core concept of the COSTproject revolves around developing and delivering new energy harvesting thermoelectric bulk materials, based on sulphides with complex structures. The scientific approach is based on the investigation of complex crystal structures including synthesis, powder processing, densification techniques, structure and microstructure analyses, transport properties measurements and modeling. This project at Technology Readiness Level (TRL) 2-3, focused on efficient thermoelectric materials, is a first step to future technological and industrial investigations, including module design, conception, and system integration. The project is then at the forefront of fundamental and applied research, and fits well with the “EMC3Attractivité” research type financing instrument.
1.2.Positionnement national et international
The COST exploratoryresearch project, focusedon the familyof ternary/quaternary complex sulphides and more preciselyontheir thermoelectric properties,is unique in France. No other research team is leading such study, even it is worth mentioning some work donearound thederivatedChevrel phases(molybdenum clusters) by the “Solid State Chemistry and Materials" team of the Instituteof Chemical Sciencesof Rennes.TheCOST project is therefore disconnected to any structure or research platform in France.
Investigations of complex sulphide structures for thermoelectricity has never been financially supported on the French territory and represents a new and innovative approach that can compete with the recent advances (publications and patents) reported by US and Japanese institutes, mostly on tetrahedrites compounds. Finding new compositions/structures with improved thermoelectric performance of sulphide compounds for medium temperature applications will serve in positioning our research activities quickly in this fast growing field.
Finally, ternary sulphide compounds are also investigated among the world for their other properties. For example, due to their ideal band gap, Cu-Sb-S and Cu-Sn-S compounds have been recently reported as excellent new candidates for both thin film and hybrid solar cells and as strong NIR absorbers in general. Advances through COST project in the synthesis of new compounds can then lead to significant breakthroughs for electronic, photovoltaic and optics applications.
Finally, CRISMAT is a leader laboratory in the field of thermoelectricity in France and in the world. It is well known for the synthesis and structure analysis of new oxide compounds (discovery of layered misfit cobaltites (best p-type oxide thermoelectric)), silicides and chalcogenides, in relation with their magnetic, electrical and transport properties. CRISMAT is notably recognized for itshigh-level technical means for materials processing (SPS, microwave sintering, etc…), structural characterization (X-ray diffraction, SEM/HRTEM analysis), and thermoelectric properties measurements (low and high temperature electrical and thermal properties).
1.3.Objectifs et caractère ambitieux et/ou novateur de la proposition de projet
Scientific and technological objectives of the project
COST has as main objective to develop high performance TE materials through the design of complex sulfosalt. The numerous compositions based on the (Cu, Ag, Zn, Fe, Cd, Hg)-Sb-S (p-type thioantimonates Sb5+, tetrahedrite) and (Cu, Fe, Cd, Hg, Zn)-Sn-S (n-type thiostannates Sn4+) systems offer a wide range of structures andtherefore TE properties (See figure 1 for example). This is the case of Cu12Sb4S13 tetrahedrite and Cu4Sn7S16 spinel derivative, whose univalent copper atoms are mainly in tetrahedral coordination, playing a primordial role in the conductivity due to the possible participation of the Cu 3d10 -4s0 orbitals to the conduction. In those systems, a possibility of copper non stoichiometry is observed, as detected for Cu4Sn7S16 showing that a part of tetrahedral copper sites is unoccupied. As a result, this suggests that structural distortions can be induced by appropriate doping and variation of stoichiometry and should influence significantly the transport properties of these materials. Besides the possibility to influence the electrical conductivity, these two series have common advantages to exhibit very low lattice-thermal conductivity, which is of capital importance for thermoelectric applications. The latter can be attributed to the large number of atoms per unit cell and a strong rattling effect in special cationic sites. Indeed, as it has been already observed for Cu12Sb4S13 and Cu4Sn7S16, the tetrahedral copper atoms vibrate with a large amplitude along a specific direction suggesting some off-plane splitting sites. Therefore, these cations vibrate with low energy in the stiff network and consequently interact with the heat carrying phonon and/or conduction carrier, as it does in caged compounds such as Skutterudites or Clathrates. On the other hand, some partial cationic occupancy (presence of cation vacancies) like in the derived Cu4Sn7S16 spinel structure, may induce phonon scattering through mass fluctuations effects, like recently observed in oxide perovskite (Nd1-TiO3, unpublished). The crystallization under certain conditions can then modify the cation occupancy on the different sites (occupied or not occupied), and strongly modify the thermal properties.
Thus, keeping this fundamental structural characteristic, their electronic properties can also be engineered by changing the composition,doping, stoichiometry, microstructures; these modifications can lead, on one hand, to a charge carrier concentration optimization and consequently to an improvement of the power factor (PF=S2/) On another hand, a band gap modification may be considered as well; that could induce an adjustment of the Fermi level position and eventually an improvement of the Seebeck coefficient.
Figure 1:Crystallographic structures of Cu12Sb4S13 and Cu4Sn7S16ternary compounds
Among the wide range of compositions doable, the first objective of this project shall be to select, synthesize and characterize the structureof the n- and p-type ternary/quaternary sulphide materials. A special focus will be given to Cu-Sb-S, Cu-Sn-S, and Cu-Fe-Sstructures with large number of atoms, which can also potentially exhibit high thermal agitation in special crystallographic sites, and cationic and/or anionic vacancies. The participation of a young researcher with expertise in structure analysis is required for characterizing accurately the crystallographic structures of the synthesized compounds using X-ray diffraction, large instrument facilities (Neutron diffraction)together with HR-TEM analysis. Whereas the X-ray/neutron diffraction will provide a description of the block-unit structure (occupancy, atomic vibration, purity), local HRTEM analysis can highlight the expected structural defects at local scale (vacancies, cation substitution/intercalation, planar defects, intergrowths…). Such approach in the understanding of structural features has rarely been conducted on such complex thermoelectric compounds and represents a promising way to identify the mechanisms behind low thermal conductivity and high ZT thermoelectric materials.
On the other hand, COST will innovate in the synthesis of the desired structures by using flash solid-state synthesis techniques like microwave synthesis and reactive spark plasma sintering. Whereas conventional techniques requires long time process, the synthesis under microwave or with plasma assisted sintering provides a new, fast and alternative method to crystallize these complex structures in a very short time. From a fundamental of point of view, these techniques can also induce special microstructures and numerous structural defects due to the high kinetic of reaction. This is of strong interest to correlate structural and microstructural features to the electrical and thermal properties. To our best knowledge, microwave synthesis and reactive spark plasma sintering have never been studied on such complex sulphide materials and represents the second objective of this COST project.
Finally, potential for COST research efforts to advance the issue of ZT in thermoelectrics is real, as selected materials are known as potential thermoelectrics since only recent years, especially for tetrahedrite compounds. In addition, all of the work done to date has not been systematically undertaken with the support of theoretical modelling, with more generally the developments following a trial-and-error approach. Materials to date have not been completely optimized, and new efficient thermoelectric ternary/quaternary compounds can be discovered. The gap to increase ZT up to 1.5 at 700K is then really possible, targetable and very attractive.
Acknowledgements:
The CRISMAT laboratory gratefully acknowledges the European Union (Feder n° 15E00111), the Regional Council of Normandie and the Ministry of Reseach (LABEX EMC3) for financial support (86 628 €, i.e. 47,34%).