OxyChalcogenides as new Thermoelectrics – OTher

This project has been carried out following several steps:
• Optimization of the doping level of the parent compound BiCuSeO. Indeed, the optimization of the thermoelectric properties requires a fine tuning of the charge carriers concentration of the material. To do so, we have studied the influence of the substitution of Bi3+ by 2+ cations.
• Influence of the crystal structure of the thermoelectric properties. BiCuSeO is a layered compound, with a distortion of the CuSe conducting layer. Using substitutions, we have studied the influence of this distortion of the electrical properties.
• Influence of the microstructure. BiCuSeO being a layered compound, the properties are anisotropic. Therefore, using a texturation process, a grain alignment is possible, which leads to change in the electrical and thermal conductivities.
• Study of the materials stability in real applications conditions, in order to assess its potential for wide scale applications.

• Optimization of the BiCuSeO-based materials, with a thermoelectric figure of merit as high as 1.4 at 923K (the figure or merit evaluates the performances of the material), which is one of the largest values ever observed for a p-type lead free thermoelectric material.
• Development of a new synthesis process, which enables the preparation of BiCuSeO based materials at room temperature under air for the first time. (previously, the synthesis of these materials required a long term annealing step at high temperature under inert atmosphere)
• We have evidenced for the first time the promising thermoelectric performances of AgBiSe2 based n-type materials, with a thermoelectric figure of merit of 1 à 773K. This result leads to new projects development and new international collaborations

At the end of this project, we have obtained one of the best p-type lead-free polycrystalline thermoelectric materials ever reported for applications in the 400-650°C temperature range. Two research directions should now be followed:
* On the fundamental point of view, the further improvement of these materials now requires a better understanding of the physics that drives their properties. A new collaborative project is under way corresponding to this research direction.
* In order to use these materials in industrial applications, technological studies are now required, dealing with the development of protective coatings to use the materials under air, the study of their mechanical properties and behaviour, and the development of electrical contacts in order to use them in conversion modules.

Our results have lead to 11 papers in good level international journal, and 3 other ones are currently submitted or in preparation. Among our results, the most significant are:
• A paper where we evidence for the first time the very promising thermoelectric performances of an n-type chalcogenide, AgBiSe2.
• A paper where we evidence that BiCuSeO-based materials can be synthesized under air at room temperature.
• A paper where we underline the links between the electronic structure and the good thermoelectric performances of BiCuSeO-based materials.

Thermoelectric generators enable the direct conversion from heat into electrical power whatever the nature of the heat source. Therefore, they provide an effective route to use waste heat originating from automobiles, incinerators and so on, to produce clean electrical power. Until the very recent years, the main drawback of these systems has been their poor energy conversion efficiency that made them unsuitable for widely used applications. However, a great effort of research has been devoted in the past decade to the development of novel materials with improved performances. Consequently, the annual number of submitted patents that deal with thermoelectric conversion systems or thermoelectric materials has grown very rapidly.
The efficiency of a thermoelectric material used for power generation increases with the so-called dimensionless figure of merit ZT defined as ZT = S²T/rl, where S is the Seebeck coefficient or thermopower, r the electrical resistivity, and l the thermal conductivity. It is generally considered that a figure of merit higher than unity is required for efficient thermoelectric energy conversion. ZT=1 for both p type and n type materials of an ideal thermoelectric device would allow for example a 10% recovery of the heavy trucks exhaust gas waste energy.
In recent years, several families of materials with ZT>1 have been developed, including for example skutterudites, magnesium silicides…, and it has been demonstrated that thermoelectric modules based on these materials can reach about 10% efficiency. However, there is still a need of more efficient thermoelectric materials which would enlarge the number of possible applications or open new markets.

During the 1990s and 2000s, oxychalcogenides materials with general formula RCuChO (R = trivalent cation, Ch = S, Se or Te) have been widely studied first as possible parent compounds for new high-Tc superconductors and then as potential p-type transparent conducting materials for optoelectronic applications, mostly in the thin film form. In the beginning of 2008, an intense research activity emerged dealing with the study of a new family of superconductors: the iron-oxypnictides. Following the discovery of superconductivity in these materials, we initiated an exploratory research in this field. We were the first to show that these materials not only exhibit, beside fascinating superconducting properties, promising thermoelectric properties. Their Seebeck coefficient can be larger than 120 µV.K-1 around 100K and they exhibit good electrical properties. As a matter of fact, we have shown that their electrical transport properties, in the liquid nitrogen temperature range, are not far from those of the best materials known to date based on BiSb alloys.
Following this discovery, we have expanded our study to the oxychalcogenides family, and we have shown that these materials exhibit very promising thermoelectric performances in the 400-650°C temperature range and that they could be used in mid-temperature thermoelectric energy converters. The main topic of this project is to study the thermoelectric properties of oxychalcogenides materials in order to assess their potential as p-type thermoelectric materials and to optimize them for applications in thermoelectric modules.