Thermopower of the Correlated Narrow Gap Semiconductor FeSi and Comparison to RuSi

Iron based narrow gap semiconductors such as FeSi, FeSb2, or FeGa3 have received a lot of attention because they exhibit a large thermopower, as well as striking similarities to heavy fermion Kondo insulators. Many proposals have been advanced, however, lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we employ realistic many-body calculations to elucidate the impact of electronic correlation effects on FeSi. Our methodology accounts for all substantial anomalies observed in FeSi: the metallization, the lack of conservation of spectral weight in optical spectroscopy, and the Curie susceptibility. In particular we find a very good agreement for the anomalous thermoelectric power. Validated by this congruence with experiment, we further discuss a new physical picture of the microscopic nature of the insulator-to-metal crossover. Indeed, we find the suppression of the Seebeck coefficient to be driven by correlation induced incoherence. Finally, we compare FeSi to its iso-structural and iso-electronic homologue RuSi, and predict that partially substituted Fe(1-x)Ru(x)Si will exhibit an increased thermopower at intermediate temperatures.Comment: 14 pages. Proceedings of the Hvar 2011 Workshop on 'New materials for thermoelectric applications: theory and experiment

Design of Thermal Contacts for High Performances Heusler-Based Thermoelectric Modules

Most researches and developments presently dealing with thermoelectric materials and devices focus on high performances and/or low cost materials. However, for practical applications, thermoelectric modules based on these materials are needed. The main challenge in the design of thermoelectric modules is the development of efficient electrical and thermal contacts. Even though studies deal with electrical contacts, former work showed that thermal contacts present a stronger effect on the module performances, mostly in the base of commercial modules. The present study deals with the design of thermal contacts for thermoelectric modules based on the low-cost Fe2VAl Heusler compound. With low-cost thermoelectric materials, the cost of modules tends to be dominated by the ceramic insulators. Therefore, new low-cost substrates need to be developed. Innovative substrates for low and medium temperature modules have been investigated. Experimental results will be presented and unravelled by FEM simulations. Results show that the optimized thermal contacts significantly enhance the performances of the thermoelectric modules

Global analysis of the assembly of Fe2VAl and metal electrode through the study of the bonding process conditions.

Fe2VAl-based thermoelectric modules are very promising for large scale applications such as industrial energy harvesting since its constituent materials are abundant, non-toxic and low cost. However, the assembly of modules based on Fe2VAl compounds requires to properly bond this thermoelectric material to high electrical conductivity material such as copper. Furthermore, the assembly must remain efficient in the entire range of temperature at which Fe2VAl compounds show interesting thermoelectric properties, which means from 300K to 700K. At these temperatures, thermal ageing may induce significant modification of the microstructure at the interface between thermoelectric material and electrical conductor. Moreover, copper is known to show poor oxidation resistance above 500K in atmospheric conditions and replacement by pure nickel or nickel-plated copper should be considered [1]. To understand the influence of the bonding process on the property of the assembly, we propose a global analysis based on characterization of the interface microstructure, measurements of surface contact resistance and Seebeck coefficient of a junction as well as the assessment of the effect of thermal ageing [2]. We tested a panel of joining technologies and discriminated some processes based on the global analysis. The most promising technique is the use of a high temperature (1000K) silver-based brazing compound under high vacuum condition (10-4 mbar). This process leads to low surface contact resistance (<10μΩ∙cm2), the appearance of a thin reactive layer at the interface, a weak change on the Seebeck coefficient and no major degradation of the assembly properties after heat treatment. Furthermore, those results were obtained for all three of copper, nickel-plated copper and nickel electrodes. Finally, this bonding process has been upscaled to manufacture a complete thermoelectric module with 36 thermoelectric legs and 37 electrodes in an induction vacuum furnace. [1] ANIEKWE, U. V. et UTIGARD, T. A. High-temperature oxidation of nickel-plated copper vs pure copper. Canadian metallurgical quarterly, 1999, vol. 38, no 4, p. 277-281. [2] ROY, G. et al. Global Analysis of Influence of Contacts on Heusler-Based Thermoelectric Modules. Journal of Electronic Materials, 2019, p. 1-13

Design of Low-cost Thermoelectric Generators for Autonomous Sensors and Actuators in Domestic Hot Water Systems

The fast emergence of the “Internet of Things” (IoT) will lead to a massive dissemination of billions of connected devices in the world during the coming years. Especially in the domestic hot water system sector’s, smart devices such as smart thermostat or intelligent thermostatic radiator valve can improve comfort while decreasing energy consumption. The main drawback of these systems is their energy dependency to batteries. This limits the system lifetime and its recyclability. An alternative to batteries is the use of energy harvesting systems that can harvest heat from the hot water and convert it in electricity using thermoelectricity. The proof-of-concept has been realised with off-the-shell component such as commercial Peltier cooler as generator limiting the performances of the system. Specific thermoelectric generators have also been designed using micro-fabrication techniques, but this solution remains costly. The present work follows a global design approach to develop a high-performance thermoelectric system for autonomous sensors and actuators in domestic hot water systems. By designing the whole system, mainly the heat sink and the thermoelectric generator, there are more degrees of freedom for optimising the performances. Moreover, the cost of the system can be dramatically decreased owing to the use of a low-cost thermoelectric materials based on earth-abundant elements, the Fe2VAl compound. The original design approach will be presented and comforted by the experimental characterisation of a prototype on a domestic hot water system test bench

Optimisation of the thermoelectric properties of Heusler Fe2VAl-based compounds through off-stoichiometry strategies

Using the thermoelectric technology for large scale applications requires thermoelectric (TE) materials that are efficient but also abundant, non-toxic, and cheap. Fe2VAl-based compounds fulfil these requirements in a temperature range that is suitable for many industrial energy harvesting applications (RT-600K). The Seebeck coefficient of such compounds can be tuned from positive to negative values by doping or by applying departures from the stoichiometric composition. Indeed, p- and n-type materials are needed in order to build TE modules. In the present work, off-stoichiometric Heusler Fe2VAl-based compounds are explored in order to find the optimised Fe-V-Al compositions for a given application, i.e. in a targeted range of temperatures. It has been shown that describing the evolution of the Seebeck coefficient as a function of the concentration in valence electrons (VEC) is not appropriate as rigid-band shift behaviours do not apply for any type of off-stoichiometry. We suggest here to represent the Seebeck coefficient as a function of the actual composition on ternary diagrams, for a given temperature. It brings the opportunity to have a quick overview of the best compositions, but also of the effect of specific off-stoichiometry strategies on the TE properties. It is worth noting that, in addition to the Seebeck coefficient, it is of primary importance to take into account the electrical conductivity and thermal resistivity when optimising the composition

Manufacture of TE-modules: joining Fe2VAl and Cu through several bonding processes.

Fe2VAl-based thermoelectric modules are very promising for large scale applications such as industrial energy harvesting since its constituent materials are abundant, non-toxic and low cost. However, the assembly of modules based on Fe2VAl compounds requires to properly bond this thermoelectric material with high electrical conductivity material such as copper. Two types of process have been investigated in the present work: copper metallization directly on Fe2VAl and brazing. Copper metallization has been performed either by diffusion bonding under pressure, physical vapor deposition or electrodeposition. Brazing has been studied by using both low and high temperature (from 300K to 700K) brazing materials in order to enlarge the application field. Very promising results have been obtained for several of those processes in terms of contact resistance measurements (<10μΩ∙cm2), element diffusion (highlighted by SEM and EDX) and mechanical resistance. Moreover, for silver-based brazing, a thin reactive layer has been observed at the surface of Fe2VAl. This layer appears owing to chemical reaction between the brazing material, copper and Fe2VAl. This phenomenon was never observed before and is supposed to have a positive effect on the contact resistance and thus on the module efficiency, as long as it does not involve too large scale elemental diffusion

Industrially Scalable Thermoelectric System for Waste Heat Recovery using Heusler-based Modules and Heat Pipes

Thermoelectricity has been proposed as a promising solution for waste heat to electrical power conversion for decades now. However, it has rarely passed the step from prototype to commercial product due to cost limitations. It is well established that thermoelectric materials and heat exchangers are the major parts of the cost [1]. We propose a solution to decrease these costs: (i) the use of low-cost modules based on the full Heusler Fe2VAl compound [2] and (ii) the use of water-filled carbon steel heat pipes [3]. In this work, a thermoelectric system for waste heat recovery is designed for an industrial case study. This design shows that costs could be decreased by a factor of two compared to a system using Bi-Te based modules and a copper finned heat exchanger. In order to validate this design, the combination of a heat pipe with TE modules has been experimentally characterised demonstrating the high heat transfer capability of heat pipes. Finally, a fully instrumented pilot thermoelectric heat exchanger based on 30 heat pipes has been developed and tested on a 450 kW hot gas test rig with gases up to 450°C. Experimental results will be presented and the next steps to reach the industrial scale will be discussed. References: [1] LeBlanc, Saniya, et al. "Material and manufacturing cost considerations for thermoelectrics." Renewable and Sustainable Energy Reviews 32 (2014): 313-327. [2] Roy, G., et al. "Global Analysis of Influence of Contacts on Heusler-Based Thermoelectric Modules." Journal of Electronic Materials (2019): 1-13. [3] Feldman Jr, K. T., and D. D. Kenney. "The compatibility of mild carbon steel and water in a heat pipe application." Advances in Heat Pipe Technology. Pergamon, 1982. 439-450