Scientific Events

Half-Heusler compounds exhibit significant potential in thermoelectric applications for power generation up to 1000 K, notwithstanding the substantial challenges posed by the cost of constituent elements and the imperative to augment the average thermoelectric figure-of-merit (zT_ave) for more practical applications. Overcoming these obstacles demands the advancement of high-performance p-type TaFeSb thermoelectric materials with diminished Ta content. This investigation systematically explores the quaternary-phase space encompassing Ta, Nb, V, and Ti to ascertain an optimal composition, namely Ta_0.42Nb_0.3V_0.15Ti_0.13FeSb. This composition is characterized by a remarkable reduction in Ta concentration coupled with an enhancement in zT, peaking at 1.23 at 973 K. Moreover, the integration of a high-mobility secondary phase, InSb, fosters enhancements in both the Seebeck coefficient and electrical conductivity, resulting in a 23% augmentation in the average power factor, while concurrently suppressing lattice thermal conductivity. The optimized composite, Ta_0.42Nb_0.3V_0.15Ti_0.13FeSb-(InSb)_0.015, achieves a peak zT value of 1.43 at 973 K and a zT_ave of 1 from 300 K to 973 K, thereby setting a precedent among p-type half-Heusler materials. Additionally, a single-leg device demonstrates a peak efficiency of approximately 8% under a temperature difference of 823 K vs. 303 K. These findings underscore the substantial potential of the proposed material design and fabrication methodologies in fostering efficient and sustainable thermoelectric applications. [more]

High entropy materials are materials with 5 or more principle components within crystalline lattices. High entropy metal chalcogenides are a recent (since 2016) development in this area that have shown exceptional promise in both thermoelectric energy conversion and electrocatalysis. However, these materials remain limited in both compositional range and characterisation. In this talk the compositional boundaries of high entropy metal sulfides will be explored, along with advanced and comprehensive characterisation techniques and our initial explorations into acidic hydrogen evolution electrocatalysis. [more]

Thermoelectric materials can realize waste heat recovery and solid-state refrigeration, providing sustainable solutions to the energy crisis and environmental pollution. The performance of thermoelectric materials is gauged by the transport of electrons and phonons. Materials with fast electron movement but slow phonon propagation would be ideal thermoelectrics. These transport behaviors of carriers are influenced by the intrinsic chemical bonding mechanism and structural defects of materials. Understanding the bonding and microstructures of materials is of paramount importance to improve their thermoelectric properties. Atom probe tomography (APT) provides a unique combination of characterizing chemical bonding and lattice defects, being a very powerful tool in the study of thermoelectric materials. It has been revealed that many of the high-performance thermoelectric chalcogenides utilize metavalent bonding (MVB), which can be distinguished from other bonding mechanisms by the unconventionally high value (>60%) of “probability of multiple events (PME)” measured by APT. Thus, many new compounds with high thermoelectric performance can be designed by tailoring their chemical bonds, and APT is an indispensable technique to corroborate the bonding transition. Owing to the high spatial and chemical resolution of APT, the local change of chemical bonding at defects such as grain boundaries and precipitates can also be detected by APT. This enables us to better understand the role of chemical bonding in regulating the electron and phonon transport across individual defects. The results in turn provide insights into the tailoring of thermoelectric properties by manipulating the local chemical bonds. For example, the thermoelectric properties of polycrystalline SnSe have been significantly improved by removing the stiff Sn-O bonds at the grain boundary as directly observed by APT. In contrast, the strong bonding connection between thermoelectric and interfacial materials enables a high-efficiency and durable thermoelectric device. Based on APT, in conjunction with other characterization techniques, we can explore some uncharted territories in the design of thermoelectric materials. [more]

Defects and Grain boundaries have a remarkable effect on the thermal and electrical transport properties of polycrystalline materials but are often ignored by prevailing physical theories. Grain boundaries and interfaces can adversely alter the properties of Power Electronics, Solar Cells, Batteries and Thermoelectrics such as interfacial electrical and thermal resistance (Kapitza resistance) and even an interfacial Seebeck effect. Interfacial thermal resistance limits the performance of power electronics because of overheating. New scanning thermal reflectance techniques can image the thermal resistance of interfaces and boundaries directly. The Thermal conductivity suppression at grain boundaries can even be imaged showing that different grain boundaries can have very different thermal resistances with high energy grain boundaries having more resistance and low energy boundaries having lower thermal resistance. Electrical grain boundary resistance can be so high in some thermoelectric materials it is the dominant property that limits zT. While small grains are usually considered beneficial for thermoelectric performance due to reduced thermal conductivity, Mg₃Sb₂ based thermoelectric materials, so far at least, contradict that trend. Indeed, atomic segregation has been recently observed at the nanometer scale in grain boundaries in many materials suggesting interfacial or complexion phases should be specifically considered when understanding nearly all thermoelectric materials. The concentration of point defects, such as vacancies, interstitial and substitutional atoms can now be predicted with DFT allowing defects to be included in phase diagram analysis for prediction of materials processing for particular properties. [more]