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N-type Mg

N-type Mg-3(Sb,Bi)(2) compounds have recently been demonstrated as promising low-cost efficient thermoelectric materials in low and intermediate temperature ranges; however, the thermal stability of this type of material still poses a great challenge for practical applications. In this work, we conduct a systematic investigation of the thermal stability of several high performing n-type Mg-3(Sb,Bi)(2)-based thermoelectric materials in both bulk and powdered forms using X-ray and neutron diffraction. It is found that the bulk sample exhibits a much slower degradation rate based on the evolution of the secondary Bi/Sb phase in comparison with the powdered sample, revealing a clear kinetic effect. Moreover, the surface of the bulk sample will gradually become Mg-poor or Bi-rich even at room temperature when exposed to air for a long time, highlighting the importance of surface encapsulation for applications. An underlying mechanism based on the Mg loss/migration is proposed to account for the property degradation. Importantly, to address the property degradation, we discuss possible solutions and propose Mg-vapor annealing as an effective approach to enhance thermal stability by suppressing the Mg loss/migration through saturating grains and grain boundaries with elemental Mg. We expect a combination of the Mg-vapor annealing and surface coating to further improve the long-term thermal stability. This work sheds light on the strategies for enhancing the long-term stability of n-type Mg3Sb2-based thermoelectrics for practical applications.

The limited stability of thermoelectric materials under operating conditions is a major inhibiting factor in the development of thermoelectric technology. With increasing awareness of the stability issue, many stability studies have been carried out during the past decade, and a wide range of characterization methods have been employed for evaluating stability. Here, we present a perspective on the most common characterization methods used in the thermoelectric literature for testing materials stability. Based on the parameter control and comparability to real-world conditions, we propose a division of stability tests into three overall levels to encourage more elaborate stability studies to be made with a high degree of real-world resemblance. For each level, we propose a reporting protocol to streamline the experimental information in stability studies, which will make comparison of results across studies much more reliable. We believe that these steps will help bridge the gap between fundamental materials science and actual application, which is necessary for better maturing thermoelectric technology towards broad commercialization.

Journal of materials chemistry / A 10(3), 1473 - 1480 (2022). doi:10.1039/D1TA09489K, In recent years hierarchical thermoelectric materials have been engineered to reach record breaking thermoelectric figures-of-merit (zT) making them attractive in green transition energy conversion applications. PbTe constitutes an archetypical example, where PbS nanoinclusions presumably scatter phonons, while leaving the electrical properties intact. For widespread use of such materials, their structural integrity and chemical stability are equally important to the thermoelectric performance. Here, we show by operando X-ray diffraction and physical property measurements that PbS nanoinclusions in PbTe are not stable at operating conditions. The PbS nanoinclusions grow in size, but surprisingly this does not diminish the thermoelectric performance. Comparison with samples without nanostructures reveals that the PbS inclusions primarily affect the electrical properties at elevated temperatures, and to a lesser degree the lattice thermal conductivity. Nanostructured materials have significantly moved thermoelectric performance limits, but if we are to develop materials by design rather than trial-and-error, it is important that the nature, effect and stability of nanoinclusions are characterized under operating conditions., Published by RSC, London [u.a.]

Powder X-ray diffraction (PXRD) has been a steadfast tool for detailed crystal structure determination in thermoelectrics materials research. PXRD provides a description of the long-range periodic order, meanwhile, local structure information can be obtained from total scattering data, i.e., by measuring both the Bragg and diffuse scattering from a sample. This method has rarely been exploited by the non-oxide thermoelectrics community. , , Treating total scattering data by the Pair Distribution Function method is a logical approach to understanding defects, disorder and amorphous components in thermoelectric materials. Moreover, in situ studies can enable an understanding of how high temperature influences the structure (and properties) at the local scale. If we can understand the local modifications in a structure prior to, during, and after restructuring as a thermoelectric material is exposed to heating cycles, then we are closer to distinguishing how we may generate materials that do not undergo specific structure reorientation processes, and/or how we may mitigate them before they occur. Here, we will present a total scattering and PDF study that probes the local structure of the Type-I clathrate Ba8Ga16Ge30. This suggests that local structure reorientations in the cage are likely to be the root cause of the degradation of the structure. This deepens our understanding of disordered clathrates, and provides evidence that the PDF technique is an effective method for probing local structure mechanisms in thermoelectric materials.

The complex host-guest structure of Type-I inorganic clathrates has been studied fervently within the CMC based on their low thermal conductivity and promising thermoelectric Figure of Merit (zT). We have recently been focused on understanding unusual features in the high temperature diffraction data collected over a number of years on Ba8Ga16Ge30 (BGG), where numerous samples have been prepared in-house using various synthesis methods. This led to a comprehensive thermal stability study of clathrate powders, where PXRD revealed amorphous components in the samples treated at high temperature in air. PDF measurements were performed on data collected from ex situ annealed BGG samples. This ex situ study (to be submitted), reveals that the seemingly subtle change in the clathrate structure and the emergence of a significant amorphous phase observed from PXRD data is likely to be the result of modifications in the cage configuration, but further data analysis is necessary to identify the specific features of the cages that are affected. Although the thermal stability of thermoelectric materials has been highlighted as a critical bottleneck in their commercialization, the aim of this work is to understand how we may stabilize TE materials through a thorough understanding of their high temperature structure reorientations and decomposition mechanisms.