Year: 2023
Type: article
Abstract: It is estimated that more than 60% of the energy is lost due to inefficient utilization, mostly in the form of waste heat. Thermoelectric materials, which can directly and reversibly convert heat into usable electricity, can significantly improve energy efficiency. The performance of thermoelectric materials is evaluated by the figure of merit, zT = S2σT/κ, which can be optimized by improving Seebeck coefficient (S) and electrical conductivity (σ) while suppressing the thermal conductivity (κ) at operating temperature (T). Entropy-engineered thermoelectrics are garnering considerable attention owing to their high conversion efficiency [1,2,3]. It is hypothesized that the introduced short-range disorder could cause lattice distortion and suppress thermal conductivity, whereas the long-range ordered atomic arrangement would provide a well-maintained electrical transport framework. However, understanding the effect of entropy on thermoelectrics remains a challenge. The rich atomic species in these materials can induce a variety of possible mixing reactions, resulting in potential structural and chemical inhomogeneity at multiple length scales, which have been rarely investigated with details. The lack of understanding can affect the evaluation of structure-property relationship for future materials design. We study an entropy-engineered IV-VI thermoelectric material and systematically characterize structural features at multiple length scales. These structural features can serve as phonon and charge carrier scattering centers and drive thermoelectric properties. Figure 1 reveals that highly ordered microdomain structures form in chemically homogeneous regions. The arrangement of polar domains induces the formation of {001} twin boundaries and {111} inversion boundaries, as revealed by electron diffraction (Figure 1b). The lattice mismatch at the domain boundaries introduces local distortion and significant strain fluctuation (Figure 1c), which suppresses lattice thermal conductivity and benefits thermoelectric performance. The structural complexity in this entropy-engineered IV-VI thermoelectric material is not only limited to micro-scale but also exists at atomic scale. Atomic disorder has been directly observed with atomic resolution HAADF imaging method (Figure 1d). Along the [110] zone axis, each projected atomic column is occupied by only cations or anions. The disordered occupation at cation sites leads to intensity fluctuation which shows strong contrast to the ordered PbTe system, where the contrast is spatially homogeneous. The intensity fluctuation was then reproduced by multislice simulation (Figure 1e) for verification [4]. Thermoelectric materials typically operate at elevated temperatures, therefore understanding temperature-dependent microstructure dynamics is pivotal for interpreting its structure-property relationship. Variable temperature X-ray diffraction demonstrates that this material crystallizes in a rhombohedral structure (R3m) at room temperature while phase transforming to cubic structure (Fm-3m) at increased temperatures. During the synthesis process, the polar <111> axes in room-temperature rhombohedral phase adopts one out of four equivalent <111> axes in the high-temperature cubic phase, causing the formation of four domain variants. In-situ heating TEM experiments (Figure 2) reveal that domain structures vanish at high temperatures, as domain variants becomes symmetrically equivalent, and reversibly form upon cooling, indicating it is a pure structural-driven process [5]. This phase and microstructural evolution are directly reflected in the temperature-dependent thermal conductivity. The characterized atomic disorder, microdomains, and temperature-driven microstructure evolution unravel the hidden structural complexity in the seemingly homogenous single-phase entropy-engineered IV-VI thermoelectric material. This highlights that macroscopic characterization techniques, such as X-ray diffraction, are insufficient for fully understanding high-entropy materials. This presentation will cover a detailed discussion of structural inhomogeneity, phase and microstructure evolution in an entropy-engineered IV-VI thermoelectric material. The observed microstructure will be correlated with temperature-dependent transport properties. At the end, chemical alloying method will be demonstrated for modulating microstructure and transport properties, which would help develop insights about the design of high-entropy thermoelectrics [6]. S/TEM analysis of characteristic domain structures and atomic disorder. (a) BF image reveals that highly ordered domain structures form and pairs of split spots observed in the (b) SAED pattern confirmss the formation of twin boundaries. Formed domains induce significant strain fluctuation at the domain boundaries as shown in (c) 4D-STEM strain analysis, where x = [002] and y = [2-20]. (d) Atomic resolution HAADF image of the disordered IV-VI material reveals significant intensity fluctuations at cation sites as compared to the spatially homogenous contrast shown in ordered PbTe. This indicates the disordered occupation exists in entropy-engineered materials. (e) Multislice simulations on ordered and disordered atomic model reproduce the experimentally observed intensity contrast and highlight the disorder at atomic scale in entropy-engineered materials. In-situ heating TEM analysis on domain structure evolution. Domain structures vanish at high temperatures and reversibly form upon cooling.
Source: Microscopy and Microanalysis
Language: English
Subfield: Materials Chemistry
Field: Materials Science
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4385071748