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Alloying is widely used as a means to fine-tune the properties of thermoelectric materials by reducing the lattice thermal conductivity. However, the effects of compositional variation on the lattice dynamics of alloy systems are not well understood, due in part to the difficulty of building realistic first-principles models of structurally-complex solid solutions. This work builds on our previous study of Sn n (S 1-x Se x ) m solid solutions (Gunn et al 2019 Chem. Mater. 31 3672) to explore the lattice dynamics of the Pnma Sn(S 1-x Se x ) system, which has been widely studied for potential thermoelectric applications. We find that the vibrational internal energy and entropy have a large quantitative impact on the mixing free energy and are likely to be particularly important in alloy systems with competing phases. The thermodynamically-averaged phonon dispersions and density of states curves show that alloying preserves the structure of the low-frequency bands of modes associated with the Sn sublattice but broadens the high-frequency chalcogen bands into a near-continuous spectrum at the 50/50 mixed composition. This results in a general reduction in the phonon mode group velocities and an increase in the number of energy-conserving scattering channels for heat-carrying low-frequency modes, which is consistent with the decrease in thermal conductivity observed in experimental measurements. Finally, we discuss some of the limitations of our first-principles modelling approach and propose methods to address these in future studies.OPEN ACCESS RECEIVED improving TE performance. In some cases the electronic and thermal transport can be almost completely decoupled [1], as in the 'phonon glass, electron crystal' concept [3]. All four parameters in equation (1) are implicit functions of temperature, requiring that materials are either optimised to produce a high peak ZT at a target operating temperature range or tuned to display a high ZT across a wide range of temperatures.Current flagship TE materials include PbTe, SnSe and Bi 2 Te 3 , all three of which display favourable electrical properties and intrinsically low thermal conductivity due to a combination of heavy elements and strongly anharmonic lattice dynamics [4][5][6]. However, PbTe and Bi 2 Te 3 are not suitable for widespread adoption due to the low abundance of Te, and there are also concerns with SnSe due to the environmental toxicity of Se. There has therefore been significant effort devoted to alternative systems including the more earth-abundant SnS [2] and metal oxides [7][8][9][10].Alloying is commonly used as a means to enhance thermoelectric performance, as a suitable choice of components can maintain or improve a favourable electronic structure while reducing k latt by introducing variation in atomic masses and chemical bond strength to promote stronger phonon scattering [11]. Pb(S, Se, Te), Sn(S, Se) and (Bi, Sb) 2 (Se, Te) 3 alloys have all been studied as thermoelectrics and the alloying shown to improve ZT [12][13][14][15][16][17]. Due to the r...
Alloying is widely used as a means to fine-tune the properties of thermoelectric materials by reducing the lattice thermal conductivity. However, the effects of compositional variation on the lattice dynamics of alloy systems are not well understood, due in part to the difficulty of building realistic first-principles models of structurally-complex solid solutions. This work builds on our previous study of Sn n (S 1-x Se x ) m solid solutions (Gunn et al 2019 Chem. Mater. 31 3672) to explore the lattice dynamics of the Pnma Sn(S 1-x Se x ) system, which has been widely studied for potential thermoelectric applications. We find that the vibrational internal energy and entropy have a large quantitative impact on the mixing free energy and are likely to be particularly important in alloy systems with competing phases. The thermodynamically-averaged phonon dispersions and density of states curves show that alloying preserves the structure of the low-frequency bands of modes associated with the Sn sublattice but broadens the high-frequency chalcogen bands into a near-continuous spectrum at the 50/50 mixed composition. This results in a general reduction in the phonon mode group velocities and an increase in the number of energy-conserving scattering channels for heat-carrying low-frequency modes, which is consistent with the decrease in thermal conductivity observed in experimental measurements. Finally, we discuss some of the limitations of our first-principles modelling approach and propose methods to address these in future studies.OPEN ACCESS RECEIVED improving TE performance. In some cases the electronic and thermal transport can be almost completely decoupled [1], as in the 'phonon glass, electron crystal' concept [3]. All four parameters in equation (1) are implicit functions of temperature, requiring that materials are either optimised to produce a high peak ZT at a target operating temperature range or tuned to display a high ZT across a wide range of temperatures.Current flagship TE materials include PbTe, SnSe and Bi 2 Te 3 , all three of which display favourable electrical properties and intrinsically low thermal conductivity due to a combination of heavy elements and strongly anharmonic lattice dynamics [4][5][6]. However, PbTe and Bi 2 Te 3 are not suitable for widespread adoption due to the low abundance of Te, and there are also concerns with SnSe due to the environmental toxicity of Se. There has therefore been significant effort devoted to alternative systems including the more earth-abundant SnS [2] and metal oxides [7][8][9][10].Alloying is commonly used as a means to enhance thermoelectric performance, as a suitable choice of components can maintain or improve a favourable electronic structure while reducing k latt by introducing variation in atomic masses and chemical bond strength to promote stronger phonon scattering [11]. Pb(S, Se, Te), Sn(S, Se) and (Bi, Sb) 2 (Se, Te) 3 alloys have all been studied as thermoelectrics and the alloying shown to improve ZT [12][13][14][15][16][17]. Due to the r...
Optimizing material compositions often enhances thermoelectric performances. However, the large selection of possible base elements and dopants results in a vast composition design space that is too large to systematically search using solely domain knowledge. To address this challenge, we propose a hybrid data‐driven strategy that integrates Bayesian Optimization (BO) and Gaussian Process Regression (GPR) to optimize the composition of five elements (Ag, Se, S, Cu, and Te) in AgSe‐based thermoelectric materials. We collect data from the literature to provide prior knowledge for the initial GPR model, which is updated by actively collected experimental data during the iteration between BO and experiments. Within seven iterations, the optimized AgSe‐based materials prepared using a simple high‐throughput ink mixing and blade coating method deliver a high power factor of 2100 μW/mK2, which is a 75% improvement from the baseline composite (nominal composition of Ag2Se1). The success of our study provides opportunities to generalize the demonstrated active machine learning technique to accelerate the development and optimization of a wide range of material systems with reduced experimental trials.This article is protected by copyright. All rights reserved
of electronic and phononic subsystems, the enhancement in a parameter can frequently deteriorate another, making the improvement of ZT very challenging. [2b-d] Based on Peltier and Seebeck effects, thermoelectric materials are widely used in up-to-date thermoelectric refrigeration and power-generation devices. [3] The schematic illustration of typical thermoelectric refrigeration and power-generation modes are shown in Figure 1. As shown in the figure, the thermoelectric module consists of a p-type and an n-type material, which are interconnected by a metallic electrical contact pads. The actual thermoelectric module is composed of an array of these couples, allowing an efficient thermoelectric energy conversion when a temperature gradient is applied. Conversely, when the current is passed through a thermoelectric module, a temperature gradient will be generated due to Peltier effect. [3] The heat could then be absorbed in the cold side and eventually being withdrawn by the sink, and therefore leads to the effect of refrigeration. In order to improve the efficiency, Lim et al. proposed a cascade refrigeration system using the thermoelectric modules. [4] In the cascade system, the heat absorption efficiency is 1.7 times better than the single system, and temperature difference of the thermoelectric module is significantly lowered to 30 °C compared to 60 °C of the single system, which made it possible to apply thermoelectric refrigerators to small capacity household applications. In 2018, a novel wearable thermoelectric generator was designed and fabricated by Kim et al. [5] This application could be easily attached to human skin for utilizing human body heat as the heat source, and thus could be used in various applications, such as self-powered wearable electrocardiography, and self-powered wireless sensor network for industry. This reflects the recent trend for developing thermoelectric technology with strong focuses on portable, flexible household and industry devices.Layered materials have long been investigated as promising thermoelectric materials in the past 50 years, from the early prototypes of PbTe and Bi 2 Te 3 . [2b] With different strengths of interlayer chemical bondings, the layered materials could be divided into two categories: typical covalent/ionic natural bulk layered materials, such as layered cobalt oxides, [7] bismuth oxyselenide, [8] artificial layered materials, such as superlattices including Bi 2 Te 3 /Sb 2 Te 3 , [9] SrTiO 3 /SrTi 0.8 Nb 0.3 O 3 , [10] GaAs/AlAs; and van der Waals layered materials including Te-based alloy, [2b] as well as artificially engineered van der Waals heterostructures like organic intercalated TiS 2 . [11] In the former case, both the intra-and interlayer chemical bondings With the continuous demands on developing renewable energy technologies to solve the global energy crisis, thermoelectric materials have attracted huge attention due to their ability to convert waste heat to useful electricity. The main advantage of layer-structured materials for thermoel...
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