Low-cost high-performance zinc antimonide thin films for thermoelectric applications

Optimization of Thermoelectric Properties and Physical Mechanisms of Cu2Se-Based Thin Films via Heat Treatment

Cu2Se is an attractive thermoelectric material due to its layered structure, low cost, environmental compatibility, and non-toxicity. These traits make it a promising replacement for conventional thermoelectric materials in large-scale applications. This study focuses on preparing Cu2Se flexible thin films through in situ magnetron sputtering technology while carefully optimizing key preparation parameters, and explores the physical mechanism of thermoelectric property enhancement, especially the power factor. The films are deposited onto flexible polyimide substrates. Experimental findings demonstrate that films grown at a base temperature of 200 °C exhibit favorable performance. Furthermore, annealing heat treatment effectively regulates the Cu element content in the film samples, which reduces carrier concentration and enhances the Seebeck coefficient, ultimately improving the power factor of the materials. Compared to the unannealed samples, the sample annealed at 300 °C exhibited a significant increase in room temperature Seebeck coefficient, rising from 9.13 μVK-1 to 26.73 μVK-1. Concurrently, the power factor improved from 0.33 μWcm-1K-2 to 1.43 μWcm-1K-2.

High-Performance and Stable (Ag, Cd)-Containing ZnSb Thermoelectric Compounds

Binary Zn-Sb-based compounds, ZnSb and Zn4Sb3, are promising thermoelectric (TE) materials because they are low-cost and earth-abundant. However, for a long time, their real applications have been limited by the low TE figure-of-merit (zT) of ZnSb and poor thermodynamic stability of Zn4Sb3. In this study, we successfully integrate both high zT and good stability in (Ag, Cd)-containing ZnSb compounds. Alloying Cd in ZnSb greatly suppresses the lattice thermal conductivity to a minimum value of 0.97 W K-1 m-1 at 300 K, while doping Ag significantly enhances the power factor to a peak value of 17.7 μW cm-1 K-2 at 500 K and reduces the bipolar thermal conductivity. As a result of the simultaneously optimized electrical and thermal transport, a peak zT of 1.2 is achieved for Zn0.698Ag0.002Cd0.3Sb at 600 K, which is comparable with the best values reported for Zn4Sb3-based compounds. Moreover, a current stress test confirms that introducing Ag and Cd does not degrade the good stability of ZnSb under an electric field. The phase composition and thermoelectric performance of Zn0.698Ag0.002Cd0.3Sb are not changed even under a high current density of 50 A cm-2, showing much better stability than Zn4Sb3. This study would accelerate the real application of ZnSb-based compounds in the field of waste heat harvesting.

Thermoelectric Materials for Textile Applications

Since prehistoric times, textiles have served an important role-providing necessary protection and comfort. Recently, the rise of electronic textiles (e-textiles) as part of the larger efforts to develop smart textiles, has paved the way for enhancing textile functionalities including sensing, energy harvesting, and active heating and cooling. Recent attention has focused on the integration of thermoelectric (TE) functionalities into textiles-making fabrics capable of either converting body heating into electricity (Seebeck effect) or conversely using electricity to provide next-to-skin heating/cooling (Peltier effect). Various TE materials have been explored, classified broadly into (i) inorganic, (ii) organic, and (iii) hybrid organic-inorganic. TE figure-of-merit (ZT) is commonly used to correlate Seebeck coefficient, electrical and thermal conductivity. For textiles, it is important to think of appropriate materials not just in terms of ZT, but also whether they are flexible, conformable, and easily processable. Commercial TEs usually compromise rigid, sometimes toxic, inorganic materials such as bismuth and lead. For textiles, organic and hybrid TE materials are more appropriate. Carbon-based TE materials have been especially attractive since graphene and carbon nanotubes have excellent transport properties with easy modifications to create TE materials with high ZT and textile compatibility. This review focuses on flexible TE materials and their integration into textiles.

Structural evolution in thermoelectric zinc antimonide thin films studied by in situ X-ray scattering techniques

Zinc antimonides have been widely studied owing to their outstanding thermoelectric properties. Unlike in the bulk state, where various structurally unknown phases have been identified through their specific physical properties, a number of intermediate phases in the thin-film state remain largely unexplored. Here, in situ X-ray diffraction and X-ray total scattering are combined with in situ measurement of electrical resistivity to monitor the crystallization process of as-deposited amorphous Zn-Sb films during post-deposition annealing. The as-deposited Zn-Sb films undergo a structural evolution from an amorphous phase to an intermediate crystalline phase and finally the ZnSb phase during heat treatment up to 573 K. An intermediate phase (phase B) is identified to be a modified β-Zn8Sb7 phase by refinement of the X-ray diffraction data. Within a certain range of Sb content (∼42-55 at%) in the films, phase B is accompanied by an emerging Sb impurity phase. Lower Sb content leads to smaller amounts of Sb impurity and the formation of phase B at lower temperatures, and phase B is stable at room temperature if the annealing temperature is controlled. Pair distribution function analysis of the amorphous phase shows local ordered units of distorted ZnSb4 tetrahedra, and annealing leads to long-range ordering of these units to form the intermediate phase. A higher formation energy is required when the intermediate phase evolves into the ZnSb phase with a significantly more regular arrangement of ZnSb4 tetrahedra.

Energy Harvesting from a Thermoelectric Zinc Antimonide Thin Film under Steady and Unsteady Operating Conditions

In practice, there are some considerations to study stability, reliability, and output power optimization of a thermoelectric thin film operating dynamically. In this study stability and performance of a zinc antimonide thin film thermoelectric (TE) specimen is evaluated under transient with thermal and electrical load conditions. Thermoelectric behavior of the specimen and captured energy in each part of a thermal cycle are investigated. Glass is used as the substrate of the thin film, where the heat flow is parallel to the length of the thermoelectric element. In this work, the thermoelectric specimen is fixed between a heat sink exposed to the ambient temperature and a heater block. The specimen is tested under various electrical load cycles during a wide range of thermal cycles. The thermal cycles are provided for five different aimed temperatures at the hot junction, from 160 to 350 °C. The results show that the specimen generates approximately 30% of its total electrical energy during the cooling stage and 70% during the heating stage. The thin film generates maximum power of 8.78, 15.73, 27.81, 42.13, and 60.74 kW per unit volume of the thermoelectric material (kW/m³), excluding the substrate, corresponding to hot side temperature of 160, 200, 250, 300, and 350 °C, respectively. Furthermore, the results indicate that the thin film has high reliability after about one thousand thermal and electrical cycles, whereas there is no performance degradation.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity

We report electron-doped AgBi₃S₅ as a new high-performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5-0.3 W m^-1 K^-1 in the temperature range of 300-800 K, which is ascribed to its unusual vibrational properties: "double rattling" phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi₃S₅. In the carrier concentration range (5 × 10¹⁸-2 × 10¹⁹ cm^-3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 m_e (m_e is the free electron mass). Samples of 0.33% Cl-doped AgBi₃S₅ prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.

High-pressure single crystal X-ray diffraction study of thermoelectric ZnSb and β-Zn4Sb3

The crystal structures of thermoelectric ZnSb and Zn₄Sb₃ have been studied by high pressure single crystal X-ray diffraction and the pressure behavior is different from thermal response.

In situ nanostructure design leading to a high figure of merit in an eco-friendly stable Mg2Si0.30Sn0.70 solid solution

The relationship between the temperature and the composition as well as the microstructure of a Sb-doped Mg₂Si_0.30Sn_0.70 solid solution was systematically studied according to the Mg₂Si–Mg₂Sn pseudo-binary phase diagram.

First principles study of defect formation in thermoelectric zinc antimonide, β-Zn4Sb3

Understanding the formation of various point defects in the promising thermoelectric material, β-Zn(4)Sb(3), is crucial for theoretical determination of the origins of its p-type behavior and considerations of potential n-type dopability. While n-type conductivity has been fleetingly observed in Te:ZnSb, there have been no reports, to the best of our knowledge, of stable n-type behavior in β-Zn(4)Sb(3). To understand the origin of this difficulty, we investigated the formation of intrinsic point defects in β-Zn(4)Sb(3) density functional theory calculations. We found that a negatively charged zinc vacancy is the dominant defect in β-Zn(4)Sb(3), as it is also in ZnSb. This explains the unintentional p-type behavior of the material and makes n-doping very difficult since the formation of the defect becomes more favorable at higher Fermi levels, near the conduction band minimum (CBM). We also calculated the formation energy of the cation dopants: Li, Na, B, Al, Ga, In, Tl; of these, only Li and Na are thermodynamically favorable compared to the acceptor Zn vacancy over a range of Fermi levels along the band gap. Further analysis of the band structure shows that Li:Zn(4)Sb(3) has a partially occupied topmost valence band, making this defect an acceptor so that Li:Zn(4)Sb(3) is indeed a p-type thermoelectric material. The introduction of Li, however, creates a more orderly and symmetric configuration, which stabilizes the host structure. Furthermore, Li reduces the concentration of holes and increases the Seebeck coefficient; hence, Li:Zn(4)Sb(3) is more stable and better performing as a thermoelectric material than undoped β-Zn(4)Sb(3).

Epitaxial growth and thermoelectric properties of c-axis oriented Bi1−xPbxCuSeO single crystalline thin films

Enhanced thermoelectric performance of c-axis oriented Bi_1−xPb_xCuSeO single crystalline thin films.

Indium selenides: structural characteristics, synthesis and their thermoelectric performances

Indium selenides have attracted extensive attention in high-efficiency thermoelectrics for waste heat energy conversion due to their extraordinary and tunable electrical and thermal properties. This Review aims to provide a thorough summary of the structural characteristics (e.g. crystal structures, phase transformations, and structural vacancies) and synthetic methods (e.g. bulk materials, thin films, and nanostructures) of various indium selenides, and then summarize the recent progress on exploring indium selenides as high-efficiency thermoelectric materials. By highlighting challenges and opportunities in the end, this Review intends to shine some light on the possible approaches for thermoelectric performance enhancement of indium selenides, which should open up an opportunity for applying indium selenides in the next-generation thermoelectric devices.

Unexpected high-temperature stability of β-Zn4Sb3 opens the door to enhanced thermoelectric performance

β-Zn4Sb3 has one of the highest ZT reported for binary compounds, but its practical applications have been hindered by a reported poor stability. Here we report the fabrication of nearly dense single-phase β-Zn4Sb3 and a study of its thermoelectric transport coefficients across a wide temperature range. Around 425 K we find an abrupt decrease of its thermal conductivity. Past this point, Zn atoms can migrate from crystalline sites to interstitial positions; β-Zn4Sb3 becomes metastable and gradually decomposes into Zn(hcp) and ZnSb. However, above 565 K it recovers its stability; in fact, the damage caused by decomposition can be repaired completely. This is key to its excellent thermoelectric performance at high temperature: the maximum ZT reaches 1.4. Molecular dynamics simulations are used to shed light on the microscopic behavior of the material.