Grain size effects on thermoelectrical properties of sintered solid solutions based on Bi2Te3

Realizing high-efficiency thermoelectric module by suppressing donor-like effect and improving preferred orientation in n-type Bi2(Te, Se)3

Thermoelectric materials have a wide range of application because they can be directly used in refrigeration and power generation. And the Bi2Te3 stand out because of its excellent thermoelectric performance and are used in commercial thermoelectric devices. However, n-type Bi2Te3 has seriously hindered the development of Bi2Te3-based thermoelectric devices due to its weak mechanical properties and inferior thermoelectric performance. Therefore, it is urgent to develop a high-performance n-type Bi2Te3 polycrystalline. In this work, we employed interstitial Cu and the hot deformation process to optimize the thermoelectric properties of Bi2Te2.7Se0.3, and a high-performance thermoelectric module was fabricated based on this material. Our combined theoretical and experimental effort indicates that the interstitial Cu reduce the defect density in the matrix and suppresses the donor-like effect, leading to a lattice plainification effect in the material. In addition, the two-step hot deformation process significantly improves the preferred orientation of the material and boosts the mobility. As a result, a maximum ZT of 1.27 at 373 K and a remarkable high ZTave of 1.22 across the temperature range of 300-425 K are obtained. The thermoelectric generator (TEG, 7-pair) and thermoelectric cooling (TEC, 127-pair) modules were fabricated with our n-type textured Cu0.01Bi2Te2.7Se0.3 coupled with commercial p-type Bi2Te3. The TEC module demonstrates superior cooling efficiency compared with the commercial Bi2Te3 device, achieving a ΔT of 65 and 83.4 K when the hot end temperature at 300 and 350 K, respectively. In addition, the TEG module attains an impressive conversion efficiency of 6.5% at a ΔT of 225 K, which is almost the highest value among the reported Bi2Te3-based TEG modules.

Effect of Starting Powder Particle Size on the Thermoelectric Properties of Hot-Pressed Bi0.3Sb1.7Te3 Alloys

P-type Bi0.3Sb1.7Te3 polycrystalline pellets were fabricated using different methods: melting and mechanical alloying, followed by hot-press sintering. The effect of starting powder particle size on the thermoelectric properties was investigated in samples prepared using powders of different particle sizes (with micro- and/or nano-scale dimensions). A peak ZT (350 K) of ~1.13 was recorded for hot-pressed samples prepared from mechanical alloyed powder. Moreover, hot-pressed samples prepared from ≤45 μm powder exhibited similar ZT (~1.1). These high ZT values are attributed both to the presence of high-density grain boundaries, which reduced the lattice thermal conductivity, as well as the formation of antisite defects during milling and grinding, which resulted in lower carrier concentrations and higher Seebeck coefficient values. In addition, Bi0.3Sb1.7Te3 bulk nanocomposites were fabricated in an attempt to further reduce the lattice thermal conductivity. Surprisingly, however, the lattice thermal conductivity showed an unexpected increasing trend in nanocomposite samples. This surprising observation can be attributed to a possible overestimation of the lattice thermal conductivity component by using the conventional Wiedemann-Franz law to estimate the electronic thermal conductivity component, which is known to occur in nanocomposite materials with significant grain boundary electrical resistance.

Modulation of Vacancy Defects and Texture for High Performance n-Type Bi2 Te3 via High Energy Refinement

The carrier concentration in n-type layered Bi₂ Te₃ -based thermoelectric (TE) material is significantly impacted by the donor-like effect, which would be further intensified by the nonbasal slip during grain refinement of crushing, milling, and deformation, inducing a big challenge to improve its TE performance and mechanical property simultaneously. In this work, high-energy refinement and hot-pressing are used to stabilize the carrier concentration due to the facilitated recovery of cation and anion vacancies. Based on this, combined with SbI₃ doping and hot deformation, the optimized carrier concentration and high texture degree are simultaneously realized. As a result, a peak figure of merit (zT) of 1.14 at 323 K for Bi₂ Te_2.7 Se_0.3  + 0.05 wt.% SbI₃ sample with the high bending strength of 100 Mpa is obtained. Furthermore, a 31-couple thermoelectric cooling device consisted of n-type Bi₂ Te_2.7 Se_0.3  + 0.05 wt.% SbI₃ and commercial p-type Bi_0.5 Sb_1.5 Te₃ legs is fabricated, which generates the large maximum temperature difference (ΔT_max ) of 85 K at a hot-side temperature of 343 K. Thus, the discovery of recovery effect in high energy refinement and hot-pressing has significant implications for improving TE performance and mechanical strength of n-type Bi₂ Te₃ , thereby promoting its applications in harsh conditions.

Comprehensive Insight into p-Type Bi2Te3-Based Thermoelectrics near Room Temperature

Bi2Te3 is a well-recognized material for its unique properties in diverse thermoelectric applications near room temperature. The considerable efforts on Bi2Te3-based alloys have been extremely extensive in recent years, and thus the latest breakthroughs in high-performance p-type (Bi, Sb)2Te3 alloys are comprehensively reviewed to further implement applications. Effective strategies to further improve the thermoelectric performance are summarized from the perspective of enhancing the power factor and minimizing the lattice thermal conductivity. Furthermore, the surface states of topological insulators are investigated to provide a possibility of advancing (Bi, Sb)2Te3 thermoelectrics. Finally, future challenges and outlooks are overviewed. This review will provide potential guidance toward designing and developing high-efficient Bi2Te3-based and other thermoelectrics.

Fermi level tuning in modified Bi2Te3 system for thermoelectric applications

The techniques of size reduction and defect engineering have attracted great interest as an effective strategy for developing new and modified thermoelectric materials with high performance. Here, we investigated the thermoelectric properties of the Bi2Te3 system modified by Fermi level tuning through the methods of nanostructuring and doping. Band structure modification and Fermi level shift contribute to the better thermoelectric performance of the material. A minimum thermal conductivity of 0.72 W K-1 was obtained at 440 K with a reduction of 50% as compared to the pristine sample. The improved transport properties along with considerably reduced thermal conductivity result in a two-fold improved figure of merit of 0.295 at 420 K for the nanostructured sample Bi0.4Sb1.6Te3 over pristine Bi2Te3.

Compressive Fatigue Behavior and Its Influence on the Thermoelectric Properties of p-Type Bi0.5Sb1.5Te3 Alloys

BiSbTe alloy is one of the most important thermoelectric materials that has been commercialized for large-scale applications in waste heat recovery and spot cooling. However, its practical application often involves complicated service conditions, such as substantial dynamic vibrational stresses as well as long-term exposure to the large thermal gradient that usually generates high thermal stress. Thus, it is of vital importance to investigate the mechanical response and the evolution of microstructure and thermoelectric performance of BiSbTe alloy under the quasi-static force and dynamic compressive stress. Herein, we elucidate the compressive fatigue behavior and its influence on the thermoelectric performance of p-type Bi_0.5Sb_1.5Te₃ materials prepared by melt spinning and subsequent plasma-activated sintering. The fatigue life was tested at various stress ratios ranging from 60 to 90%. The microstructure evaluation indicates that repetitive loading of compressive stress contributes to largely accumulated strains and dislocations near grain boundaries. With the increasing cycling numbers, the magnitude of accumulated strains reaches the critical level and leads to microcrack initiation and propagation of fatigue cracks. The cyclic compressive stress resulted in the marked degradation of thermoelectric performance in Bi_0.5Sb_1.5Te₃ material due to the strong suppression of carrier mobility by the fatigue-induced defects. This work can provide important guidance for the practical applications of p-type Bi_0.5Sb_1.5Te₃ material.

Enhancing Thermoelectric Performance of n-Type Hot Deformed Bismuth-Telluride-Based Solid Solutions by Nonstoichiometry-Mediated Intrinsic Point Defects

Bismuth-telluride-based solid solutions are the unique thermoelectric (TE) materials near room temperature. Various approaches have been applied to enhance the thermoelectric performance, and much progress has been made in their p-type materials. However, for the n-type counterparts, little breakthrough has been obtained. We herein report on enhancing thermoelectric performance of n-type bismuth-telluride-based alloys by nonstoichiometry to mediate the point defects, combined with one-time hot deformation. The improved power factor of 3.3 × 10^-3 W m^-1 K^-2 and reduced lattice thermal conductivity contribute to a high figure-of-merit, zT, of 1.2 at 450 K for n-type Bi₂Te_2.3Se_0.69 alloys with Se deficiency. The high zT is comparable to that of Bi₂Te_2.3Se_0.7 hot deformed three times, which is a practically complicated process. The results demonstrate that nonstoichiometry can be an effective and simple strategy in mediating intrinsic point defects and enhancing the thermoelectric performance of bismuth-telluride-based alloys.

New Insights into Intrinsic Point Defects in V2VI3 Thermoelectric Materials

Defects and defect engineering are at the core of many regimes of material research, including the field of thermoelectric study. The 60-year history of V2VI3 thermoelectric materials is a prime example of how a class of semiconductor material, considered mature several times, can be rejuvenated by better understanding and manipulation of defects. This review aims to provide a systematic account of the underexplored intrinsic point defects in V2VI3 compounds, with regard to (i) their formation and control, and (ii) their interplay with other types of defects towards higher thermoelectric performance. We herein present a convincing case that intrinsic point defects can be actively controlled by extrinsic doping and also via compositional, mechanical, and thermal control at various stages of material synthesis. An up-to-date understanding of intrinsic point defects in V2VI3 compounds is summarized in a (χ, r)-model and applied to elucidating the donor-like effect. These new insights not only enable more innovative defect engineering in other thermoelectric materials but also, in a broad context, contribute to rational defect design in advanced functional materials at large.

Surfactant-free scalable synthesis of Bi2 Te3 and Bi2Se3 nanoflakes and enhanced thermoelectric properties of their nanocomposites

Surfactant-free nanoflakes of n-type Bi2 Te3 and Bi2 Se3 are synthesized in high yields. Their suspensions are mixed to create nanocomposites with heterostructured nanograins. A maximum ZT (0.7 at 400 K) is achieved with a broad content of 10-15% Bi2 Se3 in the nanocomposites.