Phase transition in thermoelectric (TE) material is a double‐edged sword—it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, ...Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic‐to‐rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures‐of‐merit zT. Specifically, alloying GeTe by Sb2Te3 significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe‐based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.
The interplay between phase decomposition and athermal phase transition is leveraged in a Ge–Sb–Te ternary system to enable exquisite microstructure features by strong composition fluctuations and coexistence of rhombohedral and cubic GeTe. Specifically, alloying GeTe with Sb2Te3 significantly suppresses thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6.
Thermoelectric (TE) research is not only a course of materials by discovery but also a seedbed of novel concepts and methodologies. Herein, the focus is on recent advances in three emerging ...paradigms: entropy engineering, phase‐boundary mapping, and liquid‐like TE materials in the context of thermodynamic routes. Specifically, entropy engineering is underpinned by the core effects of high‐entropy alloys; the extended solubility limit, the tendency to form a high‐symmetry crystal structure, severe lattice distortions, and sluggish diffusion processes afford large phase space for performance optimization, high electronic‐band degeneracy, rich multiscale microstructures, and low lattice thermal conductivity toward higher‐performance TE materials. Entropy engineering is successfully implemented in half‐Huesler and IV–VI compounds. In Zintl phases and skutterudites, the efficacy of phase‐boundary mapping is demonstrated through unraveling the profound relations among chemical compositions, mutual solubilities of constituent elements, phase instability, microstructures, and resulting TE properties at the operation temperatures. Attention is also given to liquid‐like TE materials that exhibit lattice thermal conductivity at lower than the amorphous limit due to intensive mobile ion disorder and reduced vibrational entropy. To conclude, an outlook on the development of next‐generation TE materials in line with these thermodynamic routes is given.
High configurational entropy, phase‐boundary mapping, and liquid–solid ions are thermodynamic routes for designing ultralow thermal conductivity and high‐performance thermoelectrics. These conceptual and methodological breakthroughs provide new perspectives for developing next‐generation thermoelectrics.
PbTe‐based alloys have been widely used as mid‐temperature thermoelectric (TE) materials since the 1960s. Years of endeavor spurred the tremendous advances in their TE performance. The breakthroughs ...for n‐type PbTe have been somewhat less impressive, which limits the overall conversion efficiency of a PbTe‐based TE device. In light of this obstacle, an n‐type Ga‐doped PbTe via an alternative thermodynamic route that relies on the equilibrium phase diagram and microstructural evolution is revisited. Herein, a plateau of zT = 1.2 is achieved in the best‐performing Ga0.02Pb0.98Te in the temperature range of 550–673 K. Notably, an extremely high average zTave = 1.01 is obtained within 300 − 673 K. The addition of gallium optimizes the carrier concentration and boosts the power factor PF = S2ρ−1. Meanwhile, the κL of Ga‐PbTe reveals a significantly decreasing tendency owing to the defect evolution that changes from dislocation loop to nano‐precipitation with increasing Ga content. The pathway for both the κL reduction and defect evolution can be probed by an equilibrium phase diagram, which opens up a new avenue for locating high zT TE materials.
A synergic approach, including defect engineering and carrier concentration optimization, to the fabrication of Ga‐doped PbTe thermoelectrics elicits reduced thermal conductivity and an enhanced power factor. It results in an extraordinary n‐type Ga0.02Pb0.98Te alloy, whose peak zT achieves 1.3 at 673 K. A phase diagram probes the best compositional region for the n‐type Ga‐PbTe alloys, in which the dislocation loop or nano‐precipitate forms.
Incorporating dilute doping and controlled synthesis provides a means to modulate the microstructure, defect density, and transport properties. Transmission electron microscopy (TEM) and geometric ...phase analysis (GPA) have revealed that hot‐pressing can increase defect density, which redistributes strain and helps prevent unwanted Ge precipitates formation. An alloy of GeTe with a minute amount of indium added has shown remarkable TE properties compared to its undoped counterpart. Specifically, it achieves a maximum figure‐of‐merit zT of 1.3 at 683 K and an exceptional TE conversion efficiency of 2.83% at a hot‐side temperature of 723 K. Significant zT and conversion efficiency improvements are mainly due to domain density engineering facilitated by an effective hot‐pressing technique applied to lightly doped GeTe. The In–GeTe alloy exhibits superior TE properties and demonstrates notable stability under significant thermal gradients, highlighting its promise for use in mid‐temperature TE energy generation systems.
Domain density modification coupled with the incorporation of trace indium in GeTe significantly elevates the peak figure‐of‐merit (zT) to 1.3 and achieves a 3% thermoelectric conversion efficiency at 700 K. These enhancements stem from strain redistribution via hot‐pressing and carrier concentration optimization through the reduction of germanium vacancies, bypassing the need for additional doping.
The n-type I-V-VI2 AgBiSe2 features intrinsically low κ due to the anharmonicity of chemical bonds. Experimentally-determined isothermal section guides the starting compositions for the following ...AgBiSe2-based alloys. Among the undoped alloys, the Ag25Bi25Se50 exhibits a highest peak of zT∼0.75, and yet the neighboring Ag20Bi27.5Se52.5, which involves a Se-rich liquid phase, has a much lower zT∼0.3 at 748 K, respectively. With the incorporation of Ge, the (GeSe)0.03(AgBiSe2)0.97 exhibits an ultralow κ∼0.3 (W/mK), owing to the formation of Bi2Se3 nano-precipitate in the size of 20–40 nm. Additionally, the moiré fringes with a periodicity of 0.25 nm are observed in the Bi2Se3 nano-precipitate, implying the presence of local mass fluctuation and superlattice, which could further lead to enhancing phonon scattering and reduced κ. As a result, the ultra-low κ∼0.3 (W/mK) boosts the peak of zT up to zT∼1.05 in n-type (GeSe)0.03(AgBiSe2)0.97, which shows a 140% enhancement compared with that of the undoped AgBiSe2.
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Among all types of thermoelectric materials, chalcopyrite CuGaTe2 has been viewed as a promising candidate for use of thermoelectric generator due to its high figure-of-merit (zT) at the high ...temperatures. Herein the 923 K isothermal section of ternary Cu-Ga-Te system is determined, using various thermally-equilibrated Cu-Ga-Te alloys, and ternary CuGaTe2 phase is stabilized within the compositional region of 48.0–53.0 at%Te and 25.0 at%-30.0 at%Cu. Moreover, the solubility of Cu in binary Ga2Te3 and Ga3Te4 compounds at 923 K is negligible, while that in GaTe phase reaches 7.9 at%Cu. The as-determined isothermal section, depicting the phase stability regime of CuGaTe2, provides options for precisely locating the compositions of CuGaTe2-based materials that lead to promising and reproducible thermoelectric properties. A zT peak of 0.6 has been achieved in the Bridgman-grown Cu25Ga26Te49 alloy at 750 K, which is nearly eight times higher than the neighboring Cu28Ga25Te47 alloy, presumably due to the fact that the Cu25Ga26Te49 alloy, which exhibits high phase purity of CuGaTe2, has lower lattice thermal conductivity (κL∼1.3 (W/mK) and higher power factor (PF∼11.2 (μW/mK2)), comparing with that of Cu28Ga25Te47 alloy (κL∼1.8 (W/mK) and PF∼1.9 (μW/mK2)), which locates in a three-phased CuGaTe2+Cu2Te + Cu9Ga4 region, with only a slight deviation in the starting composition.
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Thermoelectric (TE) generators have come a long way since the first commercial apparatus launched in the 1950s. Since then, the β‐Zn4Sb3 has manifested its potential as a cost‐effective and ...environmentally friendly TE generator compared with the tellurium‐bearing TE materials. Although the β‐Zn4Sb3 features an intrinsically low thermal conductivity κ, it suffers from a long‐lasting structural instability issue arising from the highly mobile zinc ions. Herein, the dilute Ga dopant gives rise to the aliovalent substitution, lowers the mobile zinc ions, and optimizes the hole carrier concentration nH simultaneously. Meanwhile, the formation of nano‐moiré fringes suggests the modulated distribution of point defect that results from soluble Ga in a β‐Zn4Sb3 lattice, which elicits an ultralow lattice thermal conductivity κL = 0.2 W m−1 K−1 in a (Zn0.992Ga0.008)4Sb3 alloy. Hence, a fully dense β‐Zn4Sb3 incorporated with the dilute Ga doping reveals superior structural stability with a peak zT > 1.4 at 623 K. In this work, the aliovalent dilute doping coupled with phase diagram engineering optimizes the fluxes of moving electrons and charged ions, which stabilizes the single‐phase β‐Zn4Sb3 while boosting the TE performance at the mid‐temperature region. The synergistic strategies endow the ionic crystals with a thermodynamic route, which opens up a new category for high‐performance and thermal robust TE alloys.
Aliovalent cation substitution between Zn2+ and Ga3+ diminishes the highly mobile Zn2+ and optimizes the hole carriers, leading to an ultrahigh PF = 1.6 mW m−1 K−2. The interstitial Ga0 induces the formation of nano‐moiré fringes and thus elicits an ultralow κL = 0.2 W m−1 K−1. A light‐doped (Zn0.992Ga0.008)4Sb3 crystal attains a peak zT of 1.4 in the β‐Zn4Sb3 single‐phase region.
In this study, the Sb content within p‐type Bi2Te3 by employing phase diagram engineering is strategically tuned. This method retains the advantages of Sb doping but mitigated the brittleness ...typically seen in high‐Sb Bi0.5Sb1.5Te3 (BST). The as‐constructed phase diagram demonstrates the asymmetrical homogeneity of (Bi, Sb)2Te3, guiding focus toward developing an optimized p‐type (Bi2Te3)0.96(Sb2Te)0.04 with reduced Sb content. The resulting crystal of (Bi2Te3)0.96(Sb2Te)0.04 exhibit an exceptional peak zT of 1.3 at 303 K, surpassing the mechanical robustness of standard high‐Sb BST. Additionally, it matches the energy conversion efficiency of traditional BST, achieving 2.3% at a temperature difference ΔT of 150 K. This significant advance makes (Bi2Te3)0.96(Sb2Te)0.04 a potential competitor to the well‐established BST, thanks to its enhanced thermoelectric performance owing to the elevated carrier concentration and a less brittle nature due to the diluted Sb dopant.
Leveraging dilute doping and phase diagram engineering, the p‐type (Bi2Te3)0.96(Sb2Te)0.04 achieves a peak zT of 1.3 at 303 K with a conversion efficiency of 2.3% when integrated into a single‐leg device. This p‐type crystal retains its softness while exhibiting diminished hardness, reducing the potential for cleavage and breakages. This offers a compelling contender in comparison to the benchmark BST.
In this work, a high thermoelectric figure of merit, zT of 1.9 at 740 K is achieved in Ge
Bi
Te crystals through the concurrent of Seebeck coefficient enhancement and thermal conductivity reduction ...with Bi dopants. The substitution of Bi for Ge not only compensates the superfluous hole carriers in pristine GeTe but also shifts the Fermi level (E
) to an eligible region. Experimentally, with moderate 6-10% Bi dopants, the carrier concentration is drastically decreased from 8.7 × 10
cm
to 3-5 × 10
cm
and the Seebeck coefficient is boosted three times to 75 μVK
. In the meantime, based on the density functional theory (DFT) calculation, the Fermi level E
starts to intersect with the pudding mold band at L point, where the band effective mass is enhanced. The enhanced Seebeck coefficient effectively compensates the decrease of electrical conductivity and thus successfully maintain the power factor as large as or even superior than that of the pristine GeTe. In addition, the Bi doping significantly reduces both thermal conductivities of carriers and lattices to an extremely low limit of 1.57 W m
K
at 740 K with 10% Bi dopants, which is an about 63% reduction as compared with that of pristine GeTe. The elevated figure of merit observed in Ge
Bi
Te specimens is therefore realized by synergistically optimizing the power factor and downgrading the thermal conductivity of alloying effect and lattice anharmonicity caused by Bi doping.
For decades, the Bismuth-telluride (Bi2Te3) has been intensively studied as the thermoelectric (TE) cooler. Still, new ideas and emerging results are put forward for raising the conversion ...efficiency. Herein, we re-visit the Cu-doped Bi2Te3, and report the high zT values nearing the room temperature, for both the p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 (zT∼1.2 at 300 K) as well as the n-type (Cu2Te)0.09(Bi2Te3)0.91 (zT∼1.09 at 363 K), respectively. Given that the phase boundary mapping is essential for the optimization of high-efficiency TE materials, the isothermal section of ternary Bi-Cu-Te at 523 K is constructed, by collecting the phase equilibria information of various thermally-equilibrated alloys; it further guides the alloying directions for (Cu2Te)x (Bi2Te3)1-x and CuyBi2-yTe3, respectively. Small modulation in the stoichiometry leads to the carrier type transition. As a consequence, the lamellae composed of Bi2Te3 and Cu7Te5 precipitate along the grain boundary of the n-type (Cu2Te)0.09(Bi2Te3)0.91, resulting in the reduced κ, due to the stronger interfacial phonon scattering, and the higher PF, owing to the higher amounts of Te vacancy (VTe). On the contrary, the promising p-type (Cu2Te)0.01(Bi2Te3)0.99/Cu0.01Bi1.99Te3 features the single-phase Bi2Te3, whereas the soluble Cu introduces extra holes and might therefore promote the p-type conduction.
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