Nuanced dilute doping strategy enables high-performance GeTe thermoelectrics.

In thermoelectrics, doping is essential to augment the figure of merit. Traditional strategy, predominantly heavy doping, aims to optimize carrier concentration and restrain lattice thermal conductivity. However, this tactic can severely hamper carrier transport due to pronounced point defect scattering, particularly in materials with inherently low carrier mean-free-path. Conversely, dilute doping, although minimally affecting carrier mobility, frequently fails to optimize other vital thermoelectric parameters. Herein, we present a more nuanced dilute doping strategy in GeTe, leveraging the multifaceted roles of small-size metal atoms. A mere 4% CuPbSbTe3 introduction into GeTe swiftly suppresses rhombohedral distortion and optimizes carrier concentration through the aid of Cu interstitials. Additionally, the formation of multiscale microstructures, including zero-dimensional Cu interstitials, one-dimensional dislocations, two-dimensional planar defects, and three-dimensional nanoscale amorphous GeO2 and Cu2GeTe3 precipitates, along with the ensuing lattice softening, contributes to an ultralow lattice thermal conductivity. Intriguingly, dilute CuPbSbTe3 doping incurs only a marginal decrease in carrier mobility. Subsequent trace Cd doping, employed to alleviate the bipolar effect and align the valence bands, yields an impressive figure-of-merit of 2.03 at 623 K in (Ge0.97Cd0.03Te)0.96(CuPbSbTe3)0.04. This leads to a high energy-conversion efficiency of 7.9% and a significant power density of 3.44 W cm-2 at a temperature difference of 500 K. These results underscore the invaluable insights gained into the constructive role of nuanced dilute doping in the concurrent tuning of carrier and phonon transport in GeTe and other thermoelectric materials.

Mg Compensating Design in the Melting-Sintering Method For High-Performance Mg3 (Bi, Sb)2 Thermoelectric Devices.

N-type Mg3 (Bi, Sb)2 -based thermoelectric (TE) alloys show great promise for solid-state power generation and refrigeration, owing to their excellent figure-of-merit (ZT) and using cheap Mg. However, their rigorous preparation conditions and poor thermal stability limit their large-scale applications. Here, this work develops an Mg compensating strategy to realize n-type Mg3 (Bi, Sb)2 by a facile melting-sintering approach. "2D roadmaps" of TE parameters versus sintering temperature and time are plotted to understand the Mg-vacancy-formation and Mg-diffusion mechanisms. Under this guidance, high weight mobility of 347 cm2  V-1  s-1 and power factor of 34 µW cm-1  K-2 can be obtained for Mg3.05 Bi1.99 Te0.01 , and a peak ZT≈1.55 at 723 K and average ZT≈1.25 within 323-723 K can be obtained for Mg3.05 (Sb0.75 Bi0.25 )1.99 Te0.01 . Moreover, this Mg compensating strategy can also improve the interfacial connecting and thermal stability of corresponding Mg3 (Bi, Sb)2 /Fe TE legs. As a consequence, this work fabricates an 8-pair Mg3 Sb2 -GeTe-based power-generation device reaching an energy conversion efficiency of ≈5.0% at a temperature difference of 439 K, and a one-pair Mg3 Sb2 -Bi2 Te3 -based cooling device reaching -10.7 °C at the cold side. This work paves a facile way to obtain Mg3 Sb2 -based TE devices at low cost and also provides a guide to optimize the off-stoichiometric defects in other TE materials.

Regulating the Configurational Entropy to Improve the Thermoelectric Properties of (GeTe)1-x(MnZnCdTe3)x Alloys.

In thermoelectrics, entropy engineering as an emerging paradigm-shifting strategy can simultaneously enhance the crystal symmetry, increase the solubility limit of specific elements, and reduce the lattice thermal conductivity. However, the severe lattice distortion in high-entropy materials blocks the carrier transport and hence results in an extremely low carrier mobility. Herein, the design principle for selecting alloying species is introduced as an effective strategy to compensate for the deterioration of carrier mobility in GeTe-based alloys. It demonstrates that high configurational entropy via progressive MnZnCdTe3 and Sb co-alloying can promote the rhombohedral-cubic phase transition temperature toward room temperature, which thus contributes to the enhanced density-of-states effective mass. Combined with the reduced carrier concentration via the suppressed Ge vacancies by high-entropy effect and Sb donor doping, a large Seebeck coefficient is attained. Meanwhile, the severe lattice distortions and micron-sized Zn0.6Cd0.4Te precipitations restrain the lattice thermal conductivity approaching to the theoretical minimum value. Finally, the maximum zT of Ge0.82Sb0.08Te0.90(MnZnCdTe3)0.10 reaches 1.24 at 723 K via the trade-off between the degraded carrier mobility and the improved Seebeck coefficient, as well as the depressed lattice thermal conductivity. These results provide a reference for the implementation of entropy engineering in GeTe and other thermoelectric materials.

(GeTe)1-x(AgSnSe2)x: Strong Atomic Disorder-Induced High Thermoelectric Performance near the Ioffe-Regel Limit.

In thermoelectrics, the material's performance stems from a delicate tradeoff between atomic order and disorder. Generally, dopants and thus atomic disorder are indispensable for optimizing the carrier concentration and scatter short-wavelength heat-carrying phonons. However, the strong disorder has been perceived as detrimental to the semiconductor's electrical conductivity owing to the deteriorated carrier mobility. Here, we report the sustainable role of strong atomic disorder in suppressing the detrimental phase transition and enhancing the thermoelectric performance in GeTe. We found that AgSnSe2 and Sb co-alloying eliminates the unfavorable phase transition due to the high configurational entropy and achieve the cubic Ge1-x-ySbyTe1-x(AgSnSe2)x solid solutions with cationic and anionic site disorder. Though AgSnSe2 substitution drives the carrier mean free path toward the Ioffe-Regel limit and minimizes the carrier mobility, the increased carrier concentration could render a decent electrical conductivity, affording enough phase room for further performance optimization. Given the lowermost carrier mean free path, further Sb alloying on Ge sites was implemented to progressively optimize the carrier concentration and enhance the density-of-state effective mass, thereby substantially enhancing the Seebeck coefficient. In addition, the high density of nanoscale strain clusters induced by strong atomic disorders significantly restrains the lattice thermal conductivity. As a result, a state-of-the-art zT ≈ 1.54 at 773 K was attained in cubic Ge0.58Sb0.22Te0.8(AgSnSe2)0.2. These results demonstrate that the strong atomic disorder at the high entropy scale is a previously underheeded but promising approach in thermoelectric material research, especially for the numerous low carrier mobility materials.

Medium Entropy-Enabled High Performance Cubic GeTe Thermoelectrics.

The configurational entropy is an emerging descriptor in the functional materials genome. In thermoelectric materials, the configurational entropy helps tune the delicate trade-off between carrier mobility and lattice thermal conductivity, as well as the structural phase transition, if any. Taking GeTe as an example, low-entropy GeTe generally have high carrier mobility and distinguished zT > 2, but the rhombohedral-cubic phase transition restricts the applications. In contrast, despite cubic structure and ultralow lattice thermal conductivity, the degraded carrier mobility leads to a low zT in high-entropy GeTe. Herein, medium-entropy alloying is implemented to suppress the phase transition and achieve the cubic GeTe with ultralow lattice thermal conductivity yet decent carrier mobility. In addition, co-alloying of (Mn, Pb, Sb, Cd) facilitates multivalence bands convergence and band flattening, thereby yielding good Seebeck coefficients and compensating for decreased carrier mobility. For the first time, a state-of-the-art zT of 2.1 at 873 K and average zT ave of 1.3 between 300 and 873 K are attained in cubic phased Ge0.63Mn0.15Pb0.1Sb0.06Cd0.06Te. Moreover, a record-high Vickers hardness of 270 is attained. These results not only promote GeTe materials for practical applications, but also present a breakthrough in the burgeoning field of entropy engineering.

Enhanced Interfacial Reliability and Mechanical Strength of CoSb3-Based Thermoelectric Joints with Rationally Designed Diffusion Barrier Materials of Ti-Based Alloys.

To achieve high-performance thermoelectric (TE) devices, constructing a good interfacial connection between TE materials and electrodes is as important as having high figure-of-merit TE materials. Although CoSb3-based TE devices have received great attention for power generation recently, the limited long-term service stability is the main obstruct for their applications. In this work, we have prepared two kinds of Ti-based alloys (Ti83.7Al10.7Si5.6 and Ti74Ni26) as the diffusion barrier layer of CoSb3-based TE joints by the spark plasma sintering method and have systematically investigated their interfacial behaviors during the aging process. The performances of contact resistivity and mechanical strength for Ti74Ni26/Yb0.4Co3.8Fe0.2Sb12 TE joints are good before aging treatment but gradually deteriorate during the aging process, which should be ascribed to the phase-transition-induced negative thermal expansion in Ti-Ni alloys. On the other hand, Ti83.7Al10.7Si5.6/Yb0.4Co3.8Fe0.2Sb12 TE joints show both low contact resistivity (<10 µΩ·cm2) and high mechanical strength (>20 MPa) before and after 16-day aging at 500 °C, which is originated from the matching of the coefficient of thermal expansion (CTE) and the formation of network structures in Ti-Al-Si alloys. We have also prepared an eight-couple TE module of p-Ge0.9Sb0.1TeB0.01/n-Yb0.4Co3.8Fe0.2Sb12 and have measured its corresponding device performance. Our work has demonstrated that the matched CTE and network structures in the Ti-Al-Si alloy are key to obtain high-performance CoSb3-based TE joints for long-term service.

n-Bi2-xSbxTe3: A Promising Alternative to Mainstream Thermoelectric Material n-Bi2Te3-xSex near Room Temperature.

For decades, the V2VI3 compounds, specifically p-type Bi2-xSbxTe3 and n-type Bi2Te3-xSex, have remained the cornerstone of commercial thermoelectric solid-state cooling and power generation near room temperature. However, a long-standing problem in V2VI3 thermoelectrics is that n-type Bi2Te3-xSex is inferior in performance to p-type Bi2-xSbxTe3 near room temperature, restricting the device efficiency. In this work, we developed high-performance n-type Bi2-xSbxTe3, a composition long thought to only make good p-type thermoelectrics, to replace the mainstream n-type Bi2Te3-xSex. The success arises from the synergy of the following mechanisms: (i) the donorlike effect, which produces excessive conduction electrons in Bi2Te3, is compensated by the antisite defects regulated by Sb alloying; (ii) the conduction band degeneracy increases from 2 for Bi2Te3 and Bi2Te3-xSex to 6 for Bi2-xSbxTe3, favoring high Seebeck coefficients; and (iii) the larger mass fluctuation yet smaller electronegativity difference and smaller atomic radius difference between Bi and Sb effectively suppresses the lattice thermal conductivity and retains decent carrier mobility. A state-of-the-art zT of 1.0 near room temperature was attained in hot deformed Bi1.5Sb0.5Te3, which is higher than those for most known n-type thermoelectric materials, including commercial Bi2Te3-xSex ingots and the popular Mg3Sb2. Technically, building both the n-leg and p-leg of a thermoelectric module using similar chemical compositions has key advantages in the mechanical strength and the durability of devices. These results attested to the promise of n-type Bi2-xSbxTe3 as a replacement of the mainstream n-type Bi2Te3-xSex near room temperature.

Al-Si Alloy as a Diffusion Barrier for GeTe-Based Thermoelectric Legs with High Interfacial Reliability and Mechanical Strength.

To build high-performance thermoelectric (TE) devices for power generation, a suitable diffusion-barrier layer between the electrodes and the TE materials in a TE device is generally required for achieving good interfacial connection with high reliability, high mechanical strength but low electrical and thermal contact resistivities. GeTe-based materials have attracted great attention recently due to their high TE performance in the mid-temperature range, but studies on their TE devices are still limited. Here, we selected the Al66Si34 alloy as a diffusion barrier for GeTe-based TE legs based on the matching test of the coefficient of thermal expansion. The good connection between Al66Si34 and Ge0.9Sb0.1TeB0.01 is realized by the interfacial reaction, where the randomly distributed Al2Te3 and Ge precipitates are formed at the interface of the joint. The as-prepared interfacial electrical contact resistivity can be as low as 20.7 µΩ·cm2 and only slightly increases to 26.1 µΩ·cm2 after 16 days of aging at 500 °C. Moreover, the shear strength of the joints can be as high as 26.6 MPa and unexpectedly increases to 41.7 MPa after 16 days of aging. The thickness of the reaction layer tends to be stabilized after 8 days of aging and nearly does not change after further aging to 16 days, which may be ascribed to the drag effect from Si and the secondary Ge phases. These results demonstrate the great potential of the Al-Si alloy as a diffusion barrier for GeTe-based TE devices with high performance.

Simultaneous Enhancement of the Thermoelectric and Mechanical Performance in One-Step Sintered n-Type Bi2Te3-Based Alloys via a Facile MgB2 Doping Strategy.

The Bi2Te3-based alloys have been commercialized for the applications of energy harvesting and refrigeration for decades. However, the commercial Bi2Te3-based alloys produced by the zone-melting (ZM) method usually show poor mechanical strength and crack problems as well as the sluggish figure of merit ZT, especially for the less-progressed n-type samples. In this work, we have simultaneously enhanced the thermoelectric and mechanical performance of the one-step spark plasma sintering (SPS)-derived n-type Bi2Te2.7Se0.3 alloys just by doping a small amount of superconducting material MgB2 where Mg and B atoms can play significant roles in carrier density optimization and hardness enhancement. Besides the optimization of carrier density, the MgB2 doping can also increase the carrier mobility but decrease the lattice and bipolar thermal conductivity, leading to a peak ZT of 0.96 at 325 K and an average ZT of 0.88 within 300-500 K in the 0.5% MgB2-doped Bi2Te2.7Se0.3 (BTSMB) alloys. The peak ZT and average ZT of our optimized BTSMB samples are comparable and higher than those of the state-of-the-art commercial ZM ingot. Moreover, the optimized BTSMB sample also exhibits almost 70% enhancement in hardness compared with the ZM ingot. Our results demonstrate the great potential of the MgB2 doping strategy for mass production of SPS-derived Bi2Te3-based alloys in one-step sintering.

Stacking Fault-Induced Minimized Lattice Thermal Conductivity in the High-Performance GeTe-Based Thermoelectric Materials upon Bi2Te3 Alloying.

Materials with low lattice thermal conductivity (κlat) are crucial for the applications of thermal insulation and thermoelectric (TE) energy conversion. Stacking fault (SF)-induced phonon scattering within interfaces has been put forward theoretically by Klemens in 1950s. However, unlike other traditional defects such as point defects, grain boundaries, and dislocations, the role of SF for reducing κlat remains poorly understood and is yet to be revealed experimentally. The layered Bi2Te3 with a van der Waals gap shows different stacking structures than the nonlayered GeTe, which is used to introduce SFs into the GeTe-based alloys in this work. On the basis of the experimental and theoretical modeling results, this paper reveals the significant contribution of SF phonon scattering for minimizing the κlat. Besides the achieved extremely low κlat (∼0.39 W m-1 K-1 at 573 K), optimized carrier density and band convergence are also realized in the GeTe-based alloys upon Bi2Te3 alloying, leading to a significant high TE figure of merit ZT > 2 at 773 K and an averaged ZT > 1.4 within 300-773 K. This SF engineering strategy provides a different avenue to reduce the κlat for enhancing the performance of thermal insulation and TE materials.

Continuously Enhanced Structural Disorder To Suppress the Lattice Thermal Conductivity of ZrNiSn-Based Half-Heusler Alloys by Multielement and Multisite Alloying with Very Low Hf Content.

ZrNiSn-based half-Heusler (HH) alloys are considered very promising thermoelectric (TE) materials at intermediate and high temperatures due to their favorable intrinsic electrical properties, but they are also limited by their inherent high thermal conductivities. Numerous works have focused on reducing their thermal conductivities, especially their lattice thermal conductivities. A multielement (Ti, Hf, Nb, V, and Sb) and multisite alloying strategy for simultaneously improving the electrical properties and greatly reducing the lattice thermal conductivity of ZrNiSn-based HH TE materials is reported in this work. The continuous enhancement in structural disorder is the main factor in dramatically suppressing the lattice thermal conductivity of the materials. The use of suitable dopants also optimizes the electrical properties of the material, which is also an indispensable aspect in achieving high ZT values. As a consequence, a lowest lattice thermal conductivity κ l = 0.99 W/(m K) and a highest ZT ∼ 1.2 were obtained for Zr0.95M0.05Ni1.04Sn0.99Sb0.01 (M = Ti0.25Hf0.25V0.25Nb0.25) refined by a ball-milling process. The calculated conversion efficiency (η) of the same sample from room temperature to 873 K was close to 12%. In addition, compared with that used in other studies, the amount of Hf used in this study was greatly reduced, which means a reduction in cost. All of the findings in this study make the commercialization of ZrNiSn-based TE materials more competitive.