Zihang Liu

Manipulation of ionized impurity scattering for achieving high thermoelectric performance in n-type Mg3Sb2-based materials

Significance Higher carrier mobility can contribute to a larger power factor, so it is important to identify effective means for achieving higher carrier mobility. Since carrier mobility is governed by the band structure and the carrier scattering mechanism, its possible enhancement could be obtained by manipulating either or both of these. Here, we report a substantial enhancement in carrier mobility by tuning the carrier scattering mechanism in n-type Mg3Sb2-based materials. The ionized impurity scattering in these materials has been shifted into mixed scattering by acoustic phonons and ionized impurities. Our results clearly demonstrate that the strategy of tuning the carrier scattering mechanism is quite effective for improving the mobility and should also be applicable to other material systems. Achieving higher carrier mobility plays a pivotal role for obtaining potentially high thermoelectric performance. In principle, the carrier mobility is governed by the band structure as well as by the carrier scattering mechanism. Here, we demonstrate that by manipulating the carrier scattering mechanism in n-type Mg3Sb2-based materials, a substantial improvement in carrier mobility, and hence the power factor, can be achieved. In this work, Fe, Co, Hf, and Ta are doped on the Mg site of Mg3.2Sb1.5Bi0.49Te0.01, where the ionized impurity scattering crosses over to mixed ionized impurity and acoustic phonon scattering. A significant improvement in Hall mobility from ∼16 to ∼81 cm2⋅V−1⋅s−1 is obtained, thus leading to a notably enhanced power factor of ∼13 μW⋅cm−1⋅K−2 from ∼5 μW⋅cm−1⋅K−2. A simultaneous reduction in thermal conductivity is also achieved. Collectively, a figure of merit (ZT) of ∼1.7 is obtained at 773 K in Mg3.1Co0.1Sb1.5Bi0.49Te0.01. The concept of manipulating the carrier scattering mechanism to improve the mobility should also be applicable to other material systems.

The effect of nickel doping on electron and phonon transport in the n-type nanostructured thermoelectric material CoSbS

The effect of Ni doping on both electron and phonon transport properties of nanostructured CoSbS has been investigated in this study. We found a more than 2 times increase on figure-of-merit (ZT). The noticeable enhancement is mainly attributed to the optimized carrier concentration, high effective mass and strong electron–phonon scattering upon Ni doping. A ZT of 0.5 was achieved at 873 K together with a power factor of 20 μW cm−1 K−2 for the Ni doped CoSbS samples. The reduced lattice thermal conductivity via the strong electron–phonon scattering for Ni doped CoSbS samples is confirmed by the quantitative calculation of the various phonon scattering mechanisms according to the Callaway model.

Heat conduction in alloy-based superlattices

This work reports experimental studies of the thermal conductivity of alloy-based superlattices. Cross-plane thermal conductivity of a Si/Si/sub 0.71/Ge/sub 0.29/ (50 /spl Aring//10 /spl Aring/) superlattice is measured based on the 3/spl omega/ method. The measured thermal conductivity of this superlattice is 2-3 times smaller than that calculated from the Fourier heat conduction. This reduction in thermal conductivity is smaller than those observed in pure Si/Ge superlattices, possibly due to the smaller mismatch of the material properties between Si and Si/sub 0.71/Ge/sub 0.29/ as compared to between Si and Ge. To extend the 3/spl omega/ method for measuring both the cross-plane and the in-plane thermal conductivity of superlattices, a 2-wire 3/spl omega/ method is developed. Preliminary experimental results are reported for a AlAs/Al/sub 0.62/Ga/sub 0.38/As (455 /spl Aring//410 /spl Aring/) thick layer superlattice based on this method.

Understanding and manipulating the intrinsic point defect in α-MgAgSb for higher thermoelectric performance

Nanostructured α-MgAgSb has been demonstrated as a good p-type thermoelectric material candidate for low temperature power generation. Nevertheless, the intrinsic defect physics that impedes further enhancement of its thermoelectric performance is still unknown. Here we first unveil that an Ag vacancy is a dominant intrinsic point defect in α-MgAgSb and has a low defect formation energy, shown by first-principles calculations. In addition, the formation of an Ag vacancy could increase the crystal stability. More importantly, intrinsic point defects, namely an Ag vacancy, can be rationally engineered via simply controlling the hot press temperature, due to the recovery effect. Collectively, a high peak ZT of ∼1.3 and average ZT of ∼1.1 are achieved when hot pressed at 533 K. These results elucidate the pivotal role of intrinsic point defects in α-MgAgSb and further highlight that point defect engineering is an effective approach to optimize the thermoelectric properties.

High thermoelectric performance of α-MgAgSb for power generation

Solid-state thermoelectric devices enable the conversion of waste heat and solar energy into electricity, providing an alternative route for energy harvesting to tackle the challenges of energy sustainability. α-MgAgSb-based materials have recently gained popular attention as a new promising p-type candidate for low-temperature (between room temperature and 550 K) applications. A high figure of merit (ZT) of 1.2–1.4 at 550 K was achieved, along with experimental demonstration of a record high conversion efficiency of ∼8.5% under a cold- and hot-side temperature difference of 225 K, which holds the realistic prospect for power generation. In this review, we summarize the recent progress in both material-level understanding and device-level measurement of the α-MgAgSb system. First, the phase transitions and common fabrication methods are briefly introduced. Then, the origin of the “phonon glass electron crystal” behavior of α-MgAgSb is thoroughly elucidated with regard to some critical factors, such as the crystal structure, lattice dynamic properties, defect chemistry, microstructure, and band structure. Afterwards, the effective strategies to further enhance the ZT are illustrated, including optimizing the carrier concentration and intrinsic defect engineering. In addition, other feasible methods in theory are also presented. Finally, the conversion efficiency on a single-leg device is presented. In the outlook section, the currently unsolved questions as well as future directions and challenges for this material system are discussed.

Phase-transition temperature suppression to achieve cubic GeTe and high thermoelectric performance by Bi and Mn codoping

Significance Phase-transition behavior in thermoelectric materials is detrimental for their application in thermoelectric devices. Here we designed, and experimentally realized the high thermoelectric performance of cubic GeTe-based material by suppressing the phase transition from a cubic to a rhombohedral structure to below room temperature through a simple Bi and Mn codoping on the Ge site. Bi doping reduced the hole concentration while Mn alloying largely suppressed the phase-transition temperature and also induced modification of the valence bands. Our work provides the basis for studying phase transitions in other thermoelectric materials to optimize these materials for applications. Germanium telluride (GeTe)-based materials, which display intriguing functionalities, have been intensively studied from both fundamental and technological perspectives. As a thermoelectric material, though, the phase transition in GeTe from a rhombohedral structure to a cubic structure at ∼700 K is a major obstacle impeding applications for energy harvesting. In this work, we discovered that the phase-transition temperature can be suppressed to below 300 K by a simple Bi and Mn codoping, resulting in the high performance of cubic GeTe from 300 to 773 K. Bi doping on the Ge site was found to reduce the hole concentration and thus to enhance the thermoelectric properties. Mn alloying on the Ge site simultaneously increased the hole effective mass and the Seebeck coefficient through modification of the valence bands. With the Bi and Mn codoping, the lattice thermal conductivity was also largely reduced due to the strong point-defect scattering for phonons, resulting in a peak thermoelectric figure of merit (ZT) of ∼1.5 at 773 K and an average ZT of ∼1.1 from 300 to 773 K in cubic Ge0.81Mn0.15Bi0.04Te. Our results open the door for further studies of this exciting material for thermoelectric and other applications.

Large thermoelectric power factor from crystal symmetry-protected non-bonding orbital in half-Heuslers

Modern society relies on high charge mobility for efficient energy production and fast information technologies. The power factor of a material—the combination of electrical conductivity and Seebeck coefficient—measures its ability to extract electrical power from temperature differences. Recent advancements in thermoelectric materials have achieved enhanced Seebeck coefficient by manipulating the electronic band structure. However, this approach generally applies at relatively low conductivities, preventing the realization of exceptionally high-power factors. In contrast, half-Heusler semiconductors have been shown to break through that barrier in a way that could not be explained. Here, we show that symmetry-protected orbital interactions can steer electron–acoustic phonon interactions towards high mobility. This high-mobility regime enables large power factors in half-Heuslers, well above the maximum measured values. We anticipate that our understanding will spark new routes to search for better thermoelectric materials, and to discover high electron mobility semiconductors for electronic and photonic applications.The intrinsic origin of high-power factors observed in half-Heusler alloys remains elusive, limiting the design of new thermoelectric materials. In this work, the authors reveal it is due to weakened electron–acoustic phonon coupling, originating from crystal symmetry protection of non-bonding orbitals.

Ultrahigh Power Factor in Thermoelectric System Nb0.95M0.05FeSb (M = Hf, Zr, and Ti)

Abstract Conversion efficiency and output power are crucial parameters for thermoelectric power generation that highly rely on figure of merit ZT and power factor (PF), respectively. Therefore, the synergistic optimization of electrical and thermal properties is imperative instead of optimizing just ZT by thermal conductivity reduction or just PF by electron transport enhancement. Here, it is demonstrated that Nb0.95Hf0.05FeSb has not only ultrahigh PF over ≈100 µW cm−1 K−2 at room temperature but also the highest ZT in a material system Nb0.95M0.05FeSb (M = Hf, Zr, Ti). It is found that Hf dopant is capable to simultaneously supply carriers for mobility optimization and introduce atomic disorder for reducing lattice thermal conductivity. As a result, Nb0.95Hf0.05FeSb distinguishes itself from other outstanding NbFeSb‐based materials in both the PF and ZT. Additionally, a large output power density of ≈21.6 W cm−2 is achieved based on a single‐leg device under a temperature difference of ≈560 K, showing the realistic prospect of the ultrahigh PF for power generation.

Thermoelectric properties of Zintl compound Ca1−xNaxMg2Bi1.98

Motivated by good thermoelectric performance of Bi-based Zintl compounds Ca1−xYbxMg2Biy, we further studied the thermoelectric properties of Zintl compound CaMg2Bi1.98 by doping Na into Ca as Ca1−xNaxMg2Bi1.98 via mechanical alloying and hot pressing. We found that the electrical conductivity, Seebeck coefficient, power factor, and carrier concentration can be effectively adjusted by tuning the Na concentration. Transport measurement and calculations revealed that an optimal doping of 0.5 at. % Na achieved better average ZT and efficiency. The enhancement in thermoelectric performance is attributed to the increased carrier concentration and power factor. The low cost and nontoxicity of Ca1−xNaxMg2Bi1.98 makes it a potentially promising thermoelectric material for power generation in the mid-temperature range.

Discovery of ZrCoBi based half Heuslers with high thermoelectric conversion efficiency

Thermoelectric materials are capable of converting waste heat into electricity. The dimensionless figure-of-merit (ZT), as the critical measure for the material’s thermoelectric performance, plays a decisive role in the energy conversion efficiency. Half-Heusler materials, as one of the most promising candidates for thermoelectric power generation, have relatively low ZTs compared to other material systems. Here we report the discovery of p-type ZrCoBi-based half-Heuslers with a record-high ZT of ∼1.42 at 973 K and a high thermoelectric conversion efficiency of ∼9% at the temperature difference of ∼500 K. Such an outstanding thermoelectric performance originates from its unique band structure offering a high band degeneracy (Nv) of 10 in conjunction with a low thermal conductivity benefiting from the low mean sound velocity (vm ∼2800 m s−1). Our work demonstrates that ZrCoBi-based half-Heuslers are promising candidates for high-temperature thermoelectric power generation.Identifying new compounds with intrinsically high conversion efficiency is the key to demonstrating next-generation thermoelectric modules. Here, Zhu et al. report the discovery of p-type ZrCoBi-based half Heuslers with thermoelectric conversion efficiency of 9% and large high-temperature stability.