Achieving Compatible p/n-Type Half-Heusler Compositions in Valence Balanced/Unbalanced Mg1-xVxNiSb.

In thermoelectric and other inorganic materials research, the significance of half-Heusler (HH) compositions following the 18-electron rule has drawn interest in developing and exploiting the potential of intermetallic compounds. For the fabrication of thermoelectric modules, in addition to high-performance materials, having both p- and n-type materials with compatible thermal expansion coefficients is a prerequisite for module development. In this work, the p-type to n-type transition of valence balanced/unbalanced HH composition of Mg1-xVxNiSb was demonstrated by changing the Mg:V chemical ratio. The Seebeck coefficient and power factor of Ti-doped Mg0.57V0.33Ti0.1NiSb are -130 µV K-1 and 0.4 mW m-1 K-2 at 400 K, respectively. In addition, the reduced lattice thermal conductivity (κL < 2.5 W m-1 K-1 at 300 K) of n-type compositions was reported to be much smaller than κL of conventional HH materials. As high thermal conductivity has long been an issue for HH materials, the synthesis of p- and n-type Mg1-xVxNiSb compositions with low lattice thermal conductivity is a promising strategy for producing high-performance HH compounds. Achieving both p- and n-type materials from similar parent composition enabled us to fabricate a thermoelectric module with maximum output power Pmax ∼ 63 mW with a temperature difference of 390 K. This finding supports the benefit of exploring the huge compositional space of valence balanced/unbalanced quaternary HH compositions for further development of thermoelectric devices.

Evolution of Nanometer-Scale Microstructure within Grains and in the Intergranular Region in Thermoelectric Mg3(Sb, Bi)2 Alloys.

n-type Mg3Sb2-Mg3Bi2 alloys have been investigated as one of the most promising thermoelectric materials. To achieve high performance, a detailed understanding of the microstructure is required. Although Mg3Sb2-Mg3Bi2 is usually considered to be a complete solid solution, nanosized compositional fluctuations were observed within a matrix and in the vicinity of the grain boundary. As an inhomogeneous microstructure can be beneficial or detrimental to thermoelectric performance, it is important to investigate the evolution of compositional variations for the engineering and long-term use of these materials. Using scanning transmission electron microscopy and atom probe tomography, a Bi-rich phase and compositional fluctuations are observed in sintered and annealed samples. After annealing, the broad intergranular phase was sharpened, resulting in a greater compositional change in the intergranular region. Annealing considerably reduces the fluctuations of Bi and Mg content within the grain as observed in atom probe tomography. Weighted mobility and lattice thermal conductivity were both increased as a result of the homogenized matrix phase. The combined microstructure features of intragrain and grain boundary effects resulted in an increased thermoelectric figure-of-merit zT of Mg3Sb0.6Bi1.4. These findings imply that the optimization of thermal and electrical properties can be realized through microstructure tuning.

The effect of Mg3As2 alloying on the thermoelectric properties of n-type Mg3(Sb, Bi)2.

Mg3Sb2-Mg3Bi2 alloys have been heavily studied as a competitive alternative to the state-of-the-art n-type Bi2(Te,Se)3 thermoelectric alloys. Using Mg3As2 alloying, we examine another dimension of exploration in Mg3Sb2-Mg3Bi2 alloys and the possibility of further improvement of thermoelectric performance was investigated. While the crystal structure of pure Mg3As2 is different from Mg3Sb2 and Mg3Bi2, at least 15% arsenic solubility on the anion site (Mg3((Sb0.5Bi0.5)1-xAsx)2: x = 0.15) was confirmed. Density functional theory calculations showed the possibility of band convergence by alloying Mg3Sb2-Mg3Bi2 with Mg3As2. Because of only a small detrimental effect on the charge carrier mobility compared to cation site substitution, the As 5% alloyed sample showed zT = 0.6-1.0 from 350 K to 600 K. This study shows that there is an even larger composition space to examine for the optimization of material properties by considering arsenic introduction into the Mg3Sb2-Mg3Bi2 system.

Thermoelectric Properties of Novel Semimetals: A Case Study of YbMnSb2.

The emerging class of topological materials provides a platform to engineer exotic electronic structures for a variety of applications. As complex band structures and Fermi surfaces can directly benefit thermoelectric performance it is important to identify the role of featured topological bands in thermoelectrics particularly when there are coexisting classic regular bands. In this work, the contribution of Dirac bands to thermoelectric performance and their ability to concurrently achieve large thermopower and low resistivity in novel semimetals is investigated. By examining the YbMnSb2 nodal line semimetal as an example, the Dirac bands appear to provide a low resistivity along the direction in which they are highly dispersive. Moreover, because of the regular-band-provided density of states, a large Seebeck coefficient over 160 µV K-1 at 300 K is achieved in both directions, which is very high for a semimetal with high carrier concentration. The combined highly dispersive Dirac and regular bands lead to ten times increase in power factor, reaching a value of 2.1 mW m-1 K-2 at 300 K. The present work highlights the potential of such novel semimetals for unusual electronic transport properties and guides strategies towards high thermoelectric performance.

Metallic n-Type Mg3 Sb2 Single Crystals Demonstrate the Absence of Ionized Impurity Scattering and Enhanced Thermoelectric Performance.

Mg3 (Sb,Bi)2 alloys have recently been discovered as a competitive alternative to the state-of-the-art n-type Bi2 (Te,Se)3 thermoelectric alloys. Previous theoretical studies predict that single crystals Mg3 (Sb,Bi)2 can exhibit higher thermoelectric performance near room temperature by eliminating grain boundary resistance. However, the intrinsic Mg defect chemistry makes it challenging to grow n-type Mg3 (Sb,Bi)2 single crystals. Here, the first thermoelectric properties of n-type Te-doped Mg3 Sb2 single crystals, synthesized by a combination of Sb-flux method and Mg-vapor annealing, is reported. The electrical conductivity and carrier mobility of single crystals exhibit a metallic behavior with a typical T-1.5 dependence, indicating that phonon scattering dominates the charge carrier transport. The absence of any evidence of ionized impurity scattering in Te-doped Mg3 Sb2 single crystals proves that the thermally activated mobility previously observed in polycrystalline materials is caused by grain boundary resistance. Eliminating this grain boundary resistance in the single crystals results in a large enhancement of the weighted mobility and figure of merit zT by more than 100% near room temperature. This work experimentally demonstrates the accurate understanding of charge-carrier scattering is crucial for developing high-performance thermoelectric materials and indicates that single-crystalline Mg3 (Sb,Bi)2 solid solutions can exhibit higher zT compared to polycrystalline samples.

Revealing the Intrinsic Electronic Structure of 3D Half-Heusler Thermoelectric Materials by Angle-Resolved Photoemission Spectroscopy.

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high-resolution angle-resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half-Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first-principles calculations. Moreover, a large anisotropic conduction band with an anisotropic factor of 6 is identified by ARPES and attributed to be one of the most important reasons leading to the high thermoelectric performance of ZrNiSn. These successful findings rely on the grown high-quality single crystals, which have fewer Ni interstitial defects and negligible in-gap states on the electronic structure. This work demonstrates a realistic paradigm to investigate the electronic structure of 3D solid materials by using ARPES and provides new insights into the intrinsic electronic structure of the half-Heusler system benefiting further optimization of thermoelectric performance.

Improvement of Low-Temperature zT in a Mg3 Sb2 -Mg3 Bi2 Solid Solution via Mg-Vapor Annealing.

Materials with high zT over a wide temperature range are essential for thermoelectric applications. n-Type Mg3 Sb2 -based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures (<500 K) is compromised due to their highly resistive grain boundaries. Syntheses and optimization processes to mitigate this grain-boundary effect has been limited due to loss of Mg, which hinders a sample's n-type dopability. A Mg-vapor anneal processing step that grows a sample's grain size and preserves its n-type carrier concentration during annealing is demonstrated. The electrical conductivity and mobility of the samples with large grain size follows a phonon-scattering-dominated T-3/2 trend over a large temperature range, further supporting the conclusion that the temperature-activated mobility in Mg3 Sb2 -based materials is caused by resistive grain boundaries. The measured Hall mobility of electrons reaches 170 cm2 V-1 s-1 in annealed 800 °C sintered Mg3 + δ Sb1.49 Bi0.5 Te0.01 , the highest ever reported for Mg3 Sb2 -based thermoelectric materials. In particular, a sample with grain size >30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n-type Bi2 Te3 ) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.

Manipulating Band Structure through Reconstruction of Binary Metal Sulfide for High-Performance Thermoelectrics in Solution-Synthesized Nanostructured Bi13 S18 I2.

Reconstructing canonical binary compounds by inserting a third agent can significantly modify their electronic and phonon structures. Therefore, it has inspired the semiconductor communities in various fields. Introducing this paradigm will potentially revolutionize thermoelectrics as well. Using a solution synthesis, Bi2 S3 was rebuilt by adding disordered Bi and weakly bonded I. These new structural motifs and the altered crystal symmetry induce prominent changes in electrical and thermal transport, resulting in a great enhancement of the figure of merit. The as-obtained nanostructured Bi13 S18 I2 is the first non-toxic, cost-efficient, and solution-processable n-type material with z T=1.0.