Electronic Orbital Alignment and Hierarchical Phonon Scattering Enabling High Thermoelectric Performance p-Type Mg3Sb2 Zintl Compounds.

Environmentally friendly Mg3Sb2-based materials have drawn intensive attention owing to their promising thermoelectric performance. In this work, the electrical properties of p-type Mg3Sb2 are dramatically optimized by the regulation of Mg deficiency. Then, we, for the first time, found that Zn substitution at the Mg2 site leads to the alignment of p x,y and p z orbital, resulting in a high band degeneracy and the dramatically enhanced Seebeck coefficient, demonstrated by the DFT calculations and electronic properties measurement. Moreover, Zn alloying decreases Mg1 (Zn) vacancies formation energy and in turn increases Mg (Zn) vacancies and optimizes the carrier concentration. Simultaneously, the Mg/Zn substitutions, Mg vacancies, and porosity structure suppress the phonon transport in a broader frequency range, leading to a low lattice thermal conductivity of ~0.47 W m-1 K-1 at 773 K. Finally, a high ZT of ~0.87 at 773 K was obtained for Mg1.95Na0.01Zn1Sb2, exceeding most of the previously reported p-type Mg3Sb2 compounds. Our results further demonstrate the promising prospects of p-type Mg3Sb2-based material in the field of mid-temperature heat recovery.

Nanotwins Strengthening High Thermoelectric Performance Bismuth Antimony Telluride Alloys.

Bi2 Te3 based thermoelectric alloys have been commercialized in solid-state refrigeration, but the poor mechanical properties restrict their further application. Nanotwins have been theoretically proven to effectively strengthen these alloys and could be sometimes constructed by strong deformation during synthesis. However, the obscure underlying formation mechanism restricts the feasibility of twin boundary engineering on Bi2 Te3 based materials. Herein, thorough microstructure characterizations are employed on a series of Bi0.4 Sb1.6 Te3+ δ alloys to systematically investigate the twins' formation mechanism. The results show that the twins belong to the annealing type formed in the sintering process, which is sensitive to Te deficiency, rather than the deformation one. The Te deficiency combined with mechanical deformation is prerequisite for constructing dense nanotwins. By reducing the δ below -0.01 and undergoing strong deformation, samples with a high density of nanotwins are obtained and exhibit an ultrahigh compressive strength over 250 MPa, nearly twice as strong as the previous record reported in hierarchical nanostructured (Bi, Sb)2 Te3 alloy. Moreover, benefitting from the suppressed intrinsic excitation, the average zT value of this robust material could reach near 1.1 within 30-250 °C. This work opens a new pathway to design high-performance and mechanically stable Bi2 Te3 based alloys for miniature device development.

Defect Engineering: Electron-Exchange Integral Manipulation to Generate a Large Magnetocaloric Effect in Ni41Mn43Co6Sn10 Alloys.

A promising magnetocaloric effect has been obtained in Ni-(Co)-Mn-X (X = Sn, In, Sb)-based Heusler alloys, but the low isothermal magnetic entropy change ΔSM restricts the further promotion of such materials. Defect engineering is a useful method to modulate magnetic performance and shows great potential in improving the magnetocaloric effect. In this work, dense Ni vacancies are introduced in Ni41Mn43Co6Sn10 alloys by employing high-energy electron irradiation to adjust the magnetic properties. These vacancies bring about intense lattice distortion to change the distance between adjacent magnetic atoms, leading to a significant enhancement of the average magnetic moment. As a result, the saturation magnetization of ferromagnetic austenite is accordingly improved to generate a high isothermal magnetic entropy change ΔSM of 20.0 J/(kg K) at a very low magnetic field of ∼2 T.

Enhancing Thermoelectric Performance of Yb0.3Co4Sb12 by Synergistically Optimized Carrier Concentration and Ionized Impurity Scattering.

Previous results indicated that acceptor doping was considered an effective clue to substantially suppress electronic thermal conductivity and in the meanwhile hold a rather low lattice thermal conductivity in high Yb-filled skutterudites. However, the strength of ionized impurity scattering needs to be regulated elaborately to balance the enhanced Seebeck coefficient and the deteriorated carrier mobility. In this work, Ge doping not only synergistically modulates the Fermi energy level and strength of ionized impurity scattering to an optimal range and attains a benign power factor but also offers a valuable opportunity to further suppress κe and κ in the classic Yb0.3Co4Sb12 alloy. Since the Yb0.3Co4Sb11.75Ge0.25 sample is endowed with the most highlighted ZT value in the device application temperature range, a promising average ZT value of 1.00 across the 300-823 K is achieved, reaching up to the level of a typical triple-filled skutterudite, which is highly desirable for achieving a satisfactory theoretical conversion efficiency of ∼14.5%. Our work corroborates that the ionized impurity strength is an extremely critical benchmark to obtain desirable thermoelectric performance in the high Yb-filled skutterudites.

Enhanced Thermoelectric and Mechanical Performance in n-Type Yb-Filled Skutterudites through Aluminum Alloying.

Herein, we demonstrate a synergistic combination of novel mechanisms in aluminum (Al)-alloyed Yb0.3Co4Sb12-based thermoelectric materials to address both reduction in thermal conductivity and concomitant enhancement in power factor (PF). Upon Al alloying, CoAl nanoprecipitates are embedded in the matrix, leading to (1) significant local strain and thus intensified phonon scattering and (2) carrier injection because of interphase electron transfer. Moreover, by decreasing the Yb filling fraction, not only is the electronic thermal conductivity significantly suppressed but also the carrier concentration is modulated to the optimum range, thus resulting in the dramatically boosted PF, especially below 773 K. As a result, a peak ZT value of 1.36 at 873 K and ZTave of 0.96 from 300 to 873 K were obtained in Yb0.21Co4Sb12/0.32CoAl. Last but not the least, the mechanical properties of the Al-alloyed samples were considerably improved through CoAl precipitate hardening, offering great potential for commercial applications.

Achieving a High Average zT Value in Sb2Te3-Based Segmented Thermoelectric Materials.

A tiny amount of Mn is doped in In0.15Sb1.85Te3 sample to tailor its carrier concentration, thus boosting the power factor and suppressing the bipolar effect. Furthermore, large amounts of nanotwins are constructed to effectively scatter the phonons and reduce the lattice thermal conductivity. As a result, the zT value of Mn0.02In0.15Sb1.83Te3 is enhanced up to 1.0 at 673 K, making this material a robust candidate for medium-temperature (500-673 K) thermoelectric applications. Then combining with the low-temperature thermoelectric material Mn0.0075Bi0.5Sb1.4925Te3 previously reported by our group and using nickel as a barrier layer, a high average zT value of 1.08 during a broad temperature range from 303 to 673 K together with an Ohmic contact interface bonding is achieved in the p-type segmented leg fabricated via simple one-step sintering. Finally, the maximum theoretical conversion efficiency with a temperature difference of 370 K reaches ∼12.7%.

Enhanced Thermoelectric Properties of p-Type CaMg2Bi2 via a Synergistic Effect Originated from Zn and Alkali-Metal Co-doping.

Bi-based Zintl phase CaMg2Bi2 is a promising thermoelectric material. Here, we report that the high-concentration point defects induced by equivalent Zn doping on the Mg site significantly enhance phonon scattering and then suppress lattice thermal conductivity by 50% at room temperature. Subsequently, partial substitution of divalent calcium ions with alkali-ion doping (Li, Na, K) not only optimizes the electrical transport properties by increasing the carrier concentration but also further reduces the lattice thermal conductivity through crystal disorder. Finally, the synergistic effect of Zn and Li co-doping leads to a high ZT of ∼1.0 at 873 K and an average ZT of 0.6 between 300 and 873 K for Ca0.995Li0.005Mg1.9Zn0.1Bi1.98. This work demonstrates an instructive method to reduce the lattice thermal conductivity via doping at the Mg site, which has never been reported in the CaMg2Bi2 system. Moreover, high-performance Ca0.995Li0.005Mg1.9Zn0.1Bi1.98 alloy does not contain any toxic elements and expensive rare earth elements, which is of great significance for the development of environment-friendly thermoelectric materials.