Yintu Liu

Realizing high figure of merit in heavy-band p-type half-Heusler thermoelectric materials.

Solid-state thermoelectric technology offers a promising solution for converting waste heat to useful electrical power. Both high operating temperature and high figure of merit zT are desirable for high-efficiency thermoelectric power generation. Here we report a high zT of ∼1.5 at 1,200 K for the p-type FeNbSb heavy-band half-Heusler alloys. High content of heavier Hf dopant simultaneously optimizes the electrical power factor and suppresses thermal conductivity. Both the enhanced point-defect and electron–phonon scatterings contribute to a significant reduction in the lattice thermal conductivity. An eight couple prototype thermoelectric module exhibits a high conversion efficiency of 6.2% and a high power density of 2.2 W cm−2 at a temperature difference of 655 K. These findings highlight the optimization strategy for heavy-band thermoelectric materials and demonstrate a realistic prospect of high-temperature thermoelectric modules based on half-Heusler alloys with low cost, excellent mechanical robustness and stability.

Band engineering of high performance p-type FeNbSb based half-Heusler thermoelectric materials for figure of merit zT > 1

We report new p-type FeNb1−xTixSb (0.04 ≤ x ≤ 0.24) half-Heusler thermoelectric materials with a maximum zT of 1.1 at 1100 K, which is twice that of the ZrCoSb half-Heusler alloys. The electrical properties are optimized by a tradeoff between the band effective mass and mobility via a band engineering approach. A high content of Ti up to x = 0.2 optimizes the power factor and reduces the lattice thermal conductivity. In view of abundantly available elements, good stability and high zT, FeNb1−xTixSb alloys could be promising materials for high temperature power generation.

The intrinsic disorder related alloy scattering in ZrNiSn half-Heusler thermoelectric materials

The intrinsic structural disorder dramatically affects the thermal and electronic transport in semiconductors. Although normally considered an ordered compound, the half-Heusler ZrNiSn displays many transport characteristics of a disordered alloy. Similar to the (Zr,Hf)NiSn based solid solutions, the unsubstituted ZrNiSn compound also exhibits charge transport dominated by alloy scattering, as demonstrated in this work. The unexpected charge transport, even in ZrNiSn which is normally considered fully ordered, can be explained by the Ni partially filling interstitial sites in this half-Heusler system. The influence of the disordering and defects in crystal structure on the electron transport process has also been quantitatively analyzed in ZrNiSn1-xSbx with carrier concentration nH ranging from 5.0×1019 to 2.3×1021 cm−3 by changing Sb dopant content. The optimized carrier concentration nH ≈ 3–4×1020 cm−2 results in ZT ≈ 0.8 at 875K. This work suggests that MNiSn (M = Hf, Zr, Ti) and perhaps most other half-Heusler thermoelectric materials should be considered highly disordered especially when trying to understand the electronic and phonon structure and transport features.

Enhancing the Figure of Merit of Heavy‐Band Thermoelectric Materials Through Hierarchical Phonon Scattering

Hierarchical scattering is suggested as an effective strategy to enhance the figure of merit zT of heavy‐band thermoelectric materials. Heavy‐band FeNbSb half‐Heusler system with intrinsically low carrier mean free path is demonstrated as a paradigm. An enhanced zT of 1.34 is obtained at 1150 K for the Fe1.05Nb0.75Ti0.25Sb compound with intentionally designed hierarchical scattering centers.

Demonstration of a phonon-glass electron-crystal strategy in (Hf,Zr)NiSn half-Heusler thermoelectric materials by alloying

The general phonon-glass electron-crystal strategy is to disrupt phonon transport without affecting electron transport. Disruption of phonon thermal conductivity by alloying and nanostructuring is well known but a direct comparison of the scattering strength of electrons to that of phonons from disorder has not been made. Here we show that the point defect disorder of Zr/Hf atoms in MNiSn (M = Hf, Zr, and Ti) half-Heusler alloys effectively reduces lattice thermal conductivity as predicted from point defect scattering. However the introduced local atomic disorder produces a negligible effect on the electron scattering process and the conduction band structure. The electron scattering potential observed on the conduction band electrons is less than 0.1 eV, ten times less than that typically observed. This phenomenon can be understood from the existence of intrinsic disorder in the MNiSn system causing distinct mass and strain difference that effectively screens the effect of the induced disorder of Hf/Zr. The highest zT = 1.1 was obtained for Zr0.2Hf0.8NiSn0.985Sb0.015 at 1000 K. This substantial improvement in zT is due to alloying on the Zr/Hf site and demonstrates a dramatic improvement in TE performance that does not require nanoscale microstructures to avoid scattering of charge carriers.

Electron and phonon transport in Co-doped FeV0.6Nb0.4Sb half-Heusler thermoelectric materials

The electron and phonon transport characteristics of n-type Fe_(1−x) Co_x V_(0.6)Nb_(0.4)Sb half-Heusler thermoelectric compounds is analyzed. The acoustic phonon scattering is dominant in the carrier transport. The deformation potential of E_(def) = 14.1 eV and the density of state effective mass m^* ≈ 2.0 m_e are derived under a single parabolic band assumption. The band gap is calculated to be ∼0.3 eV. Electron and phonon mean free paths are estimated based on the low and high temperature measurements. The electron mean free path is higher than the phonon one above room temperature, which is consistent with the experimental result that the electron mobility decreases more than the lattice thermal conductivity by grain refinement to enhance boundary scattering. A maximum ZT value of ∼0.33 is obtained at 650 K for x = 0.015, an increase by ∼60% compared with FeVSb. The optimal doping level is found to be ∼3.0 × 10^(20) cm^(−3) at 600 K.

Structural transitions of ternary imide Li2Mg(NH)2 for hydrogen storage

Phase transitions and energetic properties of Li2Mg(NH)2 with different crystal structures are investigated by experiments and first-principles calculations. The Li2Mg(NH)2 with the primitive cubic and orthorhombic structure is obtained by dynamically dehydrogenating a Mg(NH2)2-2LiH mixture up to 280 °C under an initial vacuum and 9.0 bars H2, respectively. It is found that the obtained orthorhombic Li2Mg(NH)2 is converted to a primitive cubic structure as the dehydrogenation temperature is further increased to 400 °C or performed by a 36 h of high-energetic ball milling. Moreover, the primitive cubic phase can be converted to an orthorhombic phase after heating at 280 °C under 9.0 bars H2 for 1 h. Thermodynamic calculations show that the orthorhombic phase is the ground state structure of Li2Mg(NH)2. The mechanism for phase transitions of Li2Mg(NH)2 is also discussed from the angle of energy.

Structure, Magnetism, and Thermoelectric Properties of Magnesium-Containing Antimonide Zintl Phases Sr14MgSb11 and Eu14MgSb11

New Mg-containing antimonide Zintl phases, Sr14MgSb11 and Eu14MgSb11, were synthesized from high-temperature solid-state reactions in Ta tubes at 1323 K. Their structures can be viewed as derived from the Ca14AlSb11 structure type, which adopt the tetragonal space group I41/acd (No. 142, Z = 8) with the cell parameters of a = 17.5691(14)/17.3442(11) Å and c = 23.399(4)/22.981(3) Å for the Sr- and Eu-containing compounds, respectively. The corresponding thermoelectric properties were probed, which demonstrated high potential of these compounds as new thermoelectrics for their very low thermal conductivity and moderate Seebeck coefficient. Magnetism studies and theoretical calculations were conducted as well to better understand the structure-and-property correlation of these materials.

Lanthanide Contraction as a Design Factor for High-Performance Half-Heusler Thermoelectric Materials

Forming solid solutions, as an effective strategy to improve thermoelectric performance, has a dilemma that alloy scattering will reduce both the thermal conductivity and carrier mobility. Here, an intuitive way is proposed to decouple the opposite effects, that is, using lanthanide contraction as a design factor to select alloying atoms with large mass fluctuation but small radius difference from the host atoms. Typical half-Heusler alloys, n-type (Zr,Hf)NiSn and p-type (Nb,Ta)FeSb solid solutions, are taken as paradigms to attest the validity of this design strategy, which exhibit greatly suppressed lattice thermal conductivity and maintained carrier mobility. Furthermore, by considering lanthanide contraction, n-type (Zr,Hf)CoSb-based alloys with high zT of ≈1.0 are developed. These results highlight the significance of lanthanide contraction as a design factor in enhancing the thermoelectric performance and reveal the practical potential of (Zr,Hf)CoSb-based half-Heusler compounds due to the matched n-type and p-type thermoelectric performance.

Defect control in Ca1−δCeδAg1−δSb (δ ≈ 0.15) through Nb doping

Zintl phases with the nominal compositions Ca1−δREδAg1−δSb (RE = La, Ce, Pr, Nd, Sm; δ ≈ 0.15) are interesting due to their unique crystal structures and potential as high temperature thermoelectrics. Their structures generally feature the LiGaGe type structure with substantial vacancies on the Ag sites. The formation of such defects can be explained by the electronic effects, with which 18 electrons are required to stabilize the CaAgSb Zintl system. Since the substitution of Ca2+ with RE3+ will lead to electron excess, the formation of Ag defects will be an intrinsic character of such compounds to maintain their electron precise nature. In this work, the material Ca0.85Ce0.15Ag0.85Sb was selected for a detailed study on defect chemistry. In order to better understand the mechanism related to the defect formation and control in this system, we conducted a series of experiments aimed at controlling the point defects in Ca0.85Ce0.15Ag0.85Sb. This strategy was realized by intentionally doping Nb, which resulted in the discovery of a series of low defect density materials Ca0.725+xNb0.1−xCe0.15AgSb (0 ≤ x ≤ 0.05). In this work, an interesting defect controlling strategy on Zintl phases was demonstrated, which suggested the high flexibility of Ca1−δREδAg1−δSb in the optimization of thermoelectric properties.