Hanhui Xie

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.

Enhancement in thermoelectric performance of bismuth telluride based alloys by multi-scale microstructural effects

Decoupling of interdependent thermoelectric parameters was considered as a crucial strategy to enhance the thermoelectric performance of bulk materials. Here multi-scale microstructural effects have been introduced by a simple hot deformation process to obtain high-performance n-type bismuth telluride based alloys. Highly preferred orientation enables a significant improvement in in-plane electrical conductivity. The donor-like effect (an interaction of antisite defects and vacancies), which can be adjusted by varying hot deformation temperature, was also considered responsible for the remarkable enhancement in power factor. Besides, the in-plane lattice thermal conductivity was greatly reduced by in situ nanostructures and high-density lattice defects generated during the hot deformation process. The present study experimentally demonstrates a successful combination of microscale texture enhancement, atomic scale lattice defects and donor-like effect and recrystallization induced nanostructures as a new approach to improve thermoelectric properties. These effects led to a maximum ZT of 0.95 for the Bi2Te2Se1 sample hot deformed at 823 K, about 80% improvement over that without hot deformation.

Enhanced phonon scattering by mass and strain field fluctuations in Nb substituted FeVSb half-Heusler thermoelectric materials

The substitution of V by Nb in FeV1−xNbxSb (x = 0, 0.1, 0.2, 0.3, and 0.4) half-Heusler thermoelectric materials was introduced to lower the lattice thermal conductivity by the alloy scattering. A significantly reduced thermal conductivity of ∼5.5 W m−1 K−1 for the FeV0.6Nb0.4Sb was obtained at room temperature, a decrease of ∼55% compared with that of FeVSb. The disorder scattering parameters of mass fluctuation Γm and strain field fluctuation Γs were calculated to explore the dominant factors for the enhanced phonon scattering. The obtained Γm was comparable to the Γs. Therefore, the reduction of thermal conductivity in Nb substituted FeVSb compounds was attributed to both the mass and the strain field fluctuations.

Interrelation between atomic switching disorder and thermoelectric properties of ZrNiSn half-Heusler compounds

The interrelation between atomic switching disorder and thermoelectric properties in the half-Heusler alloy ZrNiSn was investigated. ZrNiSn samples were prepared by a time-efficient levitation melting and spark plasma sintering procedure. High-resolution synchrotron radiation powder X-ray diffraction shows that a single phase half-Heusler compound has been obtained. Trace impurities were detected after annealing at 970 K for 5 days. Rietveld refinements were carried out for both unannealed and annealed ZrNiSn samples to study the possible structural disorders. It is found that the generally accepted Zr/Sn antisite defects are not likely to exist. Instead, the refinements revealed interstitial fractional occupancy of Ni on the (½, ½, ½) site, which is normally empty in the half-Heusler phases, but filled in full Heusler materials. The electrical conductivity and Seebeck coefficient from 300 to 900 K of the unannealed and annealed ZrNiSn displayed no obvious distinction, and the room temperature electrical resistivity and absolute Seebeck coefficient of the annealed ZrNiSn even decreased slightly compared to those of the unannealed one, which implies no obvious Zr/Sn disorder transition during the annealing procedure. A slight increase in the thermal conductivity was observed after a long time annealing, possibly due to reduced Ni atomic disorder.

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.

Lattice thermal conductivity and spectral phonon scattering in FeVSb-based half-Heusler compounds

Alloying and grain refinement have been conducted to reduce the lattice thermal conductivity of FeVSb-based half-Heusler compounds. And the effects of point defect and boundary on phonon scattering are analyzed using the Callaway model. Point defect scattering and boundary scattering, respectively, cut out the high-frequency and low-frequency phonons. Consequently, most of the heat is carried by phonons with mean free path (mfp) in the range of 1–80 nm and 0.6–30 nm at Debye temperature and 650 K, respectively, indicating that further reduction in lattice thermal conductivity should enhance the scattering of phonons with these characteristic mfp.

Improved Thermoelectric Properties in Lu-doped Yb_(14)MnSb_(11) Zintl Compounds

The thermoelectric transport properties of Lu-doped Yb_(14)MnSb_(11) Zintl compounds have been investigated. The electrical conductivity increased and the Seebeck coefficient decreased with increasing Lu doping content, due to an increased carrier concentration. The carrier mobility behavior was in good accordance with the single parabolic band model. It is suggested that Lu doping simply shifts the Fermi energy with the band structure minimally altered. The lattice thermal conductivity roughly decreased via Lu doping. The maximum dimensionless figure of merit ZT of the Lu-doped samples was improved by ∼30% at 670 K, compared with that without doping.

Fabrication and thermoelectric properties of Yb-doped ZrNiSn half-Heusler alloys

Half-Heusler (HH) alloys constitute an important class of materials that exhibit promising potential in high-temperature thermoelectric (TE) power generation. In this work, we synthesized Zr1−x Yb x NiSn (x = 0, 0.01, 0.02, 0.04, 0.06 and 0.10) HH alloys using a time-efficient levitation melting and spark plasma sintering procedure. X-ray diffraction showed that the samples were predominantly single phased, and that the lattice constant increased systematically with increasing Yb doping ratio. The doping effects of Yb on the thermoelectric properties were studied. It was found that Yb doping consistently decreased the electrical and thermal conductivities. On the other hand, the effects of Yb doping on the Seebeck coefficient were found to be non-monotonic. The magnitude of the Seebeck coefficient (n-type) was increased upon Yb doping up to x = 0.02, above which Yb doping introduced notable p-type conduction. As a result, the room-temperature Seebeck coefficient of the x = 0.10 sample became positive although the magnitude was not high. The thermoelectric figure of merit, ZT, reached a maximum of ∼0.38 at 900 K for the x = 0.01 sample. Selective doping on the Ni and Sn sites are necessary to further optimize the TE performance of Zr1−x Yb x NiSn alloys.