Introduction of resonant states and enhancement of thermoelectric properties in half-Heusler alloys (original) (raw)
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ACS Applied Energy Materials, 2018
Despite Hf-free half-Heusler (HH) alloys being currently explored as an important class of costeffective thermoelectric materials for power generation, owing to their thermal stability coupled with high cost of Hf, their figure-of-merit (ZT) still remains far below unity. We report a state-of-the-art figure-of-merit (ZT) ~ 1 at 873 K in Hf-free n-type V-doped Zr 1-x V x NiSn HH alloy, synthesized employing arc-melting followed by spark plasma sintering. The efficacy of V as a dopant on the Zr site is evidenced by the enhanced thermoelectric properties realized in this alloy, compared to other reported dopants. This enhancement of ZT is due to the synergistic enhancement in electrical conductivity with a simultaneous decrease in the thermal conductivity, which yields ZT ~ 1 at 873 K at an optimized composition of Zr 0.9 V 0.1 NiSn, which is ~ 70% higher than its pristine counterpart and ~ 25% higher than the best reported thus far in Hf-free n-type HH alloys. The enhancement of the electrical conductivity is due to the modification of the band structure by suitable tuning of the electronic band-gap near the Fermi level, through optimized V-doping in ZrNiSn HH alloys. The reduction in the thermal conductivity has been attributed to the mass fluctuation effects and the substitutional defects caused by V-doping, which results in an abundant scattering of the heat-carrying phonons. The optimized V-doped ZrNiSn HH composition, therefore, strikes a favourable balance between cost and thermoelectric performance, which would go a far way in the realization of a costeffective (Hf-free) HH based thermoelectric generator for power generation through waste heat recovery.
Conventional Half-Heusler Alloys Advance State-of-the-Art Thermoelectric Properties
arXiv (Cornell University), 2022
Half-Heusler phases have garnered much attention as thermally stable and non-toxic thermoelectric materials for power conversion in the mid-to-high temperature domain. The most studied half-Heusler alloys to date utilize the refractory metals Hf, Zr, and Ti as principal components. These alloys can quite often achieve a moderate dimensionless figure of merit, ZT, near 1. Recent studies have advanced the thermoelectric performance of half-Heusler alloys by employing nanostructures and novel compositions to achieve larger ZT, reaching as high as 1.5. Herein, we report that traditional alloying techniques applied to the conventional HfZr-based half-Heusler alloys can also lead to exceptional ZT. Specifically, we present the well-studied p-type Hf0.3Zr0.7CoSn0.3Sb0.7 alloys, previously reported to have a ZT near 0.8, resonantly doped with less than 1 atomic percent of metallic Al on the Sn/Sb site, touting a remarkable ZT near 1.5 at 980 K. This is achieved through a significant increase in power factor, by ~65%, and a notable but appreciably smaller decrease in thermal conductivity, by ~13%, at high temperatures. These favorable thermoelectric properties are discussed in terms of a local anomaly in the density of states near the Fermi energy designed to enhance the Seebeck coefficient, as revealed by firstprinciples calculations, as well as the emergence of a highly heterogeneous grain structure that can scatter phonons across different length scales, effectively suppressing the lattice thermal conductivity. Consequently, the effective mass is significantly enhanced from ~ 7 to 10me within a single parabolic band model, consistent with the result from first-principles calculations. The discovery of high ZT in a commonly studied half-Heusler alloy obtained through a conventional and non-complex approach opens a new path for further discoveries in similar types of alloys. Furthermore, it is reasonable to believe that the present study will reinvigorate effort in the exploration of high thermoelectric performance in conventional alloy systems.
Thermoelectric Properties of the Half-Heusler Compound (Zr,Hf)(Ni,Pd)Sn
MRS Proceedings, 1998
Recent measurements of the thermoelectric transport properties of a series of the half- Heusler compound ZrNiSn are presented. These materials are known to be bandgap intermetallic compounds with relatively large Seebeck coefficients and semimetallic to semiconducting transport properties. This makes them attractive for study as potential candidates for thermoelectric applications. In this study, trends in the thermoelectric power, electrical conductivity and thermal conductivity are examined as a function of chemical substitution on the various fcc sub-lattices that comprise the half-Heusler crystal structure. These results suggest that the lattice contribution to the thermal conductivity may be reduced by increasing the phonon scattering via chemical substitution. The effects of these substitutions on the overall power factor and figure-of-merit will also be discussed.
Uncovering high thermoelectric figure of merit in (Hf,Zr)NiSn half-Heusler alloys
Applied Physics Letters, 2015
Half-Heusler alloys (MgAgSb structure) are promising thermoelectric materials. RNiSn half-Heusler phases (R=Hf, Zr, Ti) are the most studied in view of thermal stability. The highest dimensionless figure of merit (ZT) obtained is ~1 in the temperature range ~450-900 o C, primarily achieved in nanostructured alloys. Through proper annealing, ZT~1.2 has been obtained in a previous ZT~1 n-type (Hf,Zr)NiSn phase without the nanostructure. There is an appreciable increase in power factor, decrease in charge carrier density, and increase in carrier mobility. The findings are attributed to improved structural order. Present approach may be applied to optimize the functional properties of Heusler-type alloys.
Development of Thermoelectric Half-Heusler Alloys over the Past 25 Years
Crystals
Half-Heusler alloys are among the most promising thermoelectric materials. In the present review, thermoelectric properties (at 300 K and 800 K) of more than 1100 compositions from more than 220 publications between 1998 and 2023 were collected and evaluated. The dependence of the peak figure of merit, ZTmax, of p- and n-type half-Heusler alloys on the publishing year and the peak temperature is displayed in several figures. Furthermore, plots of ZT vs. the electrical resistivity, the Seebeck coefficient and the thermal conductivity at 300 K and 800 K are shown and discussed. Especially thermal conductivity vs. power factor leads to a good overview of ZT. For both p- and n-type individually separated into systems, ZTs and peak ZTs in dependence on the composition are displayed and discussed. This overview can help to find the ideal half-Heusler alloy for practical use.
Thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1.005Sn half-Heusler alloy
Journal of Applied Physics
Thermoelectrics enabling a direct conversion of waste heat into useful electricity is widely investigated for renewable energy applications. n-type half-Heusler (HH) MNiSn (M = Ti,Zr,Hf) thermoelectric (TE) elements are known as attractive semiconducting candidates for such purposes. Yet, both electronic and phonon scattering optimization are still required for fulfilling their full potential. In the current research, Ti 0.3 Zr 0.35 Hf 0.35 Ni 1.005 Sn separating into a main Ti 0.3 Zr 0.35 Hf 0.35 NiSn HH matrix and a minority full-Heusler (FH) Ti 0.3 Zr 0.35 Hf 0.35 Ni 2 Sn phase is reported. Adverse electronic effects of the metallic FH phase are nearly avoided by its small relative amount and dimension, while maintaining nearly optimal electronic TE performance along with large phonon scattering, minimizing the lattice thermal conductivity. Consequently, a very high maximal TE figure of merit, ZT, of ∼1.04 is obtained, which is among the highest ever reported for n-type MNiSn HH compounds.
Molecules
We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.
Thermoelectric properties of n-type half-Heusler compounds (Hf 0.25 Zr 0.75 ) 1–x Nb x NiSn
Acta Materialia
A series of Nb doped (Hf0.25Zr0.75)1-xNbxNiSn (x = 0-0.03) samples were synthesized and studied by arc-melting the elements to first form ingots, then ball-milling the ingots to obtain fine powders, and finally hot-pressing the fine powder to form bulk samples. Instead of the conventional Sb doping on the Sn site of the n-type half-Heusler, it was found that Nb is also an effective dopant. When the carrier concentration is optimized at 2.2% Nb, a power factor of ~47 μW cm-1 K-2 is achieved at and above 600 °C, and a peak ZT ~0.9 is achieved at 700 °C with Nb doping from 1.8 at% to 2.2 at%. An output power density of ~22 W cm-2 and a leg efficiency of ~12% are calculated for the Nb doped samples.
Transport and thermoelectric properties of Hf-doped FeVSb half- Heusler alloys
Journal of Alloys and Compounds, 2020
Nearly single-phase FeVSb half-Heusler alloys with fine grains were obtained by induction melting followed by mechanical alloying (MA), spark plasma sintering (SPS) and annealing process. Hf as a heavy element was used as dopant to present point defects aiming at decreasing the material's thermal conductivity during phonon scattering. Thermoelectric properties of the FeV 1Àx Hf x Sb samples (0.0 x 0.3) were investigated as a function of temperature in a range from 300 to 600 K. Microstructure investigations showed that grain size decreases with increasing the level of Hf doping (x). Seebeck coefficient of the parent FeVSb compound showed negative sign which refers to n-type conduction. Interestingly, the sign has changed to positive with introducing Hf in the FeVSb lattice at different concentrations (x ! 0.1), which is due to one less valence electron in Hf as compared to V. The difference between the host atom (V) and the impurity atom (Hf) in terms of mass and size has resulted in a mass fluctuation and consequently a disorder scattering. The absolute value of the Seebeck coefficient |S| of the FeVSb system was measured at 110 mV/K, while the thermal conductivity value was obtained at 10.46 Wm À1 K À1 near room temperature. A maximum power factor of 1.07 mWm À1 K À2 at 420 K was recorded. The thermal conductivity decreased rapidly upon Hf doping due to increased point defect scattering caused by Hf introducing to the FeVSb system. A maximum ZT value of 0.08 at 573 K for FeV 0.9 Hf 0.1 Sb was recorded in this study.