Synthesis of Tin-Doped FeVSb Half-Heusler System by Mechanical Alloying and Evaluation of Thermoelectric Performance (original) (raw)
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Thermoelectric Properties of Mn-Doped FeVSb Half-Heusler System Synthesized via Mechanical Alloying
Transactions on Electrical and Electronic Materials, 2018
Mn-doped FeVSb half-Heusler alloys were synthesized via a mechanical alloying process and consolidated by vacuum hot pressing. The microstructure and phase transformation of all the samples were examined by XRD and SEM. Thermoelectric properties such as the Seebeck coefficient, electrical conductivity and thermal conductivity were investigated in the moderate temperature range from 300 to 973 K. The negative value of both the Seebeck and Hall coefficients confirms the presence of n-type conductivity. The Seebeck coefficient increased with an increasing doping amount, but the electrical conductivity decreased, owing to decreasing carrier concentration. The thermal conductivity found in this experiment was quite high, possibly due to bipolar diffusion of the electronic band energy. The maximum value of the dimensionless figure of merit was achieved using a relatively high value of the Seebeck coefficient and a significantly higher value of electrical conductivity. The maximum value of ZT was observed for Fe 0.996 Mn 0.004 VSb at 468 K.
Journal of Electronic Materials, 2019
Se-doped half-Heusler compositions, FeVSb 1Àx Se x (0.03 £ x £ 0.15), were fabricated by mechanical alloying followed by vacuum hot pressing. The goal of this synthesis was to explore the effect of Se doping on the thermoelectric and transport properties of FeVSb system. A near single half-Heusler phase was found to form; however, a second phase of FeSb 2 couldn't be avoided in this process. N-type conduction was confirmed and Se acted as a donor for the FeVSb system. Lattice thermal conductivity also considerably decreased after Se doping. The absolute value of Seebeck coefficient is increased to a maximum of 126 lVK À1 at 956 K for x = 0.12, which may help to increase the figure of merit (ZT) of the FeVSb system. The figure of merit is improved by Se doping, and the improvement is possibly owing to the combined effect of fine grain structure, increased effective mass and phonon scattering at the grain boundaries. A maximum ZT of 0.27 was achieved for FeVSb 0.88 Se 0.12 at 847 K.
Electronic Materials Letters, 2018
The thermoelectric and transport properties of Ti-doped FeVSb half-Heusler alloys were studied in this study. FeV 1−x Ti x Sb (0.1 < x < 0.5) half-Heusler alloys were synthesized by mechanical alloying process and subsequent vacuum hot pressing. After vacuum hot pressing, a near singe phase with a small fraction of second phase was obtained in this experiment. Investigation of microstructure revealed that both grain and particle sizes were decreased on doping which would influence on thermal conductivity. No foreign elements pick up from the vial was seen during milling process. Thermoelectric properties were investigated as a function of temperature and doping level. The absolute value of Seebeck coefficient showed transition from negative to positive with increasing doping concentrations (x ≥ 0.3). Electrical conductivity, Seebeck coefficient and power factor increased with the increasing amount of Ti contents. The lattice thermal conductivity decreased considerably, possibly due to the mass disorder and grain boundary scattering. All of these turned out to increase in power factor significantly. As a result, the thermoelectric figure of merit increased comprehensively with Ti doping for this experiment, resulting in maximum thermoelectric figure of merit for FeV 0.7 Ti 0.3 Sb at 658 K.
Optimizing the thermoelectric performance of FeVSb half-Heusler compound via Hf-Ti double doping
• Electrical conductivity was improved due to optimizing the carrier concentration. • A reduction in thermal conductivity was achieved due to point defect scattering. • Significant enhancement in thermoelectric figure of merit (zT) was achieved. • The highest zT value at T = 873 K was achieved for Fe(V 0.8 Hf 0.2) 0.8 Ti 0⋅2 Sb sample. • Lower thermal conductivity is attributed to the Hf-Ti dual-doping. A R T I C L E I N F O Keywords: Thermoelectric performance Half-heusler alloys Hf-Ti dual-doping Disorder scattering parameters Phonon scattering Lattice thermal conductivity A B S T R A C T FeVSb-based half-Heusler (HH) compound has recently been identified as promising medium-high temperature thermoelectric (TE) materials for power generation applications. In this study, enhanced thermoelectric performance of Fe(V 0.8 Hf 0.2) 1-x Ti x Sb (x = 0.0, 0.2, 0.4, 0.5, 0.6) HH alloys by Hf-Ti dual-doping was reported studied in a temperature range from 100 to 900 K. A high content of Ti doping not only optimized the carrier concentration but also reduced the lattice thermal conductivity, which all contribute to high zT. As a result, a zT value was increased by ~20% at 873 K for Fe(V 0.8 Hf 0.2) 0.8 Ti 0.2 Sb compound. Hf-Ti dual doping significantly reduced the lattice thermal conductivity due to enhanced point defect scattering which is mainly attributed to mass fluctuations. Hence, suppressed the material's total thermal conductivity. A reduction of ~20% was obtained for the Fe(V 0.8 Hf 0.2) 0.8 Ti 0.2 Sb sample, compared with the single Hf-doped FeVSb sample and of ~80% compared to FeVSb at room temperature.
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.
Acta Materialia, 2017
A re-investigation of phase equilibria, crystal structure and homogeneity region of the Half-Heusler (HH) phase in the ternary system Nb-Fe-Sb at 600 °C has solved controversies in the literature confirming the version of Melnyk et al. ([1] J. Phase Equilibria 20(2) (1999) 113-118). For the first time transport properties of Half-Heusler (HH) compounds NbFeSb and Nb 0.85 M 0.15 FeSb (M = Ti, Zr, Hf) were studied in the full temperature range from 4.2 to 823 K. The semiconducting material NbFeSb has an electronic structure close to a metal-to-insulator transition, which leads to changes of the conductivity type with the composition as well as with increasing temperature. Ti, Zr and Hf doped NbFeSb alloys show metallic behavior and were confirmed to be high ZT p-type thermoelectric materials. Surprisingly, the lattice thermal conductivity for the Zr-doped composition was found to be higher than those of the Ti-and Hf doped materials; this effect can be explained in terms of mass and strain field fluctuations. For the first time we report experimental information on thermal expansion coefficients, specific heat and elastic moduli for these p-type compounds. The mechanical properties show a good compatibility with those previously reported for n-type HH alloys.
Journal of Physics: Energy, 2021
Half-Heusler (HH) alloys are an important class of thermoelectric materials that combine promising performance with good engineering properties. This manuscript reports a variable temperature synchrotron x-ray diffraction study of several TiNiSn- and VFeSb-based HH alloys. A Debye model was found to capture the main trends in thermal expansion and atomic displacement parameters. The linear thermal expansion coefficient α(T) of the TiNiSn-based samples was found to be independent of alloying or presence of Cu interstitials with α av = 10.1 × 10−6 K−1 between 400 and 848 K. The α(T) of VFeSb and TiNiSn are well-matched, but NbFeSb has a reduced α av = 8.9 × 10−6 K−1, caused by a stiffer lattice structure. This is confirmed by analysis of the Debye temperatures, which indicate significantly larger bond force constants for all atomic sites in NbFeSb. This work also reveals substantial amounts of Fe interstitials in VFeSb, whilst these are absent for NbFeSb. The Fe interstitials are link...
(V,Nb)-doped half Heusler alloys based on {Ti,Zr,Hf}NiSn with high ZT
Acta Materialia, 2017
Half-Heusler alloys are among the most promising materials for thermoelectric generators as they can be used in a wide temperature range and their starting materials are abundant and cheap, the latter as long as no hafnium is involved. For Sb-doped Ti 0.5 Zr 0.25 Hf 0.25 NiSn Sakurada and Shutoh in 2008 have published ZT max =1.5 at 690 K, a value that hitherto was never reproduced independently. In this paper we successfully prepared Ti 0.5 Zr 0.25 Hf 0.25 NiSn with ZT max =1.5, however, at higher temperature (825 K). As the main goal is to produce hafnium-free half Heusler alloys, we investigated the influence of niobium or vanadium dopants on Ti x Zr 1-x NiSn 0.98 Sb 0.02 , reaching ZTs > 1.2 and thermalelectric conversion efficiencies up to 13.1%. For Hf-free n-type TiNiSn-based Half-Heusler alloys these values are unsurpassed. In order to further improve our thermoelectric materials our study is completed by electrical resistivity and thermal conductivity data in the low temperature range but also by mechanical properties (elastic moduli, hardness) at room temperature. The electrical properties have been discussed in comparison with DFT calculations.
Enhanced te performance of fevsb1-xsnx half-heusler matrices using zirconia vial
Journal of Ceramic Processing Research, 2020
Thermoelectric and transport properties of FeVSb1-xSnx (0.015<x<0.055) alloys were studied with respect to types of vials (zirconia and stainless steel), Sn contents and temperature. The results were compared with the previously studied samples synthesized by using stainless-vial. All the designated compositions in current work were prepared via a mechanical alloying process using a zirconia vial. Vacuum hot pressing was conducted to consolidate the mechanically alloyed powders. F43m symmetry was being confirmed from the Rietveld refinement pattern. The phase transitions during the milling process and vacuum hot processing were investigated and the results exhibited near single half-Heusler phases with a minor portion of the second phase within the matrix. The Second phase might play a role to reduce thermal conductivity. Electrical conductivity exhibited semi-metallic behavior in all the temperature range. Carrier concentrations are found to be decreased with the increasing Sn contents and the FeVSb0.955Sn0.045 specimen showed the ZT max of 0.23 at 757 K.
Scientific Reports
A new single phase high entropy alloy, ti 2 NiCoSnSb with half-Heusler (HH) structure is synthesized for the first time by vacuum arc melting (VAM) followed by ball-milling (BM). The BM step is necessary to obtain the single phase. Local electrode atom probe (LEAP) analysis showed that the elements are homogeneously and randomly distributed in the HH phase without any clustering tendency. When the BM was carried out for 1 hour on the VAM alloy, microcrystalline alloy is obtained with traces of Sn as secondary phase. When BM was carried out for 5 h, single HH phase formation is realized in nanocrystalline form. However, when the BM samples were subjected to Spark plasma sintering (SPS), secondary phases were formed by the decomposition of primary phase. Nanostructuring leads to simultaneous increase in S and σ with increasing temperature. The micro (1 h BM-SPS) and nanocrystalline (5 h BM-SPS) alloys exhibited a power factor (S 2 σ) of 0.57 and 1.02 mWm −1 K −2 , respectively, at 860 K. The microcrystalline sample had a total thermal conductivity similar to bulk TiNiSn sample. The nanocrystalline alloy exhibited a ZT of 0.047 at 860 K. The microcrystalline alloy showed a ZT to 0.144 at 860 K, in comparison to the nanocrystalline alloy. HH alloys are ternary intermetallic compounds with three interpenetrating sub-lattices. They crystallize in MgAgAs crystal structure with F-43m symmetry. The HH alloys having a valence electron count of 18 are found to be semi-conducting in nature. Compounds such as TiNiSn and TiCoSb are some of the well-studied HH alloys for thermoelectric (TE) applications. They have a narrow band gap and have good mechanical and thermal properties over other TE systems. HH alloys exhibit high Seebeck coefficient and electrical conductivity. However, they possess a very high thermal conductivity which serves as a bottleneck in achieving high TE figure of merit (ZT) in these materials 1-3. Recent studies have thus been primarily devoted to decreasing thermal conductivity in these alloys by nanostructuring 4 , introduction of in-situ and ex-situ secondary phases 5 and solid solution approach to create point defects 6 to name a few. HH alloys have been synthesized by vacuum arc melting (VAM) 7 , induction melting 8 , solid state synthesis 9 and mechanical alloying (MA) followed by SPS (MA-SPS) 10. However, in most of these cases, long annealing time is required to obtain single phase alloys 11. In some cases, secondary phases cannot be eliminated even after long time annealing 12. Long time annealing has also proven to degrade TE properties in TiCoSb owing to the loss of Sb or introduction of structural disorder 13 .