Enhanced thermoelectric power factor of half-Heusler solid solution Sc1-xTmxNiSb prepared by high-pressure high-temperature sintering method (original) (raw)
Related papers
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.
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.
Applied Physics Letters, 2000
Half-Heusler alloys ͑MgAgAs type͒ with the general formula MNiSn where M is a group IV transition metal ͑Hf, Zr, or Ti͒ are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (Ϫ40 to Ϫ250 V/K) and low electrical resistivity values ͑0.1-8 m⍀ cm͒ both of which are necessary for a potential thermoelectric material. Results are presented in this letter regarding the effect of Sb doping on the Sn site (TiNiSn 1Ϫx Sb x ). The Sb doping leads to a relatively large power factor of ͑0.2-1.0͒ W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi 2 Te 3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at TϷ650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.
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 .
Thermoelectric properties of Sb-doping in the TiNiSn/sub 1-x/Sb/sub x/ half-Heusler system
Eighteenth International Conference on Thermoelectrics. Proceedings, ICT'99 (Cat. No.99TH8407)
Half Heusler alloys, MNiSn (M= Zr, Hf, Ti) systems, have recently been studied for their potential as new thermoelectric materials." *, 32 They have shown both high thermopower (a) values (40-250 pVK) and reasonable values of electrical resistivity, p, (0.1-8 mR-cm). However, the thermal conductivity in these systems is high for a potential thermoelectric material, on the order of 4-10 W/m-K. In an effort to reduce the thermal conductivity through alloy scattering, Sb is substituted on the Sn site with compositions TiNiSnl.xSbx where x = 0 to 0.1. With this substitution, the thermopower is only slightly reduced while the resistivity is reduced by approximately one order of magnitude resulting in a marked improvement in the power factor (a2T/p). Thermopower, resistivity, and thermal conductivity have been measured on a series of Sb doped TiNiSn samples from 10K < T < 300K. Heat capacity and Hall measurements on these same samples are measured from 2K to 350K and will be discussed. A room temperature power factor in this system has been calculated to be as high as 1.4 W/m-K, making these materials interesting for potential thermoelectric applications.
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.
Physical Review Applied
Analysis of bipolar thermal conductivity might be very useful in preliminary stages of thermoelectric materials discovery. Using its product-mobility ratio between electrons and holes-it is possible to choose the most promising compound from the series and pave the correct direction of doping. Current work presents positive verification of this approach for ScNiSb, which is anticipated to show superior mobility when tuned towards n-type behavior. In agreement with expectation, the mobility increases by an order of magnitude due to rising tellurium content in the ScNiSb 1−x Te x series. The effect is most likely driven by change of the dominant charge carriers' scattering mechanism from ionized impurity influence to point defect and acoustic phonon interaction. Simultaneously, due to a highly anisotropic conduction band, the effective mass of the carriers rises towards the n-type regime. These two effects lead to an improved thermoelectric power factor of electron-doped samples, up to 40 μWK −2 cm −1 at 740 K for ScNiSb 0.85 Te 0.15. Based on this result, we suggest n-type doping for other rare-earth-based half-Heusler compounds. Representatives of this group exhibit the smallest lattice thermal conductivity in the pristine form among any half-Heusler thermoelectrics, and are anticipated to show comparably good electrical properties to ScNiSb due to their high mobility ratio in favor of electrons.
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.
Thermal and Electronic Transport Properties of the Half-Heusler Phase ScNiSb
Materials, 2019
Thermoelectric properties of the half-Heusler phase ScNiSb (space group F-43m) were studied on a polycrystalline single-phase sample obtained by arc-melting and spark-plasma-sintering techniques. Measurements of the thermopower, electrical resistivity, and thermal conductivity were performed in the wide temperature range 2–950 K. The material appeared as a p-type conductor, with a fairly large, positive Seebeck coefficient of about 240 μVK(^−1) near 450 K. Nevertheless, the measured electrical resistivity values were relatively high (83 μΩm at 350 K), resulting in a rather small magnitude of the power factor (less than 1 × 10−3 Wm^(−1)K^(−2)) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 Wm^(−1)K^(−1) occurring near 600 K. As a result, the dimensionless thermoelectric figure of merit showed a maximum of 0.1 at 810 K. This work suggests that ScNiSb could be a promising base compound for obtaining thermoelectric materials for energy conversion at high temperatures.