Half-Heusler alloys: Enhancement of ZT after severe plastic deformation (ultra-low thermal conductivity)

Rogl, Gerda; Ghosh, Sanyukta; Wang, Lei; Buršı́k, Jiřı́; Grytsiv, A.; Kerber, Michael; Bauer, E.; Mallik, Ramesh Chandra; Chen, Xing-Qiu; Zehetbauer, M.; Rogl, P.

doi:10.1016/j.actamat.2019.11.010

Cited by 50 publications

(37 citation statements)

References 79 publications

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“…Applied on hand-ground (HM) or BM and hotpressed (HP) samples, this method leads to an enhancement of the figure of merit ZT of a TE material, because after HPTprocessing-although the electrical resistivity is enhanced and the Seebeck coefficient remains basically unchanged-the thermal conductivity is significantly lower. ZTs of filled and unfilled skutterudites, half-Heusler or Heusler alloys increase by up to 30%, as was shown by the authors (Zhang et al, 2010a;Rogl et al, 2011c;Rogl et al, 2012a;Rogl et al, 2012b;Rogl et al, 2012c;Rogl et al, 2013a;Rogl et al, 2013b;Rogl et al, 2014b;Rogl et al, 2015;Rogl et al, 2020a), Anbalagan et al (Anbalagan et al, 2014), Masuda et al (Masuda et al, 2018), and other groups as listed in Refs. (Rogl et al, 2013b;Rogl et al, 2019b;Rogl et al, 2020a).…”

Section: Introductionsupporting

confidence: 54%

“…Another possibility is severe plastic deformation (SPD), a "bottom-down" method, which produces materials with grains down to nanometer range and in addition introduces vacancies, dislocations, grain boundaries with high angles of misorientation and other defects (Zehetbauer and Estrin, 2009;Rogl et al, 2012a;Rogl et al, 2012c;Valiev et al, 2012;Rogl et al, 2013b;Valiev et al, 2016;Ivanisenko et al, 2016;Rogl et al, 2020a). SPD, actually dating back to 200 BC, when repetitive forging and folding was used to strengthen the steel for swords (Langdon, 2011;Segal, 2018), is one of the major techniques to produce materials with ultra fine grains.…”

Section: Introductionmentioning

confidence: 99%

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How Severe Plastic Deformation Changes the Mechanical Properties of Thermoelectric Skutterudites and Half Heusler Alloys

Rogl

2020

Front. Mater.

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Thermoelectric leg materials with a high figure of merit, ZT, are the essential basis to build thermoelectric generators, directly converting waste heat into electricity. Skutterudites and half-Heusler alloys are promising candidates because they can be used in a wide temperature range, the starting material is available and cheap and in addition they are environmentally friendly. Severe plastic deformation via high-pressure torsion (HPT) is a technique to achieve very fine grains in micro- or even nano size with small and high angle grain boundaries and in parallel introduces a high level of defects like vacancies and dislocations. Therefore, this method was applied not only to enhance ZT of ball-milled and hot-pressed skutterudites and half-Heusler alloys but so far was also successful to directly produce dense nanostructured bulks from skutterudite powders. Although HPT compacted samples are chemically homogeneous, they are not homogeneous with respect to the shear strain increasing from the center to the rim. HPT changes the microstructure and density and thereby not only influences the thermoelectric but also the mechanical properties. In this work an overview is given of the influence of HPT on hardness, elastic moduli, indentation fracture toughness, thermal expansion and thermal shock resistance. The corresponding properties of hot-pressed skutterudites and half-Heusler alloys are compared with those after severe plastic deformation, dependent on the processing properties and position of the specimen in respect to shear strain. Data are collected from earlier investigations of the authors and from the literature, but also newly achieved and evaluated data are included.

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Section: Introductionsupporting

confidence: 54%

Section: Introductionmentioning

confidence: 99%

How Severe Plastic Deformation Changes the Mechanical Properties of Thermoelectric Skutterudites and Half Heusler Alloys

Rogl

2020

Front. Mater.

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“…Thermoelectric materials, based on the Seebeck effect can convert heat, also exhaust heat, directly into electricity without any moving parts. There are already many materials with a high figure of merit, ZT = (S 2 T)/(ρλ) (S is the Seebeck coefficient, T is the temperature, ρ is the electrical resistivity, and λ is the thermal conductivity) reported in the literature, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] but the attempt of researchers is to further increase the figure of merit. One among many possibilities is to apply high-pressure torsion (HPT) as one of the methods of severe plastic deformation (SPD), on the ballmilled (BM) and hot-pressed (HP) sample to achieve bulk nanocrystalline materials with a high concentration of defects, not only grain boundaries but also numerous point defects and 1267 first time systematically the changes of the temperature-dependent electrical resistivity and of thermal expansion were compared.…”

Section: Introductionmentioning

confidence: 99%

Resistivity and Thermal Expansion (4.2–820 K) of Skutterudites after Severe Plastic Deformation via HPT

Rogl

Soprunyuk

Schranz

et al. 2020

Zeitschrift anorg allge chemie

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Thermoelectric materials with a high figure of merit, ZT, are the essential basis to build thermoelectric generators, which can directly convert heat into electricity. Severe plastic deformation (SPD) via high‐pressure torsion (HPT) was successfully applied to enhance ZT of ball‐milled and hot‐pressed skutterudites as well as to produce bulk nanostructured skutterudites directly from powders. SPD introduces many defects into the sample and in parallel the crystallite size is significantly reduced. During measurement‐induced heating these defects anneal partially out, and the grains grow. In this work for the first time systematically the changes of the temperature‐dependent electrical resistivity and of thermal expansion were compared. It could be shown that for p‐ and n‐type skutterudites, being hot‐pressed and HPT‐processed as well as directly HPT‐processed from compacted powder, these changes occur more or less simultaneously within the same temperature ranges. This evaluation gives a much deeper insight into the thermoelectric behavior of HPT samples under the influence of changing temperature.

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“…Fulfilling these requirements, half-Heusler alloys with a valence electron count (VEC) of 18 exhibit good thermoelectric properties at high temperatures 11,12 and have thus attracted considerable attention for power generation applications 13,14 . In the half-Heusler alloys, many different strategies have been employed to improve the thermoelectric performance besides carrier concentration optimization, e.g., point defect scattering 15 , nanostructuring 16 , phase separation 17 , band engineering 18 , and plastic deformation 19 . Consequently, several half-Heusler systems, including (Ti,Zr,Hf)NiSn 17,20,21 , (Ti,Zr,Hf)CoSb [22][23][24] , and (V,Nb,Ta)FeSb 19,25,26 , have been found to exhibit good zT values of above unity.…”

Section: Introductionmentioning

confidence: 99%

“…In the half-Heusler alloys, many different strategies have been employed to improve the thermoelectric performance besides carrier concentration optimization, e.g., point defect scattering 15 , nanostructuring 16 , phase separation 17 , band engineering 18 , and plastic deformation 19 . Consequently, several half-Heusler systems, including (Ti,Zr,Hf)NiSn 17,20,21 , (Ti,Zr,Hf)CoSb [22][23][24] , and (V,Nb,Ta)FeSb 19,25,26 , have been found to exhibit good zT values of above unity. Half-Heusler alloys with a nominal VEC of 19 have also attracted significant attention, where the intrinsic vacancies play a key role in determining the thermoelectric properties 27 .…”

Section: Introductionmentioning

confidence: 99%

Improved Thermoelectric Performance in N-Type Half-Heusler NbCoSn by Heavy-Element Pt Doping

Sanchez¹,

Luo²,

Yu³

et al. 2020

Preprint

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Half-Heusler compounds with a valence electron count of 18, including ZrNiSn, ZrCoSb, and NbFeSb, are good thermoelec-tric materials owing to favourable electronic structures. Previous computational studies had predicted a high electrical power factor in another half-Heusler compound NbCoSn, but it has not been extensively investigated experimentally. Herein, the synthesis, structural characterization, and thermoelectric properties of the heavy-element Pt-doped NbCoSn compounds are reported. Pt is found to be an effective dopant enabling the optimization of electrical power factor, simul-taneously leading to a strong point defect scattering of phonons, and thereby suppressing the lattice thermal conductivity. Annealing significantly improves the carrier mobility, which is ascribed to the decreased grain boundary scattering. As a result, a maximum power factor of ~3.4 mWm-1K-2 is obtained at 600 K. In conjunction with the reduced lattice thermal conductivity, a maximum figure of merit zT of ~0.6 is achieved at 773 K for the post-annealed NbCo0.95Pt0.05Sn, an increase of 100% compared to the undoped NbCoSn. This work highlights the important roles of the doping element and micro-structure on the thermoelectric properties of half-Heusler compounds<br><p></p>

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Half-Heusler alloys: Enhancement of ZT after severe plastic deformation (ultra-low thermal conductivity)

Cited by 50 publications

References 79 publications

How Severe Plastic Deformation Changes the Mechanical Properties of Thermoelectric Skutterudites and Half Heusler Alloys

How Severe Plastic Deformation Changes the Mechanical Properties of Thermoelectric Skutterudites and Half Heusler Alloys

Resistivity and Thermal Expansion (4.2–820 K) of Skutterudites after Severe Plastic Deformation via HPT

Improved Thermoelectric Performance in N-Type Half-Heusler NbCoSn by Heavy-Element Pt Doping

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