Research

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11 pages, 4764 KiB

Open AccessFeature PaperArticle

Crystal Structure and Thermoelectric Properties of Lightly Substituted Higher Manganese Silicides

by Yuzuru Miyazaki, Haruki Hamada, Hiroki Nagai and Kei Hayashi

Materials 2018, 11(6), 926; https://doi.org/10.3390/ma11060926 - 30 May 2018

Cited by 26 | Viewed by 3410

The dissipation of MnSi layered precipitates during solidification is critical for further enhancement of the thermoelectric properties of the higher manganese silicides. We have investigated the effects of partial substitution of V in Mn sites and of Ge in Si sites on the [...] Read more.

The dissipation of MnSi layered precipitates during solidification is critical for further enhancement of the thermoelectric properties of the higher manganese silicides. We have investigated the effects of partial substitution of V in Mn sites and of Ge in Si sites on the crystal structures and thermoelectric properties of these silicides in detail. As previously reported, a small amount of V-substitution is quite effective in completely dissipating the MnSi striations; in contrast, a small proportion of these MnSi striations always remains present in the Ge-substitution case, even in the vicinity of the Ge solubility limits. For completely MnSi-dissipated samples, domain separation of the regular and highly strained arrangements of the Si atoms is realized. This domain separation suppresses the deterioration of the carrier mobility of the partially V-substituted samples and maintains even higher electrical conductivity to yield a high thermoelectric power factor of ∼2.3 mW/K

^{2}

m at higher temperatures. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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12 pages, 7272 KiB

Open AccessArticle

Enhancing Thermoelectric Properties through Control of Nickel Interstitials and Phase Separation in Heusler/Half-Heusler TiNi_1.1Sn Composites

by Emily E. Levin, Francesca Long, Jason E. Douglas, Malinda L. C. Buffon, Leo K. Lamontagne, Tresa M. Pollock and Ram Seshadri

Materials 2018, 11(6), 903; https://doi.org/10.3390/ma11060903 - 28 May 2018

Cited by 4 | Viewed by 4898

Abstract

Thermoelectric devices, which allow direct conversion of heat into electrical energy, require materials with improved figures of merit (

z T

) in order to ensure widespread adoption. Several techniques have been proposed to increase the

z T

of known thermoelectric materials through [...] Read more.

Thermoelectric devices, which allow direct conversion of heat into electrical energy, require materials with improved figures of merit (

z T

) in order to ensure widespread adoption. Several techniques have been proposed to increase the

z T

of known thermoelectric materials through the reduction of thermal conductivity, including heavy atom substitution, grain size reduction and inclusion of a semicoherent second phase. The goal in these approaches is to reduce thermal conductivity through phonon scattering without modifying the electronic properties. In this work, we demonstrate that Ni interstitials in the half-Heusler thermoelectric TiNiSn can be created and controlled in order to improve physical properties. Ni interstitials in TiNi

_{1.1}

Sn are not thermodynamically stable and, instead, are kinetically trapped using appropriate heat treatments. The Ni interstitials, which act as point defect phonon scattering centers and modify the electronic states near the Fermi level, result in reduced thermal conductivity and enhance the Seebeck coefficient. The best materials tested here, created from controlled heat treatments of TiNi

_{1.1}

Sn samples, display

z T

= 0.26 at 300 K, the largest value reported for compounds in the Ti–Ni–Sn family. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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13 pages, 8525 KiB

Open AccessFeature PaperArticle

Fully Ab-Initio Determination of the Thermoelectric Properties of Half-Heusler NiTiSn: Crucial Role of Interstitial Ni Defects

by Alexandre Berche and Philippe Jund

Materials 2018, 11(6), 868; https://doi.org/10.3390/ma11060868 - 23 May 2018

Cited by 31 | Viewed by 3759

Abstract

For thermoelectric applications, ab initio methods generally fail to predict the transport properties of the materials because of their inability to predict properly the carrier concentrations that control the electronic properties. In this work, a methodology to fill in this gap is applied [...] Read more.

For thermoelectric applications, ab initio methods generally fail to predict the transport properties of the materials because of their inability to predict properly the carrier concentrations that control the electronic properties. In this work, a methodology to fill in this gap is applied on the NiTiSn half Heusler phase. For that, we show that the main defects act as donor of electrons and are responsible of the electronic properties of the material. Indeed, the presence of Ni_i interstitial defects explains the experimental valence band spectrum and its associated band gap reported in the literature. Moreover, combining the DOS of the solid solutions with the determination of the energy of formation of charged defects, we show that Ni_i defects are also responsible of the measured carrier concentration in experimentally supposed “pure” NiTiSn compounds. Subsequently the thermoelectric properties of NiTiSn can be calculated using a fully ab initio description and an overall correct agreement with experiments is obtained. This methodology can be extended to predict the result of extrinsic doping and thus to select the most efficient dopant for specific thermoelectric applications. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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9 pages, 4276 KiB

Open AccessArticle

Synthesis and Thermoelectric Properties of Pd-Doped ZrCoBi Half-Heusler Compounds

by Degang Zhao, Min Zuo, Lin Bo and Yongpeng Wang

Materials 2018, 11(5), 728; https://doi.org/10.3390/ma11050728 - 04 May 2018

Cited by 24 | Viewed by 4075

Abstract

In this study, n-type Pd-doped ZrCo_1-xPd_xBi (x = 0, 0.03, 0.06, 0.09) half-Heusler samples were prepared by arc-melting and rapid hot-pressing sintering. The thermoelectric properties of ZrCo_1-xPd_xBi samples were analyzed and discussed. The [...] Read more.

In this study, n-type Pd-doped ZrCo_1-xPd_xBi (x = 0, 0.03, 0.06, 0.09) half-Heusler samples were prepared by arc-melting and rapid hot-pressing sintering. The thermoelectric properties of ZrCo_1-xPd_xBi samples were analyzed and discussed. The results showed that the electrical properties of ZrCo_1-xPd_xBi, including electrical conductivity and the Seebeck coefficient, increase due to the substitution of Pd on Co site. The lattice thermal conductivity of ZrCo_1-xPd_xBi is markedly decreased because of the Pd/Co substitution. A minimum κ_L of 5.0 W/mK for ZrCo_0.91Pd_0.09Bi is achieved at 800 K. The figure of merit of ZrCo_1-xPd_xBi is boosted due to the depressed lattice thermal conductivity and the improved power factor. The highest value of figure of merit reaches 0.23 for ZrCo_0.97Pd_0.03Bi half-Heusler compound at 800 K. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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17 pages, 3425 KiB

Open AccessFeature PaperArticle

On the Phase Separation in n-Type Thermoelectric Half-Heusler Materials

by Michael Schwall and Benjamin Balke

Materials 2018, 11(4), 649; https://doi.org/10.3390/ma11040649 - 23 Apr 2018

Cited by 24 | Viewed by 5434

Abstract

Half-Heusler compounds have been in focus as potential materials for thermoelectric energy conversion in the mid-temperature range, e.g., as in automotive or industrial waste heat recovery, for more than ten years now. Because of their mechanical and thermal stability, these compounds are advantageous [...] Read more.

Half-Heusler compounds have been in focus as potential materials for thermoelectric energy conversion in the mid-temperature range, e.g., as in automotive or industrial waste heat recovery, for more than ten years now. Because of their mechanical and thermal stability, these compounds are advantageous for common thermoelectric materials such as Bi

_{2}

Te

_{3}

, SiGe, clathrates or filled skutterudites. A further advantage lies in the tunability of Heusler compounds, allowing one to avoid expensive and toxic elements. Half-Heusler compounds usually exhibit a high electrical conductivity

σ

, resulting in high power factors. The main drawback of half-Heusler compounds is their high lattice thermal conductivity. Here, we present a detailed study of the phase separation in an n-type Heusler materials system, showing that the Ti

_{x}

Zr

_{y}

Hf

_{z}

NiSn system is not a solid solution. We also show that this phase separation is key to the thermoelectric high efficiency of n-type Heusler materials. These results strongly underline the importance of phase separation as a powerful tool for designing highly efficient materials for thermoelectric applications that fulfill the industrial demands of a thermoelectric converter. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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13 pages, 14233 KiB

Open AccessArticle

Impact of Interstitial Ni on the Thermoelectric Properties of the Half-Heusler TiNiSn

by Sonia A. Barczak, Jim Buckman, Ronald I. Smith, Annabelle R. Baker, Eric Don, Ian Forbes and Jan-Willem G. Bos

Materials 2018, 11(4), 536; https://doi.org/10.3390/ma11040536 - 30 Mar 2018

Cited by 34 | Viewed by 6814

Abstract

TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi_1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation [...] Read more.

TiNiSn is an intensively studied half-Heusler alloy that shows great potential for waste heat recovery. Here, we report on the structures and thermoelectric properties of a series of metal-rich TiNi_1+ySn compositions prepared via solid-state reactions and hot pressing. A general relation between the amount of interstitial Ni and lattice parameter is determined from neutron powder diffraction. High-resolution synchrotron X-ray powder diffraction reveals the occurrence of strain broadening upon hot pressing, which is attributed to the metastable arrangement of interstitial Ni. Hall measurements confirm that interstitial Ni causes weak n-type doping and a reduction in carrier mobility, which limits the power factor to 2.5–3 mW m⁻¹ K⁻² for these samples. The thermal conductivity was modelled within the Callaway approximation and is quantitively linked to the amount of interstitial Ni, resulting in a predicted value of 12.7 W m⁻¹ K⁻¹ at 323 K for stoichiometric TiNiSn. Interstitial Ni leads to a reduction of the thermal band gap and moves the peak ZT = 0.4 to lower temperatures, thus offering the possibility to engineer a broad ZT plateau. This work adds further insight into the impact of small amounts of interstitial Ni on the thermal and electrical transport of TiNiSn. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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8 pages, 3500 KiB

Open AccessArticle

Effect of C and N Addition on Thermoelectric Properties of TiNiSn Half-Heusler Compounds

by Hwan Soo Dow, Woo Sik Kim and Weon Ho Shin

Materials 2018, 11(2), 262; https://doi.org/10.3390/ma11020262 - 08 Feb 2018

Cited by 5 | Viewed by 3470

Abstract

We investigated the thermoelectric properties of the ternary half-Heusler compound, TiNiSn, when introducing C and N. The addition of C or N to TiNiSn leads to an enhanced power factor and a decreasing lattice thermal conductivity by point defect phonon scattering. The thermoelectric [...] Read more.

We investigated the thermoelectric properties of the ternary half-Heusler compound, TiNiSn, when introducing C and N. The addition of C or N to TiNiSn leads to an enhanced power factor and a decreasing lattice thermal conductivity by point defect phonon scattering. The thermoelectric performances of TiNiSn alloys are significantly improved by adding 1 at. % TiN, TiC, and figure of merit (ZT) values of 0.43 and 0.34, respectively, can be obtained at 723 K. This increase in thermoelectric performance is very helpful in the commercialization of thermoelectric power generation in the mid-temperature range. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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Review

Jump to: Research

16 pages, 6513 KiB

Open AccessFeature PaperReview

Band Structures and Transport Properties of High-Performance Half-Heusler Thermoelectric Materials by First Principles

by Teng Fang, Xinbing Zhao and Tiejun Zhu

Materials 2018, 11(5), 847; https://doi.org/10.3390/ma11050847 - 19 May 2018

Cited by 41 | Viewed by 7346

Abstract

Half-Heusler (HH) compounds, with a valence electron count of 8 or 18, have gained popularity as promising high-temperature thermoelectric (TE) materials due to their excellent electrical properties, robust mechanical capabilities, and good high-temperature thermal stability. With the help of first-principles calculations, great progress [...] Read more.

Half-Heusler (HH) compounds, with a valence electron count of 8 or 18, have gained popularity as promising high-temperature thermoelectric (TE) materials due to their excellent electrical properties, robust mechanical capabilities, and good high-temperature thermal stability. With the help of first-principles calculations, great progress has been made in half-Heusler thermoelectric materials. In this review, we summarize some representative theoretical work on band structures and transport properties of HH compounds. We introduce how basic band-structure calculations are used to investigate the atomic disorder in n-type MNiSb (M = Ti, Zr, Hf) compounds and guide the band engineering to enhance TE performance in p-type FeRSb (R = V, Nb) based systems. The calculations on electrical transport properties, especially the scattering time, and lattice thermal conductivities are also demonstrated. The outlook for future research directions of first-principles calculations on HH TE materials is also discussed. Full article

(This article belongs to the Special Issue Half-Heusler, Silicide and Zintl-type Thermoelectric Materials)

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