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15 pages, 4678 KiB

Open AccessArticle

Are AuPdTM (T = Sc, Y and M = Al, Ga, In), Heusler Compounds Superconductors without Inversion Symmetry?

by Linus Kautzsch, Felix Mende, Gerhard H Fecher, Jürgen Winterlik and Claudia Felser

Materials 2019, 12(16), 2580; https://doi.org/10.3390/ma12162580 - 13 Aug 2019

Cited by 16 | Viewed by 4128

Abstract

Heusler compounds with 2:1:1 stoichiometry either have a centrosymmetric Cu

_{2}

MnAl structure or an Li

_{2}

AgSb structure without a centre of inversion. The centrosymmetry is always lost in quaternary Heusler compounds with 1:1:1:1 stoichiometry and LiMgPdSn structure. This presents the possibility [...] Read more.

Heusler compounds with 2:1:1 stoichiometry either have a centrosymmetric Cu

_{2}

MnAl structure or an Li

_{2}

AgSb structure without a centre of inversion. The centrosymmetry is always lost in quaternary Heusler compounds with 1:1:1:1 stoichiometry and LiMgPdSn structure. This presents the possibility of realizing non-centrosymmetric superconductors in the family of Heusler compounds. The objective of this study is to search for and investigate such quaternary derivatives of Heusler compounds, particularly with respect to superconductivity. Several compounds were identified by carrying out calculations from first principles and superconductivity was observed in experiments conducted on AuPdScAl and AuPtScIn at the critical temperatures of 3.0 and 0.96 K, respectively. All investigated compounds had a valence electron count of 27, which is also the case in centrosymmetric Heusler superconductors. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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11 pages, 2476 KiB

Open AccessArticle

Thermal and Electronic Transport Properties of the Half-Heusler Phase ScNiSb

by Karol Synoradzki, Kamil Ciesielski, Igor Veremchuk, Horst Borrmann, Przemysław Skokowski, Damian Szymański, Yuri Grin and Dariusz Kaczorowski

Materials 2019, 12(10), 1723; https://doi.org/10.3390/ma12101723 - 27 May 2019

Cited by 38 | Viewed by 5025

Abstract

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F

\bar{4}

3m) 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 [...] Read more.

Thermoelectric properties of the half-Heusler phase ScNiSb (space group F

\bar{4}

3m) 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 μV K⁻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 W m⁻¹ K⁻²) in the temperature range examined. Furthermore, the thermal conductivity was high, with a local minimum of about 6 W m⁻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. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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8 pages, 2057 KiB

Open AccessArticle

Improving the Thermoelectric Properties of the Half-Heusler Compound VCoSb by Vanadium Vacancy

by Lihong Huang, Junchen Wang, Xiaobo Mo, Xiaobo Lei, Sude Ma, Chao Wang and Qinyong Zhang

Materials 2019, 12(10), 1637; https://doi.org/10.3390/ma12101637 - 20 May 2019

Cited by 12 | Viewed by 3434

Abstract

The effects of V vacancy on the thermoelectric performance of the half-Heusler compound VCoSb have been investigated in this study. A certain amount of CoSb secondary phase is generated in the VCoSb matrix when the content of V vacancy is more than 0.1 [...] Read more.

The effects of V vacancy on the thermoelectric performance of the half-Heusler compound VCoSb have been investigated in this study. A certain amount of CoSb secondary phase is generated in the VCoSb matrix when the content of V vacancy is more than 0.1 at%. According to the results, a ZT value of 0.6, together with a power factor of 29 μW cm⁻¹ K⁻² at 873 K, were achieved for the nonstoichiometric sample V_0.9CoSb. This proved that moderate V vacancy could improve the thermoelectric (TE) properties of VCoSb. The noticeable improvements are mainly owing to the incremental Seebeck coefficient, which may benefit from the optimized carrier concentration. However, too much V vacancy will result in more CoSb impurity and deteriorate the TE performances of VCoSb owing to the increased thermal conductivity. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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14 pages, 7589 KiB

Open AccessFeature PaperArticle

The Initial Stage in Oxidation of ZrNiSn (Half Heusler) Alloy by Oxygen

by Oshrat Appel, Gil Breuer, Shai Cohen, Ofer Beeri, Theodora Kyratsi, Yaniv Gelbstein and Shimon Zalkind

Materials 2019, 12(9), 1509; https://doi.org/10.3390/ma12091509 - 9 May 2019

Cited by 16 | Viewed by 2862

Abstract

The MNiSn (M = Ti; Zr; Hf); half-Heusler semiconducting alloys have a high potential for use as n-type thermoelectric materials at elevated temperatures (~1000 K). The alloys’ durability is crucial for their commercial handling and use, and therefore it is required to [...] Read more.

The MNiSn (M = Ti; Zr; Hf); half-Heusler semiconducting alloys have a high potential for use as n-type thermoelectric materials at elevated temperatures (~1000 K). The alloys’ durability is crucial for their commercial handling and use, and therefore it is required to characterize their surface oxidation behavior and stability at the working temperature. X-ray photoelectron spectroscopy was utilized to study the surface composition and oxidation of the ZrNiSn alloy at room and elevated temperatures. It was found that during heating in a vacuum, Sn segregates to the surface in order to reduce the surface energy. Exposing the alloy to oxygen resulted mainly in the oxidation of the zirconium to ZrO₂, as well as some minor oxidation of Sn. At room temperature, the oxidation to ZrO₂ was accompanied by the formation of a thin ZrO layer at the metal-oxide interface. In contrast to TiNiSn, where most of the oxide was formed on the surface due to oxygen-enhanced segregation of Ti, and in the case of ZrNiSn, the formed oxide layer was thinner. Part of the oxide is formed due to Zr segregation to the surface, and in part due to oxygen dissolved into the alloy. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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14 pages, 8856 KiB

Open AccessArticle

Influence of Preparation Technology on Microstructural and Magnetic Properties of Fe₂MnSi and Fe₂MnAl Heusler Alloys

by Yvonna Jirásková, Jiří Buršík, Dušan Janičkovič and Ondřej Životský

Materials 2019, 12(5), 710; https://doi.org/10.3390/ma12050710 - 28 Feb 2019

Cited by 11 | Viewed by 3389

Abstract

Microstructural and magnetic properties of the X₂YZ, namely Fe₂MnSi and Fe₂MnAl, Heusler alloys have been studied from the viewpoint of technology for their production and for the Z element effect. First, arc melting was applied to produce [...] Read more.

Microstructural and magnetic properties of the X₂YZ, namely Fe₂MnSi and Fe₂MnAl, Heusler alloys have been studied from the viewpoint of technology for their production and for the Z element effect. First, arc melting was applied to produce button-type ingots from which samples in a form of 500 µm thick discs were cut. Second, planar flow casting technology yielded samples in a ribbon-form 2 mm wide and 20 μm thick. The checked area chemical compositions were in agreement with the nominal ones. Nevertheless, the darker square objects and smaller bright objects observed at the wheel side of the Fe₂MnSi ribbon sample yielded higher Mn content at the expense of Fe. The X-ray diffraction patterns of all samples have indicated L2₁ structure with lattice parameters, 0.567 (1) nm for Fe₂MnSi and 0.584 (1) nm for Fe₂MnAl, being within an experimental error independent of production technology. On the other hand, the technology has markedly influenced the microstructure clearly pointing to the larger size of grains and grain boundaries in the disc samples. From the magnetic viewpoint, both alloys are paramagnetic at room temperature without visible influence of their production. On the contrary, the low-temperature behavior of the microscopic hyperfine parameters and the macroscopic magnetic parameters exhibits differences affected by both chemical composition and microstructure. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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16 pages, 8365 KiB

Open AccessFeature PaperArticle

Surface Oxidation of TiNiSn (Half-Heusler) Alloy by Oxygen and Water Vapor

by Oshrat Appel, Shai Cohen, Ofer Beeri, Noah Shamir, Yaniv Gelbstein and Shimon Zalkind

Materials 2018, 11(11), 2296; https://doi.org/10.3390/ma11112296 - 15 Nov 2018

Cited by 17 | Viewed by 3826

Abstract

TiNiSn-based half-Heusler semiconducting compounds have the highest potential as n-type thermoelectric materials for the use at elevated temperatures. In order to use these compounds in a thermoelectric module, it is crucial to examine their behaviour at a working temperature (approximately 1000 K) [...] Read more.

TiNiSn-based half-Heusler semiconducting compounds have the highest potential as n-type thermoelectric materials for the use at elevated temperatures. In order to use these compounds in a thermoelectric module, it is crucial to examine their behaviour at a working temperature (approximately 1000 K) under oxygen and a humid atmosphere. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were utilized to study the surface composition and oxidation of the TiNiSn alloy at elevated temperatures. It was found that during heating in vacuum, Sn segregates to the surface. Exposing the alloy to oxygen at room temperature will cause surface oxidation of Ti to TiO₂ and Ti₂O₃ and some minor oxidation of Sn. Oxidation at 1000 K induces Ti segregation to the surface, creating a titanium oxide layer composed of mainly TiO₂ as well as Ti₂O₃ and TiO. Water vapor was found to be a weaker oxidative gas medium compared to oxygen. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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10 pages, 1395 KiB

Open AccessArticle

Origin of the Low Magnetic Moment in Fe₂AlTi: An Ab Initio Study

by Martin Friák, Anton Slávik, Ivana Miháliková, David Holec, Monika Všianská, Mojmír Šob, Martin Palm and Jörg Neugebauer

Materials 2018, 11(9), 1732; https://doi.org/10.3390/ma11091732 - 14 Sep 2018

Cited by 19 | Viewed by 4208

Abstract

The intermetallic compound Fe

_{2}

AlTi (alternatively Fe

_{2}

TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally [...] Read more.

The intermetallic compound Fe

_{2}

AlTi (alternatively Fe

_{2}

TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally measured ones. To unravel the source of this discrepancy, we analyze how various mechanisms present in realistic materials such as residual strain effects or deviations from stoichiometry affect magnetism. Since in spin-unconstrained calculations the system always evolves to the spin configuration which represents a local or global minimum in the total energy surface, finite temperature spin effects are not well described. We therefore turn the investigation around and use constrained spin calculations, fixing the global magnetic moment. This approach provides direct insight into local and global energy minima (reflecting metastable and stable spin phases) as well as the curvature of the energy surface, which correlates with the magnetic entropy and thus the magnetic configuration space accessible at finite temperatures. Based on this approach, we show that deviations from stoichiometry have a huge impact on the local magnetic moment and can explain the experimentally observed low magnetic moments. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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12 pages, 2294 KiB

Open AccessArticle

An Ab Initio Study of Thermodynamic and Mechanical Stability of Heusler-Based Fe₂AlCo Polymorphs

by Martin Friák, Sabina Oweisová, Jana Pavlů, David Holec and Mojmír Šob

Materials 2018, 11(9), 1543; https://doi.org/10.3390/ma11091543 - 27 Aug 2018

Cited by 14 | Viewed by 4016

Abstract

We use quantum-mechanical calculations to test a hypothesis of Glover et al. (J. Mag. Mag. Mater. 15 (1980) 699) that Co atoms in the Fe

_{2}

AlCo compound have on average 3 Fe and 3 Co atoms in their second nearest neighbor shell. [...] Read more.

We use quantum-mechanical calculations to test a hypothesis of Glover et al. (J. Mag. Mag. Mater. 15 (1980) 699) that Co atoms in the Fe

_{2}

AlCo compound have on average 3 Fe and 3 Co atoms in their second nearest neighbor shell. We have simulated four structural configurations of Fe

_{2}

AlCo including the full Heusler structure, inverse Heusler polymorph and two other phases matching this idea. The highest thermodynamic stability at T = 0 K is indeed predicted for one of the phases with the distribution of atoms according to Glover and et al. However, small energy differences among three of the studied polymorphs lead to a disordered CsCl-structure-like (B2-like) phase at elevated temperatures. The fourth variant, the full Heusler phase, is predicted to be mechanically unstable. The global magnetic states are predicted to be ferromagnetic but local magnetic moments of Fe and Co atoms sensitively depend on the composition of the first and second coordination shells. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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Review

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13 pages, 2575 KiB

Open AccessReview

The Characteristic Properties of Magnetostriction and Magneto-Volume Effects of Ni₂MnGa-Type Ferromagnetic Heusler Alloys

by Takuo Sakon, Yuushi Yamasaki, Hiroto Kodama, Takeshi Kanomata, Hiroyuki Nojiri and Yoshiya Adachi

Materials 2019, 12(22), 3655; https://doi.org/10.3390/ma12223655 - 6 Nov 2019

Cited by 9 | Viewed by 3374

Abstract

In this article, we review the magnetostriction and magneto-volume effects of Ni₂MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization [...] Read more.

In this article, we review the magnetostriction and magneto-volume effects of Ni₂MnGa-type ferromagnetic Heusler alloys at the martensitic, premartensitic, and austenitic phases. The correlations of forced magnetostriction (ΔV/V) and magnetization (M), using the self-consistent renormalization (SCR) spin fluctuation theory of an itinerant electron ferromagnet proposed by Takahashi, are evaluated for the ferromagnetic Heusler alloys. The magneto-volume effect occurs due to the interaction between the magnetism and volume change of the magnetic crystals. The magnetic field-induced strain (referred to as forced magnetostriction) and the magnetization are measured, and the correlation of magnetostriction and magnetization is evaluated. The forced volume magnetostriction ΔV/V at the Curie temperature, T_C is proportional to M⁴, and the plots cross the origin point; that is, (M⁴, ΔV/V) = (0, 0). This consequence is in good agreement with the spin fluctuation theory of Takahashi. An experimental study is carried out and the results of the measurement agree with the theory. The value of forced magnetostriction is proportional to the valence electron concentration per atom (e/a). Therefore, the forced magnetostriction reflects the electronic states of the ferromagnetic alloys. The magnetostriction near the premartensitic transition temperature (T_P) induces lattice softening; however, lattice softening is negligible at T_C. The forced magnetostriction at T_C occurs due to spin fluctuations of the itinerant electrons. In the martensitic and premartensitic phases, softening of the lattice occurs due to the shallow hollow (potential barrier) of the total energy difference between the L2₁ cubic and modulated 10M or 14M structures. As a result, magnetostriction is increased by the magnetic field. Full article

(This article belongs to the Special Issue Heusler and Half-Heusler Compounds)

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