Thermoelectric materials have attracted considerable interest for energy applications such as waste-heat recovery and energy harvesting to power Internet-of-things sensors. In recent decades, an increasing number of different strategies to increase performance have been invented and tested, including the synthesis of thin films and other high-performance multilayered structures. Although it has already been shown that the pure combination of the properties of each layer without interactions will yield worse performance compared to the best layer, a critical estimation of the size of the deviation to trace back individual properties is still missing. In this paper we derive a set of formulas to describe the total Seebeck coefficient, electrical and thermal conductivity, power factor, and zT value of a two-layer system from a simple model and elucidate the origin and size of the contribution of each layer to the total thermoelectric performance. We further show that the influence of the substrate can lead to large deviation between the measured and the film's properties, advising caution when analyzing such systems. Moreover, this model allows one to ensure that the contribution of the substrate is below a desired threshold by introducing material-related quantities ea and eA.

Full-Heusler compounds with the composition Fe 2 V 1 & minus;x Ta x Al 1 & minus;y Si y have recently shown to exhibit some of the highest thermoelectric power factors reported so far among bulk materials due to the band convergence and band gap opening caused by the V/Ta substitution. Therefore, the solubility limit of Ta and Si regarding the stability of the L2 1 phase is investigated in this study. The crystal structure and microstructure of a large number of samples is probed by X-ray diffraction as well as scanning electron microscopy and energy dispersive X-ray analysis. The results show that the Al/Si substitution significantly hampers the solubility of Ta within the Heusler structure. Furthermore, Fe 2 V 0 . 9 Ta 0 . 1 Al and Fe 2 V 0 . 95 Ta 0 . 05 Al 0 . 9 Si 0 . 1 reveal nanoscale impurity precipitates in the microstructure, together with diffuse contrasts that indicate a non-equilibrium metastable state. For that reason, different annealing conditions, varying temperature and time, have been applied to the latter and the effect on the microstructure and thermoelectric properties is investigated. It is found that additional annealing leads to further phase segregation and grain growth of the impurity precipitates, which have a detrimental effect on the Seebeck coefficient due to their metallic-like nature. They can, however, effectively reduce the lattice thermal conductivity if their average size remains below the phonon mean free path. The thermoelectric efficiency in terms of the dimensionless figure of merit ZT is increased up to ZT = 0 . 3 -0.34 at 300 K which is beyond the values previously reported for Fe 2 VAl -based bulk materials. (c) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

Full-Heusler compounds with the composition Fe2V1-XTaxAl1-YSiy have recently shown to exhibit some of thehighest power factors reported so far for bulk materials due to the band convergence and band gap opening causedby the V/Ta substitution. Therefore, the solubility limit of Ta and Si regarding the stability of the L21 phase isinvestigated in this study. The crystal structure and microstructure of a large number of samples is probed by X-raydiffraction as well as scanning electron microscopy and energy dispersive X-ray analysis. The results show that the Al/Si substitution significantly hampers the solubility of Ta within the Heusler structure. Furthermore, Fe2V0.9Ta0.1Al and Fe2V0.95Ta0.05Al0.9Si0.1 reveal nanoscale impurity precipitates in the microstructure, together with diffuse contrasts that indicate a non-equilibrium metastable state. For that reason, different annealing conditions, varying the temperature and time, have been applied to the latter and the effect on the microstructure and thermoelectric properties is investigated. It is found that additional annealing leads to further phase segregation and grain growth of the impurity precipitates, which due to their metallic-like nature have a detrimental effect on the Seebeck coefficient. They can however effectively reduce the lattice thermal conductivity if the average grain size remains below the phonon mean free path. The thermoelectric efficiency in terms of the dimensionless gure of merit ZT is increased up to ZT = 0.3 - 0.34 at 300K which is beyond the values previously reported for Fe2VAl-based bulk materials.

For Fe2VAl the temperature-dependent Seebeck coefficient S(T) and electrical resistivity ρ(T) were calculated within the framework of density functional theory (DFT).

A series of full Heusler alloys, Fe2V1-xWxAl, 0 <= x <= 0.2, was prepared and characterized, and relevant physical properties to account for the thermoelectric performance were studied in a wide temperature range. Additionally, off-stoichiometric samples with similar compositions have been included, and a 10% improvement of the thermoelectric figure of merit was obtained. The V/W substitution causes (i) a change of the main carrier type, from holes to electrons as evidenced from Seebeck and Hall measurements, and (ii) a substantial reduction of the lattice thermal conductivity due to a creation of lattice disorder by means of a distinct different mass and metallic radius upon the V/W substitution. Moreover ZT values above 0.2 have been obtained. A microscopic understanding of the experimental data observed is revealed from ab initio calculations of the electronic and phononic structure. This series of alloys constitutes the basis for thin film systems, which have recently been found to exhibit ZT values beyond those reported so far in the literature.

Thermoelectric materials transform a thermal gradient into electricity. The efficiency of this process relies on three material-dependent parameters: the Seebeck coefficient, the electrical resistivity and the thermal conductivity, summarized in the thermoelectric figure of merit. A large figure of merit is beneficial for potential applications such as thermoelectric generators. Here we report the thermal and electronic properties of thin-film Heusler alloys based on Fe 2 V 0.8 W 0.2 Al prepared by magnetron sputtering. Density functional theory calculations suggest that the thin films are metastable states, and measurements of the power factor—the ratio of the Seebeck coefficient squared divided by the electrical resistivity—suggest a high intrinsic figure of merit for these thin films. This may arise from a large differential density of states at the Fermi level and a Weyl-like electron dispersion close to the Fermi level, which indicates a high mobility of charge carriers owing to linear crossing in the electronic bands.

We report on the formation, physical-chemical, as well as elastic and mechanical properties of the novel Half-Heusler (HH) compound TaFeSb that forms during a solid-state reaction from TaSb2 and TaFe2 in the temperature range between 800 and 850 degrees C. TaFeSb behaves as a semiconductor, and changes the conductivity type either on temperature or composition. Transport properties of TaFeSb and Ta1-xTixFeSb (0 <= x <= 0.11) were measured in the temperature range from 4.2 to 823 K, and the effect of titanium on thermoelectric and mechanical properties of Ta1-xTixFeSb was investigated. The Ta/Ti substitution results in a significant increase of the thermoelectric power factor to exciting values of above 6 mW/m.K-2. In combination with a suppressed phonon thermal conductivity, due to a unique role of Ti, an enhanced figure of merit, ZT(900K) = 1.0 (for Ta0.94Ti0.06FeSb) is obtained, close to the highest values reported for Hf-free p-type HH-systems. In addition, experimental results obtained in this study are discussed and analyzed in the context of ab-initio Density Functional Theory (DFT) calculations.

Substituting V/Nb in Fe2VAl causes an initial increase of the unit cell volume as well as an increaseof the structural disorder. Although V and Nb are isoelectronically, slight changes of the electronicdensity of states N(E) right at the Fermi energy are obtained. While for a 10 % substitution of V/Nbthe absolute value of N(EF) keeps almost constant, the slope of N(E) grows. As a result, the Seebeckeffect grows in comparsion to the starting material Fe2VAl, reaching values of about 100 μ V/K at roomtemperature. The latter is one of the largest found so far for p-type substituted Fe2VAl.

A series of full-Heusler alloys, Fe_2V_1-xW_xAl, 0 ≤ x ≤ 0.2, was prepared, characterized and relevant physical properties to account for the thermoelectric performance were studied in a wide temperature range. Additionally, off-stoichiometric samples with similar compositions have been included, and a 10 % improvement of the thermoelectric figure of merit was obtained. The V/W substitution causes i) a change of the main carrier type, from holes to electrons as evidenced from Seebeck and Hall measurements and ii) a substantial reduction of the lattice thermal conductivity due to a creation of lattice disorder by means of a distinct different mass and metallic radius upon the V/W substitution. Moreover ZT values above 0.2 have been obtained. A microscopic understanding of the experimental data observed is revealed from ab-initio calculations of the electronic and phononic structure.

We report on single-crystal growth, single-crystal x-ray diffraction, physical properties, and density functional theory (DFT) electronic structure as well as Fermi surface calculations for two ternary carbides, LuCoC2 and LuNiC2. Electrical resistivity measurements reveal for LuNiC2 a charge density wave (CDW) transition at T-CDW similar or equal to 450 K and, for T > T-CDW, a significant anisotropy of the electrical resistivity, which is lowest along the orthorhombic a axis. The analysis of x-ray superstructure reflections suggest a commensurate CDW state with a Peierls-type distortion of the Ni atom periodicity along the orthorhombic a axis. DFT calculations based on the CDW modulated monoclinic structure model of LuNiC2 as compared to results of the orthorhombic parent type reveal the formation of a partial CDW gap at the Fermi level which reduces the electronic density of states from N(E-F) = 1.03 states/eV f.u. without CDW to N(E-F) = 0.46 states/eV f.u. in the CDW state. The corresponding bare DFT Sommerfeld value of the latter, gamma(CDW)(DFT) = 0.90 mJ/mol K-2, reaches reasonable agreement with the experimental value gamma = 0.83(5) mJ/mol K-2 of LuNiC2. LuCoC2 displays a simplemetallic behavior with neither CDW ordering nor superconductivity above 0.4 K. Its experimental Sommerfeld coefficient, gamma = 5.9(1) mJ/mol K-2, is in realistic correspondence with the calculated, bare Sommerfeld coefficient, gamma(DFT) = 3.82 mJ/mol K-2, of orthorhombic LuCoC2.

Among the various families of thermoelectric materials, half-Heusler and full-Heusler systems are appreciated for their excellent mechanical properties and an outstanding thermal stability. While half-Heusler materials are also known for their superior thermoelectric performance as characterized by the so-called figure of merit, ZT, reaching ZT values above 1, the thermoelectric efficiency of full-Heusler systems is still moderate and does not exceed ZT ~ 0.1 - 0.2. The latter finding is based on the unfavorable fact that the total thermal conductivity of such Heusler phases is pretty large, exceeding that of well-behaving thermoelectric materials by more than one order of magnitude. Nevertheless, the power factor of Heusler systems like those based on Fe2VA1, is comparable, or even exceeds that of well behaving and excellently performing materials based on Bi-Te. In this contribution, the influence of substitution on different lattice sites (e.g., V/W or Fe/Ni) on the thermoelectric performance is studied, both from experiments as well as from first principles DFT calculations. In addition, we show that thin film preparation of Heusler systems results in an significant enhancement of the power factor pf substantial drop of the lattice thermal conductivity and thus in an dramatic increase of the figure of merit ZT. A number of microscopic observations are accounted for to explain this boost.

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 Ti0.5Zr0.25Hf0.25NiSn Sakurada and Shutoh in 2008 have published ZTmax = 1.5 at 690 K, a value that hitherto was never reproduced independently. In this paper we successfully prepared Ti0.5Zr0.25Hf0.25NiSn with ZTmax = 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 TixZr1−xNiSn0.98Sb0.02, reaching ZTs > 1.2 and thermal-electric conversion efficiencies up to 13.1%. For Hf-free n-type TiNiSn-based half Heusler alloys these values are unsurpassed.

We present a novel type of spatial magnetic neutron spin resonator whose time and wavelength resolution can be de- coupled from each other by means of a travelling wave mode of operation. Combined with a pair of highly efficient polarisers such a device could act simultaneously as monochromator and chopper, able to produce short neutron pulses, whose wavelength, spectral width and duration could be varied almost instantaneously by purely electronic means with- out any mechanical modification of the experimental setup. To demonstrate the practical feasibility of this technique we have designed and built a first prototype resonator consisting of ten individually switchable modules which allows to produce neutron pulses in the microsecond regime. It was installed at a polarised 2.6Å neutron beamline at the 250kW TRIGA research reactor of the Vienna University of Technology where it could deliver pulses of 55μs duration, which is about three times less than the passage time of the neutrons through the resonator itself. In order to further improve the achievable wavelength resolution to about 3% a second prototype resonator, consisting of 48 individual modules with optimised field homogeneity and enlarged beam cross-section of 6 × 6cm2 was developed. We present the results of first measurements which demonstrate the successful operation of this device.