Enhancement of thermoelectric properties in nanocrystalline M–Si thin film composites (M=Cr, Mn) (original) (raw)
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Acta Materialia, 2021
Science-driven design of future thermoelectric materials requires a deep understanding of the fundamental relationships between microstructure and transport properties. Grain boundaries in polycrystalline materials influence the thermoelectric performance through the scattering of phonons or the trapping of electrons due to space-charge effects. Yet, the current lack of careful investigations on grain boundary-associated features hinders further optimization of properties. Here, we study n-type NbCo1-xPtxSn half-Heusler alloys, which were synthesized by ball milling and spark plasma sintering (SPS). Post-SPS annealing was performed on one sample, leading to improved low-temperature electrical conductivity. The microstructure of both samples was examined by electron microscopy and atom probe tomography. The grain size increases from ~230 nm to ~2.38 μm upon annealing. Pt is found within grains and at grain boundaries, where it locally reduces the resistivity, as assessed by in situ four-point-probe electrical conductivity measurement. Our work showcases the correlation between microstructure and electrical conductivity, providing opportunities for future microstructural optimization by tuning the chemical composition at grain boundaries.
Thermoelectric Properties of the Half-Heusler Compound (Zr,Hf)(Ni,Pd)Sn
MRS Proceedings, 1998
Recent measurements of the thermoelectric transport properties of a series of the half- Heusler compound ZrNiSn are presented. These materials are known to be bandgap intermetallic compounds with relatively large Seebeck coefficients and semimetallic to semiconducting transport properties. This makes them attractive for study as potential candidates for thermoelectric applications. In this study, trends in the thermoelectric power, electrical conductivity and thermal conductivity are examined as a function of chemical substitution on the various fcc sub-lattices that comprise the half-Heusler crystal structure. These results suggest that the lattice contribution to the thermal conductivity may be reduced by increasing the phonon scattering via chemical substitution. The effects of these substitutions on the overall power factor and figure-of-merit will also be discussed.
Nanostructured Thermoelectric Chalcogenides
Bringing Thermoelectricity into Reality
Thermoelectric materials are outstanding to transform temperature differences directly and reversibly into electrical voltage. Exploiting waste heat recovery as a source of power generation could help towards energy sustainability. Recently, the SnSe semiconductor was identified, in single-crystal form, as a mid-temperature thermoelectric material with record high figure of merit, high power factor and surprisingly low thermal conductivity. We describe the preparation of polycrystals of alloys of SnSe obtained by arc-melting; a rapid synthesis that results in strongly nanostructured samples with low thermal conductivity, advantageous for thermoelectricity, approaching the amorphous limit, around 0.3-0.5 W/mK. An initial screening of novel samples Sn 1−x M x Se, by alloying with 3d and 4d transition metals such as M = Mn, Y, Ag, Mo, Cd or Au, provides for a means to optimize the power factor. M=Mo, Ag, with excellent values, are described in detail with characterization by x-ray powder diffraction (XRD), scanning electron microscopy (SEM), and electronic and thermal transport measurements. Rietveld analysis of XRD data demonstrates near-perfect stoichiometries of the above-mentioned alloys. SEM analysis shows stacking of nanosized sheets, with large surfaces parallel to layered slabs. An apparatus was developed for the simultaneous measurement of the Seebeck coefficient and electric conductivity at elevated temperatures.
Applied Physics Letters, 2000
Half-Heusler alloys (MgAgAs type) with the general formula MNiSn where M is a group IV transition metal (Hf, Zr, or Ti) are currently under investigation for potential thermoelectric materials. These materials exhibit a high negative thermopower (−40 to −250 μV/K) and low electrical resistivity values (0.1–8 mΩ cm) 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 (TiNiSn1−xSbx). The Sb doping leads to a relatively large power factor of (0.2–1.0) W/m K at room temperature for small concentrations of Sb. These values are comparable to that of Bi2Te3 alloys, which are the current state-of-the-art thermoelectric materials. The power factor is much larger at T≈650 K where it is over 4 W/m K making these materials very attractive for potential power generation considerations.
Studies on Thermoelectric Properties of n-type Polycrystalline SnSe 1- x S x by Iodine Doping
Advanced Energy Materials, 2015
We prepared iodine-doped n-type SnSe polycrystalline by melting and hot pressing. The prepared material is anisotropic with a peak ZT of ~0.8 at about 773 K measured along the hot pressing direction. This is the first report on TE properties of n-type Sn chalcogenide alloys. With increasing content of iodine, the carrier concentration changed from 2.3×10 17 cm-3 (p-type) to 5.0×10 15 cm-3 (n-type) then to 2.0×10 17 cm-3 (n-type). The decent ZT is mainly attributed to the intrinsically low thermal conductivity due to the high anharmonicity of the chemical bonds like those in p-type SnSe. By alloying with 10 atm. % SnS, even lower thermal conductivity and an enhanced Seebeck coefficient were achieved, leading to an increased ZT of ~1.0 at about 773 K measured also along the hot pressing direction.
Thermoelectric properties and stability of Tl-doped SnS
Journal of Alloys and Compounds, 2019
Tin sulfide (SnS) is an analog of tin selenide (SnSe) and is a promising thermoelectric material. However, a stable and effective doping of this compound has still not been achieved. According to our observations, this is mainly due to the very low equilibrium solubility of dopants and formation of extraneous phases, which is also an important issue for photovoltaic (PV) applications. Achieving a reasonable (60%) doping efficiency of thallium (Tl) in a cation sublattice of SnSe, we explored the same doping for SnS. Hotpressed polycrystalline (PC) samples were prepared along with their single-crystalline (SC) counterparts. Samples were examined for extraneous phases by X-ray diffraction (XRD), and energy-dispersive spectroscopy (EDS). Thermal stability was determined by thermogravimetric analysis (TGA). Measurements of the Seebeck and Hall coefficient, and electrical and thermal conductivity were conducted over a temperature range of 80e775 K. The experiments suggested a very low solubility of Tl (z0.1%). Slight Tl doping resulted in a substantial improvement of the thermoelectric efficiency (ZT) of SnS and enhanced crystal quality in terms of carrier mobility. We found, however, that attempts to prepare material with a high concentration of Tl or the examination of samples at temperatures above 600 K led to chemical instability.
Materials (Basel, Switzerland), 2018
NbCoSb with nominal 19 valence electrons, and is supposed to be metallic, has recently been reported to also exhibit the thermoelectric properties of a heavily doped n-type semiconductor. In this study, we prepared Co-rich NbCoSb samples ( = 0, 0.2, 0.3, 0.4, 0.5), and their phase compositions, microstructures and thermoelectric properties were investigated. The Seebeck coefficient increased a great deal with increasing , due to decreasing carrier concentration, and the total thermal conductivity reduced mainly because of declining . Finally, a peak thermoelectric figure of merit, , was about 0.46 for NbCoSb at 973 K. This enhancement was mainly attributed to the reduction of electric thermal conductivity and the increase of Seebeck coefficient. The excess Co had effects on the carrier concentration, deformation potential and DOS effective mass . Adding an excessive amount of Co leads to a very high , which was detrimental for transport characteristics.