The role of grain boundary scattering in reducing the thermal conductivity of polycrystalline XNiSn (X = Hf, Zr, Ti) half-Heusler alloys (original) (raw)
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Effect of boundary scattering on the thermal conductivity of TiNiSn-based half-Heusler alloys
Physical Review B, 2008
TiNiSn-based half-Heusler alloys have been of significant interest for their potential as thermoelectric materials. They exhibit promising electronic transport properties as revealed through high Seebeck coefficient and moderate electrical resistivity values. The chief disadvantage of these materials is a comparatively high lattice thermal conductivity. Attempts to "tune" the lattice thermal conductivity ͑ L ͒ in these materials have led to the comparison and analysis of the thermal conductivity of two series of Ti-and Zr-based half-Heusler alloys. In the first series, Ti 1−y Zr y NiSn 0.95 Sb 0.05 , a significant reduction in L is observed, with the substitution of large concentrations of Zr ͑y ജ 25% ͒ at Ti site, which is most likely due to mass fluctuation scattering. In the second series, TiNiSn 1−x Sb x , a nonsystematic increase in L is observed, with minute amounts of Sb doping ͑x ഛ 5%͒ at the Sn site. Extensive microstructural analysis in a TiNiSn 1−x Sb x series reveals a correlation between L and the average grain diameter in these materials, which is in good agreement with theoretical predictions related to phonon boundary scattering. In addition, a comparison of the calculated phonon mean free path in each of the series of compounds shows some insight into the two different phonon scattering mechanisms.
Grain structure effects on the lattice thermal conductivity of Ti-based half-Heusler alloys
Applied Physics Letters, 2002
Half-Heusler alloys with the general formula TiNiSn 1Ϫx Sb x are currently being investigated for their potential as thermoelectric ͑TE͒ materials. A systematic investigation of the effect of Sb doping on the Sn site and Zr doping on the Ti site on the electrical and thermal transport of the TiNiSn system has been performed. Unexpectedly, lattice thermal conductivity L appears to increase somewhat randomly with small amounts (xϽ5%) of Sb doping. Subsequently, an investigation of grain structure in these Sb-doped materials has been found to correlate with the anomalous behavior of L . Furthermore, effects of submicron grain sizes on L in ball milled and shock compressed samples are also presented.
Journal of Physics and Chemistry of Solids, 2018
Nanostructuring has rejuvenated interest in thermoelectric (TE) based power generation by enabling enhanced performance in nano-grained TE materials over their bulk counterparts. In ZrNiSn based half-Heusler (HH), a promising n-type TE materials, we examine the prospects of nanostructuring by synthesizing nano-crystalline iso-electronically substituted n-type Zr 1-x Hf x NiSn HH alloys to achieve a state-of-the-art TE figure-of-merit ZT ~ 1.2 at 873K, which is ~20% higher than its bulk counterparts and corresponds to high conversion efficiency of ~9% with output power density ~7 Wcm-2 , estimated using cumulative temperature dependence model. Enhanced phonon scattering at nano-scale grain boundaries and crystal defects, arising from nanostructuring and mass defect fluctuation in Hf substituted ZrNiSn alloys resulted in significantly reduced thermal conductivity. It was found that the partial substitution at Zr site with its heavier homologue Hf causes variation in the carrier effective mass and carrier concentration, which at lower Hf concentration results in an enhancement in the power factor.
Phonon scattering effects on thermal conductivity of TiNiSn-based half-Heusler alloys
Journal of the South Carolina Academy of Science, 2008
TiNiSn-based half-Heusler alloys have been of significant interest for their potential as thermoelectric materials, which arises from their promising electronic transport properties. They exhibit high Seebeck coefficients and moderate electrical resistivity values but have comparatively high lattice thermal conductivities (κ
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.
Thermoelectric properties of Ti0.3Zr0.35Hf0.35Ni1.005Sn half-Heusler alloy
Journal of Applied Physics
Thermoelectrics enabling a direct conversion of waste heat into useful electricity is widely investigated for renewable energy applications. n-type half-Heusler (HH) MNiSn (M = Ti,Zr,Hf) thermoelectric (TE) elements are known as attractive semiconducting candidates for such purposes. Yet, both electronic and phonon scattering optimization are still required for fulfilling their full potential. In the current research, Ti 0.3 Zr 0.35 Hf 0.35 Ni 1.005 Sn separating into a main Ti 0.3 Zr 0.35 Hf 0.35 NiSn HH matrix and a minority full-Heusler (FH) Ti 0.3 Zr 0.35 Hf 0.35 Ni 2 Sn phase is reported. Adverse electronic effects of the metallic FH phase are nearly avoided by its small relative amount and dimension, while maintaining nearly optimal electronic TE performance along with large phonon scattering, minimizing the lattice thermal conductivity. Consequently, a very high maximal TE figure of merit, ZT, of ∼1.04 is obtained, which is among the highest ever reported for n-type MNiSn HH compounds.
Journal of Alloys and Compounds
The microscopic physics behind the lattice thermal conductivity of NiTiSn is investigated using first-principles-based anharmonic lattice dynamics. The calculated lattice thermal conductivity of bulk materials (5.3 W.m −1 .K −1) is in good agreement with the experimental value at the optimal working temperature (700 K), but is overestimated below this temperature. The calculated values can be strongly affected by the size of the crystalline grains. We show that the lattice thermal conductivity is dominated by the acoustic (transverse and mostly longitudinal) modes with no contribution from the optical modes. The acoustic phonons are located below 150 cm −1 and involve mainly the tin atoms. The calculated mean free path of the most heat carrying phonons is around fifty nanometers with a maximum life time of ∼100 ps. These theoretical results are a step forward in developing the experimental design of low thermal conductivity NiTiSn Heusler based materials.
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.
Molecules
We hereby discuss the thermoelectric properties of PdXSn(X = Zr, Hf) half Heuslers in relation to lattice thermal conductivity probed under effective mass (hole/electrons) calculations and deformation potential theory. In addition, we report the structural, electronic, mechanical, and lattice dynamics of these materials as well. Both alloys are indirect band gap semiconductors with a gap of 0.91 eV and 0.82 eV for PdZrSn and PdHfSn, respectively. Both half Heusler materials are mechanically and dynamically stable. The effective mass of electrons/holes is (0.13/1.23) for Zr-type and (0.12/1.12) for Hf-kind alloys, which is inversely proportional to the relaxation time and directly decides the electrical/thermal conductivity of these materials. At 300K, the magnitude of lattice thermal conductivity observed for PdZrSn is 15.16 W/mK and 9.53 W/mK for PdHfSn. The highest observed ZT value for PdZrSn and PdHfSn is 0.32 and 0.4, respectively.
Acta Materialia, 2018
With a small gap in the density of states and a substantially semiconducting behavior half Heusler alloys have drawn attention as thermoelectric materials. For this study we have selected Hf-free compounds, Ti 0.5 Zr 0.5 NiSn, Ti 0.5 Zr 0.5 NiSn (with a densification aid (DA)) and Ti 0.5 Zr 0.5 NiSn 0.98 Sb 0.02 as well their parent alloys TiNiSn and ZrNiSn as cheap thermoelectrics. Electrical resistivity, thermal conductivity and specific heat were evaluated below room temperature (4.2-300 K) in order to get insight into the mechanism of transport properties. KSEM and TEM investigations as well as DFT (density functional theory) calculations accompany this research. The fine-grained epitaxial microstructure with a large number of dislocations warrants a low thermal conductivity at ultralow values (~30 mW/cmK at 300 K) at a narrow band gap with a sufficiently high density of states at the Femi level. High order of components mixing strongly affects the stability of the solid solutions by the configuration entropy term, which causes a shrinkage of the miscibility gap. For the electronic density of states (DOS) the split Zr band and impurity Ni band induce a significant reduction of the effective energy gap and thus explain n-type of conductivity of the compounds and solid solutions studied.