Abstract
Thermoelectric devices, which allow direct conversion of heat into electrical energy, require materials with improved figures of merit () in order to ensure widespread adoption. Several techniques have been proposed to increase the 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 TiNiSn 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 TiNiSn samples, display = 0.26 at 300 K, the largest value reported for compounds in the Ti–Ni–Sn family.
Highlights
Thermoelectric materials, which convert between thermal and electric energy through solid state phenomena, have the potential to harvest waste heat, reducing energy consumption and greenhouse gas production [1]
The fine Q-space resolution of this technique allows for the evaluation of accurate lattice parameters and the observation of the asymmetry in peaks corresponding to different lattice parameters in the half-Heusler phase due to changing Ni content
Samples were quenched from a high temperature in the solid solution regime between TiNiSn and TiNi2 Sn, and the final samples were annealed at low temperature for different lengths of time
Summary
Thermoelectric materials, which convert between thermal and electric energy through solid state phenomena, have the potential to harvest waste heat, reducing energy consumption and greenhouse gas production [1]. Douglas et al have shown that hierarchical microstructural engineering through the inclusion of a semicoherent second phase in TiNiSn decreases thermal conductivity by scattering phonons at multiple length scales [3,10,15,27] In this compound, the addition of excess Ni leads to phase separation between the full- and half-Heusler upon solidification. The variability in reported zT of the half-Heusler TiNiSn is likely due to the presence of varying amounts of Ni interstitials, which are highly dependent on the processing conditions [30,33,35] In this contribution, the disorder is built in by using heat treatments to modify the occupancy of Ni on the vacant site rather than the use of additional alloying elements. Performance of TiNiSn, this work enables both the understanding of fundamental concepts behind defect-engineering and the development of high performance thermoelectric materials
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