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Register a new userHigh thermoelectric performance of half-Heusler compound BiBaK with intrinsically low lattice thermal conductivity
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Half-Heusler compounds usually exhibit relatively higher lattice thermal conductivity that is undesirable for thermoelectric applications. Here we demonstrate by first-principles calculations and Boltzmann transport theory that the BiBaK system is an exception, which has rather low thermal conductivity as evidenced by very small phonon group velocity and relaxation time. Detailed analysis indicates that the heavy Bi and Ba atoms form a cage-like structure, inside which the light K atom rattles with larger atomic displacement parameters. In combination with its good electronic transport properties, the BiBaK shows a maximum n-type ZT value of 1.9 at 900 K, which outperforms most half-Heusler thermoelectric materials.
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The electronic and transport properties of the half-Heusler compound LaPtSb are investigated by performing first-principles calculations combined with semi-classical Boltzmann theory and deformation potential theory. Compared with many typical half-Heusler compounds, the LaPtSb exhibits obviously larger power factor at room temperature, especially for the n-type system. Together with the very low lattice thermal conductivity, the thermoelectric figure of merit (ZT) of LaPtSb can be optimized to a record high value of 2.2 by fine tuning the carrier concentration.
Since their discovery around a century ago, the structure and chemistry of the multi-functional half-Heusler semiconductors have been studied extensively as three component systems. The elemental groups constituting these ternary compounds with the nominal formula XYZ are well established. From the very same set of well-known elements we explore a phase space of quaternary double ($XXY_2Z_2$, $X_2YYZ_2$, and $X_2Y_2ZZ$), triple ($X_2XY_3Z_3$) and quadruple ($X_3XY_4Z_4$) half-Heusler compositions which 10 times larger in size. Using a reliable, first-principles thermodynamics methodology on a selection of 347 novel compositions, we predict 127 new stable quaternary compounds, already more than the 89 reported almost exhaustively for ternary systems. Thermoelectric performance of the state-of-the-art ternary half-Heusler compounds are limited by their intrinsically high lattice thermal conductivity ($kappa_{L}$). In comparison to ternary half-Heuslers, thermal transport in double half-Heuslers is dominated by low frequency phonon modes with smaller group velocities and limited by disorder scattering. The double half-Heusler composition Ti$_2$FeNiSb$_2$ was synthesized and confirmed to have a significantly lower lattice thermal conductivity (factor of 3 at room temperature) than TiCoSb, thereby providing a better starting point for thermoelectric efficiency optimization. We demonstrate a dependable strategy to assist the search for low thermal conductivity half-Heuslers and point towards a huge composition space for implementing it. Our findings can be extended for systematic discovery of other large families of multi-component intermetallic semiconductors.
We explore the structural, electronic, mechanical and thermoelectric properties of a new half Heusler compound, HfPtPb which is all metallic heavy element and has been recently been proposed to be stable [Nature Chem 7 (2015) 308]. In the present work, we employ density functional theory and semiclassical Boltzmann transport equations with constant relaxation time approximation. The mechanical properties such as Shear modulus, Young modulus, elastic constants, Poisson ratio, and shear anisotropy factor are investigated. The elastic and phonon properties reveal that this compound is mechanically and dynamically stable. Pugh and Frantsevich ratio demonstrates the ductile behavior and Shear anisotropic factor reflects the anisotropic nature of HfPtPb. The calculation of band structure predicts that this compound is semiconductor in nature with band gap 0.86 eV. The thermoelectric transport parameters such as Seebeck coefficient, electrical conductivity, and electronic thermal conductivity and lattice thermal conductivity have been calculated as a function of temperature. The highest value of Seebeck coefficient is obtained for n-type doping at optimal carrier concentration. We predict the maximum value of the figure of merit 0.25 at 1000 K. Our investigation suggests that this material is n-type semiconductor.
The half-Heusler compound has drawn attention in a variety of fields as a candidate material for thermoelectric energy conversion and spintronics technology. This is because it has various electronic structures, such as semi-metals, semiconductors, and a topological insulator. When the half-Heusler compound is incorporated into the device, the control of high lattice thermal conductivity owing to high crystal symmetry is a challenge for the thermal manager of the device. The calculation for the prediction of lattice thermal conductivity, which is an important physical parameter for controlling the thermal management of the device, requires a calculation cost of several 100 times as much as the usual density functional theory calculation. Therefore, we examined whether lattice thermal conductivity prediction by machine learning was possible on the basis of only the atomic information of constituent elements for thermal conductivity calculated by the density functional theory calculation in various half-Heusler compounds. Consequently, we constructed a machine learning model, which can predict the lattice thermal conductivity with high accuracy from the information of only atomic radius and atomic mass of each site in the half-Heusler type crystal structure. Applying our results, the lattice thermal conductivity for an unknown half-Heusler compound can be immediately predicted. In the future, low-cost and short-time development of new functional materials can be realized, leading to breakthroughs in the search of novel functional materials.
Tin chalcogenides (SnS, SnSe, and SnTe) are found to have improved thermoelectric properties upon the reduction of their dimensionality. Here we found the tilted AA + s stacked two-dimensional (2D) SnTe bilayer as the most stable phase among several stackings as predicted by the structural optimization and phonon transport properties. The carrier mobility and relaxation time are evaluated using the deformation potential theory, which is found to be relatively high due to the high 2D elastic modulus, low deformation potential constant, and moderate effective masses. The SnTe bilayer shows a high Seebeck coefficient, high electrical conductivity, and ultralow lattice thermal conductivity. High TE figure of merit (ZT) values, as high as 4.61 along the zigzag direction, are predicted for the SnTe bilayer. These ZT values are much enhanced as compared to the bulk as well as monolayer SnTe and other 2D compounds.
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