26 research outputs found
Screening of thermoelectric silicides with atomistic transport calculations
More than 1000 crystalline silicide materials have been screened for thermoelectric properties using first-principles atomistic calculations coupled with the semi-classical Boltzmann transport equation. Compounds that contain radioactive, toxic, rare, and expensive elements as well as oxides, hydrides, carbides, nitrides, and halides have been neglected in the study. The already well-known silicides with good thermoelectric properties, such as SiGe, Mg2Si, and MnSix, are successfully predicted to be promising compounds along with a number of other binary and ternary silicide compositions. Some of these materials have only been scarcely studied in the literature, with no thermoelectric properties being reported in experimental papers. These novel materials can be very interesting for thermoelectric applications provided that they can be heavily doped to give a sufficiently high charge carrier concentration and that they can be alloyed with isoelectronic elements to achieve adequately low phonon thermal conductivity. The study concludes with a list of the most promising silicide compounds that are recommended for further experimental and theoretical investigations.publishedVersio
Valence band study of thermoelectric Zintl-phase SrZn_2Sb_2 and YbZn_2Sb_2: X-ray photoelectron spectroscopy and density functional theory
The electronic structure of SrZn_2Sb_2 and YbZn_2Sb_2 is investigated using density functional theory and high-resolution x-ray photoemission spectroscopy. Both traditional Perdew-Burke-Ernzerhof and state-of-the-art hybrid Heyd-Scuseria-Ernzerhof functionals have been employed to highlight the importance of proper treatment of exchange-dependent Znâ 3d states, Ybâ4f states, and band gaps. The role of spin-orbit corrections in light of first-principles transport calculations are discussed and previous claims of Yb^(3+) valence are investigated with the assistance of photoelectron as well as scanning and transmission electron microscopy
Ca_3AlSb_3: an inexpensive, non-toxic thermoelectric material for waste heat recovery
Thermoelectric materials directly convert thermal energy into electrical energy, offering a promising solid-state solution for waste
heat recovery. For thermoelectric devices to make a significant impact
on energy and the environment the major impediments are the efficiency,
availability and toxicity of current thermoelectric materials.
Typically, efficient thermoelectric materials contain heavy elements
such as lead and tellurium that are toxic and not earth abundant. Many
materials with unusual structures containing abundant and benign
elements are known, but remain unexplored for thermoelectric
applications. In this paper we demonstrate, with the discovery of high
thermoelectric efficiency in Ca_3AlSb_3, the use of elementary
solid-state chemistry and physics to guide the search and optimization
of such materials
Electron and phonon scattering in the high-temperature thermoelectric La_3Te_(4âz)M_z (M=Sb,Bi)
In this work, scattering mechanisms in the highly efficient thermoelectric material La_(3âx)Te_4 are investigated by controlling the carrier concentration via anion substitution in the nominally vacancy-free compositions La_3Te_(4âz)Sb_z and La_3Te_(4âz)Bi_z. Through a comparison of the lattice thermal conductivity Îș_L in samples with and without Sb/Bi, this work reveals that La vacancies scatter phonons very efficiently and provide a ~100% reduction in Îș_L at 575 K. The addition of Sb or Bi leads to a significant reduction in the band gap, which is observed in the temperature-dependent transport data as well as first-principles calculations. Despite this significant change to the band structure, the transport parameters of the conduction band are only slightly modified. Also, an increase in the Hall mobility is observed at high T and z, which is caused by a reduction in either the La-vacancy concentration or the electronâs effective mass. A slight increase in thermoelectric efficiency is observed for nominal La_3Te_(3.35)Sb_(0.65) at high T. Thus, the net result is a system with large thermoelectric efficiency and a tunable band gap, thereby enabling a clear example to examine the effect of band gap on thermoelectric properties
Role of the self-interaction error in studying chemisorption on graphene from first-principles
Adsorption of gaseous species, and in particular of hydrogen atoms, on
graphene is an important process for the chemistry of this material. At the
equilibrium geometry, the H atom is covalently bonded to a carbon that puckers
out from the surface plane. Nevertheless the \emph{flat} graphene geometry
becomes important when considering the full sticking dynamics. Here we show how
GGA-DFT predicts a wrong spin state for this geometry, namely =0 for a
single H atom on graphene. We show how this is caused by the self-interaction
error since the system shows fractional electron occupations in the two bands
closest to the Fermi energy. It is demonstrated how the use of hybrid
functionals or the GGA+ method an be used to retrieve the correct spin
solution although the latter gives an incorrect potential energy curve
Improved carrier concentration control in Zn-doped Ca_5Al_2Sb_6
Ca_5Al_2Sb_6 is an inexpensive, Earth-abundant compound that exhibits promising thermoelectric efficiency at temperatures suitable for waste heat recovery. Inspired by our previous study of p-type Ca_(5âx)Na_xAl_2Sb_6, this work investigates doping with Zn^(2+) on the Al^(3+) site (Ca_5Al_(2âx)Zn_xSb_6). We find Zn to be an effective p-type dopant, in contrast to the low solubility limit and poor doping efficiency of Na. Seebeck coefficient measurements indicate that the hole band mass is unaffected by the dopant type in the high-zT temperature range. Band structure and density of states calculations are employed in order to understand the carrier concentration-dependent effective mass. Ca_5Al_(2âx)Zn_xSb_6 has a low lattice thermal conductivity that approaches the predicted minimum value at high temperature (1000 K) due to the complex crystal structure and high mass contrast
Electronic structure and transport in thermoelectric compounds AZn_2Sb_2 (A = Sr, Ca, Yb, Eu)
The AZn_2Sb_2 (PÂŻ3m1, A = Ca, Sr, Eu, Yb) class of Zintl compounds has shown high thermoelectric efficiency (zT ~ 1) and is an appealing system for the development of Zintl structureâproperty relationships. High temperature transport measurements have previously been conducted for all known compositions except for SrZn_2Sb_2; here we characterize polycrystalline SrZn_2Sb_2 to 723 K and review the transport behavior of the other compounds in this class. Consistent with the known AZn_2Sb_2 compounds, SrZn_2Sb_2 is found to be a hole-doped semiconductor with a thermal band gap ~ 0.27 eV. The Seebeck coefficients of the AZn2Sb2 compounds are found to be described by similar effective mass (m* ~ 0.6 m_e). Electronic structure calculations reveal similar m* is due to antimony p states at the valence band edge which are largely unaffected by the choice of A-site species. However, the choice of A-site element has a dramatic effect on the hole mobility, with the room temperature mobility of the rare earth-based compositions approximately double that found for Ca and Sr on the A site. This difference in mobility is examined in the context of electronic structure calculations
How to verify the precision of density-functional-theory implementations via reproducible and universal workflows
In the past decades many density-functional theory methods and codes adopting
periodic boundary conditions have been developed and are now extensively used
in condensed matter physics and materials science research. Only in 2016,
however, their precision (i.e., to which extent properties computed with
different codes agree among each other) was systematically assessed on
elemental crystals: a first crucial step to evaluate the reliability of such
computations. We discuss here general recommendations for verification studies
aiming at further testing precision and transferability of
density-functional-theory computational approaches and codes. We illustrate
such recommendations using a greatly expanded protocol covering the whole
periodic table from Z=1 to 96 and characterizing 10 prototypical cubic
compounds for each element: 4 unaries and 6 oxides, spanning a wide range of
coordination numbers and oxidation states. The primary outcome is a reference
dataset of 960 equations of state cross-checked between two all-electron codes,
then used to verify and improve nine pseudopotential-based approaches. Such
effort is facilitated by deploying AiiDA common workflows that perform
automatic input parameter selection, provide identical input/output interfaces
across codes, and ensure full reproducibility. Finally, we discuss the extent
to which the current results for total energies can be reused for different
goals (e.g., obtaining formation energies).Comment: Main text: 23 pages, 4 figures. Supplementary: 68 page