121 research outputs found
Structural limitations for optimizing garnet-type solid electrolytes: a perspective
Lithium ion batteries exhibit the highest energy densities of all battery types and are therefore an important
technology for energy storage in every day life. Today’s commercially available batteries employ
organic polymer lithium conducting electrolytes, leading to multiple challenges and safety issues such as
poor chemical stability, leakage and flammability. The next generation lithium ion batteries, namely all
solid-state batteries, can overcome these limitations through employing a ceramic Li+ conducting electrolyte.
In the past decade, there has been a major focus on the structural and ionic transport properties
of lithium-conducting garnets, and the extensive research efforts have led to a thorough understanding
of the structure–property relationships in this class of materials. However, further improvement seems
difficult due to structural limitations. The purpose of this Perspective article is to provide a brief structural
overview of Li conducting garnets and the structural influence on the optimization of Li-ionic
conductivities
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
Thermoelectric properties of Sr_3GaSb_3 – a chain-forming Zintl compound
Inspired by the promising thermoelectric properties in the Zintl compounds Ca_3AlSb_3 and Ca_5Al_2Sb_6, we investigate here the closely related compound Sr_3GaSb_3. Although the crystal structure of Sr_3GaSb_3 contains infinite chains of corner-linked tetrahedra, in common with Ca_3AlSb_3 and Ca_5Al_2Sb_6, it has twice as many atoms per unit cell (N = 56). This contributes to the exceptionally low lattice thermal conductivity (κ_L = 0.45 W m^(−1) K^(−1) at 1000 K) observed in Sr_3GaSb_3 samples synthesized for this study by ball milling followed by hot pressing. High temperature transport measurements reveal that Sr_3GaSb_3 is a nondegenerate semiconductor (consistent with Zintl charge-counting conventions) with relatively high p-type electronic mobility (~ 30 cm^2 V^(−1) s^(−1) at 300 K). Density functional calculations yield a band gap of ~ 0.75 eV and predict a light valence band edge (~ 0.5 me), in qualitative agreement with experiment. To rationally optimize the electronic transport properties of Sr_3GaSb_3 in accordance with a single band model, doping with Zn^(2+) on the Ga^(3+) site was used to increase the p-type carrier concentration. In optimally hole-doped Sr_3Ga_(1−x)Zn_xSb_3 (x = 0.0 to 0.1), we demonstrate a maximum figure of merit of greater than 0.9 at 1000 K
Determining conductivity and mobility values of individual components in multiphase composite Cu_(1.97)Ag_(0.03)Se
The intense interest in phase segregation in thermoelectrics as a means to reduce the lattice thermal conductivity and to modify the electronic properties from nanoscale size effects has not been met with a method for separately measuring the properties of each phase assuming a classical mixture. Here, we apply effective medium theory for measurements of the in-line and Hall resistivity of a multiphase composite, in this case Cu_(1.97) Ag_(0.03)Se. The behavior of these properties with magnetic field as analyzed by effective medium theory allows us to separate the conductivity and charge carrier mobility of each phase. This powerful technique can be used to determine the matrix properties in the presence of an unwanted impurity phase, to control each phase in an engineered composite, and to determine the maximum carrier concentration change by a given dopant, making it the first step toward a full optimization of a multiphase thermoelectric material and distinguishing nanoscale effects from those of a classical mixture
Thermoelectric properties of Zn-doped Ca_(3)AlSb_(3)
Polycrystalline samples of Ca_(3)Al_(1)−_(x)Zn_(x)Sb_(3), with x = 0.00, 0.01, 0.02, and 0.05 were synthesized via a combined ball milling and hot pressing technique and the influence of zinc as a dopant on the thermoelectric properties was studied and compared to the previously reported transport properties of sodium-doped Ca_(3)AlSb_(3). Consistent with the transport in the sodium-doped material, substitution of aluminum with zinc leads to p-type carrier conduction that can be sufficiently explained with a single parabolic band model. It is found that, while exhibiting higher carrier mobilities, the doping effectiveness of zinc is lower than that of sodium and the optimum carrier concentration for a maximum figure of merit zT is not reached in this study. We find that the grain size influences the carrier mobility, carrier concentration, and lattice thermal conductivity, leading to improved properties at intermediate temperatures, and highlighting a possible approach for improved figures of merit in this class of materials
Effect of Isovalent Substitution on the Thermoelectric Properties of the Cu_2ZnGeSe_(4−x)S_x Series of Solid Solutions
Knowledge of structure–property relationships is a key feature of materials design. The control of thermal transport has proven to be crucial for the optimization of thermoelectric materials. We report the synthesis, chemical characterization, thermoelectric transport properties, and thermal transport calculations of the complete solid solution series Cu_2ZnGeSe_(4–x)S_x (x = 0–4). Throughout the substitution series a continuous Vegard-like behavior of the lattice parameters, bond distances, optical band gap energies, and sound velocities are found, which enables the tuning of these properties adjusting the initial composition. Refinements of the special chalcogen positions revealed a change in bonding angles, resulting in crystallographic strain possibly affecting transport properties. Thermal transport measurements showed a reduction in the room-temperature thermal conductivity of 42% triggered by the introduced disorder. Thermal transport calculations of mass and strain contrast revealed that 34% of the reduction in thermal conductivity is due to the mass contrast only and 8% is due to crystallographic strain
Band convergence in the non-cubic chalcopyrite compounds Cu_2MGeSe_4
Inspired by recent theoretical predictions on band convergence in the tetragonal chalcopyrite compounds, we have explored the influence of the crystal structure on the transport and bandstructure of different quaternary chalcopyrites. In theory, a changing lattice parameter ratio of c/2a towards unity should lead to band convergence due to a more cubic and higher symmetry structure. In accordance with this prediction, the different solid solutions explored in this manuscript show a significant impact on the electronic transport depending on the ratio of the lattice parameters. An increasing lattice parameter ratio results in an increase of the carrier effective masses which can be explained by converging bands, ultimately leading to an increase of the power factor and thermoelectric figure of merit in the class of non-cubic chalcopyrite compounds Cu_2MGeSe_4. However, the calculations via density functional theory show that the critical value of c/2a, where band convergence occurs, will be different from unity due to symmetry and chemical influences on the band structure
Bond strength dependent superionic phase transformation in the solid solution series Cu_2ZnGeSe_(4-x)S_x
Recently, copper selenides have shown to be promising thermoelectric materials due to their possible
superionic character resulting from mobile copper cations. Inspired by this recent development in the
class of quaternary copper selenides we have focused on the structure-to-property relationships in the
solid solution series Cu_2ZnGeSe_(4-x)S_x. The material exhibits an insulator-to-metal transition at higher
temperatures, with a transition temperature dependent on the sulfur content. However, the lattice
parameters show linear thermal expansion at elevated temperatures only and therefore no indication of
a structural phase transformation. ^(63)Cu nuclear magnetic resonance shows clear indications of Cu
located on at least two distinct sites, which eventually merge into one (apparent) site above the phase
transformation. In this manuscript the temperature dependent lattice parameters and electronic
properties of the solid solution Cu_2ZnGeSe_(4-x)S_x are reported in combination with ^(63)Cu NMR, and an
attempt will be made to relate the nature of the electronic phase transformation to a superionic phase
transformation and a changing covalent character of the lattice upon anion substitution in this class of
materials
Phonon Scattering through a Local Anisotropic Structural Disorder in the Thermoelectric Solid Solution Cu_2Zn_(1−x)Fe_xGeSe_4
Inspired by the promising thermoelectric properties of chalcopyrite-like quaternary chalcogenides, here we describe the synthesis and characterization of the solid solution Cu_2Zn_(1–x)Fe_xGeSe_4. Upon substitution of Zn with the isoelectronic Fe, no charge carriers are introduced in these intrinsic semiconductors. However, a change in lattice parameters, expressed in an elongation of the c/a lattice parameter ratio with minimal change in unit cell volume, reveals the existence of a three-stage cation restructuring process of Cu, Zn, and Fe. The resulting local anisotropic structural disorder leads to phonon scattering not normally observed, resulting in an effective approach to reduce the lattice thermal conductivity in this class of materials
Mechanochemical synthesis and high temperature thermoelectric properties of calcium-doped lanthanum telluride La_(3−x)Ca_xTe_4
The thermoelectric properties from 300–1275 K of calcium-doped La_(3−x)Te_4 are reported. La_(3−x)Te_4 is a high temperature n-type thermoelectric material with a previously reported zT_(max) 1.1 at 1273 K and x = 0.23. Computational modeling suggests the La atoms define the density of states of the conduction band for La_(3−x)Te_4. Doping with Ca^(2+) on the La^(3+) site is explored as a means of modifying the density of states to improve the power factor and to achieve a finer control over the carrier concentration. High purity, oxide-free samples are produced by ball milling of the elements and consolidation by spark plasma sintering. Calcium substitution upon the lanthanum site was confirmed by a combination of Rietveld refinements of powder X-ray diffraction data and wave dispersive spectroscopy. A zT_(max) 1.2 is reached at 1273 K for the composition La_(2.2)Ca_(0.78)Te_4 and the relative increase compared to La_(3−x)Te_4 is attributed to the finer carrier concentration
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