Alex Zevalkink

Thinking Like a Chemist: Intuition in Thermoelectric Materials

The coupled transport properties required to create an efficient thermoelectric material necessitates a thorough understanding of the relationship between the chemistry and physics in a solid. We approach thermoelectric material design using the chemical intuition provided by molecular orbital diagrams, tight binding theory, and a classic understanding of bond strength. Concepts such as electronegativity, band width, orbital overlap, bond energy, and bond length are used to explain trends in electronic properties such as the magnitude and temperature dependence of band gap, carrier effective mass, and band degeneracy and convergence. The lattice thermal conductivity is discussed in relation to the crystal structure and bond strength, with emphasis on the importance of bond length. We provide an overview of how symmetry and bonding strength affect electron and phonon transport in solids, and how altering these properties may be used in strategies to improve thermoelectric performance.

Defect‐Controlled Electronic Properties in AZn2Sb2 Zintl Phases

Experimentally, AZn2Sb2 samples (A=Ca, Sr, Eu, Yb) are found to have large charge carrier concentrations that increase with increasing electronegativity of A. Using density functional theory (DFT) calculations, we show that this trend can be explained by stable cation vacancies and the corresponding finite phase width in A(1-x)Zn2Sb2 compounds.

Improved carrier concentration control in Zn-doped Ca5Al2Sb6

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_(5−x)Na_xAl_2Sb_6, this work investigates doping with Zn^(2+) on the Al^(3+) site (Ca_5Al_(2−x)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_(2−x)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.

Thermoelectric properties of Zn-doped Ca5In2Sb 6

The Zintl compound Ca5Al2Sb6 is a promising thermoelectric material with exceptionally low lattice thermal conductivity resulting from its complex crystal structure. In common with the Al analogue, Ca5In2Sb6 is naturally an intrinsic semiconductor with a low p-type carrier concentration. Here, we improve the thermoelectric properties of Ca5In2Sb6 by substituting Zn(2+) on the In(3+) site. With increasing Zn substitution, the Ca5In(2-x)Zn(x)Sb6 system exhibits increased p-type carrier concentration and a resulting transition from non-degenerate to degenerate semiconducting behavior. A single parabolic band model was used to estimate an effective mass in Ca5In2Sb6 of m* = 2m(e), which is comparable to the Al analogue, in good agreement with density functional calculations. Doping with Zn enables rational optimization of the electronic transport properties and increased zT in accordance with a single parabolic band model. The maximum figure of merit obtained in optimally Zn-doped Ca5In2Sb6 is 0.7 at 1000 K. While undoped Ca5In2Sb6 has both improved electronic mobility and reduced lattice thermal conductivity relative to Ca5Al2Sb6, these benefits did not dramatically improve the Zn-doped samples, leading to only a modest increase in zT relative to optimally doped Ca5Al2Sb6.

Thermoelectric properties of Zn-doped Ca3AlSb3

Polycrystalline samples of Ca3Al1−xZnxSb3, 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 Ca3AlSb3. 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.

Thermoelectric properties and electronic structure of the Zintl phase Sr5Al2Sb6

The Zintl phase Sr5Al2Sb6 has a large, complex unit cell and is composed of relatively earth-abundant and non-toxic elements, making it an attractive candidate for thermoelectric applications. The structure of Sr5Al2Sb6 is characterized by infinite oscillating chains of AlSb4 tetrahedra. It is distinct from the structure type of the previously studied Ca5M2Sb6 compounds (M = Al, Ga or In), all of which have been shown to have promising thermoelectric performance. The lattice thermal conductivity of Sr5Al2Sb6 (~0.55 W mK(-1) at 1000 K) was found to be lower than that of the related Ca5M2Sb6 compounds due to its larger unit cell (54 atoms per primitive cell). Density functional theory predicts a relatively large band gap in Sr5Al2Sb6, in agreement with the experimentally determined band gap of E(g) ~ 0.5 eV. High temperature electronic transport measurements reveal high resistivity and high Seebeck coefficients in Sr5Al2Sb6, consistent with the large band gap and valence-precise structure. Doping with Zn(2+) on the Al(3+) site was attempted, but did not lead to the expected increase in carrier concentration. The low lattice thermal conductivity and large band gap in Sr5Al2Sb6 suggest that, if the carrier concentration can be increased, thermoelectric performance comparable to that of Ca5Al2Sb6 could be achieved in this system.

High temperature thermoelectric properties of Zn-doped Eu5In2Sb6

The complex bonding environment of many ternary Zintl phases, which often results in low thermal conductivity, makes them strong contenders as thermoelectric materials. Here, we extend the investigation of A5In2Sb6 Zintl compounds with the Ca5Ga2As6 crystal structure to the only known rare-earth analogue: Eu5In2Sb6. Zn-doped samples with compositions of Eu5In2−xZnxSb6 (x = 0, 0.025, 0.05, 0.1, 0.2) were synthesized via ball milling followed by hot pressing. Eu5In2Sb6 showed significant improvements in air stability relative to its alkaline earth metal analogues. Eu5In2Sb6 exhibits semiconducting behavior with possible two band behavior suggested by increasing band mass as a function of Zn content, and two distinct transitions observed in optical absorption measurements (at 0.15 and 0.27 eV). The p-type Hall mobility of Eu5In2Sb6 was found to be much larger than that of the alkaline earth containing A5In2Sb6 phases (A = Sr, Ca) consistent with the reduced hole effective mass (1.1 me). Zn doping was successful in optimizing the carrier concentration, leading to a zT of up to 0.4 at ∼660 K, which is comparable to that of Zn-doped Sr5In2Sb6.

Thermoelectric properties and electronic structure of the zintl-phase Sr3AlSb3

The Zintl-phase Sr3 AlSb3 , which contains relatively earth-abundant and nontoxic elements, has many of the features that are necessary for good thermoelectric performance. The structure of Sr3 AlSb3 is characterized by isolated anionic units formed from pairs of edge-sharing tetrahedra. Its structure is distinct from previously studied chain-forming structures, Ca3 AlSb3 and Sr3 GaSb3 , both of which are known to be good thermoelectric materials. DFT predicts a relatively large band gap in Sr3 AlSb3 (Eg ≈1 eV) and a heavier band mass than that found in other chain-forming A3 MSb3 phases (A=Sr, Ca; M=Al, Ga). High-temperature transport measurements reveal both high resistivity and high Seebeck coefficients in Sr3 AlSb3 , which is consistent with the large calculated band gap. The thermal conductivity of Sr3 AlSb3 is found to be extremely low (≈ 0.55 W mK(-1) at 1000 K) due to the large, complex unit cell (56 atoms per primitive cell). Although the figure of merit (zT) has not been optimized in the current study, a single parabolic band model suggests that, when successfully doped, zT≈ 0.3 may be obtained at 600 K; this makes Sr3 AlSb3 promising for waste-heat recovery applications. Doping with Zn(2+) on the Al(3+) site has been attempted, but does not lead to the expected increase in carrier concentration.

Thermoelectric properties of the Ca5Al2−xInxSb6 solid solution

Zintl phases are attractive for thermoelectric applications due to their complex structures and bonding environments. The Zintl compounds Ca(5)Al(2)In(x)Sb(6)and Ca(5)Al(2)In(x)Sb(6) have both been shown to have promising thermoelectric properties, with zT values of 0.6 and 0.7, respectively, when doped to control the carrier concentration. Alloying can often be used to further improve thermoelectric materials in cases when the decrease in lattice thermal conductivity outweighs reductions to the electronic mobility. Here we present the high temperature thermoelectric properties of the Ca(5)Al(2-x)In(x)Sb(6)solid solution. Undoped and optimally Zn-doped samples were investigated. X-ray diffraction confirms that a full solid solution exists between the Al and In end-members. We find that the Al : In ratio does not greatly influence the carrier concentration or Seebeck effect. The primary effect of alloying is thus increased scattering of both charge carriers and phonons, leading to significantly reduced electronic mobility and lattice thermal conductivity at room temperature. Ultimately, the figure of merit is unaffected by alloying in this system, due to the competing effects of reduced mobility and lattice thermal conductivity.

Thermoelectric properties and electronic structure of the Zintl phase Sr5In2Sb6 and the Ca5−xSrxIn2Sb6 solid solution

The Zintl phase Sr5In2Sb6 is isostructural with Ca5In2Sb6-a promising thermoelectric material with a peak zT of 0.7 when the carrier concentration is optimized by doping. Density functional calculations for Sr5In2Sb6 reveal a decreased energy gap and decreased valence band effective mass relative to the Ca analog. Chemical bonding analysis using the electron localizability indicator was found to support the Zintl bonding scheme for this structure type. High temperature transport measurements of the complete Ca(5-x)Sr(x)In2Sb6 solid solution were used to investigate the influence of the cation site on the electronic and thermal properties of A5In2Sb6 compounds. Sr was shown to be fully miscible on the Ca site. The higher density of the Sr analog leads to a slight reduction in lattice thermal conductivity relative to Ca5In2Sb6, and, as expected, the solid solution samples have significantly reduced lattice thermal conductivities relative to the end member compounds.