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24 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Colossal phonon drag enhanced thermopower in lightly doped diamond

Diamond is one of the most studied materials because of its unique combination of remarkable electrical, mechanical, thermal and optical properties. Using a fully self-consistent ab initio theory of coupled electron-phonon transport, we reveal another striking behavior: a huge drag enhancement of the thermopower of lightly doped diamond. Thermopower values of around 100,000 μV K –1 are found at 100 K, significantly exceeding the highest previously measured value in the correlated metal FeSb 2 , and occurring at much higher temperatures. The enormous thermopower in diamond arises primarily from exceptionally weak anharmonic phonon decay around and below 100 K that facilitates efficient momentum exchange between charge carriers and phonons through electron-phonon interactions. Exceedingly large thermoelectric power factors are also identified. Furthermore, this work gives insights into the physics of the coupled electron-phonon system in solids and advances our understanding of thermoelectric transport in the regime of strong drag.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Half-Heusler alloys as emerging high power density thermoelectric cooling materials

Abstract To achieve optimal thermoelectric performance, it is crucial to manipulate the scattering processes within materials to decouple the transport of phonons and electrons. In half-Heusler (hH) compounds, selective defect reduction can significantly improve performance due to the weak electron-acoustic phonon interaction. This study utilized Sb-pressure controlled annealing process to modulate the microstructure and point defects of Nb 0.55 Ta 0.40 Ti 0.05 FeSb compound, resulting in a 100% increase in carrier mobility and a maximum power factor of 78 µW cm −1 K −2 , approaching the theoretical prediction for NbFeSb single crystal. This approach yielded the highest average zT of ~0.86 among hH in the temperature range of 300-873 K. The use of this material led to a 210% enhancement in cooling power density compared to Bi 2 Te 3 -based devices and a conversion efficiency of 12%. These results demonstrate a promising strategy for optimizing hH materials for near-room-temperature thermoelectric applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Vacancy defect control of colossal thermopower in FeSb2

Abstract Iron diantimonide is a material with the highest known thermoelectric power. By combining scanning transmission electron microscopic study with electronic transport neutron, X-ray scattering, and first principle calculation, we identify atomic defects that control colossal thermopower magnitude and nanoprecipitate clusters with Sb vacancy ordering, which induce additional phonon scattering and substantially reduce thermal conductivity. Defects are found to cause rather weak but important monoclinic distortion of the unit cell P n n m → P m . The absence of Sb along [010] for high defect concentration forms conducting path due to Fe d orbital overlap. The connection between atomic defect anisotropy and colossal thermopower in FeSb 2 paves the way for the understanding and tailoring of giant thermopower in related materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Materials Data on Pr2FeSbO7 by Materials Project

Pr2(FeSb)O7 crystallizes in the orthorhombic Imma space group. The structure is three-dimensional. there are two inequivalent Pr3+ sites. In the first Pr3+ site, Pr3+ is bonded to eight O2- atoms to form distorted PrO8 hexagonal bipyramids that share edges with two equivalent PrO8 hexagonal bipyramids, edges with two equivalent FeO6 octahedra, and edges with four equivalent SbO6 octahedra. There are a spread of Pr–O bond distances ranging from 2.28–2.62 Å. In the second Pr3+ site, Pr3+ is bonded in a distorted body-centered cubic geometry to eight O2- atoms. There are a spread of Pr–O bond distances ranging from 2.31–2.62 Å. Fe3+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with two equivalent FeO6 octahedra, corners with four equivalent SbO6 octahedra, and edges with two equivalent PrO8 hexagonal bipyramids. The corner-sharing octahedra tilt angles range from 48–50°. There are two shorter (2.06 Å) and four longer (2.10 Å) Fe–O bond lengths. Sb5+ is bonded to six O2- atoms to form SbO6 octahedra that share corners with two equivalent SbO6 octahedra, corners with four equivalent FeO6 octahedra, and edges with four equivalent PrO8 hexagonal bipyramids. The corner-sharing octahedra tilt angles range from 46–48°. There are four shorter (2.01 Å) and two longer (2.05 Å) Sb–O bond lengths. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded to two equivalent Pr3+ and two equivalent Fe3+ atoms to form distorted OPr2Fe2 tetrahedra that share corners with six OPr2Fe2 tetrahedra and an edgeedge with one OPr4 tetrahedra. In the second O2- site, O2- is bonded in a 4-coordinate geometry to two equivalent Pr3+ and two equivalent Sb5+ atoms. In the third O2- site, O2- is bonded in a 4-coordinate geometry to two Pr3+, one Fe3+, and one Sb5+ atom. In the fourth O2- site, O2- is bonded to four Pr3+ atoms to form a mixture of edge and corner-sharing OPr4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Chapmanite [Fe 2 Sb(Si 2 O 5 )O 3 (OH)]: thermodynamic properties and formation in low-temperature environments

Abstract. of synthetic Sb 2 O 5 , MgSb 2 O 6 (analogue of the mineral byströmite), Mg[Sb(OH) 6 ] 2 ∙6H 2 O (brandholzite), and natural chapmanite [(Fe 1.88 Al 0.12 )Sb(Si 2 O 5 )O 3 (OH)]. Enthalpies of reactions, including formation enthalpies, were evaluated using reference compounds Sb, Sb 2 O 3 , Sb 2 O 5 , and other phases, with high-temperature oxide melt solution calorimetry in lead borate and sodium molybdate solvents. Heat capacity and entropy were determined by relaxation and differential scanning calorimetry. The best set of Δ f H o (kJ mol -1 ) and S o (J mol -1 K -1 ) is byströmite -1733.0±3.6, 139.3±1.0; brandholzite -5243.1±3.6, 571.0±4.0; and chapmanite -3164.9±4.7, 305.1±2.1. The data for chapmanite give Δ f G o of -2973.6±4.7 kJ mol -1 and log K=-17.10 for the dissolution reaction (Fe 1.88 Al 0.12 )Sb(Si 2 O 5 )O 3 (OH) + 6H + → 1.88Fe 3+ + 0.12Al 3+ + 2SiO$_2^0$ + Sb(OH)$_3^0$ + 2H 2 O. Analysis of the data showed that chapmanite is finely balanced in terms of its stability with schafarzikite (FeSb 2 O 4 ) and tripuhyite (FeSbO 4 ) under a specific, narrow range of conditions when both aqueous Fe(III) and Sb(III) are abundant. In such a model, chapmanite is metastable by a narrow margin but could be stabilized by high SiO$_2^0$(aq) activities. Natural assemblages of chapmanite commonly contain abundant amorphous silica, suggesting that this mechanism may be indeed responsible for the formation of chapmanite. Chapmanite probably forms during low-temperature hydrothermal overprint of pre-existing Sb ores under moderately reducing conditions; the slightly elevated temperatures may help to overcome the kinetic barrier for its crystallization. During weathering, sheet silicates may adsorb Sb 3+ in tridentate hexanuclear fashion, thus exposing their chapmanite-like surfaces to the surrounding aqueous environment. Formation of chapmanite, as many other sheet silicates, under ambient conditions, is unlikely.

58 GEOSCIENCES↗