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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Thermoelectric Properties of Co 1-x Rh x Si 0.98 B 0.02 Alloys

In our previous study, modest boron segregation at CoSi grain boundaries could drastically reduce the electrical resistivity while maintaining a large Seebeck coefficient. We attribute this behavior to a healing effect of B on microcracks without affecting the electronic structures. CoSi 0.98 B 0.02 is the optimized sample possessing the highest power factor of 60 μW K -2 cm -1 at room temperature. To reduce the thermal conductivity while maintaining the high power factor, isoelectronic substitution, i.e., Rh on Co site, is adopted. Co 1-x Rh x Si 0.98 B 0.02 alloys with x = 0.00, 0.05, 0.10, and 0.20 were prepared by an arc melting and annealing procedure. The lattice thermal conductivity decreases monotonically with increasing x, which is ascribed to enhanced phonon scattering by point defects. The lowest lattice thermal conductivity obtained for the x = 0.20 sample is less than 50% of that for CoSi at room temperature. However, all the samples still exhibit very low resistivity and large Seebeck coefficient and thus the power factor is not decreased much. The best sample is x = 0.10, showing a room-temperature power factor of 58 μW K - 2 cm -1 . The room-temperature ZT is raised by a factor of 2.3 compared with CoSi due to the power factor enhancement and simultaneous thermal conductivity reduction.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Electrical and Galvanomagnetic Properties of Extruded Samples of Bi{sub 85}Sb{sub 15} Solid Solutions with Pb and Te Impurities

The effect of tellurium impurities on the electrical conductivity (σ), Seebeck coefficient (α), and Hall coefficient R of extruded Bi{sub 85}Sb{sub 15} + 0.001 аt.% Pb is studied in the temperature range 77–300 K. It is found that doping with tellurium impurities up to 0.001 at. % compensating acceptor Pb atoms leads to a strong decrease in the electron density and, consequently, to an increase in σ and significant decrease in the values of α and R. The materials obtained can be used as the n-branch of thermoelements operating at temperatures of ~77 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Electronic and thermal transport in GeTe: A versatile base for thermoelectric materials

GeTe is a narrow-band gap semiconductor, where Ge vacancies generate free charge carriers, holes, forming a self-dopant degenerate system with p-type conductivity, and serves as a base for high-performance multicomponent thermoelectric materials. There is a significant discrepancy between the electronic and thermal transport data for GeTe-based materials reported in the literature, which obscures the baseline knowledge and prevents a clear understanding of the effect of alloying GeTe with various elements. A comprehensive study including XRD, SEM, EDS, Seebeck coefficient, electrical resistivity, thermal conductivity, and {sup 125}Te NMR of several GeTe samples was conducted. Similar Seebeck coefficient and electrical resistivity are observed for all GeTe samples used showing that the concentration of Ge vacancies generating charge carriers is constant along the ingot. Very short {sup 125}Te NMR spin-relaxation time agrees well with high carrier concentration obtained from the Hall effect measurements. Our data show that at ∼700 K, GeTe has a very large power factor, 42 μWcm{sup −1}K{sup −2}, much larger than that of any high efficiency thermoelectric telluride at these temperatures. Electronic and thermal properties of GeTe are compared to PbTe, another well-known thermoelectric material, where free charge carriers, holes or electrons, are generated by vacancies on Pb or Te sites, respectively. Discrepancy in the data for GeTe reported in literature can be attributed to the variation in the Ge:Te ratio of solidified samples as well as to different conditions of measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Study of the Thermoelectric Properties of Lead Selenide Doped with Boron, Gallium, Indium, or Thallium

Group IIIA elements (B, Ga, In, and Tl) have been doped into PbSe for enhancement of thermoelectric properties. The electrical conductivity, Seebeck coefficient, and thermal conductivity were systematically studied. Room-temperature Hall measurements showed an effective increase in the electron concentration upon both Ga and In doping and the hole concentration upon Tl doping to 7 × 10{sup 19} cm{sup –3}. No resonant doping phenomenon was observed when PbSe was doped with B, Ga, or In. The highest room-temperature power factor 2.5 × 10{sup –3} W m{sup –1} K{sup –2} was obtained for PbSe doped with 2 atom % B. However, the power factor in B-doped samples decreased with increasing temperature, opposite to the trend for the other dopants. A figure of merit (ZT) of 1.2 at 873 K was achieved in PbSe doped with 0.5 atom % Ga or In. With Tl doping, modification of the band structure around the Fermi level helped to increase the Seebeck coefficient, and the lattice thermal conductivity decreased, probably as a result of effective phonon scattering by both the heavy Tl{sup 3+} ions and the increased grain boundary density after ball milling. The highest p-type ZT value was 1.0 at 723 K.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Grain Boundary Engineering for Achieving High Thermoelectric Performance in n-Type Skutterudites

Grain or phase boundaries play a critical role in the carrier and phonon transport in bulk thermoelectric materials. Previous investigations about controlling boundaries primarily focused on the reducing grain size or forming nanoinclusions. Herein, liquid phase compaction method is first used to fabricate the Yb–filled CoSb 3 with excess Sb content, which shows the typical feature of low–angle grain boundaries with dense dislocation arrays. Seebeck coefficients show a dramatic increase via energy filtering effect through dislocation arrays with little deterioration on the carrier mobility, which significantly enhances the power factor over a broad temperature range with a high room–temperature value around 47 μW cm –2 K –1 . Simultaneously, the lattice thermal conductivity could be further suppressed via scattering phonons via dense dislocation scattering. As a result, the highest average figure of merit ZT of ≈1.08 from 300 to 850 K could be realized, comparable to the best reported result of single or triple–filled Skutterudites. Furthermore, this work clearly points out that low–angle grain boundaries fabricated by liquid phase compaction method could concurrently optimize the electrical and thermal transport properties leading to an obvious enhancement of both power factor and ZT .

36 MATERIALS SCIENCE↗
DOE OSTI2018

Temperature dependent thermoelectric properties of cuprous delafossite oxides

The use of nanostructured delafossite oxides in thermoelectric (TE) applications has attracted a great interest due to their high performance and long-term stability at elevated temperatures. Cuprous delafossites, CuMO 2 (M=Al, Cr, Fe, Ga, Mn), compared to conventional TE materials, such as Bi 2 Te 3 , PbTe and SiGe, are non-toxic and more earth abundant. In particular, CuAlO 2 compound shows a great potential for high performance thermoelectric materials. In this work, a systematic study of temperature dependent TE properties of cuprous delafossite materials, CuAlO 2 , is reported. The optimization of the TE properties has been realized by controlling nanostructure size around 80 nm CuAlO 2 powder was prepared using a solid-state synthesis method at ~1373 K in nitrogen/air atmosphere. The nanostructure size was controlled by a high energy ball milling process. Reducing the particle size of nanostructured bulk materials decouples interdependent electron and phonon transport and results in a lattice thermal conductivity decrease without deteriorating electrical conductivity. As a result, the high effective mass plays a dominant role in the high Seebeck coefficient and low electrical conductivity. The power factor reached ~0.78×10 -5 W/mK 2 at 780 K. Temperature dependent TE properties, including Seebeck coefficient, electrical conductivity, and thermal conductivity are analyzed. The processing-structure-property correlation of these materials are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Organizing Chaos: Boosting Thermoelectric Properties by Ordering the Clathrate Framework of Ba 8 Cu 16 As 30

The type I clathrate, Ba 8 Cu 16 As 30 , is reinvestigated and found to have a low-temperature polymorph mP 108-Ba 8 Cu 16 As 30 with ordered Cu and As sites. In situ temperature-dependent powder X-ray diffraction experiments guided synthetic efforts toward the synthesis of the ordered monoclinic ( mP 108) and disordered cubic ( cP 54) polymorphs with high phase purity. While a transition from mP 108-Ba 8 Cu 16 As 30 to cP 54-Ba 8 Cu 16 As 30 is not directly observed, cP 54-Ba 8 Cu 16 As 30 is stabilized through quenching from high temperatures and is confirmed through high-resolution synchrotron powder X-ray diffraction. Further, combined theoretical predictions and experimental observations of the thermoelectric properties of both polymorphs reveal that the ordering of Cu and As atoms in the clathrate framework simultaneously enhances the Seebeck coefficient and electronic conductivity by increasing the hole effective mass and reducing the electronic scattering events. Consequently, the zT of mP 108-Ba 8 Cu 16 As 30 reaches a maximum of 0.2 at 575 K, an order of magnitude higher than that of cP 54-Ba 8 Cu 16 As 30 .

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2014

Enhanced thermoelectric performance of (Ba,In) double-filled skutterudites via randomly arranged micropores

Porous (Ba,In) double-filled skutterudite materials with pore diameter about 1–4 μm were prepared by the decomposition of metastable ZnSb inclusions induced by the Zn sublimation. Transport measurements revealed that the Seebeck coefficient was increased due to the electron filtering effect induced by nanostructures in the surfaces of pores, the electrical conductivity was almost unchanged because of the percolation effect of conducted network composed of filled skutterudites, and the lattice thermal conductivity was dramatically suppressed due to the enhanced pore-edge boundary scattering of long-wavelength phonons. As a result, a maximum ZT of 1.36 was obtained, increased by 22.5% as compared to that of the bulk material with same chemical composition. This work demonstrates that by introducing porous structures is thought to be an efficient approach to improve the thermoelectric performance of bulk materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Thermoelectric properties of Zintl compound Ca 1-x Na x Mg 2 Bi 1.98

Motivated by good thermoelectric performance of Bi-based Zintl compounds Ca 1-x Yb x Mg 2 Bi y , we further studied the thermoelectric properties of Zintl compound CaMg 2 Bi 1.98 by doping Na into Ca as Ca 1-x Na x Mg 2 Bi 1.98 via mechanical alloying and hot pressing. We found that the electrical conductivity, Seebeck coefficient, power factor, and carrier concentration can be effectively adjusted by tuning the Na concentration. Transport measurement and calculations revealed that an optimal doping of 0.5 at.% Na achieved better average ZT and efficiency. The enhancement in thermoelectric performance is attributed to the increased carrier concentration and power factor. Finally, the low cost and nontoxicity of Ca 1-x Na x Mg 2 Bi 1.98 makes it a potentially promising thermoelectric material for power generation in the mid-temperature range.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2016

Research update: Prediction of high figure of merit plateau in SnS and solid solution of (Pb,Sn)S

Direct conversion between thermal and electrical energy can be achieved by thermoelectric materials, which provide a viable route for power generation and solid state refrigeration. Here, we use a combination of energetic, electronic, and vibrational first-principles based results to predict the figure of merit performance in hole doped single crystals of SnS and (Pb,Sn)S. We find high ZT values for both materials, specifically for (Pb,Sn)S along the b -axis. Both SnS and (Pb,Sn)S have excellent power factors when doped, due to a combination of increased electrical conductivity (due to doping) and a significantly enhanced Seebeck coefficient obtained by a doping-induced multiband effect. Anharmonic phonon calculations combined with a Debye-Calloway model show that the lattice thermal conductivity of both compounds is low, due to intrinsic anharmonicity, and is lowered further by the random, solid solution nature of the cation sublattice of (Pb,Sn)S. (Pb,Sn)S exhibits a high ZT plateau ranging from 1.3 at 300 K to 1.9 at 800 K. Finally, the overall ZT of the hole doped (Pb,Sn)S crystals is predicted to outperform most of the current state-of-the-art thermoelectric sulfide materials.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thermoelectric properties of DC-sputtered filled skutterudite thin film

The Yb filled CoSb{sub 3} skutterudite thermoelectric thin films were prepared by DC magnetron sputtering. The electrical conductivity, Seebeck coefficient, thermal conductivity, and figure of merit ZT of the samples are characterized in a temperature range of 300 K to 700 K. X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy are obtained to assess the phase composition and crystallinity of thin film samples at different heat treatment temperatures. Carrier concentrations and Hall mobilities are obtained from Hall Effect measurements, which provide further insight into the electrical conductivity and Seebeck coefficient mechanisms. The thermal conductivity of thin film filled skutterudite was found to be much less compared with bulk Yb filled CoSb{sub 3} skutterudite. In this work, the 1020 K heat treatment was adopted for thin film post process due to the high degree of crystallinity as well as avoiding reverse heating effect. Thin film samples of different thicknesses were prepared with the same sputtering deposition rate and maximum ZT of 0.48 was achieved at 700 K for the 130 nm thick sample. This value was between half and one third of the bulk figure of merit which was due to the lower Hall mobility.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Current transport and thermoelectric properties of very high power factor Fe{sub 3}O{sub 4}/SiO{sub 2}/p-type Si(001) devices

The current transport and thermoelectric properties of Fe{sub 3}O{sub 4}/SiO{sub 2}/p-type Si(001) heterostructures with Fe{sub 3}O{sub 4} thicknesses of 150, 200, and 350 nm have been investigated between 100 and 300 K. We observe a sharp drop of the in-plane resistivity at 200 K due to the onset of conduction along the Si/SiO{sub 2} interface related to tunneling of electrons from the Fe{sub 3}O{sub 4} into the accumulation layer of holes at the Si/SiO{sub 2} interface, whose existence was confirmed by capacitance-voltage measurements and a two band analysis of the Hall effect. This is accompanied by a large increase of the Seebeck coefficient reaching +1000 μV/K at 300 K that is related to holes in the p-type Si(001) and gives a power factor of 70 mW/K{sup 2}m when the Fe{sub 3}O{sub 4} layer thickness is reduced down to 150 nm. We show that most of the current flows in the Fe{sub 3}O{sub 4} layer at 300 K, while the Fe{sub 3}O{sub 4}/SiO{sub 2}/p-type Si(001) heterostructures behave like tunneling p-n junctions in the transverse direction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Extraordinary role of Hg in enhancing the thermoelectric performance of p-type SnTe

We report several synergistic effects in Hg alloying of SnTe to enhance the power factor and overall figure of merit ZT. Hg alloying decreases the energy separation between the two valence bands, leading to pronounced band convergence that improves the Seebeck coefficient. Hg alloying of SnTe also significantly enlarges the band gap thereby effectively suppressing the bipolar diffusion. Collectively, this results in high ZT of similar to 1.35 at 910 K for 2% Bi-doped SnTe with 3% HgTe. The solubility limit of Hg in SnTe is less than 3 mol%, and above this level we observe HgTe precipitates in the SnTe matrix, typically trapped at grain boundary triple junctions. The strong point defect scattering of phonons caused by Hg alloying coupled with mesoscale scattering via grain boundaries contributes to a great reduction of lattice thermal conductivity. The multiple synergistic roles that Hg plays in regulating the electron and phonon transport in SnTe provide important new insights into continued optimization of SnTe-based and related materials.

Tan, Gangjian↗
DOE OSTI2014

Enhanced thermoelectric performance of In-substituted GeSb{sub 6}Te{sub 10} with homologous structure

We studied the crystal structure and thermoelectric properties of polycrystalline GeIn{sub x}Sb{sub 6−x}Te{sub 10} (x = 0, 0.18, 0.3, and 0.6). Rietveld and Le Bail analyses showed that all compositions crystallized in trigonal structures with a 51-layer period. Substituting In decreased both the lattice and electronic thermal conductivity, as well as markedly increased the Seebeck coefficient. We ascribed this increase to increases in the effective mass of the carriers, likely caused by the formation of additional energy states near the Fermi level. In GeIn{sub 0.6}Sb{sub 5.4}Te{sub 10}, we found a maximum ZT of 0.75 at 710 K, 1.9 times higher than that of GeSb{sub 6}Te{sub 10}.

36 MATERIALS SCIENCE↗
NASA NTRS2014

N-type Doped PbTe and PbSe Alloys for Thermoelectric Applications

The present invention demonstrates that weak scattering of carriers leads to a high mobility and therefore helps achieve low electric resistivity with high Seebeck coefficient for a thermoelectric material. The inventors demonstrate this effect by obtaining a thermoelectric figure of merit, zT, higher than 1.3 at high temperatures in n-type PbSe, because of the weak scattering of carriers in the conduction band as compared with that in the valence band. The invention further demonstrates favorable thermoelectric transport properties of n-type PbTe.sub.1-xI.sub.x with carrier concentrations ranging from 5.8.times.10.sup.18-1.4.times.10.sup.20 cm.sup.-3.

Snyder, G. Jeffrey↗
DOE OSTI2016

High-throughput exploration of thermoelectric and mechanical properties of amorphous NbO{sub 2} with transition metal additions

To increase the thermoelectric efficiency and reduce the thermal fatigue upon cyclic heat loading, alloying of amorphous NbO{sub 2} with all 3d and 5d transition metals has systematically been investigated using density functional theory. It was found that Ta fulfills the key design criteria, namely, enhancement of the Seebeck coefficient and positive Cauchy pressure (ductility gauge). These quantum mechanical predictions were validated by assessing the thermoelectric and elastic properties on combinatorial thin films, which is a high-throughput approach. The maximum power factor is 2813 μW m{sup −1} K{sup −2} for the Ta/Nb ratio of 0.25, which is a hundredfold increment compared to pure NbO{sub 2} and exceeds many oxide thermoelectrics. Based on the elasticity measurements, the consistency between theory and experiment for the Cauchy pressure was attained within 2%. On the basis of the electronic structure analysis, these configurations can be perceived as metallic, which is consistent with low electrical resistivity and ductile behavior. Furthermore, a pronounced quantum confinement effect occurs, which is identified as the physical origin for the Seebeck coefficient enhancement.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2011

The role of boron segregation in enhanced thermoelectric power factor of CoSi{sub 1-x}B{sub x} alloys

We report on the influence of boron segregation on the thermoelectric properties of CoSi. Contrary to previous suggestions, and in stark contrast to aluminum substitution, boron does not enter the lattice on the Si site, but rather segregates to the grain boundaries in these alloys. Through a combination of x-ray diffraction, scanning electron microscope, and scanning Auger techniques, we present clear evidence of the formation of a CoB phase at the grain boundaries. Consistent with the failure of B to substitute for Si, we observe no changes in the electron concentration or the Seebeck coefficient under boron substitution. The electrical resistivity, on the other hand, displays a non-monotonic behavior with increasing boron concentration, first decreasing for small amounts of boron, before increasing at higher levels of substitution. We attribute this behavior to a combination of an initial healing effect of boron on microcracks, followed by the eventual increase in electron scattering by the secondary CoB phase at higher concentrations.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Enhanced thermoelectric performance of PbSe-graphene nanocomposite manufactured with acoustic cavitation induced defects

An order of magnitude rise in the thermoelectric (TE) performance of the PbSe, a scalable and easy-to-manufacture TE material, has been achieved by incorporating reduced graphene oxide (Gr) nanoplatelets in a PbSe/PbSeO 3 heterostructure formed by acoustic cavitation-assisted oxidation. The fabricated Gr/PbSe/PbSeO 3 nanocomposites exhibit high TE performance with an exceptionally high Seebeck coefficient coupled with low thermal conductivity. The variation in the Seebeck coefficient has been attributed to a reduction in charge carrier mobility due to the ferroelectric polarization effect. Furthermore, the increase in electrical resistivity is minimized by adding graphene. At an optimal weight fraction (0.2 wt%), graphene nano-inclusions lead to superior Seebeck coefficient values as high as ~2000 μV/K at ~500 K, providing high overall TE performance. This study shows substantial changes in the TE properties of PbSe through the incorporation of graphene and PbSeO 3 . The understanding and methodology developed in this study can be exploited for the scalable manufacturing of high-performance TE materials.

Structure-property correlations↗