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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 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Tailored semiconducting carbon nanotube networks with enhanced thermoelectric properties

Thermoelectric power generation, allowing recovery of part of the energy wasted as heat, is emerging as an important component of renewable energy and energy efficiency portfolios. Although inorganic semiconductors have traditionally been employed in thermoelectric applications, organic semiconductors garner increasing attention as versatile thermoelectric materials. Here we present a combined theoretical and experimental study suggesting that semiconducting single-walled carbon nanotubes with carefully controlled chirality distribution and carrier density are capable of large thermoelectric power factors, higher than 340 μW m -1 K -2 , comparable to the best-performing conducting polymers and larger than previously observed for carbon nanotube films. Furthermore, we demonstrate that phonons are the dominant source of thermal conductivity in the networks, and that our carrier doping process significantly reduces the thermal conductivity relative to undoped networks. As a result, these findings provide the scientific underpinning for improved functional organic thermoelectric composites with carbon nanotube inclusions.

14 SOLAR ENERGY↗
DOE OSTI2012

Thermoelectric properties and impedance spectroscopy of polycrystalline samples of the beta-gallia rutile intergrowth, (Ga,In){sub 4}(Sn,Ti){sub 5}O{sub 16}

Polycrystalline samples of Ga{sub 3}In(Sn{sub 1-y}Ti{sub y}){sub 5}O{sub 16}, y{<=}0.3 were prepared by solid-state synthesis and characterized to determine their optical properties, thermoelectric properties and electrical impedance. Using diffuse reflectance data and assuming a direct band gap, the band gap of the material ranges from 3.58 eV for y=0 to 3.74 eV for y=0.2. The dc conductivity decreased with increasing Ti content and was thermally activated, ranging from {<=}10{sup -5} S/cm at 300 Degree-Sign C to 0.03 S/cm at 1000 Degree-Sign C for Ga{sub 3}InSn{sub 5}O{sub 16}. The Seebeck coefficient was negative indicating n-type conduction. The activation energies for dc conduction and thermopower were similar, ranging from 1.4{+-}0.1 eV for y=0 to 1.8{+-}0.2 eV for y=0.2, suggesting band conduction. The thermal conductivity of Ga{sub 3}InSn{sub 5}O{sub 16} ranged from 1.1 W/m K at 500 {sup o}C to 0.75 W/m K at 800 Degree-Sign C. The highest figure of merit measured was 2 Multiplication-Sign 10{sup -4} at {approx}900 Degree-Sign C, which is much lower than desired for thermoelectric materials. A comparison of dc conductivity and impedance data indicated a substantial ionic contribution for samples containing titanium. - Graphical abstract: The thermoelectric properties of polycrystalline Ga{sub 3}In(Sn{sub 1-y}Ti{sub y}){sub 5}O{sub 16}, y{<=}0.3 were measured in air at 300-900 Degree-Sign C. The materials are broad-band n-type semiconductors with non-negligible ionic conduction for samples prepared with y>0. The thermoelectric figure of merit is much lower than desired for practical thermoelectric devices. Highlights: Black-Right-Pointing-Pointer Ga{sub 3}In(Sn{sub 1-y}Ti{sub y}){sub 5}O{sub 16}, y{<=}0.2 is a broad-band n-type semiconductor. Black-Right-Pointing-Pointer The conductivity of Ti-containing samples show evidence of an ionic as well as an electronic contribution. Black-Right-Pointing-Pointer The thermal conductivity of polycrystalline Ga{sub 3}In(Sn{sub 1-y}Ti{sub y}){sub 5}O{sub 16} decreases with increasing temperature. Black-Right-Pointing-Pointer The thermoelectric figure of merit for Ga{sub 3}In(Sn{sub 1-y}Ti{sub y}){sub 5}O{sub 16} is lower than desired for practical thermoelectric devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Lattice dynamics and thermoelectric properties of nanocrystalline silicon-germanium alloys: Lattice dynamics and thermoelectric properties of nc Si-Ge alloys

The lattice dynamics and thermoelectric properties of sintered phosphorus-doped nanostructured silicon–germanium alloys obtained by gas-phase synthesis were studied. Measurements of the density of phonon states by inelastic neutron scattering were combined with measurements of the elastic constants and the low-temperature heat capacity. A strong influence of nanostructuring and alloying on the lattice dynamics was observed. The thermoelectric transport properties of samples with different doping as well as samples sintered at different temperature were characterized between room temperature and 1000°C. A peak figure of merit zT=0.88 at 900°C is observed and is comparatively insensitive to the aforementioned parameter variations.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Polarization field engineering of GaN/AlN/AlGaN superlattices for enhanced thermoelectric properties

A novel polarization field engineering based strategy to simultaneously achieve high electrical conductivity and low thermal conductivity in thermoelectric materials is demonstrated. Polarization based electric fields are used to confine electrons into two-dimensional electron gases in GaN/AlN/Al{sub 0.2}Ga{sub 0.8}N superlattices, resulting in improved electron mobilities as high as 1176 cm{sup 2}/Vs and in-plane thermal conductivity as low as 8.9 W/mK. The resulting room temperature ZT values reach 0.08, a factor of four higher than InGaN and twelve higher than GaN, demonstrating the potential benefits of this polarization based engineering strategy for improving the ZT and efficiencies of thermoelectric materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Thermoelectric properties of p-type Ag{sub 1−x}(Pb{sub 1−y}Sn{sub y}){sub m}Sb{sub 1−z}Te{sub m+2}

The thermoelectric properties of Ag{sub 1−x}(Pb{sub 1−y}Sn{sub y}){sub m}Sb{sub 1−z}Te{sub m+2} (4≤m≤16, −0.1≤x≤0.3, 1/3≤y≤2/3, 0.2≤z≤0.4; Lead Antimony Silver Tellurium Tin, LASTT-m) compositions were investigated in the temperature range of 300 to ~670 K. All samples crystallize in the average NaCl-type structure without any noticeable second phase and exhibit very narrow bandgaps of <0.1 eV. We studied a range of m values, silver concentrations (x), Pb/Sn ratios (y), and antimony concentrations (z) to determine their effects on the thermoelectric properties. The samples were investigated as melt grown polycrystalline ingots. Varying the Ag contents, the Pb/Sn ratios, and the Sb contents off-stoichiometry allowed us to control the electrical conductivity, the Seebeck coefficient, and the thermal conductivity. The electrical conductivity tends to decrease with decreasing m values. The highest ZT of ~1.1 was achieved at ~660 K for Ag{sub 0.9}Pb{sub 5}Sn{sub 5}Sb{sub 0.8}Te{sub 12} mainly due to the very low lattice thermal conductivity of ~0.4 W/(m K) around 660 K. Also, samples with charge-balanced stoichiometries, Ag(Pb{sub 1−y}Sn{sub y}){sub m}SbTe{sub m+2}, were studied and found to exhibit a lower power factor and higher lattice thermal conductivity than the Ag{sub 1−x}(Pb{sub 1−y}Sn{sub y}){sub m}Sb{sub 1−z}Te{sub m+2} compositions. - Graphical abstract: The Ag{sub 1−x}(Pb{sub 1−y}Sn{sub y}){sub m}Sb{sub 1−z}Te{sub m+2} system defines a complex and flexible class of tunable thermoelectric class of materials with high performance.

36 MATERIALS SCIENCE↗
DOE OSTI2013

The effect of structural vacancies on the thermoelectric properties of (Cu₂Te){sub 1–x}(Ga₂Te₃) x

We have studied the effects of structural vacancies on the thermoelectric properties of the ternary compounds (Cu₂Te) 1–x (Ga₂Te₃) x (x=0.5, 0.55, 0.571, 0.6, 0.625, 0.667 and 0.75), which are solid solutions found in the pseudo-binary phase diagram for Cu₂Te and Ga₂Te₃, and possesses tunable structural vacancy concentrations. This materials system is not suitable due to the cost and scarcity of the constituent elements, but the vacancy behavior is well understood and will provide a valuable test case for other systems more suitable from the standpoint of cost and abundance of raw materials, which also possesses these vacancy features, but whose structural characterization is lacking at this stage. We find that the nominally defect free phase CuGaTe₂ possess the highest ZT (ZT=S²T/ρκ, where S is the Seebeck coefficient and ρ is the electrical resistivity κ is the thermal conductivity and T is the absolute temperature) which approaches 1 at 840 K and seems to continuously increase above this temperature. This result is due to the unexpectedly low thermal conductivity found for this material at high temperature. The low thermal conductivity was caused by strong Umklapp (thermally resistive scattering processes involving three phonons) phonon scattering. We find that due to the coincidentally strong scattering of carriers by the structural defects that higher concentrations of these features lead to poor electrical transport properties and decreased ZT. - Graphical abstract: Thermal conductivity and zT as a function of temperature for a series of compounds of the type (Cu₂Te) 1–x (Ga₂Te₃)x (x=0.5, 0.55, 0.571, 0.6, 0.625, 0.667 and 0.75). Highlights: • All the samples show p-type semiconducting behavior in the temperature dependence of the Seebeck and Hall coefficients. • The increased carrier concentration and the introduction of vacancies diminish the carrier mobility and power factor. • The low temperature k decreases significantly as the Ga₂Te₃ content increases due to increasing point defects. • The highest ZT ~ 1.0 at 840 K among the samples in this study was found in CuGaTe₂, which contains no vacancies.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Thermoelectric properties of rocksalt ZnO from first-principles calculations

Zinc oxide (ZnO) undergoes a pressure-induced structural transition from its normal ambient-pressure wurtzite (WZ) phase to a rocksalt (RS) phase around 10 GPa. A recent experiment shows that the high-pressure RS ZnO phase can be recovered and stabilized at ambient conditions, which raises exciting prospects of expanding the range of properties of ZnO. For a fundamental understanding of the RS ZnO phase, we have performed first-principles calculations to determine its electronic, phonon, and thermodynamic properties at high (20 GPa) and ambient (0 GPa) pressure. Furthermore, we have calculated its electrical and thermal transport properties, which allow an evaluation of its thermoelectric figure of merit ZT at different temperature and doping levels. Our calculations show that the ambient-pressure RS ZnO phase can reach ZT values of 0.25 to 0.3 under both n-type and p-type doping in a large temperature range of 400 K to 800 K, which is considerably lower than the temperature range of 1400 K to 1600 K where WZ ZnO reaches similar ZT values. These results establish RS ZnO as a promising material for thermoelectric devices designed to operate at temperatures desirable for many heat recovery applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Isovalent substitutes play in different ways: Effects of isovalent substitution on the thermoelectric properties of CoSi{sub 0.98}B{sub 0.02}

Boron-added CoSi, CoSi{sub 0.98}B{sub 0.02}, possesses a very high thermoelectric power factor of 60 μW cm{sup −1} K{sup −2} at room temperature, which is among the highest power factors that have ever been reported for near-room-temperature thermoelectric applications. Since the electrical properties of this material have been tuned properly, isovalent substitution for its host atoms is intentionally employed to reduce the lattice thermal conductivity while maintaining the electronic properties unchanged. In our previous work, the effect of Rh substitution for Co atoms on the thermoelectric properties of CoSi{sub 0.98}B{sub 0.02} has been studied. Here, we present a study of the substitution of Ge for Si atoms in this compound. Even though Ge and Rh are isovalent with their corresponding host atoms, they play different roles in determining the electrical and thermal transport properties. Through the evaluation of the lattice thermal conductivity by the Debye approximation and the comparison between the high-temperature Seebeck coefficients, we propose that Rh substitution leads to a further overlapping of the conduction and the valence bands, while Ge substitution only shifts the Fermi level upward into the conduction band. Our results show that the influence of isovalent substitution on the electronic structure cannot be ignored when the alloying method is used to improve thermoelectric properties.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
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 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 OSTI2015

Studies on Thermoelectric Properties of n-type Polycrystalline SnSe 1- x S x by Iodine Doping

Iodine-doped n-type SnSe polycrystalline by melting and hot pressing is prepared. The prepared material is anisotropic with a peak ZT of ≈0.8 at about 773 K measured along the hot pressing direction. This is the first report on thermoelectric properties of n-type Sn chalcogenide alloys. With increasing content of iodine, the carrier concentration changed from 2.3 × 1017 cm-3 (p-type) to 5.0 × 1015 cm-3 (n-type) then to 2.0 × 1017 cm-3 (n-type). The decent ZT is mainly attributed to the intrinsically low thermal conductivity due to the high anharmonicity of the chemical bonds like those in p-type SnSe. By alloying with 10 at% SnS, even lower thermal conductivity and an enhanced Seebeck coefficient were achieved, leading to an increased ZT of ≈1.0 at about 773 K measured also along the hot pressing direction.

36 MATERIALS SCIENCE↗
DOE OSTI2014

High-Temperature Thermoelectric Properties of the Solid-Solution Zintl Phase Eu 11 Cd 6 Sb 12-x As x (x < 3)

Zintl phases are compounds that have shown promise for thermoelectric applications. The title solid–solution Zintl compounds were prepared from the elements as single crystals using a tin flux for compositions x = 0, 1, 2, and 3. Eu 11 Cd 6 Sb 12–x As x (x < 3) crystallize isostructurally in the centrosymmetric monoclinic space group C2/m (no. 12, Z = 2) as the Sr 11 Cd 6 Sb 12 structure type (Pearson symbol mC58). Efforts to make the As compositions for x exceeding ~3 resulted in structures other than the Sr 11 Cd 6 Sb 12 structure type. Single-crystal X-ray diffraction indicates that As does not randomly substitute for Sb in the structure but is site specific for each composition. The amount of As determined by structural refinement was verified by electron microprobe analysis. Electronic structures and energies calculated for various model structures of Eu 11 Cd 6 Sb 10 As 2 (x = 2) indicated that the preferred As substitution pattern involves a mixture of three of the six pnicogen sites in the asymmetric unit. In addition, As substitution at the Pn4 site opens an energy gap at the Fermi level, whereas substitution at the other five pnicogen sites remains semimetallic with a pseudo gap. Thermoelectric properties of these compounds were measured on hot-pressed, fully densified pellets. Samples show exceptionally low lattice thermal conductivities from room temperature to 775 K: 0.78–0.49 W/mK for x = 0; 0.72–0.53 W/mK for x = 1; and 0.70–0.56 W/mK for x = 2. Eu 11 Cd 6 Sb 12 shows a high p-type Seebeck coefficient (from +118 to 153 μ V/K) but also high electrical resistivity (6.8 to 12.8 mΩ·cm). The value of zT reaches 0.23 at 774 K. The properties of Eu 11 Cd 6 Sb 12–x As x are interpreted in discussion with the As site substitution.

Kazem, Nasrin↗