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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 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

Thermoelectric Properties of P-type Skutterudites YbxFe3.5Ni0.5Sb12 (0.8 x 1)

P-type skutterudites, with nominal compositions YbxFe3.5Ni0.5Sb12 (0.8 x 1), have been synthesized by induction melting with subsequent annealing, and their thermoelectric properties evaluated from 3.5 K to 745 K to assess their suitability for thermoelectric based waste heat recovery applications. We report results for the synthesis and measurements of Seebeck coefficient (S), electrical resistivity ( ), thermal conductivity ( ), Hall coefficient (RH), and effective mass (m*/m0) of YbxFe3.5Ni0.5Sb12 (0.8 x 1). Powder x-ray diffraction and electron probe microanalysis (EPMA) show that this system has a narrow filling fraction range of x ~ 0.84 to 0.86 for Yb in the crystallographic voids. All samples show positive RH for the entire temperature range studied with carrier concentrations ranging from 9.6 1020 to 2.8 1021 cm-3 at room temperature. Relatively high values of S result in high power factors up to 17 Wcm-1K-2 at room temperature. However, large values of and a sharp reduction in the S at high temperature due to bipolar conduction prevent the attainment of high thermoelectric figure of merit.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Thermoelectric properties of Sn- and Pb-doped Tl{sub 9}BiTe{sub 6} and Tl{sub 9}SbTe{sub 6}

A variety of substitutions in Tl{sub 9}BiTe{sub 6} and Tl{sub 9}SbTe{sub 6} with Sn and Pb, amounting to 14 different samples, were performed by melting the stoichiometric amounts of elements at 923 K, followed by slow cooling. The pulverized powders were sintered using the hot-pressing technique. All samples were of single phase according to the powder X-ray diffraction patterns. Thermoelectric property measurements were performed to investigate the effects of Sn- and Pb-doping on the electrical conductivity, Seebeck coefficient, and thermal conductivity. Increasing the concentration of the dopants caused increases in electrical and thermal conductivity, while decreasing the Seebeck coefficient. Tl{sub 9}Bi{sub 0.90}Pb{sub 0.10}Te{sub 6} and Tl{sub 9}Bi{sub 0.85}Pb{sub 0.15}Te{sub 6} exhibited the highest power factor. The changes in lattice thermal conductivity were minor and did not follow a clear trend. Competitive ZT values were obtained for Tl{sub 9}Bi{sub 0.95}Sn{sub 0.05}Te{sub 6}, Tl{sub 9}Bi{sub 0.95}Pb{sub 0.05}Te{sub 6}, Tl{sub 9}Sb{sub 0.97}Sn{sub 0.03}Te{sub 6}, and Tl{sub 9}Sb{sub 0.95}Pb{sub 0.05}Te{sub 6}, namely 0.95, 0.94, 0.83, and 0.71 around 500 K, respectively. Higher dopant concentrations led to lower ZT values.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Divalent doping-induced thermoelectric power factor increase in p-type Bi 2 Te 3 via electronic structure tuning

We use first-principles calculations to reveal the effects of divalent Pb, Ca, and Sn doping of Bi 2 Te 3 on the band structure and transport properties, including the Seebeck coefficient, α, and the reduced power factor, α 2 σ/τ, where σ is the electrical conductivity and τ is the effective relaxation time. Pb and Ca additions exhibit up to 60%–75% higher peak α 2 σ/τ than that of intrinsic Bi 2 Te 3 with Bi antisite defects. Pb occupancy and Ca occupancy of Bi sites increase σ/τ by activating high-degeneracy low-effective-mass bands near the valence band edge, unlike Bi antisite occupancy of Te sites that eliminates near-edge valence states in intrinsic Bi 2 Te 3 . Neither Pb doping nor subatomic-percent Ca doping increases α significantly, due to band averaging. Higher Ca levels increase α and diminish σ, due to the emergence of a corrugated band structure underpinned by high-effective-mass bands, attributable to Ca–Te bond ionicity. Sn doping results in a distortion of the bands with a higher density of states that may be characterized as a resonant state but decreases α 2 σ by up to 30% due to increases in the charge carrier effective mass and decreases in both spin–orbit coupling and valence band quasidegeneracy. These results, and thermal conductivity calculations for nanostructured Bi 2 Te 3 , suggest that Pb or Ca doping can enhance the thermoelectric figure of merit ZT to values up to ZT ~ 1.7, based on an experimentally determined τ. Our findings suggest that divalent doping can be attractive for realizing large ZT enhancements in pnictogen chalcogenides.

36 MATERIALS SCIENCE↗
NASA NTRS2002

Synthesis and thermoeletric properties of Co(sub (1-x)ni)(sub X)P(sub 3) and CoAs((sub 3-x))P(sub x) skutterudites

Two types of promising phosphide skutterudite materials, Co(1-x)NixP3 (x= 0.025 to 0.70) and CoAs(3-x)Px (x=0.5 to 0.10), have been synthesized and their transport properties measured. These compounds were prepared using a direct synthesis technique. The samples were hot pressed and analyzed by electron microprobe microscopy. Hall Effect measurements were conducted to determine the electrical conductivity, mobility and carrier concentration. In addition, Seebeck voltage and thermal conductivity measurements were performed. The thermoelectric properties are presented anddiscussed as a function of temperature up to 1273 K. The thermal stability of the primary CoP3 was examined in a static vacuum under isothermal and in-gradient conditions. The effect of the presence of 1 atm of a cover gas on the material loss rate was analyzed.

thermoelectric↗
DOE OSTI2017

Thermoelectric transport properties of polycrystalline SnSe alloyed with PbSe

Single-crystal SnSe has been found to exhibit exceptional thermoelectric performance, but the efficiency of polycrystalline samples is still far from satisfactory. In this paper, with an intention to effectively suppress heat conduction and minimally affect hole transport, we alloyed p-type polycrystalline SnSe with PbSe. Single-phase Sn 1-x Pb x Se solid solutions were formed up to x ≈ 0.12. The lattice thermal conductivity was reduced from 1.4 to 0.85 W m -1 K -1 by 12 at. % PbSe alloying due to strain and mass fluctuations. Interestingly, the Seebeck coefficient and carrier concentration were nearly unchanged by Pb substitution, indicating a constant effective mass and an undisrupted valence band maximum. A peak figure of merit (ZT) of 0.85 at 800 K was obtained in the x = 0 sample, and relatively high performance was also achieved in solid solutions. Finally, a concise model was developed involving multiple carrier scattering mechanisms, capturing the dependence of the mobility on composition and temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2019

High-Throughput Screening for Advanced Thermoelectric Materials: Diamond-Like ABX 2 Compounds

High-throughput (HTP) calculations are a highly promising direction for the discovery of novel functional materials. Here we use an HTP framework to investigate the electronic structures and p-type thermoelectric properties of the ABX 2 compounds with diamond-like structures. We show application of HTP both to identify compounds and also to identify underlying trends. 65 entries out of 84,908 in Materials Informatics Platform are selected for this study. The electronic structures and chemical bonding analyses reveal that there exists a general conductive network consisted of the anion X sublattice, which dominates the electrical transport properties of the compounds. Electrical and thermal transport properties of the 41 pnictide and chalcogenide compounds with sufficient band gaps are studied. Pnictide compounds have relatively smaller Seebeck coefficients than the chalcogenide compounds. This is due to the smaller effective masses around the valence band maxima. The electrical conductivities and power factors, however, are better in pnictide compounds. This is because pnictide compounds have high electronic group velocities and electronic relaxation times. Combined with the predictions of lattice thermal conductivities based on the Slack model, 12 novel p-type and n-type ABX2 materials with high ZT values are predicted.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Thermoelectric properties of p-type PbTe/Ag{sub 2}Te bulk composites by extrinsic phase mixing

We investigated the thermoelectric properties of PbTe/Ag{sub 2}Te bulk composites, synthesized by hand milling, mixing, and hot press sintering. From x-ray diffraction and energy dispersive x-ray spectroscopy measurements, we observed Ag{sub 2}Te phase separation in the PbTe matrix without Ag atom diffusion. In comparison with previously reported pseudo-binary (PbTe){sub 1−x}(Ag{sub 2}Te){sub x} composites, synthesized by high temperature phase separation, the PbTe/Ag{sub 2}Te bulk composites fabricated with a low temperature phase mixing process give rise to p-type conduction of carriers with significantly decreased electrical conductivity. This indicates that Ag atom diffusion in the PbTe matrix changes the sign of the Seebeck coefficient to n-type and also increases the carrier concentration. Effective p-type doping with low temperature phase separation by mixing and hot press sintering can enhance the thermoelectric performance of PbTe/Ag{sub 2}Te bulk composites, which can be used as a p-type counterpart of n-type (PbTe){sub 1−x}(Ag{sub 2}Te){sub x} bulk composites.

36 MATERIALS SCIENCE↗
DOE OSTI2017

High thermoelectric power factor in two-dimensional crystals of Mo S 2

The quest for high-efficiency heat-to-electricity conversion has been one of the major driving forces toward renewable energy production for the future. Efficient thermoelectric devices require high voltage generation from a temperature gradient and a large electrical conductivity while maintaining a low thermal conductivity. For a given thermal conductivity and temperature, the thermoelectric power factor is determined by the electronic structure of the material. Low dimensionality (1D and 2D) opens new routes to a high power factor due to the unique density of states (DOS) of confined electrons and holes. The 2D transition metal dichalcogenide (TMDC) semiconductors represent a new class of thermoelectric materials not only due to such confinement effects but especially due to their large effective masses and valley degeneracies. Here, we report a power factor of MoS2 as large as 8.5 mW m –1 K –2 at room temperature, which is among the highest measured in traditional, gapped thermoelectric materials. To obtain these high power factors, we perform thermoelectric measurements on few-layer MoS 2 in the metallic regime, which allows us to access the 2D DOS near the conduction band edge and exploit the effect of 2D confinement on electron scattering rates, resulting in a large Seebeck coefficient. We conclude the demonstrated high, electronically modulated power factor in 2D TMDCs holds promise for efficient thermoelectric energy conversion.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Al content and annealing on the phases formation, lattice parameters, and magnetization of A l x F e 2 B 2 ( x = 1.0 , 1.1 , 1.2 ) alloys

AlFe 2 B 2 is a ferromagnet with the Curie temperature around 300 K and has the potential to be an outstanding rare-earth free candidate for magnetocaloric applications. However, samples prepared from the melt contain additional phases which affect the functional response of the AlFe 2 B 2 phase. Here, we report on the effects of Al content in samples with the initial (nominal) composition of Al x Fe 2 B 2 where x=1.0, 1.1, and 1.2 prepared by arc-melting followed by suction casting and annealing. The as-cast Al x Fe 2 B 2 alloys contain AlFe 2 B 2 as well as additional phases including the primary solidifying FeB and Al 13 Fe 4 compounds which are ferromagnetic and paramagnetic, respectively, at 300 K. The presence of these phases makes it difficult to extract the intrinsic magnetic properties of AlFe 2 B 2 phase. Annealing of Al x Fe 2 B 2 alloys at 1040°C for 3 days allows for reaction of the FeB with Al 13 Fe 4 to form the AlFe 2 B 2 phase, significantly reduces the amount of additional phases, and results in nearly pure AlFe 2 B2 phase as confirmed with XRD, magnetization, scanning electron microscopy, and electronic transport. The values of the magnetization, effective magnetic moment per Fe atom, specific heat capacity, electrical resistivity and Seebeck coefficient for the AlFe 2 B 2 compound have been established.

36 MATERIALS SCIENCE↗
NASA NTRS2015

Analysis of Advanced Thermoelectric Materials and Their Functional Limits

The world's demand for energy is increasing dramatically, but the best energy conversion systems operate at approximately 30% efficiency. One way to decrease energy loss is in the recovery of waste heat using thermoelectric (TE) generators. A TE generator is device that generates electricity by exploiting heat flow across a thermal gradient. The efficiency of a TE material for power generation and cooling is determined by the dimensionless Figure of Merit (ZT): ZT = S(exp. 2)sigmaT/κ: where S is the Seebeck coefficient, sigma is the electrical conductivity, T is the absolute temperature, and κ is the thermal conductivity. The parameters are not physically independent, but intrinsically coupled since they are a function of the transport properties of electrons. Traditional research on TE materials has focused on synthesizing bulk semiconductor-type materials that have low thermal conductivity and high electrical conductivity affording ZT values of 1. The optimization of the σ/κ ratio is difficult to achieve using current material formats, as these material constants are complementary. Recent areas of research are focusing on using nanostructural artifacts that introduce specific dislocations and boundary conditions that scatter the phonons. This disrupts the physical link between thermal (phonon) and electrical (electron) transport. The result is that κ is decreased without decreasing σ. These material formats give ZT values of up to 2 which represent approximately 18% energy gain from waste heat recovery. The next challenge in developing the next generation of TE materials with superior performance is to tailor the interconnected thermoelectric physical parameters of the material system. In order to approach this problem, the fundamental physics of each parameter S, sigma, and κ need to be physically understood in their context of electron/phonon interaction for the construction of new high ZT thermoelectric devices. Is it possible to overcome the physical limit imposed by of the effect of phonon lattice oscillation and energetic electrons towards thermal conductivity? Is the Seebeck coefficient, based on the difference in voltage over temperature gradient ( deltaV/deltaT), an intrinsic parameter of each material? All these parameters were manipulated using nano-bridge and twin-lattice structural concepts at the NASA Langley Research Center. This talk will review the current trend of TE research to optimize the ZT and discuss about new approaches on increasing ZT within functional limits of each parameter.

Kim, Hyun Jung↗
DOE OSTI2012

Effects of Se substitution on the thermoelectric performance of n-type Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x} skutterudites

Highlights: ► The simple solid state reaction technique was employed to prepare Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x} skutterudites. ► The thermal conductivity decreases gradually with the increasing Se content. ► Doping with moderate Se is an effective way to enhance the thermoelectric performance of Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x}. ► The highest ZT of 1.11 at 800 K is obtained for the Co{sub 4}Sb{sub 11.3}Te{sub 0.58}Se{sub 0.12} sample. -- Abstract: A series of double-substituted Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x} skutterudites have been fabricated by combining the solid state reaction and the spark plasma sintering method, and the effects of Se substitution on the thermoelectric properties are characterized by measurements of the electrical conductivity, the Seebeck coefficient and the thermal conductivity in the temperature range of 300–800 K. Doping Se into the Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x} matrix suppresses the carrier concentration, and the electrical conductivity actually decreases with the Se content. However, moderate Se doping is effective in enhancing the thermoelectric performance of the n-type Co{sub 4}Sb{sub 11.3}Te{sub 0.7−x}Se{sub x}, because of the resulted dramatically decreased thermal conductivity. Analyses indicate that the heightened point-defect scattering induced by Se doping together with the electron–phonon scattering induced by Te doping is responsible for the reduction of lattice thermal conductivity of these compounds.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Chemical intuition for high thermoelectric performance in monolayer black phosphorus, α-arsenene and aW-antimonene

Identifying materials with intrinsically high thermoelectric performance remains a challenge even with the aid of a high-throughput search. Here, using a chemically intuitive approach based on the bond-orbital theory, three anisotropic 2D group-V materials (monolayer black phosphorus, α-arsenene, and aW-antimonene) are identified as candidates for high thermoelectric energy conversion efficiency. Concepts, such as bond length, bond angle, and bond strength, are used to explain the trends in their electronic properties, such as the band gap and the effective mass. Our first principles calculations confirm that high carrier mobilities and large Seebeck coefficients can be obtained at the same time in these materials, due to complex Fermi surfaces originating from the anisotropic structures. An intuitive understanding of how the bonding character affects phonon transport is also provided with emphasis on the importance of bonding strength and bond anharmonicity. High thermoelectric performance is observed in these materials. In conclusion, our approach provides a powerful tool to identify new thermoelectric materials and evaluate their transport properties.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2017

Analysis of thermocapacitive effects in electric double layers under a size modified mean field theory

Thermodynamics of the electric double layer has received renewed interest for its potential applications in low-grade waste heat harvesting and reversible heating/cooling in supercapacitors. We apply a size-modified mean field theory to analytically capture the influence on the pseudo-Seebeck coefficient S = ∂φ 0 /∂T) σ of different factors, including the electrode potential φ 0 , asymmetry in ion sizes, and ion concentration, under a fixed electrode surface charge σ. The pseudo-Seebeck coefficient is predicted to scale as φ 0 /T at low electrode potentials, but it reaches limiting values when the electrode potential exceeds crossover values due to the steric effect. The qualitative behavior changes substantially, however, when the temperature dependence of the permittivity is taken into account. The pseudo-Seebeck coefficient S is then predicted to scale linearly with φ 0 even at high electrode potentials, significantly over-predicting the experimental values. Lastly, this suggests a strong influence of phenomena not captured in the mean field theory, such as deviation of local effective permittivity from the bulk value, thermally facilitated adsorption or desorption of ions on electrode surfaces, and weakening of ionic associations with temperature.

42 ENGINEERING↗
DOE OSTI2019

High Thermoelectric Power Factor and ZT in TbAs:InGaAs Epitaxial Nanocomposite Material

Lanthanide monopnictide (Ln-V) nanoparticles embedded within III–V semiconductors, specifically in In 0.53 Ga 0.47 As, are interesting for thermoelectric applications. The electrical conductivity, Seebeck coefficient, and power factor of co-deposited TbAs:InGaAs over the temperature range of 300–700 K are reported. Using Boltzmann transport theory, it is shown that TbAs nanoparticles in InGaAs matrix give rise to an improved Seebeck coefficient due to an increase in scattering, such as ionized impurity scattering. TbAs nanoparticles act as electron donors in the InGaAs matrix while having minimal effects on electron mobility, and maintain high electrical conductivity. There is further evidence that TbAs nanoparticles act as energy dependent electron scattering sites, contributing to an increased Seebeck coefficient at high temperature. These results show that TbAs:InGaAs nanocomposite thinfilms containing low concentrations, specifically 0.78% TbAs:InGaAs, display high electrical conductivity, reduced thermal conductivity, improved Seebeck coefficient, and demonstrated ZT of power factors as high as 7.1 × 10 -3 W K -2 m -1 and ZT as high as 1.6 at 650 K.

36 MATERIALS SCIENCE↗
DOE OSTI2016

First-principles calculations of thermoelectric properties of TiN/MgO superlattices: The route for an enhancement of thermoelectric effects in artificial nanostructures

We present the thermoelectric properties of TiN/MgO superlattices employing first-principles calculation techniques. The Seebeck coefficients, the electrical conductances, the thermal conductances, and the figure of merit are investigated employing electrical and thermal transport calculations based on density functional theory combined with the nonequilibrium Green's function and nonequilibrium molecular dynamics simulation methods. The TiN/MgO superlattices with a small lattice mismatch at the interfaces are ideal systems to study the way for an enhancement of thermoelectric properties in artificial nanostructures. We find that the interfacial scattering between the two materials in the metal/insulator superlattices causes the electrical conductance to change rapidly, which enhances the Seebeck coefficient significantly. We show that the figure of merit for the artificial superlattice nanostructures has a much larger value compared with that of the bulk material and changes drastically with the superlattice configurations at the atomistic level.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Photocurrent in carbon nanotube bundle: Graded Seebeck coefficient phenomenon

Here, polarized photovoltage of a suspended aligned carbon nanotube (CNT) bundle under uniform optical irradiation is discovered without additional structural modification or bias voltage. Such a phenomenon is very surprising considering the metallic behavior of the overall bundle and zero temperature difference between ends. The photovoltage characteristic time is found similar to the thermal response time under step Joule heating and implies a relation to the thermal behavior of the CNT bundle. A similar thermoelectric voltage is also observed during step Joule heating. Localized laser heating and scanning along the axial direction of the bundle uncovers a linear spatial variation of the local Seebeck coefficient. The Seebeck coefficient linearly decreases from root to tip of the CNT bundle with a rate of a few - µV∙K –1 ·mm –1 . Deep investigation in both the microscopic and macroscopic structures of the CNT bundle reveals that the local alignment of CNT assemblies rather than the minor defects in individual CNTs brings about this linear distribution of Seebeck coefficient in space. The finding presents a new way for direct photon-to-electric energy conversion via Seebeck coefficient grading in CNT structures.

42 ENGINEERING↗
DOE OSTI2013

Electrical property measurements of Cr-N codoped TiO2 epitaxial thin films grown by pulsed laser deposition

The temperature dependent resistivity and thermo-electric power of Cr-N codoped TiO2 were compared with that of single element N and Cr doped and undoped TiO2 using epitaxial anatase thin films grown by pulsed laser deposition on (100) LaAlO3 substrates. The resistivity plots and especially the thermoelectric power data confirm that codoping is not a simple sum of single element doping. However, the negative sign of the Seebeck coefficient indicates electron dominated transport independent of doping. The narrowing distinction among the effects of different doping methods combined with increasing resistivity of the films with improving crystalline quality of TiO2 suggest that structural defects play a critical role in the doping process.

co-doped↗
DOE OSTI2020

Optical conductivity of the type-II Weyl semimetal TaIrTe 4

TaIrTe4 is an example of a candidate Weyl type-II semimetal with a minimal possible number of Weyl nodes. Four nodes are reported to exist a single plane in k-space. The existence of a conical dispersion linked to Weyl nodes has yet to be shown experimentally. Here we use optical spectroscopy as a probe of the band structure on a low-energy scale. Studying optical conductivity allows us to probe intraband and interband transitions with zero momentum. In TaIrTe 4 , we observe a narrow Drude contribution and an interband conductivity that may be consistent with a tilted linear band dispersion up to 40 meV. The interband conductivity allows us to establish the effective parameters of the conical dispersion; effective velocity v = 1.1 · 10 4 m/s and tilt γ = 0.37. The transport data, Seebeck and Hall coefficients, are qualitatively consistent with conical features in the band structure. Quantitative disagreement may be linked to the multiband nature of TaIrTe 4 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗